Cement Manufacturing Process: How the Cement is Made?

The entire manufacturing process in a modern plant is now controlled through a microprocessor based 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 controlled centrally in a single control room and the plant employs minimum of manpower as compared to previous plants constructed prior to 1980.

The modern plants have also taken adequate care to prevent the environmental pollution and dust nuisance to its surrounding areas. The cement mills have electro­static precipitators (ESP) installed to check the dust emission. The bag filters and glass bag houses are located at various locations to prevent dust emission and to ensure healthy and hazard-free atmosphere.

Cement Manufacturing Process: How to Manufacture Ordinary Cement?

Following three distinct operations are involved in the manufacture of normal setting or ordinary or Portland cement:

(1) Mixing of raw materials

(2) Burning

(3) Grinding.

1. Mixing of Raw Materials:

The raw materials such as limestone or chalk and shale or clay may be mixed either in dry condition or in wet condition. The process is accordingly known as the dry process or the wet process of mixing.

(I) Dry Process (Modern Technology):

In this process, the raw materials are first reduced in size of about 25 mm in crushers. A current of dry air is then passed over these dried materials. These dried materials are then pulverised into fine powder in ball mills and tube mills. All these operations are done separately for each raw material and they are stored in hoppers.

They are then mixed in correct proportions and made ready for the feed of rotary kiln. This finely ground powder of raw materials is known as the raw mix and it is stored in storage tank. Fig. 6-1 shows the flow diagram of mixing of raw materials by dry process.

The dry process has been modernised and it is widely used at present because of the following reasons:

(i) Competition:

At present, several dry process cement plants are vying or competing with each other. The cement consumers in general and the practicing civil engineers in particular are greatly benefited by such competition.

(ii) Power:

The blending of dry powders has now perfected and the wet process, which required much higher consumption of power, can be replaced with confidence.

(iii) Quality of Cement:

It is found that the quality of the production no longer depended on the skilled operators and workmen because temperature control and proportioning can be done automatically through a centralised control room.

(iv) Technology:

There has been several advances in instrumentation, computerisation and quality control. The application of the modern technology has made the production of cement by dry process more economical and of superior quality.

Following is the procedure of manufacture of cement by the dry process using modern technology:

(a) Most of the cement factories are located very close to the limestone quarries. The boulders upto 1.2 m size are transported in huge dumpers upto 300 kN capacity and dumped into the hopper of the crusher.

(b) The hammer mill crushers of single stage are now used for crushing as against the time consuming two stage crushers used in earlier plants. The crushed limestone now of 75 mm size is moved from the crusher by a series of conveyors for stacking. The modern stacker-reclaimer system is now in use in most of the modern plants.

The stacker helps in spreading the crushed materials in horizontal layers and the reclaimer restricts the variation of calcium carbonate in crushed limestone to less than 1% thereby minimizing quality variation in the materials.

(c) The argillaceous or clay materials found in the quarry are also dumped into the crusher and stacked along with the limestone.

(d) The crushed materials are checked for calcium carbonate, lime, alumina, ferrous oxide and silica contents. Any component found short in the quarried materials is added separately. For instance, if silica content is less, the crushed sandstone is separately transported to the raw material hopper.

In a similar way, if limestone is found to contain less content of lime, the high grade limestone is crushed and stored separately in the raw material hopper.

(e) The additive material and crushed limestone are conveyed to the storage hoppers. The raw materials are fed to the raw mill by means of a conveyor and proportioned by use of weigh feeders which are adjusted as per the chemical analysis done on the raw materials taken from the hoppers from time to time.

(f) The materials are ground to the desired fineness in the raw mill. In some of the modern plants, the high efficiency vertical grinding mills are installed. The fine powder which emerges as a result of the grinding in the raw mill is blown upwards, collected in cyclones and fed to the giant sized continuous blending and storage silo by use of aeropole.

The advantage of these silos is that one stage of pumping is eliminated which was inevitable in the traditional pattern of different silos for blending and storage.

(g) The material is dropped merely by gravity from the blending to the storage silo thereby conserving power.

