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Refractories for Heatingand Melting Furnaces

Refractories for HeatingRefractories for Heatingand Melting Furnacesand Melting Furnaces

Refractories for Heatingand Melting Furnaces

ntroduction - Refractories are used to Iprevent heat loss from the furnace in which solid or liquid metal is held. Normally oxides of Al, Si, Ca, Mg and Fe are present in these materials. Oxide of iron (Fe) is present as impurity. These are porous materials and the extent of porosity decides the heat insulation properties of the refractories. More porosity means better heat insulation characteristics. They help in maintaining required temperature of the metal without appreciable heat loss to the surroundings. Different types of refractories are used in various zones of the furnaces, e. g. walls, roof, hearth, combustion chamber etc. The selection of the refractories to achieve a successful performance would depend upon the areas of application in the furnace and the service conditions. One of the major criteria influencing the choice would be

the overall cost so as to achieve the optimum economic performance. Mainly, operating temperature, working atmosphere should be taken into consideration while selecting the type of refractory and the thickness of each type of refractory. The group of furnaces used for reheating and heat-treatment covers a wide field in the processing of both ferrous and non-ferrous metals. A "re-heating" furnace is utilised to raise the temperature of the metal to prepare it for hot working (shaping), while the "heat-treatment" furnace is used for stress relieving and for changing the physical properties of the metal after the product has attained its final shape. In the present article the author discusses the applications of various refractories for heating as well as some of the melting furnaces.

Types of refractories

In general the refractories used in the heating and heat treatment furnaces are high alumina bricks (minimum 60%

Al2O3) which are closest to the source of heat which may be electric heating elements or burning gases at high temperature. These brick may support the electric heating elements. Followed by these bricks are fireclay bricks (30 – 55% Al2O3) which withstand lower temperatures. Then there is one or more layer of insulating bricks known as hot face and cold face insulation bricks depending upon the temperature of interface between the brick layers. The insulation bricks have high porosity and excellent insulating properties. They are light weight and soft such that cutting of these bricks is possible with hand held hacksaw blade. The important specifications of the bricks used in the furnace takes into account Service temperature, Composition i.e. Al2O3, SiO2, CaO, MgO and FeO percentages, Refractoriness, Bulk Density ( gms/cc ), Apparent Porosity, Cold crushing strength, Pyrometric cone equivalent (PLC) to know melting temperature range and Thermal

- Dhirajlal K. ChauhanConsultant

Email : chauhan_dhirajlal@yahoo.com

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conductivity. Some refractories are made of silicon carbide which have good wear resistance and are suitable to work in carbon rich atmosphere furnaces. The furnace has walls, roof and hearth (bottom) as main parts apart from door. The shape and size of refractory bricks will depend upon size of the furnace, location of bricks in the furnace and thickness of the brick lining. Nature of refractories in each region is briefly described below.

Walls : The quality of the refractory as well as the wall thickness would depend on the operating environment of the furnace, wall height as well as the degree of conservation of heat desired.

Roof : The furnace roof is usually made with sprung arch or suspended construction. Suspended roof construction is more in practice in recent years. The present trend, particularly in USA and other western countries, is to replace shaped bricks in the roof by fully monolithic roofs installed insitu or use of large precast blocks.

Hearth : The hearth (Bottom portion) of the furnaces is the total area available to carry the stock and is subjected to the maximum mechanical abuse, slag and scale attacks, load at high temperatures etc. In continuous furnaces soaking zone has high temperature condition. For this region both basic and high alumina (80-90% Al2O3) refractories have been successfully used. Fusion cast bricks give better performance, but the relatively high cost precludes its use extensively. The introduction of ceramic fibres which can withstand quite high temperatures have immensely increased the possibility of the

extensive use of such materials in furnace constructions.

Examples of heat treatment furnaces with refractory lining

Refractories for melting furnaces

These refractories are designed to perform at higher temperatures than heat

1- Heating elements, 2 - Alumina bricks, 3- Insulation bricks, 4 - Hearth plateFigure 1. Muffle furnace

PIT TYPE NORMALISING FURNACE

Figure 2. Pit furnace

treatment furnaces. The refractory lining in contact with refining slag may be acidic (Higher SiO2 content) or basic (Higher CaO and/or MgO content).

