process analysis and energy efficiency improvement on portland limestone cement grinding circuit.pdf

7/24/2019 Process analysis and energy efficiency improvement on Portland limestone cement grinding circuit.pdf

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Abstract

Worldwide cement production is a high energy consuming industry; 90% is thermal and 10% is

electrical energy. This is the third most anthropogenic related carbon dioxide emitting industry in

the world. With a rising price of energy and a growing emphasis on environmental issues the

cement industry is facing significant challenges to both remain a competitive and sustainable.

Composite cement manufacturing is one alternative that is used reduce energy use and greenhouse

gas emissions. The dry grinding process used for finished product represents 40-50% of electrical

energy consumption. It is a very inefficient process generally ranging around 1% efficient.

This research evaluated the process of a typical Portland cement grinding circuit in order to identify

inefficiencies in the process and how the operating parameters may be changed in order to improve

the systems performance. Tests were conducted using samples from a B.C. cement producer and

results analyzed in order to characterize and build a high accuracy model that can be used as a

bench marking tool. Representative sampling and mass balance were performed on the circuit

using real steady state operative conditions data provided by process plant managers.

Major research findings are:

Air separator efficiency is rated 46.06% efficiency at fractions below 35 microns.

High dust load feed and agglomeration are the main reasons for this low separator efficiency.

Agglomeration effect is related to overgrinding, high energy impacts and the use of limestone.


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Whiten model is an adequate tool to fit and correct experimental data on cement air separators

and to provide quantification of operating factors to evaluate the separation process.

Low grinding kinetics at ball mill compartment 01, suggests improper size grinding media

selection and high wear rate for the case studied (for media and liners).


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Table of contents

Abstract................................................................................................................................ .......... ii

Table of contents .......................................................................................................................... iv

List of tables.................................................................................................................................. xi

List of figures................................................................ .............................................................. xiii

List of abbreviations ................................................................ .................................................. xvi

Acknowledgements ................................................................ ................................................... xvii

Dedication ................................................................................................................................. xviii

Chapter 1: Introduction ................................................................ ................................................1

1.1 Circuit 03 cement production.......................................................................................... 2

1.2 Background..................................................................................................................... 3

1.3 Thesis objective .............................................................................................................. 7

1.4 Thesis outline .................................................................................................................. 8

Chapter 2: Literature review........................................................................................................9

2.1 Introduction to cement industry...................................................................................... 9


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2.2 Composite cement manufacturing ................................................................................ 10

2.2.1 History of the use of limestone additions ................................................................. 11

2.2.1.1 Europe............................................................................................................... 11

2.2.1.2 North America .................................................................................................. 13

2.3 Finished grinding and quality of cement ...................................................................... 14

2.3.1 Agglomeration .......................................................................................................... 19

2.4 Grinding technologies in cement industry .................................................................... 19

2.4.1 Ball mill .................................................................................................................... 20

2.4.2 High pressure grinding roll ....................................................................................... 22

2.4.3 Vertical roller mill (VRM)........................................................................................ 23

2.4.4 Horizontal roller mill ................................................................................................ 25

2.5 Improving grinding efficiency in closed circuit cement ball mill................................. 26

2.5.1 Grinding aids............................................................................................................. 27

2.5.2 Optimum media ball size and mixing ratio............................................................... 28

2.5.3 Fill factor of grinding media..................................................................................... 29


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2.5.3.1 Load power ....................................................................................................... 30

2.5.3.2 Blank height measurement................................................................................ 30

2.5.3.3 Run-time ........................................................................................................... 31

2.5.3.4 Ground tonnage................................................................................................. 31

2.5.3.5 Grinding efficiency ........................................................................................... 31

2.5.4 Pre-grinding of raw addition material....................................................................... 32

2.5.5 Air classifiers ............................................................................................................ 32

2.5.5.1 Performance of separators................................................................................. 36

2.5.5.2 High efficiency on separators ........................................................................... 37

2.5.6 Circulating load......................................................................................................... 40

2.6 Impact on carbon dioxide emissions............................................................................. 41

2.7 Modeling and simulation of Portland cement circuits .................................................. 45

2.7.1 Model of performance curves in separators.............................................................. 46

2.7.2 Model of two compartment ball mills....................................................................... 50

Chapter 3: Experimental procedures.........................................................................................54


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3.1 Sampling and data gathering......................................................................................... 54

3.2 Bond standard ball mill grindability test....................................................................... 55

3.3 Breakage distribution function test and estimation using BFDS software ................ 55

3.3.1 Clinker breakage function estimation test procedure................................................ 56

3.4 Particle size distribution analysis.................................................................................. 57

3.4.1 Rosin-Rammler distribution...................................................................................... 57

3.4.2 Whitens model for high efficiency separators .......................................................... 58

3.5 Selection function and its estimation using numerical grinding optimization tools in

language C (NGOTC) software ............................................................................................. 59

3.5.1 Back calculation of selection function from continuous mill data ........................... 59

Chapter 4: Results and discussion..............................................................................................61

4.1 Introduction................................................................................................................... 61

4.1.1 Particle size distribution and production................................................................... 63

4.1.1.1 Fresh feed size distribution............................................................................... 63

4.1.1.2 Air separator size distribution........................................................................... 67


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4.1.1.2.1 Separator partition curve ............................................................................. 69

4.2 Ball mill, Bond work index, breakage and selection function...................................... 77

4.2.1 Work index................................................................................................................ 78

4.2.2 Breakage function ..................................................................................................... 80

4.2.3 Selection function ..................................................................................................... 82

4.2.3.1 Grinding kinetics at compartments 01 and 02 .................................................. 82

4.2.4 Savings estimations................................................................................................... 85

Chapter 5: Major research findings and conclusions...............................................................87

5.1 Major research findings ................................................................................................ 87

5.2 Conclusions................................................................................................................... 89

5.3 Recommendations for future work ............................................................................... 91

References.....................................................................................................................................93

Appendices....................................................................................................................................96

Appendix A : Standard Bond work index calculation .............................................................. 96

A.1 Procedure ....................................................................................................................... 97


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A.2 Sample preparation ........................................................................................................ 97

A.3 Particle size analysis of the feed .................................................................................... 97

A.4 Feed bulk density ................................................................ ........................................... 98

A.5 Performing the grinding test ................................................................ .......................... 99

A.6 Conditions for closure .................................................................................................. 101

A.7 Particle size analysis of the product ................................................................ ............. 101

A.8 Bond test grindability calculations................................................................ ............... 102

Appendix B : Work indices..................................................................................................... 104

B.1 Clinker 100%................................................................................................................ 104

B.2 Limestone ..................................................................................................................... 105

B.3 Clinker 95%/Limestone 5% ......................................................................................... 106

B.4 Clinker 88%/Limestone 12% ................................................................ ....................... 107

B.5 Clinker 60%/Limestone 40% ................................................................ ....................... 108

Appendix C : Particle size distribution ................................................................................... 109

C.1 Fresh feed to ball mill................................................................................................... 109


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C.2 Air separator streams (data provided by plant) ............................................................ 110

C.3 Fresh feed and circulating load feed to ball mill.......................................................... 111

Appendix D : Bond equation for modeling throughput and savings ...................................... 112

Appendix E : Specific rate of breakage .................................................................................. 113


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List of tables

Table 1-1: Cement types and production at plant site 2014................................ ............................ 1

