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Callistus Obunadike MSc Thesis Nov. 2019 DOI: https://doie.org/10.0201/Thesis.2023724402
Master Thesis
The Influence of chemical-mineralogical
composition on High-Pressure Comminution
of Cement Clinker
Obunadike Callistus Ebuka
Faculty: Geosciences, Geotechnical und Mining
Department: Sustainable Mining and Remediation Management
Matriculation Number: 58407
November 2019
Supervisor 1:
Prof. Dr. Carsten Drebenstedt
Supervisor 2:
Prof. Dr.-Ing. Urs. Peuker
Supervisor 3:
Dr.-Ing. Thomas Mütze
Supervisor 4:
Lieven Schützenmeister(M.Sc)
https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
Declaration
I hereby declare that this thesis represents my original work which was done after the
registration for the degree of Master in Sustainable Mining and Remediation Manage-
ment, Department of Mining, Faculty of Geosciences, Geotechnical and Mining at
Freiberg University of Mining and Technology has not been previously included in any
other thesis submitted to this or any other Institution for a degree, diploma, or other
qualification. The ideas directly or indirectly taken from foreign sources were rightfully
acknowledged with reference to the original source.
Freiberg, den 30.09.2019
Obunadike Callistus
https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
Acknowledgement
Firstly, I would like to appreciate and thank all my supervisors Prof. Dr.-Ing. Urs.
Peuker, Dr.-Ing. Thomas Mütze and Mr. Lieven Schützenmeister for their great support
and their cherished advice.
In addition, my endless thanks extend to Prof. Dr. Carsten Drebenstedt, the program
coordinator who accepted my application for a second master’s degree program. Indeed,
it’s a dream come through to be alive and experience a wonderful day like this. I will
definitely hold the spirit high.
Furthermore, I won’t forget the pertinent and vital role that Mr. Lieven Schützenmeister
played during my laboratory and result (data) analysis. Indeed, his practical and field
experience coupled with his integral field knowledge helped me a lot during difficult
laboratory and data interpretation sections. In addition, I would like to appreciate Na-
talie Hering for her time and assistance while editing my thesis.
I would like to extend my appreciation to the technical staff at the Institute for mechani-
cal processing engineering and technology. They provided a friendly and cooperative
atmosphere during the laboratory and practical work.
Last but not the least, my sincere appreciation goes to my adorable family for their un-
ending love and encouragement. Honestly, their support went far beyond this master
thesis and gave me the necessary strength to endure till the end. I really appreciate the
patience and care shown to me by my beloved wife Mrs Chioma Obunadike and my
intelligent daughter Munachimso Nneamaka Obunadike. Indeed, their love and prayers
made me to believe in myself, during difficult and lonely times.
To sum up, I wouldn’t forget to thank the Almighty God, for the wisdom and strength,
bestowed on me during this thesis. Thank you Lord for creating the celestial and spiritu-
al freedom that was necessary during the hard times.
A thousand thanks to all my dear friends and family.
I
https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clink
Table of Contents
Declaration ........................................................................................................................ 2
Acknowledgement............................................................................................................. 3
TABLE OF FIGURES ...................................................................................................... 4
LIST OF TABLES ............................................................................................................ 6
ABBREVIATIONS .......................................................................................................... 7
1 Introduction .................................................................................................................... 1
2 Literature Review ........................................................................................................... 4
2.1 Stress and strain ....................................................................................................... 4
2.2 Breakage Funadamentals ......................................................................................... 7
2.2.1 Particle Breakage Mechanisms ....................................................................... 11
2.2.2 Comminution of particle beds......................................................................... 13
2.2.3 Principle of Breakage Mechanics ................................................................... 15
2.2.4 Energy Analysis .............................................................................................. 15
2.2.5 Compaction ..................................................................................................... 18
2.2.6 Back Strain ...................................................................................................... 19
2.3 Goals of Comminution .......................................................................................... 20
2.3.1 Comminution Law .......................................................................................... 21
2.4 Phase Composition of Cement Clinkers ................................................................ 23
2.4.1 Analysis of mineral phases using X-ray diffraction (XRD) ........................... 25
3 Experiments and Methods ............................................................................................ 27
3.1 Equipment and steps .............................................................................................. 27
3.1.1 Piston-die Test ................................................................................................ 27
3.1.2 Rollbock machine ........................................................................................... 29
3.1.3 Particle Size Analysis and Sieving ................................................................. 30
3.1.4 Analytical sieving ........................................................................................... 31
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
3.1.5 Laser Diffraction ............................................................................................. 37
3.1.6 Ball mill according to Zeisel Grindability Approach ..................................... 38
3.2 Simulation of grinding circuits .............................................................................. 40
3.2.1 Application of comminution and simulation of grinding circuit .................... 41
3.3 X-ray Diffractometry and Scanning Electron Microscope .................................... 41
3.4 Mineral Liberation Analysis (MLA) ..................................................................... 42
4 Results and Discussion ................................................................................................. 44
4.1 X-Ray diffractometry ............................................................................................ 44
4.2 Mineral Liberation Analysis Software (MLA) ...................................................... 45
4.2.1 Mineral Grain Size Distribution Curve from MLA ........................................ 47
4.3 Ball milling ............................................................................................................ 49
4.4 Piston-die Press ..................................................................................................... 55
5 General Discussion....................................................................................................... 65
6 Summary ...................................................................................................................... 67
7 Abstract ........................................................................................................................ 68
Publication bibliography ................................................................................................. 69
Appendix ......................................................................................................................... 80
Equipment Used .......................................................................................................... 80
A: PSD ANALYSIS .................................................................................................... 81
B: RESULTS OF THE LASER DIFFRACTION ....................................................... 82
B1: Laser Diffraction Analysis (Clinker 1) ................................................................. 82
B2: Laser Diffraction Analysis (Clinker 4) ................................................................. 83
C: RESULTS OF THE ZEISEL TEST GRINDABILITY ......................................... 85
C1: Measurement of Specific Surface Blaine and Energy Consumption ................... 85
D: CHEMICAL AND MINERAL COMPOSITION OF THE ANALYSED
CLINKERS USING MLA .......................................................................................... 87
D1: Mineral Constituents and Formulas of the analysed Cement Clinkers ................ 87
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
D2: X-RAY DIAGRAM REPRESENTING THE PHASE FORMATION OF K1 AT
DIFFERENT TEMPERATURES ............................................................................... 88
D3: X-RAY DIAGRAM REPRESENTING THE PHASE FORMATION OF K4
AT DIFFERENT TEMPERATURES ..................................................................... 89
E: Modal mineralogy and Particle size distribution plotted with MLA-data ......... 90
E1: Mineral grain size distribution plotted with MLA-data .................................... 91
E2: Particle size distribution after filtratrion for particle range of 0-90m ............. 92
E3: Particle size distribution after filtratrion for particle range of 90-500m ......... 94
F: DIADEM SKRIPT ................................................................................................. 95
F1: Skript used by Diadem to generate Energy absorption. .................................... 95
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
TABLE OF FIGURES
Figure 1: showing different steps in particle breakage (Mütze 2012). ............................. 4
Figure 2: Stress-strain curves for a) linear and b) non-linear elastic (plastic) deformation
behavior adapted ............................................................................................................... 5
Figure 3: Stress-strain curves for a) Elastic-plastic and b) Viscos-elastic deformation
(Mütze 2012). .................................................................................................................... 7
Figure 4: Influence of contact geometry on breakage of sphere and irregular shaped
particles ............................................................................................................................. 7
Figure 5: Stages of the crack process modified after (Mütze 2012). ................................ 8
Figure 6: Particles Breakage Mechanisms adopted from (Rao 2011)............................. 10
Figure 7: Coordination number and pore number for spheres of the same size modified
(Amelung 2018). ............................................................................................................. 12
Figure 8: Bulk density distribution (g/cm³) in a material bed axially loaded from above
modifies after (Schatt 1988). ........................................................................................... 13
Figure 9: Particle size distribution of quartz after single-particle and inter-particle
comminution (Kleeberg 2007). ...................................................................................... 14
Figure 10: A typical force displacement curve for energy analysis (Khanal 2005). ....... 16
Figure 11: Force-displacement curve modified after (Mütze 2012). .............................. 17
Figure 12: : Sub-processes during compaction of a particle bed adapted from (Schubert
2003). .............................................................................................................................. 18
Figure 13: Plastic deformation during loading adapted from (Schubert 2003). ............. 19
Figure 14: Force-displacement curve with corrected press travel scorr from a linearly
(Oettel 2002). .................................................................................................................. 19
Figure 15: The effect of pressure on the particle size distribution curve of cement
clinkers (Mütze 2012). .................................................................................................... 22
Figure 16: Relationship between Energy Input and Size Reduction in Comminution
(after Hukki 1961). .......................................................................................................... 23
Figure 17: Schematic representation of the Shimadzu hydraulic piston-press modified
after (Schulze 2019). ....................................................................................................... 29
Figure 18: Rollbock machine from MVAT (ITUN) Institute Bergakademie. ................ 29
Figure 19: Dimensions of rubber stoppers adapted from (Schulze 2019). .................... 30
Figure 20: a) Particle sizes sorted into different classes and b) particle sizes interval
(Mütze 2012) ................................................................................................................... 30
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
Figure 21: Particle size distribution curve showing x50 adapted from (Mütze 2012). ... 34
Figure 22:Comparative sum function of different particle size distribution curves
adapted from (Mütze 2012). ............................................................................................ 34
Figure 23: RRSB distribution curve adapted from (Mütze 2012)................................... 36
Figure 24: a) Illustration of sieve analysis [Q3(x)] adapted from (Mütze 2012) and b)
mesh seive (R10) used for the particle size distribution of the analysed cement clinkers.
