process analysis and energy efficiency …
TRANSCRIPT
PROCESS ANALYSIS AND ENERGY EFFICIENCY IMPROVEMENT ON
PORTLAND LIMESTONE CEMENT GRINDING CIRCUIT
by
Sixto Humberto Aguero
B.S. (Mechanical Engineering), Universidad Nacional Autonoma de Honduras, 1992
MASc (Energy Management), New York Institute of Technology, 2008
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF
MASTER OF APPLIED SCIENCE
in
THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES
(Mining Engineering)
THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)
April 2015
© Sixto Humberto Aguero, 2015
ii
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 system’s 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
v
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
x
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
xi
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
xiii
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–
Gaudin–Schuman 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
xiv
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 α ................................................................ 48
Figure 2-22: Variation of efficiency curve with β ........................................................................ 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, β 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
xvi
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
xvii
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.
xviii
Dedication
In memory of Professor John A. Meech, who devoted his professional life to the development of
sustainable mining and a passion for teaching.
1
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
2
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.
3
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.
4
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 850 – 1100 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
5
consumed in a conventional cement production process is typically 95 – 110 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 significantly, 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
6
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.
7
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
8
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
10
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
11
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).
12
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
13
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
14
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.
15
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
15–32 µm 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).
16
Figure 2-3: Relationship between compressive strength and uniformity factor according to Gates–
Gaudin–Schuman plotting
(Celik , 2009)
Composite cements produced by grinding clinkers at a Bond work index of 13 kWh/t with
limestone have several benefits. Limestone is softer than clinkers having a bond grindability Bond
work index average, of 4.6-12.61 kWh/t (Bhatty, 2011) and therefore requires less energy to grind
to the same fineness. Figure 2-4 shows the energy required to grind each of the two materials to
obtain various specific surface areas. It can be observed that energy required to grind limestone is
much less compared to clinker to obtain the similar specific surface area.
17
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)
18
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).
19
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 (Tamás, 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
20
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 32–35 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).
21
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).
22
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 µm. 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).
23
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).
24
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
25
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
26
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
27
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 Sika™ Polycarboxylate ether polymers grinding aid on energy use
(Sica, 2015)
Scenario / feed ratio % Production
(tph)
Grinding energy use
(kWh/t)
Reduction inEnergy *
%
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
28
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)
29
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.
30
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).
31
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.
32
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
33
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)
34
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)
35
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 of bypass is agglomeration due to
van der Waal effects (Tamás, 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).
36
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 (δ). 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.
37
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 (Tamás, 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
38
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.
39
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 separator’s 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).
40
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%.
41
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)
42
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
43
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.
44
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
(Aguero & Meech, 2014)
45
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 emissions reduction with PLC
(Aguero & Meech, 2014)
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
46
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. Whiten’s 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 Whitten’s 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, %
β : Parameter that controls the initial rise of the curve in fine sizes (Also called fish hook)
β* : Parameter that preserves the definition of d=d50 When E= (1/2) C
47
d : size, mm
d50c : Corrected cut size, mm
α : Sharpness of separation
Figure 2-20: Efficiency to overflow vs size
(Napier-Munn et al., 1996)
Alpha (α) 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 “α” means a better and sharper (steeper cut) this can be seen on Figure 2-21.
48
Figure 2-21: Variation of efficiency curve related to α
(Napier-Munn et al., 1996)
Beta (β) has been identified also a “fish hook” because of its prolonged shape especially on fine
fractions (below 45 microns). It has been associated with agglomeration in cement classification
operations. Napier-Munn (1996) high values represents high agglomeration and/or high feed rate
to separator and shown on Figure 2-22.
49
Figure 2-22: Variation of efficiency curve with β
(Napier-Munn et al., 1996)
Air separator studies have shown there is a relationship which is directly proportional between
bypass and separator feed load as shown on Figure 2-23. Also, there is evidence of cut size is
proportional to airflow and inversely proportional to rotor speed (Altun & Benzer, 2014).
Figure 2-23: Relation between (a) feed/bypass (b) and dust loading/bypass
(Altun & Benzer, 2014)
50
2.7.2 Model of two compartment ball mills
Two compartment dry ball mill used on cement finishing process can be modeled as multiple mills
in series. The classifier diaphragm located between two chambers can be modeled as a screen this
is shown on Figure 2-24. These findings were analyzed using industrial scale experiments (Benzer
et al., 2001).
Figure 2-24: (a) Two compartment ball mill (b) model circuit array
(Farzanegan et al., 2014)
Perfect mixing models are based on the principle that the contents of the mill are fully mixed. They
are represent by either one perfectly mixed segment or a number of perfectly mixed segments in
series (Gupta & Yan, 2006).
51
The mathematical equation shown in Equation 4 describes the process of comminution of a particle
for a ball mill model. This relation uses feed and product matrices calculated for the breakage and
selection function (Gupta & Yan, 2006):
= ∗ ∗ + ( − ) ∗Equation 4: Basic equation model for open circuit (Gupta & Yan, 2006)
Where:
P : Product vector for size distribution (mass)
B : Breakage function
S : Selection function
F : Feed rate matrix
I : Unit diagonal matrix
Many circuits operates in closed circuit where a classification device separates particles that need
more grinding (coarse) and sends material to the finished product (fines). This is shown on Figure
2-25. Feed and Products are denoted by F and P respectively, q the size distribution of classifier,
B, S and C the breakage, selection and classifier functions respectively. All terms are considered
as vectors.
52
Figure 2-25: Comminution on a closed circuit
(Gupta & Yan, 2006)
Equation 5 shows the classification effect on operating in a closed circuit modifies to a new
equation that is shown below (Gupta & Yan, 2006).
= ( − ) ∗ ( ∗ + − ) ∗ [ − ∗ ( ∗ + − )]¯ ∗Equation 5: Equation model for a closed circuit
Where:
P : Product vector for size distribution (mass)
B : Breakage function
S : Selection function
C : Classification function
F1 : New feed
53
F2 : Mixed feed with classifier coarse and new feed
q : Size distribution of classifier
I : Unit diagonal matrix
54
CHAPTER 3: EXPERIMENTAL PROCEDURES
This chapter brings details about the experimental tests, sampling and software programs used to
characterize the cement samples used in this study. These tests include the Bond standard ball mill
test, breakage function test and the use of software for the determination of the selection function,
Whiten model and Rosin-Rammler particle size distribution plot.
