The crystallisation of alkaline-earth metal tungstates from metal chloride melts: solubility – temperature phase diagram studies

The solubility vs temperature phase diagrams, for magnesium, calcium, strontium and barium tungstate solutions in lithium, sodium, potassium chloride melts and in the alkalineearth metal chloride melts, have been analysed. The solution in the alkali metal chloride melts are pseudo-binary reciprocal ternary mixtures; the more soluble barium tungstate systems show small deviations from ideality, while the less solube magnesium tungstate systems show marked deviations from ideality, especially in sodium and potassium chloride melts. These deviations are related to electrostatic effects in solution, that depend on some function of reciprocal solute and solvent cation radii. The solutions in the alkaline-earth metal chlorids melts are binary mixtures with no solvate formation; these systems show some from ideality at low mole fractions (x = 0.1 to 0.25) but they are practically ideal at mole fractions from 0.25 to 0.6.     

The crystallisation of alkaline-earth metal silicates and titanates from metal chloride melts solubility-temperatur phase diagram studies

The solubility-temperature phase diagrams have been analysed for calcium and barium silicate solutions and for calcium, strontium and barium titanate solutions in the metal chloride melts. These systems are typical binary metal salt mixtures with no solid solution or compound formation. Activity coefficients were estimated from log solubility v reciprocal temperature plots and the anion polymerisation numbers were estimated from these coefficients. The metal silicate systems are non-ideal at silicate mole fractions from 0.1 to 0.8: the solubilities at any temperature are appreciably greater than the ideal values determined from the temperatures and enthalpies of metal silicate fusion: the calcium silicate systems contain upto 50 percent trisilicate chains, while the barium silicate systems contain up to 100 percent trisilicate chains and significant amounts of tetrasilicate chains. The metal titanate systems are practically ideal at titanate mole fractions from 0.01 to 0.3 and possibly upt < 0.6; the solubilities at any temperature are then determined by the temperatures and enthalpies of metal titanate fusion, only.     

The crystallisation of alkaline-earth metal tungstates (molybdates,chromates, and sulphates) from metal chloride melts

This paper presents studies on the crystallisation of alkaline-earth metal tungstates, molybdates, chromates and sulphates by slow cooling of solutions in lithium chloride and alkaline-earth metal chloride melts at 600° to 1100°C. Solubility — temperature diagrams were prepared for this temperature range. The effects of solute — solvent interaction, crystallisation temperature range and rate of cooling on crystal form and size were investigated. Final crystal size increases with reduced rate of cooling and with increasing crystallisation temperature; barium sulphate crystallisation from lithium chloride melt is anomalous.     

The crystallisation of alkaline-earth metal metasilicates from metal chloride melts: Preliminary experimental studies

The crystallisation of alcaline-earth metal metalsilicates was studied by slow cooling of saturated solutions in the metal chloride melts at T₀ = 830° to 1300°C, down to ambient temperature; cooling rates were varied from 20° to 200°C hr⁻¹. Calcium metasilicate crystallised as β-CaSiO₃ prisms, strontium metasilicate as α-SrSiO₃ platelets and barium metasilicate as α-BaSiO₃ platelets. Final crystal lengths increased with reduction in cooling rate and with increase in initial crystallisation temperature.     

Kinetics of crystal growth of strontium tungstate from solutions in sodium tungstate melts by continuous cooling

The crystallisation kinetics of strontium tungstate from unstirred saturated solutions in sodium tungstate melts was studied by continuous cooling from initial crystallisation temperatures T₀ = 1000° to 800°C to room temperature at cooling rates R_T = 0.67° to 3.3°C min⁻¹. The main crystal growth was diffusion rate-controlled; the final crystal growth was rate-controlled by the development rate of excess solute concentration. The estimated diffusion rate constant (k_D) values increased with cooling rates and initial crystallisation temperatures. They are higher than the rate constants for diffusion-controlled growth of calcium tungstate from sodium tungstate melts, but very much smaller than those for strontium tungstate from lithium chloride melts.     

