Defluoridation performance comparison of aluminum hydroxides with different crystalline phases

Aluminum hydroxide is an eye catching and extensively researched adsorbent for fluoride removal and its defluoridation performance is closely related to the preparation method and crystalline phase. In this research, the defluoridation performances of aluminum hydroxides with different crystalline phases are compared and evaluated in terms of fluoride removal capacity, sensitivity to pH values and residual Al contents after defluoridation. It is found that the defluoridation performance of different aluminum hydroxides follows the order of boehmite > bayerite > gibbsite. The fluoride adsorption on aluminum hydroxides follows pseudo-second-order kinetic model and Langmuir isotherm model, and the maximum defluoridation capacities of boehmite, bayerite and gibbsite are 42.08, 2.97 and 2.74 mg m−2, respectively. The pH values and FTIR analyses reveal that the ligand exchange between fluoride and surface hydroxyl groups is the fluoride removal mechanism. Different aluminum hydroxides have different surface hydroxyl group densities, which results in the different defluoridation capacities. This work provides a new idea to prepare aluminum hydroxide with outstanding defluoridation performance.

Simulating the Use of a Smelter Off-Gas in the Precipitation Stage of the Pedersen Process

The Pedersen process is an alumina production process, which combines pyrometallurgical and hydrometallurgical methods. In the pyrometallurgical stage, limestone is calcined and CO2 is generated. This off-gas can be captured with a high CO2 concentration. At the end of the hydrometallurgical process, aluminum hydroxides, like bayerite, are precipitated using CO2. In this paper, experimental work on precipitation of aluminum hydroxides through the addition of a mixture of CO2, O2 and N2 is presented. The parameters varied, as were the percentages of each gas and the temperature. The indicators measured were the time until the beginning of precipitation and the time that the precipitation lasts. These tests simulate the use of a smelter furnace off-gas in the precipitation stage of the Pedersen process and have shown promising results.

Use of Different Process Gases for Manufacturing Isolating Alumina Coatings by Flame Spraying with Cords

This study assesses the quality of flame-sprayed alumina coatings produced from recently developed alumina cord using argon and compressed air as atomizing gases. Coatings of different thicknesses were deposited on aluminum substrates and then analyzed using optical microscopy, X-ray diffraction, and resistivity measurements. The coatings, particularly those sprayed with argon, had fine microstructure and higher surface and volume resistivity than flame-spray coatings made from alumina cord in the past. They were also found to have higher alpha phase content than plasma-sprayed coatings, regardless of the atomizing gas used. The effect of humidity and the possible formation of aluminum hydroxides are also addressed.

PROSPECTS FOR THE APPLICATION OF COPPER MELTING SLAG IN ROAD CONSTRUCTION

The article discusses the option of strengthening the soils of the roadbed of highways with complex binders based on granular slag of copper smelting production and air lime. The characteristics of the investigated slag, photomicrographs of its surface, as well as the chemical and phase compositions are presented. A detailed description of the X-ray diffractogram of the studied slag with the processing of the obtained reflections using the ICDD PDF-2 database is presented. Calculations of hydrolytic equilibria were carried out using the main phases found from the data of X-ray phase analysis. It was found by the calculation method that at pH = 11-12 in the lime-slag mixture decomposition of iron silicates and aluminosilicates occurs with the formation of gels of iron and aluminum hydroxides, as well as amorphous silica. On the basis of the obtained complex binder, standard samples of strengthened soil based on loam were made and tested to determine the physical and mechanical properties. The results of the experiments carried out create an opportunity for obtaining hardened soils based on a complex mineral binder with a compressive strength of up to 2.2 MPa.

New atmospheric corrosion inhibitor of aluminum alloy D16

The application of mixed corrosion inhibitor (CI), which is an equimolar composition of oleoyl sarcosinate (SOS) and sodium flufenamate (SFF), for protection of D16 aluminum alloy from atmospheric corrosion has been studied. The polarization measurements used to assess the effectiveness of preliminary passivation of the alloy with solutions of SOS, SFF and their composition showed significant advantages of mixed CI. The XPS method was used to study features of CI adsorption on the surface of D16 alloy. It has been established that upon adsorption of SOS and SFF separately a monolayer is formed, firmly bonded to the alloy surface, thickness of which is not exceeding 2.6—3.2 nm. After the joint adsorption of these CI, the layer thickness reaches 12—20 nm. The composition of this layer includes a considerable amount of Al3+ ions (~20%) related to their compounds with SFF and SOS, as well as to aluminum hydroxides. A possible mechanism for the formation of such a protective layer is proposed. The results of corrosion tests in a humid atmosphere with daily water condensation on samples of D16 alloy confirmed the high protective ability of the mixed CI film.