......................................................................................................................................... 36
Figure 25: Sectional view of the grinding bowl with balls and die in mesh and grinding
bowl adapted from (Fleiger et al. 2012). ......................................................................... 38
Figure 26: Comparison of modal mineralogy composition of both clinkers .................. 46
Figure 27: Mineral grain size distribution a) CXS and b) C3A ....................................... 47
Figure 28: Particle and mineral grain size distributions for the analysed clinkers. ........ 48
Figure 29: Zeisel grindability curve for a) K1 and b) K4 ................................................ 51
Figure 30: Product curves after piston die circuit simulation using data from sieve
analysis and laser diffraction. .......................................................................................... 58
Figure 31: XRD pattern (Co Kα radiation) for Clinker 1 (K1) showing severe overlap of
major phases and different components after chemical seperation. ................................ 88
Figure 32: XRD pattern (Co Kα radiation) for Clinker 4 (K4) showing severe overlap of
major phases and different components after chemical seperation. ................................ 89
Figure 33: Mineral constituents of the analysed clinkers using MLA-data .................... 90
Figure 34: Particle size distribution curve for K1 and K4 based on MLA-data (sieve size
unit in equivalent ............................................................................................................. 90
Figure 35: Mineral grain size distribution curve for major mineral phases based on
MLA-data. ....................................................................................................................... 91
Figure 36: Mineral liberation by particle composition curve for calcium-silicate mineral
phase. ............................................................................................................................... 91
Figure 37: Particle size distribution curve for filtered sieve sizes ranging from 0 – 10
m ................................................................................................................................... 92
Figure 38: Particle size distribution curve for filtered sieve sizes ranging from 10 – 90
m ................................................................................................................................... 93
Figure 39: Particle size distribution curve for filtered sieve sizes ranging from 90 – 500
m ................................................................................................................................... 94
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
LIST OF TABLES
Table 1: Classification of particle arrangement and confinement according to (Schönert
1996) ............................................................................................................................... 13
Table 2: Test parameters originally set by Zeisel modified after (Fleiger et al. 2012) ... 39
Table 3: Showing detected mineral phases from X-Ray diffraction determined by means
of Rietveld method .......................................................................................................... 44
Table 4: Measured concentrations of composite minerals in analysed clinkers using
different analytical methods ............................................................................................ 46
Table 5: Factor C(φ) [kW/(th)] as related to mill loading factor and type of grinding
media adapted.................................................................................................................. 50
Table 6: Empirical calculated values for energy required from grindability, conversion
factor (transfer func- ....................................................................................................... 54
Table 7: Different specific energy absorption recorded during the piston-die test ......... 56
Table 8: Showing the initial mass and the passing mass (g) of the analysed clinker
above and below 100m ................................................................................................. 57
Table 9: Showing the initial mass and the passing mass (g) of the analysed clinker
above and below 100m ................................................................................................. 57
Table 10: Empirical calculated values for energy required by HPGR according to
piston-die press data ........................................................................................................ 63
Table 11: Comparison between the calculated parameters of different clinkers based on
HPGR and ....................................................................................................................... 64
Table 12: Showing the particle size distribution for the measured mass (g) at each mesh
sieve size ......................................................................................................................... 81
Table 13: Showing the particle size distribution for the measured mass (g) at each mesh
sieve size ......................................................................................................................... 81
Table 14: Showing the values obtained for clinker 1 through laser diffraction analysis 82
Table 15: Showing the values obtained for clinker 4 through laser diffraction analysis 83
Table 16: Showing the recorded and measured values of mean resistance, Blaine, energy
consumption for .............................................................................................................. 85
Table 17: Different chemical and mineralogical constituents of the analysed clinkers
using MLA software ....................................................................................................... 87
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
ABBREVIATIONS
General Abbreviation
Modulus of elasticity
C (
)
Filling ratio
L/D
Length – diameter ratio
SEM
Scan electron microscopy
MLA
Mineral liberation analysis
XRD
X-ray diffraction
MPa
Mega pascal
Mi
Material index related to ore’s breakage property
C3S
Tricalcium silicate
C2S
Dicalcium silicate
C3A
Ca-Aluminate
C/S
Calcium-Silicate
C2AS
Ca-Al-Silicate
C2F2-xAx/ C4AF
Ca-Al-Ferrite
Em,z
Energy required to grind clinkers to certain fineness during
Zeisel test
Em,BM
Energy required to grind clinkers to a certain fineness dur-
ing ball milling
ft
Conversion factor / transfer function
R90
Fineness value of mesh size 90 µm in percent
X63.2
Position parameter
VBM
Mill volume
mB
Mass of the grinding media
m
Width of the particle distribution / specific throughput
M
Throughput
s
Thickness of cakes / working gap (mm)
P
Power consumption of the ball mill (kW)
Pshaft
Power consumption of shaft
Pz
Power consumption (counter)
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3.6
Conversion factor for s to h and m to mm
1000
Conversion factor of m to mm for αk.
1000
Conversion factor kN - N for Spec. grinding pressure.
K
Expansion constant (0.2)
D
Diameter of the ball mill/rolls (m)
L
Length of the ball mill/rolls (m)
Q(x,Esp)
Particle size distribution function as a function of specific
energy input
Q(x)
Particle size distribution
Fsp
Specific grinding force (N/m²)
F
Grinding force [kN]
TKIS
Thyssenkrupp Industrial Solution AG
Greek Abbreviations
Relative speed
Critical speed
Density of cakes [t/m3]
Density of feed [t/m3]
ρmax
Maximum specific grinding pressure (MPa)
Clinker subscript (K1 or K4)
Spec. free boundary energy
Lattice constant
σ
Tensile stress
ԑ
Strain
αk
Angle of compression [rad]
Quantity proportions
Ɵ
Standard compaction
U
Recycling load factor/Circulation factor [cm2/g]
ψ
circumferential speed of rolls [m/s]
Β
Angle of force [°]
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
1 Introduction
The objective of this thesis is to compare the energy required for comminution of se-
lected cement clinkers according to results obtained from piston-die press and ball-mill.
In addition, the influence of different mineral phases on the grindability of cements
clinkers aims at clarifying any complexity encountered during scale-up of HPGR.
Comminution and grindability in cement and mineral processing operations are based
on different techniques used in measuring comminution properties. Recently the im-
portance of achieving cost-efficient and effective comminution, with regards to energy
consumption, has been emphasized due to increase in the cost of electricity (Horst and
Bassarear 1976). The grindability of cement clinkers does not only depend on the clink-
er grain size but rather on other factors like the chemical and mineralogical composition
which indirectly affects the energy required during the comminution. According to
Olisah (2019), When material particles are stressed without breaking the tend to develop
micro-cracks which should make subsequent comminution easier and more efficient.
The breakage of particle during comminution processes leads to the disintegration of the
atomic and molecular masses holding or binding the stressed materials (Lea 2004;
Tavares 2005). Thus, this separation or disintegration is mainly influenced by the struc-
ture and geometry of different phases as well as by natural fissure inside the lattice
structure which normally reflects as stress peaks (Fleiger et al. 2012). The distribution
and number of these peaks varies significantly depending on the deposit or previous
processing and thus, the general physical quantities like binding energies or energy bal-
ances during crack propagation cannot be used for the effective characterization of ma-
terial for industrial comminution processes (Chen and Juenger 2011; Fleiger et al. 2012;
Duxson et al. 2007).
This problem is solved by the application of technical grindabilities which relate applied
energies to achieved fineness for defined quantities of material (Fuerstenau and
Vazquez-Favela 1997.; Fleiger et al. 2012). It has to be accounted for, that grindability
test determine a system property and not a material property, henceforth, the transfer of
energy to a material strongly influences the outcome of comminution (Kodali et al.
2011; Fleiger et al. 2012; Austin 2002). Most commonly used grindability test for ce-
ment and raw material components are test according to Zeisel and Bond (Fleiger et al.
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2012). Different factors like preparation of test material, differences in the testing de-
vice’s design, process parameters, numerical evaluation of the results and measurement
technique for the determination of the power consumption greatly influences the result
and outcome of grindability test and in most cases most industries overlook these fac-
tors through inconsistency in their regulation procedure (Fuerstenau et al. 1990;
Kosmatka et al. 2008 ,2002; Fleiger et al. 2012). The layout for grindability was devel-
oped by Zeisel in 1953 and has been further modified by other researchers. However,
the basic principle of the power measurement, which actually is a torque measurement
at constant speed of revolution, remained the same (Zeisel 1953; Fleiger et al. 2012).
The grinding of cement clinker has been in practice for decades and therefore a large
number of characterization and equipment design have been developed. However, im-
minent malfunctions practically occur during operations. Therefore, it implies that the
comminution of cement clinker has not yet been fully researched (Makowlew 2017).
Due to increasing and demanding energy consumption in comminution or grindability
of cement clinkers, new efficient and energy conserving comminution equipment are of
great demand. Since last 3 decades, HPGR has been integrated in grinding circuits and
it has increased their efficiency (Camalan and Önal 2016; Schulze 2019; Schützenmeis-
ter 2017). High-pressure grinding rolls usually consist of a set of rotating rolls through
which the samples (i.e. a bed of particles) are inserted and ground with high external
pressure exerted on the particle bed (Schönert 1988). Recently, the use of HPGR prior
to ball mills in cement clinker grinding, serves as to minimize the energy consumption
in conventional ball mill alignments (Schönert 1988; Schönert et al. 1990).
(Patzelt et al. 1995) stated that certain HPGR-ball mill arrangements consume 30% less
energy as against that of ball mills. Therefore, HPGR is currently being adopted by
most cement manufacturing companies (Von Seebach et al. 1996). Most researchers
have integrated and linked the energy-efficient use of HPGR and ball mill due to two
main reasons. According to (Fuerstenau et al. 1999) when low degrees of reduced size
ratios were taken in to consideration, it was discovered that HPGR consumed lower
energy as a result of its compressive loading mechanism as against its counterpart (i.e.
ball-mill).The use of HPGR before ball mill operation is very vital and essential due to
this advantage. In other words, it is considered as a pre-grinder. Another major reason
for high efficiency and performance of HPGR equipment was linked with external
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stresses or forces applied by the rolls on the particle bed. Most researchers and scientist
concluded that “the effect of these external stresses during HPGR operation as damag-
ing of the particles at various degrees by high inter-particular stresses that induce in
particles microcracks” (Kodali et al. 2011; Patzelt; Patzelt et al. 1995; Ozcan and
Benzer 2013; Ghorbani et al. 2011) .
(Souza et al. 2008) stated that these damaged and comminute particles tend to grind
from a certain feed size to a certain product size with less energy requirement owing to
these microcracks in subsequent ball milling. The combination of energy intensive and
poor performance of comminution process implies that there is a great opportunity for
significant energy and economic savings by the improvement of this process (De 1995;
Rosario 2010). Even small improvements to the power utilization efficiency can have a
significant influence on the economic performance of a plant (Wikedzi 2018). There-
fore, intensive research and development will contribute to improving these processes
for the long-term feasibility of the mining and cement industry and thus, represents the
main motivation for the present investigation.
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2 Literature Review
The theoretical basics are intended to give the reader an overview of the topics on dif-
ferent sections as follows: stress and strain (Section 2.1), breakage fundamentals (Sec-
tion 2.2), goals of comminution (Section 2.3) and analysis of mineral phases using XRD
(section 2.4). They are intended to provide a basis which makes it possible to under-
stand and evaluate the results.
2.1 Stress and strain
Stress is simply the amount of pressure exerted on an object like ore body or solid min-
erals. Firstly, in order to gain a better understanding an overview of particle breakage
mechanisms is detailed in section (2.2.1). Further clarification of stress behaviour is
given for a single particles breakage before considering the stress in a particle bed. Fig-
ure 1 shows the stress as a combination of external load and energy input leading to
deformation and strain (breakage).
Figure 1: showing different steps in particle breakage (Mütze 2012).
(Rumpf 1965) divided the stress into four stress mechanisms, by which energy can be
introduced into the material to be crushed. In this way strain can be carried out. The four
different mechanisms are listed below.
1. Quasi-static compressive stress (compression)
• two stressing tools/surfaces
• v < 5 m/s
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• stress velocity and energy interdependent from each other
2. Shear
• compression superimposed.
3. Dynamic compressive stress: punch
• two stressing tools/surfaces
• v > 5 m/s
• stress velocity and energy interdependent from each other
4. Dynamic compressive stress: impact
• one stressing tool/surface.
• v > 20 m/s
• stress velocity and energy not interdependent from each other
(Schubert 1997) modifies this division by classifying stress into dynamic and static
compressive stress. Static compressive stress works under pressure of form, uniaxial
and with low relative speeds (< 5 m/s) and the stress energy is independent of the load
speed. A stress leads to an energy input via the contact points, whereby a three-
dimensional state of stress is formed in the particle and thus deforming the material. The
material behaviour during deformation can be determined with the help of stress-strain
curves. This stress-strain curves are separated by different borderlines and their behav-
iour differs with respect to the borderlines e.g. elastic, plastic and viscoelastic (Figs. 2a
and 2b) (Czichos 2000).
