Preparation of gelatinous aluminium hydroxide from aqueous solutions of organic aluminium salts by reaction with alkali

The preparation of gelatinous aluminium hydroxide from aqueous solutions of the formate, citrate or tartrate of aluminium on addition of aqueous sodium hydroxide has been investigated under different conditions. The precipitates from aqueous solutions of aluminium formate consist of amorphous aluminium hydroxide, pseudoboehmite, bayerite, hydrargillite or their mixtures, depending on the condition of preparation. A mixed solution of aluminium citrate and sodium hydroxide yields no solid-phase product without aging on heating to enhance the hydrolysis of aluminium citrate. The resulting precipitate exists as pseudoboehmite, independent of the preparation conditions. In contrast, aluminium tartrate is not hydrolysed to precipitate aluminium hydroxide even by aging on heating.     

Preparation of aluminium hydroxide by reacting sodium aluminate solutions with mineral acid

The preparation of aluminium hydroxide from sodium aluminate solutions by varying the alkaline concentration at a fixed molar ratio of Na₂O: Al₂O₃ has been investigated on addition of hydrochloric or nitric acids. The resulting materials have been examined by X-ray diffraction study, infrared spectrophotometry, thermogravimetrical and differential analyses. As a result, it was found that precipitates generally exist as pseudoboehmite or bayerite, but the formation of bayerite is enhanced by a low rate of addition of acid, higher temperature or higher alkaline concentration. In this case, the composition of the precipitates was little influenced at different molar ratios of Na₂O: Al₂O₃. When the precipitates from sodium aluminate solutions at low and higher alkali concentrations were aged in the mother liquors, pseudoboehmite therein transformed to hydrargillite and bayerite, respectively.     

The nature of pseudoboehmite and its role in the crystallisation of amorphous aluminium hydroxide

Pseudoboehmite samples were prepared by ageing the amorphous hydroxide obtained by the hydrolysis of aluminium s-butoxide using water or aqueous ethanol. Further ageing resulted in the formation of a mixture of crystalline trihydroxides. The two re-crystallisation stages are accompanied by similar but opposite changes in the volume of the aqueous suspension but pseudoboehmite can only be identified by X-ray diffraction in a separated xerogel. Crystallinity of the latter was shown to be enhanced by preparation in 80% ethanol but at concentrations providing only a little more than the water needed for hydrolysis of the alkoxide a new modification of pseudoboehmite was observed. X-ray line broadening and displacement indicated that it possessed an even greater degree of lattice strain than exists in the common form. The composition and specific surface of a number of pseudoboehmite xerogels suggest that the major part of the water, in excess of that in the basic structures AlOOH, is present in a surface monolayer with a minor amount responsible for lattice strain.     

Formation and reactivity of oxides and basic salts. II. Radio-isotope studies of some calcareous materials

Rates of calcium exchange between solid and solution in the ageing of hydrated lime and of calcium carbonate samples have been determined using calcium-45 radio-isotope. After a comparatively rapid calcium uptake of about 1 molecular layer at the hydrated lime surface, the exchange rate diminishes as the ions need to diffuse more deeply into the hydrated lime structure. Results for calcium carbonate samples of widely different specific surfaces are compatible with very slow ageing in water. Rates of gypsum precipitation from radioactive calcium chloride and nitrate solutions have been compared in neutral and acidic media and in the presence of albumen. Lower gypsum saturation ratios are established ultimately in neutral rather than acidic media, particularly in solutions containing no salts other than gypsum. Solid hydrated lime reacts rapidly with dilute sulphuric acid, aluminium and magnesium sulpnates, magnesium chloride or sea-water, releasing calcium ions into solution. The radioactivities of the solutions decrease subsequently as gypsum separates out or calcium ions are adsorbed by the finely-divided magnesium hydroxide precipitated. At higher magnesium salt concentrations, the calcium-45 radioactivity does not initially increase so sharply. This is ascribed to basic salt formation preventing all of the lime from going quickly into solution.     

