Incineration is done in a rotary kiln. A rotary kiln is made up of steel tubes with a diameter of 250 to 300 cm. The length varies from 90 to 120 meters. It is laid at a gradient of 1 in 25 to 1 in 30. The kiln is supported at intervals by masonry columns. A refractory lining is provided inside the kiln. It is arranged so that the kiln rotates about its longitudinal axis at 1–3 revolutions per minute. For the wet process, the corrected slurry is charged into the rotary kiln. Coal in fine form, fuel oil and gas are common fuels for burning these kilns. The portion of the kiln near its upper end is known as the dry zone and in this zone the slurry water evaporates. As the slurry enters the next zone, the temperature increases where carbon dioxide evaporates from the slurry. Small lumps called nodules form at this stage. These nodules gradually pass through zones of increasing temperature and eventually reach the combustion zone where the temperature is about 1,500 °C. In the burned area, a calcined product forms and the nodules turn into small, hard, dark, blue-green balls called clinkers. Clinkers vary in size from 3 to 20 mm. Small size rotary kilns are provided for cooling the clinkers and the cooled clinkers at a temperature of 95°C are collected in suitable sized containers.
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