University of Nigeria Virtual Library

Serial No

Author 1

OBIKWELU, D. O. N.

Author 2

Author 3

Title

Introduction to Science and Engineering of Refractories

Keywords

Description

Introduction to Science and Engineering of Refractories

Category

Engineering

Publisher

Amdi-Niel Publishers Company

Publication Date

September, 2002

Signature

INTRODUCTION TO SCIENCE

AND

ENGINEERING OF REFRACTORIES

D.O.N. OBIKWELU B.Sc (Hons), M.Sc. Ph.D, FNMS, FNSE

INTRODUCTION TO SCIENCE AND ENGINEERING OF REFRACTORIES

Copyright O September, 2002.

DANIEL 0. N. OBlKWELU

All rights reserved

No part of this book may be reproduced, stored in retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise without t h e prior permission of the Author.

ISBN: 36646

Printed in Nigeria by: Emeh & Sons Press 43c Ekurede-ltsekiri Road Warri.

Publishers AMDI-NIEL PUBLISHING COMPANY 2 Ugbisi Close Orhuwhorun, Warri.

ABOUT THE AUTHOR

D.O.N. Obikwelu graduated from the University of Nigeria, Nsukka with Second Class Honours Upper Division in 1975 and became a lecturer in the Institute of Management and Technology, Enugu in 1977.

In the same year, he proceeded to the United States of America for further studies with a Federal Government Scholarship. He obtained M.S. with Merit Award from the Wayne State University, Detroit, Michigan in 1979 and earned his Ph.D from Michigan State University E. Lansing Michigan in 1982 with excellent academic records. As a result of his excellent performance he was elected into the famous Sigma Xi Honour Society - The Scientific Research Society founded in 1886 by eminent scientists including Albert Einstein. He won the Amax Cash Award from Wayne State University as the best MS graduand in his set and worked on a research project with the America Navy Fellowship Award in Michigan State University. He was an Instructor in Engineering Communications and Strength of Materials Laboratory in Michigan State University.

He was employed by the then Anambra State University of Technology in the Department of Metallurgical and Materials Engineering. In 1984 he joined the Delta Steel Integrated Company at Ovwian - Aladja, Delta State as a Principal EngineerIAsst. Chief Engineer (Refractories) and rose to the post of Chief Engineer in 1988. He disengaged from the Delta Steel Company Limited after 12 years of varying industrial experiences and became a Senior Lecturer in the Department of Metallurgical and Materials Engineering, Federal University of Technology, Owerri. He was a Senior lecturer in the Department of Mechanical Engineering, University of Port Harcourt. Currently, he works with the SHELL Petroleum Development Company, Nigeria Ltd.

He has published many research papers in the Proceedings of the Nigerian Metallurgical Society proceedings and German International Conference Proceedings. He is the author of Differential Equations and Applications.

He is a Fellow of The Nigerian Metallurgical Society. Dr. D.O.N. Obikwelu is married with children.

PREFACE

This text on Refractories is written for practising Engineers, University students and other technical people in the high temperature industries. The main target is the practical man in the Iron and Steel, Petroleum, Gas, Cement and other industries where operating temperatures are above 200°C.

The text equips all those involved in furnace maintenance, refractory handling and storage with the basic knowledge of the Science and Engineering of refractories. Knowledge of elementary chemistry and mathematics is assumed.

It covers all types of refractories and their industrial applications, preparation of refractories and the basic manufacturing processes, tools and equipment for furnace maintenance, scaffolding, platforms and arches in furnace maintenance, handling, packaging, transportation and storage of refractory materials, specific applications to various geometrical shapes and appendices on the location of potential refractory clay deposits in Nigeria, key numbers for handling purposes, expansion joint dimensions, standards for testing refractory materials and applications of refractories in some industries.

Science and Engineering of Refractories formed the course material, the author delivered to the Technical Management and Staff of Warri Refining and Petrochemical Company (WRPC) in Warri, Delta Glass Company, Ughelli, and University undergraduates.

The author combines his many years experience in the use of refractory materials in the Industry with his teaching ability to endear practitioners in refractories handling and application to the Science and Engineering of refractory materials.

D.O. N. OBlKWELU

ACKNOWLEDGEMENT

The author is indebted to the staff of Warri Refining and Petrochemical Company (WRPC) and Delta Glass Company Ltd Ughelli, who participated in the Short Course on Refractories Engineering, Handling and Furnace Maintenance, for their useful contributions.

I also wish to thank Messrs Onodu, Panama and Engr. M. 0 Akudolu and the staff of the Unified Refractories Department of Delta Steel Company Ltd., Ovwian-Aladja for their support.

I especially wish to express my immense gratitude to the following companies for their support: The SHELL Petroleum Development Company of Nigeria Ltd., Warri Refining and Petrochemical Company Ltd., Delta Glass Company Ltd., and Delta Steel Company Ltd.

CONTENTS

INTRODUCTION CHAPTER 1

CHAPTER 2

CHAPTER 3

CHAPTER 4

CHAPTER 5

CHAPTER 6 CHAPTER 7

CHAPTER 8

CHAPTER 9

CHAPTER 10

CHAPTER 11

CHAPTER 12

CHAPTER 13

Classification of Refractory Materials Service Properties of Refractory Materials Properties of Refractory Materials Siliceous and Alumino-Silicate Refractories and lndustrial Applications Magnesite Refractories and lndustrial Applications Carbon, Zirconia and Chromite Refractories and lndustrial Applications Refractories for the Glass Industry Refractory Concretes, Pastes and Mortars and lndustrial Applications Special Refractory Bricks and Refractory Insulating Materials Preparation of Refractories and Basic Manufacturing Processes Tools and Equipment for Furnace Maintenance Scafolding, Platforms and Arches in Furnace Maintenance Handling, Packaging, Transportation and Storage of Refractory Materials Refractory Materials: Specific Applications References Appendix Index

I DEDICATION

This book is dedicated to Victoria, Nwanne,

I Kene, Emeka, Chi and Azu

INTRODUCTION

Texts on "Refractory Materials" are in most cases too theoretical, too simplified or simply a collection of case studies from in-plant experiences acquired over the years in handling refractories and furnace materials or briefs on advertisement catalogues by manufacturers of refractory materials or a collection of research studies on inorganic oxide systems. As a result there is no text on refractory materials that can be adjudged complete in itself, in furnishing in one volume necessary information that satisfies the interests of the shop floor man, the supervisor, the middle management staff, the management staff, policy makers in a high temperature industry and UniversityIPolytechnic Students on refractory materials.

In this text many years of experience in a high temperature industry and teaching in the University and Polytechnics ionverge to produce a text that will attempt to achieve this objective.

It is believed that first refractory bricks were made of ceramic clay because it is plastic and very easy to work, and permanently shaped parts could be produced from it by firing. Refractory materials were used early in history by the Phonicians, or the Chinese and Nigerians (the NOK culture) for the smelting of ores.

A material is refractory if it has a very high melting point in addition to having properties (physical, chemical, mechanical, thermal etc), that make it suitable for uss in furnaces, kilns, reactors and in any other form of high temperature vessel. (Table I). These properties will make the material easily applicable to the vessel so that the material sustain as many campaigns

or production cycles as possible without breaking down to make the use of the materials highly economical. Thus refractories are heat resistant materials.

All vessels containing charges to be subjected to temperatures in excess of 200°C must be internally lined with the appropriate refractories to prevent the vessel shell, mainly steel from undue expansion and melting.

TABLE 1: MELTING POINT AND OTHER PROPERTIES OF REFRACTORY MATERIALS

Refractory Type

Quartz Cristobalite Tridymite

Bauxite Corundum

Chromic oxide

Refractory materials are therefore used in high temperature processes because of their heat resistant properties and stability at high temperatures. They constitute the backbone of Petroleum, Iron and Steel, Glass and Cement Industries.

Chromite Kaolinite Forsterite

Chemical Formula

Si02 SiOz Si02

AI2O3.2H20 AI2O3

Cr203

Fe0.Cr203 AI2O3.2SiO2.2H20 Mg2 SiO,

Spe~ific Gravity

2.65 2.32 2.26 - 2.28 2.55 3.95- 4.10 5.2

Melting or Dissociation

Point (OC) 171 0 171 0 1910

- 2050

21 50-2430

4.32-4.57 2.60-2.63 3.191-3.33

21 80 - 1890

CHAPTER ONE

CLASSIFICATION OF REFRACTORY MATERIALS: I ' a ' Refractory materials are classified in various

ways, namely: - (i) , By their properties and characteristics ,

. (ii) By their chemical and mineralogical compositions

(iii) By the chemical property of the refractory oxide

Experience with the use of refractories show that none of these ways of classification is complete by

: itself. Classification of refractories is as follows: r

. (a) By properties like refractoriness (to be defined later): - (i) Common refractoriness (1 580 - 1 770°C) (ii) High reiractoriness (1 770 - 2000°C)

- (iii) Highest refractoriness (above 2000°C)

Table 2: shows this type of classification.

TABLE 2 : CLASSIFICATION BY REFRACTORINESS:

Refractory Type

Silica Fire Clay Chromic Oxide Clay-graphite Zircon Magnesite Chrome-magnesite Coke

Refractoriness (OC) -

1,700 1,730 1,700 1,900 1,900 - 2000 2,000 2,000 2,500

(ii)

(iii)

(v i i)

(viii)

Ox)

By chemical and mineralogical composition: Siliceous in which the refractory base is Si02 - the quartzites, sandstone, mica sheet, siliceous fireclays of at least 75% Si02. Alumino-Silicate in which refractory cz~mponents are AI2O3 and SiO2.AI2O3 (15 - 48%), S O 2 and other oxide, the rest fireclay, semi-acid refractories are in this group. Alumina in which the basic refractory oxide, A1203 is more than 48%. Bauxite, Diaspore, Sillimanite, Kyanite (Cyanite), andalusite, tubular and fused alumina are in this group Magnesium-Silicate g rmp of formula 2R.Si02 in which R may be MgO, CaO, FeO or MnO. They are mainly Olivines .(mixture of forsterite) (2Mg0.Si02) and fayalite (2Fe0.Si02), Serpentines (2Mg0.2Si02.2H20). The refractory product in this group is usually forsterite. Magnesia-Lime group in which the refractory base is MgO of at least 80% content. In this group are natural magnesite, MgC03, Magnesia, MgO, brucite, dolomite (CaCo3.MgC03); Chromite group in which the refractory base is .' Cr203 occurring ' as spinel R0.R203' (Fe0.Mg0.Cr203). Chromite-chrome-magnesite, magnesite chrome are in this group. Carbon in which the base is Carbon. Coke, graphite, coal, tar refractories are in this group. Carbide refractories contain refractory compounds of metal carbide MeC - type. Oxide refractories are composed mainly of pure oxides - A1203, MgO, BeO, etc.

(c) By the chemical property of the refractory oxide: In this classification all refractories are divided into - (ii) Acid - Si02 base

4 *' (iii) Inert or neutral - AI2O3 base

(iv) Basic - MgO, CaO base.

With these classifications, the Refractories user is able to list all refractory materials in his plant and classify them properly.

QUESTIONS : 1. ' What do you understand by the term "refractory

material"? 2. List all the refractory materials used in a named

plant and classify them properly according to only one of the four groups of refractories.

3. If you are not able to classify satisfactorily all the refractory materials, state the possible reasons.

4. What, in your mind, should be done to facilitate satisfactory classification of all the refractory materials in a named plant.

CHAPTER TWO

SERVICE PROPERTIES OF REFRACTORY MATERIALS

The life and efficiency of a furnace plant largely depend on the suitable choice, the quality and the correct installation of the refractory furnace material. All these parameters depend on the peculiar properties of refractory materials. In this chapter these properties which cut across all types of refractory materials are discussed to facilitate the understanding of subsequent Chapters.

2.2 PROPERTIES OF REFRACTORY MATERIALS: Important service properties of refractory materials with the relevant service conditions in Industrial furnaces, reactors, kilns are shown in Table 3.

TABLE 3: (after Didier): Service Conditions and Properties of Refractory Materials

-- T v ~ o f Condition

- A

i) Thermal Service Properties Refractoriness, Refractoriness Under Load, Softening Under Load, Creep Strength, Hot transverse Strength, Thermal expansion, Permanent linear Change (after shrinkage and after expansion),

I Crushing strength, Abrasion

(i) Refractoriness: This property of a refractory material is one of

the most important factors in the application of refractory materials. As refractory materials are a conglomerate of inorganic oxides, a specific melting point is not of practical importance rather "a softening region" is defined. This region is determined with the help of comparative ceramic specimens of known softening behaviour, seger cones using truncated triangular pyramids-pyroscopes cut or shaped from the particular refractory material to be tested. Details of the technique for measuring this property are specified in German Standards publication DIN 51 063 and American standards 'for testing materials ASTM C24 (See Appendix IV).

Refractoriness is therefore the ability of the refractory material to retain its mechanical strength at high temperature with no load applied. This property is represented with a temperature at which the material "softens and thus crumbles" and hence not fit for use. Thus refractoriness determines the possibility of using a

iv) Chemical

refractory material under specific temperature condition. It depends on the chemical composition of the refractory material and amount of impurities in the

resistance, modulus of rupture and modulus of deformation, Porosity and true density. Chemical composition, mineral structure and crystal formation, pore size distribution and pore type, gas permeability, resistance to slag, glass flow, gas and vapours.

material. In real application, refractories break down at temperatures below their refractoriness mainly due to the reactions between the refractories and slags and also dust or less frequently due to insufficient thermal stability and low mechanical strength resulting from perhaps poor storage, shelf life or effect of the climatic conditions in the area. For example silica refractory has refractoriness of 1710°C to 1750°C but begins to break down in furnaces at 1200" to 1300°C. For that reason refractoriness index for a refractory material cannot be the only parameter for determining the suitability of a refractory material for application in furnaces or other furnace elements.

(ii) Refractoriness UnderLoad (RUL): This property is significant because most

refractory applications involve constant or -varying stresses being subjected to the refractory material. Each refractory material especially bricks has a characteristic softening curve as shown in fig. 1

5 %

Fig. 1: Refractoriness - Under Load curves 1 Fireclay 2. Sillimanite 3. Chrome-magnesia 4. Silica (crown) 5. Silica (coke oven) 6. Magnesia - chrome

Details of the method of determining RUL is described in DIN 51 064, The initial temperature (t)

; indicating the onset of softening is indicated by the :i intersection of the curve and a horizontal line which is

3mm below the end point of the curve corresponding to about 0.6% compression. The final temperature (tJ10) is reached when the curve has dropped 10mm below the end point. Should the sample collapse, the final temperature is given as tb (breakage temperature). As RUL is measured in a reducing atmosphere because of the furnace conditions, it does not give an accurate indication of the application limit but only an approximation. Softening under load test done in an oxidizing atmosphere, according to DIN 51 053, is done to eliminate errors as a result of the inherent expansion of the test equipment when RUL is tested. Initial temperatures (t,) are listed below in Table 4.

TABLE 4: 1, POINTS FOR SOME REFRACTORY MATERIALS

Brick Type Fireclay brick corundum brick (approx. 65% AI2O3 II 85% " 11 99% "

Mullite brick approx. 72% AI2O3 Sillimanite brick approx. 65% AI2O3 Silica for coke oven Magnesia brick, low-iron C2S - bonded Dolomite brick Chromite brick Chrome-magnesia brick Direct-bonded chromite-magnesia Silicon Carbide (85 - 90%)

Zircon Silicate Fused Silica not devitrified devitrified to cristobalite Fused mullite

4400 >A700 21750 no softening

i i i) CREEP IN COMPRESSION: This is the measure of the ability of a refractory

material to withstand compressive stresses at a particular temperature over a long period of time. Creep-in-compression phenomenon is common in the Petroleum, Glass and Steel industries. Creep in compression test is described in detail in DIN 51 053 and this property depends on the chemical composition, mode of preparation and other material properties of the refractory material.

iv) -HOT TRANSVERSE STRENGTH: Refractory materials should be resistant to

bending stresses to avoid deformation at high temperatures. Table 5 shows some refractory material types and their hot transverse strength (in ~ l m m ~ ) :

TABLE IV: HOT TRANSVERSE STRENGTH

Refractory type

Low-iron magnesia bricks High-iron magnesia bricks Direct-bonded chrome- magnesia bricks High alumina bricks (75% A1,0,\

Test Temperatures 1 2OO0C

14 12

10

25

1 400°C 11 5

5

18

1 400°C 8 - 12 0.5 - 1

3

7

v) THERMAL EXPANSION: All bodies experience a change in volume under

the influence of temperature. Fig. 2 shows the reversible linear expansion of some refractory bricks. The expansion curve of most refractory bricks are approximately straight but silica bricks depict irregular expansion curve and strong thermal expansion in the temperature range up 700°C.

By changes in structure or in firing methods, the curves can be modified to certain limits. Bricks with particularly high expansion such as magnesia bricks or irregular expansion within a certain temperature range such as silica bricks are very susceptible to thermal shock, caused by power fluctuation or outage.

In-practical applications this property is important as the effect of expansion, must be taken into account during the construction of large installations, if not edge pressure and premature spalling of the brick ends must be expected. Hence spacers are used in the lining of steel furnace vessels.

