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PYROPROCESSING IN CEMENT INDUSTRY

We were discussing “Cements and its types” and " Raw materials for manufacturing of cements” in our previous posts. Today we are going to start here one very important topic i.e. Pyroprocessing in cement industry. We will understand the various terminologies and engineering concepts used in cement technology with the help of this category.

Further we will go for  discussion of chemistry of cement, hydration of cement, properties of cement compounds, selection of the process, process technology, burning technology, firing technology, clinker cooling and grinding, cement packing and dispatch, pollution control and much more facts about cement technology in our next post in this category of cement technology.

Chemical, physical and mineralogical aspects of cement burning

Cement clinker is produced by heating the raw material to a temperature of about 14500C, so that clinkering occurs. The process requires an oxidation atmosphere in the kilns producing greyish-green clinker. The important chemical-physical processes which occur are as mentioned here

Drying

The water that is present as ‘free’ moisture in the raw-meal, or has been added to it, is driven out at temperature ranging up to 2000C.

Dehydration

Between 100 - 4000C, the clay minerals give off their absorptively bound water. At higher temperature, around 400 – 7500C, the chemically combined water is also expelled.

            Al4 [(OH) 8 Si4O10] ----------> 2 (Al2O3. 2SiO2) + 4 H2O
 The dehydration is affected by the type of clay mineral, admixtures, particle size etc.

Decomposition 0f carbonates

CaCO3 is decomposed at a temperature of about 8960C and onwards.
            CaCO3 ---------> Ca O + CO2
This temperature value relates to pure calcite. In actual practice, the reaction begins between 550 -6000C. The thermal dissociation of MgCO3, of much less importance, begins at a lower temperature.

Solid reactions

The solid reactions result in the formation of C2S, C3A, C4AF, which start at around 8000C. These proceed very slowly, but can be speeded up by reduction of particle size, raising burning temperature, crystal lattice distortions etc.

Reactions in the presence of liquid phase 

Clinkering or sintering occurs at a temperature of about 12800C. With further rise in temp., the proportion of liquid phase increases to 20 - 30 % at around 14500C. At these temperatures, the main component of Portland cement clinker is formed - C3S

Reactions during cooling

Rapid cooling prevents loss of C3S which is important for strength development. It also yields better grindability of clinker, slower setting of cement because of inter grown finely crystalline aluminate and ferrite phases and better soundness.

Kiln temperature profile

Zone
Temperature range
Reaction sequence
I
Up to 2000C
Evaporation (slurry drying)
II
200 - 7500C
Preheating (dehydration and de-hydroxylation)
III
800 - 11000C
Decarbonisation (calcinations)

Around 8000C
Formation of CaO.Al2O3, 2CaO.Fe2O3 and              2CaO.SiO2 starts.

800 - 9000C
Formation of 12CaO.7Al2O3 starts.

900 - 11000C
2CaO.Al2O3.SiO2 forms and decomposes again
 Formation of C3S and C4AF starts and all CaCO3 decomposes
IV
1100 – 13000C
Solid phase reactions
Formation starts of major part of C3A and C4AF. Content of C2S reaches maximum. First liquid formation starts
V
1300 - 14500C
Sintering
Formation of C3S with progressive disappearance of free lime
VI
1300 – 10000C
Cooling

This table indicates broadly the mode of cement compound formation. Overlapping of some of the stages may occur and any non homogeneity in the raw meal will tend to increase it.

Clinker characteristics

 The clinker characteristics that are significant in achieving a quality product are as mentioned here
  1. Appropriate bulk chemical composition
  2. Formation of hydraulically active phases - polymorphic forms of alite and belite
  3. Optimum proportion of different phases - alite --55 - 65 %
Aluminate phase --10 - 11 %
Ferrite phase -- 12 %,  
Belite -- rest
  1. Optimum grain growth ~ 30 u
  2. Proper micro structural development - prismatic / tabular crystals of alite, round of belite
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We will see other topic i.e. chemistry of cement manufacturing  in our next post in the category of Steel and cement technology.

I am very thankful to Mr. Subrata Bhaumik, Independent cement consultant, for providing such beautiful information and contents about cement technology.

Mr. Subrata Bhaumik has more than 50 (Fifty) Years (1965 - 2016) of Experience in Cement and other related Industry covering more than 100 assignments in cement plants with capacities ranging from 100 tpd to 10,000 tpd in India and abroad involving visit to 25 countries overseas in connection with work.

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