The Coprecipitation of Magnesium Iron (III) Hydroxide Powders from Aqueous Solution: Precipitate Compositions and Coprecipitation and Ageing Mechanisms

Series of magnesium iron(111) hydroxides were coprecipitated at ambient temperature from different mixed metal cation solutions, at C_{M tot} = 0.1 M and Mg/Fe₂ ratios from 4 to 1/4, with sodium hydroxide solution. The relevant single precipitations and the coprecipitations were monitored by potentiometric (pH) titration and the final coprecipitate compositions were examined by chemical analysis, infra-red spectrophotometry and thermal analysis. The coprecipitates onto aged α-FeOOH were simple mixtures of α-FeOOH and Mg(OH)₂. The main coprecipitates were either «molecular inclusion mixtures» of microcrystalline α-FeOOH with excess Mg(OH)₂ or similar mixtures of Mg(OH)₂ with excess α-FeOOH and some ten-twenty percent (relative to the Mg content) of magnesium hydroxo-ferrate(III) hydrate Mg[Fe(OH)₄]₂ · h H₂O. The coprecipitates aged in alkaline magnesium hydroxide suspension at 20 °C and 40 °C were mixtures of Mg(OH)₂, α-FeOOH and forty to ninety percent (relative to the Mg content) of Mg[Fe(OH)₄]₂ · h H₂O. The related α-FeOOH NaOH and α-FeOOH Mg(OH)₂ equilibria and the different coprecipitation mechanisms are discussed.     

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 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.     

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 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 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 Zinc Aluminium Hydroxide Powders from Aqueous Solution with Sodium Hydroxide: Precipitate Compositions and Coprecipitation Mechanisms

Zine aluminium hydroxides were coprecipitated from different mixed metal cation solutions, at C_{M tot} = 0.1 M and at Zn/Al₂ ratios from 1 to 4, with sodium hydroxide solution. The coprecipitations were monitored by potentiometrie (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 partially redissolved on further addition of sodium hydroxide (to form hydroxoaluminate anion) and zinc aluminium hydroxide coprecipitates were formed continuously at pHs from 5.5–6 to above 9. Their compositions were similar to the magnesium hydroxoaluminate coprecipitated from magnesium aluminium solutions. At Zn/Al₂ ratio = 1, the main phase was probably Zn(H₂O)_n [Al(OH)₄]₂; at Zn/Al₂ ratio = 2, the main phase was probably Zn₂(H₂O)_n [Al₂(OH)₁₀], whereas at Zn/Al₂ ratio = 4, the main phase was probably Zn(H₂O)_n(OH)₄[Al₂(OH)₁₀].     

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 transition metal oxalate powders from aqueous solution. Crystal numbers and final sizes

The precipitation of manganous, ferrous, cobalt, nickel and copper oxalate hydrates was studied from equivalent solutions of concentrations from 0.001 to 0.3 M at pHs 7 to 6, by optical microscopy and other methods. Crystals growth started after induction periods: the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The crystal numbers of the final precipitates depended on the number of nuclei (and crystallites) formed during the induction periods. At medium to high supersaturations, crystal numbers increased with increasing initial metal oxalate complex ion concentrations according to the relation. N = N₁C_mox^β, where β was 5. The N values increased in the order Mn ≪ Fe < Co < < Ni < Cu. The final crystal lengths, in this range, then decreased with increasing metal oxalate complex ion concentrations according to the relation l_fin = l₁/C_mox^γ, where γ was 1.3. For precipitations from solution of any concentration, smaller crystals were generally obtained in the precipitates of the metal oxalate of lower solubility; nickel oxalate precipitations were the exception to this.     