The crystallisation of alkaline-earth metal tungstates from sodium tungstate melts

This paper presents studies on the crystallisation of alkaline-earth metal tungstates by slow cooling of saturated solutions in sodium tungstate melts at 700 to 1000 °C. Solubility-temperature diagrams were prepared for this temperature range. The effects of variation in the initial crystallisation temperature, cooling rate and metal salt structure on crystal number and size were investigated. Final crystal lengths increased with increase in the initial crystallisation temperature and decreased with increasing rate of cooling. Crystals grown from sodium tungstate melts at any temperature were generally smaller than those grown from lithium chloride melts (at the same temperature): they were similar in size to crystals grown from the metal chloride melts but crystallised at temperatures 150 ° to 250 °C lower.     

The crystallisation of alkaline-earth metal phosphates from alkaline-metal halide and alkali metal phosphate melts: Some preliminary solubility-temperature phase diagram studies

The solubility v temperature phase diagrams, for magnesium and calcium meta-, pyroand orthophosphate solutions in the alkalineearth metal halide melts and in different alkali metal phosphate melts, have been analysed: these are the pseudo-binary sections of the ternary P₂O₅ MO MX₂ and P₂O₅ MO Na(K)₂O systems at MO/P₂O₅= 1, 2, and 3, respectively. The temperature ranges and yields for crystallisations of alkalineearth, metal phosphates from these melt solutions are discussed.     

The Crystallisation of Calcium Molybdate and Tungstate from Lithium Chloride Melts by Continuous Cooling. The Kinetics of Crystal Growth in Alumina Crucibles

The kinetics of crystallisation of calcium molybdate and tungstate from unstirred supersaturated solutions in lithium chloride melts — in alumina crucibles — was studied by continuous cooling from initial temperature T₀ = 800°C down to room temperature at cooling rates R_T = 20° to 200°C hr⁻¹. The solutions were analysed chemically and the crystals were examined by optical microscopy. Crystal growth started practically immediately after the onset of cooling: at first, the amount of material deposited onto crystals was far less than the amount of excess solute developed within the supersaturated solutions but crystallisation rates then increased as the crystal sizes increased. Then, after some time t* (at about seventy percent crystallisation), all excess solute was deposited onto growing crystals.     

The annealing of alkaline-earth metal polymetaphosphate powders (I). Crystallisation and sintering processes

Amorphous barium, strontium, calcium and magnesium polymetaphosphate powders (MP₂O₆)_n, n = 20 were prepared by dehydration of the corresponding polymetaphosphate hydrate precipitates. These powders were annealed by different continuous and isothermal heat treatments over the temperature range 450° to 700 °C, the glass transition temperatures T_g to above (T_g + 120) °C. The morphologies at different degrees of crystallisation were studied by scanning electron microscopy. For the main crystallisation process (ten to sixty-seventy percent crystallisation), the powder particles retained their original pea-pod form; then after seventy percent crystallisation, these crystallised particles sintered laterally to lozenge-shaped twin-hexagonal crystals of lengths 0.5 to 3 μm. Differential thermal analysis confirmed that a markedly exothermic crystallisation process (overall enthalpy changes from about 30 to 45 kJ mol⁻¹) was occurring within the powder particles. Crystallisation rates varied from < 0.005 min⁻¹ at temperatures near T_g to > 0.5 min⁻¹ at higher temperatures; the activation energies for this process varied from 360 to 560 kJ mol⁻¹. The completely annealed crystals were studied by scanning electron microscopy, X-ray diffraction and further differential thermal analysis to 1000 °C. The X-ray diffraction d value patterns, the fusion temperatures and the enthalpies of fusion were all in close agreement with the literature values for the corresponding beta alkaline-earth metal polymetaphosphates prepared by melt crystallisation.     