Figure 2: Stress-strain curves for a) linear and b) non-linear elastic (plastic) deformation behavior adapted
from (Mütze 2012).
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The area below the curve is a measure for the energy (E) and is calculated according to
equation 1.0. Where σ = tensile stress and ԑ = strain and E = modulus of elasticity.
1.0
The linear elastic deformation behaviour is typical for a crystalline material (e.g. quartz-
ite and glass). (Mütze 2012) explained that strain is reversible, time-independent and it
is proportional to the exterior load (Fig. 2a). A non-linear elastic deformation is typical
for rubber, but the strain is not proportional to the exterior load and they are also time-
independent (Fig. 2b). The elastic linear deformation is characterized by Hooke’s law
according to equation 1.1.
1.1
An inelastic deformation is characterized by the fact that the load curve differs from the
relief curve and some part of the energy is dissipated for example amorphous materials.
In addition, they are time-independent (Mütze 2012). Plastic deformation behaviour is
influenced by time independence and irreversibility. The deformation remains constant
due to the shifted bonds between the atoms after reducing the load e.g., metals (Fig. 3a).
Materials that are affected by time- and temperature effects are known as viscoelastic
deformation. Embrittlement in viscoelastic deformation are enhanced by high stress
velocities and/or low temperature e.g. plastics, rock salts and crystals (Fig. 3b) (Mütze
2012). The viscoelastic deformation can be deduced from equation 1.2 as shown below:
ד
1.2
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Figure 3: Stress-strain curves for a) Elastic-plastic and b) Viscos-elastic deformation (Mütze 2012).
The influence of structure and contact geometry on breakage phenomena is shown in
figure 4. It shows the impact of compressive stress on both shapes i.e. spheres and ir-
regularly shaped particle respectively and the resultant impact after the load was re-
moved. 4(a) and (b) are under compressive stress while 4(c) and (d) shows the impact
after the stress was removed (Mütze 2012).
Figure 4: Influence of contact geometry on breakage of sphere and irregular shaped particles
(Khanal 2005).
“According to (Schönert 1996) “In the single particle situation no particle interferes
with another one. This can be satisfied if only one particle is pressed or a layer of iso-
lated particles of a narrow size fraction (size ratio < 1.26). Each particle contacts only
the two working surfaces. The distance between two particles should be larger than
about twice the particle size”.
2.2 Breakage Fundamentals
To crack a solid, stresses must be exerted on the body to overcome the bonds at the
atomic level. Practically, there is never an ideal lattice, although solid material has all
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kinds of macroscopic, microscopic or sub-microscopic errors which lead to a lower
breaking stress (Schönert 1996; Schubert 1997; Schubert 1988).
At these so-called particles with in-homogeneities there are local stress peaks with a
high energy concentration, which is why the crack forms at these weak points and
spreads further (Schubert 1988). According to (Mütze 2012) the extent of crack propa-
gation depends only on magnitude of energy absorption. He further stated that geomet-
rical similar crack propagation results to self-similar breakage functions for a certain
stress situation.
The first stage is known as crack formation, which is followed by a transition from the
static crack to the moving crack called crack initiation. This is followed by crack propa-
gation, which can be divided into stable and unstable cracks. With stable crack propaga-
tion, energy must constantly be supplied from the outside for crack to occur while dur-
ing unstable crack propagation, stored elastic energy is constantly converted into crack
energy. This is followed by a macroscopic brittle crack see Figure 5.
Figure 5: Stages of the crack process modified after (Mütze 2012).
The stable crack propagation requires a constant energy supply from the outside, if suf-
ficient energy from the elastic stress field is available for the formation of new cracks,
i.e., if the elastic deformation energy is greater than or equal to the surface energy, un-
stable crack propagation takes place. Since this crack propagation form at a high speed
(> 1000 m/s), it will lead to a brittle crack. In addition, the unstable crack propagation
occur after a stable crack propagation takes place (Schubert 1997; Mütze 2012).
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Characteristic crack phenomena and other characteristic values derived from them are
used to describe the success of comminution (Schulze 2019). Broken products influ-
ences themselves much more in the particle bed and therefore the amount of energy
going into friction losses increases (Austin et al. 1984). However, a better reduced size
of particle bed can be achieved by the addition of steel balls during comminution there-
by increasing the inhomogeneity of the stress field (Rosario 2010; Rao 2011).
In order to achieve a higher efficiency while selecting comminution machines, it is very
vital to know the crack mechanism of a specific feed, proper design and selection of
comminution (Wikedzi 2018). Without external force on a bed, crack cannot take place,
therefore for crack to occur an external force must be applied on a particle. Application
of external force leads to development of stress on a particle however, when this stress
exceeds the ultimate stress, the particle will break (Rao 2011).
The crack dimension depends greatly on the nature of the particle material, the inner
structure and on the applied force (Rumpf 1965; Schönert 1996). Presently, the commi-
nution devices used in particle breakage apply various types of forces (e.g. compressive,
shear, impact or abrasion) to the assemblages of particles (Wikedzi 2018). The first
breaking point of a particle denotes its strength limit. (Fuerstenau et al. 2004) stated that
the breaking strength is force per unit area of a cross section at the point of first crack,
whereas breaking energy is the work that needs to be applied for crack to occur.
According to (Wikedzi 2018) “materials can be classified as either ductile or brittle. A
ductile material, when stressed to failure, will normally break into two pieces. Stressing
of a brittle material will result in shattering, or breakage into many pieces of different
sizes of which the crack paths cannot be controlled. Because feeds behave as a brittle
material, the pattern of breakage presents problems in grinding by attempting to create
crack within specified limits without having any control over the crack process”.
(Schlanz 1987) illustrated that real particles are irregularly shaped, therefore loading of
the particles by external forces is not uniform, but instead, it is achieved through contact
points.
In addition, Figure 6 shows different mechanism of particle crack which includes:
a. Impact or compression (application of normal force)
b. Chipping (application of oblique force) and
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c. Abrasion or attrition (application of parallel force)
The particle must be subjected to a state of strain irrespective of the mechanism in-
volved to initiate the propagation of cracks. For the state of strain to be initiated, energy
bigger than or equal to the stored strain energy of the particle must be available or sup-
plied. According to (Schlanz 1987) the determinant factors for the amount of energy
required to initiate the propagation of cracks are as follows:
d. Presence of pre-existing cracks or flaws
e. Degree of plastic flow in the solid vs. complete brittleness
f. Geometry and rate of stress application
Figure 6: Particles Breakage Mechanisms adopted from (Rao 2011).
There are two distinct size ranges when an irregular particle is cracked by compression
firstly, coarse particles resulting from the induced tensile failure and lastly, fines from
compressive failure near the points of loading or by shear at projections (Wikedzi 2018;
Unland and Al-Khasawneh 2009). By minimizing the area of loading which is mainly
done in compressive crushing machines using corrugated crushing surfaces, the amount
or number of fines produced can totally be minimized or reduced (Wills 2006; Wills
and Atkinson 1993; Veasey and Wills 1991).
Because of fast loading in impact breaking, a particle experiences a higher average
stress while undergoing strain than is required to achieve simple crack and thus, breaks
apart rapidly which is as a result of tensile failure (Wikedzi 2018; Shan et al. 2014; Ra-
shidi et al. 2017). (Wikedzi 2018) stated that several areas in the particle are overloaded
and results to a comparatively large number of particles with a wide size distribution.
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Impact or compressional stress causes immediate crack with no residual stresses
(Wikedzi 2018; Mwanga et al. 2015).
Abrasion crack occurs when a force (i.e., shear force) acts parallel to the surface of the
particle. As a result of inadequate energy applied on the particle, localized stress occurs,
and a small area is cracked to produce very fine particles. Particle-particle interaction
leads to production of abrasion, which normally occurs if a crusher is loaded too fast,
contacting particle thus, increasing the degree of compressive stress and hence shear
failure (Wills 2006; Rao 2011).
“In practice, these events do not occur in isolation. For example, when particles are
crushed by compression in a jaw crusher, coarse particles will be produced resulting
from the induced tensile stress, while, fine particles will be produced from compressive
stress near points of loading and by attrition due to particles interaction” (Rao 2011).
2.2.1 Particle Breakage Mechanisms
For a particle to break it has to be in contact with a surface and afterwards an external
force needs to be applied (Liu 1994). The resultant effect after the removal of applied
force leads to deformation and breakage (Bérubé and Marchand 1984). For a particle to
break, the stress energy must be equal to or higher than the comminution energy (Schu-
bert 1988).
Single particle breakage
In single particle breakage, the particles do not interfere with one another. This could
only be achieved if only one particle is pressed or a layer of isolated particles of a nar-
row size fraction (size ratio < 1.26) and only the two working surface should be in con-
tact with the single particle (Schönert 1996). The single particle breakage is mostly car-
ried out in two ways a) by moving single wall against the specimen and b) by moving
both walls against the specimen and these two tests are known as diametrical compres-
sion tests (Khanal 2005).
Stress on a particle bed
In a particle bed there are voids between the particles. With fine or moist materials, the
voids in the structure become larger due to adhesive forces. When a particle bed is load-
ed, the force is transmitted via the individual particle contacts (Schulze 2019). Depend-
ing on where the particle is in the particle bed, it has different contact points. Particles at
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the edge of the particle bed have fewer contacts because they are bounded by a wall
from one side. Particles in the middle have many load application points due to many
particle contacts.
(Mock 2015) stated that the number of contact points with the neighboring particles is
given by the coordination number (k). The coordination number depends on the ratio
between the void volume and the solid volume, also known as the pore number. Figure
7 shows an example of this using monodisperse spheres. The smaller the void volume,
the larger the particle contacts, and the greater the resistance to further compression
(Amelung 2018). In addition, more contacts lead to higher friction losses and the greater
the number of contacts, the more the stress entered is distributed over the contacts,
which is why the breaking strengths can no longer be achieved locally (Schützenmeister
2017).
Figure 7: Coordination number and pore number for spheres of the same size modified (Amelung 2018).
Figure 8 shows an example of the density distribution in g/cm³ in a stressed particle bed.
The highest density and thus the highest stresses are at the edge of the press plunger.
The further away the particle is from the edge of the ram and from the center of the par-
ticle bed, the lower the density (Schatt 1988).
Cordinations number
(k)
Vvoid/Vs
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Figure 8: Bulk density distribution (g/cm³) in a material bed axially loaded from above modifies after (Schatt 1988).
2.2.2 Comminution of particle beds
Four different classification of particles arrangement and confinement according to
(Schönert et al. 1990) are: (i) single particle (ii) one-particle layer (iii) particle bed and
(iv) ideal particle be. According to (Schönert et al. 1990) a contact can be created be-
tween two neighboring particles or between a particle and a solid surface. The resultant
effects of these contacts create different deformation and stress distribution in the con-
tact volume and breakage. The term particle bed is applied for all arrangements with
particle contacts or particle directly normal to one working surface (N.B: multi-particle
layer belongs to this class). The analysis of this thesis was done using particle beds of
monodisperse sizes (0.8-1mm).