3.1 Sampling and data gathering
The circuit evaluated is identified as Circuit 03 which consists of a vertical pre-grinder, a ball mill,
a high efficiency separator and several feeders of limestone, clinker and gypsum. A series of 22
samples each with 50 kg from this circuit were taken and provided by the plant. The description
of sampling points and sample details are listed below on Figure 3-1.
Figure 3-1: Circuit 03 sampling points
55
3.2 Bond standard ball mill grindability test
The purpose of the standard Bond ball mill grindability test is to determine the Bond ball mill work
index (BWI), which can be compared with the work indices of known materials to evaluate
grinding efficiency or mill design.
The Bond work index is a measurement of the power required to reduce feed with a given 80
percent passing size (d80) to product with a specified 80 percent passing size (d80). A complete
procedure on the Standard Bond work index test is included in the Appendix A.
3.3 Breakage distribution function test and estimation using BFDS® software
The breakage function is a material specific property and denotes the relative distribution of
fragments after breakage of a monosize sample. It is almost found to be independent of initial size
and usually is expressed in a matrix array in order to perform a further modelling using population
balance methods. The breakage distribution mechanism is represented in Figure 3-2.
56
Figure 3-2: Representation of the distribution of particle breakage
(Gupta & Yan, 2006)
3.3.1 Clinker breakage function estimation test procedure
For the estimation of the breakage function during this research, the following procedures were
implemented:
Preparation of 300 g of monosize clinker sample. The monosized fractions for this test
were 1.400, 1.000, 500, 355 and 150 microns.
A standard Bond ball mill was used for breaking the sample.
Grinding at intervals was selected of 0, 5, 10, 25 and 30 seconds.
Total content of sample removed after each time taking care to not lose any material and
the screened using at √2 factor screens for size distributions.
After screening is done, mill sample is returned for further grinding on the next time period.
57
Test is finished after obtaining 50% passing of the reduction of initial size fraction on the
top size screen.
The use of a computer program is required to perform and apply any correction factors to
account for any re-breakage that has occurred during the test. The software used is a
BFDS® (Breakage function determination Software). This software is able to calculate the
normalized breakage function using Berube’s, Herbst/Fuerstenua and modified
Herbst/Fuerstenau methods (Farzanegan, 2015).
3.4 Particle size distribution analysis
A size distribution is a quantitative representation of the proportion of particles in a sample. Results
from particle size distribution are presented using algebraic forms to find the best fit for the
experimental parameters found for several feed, rejects and products. Results have been reported
in a log size axis to avoid congestion in representing the values and increasing the resolution of
certain small particles areas in the plot.
3.4.1 Rosin-Rammler distribution
The use of presenting data in cumulative percent retained exposes features of the data which are
often suppressed or entirely hidden in the cumulative passing form.
This algebraic distribution describes the mass or volume distribution function in an exponential
form. This is suitable especially for very fine ground materials. Resolving the exponential by the
58
use of logarithms helps to expand the fine and coarse ends of the size range and compress the mid-
range. The distribution is shown in Equation 6.
= 100 exp − ᵇEquation 6: Rosin-Rammler distribution
Where,
R= cumulative mass retained on size x, %
x1 = size parameter, mm
b = uniformity factor
Rosin-Rammler will be used specially on the finished product representation in order to evaluate
the uniformity factor as a quality control parameter.
3.4.2 Whitens model for high efficiency separators
The most precise method to fit the curve of a classification device is the Whiten’s approach,
because of its precision using the least sum of squares of deviations especially for the fines sizes
when compared against others like Tromp curve.
The equation is the same described in Chapter 2 on modeling and simulation and listed here again.
59
= ∗ 1 + ∗ ∗ ∗ 50 ∗ (exp ) − 1(exp ∗ ∗ ∗ 50 + exp − 2)3.5 Selection function and its estimation using numerical grinding optimization tools in
language C (NGOTC®) software
The selection function or specific rate of breakage is the probability of the breakage of certain
particulate on a breakage process and is a measure of the grinding kinetics. This is a machine
specific property. For modeling it is represented by a matrix and can be estimated by analyzing
the data for a plant process.
NGOTC® is a software tool dedicated to calculate some variables including the selection function.
The software was used to back calculate the selection function based on real breakage plant data
at ball mill 03 compartments 01 and 02.
3.5.1 Back calculation of selection function from continuous mill data
The estimation of selection function using NGOTC program consist of an algorithm to back
calculate a set or a vector of selection function elements based on a set of input data. The selection
function elements are back calculated by trial and error using a bisection search procedure. The
selection function elements are back calculated sequentially i.e. first the selection function element
for the top size class is determined, and then using the estimated value, the selection function of
the second size class is estimated. The core of the algorithm is in fact a single ball mill simulator
which produces product size distributions. The criterion to stop the iteration process is the
60
difference between the measured and predicted mass of current size class, which must be within a
tolerance interval set by the user. The estimated selection function elements, then, can be used as
input to other modules.
61
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Introduction
Results obtained from testing and the calculation of different variables from samples are presented
and discussed in this chapter. Special emphasis is made on evaluating the current cement
production circuit 3. Modeling to air separator will be briefly presented and discussed. All samples
and operating condition parameters, reported values presented in this chapter and attached in the
appendix were provided by the cement company surveyed. Data and samples gathered were taken
before and during the plant crash stop on circuit 03 on May 2014. The ball mill and air separator
design and operating condition details of circuit 03 equipment on producing type 10 cement (3%
limestone) can be seen on Table 4-1.
