Long dry kilns

However, they work well when the feed particles are large. The reason for the relatively shorter length is that the feed is dry with a moisture content the same as granular solids rather than slurry. Applications include lime kilns and lightweight aggregate kilns where the mined stones are crushed to about 1.3-5 cm (0.5-1.5 in.) before feeding them into the kiln. 

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Dendritic periclase Relatively fast cooling from temperatures above 1500 = C, long dry kiln (DeHayes, Grady, and Vidergar, 1986)... 

Legate, J.H., "Lake Ontario Cement s Experience with Various Raw Materials on a Long Dry Kiln and a Four Stage Preheater Kiln," Proceedings of the 9th International Conference on Cement Microscopy,Inter-national Cement Microscopy Association, Reno, Nevada, 1987, pp. 228-233. 

These are shorter than wet kilns with lengths on the order of 90-120 m (about 300-400 ft). For long dry kilns, as with wet kilns, the drying, preheating, and calcination all occur in the one single vessel (Figure 1.6). 

After the blends have been prepared (either in the dry or wet process), these materials are fed at a uniform rate into a long rotary kiln. The materials are gradually heated to a liquid state. At temperatures up to about I,600°F the free water evaporates, the clay minerals dehydroxylate and crystallize, and CaCO, decomposes. At temperatures above 1,600°F the CaCO, and CaO react with aluminosilicates and the materials become liquids. Heating is continued to as high as 2,800°F. 

The drying times may range from days to months. Wood, for example, is dried in hot air kilns from weeks to months, depending on the size of the pieces to be dried and the type of wood species. Superheated steam drying under vacuum conditions enhances drying rates as well as product quality. Only batch dryers are suited for these long drying time requirements. 

The secondary air requirements of wet-process kilns or long dry-process kilns with a specific heat consumption of 5.0—5.5 GJ/t of clinker (1200—1300 kcal/t) is in the region of 1.3-1.5 Nm /kg of clinker. 

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

It is of firebrick. Fig. 558 shows the kiln in elevation, Above is a hot-floor, on which the charcoal is dried before it is strongly heated. The hot floor is seen also in tire longitudinal section, whore it is shown to be heated by the flux. which comes from the furnace on its way to the chimney. The kiln represented here contains twelve chambers for the charcoal, each two feet long and throe inches broad. 

Newcastle-upon-Tyne also carries on an extensive trade in the manufacture of fire-bricks from the somo days above enumerated, and the preparation of which is as follows —The clays, alter exposure to the atmosphere for somo time, aro removed to the clay-mill, and ground up with fragmento of the some clay previously burnt, foern whence it is token to. the pug-mill foT mastication, before it is taken to tho moulder. The bricks are then laid out to dry, and afterwards burnt in a kiln about fifteen feet long by fourteen feet broad, and ten feet high. Each kiln will burn 24... 

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... 

With a slow-drying impermeable species a fan speed as low as 1.5-2.0 m s may be sufficient as the moisture eontent at the surface drops quickly below the fibre saturation point - no mass flow - and there is no point in installing overly powerful fans just to strip off surface moisture for the first few minutes of a long kiln schedule (>14 days) thereafter the slow rate of transfer of moisture from the centres of the boards to their surfaces becomes more important than the rate of evaporation. 

While collapse-prone and impermeable lumber may require long, slow air-drying, elsewhere the trend is overwhelmingly toward investment in fast, eontrolled drying with kilns. Kiln-drying requires technical skill and considerable eapital investment. However it offers distinctive benefits over air-drying. 

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