Studies in the carbothermic reduction of phosphogypsum
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INTRODUCTION
Another method of reduction of gypsum to calcium sulfide is by using carbon as a reducing agent. The reduction can be carried out using different types of carbon such as coke, lignite, charcoal, wood charcoal, and graphite between 900 ~ to 1000 ~ Gypsum is reacted with coke in a rotary kiln when the following sequence of reactions takes place in the successive zones of the kiln depending on the temperature: 18-~u
GYPSUM is obtained in large quantities as a by-product phosphogypsum from phosphoric acid plants. Here, sulfuric acid is reacted with phosphate rock to get phosphoric acid. Gypsum is also produced in large quantities from flue gas desulfurization units and thermal power plants, where lime is used as a throwaway SO2 absorbent for pollution control purposes. At present, by-product gypsum is used in the manufacture of cement, partition blocks, sheets and tiles, insulation boards, and plaster of paris. Gypsum is also used in other industries like pharmaceuticals, textile, asbestos products, paints, e t c . and as fertilizer for soil conditioning. The trapped sulfur value, however, is not at all recovered for use again as a sulfur source in various chemical synthesis processes. Even though the by-product gypsum has been utilized for the above-mentioned purposes, the supply of waste gypsum is quickly overcoming its demand and billions of tonnes of it is stockpiling. I1,21 Sometimes the disposal of calcium sulfate also poses an expensive environmental challenge, so the need of the time is to welcome the development in research to change the fate of calcium sulfate from a waste disposal material to a potentially valuable mineral commodity. At present, the main chemical use of phosphogypsum is in the manufacture of sulfuric acid. During this process, the first main step involved is the decomposition of calcium sulfate to calcium sulfide which is achieved by reducing it at high temperatures either with coke, coal, or with reducing gases. In most of the studies, the reduced products are treated with steam to get hydrogen sulfide and calcium oxide as final products. In some cases, the gypsum is directly decomposed at very high temperatures to get sulfur dioxide and calcium oxide. This sulfur dioxide is further used for the sulfuric acid preparation. The reduction of phosphogypsum to calcium sulfide is achieved by its reaction at high temperatures with reducing gases like carbon monoxide, natural gas, hydrogen, water gas, town gas, and steam. This reduction suffers from two main disadvantages: (a) a large excess of gaseous reductants is required and (b) the nature of solid and gaseous products is quite varied depending upon the composition of reducing gases and reaction conditions. [3 7]
Carbothermic reduction of phosphogypsum or gypsum has the advantage of giving definite specified product, depending on the reaction temperature and gypsum carbon ratio. 1~3,~4-1sl Thus, to achieve CaS as an exclusive product, the reaction should be carried out at low temperatures and in the presence of stoichiometri
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