62
Table 4-1: Circuit 03 equipment design specifications and operating condition on type 10 cement
Ball Mill
Diameter (m) 4.42
Operating power (kW) 3,800
Length first chamber (m) 4.17
Length second chamber (m) 9.14
Ball load first chamber % 22.78
Ball load second chamber % 27.15
Top ball size first chamber (mm) 63.5
Top ball size second chamber (mm) 32
Air Classifier
Manufacturer FLSmidth OSEPA 3000
Volumetric air flow (m3/h) 130,000
Rotor speed (rpm) 205
Installed power (kW) 223
Feed tonnage (tph) 390
Dust Load (kg/m3) > 2.6
63
4.1.1 Particle size distribution and production
4.1.1.1 Fresh feed size distribution
Fresh feed for the purposes of this study is defined as the feed (clinker, gypsum and limestone) to
the ball mill excluding the circulating load (rejects from the air classifier). Joint feed is the feed to
the ball mill considering the addition of the circulating load to the fresh feed.
The average production on Type 10 cement on circuit 03 is 125.0 tph with a circulating load of
211%. The size particle of clinker is reduced initially at a vertical pre-grinder (CKP) with a d80
size from 15.0 mm to 1.98 mm before feeding the ball mill. Limestone and gypsum are fed directly
to the ball mill at d80 size of 13.70 and 61.14 mm respectively. The combined fresh feed has a joint
d80 3.29 mm. Particle size distribution individually for limestone, gypsum and clinker before and
after CKP are shown in Figure 4-1.
64
Figure 4-1: Particle size distribution of circuit 03 fresh feed and clinker at CKP
The weight proportions of fresh feed entering the ball mill: clinker 91%, limestone 3% and gypsum
6%. The ball mill product, with a flow rate of 390 tph, is fed to a high efficiency separator by a
bucket elevator where oversize rejects (circulating load) are sent back to ball mill for further
grinding and undersize products are sent to a bag house filter to be recovered, transported and the
finished product stored as detailed on Figure 4-2.
Specific energy consumption of the ball mill varies between 28.9 and 30.7 kWh/t. CKP is used
just for pre-grinding clinker before feeding the ball mill. It has a specific energy consumption that
varies between 5.00 to 5.42 kWh/t.
Finished product is recovered after being classified in a high efficiency separator at a rate of 125.2
tph and generates a d80 size of 0.0268 mm as seen on Figure 4-2.
0.0
20.0
40.0
60.0
80.0
100.0
0.1 1.0 10.0 100.0
CUM
pas
sing,
%
Size, mm
limestone clinker before CKP Clinker after CKP Gypsum
65
Figure 4-2: Current d80 feed and product size at circuit 03
Based on the assessment of the samples the following results can be obtained:
The combined 125 tph of fresh feed to the ball mill (excluding circulating load) has a d80 of
3.29 mm. This is the result of mixing 91.37% clinker at d80 of 1.98 mm, 2.95 % raw limestone
with d80 of 13.70 mm and 5.68 % gypsum with a d80 of 61.15 mm.
The average circulating load (rejects from air separator) is 264.8 tph with a d80 of 0.063 mm,
and represents a circulating load of 211%.
66
Vertical pre-grinder CKP has a size reduction ratio of 7.6 .The feed of clinker to the CKP
has a d80 of 15.05 mm, and the product a d80 of 1.98 mm.
Specific average energy of ball mill and vertical roller pre-grinder CKP is 30.77 and 5.42
kWh/t respectively, adding up to a total specific energy of 36.19 kWh/t.
There is a potential opportunity to optimize the fresh feed by reducing the limestone size
from a d80 of 13.70 to 1.98 mm at the CKP (currently just clinker is brought to CKP). The
limestone has similar feed size that can be brought without making major modifications to
current CKP. The calculated fresh feed size d80 by pre-grinding limestone through CKP
and replacing the feed by 3% limestone has been calculated as d80 of 2.98 mm.
The estimated fresh feed size d80 by pre-grinding limestone through CKP and replacing the
feed by 12% limestone has been calculated as d80 of 2.38 mm. (this is when producing 12%
PLC).
Composite cement manufacturing cautions. When planning to increase the use of raw
limestone on composite cement manufacturing (PLC) extreme care should be taken on the
operating parameters of air separator, because of the change of densities and uniformity
factor of the mixture of the feed (limestone and clinker) that will behave differently. The
use of extra grinding aid could be required in order to reduce agglomeration due to
increased limestone use.
67
4.1.1.2 Air separator size distribution
The material milled at the ball mill fed the air separator at a rate of 390 tph with a d80 of 0.050
mm. The oversize rejected particles with a d80 of 0.064 mm, are cool down and returned to the ball
mill inlet as a circulating load. The undersize fines or finished product with a d80 of 0.027 mm are
cooled down and sent to a bag house filter to be recovered and stored as finished product. The
particle size distribution can be seen in Figure 4-3.
Figure 4-3: Particle size distribution on air separator
The calculated efficiencies of air separator at size fractions of 0.020, 0.035 and 0.045 mm are
52.24, 64.74 and 45.43 % respectively. This calculation is based on the mass flow of finished
product divided by the mass flow of the feed for each fraction, and shown in Equation 7.
0
20
40
60
80
100
0 20 40 60 80 100 120 140 160 180 200
Cum
Pas
s, %
Size, microns
Separator Product Separator Feed Separator Rejects
68
= 100( × )( × )Equation 7: Efficiency to overflow
Where Woi, Wfi are the proportions by weight of material of size “ith” in the overflow and feed
solids respectively, and Mo, Mf are the total solids mass flowrates of the overflow and feed streams
respectively.
The air separator rejects 109.89 tph of material in fraction less than 0.035 mm (most important
fraction for strength development), these fractions returns to the ball mill for overgrinding instead
of being recovered as finished product. This is an indication of a high degree of inefficiency at the
separator representing 46.06% efficiency, mainly caused by high dust load feed and agglomeration
of fine particles to the feed of separator and shown on Figure 4-4.
Figure 4-4: Mass balance at air separator
69
4.1.1.2.1 Separator partition curve
In order to evaluate the total performance of the separator with regard to particle sizes the use of
the partition curve for the selectivity process or probability of rejection for the whole system was
used. The measured and predicted efficiency curves are shown in Figure 4-5.
Data from the feed, rejects and product to the separator were provided by the plant. This data were
used to calculate the actual efficiency curve (also known as Tromp curve) data points. Then,
Whiten model was used to fit the Tromp curve obtained based on real plant data. The purpose was
to obtain a smooth curve that represents the most accurate description of the process by adjusting
the experimental errors and process instabilities.
