Solubility of Minerals

Physicochemical properties and stability of minerals decide many vital processes relevant to their treatment. The solubility of minerals in various media and oxidation-reduction reactions involving minerals and various reagents are all very significant in the technology of mineral raw material processing. 

Very few generalized computer-based techniques for calculating chemical equilibria in electrolyte systems have been reported. Crerar (47) describes a method for calculating multicomponent equilibria based on equilibrium constants and activity coefficients estimated from the Debye Huckel equation. It is not clear, however, if this technique has beep applied in general to the solubility of minerals and solids. A second generalized approach has been developed by OIL Systems, Inc. (48). It also operates on specified equilibrium constants and incorporates activity coefficient corrections for ions, non-electrolytes and water. This technique has been applied to a variety of electrolyte equilibrium problems including vapor-liquid equilibria and solubility of solids. 

In chemical analysis, we encounter solubility in precipitation titrations, electrochemical reference cells, and gravimetric analysis. The effect of acid on the solubility of minerals and the effect of atmospheric COz on the solubility (and death) of coral reefs are important in environmental science. 

Monnin C (1990) The influence of pressure on the activity coefficients of the solutes and on the solubility of minerals in the system Na-Ca-Cl-SO4-H2O to 200 °C and 1 kbar, and to high NaCl concentration. Geochim Cosmochim Acta 54 3265-3282... 

Millero F.J. (1982b) The effect of pressure on the solubility of minerals in water and seawater. Geochim. Cosmochim. Acta 46,11-22. 

The solubility of mineral salts may be enhanced with the formation of ion pairs, a salting-in effect, requiring the inclusion of ion speciation effects. Conversely, there is commonly a salting-out effect of dissolved constituents across a salinity gradient. This can be particularly important when examining more hydrophobic organic compounds (HOC), such as aromatic hydrocarbons in estuaries. 

The values listed are mostly representative of low-temperature mineral solubility. Use these values only as a guide. The absolute solubility of minerals in fresh waters is a complex process. 

Comparison of Eqs. 13.8 and 13.9 with Eqs. 13.1a and 13.2a, respectively, indicates that the solubility of Q-S and Cs S is much higher than that of CS. On the other hand, by similar calculations, one may also show that the solubility of minerals such as Cm, where m < n, is lower than that of C S. In fact, C S and C A have the right combination to form ceramics at room temperature, and the rest of the minerals are either too soluble or insoluble. 

The reactions of cyanide to various minerals are summarized in a report by Hedley and Tabachnik (H21). The conditions for easy extraction of gold from its ore include a high degree of liberation of the gold minerals, a cyanide solution free of impurities, and a sufficient amount of oxygen available in the solution during the reaction. Quartz, silicate minerals, and alkali metal carbonates are relatively insoluble in cyanide solutions. The other metals that are normally associated with most gold and silver ores cause complications in the chemistry of cyanidation. The solubilities of minerals and metals in cyanide solutions are presented in Table II. 

Pokrovskii V. A. and Helgeson H. C. (1995) Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures the system AI2O3-HsO-NaCl. Am. J. Sci 295, 1255-1342. 

Sverjensky D. A. (1987) Calculation of the thermodynamic properties of aqueous species and the solubilities of minerals in supercritical electrolyte solutions. In Thermodynamic Modeling of Geological Materials Minerals, Fluids, and Melts, Reviews in Mineralogy (eds. I. S. E. Carmichael and H. P. Eugster). Mineralogical Society of America, Washington, DC, vol. 17, pp. 177-209. 

One approach to determine the reliability of geochemical codes is to take well-defined input data and compare the output from several different codes. For comparison of speciation results, Nordstrom et al. (1979) compiled a seawater test case and a river-water test case, i.e., seawater and river-water analyses that were used as input to 14 different codes. TTie results were compared and contrasted, demonstrating that the thermodynamic databases, the number of ion pairs and complexes, the form of the activity coefficients, the assumptions made for redox species, and the assumptions made for equilibrium solubilities of mineral phases were prominent factors in the results. Additional arsenic, selenium, and uranium redox test cases were designed for testing of... 

Further explanations of these geological processes must be left to more specialized sources. Such explanations are based on concepts treated later in this text. For example, modem acid-base theory helps explain the different solubilities of minerals in water or molten rock and their resulting deposits in specific locations. The divisions illustrated in Figure 1-9 can be partly explained by this theory, which is discussed in Chapter 6 and used in later chapters. 

In sedimentary basins, the gradient of groundwater density resulting from the change in solubility of minerals with temperature, is very small (Bj0rlykke... 

