CaSO4 • 2H2O

Gay, P., 1965. Some crystallographic studies in the system CaSO4—CaSO4"2H2O. Mineral. Mag., 35 347-362. 

By the 1983 start-up, Tampa Electric intends to meet the commercial specifications for wallboard use, as well as for briquets for local cement plants. Gypsum rock from Spain is presently imported by the local cement plants. Fully oxidized gypsum, 93 + % CaSO4 2H2O, low in chlorides and magnesia, will be produced. 

The reaction of the phosphate rock to produce gypsum, CaSO4 2H2O, may be illustrated by Eq 1... 

Thermal energy for drying flue gas gypsum with a moisture content of 10% 130.000 kcal/ton CaSO4 2H2O dry 550.000 kJ/ton CaSO4 IHjO dry. 

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). 

When the activity of each species in a reaction is known, we can determine the temperature (or temperatures) at which the reaction is in equilibrium. As an example, we calculate the temperature at which gypsum (CaSO4-2H2O) dehydrates to form anhydrite (CaSO4). The RXN commands... 

If the fluid had been initially richer in calcium than carbonate (AfCa++ > MHCOj), as noted by Hardie and Eugster (1970), it would have followed a distinct reaction path. In such a case, calcite precipitation would deplete the fluid in carbonate, allowing the calcium concentration to increase until gypsum (CaSO4 2H2O) saturates and forms. The point at which the calcium and carbonate are present at equal initial concentration (A/Ca++ = A/Hcoj ) is known as a chemical divide. 

Ca++ and SO4 accumulate in the fluid, eventually causing gypsum (CaSO4-2H2O) to saturate and precipitate. At this point, the overall reaction becomes... 

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