Figure 4-5: Separator efficiency reports to product fit using Whiten model
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
0 20 40 60 80
Repo
rts t
o Pr
oduc
t, %
size, µm
Tromp Whiten
α = 1.04105β = 1.61607
d50C = 33.67 µmd50 = 30.36 µmBypass = 15.0 %
70
The area of the graph circled in red represents the “fish hook” region (Altun & Benzer, 2014),
(Napier-Munn et al., 1996), this area is an indicative of loss of efficiency during the separation
process and is related to agglomeration especially at finer fractions.
Table 4-2: Separator efficiency parameters
Parameter Plant Data value Optimum values Reference
d50c 0.030 mm
D-limit 0.009 mm
By-pass(Delta), % 15.0 10 - 15
(FLSmidth, 2014)Imperfection 0.42 0.2 < I < 0.3
Sharpness, % 42.51 52-58
Alpha value, α 1.04105 4.0(Napier-Munn et al., 1996)
Beta value, β 1.61607 0.0
According to Altun & Benzer (2014), the most effective model fitting is the Whiten model as it
has the least sum of squares deviation. The following findings are related to the fit of Whiten
parameters:
The Tromp curve reported by the plant has a strong fit with respect the whiten model at all
sizes. Below 0.010 mm the model provided by plant shows a drop which decreases as size is
reduced. This difference in both Tromp and Whiten curves is a result of experimental errors
during sampling and analysis which is expressed in the Tromp graphic but corrected by the
squares differences at the Whiten curve.
71
The region circled in red on Figure 4-5 represents the “agglomeration effect” or fish hook.
This causes high tonnage of material from the feed going to the rejects (100.43 tph of fraction
below 0.035 mm). Agglomeration is the result of overgrinding on the ball mill and high energy
impacts with larger grinding media impacts (Tamás, 1983). These over ground particles
become to each other by weak electrical forces (van der Waals forces) creating a larger particle
that behaves, and also selected, as a coarse particle in the separator (Tamás, 1983).
Some parameters calculated by the Whiten model are: Alpha “α” related to the sharpness of the air
separator, Beta “β” related to the effect of agglomeration and listed in,
Table 4-2 with their comparative recommended values (Napier-Munn et al., 1996).
A good Alpha “α” factor value should be around 4 (Napier-Munn et al., 1996). The separator
reported sharpness factor is 1.04105, denoting a low sharpness value. This low sharpness
factor can be related to some operating conditions of the feed distribution at air separator i.e.
air to feed ratio (has an impact on throughput), speed of rotor (has an impact of size of
classification) (Benzer et al., 2001).
The low Alpha “α” factor is double confirmed when comparing against the sharpness value
of 42.51. FLSmidth suggest a good value to be between 52-58%.
The mass flow of material through separator with particles sizes between 0.003 to 0.035 mm
(fractions related to strength development) are: 181.41, 87.58 and 93.81 tph for feed, reject
and finished product respectively. This balance represents an average of 51.71%
(93.81/181.41) efficiency related to the final product at fractions below 0.035 mm. According
72
FLSmidth who is the manufacturer of air separator this efficiency should be at a maximum of
85% for this fraction.
The uniformity factor “n” for the finished cement product has a value of 1.30 (this is the slope
of Rosin-Rammler graph). This value represents a high quality product (narrow) with a high
surface area and is the result of the low cut off point of the air separator. But because of
agglomeration and low sharpness the cost of producing this quality product is high, resulting
in lower production throughput, increase of recirculating load and higher energy production
cost.
There is a close relationship between the effect of β (parameter that controls the initial rise of
the curve in fine sizes and related to agglomeration effect) and C (fraction subject to real
classification). This relation was recently published by Altun and Benzer (2014) when
comparing several high efficiency air separators operating with cement and they found that
the cause is related to operating factors. Low values of β represents high values of real
classification (β is related proportional to the bypass of particles). Data from the plant
evaluated have been related to these findings and shown in Figure 4-6. The finding during this
research have similar trend found by Altun and Benzer (2014).
The mass of particles between 0.001 to 0.010 mm and circled red on Figure 4-5 (these fractions
are related to early setting of cement) that goes through the separator are: 81.6, 29.25 and
52.35 tph for feed, reject and finished product respectively. The 29.25 tph of fraction below
0.010 mm that are rejected from the air separator and sent back to the ball mill for
comminution are of especial importance, because these particles have high probability of
73
being overground (mainly at compartment 02) and causing agglomeration and coating effect
on grinding media.
Results for Beta “β” and “C” (fraction to real classification) values obtained at the plant
showed a good relationship and similar trend when compared with results from Altun and
Benzer (2014) and shown on Figure 4-6. A high value of Beta “β” are related to agglomeration,
high dust load, and has a decrease and the real fraction of particles due to classification “C”.
The β result of 1.61607 obtained from the Whiten model and shown in Figure 4-6, is an
indication of the high agglomeration effect. This agglomeration effect can be related to
overgrinding and to the physicochemical properties of limestone (Altun & Benzer, 2014;
Tamás, 1983).
74
Figure 4-6: Correlation between C, β parameters and relation to plant data
(Altun & Benzer, 2014)
Altun & Benzer (2014) found a relation between the bypass of particles and dust load this
relation is shown in Figure 4-7. Bypass is calculated from the Whiten model by subtracting
100 from the obtained “C” value (Altun & Benzer, 2014). High values of bypass are the result
of high dust loading or/and low air flow throw separator feed; this can be deducted from Figure
4-7.
The bypass value obtained from Whiten model is 11.7%. By using Altun and Benzer (2014)
relationship shown on Figure 4-7, it is feasible to relate the dust loading to the separator giving
a value higher than 2.6 kg/m3. Dust load should be reduced from actual 2.6 to optimally 2.0
75
kg/actual cubic meter to the feed of separator. Three main results from reducing the dust load
on the separator feed will happen. First, improvement of separator efficiency by sending more
finished particles to product, therefore increasing the production rate. Second, the rejects rate
will be reduced on the bypass and simultaneously increasing the D80 feed size of the circulating
load to the ball mill. Third, fresh feed will be increased and in junction with upgraded
circulating load will increase the grinding kinetics on compartment 01 and reduce the high
energy impacts and agglomeration effect on overgrinding small particles rejects.