Thermodynamic analysis of solute-solvent interactions in the solubility-temperature relations of alkaline-earth metal tungstates and sodium tungstate

This paper presents a study of thermodynamic analysis of the solubility-temperature phase diagrams for solutions of calcium, strontium and barium tungstate in sodium tungstate melts in the temperature range 660 to 1200 °C. At temperatures 1000 °C and above, the solutions were ideal but below 1000 °C the solutions became non-ideal and the non-ideality increased with decreasing temperature. At any mole fraction concentration of the solute the excess free energies of mixing and the activity coefficients increased in the order CaWO₄ > SrWO₄ > BaWO₄, whereas the excess chemical potentials decreased in the order CaWO₄ < SrWO₄ < BaWO₄.     

The precipitation of iron (II) and nickel oxalates with alkaline-earth metal oxalates: Induction periods,nucleation rates and mechanisms

The coprecipitations of magnesium (and barium) iron(II) and nickel oxalate dihydrates were studied from excess magnesium and barium oxalate solutions. Nucleation rates were estimated from the induction periods for the coprecipitations. The nucleation rates in excess magnesium oxalate solutions first decreased with increasing excess oxalate anion concentration to minimum values and then increased with increasing magnesium cation concentration. At low to intermediate Mg/Fe(II) (and Mg/Ni) ratios the main nuclei were FeOx (and NiOx); at intermediate Mg/Fe(II) (and Mg/Ni) ratios the main nuclei were probably MgOx FeOx (and MgOx NiOx) mixtures and/or solid solutions of compositions Mg_αFe_{1 α}Ox (and Mg_αNi_{1 α}Ox); at high Mg/Fe(II) (and Mg/Ni) ratios the main nuclei were MgOx. The nucleation rates in excess barium oxalate solutions were similar to those for the precipitation of barium oxalate from supersaturated equivalent solutions. The main nuclei in most systems were BaOx.     

The Precipitation of Sparingly-soluble Alkaline-earth Metal and Transition Metal Oxyanion Salts from Aqueous Solution at Ambient Temperatures to 100 °C: Anhydrous and Hydrated Precipitate Phases

The anhydrous and hydrated phases of the sparingly-soluble barium, strontium, calcium, magnesium, lead(II), mercury(II), cadmium, zinc, copper(II), nickel(II), cobalt(II), iron(II) and manganese(II) salts of series of inorganic and organic oxyanions, precipitated from aqueous solution at ambient temperatures to 100 °C, have been tabulated and classified. The alkaline-earth and transition metal chromates, molybdates and tungstates are mainly anhydrous. The alkaline-earth and transition metal carbonates, orthophosphates, orthoarsenates, sulphates, selenates, bromates and iodates show mixed character. The precipitated barium, some strontium, lead(II) and mercury(II) salts are anhydrous: many of the other salts are precipitated as hydrates and the hydration numbers generally increase from one-half to three with decrease in the cation radius. All the alkaline-earth and transition metal oxalates (except lead oxalate) are precipitated as hydrates (below 100 °C). The hydration numbers at ambient temperatures generally increase from two to three-four but there is no clear inverse variation with cation radius. These results are explained by Johnson's thermodynamic treatment for the free energies of solution and precipitation (and crystallisation) of ionic salts.     

Precipitation of Alkaline-Earth Metal Silicate Hydrates from Aqueous Solution: Ionic Equilibria,Crystalline Phases and Precipitation Mechanisms

The precipitations of magnesium, calcium, strontium, barium and zinc silicate hydrates from aqueous solutions and suspensions at ambient temperature to 200 °C are surveyed. The relevant ionic equilibria (silicate and polysilicate anion formation, hydroxocation formation, alkaline-earth metal silicate hydrate and hydroxide precipitation from supersaturated solution) that may influence these precipitations are examined. - The microcrystalline and crystalline phases precipitated in systems of different cation/silicate compositions and temperatures are tabulated. Precipitation mechanisms are analysed.     

