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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. grinding and drying of coal in cement industry. We will understand the various terminologies and engineering concepts used in cement technology with the help of this category. Before reading this post, please find link “Raw meal homogenization and storage in cement industries”. 

Further we will go for discussion of properties of 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.

Grinding and Drying of Coal

The preparation of coal in cement works comprises the grinding and drying of raw coal delivered to the works. In cases where the coal consumption rates are high and coal from different sources are used, it is advantageous to blend the various coals in conjunction with stockpiling so as to obtain a fuel that is physically and chemically as closely uniform as possible and thus, to achieve well - balanced kiln operating conditions.

The raw coal has moisture content between 4 - 12 % by wt. and it is dried, in conjunction with grinding, to a residual moisture content of 0.5 - 1.5 % which is suitable for firing.
The fineness to which coal should be ground depends on its flammability and combustion rates which are in turn, governed by the ash and volatile matter content.

The drying and grinding of coal in cement industries is done by any one of the two systems:

-          Drying and grinding by air swept ball mill
-          Drying and grinding by vertical roller mill

The choice of the coal grinding system depends on various aspects, like, size of coal lump, ash content, grindability, volatile components, abrasiveness, possibility of explosion, moisture content etc.

The advantages / disadvantages of ball mill are

-          Greater safety of operation when using unknown coal
-          Simplicity of operation
-          Lesser maintenance cost
-          Higher investment cost
-          Higher energy cost

The advantages / disadvantages of vertical roller mill are

-          Lesser investment cost.
-          Lesser retention time in mill circuits
-          Lesser energy requirements
-          More suitable for coal from different sources having different hardness.
-          Coal with high moisture content can be dried during grinding.
-          Difficulty in replacement of wear parts
-          Danger of dust deposition below the grinding table
-          Fluctuating output fineness due to short term feed fluctuations.

Storage of Coal:

Coal has the property of absorbing oxygen from air. This is associated with heat evolution. If heat is not given off at a sufficiently rapid rate, self - ignition may occur over a prolonged period of storage. During this period, the temperature gradually rises to above the critical value of 70 - 80 C.
The danger is especially great in fresh coal from the mines and in coal that has been crushed so that substantial increase in reactive area has occurred.

For long term storage, coal should be deposited in layers which are each well compacted with aid of rollers, so as to minimize the entry of atmospheric oxygen to the interior of the pile. Alternatively, the coal should be deposited in thin layers and as loosely as possible so that heat evolved by oxidation can be quickly dissipated.

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We will see other topic i.e. "Burning technologies in cement manufacturing process" and “Safety requirement in manufacturing process of cement” 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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