Working of Rotary kiln

A rotary kiln is a high-temperature processing device used to heat materials to high temperatures (calcination) in a continuous process. Materials produced using rotary kilns include:

• Cement

• Lime

• Refractory materials

• Metakaolin

• Titanium dioxide

• Alumina

• Vermiculite

• Iron ore pellets

They are also used to roast various sulfide ores prior to extraction of metals.

How it works: 

The kiln is a cylindrical vessel, slightly inclined to the horizontal, which rotates slowly around its longitudinal axis. Process raw materials are fed into the upper end of the cylinder. With the rotation of the kiln body, the material gradually moves to the lower end, and may be stirred and mixed to a certain extent.Hot gases are passed along the kiln, sometimes in the same direction as the process material (cocurrent), but usually in the opposite direction (countercurrent).The hot air can be generated in an external furnace or by a flame inside the kiln.This flame shoots out of a burner tube (or "ignition tube") that acts like a large Bunsen burner.Fuel can be natural gas, petroleum, petroleum coke powder or coal powder.

Construction

The basic components of a rotary kiln are the shell, refractory lining, support wheels (support rings) and drums, drive gears and internal heat exchanger.

Kiln shell:

It is made of rolled mild steel sheet, usually between 15 and 30mm thick, welded to form a cylinder up to 230m in length and 6m in diameter.The upper limit of the diameter is determined by the tendency of the shell to deform under its own weight into an elliptical cross-section, thus bending during rotation.The length is not necessarily limited, but if the kiln is very long, it will be difficult to cope with length changes (usually about 0.1 to 0.5% of the length) when heating and cooling.

Refractory lining:

The purpose of the refractory lining is to insulate the steel shell from the high temperatures inside the kiln and to protect it from the corrosive nature of the process materials.It may consist of refractory bricks or cast refractory concrete, or it may not be present in areas of the kiln below about 250 °C.The choice of refractory material depends on the temperature in the kiln and the chemical properties of the material being processed in some process, Cement, for example, prolongs the life of refractory materials by maintaining a coating of the processed material on the refractory surface.The lining thickness is generally in the range of 80-300mm. A typical refractory is capable of maintaining a temperature drop of 1000 °C or more between its hot and cold faces.Shell temperatures need to be kept below around 350 °C to protect the steel from damage, and continuous infrared scanners are used to give early warning of 'hot spots' indicating refractory failure.