Processing of Chromite Overburden by Soda Roasting to Recover Chromium as Sodium Chromate

Abstract

Sukinda overburden is dumped with chromite ore tailing which contains about 17% Cr2O3. The other constituent with overburden is iron, silica and alumina. Because of its complex nature and large amount of gangue material it is not suitable for the extraction of valuable chromium by conventional route. The author of the present investigation has tried to recover chromium as sodium chromate from chromite overburden. In this investigation the ore is mixed with sodium carbonate. The mixture is heated in a raising hearth furnace at 1123 K. Before roasting, TG/DTA analysis has been done to fix the operating temperature. After roasting at 90 min, the roasted product is ground and sieved. The rate-determining step of roasting for the conversion into sodium chromate has also been investigated and it shows that the required activation energy at the temperature range 923-1123 K is 60 kJ/mole. The roasted product is leached in hot water condition at an optimum pulp density of 1:5 to obtain the chromium recovery. After leaching, solid liquid separation is done. The leach liquor containing sodium chromate is vaporized and a bright yellow crystal of sodium chromate is obtained. A recovery of 85% chromium is obtained at the optimum condition and the purity of sodium chromate is found to be 99.5%. This chromate is suitable for the use in chemical and tanning industries. A portion of ore, roasted product and leached residue are subjected to XRD and EPMA studies to know the mode of conversion and phases present in different stages of reaction.