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INTRODUCTION

In a conventional shaft kiln, it is well known that size of limestone should be sieved to 30 - 80 mm in order to have combustion gas flowing through the high bed of limestone.
However, nearly 45% of mined limestone usually goes to small size, less than 30mm, which cannot be processed in conventional shaft kilns. Rotary kilns and fluidized bed calciners have been applied to deal with such undersize limestone so far; nevertheless, their low thermal efficiency and high construction cost are not economical for production of small or medium scale.

The CHISAKI LIME KILN was developed to calcine limestone and other inorganic materials of 10 - 35 mm by application of moving bed principle, assuring the high thermal efficiency and reasonable construction cost. More than 30 units have been constructed and under operation as scheduled, making good use of undersize limestone and dolomite with high thermal efficiency.


According to academic investigations, necessary time for complete calcination of a limestone is proportional to the square of its diameter. Thus, 1/4 of the residence time is enough in the calcination zone for 20 mm limestone, compared with 40 mm one. At a given throughput of limestone, volume of calcination zone in a shaft kiln could be drastically reduced when limestone of small size can be used.

In addition, a bed of 20 mm solids has twice of surface area for heat transfer compared with a bed of 40 mm solids, and heat transfer coefficient for the former is 50% larger than the latter. Therefore, better heat exchange is surely carried out in the moving bed of 20 mm solids.

Serious drawbacks of the smaller size usually encounter in any shaft kiln of conventional design. First one is the high pressure drop, which hinders the necessary gas stream to flow through the bed. Second, mal-distribution of flowing gas is apt to occur, resulting in poor contact between solids and gas streams. Conventional shaft kilns have hardly overcome the above two barriers, proving it inevitable to use limestone of such large size as mentioned above.

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