Cement is a hydraulic powder material, which reacts with water to produce strength-bearing lattices. The mixture of aggregates, cement and water is concrete. The strength and durability of concrete makes it one of the most useful materials developed by man. The chemistry and mineralogy of cement is complex. In simple terms, cement is a manmade mineral structure created at high temperatures, mainly comprising lime (CaO), Silica (SiO2) and oxides of aluminium and iron (Al2O3 and Fe2O3).
The cement making process can be divided; first "clinker" is made at temperatures of 1400 oC. Then the clinker is milled with other minerals to produce the powder we know as cement.
The raw materials, used to produce clinker includes some naturally occurring minerals and some materials available as waste streams from other industries. The most common combination of ingredients is limestone (for calcium) coupled with much smaller quantities of clay, shale and sand (as a source of silica, aluminium and iron). Other "alternative" raw materials such as mill scale, fly ash and slag are brought in from other industries. Plants generally rely on nearby quarries for limestone to minimise transport.
Rock blasted from the quarry face is transported to the primary crusher where large "run of mine" rocks are broken into pieces of approximately 100mm. Generally the other raw materials do not require crushing. The raw materials are then proportioned to the correct chemical balance and milled together to a fine powder, "rawmeal. To ensure high quality of cement, the chemistry of the raw materials and rawmeal is very carefully controlled. Kiln exhaust gases are used in the rawmill to dry the raw materials. In some gases with wet materials, additional heat sources are required for drying.
Materials are also homogenised to ensure consistency of product quality.
Rawmeal is the feed material for the high temperature process in the kiln system. "Preheating" is the first part of this system. A pre-heater is a series of vertical cyclones. As the raw meal is passed down through these cyclones it comes into contact with the swirling hot kiln exhaust gases moving in the opposite direction and as a result heat is transferred from the gas to material. This pre-heats the material before it enters the kiln so that the necessary chemical reactions will occur more quickly and efficiently. By retaining energy from the exhaust gases, energy is saved. Depending on the raw material moisture, a kiln may have 3 to 6 stages of cyclones with increasing heat recovery with each extra stage.
The calciner is a combustion chamber at the bottom of the preheater above the kiln back-end. Up to 65% of the total energy needs of the kiln system can be supplied to the calciner. Calciners allow for shorter rotary kilns and for the use of lower grade alternative fuels. Calcination is the decomposition of CaCO3 to CaO, which releases CO2. These process emissions comprise 60% of the total emission from a cement kiln. The combustion of the fuel generates the rest.
Raw meal, more accurately termed "hot meal" at this stage then enters the rotary kiln. The kiln is the world's largest piece of industrial equipment. Fuel is fired directly into the rotary kiln and ash, as with the calciner, is absorbed into the material being processed. As the kiln rotates at about 3-5 revolutions per minute, the material slides and tumbles down through progressively hotter zones towards the flame. Coal, pet coke, natural gas and more increasingly alternative fuels such as plastic, solvents, waste oil or meat and bone meal are burned to feed the flame which can reach as high as 2000oC.
This allows the materials to become partially molten as the intense heat causes the chemical and physical changes that transform the raw feedstock into a material called clinker. Expressed at its simplest, the series of chemical reactions converts the calcium and silicon oxides into calcium silicates, cement primary constituent. At the lower end of the kiln, the raw materials emerge as a new substance â€“ red-hot particles called clinker.
There are two basic types of cement production processes and a number of different kiln types. Cement production is either "wet" or "dry", depending on the water content of the material feedstock. The wet process was the original rotary kiln process developed at a time when material handing of slurries was more developed than those of dry powders. It is still being used to process very wet raw materials and allows for easier control of the chemistry. However, it has much higher energy requirements due to the amount of slurry water that must be evaporated before calcinations can take place. The dry process avoids the use of slurry material and as a result is far less energy intensive. Modern day kilns are much more efficient than the kilns of old.
The clinker tumbles onto a grate cooled by forced air. Once cooled the clinker is ready to be ground into the grey powder known as Portland cement. To save energy, heat recovered from this cooling process is re circulated back to the kiln or preheater tower.
The clinker is then ground with other mineral components to cement: Gypsum is used to control the setting time of the product; slag, fly ash and gypsum can also be used to control other properties of the cement. The use of these materials reduces the total carbon footprint of the cement. In some countries, these materials are traditionally used at the concrete manufacturing plant with high clinker content cement supplied by the cement plant. Traditionally, ball mills have been used for cement milling. In recent years technologies with better energy efficiency have been developed. Compound mill systems include pre-crushing and sophisticated separator systems to reduce electricity consumption. Vertical cement mills mill the material in a roller mill with reduced electricity consumption also.
From the grinding mills, the cement is conveyed to silos for shipment. Most cement is shipped in bulk by trucks, rail or barge. A small percentage of the cement is bagged for customers who need only small amounts or who have special needs.