Table 1-2: Production circuit data in the studied cement plant in B.C ........................................... 2

Table 2-1: Type designation for Canadian Portland Cement and ASTM equivalent ................... 10

Table 2-2: European standard related composite cement ............................................................. 12

Table 2-3: Electricity consumption during production of ordinary Portland cement................... 26

Table 2-4: Effect of Sika Polycarboxylate ether polymers grinding aid on energy use............. 27

Table 2-5: Distinctive features of separators of different designs ................................................ 39

Table 2-6: Influence of circulating load and type of separator on mill efficiency ....................... 40

Table 2-7: Fuel savings and CO2 emissions reduction with PLC ................................................. 45

Table 4-1: Circuit 03 equipment design specifications and operating condition on type 10 cement

....................................................................................................................................................... 62

Table 4-2: Separator efficiency parameters .................................................................................. 70

Table 4-3: Ball mill grinding media charge details for C1 and C2............................................... 78

Table 4-4: Work indices for research samples.............................................................................. 79


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Table 4-5: Clinker average breakage function.............................................................................. 81

Table 4-6: Savings estimate on electricity for fresh feed size reduction at ball mill 03............... 86

Table 4-7: Profit estimate on increase in throughput at air classifier circuit 03........................... 86

Table 5-1: Summary of current and modeled separator parameters ............................................. 91


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List of figures

Figure 1-1: Circuit 03 in the studied B.C plant............................................................................... 3

Figure 1-2: Global production of cement........................................................................................ 4

Figure 2-1: Clinker (left) and finished cement (right) .................................................................... 9

Figure 2-2: Data on cement types produced in Europe................................................................. 13

Figure 2-3: Relationship between compressive strength and uniformity factor according to Gates

GaudinSchuman plotting ................................................................ ............................................ 16

Figure 2-4: Grindability of clinker and limestone ................................................................ ........ 17

Figure 2-5: Grindability of limestone - clinker cement mixtures ................................................. 17

Figure 2-6: Uniformity factor of inter-ground cement mixtures .................................................. 18

Figure 2-7: Two-compartment tube ball mill. A- Compartment 01, B- Compartment 01/02 and

separating diaphragm ................................................................ .................................................... 21

Figure 2-8: High pressure grinding roll ........................................................................................ 23

Figure 2-9: Vertical roller mill...................................................................................................... 24

Figure 2-10: HoroMill schematic diagram ................................................................................... 25

Figure 2-11: Optimum ball mill void filling ................................................................ ................. 30


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Figure 2-12: First generation air separator................................ .................................................... 33

Figure 2-13: Forces balance in an air separator ............................................................................ 34

Figure 2-14: Typical tromp curve ................................................................ ................................. 37

Figure 2-15: Total global industry direct greenhouse gases emission.......................................... 41

Figure 2-16: The IEA/CSI blue map for CO2 emissions reduction .............................................. 42

Figure 2-17: Average kg of CO2 released per ton of cement produced........................................ 43

Figure 2-18: Material balance - no raw limestone addition.......................................................... 44

Figure 2-19: Material balance - PLC with 5% limestone addition ............................................... 44

Figure 2-20: Efficiency to overflow vs size.................................................................................. 47

Figure 2-21: Variation of efficiency curve related to ................................................................ 48

Figure 2-22: Variation of efficiency curve with ................................................................ ........ 49

Figure 2-23: Relation between (a) feed/bypass (b) and dust loading/bypass ............................... 49

Figure 2-24: (a) Two compartment ball mill (b) model circuit array ........................................... 50

Figure 2-25: Comminution on a closed circuit ............................................................................. 52

Figure 3-1: Circuit 03 sampling points ................................................................ ......................... 54


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Figure 3-2: Representation of the distribution of particle breakage ............................................. 56

Figure 4-1: Particle size distribution of circuit 03 fresh feed and clinker at CKP........................ 64

Figure 4-2: Current d80 feed and product size at circuit 03........................................................... 65

Figure 4-3: Particle size distribution on air separator................................................................... 67

Figure 4-4: Mass balance at air separator ..................................................................................... 68

Figure 4-5: Separator efficiency reports to product fit using Whiten model................................ 69

Figure 4-6: Correlation between C, parameters and relation to plant data ................................ 74

Figure 4-7: Effect of separator dust loading on bypass and relation to plant data........................ 76

Figure 4-8: Relation between sharpness and dust loading............................................................ 77

Figure 4-9: Clinker breakage function at normalized size............................................................ 80

Figure 4-10: Selection function for compartments 01 & 02 on 100% clinker.............................. 83


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List of abbreviations

Abbreviation Description

ASTM American Society of Testing Material

BM Ball Mill

CKP Chichibu Kawasaki Pre-grinder

CSA Canadian Standard Association

CSI Cement Sustainable Initiative

EIA U.S Energy Information Administration

HPGR High Pressure Grinding Roll

IEA International Energy Agency

PC Portland Cement

PCA Portland Cement Association

PFC Portland Flyash Cement

PLC Portland Limestone Cement

PPC Portland Pozzolan Cement

PSD Particle Size Distribution

RPM Revolutions Per Minute

SSA Specific Surface Area

VRM Vertical Roller Mill


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Acknowledgements

I would like to express my deep appreciation to Dr. John Meech who trusted and gave me the

opportunity of coming back to college. I would like to acknowledge Dr. Marcello Veiga, for

adopting me as my co-supervisor and Dr. Akbar Farzanegan, for his right technical guidance and

support throughout this research project.

My gratitude for the support of all the sponsors involved in providing the logistics and funds

necessary for this study.

The assistance of Lo Pius, Aaron Hope, Maria Liu, Leslie Nicholson, Amit Kumar, Nawoong

Yoon and Mike McClintock are deeply acknowledged.

Finally, I would like to thank my loving wife and two children for their support during my student

duties.

This research has been funded by NSERC (Natural Sciences and Engineering Research Council

of Canada) as an Engage Grants program whit a main purpose of the engage Grants programs the

interest of industry as partners on research activities related to production improvement, new

technology/products development and knowledge transfer within universities.


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Dedication

In memory of Professor John A. Meech, who devoted his professional life to the development of

sustainable mining and a passion for teaching.


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CHAPTER 1: INTRODUCTION

The following research has been performed at a Portland cement plant located in the province of

British Columbia, Canada. This plant produces three types of cements: Type 10 (GU) is a general

use hydraulic cement with 95% clinker and 5% gypsum, Portland Limestone Cement (PLC) with

a composition up to 12% limestone addition, 5% gypsum and 83% clinker and Type III cement

that is a high early strength hydraulic cement with average productions. Gypsum is usually added

up to 5% in each cement type in order to control the rate of setting of the cement (Bhatty, 2011).

A detail on production per type of cement is shown on Table 1-1.

