Phase diagram of the NaCl–MgCl<Subscript>2</Subscript>–H<Subscript>2</Subscript>O system at 25–75°C and its application for MgCl<Subscript>2</Subscript> · 6H<Subscript>2</Subscript>O purification

The co-saturation line for the solid phases NaCl_(s) and MgCl₂ · 6H₂O_(s) in aqueous solution has been measured by a phase equilibrium at various temperatures. It was found that the Y _b (Y _b = w(NaCl)/(w(NaCl) + w(MgCl₂))) value of the co-saturation line increase with increasing temperature. A new recrystallization approach has been suggested for the purification of MgCl₂ · 6H₂O_(s) containing quite amount of impurity NaCl, i.e., dissolving the crude sample at low temperatures, followed by evaporating and phase separating at high temperatures. Applying the proposed approach a crude MgCl₂ · 6H₂O_(s) sample can be purified to the level of Y _b = 0.17% by only one crystallization process.     

Solubilities of salts in the ternary systems NaCl + CaCl<Subscript>2</Subscript> + H<Subscript>2</Subscript>O and KCl + CaCl<Subscript>2</Subscript> + H<Subscript>2</Subscript>O at 75°C

The solubility in the NaCl-CaCl₂-H₂O and KCl-CaCl₂-H₂O systems were determined at 75°C and the phase diagrams and the diagram of physicochemical property vs composition were plotted. One invariant point, two univariant curves, and two crystallization zones, corresponding to potassium chloride, dihydrate (CaCl₂ · 2H₂O) showed up in the phase diagrams of the ternary systems. The mixing parameters θ_{M, Ca} and Ψ_{M, Ca, Cl} (M = Na or K) and equilibrium constant K _sp were evaluated in NaCl-CaCl₂-H₂O and KCl-CaCl₂-H₂O systems by least-squares optimization procedure, in which the single-salt Pitzer parameters of NaCl, KCl, and CaCl₂ β⁽⁰⁾, β⁽¹⁾, β⁽²⁾, and C ^Φ were directly calculated from the literature. The results obtained were in good agreement with the experimental data.     

Solubility in the Na,Ca∥CO<Subscript>3</Subscript>,HCO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 25°C

Solubility in the Na,Ca∥CO₃,HCO₃–H₂O system is studied; its phase diagram at 25°C is plotted.     

Solubility in the LaCl<Subscript>3</Subscript>-GdCl<Subscript>3</Subscript>-HCl-H<Subscript>2</Subscript>O system at 25°C

Solubility has been studied in the LaCl₃-GdCl₃-HCl-H₂O quaternary water-salt system at 25°C along the 40±0.2% HCl section; the system is of the eutonic type. The composition of the eutonic solution is as follows: LaCl₃·7H₂O-4.50, GdCl₃·6H₂O, 0.52; HCl, 37.99; and H₂O, 56.99 wt %.     

Measurement of mineral solubilities in the ternary systems NaCl−ZnCl<Subscript>2</Subscript>−H<Subscript>2</Subscript>O and MgCl<Subscript>2</Subscript>−ZnCl<Subscript>2</Subscript>−H<Subscript>2</Subscript>O at 373 K

In this work, the solubilities of the salt minerals and the densities of solution in two ternary systems sodium chloride–zinc chloride–water and magnesium chloride–zinc chloride–water were measured at 373 K using an isothermal solution saturation method. Based on the determined equilibrium solubility data and the corresponding equilibrium solid phase, the phase diagrams and density diagrams of the two systems were plotted. The results show that the two ternary systems are complex and the eutectic points, the univariant solubility curves and the solid crystalline phase regions are shown and discussed. The phase diagram of the ternary system NaCl?ZnCl₂?H₂O at 373 K is constituted of two eutectic points, three univariant solubility curves and three solid crystalline phase regions corresponding to NaCl, ZnCl₂ and 2NaCl · ZnCl₂. And the phase diagram of the ternary system MgCl₂?ZnCl₂?H₂O at 373 K includes two eutectic points, three univariant solubility curves and three solid crystalline phase regions corresponding to MgCl₂ · 6H₂O, MgCl₂ · ZnCl₂ · 5H₂O and ZnCl₂. The experimental results were simply discussed.     

Interaction of components in the NaOH-TiO<Subscript>2</Subscript> · H<Subscript>2</Subscript>O-H<Subscript>2</Subscript>O system at 25°C

Solubilities in the NaOH-TiO₂ · H₂O-H₂O system at 25°C were studied. The solubility isotherm was found to have two maxima. The formation of two compounds, Na₂Ti₅O₁₁ · 10H₂O and Na₂Ti₃O₇ · 7H₂O, was established.     

Solubility in the Na,Са||SO<Subscript>4</Subscript>,СО<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 25°С

Solubility at the invariant points of the Na,Са||SO₄,СО₃–H₂O system at 25°C was studied, and a solubility diagram at this temperature constructed.     