%

Fig. 2: thermal expansion of refractory bricks: 1. Magnesia 2. Chrome-magnesite

3. Chromite 4. Silica 5. Zirconia 6. Corundum 99% AI2O3 7. Corundum 90% AI2o3 8. Fireclay 9. Sillimanite 10. Zircon 11. Silicon Carbide

vi) PERMANENT LINEAR CHANGE: (After Shrinkage and After Expansion) In an operating plant, refractory materials are

subjected to high temperatures and during "shutting off t to "idling" state or fully down, subsequent cooling occurs. Linear changes often remain which are described as after-shrinkage. A brick with a very strong after-shrinkage shoule have enlarged joints, loose brick work that is not gas tight. A very strong after-expansion should be compensated to avoid the destruction of the, .

brickwork bond through pressure. The permanent linear change. of refractory materials can be influence by controllhg the burning of the raw material and the firing of the bricks so that an equilibrium is attained at the required temperature. Fireclay bricks and magnesia bricks, if they are underfired, have a tendency to after- shrinkage and silica bricks to after-expansion. The application limit of insulating bricks is mainly determined by the permanent linear change.

Vii) THERMAL SHOCK RESISTANCE OR THERMAL STABILITY Intermittent operation of the furnace subject

refractory materials to thermal shock leading to the spalling of the materials in layers or reduction of the strength of the brick structure and hence total fracture of the refractory materials. This important property is measured according to DIN 51 068 parts 1 and 2 - based on the water-quenching method and air-

quenching method. Listed below are . refractory material.

TABLE 6: THERMAL STABILITY REFRACTORY MATERIAL

test on some

OF SOME

Usually the thermal stability is finished when the sample has lost more than 20% of its mass. It is advised that parameters like thermal or temperature conductivity, thermal expansion and notched bar strength which can be determined easily can be used to provide better values for the behaviour of refractory bricks under thermal shook.

As thermal stability of refractory material often decreases with increasing firing levels, it is known that in practice that these products which retain their thermal stability even after higher firing temperature or operating temperature are most useful in practice.

: '

viii) THERMAL CONDUCTIVITY Thermal conductivity of refractory material

depends on temperature, chemical composition of the raw material for the refractory product, the mineralogical structure of the brick mix, true porosity and pore size, firing temperature and grain structure.

MATERIAL

Low alumina fireclay bricks Plastic pressed fireclay bricks Dry pressed fireclay bricks Corundum bricks Mullite bricks Silicon carbide bricks

NO. OF QUENCHING BEFORE COLLAPSE

> 5 > lo >15 >15 >20 >30

Fig 3 illustrates the variation of this property for various refractory materials. -

8. Magnesia-Chorome 9. Zircon Silica

14. Silicon Carbide 60 15. Silicon Carblde 90 16. Graphite compared to

200 600 1000°C

Fig. 3: Thermal conductivity of fired refractory bricks

Thermal conductivity of crystalline component like magnesia, chrome - magnesia, corundum and zircon depict negative temperaturelthermal conductivity gradient, while refractory materials with high proportion of amorphous or glassy phase have positive and small gradient - thus crystalline substances have higher thermal conductivity than amorphous ones. Thermal

Conductivity in refractories except in magnesite and forsterite increases with temperature but generally decreases with increasing porosity.

ix) SPECIFIC HEAT (C): This property of refractory materials depends on

temperature and the material. It defines the amount of energy required to raise the temperature of one gram of the refractory material by one degree Kelvin. Compared with that of water (C = 4.19), the specific heat of refractory material is about 0.25. Values of specific heats for refractory material are listed in Table 7.

SPECIFIC HEAT (KJ1Kg.k) OF VARIOUS REFRACTORY MATERIALS

TABLE 7: .. .

x) BULK DENSITY

;

The bulk density cf a refractory material is necessary in order to calculate the stored heat (thermal capacity) of a refractory material. Thermal capacity = (bulk density x specific heat). Bulk density = mass (including pore space)

volume

MATERIAL (BRICKS) Silica Fireclay Corundum (99% ,41203)

Zirconia Silicon Carbide Magnesia Chromite Forsterite

Using bulk density, one can calculate both true and apparent porosity.

Bulk density value for various refractory material are listed in Table 8: TABLE8: BULK DENSITY VALUES FOR SOME

REFRACTORY MATERIALS:

(BRICKS) DENSITY

Chrome- 3.0 - 3.2 magnesite

> ' . 3.0 - 3.2

Corundum (90% AI203)

MATERIAL I Fused Silica 1 2.0

Fireclay 1.9 -2.0' plastic pressed

Silica 1.8 -1.9

xi) THERMAL CAPACITY AND TEMPERATORE CONDUCTIVITY: The thermal capacity (bulk density x specific

heat) can be used to calculate the heat stored (W) in a square meter of a refectory furnace wall of thickness, S:

W = bulk density x spec ht x s [y - 0 , ) . .

Where: W = stored heat in KJ I~ ' S = wall thickness in m t j , = Inside temp. of furnace wall in O C

0, = external surface temp. of furnaces wall in OC 0, =air temp. in O C.

With intermittent heat flow e.g. in checker work, , not only the thermal conductivity of the refractory :i material is of interest but also the ratio of the thermal

conductivity to the thermal capacity which is important for the progression of the temperature changes and is described as Temperature Conductivity

- - Thermal conductivity x l o3 Bulk density x spec. heat

[&I ..... (2)

xii) CRUSHING STRENGTH

This property is a measure of the mechanical ' strength of the refractory materials with which the

material withstands compressive forces and mechanical wear in the furnace. A brick with high crushing strength is more resistant to impact from bars or during removal of slag or process deposit than refractory of low crushing strength. The crushing strength of dry pressed fireclay brick is generally higher than that of plastic pressed bricks because of the higher content of fireclay and the low porosity.

High firing temperatures improve crushing strength of refractory materials but realizing that vitrification accompanies higher firing temperature thermal shock resistance could be impaired. A high cold crushing strength does not always indicate poor thermal shock resistance.

Plastic pressed fireclay brick has a cold crushing strength of 10 - 15 N/mm2 at least, dry pressed firecla Y bricks 30 - 50 plus N/mm2, silica bricks 20 - 30 Nlmm , high-alumina and iron-rich magnesia bricks 30 - 150~/mm* and silicon carbide and high AI2O3 content refractories 200 plus N/mrn2. Insulating bricks with high

porosity as expected have low cold crushing strength of 2 - 10 ~ / m m ~ .

In order to evaluate the behaviour of .refractory materials at service temperatures, the hot crushing strength of refractory materials is sometimes determined according to Standard Tests as described for example in DIN 51 067.

xiii) ABRASION RESISTANCE Refractory bricks are not only subjected to

pressure but also to abrasive attack of the solid charge materials, fast moving gases with fine solid dust particles as they pass over the brickwork or rammed refractory masses in furnaces, shaft kilns, coke ovens, rotary kilns, combustion chambers, heaters, reactors. Listed below in Table 9 are abrasion resistance and resistance to sandblasting.

The sandblast test simulates the impinging effect of solid particles in gas on refractory bricks and masses. The abrasion factor is affected much when the refractory surface has been altered by chemical influences in the vessel. a

TABLE 9: ABRASION RESISTANCE VALUES OF '

SOME REFRACTORY MATERIALS

b

Acid Resistant Brick

2.15 c 16

> 80

0.1 0

I 'Bulk Density True porosity 1 Cold Crushinq Strength Nlmm Abrasion factor cm3/cm2 Sandbiact T A I - - . - -

Plastic Pressed Fireclay Brick

1.95 < 28

> 20

0.81 .

Silica Brick Dense, coarse Grained

1.87 c 23

> 3tj

0.66 .-

DV Pressed Fireclay Brick

2.15 c 21

> 30

0.40

xiv) POROSITY AND DENSITY Low porosity in refractory products improves

mechanical strength and other properties of refractory materials except in the case of insulating bricks which are specially made with high porosity for a function.

True Porosity = Total pore space (open and sealed pores) Volume

and True Porosity = Density - bulk density X 100% Density

The density is the quotient of mass and volume less pore space. The life of refractories is largely dependent on their porosity which may vary from 1 Oh in

. melted refractories to 80% in ins.ulating refractory materials.

At a higher apparent porosity, a refractory material is less resistant td the eroding action of slag and solid particles which can penetrate through pores inside the materials.

The apparent porosity includes only those pores, which can be infiltrated by water and not the sealed pores. For most fireclay bricks the difference between true porosity and apparent porosity is about 20%. In general, basic bricks have sealed porosity of less than 1%. In highly sintered bricks and acid - resistant bricks, the difference between true porosity and apparent porosity can be considerably higher, porosity measurements are described in the German Standards, DIN 51 057.

xv) CHEMICAL COMPOSITION The chemical composition of refractory is of

great importance with respect to attack by slag, glass melt, flue dust and vapour. When there is limited rate

of chemical reaction between the refractory material and the slag or flue gases in a vessel, the refractory material is more resistant. Therefore where acid slag or vapour is expected acid bricks are used and basic bricks where basic slag or vapour is expected.

In addition to the chemical reaction between refractory materials and foreign substances in the furnace atmosphere "contact reactions" also occur between refractory materials of different chemical composition. 'Contact reactions" are serious at operating temperatures above 1600°C. To avoid these reactions at high operating temperatures refractory materials are differentiated into three groups, viz:

Acid Group Basic Group Inert Group Fused Silica 99% SiOz Dolomite bricks Carbon bricks Silica bricks Magnesia bricks High alumina bricks 94 -97% SiOz (72 - 100% AlzO,) Silicon Carbide bricks Chrome-magnesia

bricks Zircon cristobalite Zircon silicate bricks Forsterite bricks Chromite bricks

Refractories in each group used alone can withstand temperatures of 1600°C and above without strong contact reaction occurring. The bricks high up in each group are characteristic of each group while those listed last are transitional and exhibit certain variation in chemical behaviour. Bricks in the inert group can be heated to 1600°C in contact with both acid and basic group.

In high AI2O3 refractories, only solid phases namely corundum and mullite occur in the pure system up to 1840°C. Above 1840°C, mullite forms a melting phase whilst corundum remains solid up to about 2000°C, thus Corundum could exist in contact with acidic or basic refractory materials unless liquid slag or

aggressive gases are present to accelerate contact reactions.

xvi) PORE SIZE DISTRIBUTION AND TYPES OF PORES The mechanical strength of refractory materials

is largely determined by the porosity. Other important properties, in particular the behaviour during chemical attack by slag, glass melt, gas, vapour are influenced by the size, shape, number and distribution of pores.

The true porosity of a refractory brick is composed of sealed pores and open pores, the latter being permeable or impermeable. Apparent porosity is-

.' composed of the open pores and closed pores ,. compared to the total volume of the article. The content

of open pores is calculated from the water absorption and by using a water-air di5placement method; the open pores are classified as permeable or effective and impermeable pores.

xvii) RESISTANCE TO SLAG, GLASS MELT, GAS AND VAPOURS In the Steel, Glass and Oil Cracking plants,

. destructive agents come into contact with refractory materials and render the refractory materials unserviceable. Resistance of refractory materials to these destructive agents can be simulated in order to make predictions as to the suitability of brick types for certain applications.

Brick destruction is hot only caused by liquid slag and glass melts but also by gases, vapour as illustrated below: - at 400' - 500°C, fireclays contain iron oxide, by

catalytic action of iron oxide the carbon monoxide is decomposed and carbon is deposited inside brick

with the consequent weakening effect and subsequent failure of the brick structure.

- above 900°C, methane in the presence of iron oxide deposits carbon on the brick and weakening the brick structure.

- at about 1000°C: and above, reducing and oxidizing atmosphere influence the chrome variable oxidation state. This influence weakens the chrome bearing refractories, destroying its brick structure.

- in an oxidizing atmosphere containing steam at 900 -1 100°C silicon carbide products are destroyed.

- alkalis in vapour form affect the refractoriness and sintering of atter shrinkage bricks.

- in glass industries, regenerators, melting furnaces, etc: fireclay refractories absorb Na20 or K20 from alkali atmosphere to form low-melting point compounds causing thin layers to spall off because of the increase in volume.

- in oil cracking plants fireclay, sillimanite and silica bricks break down because of the incursion into their structure by V2O5 and Na2S04 from some oil ashes.

Refractory materials containing free CaO can disintegrate completely even before installation, - because of the steam in the air which hydrates CaO to' Ca(OH)2 with increase in volume. In magnesia without free CaO such hydration occurs In an atmosphere containing steam to form Mg(OH)2 at a temperature in which Mg(OH)2 cannot decompose.

Subsequent chapters will deal in detail w;th various refractory classes.

QUESTIONS

1. List the key service properties of refractory materials and specify the properties expected in a named refractory material for a reactor whose service temperature is 1400°C in an acidic medium with flue gases ladened with particles.

2. State the major causes of cracks and refractory failure in the Oil cracking and Glass industries.

CHAPTER THREE

SILICEOUS AND ALUMINO-SILICATE REFRACTORIES AND INDUSTRIAL APPLICATIONS

In the classification of refractories in Chapter One, siliceous refractories are Si02 base of at least 75% Si02 content while the alumino-silicates contain (15 - 48%) AI2O3 and the rest Si02 and the refractory oxides. (I) Siliceous refractory materials are the most

abundant on the earth's surface in its raw state. They are made up of quartzites containing at least 98% silica (Si02). Silica may exist in three modifications namely - quartz, tridymite and cristobalite which transform into each other depending on temperature and other conditions. Each of these modification has an o~ (high temperature) and p (low temperature) form which can transform into each other, Fig. 4.

These transformations involve volume changes which should be considered in the manufacture and use of silica bricks. Silica brick should mainly contain tridymite as this modification gives the least volume changes. Volume changes during transformation of quartz to tridymite, cristobalite and fused silica are listed in Table 10.

p - Cristobalite

IS@ - 270 '~ ? 2.8%

CC - Crlstoballte

573Oc

p - ~uar tz ,~ - 7.0%

a - Trldymlte

$ - Trldymite

I I 117% + 0.2%

6- Tridymite

Fig. 4: Polymorphic transformation in Silica with positive and negative volume changes

Table 10: VOLUME CHANGES DURING TRANSFORMATION FROM QUARTZ TO TRIDYMITE, CRISTOBALITE AND FUSED SILICA

This volume effect can cause considerable stress and pressure in the brick structure and in the brick work. Manufacturers of silica base refractory bricks should take this property into consideration in the firing schedule and manufacture of this type of bricks.

The general diagram for the manufacture of silica refractories is illustrated in Fig. 4. In most cases edge mills are used for mixing and special bonding agents generally 2% slaked lime in liquid form (lime wash) and some sulphite solution as a binder are added at the same time. The friable mix is shaped on screw presses, rotary table presses or hydraulic presses, complicated shapes are rammed by hand. Drying takes only a short time, one to two days, as lime-bonded bricks are not affected by drying.

Silica bricks are fired at temperature about 1450 '~ with longer holding time at highest temperatures, preferable in annular kilns. As transformation in silica take place erratically, cooling of finished products should be carried out slowly or the bricks can shatter.

Modification

Quartz Tridymite Cristobalite Fused Silica

Specific Volume

0.377 0.442 0.431 0.454

True Density

2.65 2.26 2.32 2.20

Volume Increase in % - + 17.2 +14.3 +20.4

The reversible thermal expansion depends also on the mineral composition. As shown in Fig. 4, tridymite and cristobalite do not expand uniformly during heating but exhibit erratic changes in length occurring suddenly during heating and cooling.

Quartz shows such transformation at 573'~, tridymite at 115'~, and cristobalite between 2 2 5 ' ~ and 270'~. Well-transformed silica bricks contain little or no residual quartz thus their behaviour with temperature largely depends on the ratio of cristobalite to tridymite.

During heating up, silica bricks expand rapidly with the total reversible expansion being completed at 800 - 1 OOO'C, Fig. 5b.

0 200 400 600 800 '1000 oc

Fig. 5a: Thermal expansion of various Silica modification

Fig. 5b: Thermal expansion of an incompletely (I) and a completely (2) Transformed Silica Brick at rate of 2 - 5' per min.

Therefore silica bricks are resistant to temperature variations above 800°C (red heat) but very susceptible below this temperature because of the erratic volume expansion. For this reason, sufficient time must be allowed for heating furnaces up to about 800°C. Glass tank furnaces with a silica crown are generally heated up within 8 - 18 days, the individual heating-up periods depending on the size of the furnace.

Silica bricks and masses are used mainly in the following fields of application - - Glass melting furnaces - Coke and gas producing plant - Hot blast stoves.

At temperatures around 1700°C, the working surface of bricks in the roof lining of silica-base roof bricks becomes covered with melting dust and splashes of slag which contain much of iron and calcium oxides. Calcium oxide reacts with Si02 of the silica to form free running melt. Part of this melt drops down and the other penetrates into the brick and reacts with silica.

After 15 to 20 campaigns of normal operation four zones form in the silica brick (beginning from the working surface): grey, black, brown and invariable. The grey zone is denser and is composed of mainly cristobalite, with a small amount of glasslike mass and magnetite. The extent of the grey zone determines the durability of the bricks, the deeper this zone the longer the life of the bricks. The black zone contains predominantly tridymite, it has greater amount of fluxes than the grey zone and its refractoriness is.lower than that of the core.

The brown zone contains tridymite sand a small amount of glass-like mass and iron oxides. During operation the zones shift gradually deeper and the core becomes thinner. Silica in regenerator checker work is worn off mainly due to sweating as the melting dust forms a fusible nielt on the brick surface. Silica brick luckily retains its mechanical strength even after its volume has reduced appreciably due to melting.

(11) Alumino-Silicate Refractories: While silica bricks consist largely of single oxide

component, alumino-silicate refractories are made up of AI2O3 (1 5 - 48%) silica (not more than 65%). Depending on the content of Si02 and AI2O3 (Fig. 6) alumino-

silicate refractories are classed as semi-acid (1 5 - 30% AI2O3), fireclay (30 - 45% AI203) and high alumina type (over 45% AI2O3) and the rest silica.