Table 1-1: Cement types and production at plant site 2014

Cement type Annual Production tonnes/year

Type 10 853,989

PLC 181,594

Type III 43,425

The production of finished cement is performed at three grinding circuits identified as circuits 01,

02 and 03. Circuits 01 and 02 are operated individually in a closed circuit grinding dry clinker and

additives each at a two compartment ball mills with a high efficiency air separator that classifies

the finished product and rejects. Circuit 03 has a vertical pre-grinder that condition the clinker feed

by reducing its size feeding to the ball mill. Limestone and gypsum are fed directly to a ball mill

that operates in a closed circuit with a high efficiency air separator. For the purpose of this study

the research was focused on Circuit 03, since this circuit has the highest production capacity and


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the potential for energy improvements like the current operation of Chichibu Kawasaki Pre-grinder

(a vertical grinder) that process clinker and can be used also to reduce limestone. A summary at

this B.C cement plant showing specific energy use, production rate and limestone addition may be

found in Table 1-2. Blaine surface area is a quality parameter on finished cement that is related

with early strength development and water/clinker ratio should be brought in all these three circuits

to 3,700 cm 2/g to assure an adequate strength on finished product. The low specific energy

consumption (energy in kWh used to produce a ton of finished cement) of Circuit 03 is basically

due to the current operation of the pre-grinder before the ball mill.

Table 1-2: Production circuit data in the studied cement plant in B.C

Circuit Production rate, t/h Specific energy, kWh/t Limestone addition , %

01 106 38.35 9.5

02 106 42.48 12.5

03 101-130 30.0-36.901 4.0-13.0

Average BM systems 32-37 *

(Seebach, 1996)* At Blaine 3,000-3,200 cm2/ g1 without including specific energy from CKP pre-grinder

1.1 Circuit 03 cement production

Circuit 03 is composed of a vertical roller mill pre-grinder, feeding a two compartment ball mill

operated as a closed circuit with a third generation high efficiency Osepa separator that rejects the

oversize particles to the ball mill and separates the finished product. This circuit is used to produce

mainly type 10 cement (4% limestone) and is shown on Figure 1-1.


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Figure 1-1: Circuit 03 in the studied B.C plant

1.2 Background

Cement is a key material used for the construction in housing and infrastructure. According to the

International Energy Agency report (IEA, 2009) approximately 3.6 billion tonnes of cement was

produced worldwide. The global production of cement actual and projected is shown in Figure

1-2.


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Figure 1-2: Global production of cement

(EIA, 2009)

The cement industry worldwide is facing challenges to conserve material and energy resources,

and a demand to reduce CO2 emissions. According to Sustainable Cement Initiative CSI

(Schneider et al., 2011) the main alternatives for cement producers are the increase in energy

efficiency, clinker substitution and the use of alternative fuels.

Cement production is an energy intensive process requiring an energy input of 8501100 kWh/t

of cement produced (Harder, 2003) and is the third most intensive anthropogenic industry related

in terms of carbon dioxide (Abdel-Aziz et al., 2014). The thermal energy in cement production

represents approximately 90% of the total specific energy consumption with major fuel sources

ranging from coal, fuel oil to alternative residual fuels such as biomass, animal wastes and

discarded tires. Electrical energy accounts for the remaining 10% of the total specific energy

consumption. The selection of the fuel source is primarily based on the cost. The electrical energy


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consumed in a conventional cement production process is typically 95110 kWh/t. The process

of comminution, crushing and grinding of cement raw materials and finished cement, accounts for

70% of the total electrical energy. The grinding stage for clinker and other additives accounts for

approximately 40 to 50% of total electrical energy consumption (Harder, 2003).

Despite a high specific energy demand, two-compartment tube ball mills with an air classifier in

closed circuit have been used for the finish grinding of cement for over 100 years due to their good

reliability and favorable physical and chemical properties of the cement product such as a narrower

particle size distribution (Aguero & Meech, 2014). Unfortunately, ball mills are one of the lowest

energy efficiencies of all the grinding mills. Ball mills suffer from considerable energy loss

(approximately 98%) in the form of heat due to friction and collision in the tumbling mass of balls

which transfers input energy to an unconfined bed of particles (Duda, 1976). Numerous impacts

are required to produce effective breakage. Due to the high energy demand and the inherently low

energy efficiency of conventional ball mill grinding, the cement industry is continually searching

for new ways to reduce the energy use by improvements in mill design and circuit configuration.

In recent years, the use of alternative fuels has already increased signi ficantly, however the

potential for further improvements still exists. In cement, the reduction of the clinker during

finished cement grinding by substitution with some specific materials having properties similar to

clinker (such as limestone, pozzolan and blast furnace slag) remains a key priority. Remarkable

progress has already been made in this area. Nevertheless, appropriate materials are limited by

their regional availability. New materials might be able to play a significant role as cement

constituents in the future, such as the use of synthetic pozzolan (waste material recovered from


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combustion residues having an SiO2/CaO ratio greater than 1,e.g. waste incineration/power plants

tails, and having alkali oxides in amounts exceeding 1.5% by weight). Currently, the safe

proportion of replacing clinker by alternative materials, such as raw limestone is up to 10%

(Ramezanianpour et al., 2009), but the maximum extent of the substitution of pozzolanic and

limestone additives still needs to be evaluated.

Initiatives to reduce the carbon footprint, reduce electrical use and the use of different additives to

the clinker are trending in cement industry worldwide. Portland Limestone Cement (PLC) is

progressively becoming a common product in the industry. PLC is produced by inter-grinding

cement clinker, raw limestone and gypsum. The replacement of clinker with raw limestone in

Portland cement production has resulted in a proportional reduction in the amount of fuel usage

and CO2 emissions associated with cement production (Nisbet, 1996)

The following study has been performed at a Portland Cement Plant in the province of British

Columbia, Canada with an annual production of 85,989 t of type 10 or General Use cement (GU),

181,594 t of Portland Limestone Cement and 43,425 t of Type III cement. Finished grinding of

cement is performed using three production circuits: 01, 02 and 03. Each of these circuits has a

ball mill which operates with high efficiency separators in closed circuit.

This research is focused on circuit 03 because of a greater potential to improve circuit efficiency.

Circuit 03 has the highest production capacity and is designed to operate in series with a vertical

roller miller pre-crusher and individual storage capacity.


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In order to characterize the grinding behavior of different parts of the circuit, particle size

distribution (PSD) tests, breakage function, selection function and Bond grindability work index

(Wi) tests have been carried out (Drosdiak, 2013).

1.3 Thesis objective

The primary objective of this research is to evaluate circuit 03 of a B.C Portland cement plant in

order to identify energy inefficiencies and propose findings to improve operating parameters. To

provide recommendations on improving the efficiency of the circuit, in order to accomplish this

target the following secondary objectives have been defined,

Analyze representative samples taken from the production circuit, their posterior analysis

at the lab, interpretation of process and production data provided by the plant.

Evaluate the breakage kinetics mechanisms on the two compartments of the ball mill

identifying the breakage and selection function.

Determine the classification function and performance of air efficiency separator using

precise models, identify its probable causes and compare against normalizable

bibliographic data.

Define recommendations to improve the system and suggest possible areas of further

research.

Create a baseline for future energy improvements by the use of computer simulation

software in order to become a benchmarking reference for further research for the


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improvement of Portland cement grinding circuits for ordinary and PLC cement

manufacturing.

1.4 Thesis outline

Chapter 2 provides a literature review on a brief overview of the worldwide cement industry,

current technologies used in the grinding process and key factors involved with regards to the

quality of cement. This chapter also discusses the importance of using PLC cements and its impacts

on greenhouse gas emissions. The chapter includes the modeling and simulation principles for air

separators and ball mills and examines researches on relating operating parameters on air

classifiers.

Chapter 3 discusses the experimental procedures used for testing the different samples and also

the description of software used to calculate parameters such as: uniformity factor, breakage

function and selection function.