The precipitation of barium-, strontium- and calcium molybdates from aqueous solution: Kinetics of the crystal growth

The kinetics of precipitation of barium, strontium and calcium molybdates, from supersaturated aqueous solutions of initial solute concentrations C₀ = 0.0004 to 0.003 M, C₀ = 0.002 to 0.015 M and C₀ = 0.1 to 0.06 M, respectively, were studied at 25° by conductivity measurements and chemical analysis. Nucleation occurred during induction periods and continuous crystal growth then took place onto the crystallites formed during the induction periods. The growth rates at any time were expressed, in terms of degree of crystallisation, by the relation . The rate constants (kα) for the crystal growth of barium, strontium and calcium molybdates at 25° were 7700, 200, and 8.7 sec⁻¹ M⁻², respectively.     

The precipitation of alkaline-earth metal polymetaphosphate hydrate powders from aqueous solution. Nucleation and crystal growth processes,final precipitate compositions and crystal sizes

The precipitation of barium, strontium, calcium and magnesium polymetaphosphate hydrates was studied from aqueous solutions of initial metal salt concentrations from 0.001 to 3 M at 20 °C; equivalent sodium polymetaphosphate solutions were added to the alkaline-earth metal chloride solutions. Precipitate compositions were determined by chemical analysis, paper chromatography, potentiometric analysis, thermogravimetric and differential thermal analysis and infra-red spectrophotometry; final crystallite morphologies and sizes were studied by scanning electron microscopy and X-ray powder diffraction. Nucleation rates and nucleus numbers (at the end of the induction periods) were very high; crystal numbers varied from 10¹⁴ to 10¹⁵ at the critical concentrations to above 10¹⁷ per 1. solution. Crystal growth rates were also very high and varied as the fourth power of the initial metal salt concentration. High molecular-weight metal polymetaphosphate hydrates were precipitated from the more dilute solutions (0.001 to 0.025 M) while increasing amounts of the more soluble intermediate and low molecular-weight products were precipitated from the more concentrated solutions. Washing with cold water removed the tri- and tetralinear and cyclic phosphate products. The magnesium salts were not precipitated even from 3 M aqueous solutions. The precipitates from aqueous (NaPO₃(I))_n (n = 12) solutions had the compositions (BaP₂O₆ · 2.5 H₂O)₆, (SrP₂O₆ · 3 H₂O)_n and (CaP₂O₆ · 4 H₂O)_n while the magnesium salt precipitate from 20 percent aqueous acetone solution had the composition (MgP₂O₆ · 4 H₂O)_n, the precipitate n values varied from 19 to 13. The precipitates from aqueous (NaPO₃(II))_n (n = 20) solutions contained 0.5n to n additional adsorbed water molecules; these precipitate n values varied in turn from 40 to 26. The final precipitate powders consisted of ‘spherules’ of highly microcrystalline or amorphous polymer glass; the spherule diameters were about 0.2 μm at the critical concentrations and decreased to below 0.05 μm with increasing solution concentrations.     

The precipitation of alkaline-earth metal and transition metal ‘oxinate’ powders from aqueous solution. Crystal numbers and final sizes

The precipitation of barium strontium, calcium, magnesium, zinc, cadmium and lead, manganese, cobalt, nickel and copper 8-quinolinolates (‘oxinates’) was studied from equivalent solutions, at pHs from 4.5 to 10, by optical microscopy: the metal cation and overall ‘oxinate’ with ‘oxine’ concentrations were varied from 0.0002 to 0.020 M (while the mean metal oxinate concentrations varied from 10⁻⁷ to 0.001 M). Crystal growth started after induction periods; the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The final precipitate crystal numbers depended on the number of nuclei formed during the induction periods. Crystal numbers at medium to high supersaturations increased with increasing initial metal oxinate concentration according to the relation, The final crystal lengths in this supersaturation range then decreased (from maximum values) with increasing initial mean metal oxinate concentration according to the relation, For precipitation from solutions of any concentration at any pH, smaller crystals were generally obtained in the precipitates from solutions of the metal oxinate of lower solubility.     

