Solubility gypsum

For purposes of illustration we will assume complete dissociation of the gypsum. The system may then be described water dissociation 

To determine the solubility of gypsum in a sodium chloride solution, the NaCl solubility equation is also necessary  

These three equilibris result in the following equilibrium K equations in a water-gypsum-sodium chloride system model  

Since complete dissociation is being assumed for the gypsum and sodium chloride, the possible formation of complexes such as NaSO and HSOh is disregarded. Given the initial moles of water, HaOfin). gypsum as moles of CaSO. CaSOndn), and sodium chloride. NaCl(in), in the solution, the model must determine nine unknowns  

Either way, the model still needs six more equations in order for the Newton-Raphson method to be able to solve the system. These would be elemental and electroneutrality balances  

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The activities of the free ions remain roughly constant with NaCl concentration, and their concentrations increase only moderately, reflecting the decrease in the B-dot activity coefficients with increasing ionic strength (Fig. 8.3). Formation of the complex species CaCl+ and NaSOj drives the general increase in gypsum solubility with NaCl concentration predicted by the B-dot model. 

In the HMW model, in contrast, Ca++ and SO4 are the only calcium or sulfate-bearing species considered. The species maintain equal concentration, as required by electroneutrality, and mirror the solubility curve in Figure 8.6. Unlike the B-dot model, the species activities follow trends dissimilar to their concentrations. The Ca++ activity rises sharply while that of SO4 decreases. In this case, variation in gypsum solubility arises not from the formation of ion pairs, but from changes in the activity coefficients for Ca++ and SO4 as well as in the water activity. The latter value, according to the model, decreases with NaCl concentration from one to about 0.7. 

Temperature dependency of gypsum solubility in well water... 

Element Average, itg/g Leached. Absorbed. %of total Axial ratios. A/A gypsum Solubility, % as element Compounds Whose Axial Ratios and Solubilities Were Considered... 

The sulfate co-precipitates with the calcium sulfite, resulting in a mixed crystal (or solid solution) of calcium-sulfur salts. Gypsum is not formed. The relatively high sulfite concentrations in the solution prevent soluble calcium concentrations from reaching the levels required to exceed the gypsum solubility product, and the system operates unsaturated with respect to calcium sulfate. 

Data derived from gypsum solubility in sodium chloride, sodium sulfate, and calcium chloride solutions were used in addition to calcium sulfate solutions. 

The three models have been compared to calculations of the solubility of gypsum in 0.5 molal NaCl with varying concentrations of Na2S04 ( ) (Figure 3). Results of the WATEQ and amended WATEQ models were not similar to those of the Harvie and Weare model. However, results of the fit model were similar to those of the Harvie and Weare model. The fit model and the Harvie and Weare model used gypsum-solubility measurements to fit model parameters. 

Figure 3. Comparision of gypsum solubilities calculated by the Harvie and Weare model (23) and ion-association aqueous models in 0.5-molal NaCl solutions with varying concentrations of Na2S04.

Figure 2.29 Gypsum effective solubility vs. concentration of some electrolytes in water [Kraynov et al, 2004]. Figure shows decline in gypsum solubility in the presence of calcium chloride, as a result of competition for calcium.

Consequently, the lowering of calcinm hydroxide solubility together with the increase of gypsum solubility is observed [1, 5, 6],... 

Adler, M.S. J. Glater, J.W. McCutchan, "Prediction of gypsum solubility and scaling limits in saline waters", j. Chem. Eng. Data, v24, 3, PP187-192 (1979)... 

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