Calcium sulfate

 

Calcium sulfate

Chemical formula	CaSO4
OTP appearance	white solid
Molar Mass(g/mol)	136.14
Density(g/cc)	2.96
Melting Point(°C)	1460
Solubility in water(g/L)	2.1
NFPA 704	0 1 0

Uses

Primary

• The γ-anhydrite form (1-5 molar percent water) is the primary component of gypsum plaster

Natural occurrence

• -dihydrate occurs naturally as gypsum

Production

Extraction

from gypsum

• Heat gypsum to over 180°C producing anhydrite

Synthesis

as a byproduct

• During the production of phosphoric acid:

  Ca₃(PO₄)₂ + 3 H₂SO₄ → 2 H₃PO₄ + 3 CaSO₄

from chalk

• Combine sulfuric acid and calcium carbonate

  CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Disposal

Calcium sulfate is not a terminal state for sulfate or calcium. Calcium sulfate may be converted to sodium sulfate and either calcium hydroxide or calcium carbonate by combining it with monovalent alkali metal salts while maintaining low temperatures, large amounts of water, and strong intermixing:

CaSO₄(s) + 2 NaOH(aq)

{

→

≤ 18°C strong mixing}

Ca(OH)₂(aq) + NaSO₄

CaSO₄(s) + Na₂CO₃(aq)

{

→

cold, strong mixing}

CaCO₃(aq) + Na₂SO₄ // may also work with bicarbonate, producing bisulfate

See Also

• Gypsum