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)₈Si₄O10] ----------> 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

Appropriate bulk chemical composition
Formation of hydraulically active phases - polymorphic forms of alite and belite
Optimum proportion of different phases - alite --55 - 65 %

Aluminate phase --10 - 11 %

Ferrite phase -- 12 %,

Belite -- rest

Optimum grain growth ~ 30 u
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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