Caustic Ratio Research Articles

Extraction and value-added utilization of alumina from coal fly ash via one-step hydrothermal process followed by carbonation

Coal fly ash (CFA), containing 25–50 wt% of Al2O3, is a typical and massive secondary aluminum resource. However, alumina recovery from the CFA by traditional processes is very difficult due to 40–60 wt% SiO2 content and low Al2O3/SiO2 mass ratio. In this study, a “one-step” hydrothermal process was proposed to extract Al2O3 from CFA; a mixture of lime and NaOH was used to enhance the dissolution and separation of Al/Si components. The results showed that 60.14% Al2O3 leaching could be obtained for treatment at 210 °C temperature for 3 h in the presence of 30 g/L NaOH and CaO/SiO2 molar ratio of 1.0. During the hydrothermal process, the Si- and Al-bearing components of CFA were sufficiently decomposed in lime and NaOH mixture; the dissolved silicate ions were then combined with calcium ions to form tobermorite, while the aluminate ions were concentrated in the liquor, thereby achieving selective leaching of alumina from CFA. Owing to the limited solubility, part of the aluminate ions was also converted into aluminiferous zeolite, hence lowering the alumina leaching ratio. The lixivium was of low caustic ratio that was favorable for the preparation of high-value pseudo-boehmite via carbonation process at 80–90 °C with the ultimate pH of 9.5–10.0.

Formation behavior of tricalcium aluminate hexahydrate in synthetic sodium aluminate solution with high alkali concentration and caustic ratio

The formation kinetics and mechanism of tricalcium aluminate hexahydrate (TCA) in synthetic sodium aluminate solution with high alkali concentration (caustic Na2O from 235 to 295 g·L−1) and caustic ratio (from 3 to 7) were studied by XRD, FTIR, SEM-EDS and PSD methods. Over high caustic alkali concentration, caustic ratio, lime addition and reaction temperature inhibit the formation of TCA in high-alkali aluminate solution. The optimum synthesis conditions of TCA were established, and the corresponding formation efficiency of TCA and the caustic ratio of solution after reaction are 77% and 21 respectively. The formation process of TCA is controlled by the internal diffusion, and the apparent activation energy is 6.03 kJ·mol−1. The formation kinetic equation of TCA is: 1 − 2α/3 − (1 − α)2/3 = 1.11 × 10−3 × e−6030/RT×t. The synthesis product at equilibrium is mainly comprised of TCA as well as minor Ca(OH)2 and CaCO3, and the morphology of TCA is agglomerates of cube particles, which is quite different from those in low and medium alkali solution.

Alkaline digestion behavior and alumina extraction from sodium aluminosilicate generated in pyrometallurgical process

In pyrometallurgical process, Al- and Si-bearing minerals in iron and aluminum ores are easily transformed into sodium aluminosilicates in the presence of Na2O constituents, which alters the leaching behaviors of Al2O3 and SiO2. It was confirmed that sodium aluminosilicates with different phase compositions synthesized at various roasting conditions were effectively digested in the alkaline digestion process. Under the optimum conditions at temperature of 100–120°C, liquid-to-solid ratio (L/S) of 10:2 mL/g, caustic ratio of 4, and Na2O concentration of 240 g/L, the actual and relative digestion ratio of Al2O3 from the synthesized sodium aluminosilicates reached maximums of about 65% and 95%, respectively, while SiO2 was barely leached out. To validate the superior digestion property of sodium aluminosilicate generated via an actual process, the Bayer digestion of an Al2O3-rich material derived from reductive roasting of bauxite and comprising Na1.75Al1.75Si0.25O4 was conducted; the relative digestion ratio of Al2O3 attained 90% at 200°C.

