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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 6 m × 60 m (20 ft × 197 ft) kiln, including refractories and feed, is around 1,100 t (2,400,000 lb), 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.
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below approximately 250 °C (482 °F). 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 80 to 300 mm (3 to 12 in). 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 350 °C (662 °F) to protect the steel from damage, and continuous
350:, 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.
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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
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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
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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.
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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
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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
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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
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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
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to start the kiln with a large eccentric load. A 6 m × 60 m (20 ft × 197 ft) 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
262:. 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.
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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
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plate, usually between 15 and 30 mm (0.6 and 1.2 in), welded to form a cylinder which may be up to 230 m (750 ft) in length and up to 6 m (20 ft) in diameter.
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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
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Grzella, Jörg; Sturm, Peter; Krüger, Joachim; Reuter, Markus A.; Kögler, Carina; Probst, Thomas (2005).
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lining, support tyres (riding rings) and rollers, drive gear and internal heat exchangers.
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scanners are used to give early warning of "hot-spots" indicative of refractory failure.
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General layout of a direct fired countercurrent rotary kiln used in cement manufacture
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Exhaust gases contain dust, and there may be undesirable constituents, such as
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Rotary kiln (large rust-colored horizontal tube at center right) at a
Wyoming
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160:) in a continuous process. Materials produced using rotary kilns include:
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for this may be gas, oil, pulverized petroleum coke or pulverized coal.
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756:. Amsterdam; Boston: Elsevier/Butterworth-Heinemann (2008),
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Rotary kiln body, with drive gear and 2 tyres (riding rings)
726:(7th Ed), McGraw-Hill (1997), sections 12.56-12.60, 23.60,
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Rotary kilns: transport phenomena and transport processes
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may go to waste, or may enter a preheater, which further
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The basic components of a rotary kiln are the shell, the
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and ferrous sands : size between 0.05 and 0.5 mm.
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device used to raise materials to a high temperature (
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Kiln tyre closeup showing typical chair arrangement
49:. Unsourced material may be challenged and removed.
389:Direct reduction processes based on a rotary kiln
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722:R. H. Perry, C. H. Chilton, C. W. Green (ed.),
278:0.5% of the length) if the kiln is very long.
717:General and cited sources and further reading
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338:Heat exchange in a rotary kiln may be by
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109:Learn how and when to remove this message
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258:The rotary kiln was invented in 1873 by
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724:Perry's Chemical Engineers' Handbook
452:Preferred iron content of ore (% Fe)
379:Differences according to the process
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47:adding citations to reliable sources
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695:. John Wiley & Sons. p. 7.
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677:. UM Libraries. 1900. pp. 3–.
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321:. This must have high starting
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34:needs additional citations for
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426:Consistency of kiln discharge
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334:Internal heat exchangers
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221:Principle of operation
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609:Examples of processes
476:Size of ore feed (mm)
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16:Pyroprocessing device
783:at Wikimedia Commons
741:, CHS Press (1998),
674:The Michigan Technic
43:improve this article
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636:Krupp-Renn Process
557:Oxygen removal (%
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210:a wide variety of
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807:Firing techniques
779:Media related to
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752:Boateng, Akwasi,
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732:978-0-07-049841-9
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405:Extraction Point
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282:Refractory lining
260:Frederick Ransome
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99:December 2019
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58:"Rotary kiln"
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54:Find sources:
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32:This article
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781:Rotary kilns
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242:Construction
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175:Refractories
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127:cement plant
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41:Please help
36:verification
33:
433:semiliquid
360:exhaust gas
195:Vermiculite
158:calcination
150:rotary kiln
802:Lime kilns
791:Categories
594:12 %
551:1400-1500
548:1200-1300
344:convection
340:conduction
312:Drive gear
272:mild steel
266:Kiln shell
248:refractory
180:Metakaolin
69:newspapers
659:Citations
545:900-1100
495:< 0.2
348:radiation
707:ilmenite
647:See also
616:Highveld
542:600-900
532:2.8-3.0
486:< 10
483:< 20
480:< 20
444:pig iron
289:infrared
208:roasting
200:Iron ore
640:Basset
600:>90
492:< 5
438:clinker
254:History
234:. The
212:sulfide
202:pellets
190:Alumina
83:scholar
760:
745:
730:
632:Krupp
624:Elkem
613:Lurgi
597:20-70
503:CaO/Al
470:50-67
467:25-45
464:55-63
461:30-60
456:30-60
442:liq. (
436:sol. (
430:solid
323:torque
165:Cement
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797:Kilns
693:(PDF)
630:SL/RN
621:LARCO
489:5-25
152:is a
90:JSTOR
76:books
758:ISBN
743:ISBN
728:ISBN
705:For
603:100
529:0.3
346:and
236:fuel
227:kiln
225:The
215:ores
170:Lime
62:news
627:RN
618:Udy
371:or
45:by
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571:Fe
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