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 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).     

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 Carbonate Powders from Aqueous Solution. Crystal Numbers and Final Sizes

The precipitations of magnesium carbonate trihydrate, basic magnesium carbonate and calcium, strontium and barium carbonates was studied from equivalent solutions of concentrations from 0.0005 M to 1M, at pHs from 10 to 7, by optical microscopy and other methods. Crystal growth started after induction periods: the precipitations of the more sparingly-soluble metal carbonates — mainly studied at medium to high supersaturation — were homogeneously nucleated while the magnesium carbonate trihydrate precipitations — studied at low supersaturations at pH ≦ 7.6 — were heterogeneously nucleated. The crystal forms and numbers of the final precipitates depended on the type and numbers of nuclei (and crystallites) formed during the induction periods. Crystal numbers generally increased with increasing initial mean metal carbonate concentration according to the relation N = N₁C_MCO3^β; β was 3 for the metal carbonate precipitations and β was 4 for the basic magnesium carbonate precipitations. N₁ values increased in the order basic MgCO₃ (at pH ≧ 9), or MgCO₃ · 3H₂O (at pH ≦ 7.6) < CaCO₃ < SrCO₃ < BaCO₃. The final crystal lengths then generally decreased, from maximum values, with increasing initial concentration according to the relation l_fin = l₁/C^γ, where γ was 0.7 and 1.0. For precipitation at any concentration and pH, smaller crystal sizes were generally obtained in precipitates from solutions of the metal carbonate of lower solubility.     

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.     

层状氢氧化镁铝的合成及其对生物柴油中游离脂肪酸吸附的研究

在水热条件下,用硫酸镁、硫酸铝、氢氧化钠和纯碱按照一定的比例制备层状氢氧化镁铝,并利用XRD、热重分析仪、红外光谱、扫描电镜等手段分析表征样品,研究了层状氢氧化镁铝在不同温度下的焙烧产物对生物柴油中游离脂肪酸的吸附性能.结果表明:制得的层状氢氧化镁铝对游离脂肪酸的吸附性能良好,在焙烧500℃的条件下吸附量可达到663 mg/g,并且可以焙烧再生.     

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 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.     

六角片状氢氧化镁(001)晶面优先生长条件的研究

以常温工业合成氢氧化镁为原料,采用水热法制备六角片状氢氧化镁阻燃剂.研究了水热条件对氢氧化镁(001)晶面生长的影响并从生长基元的角度对其进行了解释.采用SEM、XRD、激光粒度仪和接触角测试仪对水热处理后的氢氧化镁进行表征.结果表明,原料初始粒度越小越有利于(001)晶面生长,当水热介质中碱浓度为6～8mol/L、水热温度为140～180℃时,氢氧化镁(001)晶面的生长速度最快,当氢氧化镁的含量大于15wt;时会明显抑制(001)晶面的生长.     

The Catalysed and Uncatalysed Nucleation of Gypsum Crystals from Aqueous Solution

Gypsum (Calcium Sulphate Dihydrate) has been precipitated by mixing aqueous solutions of calcium chloride, sodium sulphate and sodium chloride at 25°C. In the absence of solid gypsum, classical nucleation theory predicts rates reasonably well only at the expense of some adjustable parameters like surface energy term. This has been attributed to the difficulties associated with the application of equilibrium thermodynamics to a process of kinetic origin. The presence of solid matter acts as a catalyst and decreases the induction period. In this case, the habit as well as the quantity of gypsum added greatly influences the induction period. Possible reasons for this have been given.     

Precipitation of Alkaline-earth Metal Hydroxoberyllate,Hydroxoaluminates and Allied Salts from Aqueous Solution: Ionic Equilibria,Crystalline Phases and Precipitation Mechanisms

The precipitations of alkaline-earth metal hydroxoberyllates and zincates, hydroxochromates (III), ferrates (III), aluminates and gallates, hydroxotitanates, zirconates and stannates (IV) and hydroxoantimonates (V) from different aqueous solutions are surveyed. The relevant ionic equilibria (hydroxoanion formation, precipitation of hydroxosalts, amphoteric metal hydroxides and alkaline-earth metal hydroxides) that may influence these precipitations are examined. The crystalline phases precipitated from different systems are tabulated and precipitation mechanisms are analysed.     

The Slow Precipitation of Sparingly-soluble Metal Salt Powders from Aqueous Solution: Mathematical Relations for the Kinetics of the Crystal Growth Processes in Different Types of Precipitation

The following types of slow precipitation, of sparingly-soluble metal salts from aqueous solution, have been analysed: precipitations without an induction period – (i) with very few microcrystallites or seeds, (ii) with microcrystallites and first/second order surface or diffusion rate-controlled growth, (iii) with very many microcrystallites; precipitations with significant induction periods – (i) with very few microcrystallites or seeds, (ii) with microcrystallites and first/second order surface or diffusion rate-controlled growth, (iii) with very many microcrystallites. Generally, in the early stages of precipitation, growth rates depend on the rate of addition of metal salt ions to the system and on the rate constant for crystal growth; in this stage, rates increase from low values to optimum values: in the later stages of precipitation, the metal salt ions are used up as rapidly as they are added to the system and growth rates then depend only on the rate of addition and on the number of microcrystallites or seeds; in this range, rates gradually decrease to low values. Mathematical relations, for the variation of crystal size with precipitation time at different stages of growth, have been derived for each of the above types of precipitation.     

