Question

1. Name two supplementary materials and explain where they come from .

2. Discuss the differences , advantages and disadvantages of the wet and dry process of cement production.

3. List the main thermal zones in a cement kiln. Briefly describe the processes in each zone.

Answer 1

1. Fly Ash and Ground Granulated Blast Furnace Slag are 2 supplementary materials.

They are used to improve cement/concrete properties like workability, durability and strength.

a) Fly Ash: It is non combustable particulates removed from the flue gases which is formed due to burning of the coal in coal fired furnaces at power generation plants.

b) Ground Granulated Blast Furnace Slag: It is obtained as a byproduct from blast furnace where it on ore is reduced to pig iron. This slag is rapidly cooled using water and ground to fines.

2. Difference between wet and dry process

In dry process, the calcareous materials (lime stone) and argillaceous materials (clay) are separately ground in ball mills or jaw crushers and then stored in silos. Later, they are mixed in proportions and pass through rotary kiln to form clinker and then ground with small addition of gypsum to form cement.

In wet process, calcareous materials are crushed and stored in silos. Argillaceous materials are washed and stored in basins. Then, this limestone and wet clay are ground in ball mill/tube mill to form slurryslurry and stored. Later, passed through rotary kiln to form clinkers(lumbs) and ground with gypsum to form cement.

Dry process

Advantage	Disadvantage
Small kiln is suitable	Difficult to control mixing of raw material,
Low fluel consumption(100kg/coal/tonne of cemetery)	Low quality product is obtained
Low cost of production	High capital cost due to use of blenders

Wet process

Advantage	Disadvantage
Homogenous mix is easily obtained as it is easy to control mixing process	Large kiln needed
High quality product is obtained	High fuel consumption (350kg/ton of cement)
Low capital cost	High cost of production

3. Thermal zones in cement kiln:

• Calcining zone
• Upper transition zone/Safety zone
• Sintering zone
• Lower transition zone/Cooling zonezone

a) Calcining zone: Near inlet. Dehydration takes place in the case of wet process. Calcination/decarbobation process produces free limelime (CaO) from lime stone. C₂S starts to form.

b) Upper transition zone: Transition between calcining and sintering phase. Fines particles undergo complete calcination here and start to react with alumina, silica or iron to form clinker compounds.

c) Sintering zone: C₃​_​A, C₄AF clinker compounds are formed. C_{​​​​​​2}S combines with lime(CaO) to form C₃S. High temperature zone > 1400⁰C with liquid clinkers.

d) Cooling zone: This zone affect quality of clinker as crystallization of clinkers occur. For rapid cooling, this zone is close to outlet.