According to the air separator’s manufacturer, the equipment is designed to handle 2.0-2.5
kg/m3 efficient (this is the feed density of material to the air separator), but the actual load of
fines and agglomeration effect on the feed is high for this specific operation.(FLSmidth, 2014).
76
Figure 4-7: Effect of separator dust loading on bypass and relation to plant data
(Altun & Benzer, 2014)
A relation between sharpness and dust load is shown in Figure 4-8, it can be seen the actual
levels (red) and manufacturer suggested levels (green), it can be deducted that at lower dust
load there is a higher sharpness factor.
77
Figure 4-8: Relation between sharpness and dust loading
(Altun & Benzer, 2014)
4.2 Ball mill, Bond work index, breakage and selection function
A series of standard bond ball mill tests, breakage and selection functions were performed on the
samples in order to categorize the different properties of the cement samples. The details of the
parameters of the charge of the ball mill for compartment 01 and 02 are show in Table 4-3.
78
Table 4-3: Ball mill grinding media charge details for C1 and C2
Compartment 01 has a 22.7% grinding media load and an equivalent ball size of 47.49 mm whereas
Compartment 02 has a 26.18% grinding media load and an equivalent ball size of 18.54 mm.
4.2.1 Work index
The results of the Bond work index analysis conducted with the samples of limestone, clinker and
different proportions of limestone, clinker and gypsum are shown on Table 4-4. The following
results obtained are discussed below:
79
Table 4-4: Work indices for research samples
SampleWork Index calculated for
this research (kWh/t)
Reference data
Work Index (kWh/t)
(Bhatty, 2011)
100 % Limestone 5.29 4.60 - 12.61
100 % Clinker 11.85 9.15 - 16.19
95/5 % C/L 11.12 ---
88/12 % C/L 10.86 ---
60/40 % C/L 9.46 ---
Limestone used at the plant site has a Bond work index of 5.29 kWh/t. This value categorizes
the sample close to the lower ranges of another limestone found on bibliographic references
(Bhatty, 2011). This can be one reason which limestone can be responsible for high rates of
agglomeration in the air separator and also to a fast selective breakage rate on the ball mill.
According Beke (1983) Limestone has “free crystal movements side by side and a great scatter
of sizes” that makes limestone a highly grindable material.
Clinker with 13.03 kWh/t can be characterized as material with a medium work index in
comparison with other clinkers shown in Table 4-4. By adding more proportions of softer
limestone to clinker, there is a visible reduction on Bond work index. For 5%, 12% and 40%
limestone addition Bond work index is 11.12, 10.86 and 9.46 respectively. This should be
taking into consideration when planning the production of PLC with higher limestone contents
80
than 3%. This can increase the agglomeration effect on the separator due to softer limestone
and selective grinding in the ball mill at higher limestone additions. The addition of grinding
aids after improving air efficiency classifier should be required in order to improve efficiency.
4.2.2 Breakage function
Breakage function is a material related property, and by definition is the distribution of sizes from a single
particle breakage event. The breakage function values for clinker were calculated from five different size
fractions from 1.4 to 0.325 mm. Results for the size normalized breakage function are shown at Figure 4-9
and the average breakage function values are also in Table 4-5 (Farzanegan, 2015).
Figure 4-9: Clinker breakage function at normalized size
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Cum
ulat
ive
Brea
kage
func
tion
Normalized size
1400 500 150 1000 355 average
81
Table 4-5: Clinker average breakage function
Size,
mm
Normalized
Size
Average cumulative
breakage function
calculated for this research
Breakage
function from bibliography
(Farzanegan, 2015)
1.400 1.00 1.00 1.0
1.000 0.71 0.47 0.54
0.710 0.51 0.29 0.19
0.500 0.36 0.20 0.12
0.355 0.25 0.14 0.09
0.250 0.18 0.10 0.07
0.150 0.11 0.07 0.06
0.106 0.08 0.07 0.05
0.075 0.05 0.06 0.05
0.063 0.05 0.05 0.04
0.045 0.03 0.03 0.03
Results shows that the breakage function related to the clinker evaluated is normalizable and
similar to other clinker references found.
82
4.2.3 Selection function
Selection function is a machine related property, represents the breakage kinetics of the system.
For this research selection function has been back calculated using real plant data. Selection
function has been estimated for each of the two ball mill compartment using NGOTC software.
Selection function or specific rate of breakage provides information on how breakage kinetics are
evolving inside each compartment and how to improve them (Farzanegan, 2015).
4.2.3.1 Grinding kinetics at compartments 01 and 02
Compartment 01 is the coarse grinding media compartment, and it receives a mixture of fresh ball
mill feed and rejects (circulating load) from separator. This mass balance represents 125 and 264.8
tph respectively accounting for a total ball mill feed mass of 389.8 tph with a feed size d50 of 0.05
mm. The following discussion is related to these two compartments:
For a feed size to compartment 01 d80 of 0.207 mm, the top calculated grinding media size
diameter should be 10.29 mm according Allis Chalmers’s formula (FLSmidth, 2014).
The actual compartment 01 equivalent grinding media diameter is 47.49 mm and as shown in
Table 4-3. This larger grinding media diameter selection of 47.49 mm instead of 10.29 mm,
makes a high energy impact environment, creating an early agglomeration condition
according Beke (1983).
The specific rate of breakage (on 100% clinker) for compartment 01, and represented in solid
black line and shown on Figure 4-10 denotes a drop in grinding kinetics for sizes larger than
1.0 mm. and is related to an undersized grinding media.
83
Figure 4-10: Selection function for compartments 01 & 02 on 100% clinker
The particles smaller than 1.0 mm represent 87.8 % (342.4 tph) of the feed. The grinding media
selected to grind these size range is not efficient for these fractions, its grinding kinetics is
reduced creating an increased wear to the media and high energy impacts (this condition was
confirmed by the processing plant personnel).
In order to improve the grinding kinetics on compartment 01, there should be before the ball
mill a further and efficient size reduction of the limestone and gypsum top size feed. After
optimizing the feed the proper grinding media selection should be calculated.
84
The reason for the tendency on using high top grinding media is for reducing the high fresh
feed size of fresh limestone and gypsum (on composite feed to ball mill).
