Preparation of sodium aluminate from the leach liquor of diasporic bauxite in concentrated NaOH solution

A new process to produce monosodium aluminate hydrates (MAH) by fast crystallization from the leach liquor of a diasporic bauxite in concentrated NaOH solution is presented. The crystallization of MAH was carried out easily compared to the precipitation of gibbsite and the effect of agitation, initial concentration of sodium aluminate, seed amount and the presence of red mud were systematically studied in a batch crystallizer. The apparent kinetics of crystallization followed a second order rate law with an apparent activation energy for MAH crystallization of 38.0 kJ/mol which implies a surface-diffusion controlled mechanism. X-Ray diffraction and scanning electron microscopy identified the structure of MAH as Na(2)O center dot Al(2)O(3)center dot 2.5H(2)O with a flake crystal morphology. The molar ratio a of Na(2)O to Al(2)O(3) in the MAH products was < 1.2 after a simple wash by dilute sodium aluminate. (C) 2009 Elsevier B.V. All rights reserved

Cryst. Growth Des.

The induction time for monosodium aluminate hydrate (MAH) crystallization from supersaturated solution at 333 to 373 K is systematically investigated by experiment and the primary nucleation is also identified according to the classical nucleation theory. The truncated quadrangular orthopyramid and the twin/composite truncated pyramid crystals of MAH except for the tabular crystals and octagonal platelets obtained in the research are presented for the first time; furthermore, the supersaturation and temperature of the nucleation are experimentally studied. The interfacial free energies gamma of truncated pyramid and octagonal platelet MAH crystals in the supersaturated sodium aluminate solution are conducted, and then the octagonal platelet MAH crystal is considered more stable in the solution than the truncated pyramid crystals. The two-dimensional (2D) mediated growth mechanism for MAH crystallization is suggested based on the regression of experimental data of nucleation.The induction time for monosodium aluminate hydrate (MAH) crystallization from supersaturated solution at 333 to 373 K is systematically investigated by experiment and the primary nucleation is also identified according to the classical nucleation theory. The truncated quadrangular orthopyramid and the twin/composite truncated pyramid crystals of MAH except for the tabular crystals and octagonal platelets obtained in the research are presented for the first time; furthermore, the supersaturation and temperature of the nucleation are experimentally studied. The interfacial free energies gamma of truncated pyramid and octagonal platelet MAH crystals in the supersaturated sodium aluminate solution are conducted, and then the octagonal platelet MAH crystal is considered more stable in the solution than the truncated pyramid crystals. The two-dimensional (2D) mediated growth mechanism for MAH crystallization is suggested based on the regression of experimental data of nucleation

Crystengcomm

The primary nucleation of sodium carbonate in caustic liquor was monitored using a focused beam reflectance measurement (FBRM) during evaporation in a batch evaporator. The results obtained by the FBRM system showed that there were two abrupt increases, and the crystals obtained from the two increases exhibited different morphologies. The content of calcium in the first abrupt crystal was much higher than that in the stock solution. Calcium ions were found to have an inhibitive effect on sodium carbonate nucleation.The primary nucleation of sodium carbonate in caustic liquor was monitored using a focused beam reflectance measurement (FBRM) during evaporation in a batch evaporator. The results obtained by the FBRM system showed that there were two abrupt increases, and the crystals obtained from the two increases exhibited different morphologies. The content of calcium in the first abrupt crystal was much higher than that in the stock solution. Calcium ions were found to have an inhibitive effect on sodium carbonate nucleation

Ind. Eng. Chem. Res.

