Gypsum precipitation

The initial compositions of both the infiltrating water and the solid materials may change due to their interaction, which in turn may affect the solubility and the pathway of dissolution-precipitation processes with time. When a particular component of the dissolved solution reaches a concentration greater than its solubility, a precipitation process occurs. Table 2.1 includes the solubility of selected sedimentary minerals in pure water at 25°C and total pressure of 1 bar, as well as their dissolution reactions. All of the minerals listed in Table 2.1 dissolve, so that the products of the mineral dissolution reactions are dissolved species. Figure 2.2 shows the example of gypsum precipitation with its increasing concentration in a NaCl aqueous solution. 

The concentration trends of the major species excluding Mg are plotted here. Computer modeling of the chemical analyses (assuming Fe is present as ferric iron) suggests that the drop in sulfate is caused by gypsum precipitation and that decreases in the concentrations of Al, Si, and Fe may result from supersaturation and consequent precipitation of phases, including allophane, alunite, basaluminite, Al(OH)3(am), SiOjfam), ferrihydrite, jarosite, and jurbanite. 

The dissolution of calcite also results in higher calcium concentration and leads to gypsum precipitation ... 

Precipitation of Gypsum at Constant Temperature. This example simulates diffusion of Ca (aq) and S04 (aq) into a domain where the two ions react and gypsum precipitates according to the reaction... 

Under the isothermal condition imposed in this example, gypsum precipitates only at the boundary a = 0 in both constant porosity and variable porosity cases. Significant changes of porosity occur at this location in the variable porosity case. Figure 5 shows the accumulation of gypsum at this location at times up to 10 seconds for the two cases. The results are qualitatively similar to the results of the previous example. In the case of constant porosity the volume of precipitated gypsum exceeds available pore space at times greater than about... 

Figure 6. Porosity at boundary x = 0 for gypsum precipitation, temperature 25 °C.

In the subsoils of arid and semiarid soils, Ca commonly precipitates as cakite (CaCC>3) rather than being leached away. It is found as indurated layers (caliche and other local names) in many arid soils and as more diffuse CaC03 in Aridisols and Mollisols. Precipitation of CaCCTj in soils is affected by the rates of soil water movement, CO2 production by roots and microbes, CO2 diffusion to the atmosphere, and water loss by soil evaporation and plant transpiration. CaCC>3 layers are also derived from upward movement and evaporation of Ca-rich waters. Calcium carbonate accumulations can amount to as much as 90% of the mass of affected soil horizons. Gypsum precipitates in some arid soils, despite being about 10 x as water soluble as Ca carbonate. 

Figure 18.7 shows the minerals that precipitate over the reaction path (Table 18.3 lists their compositions), and Fig. 18.8 shows how fluid chemistry in the calculation varies. Initially, dolomite and gypsum precipitate. When the fluid is concentrated about ten-fold, the decreasing water activity causes the gypsum to dehydrate... 

Example 2.13. From a solution, which contains 7.01-10 mole-l of and 3.50-10 mole l of SO, gypsum precipitates during evaporation. How will the solutions composition change in the course of evaporation Here, Ca + content is 2 times that of. At the shrinkage of the solution volume n times only due to evaporation, N moles of gypsum forms. It means that from the solution are removed N moles of Ca and. Then the changed ion concentrations will become. 

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. 

Sulfates barytes (DIN ISO 3262-2) gypsum precipitated barium sulfate (DIN 55911)... 

Emission control systems in a power plant also require water. Hue gas desulfurization (FGD) systems in coal-fired power plants remove SO2 from the exhaust flue gas by reacting it with lime or limestone in a wet scrubber system generating wastewater that must be treated before discharge. FGD wastewater treatment involves removal of the gypsum precipitate, adjustment of the pH, and the removal of any toxic dissolved metals. This is typically accomplished using a combination of gravity-driven processes, chemical floc-culants, absorption, and biological processes. Membrane approaches are currently not economically competitive for this application [58],... 

Sarig, S. and Mullin, J.W. (1982) Effect of trace impurities on caldum sulphate (gypsum) precipitation. Journal of Chemical Technology and Biotechnology, 32, 525-531. 

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