Table 1: Classification of particle arrangement and confinement according to (Schönert 1996)
Group
Size dispersion
Confinement
1. single-particle situation
Monodisperse
None
2. one-particle layer
Monodisperse
None
3a. particle bed
Monodisperse
close or wide
3b. p.b. without direct contact of max. particle
Polydisperse
close or wide
3b. p.b. with direct contact of max. particle
Polydisperse
close or wide
4. ideal particle bed
Monodisperse
Close
Height (mm)
Distance from middle point (cm)
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In contrast to single particle comminution, the particles influence each other during
comminution in particle beds (Schönert and Aziz 1980; Kleeberg 2007). A transitional
situation between single-particle and particle bed is referred to as a single grain layer.
When several particles of the same size “x” are stressed there with sufficient distance >
2x to each other, there is also no influence while at distance of < 2x, the particles influ-
ence each other to break (Schönert and Aziz 1980; Aziz 1979.).
Figure 9: Particle size distribution of quartz after single-particle and inter-particle comminution (Kleeberg 2007).
Figure 9 compares the particle size distribution of a quartz fraction after a single particle
and particle bed load. For stress under the single particle, only the broken particles are
shown while for particle-bed, the curve consists of a mixture of crushed and uncrushed
material (Mütze 2012). The distribution is more coarse-grained, since in a particle bed
the energy distribution takes place via a larger number of particle contacts and fewer
cracks occur. In wide distributions, the small particles protect the larger ones by dissi-
pating the energy, which makes the comminution of the large particles more difficult
(Hoffmann et al. 1976).
The main factors influencing the comminution of particle bed are the stress intensity,
the particle size and shape and the width of the initial distribution, the material, stress
speed and the dimensions of the material bed. (Göll 1975) investigated various material
influences during comminution. The most significant influence on the comminution was
the stress intensity.
Single particle
Particle bed
Feed
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2.2.3 Principle of Breakage Mechanics
For the comminution of solids, the application of stresses by external forces necessary
to overcome the bonding forces on atomic level. Equation 1.3 can be used to overcome
a uniform stress under an ideal lattice condition. Theoretical crack stress required for the
bond forces can be determined (Schubert 1988).
Where
= modulus of elasticity
= Spec. free boundary energy
= lattice constant
1.3
However, since the specific free boundary energy is difficult to determine, the theoreti-
cal breaking stress is derived from the following relation from equation 1.4 (Schubert
1988).
1.4
(Schubert 1988) stated that the real strength, however, is much lower due to inhomoge-
neity.
2.2.4 Energy Analysis
Since comminution is an energy consuming process. The energy analysis is the vital
process variable used in evaluating and analyzing the efficiency of a crushing system
(Bernotat and Schönert 1988). Deformation of particles occurs during crushing (extent
of deformation depends on the particle) and cracks (generation of new surfaces) when it
meets the failure criterion (Khanal 2005).
Stressing energy, breakage energy, mass related energy and surface related energy are
among different types of energy evaluated during comminution. (Bernotat and Schönert
1988) classified input energy into two categories – energy at the breakage point and
energy above the breakage point. Breakage energy/energy of fracturing is term used for
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energy supplied till the primary breakage, which is related to mass of the particle see
Equation 1.5
Breakage energy (WM)
1.5
Where: WM is breakage energy for material having mass (i), while ʃ Fds is the area un-
der the force - displacement curve.
Figure 10: A typical force displacement curve for energy analysis (Khanal 2005).
(Bernotat and Schönert 1988) stated that energy is still supplied immediately after the
primary breakage in order to obtain the desired particle size distribution (see Fig. 10).
Primary breakage is similar in the case of material testing. Where the breakage point is
noted to define the failure limit of the material and post breakage is a real comminu-
tion(Schönert 1991).
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Figure 11: Force-displacement curve modified after (Mütze 2012).
The loading of particle beds is described by force-displacement curves, such as Figure
11, which are recorded during loading and unloading of the sample observed by (Mütze
2012). From these force-displacement curves, rearrangement, compaction and commi-
nution processes can be read and derived. According to (ProcessNet 2018) compression
occurs due to both rearrangement and comminution. The first stage of compression is
dominated by rearrangement while the second stage is governed by comminution. Nev-
ertheless, both events (rearrangement and comminution) always happen at the same
time. Therefore, the phases cannot be clearly distinguished see Figure 11. The maxi-
mum pressing pressure ρmax is calculated according to equation 1.6 by dividing the max-
imum force Fmax by the cross-sectional area of the particle bed (Apb).
1.6
Another important parameter is the mass-specific energy absorption (Em) in equation
1.6.1. It is the ratio of the energy absorbed by the material to the mass of solid matter
stressed.
1.6.1
Load
Fs(s)
Relieve
Fr(s)
Pressure F
Displacement s
Height of particle bed h
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Where
EV = volume-specific energy absorption
m = Mass
ρs = Solids density
FB(s) = Force-displacement curve during loading
FE(s) = Force-displacement curve during unloading
Energy absorption is composed of the energy consumption of all micro processes, such
as energy absorption up to break, friction losses, and losses due to plastic deformation
(Schulze 2019). The composition is different, depending on the material. Clinker is a
brittle refracting material, which is why plastic deformation plays a subordinate role,
thus, it implies that deformation has no high proportion of compaction in cement clink-
ers. With decreasing particle size, the proportion of plastic deformation increases. The
proportion of friction losses depends on the width of the distribution, the number of
contacts and the size of the particles (Schubert 1984).
2.2.5 Compaction
When a particle bed is subjected to pressure, compaction occur see Figure 12. The parti-
cles rearrange and break. The result is a denser packing and the solidification increases
(Oettel 2002). Compaction is widespread in industry and is used in pharmacy, biology
and technology (Schäfer 2007; Zwan and Siskens 1982).
Figure 12: : Sub-processes during compaction of a particle bed adapted from (Schubert 2003).
The first sub-process runs at low-press pressures. The particles arrange themselves by
sliding off and turning around and first of all the spaces in the order of the primary par-
ticles are filled up (Denny 2002). The frictional forces between the particles must be
overcome and elastic deformations of individual particles can occur. The fluid is dis-
placed from the pores (Zwan and Siskens 1982). If the stress is increased, cracks can
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appear within the particle, which in turn can lead to breakage. The fragments then fill
further voids that are smaller than the initial particle size. During this partial process, the
fluid is also displaced or partially enclosed and compacted.
Figure 13: Plastic deformation during loading adapted from (Schubert 2003).
Plastic deformation as shown in Figure 13 can be seen at small particle sizes, high tem-
peratures and low compaction speed. The point contacts become surface contacts, which
leads to higher friction forces and lower compaction. (Schubert 1988; Cooper and Eaton
1962; Mütze 2012; Heinicke 2012).
2.2.6 Back Strain
When the particle bed is relieved, the bed expands to a height sEnd. This is due to re-
versible elastic deformation and irreversible rearrangements in the particle bed (Müller
1989). (Oettel 2002) adjusted the back strain in the range of 0.15 - 0.85 Fmax with a
straight line. (Makowlew 2017) approximated the range to 0.1 - 0.9 Fmax for ce-
mentclinker with a straight line. From this follows the corrected value Skorr by which the
pure elastic back strain can be inferred (see Fig.14).
Figure 14: Force-displacement curve with corrected press travel scorr from a linearly (Oettel 2002).
Preß-Load F in kN
Preß-distance s in mm kN
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The difference between the corrected value and the actual measured value at the end of
the experiment indicates the irreversible rearrangement processes. The elastic back
strain increases with increasing loading speed (Schubert 1997; Schubert 1988; Schubert
1984).
2.3 Goals of Comminution
The aim of comminution is to achieve an efficient cracking mechanism for a certain
fragment with a suitable particle size distribution and indirectly liberating aggregates
with minimal energy input (Gay 2004; Montes et al. 2018). Crushing can be grouped
into coarse and fine crushing (milling). The whole process of crushing or comminution
is defined by the feed size and the output size (Mwanga 2014). In general, comminution
is a desired activity, it is vital to have a suitable fragment size and distribution for fur-
ther use (Gay 2004).
Comminution is the mechanism that involves the reduction of materials by crushing,
blasting and grinding to a smaller particle size required for downstream processing.
Crushing helps to reduce the size of a material to a level that grinding can be attained
(Fuerstenau et al. 2004; Wills 2006; Masuda et al. 2006; Antonyuk et al. 2005). The
pressure pot are usually loaded with a certain amount of feed (2m), which is reduced to
10-200 mm size and subsequently the crushed product is fed to grinding processes and
it is further reduced to 2mm – 74mm (Rao 2011). Different types of blasting, grinding
and crushing equipment have been used in various industries as a technical and mechan-
ical way of producing particulate solids. There are complexities in the working princi-
ples of these equipments and thus, different companies adopt different principle.
According to (Grimble et al. 2010) the mechanism of size reduction by crushers is based
on the slow compression of large particles against rigid surfaces or by particle impact
against surfaces in a rigidly constrained motion path. Crushing is mainly a dry process
and it is performed in different stages with small reduction ratios, ranging from 3 to 6 in
each stage (Wills 2006).
“Furthermore, most grinding is performed in rotating cylindrical steel vessels known as
tumbling mills. These mills usually contain a charge of loose crushing bodies (grinding
medium) which is free to move inside the drum, breaking the ore with the combination
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of impact, attrition and abrasion forces, producing the specified product size” (Wills
2006; Yildirim and Prezzi 2011; Wikedzi 2018).
Nowadays, HPGR is generally accepted as a reliable and cost-effective machine for
comminution of cement clinkers and hard rocks and as an alternative for SAG miils,
attention of most industries is now focusing on the metallurgical benefits (Wolfgang et
al. 1997). (Michaelis 1988) stated that HPGR technology has demonstrated several mer-
its which include reduction in comminution power consumption, low operational cost,
grinding of hard rocks relative to SAG-Ball milling.
2.3.1 Comminution Law
It is very important to correctly evaluate the comminution energy during the design,
operation and the control of processes involved during comminution (Wikedzi 2018).
The prevailing challenge encountered remains that most of the energy input during
comminution is absorbed by the machine itself or lost due to friction, leaving a small
fraction of the total energy for the crushing and breaking of the sample material (Wills
2006; Morrell 1996). According to (Wills 2006) it is expected that there is a relation-
ship between the energy required to break the sample-material and the new surface pro-
duced during this process, thus, the energy required in creating of the new surface needs
to be known in order to determine the relationship.
(Mütze 2012) stated that for comminution to be progressive and successful in produc-
tion of cement, the energy utilization needs to be minimal. The evaluation of comminu-
tion is done through the evaluation of particle size distribution before and after commi-
nution. The key figures in the evaluation of comminution are selection function, break-
age function, grain enrichment curve, increase in surface area and energy utilization
(Mütze 2012). The higher the pressure, the finer the material see Figure 15 (Heckel
1961; Mütze 2012).
(Touil et al. 2008) stated that breakage process is characterized by two major basic
functions: a selection function that represents the fractional rate of breakage of particles
in each size class; and a breakage function that gives the average distribution of daugh-
ter fragments resulting from primary breakage event. In other words, selection function
is the probability of every size fraction to break at certain energy inputs. Specific selec-
tion function obtained during the grinding tests, reflects the size reduction energy effi-
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ciency on the energy consumed to produce a desired Blaine surface area(Touil et al.
