Cement production involves the fusing together at high temperature of a precisely controlled blend of very finely ground limestone and shale. The resultant clinker is then finely ground with the addition of about 5% of gypsum. Platin Works utilises the modern dry production process. The consequent elimination of large quantities of mixing and blending water, which characterised the older wet process, enables savings to be made in fuel traditionally used to drive off moisture. Click to view enlarged production process diagram The kilns at Platin can operate using either pulverised coal, petroleum coke or oil and the choice of fuel is dictated largely by cost considerations and availability.

Limestone consisting almost entirely of calcium carbonate (CaCO3) and shale containing a high proportion of silica (SiO2) with lesser amounts of alumina (Al2O3) and ferrous oxide (Fe2O3) provide the essential constituents for the manufacture of cement. Separately crushed and stored, the materials are then blended in proportion of about 4:1 limestone:shale and ground to very fine powder know as raw meal. The care and attention given to eliminating natural variations at this stage has a critical influence on the consistency and performance of the finished product. During this operation hourly checking of chemical composition and sophisticated blending techniques play a very important role.

Burning takes place in the rotary kiln. This is a large, long, welded steel tube lined with refractory materials and inclined slightly towards one end. It rotates slowly and continuously about it’s longitudinal axis and is fired by pulverised coal, petroleum coke or oil at it’s lower end. Raw meal is preheated by kiln exhaust gases before entering the upper end of the kiln. The rotation of the unit gradually conveys the powder towards the firing zone. Initially carbon dioxide is driven off. As the temperature continues to rise the original raw meal undergoes further changes until at temperatures approaching 1500°C a white hot mass with a complex chemical composition is produced. On cooling, this has the appearance of dark grey irregular gravel and is called cement clinker.

All that is now required is to grind the clinker to a very fine powder in a ball mill. During this operation – achieved by the tumbling action of many tons of steel balls and cylindrical bodies within a rapidly rotating steel cylinder – about 5% of gypsum is interground. This controls the setting time of the cement to a level appropriate for practical site use. This finished product is then stored in large silos to await despatch either in bulk, or packed in paper sacks.