Portland cement can be manufactured by intimately mixing together calcareous and argillaceous stones containing silica, alumina, and iron oxide etc. and burning it in large rotary kilns at a temperature of approximately 1400°C to 1500°C. At his temperature the materials partially fuse, forming clinker. This clinker is cooled and ground to fine power. Usually it is gray or brownish in colour.
The manufacture of Portland cement consists of the following three distinct processes: 1. Mixing 2. Burning 3. Grinding.
Process # 1. Mixing:
Mixing can be done either by dry process or wet-process.
In this process, the raw materials, lime stone and clay etc. are mixed in calculated proportions, crushed and ground in tube mills. The finely ground mixture is stored in silos and burnt as per need in the kiln and ground. Sometimes the crushed power is mixed with water and balls are prepared of the crushed material. Then these balls are burnt at clinker temperature in suitable hearths. The clinker formed is cooled and ground to specified fineness. This process has proved costlier.
In this process the raw materials are crushed in roll mills and mixed in proper proportion with water to form slurry. The slurry is ground to form a smooth liquid in a huge horizontal rotating cylindrical mill. This slurry is stored in silos and proportion of materials checked. The correct chemical composition is adjusted by adding pure slurry of different materials prepared separately. The adjusted slurry is stored in silos to be fed into rotary kiln as and when required. This process ensures better control on chemical composition of slurry. The slurry is of creamy consistency having 35 to 50% water content.
Process # 2. Burning:
The fine slurry or dry mixture is burnt in rotary kilns. A rotary kiln is a steel tube upto 7.5 m in diameter and 230 m long, usually the dimensions of a rotary kiln vary from 2 to 3 m in diameter and 30 to 100 m in length. It is kept inclined at a gradient of 1 in 25 to 1 in 30 with the horizontal rotating once every minute. This kiln is lined from inside with refractory material.
The slurry is fed at the upper end of the kiln and it gradually descends towards the flame burners at the lower end of the kiln. As the slurry descends, it comes in contact with hot gases and dries up forming small lumps or “nodules”. These nodules gradually move downwards through rising temperature till they reach burning zone at a temperature of 1500°C to 1700°C.
At this temperature about 20-30% of the constituents of slurry become liquid and recombination of lime-silica- alumina mixture takes place, and the resulting mass fuses into balls known as clinker. At this temperature, the clinker is red hot emitting white rays of light. The flame is produced by burning pulverised coal or oil.
Now the clinker is discharged into another rotary tube which resembles the rotary kiln, but shorter in length having about 12 coolers. Various types of coolers are used in these tubes. The air sent by coolers becomes hot by exchanging heat from clinker. This hot-air is allowed to pass through the rotary kiln, which helps to heat the clinker. The largest existing capacity rotary kiln produces 3600 tonnes clinker per day. The cool clinker characteristically is black, glistening (shining brightly) and hard. The density of clinker is about 1100 to 1300 kg/per m3.
Effect of Rate of Cooling of Clinker on Strength of Cement:
It has been observed that the rate of cooling of clinker affects considerably the strength properties of cement. It has been observed that a moderate rate of cooling of clinker in the rotary cooler gives about 3.7% higher strength of cement at 28 days than quick cooling and about 11.1% less strength at slow cooling. The results based on the experiments carried out by ENKEGAARD are shown in Table 2.1.
From table 2.1, it can be seen that the moderate rate of cooling of clinker in the rotary cooler results in higher strength of cement.
Moderate rate of cooling may be taken that rate at which the clinker temperature from 1200°C to 500°C is reduced in about 15 minutes and from 500°C to normal atmospheric temperature in about 10 minutes.
The rate of cooling of the clinker influences the degree of crystallisation, size of the crystal and the quantity of amorphous materials present in the clinker. The properties of this amorphous material for similar chemical composition are found different from a crystalined one.
Process # 3. Grinding:
The grinding of the clinker is done in ball mills, consists several compartments with progressively smaller steel balls. The diameter of balls may vary from 5 cm to 13 cm. Due to the impact of steel balls, the clinker breaks into small pieces. The product obtained in ball mills is ground in tube mills. The diameter and length of tube mill are 2-5 m and 10 m respectively. The diameter of balls used in this mill is 2.5 cm.
At the time of grinding a small quantity of gypsum (CaSO4H2O) is added to the clinker to prevent flash setting of the cement. Usually gypsum is added from 2 to 4% by weight of clinker depending upon the quantity of C3A and alkali content of cement. If the quantity of C3A is not more than 5% then gypsum should not be added more than 2.5%, if C3A is more than 5%, then 3% gypsum should be added, but in no case it should be more than 4%.
The flow diagram of cement manufacture is given below:
In dry process 100 kg coal is required per ton of cement, while in wet process 350 kg coal is required for the same quantity of cement. In 1998 there were 173 large cement plants in India. Out of 173 plants 49 operated on wet-process, while 115 operated on dry process and 9 on semi dry process.