Calcium sulphate (CaSO4) is a chemical compound made from calcium, oxygen and sulphur. The solid compound is inorganic with a molecular weight of 136.14g/mol and a melting point of 2,660°F (1460°C).  Calcium sulphate is insolvent in water and most of the organic solvents. The compound is also known as anhydrous gypsum.

The chemical compound exists in three forms:

  • CaSO4 (anhydrous calcium sulphate); the anhydrous form of CaSO4 is available in two states: soluble anhydrite and insoluble anhydrite.
  • CaSO4·2H2O (calcium sulphate dihydrate); it’s also known as precipitated calcium sulphate, mineral white, native calcium sulphate, light spar and terra alba.
  • CaSO4·1/2H2O (calcium sulphate hemihydrate); famously referred to as dried gypsum and plaster of Paris.

The three forms of calcium sulphate have different physical properties despite having similar chemical properties. Calcium hemihydrate and anhydrous calcium sulphate are odourless and found in two states, crystalline solid or powder form. The calcium sulphate dihydrate also occurs in two forms, either white lumps or powder. When you heat both calcium sulphate dihydrate and calcium sulfate hemihydrate, they are converted to an anhydrous state.

CaSO4·2H2O → CaSO4 + 2H2O.

Sources of Calcium Sulphate

Calcium sulphate can be easily found from naturally occurring anhydrite or gypsum.  Anhydrite occurs in many locations around the world in the form of evaporites. Evaporite is water-soluble, natural, mineral sediment resulting from crystallisation and concentration by evaporating an aqueous solution. There are two main types of evaporites: marine and nn-marine. Marine evaporates are commonly referred to as ocean deposits, while non-marine evaporites are located in standing water bodies such as  lakes.

Evaporates are sedimentary rocks formed by chemical sediments. When mining for calcium sulphate, evaporites are extracted from deep and open cast mines. The annual production of gypsum in the world is 127 million tons. There are other commercial means of producing Calcium sulphate (CaSO4).

There are also several chemical processes whereby calcium sulphate is produced as a by-product. These processes include:

  • The production of phosphoric acid (H3PO4) from phosphate rock; Ca3 (PO4)2 (calcium phosphate is treated with H₂SO₄ (sulphuric acid) and calcium sulphate precipitate.
  • In flue-gas desulphurisation (FGD); the exhaust gases from the fossil fuel power stations are scrubbed to lower their sulphur oxide levels. This process is carried out by injecting fine lime or limestone. The chemical reaction produces an impure form of CaSO3·x (H2O) (calcium sulphite). The calcium sulphite then oxidised to form calcium sulphate when in storage.
  • Hydrogen fluoride (HF); calcium fluoride () is reacted with H₂SO₄ (sulphuric acid). The chemical reaction precipitates calcium sulphate.
  • Zinc refining; the co-precipitation of heavy metals like barium, zinc sulphate (ZnSO4) is treated with calcium hydroxide or hydrated lime (Ca (OH) 2.). One of the final products is calcium sulphate (CaSO4).
  • The last process of recovering calcium sulphate (CaSO4) from scrap drywall located at construction sites.

The above processes concentrate any radioactive element present in the calcium sulphate compound. Regardless of the method you use to get calcium sulphate, it’s crucial to take the necessary precautions and protective measures.