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8/10/2019 Manufacture of Cement
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THE CEMENT FACTORY
1. MANUFACTURE OF CEMENT
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The Cement factory
1. Manufacture of Cement
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Table of Contents
1 MANUFACTURE OF CEMENT.............................................................3
1.1 ....MORTAR , CEMENT AND CONCRETE .............................................................3
1.2 ....FROM R AW MATERIAL TO CEMENT ............................................................3
1.3 ....THE MANUFACTURING PROCESS..................................................................4
1.4 ....R AW MATERIALS FOR CEMENT MANUFACTURING .......................................5
1.5 ....R AW MIX MODULI .......................................................................................7
1.6 ....IGNITION LOSS ............................................................................................8
Table of Figures
Figure 1 From Raw Materials to Cement ................................................................4
Figure 2. The Cement Process .................................................................................5
Figure 3 Normal Limits in Raw Mix and Clinker ....................................................6
Figure 4 Raw Material Composition .......................................................................7
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1 MANUFACTURE OF CEMENT
1.1 Mortar, cement and concrete
When limestone, which is composed mainly of calcium carbonate CaCO3, is heated
to temperatures above 700-800oC it will decompose into gaseous carbon dioxide,
C02, and solid calcium oxide or burnt lime, CaO, according to the process
CaCO3 + Heat ↔ CaO + C02
Burnt lime slaked with water will harden when left under atmospheric conditions by
absorbing C02 and thereby reversing the above process. Slaked lime mixed with sand
and used, as a mortar between building bricks has been known since antiquity.
If a small percentage of clay is burnt together with the limestone a different type of
binder will develop, which will hydrate and harden due to the direct influence of
water. This is the hydraulic building material, which we know and use today under
the name of cement.
The Romans already more than 2.000 years ago knew cement making, but the
knowledge was lost with the fall of their empire. It was not until the 19th century,
starting with the British bricklayer John Aspdin's patent in 1824, that manufacturing
of cement was taken up again as an industry.
Cement was originally manufactured in vertical shaft kilns and it was not until the
invention of the rotary kiln and the tube mill that large-scale industrial exploitation
be-came possible.
Cement is mainly used in the form of concrete, i.e. cement to which is added sand or
stones as filler during casting. Concrete has high compression strength but must be
rein-forced with iron bars or similar to obtain acceptable tensile strength.
1.2 From Raw Material to Cement
Cement is manufactured from 75-80% limestone and 20-25% clay, or from rawmaterials containing the same chemical constituents. The raw materials are quarried
and crushed after which they are mixed in the correct proportions. The raw mix is
then ground in a raw mill and subsequently burned in a rotary kiln at a temperature
around 1.450oC.
During the burning process, the raw materials will undergo a number of very
complex chemical reactions and will eventually leave the kiln as cement clinker, i.e.
agglomerates of clinker minerals. The main chemical reactions in the rotary kiln will
be dealt with in the chapter "Clinker burning".
The final product - cement - is obtained by grinding the clinker to a fine powder, in acement mill together with some 3-4% of gypsum. The gypsum is a necessary
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1. Manufacture of Cement
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additive in order to retard the setting time of cement. Without the addition of
gypsum, the cement would harden immediately with the addition of water.
Figure 1 From Raw Materials to Cement
1.3 The manufacturing process
The original manufacturing method to produce cement was based on the so-called wet process. In this process the raw mix is ground with addition of up to 40% water
and the finished-ground raw mix leaves the mill and is fed to the mill as slurry. The
process is characterised by simple and uncomplicated installations. Also the
operation of raw mill and the kiln is fairly straightforward. Furthermore, it is easy to
mix - homogenise – the slurry and, finally, the wet process does only produce limited
amounts of dust and was therefore well suited for the early, primitive, de-dusting
facilities.
The energy crisis in the 1970's accelerated the development of the dry process, which
today is used in almost all modern cement plants. The obvious advantage of the dry
process is the fuel saving in the kiln, since there is no water to evaporate from the
kiln feed. Whereas the energy consumption per kilo clinker using the wet process is
in the range of 5,2-5,3 MJ (approx. 1.250 kcal), the dry process energy consumption
is no more than 3 MJ (approx. 700 kcal).
The dry process kiln is considerably shorter than a wet kiln, but both the kiln and
the raw grinding plants are more complicated both in installation and operation.
Furthermore, compared to the fluid slurry, it is much more complicated to
homogenise dry raw meal and the kiln requires highly efficient de-dusting.
Today, all new plants are based on the dry process and many old wet plants are either replaced or, if suitable, converted to dry or semi-dry production.
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Figure 1. The Cement Process
1.4 Raw materials for cement manufacturing
Cement is made primarily from burnt lime, CaO, and oxides of silica, SiO 2, alumina,
Al203 and iron, Fe203.