The molar volume of ions is based on the convention that the molar volume of ion equals zero at all temperatures. This assumption leads to molar volumes of -0.4 cmVmol for Li and 36 cmVmol for I", for example. Molar volumes of ions increase with increasing temperature. A V° is negative for most ionization and dissolution reactions so that pressure generally increases the progress of ionization reactions and the solubility of minerals. 

Pressure generally increases the solubility of minerals. An accurate evaluation of the pressure effect on solubility may require that we consider both molar volumes and compressibilies of reactants and products. In the following problem we compare the solubility of celestite (SrS04) in a surface water at 1 bar pressure and 25°C to its solubility in groundwater at 6000 ft depth at a temperature of 75 C and pressure of 180 bars. The reaction of interest is SrS04(celestite) = + SO ". To... 

An increase in pressure always leads to an increase in the solubility of minerals. Why In what natural water-rock environments is the effect of pressure on important ... 

In high-salinity waters such as seawater, both ion-pairing and activity-coefficient effects (see Chap. 4) increase the concentrations of species limited by the solubility of minerals. For example, in pure water saturated with respect to calcite, the molal solubility product ZmCa x ZmCOf" = 10 whereas in seawater this product equals 10 If the concentration of carbonate is constant, this corresponds to a 250-fold increase in the concentration of dissolved calcium in seawater relative to that in pure water. 

Complexing tends to increase the solubility of minerals that contain the species being com-plexed. Complexing of a species may also enhance or inhibit its adsorption and will usually affect its toxicity and bioavailability. 

It was mentioned earlier that complexing increases the solubility of minerals and thus the mobility of the species involved. This is evident from Fig. 3.14, which shows the solubility of thorian-ite (Th02) as a function of pH in pure water and in the presence of inorganic and organic complexing ligands. The lower, crosshatched curve is the solubility in pure water. At low pH the curve corresponds to the reaction... 

Many environmentally important chemicals are transported as complexes in natural waters. Complexes may increase or decrease the toxicity and/or bioavailability of elements. Complexation increases the solubility of minerals and may increase or decrease the adsorption of elements. The major monovalent and divalent cations and anions (especially > 10 m) form outer-sphere complexes or ion pairs, in which the bonding is chiefly long-range and electrostatic. Ion pairs are unimportant in dilute fresh waters, but become important in saline waters such as seawater. Minor and trace ions such as Cu, Fe +, Pb +, and Hg are usually complexed, and occur in inner-sphere complexes, which are usually much stronger complexes than the ion pairs. Written in terms of Gibbs free energy. 

Metal complexation in solution can have the effect of increasing the apparent solubility of minerals. A case in point is the oxalic acid—A1 hydroxide system. Oxalic acid, naturally occurring in soils as a product of biological activity, is a relatively strong dicarboxylic acid that dissociates readily ... 

Lassin, A., Azaroual, M., and Mercury, L. (2005) Geochemistry of unsaturated soil systems aqueous speciation and solubility of minerals and gases in capillary solutions, Geochim. Cosmochim, Acta 69, 22, 5187-5201... 

Tsp is called solubility product of the mineral and depends upon the ionic strength and on the temperature. If other reactions such as hydrolysis of cations M"" " and proton-addition of anions take place simultaneously, the solubility of minerals will be affected accordingly. A conditional solubility product can then be applied ... 

For solution compositions in which the virial coefficients are well defined, the mineral phase boundaries, ionic activity, and the activity of water, can be modeled remarkably well. Numerous applications are already benefiting from the existing database, such as the ability to predict the solubility of minerals in brine environments. Despite the advancement in the description of high ionic strength solutions, the incompatibility of the ion-pairing model and the semi-empirical... 

Once the distribution of mass among the various aqueous species in solution has been calculated, there are a number of different geochemical modeling options available (Fig. 1). The most commonly used option is to increase the temperature and pressure and re-calculate the distribution of species and saturation states of minerals, as initially discussed by Kharaka and Mariner (1977). The effects of increasing pressure on the complexation of the aqueous species was not included at that time but has now been added based on the correlations of Aggarwal et al. (this issue). Including this correction is important because the dissociation constant of complexes can vary by a factor of three over 1 kilobar. The pressure correction for aqueous complexes results in reducing the enhanced solubility of minerals from that predicted without this correction (22). 

As first suggested by Bottinga (1968), this discrepancy may be due to the nonideality of H2O under supercritical conditions, and the effect of this non-ideality on the vibrational frequencies of the H2O molecule. Another possible reason for the discrepancy is the solubility of minerals in water at elevated pressures and temperatures. Thus, partition function ratios of H2O derived empirically from mineral-H20 experiments may be different from those of a pure H2O fluid at comparable P-T conditions (Hu and Clayton, in press). 

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