Crystallisation processes in aluminium hydroxide gels. IV. Factors influencing the formation of the crystalline trihydroxides

The formation of crystalline aluminium trihydroxides from aqueous suspensions of pseudoboehmite, obtained by the hydrolysis of aluminium s-butoxide, has been examined by X-ray diffractometry, nitrogen adsorption and dilatometry. The results obtained have been interpreted in terms of a mechanism involving dissolution of the pseudoboehmite. For those suspensions with which the recrystallisation was followed dilatometrically, first-order kinetics were found, suggesting that random nucleation of trihydroxide crystals is involved. The rate of recrystallisation was decreased by the addition of organic solvents to the aqueous suspension and also, initially, by an increase in temperature. At the same time the proportion of gibbsite produced in admixture with bayerite was reduced and that of nordstrandite increased. It is concluded that the retardation is due, at least in part, to an increase in the crystallinity of pseudoboehmite which has been found to accompany the introduction of the organic solvents. The increased crystallinity must result in a decrease in the solubility and rate of solution of pseudoboehmite; under these conditions nordstrandite formation appears to be promoted. Neutron activation and spectroscopic analysis of hydroxide samples showed that storage of aqueous suspensions in glassware resulted in pick-up of sodium and silica at the ppm level. The changes in these levels as recrystallisation occurred suggested that retention of the products of glass dissolution was occurring in a way which depended on the surface area of the hydroxide. No variation related to the phase composition of the trihydroxide mixtures was observed.     

Studies of the crystallisation of gallium hydroxide precipitated from hydrochloric acid solutions by various alkalis

Gallium hydroxide precipitated from aqueous gallium chloride solution on addition of alkalis at varying pH have been aged in the mother liquors for different periods of time. The resulting precipitates have been examined by X-ray diffraction and i.r. spectrophotometry. It is found that the precipitate, separated immediately from the mother liquor after preparation, exists in an amorphous type and transforms to α-gallium oxyhydroxide during aging independent of pH.     

Editorial

This work deals with the preparation of aluminium hydroxide gels by precipitation from pure and mixed solvents in the presence of propylamine acting as proton acceptor in the hydrolysis reaction. As solvents were used water, ethanol and 50% v/v water-ethanol and water-acetone mixtures. Pure acetone could not be used because the aluminium salt used (AlCl₃.6H₂O) was not sufficiently soluble in it. The samples were freeze-dried and activated to temperatures up to 773 K. The samples prepared from aqueous solutions showed the structure of poorly crystallized pseudoboehmite, while the sample prepared from pure ethanol was nearly amorphous and presented the highest specific surface area of all the samples (570 m² g^?1 for thermal activation at 473 K). The sample prepared from aqueous acetone exhibited the highest porosity (about 4 cm³ g^?1) consisting of pores of nearly uniform size. This porosity remained almost constant throughout the range of thermal activation temperatures.     

Incorporation of aluminium in calcium-silicate-hydrates

Calcium silicate hydrate (C-S-H) was synthetized at 20 °C to investigate the effect of aluminium uptake (Al/Si = 0–0.33) in the presence and absence of alkalis on the composition and the solubility of a C-S-H with a Ca/Si equal to 1.0.C-S-H incorporates aluminium readily resulting in the formation of C-A-S-H at Al/Si ≤ 0.1. At higher Al/Si ratios, in addition to C-A-S-H, katoite and/or stratlingite are present. Aluminium is mainly taken up in the bridging position of the silica dreierketten structure, which increases the chain length. The aluminium uptake in C-S-H increases with the aqueous aluminium concentrations.The presence of potassium hydroxide leads to higher pH values, to the destabilisation of stratlingite and to higher dissolved aluminium concentrations, which favours the aluminium uptake in C-S-H. Potassium replaces partially the calcium ions on the surface and interlayer, thus leading to more negative surface charge and to shortening of chain length.     

Influence of different anions on the behaviour of aluminium in aqueous solutions

The influence of chloride, sulfate and perchlorate anions on the behaviour of native oxide layers on aluminium is investigated using electrochemical techniques. Due to its influence on the open circuit potential and the cathodic side of the polarization curve the oxygen concentration has been carefully controlled. Two kinds of attack on a commercially pure aluminium (99.5 wt %) have been observed. In all the investigated 0.5 M Cl^–, 0.5 M ClO^– ₄ and 0.5 M SO^2– ₄ aqueous solutions the metal is corroded around the iron and silicon containing precipitates, but only in Cl^– and ClO^– ₄ solutions is crystallographic pitting observed. Comparison with high purity aluminium (99.99 wt %) shows that pitting corrosion is not influenced by the presence of impurities in the aluminium alloys, but by the presence of anions in solution. The pH and/or oxygen concentration determine whether or not the pitting potential coincides with the corrosion potential.     