The Precipitation of Alkaline-Earth Metal Oxalate Powders from Aqueous Solution. Crystal Numbers and Final Sizes

The precipitation magnesium oxalate dihydrate, calcium oxalate monohydrate, strontium oxalate monohydrate and barium oxalate hemihydrate was studied from equivalent solutions of concentrations from 0.001 M to 0.5 M, at pHs from 7 to 6, by optical microscopy and other methods. Crystal growth started after induction periods: the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The crystal form and numbers of the final precipitates depended on the type and number of the nuclei (and crystallites) formed during the induction periods. Crystal numbers at medium to high supersaturation, increased with increasing initial mean metal oxalate concentrations according to the relation, N = N₁c; β was 5 for calcium oxalate precipitations and β was 6 for the other metal oxalate precipitations. The N₁ values increased in the order MgC₂O₄ · 2 H₂O < BaC₂O₄ · 1/2 H₂O < SrC₂O₄ · H₂O < CaC₂O₄ · H₂O. The final crystal lengths, in this supersaturation range, then decreased (from maximum values) with increasing initial concentrations according to the relation, l_fin = l₁/C_MOx^β, where γ was 1.3 to 1.6. For precipitations from solution of any concentration at any pH, smaller crystals were obtained in the precipitates of the metal oxalate of lower solubility.     

Influence of complex formation upon inclusion of Mn(II), Co(II), Ni(II), and Cu(II) in ZnC2O4·2H2O

The inclusion of 3d‐impurities Mn(II), Co(II), Ni(II) and Cu(II) in a crystalline precipitate of ZnC₂O₄·2H₂O is investigated. This study is a part of the systematic one deal with the mechanism of inclusion of 3d‐ions in sparingly soluble oxalate systems. The experiments are carried out in bi‐ end multi‐component systems at two different mediums – one with deficiency of oxalate ions, another with excess. The insertion of 3d‐ions upon mass crystallization of ZnC₂O₄·2H₂O does not proceed by a simple ionic substitution. The results show that the inserted amount of impurity depends on some physicochemical characteristics of the neutral monooxalato complexes [MnC₂O₄]^o, [CoC₂O₄]^o, [NiC₂O₄]^o and [CuC₂O₄]^o. Good agreement between included impurity and the concentration of its complex in the solution is established. The stability constant of monooxalato complex affects the impurity inclusion. This effect depends on the medium nature. In the deficiency of oxalate ions the factor determining the inclusion is thermodynamic one – stability of monooxalato complexes. In the excess of oxalate ions inserted amount depends on kinetic factor – the formation rate of these complexes. In the term of that the insertion of Mn(II) is definitely different in the two mediums while that of the Ni (II) does not depend on the medium. The copper shows deviation from overall dependence in the two mediums due to the Jahn‐Teller distortion. Its double decreasing insertion in the excess of oxalate ions is related with stabilization of [Cu(C₂O₄)2]^2‐. The conclusions presume that by varying the background medium and taking in view the ions present in the solution, the amount of inserted impurities can be predicted and controlled. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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 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 Precipitation of Alkaline-Earth Metal Polyacrylate Hydrate Powders from Aqueous Solution: Ionic Equilibria,Precipitate Compositions and Morphologies

The precipitation of barium, strontium, calcium and magnexium polyacryate hydrates was studied from equivalent aqueous solutions of initial concentrations 0.03 M to 0.30 M at ambient temperature: sodium polyacrylate (m. wt = 30,000 to 300,000) was added to metal chloride solution. The final yields of precipitate increased with decreasing solubility and/or peptisation (Ba > Sr > Ca > Mg) and increasing molecular weight of the polyacrylate; the precipitates had the compositions [BaA(COO)₂ · 1–2H₂O]_n, [SrA(COO)₂ · 1–2 H₂O]_n, [CaA(COO)₂ · 2 H₂O]_n and [MgA(COO)₂ · 2 H₂O]_n. The final yields of the precipitates from sodium polyacrylate solutions were far lower and these had the compositions [Na_2αM_1-αCA(COO)₂ · 2 H₂O]_n; A =  CH CH₂ CH . The metal polyacrylate hydrate powders consisted of microcrystalline ‘spherules’; their average diameters were from 0.03–0.05 μm (for lower m. wt products) to 0.01 to 0.02 μ,m (for higher m. wt products).     

