The effect of salt-phase composition on the rate of soda-ash roasting of chromite ores

Abstract

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.