Process Of Manufacture Of Cement With Flowchart

PORTLAND CEMENT

Cement

          Cement is a material with adhesive and cohesive properties which make it capable of bonding minerals fragments into a compact whole.

        For constructional purposes, the meaning of the term "cement" is restricted to the bonding materials used with stones, sand, bricks, building stones, etc.

        The cements of interest in the making of concrete have the property of setting and hardening under water by virtue of a chemical reaction with it and are, therefore, called hydraulic cement.

         The name "Portland cement" given originally due to the resemblance of the color and quality of the hardened cement to Portland stone – Portland island in England.

Manufacture of Portland cement

Raw materials 

• Calcareous material – such as limestone or chalk, as a source of lime (CaO).

• Clayey material –        such as clay or shale (soft clayey stones), as a source of silica and alumina.

Methods of cement manufacturing

1. Wet process : Grinding and mixing of the raw materials in the existence of water.

2. Dry process : Grinding and mixing of the raw materials in their dry state.

The process to be chosen, depend on the nature of the used raw materials:

1. Wet process : The percentage of the moisture in the raw materials is high.

2. Dry process :

• The raw materials is so hard (solid) that they do not disintegrate by water

• Cold countries, because the water might freeze in the mixture

• Shortage of the water needed for mixing process.

1. Wet process

1. When chalk is used, it is finely broken up and dispersed in water in a washmill. The clay is also broken up and mixed with water, usually in a similar washmill. The two mixtures are now pumped so as to mix in predetermined proportions and pass through a series of screens. The resulting – cement slurry – flows into storage tanks.

2. When limestone is used, it has to be blasted, then crushed, usually in two progressively smaller crushers (initial and secondary crushers), and then fed into a ball mill with the clay dispersed in water. The resultant slurry is pumped into storage tanks. From here onwards, the process is the same regardless of the original nature of the raw materials.

3. The slurry is a liquid of creamy consistency, with water content of between 35 and 50%, and only a small fraction of material – about 2% - larger than a 90 µm (sieve No. 170).

4. The slurry mix mechanically in the storage tanks, and the sedimentation of the suspended solids being prevented by bubbling by compressed air pumped from bottom of the tanks.

5. The slurry analyze chemically to check the achievement of the required chemical composition, and if necessary changing the mix constituents to attain the required chemical composition.

6. Finally, the slurry with the desired lime content passes into the rotary kiln. This is a large, refractory-lined steel cylinder, up to 8 m in diameter, sometimes as long as 230 m, which is slightly inclined to the horizontal.

7. The slurry is fed in at the upper end while pulverized coal (oil or natural gas also might be used as a fuel) is blown in by an air blast at the lower end of the kiln, where the temperature reaches about 1450P o PC.

8. The slurry, in its movement down the kiln, encounters a progressively higher temperature. At first, the water is driven off and COR 2R is liberated; further on, the dry material undergoes a series of chemical reactions until finally, in the hottest part of the kiln, some 20 to 30% of the material becomes liquid, and lime, silica and alumina recombine. The mass then fuses into balls, 3 to 25 mm in diameter, known as clinker. The clinker drops into coolers. 

2. Dry process 

         The raw materials are crushed and fed in the correct proportions into a grinding mill, where they are dried and reduced in size to a fine powder. The dry powder, called raw meal, is then pumped to a blending silo, and final adjustment is now made in the proportions of the materials required for the manufacture of cement. To obtain a uniform mixture, the raw meal is blended in the silo, usually by means of compressed air.

      The blended meal is sieved and fed into a rotating dish called a granulator, water weighing about 12% of the meal being added at the same time. In this manner, hard pellets about 15 mm in diameter are formed.

          The pellets are baked hard in a pre-heating grate by means of hot gases from the kiln. The pellets then enter the kiln, and subsequence operations are the same as in the wet process of manufacture.

Grinding of the clinker 

    

     The cool clinker (produced by wet or dry process), which is characteristically black and hard, is interground with gypsum CaSOR 4R.2HR 2RO in order to prevent flash setting of the cement, and to facilitate the grinding process. The grinding is done in a ball mill. The cement discharged by the mill is passed through a separator, fine particles being removed to the storage silo by an air current, while the coarser particles are passed through the mill once again.

REFERENCE:

1.http://www.uotechnology.edu.iq