| Isothermal evaporation crystallization is a simple and cost-effective technique that can be used to extract valuable resources from salt lakes containing the ions Li+, Na+, K+, Mg2+, Cl-, SO42-, and B4O72-. The theoretical basis of isothermal evaporation crystallization lies in the thermodynamic equilibrium and non-equilibrium solubility phase diagrams of related multi-component systems. This method takes advantage of the natural climate in the area of the Qarhan salt lake, i.e., low humidity and abundant solar energy. After several steps of solar pond evaporation, the complicated salt lake system containing sulfur type becomes a relatively simple system that can be approximately represented by Li+, Mg2+//Cl-, SO42--H2O. In the last stage of the natural evaporation process, the mass ratio of Mg:Li in the solution decreases to approximately 20. If lithium salts out as Li2SO4·H2O(s) along with MgSO4·nH2O(s) during the evaporation process, the loss of lithium salt will increase abruptly. Chemical engineers who work with salt lakes informed us that the loss ratio of lithium could be as high as 50% in a realistic solar pond process (personal communication). To understand the loss mechanism and to develop a new evaporation process to avoid this substantial loss of lithium, we need a more thorough understanding of the phase diagram of the quaternary reciprocal system Li+, Mg2+//Cl-, SO42--H2O, especially in the zone describing the formation of solid Li2SO4·H2O(s). In fact, this quaternary system and its ternary sub-systems have been widely investigated by several research groups. However, the reported results are contradictory with each other, so the new technology lacks the basic phase diagram to support. To obtain the accurate experimental data, we improve the experimental setup, the analytical method, the initial material, the time to equilibrium, then comprehensively determined the solubilities for the ternary system MgCl2-MgSO4-H2O at 298.15 K,323.15 K and 348.15 K, and the quaternary reciprocal system Li+, Mg2+//Cl, SO42--H2O at 298.15 K by an isothermal dissolution method. A Pitzer-Simonson-Clegg thermodynamic model was chosen to simulate the properties of the sub-binary and sub-ternary systems and to predict the solubility phase diagram of the quaternary system. The results of the modeling are in reasonable agreement with the experimental data. Main research details are as follows:(1) In the ternary system MgCl2-MgSO4-H2O at 298.15 K, there are six solubility branches, corresponding to the solid phases MgSO4·nH2O(s)(n= 7,6,5,4, 1) and MgCl2·6H2O(s), the phase field of MgSO4-H2O(s) overlaps with the phase fields of MgSO4·4H2O(s) and MgSO4·5H2O(s), which indicates that MgSO4·nH2O(s) (n= 7,6,1) and MgCl2·6H2O(s) are stable phases, MgSO4·4H2O(s) and MgSO4·5H2O(s) are metastable phases in the ternary system. We make sure the phase MgSO4·H2O(s) exists in the ternary system at 298.15 K by using two methods in the experimental process.(2) In the ternary system MgCl2-MgSO4-H2O at 323.15 K, there are five solubility branches, corresponding to the solid phases MgSO4·nH2O(s) (n= 6,5,4,1) and MgCl2·6H2O(s), the phase field of MgSO4·H2O(s) overlaps with the phase fields of MgSO4·4H2O(s) and MgSO4·5H2O(s), which indicates that MgSO4·nH2O(s) (n= 6, 1) and MgCl2·6H2O(s) are stable phases, MgSO4·4H2O(s) and MgSO4·5H2O(s) are metastable phases in the ternary system. Balarew et al.[30] recognized that the phase MgSO4·4H2O(s) is a stable phase in this ternary system at 323.15 K, which is inconsistent with our results.(3) In the ternary system MgCl2-MgSO4-H2O at 348.15 K, there are four solubility branches, corresponding to the solid phases MgSO4·nH2O(s) (n= 6,4,1) and MgCl2·6H2O(s), the phase field of MgSO4·H2O(s) overlaps with the phase fields of MgSO4·4H2O(s) and MgSO4·6H2O(s), which indicates that MgSO4·H2O(s) and MgCl2·6H2O(s) are stable phases, MgSO4·4H2O(s) and MgSO4·6H2O(s) are metastable phases in the ternary system. The phase field size of MgSO4·H2O(s) in this work is larger than that determined by Balarew et al.,[30] but is consistent with the calculated results by Voigt et al.[30,31](4) In the quaternary system Li+, Mg2+//Cl-, SO42--H2O at 298.15 K, there are sixteen solubility co-saturated lines, corresponding to the solid phases MgSO4·nH2O(s) (n= 7,6,5,4,1), MgCl2·6H2O(s), Li2SO4·H2O(s), LiCl·MgCl2-7H2O(s) and LiCl·H2O(S). This report describes that the equilibrium solid phases MgSO4·H2O(s) and MgSO4·4H2O(s) have been found to exist for the first time in this quaternary system. However, the phase field of MgSO4·H2O(s) overlaps with the phase fields of MgSO4·4H2O(s) and MgSO4·5H2O(s), as that in the ternary system MgCl2-MgSO4-H2O at 298.15 K, which indicates that MgSO4·4H2O(s) and MgSO4·5H2O(s) are metastable phases, while the other phase fields are stable phases in the quaternary system.(5) A Pitzer-Simonson-Clegg thermodynamic model was chosen to simulate the selected water activity and solubility data of the sub-binary and sub-ternary systems at multi-temperature, the binary and ternary model parameters are obtained and correlated as linear functions versus temperature, then we calculate the thermodynamic properties of the binary and ternary systems by using these parameters, the results of the modeling are in reasonable agreement with the experimental data. Then we predict the solubility phase diagrams of the quaternary system at 298.15 K,323.15 K and 348.15 K. We find that the predicted results agree well with our experimental ones at 298.15 K, which indicates that our experimental data are accurate, the model parameters are reliable and the model has a good prediction ability. Moreover, we have predicted the equal-scale lines of the water content in the quaternary system at 298.15 K, which helps to guide the isothermal evaporation crystallization process and the sequence of salting out. |
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