Calcium Sulfate

Over 200 million tonnes per annum of calcium sulfate waste is produced by the phosphate industry worldwide. Heating gypsum to between 100 C and 150 C (302 F) partially dehydrates the mineral by driving off approximately 75% of the water contained in its chemical structure. The partially dehydrated mineral is called calcium sulfate hemihydrate or calcined gypsum (commonly known as plaster of Paris ) (CaSO 4 nH 2 O), where n is in the range 0.5 to 0.8. 4 In a fire, the structure behind a sheet of drywall will remain relatively cool as water is lost from the gypsum, thus preventing (or substantially retarding) damage to the framing (through combustion of wood members or loss of strength of steel at high temperatures) and consequent structural collapse. This reaction is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement. On heating to 180 C, the nearly water-free form, called -anhydrite (CaSO 4. nH 2 O where n=0 to 0.05) is produced. On heating above 250 C, the completely anhydrous form called -anhydrite or natural anhydrite is formed. In addition to natural sources, calcium sulfate is produced as a by-product in a number of processes: This is particularly the case with the phosphate by-product, since phosphate rocks naturally contain actinides.

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