Thermodynamic calculation of the effect of polymorphism and pH value on the solubility of aluminium oxide and its hydrates in aqueous media

Экспериментальное определение истинной равновесной растворимости гидроксидов и оксидов алюминия затруднено характерной склонностью их к образованию золей, а также из-за медленного установления гетерогенного химического равновесия и низкой величины растворимости в водных средах [Школьников, 2008, 2009]. В статье на основе результатов тщательного медленного кондуктометрического и потенциометрического титрования 0,0010 М раствора Al(NO3)3(pH0 = 3,00) раствором 0,050 (0,100) М NaOH при 20 °С определен cостав устойчивых гидроксокомплексов Al(ОН)n где n = 1, 2, 3, 4. Выполнен критический анализ информации о термодинамических свойствах оксидов, гидроксидов и гидроксокомплексов алюминия. Теоретически рассчитано влияние pH в интервале 3–14 на равновесные молярные доли катиона Al3+ и его моноядерных гидроксокомплексов и на равновесную растворимость различных модификаций оксида алюминия и его гидратов при 25 °С. Для большинства модификаций Al2O3 расчет проведен впервые. Экспериментально определенная растворимость твердой фазы Al(OH)3 [Gayer, Thompson, Zajice,1958] близка в слабокислой среде к рассчитанной авторами растворимости нордстрандита β-Al(OH)3, а в щелочной среде – байерита α-Al(OH)3. Растворимость оксидов и гидроксидов алюминия в чистой воде определяется содержанием растворенных нейтральных комплексов Al(OH)3 (молярная доля ~ 0,93) и зависит от структуры, уменьшаясь на три порядка при переходе от аморфных к термодинамически стабильным модификациям α–Al2O3, α–AlO(OH) и γ – Al(OH)3. Рассчитаны минимальная молярная растворимость (от 2·10–8 для α-AlO(OH) до 4·10–3 моль/л для аморфного Al2O3) и оптимальная величина pH (7,15) наиболее полного осаждения оксидов и гидроксидов алюминия в водных средах при 25 °С. Experimental determination of the true equilibrium solubility of aluminum hydroxides and oxides is complicated by their characteristic tendency to form sols, as well as due to the slow establishment of heterogeneous chemical equilibrium and low solubility in aqueous media [Shkol`nikov, 2008 and 2009]. In the article, based on the results of careful slow conductometric and potentiometric titration of 0.0010 M Al(NO3)3 solution (pH0 = 3.00) with solution 0.050 (0.100)M NaOH solution at 20 °C, the composition of stable hydroxocomplexes Al(OH)n where n = 1,2,3,4 is determined. The critical analysis of information about the thermodynamic properties of aluminum oxides, hydroxides, and hydroxocomplexes is performed. The effect of pH in the range 3-14 on the equilibrium molar fractions of the Al3+ cation and its mononuclear hydroxocomplexes and on the equilibrium solubility of various modifications of aluminum oxide and its hydrates at 25 °C was theoretically calculated. For most Al2O3 modifications the calculation was performed for the first time. The experimentally determined solubility of the solid phase Al(OH)3 [Gayer, Thompson, Zajice, 1958] is close in a slightly acidic medium to the calculated solubility of nordstrandite β-Al(OH)3, and in an alkaline medium – bayerite α-Al(OH)3. The solubility of aluminum oxides and hydroxides in pure water is determined by the content of dissolved neutral Al(OH)3 complexes (molar fraction ~ 0.93) and depends on the structure, decreasing by three orders of magnitude during the transition from amorphous to thermodynamically stable modifications of α-Al2O3, α-AlO(OH) and γ-Al(OH)3. The minimum molar solubility (from 2·10–8 for α-AlO(OH) to 4·10–3 mol/l for amorphous Al2O3) and the optimal pH(7.15) for the most complete deposition of aluminum oxides and hydroxides were calculated in aqueous media at 25 °C.