Reactivity of lime and related oxides. XVII. Sulphating of lime with magnesium and aluminium sulphates

Lime water and solid hydrated lime have been sulphated with magnesium and aluminium sulphate solutions. Changes in phase composition and sulphating rates are followed from pH (glass electrode) and conductivity measurements. The crystallinities of the products have been compared with calcium sulphate dihydrate precipitated directly from solution by double decomposition and subsequently aged. The reactions with the solid hydrated lime are not appreciably impeded by the insoluble products, magnesium and aluminium hydroxides, which are very finely divided. These compounds provide additional surface on which most of the gypsum can form a gel when the gypsum solution saturation ratios have become sufficiently high. The small amounts of gypsum ultimately separating out from the lime water have crystallinity inferior to the larger amounts formed from the solid hydrated lime. Both are still inferior to gypsum from acid-sulphated lime, and are more comparable with gypsums precipitated from calcium nitrate in acidic media containing concentrations (±0.1 %) of albumen, or from calcium chloride in neutral media. Slower gypsum precipitation from calcium chloride with sulphuric acid (as in the manufacture of ‘Pearl hardening’) gives a more consistent degree of crystallinity.     

Sol–gel synthesis of YAG/Al2O3 long fibres from water solvent systems

Y₃Al₅O₁₂(YAG)/Al₂O₃ long fibres were prepared by a sol–gel method using water as the solvent. They were synthesized from aluminium nitrate and chloride solutions, aluminium salt, aluminium metal and Y₂O₃. The starting materials were dissolved by refluxing at 100°C for 2–18 h and were then condensed. The fibre spinnability was examined by a hand drawing method using a glass rod. In the nitrate solution system, the composition range available for fibre preparation was very limited because nitrate ions decomposed during the refluxing, raising the solution pH and precipitating the Y component. On the other hand, the composition range of the fibres prepared from the chloride system was 0/10⩽YAG/Al₂O₃⩽6/4 (volume ratio) and was wider than that from the nitrate system. The YAG/Al₂O₃ fibres prepared by firing at 1300°C became denser with faster heating rates. The grain size in the fired fibres was small, below the firing temperature at 1400°C, but increased greatly above that temperature.     

The characterisation of aluminium hydroxide gels by nitrogen adsorption

Aluminium hydroxide gels with a range of textures have been prepared by the hydrolysis of aluminium s- butoxide with water, aqueous ethanol and aqueous glycerol. Crystallisation was permitted to take place to varying extents by ageing of precipitates in their mother liquors at room temperature. Adsorption isotherms on the outgassed gels were of Types II and IV of the Brunauer classification. An examination of the data by plotting reduced isotherms (V/V_m versus P/P_o) revealed the existence of reversible capillary condensation on some of the hydroxides, although the isotherms were of Type II. The Lippens de Boer t method has been found useful in locating the onset of capillary condensation in an adsorption isotherm, but the t-plot can be misleading with samples that have a spread of pore dimensions extending into both the micro- and transitional-pore regions. This situation appears to arise in pseudoboehmites aged for extended periods so that cementation is extensive. The principal factors influencing the texture of the gels appear to be the extent of crystallisation and the amount of inter-particle cementation. Thus amorphous material prepared in water was found to be porous whereas that precipitated in glycerol, which apparently protects the surface of the primary particles, was non-porous (or macro-porous). Transitional pore systems with the best definition were obtained with pseudoboehmites, especially with those formed by ageing in aqueous ethanol. In water, the ready conversion of pseudoboehmite to low-area, non-porous, bayerite is a complicating factor.     

Hydrated aluminium sulfate precipitation by enzyme-catalysed urea decomposition

Hydrated basic aluminium hydroxide precipitates were obtained from aluminium sulphate solution at room temperature and at 92°C by using urea. Enzyme urease was used to decompose urea at room temperature. It was observed that precipitates obtained at room temperature were much finer than those obtained at 92°C but they had similar chemistry. On calcination at 1200°C, each precipitate obtained at room temperature converted to a discrete α-Al₂O₃ crystal whereas that obtained at 92°C converted to a rather densely packed agglomerate of α-Al₂O₃ crystals. As a result of this, α-Al₂O₃ powder obtained from 25°C precipitates showed much better sintering behaviour.     

