The coprecipitation of magnesium hydroxide aluminium hydroxide powders from aqueous solution with sodium hydroxide: Precipitate compositions and coprecipitation mechanisms

Magnesium aluminium hydroxides were coprecipitated from different mixed metal cation solutions — at total C_M = 0.1 M and Mg/Al₂ ratios from 1 to 6 — with sodium hydroxide solution at ambient temperature, with different pre-ageing conditions for the aluminium hydroxide pre-precipitate. The coprecipitations were monitored by potentiometric (pH) titration and the final precipitate compositions were examined by chemical analysis, infrared spectrophotometry and thermal analysis. Magnesium hydroxide was coprecipitated onto completely recrystallised aluminium hydroxide as a simple mixture. Generally, with no to three days pre-ageing, microcrystalline aluminium hydroxide was first precipitated at pH about 4; this then partially redissolved on further addition of sodium hydroxide (to form hydroxoaluminate anion) and magnesium aluminium hydroxide coprecipitates were formed continuously at pHs from 8.0–8.7 to 12.0–12.5. Their compositions were similar to the magnesium hydroxoaluminate hydrates formed by direct precipitation from high pH sodium hydroxoaluminate solutions.      

The coprecipitation of magnesium hydroxoaluminate hydrates from aqueous solution with ammonium hydroxide: Precipitate compositions and coprecipitation mechanisms

Magnesium hydroxoaluminate hydrates were coprecipitated from different mixed metal cation solutions at Mg/Al₂ ratios from 1 to 4 by ammonium hydroxide. The coprecipitations were monitored by potentiometric titration and the final precipitate compositions were examined by chemical analysis, X-ray diffraction, infra-red spectrophotometry and thermal analysis. The process of coprecipitation was similar to that for coprecipitation with sodium hydroxide but large excess of ammonium hydroxide was required for complete reaction at pHs from about 8 to 10.

• At Mg/Al₂ = 1, the main phase was probably Mg(H₂O)_h [Al(OH)₄]₂;
• at Mg/Al₂ = 2, the main phase was probably Mg₂(H₂O)_h [Al₂(OH)₁₀];
• at Mg/Al₂ = 4, the main phase was probably (MgOH₄) (H₂O)^h [Al₂(OH)₁₀].

     

The coprecipitation of magnesium chromium hydroxide powders from aqueous solution: Precipitate compositions and coprecipitation mechanisms

Magnesium chromium (III) hydroxides were coprecipitated at ambient temperature from different mixed metal cation solutions — at C_Mtot = 0.1 M and Mg/Cr₂ ratios varying from 1 to 4 — with sodium hydroxide solution. The coprecipitations were monitored by potentiometric (pH) titration and the final precipitate compositions were examined by chemical analysis, i.r. spectrophotometry and thermal analysis. Generally, microcrystalline chromium(III) hydroxide was first precipitated at pH about 5; this material then redissolved on further addition of hydroxyl ion to form hydroxochromate(III) anion and magnesium chromium hydroxide coprecipitates were then formed continuously (at OH/Cr ratios from 4 to 10) at pHs from 9.5–10 to about 11. The coprecipitates from Mg/Cr₂ = 1 systems was predominately magnesium hydroxochromate hydrate. The coprecipitates from Mg/Cr₂ = 2 to 4 systems were mixture or solid solutions of magnesium hydroxochromate hydrate with increasing amounts of magnesium hydroxide. The ionic equilibria involved in different coprecipitations are discussed.     

