Thermodynamic model for equilibrium solubility of gibbsite in concentrated NaOH solutions

The thermodynamic properties of the most important NaOH-NaAI（OH）4-H20 system in Bayer process for alumina production were investigated. A theoretical model for calculating the equilibrium constant of gibbsite dissolved in sodium hydroxide solution was proposed. New Pitzer model parameters and mixing parameters for the system NaOH-NaAI（OH）4-H20 were yielded and tested in the temperature range of 298.15-373.15 K. The results show that the proposed model for calculating the equilibrium constant of gibbsite dissolution is applicable and accurate. The obtained Pitzer model parameters of β（0）（NaAl（OH）4）、β（1）（NaAl（OH）4）和CΦ（NaAl（OH）4）,Al（OH）4 for NaAI（OH）4, the binary mixing parameter of θ（OH-Al（OH）4-） with OH-, and the ternary mixing parameter of ψ（Na＋OH-Al（OH）4-） for AI（OH）4- with OH- and Na＋ are temperature-dependent. The prediction of the equilibrium solubility of gibbsite dissolved in sodium hydroxide solution was feasible in the temperature range of 298.15-373.15 K.

Dissolution kinetics and mechanism of gibbsitic bauxite and pure gibbsite in sodium hydroxide solution under atmospheric pressure

The crystal structure, morphology, dissolution kinetics and mechanism of gibbsitic bauxite and pure gibbsite in Na OH solution under atmospheric pressure were systematically investigated by XRD and SEM. The results show that the size of single crystal of gibbsite in gibbsitic bauxite is smaller than that in pure gibbsite, but the interplanar distance is larger than that of pure gibbsite, which result in more defects in the crystal and less energy needed to dissolve in alkaline solution for the gibbsitic bauxite. The dissolution kinetic equations of gibbsitic bauxite and pure gibbsite were established, and the corresponding activation energies were calculated to be 99.144 and 115.149 k J/mol, respectively.

Non-isothermal thermal decomposition kinetics of high iron gibbsite ore based on Popescu method

The thermal decomposition kinetics of high iron gibbsite ore was investigated under non-isothermal conditions.Popescu method was applied to analyzing the thermal decomposition mechanism.The results show that the most probable thermal decomposition mechanism is the three-dimensional diffusion model of Jander equation,and the mechanism code is D3.The activation energy and pre-exponential factor for thermal decomposition of high iron gibbsite ore calculated by the Popescu method are 75.36 kJ/mol and 1.51×10-5 s-（-1）,respectively.The correctness of the obtained mechanism function is validated by the activation energy acquired by the iso-conversional method.Popescu method is a rational and reliable method for the analysis of the thermal decomposition mechanism of high iron gibbsite ore.

Crystallization of gibbsite from synthetic chromate leaching solution in sub-molten salt process

For the clean and economical production of chromium compounds, it is crucial to remove aluminates from chromate alkali solutions and utilize aluminum-containing compounds. In this work, carbonization was used to remove aluminates from a synthetic chromate leaching solution containing a high K2O/Al2O3 mole ratio. The influence of reaction temperature, carbonization time, flow rate of carbon dioxide, and seed ratio on the precipitation of Al was investigated. The optimal output was obtained under the following experimental conditions： a reaction temperature of 50 °C, a carbonization time of 100 min, a carbon dioxide flow rate of 0.1 L/min, and a seed ratio of 1.0. Gibbsite was obtained following carbonization. The structure and morphology of the gibbsite were characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, and laser particle size analyzer. The particle size distribution and morphology of the gibbsite were significantly influenced by the experimental conditions. The gibbsite had a mean particle size （d50） of 16.72μm. The thermal decomposition of the gibbsite was analyzed by XRD and the decomposition path was determined. The obtained coarseα-Al2O3 precipitate, which contains 0.08% Cr2O3 and 0.10% K2O, was suitable for subsequent utilization.

The Most Probable Mechanism Function and Kinetic Parameters of Gibbsite Dissolution in NaOH

Determination of probable mechanism function and kinetic parameters is important to hydrometallurgical kinetics.In this work,the most probable mechanism function and kinetic parameters of gibbsite dissolution in NaOH solution are studied.The sample,the mixture of synthetic gibbsite and sodium hydroxide solution,was scanned in high-pressure differential scanning calorimetry(DSC) equipment with the heating rate of 10 K·min-1. Integral equation and differential equation of non-isothermal kinetics were solved to fit the data related to DSC curve.According to the calculation results,the most probable mechanism function for pure synthetic gibbsite dissolution in sodium hydroxide solution is presented based on the optimum procedure in the database of the mechanism function.The apparent activation energy obtained is(75±1) kJ·mol-1,the frequency factor is 10 8±1mol·s-1,and the reaction is a second order reaction.

