Topicg2ggRawgMaterialgPreparation
CoursegLBg2.4ggRawgMealgHomogenizationgSystems
VDZgOnlinegCourses // Cement
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German Cement Works Association
Research Institute of the Cement Industry
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Phone: +49 211 45 781
Fax: +49 211 45 78296
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Issued: 19th May 2015
Contents
1 Introduction....................................................................................................... 1
2 Design and Operating Principle ...................................................................... 1
2.1 Aeration Mixing Process .................................................................................... 1
2.2 Tangential Mixing Silo ....................................................................................... 4
2.3 CF Silo from FLSmidth A/S............................................................................... 4
2.4 Multi-Cell Silo .................................................................................................... 5
3 Health and Safety Practices ............................................................................. 5
4 Operation and Quality ..................................................................................... 6
5 Environmental Protection ................................................................................ 8
6 Questions on Course LB 2.4 Raw Meal Homogenization .......................... 9
Solutions............................................................................................................................... 10
Glossary ............................................................................................................................... 11 i
Index..................................................................................................................................... 12
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1 Introduction
In most industrialized countries cement clinker is now predominantly produced by the dry
process. This process requires very uniform (homogeneous) raw meal. Advances in the
field of aerodynamics and pneumatics have made it possible to produce dry raw meal that
has a homogeneity similar to that of the raw slurry in the wet process.
In order to produce a homogeneous raw material mixture, the material is first mixed in
blending beds. In the grinding plant the material then undergoes a second homogeniza-
tion of the chemical composition. However, in most cases this is not sufficient to satisfy
the strict requirements of the burning process, so it is then necessary to mix the largest
possible quanitities of raw meal with each another.
Processes have therefore now been developed that permit extensive, cost-effective, ho-
mogenization (uniform mixing) of the raw meal, and produce appropriate silo plants with
equipment for mixing and storing the raw meal.
In this course you will learn about the key methods of raw meal homogenization. In 1
particular, you will be made familiar with pneumatic homogenization in raw meal silos.
Course Summary
2 Design and Operating Principle
In the cement industry, blending or homoginizing silos are used for homogenizing the raw Homogenization
meal and for intermediate kiln feed storage between the raw grinding system and the kiln
system in order to decouple these plant sections.
As already mentioned, the cement raw materials are normally homogenized in a number
of successive process steps, which should even out the natural variations in individual
rock layers and deposits as much as possible. Homogenization also serves to offset any
segregation effects by which larger pieces of material come to lie at the base with smaller
particles on top.
1 Blending Bed: The crushed material is first piled up and spread out in blending beds
so that it is mixed as thoroughly as possible.In this way e.g. a truckload of material is
distributed in a layer over the entire surface of the blending bed.
2 Reclamation and Raw Mill: The raw material is then reclaimed from the blending
bed across the layers, ground in the raw mill and stored in the raw meal silos.
3 Storage in the Raw Meal Silo: The material is further homogenized in the silo by
carefully controlled deposition of the raw meal i.e.by filling from constantly changing
positions in the top of the silo.
4 Aeration Mixing Process and Discharge: Variations in the chemical composition
of the raw meal are further reduced by subsequent mixing using compressed air and
carefully controlled extraction.
Methods for storing raw meal, homogenization using aeration and different strategies for
extracting the meal from the silo will now be discussed.
Course Summary
2.1 Aeration Mixing Process
The aeration mixing process (pneumatic homogenization) is the most popular homog- Pneumatic
enization process. The aim of the process is to generate a fluidized bed in the silo that can Homogenization
then be used to achieve the highest possible level of uniformity of the raw meal. To this
end, the raw meal is loosened by introducing compressed air through the aeration base.
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2 Design and Operating Principle
Aeration of different sections of the base in turn causes the raw meal in the silo to cir-
culate and swirl, thus achieving effective, thorough mixing. In this process, one or more
segments function as mixing segments (high air volume, fluidized bed), whereas the other
segments are only lightly aerated (loosening, slight fluidization).
The fluidized bed generates vertical circulation of material and a high degree of mixing.
Since the quasi-fluid raw meal has a lower density than the surrounding non-fluidized
material it ascends to the top. The segments are supplied in turn with mixing air in a fixed
cycle to achieve homogeneous mixing of the material in the silo (Fig. 2.1-1). If the mixing
air is fed to the silo in pulses, the raw meal is fluidized even more efficiently. This is more
expensive and complex, but results in a higher level of homogeneity.
The segments are supplied with air separately from a mixing air compressor and an aer-
ation compressor. The volume of mixing air that is introduced into the mixing segments
accounts for approximately 85 % of the total air flow. Depending on the number of seg-
ments, a distinction may be made between the Fuller process (also called the quadrant
system), the Polysius homogenizing system (nine segments) and the octant system (eight
2 segments).