The coprecipitation of calcium aluminium hydroxide (calcium hydroxoaluminate hydrate) powders from aqueous solution with sodium hydroxide: Precipitate compositions and percipitation mechanisms

Calcium aluminium hydroxides were coprecipitated from different mixed metal cation solutions — at total C_M = 0.1 M and Ca/Al₂ ratios from 1 to 4 — with sodium hydroxide solution at ambient temperature. The coprecipitations were monitored by potentiometric (pH) titration and the final coprecipitate compositions were examined by chemical analysis, infra-red spectrophotometry and thermal analysis Generally, microcrystalline aluminium hydroxide was first precipitated at pH about 4; this then redissolved on further addition of sodium hydroxide to form hydroxoaluminate anion and polyanion and calcium aluminium hydroxide coprecipitates were formed continuously at pHs from about 9 to above 12. Their compositions were similar to the calcium hydroxoaluminate hydrates formed by direct precipitation from high pH sodium hydroxoaluminate solutions. At Ca/Al₂ ratio = 1, the main phase was probably Ca₂(H₂O)_h[Al₂(OH)₄]₂ with some Al(OH)₃; At Ca/Al₂ ratio = 2, the main phase was probably Ca₂(H₂O)_h[Al₂(OH)₁₀] dehydrating to Ca₂[Al₂O(OH)₈]; At Ca/Al₂ ratios = 3–4, the main phase was Ca₂(H₂O)_h[Al₂(OH)₁₀] with increasing amounts of Ca₄(H₂O)_h(OH)₄[Al₂(OH)₁₀] and 5–10 percent adsorbed or post-precipitated Ca(OH)₂.     

The coprecipitation of magnesium aluminium hydroxocarbonate powders from aqueous solution: Precipitate compositions and coprecipitation mechanisms

Magnesium aluminium hydroxocarbonate hydrates were coprecipitated from mixed metal nitrate solutions, at total C_M = 0.2 M and Mg/Al₂ = 1 ratio, with four sodium hydrogen carbonate-sodium carbonate solutions (of pH 8.1 to 11.5) at ambient temperature. The course of precipitation was monitored by potentiometric (pH) titration, and the compositions of the primary and final precipitates were determined by chemical analysis, infrared spectrophotometry and X-ray diffraction. Precipitation generally occurred through three stages, primary precipitation (of low CO₃ aluminium hydroxocarbonates) at low pH with evolution of carbon dioxide, their dissolution by complexing to form hydroxocarbonatoaluminate anions and then secondary precipitation of the final coprecipitate at higher pHs. The final product from coprecipitation by sodium hydrogen carbonate solution (pH 8.1) was mainly the magnesium hydroxocarbonatoaluminate ‘MAHC I’; the final products from coprecipitation by sodium hydrogen carbonate-sodium carbonate solutions (pH 9.4 and 10.3) were ‘MAHC I’/‘MAHC II’ mixture and ‘MAHC II’/‘MAHC I’ mixture whereas the final product from coprecipitation by sodium carbonate solution (pH 11.5) was a complex mixture if ‘MAHC II’ with ‘MAHC I’ and ‘MAHC III’;

• ‘MAHC I’ was probably Mg₂[Al₄(OH)₁₀(CO₃)₃] · hH₂O,
• ‘MAHC II’ was probably Mg[Al₂(OH)₄(CO₃)₂] · h H₂O whereas
• ‘MAHC III’ was probably Mg[Al₂(OH)₆CO₃] · h H₂O.

     

The coprecipitation of magnesium nickel hydroxide solid solutions from aqueous solution: Precipitate compositions and precipitation mechanisms

Magnesium nickel hydroxides (solid solutions) were coprecipitated from different mixed metal cation solutions (overall concentration 0.1 M) and from hydroxide solution (0.1 M). The course of different coprecipitations was monitored by potentiometric (pH) titrations. Final Coprecipitate compositions were determined by chemical analysis, infra-red spectrophotometry and thermal analysis. The ionic equilibria involved in different coprecipitations and the precipitation mechanisms are discussed.     

