The soda-ash roasting of chromite ore processing residue for the reclamation of chromium
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NTRODUCTION
CHROMIUM chemicals are commercially produced via the alkali roasting (soda-ash roasting) of chromite ores with sodium carbonate. The chromite mineral is a solid solution of MgO⭈Al2O3, FeO⭈Cr2O3, MgO⭈Cr2O3, and MgO⭈Fe2O3 spinels. During the soda-ash roasting reaction, sodium carbonate is mixed with the chromite ore and heated at temperatures around 1423 K in air. The sodium oxide produced as a result of the decomposition reaction of Na2CO3 reacts with the chromium ion (Cr3+) from the chromite spinel in an oxidizing atmosphere and forms water-soluble sodium chromate.[1,2] The overall reaction can be described by (Fe,Mg)[Cr,Al,Fe]2O4 ⫹ 2Na2CO3 ⫹ 3O2 → 2Na2CrO4 ⫹ (Fe,Mg)[Al,Fe]2O4 ⫹ 2CO2
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After the reaction, sodium chromate is leached out in water. The maximum efficiency of the sodium chromate formation, achieved so far by using the alkali roasting process, is less than 85 pct. Unreacted chromium oxide remains with the refractory oxides and ends up in the waste stream, for landfills. The main thrust of the previous research on soda-ash roasting[1,2,3] has been to optimize the process parameters for improving the kiln performance. Various methods, such as pellet roasting[4] and duplex pellet technology,[5] were investigated.
M.P. ANTONY, formerly Post-doctoral Research Fellow, with the Department of Materials, University of Leeds, is Scientist, SO/F, Fuel Chemistry Division, IGCAR, Kalpakkam-603102, India. V.D. TATHAVADKAR and C.C. CALVERT, Postdoctoral Students, and A. JHA, Professor, are with the Department of Materials, University of Leeds, Leeds L2W 9JT, United Kingdom. Manuscript submitted November 2, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
Techniques such as sodium hydroxide leaching[6,7] and the hydrometallurgical extraction of chromate from lean chromite ores[8] were also reported. The lime-based process, in which lime is added in the roasting charge, was developed for the roasting of MgO- and SiO2-rich chromite ores.[1] This process enhanced the conversion efficiency dramatically and also improved the operational conditions of the kiln. However, the traditional method of sodium chromate extraction done on a commercial scale is based on soda-ash roasting either in a kiln or in a rotary hearth furnace. The residue generated from the lime-based process contains 0.1 to 0.2 pct hexavalent chromium and the treatment to reduce the Cr6+ concentration from this residue is very difficult. The landfill sites of these residues are now major sources of Cr6+ contamination.[9] In the alkali roasting process, alumina and magnesia are two critical components; they are invariably abundant with the chromite ores. Alumina and magnesia in the presence of silica increase the soda consumption and form complex sodium aluminum silicates.[3] The formation of a binary Na2CO3-Na2CrO4 liquid phase is also reported.[9] A thin film of the binary liquid covers the chromite grains and retards the diffusion of oxygen to the reaction interface. As a result of the combined effects of the presence of silica and the binar
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