2008).
Figure 15: The effect of pressure on the particle size distribution curve of cement clinkers (Mütze 2012).
(Mütze 2012) recorded that geometrical similar particles of the same strength show self-
similar breakage behavior while (Schönert 1996) noted that increase of the specific sur-
face (∆SM) can be used as an integral measure, but usually very little amount of the fine
material determines the surface, which implies that ∆SM does not indicate size-reduction
effect in middle and coarse range (Mütze 2012; Makowlew 2017; Schönert 1996).
Breakage function does not depend on changes in the fine-grain range. A particle can be
broken but still so large that it fits into the initial fraction (Mütze 2012).
Figure 15 shows that at different applied pressures, the size of the particle differs. At
175MPa, the particle size distribution of the analysed clinker was finest among the four
clinkers and vice versa. (Francioli 2015) stated that semi-empirical energy-size reduc-
tion relationships were previously proposed by Bond, Rittinger and Kick. However,
these laws are not applicable over a wide range of sample sizes (Rao 2011). In addition,
various researchers have been able to show that the relationships of Bond, Rittinger and
Kick were for single general equations rather than large particles. The relationship be-
tween energy and particle size is a combination of the three laws though each law is
applicable at a particular region for each comminution law see Figure 16 (Wikedzi
2018).
0
20
40
60
80
100
110 100 1000
Q3(x) in %
xin μm
0,08… 0,1 mm
0,08… 0,1 mm 50 MPa
0,08… 0,1 mm 100 MPa
0,08… 0,1 mm 175 MPa
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Figure 16: Relationship between Energy Input and Size Reduction in Comminution (after Hukki 1961).
Theoretically, Bond’s law was a successful assumption (Bond 1952). Though, it is pres-
ently seen as an empirical relation that provides acceptable fit to results obtained in
grinding experiments. However, this law is mainly used as a tool for sizing crushing and
grinding equipment. (Francioli 2015) recognized that this methodology could have up to
20% discrepancies in respect to the actual energy consumption for ball mills and much
higher in crushers. “The probability of breakage in comminution is high for large parti-
cles, and rapidly diminishes for fine sizes (Wikedzi 2018; Francioli 2015; Bye 1999).
2.4 Phase Composition of Cement Clinkers
Clinker is the main component of cement, which comprises of different crystal phases
including tricalcium silicate, dicalcium silicate, aluminate and ferrite. Tricalcium sili-
cate and dicalcium silicate are calcium silicate mineral phases. According to (Albats et
al. 1992) Alite is a tricalcium silicate phase (Ca3SiO5) and Belite a dicalcium silicate
phase (Ca2SiO4). Aluminate phase, formed by pure CaO and Al2O3, is a tricalcium alu-
minate phase (Ca3Al2O6) while the ferrite phase, formed by pure CaO, Al2O3 and Fe2O3,
is a tetracalcium aluminoferrite (Ca4Al2Fe2O10) (Mohan and Glasser 1977). The phase
composition of the clinker depends on the chemical composition of the clinker and the
calcining procedure (Suherman et al. 2002). The formation of clinker phase composition
is mainly influenced by several external factors depending on the synthesis conditions
and internal factors such as the composition of the source raw mixes (Xing and
Fengjuan 1992; Albats et al. 1992).
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According to (Telschow 2012) the main constituents of cement is clinker otherwise
known as Portland cement clinker, it consists of mostly 40 -80 wt. % tricalcium silicate,
10 -50 wt. % dicalcium silicate, 0 -15 wt. % tricalcium aluminate and 0 -20 wt. %
tricalcium aluminate ferrite. The composition of rapid burnt tricalcium silicate is uni-
form than slow burning tricalcium silicate. Tricalcium silicate from rapid burning has
lower absolute values of C/S when compared to dicalcium silicate. Although in general
dicalcium silicate crystals are more uniform in composition than tricalcium silicate
crystals (Albats et al. 1992). In addition, the C/S ratio of dicalcium silicate is higher at
rapid heating with greater impurities and the higher, the MgO in cement clinkers, the
higher the impurity content of the silicate phases (Albats et al. 1992). Mg+ replaces Ca2+
and by so doing, impurities are incorporated in the voids of the cement clinkers (Albats
et al. 1992). Observation shows that impurities in the volume of tricalcium silicate
crytals are not uniformly distributed. According to (Albats et al. 1992) fine crystals
(10*30m) are distinguished from coarser ones (60*100m) at any burning tempera-
tures through higher C/S ratio, with great impurities content. Regardless of the chemical
composition of source mixes, their dispersity, burning temperature, crystal sizes and
their surface layers have a higher C/S ratio, and a greater content of impurity oxides,
mainly through Al2O3 and Fe2O3 (Albats et al. 1992).
During the burning of cement clinker dicalcium silicate phase in the clinker is formed at
800 - 1250°C by solid reaction while tricalcium silicate phase is formed by solid-liquid
reaction after melt had formed above 1250°C (Telschow 2012). According to (Telschow
2012) the cooling of tricalcium silicate clinker phase is thermodynamically stable only
at temperature above 1250 °C.
A higher temperature of about 1450°C should be maintained, so that the avaiilable C2S
would react with CaO to form C3S and thus enhancing burnability (Xing and Fengjuan
1992). According to (Mohan and Glasser 1977) at temperature > 1250 °C, dicalcium
silicate (C2S) reacts with CaO to form tricalcium silicate (C3S) while at temperature <
1250 °C, C3S decomposes to C2S, C2A and C4AF.
(Hornian and Regourd 1980) stated that among four major constituents’ minerals of
cement clinker, dicalcium silicate shows the strongest resistance to fracture. Industrial
1250°C
Dicalcium silicate (C2S) + CaO Tricalcium silicate (C3S)
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cement clinker nodules mostly show remarkable differences in chemical composition,
bulk density, texture and colour between the inner and outer parts (Maki et al. 1989).
Dicalcium silicate particle grains usually aggregate to form large clusters in the inner
part as opposed to the uniformly distributed grains in the outer parts. Dicalcium silicate
clusters originate from inhomogeneity of raw mixes and the grindabilities of cement
clinkers is explained in terms of pore structure (Maki et al. 1992).
According to (Butt and Timashev 1974), macro- and micro pores, with a distinction at
1m, are effective for grinding in the region below and above a Blaine specific surface
area of 1600–1900cm2/g respectively. Microcracks are mainly observed within tricalci-
um silicate crystals and it is termed “parting” in mineralogical terms (Maki et al. 1992).
(Hornian and Regourd 1980) explained the outset of microcracks to the anisotropic con-
traction caused by the differences in thermal expansion coefficient between the tricalci-
um silicate crystals and the crystalline interstitial materials during cooling. (Hornian and
Regourd 1980) experimented and found out that on quenching tricalcium silicate in
water at 1450°C the interstitial liquid was transformed into glassy state and very few
cracks occurred due to the smaller difference in thermal expansion coefficients while
microcracks were formed in abundance on quenching tricalcium silicate in water at
1250°C although the interstitial liquid temperature had crystallized already. Thus, the
occurrence of microcracks is deeply influenced by crystallization of the interstitial liq-
uid as well as the degree and extent of thermal stress (Maki et al. 1989). According to
(Maki et al. 1992), a large and sudden volume shrinkage during crystallization, which is
assumed to exceed the total volume change because of cooling down to ambient tem-
perature, exerts large tensile stress on the tricalcium silicate crystals and thus results to
the occurrence of microcracks.
2.4.1 Analysis of mineral phases using X-ray diffraction (XRD)
X-ray diffraction is used for both qualitative and quantitative phase analysis of cement
clinker (Copeland and Robert 1958; Kristmann 1977b, 1977a; Aldridge 1982b, 1982a).
Quantitative phase analysis of clinker is based on discrete peak intensities. The complex
structure of clinker, resulting in overlapping reflections, limits the use of discrete peak
intensities for traditional analysis (Suherman et al. 2002). (Aldridge 1982a) discovered
that an interlaboratory comparison of cement clinker phases using discrete-peak analysis
did not yield satisfactory results. Further research according to (Aldridge 1982b)
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showed that greater attention during sampling grinding and sample preparation yielded
more precise phase quantification of clinker phases. XRD is the best direct method for
quantitative analysis of clinker (Suherman et al. 2002). XRD is the only analytical
method that can directly provide the phase abundancies in clinker (Kristmann 1977a).
The limitations imposed by peak overlap in analyzing cement clinkers by discrete peak
intensity methods can be overcome using rietveld multiphase refinement methods (Tay-
lor and Aldridge 1993; Meyer et al. 1998).
The amorphous constituent in cement clinker can be determined from X-ray diffraction
data using internal and external standards methods and Rietveld refinement (Suherman
et al. 2002). The preparation of samples for laboratory X-ray diffraction is the most per-
tinent step for a good quantitative analysis (Jenkins and Snyder 1996; Bish and Reyn-
olds 1989). In addition, to extract accurate information from a powder pattern, two pa-
rameters have to be controlled carefully: i) the peak intensities should be those expected
from the crystal structure and phase assemblages, which implies that a sufficient large
number of crystallites contributing to each reflection of every phase should be bathed by
the X-rays; and ii) the position of the diffraction maxima should be at right place, which
indicates good diffractometer alignment and good sample mounting practice (Bish and
Reynolds 1989; Jenkins and Snyder 1996; Whitfield and Mitchell 2009; Elton and Salt
1996). It is very necessary to obtain diffraction peaks of reproducible intensity without
altering the sample or inducing too much preferred orientation. The particle statistics is
of the outmost importance in quantitative studies (Whitfield and Mitchell 2009; Elton
and Salt 1996) see Appendix D2 and D3.
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3 Experiments and Methods
This chapter details more on different techniques and methods applied during the analy-
sis of the cement clinkers. Section 3.1 explained the equipment and steps adopted dur-
ing the experiment, section 3.2 gives information about the simulation of grinding cir-
cuit, section 3.3 talks about SEM and X-ray diffractometry and 3.4 explained MLA.
This experiment was done to examine the amount of energy and power required to grind
selected cement clinker using piston-die press and ball mill while the XRD was used to
analyse the influence of mineral phases on the grindability which directly influences the
amount of energy required during comminution and grindability. MLA was subsequent-
ly used to interpret different concentrations of mineral phases and the mineral grain size
distribution curve for the selected clinkers. For the piston die press and ball-mill meth-
ods, the experiments were done using four cement clinkers (K1 - K4). But it was later
limited to two clinkers (K1 and K4), because of the similarities among clinkers of K1, K2
and K3. The size range of the analysed cement clinkers ranges from 0.8 – 1mm (mono-
disperse).
3.1 Equipment and steps
In this section data about the tests with the hydraulic piston-die press and ball mill pro-
cess are explained. A description of the test design and procedure is illustrated and ex-
plained. Also, the steps adopted for particle size distribution analysis, sieve analysis and
laser diffraction of the analysed cement clinkers were further discussed in the sub-
sections.
3.1.1 Piston-die Test
A Shimadzu UH-500 kNA hydraulic piston press from the Institute of Thermal-, Envi-
ronmental- and Resources Engineering was used for the piston-die press.