Chapter 4 provides a description of the clinker used based on the work index, and other lab analyses

and their relation within the existing circuit. The chapter also discusses operating parameters and

effects on the process.

This thesis is concluded in Chapter 5 with a list of the major findings and recommendations for

future work.


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CHAPTER 2: LITERATURE REVIEW

2.1 Introduction to cement industry

Cement is produced from a process of calcining a mixture of limestone and clay minerals with

some minor components as: iron ore, bauxite and sand. Raw materials are brought to a rotating

kiln with a temperature ranging from 1,3000C to 1,5500C (Bhatty, 2011). This sintering

temperature generates a new product is defined as clinker shown at Figure 2-1.

Figure 2-1: Clinker (left) and finished cement (right)

(www.nachi.org)

The final cement product is usually produced when the clinker is ground with usually 5% of

gypsum. The grinding process target is to produce a fine powder with 80% particle passing size

(d80) of 30-40 microns. According to ASTM C-150 specifications (ASTM, 2011) there are eight

different types of Portland cement: Type I, IA, II, IIA, III, IIIA, IV and V and is based on: particle


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fineness, chemical content, reactivity, early strength and final use. In Canada the types are related

to the Canada Standard Association (CSA) A3000-03 standards and specified as type GU (General

Use and suitable for all operations), MS (moderate sulfate), MH (moderate heat), HE (high early

strength), LH (low heat cement) and HS (high sulfate) detailed description and equivalents are

shown in Table 2-1.

Table 2-1: Type designation for Canadian Portland Cement and ASTM equivalent

(CSA, 2013)

2.2 Composite cement manufacturing

Composite cement refers to the addition of raw mineral additives in the final clinker grinding

process that reduces the use of clinker in the finished product. Usually cement manufacturers allow

substitution of 5% to 35% of the clinker. The substitutes can be limestone, blast furnace slag, coal

fly-ash, synthetic or natural pozzolan also identified as PLC (Portland Limestone Cement), PFC

(Portland fly-ash cement) and PPC (Portland Pozzolan Cement). There are economic,

environmental, and technical advantages related to the manufacture of composite cements. M In

Canada manufacturers are slowly accepting the manufacture of composite cement after several


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well-publicized trials and rigorous testing and standards development such as the European

Standard EN 197-1 (EN, 2011) , ASTM C595 (ASTM C595, 2014), and Canadian CSA

A3000/3001 (CSA, 2013) . Economic benefits relate to the fact that less fuel per unit of cement

product is required in calcining since the clinker is replaced with a raw composite material thus

reducing the thermal energy required to manufacture clinker. Environmental benefits are related

to the emissions of greenhouse gases from combustion sources.

2.2.1 History of the use of limestone additions

2.2.1.1 Europe

There have been several early documented experiences on the use of limestone addition in cement

manufacturing. In Europe, a number of countries allowed different percentages of limestone prior

to the adoption of the European Standard EN 197-1 (EN, 2011) . For example, in Germany cements

with 20% limestone were produced by Heidelberg Cement as early as 1965 for specialty

applications in industry (Nokken et al., 2007).

In the 1987 draft of EN 197-1, a cement designated as PKZ (Portland Kalkstein Zement) was

composed of 85+/-5% clinker and 15+/-5% limestone (Nokken et al., 2007). By 1990, 15+/-5%

limestone blended cements were reported to be commonly used in Germany. In the United

Kingdom, BS 7583 (BS, 1996) allowed up to 20% limestone cement in 1992. European Standard

number EN 197-1 now allows all of the 27 common types of cement to contain 5% Minor

Additional Components (MAC) or mineral additives, which most typically are either limestone, or

pozzolan as shown in Table 2-2 (EN, 2011).


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Table 2-2: European standard related composite cement

(EN, 2011)

According to Table 2-2, six different types of cement allow higher amounts of limestone in two

clinker replacement levels, CEM II/A-L and CEM II/A-LL (6 - 20% limestone). The use of CEM


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II limestone cements has grown from 15% in 1999 to 31.4% in 2004 and is now the single largest

type of cement produced in Europe as shown in Figure 2-2.

Figure 2-2: Data on cement types produced in Europe

(Nokken et al., 2007)

2.2.1.2 North America

In Canada, CSA (Nokken et al., 2007) has allowed up to 5% limestone addition in the clinker to

make composite cements and defined as Type GU under CSA 3,001 designation since 1983 (CSA,

2013). This was related to the presentation of data from the Portland Cement Association in Canada

to CSA that 5% limestone had no detrimental effect on concrete properties based on several studies

(Sohoni et al., 1991). There have been attempts to allow a maximum of 12% of limestone addition


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in the clinker grinding circuit, which is related to market driven forces and new regulatory

government standards in Canada.

2.3 Finished grinding and quality of cement

Grinding of clinker with additives is the final part of the cement production. It has a great impact

on finished product quality. Specific surface area (SSA), particle size distribution (PSD) and

uniformity factor (n) are important physical parameters affecting cement service properties. These

parameters define the proportion of fine and coarse particles in the cement. Grinding technologies

(Ball mill, VRM, HPGR or Horomill) have different effects on the particle (Celik, 2009). It is not

effective to excessive grinding in order to obtain a large surface area. The ground product must

follow certain criteria relative to its particle size distribution in order to ensure the hardening

process. According to Duda (1976) , the technology of grinding clinker is based on the following

aspects:

The particle size fraction from 3 to 30 microns is conductive to the most strength development

of the cement.

The particle size fraction below 3 microns contributes to the initial strength only. This particle

fraction hydrates faster and after one day results in the highest compressive and flexural

strengths.

The fraction above 60 microns hydrates slowly and does not have significant contribution to

the strength of the cement.


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The particle size distribution controls some cement quality parameters such as water demand,

setting and hydration reaction (Schiller & Ellerbrock, 1992), heat release, capillary porosity

percolation, diffusivity, shrinkage and microstructure (Bentz, 1999).

The Uniformity factor, n, is defined by the slope of the graph representing the size distribution

using Rosin-Rammler mathematical function (Gupta & Yan, 2006) is shown on Figure 2-6. It

defines the size distribution as narrow (sharp cut with a high slope or high uniformity value) or

wide (prolonged slope with a low uniformity value). It is already established that narrow and

wide particle size distributions under Rosin-Rammler plot have different influences upon cement

properties. Wider particle size distribution increases packing density and decreases water demand,

while a narrower particle size distribution gives higher hydration rates for equal specific surface

area (Celik, 2009). A narrow particle size distribution, produced by closed circuit grinding with

high efficiency separators, influences both cement paste and concrete properties (Sumner, 1989).

For samples having a constant position parameter, the 28 days strength remained unchanged even

for increased slope. This is because the position parameter of a cement sample lies in the range of

1532 m which is the determinant particle size range for strength development (Ellerbrock,

1985).

The relationship between the compressive strength and uniformity factor according to Gaudin-

Schuman is shown in Figure 2-3. Higher compressive strength is obtained when the value of

uniformity factor n is higher than 2 (narrow slope). Lower than 2 (wide slope) the compressive

strength is almost a constant (Celik, 2009).


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Figure 2-4: Grindability of clinker and limestone

(Opoczky, 1996)

As the content of limestone increases, the energy required to produce the same fine product

decreases under controlled lab conditions and shown in Figure 2-5 . It can be deducted from

reference (Opoczky, 1996) and Figure 2-5 that replacement of clinker with a material of lower

work index as limestone decreases the mixed grindability and thus the energy usage.

