The Rotary Kiln Evolution

This chapter introduces the reader to rotary kilns as an alternative to other unit operation devices. Here, the history and evolution of the rotary kiln and some processes that have found applications over the years are presented. 

Fundamentally, rotary kilns are heat exchangers in which energy from a hot gas phase is extracted by the bed material. During its passage along the kiln, the bed material will undergo various heat exchange 

1 Comparison of the Rotary Kiln with Other Contactors 

Vertical Kilns Horizontal Kilns Other/Mixed 

Traditional shaft-type kilns Conventional long wet, dry rotary kilns Fluidized-bed type kilns 

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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... 

To determine the extent of bloating or expansion in an industrial rotary kiln, one must carry out laboratory tests using bench scale furnaces for the evolution kinetics and further correlation tests in a pilot rotary kiln for appropriate temperature profiles. The temporal events determined are, in turn, used to plan quarry operations for product quality control. The same data may also be useful in developing a mechanistic mathematical model that can predict temperature distribution and density changes in the raw material as they journey through the kiln (Boateng et al., 1997). Such tools have proven to be useful for the control of product quality as new mines are explored or even as different strata of the existing mine are explored for feedstock. Some of these time-temperature histories are discussed herein. 

In view of this evolution, the present chapter will be concerned only with rotary kiln systems. 

Raw Materials. The basic raw materials limestone and coke or coal (qv) should be high quaHty. Limestone should contain a minimum of 95—97% CaCO and a maximum of 1.5% MgCO, 1—1.5% Si02, 1% Fe202 plus AI2O2, and 0.006% phosphoms (see Lime and limestone). The limestone is first converted to lime in a rotary or vertical shaft kiln. The lime is screened to eliminate fines that interfere with the evolution of carbon monoxide in the smelting process. 

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