Mechanical & hydraulic design of Shimadzu UH-500kNA:
The lower crosshead is guided by two fixed spindles, which are connected to the ma-
chine base. The electric motor usually moves the cross head. The upper part of the
crosshead is directly connected to the machine table through the two loading columns.
The machine table is fixed to the hydraulics cylinder’s piston, which allows load trans-
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mission to the upper cross head. The limit switches help to stop or off the oil pump
when approaching the crosshead. The hydraulic unit consists of filter, oil tank, oil
pump, hydraulic and electro-hydraulic servo valve while the entire unit is usually locat-
ed in the lower part of the controller (Kalala et al. 2011).
A schematic structure of “Shimadzu Typ (UH-500kNA) hydraulic piston-press” can be
seen in Figure 17. The load cells and displacement sensors are arranged under the piston
plate. The force measurement has an error tolerance of 3% Fmax, which corresponds to
15kN for this press. The displacement measurement has an error of 10-6 m. The piston
plate has a diameter of 57 mm and a height of 51 mm. It is attached to the pressure
plate. The piston-die press is mounted on the traverse; with this it moves towards the
piston plate in the direction of the traverse. Once the setup is complete, the cement
clinker is loaded into piston-die plate, using a small spherical metal the loaded clinkers
are well spreaded and the height (mm) between the clinker and the top of the piston-
plate is recorded using a meter rule. The next stage is to input all the necessary parame-
ters using software (LabMaster), the load was set at 0.5kN and the position (mm) is set
close to the value of the difference between the clinker and the top of the piston-plate.
The parameters for moving the piston-die were set at 500N/s for (speed to start position)
and 150N/s for (speed to second position), with a maximum pressure of 150MPa. The
measuring signal is recorded with a frequency of 50 Hz and the force-displacement
curve is recorded using the software (LabMaster). The inherent elastic deformation of
the system is 0.587m/kN and is considered during the evaluation of force-displacement
curves.
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Figure 17: Schematic representation of the Shimadzu hydraulic piston-press modified after (Schulze 2019).
3.1.2 Rollbock machine
Dispersion of the agglomerates (crushed clinkers) was done using a rollbock machine
see Figure 18. The rollbock machine helps to de-agglomerate the sample at 100 rpm for
about 20mins. The sample is placed in a cylindrical porcelain vessel with a height of
112 mm and 106 mm inside diameter. For a better dispersion of the agglomerates, 10
big and 10 small rubber stoppers were added together into the cylindrical porcelain ves-
sel see Figure 19. The height of the rubbers is 19mm while the diameter was 17mm and
14mm respectively.
Figure 18: Rollbock machine from MVAT (ITUN) Institute Bergakademie.
Traverse
Piston-die press
Piston/compreession
top
Piston plate
Cylindrical porcelain
vessel
Upper cross
head
Lower cross
head
Machine table
Hydraulic cylinder
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Figure 19: Dimensions of rubber stoppers adapted from (Schulze 2019).
3.1.3 Particle Size Analysis and Sieving
According to (Mütze 2012) particle sizing uses different analyzing methods based on
different physical principles: a) laser diffraction (interaction of particles with laser light
by correlating the scattered pattern to the particle size), automated image analysis (cap-
turing of individual particle images by high speed camera) and analytical sieving.
The crushed and dispersed clinkers were sieved with analytical sieves from (Haver and
Boecker). Firstly, definition of the size classes (fractions) used in classification (sort) of
the collective particle needs to be done (see Fig. 20a). (Mütze 2012) stated that there is
the possibility of the analyzing method already predefined. For instance, the mesh sizes
of the sieves are already known during this thesis (i.e. R10 sieve classes ranging from
0.1mm – 0.8mm sieve sizes). The particle system contains different particles sizes.
Figure 20: a) Particle sizes sorted into different classes and b) particle sizes interval (Mütze 2012)
(a)
(b)
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According to (Mütze 2012) since most of the particles are small in size, it leaves the
technical processes a great task by dealing with lots of particles. “Therefore the particles
are no longer described with their individual properties but with their collective proper-
ties”(Mütze 2012). This is achieved by applying statistical methods. The statistical step
on how many particles of one size (i.e. within size interval) are present in the collective
particle is illustrated in figure 20b.
3.1.4 Analytical sieving
The analysed cement clinker was obtained from Thyssenkrupp Industrial Solutions AG
and with the help of the analytical sieve shakers from the company (Haver and Boecker)
the cement clinkers were classified into narrow distributed fractions. To investigate the
grinding progress, the grounded products were analytically sieved using R10 sieve se-
ries. (Schubert 2003) stated that as the particle size decreases, the sieving deteriorates
because the adhesive and friction forces exceed the sieving forces. In a sieve analysis,
the particle size is determined by the mesh size. As the particle size decreases, the siev-
ing deteriorates because the adhesive and friction forces exceed the sieving forces
(Schützenmeister 2017). (Mütze 2012) illustrated how sieve analysis is conducted by
using a sieve tower see Figure 24. A low layer thickness on the sieve also causes weak
sieving forces. Below a particle size of 60 µm, the quality of the sieving is poor, as the
resulting air currents swirl up the fine particles (Batel 1957).
The sieve tower consists of stack of sieves, with different mesh sizes piled above each
other and the coarse fraction remains on the sieve, while the fines portion passes
through the openings. According to (Mütze 2012) the fines of the largest screen become
the feed of the next finer sieve, and so on (see Figure 20a). Finally, the feed is split in
several fractions Δmi as shown in figure 20a. The size classes are given by the mesh size
of the sieves used and thereafter we generate data set: xi – xi-1 and Δmi.
Definition of Size Classes:
From the analysis, the (relative) mass of the particle system with definite size between
two mesh sizes can be known. In this order, one has to respect the definition of the size
classes (R10). The class is defined and named by its upper limit. The size class with up-
per limit is denoted with (xi) and it’s mass fraction (Δmi) belonging to this cut size
(Mütze 2012). The width of the size class with the upper cut size xi is (Δxi). This ap-
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proach can be generalized to other characteristic quantities r (i.e. surface, number,
length). In this case, the quantity (Δµi) is being referred to, which belongs to xi and Δxi
see Appendix A2; Table 11 & 12.
Cumulative Sum Distribution:
The particle size distribution PSD is usually plotted as cumulative sum distribution. The
cumulative sum is the relative quantity (e.g., relative mass, relative number) having the
property being smaller than the size (xi). For instance, if Q3 (xi = 50 µm) = 0,633; it
means that 63.3 wt.% of the material comes from particles ≤ 50 µm (see Appendix A2;
Table 11 & 12).
1.7
1.7.1
Relative quantity [Q3 (xi)]:
The relative quantity sums up all mass fractions Δmi to determine mi, which is the cu-
mulative mass of all particles below the size (xi). If this mass (mi) is divided by the en-
tire mass (m) of the initial sample, the quotient mi/m is the relative mass smaller that the
corresponding particle sizes see Appendix A2.
1.7.2
1.7.3
Cumulative sum – plotting the relative quantity and homogeneous function:
The relative mass is often plotted as cumulative sum by adding up the relative mass in
all size classes smaller that the corresponding particle size Q3 (xi). It is a dimensionless
parameter but it is usually represented in (%). (Mütze 2012) stated that it is possible to
adjust the plotted diagram of the cumulative sum to generate a homogeneous curve.
Therefore, the data points for Qr(xi) are plotted as a function of (xi). The connecting
curve is then the homogenous function Qr(x) and its properties are expressed mathemat-
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ically as shown in equations 1.7.4 and 1.7.5. For the data and results used during the
calculation see Appendix A2.
1.7.4
1.7.5
Density distribution:
The second established form to plot the PSD is the density distribution. It looks at the
individual quantities in the size classes. Since the size classes (∆xi) can have different
width, the relative mass (∆mi/m) is related to the size of the interval (∆xi). This finally
gives the value for the density function q3(xi) see Appendix A2.
1.7.6
1.7.7
Characteristic values of PSD:
In technical processes it is often necessary to use only one parameter as representative
for the entire distribution. The most common parameter is the median value (x50,r). The
particles smaller than the median value represent 50% of the characteristic quantity. The
particle size distribution thus is separated into two partitions by the median value. The
median value indicates that 50% of the mass comes from the particle below this size
(Mütze 2012).
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Figure 21: Particle size distribution curve showing x50 adapted from (Mütze 2012).
Comparing cumulative sum functions for different quantities:
For all size distributions representing the different quantities r the lower (xmin) and
upper (xmax) particle size are set. The differences are expressed in the shape of the
curve see Figure 22:
• The number distribution weights a small particle with the same impact than a
large one.
• The mass distribution weights a large particle with the size to the power to three.
• Thus the higher r becomes the larger becomes x50,3.
Figure 22:Comparative sum function of different particle size distribution curves adapted from (Mütze 2012).
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Analytical mathematical function to describe PSD.
After measuring a PSD, the information obtained is used for quantitative calculations
(e.g., the prediction of change in PSD due to breakage events). One possibility is to use
the entire set of information [all size classes (xi) and quantities (µi)]. This results in a
population balance approach. The second way uses analytical functions to represent the
PSD. The analytical function can further be transformed with a process function, which
represents the change by breakage or agglomeration. The analytical function adopted for
this thesis was based on RRSB-distribution because it is a function that is commonly
used in the cement industry for finished (grinded) products.
Rosin, Rammler, Sperling and Bennett (DIN 66145) Function.
The RRSB is a function, which uses two parameters to fit the PSD. The parameter (m)
represents the information on the width of the PSD and the parameter (x63.2) represents
the particle size. The mathematical form of the RRSB-function does give neither a min-
imal nor maximal value for x (Mütze 2012). The RRSB distribution uses a special prob-
ability grid. In the RRSB-grid the PSD becomes a straight line, if it follows the RRSB
function. The scaling of the ordinate is strongly non-linear due to the double-
logarithmic partition. This can be used to deduce the size parameter x63.2 (see Fig. 23).
The RRSB distribution has its origin in the field of fine comminution. The RRSB-
distribution is mathematically a Weibull-distribution and it is often used for the descrip-
tion of breakage events (Mütze 2012). See equation 1.7.8a.
Transformation fit into RRSB-grid.
1.7.8a
1.7.8b
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Figure 23: RRSB distribution curve adapted from (Mütze 2012).
Figure 24: a) Illustration of sieve analysis [Q3(x)] adapted from (Mütze 2012) and b) mesh sieve (R10) used for the
particle size distribution of the analyzed cement clinkers.
Sieve analysis is commonly used to assess the particle size distribution of a dry material
(cement clinker) by allowing the material to pass through a series of sieves with proper
arrangement ranging from bigger sieves on top to the smaller sieves below and at the
end weighing each sieve to record the exact amount of materials (g) retained after the
applied pressure (i.e. through mechanical or electronic shaker) is stopped. Each succes-
sive sieve analysis was conducted for 10 mins with amplitude of 2m (see Fig. 24a and
b).
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Procedures for the sieve analysis of cement clinkers
• Weighing of total sample (clinker) to the nearest 0.1g using KERN / 440-35N /
max. 400g, d = 0.01 g. Recording the weight and assigning a name to avoid error
due to complexity.
