Cooling rate of clinker as described in different cement books and guidelines for
better quality of clinker production.
per minute
Fast cooling of clinker after burning at 1450oC is very important as the phases
produce can be preserved and quality is maintained. The direct benefits are
The crystal sizes are smaller, which will make easier for the clinker
grinding.
The C3S mineral is bigger when its formation reaction gets frozen more
effectively.
More MgO is present in clinker phases, It is verified that as a result of a
fast cooling, the MgO solidifies in glassy state in very small sizes (<5
microns) which not generate expansion in cement. Otherwise, by slow
cooling, will acquired bigger sizes that will affect expansion (Pericles
formation)
The C3A is less reactive to the setting time, decreasing the quick setting
or false setting.
If the clinker is sieved in 1.0 mm screen, the %age of passing will give the
classification of clinker quality as tabulated.
The clinker that passes through 1 mm in the order of 15 to 20% is
called Optimum clinker.
It has been seen that the clinker phases, as well as the melt formed during
sintering, get effected by the rate of Cooling. It is reported that the best clinker
is obtained by cooling slowly to 1250 degree centigrade followed by rapid
cooling. The effects of the cooling rates on the clinker phases and their
properties are summarized above and demand for rapid cooling (18 – 20oC) has
been suggested.
Effect of different parameters on slow and fast cooling is given below, slow
cooling 4 to 5 degree per minute and fast cooling is 18 to 20 degree per
minut
The Kiln System
Reactions at Different Stages
1. Temperature of 900 to 1160 degree C :
C2AS and C12A7 formation started.
At 1160 degree C begins the massive C4AF formation and therefore the liquid
phase.
Under fluoride presence , the C2S becomes stable at 1160 degree C.
At 1160 degree Centigrade, C3A + C12A7 is formed.
1. At 1200 to 1338 degree centigrade
All the C2S has been formed and the C2AS and quartz have dis appeared , and
C3S begins.
At 1250 degree centigrade C12A7 has become C3A and the C3S becomes
stable.
At 1338 degree centigrade the C3A and C4AF have been melted. Acquiring an
eutectic balance,
β-C2S -> α- C2S
when melting C3A and C4AF begins appearing C3S.
at 1338 degree centigrade Free CaO + C3A + C4AF + α-C2S + Some of C3S
At 1450 degree centigrade the sintering happens, almost all the liquid phases
has been formed, at 1450 degree centigrade it is completed always at the
nodulization zone.
Main reaction in the sintering is
Clinkering C2S + Free CaO -> C3S
Cooling : The freezing reaction is constituted keeping the C3S and C2S solid
phases contained towards the crystallization of the melted C3A and C4AF phases,
being critic reaching the 1170 degree centigrade since at this temperature the
C3S and C2S decomposition is maximum.
Between 1200 to 1450 OC
Between 1260 to 1310 OC, first formation of melt takes place. In the presence of liquid melt,
formation of C3S takes place by liquid phase sintering. Much of C2S and nearly all the lime
react in the presence of the sintering liquid to form C3S,
CaO + 2CaO.SiO2 → 3CaO.SiO2 (C3S).
The material nodulises to form clinker
During cooling –
The liquid crystallizes giving mainly aluminate and ferrite. Polymorphic transformations of the
alite and belite occur.
Thus, during the process of cement manufacture, the following operations take place at the
temperatures indicated;
1. Slurry preheat and evaporation of free and chemically combined moisture– 100 -
550OC,
2. Preheat and evolution of CO2 from the carbonates (calcinations) – 805 OC,
3. Formation of phases and clinkerisation– 800 to 1400 OC,
4. Clinker discharge and cooling.
Derivation of the LSF (Lime Saturation Factor)
LSF (Lime Saturation Factor) is the theoretical point in the C-S-A-F system
where there is just enough CaO present to react completely to form C3S from
C2S at 1450oC under equilibrium conditions. At this point the C2S content of
the clinker would theoretically be 0.