Fireclay Refractories Fireclay refractories are prepared from refractory

clays, Kaolins (China clay) AI2O3.2SiO2.2H20 and varying amounts of Fe203, CaO, M ~ O and alkalies

1. SiO* + melt 2. Melt 3. Mullite + melt 4. Corundum +;melt 5. Mullite + melt 6. Corundum + melt 7. SiO, + melt 8. Mullite

0 10 20 30 40 50 60 70 80 90 100 SiO, Weight % AI,O, AI,O,

Fig. 6: Si02 - A1203 Phase Diagram

The total quantity of these fluxing agents which lower the melting point should be at most 5 - 6%. The most important characteristic property of clays is plasticity, binding capacity and sintering capacity. The plasticity is due to the fact that clay particles when moistened with water are enveloped by a liquid film having a great surface tension and high viscosity, this ensures that particles can slip relative to one another and at the same time the bonding between them is retained. Refractory clay shrinks substantially on

roasting which results in cracking. To reduce this contraction and obtain a firebrick of accurate size and shape the clay must be made leaner with prefired, volume-stable clay i.e. fireclay. Only clay-containing sand which has many lean components, naturally can be shaped directly and used in an unfired condition. Roasted clay is called f~reclay or chamotte.

Clays lose their plasticity on drying at 1 10°C but regains this property on addition of water, The plasticity of clays lowers with increasing temperature and disappears entirely on heating above 450°C. When heated up to 800°C clays are sinterec! into a stone like grog. W ~ t h further heating, the clay grog softens owing to the appearance of liquid phase in it. The alumina of the grog recrystallizes from y-form to a-form with the' formation of mullite 3AI2O3.2SiO2 whose grains grow coarsec at 1200°C and form a splice.

After firing in the manufacturing plant, the fireclay bricks consist of mullite, cristobalite, residual quartz and glass. As with many other products manufactured according to heavy clay ceramic methods, the mineral components are present in unbalanced proportions. Only after the fireclay bricks have been installed in an industrial furnace can a balance be expected in the brick zone facing the flame. With rising temperature and longer period under the effect of temperature, the mullite content of cristoSalite and quartz is reduced rapidly and disappears totally above 1400 - 1500°C. The fireclay bricks now consist only of mullite and a viscous glass which in addition to silica and some AI2O3 comprises in particular alkalies and other fluxing agents.

In fireclay bricks of plastic refractory clay the amount of glassy phase after firing at 1400°C is about

40 - 55%. This can be reduced by using clay which is - low in fluxing agents and low in alkali.

The application range for fireclay refractories is as follows:

Furnace construction Glass industries Foundry furnaces Indurating furnaces Reformer boxes Coke oven and gas plants Reformed gas boilers Chimneys Steam boilers, etc.

High Chamotte Refractories These are derived from a mass containing 80 to

95% chamotte (fireclay) and 20 to 5% refractory clay. Glues and electrolytes are added to the mixture to increase the binding capacity. They have high compressive strength (298 KN/cm2), low porosity and high thermal stability (up to 100 cycles).

Semi-Acid Refractories These are made from clays containing 15 to 30%

A1203 with leaning additions of chamotte and quartz sand. Semi-acid refractories have lower refractoriness and high density than fireclay refractories and a low shrinkage of only (1 - 1.5%).

(Ill) High Alumina Refractories High alumina refractories are those containing

more than 45% AI2O3. Raw materials for their manufacture are andalusite (A1203.Si02), Sillimanite (AI2O3.SiO2), cyanite (AI2O3.SiO2), diaspore

(A1203. nH20), baux~te (A1203. nH20), hydrargillite (AI2o3.3ti20), and corundum (AI203). Synthetic raw materials include sintered mullite, '~sed mullite, calcined alumina, sintered corundbm and fused corundum.

In the manufacture of high alumina refractories pure kaoiins and organic additives are used as binders and preliminariiy roasted materials (grog). The thermal stability of high alumina refractories is quite high, 160 cycles, the temperature of deformation under load is also qcite high (1500 - 1600°C). They are stable to the action of slags at temperatures below 1300°C and to the action of acidic slags above that temperature. Sillimanite bricks without high lime and iron oxide combine -this slag resistance with stability to -frequent temperature shocks.

Although the minerals cyanite. sillimanite and andalusite have the same chemical formula, AI2O3.SiO2, their crystal structure differs widely according to their formation and origin. Their properties, for example, density depend on their mineralogical origin. During firing these minerals change into mullite and liquid phase at different temperatures.

Fused Mullite articles are made by charging a batch of sillimanite, coke and steel scrap into electric smelting furnace. During the melting process, sillimanite is decomposed and forms silicon and mullite:

3(AI2O3.SiO2) + 2C + Fe = FeSi + 3AI2O3.2SiO2+ 2C0 ... ... . .

The heavy FeSi sinks to the furnace hearth and the slag mostly mullite floats in the upper layer and is tapped into a mould where it is cooled for 4 to 10 days. The articles are then extracted from the moulds and ground to specified dimensions. Fused mullite has a

low porosity (1 - 2%), high refractories (1850°C), and the temperature of deformation under load of 1700°C. The ultimate compressive strength may be up to 2 9 . 5 ~ ~ l c m ~ .

Bauxite is a rock consisting largely of alumina hydrates (diaspore and gibbsite). Only bauxite with a high AI2O3 content and low iron oxide content is suitable for refractory purposes. Because bauxite gives off its water during heating, accompanied by a marked reduction in volume, it is generally sintered at the mine site. During this sintering, corundum and mullite are formed together with liquid phase and low melting titanates. Despite these low melting phases bauxite can be densified to a compact sinter at high temperatures.

Synthetic mullite or corundum-free sillimanite is. preferable for use in certain applications because sillimanite bricks made from raw sillimanite containing corundum are not volume stable, despite prefiring. This is because during extended periods of use recrystallization of mullite occurs along with a increase in volume. Bauxite bricks are used in electric furnace roofs, ladles the Steel Industry. Because of the requirements of high purity and service life of' refractories iri the glass industry, mullite bricks are used for the superstructure of glass tanks and tank linings.

Pure direct bonded corundum bricks based on fused or sintered corundum can be economically used in those areas where high thermal stresses are expected.

QUESTIONS

1) State the basic polymorphic formations of siliceous refractories. What are the disadvantages of these formations in the industrial applications of these refractories? What are the possible ways of controlling these disadvantages?

2) State the key properties of high alumina bricks and the possible areas of application in a glass or oil cracking plants.

CHAPTER FOUR

MAGNESITE REFRACTORIES AND INDUSTRIAL APPLICATIONS

Magnesia refractories contain at least 89% Magnesia (MgO). They are produced roasted, unroasted or fused. The starting material for the manufacture of magnesite refractories is mineral magnesite (MgC03) or seawater which contains magnesium chlorides and sulphides.

The main component of magnesite bricks is the mineral periclase (MgO). Its properties determine the beh,aviour of the brick material to a large extent. The most important properties are as follows:- :. . - High melting point about 2800°C

. - High thermal conductivity 5 W1K.M. ; - High resistance to basic slags - High thermal expansion 2.0% at 1400°C - No change of modifications.

Raw magnesite is roasted at 550' - 650°C thus; MgCOs , MgO + C02

Magnesia, MgO is present in the form of fine crystals of periclase. This loose structure makes magnesite very liable to hydration. With increasing roasting temperature, periclase grains grow coarser and magnesite is sintered. Upon roasting at 1650°C magnesite loses its ability to react with water. The general scheme of manufacture of magnesite refractories is shown below:

Raw Magnesite Stone I

I Crushing I

Roasted Magnesite Stone s Crushing u Grinding u

I Charge Preparation I I Curing of Moulding Mass I

I Pressing I

Drying I Sorting and Storage of Final Products

Fig. 7a: General Diagram of Manufacturing Magnesite Refractories

In addition to the main component, MgO, sintered magnesite and sintered magnesia contain varying amounts of Fe203, AI2O3, CaO and SO2. As well as periclase (MgO) the following silicates and fenites occur as secondary components depending on the chemical composition of the sinter:

Mineral

Periclase Forsterite

Formula Abbreviation

Monticellite

Silicate

Melting Temp.

"C MgO 2Mg0.Si02

Merwinite Dicalcium

The properties of magnesia bricks and thus their production also, depend on the final application. Commercial magnesite bricks have a high cold crushing strength and low porosity and soften under load in an oxidizing atmosphere at about 1400°C. Special low-iron magnesia bricks have gained importance in recent years. In order to achieve a better thermal shock resistance, they are manufactured from magnesia sinter low in fluxing agents and to a special grading. This type of brick is used in the checker work in the glass industry and as wear-resistant lining in pig iron mixers. Addition of 5 to 12% A1203 increases the thermal stability and

Ca0.Mg0.Si02

M MzS

2800 1890

CMS

1575 21 30

I

1495

3Ca0.Mg0.2Si02 2Ca0.Si02

C3MS2 c2s

slag resistance of magnesite refractories owing to the formation of spinel MgO. AI2O3. TO further improve slag resistance, low-iron magnesia bricks are impregnated with tar of a high pitch content or with pitch. Under vacuum, practically all brick pores can be impregnated with tar thus the tar content of bricks of normal porosity is around 5 - 6 weight %. Thermal shock resistance is achjeved by adding small amounts of chrome ore.

Magnesite bricks are used for lining walls, hearths of open hearth and electric - steel making furnaces, mixers, bottoms of soaking pits and continuous furnaces. Various powders (magnesite, iron scale, their mixtures etc.) are used to ensure better weldability between bricks. Magnesite bricks are prone to sweating as well as cracking. When the working layer of brickwork becomes saturated with ferric and silicon oxides its refractoriness diminishes and sweating of the lining takes place. - Sintered magnesite refractory materials are used as heat storage materials because of their specific heat and high thermal conductivity.

- Magnesia bricks containing chrome ore are manufactured by a method shown in fig.' 7. The firing temperature depends on the degree of purity of the raw materials and is between 1500 and 1800°C. Magnesia bricks with chrome are highly slag resistant and possess good thermal shock resistance.

Magnesia chrome bricks and chrome magrlesite bricks generally do not wear like silica bricks by melting and dripping but by spalling. Alternating oxidizing and reducing atmospheres aggravate the attack on the bricks. With numerous spinel

reactions, the hot brick zones swell to a greater or lesser degree (bursting) and in addition low viscosity melts migrate behind the hot working zone of the bricks and penetrate into the cooler brick zones thus densifying that part leading to spalling during temperature fluctuations.

- Forsterite Bricks The principal component of forsterite bricks is the

magnesium orthosilicate (2Mg0.Si02) with melting point of about 1 890°C from the mineral olivine.

Forsterite bricks can withstand a refractoriness under a load of 0 . 2 ~ l m m ~ at temperatures up to 1700°C. They have a remarkable slag resistance to iron containing slag up to 1-400°C and are hardly attacked by basic slag but susceptible to acidic slag. In glass tanks forsterite bricks are stable. They also resist sulphate attacks when forsterite bricks are free from periclase. Forsterite bricks react with silica and alumina containing bricks thus a protective intermediate inert chromite bricks must be provided in vessels where these bricks are used together.

High temperature fluctuations and variable oxidizinglreducing atmospheres cause loosening and eventual collapse of furnace structures made of forsterite bricks.

- Dolomite Bricks These consist of sintered dolomite containing

mainly CaO and MgO evenly distributed. Its behaviour depends on the proportion of free lime which makes the materials susceptible to hydration.

Dolomite refractories contain at least 40% CaO and 35% MgO, they also contain 3 0 2 , Fe203, AI2O3 and other minor mixtures. Free CaO retains its ability to react with water and increase in volume even after high temperature roasting (at 1550°C - 1650°C). This tendency is controlled by reacting CaO 'with'dicalcium or tricalcium silicates which db not react with water and therefore cause no volume changes.

Dolomite, magnesite or their mixture and 4 to 11% tar are the materials for the production of dolomite refractories. Tar dolomite refractories do not experience the transformation of 2Ca0.Si02 from p-form to y -form which may cause volume expansion. Tar dolomite refractories are resistant to basic slag action. They are used for the lining of oxygen converters and the walls and hearths of electric steel making furnaces.

QUESTIONS

1. List the main properties of magnesite bricks that make them better high temperature materials than silica bricks.

CHAPTER FIVE 1

CARBON, ZIRCONIA, CHROMIC, CARBIDE AND CARBORUNDUM REFRACTORIES

(i) CARBON REFRACTORIES ~ h e s e class of refractories are manufactured

from coke, anthracite or graphite, dehydrated coal tar, refractory clay and organic glues. They have a high refractoriness and high temperature of softening under load, resist well the action of slags and have high thermal stability. Their serious demerit is that they can be oxidised at temperature above 600°C, thus carbon

- refractories can only be used in. reducing or inert atmospheres.

Carbon refractories are divided into coke and graphite refractories. Coke refractories are prepared from coke, coal tar, pitch, anthracene oil and bitumen. The mixture is moulded into bricks or blocks. Coke refractories are roasted at 1400 to 1400°C in crucibles or capsules with a coal dust filling. The refractoriness of the final products is above 2500°C.

Graphite refractories are ' the claylgraphite materials. The production mass is made of 35 percent flaky graphite, 35 to 45 percent refractory clay and 10 to 40 percent fireclay. Moulded products are roasted in coal-fired capsules at 800 to 1000°C or at 1300°C to 1350°C. The refractoriness of the clay-graphite products is 1900°C and their thermal stability 150 200 water cycles.

Carbon refractories are used in the lining of th earth and wall of blast furnaces. The inner (working lining of the walls is laid of coke products and the outer

lining of walls (in the zone of coolers) and the hearth are laid of graphite products which have a higher heat conductivity, Carbon refractories are destroyed through oxidation by molten oxides (FeO, MnO), water vapours and carbon dioxide. In a reducing atmosphere at a temperature of 816OC carbon lining has been found to suffer from alkali metal compounds (Na2C03, K2CO3, etc.). Carbon refractories are also used in furnaces for smelting certain non-ferrous metals for manufacture of calcium carbide, ferroalloys etc. Some non-ferrous metals and their alloys are smelted in clay-graphite crucibles, which are gradually destroyed as their graphite is being burned off partially and also washed off by molten metal.

. .

(ii) ZIRCON REFRACTORIES The refractory. base in this type of refractory is. Zirconium dioxide, Zr02 with melting point 2700°C. This group includes Zirconia and Zircon refractories.

Zirconia Refractories: The minerals for the manufacture of zirconia refractories are baddeleyite and zircon are with 80 to 90 percent Zr02. The raw materials are usually contaminated with SO2, Fe203, AI2O3 and other oxides which form fusible entecticks or chemical compounds and thus lower refractoriness of final products.

Zirconia refractories have a high refractoriness around 2500°C, fair thermal stability (more than 25 cycles) and zre slag resistant especially to the action of acid slags. At high temperatures (near 2000°C), Zirconia is unstable to basic slags and can react with nitrogen arid carbon thus form~ng brittle nitrides and carbides.

Zircon Refractories: These refractories contain 59 to 63 percent Zr02 and 32 to 34 percent SiOz. The refractoriness of Zircon refractories is 1900 to 2000°C. They are stable to molten chlorides but are easily destroyed by fluorides. Zircon refractories are used in the lining of salt baths for steel quenching and also for making sleeve inserts for continuous casting of steel; the latter are inferior to high-alumina products because of their greater brittleness.

(iii) CHROMIC REFRACTORIES This group of refractories is based on chromite mineral FeO-Cr203 which is quite stable to acid and basic slags. The group includes the following types of refractories: (a) Chromite refractories (containing about 30 percent Cr203 and 24 percent MgO. (b) Chrome-Magnesite refractories (15 - 30 percent Cr203 and 45 - 60 percent MgO). (c) Magnesite-Chromite refractories (8 - 15 per cent Cr203 and 65 - 70 per cent MgO).

The main starting material for making chromite refractories is chromite ore whose content in the batch amounts to 85 - 90 perceid. Chromite bricks have narrow application because of its low temperature of deformation under load. Chromite is used more often for ramming. Chromite mass is used in the lining of cooled surfaces in furnace and steam boilers. The mass contains 92 to 97 percent fire-ground chromite, and 3 to 4 percent refractory clay, 3 to 6 percent soluble glass are added to the mixture. The refractoriness of the mass is 1700°C.

Chrome-magnesite refractories are prepared from 50 to 60 percent chromite and 40 to 50 percent roasted magnesite. They have a high refractoriness (above 1920°C) and are quite stable to basic slags. They do not reach with silica at temperatures up to 1650 - 1 700°C.

(iv) CARBIDE REFRACTORIES This group of refractories is based on metal

carbides characterized by a high .refractoriness and high melting points. The melting points of selected carbides as listed below:

ZrC ... . . . . . . . . 3530°C TaC ............. 3800°C NbC .......... 3500°C WC .. . . . . . . . . . . . 26OO0C Tic ... . . . . . . . . . 3140°C ThC . . . . . . . . . . . . . 2625OC

The main demerit of carbides is that it is oxidized a at high temperatures and this limits its field of

application. Carbides are used to manufacture cermets (metal ceramic products) which possess high strength at high temperatures and have found application in nuclear and rocket engineering, gas-turbine engines.

(v) CARBORUNDUM REFRACTORIES This class of refractories is based on Sic and is made up of carbofrax and refrax products. The former is made from 80 - 90 percent crystalline carborundum and 10 to 20 percent refractory clay. Carbofrax refraciories high mechanical strength, hardness, heat conductivity thermal stability. Their demerit is that they are oxidized at temperatures above 130O0C and have poor resistance to basic slags.