Figure 2-5: Grindability of limestone - clinker cement mixtures

(Opoczky, 1996)


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The particle size distribution of any mixture of harder and softer constituents inter ground together

is affected by each of its respective grindabilities (Schiller & Ellerbrock, 1992). On Figure 2-6,

Portland limestone cement gave a wider particle size distribution (lower slope) than other cements

that were inter-ground with fly ash or natural pozzolan due to the softness of limestone compared

to fly-ash and pozzolan. PC represents Portland Cement 95% clinker and 5% gypsum.

Figure 2-6: Uniformity factor of inter-ground cement mixtures

(Voglis, 2005)

When producing Portland limestone cements in order to provide equivalent compressive limestone

strengths (compared to just Portland cement) the grinding time required is increased and having a

greater surface area to obtain the targeted compressive strength, this by having to increase the

energy use. It can be deducted from Figure 2-5 that the grain size was considerably reduced mainly

due to the effect of over grinding limestone which was caused by the presence of a harder clinker

grinding media that abrades easily the soft limestone (Opoczky, 1996).


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Tsivilis et al. (1999) evaluated the production of fines when grinding clinker and limestone for

various times. In general, the clinker is concentrated in the coarser fraction because it is more

difficult to grind it than limestone. Limestone reports to the finer fraction at an early grinding stage

as the harder clinker abrades the limestone and returns to the mill in the circulating load as harder

concentrated circulating load.

2.3.1 Agglomeration

Overgrinding the cement in a ball mill can have a negative impact on production as energy

increases due to the agglomeration effect and the drag of fine particles. This is a reason why size

classification is important at the circuit. Agglomeration is the bonding of small particles one to

another under the Vander Wall forces principle. It is a consequence of breakage energy oversupply

by the effect of grinding impacts of high energy level generally from higher grinding media size

impacts (larger balls), temperature and the crystals structure of material ground. Limestone is

considered a highly agglomerative material (Tams, 1983). Agglomeration can be reduced by

improving separator efficiency and reducing the oversized grinding media and the use of grinding

aids.

2.4 Grinding technologies in cement industry

Grinding systems in cement industry play an important role in the particle size distribution and

particle shape. This affects the reactivity of the clinker and the temperature dependence of

dehydrating gypsum that is ground together with the clinker. These factors affect the mortar


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properties of the cement product such as water demand, initial and final setting times and strength

development (Celik, 2009).

Ball mills have been used as the main grinding equipment for finished cement production for over

100 years. Although simple to operate and cost competitive relative to other technologies, the low

efficiency of ball milling is one of the main reasons for the development of more efficient grinding

processes in recent years. Vertical Roller Mills (VRM), High Pressure Grinding Rolls (HPGR),

Vertical Shaft Impact crushers (VSI) and more recently, the Horizontal Roller Mill (HOROMILL)

(in which energy consumption is substantially reduced) has resulted in an improvement between

45-70% in specific energy related a typical ball mill (Seebach, 1996).

2.4.1 Ball mill

Ball mills or tubular mills are built with diameters up to 6.0 m and lengths up to 20 m; the drive

ratings today are as high as 10,000 kW with stable operation and maintenance of a ball mill is

relatively simple. The maintenance cost and the capital cost are relatively low compared to other

technologies. Due to the high levels of operational reliability and availability (~95%) ball mills

remain the most frequently applied finishing grinding unit in cement plants. Compared with newer

milling devices as VRM, HPGR, ball mills have the highest specific power consumption and the

lowest power utilization (about 3235 kWh/ton depending on the material hardness and to a

fineness between 3,000-3,200 cm2/gr) (Seebach, 1996) Most of the energy is lost as heat from the

collision of the steel balls among themselves and against the mill walls (Duda, 1976).


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Portland cement production is usually finished using a two compartment ball mill as shown in

Figure 2-7. First compartment or chamber 01 is known as the coarse chamber and in the second

compartment material is finely ground. Between the two compartments there is a classification

diaphragm that screens the fine form the coarse material.

Figure 2-7: Two-compartment tube ball mill. A- Compartment 01, B- Compartment 01/02 and

separating diaphragm

Generally on cement mills, the product is ground dry in a ball mill has a relatively wider particle

size distribution; hence it is required to operate the ball mill in closed circuit with a size classifier

with an efficient or sharp cut of size separator. This happens especially when high levels of fines

are generated, when mixtures have low Bond work index or grinding materials that have a tendency

to agglomerate due to overgrinding effect. The circulating loads range from 100% up to 600% that

are established based on the grindability of the new feed, the cut size, and the required product

fineness in relation to reaching the adequate cement strength (Duda, 1976).


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The energy efficiency of dry ball-mill grinding of cement depends on factors such as: ball charge

fill-ratio, mill length/diameter ratio, size distribution of the ball charge, operating conditions of the

air separators, air flow through the mill, production rate, use of grinding aids and the hardness and

fineness of the feed and product (generally referred to as the Work Index (kWh/t) and the F80 and

P80 sizes respectively) (Gupta & Yan, 2006).

2.4.2 High pressure grinding roll

In High Pressure Grinding Roll (HPGR), the material is reduced by a highly compressive stress

created by two counter-rotating rolls (one fixed and another floating). This creates a critical

fracture process that presses the material into a compact flow area. This flow area is shown in

Figure 2-8. The grinding pressure between the rolls is 50 to 350 MPa, and the circumferential

speed of the rolls varies between 1 and 2 m/s (Rosemann & Ellerbrock, 1998) on the grindability

characteristics of the feed and the pressure applied to the roll, the compacted cake (consisting of

over 70% solids by volume) has a fine fraction below 90 m. Up to 40% of these fines must be

recovered by de-agglomeration of the compacted cake using another de-agglomerating device. The

specific power utilization is between 14.6-19.8 kWh/t at a Blaine area 3,000-3,200 cm2/g

(Seebach, 1996). HPGR are reported to be 45-60% more efficient than ball mills (Seebach, 1996).


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Figure 2-8: High pressure grinding roll

(KHD Humboldt Wedag, 2011)

Trouble-free operation of an HPGR depends to a great extent on ensuring proper moisture below

3% and the maximum particle size of the material should not exceed 1.5 to 2 times the gap width.

Feed is distributed evenly along the rolls; and foreign material (scats) is not allowed to pass into

the rolls and is captured using a magnetic separator system. HPGR is covenient to comminute

materials that are not overly fine and have low moisture content. Material above 3% moisture must

be pre-dried before feeding to the rolls. HPGR can be integrated into various circuits

configurations in new and existing grinding plants to increase the output of plants that have only

ball mills with precrushing before a ball mill (Seebach, 1996).

2.4.3 Vertical roller mill (VRM)

VRMs with integrated classifiers have been used successfully for many years in cement plants to

grind and simultaneously dry raw materials with moisture contents up to 20% by weight (Seebach,

1996). Their production can be as high as 400 tph and have a drive power of 11.5 MW (S.L, 2014).


