Computer programs CALCITE

In the computer program input (Table 10.2), the column in Figure 10.2 is discretized into 40 cells, each 20 m long, and each containing specific amounts of calcite, kaolinite,... 

While the activities can fairly easily be calculated in the case of the open systems due to the fixed pressure of atmospheric CO2 at 10 - atm, calculations for the closed systems are relatively tedious. A computer program is developed for calculating the activities of various dissolved species from available thermodynamic data, taking into consideration the effects of other dissolved species and solid phases in equilibrium with the system. Figs. 3.4a and 3.4b show the distribution of various species as a function of pH for the closed and open systems, respectively. The marked effect of atmospheric CO2 on the solubility of calcite can be seen by comparing the data in these diagrams. This effect is particularly noticeable for Ca " ", HCO and activities. Since Ca plays an important role in calcite flotation systems, the possible role of CO2 as well as other interfacial species on flotation due to this effect should be noted. 

Table 9.3 Model calculation applying the computer program PHREEQC (Parkhurst 1995) to a sample of ocean water in a water depth of approximately 1000 m, near the equator. The constant of the solubility product for calcite is corrected for temperature and pressure. The decomposition of organic substance due to the presence of free oxygen in the water column has been included.

This simplified equilibration process holds strictly for a cubic crystal in a general non-cubic case, the volume, static pressure and bulk modulus have to be replaced by the strain components (related to unit-cell parameters), stress components and elastic constants, respectively. The computer program PARAPOCS[17] performs the lattice-dynamical, thermodynamical and quasi-harmonic calculations in the general tensorial formalism, and has been used to obtain all results reported below for calcite and aragonite. 

Dosages for scale inhibitors should be applied as a function of a driving force for scale formation and growth (e.g., calcite saturation level), temperature as it affects reaction rates, pH as it affects the dissociation state of the inhibitor, and time. A version of the computer program allows the development of mathematical models for the minimum effective scale inhibitor dosage as a function of these parameters driving force, temperature, pH, and time. 

In another example, five test cases were computed by PHREEQE and EQ3/6 and the same thermodynamic database was run for each program (INTERA, 1983) to test for any code differences. The five examples were speciation of seawater with major ions, speciation of seawater with complete analysis, dissolution of microchne in dilute HCl, reduction of hematite and calcite by titration with methane, and dedolomitization with gypsum dissolution and increasing temperature. The results were nearly identical for each test case. Test cases need to become standard practice when using geochemical codes so that the results will have better credibility. A comparison of code computations with experimental data on activity coefficients and mineral solubilities over a range of conditions also will improve credibility (Nordstrom, 1994). 

The energy parameters were fitted to equilibrium structural data (cell constants and atomic fractional coordinates), elastic constants, vibrational frequencies at the centre of the Brillouin zone (wave vector K=0) and, for the SM case only, dielectric tensor components, using the THBFIT program[24]. The input data are all the experimental quantities referred to above, and initial trial values for the energy parameters to be optimized. At first specific potentials were fitted in this way separately on calcite and aragonite then some of them were used to compute properties of the other polymorph, so as to check their generality and transferability. 

---