Rotary kiln

A rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process.

Materials produced using rotary kilns include: They are also used for roasting a wide variety of sulfide ores prior to metal extraction.

As the kiln rotates, material gradually moves down toward the lower end, and may undergo a certain amount of stirring and mixing.

The basic components of a rotary kiln are the shell, the refractory lining, support tyres (riding rings) and rollers, drive gear and internal heat exchangers.

Upper limits on diameter are set by the tendency of the shell to deform under its own weight to an oval cross section, with consequent flexure during rotation.

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 properties of the process material.

The refractory selected depends upon the temperature inside the kiln and the chemical nature of the material being processed.

The shell temperature needs to be maintained below around 350 °C (662 °F) to protect the steel from damage, and continuous infrared scanners are used to give early warning of "hot-spots" indicative of refractory failure.

Tyres, sometimes called riding rings, usually consist of a single annular steel casting, machined to a smooth cylindrical surface, which attach loosely to the kiln shell through a variety of "chair" arrangements.

The tyre rides on pairs of steel rollers, also machined to a smooth cylindrical surface, and set about half a kiln-diameter apart.

The bearings of the rollers must be capable of withstanding the large static and live loads involved and must be carefully protected from the heat of the kiln and the ingress of dust.