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The feed is comminuted by pressure and friction between a horizontal rotating table and 2 to 4

grinding rollers hydraulically pressed against the table as shown in Figure 2-9. Nowadays, the

grinding rollers have diameters as large as 2.5 m. The material being ground is carried by

pneumatic and mechanical transport to the classifier located in the same housing directly above

the grinding chamber. The classifier tailings (over-size rejects) are recycled back into the grinding

chamber together with the fresh material. The grinding elements and mill settings are modified to

grind harder materials such as clinker and granulated blast furnace slag. Power use is between 26-

29 kWh/t when grinding to a Blaine 3,300 cm2/g using a VRM (Seebach, 1996).

Figure 2-9: Vertical roller mill

Vertical roller mills integrate the grinding, drying and separation processes into one unit. This

integration makes the VRM competitive in terms of specific electrical power consumption


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compared against other technologies. According Seebach VRM are 50% more efficient than ball

mills when comparing kWh/t used to grind same product under similar service properties

(Seebach, 1996).

2.4.4 Horizontal roller mill

The horizontal tube (or horizontal roller) mill has a length/diameter ratio around 1.0 and is

supported and driven on axial bearings. A solid single armored grinding roller is pressed

hydraulically against the rotating inner drum surface within a cylindrical grinding zone as shown

in Figure 2-10. The pressure is much lower than HPGR and is comparable to VRM. No compacted

cake is produced that requires further deflaking. The grinding roller is supported on bearings

outside the grinding tube. Internal fittings are subjected to heavy wear, however wear of the

grinding elements is still lower with VRM. Power consumption on horizontal roller mill when

compared against a ball mill is reduced by 10 to 25 kWh/t of cement depending on clinker

grindability and Blaine specific surface area (Aguero & Meech, 2014).

Figure 2-10: HoroMill schematic diagram


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2.5 Improving grinding efficiency in closed circuit cement ball mill

Final grinding of the cement is the most energy demanding part of the manufacture process

consuming almost 50% of the electrical energy (Bhatty, 2011). On a plant averaging consumption

of 110 kWh/t (electrical energy), use can be broken down according each main consumption

process as shown in Table 2-3. Cement grinding circuits operate more efficient in closed circuit

configurations. Now with the implementation of high efficiency size separators, a more precise

particle size cut product can be obtained, improving the quality of the cement. The separator

configuration can be arranged in different ways but it is usually related to the conserve heat by

sending back or recirculating clean hot air from the grinding process (FLSmidth, 2014).

Table 2-3: Electricity consumption during production of ordinary Portland cement

(Nisbet, 1996)

Process % kWh/t of cement

Quarry 5 5.5

Raw mix preparation 17 18.7

Pyro-processing 29 31.9

Finish Grinding 49 53.9

Total 100 110.0

Low-cost energy improvements can often be achieved with existing equipment with minor changes

such as the use of grinding aids and optimization of grinding media, improving the size separator

efficiency and the use of classifier liners (FLSmidth, 2014). These improvements can readily be


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determined from circuit production surveys (particle size distributions, work indices) to define

equipment baseline energy levels for process stages and specific energy (kWh/t) of individual

stages and materials to identify potential improvements in production strategies (Aguero & Meech,

2014).

2.5.1 Grinding aids

Grinding aids are chemical additives used to improve the production efficiency of cement plants

and energy consumption by reducing the boundary surface forces (Sohoni et al., 1991). Table 2-4

shows the benefits of addition of grinding aid to the energy usage.

Table 2-4: Effect of SikaPolycarboxylate ether polymers grinding aid on energy use

(Sica, 2015)

Scenario / feed ratio % Production

(tph)

Grinding energy use

(kWh/t)

Reduction in

Energy *

%

No grinding aid 80 50.2 0.0

0.018% grinding aid 85 47.3 4.3

0.035% grinding aid 90 44.7 8.6

* includes the energy associated with manufacturing the polymer

These products enhance particle size distributions as well as powder "flowability" of the finished

cement (Opoczky, 1986). Two important mechanisms have been put forward to explain the action

of various grinding aids. Sohoni et al. (1991) explained a mechanism known as the "Rehbinder

Effect" which is based on the assumption that the action of grinding aids depend on the reduction


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of the specific surface free energy of freshly ground material through the adsorption of a surface-

active chemical. By reducing the surface free energy, the grinding aid helps propagate micro-

cracks of fractured particles from impact that prevents the particles from binding together. This

mechanism also helps to explain the lack of coating on balls and mill liners with a fine particle bed

that absorbs impact energy. Beke (1983) has added to this mechanism with the idea that adsorption

of a grinding aid causes induced mobility of near-surface dislocations causing a lower hardness.

2.5.2 Optimum media ball size and mixing ratio

Various formulas have been proposed by different researchers for optimum ball size. Based on

these formulas and a number of empirical rules, mixing phenomenon in the mill has been studied.

In recent years, the ball size has decreased due to the adoption of improved classification liner

configurations and to the use of a pre-grinder (Asia Pacific Partnership, 2011). According to Asia-

Pacific Partnership on Clean Development & Climate Cement Task Force (Asia Pacific

Partnership, 2011), the percentage of tube mills using the smallest ball size of 17 mm has increased

from 10% to 80% from 1979 to 1991.

A critical speed must be maintained in order to avoid centrifugation of grinding media during

comminution. It is calculated according to Equation 1 and the mill speed should be around 60 to

80% of that critical speed.

Nc =42.3/ (D) 0.5

Equation 1: Critical speed (Bhatty, 2011)


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Where:

Nc = Mill critical speed, RPM

D= Ball mill diameter, meters

2.5.3 Fill factor of grinding media

Fill factor is the percent volume of a ball mill occupied by the grinding media. The fill factor of

grinding media greatly affects the grinding capacity and power consumption of a mill. For cement

grinding, the optimum value is around 26-30% (FLSmidth, 2014) . To keep the fill factor

appropriate for high grinding efficiency, continuous replenishment of grinding media is necessary

to compensate for the abrasion of the media. There are five methods to determine the media

replenishment time. The graph effect of fill ratio on mill efficiency can be seen on Figure 2-11.


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Figure 2-11: Optimum ball mill void filling

(Bhatty, 2011)

2.5.3.1 Load power

The baseline power draw of the mill is an indication of the ball fill charge level. This can be used

as a set-point to maintain the fill factor on target. This method gives less variability in the fill factor

over time, but it must be periodically checked using one or more of the remaining techniques.

2.5.3.2 Blank height measurement

This method relies on a visual estimate of the height of the ball charge in the mill when it is

shutdown. It is typically done once per shift or per day. The required addition of balls is calculated

from a fill formula based on the geometry of the charge (Gupta & Yan, 2006).


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2.5.3.3 Run-time

Based on the time the mill is turning under load, balls are added periodically based on a formula

to predict ball wear rates. This method is not particularly accurate.

2.5.3.4 Ground tonnage

Based on the tonnage rate of ore being ground over a period of time, balls are added periodically

based on a formula to predict ball wear rates. This formula is generally a little more accurate than

that based on run-time.

2.5.3.5 Grinding efficiency

Based on grinding efficiency measured periodically calculated using average power draw, total

ore processed, and the average particle sizes of feed and product over the time period in question,

a formula is applied that predicts ball wear rates, this formula is shown in Equation 2. Based on

the calculations grinding balls are added periodically (shift or daily basis) (Moly-Cop, 2012) .

Wt = d (mb)/d (t) = - km Ab

Equation 2: Ball mill media charge wear rate

Where:

Wt = mass wear rate, kg/hr

mb = ball weight, kg; after t hours of being charged into the mill.