Solubility in the Na,СаǁSO<Subscript>4</Subscript>,СО<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 0°С

Solubility at the invariant points of the Na,Са∥SO₄,СО₃–H₂O system at 0°C was studied, and a solubility diagram at this temperature constructed.     

Solubility in the system 2HCOONa + CaCl<Subscript>2</Subscript> ⇄ (HCOO)<Subscript>2</Subscript>Ca + 2NaCl–H<Subscript>2</Subscript>O

Phase and conversion equilibria in the quaternary reciprocal water–salt system 2HCOONa + CaCl₂ ? (HCOO)₂Ca + 2NaCl–H₂O at 25°C were studied for the first time to determine the concentration parameters of the production of calcium formate from sodium formate.     

Phase equilibria in system LiCl–NaCl–H<Subscript>2</Subscript>O at 308 and 348 K

The solubilities and densities of the solutions in the ternary system LiCl–NaCl–H₂O at 308 and 348 K were determined by the method of isothermal dissolution equilibrium. There are one invariant point, two univariant isotherm dissolution curves, and two crystallization regions corresponding to monohydrate (LiCl · H₂O) and NaCl, respectively. This system at both temperatures belongs to hydrate type I, and neither double salt nor solid solution was found. A comparison of the phase diagram for the ternary system at 273–348 K shows that the area of crystallization region of LiCl · H₂O is decreased with the increasing of temperature, while that of NaCl is increased obviously. The solution density of the ternary system at two temperatures changes regularly with the increasing of LiCl concentration.     

Solubility in the PrCl<Subscript>3</Subscript>-SmCl<Subscript>3</Subscript>-HCl-H<Subscript>2</Subscript>O System at 25°C

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 the diagonal sections of the 2KCl + Ca(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> ⇆ 2KNO<Subscript>3</Subscript> + CaCl<Subscript>2</Subscript>–H<Subscript>2</Subscript>O system

Solubility data in the diagonal sections of the quaternary reciprocal 2KCl + Ca(NO₃)₂ → 2KNO₃ + CaCl₂–H₂O system at 25 and 15°C are presented. It has been shown that the quaternary system has no stable diagonal at the studied temperatures, but contains a stable pair of salts, namely, potassium nitrate and calcium chloride. The obtained data can be used to optimize the thermal and concentrational parameters of the synthesis of potassium nitrate from calcium nitrate and potassium chloride.     

Formation of binary salts in the system LiCl–CsCl–H<Subscript>2</Subscript>O

In the ternary system LiCl–CsCl–H₂O the binary salt LiCl·2CsCl·4H₂O was formed. Its structure was proved by the X-ray structural analysis. The binary compound formed in the aqueous solution by the structurally-forced insertion mechanism.     

Phase equilibria in a ternary fullerenol-d(C<Subscript>60</Subscript>(OH)<Subscript>22–24</Subscript>)–SmCl<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 25°C

The solubility in a ternary fullerenol-d (C₆₀(OH)_22–24)–SmCl₃–H₂O system at 25°C is studied via isothermal saturation in ampules. The solubility diagram is shown to be a simple eutonic one that consists of two branches corresponding to the crystallization of fullerenol-d (C₆₀(OH)_22–24 · 30H₂O) and samarium(III) chloride SmCl₃ · 6H₂O crystallohydrates and contains one nonvariant eutonic point corresponding to saturation with both crystallohydrates. The long branch of C₆₀(OH)_22–24 · 30H₂O crystallization shows the effect of fullerenol-d salting out of saturated solutions; in contrast, the short branch of SmCl₃ · 6H₂O crystallization shows the pronounced salting-in effect of samarium(III) chloride.     

Solid–Liquid Metastable Phase Equilibria in the Ternary System (MgCl<Subscript>2</Subscript> + MgSO<Subscript>4</Subscript> + H<Subscript>2</Subscript>O) at 323.15 K

The solubilities and the densities in the aqueous ternary system (MgCl₂ + MgSO₄ + H₂O) at 323.15 K were determined by the isothermal evaporation method. The phase diagram was drawn for this system at 323.15 K. The phase diagram consists of two invariant points, three univariant curves, and three crystallization regions corresponding to bischofite (MgCl₂ · 6H₂O), tetrahydrate (MgSO₄ · 4H₂O) and hexahydrite (MgSO₄ · 6H₂O). Neither double salts nor solid solution was found. Based on the Pitzer and Harvie–Weare (HW) model, the solubility equilibrium constants for the salts were fitted with the solubilities in this research work, and the solubilities of the ternary system at 323.15 K were calculated. Comparisons between the calculated and measured solubilities show that the predicted data agree well with the experimental results.     