Precipitation of spherical boehmite from concentrated sodium aluminate solution by adding gibbsite as seed

The precipitation of spherical boehmite was studied by surface energy calculations, measurements of precipitation ratios, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The surface energy calculation results show that the(001) and(112) planes of gibbsite surfaces are remarkably stable because of their low surface energies. In addition, the(010) plane of boehmite grows preferentially during precipitation because of its low surface energy. Thus, we propose a method to precipitate spherical boehmite from a supersaturated sodium aluminate solution by adding gibbsite as seed in a heterogeneous system. In this method, gibbsite acts as the preliminary seed and saturation modifier. The results show that the fine boehmite first nucleates on the(001) and(112) planes of gibbsite and then grows vertically on the(001) and(112) basal planes of gibbsite via self-assembly, thereby forming spherical boehmite. Simultaneously, gibbsite is dissolved into the aluminate solution to maintain the saturation for the precipitation of boehmite. The precipitation ratio fluctuates(forming an M-shaped curve) because of gibbsite dissolution and boehmite precipitation. The mechanism of boehmite precipitation was further discussed on the basis of the differences in surface energy and solubility between gibbsite and boehmite. This study provides an environmentally friendly and economical method to prepare specific boehmite in a heterogeneous system.

Effect of oxalate on seed precipitation of gibbsite from sodium aluminate solution

The precipitation performance and kinetics of gibbsite from sodium aluminate solution with different sodium oxalate concentrations as well as the corresponding influence mechanism of oxalate during the seed precipitation process were systematically investigated by physicochemical properties test,using SEM and Raman spectra.As the concentration of sodium oxalate increases,both the precipitation rate and particle size of gibbsite decrease.The presence of sodium oxalate not only increases the viscosity of sodium aluminate solution,but also promotes the transformation of Al(OH)4^? to Al2O(OH)6^2?.The overall reaction rate constant decreases and the apparent activation energy of gibbsite increases with the increasing sodium oxalate concentration,the rate controlling step of which is chemical reaction.The needle-like sodium oxalate precipitates on the gibbsite crystals and covers the active Al(OH)3 seed sites,which leads to the lower precipitation rate and the finer particle size of gibbsite during the seed precipitation process.

Effect of tetracarbon additives on gibbsite precipitation from seeded sodium aluminate liquor

Effects of tetracarbon additives, 1-butanol, 1,4-dioxane and tetrahydrofurane on gibbsite precipitation from caustic seeded sodium aluminate liquor were investigated. The additive was charged into supersaturated seeded sodium aluminate liquor, then the precipitation ratio of aluminate liquor in 10 h was evaluated and compared, and the particle size distribution of product was measured. The Mulliken atomic charge of oxygen in the additive molecule was calculated with DMo13 program. The results show that 1-butanol has little effect on gibbsite crystallization, while the precipitation ratios under the effect of 1,4-dioxane and tetrahydrofurane are 1.7% and 3.6% higher than that of the blank, respectively. The agglomeration efficiency of the product is also enhanced obviously by the addition of 1,4-dioxane and tetrahydrofurane. The precipitation ratio is inversely proportional to Mulliken atomic charge of oxygen atom, which implies that functional group in the additive molecule is involved in the gibbsite precipitation process more fundamentally than carbon chain.

Adjustment on gibbsite and boehmite co-precipitation from supersaturated sodium aluminate solutions

Gibbsite is the usual precipitation product from alumina refineries with either Bayer or sintering process.However,the advantage of boehmite precipitation over gibbsite precipitation is the significant energy saving in the subsequent calcination step.The current investigation takes a pragmatic approach to measure precipitation ratios,determine product phase,morphology and particle size distribution,and assess the impacts and adjustment capability of main parameters such as seed,temperature,ethanol medium,and supersaturation on the precipitation kinetics and alumina hydrate type during co-precipitation process.The results clarify that gibbsite and boehmite both can be precipitated from supersaturated sodium aluminate solutions simultaneously,and the competitive formation between Al(OH)3 and γ-AlOOH determines the main precipitate phases from pregnant liquor.Boehmite seeds,high temperature and ethanol addition can promote the boehmite precipitation and improve the mass fraction of boehmite in products.Co-precipitation changes the multimodal distribution of seeds to a normal and well distribution of products,and the particle size is more than several times that of seeds.

