Modeling precipitation from concentrated solutions with the EQ3/6 chemical speciation codes

• Published in: Computers & chemistry 1998-01, Vol.22 (5), p.419-427
• Main Authors: Brown, Lee F., Ebinger, Michael H.
• Format: Article
• Language: English
• Subjects: chemical speciation codes; concentrated solution precipitation; EQ3/6 codes; equilibrium codes; modeling precipitation
• ISSN: 0097-8485
• DOI: 10.1016/S0097-8485(97)00056-9

Four simple precipitation problems are solved to examine use of numerical equilibrium codes. The study emphasizes concentrated solutions, assumes both ideal and non-ideal solutions, and employs different databases and different activity-coefficient relationships. The study uses the EQ3/6 numerical speciation codes. Results show satisfactory agreement between solubility products calculated from free-energy relationships and those calculated from concentrations and activity coefficients. Most material balances are also satisfactory, but the modeling of an evaporation campaign exhibits serious deficiencies in the balances. Precipitates show slightly higher solubilities when solutions are regarded as non-ideal than when considered ideal, agreeing with theory. A code may or may not predict precipitation from a solution dilute in the precipitating species, depending on the database or activity-coefficient relationship used. In solutes remaining after precipitations there is little consistency in calculated concentrations and activity coefficients. They do not appear to depend on the database or activity-coefficient relationship used. These results reinforce warnings in the literature about perfunctory or mechanical use of numerical speciation codes.