Refrax refractories are carborundum with amorphous binder. The products roasted

prepared from crystalline carborundum added as a at 2000°C in a reducing

atmosphere have better properties than carbofrax ' products but are more expensive.

Carborundum refractories are used for making '; recuperator tubes, heaters of resistance furnaces, " crucibles, etc.

QUESTIONS

What are the major limitations of carbon, carbide and chromic refractories iwindustrial applications?

CHAPTER SIX

REFRACTORIES FOR THE GLASS INDUSTRY

Earlier in this book, refractories have been classified, essential service properties and refractory types discussed in detail.

In this chapter emphasis will be placed on essential aspects of refractories, used in the Glass Industry. Stringent requirements, like shock resistance, high purity, long service life, high thermal stresses, resistance to chemical and mechanical attack of melts in contact with the refractories, for the Glass Industry, constitute challenges to the refractory specialist. :These conditions occur mainly in the glass tanks, as a result, special bricks should be used for the glass tank.

Fireclay machine-pressed fired bricks and insulating bricks are used in the deep layers of the furnaces foundation and outer layers of the recuperator. Glass tank furnaces have silica crown refractories. It is worth noting, as shown in Figures 4 and 5 that during heating-up, silica bricks expand rapidly with the total reversible expansion being completed at 800 - 1 O'OO°C. Thus they are resistant to temperature variations above 800°C (red heat) but very susceptible below this temperature because of the erratic volume expansion. For this reason sufficient time must be allowed for heating furnaces up to about 800°C and in the glass industry it is recommended that, depending on the furnace size, the silica crown are generally heated up within 8 - 18 days. In the manufacture of silica bricks for the glass industry, coke ovens and hot blast stoves, extremely dense bricks low in fluxing agents are aimed

at. The raw materials for glass furnace silica brick manufacture are selected and should contain the same quantity of viscous melt at 1650°C as there is in normal silica bricks at 1 650°C.

This imparts a considerably higher temperature resistance before the bricks fail through melting or dripping.

In the Si02 - AI2O3 phase diagram, Fig. 6, the only stable compound in the system is mullite. Mullite decomposes to corundum and liquid phase at about 1 840°C. Mullite in some form is used in the glass furnaces.

The uniqueness of the glass industry as mentioned above demands the use of electrically fused or fusion cast :products and extruded fusion products:. Such products are compact, extremely dense with very low or near zero porosity. They resist corrosion and erosion.

Fusion cast products differ from extruded fusion products. The former type has high mechanical strength, high refractoriness and resistance to melts or slag. Its advantages are low thermal shock resistance, high thermal conductivity, and the formation of cavities inside the bricks. They are produced by using the raw' materials in an electric furnace at temperatures between 1800 - 2100°C and pouring the melt into preheated sand or graphite moulds. Fusion cast products are classified thus - high A1203 content, mainly corundum, high MgO and Cr203 content and high AI2O3 and Zr02 contents. Many glass melting furnaces are constructed mainly of fusion blocks but this had been ~rneconomical because of high material costs. Preference is given to special tank blocks manufactured by the ceramic process.

Fusion cast products are made in required shapes cut from the extruded blocks. They include those fused refractory bricks which consist of over 98% SO2 and made in electric furnaces from quartz sand. Fused silica has a milky, sometimes greyish-white colour because of the enclosed air bubbles. Of 211

refractory products, fused silica bricks have the lowest expansion coefficient, 0.55 x 1 0 ' ~ and for. this they are ideal for furnace construction. They can be heated rapidly to high temperatures but cannot withstand high compressive stresses at that temperature because it is an under cooled melt. After long heating periods above

: 1500°C fused silica devitrifies to cristobalite which is fragile during cooling in the lower temperature range because of the. modification change. Fused silica IS - .

used for glass tank, gathering openings and other working structures. They are used as arches ouer the doghouse of white glass tanks and as curtain bricks and can be easily repaired.

Fused corundum, sintered corundum and fused mullite are finding applications in the Glass Industry. For instance, fused mullite bricks are used for the superstructure of the glass tanks and tank linings. Pure

fused or ' direct bonded co'rundum bricks based on sintered corundum can also be used economically in those areas where high thermal stresses are expected.

Zirconia (Zr02) and Zircon~um silicate (mineral name, zircon) have applications in the glass industry. Zirconia with a very high melting point of 2700°C has a tendency, similar to Si02 to be polymorphic. These modifications occur with very different volume changes, for this reason Zirconia must be stabilised by the addition of MgO or CaO before it can be used as a raw material for refractory purposes. Because of its low

49

thermal conductivity and relatively high thermal expansion, the thermal shock resistance of Zirconia is not very high.

Zircon contains about 65% Zr02 and 33% Si02 and unlike Zirconia is stable up to the decomposition temperature of about 1800°C. Zircon bricks have excellent thermal shock resistance because of its much higher thermal conductivity than Zirconia. Zircon bricks are used in the glass melting tank furnaces. Between the burners in the superstructure few courses of Zircon bricks absorb streams of melted silica and transform them' into harmless, viscous melts. Bricks manufactured fro& Zircon and quartz material are often used as a hot repair material. These Zircon cristohalite bricks are particularly resistant tothermal shock.

Special low-iron magnesia bricks have gained importance in recent years in the Glass Industry. They are manufactured from the magnesia low in fluxing agents and they have good thermal shock resistance and are used in the checkerwork of the Glass Industry.

1. List the key refractory materials used in the glass melting furnace.

2. An operator in the glass melting furnace just ran to you, the plant engineer, to report that there was a break-out on the glass melting pot. State properly how you would solve the problem.

CHAPTER SEVEN

UNSHAPED REFRACTORY PRODUCTS

This group of refractory materials is composed of refractory raw materials and bonding agents. Unshaped refractories are composed of fundamentally the same raw materials as bricks and the same selection criteria apply to both.

Depending on the application techniques there are mainly three major classes of unshaped refractories, viz;

(i) Mixes for monolithic structures and repairs (ii) Materials for laying and jointing (iii) Materials for coating and surface protection.

( i ) MIXES Generally refractory mixes solidify and sinter on

installation. Basic mixes consist mainly of megnesia, chrome ore and sintered chromite and used largely for repair and maintenance. Mixes are classified according to the water' content during application and the type' of setting as shown on Table 11. It has been found that after 5 hour firing, cubic specimens have the following maximum drying and firing shrinkage:

TABLE 11: CLASSIFICATION OF MIXES

MIX

Ramming Mixes

Plastic Mixes

- Refractory castables

Gunning and slinging mixes

NATURE

-- Dry ramming Mixes moist crumbly ramming mixes.

Plastic crumbly mixes '

Plastic lumpy mixes Dense refractory castables refractory light weight casta bles Moist, crumbly ramming mixes dense refractory casta bles refractory light weight casta bles

-- WATER

CONTENT 0 N

APPLICATION

-- TYPE OF SETTING

Ceramic

Chemical and lor Ceramic

Chemical and/ or

Ceramic

hydraulic and sometimes chemical

Chemical Hydraulic ceramic

START OF SETTING AT TEMP.

OF

4pprox. 1 00o0c

1 1 o0c

approx. 1 0oo0c 1 1 ouc

approx. 1 0ooQc

room temp.

dependel on type of bondmg

Ramming mixes 2% linear Plastic mixes 3% linear

, Refractory castables 1.5% linear ?1 #4

(i) RAMMING MIXES and the so-called dry- ramming mixes (fritting mixes) are mixes of a crumbly cdnsistency before installation which 'can contain bonding agents. They harden under the influence of heat above room temperature.

Ramming mixes can be used as delivered or water and other liquids can be added. They can be rammed by hand or with ramming machine.

* (ii) PLASTIC MIXES These arecrumbly or lumpy materials ready for :

use, consisting of granular refractory materials and plastic bonding clays and can contain other additions of chemical bonding agents. Less energy is required in compacting plastic mixes than ramming mixes. Templates are not required for plastic lumpy materials. The lumpy mixes are normally sealed in plastic foil. At the time of use the foil is unpacked on a clean surface and divided into pieces according to requirement. The wall is rammed in'layers in the direction of construction. To compensate for shrinkage and expansion, narrow joints about lOmm deep as for ramming mixes must be cut horizontally and vertically at 300 - 400mm intervals at the hot side. Better and quicker drying can be achieved with ventilation holes rodded at 200 - 300mm intervals.

(iii) REFRACTORY CASTABLES These are mainly granular refractory additives

and bonding agent which after water or another mixing

Ramming mixes 2% linear Plastic mixes 3% linear

, Refractory castables 1 -5% linear .\

(i) RAMMING MIXES and the so-called dry- ramming mixes (fritting mixes) are mixes of a crumbly cdnsistency before installation which 'can contain bonding agents. They harden under the influence of heat above room temperature.

Ramming mixes can be used as delivered or water and other liquids can be added. They can be rammed by hand or with ramming machine.

* (ii) PLASTIC MIXES These are crumbly or lumpy materials ready for

use, consisting of granular refractory materials and plastic bonding clays and can contain other additions of chemical bonding agents. Less energy is required in compacting plastic mixes than ramming mixes. Templates are not required for plastic lumpy materials. The lumpy mixes are normally sealed in plastic foil. At the time of use the foil is unpacked on a clean surface and divided into pieces according to requirement. The wall is rammed in'layers in the direction of construction To compensate for shrinkage and expansion, narrow joints about lOmm deep as for ramming mixes must be cut horizontally and vertically at 300 - 400mm intervals at the hot side. Better and quicker drying can be achieved w~th ventilation holes rodded at 200 - 300mm intervals.

(iii) REFRACTORY CASTABLES These are mainly granular refractory additives

and bonding agent which after water or another mixing

liquid has been added, solidify and harden without drying at room temperature. They are only supplied dry and are mixed for use at the installation site. Refractory castables are used above 200°C or 350°C, below these temperatures normal construction castables are used.

Refractory castables are generally installed by pouring and are compacted by vibration, ramming is only used in special cases. Refractory light weight castables (total porosity at least 45 Vol.%) are only poked lightly in order to guarantee the subsequent insulation effect.

(iv) GUNNING AND SLINGING MIXES These mixes are named according to the method

of application. For gunning, for in-situ hot mechanised repair method of reactors or rational lining method for tubular reactors, an adequate supply of water and . compressed air must be guaranteed. Gunning machines are used for both repair method and the rational lining of tubular reactors. Slinging machines are used for applying slinging mixes in reactor vessels. A suitable slinging machine has a material feedhopper and a rapidly rotating wheel or the usual so-called: slinger machine used, for example, for lining steel mill ladles. Detailed account of these machines will be given In a subsequent chapter.

(v) LAYING AND JOINTING MATERIALS (MORTARS, MASTICS, ADHESIVES

These materials determine the durability of brickwork in high temperature applications. The clmracteristics, chemical compositions, laying technique ,,

I . and grain size of these materials should of esserlce I,

match with those of the bricks. The solid substance of the mortars and mastics should have good water

retention properties without precipitation to avoid .: premature water penetration of the bricks after laying

which can cause the mortar to fuse to the bricks. Refractory mortars are dry material mixes which

are mixed with water to a workable consistency at the site. Setting is either ceramic hydraulic or chemical.

Refractory mastics are always mixed to the correct consistency with a special liquid in place of water, and are supplied ready for use or with the mixing liquid separate. At both low and high temperature they ensure a tight brickwork structure and a considerable increase in strength. Compared to ceramically setting mortar they offer better resistance to corrosive attack such as slay attack than the brickwork itself thus there is no premature wear of the joints. . .

Adhesives are fiber mats and insulating fabrics used for attaching insulating fabrics to metal walls or brickwork. They contain a very fine granular substance and a chemical bonding or adhesive agent which ensures gas tightness of the brickwork. Before application the surfaces are cleaned carefully and wetted with the aqueous solution of soluble glass. The adhesive must possess the appropriate refractoriness, slag resistance and other properties expected for a particular application.

(vi) MATERIALS FOR SURFACE PROTECTION These materials have a higher mixing liquid

content than refractory mortars and mastics because of the specialized application methods - painting, spraying or trowelling. They are used in protecting steel paths against premature corrosion and scaling or to shield refractory lining from gas and dust and to prevent

coatings and turbulence by providing a smoother surface in pipelines with high gas velocities.

Different application methods require varying amounts of water as shown below:

TABLE 12: TYPE OF BONDING AND WATER MIXING FOR LAYING AND JOINTING MATERIALS

of Chemical Chemical Chemical Chemical i n 1 C;ramic 1 Hydraulic 1

h draulic ceramic

P.lixing Liquid1 water

QUESTIONS

1) List all the unshaped refractory materials and mode off preparation.

2) Select the appropriate laying and jointing materials for a reactor lined with magnesite bricks stating clearly your reasons.

Unshaped refractory and semi- refractory materials for laying and. jointing shaped refractories.

Unshaped refractory and semi- refractory material for sealing shaped and

Refractory mortar 10-25'% '

unshaped Putty plaster 15-30%

Refractory mastic 12-30%

xoduct Coatings

30-60% :.

CHAPTER EIGHT

I

;i SPECIAL REFRACTORY BRICKS AND REFRACTORY INSULATING MATERIALS

(i) SPECIAL REFRACTORY BRICKS: Some bricks are made by cutting required

shapes from extruded blocks while others are fusion cast. Fusion cast products are very compact, dense (porosity is extremely low or nearly zero) and produced by fusing the raw material mix in the Electric Arc furnace at temperatures between 1800 - 2,100°C and pouring the melt into preheated sand or graphite moulds.

2 . The fusion cast products are classified according to material composition namely:

; (a) Bricks with a high AI2O3 content: Main component corundum, "P - alumina". (b) Bricks with a high MgO and Cr203 contents: Main components periclase and spinel. (c) Bricks with a high AI2O3 and Zr02 - content: Main component corundum and baddeleyite.

Fusion cast products are largely used in the glass industry and to some extent in the steel industry where high mechanical and chemical stresses and low thermal shock are expected. In some glass melting furnaces the entire tank is constructed with fusion cast blocks.

Extruded fusion products like fused silica products are also available for special application in the glass tank lining, for low-alkali glass (borosilicate and optical glasses) production. They are also used in making nozzles for the continuous steel casting process. Extruded fusion silica products cannot

withstand high compressive stresses at high temperature. After long periods of heating above 1 150°C fused silica devitrifies to cristobalite which is fragile during cooling.

(ii) REFRACTORY INSULATING MATERIALS sThese materials contain high pore volume thus

considerable quantity of air with its very low thermal conductivity, thus refractory insulating materials are solely for thermal insulation of furnaces and other combustion equipment.

Production of insulating bricks requires the use of porous raw materials, addition of volatile or combustible materials and addition of foaming agents to the material mix. .. .

Insulating materials can be classed into two groups namely:

Insulating bricks with an application temperature of 900°C to 1100°C (Vermiculite bricks, Kieselguhr bricks, perlite bricks etc), those with an application temperature above 1 100°C (light weight fireclay bricks, light weight silica bricks, light weight sillimanite bricks etc.) The first group of insulating bricks are used as' back-up biickwork because of their low refractoririess* while the second group can be used in direct contact with flame provided that the furnace walls are not under mechanical stress, there is no flue dust or slag attack and the operating temperature does not exceed the respective application limit. Insulting bricks are classified thus:

TABLE 13: LIGHTWEIGHT REFRACTORY BRICKS

/ FIRECLAY Temperature O C 1300-1 350

Bulk density glcc

QUARTZITE 1550-1 550

0.8-0.9, 1.0-1.1

Cold crusting Strength ~1rnt-n~

Lining techniques of Insulating bricks involve use of the two-layer bricks during pressing in which a refractory material of normal porosity and strength is combined with an insulating material to form a

3 - 4

composite brick. The materials-of these two bricks must.

Refractormess under load 1 0.025 ~1rnt-n~ I

in all standards be compatible ' physically and chemically.

Natural thermal insulating materials are asbestos, diatomite, etc. Asbestos is employed in the form of cardboard, paper, wool, rope and as a component of various insulating pastes. It can withstand temperatures up to 500°C and then becomes dehydrated and breaks to powder.

Ceramic fibers have become popular as thermal insulating materials. These fibers are produced from Kaolin clays and synthetic products in the Si02 - AI2o3

system and depending on their properties they can be used in any appropriate application. Fibers are used either with or without a binder to produce matting, felt, paper, vacuum-formed parts, ropes and prefabricated parts. The fiber can be cut to size and with the aid of pins or anchors of heat resisting steel they can be attached safely.

A vacuum-formed blanket (47% A12031 52% Si02 - AI2O3) has linear shrinkage value of 2.5% after 24 hours heating, operating temperature of 126O0C1 melting oint of 1760°C, bulk density approximately !? 2lOkglm and thermal conductivity at 1000°C of 0.19 W1K.M.

Insulating castables, blanket and belt have wide applications in the petroleum industries and iron and steel industries.

QUESTION

Outline the areas where refractory insulating materials are used in your plant or a named plant. Specify the type' of material and justify their application in such areas.

CHAPTER NINE

\ i1 PREPARATION OF REFRACTORIES AND BASIC

MANUFACTURING PROCESS

(a) PREPARATION OF MASSIVE AND GRANULAR REFRACTORIES: .

(1) MASSIVE REFRACTORIES:

(i) Raw State: Sandstone and mica schist are invariably used in the form of large blocks sawed or split

; from the raw rock and further shaped as needed on the . job. .. .

(ii) Burned Products: Fired brick and shapes of all types of refractories donstitute the bulk of refractories used in the massive form. The processes of producing these refractories are basically similar, Although the nature of raw materials and the wide variety of properties desired in the final products have caused the adoption of a number of variations in these processes.