The precipitation of aluminium hydroxocarbonates powders from aqueous solution: Precipitate compositions and precipitation mechanisms

A series of aluminium hydroxocarbonate hydrates were prepared by precipitation from aluminium nitrate solution, with five sodium hydrogen carbonate-sodium carbonate solutions of different pH, at ambient temperature. The course of precipitation was monitored by pH measurement and the final precipitate compositions were determined by chemical analysis, infra-red spectrophotometry, X-ray diffraction and thermal analysis. Precipitation generally occurred through three stages, primary precipitation of materials with low carbonate content at low pH with evolution of carbon dioxide, their dissolution to form hydroxcarbonato anions and then secondary reprecipitation of the final products at higher pH. These materials were mixtures of polyhydroxoaluminium carbonate hydrates of general composition Al_n(OH)_3n-2CO₃ · hH₂O (where n = 2, 4, 6 and h = 6–8); their CO₃ content increased with increasing pH and carbonate anion content of the precipitant solution.     

The coprecipitation of Magnesium Nickel Oxalate Dihydrate Powders (solid solutions) from aqueous solution: Precipitate compositions and Cporecipitate Mechanisms

The coprecipitation of magnesium nickel oxalate dihydrates from mixed metal cation solutions was monitored by conductivity measurements. A series of coprecipitates was then prepared from 0.2 M solutions with Mg contents varying from 0.2 to 0.9 total metal cation and their compositions and structures studied by chemical analysis, infra-red spectrophotometry, thermogravimetric analysis and detailed differential calorimetry. The coprecipitates with upto 20 percent Mg content were solid solutions with structures similar to nickel oxalate dihydrate, the coprecipitates with 20 to 80 percent Mg content were probably mixed solid solutions while the coprecipitates with over 80 percent Mg content were solid solutions with structures similar to magnesium oxalate dihydrate. The ionic equilibria in supersaturated magnesium nickel oxalate solutions were analysed and mechanisms are proposed for coprecipitations from solutions of different Mg/(Mg + Ni) ratios.     

The dissolution and crystallisation of amphoteric metal hydroxides from sodium hydroxide solutions: Ionic equilibria,crystalline phases and crystallisation mechanisms

The dissolution and recrystallisation of beryllium, zinc, cadmium and tin (II) hydroxides and chromium (III), iron(III), aluminium, scandium, yttrium, gallium and indium hydroxides, from sodium hydroxide solutions of concentrations C = 1 to 20 M at ambient temperatures, are surveyed. The different inic equilibria in metal hydroxide-sodium hydroxide systems are examined: the phases crystallising from different sodium hydroxide solutions are tabulated and crystallisation mechanisms are analysed.     

The nucleation and precipitation of nickel ammonium sulphate crystals from aqueous solution

A precipitation study has been made with nickel ammonium sulphate produced by mixing aqueous solutions of its constituent salts. Rates of nucleation, as indicated by the induction period, were measured for both agitated and non-agitated systems over the temperature range 0–35 °C. The nucleation rate increases with increases in agitation and temperature but supersaturation has the dominant effect, as predicted by classical nucleation theory. However, attempts to analyse the results in accordance with classical theory were not entirely successful, but it is shown how the assumption of (a) a variation of crystal surface energy with temperature and (b) the influence of heterogeneous nucleation can account for the discrepancies.     

The thermal decomposition of aluminium hydroxide doped with Fe(III) ions

The thermal decomposition of various aluminium hydroxides doped with Fe(III) ions has been studied using ESR. The transition aluminas (η, γ, ϰ-Al₂O₃) formed at low calcination temperatures show the ESR-spectrum characteristic for Fe(III) ions incorporated in low crystalline materials. Spectra of doped reference compounds (ZnAl₂O₄, MgAl₂O₄, (dis)ordered LiAl₅O₈) showed that disorder in the cation sublattice could be responsible for this feature. The ESR spectra of aluminas (θ, x, α-Al₂O₃) formed at high calcination temperatures reveal well-defined and characteristic fine structures. Comparison with reference compounds (β-Ga₂O₃, α-Ga₂O₃) allows to draw conclusions as to the structure. The quality of the spectrum of δ-Al₂O₃ formed in the dehydration sequence of boehmite, was in between that of the low and high temperature aluminas. In order to explain some of the features observed in the various dehydration sequences, a discussion based on the coordination of the anions along lines pointed out by GORTER , has been put forward. To this end, the regularity of the anion coordination is followed within the special packing sequences of the anion layers in the dehydration products.     

