Calcium sulfate, from

Extraction of Nonmetallic Inorganic Compounds. Phosphoric acid is usually formed from phosphate rock by treatment with sulfuric acid, which forms sparingly soluble calcium sulfate from which the phosphoric acid is readily separated. However, in special circumstances it may be necessary to use hydrochloric acid ... 

The abatement of fluorine emissions and disposal of by-product calcium sulfate from phosphoric acid plants are environmental concerns. 

If the hydrogen could be reduced, the coproduction of hydrogen and valuable side products, eg, sulfur, sulfuric acid, and calcium sulfate, from H2S could become economically competitive. 

Calcium sulfate is formed as a byproduct in industrial processes such as flue gas desulfurization and the production of zinc, fluoride, organic acids and phosphoric acid, in amounts of many million tons per year. In this study the attention is focussed on calcium sulfate from the production of phosphoric acid for fertilizer applications. It is precipitated, from solution after digestion of phosphate ore by addition of sulfuric acid according to [1] ... 

Figure 11.25 This plasterboard is made using calcium sulfate from an FGD plant.

The kinetics and the variables affecting initial separation of calcium sulfate from phosphoric acid as the hemihydrate have been carefully examined [35]. The value of separating the phosphate dissolution stage (rapid stirring) [36], from the calcium sulfate crystallization step via the hemihydrate (slow agitation) has been tested on a laboratory scale [37]. 

Ions in Precipitation Reactions. Grade 10 students of German academic high schools have learnt the atomic model and about the idea of the ion and ionic bonding in their chemistry lessons. These students saw precipitates of calcium sulfate from saturated salt solutions and have been asked to imagine the smallest particles in these solutions before and after the precipitation [6]. The expected ion symbols of the initial solution were correctly supplied in 50% of the cases. However, the other half of the student group has shown misconceptions of salt molecules or of electron transition in the formation of ions from atoms . With respect to the precipitation product, only 30% of the students provided acceptable structural models, the amount of misconceptions grew to 70% [6]. 

Sarkar and Samet (1994), utilizing x-ray diffraction and light and electron microscopy, concluded that an abundance of a potassium-calcium sulfate (from excessive insufflation) and unusually large belite crystals (from long residence times) were responsible for low... 

Among the naturally occurring sulfate ores, only gypsum has ever been of any importance as a raw material for sulfur production, and only in the direct production of sulfur dioxide for sulfuric acid manufacture. Byproduct calcium sulfate from phosphoric acid production (phosphogypsum) has been used to produce sulfuric acid. Elemental sulfur has never been extracted from sulfates on any significant scale because of high energy costs. Experimental work has been done on both chemical [721... 

The major anionic impurity in most brine systems is sulfate. Control of its concentration is an issue mostly in membrane cells. In the diaphragm-cell process, sulfate passes with the rest of the anolyte into the cathode side of the cells. It can be separated from caustic soda in the evaporators and purged from the system as Glauber s salt This is covered in Section 9.4.2.1. Mercury cells are least sensitive to sulfate. Its concentration is frequently allowed to build to the point where dissolution of calcium sulfate from the salt is inhibited. The greatest problem then caused by the sulfate is a reduction in the solubility of NaCl or KCl. 

A. Austin, et al. Precipitation of Calcium Sulfate from Sea Water at High... 

Ainsworth, R.O., "Dissociation constant of calcium sulfate from 25 to SO°C", J. Chem. Soc. Faraday Trans I, v69, ppl028-1032 (1973)... 

Further applications are the examination of pigments, the detection of CaS04 used as a filler in paper (after ashing), the detection of adulteration of flour with gypsum, and the differentiation of superphosphate of lime (a mixture of monobasic calcium phosphate and calcium sulfate) from phosphate fertilizers containing no CaS04. 

Clifton, J. R., Thermal Analysis of Calcium Sulfate Dihydrate and Supposed a and P Forms of Calcium Sulfate from 25 to 500°C, J. Res. Natl. Bur. Stands-A, Phys. and Chem., Vol. 78A (1972)... 

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