Therefore, one can only expect C3S, C3A and C4AF to exist on the C-S-A-F
Quaternary phase diagram. While this is certainly true for C3S and C4AF, it turns
out that a significant amount of SiO2 is incorporated into the C3A at this point
and its actual composition is closer to C3.31 AS0.39 (i.e. 3.31 moles of CaO, one
mole of Al2O3 and 0.39 moles of SiO2)
Therefore, assuming that the minerals existing at the point of lime saturation
are C3S, C4AF and C3.31 AS0.39 ,one can the say that the sum of the oxides, SiO2,
Fe2O3 and Al2O3in these minerals, expressed as a molar fraction of the CaO, will
be equal to unity.
Now, given that the minerals existing at the point of lime saturation are C3S,
C4AF and C3.31 AS0.39 , one can calculate the molar weight ratios of SiO2/CaO,
Fe2O3/CaO and Al2O3/CaO in these minerals.
For C3S;
SiO2/CaO = (60/(3 x 56)) = 0.357
Al2O3/CaO = (0/(3 x 56)) = 0.000
Fe2O3/CaO = (0/(3 x 56)) = 0.000
For C4AF;
SiO2/CaO = (0/(4 x 56)) = 0.000
Al2O3/CaO = (102/(4 x 56)) = 0.455
Fe2O3/CaO = (160/(4 x 56)) = 0.714
For C3.31 AS0.39;
SiO2/CaO = (0.39 x 60)/(3.31 x 56)) = 0.126
Al2O3/CaO =(102 /(3.31 x 56)) = 0.549
Fe2O3/CaO = (0/(3.31 x 56)) = 0.000
Let the relative multipliers for SiO2, Al2O3 and Fe2O3 required for lime saturation
be “a”, “b”and “c”respectively. Now, using the molar weight ratio, one can write
three equations (one for each mineral) which are all true at the lime saturation
point.
The three equations take the form;-
[a x (SiO2/CaO)] + [b x (Al2O3/CaO)] + [c x (Fe2O3/CaO)] = 1
For C3S (a x 0.357) + (b x 0) + (c x 0) = 1
For C4AF (a x 0) + (b x 0.455) + (c x 0.714) = 1
For C3.31 AS0.39 (a x 0.126) + (b x 0.549) + (c x 0) = 1
Solving the equations
a = 2.8 ( Which is the LSF coefficient for SiO2)
b = 1.18 (Which is the LSF coefficient for Al2O3)
c = 0.65 (Which is the LSF coefficient for Fe2O3)
(2.8 x SiO2/CaO) + (1.18 x Al2O3/CaO) + (0.65 x Fe2O3 /CaO) = 1
or, 2.8 x SiO2 + 1.18 x Al2O3 + 0.65 x Fe2O3 = CaO
or, CaO/(2.8 x SiO2 + 1.18 x Al2O3 + 0.65 x Fe2O3) = 1
Which is the derivation for LSF(Lime Saturation Factor).
Raw mix design
RAW MIX DESIGN
(BASED ON REQUIRED PHASE VALUE IN CLINKER)
The requirement of quality clinker with known phase values is most for
producing quality cement. This raw mix design is prepared taking the required
phase value of clinker into consideration.
STEP-1
Calculate the molecular weight of different oxides & phases
CaO = 40 + 16 = 56
SiO2 = 28 + (2 x 16) = 60
Al2O3 = (2 x 27) + (3 x 16) = 102
Fe2O3 = (2 x 56) + (3 x 16) = 160
[C3S] = 3 x CaO+SiO2 = (3 x 56) + 60 = 228
[C2S] = 2 x CaO+SiO2 = (2 x 56) + 60 = 172
[C3A] = 3 x CaO+Al2O3 = (3 x 56) + 102 = 270
[C4AF] = 4 x CaO+Al2O3+Fe2O3 = (4 x 56) + 102 + 160 = 486
STEP-2
Requirement of %age of Phases in Clinker
It should be ensured that the total should be 100.