The raw mix compounds undergo a chemical change during their passage of the high
temperature kiln and are thereby transformed into clinker minerals.
The main clinker mineral, CaO, is not directly available in nature, but is obtained
from limestone or chalk as CaCO3, which decomposes during the calcining phase in
the rotary kiln.
The oxides of silica, alumina and iron are naturally present in clay, sand shale and
marl. If one of the oxides, most often the iron oxide, occurs in insufficient quantity in
the clay, a corrective material must be added to the raw mix.
Fly ash precipitated in dust filters of coal fired power stations is finding increasing
application in the manufacture of cement. Fly ash consists of iron- and alumina-rich
silicates and may to a certain extent substitute the clay component in the raw mix. Its
chemical composition is usually within 35-55% SiO2, 20-30% Al203 and 3-30%
Fe203. Besides, elements like free CaO, MgO, S03 and alkalis K 20 and Na20 may also
occur.
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mix composition. Approximate compositions of typical cement raw materials are
shown in Figure 1.4.
RAW MATERIAL % CaCO3 % Sio2 % Al2O3 % Fe2O3
Rich Limestone 100-96
Lean Limestone 96-90
Limey Marl 90-75
Marl 75-40
0-30 0-10 0-5
Clayey Marl 10-4
Clay 4-0
Fly Ash 2-10
35-70 10-25 5-10
Bauxite 10-16
Iron Ore 45-60
Bl. Furn. Slag 50-70
Pyrite 60-90Figure 2 Raw Material Composition
1.5 Raw mix moduli
Some of the clinker minerals primarily give strength to the cement while others act
as a necessary matrix for the strength-carrying minerals. It is important for the
quality of the cement as well as for the fuel economy and the lifetime of the lining in
the kiln that the proportions between the compounds of the raw mix are correct. This
composition compatibility is governed by some so-called moduli.
Correct formation of the matrix is mainly governed by the relationship between the
silicates and the alumina-ferro oxides - the silica modulus or silica ratio, MS - and by
the relationship between the oxides of aluminas and iron - the alumina modulus or
alumina ratio, MA. We have thus :
( )5,38,13232
2−
+
= NormallyOFeO Al
SiO MS
( )5,35,132
32−= Normally
OFe
O Al MA
The main reaction during the burning phase is the fusion between the basic CaO and
the acid oxides and it is essential that there should be sufficient CaO in order to
combine completely with these. On the other hand, a surplus of CaO must be avoided
since it is harmful to the cement.
The relationship between the total CaO and the total of acid oxides to which it is able
to bind, is the most important single parameter for the cement. The relationship iscalled the Lime Saturation factor and is expressed as
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[ ]%10065,08,18,2 32322
×
×+×+×
=
OFeO AlSiO
CaO LSF
The LSF value is normally in the range of 90 - 100%.
Sometimes the hydraulic modulus MH, expressing a direct relationship between the
ha-sic oxide CaO and the total of the acid oxides, is encountered. The MH is
expressed as
32322 OFeO AlSiO
CaO MH
++
=
The MH is normally in the range of 2,0 - 2,2. The MH modulus has today been
replaced by the LSF, but may still be encountered.
The relationship between the different moduli, clinker quality and burning economy
will be clarified in the chapter covering clinker burning.
1.6 Ignition loss
The raw materials undergo a reduction in weight during burning, partly from loss of
C02 during the calcination of CaCO3 in the limestone and partly from evaporation of
inherent water in the clay. This reduction must be taken into account whencalculating the total consumption of raw materials.
The loss can be calculated from the atomic weights, since this weight remains
unchanged during a chemical reaction. The approximate atomic weights are
Calcium, Ca 40
Carbon, C 12
Oxygen, 0 16
from which the following is derived :
Ca + C + O3 = Ca + O + C + O2
40 + 12 + 48 = 40 + 16 + 12 + 32
and thus
100 parts of CaCO3 = 56 parts of CaO + 44 parts of C02
The clay component loses normally approx. 7% inherent water during burning.
Therefore I kg of a raw mix consisting of 76% CaCO3 and 24% clay totals anignition loss as follows :
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from the CaCO3 : 0,76 × 0,44 = 0,3344 kg C02
from the clay : 0,24 × 0,07 = 0,0168 kg H20
ignition loss : 0,3512 kg total
In other words, 1 kg of raw mix yields a clinker production of (1,0000 - 0,3512) =
0,6488 kg or, alternatively, the manufacture of 1,0 kg clinker will require 1,54 kg of
raw mix.
The ignition loss for cement raw materials is normally around 0,35 to which must be
added a certain amount of dust loss. For first estimates it is common to calculate with
a necessary quantity of 1,6 kg of raw mix for each kg clinker produced. Thus
C R PP ×= 6,1