A mass flow of 225.76 tph corresponding to 57.8% of the comminuted mass on compartment
01 is below 0.063 mm, representing a high portion of the mass with a low grinding kinetics.
These fractions are transported rapidly with low reduction ratio to internal diaphragm and to
compartment 02. This effect is mainly caused by a larger top size grinding media and the
saturation of very fine particles rejected from air separator.
Compartment 02 has a feed size d80 of 0.084 mm. In addition 96.11% of the feed comprises
particles below 0.15 mm showing high percentage of the amount of small particles.
Selection function for compartment 02 is shown in Figure 4-10 (represented by a solid blue
line) and denotes good reduction of particles between 0.001 to 0.1 mm. (less than 1% of the
particles on sizes 0.1-1.4 mm are comminuted on compartment 02). This condition creates a
perfect comminution environment for particles rejected from the separator which reduces its
size to an even smaller size efficiently.
In compartment 02, particles below 0.01 mm have a high tendency for agglomeration and
shown in Figure 4-5, because of overgrinding and further rejection in the grinding circuit.
It was estimated that 62.92 % of the feed to compartment 02 consists of particles below 45
microns, showing a huge amount of particles that should have been separated efficiently at the
air separator instead of being rejected.
The overgrinding in compartment 02 of particles smaller than 0.003 mm is most likely related
to the use of raw limestone (Ludmilla Opoczky, 1996). Particles are rejected from the separator
85
and is the main reason of the agglomeration effect (Tamás, 1983). An efficient air separation
is required followed by the use of grinding aid.
By designing the right grinding media size according to joint feed. It is expected that the
specific breakage rate of compartments 01 and 02 be increased and that the difference between
both rates be reduced. Another benefits from selecting the right grinding media size is the
reduction of wear rate and damaging internal parts of ball mill for direct impacts between
grinding media and ball mill surfaces.
4.2.4 Savings estimations
Based on Bond formula there is possible to calculate the production increase and the use in specific
energy for the ball mill by reducing the fresh feed size using the CKP. The effect of classifier is
not modeled, just the particle size reduction. As shown in Table 4-6, three scenarios under two
different conditions are calculated for a 3% and 12% limestone substitution under three reduction
scenario: first is (just clinker through CKP), second is (clinker + limestone through CKP) and third
is (clinker + limestone throw CKP and gypsum trough vertical shaft impactor).
86
Table 4-6: Savings estimate on electricity for fresh feed size reduction at ball mill 03
Savings in CAD/yr.
Scenario 01 Scenario 02 Scenario 03
3% limestone 0 13,641.56 12,651.69
12% limestone 0 31,030.74 38,805.25
* Assumptions: production of 800,000 t/yr. and cost of electricity at $0.03/kWh.
The calculated profit on recovering fines in air separator at different percentages from the rejects
is shown on Table 4-7.
Table 4-7: Profit estimate on increase in throughput at air classifier circuit 03
Recovery of fines
1% 5% 10%
CAD/yr. 130,305 490,560 1,177,344
* Assumptions: profit of $14/t and plant availability factor 0f 85%
87
CHAPTER 5: MAJOR RESEARCH FINDINGS AND CONCLUSIONS
5.1 Major research findings
The main objective of this research was to evaluate the process at grinding circuit 03 of a cement
producing plant by quantifying the most related causes of inefficiency. The objective was to obtain
a relationship of the operational parameters, efficient models, comparing the results with
bibliographical references. This assessment provides a series of recommendations to improve
efficiency of the system. The following major findings from this research are as follows:
Air separator operating with low efficiency. Separator is performing inefficiently with values
rated at 64.74, 52.24 and 45.43 % on fractions of 0.020, 0.035 and 0.045 mm respectively, this
range of fractions are fundamental for the strength development of cement. This operational
condition makes that a high amount of fine particles, about 100.4 tph (on fraction below 0.035
mm) are being rejected and sent back to ball mill for overgrinding. This bypass from the feed
to rejects condition can be confirmed by the use of at least two comparative factors that defines
optimum operating values:
- Alpha sharpness value using Whiten model. According to references (Altun & Benzer,
2014), this model is the most precise because provides the least square difference
related to other methods. The value of alpha obtained by fitting the Whiten model to
Tromp curve for the current separator has a factor of 1.0411. An efficient separator
should have an alpha value around 4.0 according to references available (Napier-Munn
et al., 1996).
88
- Sharpness by Tromp curve slope. Tromp or selectivity curve is defined as the
probability of defined partitions or portions of feed that reports to the products or
rejects. The Tromp sharpness obtained from the evaluated separator is 42.51%. Based
on the literature (FLSmidth, 2014; Bhatty, 2011), it is recommended values between
52-58%.
Factors affecting separation efficiency. Two major causes for the low separation efficiency has
been found: low sharpness and high agglomeration levels on the separator caused mainly by,
- Overloading separator feed. It is estimated that dust load density greater than 2.6
kg/actual cubic meter of air is currently being fed into the separator. According to
equipment manufacturer this feed rate should be between 2.0 and 2.5 kg/actual cubic
meter of air.
- The reduction of the dust load on the feed will improve on the separator’s sharpness.
Altun & Benzer (2014) found that the model that represents sharpness “α” and dust
load “DL” is: α = 4.2044( . ) and shown Figure 4-8, it is deducting that by
reducing the dust load the sharpness is increased.
Agglomeration: This high agglomeration phenomenon is the effect related to mainly two
operating conditions: the first is by the use of oversized/larger grinding media on compartment
01 creating high energy impact environment inside the mill allowing agglomeration of fine
particles. This group of agglomerated particles will attach one to another behaving like one
bigger particle and once inside the air classifier will be rejected as a big particle. The second
condition is related to overgrinding the particles under high breakage rates (especially on
89
compartment 02). This agglomeration effect is aggravated by the use of limestone as a mineral
additive on composite cements manufacturing. Limestone with a low Bond work index of 5.29
kWh/t is comminuted at higher grinding kinetics and reduced faster than clinker producing fine
fractions of limestone that creates a narrower uniformity factor “n” slope during the grinding
stage. This condition creates over expenditure of energy and the throughput is reduced by the
high recirculation ratio and the agglomeration effect discussed in the first condition previously.
