Gypsum equilibria

G35. MacDonald, G.J.F., "Anhydrite-gypsum equilibrium rdations", Amer. J Sci, v251, PP884-898 (1953)... 

The equilibrium constant is small, indicating a slightly soluble salt, as we would expect for a naturally occurring mineral like gypsum. 

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 (CaS04-2H20). Of the two minerals, halite is the more soluble and hence more of it must precipitate for the fluid to approach equilibrium. 

Fig. 8.7. Molal concentrations m, and activities a, of calcium and sulfate species in equilibrium with gypsum at 25 °C as functions of NaCl concentration, calculated using the B-dot equation (left) and the hmw activity model (right).

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 (CaS04 2 H2O) dehydrates to form anhydrite (CaS04). The RXN commands... 

Figure 3. Equilibrium SOi partial pressure as a function of pH and magnesium concentration for liquors saturated with calcium sulfite and gypsum at 50°C with...

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. 

The equations required to calculate the effect of pressure and temperature on AG are modified from Equation (7.43) to include a term for each pressure at any temperature T. For example, for the gypsum-anhydrite equilibrium,... 

The equilibrium diagram (2, p. 274 6) for the species in Equation (13.16) is shown in Figure 13.7. If gypsum and anhydrite are both under liquid water at 1 bar, then equUibrium can be attained only at 40°C (see Fig. 13.7). If the hquid pressure is increased, and the rock formation is completely impermeable to the hquid phase, so that the pressure on the fluid phase is equal to the pressure on the sohd phase, then the temperature at which the two solids, both subject to this hquid pressure, are in equUibrium is given by the curve with positive slope on the right side of Figure 13.7. Thus, the right curve apphes to any situation in which Pp is equal to Ps- Under these conditions, the net AV , for the transformation of Equation (13.16) is 36.14 29.48 = 6.66 cm [see Equations (13.18) and (13.19)],... 

Figure 20.5. Effect of NaCl concentration on the equilibrium temperature of the anhydrite-gypsum reaction at 1 bar. Data from Ref. 16.

Further measurements were made by Khan (16) and Mukhopadhyay (17). A large excess of either gypsum or hemlhydrate crystals was added to an acid solution of measured composition. This was agitated at constant temperature until equilibrium was reached. Duplicate runs were carried out both from undersaturated and supersaturated solutions. After settling, the solution was analysed for calcium content by atomic absorption. 

A,A, A" Calcium concentrations, operating, satd. with gypsum, satd. with hemlhydrate (Figure 4). nucleatlon rate, /min. 1 o slurry, calcium concentration g Ca /I soln. under saturation, g Ca /I soln. equilibrium calcium solubility, g Ca Vg soln. equlHbrium calcium concentration at transition temperature g Ca /g soln. 

In this case study, the selected phases are pyrite, amorphous FeS, calcite (present in limestones in the roof strata Fig. 5), dolomite (possibly also present in the limestones), siderite (which occurs as nodules in roof-strata mudstones), ankerite (present on coal cleats in the Shilbottle Seam), melanterite and potassium-jarosite (representing the hydroxysulphate minerals see Table 3), amorphous ferric hydroxide (i.e., the ochre commonly observed in these workings, forming by precipitation from ferruginous mine waters), and gypsum (a mineral known to precipitate subaqueously from mine waters with SO4 contents in excess of about 2500 mg/L at ambient groundwater temperatures in this region, and with which most of the mine waters in the district are known to be in equilibrium). In addition, sorption reactions were included in some of the simulations, to contribute to the mole transfer balances for Ca, Na, and Fe. 

The FREZCHEM model is a chemical equilibrium model. For a reaction such as gypsum dissolution... 

The goal of this research was to improve activity coefficient prediction, and hence, equilibrium calculations in flue gas desulfurization (FGD) processes of both low and high ionic strength. A data base and methods were developed to use the local composition model by Chen et al. (MIT/Aspen Technology). The model was used to predict solubilities in various multicomponent systems for gypsum, magnesium sulfite, calcium sulfite, calcium carbonate, and magnesium carbonate SCU vapor pressure over sulfite/ bisulfite solutions and, C02 vapor pressure over car-bonate/bicarbonate solutions. 

In comparison to this calculation, the dissolution of gypsum in distilled water shall now be modeled by means of PHREEQC The input is very simple as it concerns distilled water and thus, the SOLUTION block contains only pH = 7 and temperature = 20 °C. To force equilibrium with gypsum, the keyword EQUILIBRIUMPHASES and the saturation index of 0 are used. 

The existing input file is extended by setting up equilibrium not only with pyrite but also with calcite. 2.621 mmol of calcite dissolve. The amount of pyrite dissolved is the same as in the absence of calcite (1.347 mmol). The pH value of 7.58 is in the neutral range. Thus, to neutralize the pH approximately 2 moles of calcite must be added for every mol of pyrite. The saturation index of gypsum is still clearly undersaturated (SI = -1.09), i.e. that gypsum is not a limiting mineral phase and hence the sulfate contents stay more or less invariable. 

The increased C02 partial pressure is implemented using the logarithm of its partial pressure in bar under the key word EQUILIBRIUMPHASES. For a partial pressure of 10 vol% C02 considerable more calcite must be available in the heap, since a significant amount of the C02 is used for the dissolution of calcite. To reach equilibrium now 6.288 mmol of calcite are needed. For one mol of pyrite 4.7 mol of calcite must be added. The pH value is with 6.65 compared to 7.58 lower by almost one order of magnitude. Again no saturation is reached for gypsum. 

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