Intensifying gibbsite precipitation from sodium aluminate solution by adding a mixed seed

Gibbsite precipitation from sodium aluminate solution was intensified by adding mixed industrial and self-prepared active seeds,and its mechanism was researched preliminarily.The interfacial properties of seed/aluminate solution were determined for separate industrial and active seed.Contact angles of seed/aluminate solution and the specific surface area of seeds were respectively measured by sessile drop and BET method,and the morphology and particle size of precipitates were recorded by SEM and laser diffraction.The results show that,compared with the industrial seed,the active seed has a better wettability,lower interfacial tension,and larger specific surface area,being conducive to enhancing gibbsite precipitation from sodium aluminate solution.SEM analysis of the precipitates indicates that the embedment and accumulation/agglomeration of extremely fine particles on the surface of coarse industrial seed can effectively control the content of fine particles in the precipitation product.With extra 3.1–4.6 g/L active seed,the gibbsite precipitation ratio was increased by 3.23%–3.92%.Moreover,the mass percentage of particles<45μm in precipitation product has even a slight decrease compared with that for the traditional precipitation product or of the industrial seed itself.The result presented is favorable to developing an intensified gibbsite precipitation process for commercial alumina manufacture.

KINETICS OF THERMAL DECOMPOSITION OF SYNTHETIC GIBBSITE

The thermal decomposition of synthetic gibbsite has been investigated by means of DSC and X-raydiffraction methods.The mechanism of thermal decomposition and kinetic parameters were determined at tem-peratures ranging from room temperature to 400℃.

THE DIGESTION OF SYNTHETIC GIBBSITE

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The first discovery of aggegates of gibbsite and metahalloysite with unique morphology

A kind of worm-like aggregates, consisting of very tiny curving tubes, was first found in a residual kaolin deposit at mine 414, Jiangxi Province. The results of several mineralogical analyses show that the single curving tube is composed of gibbsite and metahalloysite minerals. This particular shape of mineral aggregates has not been reported before.

Petrological and Statistical Studies of the Limbiko Bauxite Deposit, Republic of Guinea

The increasing demand for Aluminium pushes the miners to multiply the research in order to answer this demand. The objective of this work is to characterize the bauxitic deposit of Limbiko. The methodology consists in collecting samples, and establishing stratigraphic logs, sections and geological maps. The analyses were carried out in the laboratory of the Company des Bauxites de Guinea. Statistical methods were used to process the geochemical data. The study area is characterized by sedimentary formations of the Devonian, in which the dolerites of the Mesozoic were injected. It is on these Devonian and Mesozoic formations that the Limbiko bauxite deposit was developed. The stratigraphic logs show the succession of formations. The parent rock is surmounted by an alteration crust. The petrographic study is based on the bauxitic and transitional zones characterized by ferruginous laterites. Both zones contain some ferriplantite. Mineralogical analysis shows the presence of gibbsite, goethite, alumogoethite, rutile and clay minerals. The geochemical study of the major elements shows that the more the contents of SiO2, Fe2O3 decrease, the more the content of Al2O3 increases and those of TiO2, Al2O3 increase in the same direction. The PCA confirms the mineralogical results by classifying the samples into clay, bauxite and ferrite.

Thermodynamics study on leaching process of gibbsitic bauxite by hydrochloric acid

For the low-grade gibbsitic bauxite,the leaching rate of alumina is very low during the Bayer process.The acid leaching method is attracting more attention,and the hydrochloric acid leaching was developed rapidly.The mineral composition and chemical composition were investigated by X-ray diffraction analysis and semi-quantitative analysis.The thermodynamics of leaching process was analyzed.The results show that the major minerals in the bauxite are gibbsite,secondly goethite and quartz,anatase and so on.The acid leaching reactions of the bauxite would be thermodynamically easy and completed.Under the conditions that ore granularity is less than-55 μm,the L/S ratio is 100：7,and the leaching temperature is 373-383 K,the leaching time is 120 min and the concentration of HCl is 10%,both the leaching rates of Al and Fe are over 95%.The main composition of leaching slag is SiO2 which is easy for comprehensive utilization.

