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The solubility was studied for compounds in the system Zr(SO4)2–K2SO4–H2SO4–H2O in the section at 10 wt % H2SO4 under isothermal conditions at 25°C in the cuts at a constant solvent content of 60 wt % (H2O + H2SO4). Equilibrium in the system was reached for 16 h while continuously stirring. In the section at 10 wt % H2SO4, equilibrium solid phases are the compounds K2Zr(SO4)3 ⋅ 2H2O, K3Zr(OH)(SO4)3 ⋅ 2H2O, K6Zr(SO4)5 ⋅ 3H2O, and β-K2SO4. The recovered compounds were investigated by X-ray powder diffraction, crystal optical, thermal analyses and IR spectroscopy. It was found that the solubilities of the potassium zirconium sulfates are much lower than those of the sodium zirconium compounds. The results are of practical importance for sulfuric acid treatment of eudialyte, a zirconium-containing mineral, huge reserves of which occur in the Kola Peninsula.

The solubility was studied in the system NaCl–AlCl3−SrCl2–HCl–H2O of the eutonic type at 25°C in the section 28 wt % HCl. It was determined that it is possible to derive equations of the solubility surface of crystallizing salts and to calculate the composition of the eutonic solution.

Solubility was studied in the system NaCl–AlCl3–HCl–H2O at 25°C in the section 28 wt % HCl. The system is of the eutonic type and has an extensive sodium chloride crystallization region. The composition of the eutonic solution is the following, wt %: NaCl, 0.47; AlCl3 • 6H2O, 8.88; HCl, 25.38; and H2O, 65.27. The lines of saturated solutions were approximated by polynomial equations.

The solubility in the quaternary water–salt system Zr(SO4)2 · 4Н2О–Na2SO4–H2SO4–H2O at 25°C was studied. It was found that, in the system, there is crystallization of not only Na2SO4 and Zr(SO4)4 · 4H2O, but also sodium sulfate zirconates Na2Zr(SO4)2(OH)2 · 0.3H2O, Na4Zr(SO4)4 · 3H2O, and Na2Zr(SO4)2 · 3H2O and two new compounds, S1 and S2, which are presumably Na2ZrO(SO4)2 · 2H2O and Na2Zr2O2(SO4)3 · 6H2O.

The solubility at 25°C in the NdCl3-SmCl3-HCl-H2O and NdCl3-GdCl3-HCl-H2O quaternary water-salt systems has been studied in 40% hydrochloric acid sections. The NdCl3-SmCl3-HCl-H2O system represents a continuous series of solid solutions (type I solid solutions). The NdCl3-GdCl3-HCl-H2O system is a system with peritonic discontinuity (type V solid solutions). The discontinuity point of the peritonic solution has the following composition (wt %): NdCl3 · 6H2O, 1.13; GdCl3 · 6H2O, 0.11; HCl, 39.5; H2O, 59.26.

The solubility in the quaternary water-salt systems LaCl3-NdCl3-HCl-H2O (1) and LaCl3-PrCl3-HCl-H2O (2) at 25°C was studied in the section of 40 wt % hydrochloric acid, a system with a eutonic discontinuity. The composition at the point of discontinuity for the eutonic solution is the following. In system 1: 4.67 wt % LaCl3 · 7H2O, 0.37 wt % PrCl3 · 7H2O, 37.98 wt % HCl, and 56.98 wt % H2O; in system 2: 4.37 wt % LaCl3 · 7H2O, 0.93 wt % NdCl3 · 6H2O, 37.88 wt % HCl, and 56.82 wt % H2O.

Solubilities were studied in the NaCl-ZrOCl2-HCl-H2O water-salt system along the 28 wt % HCl section at 25, 50, and 75°C. This is a eutonic system. The compositions of the eutonic solution are as follows (wt % NaCl, ZrOCl2 · 8H2O, HCl, and H2O, respectively): at 25°C, 0.26, 1.88, 27.4, and 70.46; at 50°C, 2.82, 2.68, 26.46, and 68.04; and at 75°C, 1.9, 16.1, 22.96, and 59.04.

The solubility in the PrCl3-SmCl3-HCl-H2O quaternary water-salt system was studied at 25°C along the 40% hydrochloric acid section (a system with solid solutions and discontinuity). The composition of the discontinuity point was as follows: PrCl3 · 7H2O, 2.33; SmCl3 · 6H2O, 0.11; HCl, 40.05; and H2O, 57.51 wt %.

Solubility in quaternary water-salt systems AlCl3-SrCl2-HCl-H2O (I) and NaCl-SrCl2-HCl-H2O (II) at 25°C along the 28% HCl section of the eutonic-type systems has been investigated. The compositions of eutonic solutions are the following (wt %): in system I, 3.11 AlCl3 ·6 H2O, 7.57 SrCl2 · 6H2O, 22.21 HCl, and 57.11 H2O; and in system II, 2.3 SrCl2 · 6H2O, 1.7 NaCl, 26.88 HCl, and 69.12 H2O.

The solubility in the PrCl3-SmCl3-HCl-H2O quaternary water-salt system was studied at 25°C along the 40% hydrochloric acid section (a system with solid solutions and discontinuity). The composition of the discontinuity point was as follows: PrCl3 · 7H2O, 2.33; SmCl3 · 6H2O, 0.11; HCl, 40.05; and H2O, 57.51 wt %.