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Gypsum - CaSO4 2H2O


T he Gypsum is an evaporite mineral most commonly found in layered sedimentary deposits in association with halite, anhydrite, sulfur, calcite and dolomite. Gypsum (CaSO4.2H2O) is very similar to Anhydrite (CaSO4). The chemical difference is that gypsum contains two waters and anhydrite is without water. Gypsum is the most common sulfate mineral. Gypsum is available in two forms, viz, mineral gypsum and by-product gypsum.The mineral gypsum is mined from land deposits.By-product gypsum includes marine gypsum, phosphor-gypsum, fluoro-gypsum, boro-gypsum, scrubber gypsum,etc. Marine gypsum is recovered from sea-water as a by-product while producing common salt, whereas the other types are obtained as by-product from different chemical plants. The quality of mineral gypsum available in India is very inconsistent and the percentage of calcium sulphate varies even in the same deposit. The ratio between high grade gypsum containing above 80% calcium sulphate content and the rest is generally 1:3. The quality of gypsum also depends on the size of the particles, the large particles generally have higher percentage of calcium sulphate. The average difference in calcium sulphate content between lumps ( above 5 cm) and fines(5 cm and less ) has been found to vary considerably and is generally of the order of 6 percent. Investigations have further shown that it is due to more friable material containing silica which crumbles, leaving lumps (above 5 cm) which is more concentrated in calcium sulphate. In gypsum calcium or magnesium carbonate, chlorides, other sulphate minerals, clay minerals or silica are considered as deleterious constituents. As a result most mine production of gypsum will have the purity ranging between 70 and 95%. Often it is used as mined, although in certain cases, one or more methods of mineral beneficiation are employed to upgrade the product.
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