The influence of hydrating agents on the hydration of industrial magnesium oxide

BACKGROUND: The influence of different hydrating agents on the pH of the hydrating solutions, rate of hydration of MgO to Mg(OH)₂ and product surface area was studied as a function of temperature of hydration. Hydrating agents used were aqueous solutions of ammonium chloride, magnesium acetate, magnesium nitrate, nitric acid, acetic acid, magnesium chloride, sodium acetate and hydrochloric acid and distilled water as control. These were chosen to determine either the effect of addition of a common ion, the effect of changing the solution pH or due to the presence of an acetate ion, found earlier to have a beneficial effect on the hydration of MgO. RESULTS: There was no significant difference in the hydration behaviour of the hydrating agents up to 50 °C, where less than 10% of magnesium hydroxide was formed. The amount of hydroxide increased at temperatures above 60 °C. When compared with the hydration in water, all the hydrating agents, with the exception of sodium acetate, showed a significant increase in the degree of hydration. Sodium acetate formed the lowest amount of magnesium hydroxide, ranging between 1.2 and 12.2% magnesium hydroxide. The largest percentage (56.7%) of magnesium hydroxide was formed from hydration in magnesium acetate. CONCLUSION: It seems that MgO hydration is a dissolution‐precipitation process controlled by the dissolution of MgO. The increased degree of hydration in magnesium acetate is possibly due to the presence of acetate and Mg²⁺ ions. Copyright © 2010 Society of Chemical Industry     

Adsorption behavior of fluoride ions using a titanium hydroxide-derived adsorbent

The removal behavior of fluoride ions was examined in aqueous sodium fluoride solutions using a titanium hydroxide-derived adsorbent. The adsorbent was prepared from titanium oxysulfate (TiOSO₄·xH₂O) solution, and was characterized by X-ray diffraction, scanning electron microscopy, thermogravimetry-differential thermal analysis, Fourier transform infrared spectrum and specific surface area. Batchwise adsorption test of prepared adsorbent was carried out in aqueous sodium fluoride solutions and real wastewater containing fluoride ion. The absorbent was the amorphous material, which had different morphology to the raw material, titanium oxysulfate, and the specific surface area of the adsorbent (96.8 m²/g) was 200 times higher than that of raw material (0.5 m²/g). Adsorption of fluoride on the adsorbent was saturated within 30 min in the solution with 200 mg/L of fluoride ions, together with increasing pH of the solution, due to ion exchange between fluoride ions in the solution and hydroxide ions in the adsorbent. Fluoride ions were adsorbed even in at a low fluoride concentration of 5 mg/L; and were selectively adsorbed in the solution containing a high concentration of chloride, nitrate and sulfate ions. The adsorbent can remove fluoride below permitted level (< 0.8 mg/L) from real wastewaters containing various substances. The maximum adsorption of fluoride on the adsorbent could be obtained in the solution at about pH 3. After fluoride adsorption, fluoride ions were easily desorbed using a high pH solution, completely regenerating for further removal process at acidic pH. The capacity for fluoride ion adsorption was almost unchanged three times after repeat adsorption and desorption. The equilibrium adsorption capacity of the adsorbent used for fluoride ion at pH 3 was measured, extrapolated using Langmuir and Freundlich isotherm models, and experimental data are found to fit Freundlich than Langmuir. The prepared adsorbent is expected to be a new inorganic ion exchanger for the removal and recovery of fluoride ions from wastewater.     

Adsorption off Lauryl Sulfate from Electrolyte Solutions on Cobalt Hydroxide

Abstract

The adsorption of lauryl sulfate on cobalt(II) hydroxide from solutions of inorganic salts has been found to change as follows: without salt ≥ chloride ≥ bromide ≥ nitrate ≥ sulfate ≥ phosphate Adsorption in the presence of acids increases in the following series: without acid ≥ hydrochloric ≥ sulfuric ≥ phosphoric, and is higher than in the presence of salts. The surface concentration of lauryl sulfate adsorbed from a suspension without electrolyte is about 0.9 (μmol/ m². The surface coverage of hydroxide by lauryl sulfate is about 1/5 of a monolayer, but it decreases in the presence of electrolyte. The law of mass action was applied to characterize the effect of solution composition on the adsorption of lauryl sulfate on cobalt(II) hydroxide.     