The nucleation, growth, and agglomeration of monosodium aluminate hydrate (MAH) crystallization in concentrated sodium aluminate solutions were investigated in a steady-state mixed-suspension-mixed-product-removal (MSMPR) crystallizer, and the mechanism was analyzed in detail. The crystal growth was diffusion- and surface-integration-controlled, and the secondary nucleation, as a result of crystal-agitator and crystal-crystallizer collisions, was determined further. The agglomeration kernel, expressed in terms of mean residence time, growth rate, and suspension density, was found to have a positive order of about 0.52 in the suspension density, indicating that the agglomeration kernel increased at higher frequencies of collisions between particles. The growth rate of MAH was found to be higher than that of gibbsite in active NaAl(OH)(4)-NaHCO3 systems, but the nucleation rate of MAH was lower than that of gibbsite in seeded-hydrolysis processes.The nucleation, growth, and agglomeration of monosodium aluminate hydrate (MAH) crystallization in concentrated sodium aluminate solutions were investigated in a steady-state mixed-suspension-mixed-product-removal (MSMPR) crystallizer, and the mechanism was analyzed in detail. The crystal growth was diffusion- and surface-integration-controlled, and the secondary nucleation, as a result of crystal-agitator and crystal-crystallizer collisions, was determined further. The agglomeration kernel, expressed in terms of mean residence time, growth rate, and suspension density, was found to have a positive order of about 0.52 in the suspension density, indicating that the agglomeration kernel increased at higher frequencies of collisions between particles. The growth rate of MAH was found to be higher than that of gibbsite in active NaAl(OH)(4)-NaHCO3 systems, but the nucleation rate of MAH was lower than that of gibbsite in seeded-hydrolysis processes

Current efficiency of Ga electrodeposition under different anions concentrations

The gallium electrodeposition from alkaline solution has a very low current efficiency, the reason for which is still not quite understood. The effects of electrode materials used for gallium electrodeposition, as well as the effects of NaOH concentration and the anions concentrations in the solution, including SO_4~(2-), SiO_3~(2-), CO_3~(2-), AlO_2-, F~-, and Cl~-, on the deposition were analyzed in this study. The suitable materials of SUS316–SUS316 were suggested for the gallium electrodeposition. Based on the electrode couples, the NaOH concentration of 4 mol·L~(-1) for gallium electrodeposition exhibits the greatest current efficiency. Moreover, the current efficiency would decrease in the electrolyte along with the increasing concentration of the anions, except that, 0.2 mol·L~(-1) Cl~- in the solution slightly improves the current efficiency of gallium electrodeposition. Moreover, the gallium deposited on the cathode from the solution with 0.6 mol·L~(-1) SiO_3~(2-) appears tiny black in color and coarse. Meanwhile, SUS304 is shown to be not suitable to be used as cathode for the gallium electrodeposition from the alkaline solution

The solid-liquid separation behaviors of the typical leach slurries in the alkaline processes for alumina

The filtration kinetics and the cake properties of the alumina leach slurry were investigated. Four kinds of alumina leach slurry were prepared by the hydro-chemical process to treat the bauxite, the Bayer process, the Bayer process with less lime, and the hydro-chemical process to treat the Bayer red mud. The filtration kinetics was regressed by filtration test method at constant pressure. The diversity of the solid-liquid separation behaviors indicates that the digestion conditions play an important role in the filtration process. The composition and phase, the size distribution, the shape and the surface properties of the solid particles as well as the viscosity of filtrate were measured, and their effects on the filtration resistance and the cake compressibility were confirmed. Results indicate that the fine size and the broad size distribution of the particles lead to a high cake resistance. And the substances containing Fe or Ti exhibit a noticeable influence on the cake characteristics. Methods of filtration optimization through changing digestion conditions were recommended. (C) 2017 Elsevier B.V. All rights reserved.</p

The phase transition in Bayer red mud from China in high caustic sodium aluminate solutions

The phase transitions of the main substances in Bayer red mud in high caustic sodium aluminate solutions were studied. Without addition of lime, cancrinite was not found to transform to CaNaHSiO4 even with 16 wt.% CaO present in the original residue up to 270 degrees C. However, this transition was verified to be a fast reaction which was completed in just 10 min with the temperature higher than 240 degrees C after adding lime, thus enabling the extraction of Al2O3 from the red mud. Whether additional CaO was supplemented or not, the isomorphous substitution of Fe to Al atoms occurred in hydrated andradite as long as the temperature was more than 240 degrees C, providing another way to extract Al2O3 from Bayer red mud. Accordingly the A/S (weight ratio of solid Al2O3 to SiO2) of red mud would reduce to 0.134 after the complete transition from Ca2.93Al1.97Si0.64O2.56(OH)(9 44) to Ca-3(Fe0.87Al0.13)(2)(SiOL4)(1.65)(OH)(5.4) in our study, assuming no other Al or Si containing phases. Furthermore, the addition of CaNaHSiO4 seed was confirmed to accelerate both the transition of cancrinite to CaNaHSiO4 and the reaction of isomorphous substitution in the andradite. (C) 2013 Elsevier B.V. All rights reserved

