Calcium sulfate

Structural formula
General
Surname	Calcium sulfate
other names	Anhydrite E  516 Gypsum (dihydrate)
Molecular formula	CaSO 4
Brief description	white odorless solid
External identifiers / databases
CAS number 7778-18-9 (anhydrous) 10034-76-1 (hemihydrate) 10101-41-4 (dihydrate) 13397-24-5 (hydrate) PubChem 24497 Wikidata Q407258
properties
Molar mass	136.11 g mol −1
Physical state	firmly
density	2.96 g cm −3
Melting point	The substance decomposes when heated (decomposition temperature at 1450 ° C)
solubility	Slightly soluble in water (approx. 2.4 g l −1 at 20 ° C)
safety instructions
GHS labeling of hazardous substances no GHS pictograms H and P phrases H: no H-phrases P: no P-phrases
MAK	DFG : 1.5 mg m −3 Switzerland: 3 mg m −3 (measured as respirable dust )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

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

Occurrence

Anhydrite

plaster

Calcium sulphate occurs naturally in the form of the minerals anhydrite CaSO ₄ , gypsum Ca [SO ₄ ] · 2H ₂ O (dihydrate) and bassanite Ca [SO ₄ ] · ½H ₂ O (hemihydrate) in evaporites .

Extraction and presentation

It occurs as a dihydrate in many wastewater treatment processes when it comes to the neutralization of sulphate-containing process wastewater or sulfuric acid pickling, and in large quantities also in flue gas desulphurisation together with calcium sulphite .

Because calcium sulfate is a secondary product in many chemical processes (usually in the form of gypsum), for example in citric acid production , there is no need for targeted industrial production on a large scale. The so-called phosphogypsum formed in the production of phosphoric acid is u. a. contaminated with uranium and a problem waste . The classic process is precipitation from sulfuric acid water with milk of lime or limestone :

${\displaystyle \mathrm {H_{2}SO_{4}+CaCO_{3}\longrightarrow CaSO_{4}+H_{2}O+CO_{2}} }$

Calcium sulfate modifications

• Alpha hemihydrate (CaSO ₄ • ½ H ₂ O) is produced in a closed vessel ( autoclave ) under a wet steam atmosphere or without pressure in acids and aqueous salt solutions. It is the starting material for harder plasters (type III, IV and V) and requires less water but more time to set .
• β-hemihydrate (CaSO ₄ • ½ H ₂ O) is created when burning in an open vessel under normal atmosphere. When mixed with water, hydration to the dihydrate occurs within minutes. It is the raw material for the softer plasters.

In the case of α- and β-hemihydrate, it is not a question of different modifications, but rather different crystallinity states of the hemihydrate structure.

• Anhydrite III (CaSO ₄ • x H ₂ O; 0 <x <0.5) arises from the hemihydrate at temperatures up to 300 ° C. In the presence of water, including humidity, hemihydrate is formed very quickly.
• Anhydrite II _s (CaSO ₄ ) is formed at temperatures between approx. 300 to 500 ° C, the _s stands for "poorly soluble". When mixed with water, hydration occurs within hours and days.
• Anhydrite II _u (CaSO ₄ ) is formed from anhydrite II _s at temperatures of 500 to 700 ° C , the u stands for “insoluble”.
• Anhydrite I (CaSO ₄ ) is the high temperature modification of CaSO ₄ , it forms at 1180 ° C.

properties

Calcium sulfate is a white, odorless solid that is sparingly soluble in water and decomposes from a temperature of around 1450 ° C, producing calcium oxide and sulfur trioxide . The water of crystallization of the dihydrate takes place at 125–130 ° C, that of the hemihydrate at temperatures greater than 163 ° C. Calcium sulphate is dissolved in mineral-containing spring, drinking and tap water and, together with calcium chloride and the corresponding salts of magnesium, forms the permanent (permanent) water hardness . It crystallizes out as dihydrate (gypsum) from aqueous solution at room temperature. This crystallizes monoclinically in the space group A 2 / a (space group no. 15, position 4) with the lattice parameters a  = 6.52  Å ; b  = 15.18 Å; c  = 6.29 Å and β = 127.4 ° as well as 4 formula units per unit cell . The hemihydrate bassanite crystallizes monoclinically in space group I 2 (No. 5, position 3) 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 area of ​​application of the calcium sulphate phases is in the building materials sector (see use of gypsum ).

In the food industry, calcium sulfate is a by-product in the production of tartaric acid and is approved as a firming agent , acidity regulator and carrier. Among other things, it is used as a coagulant in the production of tofu .

Calcium sulphate as a drying agent Drierite with moisture indicator

Anhydrous calcium sulfate is also traditionally used as an inexpensive and versatile drying agent in chemical laboratories , where cobalt (II) chloride can be added as an indicator for the water content. When drying organic solvents, calcium sulfate is only moderately effective: 1 g calcium sulfate binds less than 0.05 g water ( calcium chloride up to 0.2 g), so that calcium sulfate is not good for drying relatively “wet” solvent-water mixtures or only for the Pre-drying is suitable.

Individual evidence

1. ↑ Entry on E 516: Calcium sulphate in the European database for food additives, accessed on June 27, 2020.
2. ↑ ^{a b c d e f g h i} Entry on calcium sulphate in the GESTIS substance database of the IFA , accessed on December 22, 2019 (JavaScript required)
3. ↑ Swiss Accident Insurance Fund (Suva): Limit values ​​- current MAK and BAT values (search for 7778-18-9 or calcium sulfate ), accessed on November 2, 2015.
4. ↑ Heinz M. Hiersig : Lexikon production engineering, process engineering. S. 805, ISBN 978-3-18-401373-8
5. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 97th edition. (Internet version: 2016), CRC Press / Taylor and Francis, Boca Raton, FL, Properties of the Elements and Inorganic Compounds, pp. 4-54.
6. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . Walter de Gruyter, Berlin, p. 628.
7. ^ Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 393 .
8. ↑ American Mineralogist Crystal Structure Database - Bassanite (English, 2001)
9. ↑ handbookofmineralogy.org - Mineral data sheet Bassanite (English, PDF 65.8 kB)
10. ↑ E 516 - Calcium sulfate. In: das-ist-drin.de. snoopmedia GmbH, accessed on February 22, 2020 . 
11. ↑ Martin Bertau, Armin Müller, Peter Fröhlich, Michael Katzberg: Industrielle Inorganische Chemie . John Wiley & Sons, Weinheim 2013, ISBN 978-3-527-33019-5 , pp. 550 ( limited preview in Google Book Search).
12. ↑ David R. Burfield: Desiccant efficiency in solvent and reagent drying. 9. A reassessment of calcium sulfate as a drying agent . In: The Journal of Organic Chemistry . tape 49 , no. 20 , 1984, pp. 3852-3854 , doi : 10.1021 / jo00194a043 .