The precipitation of barium chromate from aqueous solution (with rapid mixing): Kinetics of the crystal growth

The kinetics of precipitation of barium chromate from well-stirred aqueous solutions of initial solute concentrations C₀ = 0.0001 to 0.0010 M (supersaturations 8 to 80) was studied at 25 °C by conductivity measurements and chemical analysis. Nucleation occurred during induction periods and regular crystal growth then took place onto the crystallites formed during the induction periods. The crystal growth was rate-controlled, in this range, by the rate of deposition of metal salt ions onto the growing crystal surfaces. This rate, at any time, then depended on both the overall surface area (A_t) and on the residual excess solute concentration (ΔC_t) in solution according to the relation, the growth rate expressed in terms of degree of crystallisation was then The rate constant (K_α) for the crystallisation of barium chromate at 25 °C was 1.5 10⁶ sec⁻¹ M⁻².     

Influence of solution constituents, solution conditioning and seeding on the crystalline phase of aluminium hydroxide using in situ X-ray diffraction

Filtering, drying and aging of crystalline solids can affect crystalline phase(s) and therefore ex situ X-ray diffraction is not necessarily indicative of the in situ crystalline components. This paper examines some of the factors affecting the crystallised phase of aluminium hydroxides in highly caustic solutions. Caustic aluminate solutions were prepared using several different methodologies. Where solutions were prepared from aluminium metal, gibbsite and bayerite were observed to be the crystallisation products. Where gibbsite was used as the aluminium source in the liquors, the crystallised phase was found to be a function of solution conditioning. Solutions prepared at 100°C and at atmospheric pressure crystallised to form only gibbsite. These same solutions subsequently heated to 160°C for 16 h crystallised as gibbsite and bayerite. It has been concluded from these results that the caustic aluminate solutions made using gibbsite were not dissolved on the molecular scale (although optically clear) if the solutions were heated to only 100°C. Further heating at elevated temperatures completed the dissolution process so that the species in solution were similar to those found in solutions made from aluminium metal. Solutions seeded with either bayerite or gibbsite crystallised to form only gibbsite. Only crystallisation in the bulk solution and not on the seed surfaces was recorded. In some cases seeding altered the expected crystallisation products, i.e. from gibbsite and bayerite to only gibbsite.     

The precipitation of calcium hydroxoaluminate hydrate powders from sodium hydroxoaluminate solutions: Precipitate phases and precipitation mechanisms

Calcium hydroxoaluminate hydrates were precipitated from different sodium hydroxoaluminate and hydroxoaluminate-excess hydroxide solutions at ambient temperature (at C_Al = 0.1 to 0.3 M and at XS OH/Al = 0 to above 8). The precipitations were monitored by potentiometric (pH) measurements. Precipitate morphologies were examined by optical microscopy and precipitate compositions were determine by chemical analysis, infra-red spectrophotometry and thermal analysis. Generally at OH/Al ratios of 4 to 4.5 (XS OH/Al = 1 to 1.5), the compound 2 CaO · · Al₂O₃ · 8 H₂O (C₂AH₈) was precipitated with some aluminium hydroxide; then at OH/Al ratios of 5 to above 11 (XS OH/Al = 2 to above 8), the compound 2 CaO · Al₂O₃ · 8 H₂O was precipitated with increasing amounts of the compound 4 CaO · Al₂O₃ · 13 H₂O (C₄AH₁₃).     