(iii) Mininq of Raw Materials: The first step is the mining of raw materials. Mining methods depend on the location, size, and uniformity of the deposit. Some deposits are worked underground while open-pit method is used for large uniform surface deposits e.g. plastic clays and dolomites.

(iv) Purifying Operations: Purifying Operations which follow mining may involve mere visual selection, weathering to remove soluble salts or simple washing operation as applied with some flint clays and

quartzites. Raw material beneficiation for example flotation process for the removal of lime and dolomite impurities from natural magnesites is also practised.

(v) Crushing, Grinding, Screening and Mixing: These are important operations which to a large

extent control the density, porosity, strength, spalling resistance and thermal characteristics of the bricks. Grinding must be varied for different raw matereials and mixes. Screening is used as a means of controlling the particle size which in turn, is one of the means of controlling the significant properties of porosity and bulk density of brick.

Mixingmay consist of adding fines to coarse material of the same kind to control:the grain or particle size, or of adding one material to another, e.g. lime to quartzite and plastic clay to flint clay to act as binders, burned clay or refractory to plastic mix to- control shrinkage or magnesite to olivine, magnesite to chrome ore, bauxite to clay to change the composition and character of the brick.

(vi) Forming Operations: These may be carried out: by hand moulding; extrusion; extrusion and pressing; dry pressing; isostatic, vibratory and impact pressing; and pneumatic ramming. The moist mix for these operations ranges from (12% to 20% moisture),

Refractories are burned or fired in periodic kilns or funned kilns for the following reasons: (a) to drive off hygroscopic and combined water; ( 5 ) to hring about desired chemical changes namely driving off Con and oxidizing iron and sulphur :t compour~ '~ , organic matter, etc;

c) to effect transformations of the mineral constituents and convert them to most desirable stable forms; and (d) to effect necessary combinations and vitrification of bonding agents.

(vii) Chemically Bonded Products: In the chemical- bonding process, a suitable bonding material such as lignin liquor, magnesium chloride, or sulphuric acid is mixed with the raw materials and the bricks are formed under pressure, sometimes exceeding 10,000 pounds per square inch. For bricks that will be metal encased, the process of preparation involves pressing of the steel jacket onto the brick during the actual forming operation. Drying is below 500°F to develop proper strength.

(viii) Tar-Bonded- Products: These are made from dead burned dolomite, mixtures of dolomite and magnesite or magnesite by mixing the sized materials with approximately 6% of tar or pitch at 250°F to 303OF and pressing the hot mixture in a conventional brick press. Many of these products are given final heat treatment at 400°F to 600°F (tempering) to set the tar- bond and thus avoid softening or slumping of the brick in service.

(ix) Refractory Concrete Products: This is made of sized refractory aggregates, such as calcined flint clay; crushed fire clay brick fats, chrome ore, or various types of high alumina products and a hydraulic bond usually calcium aluminate cement. Aggregate-cement ratios may vary from 4 to 1 to 10 to 1. Calcium aluminate cements are employed because of their rapid setting

and their ability to retain their water of hydration, and hence their bonding power to considerable higher temperatures than Portland cement.

(x) Electrocast Products: The electrocast process consists of mixing raw materials of high purity in proper proportion, heating them to complete fusion in an electric arc furnace. The products include combinations of chrome, magnesia, alumina and zirconia. Fused mullite is produced by this method. Electrocast products have advantages like complete lack of voids, narrow range of melting, and better control of chemical properties. Disadvantages include high cost, poor resistance to thermal shock and continued growth at high temperatures. . .

2. GRANULAR REFRACTORIES: (i) Raw State: The refractories used in the raw state in granular form are clays, chrome ore and dolomite. (ii) Fired Products: Fired granular products include dolomite, magnesite, and other special basic refractories. Two types of dolomite are recognizes in this category. Calcined dolomite; produced by heating the natural rock to a temperature above its decomposition temperature (about 2000°F). The product consists of crushed porous granules which are very active chemically. Dead burned or clinkered dolomite is a product calcining or dead-burning dolomite at a temperature of approximately 3000°F usually in a rotary kiln using iron oxide as a dead burning agent.

Special granular refractories are used principally for basic ramming and patching materials. These are carefully-sized chemically-bonded preparations

composed of dead-burned magnesites or magnesia dolomite mixtures (MgO = 60%, 40% Lime and Silica).

Another group of special granular refractories is a the fireclay-base plastic refractories. These consist generally of sized mixtures of raw and calcined clays, often containing sodium silicate or organic bonds, which are shipped wet for applications where a rammed refractory may substitute for brick work. Such plastic refractories may also be made with super-duty clays, Kyanite or other high-alumina materials as additions.

Castables are also granular refractories since they are sold ready for mixing with water.

* 3. FINELY DIVIDED OR PULVERIZED PRODUCTS (i) Raw State:. Clays for bricks laying and various .. .

bonding purposes are in this raw, finely-divided state.

(ii) Processed Products: These include the various heat-setting and air-setting cements and mortars. They are produced for laying every type of brick, and, hence may, have as many different base materials, the principal ones being silica, fire-clay, chrome, magnesite, and high alumina material. Silica cement or silica fireclay, is usually made of finely-ground quartzite or '

sand to which is added about 10% plastic clay and possibly on organic binder. Various types of phosphate binders are also used. Chrome and magnesite cements may contain small clay additions for workability, and like fireclay cements, may also contain sodium silicate or organic binders.

Fireclay and high alumina cements differ from plastic fireclay refractories chiefly in respect to fineness as they also consist of mixtures of raw and calcined clays and high alumina materials. The heat setting varieties rely on vitrification for bonding while the air-

setting types usually contain liquids or dry sodium silicate and hence may be shipped wet or dry.

Fig. 7B: Manufa~ture of dry pressed refractory fireclay bricks:; 1.) raw material storage 2.) clay crusher 3.) clay mill 4.) drying towerlclassifier 5.) storage silos 6.) mixing of liquid components 7.) coarse crusher 8.) fine crusher 9.) bucket elevator 10.) screen I 1 .) feed hopper 12. ball mill 13.) air classifier 14.) weighing scales 16.) dosing of liquid components 17.) mixer 18.) screw press 19.) hydraulic press a 20.) hand shapinglrammer 21.) kiln car 22.) tunnel dryer. 23.) tunnel kiln 24.) brick storagelloading

B. BASIC MANUFACTURING PROCESS: Generally the manufacture of dry pressed

refractory fireclay bricks as shown in fig. 7b above includes the following production processes:

Crushing - Classification

)

Mixing 1 )Preparation 14

Shaping ) $ 4

- D,ying 1 - Firing 1

(i) Preparation: Preparation includes three individual processes - crushing and grinding the raw materials, classification of the crushed material by screening or grading, mixing the graded materials to obtain mouldable mix. Crushing process involves coarse crushing, fine crushing and grinding. Various machines used in the crushing process are for coarse crushing: jaw crusher, impact crusher cone crusher, roll crusher, for fine crushing impact crusher, cone crusher, roll crusher, for grinding: ring-roll mill, ball mill, vibratory mill.

Choice of machines for crushing depends on the hardness and-toughness of the raw material, tie required degree of crushing; grain size, shape etc. Classification of the crushed material is generally done on vibrating screens using one or more screen sizes according to the number of grades required. To ensure efficient grading the material should be free from moisture; especially - fine screening when screens are often heated to prevent clogging.

(ii) Shaping: To produce a mouldable mix, the components are fed into a mixer according to specific raw material and grain size batches, a bonding agent is added and the mix is homogenized, usually in intermittent mixers. The edge mill generally produces a mix which is suitable for bricks with dense structure. In order to achieve uniformity and accuracy, the separate preparation stages in modern plant are largely automated using electronic control equipment. The physical properties of a refractory material

are mainly determined by the grain structure of the mix. This applies particular to those products which are manufactured without or with only a low proportion of bonding agent for example magnesia bricks, hard fireclay bricks.

Bricks are shaped in various ways - normal and key depending on the configuration of the vessel. All bricks are supplied as machine- pressed shapes and preferably in a standard size. Mechanical or hydraulic presses are used to manufacture pressed shapes. Screw presses for special shapes and rotary table presses for straights are the modern practises.

For the chemically and hydraulically- bonded mixes are compressed into shapes by .

vibrators and simultaneous application of a relatively low pressure Pre-fabricated refractory parts or blocks can be produced by this method.

An effective method for the manufacture of high quality special refractory is called isostatic pressing method in which a flexible rubber moulds are f~lled with fine ceramic powder mix. The rubber mould is closed and subjected to pressure uniformly applied in an autoclave via a compressed liquid. A pressure up to 300Nlmm2 can be applied w~ th isostat~c presses and these are used for manufacturmg large sized refractory blocks, special shapes like nozzles, tubes and similar items

(iii) Drying: Drying accompanies shaping to remove water or other liquids added during shaping. Only completely dry shapes can be fired without danger of cracking. An efficient

drying process produces bricks that no longer change in shape during drying. Drying process can last from several days to several weeks according to brick size and consistency of the mix.

Large shapes for example glass tank blocks sledges are dried under controlled humidity conditions. Pressed bricks are dried in drying chambers or drying tunnels using waste heat from the firing kilns. In modern factories the pressed green products are loaded on tunnel kilns car immediately after pressing and after drying in the drying tunnel, they are moved to the tunnel kilns. This saves money and avoids

- . . damage caused by handling. . .

(iv) Firing: According to brick type, the dried shapes are fired at temperatures in excess of 1 OOO°C. During this process, volatile components such as residual water, water of hydration, Co2, SOs etc are released. Brick structure is formed as result of reactions, recrystallization, transformation or the formation of a glassy phase. As these processes are connected with contraction or expansion, the dimensions of the refractory brick often change during firing and this must be allowed for when the moulds are made.

The firing temperature for the most important materials group are as follows: Fireclay bricks 1250 -1 5OO0C Silica bricks 1450 - 1 5OO0C High alumina bricks 1500 - I 8OO0C Magnesia bricks 1500 - 1 8OO0C

The total firing period including preheating and cooling is from about 3 days for tunnel kiln to about 3 weeks for annular or periodic kilns. For tunnel kilns, the heat required per kg of bricks is about 3500 KJlkg for fireclay bricks about 6000 KJIkg for magnesia bricks about 12000 KJlkg for high-fired magnesia or corundum bricks.

At the same temperatures the heat required with periodic or annular kilns is about 1.5 - 3 times as much. Preferable heating source is liquid and gaseous fuels.

During firing, the flames should surround the bricks on all sides if possible in order to achieve uniform setting. The setting of tunnel kiln cars must be carried out with particular care.

The firing time depends not only on the type of kiln but also on the different types of brick, and this must be taken into account in kiln operation. In addition, it must be noted if firing is to be oxidizing or reducing. Silicon carbide bricks, for example have to be fired in an oxidizing atmosphere up to a certain temperature or carbon cores can occur, resulting in a reduction in quality.

In order to obtain brickwork with tight joints, . many refractory products must be ground after 'firing. This applies to glass tank blocks, magnesia-mixed blocks and others. The total manufacturing time for fired refractory bricks according to brick shape and type of firing can be'between 10 days and 15 weeks.

QUESTION -..

Outline the major steps in the mawfacture of silica - ',! base refractories.

CHAPTER TEN

TOOLS AND EQUIPMENT FOR REFRACTORY APPLICATIONS

INTRODUCTION Tools and equipment for refractory applications

range from simple to complex and they constitute the basic tools a refractory worker needs to ensure effective application of refractory materials.

These tools are expected to be. - (i) Robust (ii) Capable of withstanding much use, wear and

tear (iii) :.Capable of giving many years of service .

(iv) Capable of serving specific purpose in refractory applications satisfactorily and being used for the job for which particular tool was intended.

In procuring refractory tools quality and renowned manufacturers of such tools are the key factors to be considered. A high quality tool manufactured by reputable companies is the key to efficient refractory works. Fig. 8 shows some of the tools and equipment for refractory works.

TOOLS AND THEIR APPLICATION (i) Trowel

This simple tool consists of a steel blade and shank into which a wooden handle is fixed. It is used in lifting and spreading refractory mortar or mix onto a reactor wall surface and in forming joints. It is also used in cutting or shaping bricks.

The pointing trowels are used-for placing mortar into Joint.

(ii) Spirit Level This is available in various lengths. The shorter length is used for adjusting small works while the longer ones are used in conjunction with the straight-edge for obtaining horizontal surface.

(iii) Club Hammer It is a heavy tool (1 to 2kq in weight) used for cutting and removing bricks or cutting holes. It is used in conjunction with bolster and chisel,

(iv) Bolster. .. . This is used in cutting bricks. The edge of the

; tool is placed on the portion of the bricks to be cut and a sharp blow is with a hammer on the end of the steel handle of the bolster is usually sufficient to split the bricks.

(v) Mallet It is a hammer usually of a wooden handle only or wooden handle with rubber head for striking bricks into position in reactor vessels. Other simple tools include shovels, wooden float, chisel, metre rule, tapes, etc.

EQUIPMENT AND THEIR APPLICATIONS (i) Abrasive Cut-Off Machines

These are used for cutting bricks especially alumina, magnesite and silica bricks. The machines are used for wet cutting (diamond tipped disks) and dry

cutting (synthetic resin bonded and fibre reinforced cutting disks).

(ii) Air Compressors These are used to build up necessary air

pressure for operating machines used in refractory works, like demolishing hammers, hand grinders, ramming hammers, vibrators and drills.

(iii) Stud Welding Machines Using refractory mixes in reactors involves bolts

and expanded anchors thus stud welding machines are used in refractory works in welding these bolts and expanded anchors to reactor shells.

:. . .. .

(iv) Mortar Twirling Stick In mixing refractory mortars, there is usually

some mortar which separate from the mortar mix. The mortar twirling stick is used for mixing this settled mortar and is mostly driven by a compressed air hand drilling machine.

(v) Compressed Air Hammer This is either heavy or light for breaking up

refractory lining.

Bricklayer's hammer Cutting hammer

Mallet Rubber hammer

Flat chisel Pointed chisel

Smoothing chisel

Smoothing chisel

Mortar tdgirling-sticks

7.5

Mortar pan

Trowel fr-

:. . vi) Ramming Hammer $ . .

Light weight ramming hammers for densifying refractory masses.

(vii) Vibrator Or Jotter This equipment is used for vibrating refractory

castables or mix poured into a mould.

(viii) Demolishing Machine Fast demolition of refractory' lining especially

brick work is accomplished by this machine which is fixed to a compressed air line.

(ix) Mixing Machine This is an electrically or petrol engine driven

equipment for mixing refractory mortar or castables.

(x) Gunning Machine This important unit is one of the key equipment

for refractory works and insitu repair of refractory lining

8 in some reactor vessels especially in the steel industry. An Aliva-made gunning unit is designed for the dry mix principle. The dry refractory mix having a moisture content of 5% maximum is fed via the feeding hopper incorporating an agitator through the mouth of the upper joint plate into the bypassing rotor chambers. The refractory mix is transported continuously to the outlet in the lower joint plate and thus into the stream of compressed air. From here it is blown pneumatically through the material hose or pipe to the spraying nozzle and the point of application respectively. Water is always added to the dry mix at the nozzle. Serviceloperation manuals for running machines should be used for the operation maintenance of these machines. : (xi) Slinainq Machine

This is another important equipment for repairing refractory lined vessel. Like the gunning machine the input refractory material is in the form of a mix. In the use of this equipment as in gunning, correct adjustment of air, water and the refractory mix is important. For these machines serviceloperation manuals should be well studied to ensure effective slinging operation and proper maintenance of the machines.

(xii) Anchor Svstems In all refractory construction brick laid, rammed,

cast or sprayed walls and roofs except in self supporting arches, anchors and supporting elements are essential to distribute weight and exert tensile forces. Anchors are heat resistant metals shaped in such a way to exert effective and enough tensile stresses on the refractory mix. Whether they are

expanding anchors or slot bolts they are welded to the reactor vessel shell in order to exert adequate grip on 1 the refractory material.

Anchor applications in Refractory works Anchor for bricks for bricklaid wall: These special bricks have openings for these anchors (heat resistant amps). With these clamps the bricks are suspended by hooks fixed on the furnace shell.

- Anchors for suspension bricks for bricklaid roofs and flues: The bricks have adapted hooks or loops and are hooked directly to the roof construction. There are special roof suspension bricks which can be pushed on bearing rails and also combination of suspension and T-Shape bricks.

- Anchors for masonry made of rammings and castables: The bricks are fitting with clamps and can be suspended by hooks, loops, chains, etc. from the furnace construction.

- metal anchors for masonry made of castables and sprayed masses: These are corrugated anchors, expanding anchors and slot bolts. They are welded to the furnace construction by automatic welding machines. There are also heat resisting cast steel anchors that are fixed to the furnace wall with special suspension devices.

- Consoles for masonry made of ramming masses: They are heat resistant cast steel and hooked into especially developed wall holders which are tightly connected to the furnace wall.

1 i

! (xiii) INSTALLATION OF ANCHORS The suspension bricks in the bricklaid walls will

be installed in such a way that the system is movable to a reasonable extent to allow for thermal expansion. The suspender clamp is laid into the opening of the brick without mortar. It can be wrapped with refractory paper. The anchor bricks must be installed in such a way that it almost flushes with the rammed surface or lOmm below the rammed surface. The anchor must not project out of the ramming mass or be covered. The anchor system as said earlier must stay under tensile stress and this is achieved by wedging the bricks and the operating fastenings. The wedges must be removed after the finish.

Tools, equipment and anchoring systems are I shown in Figs 8 and 9.