C3S = 50%
C2S = 24%
C3A = 7%
C4AF = 11%
MgO = 4%
Free CaO = 2%
Others = 2%
Total = 50% +24% +7% +11% +4% +2% +2% = 100%
STEP-3
Clinker Composition required
The clinker composition required for achieving the target phase values is to be
calculated
CaO required =>
+2
=50 x (3 x 56/228) + 24 x (2 x 56/172) + 7 x (3 x 56/270) + 11 x (4 x 56/486)
+2 = 63.87 %
SiO2 required =>
= 50 x (60/228) + 24 x (60/172) = 21.52 %
Al2O3 required =>
= 7 x (102/270) + 11 x (102/486) = 4.95 %
Fe2O3 required =>
= 11 x (160/486) = 3.62 %
The ultimate clinker analysis is as follow
SiO2 = 21.52%
Al2O3 = 4.95%
Fe2O3 = 3.62%
CaO = 63.87%
MgO = 4%
F/CaO = 2%
Others = 2%
STEP-5
Let us Calculate the different module values,
Lime Saturation Factor (LSF) =>
=93.17 (Okay)
Silica Modulus =>
= 2.51(Okay)
Alumina Modulus =>
= 1.36(Okay)
Liquid content at 1450 degree centigrade =>
(3.0 x Al2O3) + (2.25 x Fe2O3)+ MgO + (Na2O + K2O)
= 27.5 %(Okay)
STEP-4
Consideration of coal ash mixing in clinker,the quality of the coal is taken into
consideration.
Coal NCV used for clinker production = 4000 Kcal/kg of coal
Ash of coal used = 35%
Heat required for producing 1 kg of clinker = 900 Kcal/kg of clinker
So percentage of coal used = 900 x 100/4000 = 22.5%
Ash absorption by clinker say it is 100%
Coal ash absorption = 22.5 x 35 /100 = 7.88%
STEP-5
Coal ash analysis is
SiO2 = 60%
Al2O3= 25%
Fe2O3 = 12%
CaO = 1%
MgO = 1%
Others= 1%
Oxides contribution from coal ash to Clinker
SiO2 = (7.88 x 60)/100 = 4.73%
Al2O3 = (7.88 x 25)/100 = 1.97%
Fe2O3 = (7.88 x 12)/100 = 0.96%
CaO = (7.88 x 1.0)/100 = 0.08%
MgO = (7.88 x 1.0)/100 = 0.08%
STEP-6
Now, the oxides %age in loss free basis of clinker can be calculated as
CaO = (63.87 – 0.08) x 100 /(100 – 7.88) = 69.20%
SiO2 = (21.52 – 4.23) x 100 /(100 – 7.88) = 18.80%
Al2O3 = (4.95 – 1.97) x 100 /(100 – 7.88) = 3.20%
Fe2O3 = (3.62 – 0.96) x 100 /(100 – 7.88) = 2.90%
STEP-7
From raw material analysis we have to get these values by using different
proportion of the raw materials. We have to go for any three oxides, forth value
will come as default value.
Either SiO2, CaO and Fe2O3 or SiO2, CaO and Al2O3
Case-1 — If Limestone, Clay and Laterite as Raw material than SiO2, CaO and
Fe2O3will be considered for balancing and Al2O3 calculated as default.
Case -2 –If Lime stone, Clay and Bauxite as Raw material than SiO2, CaO and
Al2O3will be considered for balancing and Fe2O3 calculated as default.
Consider the case -1 in this paper for calculation purpose.
STEP-7
Conversion of raw material analysis from as received to dry basis for calculation.
Raw material analysis as received basis.
Find the Raw material analysis in Loss Free Basis
The analysis on loss free basis of Raw Material is as follow Calculated from given
below formula.
Oxide loss free% = Oxide%/(100 – %LOI)
STEP-8
Calculation of %age of raw material to achieving the target analysis results.
Required oxides from Raw materials: = CaO =69.2, SiO2 =18.8, Fe2O3 = 2.9
% age Limestone used = “A”
% age Clay used = “B”
% age Laterite used = “C”
CaO balance
(80.8 x A) + (1.06 x B) + (1.05 x C) = 69.2 ——–(a)
SiO2 balance
(9.66 x A) + (76.43 x B) + (37.89 x C) = 18.8 ——-(b)
Fe2O3 balance
(1.33 x A) + (8.49 x B) + (36.84 x C) = 2.9 ——-(c)
Solving The equation a , b and c , we will get
A = 85.5 %, B = 13 % and C = 1.5 %
We have to prepare a raw-mix with 85.5% Lime stone, 13 % Clay and 1.5%
Laterite to achieve desired values of phases in Clinker.
