KILN. 492 KILN. employed cliiefly in England. They are ordinarily iiilenniltent in operation, but by having two or more drying eliambers attached to a single kiln, so that some are receiving the hot gases while others are being strippeu ot the dry mixture and re- charged with fresh wet mixture, they may he made to operate continuously. Continuous cham- ber kilns are employed only in a few works in France. Conlinuous Icilns are made in two general forms, which may be defined as vertical chamber kilns and ring kilns. The Dictzsch kiln is a well- known exani]dc of the vertical cliamber type. It consists internally of three coimeeting chamljers. .t the top is the heating chamber, into which the raw mi.xture is led by one or more charging eyes. This heating chamber connects by means of a horizontal pa.ssagc with a second vertical chamber, which is called the crucil)le. and in which the calcination of the clinker takes place. Below the crucible is the cooling ehamlicr in which the burned clinker from the crucible is cooled by the current of air which passes up through it from the drawing eye below. The ring kiln or Holl'nuin kiln consists of a ring-shaiied chamber surrounding a central sliaft connecting with a chimney. This ring-shaped chamber has flues at regular intervals, which connect with the chimney, and doors are ])laced in the outer walls at corresponding regular intervals. This arrange- ment of lines and doors makes it possible to divide the large annular cl;ai;i1icr inlo several — generally from 15 to l)0 — sfparale compartments, each of which has a flue connecting with the chimney, and a door opening to tlie outside. The burning proceeds pro;;ressively from one chamber to another around the ring. Continuous kilns of the vertical chamber and ring types are used chiefly in Continental Europe. Jiotarii kilns for burning cement are distinctly an American development, although the device was invented in England. A rotary kiln consists of a steel or iron c.ylinder lined with fire-brick or some other refractory substance to resist the heat, and mounted on roller bearings, generally 60b'~ Fig. 1. ROTARY CEMEST-KII.N. placed at two points near the ends. A circum- ferential rack on the shell enables the cylinder to be rotated by worm-gearing or by a sprocket chain. The head ot the cylinder is inclosed by the chimney in such a manner as to be free to rotate, and is somewhat higher than the front, vhich is covered by a movable cap, tlirotigh which the crude oil or powdered coal is forced .by air- pressure. The raw material, eitlier dry or wet, is fed into the upper end of the kiln, and grad- ually works toward the bottom, in consequence of the inclinatirm and the rotary movement of the cylinder. Calcination takes place during the passage. AH four of the classes of kilns described are used for burning Portland cement. Xatural cement is burned in a dome kiln much resembling those used for burning Portland cement, but smaller in size and somewhat fatter in shape. and the operation is continuous instead of inter- mittent. See Cement. Brick-Kilns may be temporary or iicrmanent. The former are built up of the bricks themselves, generally in a number of rows of parallel arches, which, may contain as many as 40,000 brick and be 40 cour.ses in height. Burnt brick arc placed around the outer sides and on top of the kiln, the walls being daubed with mud. Openings are left at the top to jK'rmit the escape of steam arising from the liberated moisture in the brick. l*"ires are started in the windward end of each b?^^^?^i??^b^?^^ W/////////////////. Fig. 2. DOWN-IUIAUGIIT BRICK-KILN. arch, then at the other eiul, and allowed to ap- proach each other sluwly, several days sometimes being required. I'lie doors are closed and sealed to prevent the entrance of air. Wood, coal, or oil may be used for fuel, the oil requiring special burners, and bein.g used but comparatively little. Permanent hrick-kitns have fixed side walls, but may be open or closed at the fop. If open, the fires extend beneath the whole length of the charge, the gases j)assing U])ward, much as in temporary kilns. If closed, the fires are at one end. Both up-draught and down-draught are used with the permanent, closed-top kilns. In up- draught kilns the (barging holes are above the level of the fire-holes, the latter being on the out- side. In the down-draught kilns the gases enter through the flues ranged around the inside of the kilns, ('ontinuoiis brick-kilns have a series of connecting chambers fired in succession, the gases passing from the first to the second, and so on. Filling, burning, and emptying goes on in differ- ent chambers at the same time. Firebrick and refractoiv ware are frequently burned in down- draught kilns, which must be lined with fire-brick. Burning requires five or six days, and cooling several days more. Paving-brick, in the Middle West, are generally burned in the down-draught kilns. The tempprature while burning ranges from 1600 to 2300 degrees Fahrenheit. The burn-
