A rotary kiln is a pyroprocessing device used to raise materials to a high temperature (

calcination Calcination is thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), generally f ...

) in a continuous process. Materials produced using rotary kilns include: *

Cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

* Lime *

Refractories In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

* Metakaolin *

Titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...

Alumina Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...

Vermiculite Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently; commercial furnaces can routinely produce this effect. Vermiculite forms by the weathe ...

Iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the f ...

pellets They are also used for

roasting Roasting is a cooking method that uses dry heat where hot air covers the food, cooking it evenly on all sides with temperatures of at least from an open flame, oven, or other heat source. Roasting can enhance the flavor through caramelizat ...

a wide variety of

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

s prior to metal extraction.

Principle of operation

The

kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or Chemical Changes, chemical changes. Kilns have been used for millennia to turn objects m ...

is a cylindrical vessel, inclined slightly from 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, 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 may be 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 A Bunsen burner, named after Robert Bunsen, is a kind of ambient air gas burner used as laboratory equipment; it produces a single open gas flame, and is used for heating, sterilization, and combustion. The gas can be natural gas (which is main ...

. The

fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work (physics), work. The concept was originally applied solely to those materials capable of releasing chem ...

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

Construction

The basic components of a rotary kiln are the shell, the

refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

lining, support tyres (riding rings) and rollers, drive gear and internal heat exchangers.

History

The rotary kiln was invented in 1873 by Frederick Ransome. He filed several patents in 1885-1887, but his experiments with the idea were not a commercial success. Nevertheless, his designs provided the basis for successful kilns in the US from 1891, subsequently emulated worldwide.

Kiln shell

This is made from rolled

mild steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...

plate, usually between , welded to form a cylinder which may be up to in length and up to in diameter. 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. 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. It may consist of refractory bricks or cast refractory concrete, or may be absent in zones of the kiln that are below approximately . The refractory selected depends upon the temperature inside the kiln and the chemical nature of the material being processed. 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 . A typical refractory will be capable of maintaining a temperature drop of 1000 °C (1,800 °F) or more between its hot and cold faces. The shell temperature needs to be maintained below around to protect the steel from damage, and continuous

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

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. These require some ingenuity of design, since the tyre must fit the shell snugly, but also allow thermal movement. The tyre rides on pairs of steel rollers, also machined to a smooth cylindrical surface, and set about half a kiln-diameter apart. The rollers must support the kiln, and allow rotation that is as nearly frictionless as possible. A well-engineered kiln, when the power is cut off, will swing pendulum-like many times before coming to rest. The mass of a typical kiln, including refractories and feed, is around , and would be carried on three tyres and sets of rollers, spaced along the length of the kiln. The longest kilns may have 8 sets of rollers, while very short and small kilns may have none. Kilns usually rotate at 0.5 to 2 rpm. The Kilns of modern cement plants are running at 4 to 5 rpm. 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. Since the kiln is at an angle, it also needs support to prevent it from walking off the support rollers. Usually upper and lower "retaining (or thrust) rollers" bearing against the side of tyres prevent the kiln from walking off the support rollers.

Drive gear

The kiln is usually turned by means of a single Girth Gear surrounding a cooler part of the kiln tube, but sometimes it is turned by driven rollers. The gear is connected through a gear train to a variable-speed

electric motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...

. This must have high starting

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

to start the kiln with a large eccentric load. A kiln requires around 800 kW to turn at 3 rpm. The speed of material flow through the kiln is proportional to rotation speed; a variable-speed drive is needed to control this. When driving through rollers, hydraulic drives may be used. These have the advantage of developing extremely high torque. In many processes, it is dangerous to allow a hot kiln to stand still if the drive power fails. Temperature differences between the top and bottom of the kiln may cause the kiln to warp, and refractory is damaged. Hence, normal practice is to provide an auxiliary drive for use during power cuts. This may be a small electric motor with an independent power supply, or a

diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

. This turns the kiln very slowly, but enough to prevent damage.

Internal heat exchangers

Heat exchange in a rotary kiln may be by

conduction Conductor or conduction may refer to: Biology and medicine * Bone conduction, the conduction of sound to the inner ear * Conduction aphasia, a language disorder Mathematics * Conductor (ring theory) * Conductor of an abelian variety * Condu ...

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

and

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

, in descending order of efficiency. 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. These may consist of scoops or "lifters" that cascade the feed through the gas stream, or may be metallic inserts that heat up in the upper part of the kiln, and impart the heat to the feed as they dip below the feed surface as the kiln rotates. The latter are favoured where lifters would cause excessive dust pick-up. The most common heat exchanger consists of chains hanging in curtains across the gas stream.

Other equipment

The kiln connects with a material exit hood at the lower end and ducts for waste gases. This requires gas-tight seals at either end of the kiln. The

exhaust gas Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through ...

may go to waste, or may enter a preheater, which further exchanges heat with the entering feed. The gases must be drawn through the kiln, and the preheater if fitted, by a fan situated at the exhaust end. In preheater installations which may have a high pressure-drop, considerable fan power may be needed, and the fan drive is often the largest drive in the kiln system. Exhaust gases contain dust, and there may be undesirable constituents, such as

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

hydrogen chloride The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...

. Equipment is installed to scrub these from the gas stream before passing to the atmosphere.

Differences according to the process

Kilns used for DRI production

Citations

General and cited sources and further reading

* R. H. Perry, C. H. Chilton, C. W. Green (ed.), ''Perry's Chemical Engineers' Handbook'' (7th Ed), McGraw-Hill (1997), sections 12.56-12.60, 23.60, . * K. E. Peray, ''The Rotary Cement Kiln'', CHS Press (1998), . * Boateng, Akwasi, ''Rotary kilns: transport phenomena and transport processes''. Amsterdam; Boston: Elsevier/Butterworth-Heinemann (2008), .

External links

* {{Authority control Kilns Lime kilns Firing techniques