Continuous kilns

Calcination. Calcination involves a low (<1000° C) temperature soHd-state chemical reaction of the raw materials to form the desired final composition and stmcture such as perovskite for BaTiO and PZT. It can be carried out by placing the mixed powders in cmcibles in a batch or continuous kiln. A rotary kiln also can be used for this purpose to process continuously. A sufficiendy uniform temperature has to be provided for the mixed oxides, because the thermal conductivity of powdered materials is always low. 

In the Netherlands, the continuous kiln is mostly applied. We distinguish two principles here the bricks pass through the fire or the fire moves across the stationary bricks. The former principle is applied in tunnel kilns. Nowadays these are about 100 metre long and up to 8 metre wide. 

Calcination can be carried out by placing the mixed powders in shallow saggers in a batch or continuous kiln. The saggers may need to be closed if any of the constituents are volatile, as is the case with lead oxide. The container surfaces in immediate contact with the powder must not react with it both to avoid contamination and to permit reuse of the sagger. The thermal conductivity of powdered materials is always low, so that a sufficiently uniform temperature can only be obtained through a depth of a few centimetres when the period at maximum temperature is, as is usual, only 1 or 2h. 

In all designs involving "acid brick," provision must be made for the irreversible growth of North American brick which takes place over an extended period. This growth or "swelling" was not much noted in "acid brick" prior to about 1954, when manufacturers stopped using "beehive" ovens and started using continuous kilns. 

Fig. 6.7-50 A continuous kiln for the drying and firing of load-bearing...

Fig. 6.7-50 A continuous kiln for the drying and firing of load-bearing stmctural bricks (courtesy Eisenmann, Boblingen, Germany) Fig. 6.8-1 Graph showing the development of worldwide iron ore pelletizing capacity during the first 40 years (1950-1990) and projections at that time for further growth [B.48]...

The earliest continuous kilns (also known as running kilns or draw kilns) were fired using the mixed-feed principle (Fig. 16.3). Alternate layers of limestone and fuel (wood or coal) were charged into the top of the kiln and lime was removed through the drawing door. 

Searle [16.2] describes a large number of designs of continuous kilns, categorising them into ... 

Continuous Kiln. A kiln in which the full firing temperature is continuously maintained in one or other zone of the kiln. There are two types annular KILN (q.v.) and tunnel kiln (q.v.). Continuous Vertical Retort. A type of gas retort, built of silica or siliceous refractories. Coal is charged into the top of the retort, coke is extracted from the bottom, and town gas is drawn off, the whole operation being continuous (cf. HORIZONTAL retort). Continuous vertical retorts are also used in the zinc industry, in which case they are built of silicon carbide refractories. 

Semi-continuous Kiln. A transverse-arch KILN (q.V.) having only a single line of chambers, so that when the firing zone reaches one end of the kiln the process of fire travel must be re-started at the other end. Kilns of this type were never common. 

Tunnel Kiln. A continuous kiln of the type in which ware passes through a stationary firing zone near the centre of the kiln. In the most common type of tunnel kiln the ware is placed on the refractory-lined deck of a car, a continuous series of loaded cars being slowly pushed through a long, straight, tunnel. 

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