• Arrangement of the sieve set, with the largest opening on top (0.8mm) and the
lowest below (< 0.1mm), pouring of the aggregate (crushed cement clinker) onto
the top sieve.
• Separation of the material into a series of particle sizes using mechanical sieve
shaker.
• Setting of the time and amplitude of the mechanical sieve to 10 mins and 2m re-
spectively.
• Waiting for the required shaking time for different clinkers to be properly sepa-
rated into their various mesh-sieve sizes.
• After the completion of the sieve analysis, materials > 100m (0.1mm) and <
100m (0.1mm) are recorded for the plotting of particle size distribution graph
see Appendix A1 Table 9 and 10.
3.1.5 Laser Diffraction
The material < 100µm is examined with the HELOS laser diffraction device from Sym-
patec. The test was carried out wet (suspended in ethanol), this is because wet testing
results in slightly finer particle size distribution (i.e. the samples are stirred in ethanol
which makes available agglomerates broken). By means of a wet comparison measure-
ment, agglomerates still present in the fine range are detected (Schulze 2019; Taylor
1997). To maintain comparability with other work, the dry measurement is used. The
values are then calculated to obtain a sum distribution see Figure 31.
Procedure for laser diffraction:
In laser diffraction, the particle size is determined by the scattering of the light on the
particles which results to diffracted images. Measured spheres thus consist of concentric
circles of different light intensity. The diffraction image of irregularly shaped particles
depends on the orientation and angles of the particle (i.e. the diameter of a sphere-
equivalent with the same intensity). For the mathematical back calculation the deter-
mined particle size distribution is smoothed (Schubert 2003; Schlosser 1959). Due to
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the different type of particle size in sieve analysis and laser diffraction, a “kink” occurs
when the curves are added together (Napier-Munn 2005). The curves do not exactly
overlap because of the sphere-equivalence which produces finer results of laser diffrac-
tion particles compared to sieving particle (Schützenmeister 2017).
3.1.6 Ball mill according to Zeisel Grindability Approach
Zeisel developed a new setup device suitable for materials harder than coal based on the
existing device of Hardgrove (Quist 2017). The encountered limitations of Hardgrove
device (i.e., achievable fineness and missing of grinding possibility of direct measure-
ment of grinding work) were overcame by Zeisel through the modification of grinding
bowl’s shape and torque measurements installation. Thereafter, particle size distribu-
tions comparable to industrial processes were achieved (Fleiger et al. 2012; Quist 2017).
Design of the testing device
It consists of a grinding bowl with a slight narrow grinding path compared to the
Hardgrove test in which usually eight steel balls roll upon the testing material (cement
clinker). The rotating die applies a load to the balls, thereby causing comminution due
to pressure and friction.
Figure 25: Sectional view of the grinding bowl with balls and die in mesh and grinding bowl adapted from (Fleiger et
al. 2012).
Figure 25 shows sectional view of the bowl and the stamp. After assigned number of
revolutions, comminution process is interrupted, and measurement of fineness or parti-
cle size distribution is recorded. Due to this, a function for the specific energy consump-
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tion in kWht-1 depending on the fineness or specific surface is calculated. According to
(Zeisel 1953) the result is only affected by the fraction of the initial material while other
parameters (i.e. speed of revolution or load of the die) influences the achieved fineness
but do not alter the grindability.
Test parameters according to Zeisel
Zeisel test is carried out using the specific surface according to Blaine for the classifica-
tion of results obtained from grindability. (Fleiger et al. 2012) stated that the Zeisel test
and determination of specific surface should be conducted in an air-conditioned envi-
ronment of 20°C and 60% humidity. Zeisel test consists of several grinding steps till
specific surface of about >3000cm2g–1 is attained. Each step is determined and the dura-
tion is about one minute [e.g. limestones takes 0.5-1min; clinker takes 3-4 mins] (Flei-
ger et al. 2012; Zeisel 1953).
Table 2: Test parameters originally set by Zeisel modified after (Fleiger et al. 2012)
Parameter
Zeisel
Revolution
200 min 1
Die Load
26kg
Sample weight
30.25 g
Grinding steps
-
Measurements grinding results
Blaine [cm2 g-1]
Targeted fineness
3000-4000 cm2 g-1
Test material size fraction
0.25-1mm
Temperature
20°C
Humidity
60%
High precision power measurement according to Zeisel
The most outstanding modification of Zeisel’s test layout was the integration of power
measurement. The grinding work can be calculated by determining the torque acting on
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the pivot-mounted grinding bowl during comminution: Thus, the following integral are
used for the calculation of grinding work (Fleiger et al. 2012; Morrell 2004)
1.8
Where: W is the grinding-work (Nm), Md the torque (Nm) and the angular velocity (s-
1). According to (Bernutat 1961) grinding work can be calculated from eqn. 1.8.1
1.8.1
Where n is the revolving speed (rpm). FF is the elastic force (N) and l is the lever arm
(m). (Fleiger et al. 2012) stated that in consideration of the sample mass ms (g), the spe-
cific energy consumption in kWht-1 can be calculated using equation 1.8.2
1.8.2
During the grinding process, the power (P) can be derived. Therefore, the grinding work
is then calculated by numerical integration of the measured power values over time ac-
cording to equation 1.8.3 (Fleiger et al. 2012; Zeisel 1953).
1.8.3
The angular deflection is measured 20 times per second, which leads to a time step
width ∆t of 50ms. The numerical integration can be performed using the constant step
width based on equation 1.8.4
1.8.4
Due to the inertia of the system, the step width of 50ms is more than sufficient to re-
solve the movement of the grinding bowl. Finally, the specific energy consumption can
be calculated by equation 1.8.2 (Zeisel 1953; Fleiger et al. 2012).
3.2 Simulation of grinding circuits
This chapter explains the procedure employed during the simulation of grinding circuits
based on adaptation of High-Pressure Grinding Rolls with piston-die press.
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3.2.1 Application of comminution and simulation of grinding circuit
The steps applied during the comminution and simulation of grinding circuits is listed
below:
• Initial material: 0.8 – 1 mm cement clinker
• The grinding process is explained in section 3.1.1 and one cycle commi-
nution process.
• Grinding pressure in each cycle: 150 MPa
One cycle comminution process
Various steps were applied during the one cycle comminution process. The following
input variables were inserted into the software (LabMaster) correctly, before stressing
the cement clinker using a piston-die. The amount of the measure sample used was 140g
monodisperse size. These clinkers are filled in a pressure pot using a funnel, to ensure
uniform mixture of all clinker grains. After the filling, a metal disc is used to ensure
uniform surface, the height between the upper edge of the pot and bulk material surface
is measured using a calibrated ruler, which would be required during the calculation of
power and diameter based on “Thyssenkrupp Industrial Solution AG and Polycom Pro-
cessing guideline”. The clinker samples are stressed with initial pressure of 150MPa.
After the stressing, the crushed products are weighed and deagglomerated using “Roll-
bock machine” (see chapter 3, section 3.1.2). The next step after deagglomeration is
weighing of different size particles using R10–seive series, products < 100 µm (fine ma-
terials) are subjected to laser diffraction while those > 100 µm are re-mixed again by
adding fresh clinkers till a mass of 140g is attained. The re-measured products would be
stressed repeatedly until “stop criterion” is achieved. Thus, stop criterion implies that
the mass of finished product < 100 µm does not change any more. This is also known as
“constant operating point”. A constant operating point is reached as soon as the mass <
100 µm changes by < 3 % between two consecutive cycles (see Table 8 and 9).
3.3 X-ray Diffractometry and Scanning Electron Microscope
In order to examine thin and polished sections of mineral samples, a conventional opti-
cal microscope is required. However, the use of quantitative automated mineral analys-
ers like SEM (scanning electron microscopy and energy dispersive X-ray analysers is
presently employed (Wills and Finch 2016; Wikedzi 2018).
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The qualitative and quantitative determination of the crystalline and partially amorphous
phases in the supplied crystalline and partially crystalline materials. There are two sam-
ples each prepared: one sample of 100% sample material and a second sample of 80%
sample and 20% internal standard for the quantification of phases using the Rietveld
method. According to (Leng 2008) X-ray diffractometry (XRD) was traditionally known
as X-ray powder diffractometry, it is among the mostly used technique in materials char-
acterization. Two different methods were adopted during the XRD analysis namely:
XRD Thyssen (XRD analytical method from Thyssenkrupp Industrial Solution AG) and
XRD IKGB (XRD analytical method from Institute for Ceramics, Glass and Building
Materials Technology, TuBaF). XRD measurements were conducted using X’Pert Pro
MPD with Pixcel detector X-ray diffractometer. The XRD analysis helped to determine
the mineralogical phases of the cement clinkers. 1500 W’ Cu-anode tube, operated at 40
kV/40 mA. Aperture slits of 1° were used for the incident beam, while the post-sample
anti scatter slit and detector slit were fixed at 1 and 0.15° respectively. The soller slit lo-
cated in front of the detector reduce the axial divergence of the X-ray beam. A post-
specimen graphite monochromator was used to remove the Co Kα radiation. A NaI scin-
tillator with pulse height analyzer was used as the detector. The XRD data were collect-
ed with a step size of 0.0131° 2θ over the goniometer range of 7.5–90° (2θ) and count-
ing time of 30 seconds per step. The samples were rotated during measurement. The
diffraction data from the diffractometer were converted into ASCII files suitable for
Rietveld refinement and thus the evaluation of discrete peak was made by reference
patterns in an evaluation program supplied by the manufacturer of the equipment see
Appendix D2 – D3.
The scanning electron microscope (SEM) uses a focused beam of high-energy electrons
to generate a variety of signals at the surface of the clinkers. These signals derived from
electron-sample interactions reveal information about geometry, chemical composition,
crystalline structure and orientation of the clinkers. SEM requires plane cuts of particles
samples embedded in epoxy resin.
3.4 Mineral Liberation Analysis (MLA)
The application of MLA implementation of SEM-based image analysis helps to over-
come challenges of identification of trace mineral phases within the clinkers and also
the quantification of their respective concentrations (Bachmann et al. 2017).
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
The quantitative mineralogical analyses using MLA were conducted on the selected
cement clinker. The analysis of the mineral grain size distribution, particle size distribu-
tion based on equivalent circle diameter and the mineralogical phases present in the
analysed clinkers were done using MLA image viewer and MLA data viewer (i.e. MLA
Image Processing software version 3.1.4). The data and image viewer of this software
allows one to compare different minerals or clinkers phases in terms of their concentra-
tion and distribution of these mineral. In addition, the modal mineralogy, particle size
distribution, mineral reference and mineral grain size distribution allows one to observe
the dominant/recessive mineral phases and particles present in the analysed cement
clinkers. The image viewer helps to examine the accurate and precise distribution of
various mineral’s composition by using the particle line up feature. Thus, allowing one
to see different mineral grain sizes, shapes, and boundaries. For more clarity and better
representation of the mineral composition, this software allows one to group and un-
group minerals into their suitable classes, for easy classification and identification of
minerals.
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
4 Results and Discussion
The evaluation of the experiment is arranged in relation to various steps applied during
the analysis of cement clinkers. Firstly, X-ray diffractometry analysis in section 4.1,
followed by mineral liberation analysis (section 4.2), the ball milling according to
Zeisel test is shown in section 4.3, while section 4.4 represents the particle size distribu-
tion curve in section and the scale-up of HPGR using piston-die press in section.