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Ab = exposed ball area, m2

km = mass wear rate constant, kg/hr/m2.

2.5.4 Pre-grinding of raw addition material

Most ball mills operates in dry condition to grind cement have two chambers, one for coarse and

one fine grinding. Ball size and distribution of sizes are designed and adjusted to take into account

raw material feed conditions and mill dimensions. However, energy efficiency in the coarse

chamber is generally lower than in the second chamber. Furthermore, some plants feed cement

additives at particles sizes well above the maximum size (i.e. 12 mm) that a ball mill can process

(FLSmidth, 2014). This can significantly limit the ability to improve both coarse and fine grinding

performances. Recently, a new system has been applied in which a pre-grinder (VRM or HPGR)

is installed to perform coarse grinding ahead of an existing tube mill, which then is exclusively

used for fine grinding. This system greatly reduces total specific power consumption and can

improve production as well (Seebach, 1996).

2.5.5 Air classifiers

Air separator is a key component for close circuit efficiency. The performance of a grinding plant

depends on the type of grinding technology. Modern grinding equipment incorporates air

separators to comminution devices in the upper part of equipment as the Vertical Roller Mills. The

dispersion separator (static or dynamic), is the most used separator in the cement industry. A


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distribution plate at the feed inlet is used to disperse evenly the feed into the separator (FLSmidth,

2014).

The operation principle is based on: the action of an air current of certain velocity upon a mass

particle is proportional to the projected surface presented by this particle to the air current, so the

square of the average diameter size of the particle. The action of the force of gravity upon a mass-

particle is proportional to the volume, in other words to the cube of the mean dimensions of the

particle. Therefore the effect of the gravity increases faster than that of an air current of constant

velocity. If these two forces are concurrent, the gravity will prevail over the effect of the air current

as particle dimensions increase. On the other hand a properly adjusted air current will oppose the

force of gravity and lift up the smallest mass particles (Duda, 1976). A diagram of an air separator

is shown in Figure 2-12.

Figure 2-12: First generation air separator

(FLSmidth, 2014)


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The separator inlet feed passes through the feed spout and drops by gravity to the distribution plate.

The drive operates a rotating fan that promotes a continuous circulating internal air current (defined

as circulating air separator) and the distribution plate disperses the feed evenly into the separator.

Materials leaving the distribution plate are acted upon three forces as shown in Figure 2-13:

1. - The centripetal force, Fc

2. - The force of drag, Fd.

3. - The force of gravity, Fg.

Figure 2-13: Forces balance in an air separator

(FLSmidth, 2014)


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Air velocity, volume of air, density of material, particle size feed and speed of rotation are

important factors in the separating fines to coarse particles. The distribution plate must exert to the

particles a centripetal force of adequate magnitude to send the particles to the classification zone

faster than the new feed is received at the air material entry. As more density material and larger

particles are sent to the outer body of separator and the particles centripetal force is decreased.

They settle because of gravity. If the particles hits the body of cyclone wall the effect will force

the particles into the rejects (Duda, 1976). Some small particles are entrained between larger

particles causing a bypass effect to the rejects. Another cause ofbypass is agglomeration due to

van der Waal effects (Tams, 1983).

Undersize particles (finished product) are dragged up to the cut cyclone size and lifted by the

ascending air current and passing between the blades of rotating fan. Underneath the separating

zone the return air vanes are located to improve separation. The separation of the fines from the

descending air current in the outer separator cone is performed by decreasing the air velocity as

well as by the change air current. Because of the low rate of descent of the smallest particles, these

fractions are always suspended in the air stream and therefore a portion of finished product is

continuously circulating resulting in fraction of the fines comes into separator rejects.

Main fan, auxiliary fan and dispersion plate are mounted in a common shaft. The auxiliary fan acts

against the intake air current caused by the main fan. This counteraction can be controlled by the

number of blades of the auxiliary fan. A large number of blades cause a stronger counteraction.

An adjustment in the number of blades is necessary when switching to other types of cement

(Duda, 1976).


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Another possibility for fine adjustment of dynamic separators is by the use of horizontal control

valves which make it possible to change the cross section of the ascending air current. By adjusting

the control air valve is possible to strangle the air stream and shift the classification boundary

closer to the fines.

2.5.5.1 Performance of separators

The performance of any type of separators is determined using the tromp curve (also identified as

partition curve, selectivity curve and probability curve). Tromp curve is a graphic that

combines the fractions in a sample and the fractions of particles of different sizes in the feed going

to coarse (rejects) or fines (product). Some important parameters are depicted on the Tromp curve:

Cut Size (x50). Defined as the size of particles with equal distribution in the fine and as the

coarse fraction. This value is adjusted by setting the speed according the product size required.

Sharpness factor (k). Is a measure for the steepness of the tromp curve. It is calculated as the

size of particles of which 25% pass into the rejects divided by the size of which 75% pass

k=D25%/D75%. A good separator has values between 0.52-0.58

Delta or bypass (). The lowest point on the curve, indicating the amount of bypass of good

product reporting to the rejects. This value should be between 10-15%. Bypass is affected by:

agglomeration resulting from over ground small particles, poor dispersion at distribution plate

and by the dragging effect of circulating larger particles with air into the rejects.

A typical tromp curve for the rejects parts with plots for bypass, cut size and sharpness is shown

in Figure 2-14.


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Figure 2-14: Typical tromp curve

(www.thecementgrindoffice.com)

Agglomeration is the electrical attachment of very fine particles as a consequence of van der Waals

forces (on the order of 40-400 kJ/mol) in which charges of the crystal lattice suffer structural

changes. Overgrinding and high impact energy events are some of the effects of agglomeration.

This condition can be reduced by the use of a grinding aid (Tams, 1983).

2.5.5.2 High efficiency on separators

High efficiency size separators improve the process by the application of air vortex allowing the

centrifugal and drag forces to interact effectively and perform a good classifying function. The


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control of a particle motion is essential to the improvement of the separator performance. Limiting

the random motion of particles allows more fines to be removed from the mill. Also they operate

with cooler air, allowing a reduction in temperature of the finished product and rejects and the

mill.

High efficiency separators reduce the energy consumption by: first removing the fines from the

system sending the fines to the finished product preventing the fines from returning to mill and

cause overgrinding, second by controlling the fines that cause a cushioning effect on breakage in

the mill. On average high efficiency separators reduces the specific power consumption in finish

grinding by 20-30% (Brugan, 1988).

Modern separator manufacturers suggest that the fraction of fines in the distribution loading feed

should be 2.0- 2.5 kg of feed per actual cubic meter of air. For the finished product the effective

transport concentration value should be between 0.75-0.85 kg of product per actual cubic meter of

air (FLSmidth, 2014).

The separator finished product or efficiency is related to the amount of rejects which pass into the

fines. The criterion for separator capacity is the amount of fines present into the rejects.

The types of separators and characteristics of air classifiers according the generation type used in

the cement industry is shown in Table 2-5.


