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Saturday, 17 December 2016

CEMENT MANUFACTURING PROCESS: BURNING TECHNOLOGY

We were discussing “Cements and its types” and “Material preparation technology in cement manufacturing” in our previous posts. We have also see the concept of cement manufacturing process and its selection in our recent post. 

Today we are going to start here one very important topic i.e. burning technologies in cement manufacturing process. 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 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.

Cement burning technology

The kiln of the pyro - processing plant is the heart of the cement plant. Cement manufacturing processes, termed according to the condition of the feed entering the kiln, are Wet, Dry, Semi - wet and Semi - dry.

The rotary kiln used in the cement burning may be classified into various types as shown below 

Type of kiln
Dimension (m)
Heat consumption
kiln load

length
dia.
kcal / kg
t / m3
Long wet process
80 - 230
3 - 7.6
1200 - 1450
0.4 - 0.8
Long dry process
70 - 230
3 - 7.0
850 - 950
0.6 - 1.0
Short dry process (with suspension- preheater + precalciner)
40 - 110
3 - 6.0
720 - 900
1.4 - 2.2

Wet process

Wet processes require long rotary kiln (L / D ~ 40) and degree of kiln filling not more than 17 %. Practically, it should be ~ 13 % as a higher value may impair heat transfer. 

Slurry dewatering may be done mechanically by using proper filters (drum disc filter or filter press) or chemically using thinners. Thinners are ions and molecules which are adsorbed on the surface of raw mix particles preventing their agglomeration, thus, reducing internal friction and increasing flowability of slurry. Chain systems installed inside the kiln bring about dewatering of slurry. There are two types of systems - Garland chain and Chain curtain. 

Two other installations in the kiln are slurry preheater located before the chain section and slurry dryer located behind the chain section to remove the remaining water and preheat the dried raw mix.

Dry Process   

The dry process kiln can be of the following types:
1. Long dry kiln with internal /external heat exchanger
2. Kiln with suspension preheater
3. Kiln with suspension preheater and precalciner

Long dry kiln is easy to operate as it is not very sensitive to upset conditions as a result of high chloride or alkali content in the raw mix. However, it is not very economical from the point of view of fuel economy. 

The heat liberated in the rotary kiln can, besides being used for the actual processing of the feed material, advantageously be utilized for preheating the material. to promote this, the inlet zone of the kiln may be equipped with suspended chains made of heat resistant steel or ceramics which assist heat exchange. All such devices aim to provide a large contact surface area between the hot gases and the kiln feed material in order to promote heat exchange.

Devices external to the kiln and installed at the feed end, where hot exit gases flow through them, preheat the feed material and, if necessary, dry it. With the dry process, the drying operation is unimportant; what is important is the preheating attainable in suitable devices.

Cyclone Preheater

In cyclone preheater system, an efficient heat transfer takes place between the finely dispersed raw material powder falling downwards and the hot gases flowing upwards. With this preheater it has been able to obtain exit gas temperature of about 320 - 350 C. 

Due to efficient heat transfer between raw mix and hot gases, it is possible to achieve about 25 - 30 % calcination of raw mix in the preheater before entry into the kiln. Thermal efficiency of 800 - 900 kcal /kg of clinker are possible with this system. 

 Now days, the kiln plays use low pressure drop cyclones in the preheater system. The gain in pressure drop enables to design the system with increased number of cyclone stages. Cyclone preheaters with four or five stages are used in the cement plants.

Preheater Bypass

The volatilized alkalis in the kiln (at 1470 F) condense on the colder kiln feed (stages 3 and 4). The condensed alkali arrives with the preheater raw mix and volatilize again in the high temperature zone of the kiln. This causes the internal alkali circuit.

Hence a part of the gases is diverted through the bypass valve to lower the alkali content in the clinker. This, however, has the following disadvantages:

Increased investment cost.
Negative influence on kiln’s heat economy due to increased heat consumption

Shaft Preheater

In this system, heat exchange is accomplished by the raw mix descending down a vertical shaft against the counter current hot gases. Different designs of shaft preheaters are available with thermal efficiency ranging from 850 - 950 kcal / kg. The raw material is calcined to about 25 ~ 30 % the shaft preheater has found its operation abroad with smaller cement units producing 1600 tpd or lower, because of its self - supporting construction and hence, lower civil costs.


We will see in our next post “Pre calciner in cement plant “with continuation of firing technology

Cooling of the shell


Water cooling of the burning zone of the shell has a protective influence on the lifetime of the refractory; it extends the life by 1.5 - 2.0 times and facilitates the formation of a protective coating on the refractory.


If proper cooling of the shell is not provided for, a thin layer of low viscosity liquid clinker phase is formed which fails to provide the protective coating.



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We will see other topic i.e. “Basic properties of cement compounds ” 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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