Oversized grinding media. The grinding media size design diameter used in compartment 01
is larger than the optimum for the current feed. This is based on Allis Chalmers formula that
gives a 10.29 mm diameter instead of the current 47.49 mm. The effects of this oversizing are
the expenditure of energy especially on top grinding media, probably increasing the wear rate
of media and explaining the fast load degrading reported from last two mill surveys (February
and May 2014). This oversizing is mainly defined by the fresh feed top size of raw limestone
and gypsum which are d80 of 13.7 and 61.4 mm respectively.
5.2 Conclusions
The following research is unique because of the following results obtained:
The use for the very first time at this plant of different tools like: Whiten model in analyzing
more precisely the process of air separation, the use of breakage/selection function to evaluate
the grinding kinetics at the ball mill and the application of updated researches on the
optimization of finished grinding in the cement industry.
90
Based on Altun and Benzer (2014) who investigated the relationship of some operating
parameters on different high efficiency classifiers and FLSmidth (air separator manufacturer)
the following upgrades can be estimated using Altun & Benzer models (Altun & Benzer,
2014):
- Sharpness “α” can be modeled by the use of the following relation α = 4.2044 ∗ .DL is dust load (density of the feed to separator) that according manufacturer should be
between 2.0 and 2.5 kg/m3. Modeling to get the most efficient dust loading of 2.0 kg/m3 it
is obtained a sharpness factor α=1.7948
- Bypass “100-C” can be modeled by the use of the relation 100 − C = 10.467 ∗ .DL is dust load (density of the feed to separator) that according manufacturer should be
between 2.0 and 2.5 kg/m3. Modeling to get the most efficient dust loading of 2.0 kg/m3 it
is obtained a sharpness C=72.04% and Bypass=27.96%
- The parameters “β” and “β*” can be modeled by the use of the relationβ = −0.0422( ) + 4.0907andβ∗ = 0.9878(β) + 0.8516
obtaining the following values β=1.05 and β*=1.88
Obtained values for all the modelled parameters are summarized in Table 5-1.
91
Table 5-1: Summary of current and modeled separator parameters
Parameter Current plant valueModeled best value
at dust load DL=2.0
α 1.04105 1.7948
β 1.61607 1.05
β* 0.24271 1.88
C 63.67 72.04
Bypass 36.33 27.96
DL >2.6 2.0
CL 211 100
There is an increase in efficiency on operative air separator parameters modeled by
adjusting the dust load to manufacturer design parameters.
In order to calculate an accurate improvement on the grinding circuit, it will be required the
use of a calibrated simulation program that integrates all the different components of the circuit
and its interactions and relate it to its impact on quality and production.
5.3 Recommendations for future work
Circuit simulation. The use of a calibrated simulation program is highly recommended as an
optimizing tool for this operation. A simulator will integrate all the key process components
of the circuit and will provide a low cost modeling of changing several variables on searching
for the right configuration.
92
Composite cement manufacturing cautions. When planning to increase the use of raw
limestone on composite cement manufacturing (PLC) extreme care should be taken on the
operating parameters of air separator, because the change of densities and uniformity factor of
the mixture of the feed (limestone and clinker) may kame the separator to behave differently.
The use of extra grinding aid reagent could be required in order to reduce agglomeration due
to increased addition of limestone.
Further research on the agglomeration effect during grinding and its implications should be
developed, especially if limestone content is expected to be increased in composite cement
manufacturing.
93
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APPENDICES
Appendix A: Standard Bond work index calculation
The purpose of the standard Bond ball mill grindability test is to determine the Bond Ball mill
Work Index (BWI), which can be compared with the work indices of known materials to evaluate
grinding efficiency or mill design.
The Bond work index is a measurement of the power required to reduce feed with a given 80
percent passing size (d80) to product with a specified 80 percent passing size (d80).
This procedure assumes a standard 100 mesh (150 µm) closing screen size. The Bond ball mill is
composed of a steel shell with internal dimensions of 12-inch (30 cm) diameter x 11-inch (28 cm)
length. The shell has rounded corners, a smooth liner and no lifters. The feed hatch consists of a
removable cover plate on the curved surface of the mill. The mill is set to operate at 70 rpm. The
ball charge consists of 20.125 kg of steel balls ranging from approximately 37 cm to 15 cm in
diameter.
Tolerance Ball Size Number Weight, ginch mm of Balls Total Avg. Ball
Max 1.50 38.1Avg 1.45 36.8 43 8,809 204.9Min 1.25 31.8Max 1.25 31.8Avg 1.17 29.7 67 7,215 107.7Min 1.06 26.5Max 1.06 26.5Avg 1.00 25.4 10 670 67.0Min 0.88 22.4Max 0.88 22.4Avg 0.75 19.1 71 2,003 28.2Min 0.63 16.0Max 0.63 16.0Avg 0.61 15.5 94 1,428 15.2Min 0.53 13.2
Total 285 20,125
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A.1 Procedure
The procedure for a Bond ball mill grindability test depends on the following variables. The
variables specified in the work order instructions for each test are as follows:
• Closing screen size: default is 100 mesh (150 µm)
• Cycle 1 revolutions: selected based on known sample hardness; default is 100 cycles but
it typically corresponds to the closing screen size (e.g. 100 mesh corresponds to 100 cycles).
A.2 Sample preparation
Prepare 10 kg of sample to have 100% passing a 6 mesh screen. Split into twelve different feeds
and store each split.
A.3 Particle size analysis of the feed
Weigh one “Feed” charge and record the weight on the manual worksheet. Screens to be used
according based on the closing screen size as specified in the test work order. Using the selected
screen stack, add the sample and shake using the dry Ro-Tap® machine for 15 minutes. Weigh
each size fraction on the manual worksheet. Recombine all fractions, bag, label (Feed PSA Reject)
and set aside. Rejects may be used as supplementary feed for the grind test if needed.
98
The following conditions must be accomplished on the first stage before continuing to the
following stage:
i. The feed d80 should be between 2,200 µm and 2,500 µm.
ii. The feed size analysis should show less than 20% passing the closing screen size.
