Theoretical derivation of schemes for complete phase diagrams of two- and three-­component systems provides a basis for formulation of the development of experimental studies of water-salt systems under superambient conditions. The results an investigation of ternary water-salt systems using this approach are discussed herein.

The phase equilibria and critical phenomena in the Na2SO4–Na2B4O7–H2O system with the boundary subsystems Na2B4O7–H2O (type 1 with the phase separation of the solutions) and Na2SO4–H2O (type 2) at 350–460°C and a pressure of up to 100 MPa are studied. It is shown that there exist two three-phase regions L1–L2–S, which are adjacent to two water–salt subsystems and divided by a fluid region, at temperatures ranging from 440°C (the critical point Q (L1 = L2–S) of the type 2 binary system) to ~455°C (at which these regions unite). The ternary system has two critical curves in saturated solutions. The curves originate from different critical points of the type 2 binary subsystem, point p (G = L–S) and point Q (G = L1–L2–S), respectively, and end at the end ternary critical points pR (G = L1–L2–S) and QN (G–L1 = L2–S), respectively. Between the last two points, there is a four-phase equilibrium (G–L1–L2–S), which, according to the visual observations in the sealed quartz ampules, exists in the temperature range 350–380°C.

The solubilities of salts in the MgSO4–H2O and CdSO4–H2O systems are studied at 276–500°C using various methods (visual examination of phase transformations in water–salt mixtures in sealed quartz ampoules, collecting solution samples and performing chemical analysis, and measuring p–V–T dependences in an autoclave) to determine temperature dependence and classify the compounds as one the of two types (type 1 or type 2) according to the classification of inorganic salts. In the studied temperature range, the solubility of these salts is very low at pressures below 157 MPa and decreases when approaching the critical point of water. The systems under study are found to be classified as type 2 water–salt systems having the critical point (G = L – S) lying near the critical point of water. No separation of fluid into two liquid phases was detected at temperatures below 500°C and pressures below 157 MPa, thus indicating that the second critical point (L1 = L2 – S) has very high parameters. The resulting data can be used for elaborating methods for raw material processing.

Phase equilibria in the binary system ZnSO4–H2O at temperatures of 240–450°C and pressures to 34 MPa were studied by various methods (visual observation of phase transformations in water–salt systems of various compositions (30–60 wt%) in thick-walled quartz ampules (3 mm i.d.), taking samples of solutions and their chemical analysis, and autoclave measurements of P–V–T parameters in phase transitions). It was shown that the decrease in the ZnSO4 solubility with increasing temperature, which is observed to 350–370°C, gives way to its abrupt increase with further heating; i.e., the negative temperature coefficient of solubility, which is characteristic of type 2 systems, changes to a positive one, as in type 1 systems. The ZnSO4–H2O system is of type 1 complicated by phase separation of saturated (and unsaturated) solutions, which leads to the sharp increase in the salt solubility above 375°C.

Phase equilibria were studied in aqueous solutions of sodium and potassium phosphates at elevated temperatures (to 425°C) and pressures (to 63 MPa). Experimental data were obtained on the transformation of gas–liquid equilibria into heterogeneous equilibria of two liquid phases with increasing pressure, which ends in the critical phenomena of immiscibility (L 1 = L 2 ). Based on the experimental and literature data, it was concluded that the binary water–salt systems containing KH 2 PO 4 , K 2 HPO 4 , K 3 PO 4 , NaH 2 PO 4 , and Na 2 HPO 4 are characterized by phase diagrams of the first type [1, 2], which have two critical invariant end points: N (G–L 1 = L 2 ) and R (G = L 1 –L 2 ).

