Limestone calcination in a rotary kiln
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I.
INTRODUCTION
ABOUT21 million tonnes of lime is produced annually in North America primarily for use as a flux in steel making. 1,2 Roughly 80 pet of this lime is produced in over 180 rotary kilns using crushed limestone as feed. About 7 pct is used in the pulp and paper industry partly as a makeup to kilns that calcine precipitated CaCO3 from the chemical recovery cycle of the Kraft process. The efficiency of design and operation of rotary kilns for limestone calcination has improved in an evolutionary manner over many decades of use, but recent rapid increases in fuel prices have prompted interest in a better understanding of the calcination process. Although a wealth of literature exists on the calcination of limestone, little of it is of direct use for rotary-kiln design and operation. The basic calcination reaction CaCO3 ---> CaO + COs
[1]
is endothermic with a heat of reaction of 1.70 MJ/kg CaCO3 at 1173 K 3, the dissociation temperature. Fuel consumption in the calcination process varies over a wide range depending on several factors: whether dry stone or mud is fed to the kiln, the kiln design, the presence of preheaters, recuperators and coolers, the type and thickness of refractories used to line the kiln, and the scale of operation. For rotary kilns fed with dry stone, reported energy requirements 4 are in the range 6 to 8 GJ/tonne CaO, which compares to the heat of reaction of 3.04 GJ/tonne CaO. Thus, there is considerable scope for energy savings if the heat-exchange process can be made efficient. This paper describes experimental studies of calcination and heat flow in a pilot rotary kiln. The effects of operating variables such as feed rate, rotational speed, kiln inclination angle, pct fill, and particle size on the temperature, heat flow and calcination fields within the kiln, and on the lime quality are described. Experiments have been done on several different high calcium limestones, and on a dolomitic limestone. A. P. WATKINSON, Professor, Department of Chemical Engineering, and J.K. BRIMACOMBE, Stelco Professor of Process Metallurgy, Department of Metallurgical Engineering, are both with the University of British Columbia, Vancouver, BC V6T lW5, Canada. Manuscript submitted October 22, 1981. METALLURGICALTRANSACTIONS B
An overall goal of this work has been to relate the observed temperature and calcination patterns to the heattransfer processes which govern the performance of the kiln, and to provide realistic calculation methods for kiln design. To this end experimental studies of effects of burner conditions, bed mixing, kiln internals, oxygen enrichment, and sulfur transfer have also been carded out, and will be reported separately.
H.
EXPERIMENTAL
The rotary kiln shown in Figure 1 is described in detail elsewhere. 5 It is 5.5 m long x 0.61 m O. D. x 0.406 m I.D. and fired by a premix natural gas burner. The kiln is lined with 9.2 cm of Plicast Tuff-lite over a 2 mm layer of insulating fiber. Dry limestone is fed at up to 86 kg per hour via a belt feeder into the kiln and flows co
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