Cement and Lime Kilns case study

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Cement and Lime Kilns

PROCESS DESCRIPTION
Cement, lime and gypsum manufacturing processes have in common the mixing of inorganic minerals calcined at high temperatures typically using rotary kilns (vertical kilns are also used).

CEMENT
Regular Portland cements are the usual products for general concrete construction. The raw materials, limestone or chalk, together with clay or shale are mixed and fed to a large rotary kiln at temperatures up to 2600°F (1427°C). The mix can be taken through a number of preheater and precalciner stages before being charged into the rotary kiln.

Kilns are normally fired with pulverized coal or gas at one end. Raw materials are fed to the kiln at the back end furthest from the burners. Travelling through the kiln, the ingredients are progressively heated. Here flue gas flows in the opposite direction. The kiln can be divided into three zones - drying, calcining and burning. Drying removes the water from the mix; calcining drives off the carbon dioxide;

burning sinters and partially fuses the ingredients into lumps known as clinkers. These clinkers are then cooled and pulverized into fine powder. Some gypsum is used to control the speed of setting when water is added. After the kiln, the flue gas passes through various heat recovery stages, then to electrostatic precipitators for final clean up before being discharged to the stack.

Some cement kilns are the wet 'slurry' type. Slurry of raw materials is fed directly into the kiln. The temperature at the inlet is considerably less than the dry feed process. Also the kiln is much longer to allow sufficient time for drying before calcination. Figure 1 is a schematic of a typical cement plant.

The tremendously high temperature sustained by cement kilns and the long transit time are ideal for the disposal of material such as tires or hazardous wastes.

LIME
Lime or quicklime are the common names for calcium oxide (CaO), a gray-white powder. Either directly or indirectly lime and limestone are used in more industries than any other natural substance. It is used in the manufacture of glass, cement, brick, pulp/paper, steel, aluminum, magnesium and poultry feed. It also is used in processing cane and sugar beet juices. Lime kilns are associated with every kraft pulp mill and often used in steel mills.

Since lime is manufactured from quarried limestone, lime plants are located near these deposits. The crushed raw material is fed to a rotary or vertical kiln where carbon dioxide is driven off to produce lime. The use of lime for any particular process depends on its composition and physical properties, which are controlled by the selection of the limestone and manufacturing process details. The quality and color affect the suitability of use and price.

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