(h) The material is then once again pumped using an aeropole into the preheater. The most modern preheaters have five stages. The temperature of the material fed from the top is increased in stages from 60°C to 850°C as hot gas at temperature of 1000°C is blown against the falling gradient.

(i) The material from the bottom of the preheater is fed to the rotary kiln. Due to the use of multi-stage preheaters in the modern plants, the length of rotary kilns is considerably reduced thereby resulting in saving of maintenance cost and power requirements.

(II) 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 manufacturers and the govt. policies till early 1982.

All the cement plants set up after 1980 use the dry process for the manufacture of cement. In this process, the calcareous materials such as limestone are crushed and stored in silos or storage tanks. The argillaceous material such as clay is thoroughly mixed with water in a container known as the wash mill. This washed clay is stored in basins.

Now, the crushed limestone from silos and wet clay from basins are allowed to fall in a channel in correct proportions. This channel leads the materials to grinding mills where they are brought into intimate contact to form what is known as the slurry. The grinding is carried out either in ball mill or tube mill or both.

The slurry is led to correcting basin where it is constantly stirred. At this stage, the chemical composition is adjusted as necessary. The corrected slurry is stored in storage tanks and kept ready to serve as feed for rotary kiln.

Fig. 6-2 shows the flow diagram of mixing of raw materials by the wet process.

It is thus seen that in case of mixing of raw materials by dry process, the raw mix is formed and in case of mixing of raw materials by wet process, the slurry is formed. The remaining two operations namely, burning and grinding, are the same for both the processes.

2. Burning:

The burning is carried out in a rotary kiln as shown in fig. 6-3. A rotary kiln is formed of steel tubes. Its diameter varies from 2.50 m to 3 m. Its length varies from 90 m to 120 m. It is laid at a gradient of about 1 in 25 to 1 in 30. The kiln is supported at intervals by columns of masonry or concrete. The refractory lining is provided on the inside surface of rotary kiln. It is so arranged that the kiln rotates at about one to three revolutions per minute about its longitudinal axis.

The corrected slurry is injected at the upper end of kiln. Fig. 6-3 shows the rotary kiln for the wet process. The hot gases or flames are forced through the lower end of kiln.

The portion of the kiln near its upper end is known as the dry zone and in this zone, the water of slurry is evaporated. As the slurry gradually descends, there is rise in temperature and in the next section of kiln, the carbon dioxide from slurry is evaporated.

The small lumps, known as the nodules, are formed at this stage. These nodules then gradually roll down passing through zones of rising temperature and ultimately reach to the burning zone, where temperature is about 1400°C to 1500°C.

In burning zone, the calcined product is formed and nodules are converted into small hard dark greenish blue balls which are known as the clinkers.

In the modern technology of dry process, the coal brought from the coal fields is pulverised in vertical coal mill and it is stored in silo. It is pumped with required quantity of air through the burners. The preheated raw materials roll down the kiln and get heated to such an extent that the carbon dioxide is driven off with combustion gases. The material is then heated to temperature of nearly 1400°C to 1500°C when it gets fused together. The fused product is known as the clinkers or raw cement.

The size of clinkers varies from 3 mm to 20 mm and they are very hot when they come out of burning zone of kiln. The clinker temperature at the outlet of kiln is nearly 1000°C. A rotary kiln of small size is provided to cool down the hot clinkers. It is laid in opposite direction as shown in fig. 6-3 and the cooled clinkers having temperature of about 95°C are collected in containers of suitable sizes.

3. Grinding:

The clinkers as obtained 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 added.

The gypsum controls the initial setting time of cement. If gypsum is not added, the cement would set as soon as water is added. The gypsum acts as a retarder and it delays the setting action of cement. It thus permits cement to be mixed with the aggregates and to be placed in position.

The grinding of clinkers in modern plants is carried 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 collected 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 of cement contains 50 kg or 500 N or about 0.035 m³ of cement.

These bags are automatically discharged from the packer to the conveyor belts to different loading area. They are carefully stored in a dry place. Fig. 6-4 shows the flow diagram of burning and grinding operations.