ROTARY KILN

The diameter of the rotatory kiln is 6 - 11.8 feet.

Length varies from 60-400 feet

The rotatory kiln is rotated at the speed of 95 seconds for one rotation

The maximum temperature that can be maintained in the rotatory kiln is 1030 degrees celsius 

The lime obtained after calcination is taken out of the kiln at a constant temperature of 900 degrees celsius

The size of limestone calcinated in the rotatory kiln is 0.25 - 2.5 inches

The hot air is used as secondary combustion fuel

A separate fan takes the hot air to the outlet to reduce the temperature required for the looking of quicklime

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: 

Cement 

Lime 

Refractories 

The kiln is a cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its longitudinal axis. 

The process feedstock is fed into the upper end of the cylinder. 

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

Hot gases pass along the kiln, sometimes in the same direction as the process material (co-current), but usually in the opposite direction (counter-current). 

The hot gases may be generated in an external furnace or generated by a flame inside the kiln. 

Such a flame is projected from a burner pipe (or "firing pipe") which acts like a large bunsen burner. 

The fuel for this may be gas, oil, pulverized petroleum coke or pulverized coal. 

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. 

The rotary kiln was invented in 1873 by Frederick Ransome. 

Kiln shell 

This is made from rolled mild steel plate, usually between 15 and 30 mm thick, welded to form a cylinder that may be up to 230 m in length and up to 6 m in diameter. 

Upper limits on a 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. Length is not necessarily limited, but it becomes difficult to cope with changes in length on heating and cooling (typically around 0.1 to 0.5% of the length) if the kiln is very long. 

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 properties of the process material. In some processes, such as cement, the refractory life is prolonged by maintaining a coating of the processed material on the refractory surface. 

The thickness of the lining is generally in the range of 80 to 300 mm. A typical refractory will be capable of maintaining a temperature drop of 1000 °C or more between its hot and cold faces. The shell temperature needs to be maintained below around 350 °C to protect the steel from damage, and continuous infrared scanners are used to give early warning of "hot spots" indicative of refractory failure. 

Tyres and rollers 

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 longest kilns may have 8 sets of rollers, while very short kilns may have only two. Kilns usually rotate at 0.5 to 2 pm, but sometimes as fast as 5 rpm. The Kilns of most modern cement plants are running at 4 to 5 rpm. 

Drive gear 

The kiln is usually turned by a single Girth Gear surrounding a cooler part of the kiln tube, but it is sometimes turned by driven rollers. The gear is connected through a gear train to a variable-speed electric motor. This must-have high starting torque to start the kiln with a large eccentric load. A 6 x 60 m kiln requires around 800 kW to turn at 3 rpm. 

Internal heat exchangers 

In descending order of efficiency, heat exchange in a rotary kiln may be by conduction, convection, and radiation. In low-temperature processes, and in the cooler parts of long kilns lacking preheaters, the kiln is often furnished with internal heat exchangers to encourage heat exchange between the gas and the feed. The most common heat exchanger consists of chains hanging in curtains across the gas stream.