Phase diagrams and thermochemical modeling of salt lake brine systems. V. Li+-Na+-K+-Mg2+-Ca2+-SO42−-H2O system

This paper is part of a series of studies on the development of a multi-temperature thermodynamically consistent model for salt lake brine systems. The objective of this study is to extend the sulfate binary models published in our previous study to multi-component systems, i.e. Li+-Na+-K+-Mg2+-Ca2+-SO42−-H2O and its sub- ternary, quaternary and quinary systems. The model is based on our revised Pitzer–Simonson–Clegg equations and the CALPHAD-type thermodynamic framework. Ternary PSC mixing parameters of 10 systems and standard Gibbs energy of double salts occurred in these ternary systems are determined by fitting ternary solubility data at various temperatures. The standard Gibbs energy of triple salts 2Li2SO4·Na2SO4·K2SO4(cr) and polyhalite are determined by fitting their solubility data in corresponding quaternary systems (i.e. Li2SO4+Na2SO4+K2SO4+H2O and K2SO4+MgSO4+CaSO4+H2O). The ternary model reproduces these fitted data well for most systems. With the help of these ternary models, complete phase diagrams of these ternary systems are critically constructed from the lowest eutectic temperature to approximately 373.15 K. In addition, phase diagrams of 10 quaternary systems and invariant points in a quinary system are predicted using the PSC parameters determined from binary and ternary systems. These results are consistent with most available experimental data, indicting a very good predictive ability of the model for multi-component systems. In the following study of our series work, we will combine the multi-component sulfate part and our previously published multi-component chloride part to achieve a complete model for the core salt brine system Li+-Na+-K+-Mg2+-Ca2+-SO42−-Cl−-H2O.