Cement kilns produce hydraulic cements, including Portland cement, by heating a mixture of calcium carbonate, clay or shale, and other minerals. The resulting clinkers are cooled and ground into powder. Modern kilns are continuous and require a lot of space. Emissions and dust are produced during the process, and filtration systems are used to reduce pollution.
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 that is used to patch kilns used in pottery making. Few people have the opportunity to interact with a concrete kiln in their lifetime, although they may use concrete products every day. When visiting a cement kiln, the first thing people usually notice is the size, with cement kilns designed for continuous processing of huge volumes of raw materials.
Cement kilns are used in the production of so-called “hydraulic cements”, cements that are mixed with water to create a material that can be used for binding. Portland cement is probably the most famous form of hydraulic cement, although numerous 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 aggregate such as crushed rock with cement to form a very strong and durable mix.
The process of firing materials for cement begins 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 heated to temperatures around 2,642 degrees Fahrenheit (1,450 degrees Celsius), triggering a chemical reaction that leads to the formation of calcium silicate. Calcium silicate precipitates in the form of “clinkers” which are cooled before being ground into powder. The resulting powder is cement, and can be further modified with specific minerals for particular desired traits.
The first cement kilns were designed for batch operations. Modern kilns are continuous, slowly feeding the mix onto a conveyor so that the cement kiln can be kept in continuous operation. From an efficiency standpoint continuous operations are preferable, and are also easier on the equipment, since the furnace does not have to be heated and cooled repeatedly. They also require a lot of space.
Making cement can be a very messy process. Numerous emissions are produced during kiln firing and many cement kilns are equipped with air purifiers to trap as many pollutants as possible. Grinding clinker also generates a large amount of dust, which can be irritating if it gets into the respiratory tracts of people and animals. Filtration systems are used in cement grinding plants to reduce dust spillage, but it can be difficult to contain.
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