FOrmation and reactivity of oxides and basic salts. I. Basic magnesium chlorides and sulphates

Reactions of lime and magnesia with magnesium chloride and sulphate have been studied in relation to industrial processes, namely, extraction of magnesium from sea-water and production of Sorel-type cements. Variations in phase composition, surface area and crystallite and aggregate sizes of the products have been correlated with experimental conditions. The solid limes react rapidly with the more dilute magnesium salt solutions, and the precipitated magnesium hydroxides are much more finely-divided. Basic magnesium salts initially formed are decomposed subsequently to hydroxide as the pH becomes stabilised at higher values. Greater concentrations of magnesium chloride give more extensive basic salt formation and prevent all of the lime going quickly into solution. Similar products are given with solid magnesium hydroxide and magnesium chloride and sulphate solutions. The Sorel-type cements obtained from magnesium oxide and the more concentrated chloride solutions set appreciably, even before loss of surface through final ageing. They show two distinct phases: (a) irregular grains of 10–50 μm diameter, and (b) acicular or tabular crystals up to 20μm long, mainly in radiating growths. By contrast, the magnesium oxysulphate crystals are not so elongated and the cements set with considerable loss of surface area, giving interlocking structures in which 20–50 μm-diameter grains are often ‘fused’ together as larger 400–500 μm aggregates.     

Effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte

The effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte has been investigated through thermodynamic analysis and corrosion experiments. The solution chemistry of aluminium, zinc, and iron in aqueous solution in the absence and in the presence of fluoride ions was studied with the construction of the Eh-pH diagrams for the Al–F–H₂O, Zn–F–H₂O and Fe–F–H₂O systems at 25°C. In the presence of fluoride ions, aluminium can form a series of aluminium-fluoride complexes depending on the fluoride concentration and pH whereas zinc and iron can form soluble or insoluble metal-fluoride complex species only at relatively high fluoride concentration and at higher pH values. Experimental results show that in the presence of fluoride ions, the corrosion of pure aluminium in sulphuric acid is due to uniform dissolution and the reaction rate depends on the fluoride concentration. In zinc electrolyte containing fluoride ions, zinc deposits onto the pure aluminium substrate spontaneously and the amount of deposited zinc also depends on the fluoride concentration. On the other hand, the presence of iron in the Al–Fe alloy accelerates the corrosion of aluminium in H₂SO₄ and zinc electrolyte significantly and prevents the deposition of zinc on the aluminium surface. The effect of fluoride ions on zinc adherence to the aluminium is also discussed.     

Extraction of Copper(II) Ions from Chloride and Sulphate Solutions Using Hydrophobic Pyridyl Ketoximes

Hydrophobic pyridyl ketoximes: 1-(2-pyridyl)tridecan-1-one oxime, 1-(3-pyridyl)tridecan-1-one oxime and 1-(4-pyridyl)tridecan-1-one oxime have been synthesized and investigated as extractants of copper(II) ions. Removal of metal ions was conducted from chloride, sulphate, and sulphate/chloride solutions. The influence of pH of aqueous solutions, copper(II), chloride, and sulphate ions and ligand concentration for extraction process were studied. Copper(II) extraction by hydrophobic 2-, 3-, and 4-pyridyl ketoximes from sulphate solutions is not possible. However, addition of chloride ions to initial sulphate media enables metal removal. The oxime of 1-(2-pyridyl)tridecane-1-one was determined as the strongest extractant of the tested oximes, but metal stripping was impossible. For the rest of the studied extractants the stripping process could be done using water or diluted mineral acid.     

Tungstate induced corrosion of some aluminium alloys in acidic chloride solution — An approach for controlling it with morpholine

The corrosion behaviour of some aluminium alloys (1060, 1100 and 3003) in acidic chloride solution (pH=1) have been studied in the presence of various concentrations of tungstate ions. The alloy most vulnerable to the effects of tungstate ions is 1100 followed by 1060 and 3003 alloys. After six hours of immersion the alloys exhibit 6 to 8 times higher corrosion rates compared to that in the blank solution containing only chloride ions. The experiments suggest that the tungstate ion stimulates the corrosion of aluminium by acting as a cathodic depolarizer. Morpholine effectively arrests the dissolution of the alloys in the blank electrolyte as well as in the presence of tungstate ions. Morpholine polarizes local cathodic sites to act as an inhibitor. Tungstate ions are not adsorbed on the surface in the presence of morpholine and a synergistic effect is obtained at higher concentrations of morpholine. A Langmuir adsorption isotherm has been used to explain the data.