Phase equilibria were studied at temperatures 475–520°C and a pressure to 130 MPa in the ternary system Na2SO4–NaCl–H2O with boundary binary subsystems of two types. In type 1 subsystems (the NaCl–H2O subsystem in this work), there are no critical phenomena in saturated solutions. Type 2 subsystems (the Na2SO4–H2O subsystem in this work) have terminal critical points p (G = L – \({S_{N{a_2}S{O_4}}}\)) and Q (L1 = L2 – \({S_{N{a_2}S{O_4}}}\)). It was shown that the ternary system contains two regions of three-phase equilibria ((G–L–S) and (L1–L2–S)), divided by a two-phase fluid region (F – \({S_{N{a_2}S{O_4}}}\)), and two types of monovariant critical curves ((G = L – \({S_{N{a_2}S{O_4}}}\)) and (L1 = L2 – \({S_{N{a_2}S{O_4}}}\))). With increasing temperature, these three-phase regions approach each other until the two-phase fluid equilibrium vanishes and the monovariant critical curves meet at a binary homogeneous critical point (G = L–S ⇔ L1 = L2 – \({S_{N{a_2}S{O_4}}}\)) at a maximal temperature of ~495°C and a pressure of ~75 MPa.

Supercritical phase equilibria in the ternary system K2SO4–KOH–H2O at 420–500°C and up to 130 MPa pressure with binary boundary subsystems of different types are studied. The binary subsystem of type 1 features no critical phenomena in saturated (l = g) aqueous solution and no phase separation (l1–l2) (KOH–H2O); the binary subsystem of type 2 is characterized by immiscibility of the liquid phase and has two critical end-points \(p(g = l-_{S_{K_{2}SO_{4}}})\) and \(Q(l_{1} = l_{2}-_{S_{K_{2}SO_{4}}})\) in saturated aqueous solution (K2SO4–H2O). The ternary system has two three-phase equilibria (g–l–s) and (l1–l2–s), separated by a two-phase supercritical fluid region \((fl-_{S_{K_{2}SO_{4}}})\), and two types of monovariant critical curves \((g=l-_{S_{K_{2}SO_{4}}})\) and \((l_{1}=l_{2}-_{S_{K_{2}SO_{4}}})\). The three-phase regions approach each other upon temperature increase up to the point where the two-phase supercritical equilibrium disappears, and the two mentioned monovariant critical curves are joined into a double homogeneous critical point \((g=l-_{S_{K_{2}SO_{4}}} \leftrightarrow l_{1} = l_{2}-_{S_{K_{2}SO_{4}}})\) at maximum temperature ~445°C and 51–52 MPa.

Phase equilibria were studied at 450 and 470°C in the Li2SO4-LiCl-H2O ternary system with boundary binary subsystems of various types: type 1 subsystem LiCl-H2O and type 2 subsystem Li2SO4-H2O with critical points \(p\left( {G = L - S_{Li_2 SO_4 } } \right)\) and \(Q\left( {L_1 = L_2 - S_{Li_2 SO_4 } } \right)\). It was shown that, in the system, there are two regions of three-phase equilibria (G-L-S) and (L1-L2-S), which are separated by a fluid region \(\left( {F - S_{Li_2 SO_4 } } \right)\), and also there are monovariant critical curves of two types \(\left( {G = L - S_{Li_2 SO_4 } } \right)\) and \(\left( {L_1 = L_2 - S_{Li_2 SO_4 } } \right)\). With increasing temperature, both three-phase regions and critical curves respectively approach each other. The fact that the p-T parameters of different critical curves near 50–51 MPa and 470°C coincide, whereas the compositions of the critical solutions are essentially different, indicates that there may emerge a four-phase equilibrium \(\left( {G - L_1 - L_2 - S_{Li_2 SO_4 } } \right)\) and there is no double homogeneous critical point \(\left( {G = L - S_{Li_2 SO_4 } \Leftrightarrow L_1 = L_2 - S_{Li_2 SO_4 } } \right)\) in this ternary system.

The system LiOH—H 2 O is characterized by immiscibility phenomena both in equilibrium with vapor and solid saturated solutions or without the solid phase. The solubility of LiOH increases as a result of immiscibility phenomena at ~355°C and further increases above this temperature up to the melting point. The immiscibility region (g–l 1 –l 2 ) is terminated by the critical phenomena (g = l 1 –l 2 ) at ~390°C. A separation of liquid phases is observed at elevated pressures (at ~39 MPa) and temperatures up to 425°C.