Hydrometallurgy

The decomposition of NaCaHSiO4 in sodium aluminate solution was investigated. NaCaHSiO4 transformed to sodalite and katoite during decomposition. Parameters, such as Na2O concentration, reaction time, temperature and Al2O3 concentration, had a significant influence on the NaCaHSiO4 decomposition. The optimal Na2O concentration and reaction time for the NaCaHSiO4 decomposition were 150 g/L and 105 min, respectively. The extent of NaCaHSiO4 decomposition largely depended on Al2O3 concentration in sodium aluminate solution. Reaction products were composed of thin flakes with a loose structure at 50 g/L of Na2O concentration, and were agglomerates with a compact surface composed of tabular and rod crystals at 150 g/L of Na2O concentration. (C) 2013 Elsevier B.V. All rights reserved.The decomposition of NaCaHSiO4 in sodium aluminate solution was investigated. NaCaHSiO4 transformed to sodalite and katoite during decomposition. Parameters, such as Na2O concentration, reaction time, temperature and Al2O3 concentration, had a significant influence on the NaCaHSiO4 decomposition. The optimal Na2O concentration and reaction time for the NaCaHSiO4 decomposition were 150 g/L and 105 min, respectively. The extent of NaCaHSiO4 decomposition largely depended on Al2O3 concentration in sodium aluminate solution. Reaction products were composed of thin flakes with a loose structure at 50 g/L of Na2O concentration, and were agglomerates with a compact surface composed of tabular and rod crystals at 150 g/L of Na2O concentration. (C) 2013 Elsevier B.V. All rights reserved

Recovery of Alkali from Bayer Red Mud Using CaO and/or MgO

Recovering alkali from Bayer red mud is crucial for storage security, resource utilization and environmental protection. In this study, the addition of MgO and/or CaO was conducted to recover alkali from red mud with a hydrothermal method for the first time. A synergistic result with a residual Na2O/SiO2 weight ratio of 0.03 was obtained by adding the blend of CaO and MgO at an appropriate temperature. MgO was found to be more temperature-dependent than CaO when substituting Na2O from red mud due to their different hydration processes. The alkali recovery was controlled by a reaction at a temperature of <200 degrees C and by internal diffusion at a higher temperature for MgO, but controlled by internal diffusion for CaO in the whole temperature range studied. The formation of hydrotalcite-like compounds with a loose structure was verified with the help of XRD, FTIR, and SEM-EDS. It was proved that both the reaction kinetics and the characteristics of solid products have a significant influence on the recovery of alkali

Crystallization of monosodium aluminate hydrate from a concentrated aluminate solution containing silica

The influence of silica content in concentrated aluminate solution on the crystallization of monosodium aluminate hydrate (MAN) has been investigated systematically. The resultant crystals were found to be round flakes, with a high silica concentration significantly inhibiting the crystallization. The conversion ratio and the average particle size of MAN crystals decreased with increasing SiO2 concentration in solution. The silicate impurity phases which crystallized simultaneously with MAN were identified as NaAlSiO4 and Na2SiO3 from low silica solution. Ca3Al2Si3O12, Ca3Al2(SiO4)(2)(OH)(4), and Na4Ca4(Si6O18) we obtained when there was a high silica concentration. These phases have a much higher silica content than the more common desilication product Ca3Al2(SiO4)(n)(OH)(12-4n) from the Bayer process for alumina production where n is usually below 0.8. (C) 2012 Elsevier B.V. All rights reserved