Non-Kossel crystals: Calcium and magnesium oxalates

The rate of movement of elementary growth layers at faces of CaC₂O₄ · H₂O and MgC₂O₄ · 2H₂O crystals is measured in situ by atomic force microscopy under kinetic growth conditions at a constant supersaturation with a varying excess of cations or anions in the solution. It is shown that, as the deviation of the Ca²⁺/C₂O ₄ ^2? (or Mg²⁺/C₂O ₄ ^2? ) ratio from unity increases, the rate of movement of the growth layers decreases abruptly and nonlinearly. A model describing this effect is discussed.     

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.

     

Controls on Morphology of Analcime-Pollucite in Natural Minerals,Synthetic Phases,and Nuclear Waste Products

Crystals of the analcime-pollucite series (NaAlSi₂O₆ · H₂O CsAlSi₂O₆) exhibit tetragon-trioctahedral {211} and hexahedral {100} morphology, with rare modification by tetrahexahedral {210} or rhombic dodecahedral {110} forms. Under weakly acidic to weakly basic conditions {100} is dominant; with increasing alkalinity (cation concentration), basicity (pH), and temperature {211} is dominant over {100}, with minor development of {210}. The morphology does not appear to be influenced by broad ranges in Si/Al (from 2.0 to 2.6 in the natural minerals) and Na-Cs contents between analcime and pollucite, by substitutions of univalent cations Li, K, Rb, and Tl, or by small amounts of divalent cations Mg, Mn, Ni, Zn, Fe. Larger amounts of Ca, however, and possible substitutions for framework Al and Si seem to produce a more complex morphology.     

High-pressure Synthesis and Properties of Manganates with Ilmenite Structure

A²⁺Mn⁴⁺0₃(A  Ni, Co, Mg, Cu) manganates with ilmenite structure have been obtained by solid phase synthesis at high pressure and temperature. The synthesis conditions for single phase samples have been determined, and investigations of the magnetic, electric, magnetooptic, and resonance properties were carried out.     

Direct and outer-sphere coordination of the magnesium ions in the crystal structures of complexes with 2-furancarboxylic acid (I) and 3-furancarboxylic acid (II)

(I) Mg₂C₂₀H₂₄O₁₈ monoclinic,PT,a=10.760(2) Å,b=11.052(2) Å,c=12.822(3) Å, α=105.31(3)^o, β=98.18(3)^o, γ=91.59(3)^o,Z=2. (II) MgC₁₀H₁₄O₁₀, monoclinic,C2/c,a=30.817(6)Å,b=10.499(2)Å,c=9.000(2)Å, β=91.31(3)^o,Z=8. Magnesium in complexes with furoic acids reveals two ways of coordination: direct, when furoic anions are bonded to Mg²⁺ in an ionic fashion and outer-sphere, when cations bind water in the first coordination sphere and furancarboxylic ligands are hydrogen bonded to the water molecules. This results in the formation of three bridging systems: ?Mg?O_carboxyl?C?O_carboxyl?Mg?, ?Mg?O_water ?O_carboxyl?C?O_carboxyl?C?O_carboxyl?Mg?, and ?Mg?O_water?O_carboxyl?C?O_carboxyl?O_water?Mg?. Magnesium 2-furancarboxylate (I) is dimeric, while magnesium 3-furancarboxylate (II) exhibits a polymeric structure.     

Influence of Fe/Ba ratio,basic agent and calcination temperature on the physical properties of nanocrystalline barium hexaferrite thin films prepared by a sol–gel method

In this work nanocrystalline BaFe₁₂O₁₉ thin films have been prepared on the Si (1 1 0) substrates by a sol–gel method using the aqueous solution of metal nitrates. The efforts have been done to decrease the calcination temperature and to reduce the crystallite size of the single-phase barium ferrite thin films. The precursor solutions were primed with the various Fe/Ba ratios and two kinds of the basic agents, and the coated films were heat treated at the different temperatures. The thin films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The effects of calcination temperature, molar ratio of Fe/Ba and basic agent on composition, crystallites size and morphology were also investigated.