Phase equilibria were studied in the Li 2 SO 4 –LiCl–H 2 O system with boundary subsystems of various types: type 1 subsystem LiCl–H 2 O and type 2 subsystem Li 2 SO 4 –H 2 O. The ternary critical curves (G=L–S) and (L 1 =L 2 –S) originating at the terminal critical points p and Q of the type 2 binary system do not combine to form a single curve with the binary homogeneous critical point (G=L–S) ⇔ (L 1 =L 2 –S) but end at the invariant points (G=L 1 –L 2 –S) and (G–L 1 =L 2 –S) connected by the monovariant four-phase curve (G–L 1 –L 2 –S) at a temperature below 490°C. It was found that, at 490–520°C, there are no critical phenomena in the solutions, and the solubility of each of the salts varies continuously from the solubility in the binary aqueous system to the solubility in an anhydrous melt. Above the temperature of the four-phase equilibrium (G–L 1 –L 2 –S), a region (G–L 1 –L 2 ) of phase separation of unsaturated solutions in the presence of gas is observed, which is bounded by the monovariant critical curves (G=L 1 –L 2 ) and (G–L 1 =L 2 ). With increasing temperature, this region shrinks and collapses at the tricritical point (G=L 1 =L 2 ) at ~520°C.

Experimental studies of binary and ternary water-salt systems containing NaF or Li2CO3 demonstrate that these salts belong to compounds of the 2nd type. The compounds are characterized by a negative temperature coefficient of salt solubility in water, critical phenomena in salt-saturated aqueous solutions, and a wide region of fluid equilibria, with supercritical fluid solutions being homogeneous within this region at any pressure. However, the behavior of these two salts of the 2nd type is different from that of alkali metals sulfates and carbonates, which is complicated by the immiscibility phenomena in stable and metastable states. It is concluded that the immiscibility phenomena in the hydrothermal binary systems with NaF and Li2CO3 are absent, while the second critical Q end-point appears in solid saturated solutions at a very high temperature and pressure (above 500°C and 150 MPa, correspondingly).

Experimental studies of water solubilities of Li2WO4 at temperatures up to 450°C and pressures up to 147 MPa show that this salts with water forms a type 2 binary system, where the solubility of the salt under the vapor pressure decreases as temperature rises to end at the critical end-point p (g=l-s) near the H2O critical temperature. High-temperature Li2WO4 solutions are shown to not react with silica, but they cause corrosion of metallic walls of an autoclave accompanied with gas evolution and formation of dark-colored solid corrosion products.

Phase equilibria in Li2SO4-KLiSO4-H2O and K2SO4-KLiSO4-H2O ternary systems, which contain type 2 salts (Li2SO4, KLiSO4, and K2SO4), were studied at temperature of 380 to 400°C and pressures up to 90 MPa. Homogeneous supercritical (SC) fluids, which propagate from type 2 water-salt subsystems into a three-component region, change to heterogeneous liquid-phase equilibria as a result of the transformation of metastable immiscibility regions into stable equilibria. The heterogenization of SC fluids starts with critical phenomena in saturated solutions. In the K2SO4-KLiSO4-H2O system, the monovariant critical curve \((l_1 = l_2 - s_{KLiSO_4 } )\) reveals a temperature maximum at the invariant double critical end point (DCE).

Phase equilibria in the Na2CO3-NaCl-H2O and Na2CO3-Na2WO4-H2O ternary systems formed by type 1 salts (NaCl, Na2WO4) and a type 2 salt (Na2CO3) were experimentally studied at temperatures from 425 to 500°C and pressures from 30 to 160 MPa with the contents of type 1 salts from 10 to 30 wt %. Transition from supercritical homogeneous fluid equilibria of the Na2CO3-H2O system to heterogeneous equilibria of the title ternary systems was studied in the presence or absence of liquid phase immiscibility in the type 1 subsystems.