4.1 X-Ray diffractometry
The mineral phases from X-Ray diffraction were further sorted and arranged using
Rietveld multiphases refinement method (see Table 3). The X-Ray diffractometry anal-
ysis confirms the presence of calcium silicate, calcium aluminate, calcium alumino-
ferrite and other minerals in the analysed cement clinkers. The results showed that the
mineral content in K1 and K3 are similar while that of K4 is different. In addition, the
minerals phases of clinkers K1 to K3 showed a great similarity and thus further analysis
were conducted with respect to two clinkers namely: K1 and K4.
Table 3: Showing detected mineral phases from X-Ray diffraction determined by means of Rietveld method.
Mineral Phases
Content (%)
Clinker 1
Clinker 2
Clinker 3
Clinker 4
C3S
47.1
24.7
28.8
-
C2S
10.3
11.6
10.2
27.9
C3A
2.6
4.9
4.8
1.6
C3AF
4.5
4.8
4.0
1.9
Ye'elimit
-
-
-
19.6
C2AS
-
-
-
0.4
C12A7
-
-
-
2.9
Anhydrit
-
-
-
4.7
Quartz
-
-
2.7
1.3
Amorph
35.5
43.5
46.0
39.7
According to the result from X-Ray diffraction, tricalcium silicates (C3S) mineral is the
major difference between other clinkers and clinker 4. The mineral phases in the ana-
lyzed cement clinkers indicates that C3S > C2S > C3A/ C3AF except for C3S in clinker 4.
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
According to (Dunstetter et al. 2006) most of the analysed cement clinkers consists
mainly of tricalcium silicate, dicalcium silicate, tricalcium aluminate and calcium alu-
minate ferrite. In addition, (Schützenmeister 2017) stated that the formation of the min-
eralogical phases, the grain size and the microstructure are highly influenced by the
burning process and the subsequently cooling rate. The results showed that the tricalci-
um-silicate mineral phase (Alite) is only present in K1 with a total of 41.7% while the
highest mineral composition of dicalcium silicate (Belite) occurred in K4 (see Table 3).
The influence of these mineral phases on comminution of the analysed cement clinkers
indicates the coarse mineral grain size and the one-fourth hard-belite present in K1 con-
tributes to high energy consumption during the grindability and scaling-up. According
to (Albats et al. 1992) formation of clinker phase composition is mainly influenced by a
number of external factors depending on the synthesis conditions, and internal factors
such as the composition of the source raw mixes. Thus, the influence of mineral phase
during the grindability and comminution processes results in the disparity of consumed
energy in K1 and K4.
4.2 Mineral Liberation Analysis Software (MLA)
Comparisons were made using the MLA-data and images generated from SEM-Images.
The differences between the chemical and mineralogical composition of the analysed
clinkers (K1) and (K4) were observed. It is very vital to recognise that all measurements
for mineral grain size presented in this section are based on the Equivalent Circle Diam-
eter (ECD) a function used by MLA software for computation. Since the mineral phases
of tricalcium silicate and dicalcium silicate could not be distinguished properly by
MLA, both mineral phases were grouped as calcium-silicate minerals. The dominant
minerals in both clinkers were calcium-silicate, followed by calcium-aluminate and
calcium-aluminium-ferrite (see Fig. 26). The mineral reference shows the density and
formula of each mineral (see Appendix D1). In addition, the percentage of each mineral
constituents and the chemical formula helps in the classification and grouping of the
mineral constituents into their major classes. The mineral grain sizes were further fil-
tered through the filtration button into 3 different classes (i.e., 0-10 m, 10-90 m and
90-500 m). The filtration helped in the determination of coarser/finer mineral grain
sizes for the different particles. For instance, the particle size of 0-10 m and 10-90 m,
shows that K1 > K4 and vice versa for 90-500 m (see Appendix E2 and E3).
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
Figure 26: Comparison of modal mineralogy composition of both clinkers
The modal mineralogy shows different constituents of minerals present in the analysed
clinkers. Figure 26 clearly showed that the dominant mineral phase by percentage
weight is calcium-silicate mineral. The mineral calcium-silicate is a composition of both
tricalcium silicate and dicalcium silicate. Using the MLA data view, the minerals were
re-grouped in order to clearly represent the diagram in less complex form see Appendix
E. Since the dominant mineral phase happens to be the calcium-silicate, the concentra-
tion of calcium silicate mineral in K1 is higher than K4. Since MLA could not properly
differentiate between Alite and Belite mineral phases, the results from XRD-Thyssen
and XRD-IKGB justifies that there is no presence of Alite in K4 (see Table 4).
Table 4: Measured concentrations of composite minerals in analyzed clinkers using different analytical methods.
XRD Thyssen
MLA
XRD IKGB
Minerals (weight %)
K1
K4
K1
K4
K1
K4
Alite (C3S)
69.8
-
CxS
71.1
-
Belite (C2S)
10.2
51.8
85.2
45.06
17.6
48
Ca-Aluminate (C3A)
4.6
5.3
0.04
23.72
4
4.4
Ca-Al-Ferrite (C4AF)
14.3
2.6
10.13
1.17
7.3
3.2
Freelime (CaO)
0.7
-
1.71
-
-
-
Periclase (MgO)
0.4
0.4
0.14
0.04
0
-
0
30
60
90
Ca-Silicate Ca-Aluminate CaAl-Ferrite
Weight (%)
Modal Minerology
Klin 4 Klin 1
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The mineralogical composition is shown in Table 4 and was carried out using mineral
liberation analysis software (MLA), data generated from SEM-Images and X-ray diffrac-
tometry (see Appendix D2 – D3). The percentage composition of tricalcium silicate
shows the highest composition among all the analysed mineral phases (e.g., 69.8% with
XRD Thyssen and 71.1% XRD IKGB as shown in Table 4.
4.2.1 Mineral Grain Size Distribution Curve from MLA
MLA indicates that the dominant mineral grain sizes in the analyzed cement clinkers are
calcium silicate, calcium aluminate and calcium alumino-ferrite. The grains of CXS
show a distinct size mode of about 120 m and 80 m_ECD for K1 and K4 respectively.
The retained weight (%) versus seize size_ECD (m) curve indicates that the mineral
grain size distribution for calcium-silicate minerals of K1 are higher than K4 (see Fig.
27a). The calcium-aluminate minerals appeared to be dominant in K4 clinkers compared
to K1 except for 0-10m sieve sizes (see Fig.27b).
Figure 27: Mineral grain size distribution a) CXS and b) C3A
MLA software was used to analyse the major mineral phases for K1 and K4 (see Fig. 27
and 28). CxS in K1 has the coarsest mineral grain size distribution followed by CxS in
K4 and C3A in K1. In addition, mineral grain size distribution analysed by MLA is finer
than the particle size distribution (laser diffraction and sieve analysis), because the min-
erals were further grinded before it was analysed using the XRD and SEM. Figure 28
shows that the mineral grain size distribution for CxS, C3A and C4AF in K4 show a self-
similarity particle size distribution.
0
5
10
15
20
050 100 150
Retained Wt. (%)
Sieve Size_ECD (𝜇m)
MGSD_CXSK1
K4
0
10
20
30
40
050 100 150
Retained Wt. (%)
Sieve Size_ECD (𝜇m)
MGSD_C3AK1
K4
(a)
(b)
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
According to (Wills and Finch 2016) the liberation distribution is based on mineral free
surface since the efficiency of most of the mineral beneficiation processes (leaching or
flotation) requires at least a surface of valuable mineral to be exposed. Based on MLA
the mineral liberation by particle composition for CxS indicates that K1 are liberated
better than K4 (see Fig. 38). The mineral grain size distribution curve also indicates that
the grains of K1 > K4 and thus it indicates that the milling energy would be consumed
more in the grinding of minerals phases in K1 (see Fig.28).
Figure 28: Particle and mineral grain size distributions for the analyzed clinkers.
From figure 28, the mineral grain distribution curve indicates that particles of calcium
alumino-ferrite (C4AF) for K4 has the finest mineral grains while that of calcium silicate
(CXS) phases have the coarsest mineral grains. Generally, the fracture toughness of Be-
lite is superior to other mineral phases in cement clinkers when clustered (i.e. deterio-
rated the grindability of clinker to a considerable extent whereas grindability improves
when they do not form clusters (i.e. spherical single grains of Belite) (Maki et al. 1992).
Therefore, Belite clusters play a major important role in grinding of cement clinkers
than pores (void volume). The clinker grains of K4 are of spherical grains and thus they
do not form clusters during grinding which also results to the lesser energy consumption
unlike that of K1. Comparison between the mineral phases shows that the K4 minerals
attained fineness before K1 and therefore the pores of the minerals are in steady contact
0
20
40
60
80
100
110 100 1000
Cummulative passing (Wt%)
Sieve Size_ECD (𝜇m)
MGSD
CᵪS_K₁
CᵪS_K₄
C₃A_K₁
C₃A_K₄
C₄AF_K₁
C₄AF_K₄
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with one another making it difficult for further absorption of energy required in size
reduction at a steady pressure of 150Mpa.
4.3 Ball milling
Practically, a mill could be integrated with several chambers; first chamber consisting of
largest grinding ball, followed by second chamber and the third chamber with the
smallest grinding ball (Schützenmeister 2017). Thus, implies that the fineness of cement
clinkers decreases with respect to number of chambers present. Alternatively, there are
types whereby the plating of the inner wall is designed in such a way that the balls are
sorted by size in a single chamber. Thus, it implies that sometimes the chambers do not
necessarily need to be separated. For this thesis, the single chamber mill was used dur-
ing the grindability test. In addition, several empirical values were adopted from (Zis-
selmar 1981) which were applied during the calculation of Blaine (amount of fineness)
and R90 (i.e. based on RRSB distribution function) for grindability of K1 and K4.
According to (Zisselmar 1981) to determine the mill size, parameters such as filling
ratio [C ()] and length-diameter ratio L/D of the mill, the relationship between the ex-
perimentally determined energy required for grindability according to Zeisel and the
mill diameter (D) must be known. Therefore, in order to determine the required length
and diameter of a ball mill for grinding 200 t/h of cement I 32.5 according to DIN 1164
[Mill designed to consist of only one chamber with no species (i.e. grinding balls of
different sizes)] the following underlisted parameters were adopted from (Zisselmar
1981).
• Density of grinding media (steel balls): = 7.85 t/m³
• Uniform ball size: 35 mm
• Porosity of grinding media bulk: ε = 0.4
• Filling ratio of grinding media: φ = 0.4
• Length-diameter ratio: L/D = 3
• Relative speed: η = 0.75 or ¾
•
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https://doie.org/10.0201/Thesis.2023724402 Influence of chemical-mineralogical composition on HPC of cement clinker.
Calculation of C () according to Zisselmar is derived from Table 5. Since the ball sizes
are uniform, with value of 35 mm and the filling ration is 0.4, then C () would be 6.0
(see Table 5).
Table 5: Factor C(φ) [kW/(th)] as related to mill loading factor and type of grinding media adapted.
from (Zisselmar 1981)
Filling ratio of grinding media ()
0.1
0.2
0.3
0.4
0.5
Type of
grinding
mill
Flintstone
9.8
9.0