The precipitation of alkaline-earth metal and transition metal oxalates from aqueous solution. Induction periods and nucleation rates

The precipitation of magnesium, calcium, strontium and barium oxalates and of manganous, ferrous, cobalt, nickel and copper oxalates was studied from equivalent aqueous solutions at 22°C: the initial overall concentrations (C) generally varied from 0.001 to 0.2 M and the saturation ratios (S_mox) varied from <10 to >3000. The induction periods before the main growth surge were measured and nucleation rates were determined from final crystal numbers and induction periods. Precipitation occurred through homogenous nucleation: the critical nuclei in supersaturated alkaline-earth metal oxalate solutions were formed by aggregation of 6–8 M⁺⁺Ox⁻ ion-pairs while the critical nuclei in supersaturated transition metal oxalate solutions were formed by aggregation of 6–8 MOx complexes (to units of 3–4 M⁺⁺MOx₂⁻ ion-pairs). Over the range studied, the nucleation rates then varied with saturation ratios according to the relation, Nucleation rates at any saturation ratio decreased in the order Mg > Sr, Ba > Ca and Fe > Mn > Co, Cu > Ni; that is, generally in the order of increasing M⁺⁺–Ox⁻ and M⁺⁺–MOx₂⁻ bond strengths and increasing surface energies of the metal oxalate crystals. Induction periods decreased with increasing-concentration and saturation ratio; over The factors t _C1 and t _S1 depended in turn on the ‘rate constants’ for nucleation and growth during the induction periods and on metal oxalate solubilities.     

Growth of Barium Cerate Crystals from BaCl2 Solution

Barium cerate crystals of 1.5 mm diameter could be grown from melt solutions containing barium chloride, barium oxide (from thermolysis of barium carbonate), and cerium(IV) oxide. The maximum size of the crystals is limited by the low solubility of cerium(IV) oxide in the barium chloride melt (ca. 0.1 Mol%). The flux can be removed by leaching the melt with water. Acid solutions must not be used to avoid decomposition of the barium cerate crystals. Neither congruent melting or incongruent decomposition of the BaCeO₃ crystals could be found up to 1580 degrees centigrade.     

The crystallisation of alkaline-earth metal molybdates and tungstates from lithium chloride melts by continuous cooling. Nucleation mechanism and kinetics and induction periods

Calcium, strontium and barium molybdate (and tungstate) solutions in lithium chloride melts were crystallised in alumina and in platinum crucibles; saturated solutions were cooled from initial temperatures 700° to 800°C down to room temperature at cooling rates 40° to 200°C hr⁻¹. The nucleation and early crystal growth were investigated by chemical and differential thermal analysis and by optical microscopy studies. Crystallisation occurred through heterogeneous nucleation at low supersaturations. Heterogeneous nuclei formed slowly onto metal aluminate (and platinate) particles within the solution during induction periods from < 0.2 to 14 hr. The main growth surge then started and few new nuclei were formed. The nucleation probably terminated at times just after the times for maximum rate of formation of nuclei. Then, at any temperature, the induction periods (t ) varied inversely with cooling rate and with the rate (R_c) of development of excess solute concentration according to the relation, The parameters k₁ were related to the rate constants (k_n) for the heterogeneous nucleation. These constants in turn probably dependend on the free energy for formation of critical heterogeneous nuclei and thence on some nucleator vs solute surface energy ‘wetting’ function. The k₁ and k_n values at any temperature decreased in the order : they increased from 2 to 4 times for 100°C rise in temperature.     

The precipitation of alkaline-earth metal molybdate powders from aqueous solution. Crystal numbers,final morphology and sizes

The precipitation of barium, strontium and calcium molybdates was studied from neutral equivalent solutions of concentrations from 0.0004 to 0.4 M at 25 °C. Crystal growth started after induction periods; the precipitiations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. Barium molybdate was precipitated as tetragonal bipyramids, strontium molybdate generally as prisms and calcium molybdate as platelets. Crystal numbers at medium to high supersaturations increased with increasing inital metal molybdate concentrations according to the relation, The final crystal lengths in this range than decreased from maximum values (at the critical concentrations) with increasing initial metal molybdate concentrations according to the relation. Generally, for precipitation from solutions at any concentration, larger crystals were obtained in the precipitates of the salt of higher solubility.