Water Activity and Solubility Measurements and Pitzer Model Simulation of the MgCl<Subscript>2</Subscript>–RbCl–H<Subscript>2</Subscript>O Ternary System at 298.15 K

Water activities for the MgCl₂–RbCl–H₂O ternary system and its sub-binary MgCl₂–H₂O and RbCl–H₂O systems at 298.15 K have been measured using the isopiestic method. The measured results for the binary systems agree well with literature data. The measured equal water activity lines for the MgCl₂–RbCl–H₂O ternary system deviate from Zdanovskii’s rule. The solubilities for the MgCl₂–RbCl–H₂O ternary system also have been measured using a isothermal visual first/last crystal method, and the results are consistent with one reported set of solubility data. Pitzer’s model was selected to correlate the measured water activity and solubility data and the parameters reported in the literature are re-examined by comparing the model-calculated equal water activity lines and solubilities with experimental values measured in this work. New and reasonable parameters are obtained by fitting the water activity and solubility data measured in this work and those reported in the literature. The water activities and solubilities were simulated with the new parameters and the results show that the Pitzer model can successfully represent the thermodynamic properties and be used to calculate the solubility isotherms.     

Solid-Liquid Equilibria in the Quaternary Systems KCl–MgCl<Subscript>2</Subscript>–SrCl<Subscript>2</Subscript>–H<Subscript>2</Subscript>O and NaCl–KCl–SrCl<Subscript>2</Subscript>–H<Subscript>2</Subscript>O at 348 K

Solid-liquid equilibria in the quaternary systems KCl–MgCl₂–SrCl₂–H₂O and NaCl–KCl–SrCl₂–H₂O at 348 K were measured by the isothermal solution saturation method. The composition of the equilibrium solid phase, solubilities of salts, and densities of saturated solution in the two systems were determined. Phase diagrams, water content diagrams and solution density diagrams of quaternary systems were plotted according to experimental data. The phase diagram of the quaternary system NaCl–KCl–SrCl₂–H₂O has one invariant point, three univariant curves as the boundary of NaCl, KCl and SrCl₂ · 2H₂O. This phase diagrams were simple co-saturation type without complex salt and solid solution. For the quaternary system KCl–MgCl₂–SrCl₂–H₂O, one complex salt KCl · MgCl₂ · 6H₂O (Car) had been found in this system, consisted of five univariant curves, two invariant points and four crystallization regions of MgCl₂ · 6H₂O (Bis), KCl, SrCl₂ · 2H₂O and KCl · MgCl₂ · 6H₂O. And the densities transformation rules were simply discussed. Simultaneously, the solubilities and densities data in invariant point of the quaternary system NaCl–KCl–SrCl₂–H₂O had been compared with the experimental data of previous researchers.     

Formation of nanocrystals in the ZrO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O system

Formation of zirconia nanocrystals in the course of thermal treatment of an X-ray amorphous zirconium oxyhydroxide was studied. It was shown that the formation of tetragonal and monoclinic polymorphs of ZrO₂ in the temperature range from 500 to 700°C occurs owing to dehydration and crystallization of amorphous hydroxide. An increase of the temperature up to 800°C and higher activates mass transfer processes and, as a result, activates the nanoparticle growth and increases the fraction of the phase based on monoclinic modification of ZrO₂ due to mass transfer from the nanoparticles with the non-equilibrium tetragonal structure. Herewith, formed ZrO₂ nanocrystals with monoclinic structure have a broad size distribution of crystallites, and the average crystallite size after thermal treatment at 1200°C for 20 min is about 42 nm.     

Solubility and Ionic Interaction of the CaF<Subscript>2</Subscript>–CaSO<Subscript>4</Subscript>–H<Subscript>2</Subscript>O System at 298.1 K

Solubility isotherms of the sparingly soluble salts CaF_2(s) and CaSO₄·2H₂O_(s) in their mixed aqueous solutions have been measured at 298.1 K. It was found that the CaF_2(s) solubility decreases with increasing CaSO₄ concentration in the solution and reaches about 1/3 of the CaF_2(s) solubility in pure water in the CaSO₄·2H₂O_(S) saturated solution. A thermodynamic model was developed to predict the CaF_2(s) solubility isotherm in this system, in which the short range interactions of the species in the aqueous solution are represented by ion-association constants reported in literature, and the long range interaction, i.e., the electrostatic term, is represented by the well known Davies equation. The predicted solubility isotherm reasonably agrees with the experimental results. The contributions of the long-range term and the short-range term to the calculated solubility isotherm were investigated. It was concluded that the ionic association combining with the Davies equation is sufficient to represent the excess interaction of the CaF₂ + CaSO₄ aqueous solution at 298.15 K. This model approach could be applicable for other dilute mixed electrolyte systems in which component activity coefficients are lacking and model parameters are difficult to determine.