Horizontal kilns

Beside continuous horizontal kilns, numerous other methods for dry pyrolysis of urea have been described, eg, use of stirred batch or continuous reactors, ribbon mixers, ball mills, etc (109), heated metal surfaces such as moving belts, screws, rotating dmms, etc (110), molten tin or its alloys (111), dielectric heating (112), and fluidized beds (with performed urea cyanurate) (113). AH of these modifications yield impure CA. 

Probably the rotary horizontal kiln is the most versatile, since it allows a feed of lumps or fines of limestone or marble, or wet or dry calcium carbonate sludges (Fig. 7.1). The main component of this calcination system is a 2.5- to 3.5-m diameter by 45- to 130-m long firebrick-lined inclined steel tube. Heat is applied to the lower end of this via oil, gas, or coal burners [7]. The feed to be calcined is fed in at the top end. Slow rotation of the tube on its axis gradually moves the feed down the tube, as it tumbles countercurrent to the hot combustion gases. In this way, wet feed is dried in the first few meters of travel. Further down the tube, carbon dioxide loss begins as the temperature of the feed rises. By the time the solid charge reaches the lower, fired end of the kiln it reaches temperatures of 900-1,000°C and carbon dioxide evolution is virtually complete. Normally the temperature of the lower end of the kiln is not allowed to go much above this as it reduces the life of the kiln lining. It also adversely affects the crystal structure of the lime product since it produces a dead-burned or overburned lime. Overburned lime is difficult to slake to convert it to calcium hydroxide and raises... 

Most of the kilns prior to 1935 operated on natural draught , i.e. the combustion products were drawn through the kiln by the chimney effect of vertical kilns, often supplemented by a small chimney. As a result, output rates were low (e.g. 1.5 t/d per m of cross-section). Horizontal kilns operated on the draught produced by tall chimneys. 

Vertical Kilns Horizontal Kilns Other/Mixed... 

A calcining kiln is a horizontal steel cylinder, slightly sloped to help the coke move forward and lined with refractory brick. The raw coke is fed at the upper end, natural gas or oil is burned at the lower end, and the combustion gas flows through the kiln above and against the coke stream. 

Kilns. Reactions involving free-flowing solid, paste and slurry materials can be carried out in kilns. In a rotary kiln, a cylindrical shell is mounted with its axis making a small angle to the horizontal and rotated slowly. The solid material to be reacted is fed to the elevated end of the kiln and it tumbles down the kiln as a result of the rotation. 

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