Calcium sulfate

• Anhydrite
• E 516

• 7778-18-9 (anhydrous)
• 10034-76-1 hemihydrate
• Dihydrate 10101-41-4
• Hydrate 13397-24-5

White odorless solid

Fixed

2.96 g · cm -3

The substance decomposes on heating (decomposition temperature at 700 ° C)

Sparingly soluble in water (approx. 2 g · l-1 at 20 ° C)

1.5 mg · m-3

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Calcium sulfate is a chemical compound selected from the group consisting of calcium compounds and sulfates, which is known as the dihydrate gypsum.

Occurrence

Calcium sulfate occurs naturally in the form of the minerals anhydrite CaSO4, gypsum Ca [ SO4 ] · 2H2O (dihydrate ) and bassanite Ca [ SO4 ] · ½ H 2 O ( hemihydrate ) in front of evaporites.

Production and representation

It is obtained as a dihydrate in many wastewater treatment processes, when it comes to the neutralization of sulfate-containing process wastewater or sulfuric acid pickling, and in large quantities also in flue gas desulfurization with calcium to.

Because calcium sulfate is formed in many chemical processes (usually in the form of gypsum) as a secondary product, for example in the Citronensäureherstellung, a targeted industrial production on a larger scale is unnecessary. The resulting in the production of phosphoric acid, so-called phosphogypsum is, inter alia, contaminated with uranium and a problem waste. The traditional process is the precipitation of sulphate of water with lime or limestone:

Calcium sulfate modifications

• α - hemihydrate (CaSO4 • ½ H2O) produced in a closed vessel (autoclave) under wet steam atmosphere or pressure in acids and aqueous salt solutions. It is the starting material for harder plaster (type III, IV and V) and requires less water but more the setting time.
• β - hemihydrate (CaSO4 • ½ H2O) produced during burning in an open vessel under normal atmosphere. When mixed with water is carried out within minutes for a hydration dihydrate. It is the starting material for the softer plasters.

In the case of α - and β - hemihydrate is not different Modikikationen, but different Kristallinitätszustände the Halbhydratstruktur.

• Anhydrite III ( CaSO 4 • H 2 O x, 0
• IIs anhydrite (CaSO4 ) occurs at temperatures between about 300 to 500 ° C, the s is " soluble" for. When mixed with water, the hydration takes place within hours and days.
• IIU anhydrite (CaSO4 ) is formed at temperatures of 500 to 700 ° C from the anhydrite IIs, the u stands "insoluble" for.
• I anhydrite (CaSO4 ) is the high- temperature modification of CaSO4, it is formed at 1180 ° C.

Properties

Calcium sulfate is a white odorless solid that is sparingly soluble in water and decomposes at a temperature of 800 ° C, calcium oxide and sulfur trioxide formed. The crystal dehydration of the dihydrate is carried out at 100-130 ° C, which of the hemihydrate at temperatures greater than 120 ° C. Calcium sulphate is dissolved in mineral source, drinking and tap water and, together with calcium chloride and the corresponding salts of magnesium, the permanent ( permanent ) hardness. It crystallized from aqueous solution at room temperature as a dihydrate (gypsum). This crystallizes in the monoclinic space group A2 / a with lattice parameters a = 6.52 Å; b = 15.18 Å; c = 6.29 Å and 127.4 ° and β = 4 formula units per unit cell. The hemihydrate bassanite crystallizes in the monoclinic space group I2 with the lattice parameters a = 12.04; b = 6.93; c = 12.67 and β = 90.27 °, and 12 formula units per unit cell.

Use

The main field of application of CaSO4 phases is in the building materials sector. In the food industry, it is a by- product in the preparation of tartaric acid and has been approved as a firming agent, acidity regulator and carrier. Among other things, it is used in the manufacture of tofu.