A.4 Feed bulk density
Take two of the Feed charges and transfer the material to a 1000 mL graduated cylinder. Vibrate
the sample for 10 minutes on the Vibro-Pad in order to minimize air pockets as shown on. Record
the final volume level and the actual sample weight on the manual worksheet. Transfer and enter
data values into the computer spreadsheet. Riffle out the weight calculated by the computer
spreadsheet which should be equivalent to 700 mL. This is the feed for Cycle 1. Bag the remaining
sample, label (Feed Reject) and set aside (Gupta & Yan, 2006).
99
A.5 Performing the grinding test
Ensure that the inside of the mill is clean (i.e. no foreign material) and follow the instructions:
1. Place the ball charge in the mill according
2. Place the equivalent weight of 700 mL of the “feed charges” samples (stored on bags) in the
mill according the density calculation.
3. Secure the full cover plate with the two wing nut clamps.
4. Grind the ore for 100 revolutions.
5. At the end of the grinding during the 100 revolutions, replace the full cover.
6. Making sure that the collection pan is in place, discharge the ball mill as shown
7. Screen the product at the required closing screen size (as specified in the work order; default
100 mesh).
8. Ro-Tap® using the closing screen for 15 minutes.
9. Collect the undersize material from the screens and set aside in a labelled ‘PRODUCT’ in
plastic bag.
10. Collect the oversize material from the three screens, combine and weigh in a metal pan.
Record the weight on the manual worksheet as “Oversize #1”.
11. Re-screen the oversize material for another 15 minutes using multiple screen-pan sets if
necessary.
100
12. Collect the oversize material from the second stage of screening, weigh and record as
“Oversize #2” on the manual worksheet. Enter this value on computer spreadsheet.
13. The spreadsheet will forecast the number of revolutions and the mass of new feed to add to
the oversize material reserved in Step 12 for the next cycle. Record these two set points on
the manual worksheet.
14. Add the mass of new feed to the reserved oversize material. Verify that the actual combined
mass is equal to the original mass equivalent to 700 mL.
15. Place the combined new/oversize material in the mill and run for the determined number of
revolutions.
16. Repeat Steps 2 through 15 until all conditions for closure have been met, as described in the
following section 3.2.6.
101
A.6 Conditions for closure
All of the following conditions must be met for closure.
• Minimum of seven cycles
• A reversal in the Net Product per Revolution should be reported (calculated by the
spreadsheet; grams of undersize product per mill revolution). A reversal is a trend in the Net
Product per Rev of either up/down/up (eg. 1.91 – 1.88 – 1.92) or down/up/down (eg. 1.81 – 1.85
– 1.82).
• Less than 3% difference between the highest and lowest values of Net Product per
Revolution in the last three cycles. This assures a 250% circulating load. The following formula
is used to calculate the difference:
• Circulating load between 245 to 255% (250 +/- 5%).
A.7 Particle size analysis of the product
Combine the undersize products from the last three cycles. Blend the combined undersize product
and then riffle-split a sub-sample of approximately 200 g. Record this as the total product weight
on the manual worksheet. Select the screens to be used according to based on the closing screen
size as specified in the test work order.
Wet screen at 400 mesh and dry the +400 mesh product in a clean pan in the over. Discard
the -400 mesh material.
Using the selected screen stack, add the dried +400 mesh product and shake using the Ro-
Tap® machine for 20 minutes.
Weigh and record the weight of each size fraction on the manual worksheet.
Lowest ValueHighest Value
Difference = 1 - x 100
102
Transfer the feed and fraction weights from the manual worksheet to the spreadsheet created.
A.8 Bond test grindability calculations
The following calculations are automatically conducted by the spreadsheet.
Ore Feed Density:
weightfeedActualvolumeActualmLdensityfeedOre
700
Target Recirculation Load Weight
The target recirculation load weight, also known as the Ideal Potential Product (IPP), corresponds
to the target product weight to achieve a circulating load of 250%.
5.3700 mLFeedWeightofproductpotentialIdeal
Where 3.5 factor corresponds to 1 part (100%) target product and 2.5 parts (250%) circulating
load.
Bond Ball Mill Work Index
The BWI calculation is derived from F. C. Bond’s Third Theory of Comminution.
103.11010
5.44
8080
82.023.01
FPGprP
BWi
Where:
BWI = Bond Ball mill Work Index number in kWh/t
P1 = Aperture of the closing screen size in microns
Gpr =Average grams of undersize product per revolution from the last three cycles
103
P80 = Size at which 80 percent of the undersize product passes, in microns
F80 = Size at which 80 percent of the feed passes, in microns
104
Appendix B: Work indices
B.1 Clinker 100%
105
B.2 Limestone
106
B.3 Clinker 95%/Limestone 5%
107
B.4 Clinker 88%/Limestone 12%
108
B.5 Clinker 60%/Limestone 40%
109
Appendix C: Particle size distribution
C.1 Fresh feed to ball mill
110
C.2 Air separator streams (data provided by plant)
FINES (F) REJECT (R) FEED (A)Equivalent Corrected Corrected CorrectedSize µm % Passing % Passing % Passing150 99.70 97.08 97.87125 99.70 96.91 97.11100 99.70 94.31 95.4195 99.70 93.19 94.8390 99.70 91.82 94.1285 99.70 90.10 93.2480 99.70 87.98 92.1575 99.69 85.39 90.8070 99.63 82.22 89.1365 99.49 78.36 87.0560 98.98 73.63 84.4955 98.02 67.88 81.3450 96.62 61.10 77.5245 94.62 53.73 72.9440 91.75 46.00 67.5035 87.78 37.93 61.2730 82.38 29.98 54.5625 75.28 22.78 47.3920 66.24 17.06 39.9115 55.29 13.24 32.3910 41.81 11.05 24.779 38.66 10.69 23.138 35.30 10.29 21.377 31.67 9.63 19.456 27.68 8.74 17.325 23.24 7.67 14.904 18.30 6.37 12.133 12.83 4.85 9.022 7.15 3.23 5.711 2.11 1.64 2.63
111
C.3 Fresh feed and circulating load feed to ball mill
112
Appendix D: Bond equation for modeling throughput and savings
113
Appendix E: Specific rate of breakage