Study on kinetics of thermal decomposition of low LOI goethetic hematite iron ore

In the present study, the kinetics of thermal decomposition of hydrated minerals associated in natural hematite iron ores has been investigated in a fixed bed system using isothermal methods of kinetic analysis. Hydrated minerals in these hematite iron ores are kaolinite, gibbsite and goethite, which contribute to the loss on ignition(LOI) during thermal decomposition. Experiments in fixed bed have been carried out at variable bed depth(16, 32, 48 and 64 mm),temperature(400-1200 ℃) and residence time(30,45, 60 and 75 min) for iron ore samples. It is observed that beyond a certain critical bed depth(16 mm), 100% removal of LOI is not found possible even at higher temperature and higher residence time. Most of the solid-state reactions of isothermal kinetic analysis have been used to analyze the reaction mechanism. The raw data are modified to yield fraction reacted "α" versus time and used for developing various forms of "α" functions.f(α) is the inverse of first derivative of g(α) with respect to α. The study demonstrates that decomposition of hydrated mineral in hematite follows the chemical kinetics.The estimated activation energy values in all the experimental situations are found to high, of the order of 60 kJ/mol, reinstating that the reactions are indeed controlled by moving phase boundary and random nucleation.

Mineral structure and crystal morphologies of high-iron hydrargillite

Various characterization methods, including scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller surface-area measurements, thermogravimetry–differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy, were used to study the mineral structure and surface characteristics of high-iron hydrargillite. Gibbsite, goethite, and hematite were found to be the main mineral components of hydrargillite, whereas the goethite and hematite were closely clad to the surface of the multilayer gibbsite crystals. Compared with the synthetic gibbsite, the hydrargillite contained more structural micropores generated by the mineral evolution during the mineralization process. The gibbsite in hydrargillite contained less crystal water compared with the synthetic gibbsite, and it was a typical polymorphic structure. The isomorphous substitution of Al and Fe was observed in goethite. The dissolution-controlling step of hydrargillite was the ionic diffusion speed because of the goethite and hematite that closely covered and encapsulated the gibbsite crystals.

Controls of Soluble Al in Experimental Acid Sulfate Conditions and Acid Sulfate Soils

The controls of soluble Al concentration were examined in three situations of acid sulfate conditions: 1)experimental acid sulfate conditions by addition of varying amounts of Al(OH)3 (gibbsite) into a sequenceof H2SO4 solutions; 2) experimental acid sulfate conditions by addition of the same sequence of H2SO4solutions into two non-acid sulfate soil samples with known amounts of acid oxalate extractable Al; and3) actual acid sulfate soil conditions. The experiment using gibbsite as an Al-bearing mineral showed thatincrease in the concentration of H2SO4 solution increased the soluble Al concentration, accompanied bya decrease in the solution pH. Increasing amount of gibbsite added to the H2SO4 solutions also increasedsoluble Al concentration, but resulted in an increase in solution pH. Within the H2SO4 concentration rangeof 0.0005～0.5 mol L-1 and the Al(OH)3 range of 0.01～0.5g (in 25 mL of H2SO4 solutions), the input ofH2SO4 had the major control on soluble Al concentration and pH. The availability of Al(OH)3, however, wasresponsible for the spread of the various sample points, with a tendency that the samples containing moregibbsite had a higher soluble Al concentration than those containing less gibbsite at equivalent pH levels.The experimental results from treatment of soil samples with H2SO4 solutions and the analytical results ofacid sulfate soils also showed the similar trend.

X-Ray Diffraction, Electron Paramagnetic Resonance and Optical Absorption Study of Bauxite

The bauxite mineral obtained from Araku, Vishakapatnam district of Andhra Pradesh, India is used in the present work. Structural characterization was performed by X-ray diffraction (XRD). The mineral was found to be gibbsite in phase. The transitional metal ions present were investigated using electron paramagnetic resonance (EPR) and optical absorption spectra. The EPR results suggest that Fe3+ has replaced Al3+ in the unit cell of bauxite. The optical absorption spectrum is due to Fe3+ which indicates that it is in distorted octahedral environment. The near-infrared (NIR) spectrum is due to water fundamentals and combination overtones, which confirm the formula of the compound. The impurities in the mineral are identified using spectroscopic techniques.

Preparation and Phase Transformation Behavior of Boehmite via Heat Treatment in Water Vapor

Boehmite was prepared under heat treatment in water vapour, and the phase transformation of gibbsite heat-treated at various temperatures was investigated. The sample was characterized by scanning electron microscopy(SEM), X-ray diffraction (XRD), thermogravimetry and differential thermalanalysis (TG-DTA), fourier transform infrared (FTIR),and BET surface area.Effect of temperature on preparation was studied in the range of 155°–195°.With the increase in temperature, transformation of gibbsite into crystalline boehmites took place as indicated by the X-ray diffraction (XRD). The shape of the grains in the prepared sample was cube-like morphology.In water vapour gibbsite transform into boehmite by a dissolution - precipitation mechanism.