Solid–Liquid Phase Equilibria of Aqueous Quaternary System Na + , K + , Mg 2+ //SO 4 2 – –H 2 O at T = 298.2 and 323.2 K

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To prepare for the exploitation of deep-buried polyhalite resources, the phase equilibria of the ternary system K + //Cl − , SO 4 2− -H 2 O, Mg 2+ //Cl − , SO 4 2− -H 2 O and the quaternary system K + , Mg 2+ //Cl − , SO 4 2− -H 2 O at 363.2 K were investigated by using the isothermal dissolution method. The solubility and density of the systems were measured experimentally, and the related phase diagrams and density vs composition diagrams were plotted. The ternary systems K + //Cl − , SO 4 2− -H 2 O and Mg 2+ // Cl − , SO 4 2− -H 2 O are simple systems at 363.2 K. Both systems have one invariant point corresponding to the solid phases sylvite (KCl) + arcanite (K 2 SO 4 ) and bischofite (MgCl 2 •6H 2 O) + kieserite (MgSO 4 •H 2 O), respectively. The quaternary system K + , Mg 2+ // Cl − , SO 4 2− -H 2 O generates four double salts corresponding to carnallite (KCl•MgCl 2 •6H 2 O), kainite (KMg(SO 4 )Cl•3H 2 O), langbeinite (K 2 SO 4 •2MgSO 4 ), and leonite (K 2 SO 4 •MgSO 4 •4H 2 O), and the phase diagram consists of 7 quaternary invariant points, 14 isothermal dissolution curves, and 8 crystallization regions at 363.2 K. By comparing the phase diagram of the quaternary system at 348.2, 363.2, and 373.2 K, it can be seen that the crystallization region of K 2 SO 4 •MgSO 4 grow significantly with the increase of temperature, whereas the crystallization regions of MgSO 4 •H 2 O, KCl, and K 2 SO 4 shrink and the crystallization region of K 2 SO 4 •MgSO 4 •4H 2 O is on the verge of disappearing at 363.2 K. Based on the Pitzer−Simonson−Clegg (PSC) model, the solubility data of the ternary subsystems and the quaternary system were calculated. The results showed that the calculated values were close to the experimental values.

Solid–Liquid Phase Equilibria of the Aqueous Quaternary System K⁺, Mg²⁺//Cl^–, SO₄^2–-H₂O at T = 363.2 K

Du,

Zeng,

et al. 2024

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Solid–Liquid Equilibria of Aqueous Ternary System Na⁺, Mg²⁺//SO₄^2––H₂O at 288.2, 303.2, and 318.2 K

Yu,

Li,

Quan

et al. 2024

The phase equilibria of the ternary system Na+, Mg2+//SO4 2––H2O at 288.2, 303.2, and 318.2 K were investigated by the isothermal dissolution method. The solubility, density, and refractive index of the equilibrium liquid phase were measured, and the phase diagrams and density/refractive index versus composition diagrams were drawn. According to the multitemperature comparison of Na+, Mg2+//SO4 2––H2O at 288.2, 298.2, 303.2, 318.2, 323.2, and 348.2 K, it has found that it is a simple system at 288.2 K, while a complex system with double salt bloedite (Na2SO4·MgSO4·4H2O) formed at 298.2–323.2 K and double salts 6Na2SO4·7MgSO4·15H2O and 3Na2SO4·MgSO4 formed at 348.2 K. The solid phase of sodium sulfate and magnesium sulfate changes along with the increase of temperature: for sodium sulfate, Na2SO4·10H2O (288.2–303.2 K) and Na2SO4 (303.2–348.2 K); for magnesium sulfate, MgSO4·7H2O (288.2–303.2 K), MgSO4·6H2O (318.2, 323.2 K), and MgSO4·H2O (348.2 K). The crystallization area of Na2SO4·MgSO4·4H2O increases with the increase in temperature, which can lead to the coprecipitation of sodium and magnesium from salt lakes. Additionally, this temperature-dependent behavior is significant in the cooling crystallization method used to produce Na2SO4·10H2O and MgSO4·7H2O from the double salt Na2SO4·MgSO4·4H2O.

Solid Liquid Phase Equilibria in the Ternary System NaCl–Na₂SeO₃–H₂O and Na₂SO₄–Na₂SeO₃–H₂O at 298.15, 323.15 K

Wu,

Zheng,

Zhang

et al. 2024

At temperatures of 298 and 323 K, the solubility data for both solid and liquid phases across various systems was measured utilizing the isothermal dissolution equilibrium method at equilibrium. This investigation encompasses the binary system Na 2 SeO 3 −H 2 O as well as the ternary systems NaCl−Na 2 SeO 3 −H 2 O and Na 2 SO 4 −Na 2 SeO 3 −H 2 O. The research findings indicate that at both 298 and 323 K, the ternary system NaCl−Na 2 SeO 3 −H 2 O exhibits one invariant point, two univariant curves, and three crystallization regions. These correspond to NaCl, Na 2 SeO 3 , and a combination of them, respectively. In the context of the ternary system Na 2 SO 4 −Na 2 SeO 3 −H 2 O, there are significant differences in the equilibrium phase diagram at two different temperatures. At 298 K, the phase diagram comprises two invariant points, three univariant curves, and five crystallization regions, which correspond to