QUESTIONS

1. Name ten essential tools used by a refractory craftsman. 2. A reactor in your plant needs immediate repair but management of your industrial establishment directs that production must go on by all means. What line of action will you take as an engineer or operator of the reactor? 3. Where are anchors used in refractory applications and what are the requirements for using anchors in refractory lining?

Fig. 8: Tools for Refractory Works

80

Fig. 9(a) Anchor bricks for refractory brick lininqs

Anchor brick

Wall brick with tongue and arove

Light weight refra&o& anchor brick

Heat resistant clamplcast skel

Heat resistant flaplsteel

Heat resistant flap

I Heat resistant bolt

NOTE: Flaps are laid into the bed joints and bolts are inserted in the bore holes when brick laying

(b) Anchor bricks for monolithic lininqs High alumina anchor Heat resistant brick bricks holders

:' . Anchors for Refractory works

(c) Anchor bricks for monolithic linings High alumina anchor Heat resistant brick bricks

a I3

holders

(d) Metal Anchor for monolithic linings

Heat resistant'cast stee

! Anchor

Anchoring of light weight refractory brick linings

top view section

top view section

Anchoring of monolithic linings section to^ view

1. block insulation 2. insulating gravel 3, light weight refractory

bricks 4. heat resistant flap (5012) 5. heat' resistant bolt (El 2) 6, steel stud (0 10) 7. furnace casing

1. block insulation 2. insulation gravel 3. light weight refractory

bricks 4. heat resistant wire hook 5. steel stud (L?, 10) 6. furnace casing

1. furnace casing . . 2. block insulation

3. refractory concrete 4. metal anchor and fixing . Y flap

/ : 1 5. insulating gravel

section tor, view furnace casing light weight refractory bricks high alumina anchor brick, fixing clamp and hook ramming mix insulation gravel

Fig. 9b: Anchors i n use 84

Anchoring of refractory brick walls

top view

1. block insulation 2. refractory bricks 3. anchor bricks for fastening 4. insulating gravel

Fig. 9b: Anchors in use

CHAPTER ELEVEN

SCAFFOLDING AND ARCHES IN REFRACTORY WORKS

A. SCAFFOLDING

(i) INTRODUCTION

Since Refractory brickwork is a constructive craft, it is continually developing and changing. As furnace designs develop in an upward direction, working platforms must be provided which are extended stage by stage to enable the craftsmen to reach their work -points!.and to provide the means of conveying. materials. These platforms are called scaffolds or formwork.

Accidents are far too common in the construction industry and they are often caused by people taking unnecessary risks like: - (a) Managing with unsuitable or unsafe platforms (b) Not waiting for proper erection (c) Using unstable scaffolds (d) Interfe'ring with scaffolds without author (e) And generally ignoring the safety precautionary

measures.

(ii) THE SCAFFOLDS (BASIC REQUIREMENTS) Enough strength to carry all loads that will be placed too far apart and a single tube should not carry too much weight.

- It should be stable and should not bend or move in any direction. Sufficient bracing is provided both across the scaffold and along its elevation.

- Materials or tools are not expected to fall out of the platform. - The safety of all personnel on site is guaranteed by taking safety precautions at every point where an accident is liable to happen. - Standards shall be either vertical or slightly leaning towards the building and fixed suffciently close together to ensure the stability of the scaffold. - The distance between putlogs shall not as a general rule exceed: i.) 1 m for 31 mm planking ii) 1.5m for 38mm planking iii) 2.59 rn for 50mm planking. - Loose bricks, drain pipes, chimney pots or other

:unsuitable materials shall not be :used for the construction of scaffolds. - Putlogs shall be securely fastened to the ledgers or to the standards. Where they are supported by the wall they shall extend into the wall sufficiently to provide an adequate support. - No scaffold shall be partly dismantled unless it complies with these regulations or a notice is prominently displayed,.warning that the scaffold is not to be used, and any access is effectively'blocked.

(iii) TYPES OF SCAFFOLDS The scaffold material used on the outside of a

unit being repaired must be strong, durable, soundly erected, well braced against possible collapse and well tied into the unit to prevent its falling outwards.

Nowadays, praclr~ally all scaffolding is constructed of metal - either steel or aluminium alloy. In some special cases timber scaffolds are still used.

(a) TRESTLE SCAFFOLDS - These are varied in type and pattern, but they

are normally extremely useful in confined space or where a platform has to be quickly erected and dismantled and where great heights do not have to be reached.

Some types of trestles are used only foc small lifts of about 1.2 or 1.5m high. The most effective type is called a TRE-LEG,,

It can be used in building internal partitions. They are assembled quickly and efficiently.

(b) TUBULAR SCAFFOLDS Metal or tubular scaffolds may be constructed of

steel or .aluminium alloy tubes connected together by means of special fittings or couplings.

Aluminium alloy because of its Lightness, is very popular, though steel scaffolding is much stronger than

the alloy. There are two ways in which tubular scaffolding

may be erected:

- DEPENDENT: Where the scaffold relies on the wall for support.

INDEPENDENT: Where the scaffold is erected so that it is self supporting.

(iv) SCAFFOLDING TERMS (a) Standard: The tube used as a vertical support in a scaffold. It transmits the load to the ground and should be fixed upright or slightly leaning towards the building.

(b) Ledger: The horizontal tube which ties a scaffold longitudinally and acts as a support for putlogs. It should be fixed at right angle to the standard.

(c) Putlog: Is the tube spanning the distance from ledger to the wall of a building, upon which a scaffold board rests. . '

(d) Transom: The tube spanning across two ledgers in an independent scaffold.

(e) Raker: Is the tube which bears on the ground or an adjacent structure. .

.. . (f) Brace: The tube fixed diagonally in a scaffold to

prevent any movement.

(v) WORKING PLATFORMS AND GANGWAYS It is often necessary to provide an access to a

platform by means of a gangway. This must be constructed so as to prevent accidents by people falling off them.

They should never be too steep. If two or more boards are used, they should be braced together so that there is no uneven sagging between the boards.

(vi) THE CONSTRUCTION REGULATION This prescribes the following requirements:

(a) All working platforms and gangways shall be closely boarded, planked or plated. (b) No plank shall be projected beyond its end support by a distance more than four times its thickness, unless it is effectively secured.

(c) Every board shall rest securely and evenly on its supports, and shall also rest on at least three supports. (d) When work has to be done at the end of a wall the platform shall extend at least 610mm beyond the end wall. (e) Every board in a working platform or gangway sliall be not less than 200mm wide, or those exceeding 50mm in thickness not less than 150mm wide.

(vii) LOADS ON SCAFFOLDS A scaffold must not be overloaded, and as far as

possible the load must be evenly distributed. Any load which is transferred on, or to a scaffold,

shall be deposited without any violent shock. .. . Materials shall not be kept upon a scaffold

unless needed for working within a reasonable length of time.

(viii) INSPECTION OF SCAFFOLDS Before a scaffold is used, it must be inspected by

a competent person within 7 days. Especially if the scaffold has been subjected to adverse weather conditions which are likely to affect its stability.

Reports must be made in the register which shall show the following information: (a) The location and description of the scaffold or equipment (b) The date of inspection (c) The result of this inspection (dj The signature of the person making the inspection.

F l y r e 10 illustrates various types of scaifoiding.

tabulator steel

Wide scaffolding

.. .

wide s t r u c t

Fig. 10: Tjpes of scaffolding

B. ARCHES

(i) INTRODUCTION An Arch is a structure comprising a number of

relatively small units, bricks or masonry blocks, which are wedge-shaped, joined together with mortar, and spanning an opening to support the weight above.

Because of their wedge-like form the units support each other. The load tends to make them compact and enables them to transmit the pressure downwards to their supports.

Arches are not commonly used in modern structures. They are however, still designed for special geometries in furnaces.

. . It is, therefore, essential for the modern crafts- man to have a knowledge of traditional work and this includes understanding of the setting out, cutting and building of arches.

( i i ) DEFINITION OF TERMS IN ARCH WORKS: There are various technical terms in connection

with arches and adjacent structures as shown in the sketches, Figure .(a) Extrados: Or Back: The external or outside or upper curve of the arch. (b) Intrados: The inside (inner) curve of the arch. (c) Abutments: The outside supporting brickwork to an arch. (d) Skew-Back: The bricks or stone of the main walling cut sloping to form an abutment for an arch. (e) SPAN: The horizontal distance between the reveals of the supports. (f) Voussoirs: Are the individual wedge-shape bricks or blocks of stone which comprise an arch. The last voussoirs to be placed in position is usually the

central one and is known as the KEY BRICK or KEY STONE. (g) The Crown: The highest point of an arch at which the key brick is placed. (h) Springing Points: Are the lowest points of an arch from which the arch curve starts. (i) The Springing Line: Is a horizontal line drawn through the springing points. (j) Rise: Is the vertical distance between the springing line and the highest point of the intrados. (k) Haunch: Is the name given to the lower part of the arch from the springing line to midway to the crown. (I) The Bed Joints: The joints between the voussoirs which radiate from the centre. (m) The Turning Piece Or Centre Arch: is formed into the actual shape of the arch from a solid piece of wood. This supports the arch whilst it is being c~nstructed. Turning pieces are generally used for segmental arches which have only small span and vise. It is more economical to build up a centre from small pieces of timber when arches of dimensions greater than I m span and 75 or 100mm rise are to be formed. Arch centres have the advantage that they can be conveniently used over quite large spans (n) Impost: The projecting course or courses at the upper part of a pier or other abutment to stress the springing line; somet~mes moulded and known as a cap.

Plinth: The projecting brickwork at the bass of a (0) - wall or pier wh~ch gives the appearance of additional strength; also known as bass. (p) Thickness: The horizontal distance between and at right angles to the front and back faces. It is

sometimes referred to as the width or breadth of the soffit (intrados). (q) Face: The surface in the plane of the wall.

(iii) CLASSIFICATION OF ARCHES Arches are classified according to: -

(a) their shape (b) the materials and workmanship employed in their construction.

The more familiar forms of arches are either flat, segmental or semicircular, whilst others which are not so generally adopted are of the semi-elliptical and pointed.

i > SEGMENTAL ARCHES::.Are those with a rise less than half the span. The joints radiate from the centre which is found, by drawing lines to each, springing from the crown of the intrados and bisecting them; the intersections of the bisectors is the centre of the circle of which the arch form a part. ii) SEMI-CIRCULAR ARCH: There are four varieties of semi-circular arches - a) Gauged Semi-circular arches b) Rough brick circular arches c) Axed brick circular arches d) Purpose made brick circular arches

PURPOSE - MADE FLAT ARCH This arch differs from gauged arch type in that

wrpose-made bricks are used instead of rubbers. The jointing materials and the thickness or the joints are the same as the general walling. a ) Axed Brick Arches: Are carefully set out and the wedge-shaped bricks are all cut to shape and size

and have a pleasing appearance when finished. They are used on the fine work and may be cut from the same brick as the general face work. Or sometimes from bricks which have contrasting colour. (b) Gauged Arches: The voussoirs are accurately prepared to the shape required, and the joints are very thick, about 0.8 to.l.6mm (1.32" to 1.16" in thickness). The general arch represents the highest class of workmanship. It can only be formed satisfactorily if rubber are used. Purpose-made bricks are available for this type of arch construction. (c) .Rough Arches: This cannot be termed a true arch, since the voussoirs are bricks which are not cut to shape in any way. Each brick is set with its centre line normal to the curve of the arch, with the result that each joint between the voussoirs is wedge shaped, being wider at the extrados than the intrados, the bricks on the soffit of the arch being set together, etc.

TYPES OF ARCHES

(a) Upright Arches: are used if a plane roof is needed. They are rarely erected because they produce strong horizontal shearing forces on the abutments and therefore an expensive furnace construction would be necessary. Such arches can only be erected with shaped bricks and only have short spans. Furthermore, they can only be erected on shutterings. It is necessary to lay bricks in ringlike sections. (b) Arches with Risers: The shape of an arch with riser is determined by its span, b y the tensions occurring and by the forces acting on the abutments. The lowest rise is 10% of the span. The rise is to be elevated if higher. temperatures are expected. If necessary the rise must be increased up to 1/2 of the

span (round arch). The thickness of the arch depends on the span and on the expected wear. The. following reference values are recommended:

Span (mm) Thickness of Arch (mml Up to 800 125

2500 250 4500 37 5 7000 500

Arches with risers are lined with the corresponding skew backs and arch bricks. Attention must be paid to achieve the closest possible joints. The arches can be erected in rings or in bonds. It is advisable to erect arches with the help of template arches or plankings. The arch shuttering must be put very accurately and they must be stable. (c) . Ribbed Arches: have ribs projecting over the arch. The arch ribs are laid in rings and the intermediate areas are laid in bonds. (d) Suspended Support Arches: are the ones supported by abutments and are suspended. The ribs projecting on the upper side of the arch are further suspended. This increases durability when wear is great. (e) Monolithic Arches: are the ones erected with refractory masses. (f) Arches from light weiqht refractory bricks: are used for heat treating furnaces with crass-sections. Round arches and arches with oval or chainlike cross-section can be produced from this type of arch.

(v) SUPPORTS AND INSULATION OF ARCHES:

(a) BACKING UP OF ARCHES: All arches with risers having risers larger than

30% of the span must have backing-up at least half of the height of the arch otherwise they could collapse when the arch is stripped. (b) ARCH INSULATION: ,

If great heat builds up in the arch bricks, the arch must be insulated. Increased .thermal stresses can, under certain conditions lead to premature wear of the arch. (c) SUSPENDED ROOFS:

In contrast to self supporting arches every single brick or a certain amount of bricks of the suspend-ed. roof must be suspended with the help of special carrying elements, on rails or other devices.

' Industrial furnaces are equipped with s'uspended roofs in the following cases: - i) to cover wide spaces in furnaces especially spans of more than 3m. This is to prevent great shearing forces in the abutments. ii) If a plain-or shaped form of the roof is required due to operational reasons. 111) If parts of the roof must be exchanged in order to repair premature wear. iv) If there are many or large openings in the roof.

Special shaped bricks and suspending devices have been developed by the refractory industry for all kinds of suspended roofs, Fig. 12.

every single brick can be suspended or two or more bricks can be suspended on a metallic or ceramic bearing element. - in boiler construction it is usual to suspend brickplates from pipes

- Suspended roofs have greater gas permeability than self-supporting arches - Obviously all kinds of and shapes of suspended roofs can be lined with refractory masses, Suspension is effected by ceramic or metallic anchors.

Scaffolds and Arches are shown in Figures 10 & 11

(v) SETTING THE A-RCH When the cutting of the voussoirs is finished the

procedure for setting the arch in place is as follows: - (a) A soffit board is fixed in position and firmly strutted in the opening. (b) The course marks are transferred from the full size drawing to the top of the soffit board. (c) A length of line should be fixed, if possible at the point where the angles of the skewbacks meet. This line is used to ,check the accuracy of the angle at which each arch course is laid. (d) The brick forming the skewbacks are marked from the soffit board with a bevel, rut and bedded with the brickwork at each side of the arch. (e) Lines should be set up along the face of the arch at the top and lower face of the arch. (f) The voussoik are bedded from each side of the arch until the key brick placed in position. (3) When the arch has been checked for accuracy with a straight-edge, the joggle are filled with a neat cement grout or mixture of cement and clean washed sand 1 :I

QUESTIONS

1. What are the essential features of scaffolds and arches used in refractory works? 2. Outline reactors in your plant where scaffolding and using arches will be used for effective refractory works.

1

? ' t '

Stone Brick I

Fiq. 11 : Types of Arches

Fig. I l r AaOlES

q - e TANGENT.

STONE

TUDOR ARCH

NOTE: VOUSSOIR ON NORMAL

/ / -

COMPOUND ARCHES

l o 3

Brick lininq of arched roofs

Mononlithic lining of suspended roofs

. . L. Bottom view

1. block insulation 2. brick holder

(heat resistant cast steel) 3. anchor bricks (high alumina) 4. ramming mix or refractory concrete 5. parting line

Brick linim of suspended roofs

1 . Cerfelt insert

Fig. 12: Suspended Roofs

NOTE: The suspended light weight refractory brick blocks are four light weight prefabs. Refractory bricks are glued with refractory mortar, and the heat resistant hanger is anchored to the bricks.

Section B - B

.-

Suspended curved roof

1. Insulation concrete 2. Hanger rib 3. lnter locked hanger bricks 4. Alumina bricks 5. Anchor brick 6. Light weight refractory bricks 7. Cerfeit insert 8. frame work of the furnace 9. Beam of the hanger rib 10. Steel apron

Section A - A ., .

1 I 1. Insulation concrete . 2. Hanger rib 3. lnter locked hanger bricks

Suspended Roofs

CHAPTER TWELVE

PACKAGING, TRANSPORTATION AND STORAGE OF REFRACTORY MATERIALS

I .O CLASSIFICATION

Refractory materials come in various state, shape and sizes. There is no comprehensive internationally recognized refractory classification so a system was adopted based essentially upon a model used in surveys In the Federal Republic of Germany. ' Adaptations and additions were made in order to carter as far as possible for worldwide use and practices. This class system was used to provide a key to materials and products as an esseniial aid to the collection, processing and interpretation of data.

This classificatibn is given in the Appendix of this book. In addition to classifying each refractory quality using a four digit code, a two digit suffix was added to define the form of the materials, that is shaped or unshaped, dense or insulating, special components (e.g. plates, nozzles, etc) or fibre.

As an example, the .complete code 0103/01 would denote sand (0103) in brick form (01). Each product is clearly identified with the type of handling needed as labelled on the cover.

