Manufacture of Cement:
The cement manufacturing process consists of the following steps:
- Grinding and mixing of the raw materials.
- Burning of the mixture at a specified temperature for the correct duration.
- Grinding the consumed item called (clinker) along with gypsum.
There are two methods available for the manufacture of cement:
- Wet Process
- Dry Process
The decision between wet and dry procedures depends on certain factors:
- The physical condition of the raw materials.
- The price of the fuel.
- The local climatic condition of the factory.
In Pakistan, both Wet and Dry processes are being used for the cement. The dry process although cheaper needs excessive fine grinding. It is more suited for the hard material (Fig 3.1)
The wet process, on the other hand, is free from the dust, grinding is easier and the composition of the cement can easily be controlled.
The specific feature of the wet process is that the raw materials are prepared in water whereas in the dry process the materials are ground and mixed dry. The flowsheet diagram of the wet process for manufacturing Portland cement is given in Figure 3.3. The limestone is disintegrated in crushers. The crushed limestone is fed to a tube mill, where the limestone and the clay, introduced as a slurry from a clay mixer, are simultaneously subjected to fine grinding. From the mill, the raw material slurry is fed to a reinforced concrete reservoir, where it is stirred with agitators. The Kilns are heated using coal dust, gaseous fuel, or fuel oil. The raw material slurry is fed to the kiln from a horizontal slurry basin and inside moves through it counter-current to the hot, gaseous combustion products. Their interaction results in the successive processes of water evaporation, Mineral dehydration, limestone dissociation and chemical reaction between the basic oxides, CaO, and the components of the clay – SiO2, Al2O3, Fe2O3. Within the kiln, the process of making the clinker is accomplished in four Zones, i.e., dehydration calcination, clinkering, and cooling.
In this Zone (100°C – 500°C) the evaporation of free water from the fuel and evolution of combined water from the clay takes place (500 – 1000°C)
In this Zone evolution of CO2 from the carbonates takes place.
In the Burning Zone, the lime-rich mixture containing silica, alumina, and ferric oxide with a small percentage of other oxides is heated up to the sintering temperature. The burning of the calcined mass is completed at about 1450°C.
The process of clinker formation is completed in this Zone. Cooling of the clinker starts a few feet short of the discharge end of the kiln and is completed when it passes through the cooler and its temperature is approximately 150 – 200°C.
Clinker is finely ground in ball mills. The cement is stored in reinforced concrete reservoirs with bottoms containing air nozzles through which air is forced when the cement is being discharged. Cement is delivered to consumers in automobiles and railway cement tanks in bulk, or in paper multilayer bags Fig. (3.4)
The dry process for the manufacture of cement is preferred only in places where hard crystalline limestone and shales are available. The only advantage of this process is that the fuel consumption is low. The raw materials are first crushed together in the dry state in a multi-chamber tube mil of a fairly large diameter. The drying unit consists of a grade firing with an induced draught fan. The grinding and drying operations are carried out simultaneously. This dry powdered mixture from the grinding mill is homogenized by means of compressed air. The dry material is next burnt in a rotary Kiln as explained in the Wet process fig 3.5.