CaSO4/NaCl

Second, at a given saturation index, supersaturated minerals with high solubilities have the potential to precipitate in greater mass than do less soluble ones. Consider a solution equally supersaturated with respect to halite (NaCl) and gypsum (CaSO4-2H2O). Of the two minerals, halite is the more soluble and hence more of it must precipitate for the fluid to approach equilibrium. 

To explore the differences between the methods, we use REACT to calculate at 25°C the solubility of gypsum (CaSO4-2H2O) as a function of NaCl concentration. We use two datasets thermo. data, which invokes the B-dot equation, and thermo. hmw, based on the HMW model. The log K values for the gypsum dissolution reaction vary slightly between the datasets. To limit our... 

FIG. 7.6 Solubility of gypsum (CaSO4-2H2O) at 25°C as a function of NaCl concentration, calculated according to the Harvie-M< )ller-Weare and B-dot (modified Debye-Huckel) activity models. Circles and squares, respectively, show experimental determinations by Marshall and Slusher (1966) and Block and Waters (1968). 

Figure 7.7 shows how concentrations and activities of the calcium and sulfate species vary with NaCl concentration. In the B-dot model, there are three ion pairs (CaCl+, NaSO4, and CaSO4) in addition to the free ions Ca++ and SO4. The activities of the free ions remain roughly constant with NaCl concentration,... 

As a test of our ability to calculate activity coefficients in natural brines, we consider groundwater from the Sebkhat El Melah brine deposit near Zarzis, Tunisia (Perthuisot, 1980). The deposit occurs in a buried evaporite basin composed of halite (NaCl), anhydrite (CaSO4), and dolomite [CaMg(CO3)2], The Tunisian government would like to exploit the brines for their chemical content, especially for the potassium, which is needed to make fertilizer. 

We take as an example a 6 molal NaCl solution containing. 01 molal CaSO4. Since the only species considered for this chemical system are Na+, Cl , Ca", and SO -, we can immediately write down the species molalities nij along with their charges z, ... 

Block, J. and O.B. Waters, Jr., 1968, The CaSO4-Na2SO4-NaCl-H2O system at 25° to 100°C. Journal of Chemical and Engineering Data 13, 336-344. 

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