The entire manufacture of portland cement process in a modern plant is now control through a microprocessor base programmable logic control system to maintain a consistently uniform quality of cement and a high rate of production. The entire operation of the plant is control centrally in a single control room and the plant employs minimum of manpower as compare to previous plants construct prior to 1980.
Following three distinct operations are involve in the manufacture of normal setting or ordinary or Portland cement :
- Mixing of raw materials
Mixing of raw materials
The raw materials such as limestone or chalk and shale or clay may be mix either in dry condition or in wet condition. The process is accordingly refer as the dry process or the wet process of mixing.
Dry process ( modern technology )
In this process, the raw materials are first reduce in size of about 25 mm in crushers. A current of dry air is then pass over these dry materials. These dry materials are then pulverise into fine powder ball mills and tube mills. All these operations are do separately for each raw material and they are store in hoppers. They are then mix in correct proportions and make ready for the feed of rotary kiln. This finely ground powder of raw materials is refer as the raw mix and it is store in storage tank. The flow diagram of mixing of raw materials by dry process.
Wet process ( old technology )
In the earlier part of the century i.e., from 1913 to 1960, the wet process was used for the manufacture of cement. From 1913 onwards, the cement industry underwent a number of changes mainly to suit the requirements of the manufactures and the govt. policies till early 1982.
All the cement plants set up after 1980 use the dry process for the manufactures of cement. But the method of preparing the wet process and flow diagram of mixing of raw materials are given below :
The burning is carry out in a rotary kiln. A rotary kiln is form of steel tubes. Its diameter varies from 2.50 m to 3 m. Its length varies from 90 m to 120 m. It is lay at a gradient of about 1 in 25 to 1 in 30. The kiln is support at intervals by columns of masonry or concrete. The refractory lining is provide on the inside surface of rotary kiln. It is so arrange that the kiln rotates at about one to three revolutions per minute about its longitudinal axis.
The clinkers as obtain from the rotary kiln are finely ground in ball mills and tube mills. During grinding, a small quantity, about 3 to 4 per cent, of gypsum is add.
The gypsum controls the initial setting time of cement. If gypsum is not add, the cement would set as soon as water is add. The gypsum acts as a retarder and it delays the setting action of cement. It thus permits cement to be mix with the aggregates and to be place in position.
The grinding of clinkers in modern plants is carry out in the cement mill which contains chromium steel balls of various sizes. These balls roll within the mill and grind the mixture which is collect in a hopper and taken in the bucket elevator for storage in silos.
The cement from silos is fed to the packer machines. Most of the modern plants have electric packing plant having provision to account for the weights of empty bags of different types and to ensure a 50 kg net weight of cement bag within +- 200 g limit. Each bag contains 50 kg or 500 N or about 0.035 m3 of cement.
These bags are automatically discharge from the packer to the conveyor belts to different loading area. They are carefully store in a dry place. The flow diagram of burning and grinding operations.
Ball Mills and Tube Mills
These mills are use to carry out grinding of raw materials or mixture of raw materials or clinkers. The ball mills are use to have preliminary grinding and the tube mills are use to carry out final grinding.
The vertical section of a typical ball mill. It is in the form of steel cylinder of diameter about 2 m to 2.50 m and of length about 1.80 m to 2 m.
The cylinder is place in a horizontal position and it rotates around a steel shaft. On the inside of cylinder, the perforate curve plates are fix. The ends of these plates overlap each other. The cylinder is fill partly with steel balls of size varying from 50 mm to 120 mm.
The action of ball mill is very simple. The material to be ground is fed from the top. When the mill is rotate about its horizontal axis, the steel balls strike against the perforate curve plates and in doing so, they crush the materials. This crush material passes through an inner sieve plate and then through an outer sieve plate. It is collect from an outlet at the bottom of outer casing of mill.
It is in the form of a long horizontal steel cylinder of diameter about 1.50 m and of length about 7 m to 10 m. The cylinder is fill partly with steel balls of size varying from 20 mm to 25 mm.
The action of tube mill is similar to that of ball mill. But fine grinding is achieve due to steel balls of smaller size. A worm is provide to feed the material to the mill. The pulverised material is collect at the outlet funnel. In case of large scale production, the air separators may be employ to separate finely ground particles. In this arrangements, the current of air is use to carry away the finely pulverise particles.
To combine preliminary and final grinding, the compartment mill or multiple chamber mill may be adopt. Such a mill has different chambers or sections in which steel balls of different sizes are place. The material to be ground is allow to pass through chambers in succession. The chambers with steel balls of bigger size are place first and they are follow by chambers having steel balls of smaller size.
It is thus seen that a compartment mill combines the actions of ball mill and tube mill. It results in saving of floor space and it simplifies the grinding process. The cost of grinding also works out to be less by the installation of such a mill.