Key stages in cement production  

Stage 1 - Raw materials

These are generally combinations of limestone, shells or chalk, and shale, clay, sand or iron ore, usually mined from a quarry close to the plant where they undergo size reduction using primary and secondary crushers. When the materials reach the cement plant they are proportioned to create a cement of specific chemical composition. At this stage, the use of alternative raw materials – often the by-products of other industrial processes offers a number of advantages: less quarrying, better economics and lower environmental impact.
There are two basic methods used in Portland cement production – wet and dry. In the dry process, dry materials are proportioned, ground to a powder, blended and fed into the kiln. The wet process, in contrast, involves adding water to the proportioned raw materials and completing the grinding and blending operations in slurry form. This approach tends to increase the efficiency of the size reduction process. Furthermore, when the slurry is dried in the kiln it produces a granular form that is ideal for subsequent processing. On the other hand the removal of added water increases energy consumption in the kiln, an overriding issue that has over preceding decades driven a decline in this method. When dry processing is adopted some damping down of the feed is however necessary to control its movement through the kiln.

Stage 2 - Pre-heater

To conserve energy, most modern cement plants pre-heat raw materials before they enter the kiln, using the hot exhaust gases from the kiln itself.

Stage 3 - Kiln

The mixture of raw materials is fed into the upper end of a rotating, cylindrical kiln, which achieves temperatures in excess of 1000°C. It passes through at a rate controlled by the slope and rotational speed of the kiln. Chemical reaction inside the kiln leads to the fusion of the raw materials to produce clinker. Traditionally kiln fuels have been powdered coal or natural gas, but increasingly alternative fuels are being used. These include materials such as scrap tyres, processed sewage sludge and packaging waste. Using these materials is environmentally attractive and has a major impact on production economics as some attract a waste disposal payment

Stage 4 - Cooling/finish grinding

Clinker is discharged from the lower end of the kiln and transferred to various types of coolers. Cooled clinker is combined with gypsum and ground to a fine powder in the finish milling circuit to produce the final grade cement. Ball mills have traditionally been the most common choice for this energy intensive process but vertical roller mills are increasingly used because of the efficiency gains they offer.

Once again the use of alternative materials is environmentally and economically advantageous at this stage. Replacing fresh clinker with, for example, blast furnace slag or fly ash reduces CO₂ emissions, production costs and energy consumption.