This furnace is used for production of various types of steels including stainless steels. The refractory lining for this furnace should have combination of properties such that they can withstand high temperature and rapidly changing environment during steel production (called HEAT). Factors such as Chemical attack, mechanical wear, thermal stresses, slag chemistry, temp of slag etc are chief causes of dissolution of lining. This lining should be such that wear in different zones of the furnace is uniform and hence relining of all the portions (entire furnace) can be carried out together. This saves down time of the furnace. To achieve this objective the type of refractories and the lining thickness is chosen carefully. The selection also requires careful study of the wear pattern of the refractories in the converter. This lining is called zonal lining. Hence, the zone in which less wear

Refractories for Basic Oxygen Furnace (BOF converter)

Figure 3. Basic Oxygen Furnace (BOF) Figure 4. Cross section of Refractory

occurs can have lower thickness or lower quality of refractories zones in which more wear takes place will have highly wear resistant and costly refractories so as to have longer life. Taking into consideration all these factors following refractories are used.

A) Tar bonded dolomite, pitch bonded dolomite and pitch bonded magnesia.

B) Advanced refractories made with resin bonds, graphite and sintered or fused magnesia which are 99% pure can

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also be used.

C) Presently, the most popular refractories are magnesia carbon (Mag-C) for basic oxygen converter furnace.

Electric arc furnace

These furnaces (EAF) are very

Figure 5. A cross-section of EAF. (Three electrodes (yellow), molten bath (gold), tapping spout at left,

refractory brick movable roof, brick shell, and a refractory-lined bowl-

shaped hearth)

popular in foundries and mini steel plants for melting cast iron and various types of steels. The refractory lining for these furnaces is chosen based up on the type of metal (Cast iron or steel) and nature of slag used.

Roof : The roof of the arc furnace is one of the most affected portion as it is subjected to all kind of stresses, e.g. thermal, chemical and mechanical. The roof is exposed to very high temperature often exceeding 1700°C. It is also exposed to corrosive action of slag and metal oxides, especially iron oxide fumes which is deleterious towards the refractories. Since the design is also complicated because of the openings of the electrodes etc. it is subjected to considerable mechanical stresses. The problem has been tackled in two ways (a) by improving the quality of refractories, (b) by changes in the design of laying. Conventionally, for many years Silica Bricks were used in the EAF roof which however could not fulfil the requirements with the changed severe environments. High alumina bricks (up to 95% Al2O3) have replaced silica bricks

for quite some time.

Side walls : Refractories used are Dolomite (CaO:MgO), Magnesite (MgO), high purity magnesite, direct bonded basic bricks with or without tar impregnation and fusion cast blocks in the side walls. Now a days water cooled panels are replacing refractory material in the side walls.

Bottom/Hearth : The hearth or bottom of electric furnace actually acts as a receptacle of the liquid metal and slag. It has to withstand very arduous service condition like high temperature, slag and metal corrosion, mechanical abuse during charging, thermal stresses due to fluctuation in temperature before and after tapping etc. In the conventional tilting type furnaces lot of structural stresses are also induced during tapping when the maximum tilt is effected. Normally the bottom hearth is divided into two sections viz. sub hearth and working hearth or bottom. The sub-hearth is constructed with few layers of high fired magnesite bricks along with a fire bricks (30-55%Al2O3 & SiO2) lining against the shell for thermal insulation. The working hearth is made by ramming with either dolomite or magnesite suitably graded to provide a monolithic, joint free surface. With the use of high purity magnesite ramming mass the life of hearth could be increased considerably. The thickness of the working hearth is normally around 250-300 mm, which used to be replaced after about 5000 - 10000 heats ( Heat is a melting cycle from charging solid raw materials till tapping of refined molten metal)

depending upon the size of the furnace and operating parameters. Initially the ramming materials used to be mixed with water and then rammed into position. However, currently dry ramming materials have been developed which do not require any water. Such material can be suitable graded to give maximum packing density when put in position. It requires either ramming orvibration with a vibrator. This has an advantage as no drying time is lost and sintered layer thickness can be precisely controlled.

The refractories are a very vital part of furnaces used for heating and melting various metals and alloys. Judicious selection of nature of refractory lining in various parts of the furnace is extremely important for its efficient operation. Of course, the selection of refractories in the furnace must also take into consideration cost and time required for relining of worn out refractories.

References: 1) Refractories for Reheating and Heat treatment furnaces. 130-141

PDF. S.S. Gosh et al. http://eprints.nmlindia.org. 2) Refractories for Electric arc

furnace 37-51 PDF. B.N.Gosh . http://eprints.nmlindia.org.

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