1 2 3 4
5 6 7 8
Figure 2.1-1: Diagram of a Mixing Silo with Eight Aeration Segments (1-8) .
Design The dimensions of a homogenizing silo are determined on the basis of a filling volume
of 10 to 12 hours output from the grinding mill to ensure that the chemical variations
are theoretically evened out. The height of material in the silo should not exceed 1.5
times the silo diameter to ensure thorough mixing. The air required is approximately
10 to 13 m3 air/t raw meal and the energy consumption is 0.65 to 0.84 kWh/t raw meal.
The air pressure of the mixing air is about 2 to 2.2 bar, and that of the fluidizing air is
about 1.75 bar.
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2.1 Aeration Mixing Process
Internal fittings in the silo, such as a pressure-relief cone, can further improve the mixing
effect. Because of the inner cone, the horizontal material layers produced when the silo is
filled cannot move downwards uniformly (Fig. 2.1-2). This brings about primary mixing
of the raw meal in the silo. It is also possible to achieve improved homogenization by
providing a downstream mixing chamber. The designs vary with the different suppliers.
At this point in the online course you will watch a video instead of this image.
Figure 2.1-2: Movement of Material at the Pressure-Relief Cone .
The basic elements of pneumatic dry mixing systems are air distribution boxes (Fig. 2.1-
3) that are located on the silo base. Their main component is an air-permeable, porous
cover that may be made of ceramic plates or woven textile. Air is forced from below into
the raw meal through the porous plate, which produces fine currents of air that bring the
raw meal into a quasi-fluid (fluidized) state.
Between 50 and 80 % of the silo base is aerated depending on the type of homegeniza-
tion process and the design of the mixing silo.
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2 Design and Operating Principle
pressure of material
porous
plate
air inlet
air pressure
aeration box
Figure 2.1-3: Schematic View of an Air Distribution Box .
4 2.2 Tangential Mixing Silo
Tangential mixing silos were developed specially for the storage of fine-grained bulk
materials. The main components of a tangential mixing silo (Fig. 2.2-1) are the inner
cone (1), the aeration surface with extraction points (2) and the central mixing chamber
(3). The silo outlets are postioned under the inner cone and extend into the centre.
Figure 2.2-1: Tangential Mixing Silo. (Source: ThyssenKrupp Industrial Solutions)
Depending on the fill level of the mixing compartment, parts of the silo base are aerated,
causing the raw meal located in that part to be fluidized. In this state it is possible to
draw the meal off and transport it to the mixing chamber.
The bulk material is transformed into a fluidized bed in the mixing container, which en-
sures the homogeneity of the raw meal. The raw meal is drawn off continuously through
an outlet at the mixing container.
2.3 CF Silo from FLSmidth A/S
The base of the CF-silo (CF = controlled flow) is divided into seven identical hexagons
irrespective of the size of the silo. In the centre of each hexagon there is a discharge
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2.4 Multi-Cell Silo
opening below a steel cone. This cone protects the discharge opening from excessive
pressure and improves the flow behaviour of the raw meal. The area surrounding each
discharge opening is divided into six segments that are aerated independently, producing
6x7=42 individually and sequentially aerated surfaces in the silo base (Fig. 2.3-1).
Figure 2.3-1: Schematic View of a CF Silo. (All Rights Reserved FLSmidth A/S)
The homogenizing effect is achieved in this design by maintaining different extraction
speeds at the individual outlets. Fig. 2.3-2 shows a schematic view of this mixing effect.
Seven discharge openings are activated separately and sequentally. Selective simultane-
ously withdrawal from three different discharge openings also results in blending of the
layers in the silo.
2.4 Multi-Cell Silo
In contrast to the previous processes described, in a multi-cell silo (Fig. 2.4-1) there is no
actual mixing zone. This design consists of a number of silo cells and is used mainly for
raw meals that tend to segregate.
The underlying mixing principle is simple: the individual silos are filled one after the
other, but are emptied concurrently. The raw meal is then fed directly to the preheater.
This prevents any segregation during intermediate storage.
3 Health and Safety Practices
The following measures must be taken:
All silo doors and silo entrances must be kept properly closed and secured by padlocks.
The interior of the silo may only be entered when the silo is empty.
Only suitable, reliable people who are familiar with this work may take a vehicle
into the silo. The employer must give written permission for this (vehicle operating
permit).
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4 Operation and Quality
Figure 2.3-2: Layer Movement in a CF Silo .
Welding regulations must be observed.
The welding power source must never be placed in small, enclosed spaces on electri-
cally conductive segments.
Among other regulations, the Associations BGV C 12 regulation Silos and Sec-
tion 8 Hazardous Work of the Associations BGV A 1 regulation Basic Prevention
Principles must be observed.