2.0 PACKAGING: Most end users of refractory materials have to

buy from Vendors or the manufacturers. In the case of developing countries like Nigeria, it is imported from Overseas. The materials are packaged in unit loads that can withstand the weather and double handling involved in the shipment.

2.1 SHAPED DENSE PRODUCTS: These materials that come in the form of bricks

and blocks. Usually the materials are packed inside a carton lined with polyethlene, bounded with strips to avoid easy collapsing and positioned in a wooden pallet. In some cases the bricks are brittle and are protected with shredded wooden materials. The weight of a unit load is about 1576kg gross and 1535kg net,

2.2 UNSHAPED DENSE MATERIALS: The various types of masses are under this

category supplied in 25 to 30kg bags well protected with various layers of water proof paper. The individual bags are packed inside a carton or wooden crate of 1200 x

.. . 900 x 960mm3 .. .

2.3 PLATES: The materials are delivered in the form of plates

like the slide gates and lining boards. Usually it domes in sets made up of various shapes and sizes depending on the area of applications. Each component of a set is packed separately in a wooden crate. Wool materials or shredded cloth or shredded papers are used in- between the products to prevent' breakages during transportation. The crates are positioned on wooden pallets to form a unit load. The weight of the crate is about 2 tonnes,

2.4 SHAPED INSULATING PRODUCTS These products are supplied in pieces and

shapes like discs, plates and crests. Usually, they are placed in-between other materials. The materials are packed inside cartons and placed on top of wooden pallets.

2.5 UNSHAPED INSULATING MATERIALS The various types of mortar and sealants come

under this category. The materials are in paste form. It is initially packaged inside a tube, sealed plastic, metal drums or barrels. The tubed ones are packed inside smaller cartons containing about twelve each. The smaller cartons are then packed inside a bigger carton or wooden crate. The products in drums or barrels comes in various sizes ranging from 2kg to 50kg. The individual containers are further packed inside a wooden crate. A crate weighs about 3 tonnes and placed on top of a wooden pallet.

2.6 FIBRE MATERIALS These are semi-solid materials like felts. It

comes in sheets that are wrapped in bales. The outer surface is protected with waterproof materials. The weight of the packaged material is about 25kg.

2.7 SLEEVES, STOPPER HEADS, TUBES, NOZZLES AND PLUGS

These are solih products of various sizes and shapes depending on the application. They came in pieces inside a carton contai~ing shredded materials. The shredded materials serve as a cushion to prevent breakages during the course of transportation. The cartons as@ strapped with metal or plastic material and placed on top of a wooden pallet. The weight of each carton is about 2 tonnes.

3.0 MOVING: This involves knowing where to move the

materials to and when the materials are to be moved. The at?joilnt of materials moving at a given period of

time along a specific route, as well as the condition of the load is considered and also the point in time that materials are to be delivered to the receiving dock, the storage area and the work station are to be known well ahead of time.

3.1 STORAGE: Refractory materials generally degrade with

moisture and are stored indoors at ambient temperature. The materials are received and stored in a central warehouse or store. The warehouse represents the starting and finishing points for material handling in a plant and should be designed to provide orderly f!ow of materials into and out of the facility. Smooth, gradual slopes should be provided in drive-way approaches and proper drainage must be included.

Access for lift trucks into truck trailers or railcars may be provided by duck plates or permanent, adjustable dock levelers. Dock levelers may be of the manual, counterbalanced type, or they may be mechanically or hydraulically operated. They have safety devices to prevent them from dropping when the delivery vehicle moves away, and locks to permit cross travel for liY trucks.

Dock doors often are equipped with weather seals to prevent entry of unwanted air into or escape from the warehouse. In addition, seals help provide an element of security. Shelters also may be prov~ded to protect trucks and products during inclement weather.

Rubber bumpers and timbers on the dock face absorb the shock of a truck impact. Concrete pipe bumpers are often placed at building corners adjacent to truck manewering space.

Storage equipment is used to provide an orderly method - and accessible place - for storing refractory material. Stacking on the floor invariably wastes spac and makes retrieval of desired items a tedious tas Effective use of cube space should be considered wh planning and installing storage equipment.

Palletized unit loads, typically of refract0 materials are stored in pallet racks. Often' loads are stacked and retrieved by lift trucks operating in aisles between racks.

Additionally, Warehouses for refractory storage must be roofed on smooth wooden or tramped earthen floors. If refractories are stored outdoors, their strength may be worsened by the moisture from the atmospheric air. Compression strength .of .fireclay and magnetite is lowered by 30 percent and that of silica by 35 percent on one year of outdoor storage. Refractories are normally kept in piles up to 1.8m high with passages of at least 0.6m left between them; various kinds and grades of refractories should be kept in separate piles. Refractory powders are kept in roofed stores in bins seating on wooden pallets or wooden floors.

Materials are formally withdrawn from the warehouse when needed. The quantity to be retrieved at any point in time is predetermined to avoid congestion in the work station. At the work area some quantity or materials remaining after usage are stored within the vicinity. Efforts should be made to order only the amount of material needed for a particular campaign. The lead time should include the time for shipment if the materials are coming from overseas. Refractory materials have shelf-lives and stocking of material beyond its life will be undesirable.

4.0 METHODS The method to be employed in handling

refractory material includes the plan or scheme for the operation including the layout - and specific techniques and equipment that may be required. Table 14 provides a general guide for selecting equipment for a number of material handling applications, many of which are found in small to'medium size plants.

Labour requirements also should be specified which defines responsibility as well as manpower needs.

4.1 THE EQUIPMENT DECISION Choosing the right equipment or 'equipment

system - for a handling task can be a challenging job. A number of key factors must be kept in mind before the Selection process is attempted. - Define the Problem: It is amazing how often the real problem requiring a handling solution is not understood or fully analysed. Often the first question that should be asked is "Does this handling step really have to be performed at all?"

*TABLE 14: TYPICAL MATERIAL HANDLING APPLICATIONS I TYPE OF LOAD I LOAD 1 GENERAL 1 TRAVEL I I MATERIAL FLOW REQUIREMENTS 1 I LOAD I SIZE IWEIGHT~ SHAPE

LENGTH ~DIRECTIO~THROUGHPU lCONDlTlO [OTHERS

O M IlmC. Drums

s 7 1 h m venltai

914brnm

2286mm x lnclmd

9144mm

*The intent of this Table is to show some common conditions found in many plants or warehouse operations where refractory materials are handled. There are, of course, many material handling applications that fall beyond the arbitrary boundaries of this Table.

- Looking to the future: Note that throwing in a section of conveyor, or putting in another row of shelving, may provide a temporary solution today, but will it create more problems than it solves in the future? Equipment selection should be planned with an eye to the future. - Remember~ng the systems concept, rarely is an activity performed in a vacuum without affecting other operations or being affected by them, note that the equipment being selected should play a part in the overall goals. of the facility. It is not confined to one small corner'of the plant- An individual flat truck or iift truck is part of a total material handling system - Simplicity is the key: Don't go in for unnecessary sophistication when it is not warranted. For example, take advantage of gravity when possible. Make sure your existing equipment is fully utilized before additional investments are made. Make sure qualified personnel are available to take care of the equipment after it is purchased and in the plant. - Don't Over specify: it makes little sense to buy the most expensive heaviest-duty equipment available for a light-duty operation, or one with a short anticipated life. Likewise, whenever possible, use a standard design instead of a more costly custom piece of equipment. - @ Check the alternatives: Do not select a particular way of accomplishrng the job on the advice of just one equipment supplier. There may be better, less

expensive methods and equipment alternatives that you are overlooking.

4.2 TYPES OF HANDLING EQUIPMENT As noted previously, refractory materials fall

under units (indiv-idual items, assemblies, pallet loads) category. The handling equipment is generally classified into three groups - mobile, stationary and non-standard.

The mobile equipment are motor-powered and include the industrial trucks and demolishing machine. One of the most familiar type is the fork-lift truck, which uses a pair of forks riding on a vertical mast to engage, lift, lower, and move loads. Lift trucks may be manually propelled or powered by etectric motors, gasoline, LPG, or diesel-fueled engines. With some models, the operator walks behind the truck. On others, he rides on . the truck, in either a standing or sitting position. Lift trucks are very effective in lifting, stacking, and unloading materials from storage sacks, highway vehicles, and other equipment. They are also used for short-run transporting of goods in the plant or warehouse. Other common industrial trucks used in handling refractory materials include, box-cars, hand trucks and platform trucks.

The demolishing machine is a special purpose, equipment used in cleaning of used refractory materials inside a ladle. The head is detachable to iit either a hammer, spade or hoe and is pneumatically operated. - The stationary equipment are usu4l;. found in the work areas and include the mixing machines and cutting machines. - The non-standard equipment inciude the ramming I iiachines.

Other equipment and tools for refractory works have been treated earlier in this text.

4.3 TRAINING Industrial studies have shown that over one-fifth

of reported industrial accidents are related to handling, particularly lifting and other types of manual effort. One of the major goals of modern material handing methods is to improve safety of industrial operations. To be successful, any safety effort must include a good combination of training, equipment selection and maintenance, proper operating practices, and a formalized programme of inspection handling operations.

Training is a key ingredient in operators who are well trained in the characteristics of their equipment, and who understand their assignments, are;far less prone to be involved in an accident.

Among the two mobile equipment mentioned earlier, the lift truck operation involves a major training effort for both operators and machinists. It is a good investment, because it pays off in reduced damage to products, equipment and plant facilities. Bent rack uprights, broken pallets, and damaged walls and doors all point to a lack of proper training or supervision or both in many plants. The following are also indicators of inadequate lift truck operator training: -

High maintenance costs - operators may not be performing m t i n e checks.

Excessive product damage and customer complaints

High inc~dence of lift-truck-related accidents damage to physical plant - High level of fuel consumption

Regardless of the type of equipment involved, supervisors must be trained as well as operators. In particular, supervisors should be familiar with the following items: - - Material handling requirements in the plant - Suitability of the equipment for the materials being handled - Environmental effects on equipment operations - Effect of equipment on the environment (carbon monoxide and exhaust fumes).

Equipment service and repair requirements - Operator capabilities - Procedures for planned use and servicing of material handling equipment.

:- - Transportation vehicles and storage units for

refractory materials are shown in Fig. 13 and 14.

QUESTIONS

1. How are refractories stored both in the warehouses and in the work sites? Why are refractories not stored like other production consumables in the Warehouse? 2. A ship load of refractory materials just arrived from overseas, inspection shows that these refractory materials are mainly magnesite masses, silica mortar and plates. Describe the processes for transporting these materials to the Warehouse.

Counterbalanced rider

aisle wck

Fig. 13: Transportation vehicles for Refractory materials

Personal burden carrier

Fig. 13: Transportation vehicles for Refractory materials

Cantilever rack

Flow rack

Fig. 14: Storage Units

120

Fig. 14: Storage Units

121

Boiler wall (s~ecial construction) Top view Section

1. Furnace casing I

2. Block insulation 3. Tongue and groove bricks

; 4. Heat resistant flap (32 x 3mm) 5. Heat resistant console 6. Cerwool

Fig. 18: BOILER WALL REFRACTORY LINING

Section A - A Front View I A

1. Opening bricks (shaped bricks) 2. Permanent lining 3. Ring of alumina bricks 4. Insulating bricks 5. Asbestoes back-up board 6. Expansion joint cerfelt 7 . Lining of the pipe

Fig. 19: BRICK LINING OF OPENINGS

CHAPTER THIRTEEN

REFRACTORY MATERIALS - SPECIFIC APPLICATIONS

INTRODUCTION:

Operating conditions in the furnaces and the furnace construction systems generate stresses as such provision of joints in refractory works is essential to accommodate expansion of the furnace elements and reduce to a safe level the damage of the refractories. The width of these joints should be approximately 50 to 80% of the theoretical thermal expansion of the brickwork considered. When refractory bricks are being laid with refractory cement the joints should'be 100% of the theoretical thermal expansion of the refractory brick.

Most applications must adhere to the manufacturer's recommendations.

(ii) PLANE AND SLIGHTLY CURVED WALLS: Here brick bond is formed in such a manner that

an arch-like effect is achieved in the brickwork (up-right arches . or arches with riser). Anchorages .will- be mounted on the furrlaces shell or furnace trestle (metal or ceramic anchors) to join the furnaces shell and the lining. B ~ k k s with tongue and groove are used to build up a stable interlocked refractory brickwork.

jlii) STRONGLY CURVED OR ARCHED WALLS e.q. BOILERS

Pipes are laid with arch bricks in rings or in bonds With or without tongue and groove, Fewer bricks are requited in this foimation, rGpair work is easier because single rings can be exchanged and the lining is

more elastic and fits better deformation of the furnace shell, but whole rings can fall off if the bricks are not laid correctly.

(vi) REFRACTORY MORTARS AND REFRACTORY CEMENTS:

These jointing materials should achieve the following - compensate for irregularities between bricks, prevent the shifting within the brickwork, and effectively bond the bricks, Mortars may be ceramic or hydraulically or chemically bonded due to the type of bonding agent added to the fire grained refractory material. Ceramic bonding mortars are delivered dry and are prepared with water. They attain their strength during firing :in .the furnace and by ceramic bonding, hydraulically setting mortars are delivered dry, prepared with water, harden at room temperature and sinter during firing -in the furnace. Chemical bonding mortars begin setting at low temperatures and reach final strength during firing and by ceramic sintering.

(vii) REFRACTORY MASSES: fall also into hydraulically setting refractory cement, plastics (lumps, sheets, precdmpressed lumps) and granular masses with low or zero AI2O3 content and with or without bonding agents. The masses can be applied by ramming, casting or spraying. Generally they are cast into plankings and set with vibrations. For ramming also a planking is needed while for spraying no planking is needed. (a) Plastic masses - they can be applied without boarding. Their water content is high up 8% as a result volume stability when drying and heating is difficult. (b) Granular masses - they can be rammed with the help of planking and have low water and clay

content, such masses have low porosity, high strength, volume stability and resistance against chemical influences.

(viii) PREPARATION OF REFRACTORY MASSES: Casting: During this process the water-cement

ratio determines the strength achieved. Uniform mixture is recommended. The mixing process which is recommended (but manufacturer's instructions if available should supersede) is as follows: 9110 of the water requirement according to the bag label is put in the paddle mixer. The specified quantity of the refractory material is added, after two (2) minutes mixing the mixer should be stopped and the consistency of the mix- tested. If the consistency is quite satisfactory, the rest of the required amount of

. water could be added and then after four (4) minutes mixing, the consistency and flowability when vibrated are checked, to certify the mixture good.

(ix) APPLICATION OF REFRACTORY MASSES: Refractory masses are applied by spraying with

a dry-type gun, hot patching with a wet-type gun, patching with the trowel or ramming. - Spraying with a dry-type gun.

This technique of application requires a lot of practical experience. A prerequisite for proper spraying is the admixing of the right amount of water in the spraying nozzle. The angle of gunning, the distance of the nozzle from the wall and the spraying pressure are important factors for effective application.

Hot patching with a wet-type gun. In this process the fresh, ready mixed concrete is

pumped by compressed air in an armoured hose to the

site and then behind the shuttering where it is compacted as usual. - Patching with the trowel.

In using this technique, the mass should be of very fine grain size and mixed with refractory binders. The technique is good for the repair of refractory brickwork that have worn prematurely. Before applying this technique to the refractory work, deposits should be chipped off, loose brickwork removed and if possible a mortar coating could be applied before the ramming repair. - Ramming:

All plastic and granular masses are to be rammed with the help of a shuttering except the precompressed lumps, the so-called "plastic bricks". Plastic and granular masses are delivered ready for use. They are packed with plastic foil, in the form of sheets in packets or in air-tight bags. The packets and bags must be kept air-tight and stored in a cool and dry atmosphere. If the masses dry up they can be re- processed without loss of quality. The material is unpacked, crushed, moistened with water and rammed with pneumatic rammer. It is then crushed again and rammed' until the necessary consistency is achieved. The mixed material in the right condition is easy to form and does not stick to the hand. - Professional Planking:

For the installation of plastic or granular masses a stable shuttering consisting of planks 50mm thick is necessary. The support should consist of strong squared timber and at a maximum interval of 800mm. For large construction sections shutter systems with suspension (support) and clamping fixtures are used. This is advantageous in roof constructions.

- Rules for Ramming: When ramming masses the following rules must

be obeyed: the-base on which ramming will be done must be cleaned well. for repair work the base must be rough or dovetailed. accretions or deposits and loose parts of brickwork must be removed. as a rule ramming must be done parallel to the final surface. If this is not possible for example on floors and roofs, ramming can be carried out at an angle of 450. no cavities or layers should occur during ramming 3mm size steamholes must be punched into the lining at a depth of 150mm maximum ramming masses which do not b i d chemically (cold binding) must be covered with plastic foils immediately after the removal of the planking, to keep the material humid

Ramming of Plastic Masses: As these masses are delivered in sheets in a

cardboard, the sheets are removed and laid as a whole or in pieces on a clean base. Each of these layers is rammed with a compressed air hammer (car mallet of Ikg) and they bond. Each layer must be rammed 3 to 5 times in order to achieve a good bonding between the layers and to prevent the formation of cavities and layer-like texture. The surface of the rammed layers remains uneven to achieve a good bonding of the layers resulting in the rammed wall being homogeneous. The shuttering must be removed immediately rafter ramming. Immediately after the

stripping and roughening the shrinkage cuts in the wall, roof and floor must be made. If not indicated in the drawing they are cut vertically and horizontally at a distance of 1000mm using a joint gauge of 1-2mm thickness. The shri,ikage cuts are made up '13 of the thickness of the wall (max. depth 60mm). The necessary steam holes are punched using 3 to 4mm thick pricker with which the whole wall is penetrated by holes ascending to the back. The steam holes should be staggered and at an interval of 100 - 150mm.

