Calcium sulphate anhydrite

Kontrec J, Kralj D, and Brechevich L. Cadmium removal from calcium sulphate suspension by liquid membrane extraction during recrystallization of calcium sulphate anhydrite. Colloids Surf A Physicochem Eng Aspects, 2003 223(1-3) 239-249. 

Fluoride production consists on reacting fluorspar mineral (CaF ) with H SO (cc) to form HF and calcium sulphate. However, reactions do not achieve 100 % yield, so fluorogypsum or anhydrite (CaSO ) obtained contains small amounts of fluoride as CaF in its stmcture. The application of this kind of subproduct is well-known as constmction material, so that the flouride content has to be controlled. 

The optical properties of crystals are usually quite reliable criteria for identification but occasionally crystals have sifbmicroscopic cracks and cavities, and although appearing quite normal, give refractive indices lower than those of an entirely solid crystal. This phenomenon, which is obviously very misleading, is fortunately very rare, but has been observed in anhydrite (calcium sulphate) and calcite (calcium carbonate) prepared in the laboratory. In cases erf doubt, X-ray powder photographs should be taken—see Chapter V. 

Type S cements are Portland cements high in C3A and with suitable contents of calcium sulphate they have found little favour as they are too diHicLilt to control. The A1,0, has also been supplied in forms other than those mentioned above. Impure alunite [KAl3(S04),(0H)(,]. which occurs as a natural rock, has been used, either after calcination (V6) or uncalcined (W30). In the latter case it was mixed with Portland cement clinker, anhydrite and pfa or slag and was found to dissolve relatively slowly, thus suitably delaying the expansive reaction. 

Fig. 3.19 Summary of the sulphur cycle (oxidation states of sulphur are shown in parentheses). Solid lines show major microbial pathways broken lines show geological processes involving sedimentary organic-S, calcium sulphate (gypsum/anhydrite) andiron(II) sulphide (pyrite).

Calcium sulphate separates from water at temperatures below 65°C in the form of CaS04.2H20(gypsum), and at higher temperatures in the form of anhydrous CaS04 (anhydrite). The solubility of gypsum is about 2000 mg 1. It readily forms supersaturated solutions. 

The diversity of sources of inorganic chemicals was touched on in the introduction to this chapter and the book entitled The Modern Inorganic Chemicals Industry discusses, in detail, the production of a number of important inorganic chemicals. Comment here therefore will be restricted to a summary of world consumption (Table 2.6) and major uses of the more important inorganic raw materials, plus a brief study of the changing raw-material usage for sulphuric acid production and the reasons for this. Between 1960 and 1975 four alternatives were utilized in sulphuric acid production. They are sulphur, anhydrite (a calcium sulphate mineral), zinc concentrates... 

Several variations of the basic process have been evolved, one of the options being to precipitate the calcium sulphate as dihydrate (x = 2), hanihydrate (x= 1/2) or anhydrite (x = 0). In the most used dihydrate process, the acid is usually obtained at a concentration of about 32% P2O5. Evaporation is used to increase the concentration to 40-42% P2O5 (as required for ammonium phosphate manufacture) or 52-54% P2O5 - merchant grade (as required for superphosphate production). During... 

Gypsum plaster It is produced by mixing water with calcium sulphate hemihydrate, with the help of heat of about 300°F (150°C). When the dry plaster powder is mixed with water, it re-forms into gypsum. If plaster or gypsum is heated above 200°C, anhydrite is formed, which will also re-form as gypsum if mixed with water. 

Klaproth found that precipitated alumina is soluble in caustic potash(i789). He proved by analysis that anhydrite (then called muriacite) is calcium sulphate free from water and that calcite and aragonite are two different crystalline forms of calcium carbonate and confirmed that bitter spar (dolomite) is a compound of calcium and magnesium carbonates. 

Calcium Sulphate. See anhydrite gypsum plaster of Paris. 

Recently, the gypsum precipitates from among calcium sulphates whereas anhydrite crystallizes very rarely - the only locations of its recent crystallization are the Persian Gulf coast, lakes Elton and Inger, Death Valley and Clayton Playa (Nevada). 

Stone has been used from Neolithic to built different types of structures (e.g. housing, tombs). Among different stone types, gypsum has been commonly used from early prehistoric times. It is the most abundant sulphate mineral in the World and usually occurs in association with limestone, halite, calcite, and anhydrite. Most gypsum bed deposits are formed as a result of evaporation of seawater that has a large amount of calcium sulphate in solution. 

Gypsum is hydrous calcium sulphate having chemical composition CaS04.2H20 (cf., composition of anhydrite is CaS04). It has five varieties, namely ... 

---