4 Operation and Quality
Having a uniform raw meal quality is a basic prerequisite for fault-free operations, because
fluctuations in the chemical composition or particle size distribution can cause problems
in the preheater.
The aim of raw meal homogenisation is for the raw meal composition to fluctuate as
little as possible. The fluctuation range of the raw meal composition is often verified
using the lime saturation factor and the silica ratio.
The target value for the variation of the aforementioned parameters from a predetermined
target valued is no more than 1 to 1.5 per cent. For example, a targeted lime saturation
factor of 100 should deviate by no more than 1 to 1.5 points above or below after ho-
mogenisation.
The starting materials for the production of Portland cement clinker must contain mainly
calcium oxide (CaO) and silica (SiO2) and small amounts of aluminium oxide (Al2O3)
and iron (Fe2O3) necessary for the formation of the melt oxides.
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7
Figure 2.4-1: Multi-Cell Silo. (Source: ThyssenKrupp Industrial Solutions)
The raw material used for the clinker burning process basically consists of limestone and
clay or their natural mixture, marl (see Tab. 4.0-1). There are also so-called corrective
materials such as sand or iron oxide carriers. In recent years, moreover, secondary raw
materials were increasingly used to substitute for natural resources. These substances
usually also contain sulphur in different bonds and volumes, mainly as inorganic sulphate
(SO42-) or sulphide (S2-). In order to account for the sulphide content in the raw material
mixture, it is also necessary to be familiar with the ratio of pyrite or marcasite in the quarry.
Marcasite is a mineralogical variation of pyrite.
In order to achieve optimal kiln feed reactions, the used raw materials must be ground as
finely as possible and the raw material components must be distributed evenly. Coarse
crystal and calcite bodies diminish the burnability of the raw material and can therefore
affect the strength development.
When using high-ash fuels, the ash particles can also cause inhomogeneities in the clinker. Ash
Through increased ash input, the standard lime factor also decreases in the kiln feed while
the ratio of melt increases.
Good homogenisation of the raw meal is also important to maintain a consistent clinker Free lime
quality. High levels of free lime, for example, can be caused by insufficient homogenisa-
tion. Usually these are only temporary, but must be avoided all the same.
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5 Environmental Protection
Limestone Lime marl Clay Sand Raw meal
Loss on igni- 40 44 2 42 1- 20 +5 32 36
tion
SiO2 0,5 3 3 50 37 78 80 -99 12 16
Al2O3 + TiO2 0,1 1 1 20 6 30 0,5 3 25
Fe2O3 + 0,1 0,5 0,5 10 2 15 0,5 2 +2
Mn2O3
CaO 52 55 5 52 0,5 25 0,1 3 40 45
MgO 0,5 5 0,5 5 +5 +0,5 0,3 3
SO31) + 0,1 0,1 4 +3 +0,5 + 1,2
K2O + 0,3 +3,5 0,5 5 +1 0,2 1,4
Na2O + 0,1 +0,2 0,1 0,3 +0,5 + 0,3
8 Cl- - - - - 0,01- 0,1
F- - - - - 0,02-0,07
1) Total sul-
phur content
Table 4.0-1: Composition of raw materials and cement raw meal major and minor components.
5 Environmental Protection
Good primary homogenization and uniform metering of all raw materials and fuels must
be ensured so that fluctuations in kiln operation can be largely avoided. Uniform raw meal
also allows the specific energy consumption of the kiln to be kept substantiallly constant.
Large variations in the quality of the raw meal (e.g lime staturation factor) lead to higher
fuel consumption.
The raw material used for the clinker burning process mainly consists of limestone and
clay or marl, a natural mixture of the two. Corrective materials, such as sand or iron
oxide-bearing compounds, are also used. In recent years secondary raw materials that
replace natural raw materials have also been used increasingly. These materials generally
also contain sulfur combined in different forms and amounts.
The sulfur contained in these materials is predominantly combined inorganically as sulfate
(SO42-) or as sulfide (S2-).
Sulfur combined as sulfide may lead to increased SO2 emissions from the preheater as
a result of its high volatility. If the raw material deposit locally contains high sulfide
concentrations it is possible to even out the level of SO2 emissions by selective extraction
and appropriate mixing and homogenization of the raw materials. This is particularly the
case if the quarry does not contain homogeneous raw materials and a number of fractions
are extracted from the quarry and homogenized using a blending bed.
The extraction is generally planned on the basis of the CaCO3 content and, if appropriate,
on the basis of the content of other main constituents. However, knowledge of the distribu-
tion of the pyrites or marcasite content of the quarry is also necessary when dealing with
the sulfide content of the raw material mixture.
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6 Questions on Course LB 2.4 Raw Meal Homogenization
You can test your knowledge by answering the following questions.
