The effect of salt-phase composition on the rate of soda-ash roasting of chromite ores
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8/28/03
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The Effect of Salt-Phase Composition on the Rate of Soda-Ash Roasting of Chromite Ores VILAS D. TATHAVADKAR, M.P. ANTONY, and A. JHA The formation of a liquid phase during the early stages of the roasting reaction is a common problem in the sodium chromate manufacturing process. The molten salt phase, which is primarily constituted of a binary mixture of Na2CrO4 and Na2CO3, creates major operational problems such as the granulation and blocking of the kilns. In addition to the operational problems, it was observed that the molten salt also affects the transport of oxygen toward the reaction interface. The mechanism of the soda-ash roasting reaction has been analyzed for improving the yield of sodium chromate. It was observed that the conversion efficiency of the roasting process changed dramatically, depending on the origin and the type of the chromite ores used. Thermal and scanning electron microscopic analyses of the products of roasting were carried out to establish the reaction mechanism. It was observed that the presence of silicates in the chromite ores interferes with the formation of sodium chromate involving the binary Na2CO3-Na2CrO4 liquid. The roasting reaction proceeds in a certain crystallographic direction in the chromite spinel in the presence of a nonsilicate molten salt, whereas a complete dissolution of chromite appears to take place in the binary liquid containing silicate phases present in the ore.
I. INTRODUCTION
CHROMIUM-chemicals-producing industries have come under enormous pressure from environmental protection groups and regulations to control the transmission of Cr6 in the environment. The old landfill sites, used for the disposal of chromite ore processing residue (COPR) containing carcinogenic and mutagenic Cr6 ions, have now become a major source of ground and surface water contamination.[1] The COPR from these landfill sites is highly alkaline, with a pH in the range of 10 to 12, as it contains a significant amount of lime. In 1845, a lime-based roasting process was developed.[2] In this process, lime was incorporated in the range of 70 to 100 pct of chromite ore in the charge to control the siliceous and acid gangue minerals in the ore. The main beneficial aspects of the lime addition were the reduced consumption of sodium carbonate in the secondary reactions, e.g., the control of sodium silicate, and the enhanced extraction efficiency, which solved the major operational problems such as the globule formation and kiln chocking.[3] However, the addition of lime generates a highly alkaline COPR, which contains 1 to 2 pct Cr6 compounds. It also results in the generation of CaCrO4, which is volatile and can be easily airborne, causing a major threat to human, animal, and plant health. The treatment of the highly alkaline COPR is very difficult and expensive, which is why an alternative lime-free or low-lime roasting process was developed in the 1960s,[2] which generates less alkaline COPR. However, the age-old lime-based process is still used extenVILAS
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