Mechanisms of dc-current transfer in tris(acetylacetonato)iron(III) films

Thin tris(acetylacetonato)iron(III) films were prepared by sublimation in vacuum on p-Si substrates for electrical investigation. The prepared films were characterized by X-ray diffraction (XRD) which shows the material to exhibit polycrystalline orthorhombic structure. Thin films of the complex were prepared as a gate-insulator for metal/insulator/Si (MIS) device. The capacitance-gate voltage, C(V_g) characteristics of the constructed MIS device were measured, from which the relative permittivity and the density of the charges in the sample were extracted. The dc-electrical conduction in the complex film was studied at room temperature and in the temperature range of (293–353 K). It was found that the data follow a Poole–Frenkel (PF) mechanism for low voltages and a trap-charge-limited space-charge-limited conductivity (TSCLC) mechanism for higher voltages. The switching observed between the two mechanisms was explained. The characteristic parameters both mechanisms were also determined. It was concluded that the dc-conduction can be described by hopping between structural defects that form trap levels in localized states near the bottom of the mobility band. Therefore, the density of structural defects in the film, which are controllable by the method of preparation, is critical in determining the mechanism of current transfer.     

The Crystallisation of Aluminium Trihydroxide (Gibbsite) from Sodium Hydroxide Solution Onto Fine Seeds: Physico-chemical Studies on the Internal Structure and Agglomeration of Different Crystal Fractions

A batch of Bayer gibbsite crystals (particle size range 3 to 70 μm) was prepared by crystal growth onto fine seeds (1 to 3 μm) from sodium hydroxoaluminate/sodium hydroxid-solution. The internal structures and seed agglomeration of different fractions were examined by optical microscopy, infra-red spectrophotometry, thermal analysis and chemical dissolution analysis. These physico-chemical studies confirmed that crystal growth occurs with some seed agglomeration in the early stages, followed by growth onto the ‘agglomerates’. The agglomerated seeds contents (X) were as follows: for fraction DiD (l₀ = 6.3 μm), X = 0.44; for fraction Gi C (l₀ = 20 μm), X = 0.41; for fraction Gi B (l₀ = 38 μm), X = 0.31 and for fraction Gi A (l₀ = 55 μm), X = 0.24.     

Kinetics of Mixed Crystal K2(SO4, CrO4) Growth from Aqueous Solution

The investigation of crystal growth kinetics and phase equilibria in the system K₂SO₄–K₂CrO₄–H₂O was carried out using the method for the determination of the saturation temperature and the growth rate by microscopic observation of the seed crystal behaviour. The kinetics of mixed crystal growth can be characterized by the expression V = 450.24σ^1,6,2, where σ is the value of effective supersaturation of the solution calculated by taking into account the specific features of the phase equilibria diagram. Use of this criterion allows the control of the rate of delta crystal growth from solutions of variable composition.     

The Structure and Characterization of [1,11-Bis(2-Hydroxybenzyl)-1,4,8,11-Tetraazaundecane]Iron(III) Perchlorate

Abstract  The structure of an Fe(III) complex of reduced Schiff base is reported. The title compound, C₂₁H₃₀ClFeN₄O₆ (I), crystallizes in the monoclinic space group P2₁/n with cell constants: a = 9.988(2) ?, b = 20.430(5) ?, c = 11.415(3) ?, β = 105.480(4)°. It contains a six-coordinate FeN₄O₂ cation where the ligand is a reduced Schiff base resulting from the NaBH₄ reduction of the condensation product between salicylaldehyde and 1,4,8,11-tetraazaundecane. Due to the increased flexibility of the saturated backbone of the ligand compared to the Schiff base from which it was synthesized, the complex adopts a trans-FeN₄O₂ conformation. There is extensive hydrogen bonding between the amine H atoms and the anion O atoms. Index Abstract  The structure of an Fe(III) complex of reduced Schiff base is reported which adopts a trans-FeN₄O₂ conformation where the ligand is a reduced hexadentate Schiff base resulting from the NaBH₄ reduction of the condensation product between salicylaldehyde and 1,4,8,11-tetraazaundecane.      

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 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 Precipitation of Strontium and Lead Sulphates from Aqueous Solution. Kinetics of the Crystal Growth

The precipitation of strontium and lead sulphates from well-stirred supersaturated aqueous solutions, of initial solute concentrations C₀ = 0.001 to 0.020 M and C₀ = 0.0002 to 0.003 M respectively, was studied at 20° and 40°C by chemical analysis and optical microscopy. Nucleation occurred during induction periods and continuous regular growth then took place onto the nuclei formed during these periods. Crystallisation was complete after 4 to 48 hr. The crystal growth was rate-controlled by the rate of deposition of metal salt ions onto the growing crystal surfaces. This rate (dC/dt), at any growth 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 while the growth rate (dα/dt), expressed in terms of degree of crystallisation, was . The rate constants (k_α) for the crystal growth of strontium and lead sulphates at 20°C were 22 and 4200 sec⁻¹ M⁻² respectively — that is, greatest for the salt with least cation hydration –; these constants increased 4 to 6 times for 20°C temperature rise. The rate-determining process for the metal salt deposition was probably the ion dehydration.