A flow hydrothermal setup with a tubular reactor equipped with a plunger pump and back pressure valves is used to study the effects of scaling in the K2SO4-KCl-H2O, K2SO4-K2CO3-H2O, and Na2SO4-NaCl-H2O systems at pressures of up to 270–340 kg/cm2, temperatures of 400–600°C, and flow rates of 5.0 and 2.5 ml/min in order to establish conditions for the formation of salt plugs of type 2 (K2SO4, Na2SO4) in the flow mode at supercritical (SC) state parameters and to explore ways of eliminating such salt deposits by means of hydrothermal solvents, more specifically, high-temperature aqueous solutions of salts of type 1 (KCl, K2CO3, and NaCl). The concentrations of hydrothermal solvents sufficient to prevent the plugging of flow systems with solutions containing 0.26–0.27 mol % K2SO4 or Na2SO4 are determined, and the effects of the flow rate and chemical composition of type 1 salts on this process are studied. The results show that the phenomenon of scaling with the formation of salt plugs, which hinders the practical use of supercritical water oxidation technology, can be eliminated by adding readily soluble electrolytes, salts of type 1, to initial aqueous solution of type 2 salts.

Experimental studies of high-temperature phase equilibria in the BaCl2-NaCl-H2O made it possible to determine the conditions of heterogenization of homogeneous supercritical (SC) fluids spreading from the BaCl2-H2O binary subsystem of type 2 and to reveal the continuity of the transformation of gasliquid equilibria into equilibria of phase separation in SC environment. Comparison with the characteristics of the K2SO4-KCl-H2O system suggests that ternary systems with one volatile component formed by binary subsystems of type 2 with phase separation and of type 1 without separation have the same type of phase diagrams.

The systematic classification of complete phase diagrams for binary systems and theoretical derivation of ternary phase diagrams with one volatile component, based on the method of continuous topological transformation, are briefly discussed. Review of high-temperature experimental data for ternary water–salt systems permits to establish both the main directions of phase transformations at sub- and supercritical conditions and some details of phase behavior in stable and metastable equilibria. The greater attention is paid to the ternary systems with binary water–salt subsystems of different types, where the heterogenization of homogeneous supercritical fluid takes place. The topological schemes of ternary phase diagrams are used for interpretation of available experimental data both for critical and non-critical phase behavior in stable and metastable conditions.

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The phase equilibria in ternary systems with one volatile component and a homogeneous supercritical fluid (SCF) region in one of the binary subsystems (with two critical end-points p and Q) were analyzed. Heterogenization of SCFs, extending from the 2d′ boundary subsystem into a ternary system, starts with monovariant critical phenomena in saturated solutions (l1 = l2-s) or (g = l−s), which pass through the extremal parameters of the temperature or composition to form special critical equilibria such as double critical end-points DCEs (l1 = l2-s, g = l-s, l1 = l2-g) or double homogeneous points DHPs (l1 = l2-s ↔ g = l-s). Ternary nonvariant critical points (l1 = l2 = g, l1 = l2-g-s, and l1 = g-l2-s) appear at the intersections of the monovariant critical and noncritical curves.

This experimental study of phase equilibria in the K 2 SO 4 -K 2 CO 3 -H 2 O system at 385–500°C and pressures up to 100 MPa is directed to determine the sequence of phase transformations that generate heterogeneous supercritical fluids from the homogeneous one; the homogeneous supercritical region spreads into the ternary system from the K 2 SO 4 -H 2 O subsystem. We found that heterogenization of supercritical fluid upon addition of K 2 CO 3 starts with l 1 =l 2 critical phenomena in solid saturated solutions and is attended by amalgamation of the stable immiscibility region that spreads from the K 2 CO 3 -H 2 O system with the metastable immiscibility region that originates from the K 2 SO 4 -H 2 O system. Our experimental results and the topological analysis of phase equilibria at temperatures above the critical point of water gave us the full scenario of the phase behavior of the title ternary system in the regions of fluid equilibria, g=l and l 1 =l 2 critical phenomena, and liquid-liquid phase separation in two-, three-, and four-phase equilibria.