Making Clinker Portland Cement: a blog about the process of making cement and all its stages.
The clinker is cooled in a grate cooler, where the particles are sorted into the right size range to be used again in the kiln as part of the feed meal. This is done by having a large number of steel gratings at a slight angle to create a moving bed, in which air is drawn up through the layer of clinker. The hot clinker passes from one grating to another until it reaches the end of the cooler, where it drops onto conveyors below and is transported to storage. The cooling medium is forced draught air from the preheater outlet.
While cooling, some of the calcium hydroxide produced during clinkering reverts back to calcium carbonate and some water is lost from the clinker.
Clinker and gypsum are ground together in ball mills similar to those commonly used for grinding ore, to produce Portland cement. Clay and shale are also ground with the mixture, producing Portland cement clinker (see below).
What is Clinker?
Clinker is the raw material used to manufacturing the cement. The clinker plant comprises of feed pre crusher, feed hopper, table feeders, ball mill, packaging cement plant and material handling equipment. However, the operation of the plant consists of basically four processes viz. quarry, raw mill grinding, coal grinding and kiln-burning.
The most common combination of ingredients used to make Portland cement includes: limestone (calcium), sand or clay (silicon), bauxite (aluminum) and iron ore, and may include shells, chalk, marl, shale, clay, blast furnace slag or slate. All of these raw materials are prepared in quantity at a portland cement plant.
The cement clinker is ground to a fine powder in a ball mill and blended with gypsum to make Ordinary Portland Cement (OPC). Some specialty cements are made by inter-grinding various clinker minerals with the calcium sulfate.
OPC is the “normal” or standard cement with which most people are familiar. It is manufactured from cement clinker, gypsum and limestone. The manufacturing process is energy intensive and involves several processes and many types of equipment, including:
Crushers for crushing limestone, clay and shale
Rotary kilns for heating raw materials
Grinding mills for grinding the raw material into slurry form
Rotary coolers for cooling the clinker prior to storage
Clinker storage areas
Finish mills for grinding clinker into powder along with additives such as gypsum and limestone
Cement is made from gypsum, shale or clay and limestone. These raw materials are extracted from the quarry crushed to a very fine powder and then blended in the correct proportions. This blended raw material is called the ‘raw feed’ or ‘kiln feed’ and is heated in a rotary kiln where it reaches a temperature of about 1400 C to 1500 C. In its simplest form, the rotary kiln is a tube up to 200 meters long and perhaps 6 meters in diameter, with a long flame at one end.
The raw feed enters the kiln at the cool end and gradually passes down to the hot end, then falls out of the kiln and cools. The kiln is slightly inclined and rotates once every hour or two, constantly passing new feed in at the cool end and withdrawing clinker from the hot end. The clinker nodules are then ground with about 3 % gypsum to produce cement with a fineness typically of less than 90 microns.*
Clinker is the product of heating raw materials (limestone and clay) to 1500°C. It is a granular material with marble-sized balls formed from the combination of calcium, silicon, aluminium and iron oxides that are clinked together by heating in a kiln. The raw materials are blended by dry grinding and homogenised into a uniform mix using blending silos. Clinker is fed into the rotary kiln and is heated by a direct flame to temperatures up to 1450°C. The clinker nodules are then cooled at the cooler outlet down to 120°C. At this temperature they are brittle and break easily into smaller pieces called cement clinker. Clinker quality is influenced by raw material composition, which has to be closely monitored to ensure the correct chemical compositions for the raw mix. A typical chemical analysis of the raw materials and finished cement is shown in the table below:
| Raw Material | Cement |
| :— | :— |
| Silica (SiO2) | 20% |
| Alumina (Al2O3) | 5% |
| Iron oxide (Fe2O3) | 3% |
| Calcium oxide (CaO) | 65%
Most cement is made in a rotary kiln. Basically, this is a long cylinder rotating about its axis once every minute or two. The axis is inclined at a slight angle, the end with the burner being lower.
The rotation causes the raw meal to gradually pass along from where it enters at the cool end, to the hot end where it eventually drops out and cools. Obviously, a lot of heat is needed to drive off the water in the raw meal. This is supplied by burning fuel in the firing zone, just above the point where the meal drops out.
In addition to supplying heat, a burner has another important job. This is to dry and preheat the raw materials as they pass through it. The raw meal enters at about 200 C and leaves at about 800 C, having been thoroughly dried and heated. It then drops into a second chamber in which it is brought up to about 1350 C before dropping out into an air quench cooler (see below).
The temperature of about 1350 C is necessary for clinker making so that as much of the calcium carbonate as possible can be calcined to produce lime (CaO), producing some clinker minerals such as belite (Ca2SiO4), alite (3
The manufacture of ordinary Portland cement (OPC) is one of the most energy intensive processes in the production of construction materials, consuming about 2 to 3 GJ per tonne of clinker produced. Theoretically, it requires about 2.3 GJ per tonne of clinker to make OPC at 0% cement replacement level with 30% raw meal replacement level. However, in practice the actual energy consumption is higher than this theoretical value and ranges between 3 and 5 GJ per tonne of clinker.
The key energy-intensive steps in cement manufacture are:
* LIME BURNING (decarbonation): This is the chemical process that converts calcium carbonate into calcium oxide, by heating limestone and clay at temperatures up to 1450℃ in a kiln. This process consumes about 65-75% of total power demand for cement manufacturing process (excluding grinding).
* GRINDING: Grinding accounts for almost 40 % of the electricity consumed in the production of cement.
* FUEL: Fuel comprises 15 % to 20 % of total production costs.