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A cement kiln is a specialized kiln used in the production of cement. It should not be confused with kiln cement, a heat-resistant material which is used to patch the kilns used in pottery production. Few people have an opportunity to interact with a cement kiln in the course of their lives, although they may use cement products every day. Upon visiting a cement kiln, the first thing that people notice is usually the size, with cement kilns being designed for continuous processing of huge volumes of raw materials.
Cement kilns are used in the production of so-called “hydraulic cements,” cements which are mixed with water to create a material which can be used for binding. Portland cement is probably the most famous form of hydraulic cement, although a number of other types of cement can be made in a cement kiln. Many people are familiar with cement in the form of concrete, a material made by mixing an aggregate such as crushed rock with cement to form a very durable, strong mixture.
The process of firing materials for cement starts with mixing calcium carbonate, also known as lime, with clay or shale and other minerals to create a mixture which is poured into the kiln. The kiln is slowly brought to temperatures around 2,642 degrees Fahrenheit (1,450 degrees Celsius), triggering a chemical reaction which results in the formation of calcium silicate. The calcium silicate precipitates out in the form of “clinkers” which are cooled before being ground into powder. The resulting powder is the cement, and it can be further amended with specific minerals for particular desired traits.
Early cement kilns were designed for batch operations. Modern kilns are continuous, slowly feeding the mixture through on a conveyor so that the cement kiln can be kept in continuous operation. Continuous operations are preferable from an efficiency point of view, and they are also easier on the equipment, since the kiln does not have be repeatedly heated and cooled. They also require a lot of space.
Cement manufacturing can be a very messy process. A number of emissions are produced in the firing of the kiln, and many cement kilns are equipped with air scrubbers to trap as many pollutants as possible. Grinding the clinkers also generates a great deal of dust, which can be an irritant if it enters the airways of people and animals. Filtration systems are used in cement grinding facilities to cut down on escaping dust, but it can be difficult to contain.
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