Comparison of the Rotary Kiln with Other Contactors

As can be seen from the history of its evolution, the design and operation of rotary kilns have undergone a systematic evolution since the days of the ancient Egyptians. Improvements include reduced labor, increased productivity, mixing, heat transfer, and product quality. Mineral processing kilns can be classified as vertical, horizontal, or other miscellaneous mixed types (Table 1.). At one extreme, vertical kilns operate in the packed-bed mode whereby the material being processed (calcined) is charged from a top hopper and contained in a vertical chamber in which the static bed moves, en bloc, downward in plug flow. An example is an annular shaft kiln schematic shown in Figure 1.

Table 1. Typical Features of Rotary and Other Contact Kilns

Here the charge can be either in countercurrent or in parallel flow to the combustion gases that transfer heat to the solids (e.g., limestone) as the gas flows through the particle-gas interstices. To maximize heat and mass transfer in such devices, ample voidage within the particulate charge is necessary.This ensures uniform circulation of hot gases through the packed bed. Feed particle size and distribution must be selected to ensure an optimum voidage. Typically, particle size greater than 50mm (2 in.) is normal for shaft kilns leading to a typical charge void

Figure 1. Schematic diagram of annular shaft kiln.

fraction of about 45 percent. At the other extreme to packed beds, as encountered in vertical shaft kilns, are fluidized bed contactors or related kilns whereby the charged particles are suspended by the hot gases in a dilute phase (Figure 2.).

Figure 2. Schematic diagram of a fluidized-bed calciner.

In fluid-bed suspension kilns, the void fraction can be on the order of 60–90 percent. The hot gases perform two functions, that is, they fluidize or suspend the particles, and, at the same time, they transfer heat to the particles. Although heat transfer is extremely efficient at the gas-particle level, a tremendous amount of energy is required to keep the bed in suspension and to move the charge. Since fluidization is a function of particle size, feed particles can only be fed as fines. Additionally, because of vigorous mixing associated with fluidization, attrition and dust issues can be overwhelming. In between these two extremes are the horizontal rotary kilns that offer a distinct environment for combustion gases (freeboard) and the charge (bed). Unlike packed bed vertical kilns, some degree of bed mixing is achieved by kiln rotation and associated phenomena although not to the extent achieved by fluid-bed suspension kilns. Rotary kilns have evolved as the equipment of choice for most minerals processes because they provide a compromise between the packed and suspension bed type mode of operation thereby allowing large capacity processing with few process challenges.

In spite of the distinctions described herein, most kilns, vertical or horizontal, when used for thermal processing, for example, calcination, oxidation, reduction, and so on, in a continuous operation will have distinct zones along their axial length. These will include a preheating zone where the particles are preheated, a combustion zone that normally coincides with the location along the vessel where the combustion or the flame is situated, and the discharge or cooling zone behind the flame. The extent of the intended reaction and, for that matter, product quality, is most influenced by the conditions in the combustion zone where heat must be supplied to the solids, for example, in limestone calcination well above the dissociation temperature. For product quality purposes, it is important to ensure that the temperature in the calcining zone is uniform, with no hot or cold spots and that it can be controlled within a tolerable limit. Of all the furnace types described above, one can say that the rotary kiln offers the best potential to control the temperature profile.