CLASSIFICATION OF INSULATING MATERIALS FOR APPLICATION BASED ON CRITICAL APPLICATION TEMPERATUREv(CAT),;

CAT' 500°C . ,

Mats and plates of glass, slag, asbestos wool. Mostly stitched to cardboard. CAT 700°C Mats and plates of mineral wool pressed with binding agents andlor tacked to a wil-e mesh. CAT 800°C Calcium-silicate plates. CAT, 1 OOO°C Bricks and plates from vermiculite etc. CAT over 1 OOO°C Light weight refractory bricks and ceramic fibre mats.

In installing insulation materials, the mats and plates are cemented to the shell of the furnace or fixed with reforming clips or bolts, the bricks are laid with an insulation mortar or refractory c ?mer:t.

Light weight refractory bricks and ceramic fibre mats are used in industrial fl~rnaces for light weight

walls and light weight roofs as working lining. Light weight refractory bricks are laid with cement and fixed at various points to the wall with clamps or other devices. In roofs they are suspended in blocks with the help of special devices. Mats and plates are also not installed in walls and roofs with jointing materials, they are suspended from the furnace shell with the help of special bolts.

- Ramming of Granular Refractory Masses: These masses are mostly delivered in air-tight

50kg bags and are earth humid. The loose materials is poured into the shuttering at a height of 59 - 60mm and is rammed with a compressed air-hammer. This is done

: - by circular movements with the rammer. In this way the materials are compressed so that only a 20mm thick layer of loose material remains. On this loose layer 50 - 60mm of material are poured again and then the material is rammed etc. The direction of ramming is as with ramming plastics, parallel to the final surface and if this is not possible, ramming is done at an angle of 45'.

In contrast to plastic masses, no joints can be cut after granular masses have been installed. Therefore, the rammed sections have to be divided with dividing boards beforehand. Furthermore, it is impossible to punch steam holes.

xi GUIDELINES FOR LINING INDUSTRIAL FURNACES: Material supply and working procedure are key

factors to the successful lining of the industrial furnaces and handing over to production sector for use.

Lining:

Before lining a furnace (reactor), the important dimensions of the furnace foundation, the construction and the furnace shell must be examined. The surfaces ;io be lined must be free of contamination and deposits (scales). Loose scales must be removed by sand blasting. Depending on the season the lining is done, the refractory materials must be kept warm. As must as possible when working with refractory materials, special regulations provided by the manufacturer must be observed, The appropriate equipment must be used at all times for example paddle mixer should be used to mix refractory mortars and mixes. Quantity of water and binders should be measured with a measuring vessel. Precautions about jointing must be kept as the joints are the weak points in refractory brickwork, they must be filled entirely and must be close. In general the joints must not be wider than 3mm. Burn-out joints are arranged at sufficient intervals in the refractory works which are constructed of bricks which have a great thermal expansion, for example silica or magnesite bricks.

- Drying a

Newly lined as well as repaired refractory works must be dried before it is put into use. Drying too quickly and insufficiently can lead to serious damage to the refractories. A good drying profile should be followed. Instructions by the manufacturer should be carefully followed.

Hydraulically and chemically hardening mortars, cements and masses may be dried after they have hardened. Linings with light weight refractory bricks or ceramic fibre plates are generally not dried. Magnesite bricks, chrome-magnesite, dolomite and carbon bricks

should be installed after the back-up lining has been dried, if possible. For drying purposes hot air, steam in heating pipes, heat radiators, gas flames and open fires (wood, coke) can be used. Drying must be uniform in all parts of refractory work. Doors, Bleeder valves etc must be open to let the steam out.

WATER CONTENT OF SOME REFRACTORIES: Bricks 2 - 3% (from humid air) Mortar 20 - 25% Plastic building masses 10 - 12% Spraying masses . 6 - 8 % Cast masses 8 - 14%

HEATING AND COOLING: 2 , . The rate of heating depends on the thermal

properties of the refradories used. Generally it can be said that thick walled refractory work made from materials with high bulk density requires long heating time than thin walled refractory work made from light-weight refractory bricks. The hardening or sintering temperatures of mortars, cements and masses musj be reached during the heating and maintained for a sufficient time. During breakdown temperatures should nct fall, openings and flues must be kept closed. Cooling too quickly must be avoided to prevent cracks from forming in the refractories.

CONTROLLING THE LINING: 7 he state of the lining can be done by:

(i) Observing the lining during operation through inspection holes. s t J

( i~) Inspecting the lining durmg shutdown. (iii) Measuring tne temperature on the furnace shell.

(iv) Measuring the temperature of the refractory using pyrometer, thermoample.

(v) Controlling the thickness of the refractories (measuring the bricks or thickness of the ramm,ed mass).

(vi) Controlling the pressure of the furnace.

REPAIRS: Minor damages on the refractory lining can be

repaired by ramming or sprayinglgunning techniques using masses. Major repairs are done during a shut- down.

:. . QUESTION . .

1. Discuss Packing, Storing and Transportation of Refractory Materials in a named Industry.

2. What is the effect of poor storage and transportation on the shelf life of refractory materials?

REFERENCES

1. KINGERY et al;

2. ALPER, A.M.;

3. KINGERY, W.D.

Introduction to Ceramics Second Edition John Wiley & SONS, New York (1 975) Ed.; Phase Diaqrams: Materials Science and Technoloqv, Vol. 1 "Theory, Principles and Techniques of Phase Diagrams", Academic Press Inc. New York (1 970). Ed.; Kinetics of High Temperature Processes, Technology Press, Cambridge; Mass; and John Wiley & Sons Inc. New York (1 959).

4. M4CKENZIE, J.D. ' ~ d . ; Modern Aspects of the Titreous State, Vol. 3 Butterworth,

* London (1965) 5. Ensineering Properties of Selected Ceramic Materials,

The American Ceramic Society Columbus, Ohio, (1 966)

6. CAMPBELL, I.E. and SHERWOOD, E.M. Eds.; High Temperature Materials and Technologv, John Wiley & Sons, Inc. New York (1 967).

7 CHESTERS, J .H.; Refractories, Production and Properties, Iron and Steel Institute, London (1 973).

8. KRIVANDIN, V. and MARKOV, B ; METALLURGICAL FURNACES, Translated by V.V. Afansyev, M ir Publishers, MOSCOW (1980).

9. DIDIER, A. Famous German Refractory Manufacturing Company Manual.

10. DELTA STEEL COMPANY LIMITED, Manual.

11. OBIKWELU, D.O.N.; Viability of local clays for the Manufacture of Refractories for the Steel and Allied Industries, presented at the Technical Seminar of the Raw Materials Research and Development Council (RMRDC), Lagos ( I 989).

12. OBIKWELU, D.O.N,; The Challenge of Refractories in the Economic growth of Metallurgical lndustries In Nigeria, proceedings of the Technical Conference. of the Nigerian Metallurgical Society at Ajaokuta (1986).

13. OBIKWELU, D.O.N.; TALABI, K.O., CHUKWUOGO, Refractories Requirements in the Nigerian Steel lndustrv and the Viabilitv of Local Raw Materials, proceedings of the International Seminar on Ancilliary lndustries 5- 6 June (1986).

14. OBIKWELU, D.0 N , TALABI, K.O.; Reprocessinq of Used Refractories, proceedings of the International Seminar on Ancilliary lndustries 5-6 (June 1986)

15. OBIKWELU, D.O.N., OKORIE, B.A.; Formulation of Coal-base Insulating Compound for Application in the Metallurgical Industry, proceedings of the sixth Technical Conference of the

Nigerian Metallurgical Society 3-5 Nov. (1988).

(16) MCKAY, W.B.; Building Construction V. 1 Fifth Edition, The English Language Book Society and Longman (1971)

17. LEVIN, E.M., ROBBINS, C.R. and McMURDIE, H,F,; Phase ~ i aq rams for Ceramists, American Ceramic Society, Columbus Ohio (1964)

APPENDIX 1

NIGERIAN CLAY DEPOSITS FOR POSSIBLE REFRACTORY MANUFACTURE

SOURCE

Ozubuiu (Anambra) Enugu (Anambra) *Onibode (Og un)

Oshiele (Ogun) Orun

Okpekpe

*Isakasimta (Mg0=36.5%) (Adamawa)

39.30 42.30 Traces 18.04 0 19.31

::,: 1 53.20 1 1.45 1 21.50 1 10.70 0.36 44.57 Magnesite

QTY x 1 o6

58.30 1

Not known

4.2

0.08

Not known

6

Not known

Not known

'Ttie author of this book has developed techniques for beneficlatrng these clay deposits to acceptable levels for refractory manufacture.

1.55 20.84 Alumino- Silicate

APPENDIX II

LIST OF REFRACTORY MATERIALSIPRODUCT AND KEY NUMBERS FOR HANDLING PURPOSES

KEY NUMBER 01 Silica-Alumina System 01 01 Fused Silica 0102 Silica 0103 Sand 01 04 Pyrophyllite (Roseki) 01 05 25 to 50% Alumina 0106 50 to 75% Alumina 01 07 .75 to 90% Alumina .. .

More than 90% Alumina High-Alumina Special Products yitJ Carbon (e.g. nozzles, sliding gates, lances) High Alumina Special Products without Carbon (e.g. purge bricks) Any others (please specify) Silica-Alumina-Zirconia System Zircon-Alumina Products (e.g. ladle bricks) Zircon Products (e.g. Tundish nozzles) Zirconia Products (e.g. Tundish nozzles) Any others (please specify) Magnesia-Iron Oxide-Silica System Olivine (Forsterite) Any others (please specify) Maqnesia-Lime-Chromic Oxide Svstem Doloma; Fired (45% MgO) Doloma; Pitchbonded (45% MgO) Magnesia; Fired (1.5% Fe203, 85% MgO) Magnesia; Fired (1.5% Fe203, 85%Mg0)

Magnesia; Pitchhonded or Chen~ically Bonded (1.5% Fe203, 85% MgO) Magnesia; Pitchbonded or Chemically Bonded (1.5% FezOs, 85% Mg 0 ) Magnesia Carbon Bricks (5% C, 85% MgO) DoIomalMagnesia Mixed Bricks; Fired (45% to 90% MgO) DolomalMagnesia Mixed Bricks, Pitchbonded (45% to 90% MgO) Magnesia Chrome and Chrome Magnesia Fired Magnesia Chrome and Chrome Magnesia Chemically Bonded Any others (please specify) Maqnesia-Alumina-Chromic Oxide System Spinel (e.g. Picrochromite) .. . Any others (lease specify) Special Products Carbon Refractories Silicon Carbide Products Products with Boron Additives Products with other Additions Any others (please specify)

To typify 'the conditions of design the key number is extended as follows:

Please add

01 for shaped dense products (bricks and blocks) 02 for unshaped dense materials 03 for plates (e.g. slide gates) 04 for shaped insulating products 05 for unshaped insulating materials 06 for fibre materials 07 for sleeves, stopper heads, tubes, nozzles, plugs

Example:

Key No. Key No.

Material Desiqn

0103101 -- for sand bricks 0 IO9lO3 -- for slide gates, high alumina special

products with carbon plates.

APPENDIX Ill

USUAL EXPANSION JOINT DIMENSIONS

Refractory Material (Bricks) High Alumina Alumina-rich Alumina Silica Magnesite Chrome-magnesite Chrome ore Dolomite Carbon Zircon Silicon Carbide Light weight Refractories

Joint Dimensions 8 mmlm 7 mmlm 5 mmlm 13 mmlm 14 mmlm 10 mmlm 12 mmlm 12 mmlni 5 mmlm 5 rnmlm:. . 5 mmlm 0;- 5 mmlm (Depending on raw material)

APPENDIX IV

STANDARDS FOR TESTING OF REFRACTORY MATERIALS.

Standard Institutions include BSI (British Standards Institution), DIN (Deutshe lndustrie Normen); ASTM (American Society for Testing Materials), I S 0 (International Standards Organisation).

A. EXTRACTS FROM BS 1902: Part 1A: 1966 Section Two: Determination of Density and Porosity Section Three: . Determination of true Specific gravity. Section Four: Determination of powder Density Section Five: Determination of permeability to air Section Six: Determination of Refractoriness (P.C. E.) Section Seven: Determination of Refractoriness under Load (R. U .L.) Section Eight: Determination of the Permanent Dimensioyl Change on Reheating (Length or Volume) Section Nine: Determination of Cold Crushing Strength Section Ten: Determination of Thermal. Expansion

(1 0) Section Eleven: Determination of Resistance to Carbon Monoxide

(1 1 ) Section Thirteen: Determination of Modulus of Rupture

(1 2) Section Fourteen: Measurement of size, bulk density and warpage

BS 1902 (1) Part 2A; 1964 Chernical Analysis of

high-silica and alumino-silicate materials (2) Part 2B; 1967 Chemical Analysis of

Aluminous materials (3) Part 2C; 1969 Chemical Analysis of

Chrome-bearing materials (4) Part 2D; 1969 Determination of silica in

high silica alumino-silicate and aluminous materials by coagulation

(5) Part 2E; 1970 Chemical Analysis of Magnesites and dolomites

EXTRACTS FROM DIN STANDARDS (1) DIN 51 010 Part 1: Testing of ceramic

raw materials (1 979) (2) DIN 31 045 (1 976): Determination of

linear changes of a solid body under the influence of heat.

(3) DIN 51046 Part I : Testing of ceramic raw materials. (1976 ) Determination of tl.iermal conductivity of temperature up to 1600,C by the hot wire method.

(4) DIN 51062 Testing of Ceramic raw materials and materials. Preparation and drying of specimens for chemical. analysis.

APPENDIX V

APPLICATION OF REFRACTORY MATERIALS IN SOME INDUSTRIES

1. Boiler Plants, power stations, equipment

2. Glass Industry Melting containers (Tanks) Superstructure (crowns, ports, etc.) Plunger and feeder Regenerators Recuperators

3. Cement Industry: - Rotary kilns

4. Ceramics and Enamel Industries 5. Oil Industry:

FCC Feed preheater -

Soot Blowers Hydrotreater Reactor feed heater Catalytic Reforming stabilizer Reformed Gas Boilers Vacuum Heater Topping Heater

6. Iron arid Steel Industries: - Indurating machines - Gas Generators - Direct Reduction Shaft furnace - Reformer Boxes - Electric Arc furnaces vessels - Ladles - Tundishes - Reheating furnaces - Lime Plant rotary kilns

combustion

7. Refractory lined vessels in some high temperature industries

I 46 d

ml- -Working lining refractory units

Safety lining refractory units

Permanent lining refractory units Steel Shell

$ Refracto mass. - well b103 retnctov

Applicable Refractories (AluminoSilicate, Silica, Alumina refractories

Fig. 20: Schematic diagram of typical high temperature vessel (ladle for steel making)

Refractory Refractpry /? I Refractory

Recuperating zon~ Melting zone Refining zone Applicable ~efractories (AluminoSilicate, Silica, Zirconia, Alumina

Fig. 21: Schematic Glass Furnace System refractories

Worklng l~nlng refractory unlts Safety lmng refractory unlts

---Permanent I~ntng refractory

Applicable Refractories (Tar-Bonded Dolomite Bricks. Chrome-Magnesite)

Fig 2 2 , Typical high temperature tilting vessel (Electric Arc Furnace - Steel making)

Applicable Refractories, Silica, Alumina (Bricks, Castables, Blanket) refractories

Fig. 23: Topping Heater in the Petrorefining Unit

Fig. 24: Refractory - lined Cupola Furnace

Fig. 25: Direct Reduction Shaft Furnace (Refractory lined)

Fig. 26: Electric Arc Furnace Vessel (Refractory lined)

150

Fig. 27: Fluid Catalytic Cracking Unit {FCC) (Refractory Lined)

Fig. 28 Space Shutter Orbiter (Refractory lined) Courtesy NASA

ESTIMATES OF SOME REFRACTORY MATERIALS CONSUMPTION IN SOME HIGH TEMPERATURE INDUSTRIES IN NIGERIA (TONNES PER YEAR):

Refractories Delta Steel Company Ltd., Ovwian-

Ni eria Alumina- 18,000

Basic

Ajaokuta Steel Company Ltd., Ajaokuta In Nigeria 36,000

I

Cement Plants

-- 3.000

4,500

Mln!Steel Plants

4.000

5,900

Delta Steel Compan Ltd., Ovwian-Aladja near Warri in Nigeria Y was desinned for 10 tonnes of liquid steel per year.

eel Company Ltd., Ajaokuta, near Okene was ' 1.3 X 10 tonnes of liquid steel per year in its first

Index ............................................. Abrasion .......................................... Andalusite ......................................... Anthracene

Alumina ............................................... .................................... Aluminosilicate

................................................ Anchor

................................................ Arches .................... ......................... Bauxite .. ................................................ Bolster

............................................ Carbofrax ...................................... Carborundum

............................................ Castables .............................................. Chromic

........................................ Conductivity ................................................ Cyanite

.......................................... Cristobalite .............................................. Dolomite .............................................. Extrados ............................................. Forsterite

................................................. Impost ............................................... lntrados .............................................. Kaolinite

.................................................. Mallet ................................................ Mortars

......................................... Orthor;ilicate ............................................. Periclaise

Putlog .................................................. .................................................. Quartz

.......................................... Refractories ............................................... Slinging ;

..................................................... Stud Tar dolomlte .......................................... Thermal expansion ................................ Tridymite ..............................................

.................................................. T'rawel 'Jibratci ................................................

................................................ Zirconia