Question 6.0 A:
1.: What is the main process used to homogenize raw meal?
2.: Name and describe an important basic element of pneumatic dry mixing.
3.: Explain the operating principle of an air distribution box.
4.: What percentage of the total surface area of the base of a mixing silo is fitted with
air distribution boxes of this type?
5.: Name some homogenization processes and explain them briefly.
6.: Name and describe the safety measures to be taken when working in silos.
7.: What is the function of a raw meal homogenizing system?
Solution see p.10
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6 Questions on Course LB 2.4 Raw Meal Homogenization
Solutions
Solutions to 6.0 A:
1.: Pneumatic dry homogenization with air distribution boxes
2.: Air distribution boxes with an air-permeable, porous cover that may be made of
ceramic plates or woven textile fabric.
3.: Air distribution boxes are located in the silo base. Air is forced from below into
the raw meal through the porous plate which produces air currents that bring the
raw meal into a fluidized state.
4.: Depending on the type of homogenization process and the design of the mixing
silo 50 to 80 % of the silo base are fitted with air distribution boxes.
5.: a) Aeration mixing process: the silo base is divided into a number of segments that
are aerated separately. One or more segments operate as mixing segments (high air
flow), while the others are only lightly aerated (fluidized). This achieves vertical
circulation of material with a good mixing effect. The segments are supplied in
10 turn with mixing air.
Fuller process: quadrants
Krupp-Polysius: 9 segments
Octant process: 8 segments
b) A tangential mixing silo has an inner cone, an annular aerating surface with a
number of extraction points and a central mixing chamber. The silo outlets are
located under the inner cone and reach toward the centre.
c) CF-Silo from FLS: the silo base is divided into 7 independent hexagons. In the
centre of each hexagon there is an outlet opening under a steel cone. The area
surrounding each outlet opening is divided into 6 segments that can be aerated in-
dependently. The mixing effect is produced by the different extraction rates from
the individual outlets.
d) Multi-cell silo:
A number of adjacent silo cells are filled one after another but are emptied concur-
rently. The raw meal is fed directly to the preheater which prevents any possible
segregation during intermediate storage.
6.: a) The interior of the silo may only be entered when the silo is empty.
b) Only suitable and reliable operators who are familiar with this type of work may
take a vehicle into the silo. The employer must give written permission for this.
c) Welding regulations must be observed
d) The welding power source must never be placed in small, enclosed spaces on
electrically conductive segments
e) Association regulations must be observed
7.: To produce uniform raw meal quality as a basic requirement for continuous trouble-
free kiln operation with low fuel consumption resulting in consistently high clinker
quality
Question see p. 9
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Glossary
aeration blending
Process in which a homogenization of powders is achieved by controlled introdution of compressed air
aeration box
Air distribution box through which compressed air is introduced into the floor of a silo for the homogenization
of a raw meal
blending bed
A blending bed is produced by placing layers of raw material successively on two or more stockpiles in a recti-
linear or circular pattern. Blending beds are used for preliminary homogenization of the crushed raw stone. This
is generally required due to the large variations in the chemical compositions of the deposits.
fluidizing air
Air with a pressure of ca. 1,75 bar for maintaining the flowability of a raw meal
free lime
CaO; free lime; the fraction of CaO that has not combined with SiO2, Al2O3 or Fe2O3 in the burning process
homogeneous
ideally mixed opposite of heterogeneous
homogenize 11
to make uniform
lime saturation factor
Specifies the ratio of lime to quartz, iron oxide and aluminium oxide.
marl
Sedimentary rock that is composed approximately half of clay and half of limestone.
mixing air
Air with a pressure of 2,0-2,2 bar for controlled homogenization of raw meal in a silo
Multi-cell silo
Complex made up of several silos which are filled consecutively and emptied simultaneously without by means
of compressed air
particle size distribution
The frequency distribution of different particle sizes. A combination of different degrees of fineness is known as
a wide particle size distribution or a wide particle range.
pressure relief cone
Cone-shaped structure in the base of a silo below which withdrawal aggregates may be installed
raw meal (raw meal)
The raw materials for Portland clinker, blended and ground to a particle size of less than 0.2 mm.
secondary raw materials
Material used to replace natural raw materials
segregation
Separation of a mixture into different fractions
silica ratio
Ratio of quartz to iron and aluminium oxides.
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Index
A
aeration blending 1
aeration box 3
B
blending bed 1
F
fluidizing air 2
free lime 7
H
homogeneous 1, 6
homogenize 1
12
L
lime saturation factor 6
M
marl 7
mixing air 2
Multi-cell silo 5
P
particle size distribution 6
pressure relief cone 4
R
raw meal 1
S
secondary raw materials 7
segregation 5
silica ratio 6
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