Gypsum saturation degree

Dissolved Concentrations of Calcium and SO2 Species. The equilibrium dissolved concentrations of total calcium and SO2 (sulfite plus bisulfite) species are important because comparison of these equilibrium concentrations with actual measured values determines the degree of gypsum saturation, and hence the potential for gypsum scale formation in the scrubber. As a first approximation, the fraction gypsum saturation of a scrubber liquor, having specified pH and specified concentrations of magnesium and chloride, is proportional to the measured calcium concentration, and inversely proportional to the measured S02 concentration. 

For a liquor of known pH and magnesium and chloride concentrations, the degree of gypsum saturation can be determined by measurement of either the total dissolved calcium or the total dissolved SO2 (sulfite plus bisulfite). The chemical model has been used to obtain correlations for gypsum saturation, presented below. The correlations, Equations 7 and 9, are valid for a typical scrubbing temperature of 50 °C, and for the same ranges of pH, magnesium, and chloride as for Equations 1-4. 

The degree of gypsum saturation of a 1imestone/magnesia scrubbing liquor can be calculated from either dissolved calcium or dissolved S02 (sulfite plus bisulfite) analyses. 

Precipitation of Carbonate Salts. In the Appendix, equations are given for the calculated degrees of saturation of calcite, S3, and of dolomite, S4, based on regression of these properties at the conditions shown in Table III. Figure 7 shows the effects of pH and PCo2 for solutions that are saturated with calcite or dolomite. In each case the solutions are saturated with calcium sulfite and gypsum, and the content of sodium chloride is 200 mmoles/kg H20. 

CaS04 deposition can be controlled by keeping the concentration below the saturation value at any point in the system. This means control of the degree of concentration of the sea water in relation to the temperature. The situation is complicated by the fact that CaS04 exists in three different crystal forms that are stable in contact with solutions and by the fact that the crystal form actually deposited is more likely to be determined by kinetic considerations than by equilibrium. Thus anhydrite is the stable solid phase in all cases encountered in sea water distillation, but the actual phases found are either gypsum or hemihydrate. 

The identified evaporation stages are associated with the salt precipitation of different solubility. For this reason they require different degrees of concentration. For instance, for the saturation of water with trona after the precipitation of Ca and Mg + carbonates in Lake Magadi is needed 250-fold saturation. Only after the precipitation of sulphates, at the salinity of almost 300 g-T and higher, sodium and potassium chlorides start precipitating. For this reason, for water saturation by halite after the gypsum precipitation in Saline Valley Lake, 25-fold concentration is required. 

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