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Table 2-5: Distinctive features of separators of different designs

(Bhatty, 2011)

Evolution First generation Second generation Third generation

Nomenclature Conventional

separator

Cyclone-air separators High Efficiency

Separators

Efficiency 50-60% 60-75% 80-90%

Bypass 30% 10% 2%

Reclassification of

tailings

Not efficient due to

lack of fresh air

Better as recirculation air

contains fresh air also

More effective as fresh

air is used

Fines collection In the outer cone of

separator

In the external cyclones

attached to separators

In cyclones or bag filter

attached to mill system

Commercial

examples

Sturtevant

Turbopol (Polysius)

Cyclopol (Polysius)

ZUB (KHD-Wedag)

O-sepa (Fuller)

Sepax (FLS)

Sepol (Polysius)

Sepmaster (KHD)

The finer the particle size of the finished product, the lower the separators production capacity.

The efficiency of an air separator depends upon the type of the mill (BM, HPGR or VRM) working

with the separator. It is possible an increase production by 10-30% by replacing a poor

performance low efficient separator (first or second generation) by installing a high efficient (third

generation) (Cleemann, 1986).


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2.5.6 Circulating load

The circulating load has a big impact on mill efficiency. There is a relationship between the lower

energy use to related specific surface area and circulating load as reported by (Bhatty, 2011) and

shown on Table 2-6.

Table 2-6: Influence of circulating load and type of separator on mill efficiency

(Bhatty, 2011)

Mill Circuit Open Closed Closed Closed

Separator type none 1st gen. 1st gen 3rd gen

Product SSA, m2/kg 370 370 370 370

Rejects SSA, m2/kg n/a 220 220 90

Circulating load,% 100 300 500 300

Mill exit SSA, m2/kg 370 270 250 183

cm2/joule 22.9 26.2 26.8 27.5

kWh/t 44.9 39.2 38.3 37.4

Mill output,% 100 114 117 120

For high efficiency separators there are smaller amount of fines in the rejects and as result SSA is

lower (90 m2/kg). As result the exit from the mill (183 m2/kg) is lower giving a similar comparative

product of 370 m2/kg, but using less energy at mill (37.4 kWh/t) and increasing production of

120%.


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2.6 Impact on carbon dioxide emissions

Cement production has a large CO2 footprint due to the tremendous use of cement around the

world, it is estimated that 3.6 billion of tons were produced on 2012 (Kline & Kline, 2014)

representing the third largest CO2 emitting industry by anthropogenic sources (Abdel-Aziz et al.,

2014) as shown on Figure 2-15. The vast quantities of cement (3.6 billion t/a) used around the

world today make cement production one of the leading sources of CO2 emissions and represents

the second most used commodity in the world (Kline & Kline, 2014).

Figure 2-15: Total global industry direct greenhouse gases emission

(Abdel-Aziz et al., 2014)

There are several alternatives to reduce CO2 emissions on cement production such as,

Carbon Capture/ Sequestration (CCS)


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Clinker substitution by similar properties minerals (limestone, pozzolan)

The use of alternative fuels (biomass, used tires, industrial wastes)

Energy efficiency (lighting, high efficiency motor, compressed air optimization, and high

efficiency separators).

The Cement Sustainability Initiative (CSI), in junction with the International Energy Agency (EIA)

have developed a road map for CO2 reduction in the cement industry. The IEA/CSI blue map

has targeted for a reduction of approximately 50% in the specific CO2 emissions per ton of cement

by the year 2050 (IEA, 2009) this target is shown in Figure 2-16. 56% of the targeted reduction

will be from carbon capture and sequestration.

Figure 2-16: The IEA/CSI blue map for CO2 emissions reduction

(Kline & Kline, 2014)

The CO2 emission from cement manufacturing is caused by calcining of limestone and from the

combustion of fuels at the kiln. The amount of limestone calcined in the cement manufacture is


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relatively consistent across most cement plants. It can be decreased when alternative sources of

calcium oxide are utilized, such as slag, fly-ash and/or bottom ash (Kline & Kline, 2014).

Evaluation of the CO2 emissions in cement production excluding the emissions from electricity

and found it be approximately 680 kg CO2/t of cement produced (Kline & Kline, 2014) as shown

in Figure 2-17.

Figure 2-17: Average kg of CO2 released per ton of cement produced

(Kline, 2014)

The CO2 from combustion depends on the system efficiency and the fuel which in turn depends

on the technology and type of fuel used. Modern plants often use a 5 stage pre-calciner kiln system

with an inline raw mill for maximum thermal efficiency. The amount of clinker in the cement has

a direct impact on the specific CO2 emissions per tonne of cement produced.


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When calculating the impact on mass flows it is assumed that raw limestone added to the cement

comes from the same quarry as that used in the raw mix. If 5% raw limestone is added to the

cement replacing clinker, the amount of clinker used in the final product (95%) is decreased by

5.26% (M. Nisbet, 1996).

The material balance for the finished Ordinary Portland Cement (OPC) product without any

limestone is shown in Figure 2-18. Produce PLC with 5% limestone is shown in Figure 2-19.

Figure 2-18: Material balance - no raw limestone addition

(Aguero & Meech, 2014)

Figure 2-19: Material balance - PLC with 5% limestone addition



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However, the total limestone needed per tonne of final cement product actually decreases slightly

from 1.216 to 1.202 tonne (1.152 for the raw mix plus 0.05 added to the finishing step) (M. Nisbet,

1996).

The fuel savings and reduction in CO2 emission by adding 5% and 20% limestone is shown in

Table 2-7. It is assumed that the heat combustion of coal is 22.7 MMBtu/t of coal and CO2 emission

is 707 kg CO2/t of cement produced (Carbon Dioxide Emission Factors for Coal, 2015).

Table 2-7: Fuel savings and CO2 emissionsreduction with PLC


Coal as fuel source Units Limestone replacement

5% 20%

Fuel saved tfuel/tcement 0.014 0.043

Reduced CO2 tCO2/tcement 0.0315 0.1258

2.7 Modeling and simulation of Portland cement circuits

The modeling of a process is the use of mathematical equations to characterize an operation

accurately and to be able to simulate their impacts on production/efficiency from modifying their

different variables. The model process has been used by several authors (Benzer et al., 2001) in

the characterization of cement production models in different processes. This technique has been

used to model grinding and separation technologies, since it is a low cost and high confidence way


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to evaluate the improvements of any circuit based on powerful computers and the research over

the time from various authors (Napier-Munn et al, 1996).

2.7.1 Model of performance curves in separators

There are several equations published to model the performance of separators. The most adequate

depends on the amount of variables affecting the process. Whitens models has been categorized

as the least sum of squares of deviations in particular at fines sizes as compared to others (Altun

& Benzer, 2014). The mathematical model defined by Whittens is related to the overflow

efficiency (finished product) and is defined by Equation 3 and concepts shown graphically on

Figure 2-20.

= 1 + 50

(exp ) 1

(exp 50 + exp 2)

Equation 3: Whiten efficiency of separator reporting to overflow

Where,

Eoa : Actual Efficiency of fines to overflow, %

C : Fraction subject to real classification, %

: Parameterthat controls the initial rise of the curve in fine sizes (Also called fish hook)

* : Parameter that preserves the definition of d=d50 When E= (1/2) C


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d : size, mm

d50c : Corrected cut size, mm

: Sharpness of separation

Figure 2-20: Efficiency to overflow vs size

(Napier-Munn et al., 1996)

Alpha () is a variable from equation that is related to the slope the graph and represents the

sharpness or steepness of the efficiency curve. It can varies from values between 0.25 to 10 and

high values of means a better and sharper (steeper cut) this can be seen on Figure 2-21.

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