The precipitation of barium hydroxoaluminate hydrate powders from sodium hydroxoaluminate solutions: Precipitate phases and precipitation mechanisms

Barium hydroxoaluminate hydrates were precipitated from different sodium hydroxoaluminate solutions at 20 °C; C_Al varied from 0.1 to 0.5 M and initial Ba/Al₂ ratios ( = excess OH/Al ratios) varied from 1 to 7. Precipitate compositions were determined by chemical analysis, infra-red spectrophotometry and thermal analysis. The compound BaO · Al₂O₃ · 7 H₂O was precipitated at initial Ba/Al₂ ratios of one to well above two while the compound 2 BaO · Al₂O₃ · 5 H₂O was only precipitated over a narrow range of concentrations. The compound Ba(OH)₂ · 8 H₂O was precipitated from solutions of high hydroxide and barium ion concentrations. The ionic equilibria and precipitation mechanisms in different solutions are discussed.     

The precipitation of magnesium hydroxoaluminate hydrate powders from sodium hydroxoaluminate solutions: Precipitate phases and precipitation mechanisms

Magnesium hydroxoaluminate hydrates were precipitated from different sodium hydroxoaluminate and hydroxoaluminate-hydroxide solutions at ambient temperature, at C_Al = 0.1 M, OH/Al ratios = 4–9 and XS OH/Al ratios = 1–6. The precipitations were monitored by potentiometric (pH) measurements while the final precipitate compositions were examined by chemical analysis, infra-red spectrophotometry and thermal analysis. At solution OH/Al ratio = 4, the main precipitate phase at 20°C was Mg(H₂O)_n[Al(OH)₄]₂ admixed with some Al(OH)₃; at solution OH/Al ratio = 5, the main phase was Mg₂(H₂O)₄[Al₂(OH)₁₀]; at solution OH/Al ratio = 7, the main phase was Mg₄(H₂O)_n(OH)₄[Al₂(OH)₁₀] while at solution OH/Al ratio = 9, the main phase was Mg₆(H₂O)_n(OH)₈[Al₂(OH)₁₀] admixed with some Mg(OH)₂. These hydrates were dehydrated at 60–100°C probably to the compounds Mg₂[Al₂O₃(OH)₄], Mg₄(OH)₄[Al₂O₃(OH)₄] and Mg₆(OH)₈[Al₂O₃(OH)₄], 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 sparingly-soluble acid,neutral and basic metal salts from aqueous solution: Ionic equilibria at different pHs,precipitate phases and precipitation mechanisms

The precipitations of the sparingly-soluble acid, neutral and basic salts of the divalent cations beryllium, magnesium, calcium, strontium, barium, manganese (II), iron (II), cobalt (II), nickel (II), copper (II), zinc, cadmium, mercury (II), tin (II) and lead (II) with the inorganic anions sulphate, chromate, molybdate, monohydrogen phosphate, phosphate, and carbonate (from supersaturated aqueous solutions) are surveyed. The different types of ionic equilibria (cation hydrolysis, anion hydrolysis, ion-pair formation) that may influence these precipitations, at different pHs and ionic concentrations, are examined. The crystalline phases precipitated at different pHs are tabulated and the precipitation mechanisms (at different pHs) are analysed.     

The precipitation and hydrothermal crystallisation of alkaline-earth. Metal aluminosilicate hydrates from aqueous solution: Crystalline phases and precipitation and recrystallisation mechanisms

The preparations, precipitations and recrystallisations of magnesium and calcium aluminosilicate hydrates, from aqueous suspensions of microcrystalline silica, aluminium hydroxide (and oxohydroxide) and magnesium (calcium) hydroxides at ambient temperatures to 400 °C, are surveyed. The phases reported in systems of different Mg(Ca)/(Al + Si) and Al/(Al + Si) composition ratios are tabulated and precipitation and recrystallisation mechanisms are proposed.     

The precipitation of sparingly-soluble alkaline-earth metal and transition metal oxalates and mixed metal oxalates from aqueous solution: Ionic equilibria,crystalline phases and precipitation mechanisms

The precipitations of alkaline-earth metal and transition metal oxalates, from aqueous solution and from excess alkali metal oxalate solution, are surveyed: also the coprecipitations of transition metal oxalates together with alkaline-earth metal oxalates. The ionic equilibria that influence these precipitations are examined, the crystalline precipitate phases are tabulated and precipitation mechanisms are analysed.