ANFIS-Based Prediction of the Decomposition of Sodium Aluminate Solutions in the Bayer Process

This article presents the results of nonlinear statistical modeling of the decomposition process of sodium aluminate solution, as part of the Bayer technology for the production of alumina. Based on the data collected in 2011 and 2012 from industrial production in the Birač Alumina Factory, Zvornik (Bosnia and Herzegovina), nonlinear statistical modeling of the industrial processes was derived. The model was developed as an attempt to define the dependence of the degree of decomposition of sodium aluminate solution as a function of the input parameters of the leaching process: caustic ratio (αk) of the solution; ratio of the crystallization; content of Na2O(caustic) in the solution; the initial temperature of the solution; the final temperature of the solution; average diameter of the crystallized seeds; and duration of the crystallization process. As a tool for statistical modeling, Adaptive Network Based Fuzzy Inference System (ANFIS) was applied. The defined model using ANFIS methodology expressed a high level of fitting, and could be used to effectively predict the degree of decomposition of the sodium aluminate solution as a function of the input process under industrial conditions.

Recovery of aluminium and lithium from gypsum residue obtained in the process of lithium extraction from lepidolite

Lepidolite has an Al2O3 content of greater than 22%. Using it efficiently will be very important for the economic extraction of lithium from this low-grade lithium ore if an industrial-scale extraction process is to be developed. This paper presents a novel technique for recovering aluminium and lithium from the gypsum residue generated during the extraction of lithium from lepidolite. The alkali digestion conditions were considered based on the principles behind the Bayer process. Maximum extraction of aluminium (100%) and 96.4% extraction of lithium were recorded at caustic ratio (Na2O/Al2O3 molar ratio) of 1.7, digestion temperature of 136°C and digestion time of 60min. After digestion, aluminium hydroxide was precipitated by cooling and seeding of the digestion solution. Li+ ion was not adsorbed by aluminium hydroxide during this process. Therefore, Li could easily be recovered by adding sodium phosphate (Na3PO4). In addition, the precipitation of lithium at several different molar ratios of Li+/PO43− was performed to investigate its influence on the recovery process.

Separation of NaOH and NaAl(OH)4 in alumina alkaline solution through diffusion dialysis and electrodialysis

Alkaline solution containing NaOH and NaAl(OH)4 is produced largely during extracting aluminum from aluminum ore. Conventionally alkali and alumina-bearing components in the solution are separated by purification, filtration and precipitation, so that gibbsite precipitation is obtained and the alkaline liquor can be returned to the circuit. The feasibility of membrane separation, including diffusion dialysis (DD) and electrodialysis (ED), is investigated by batch DD and ED running with total 6.15×10–4m2 and 2.865×10–3m2 membranes, respectively. The DD running shows that the dialysis coefficient of OH− (UOH−) is at a low level (0.0028–0.0031m/h), mainly because of the hydrophobic nature of the membrane matrix and absence of a pressure difference or an electrical field as the driving force. The OH− recovery ratio is 6.3%, the Al(OH)4− leakage ratio is 0.6%, and the caustic ratio (αk) can reach up to 17.8 after 4h. As for ED running, the OH− recovery ratio is substantially enhanced, while the Al(OH)4− leakage and αk value can still meet the industrial requirements. The OH− recovery ratio, Al(OH)4− leakage ratio and αk are 48.0%, 12.6% and 7.9, respectively, when OH− concentration is ~1.55mol/L in the feed chamber, and the ED current density is 350.0mA/cm2. The current efficiency is 62.5% and the energy consumption is 12.43kWh/kg, relatively high because of use of a 0.01m plexiglas spacer between two neighboring membranes, the increase of the solution temperature which is caused by the high current density. Overall, the ED process is a more effective approach to treat the alumina alkaline solution. Much higher alkali concentration and recovery ratio can be achieved in a short time because of the higher permeability, leading to higher treatment capacity for ED device. The DD process, on the other hand, has advantages of low energy consumption, low membrane fouling and environmental friendliness.

Recovery of alumina from circulating fluidized bed combustion Al-rich fly ash using mild hydrochemical process

To utilize CFBC Al-rich fly ash, a mild hydrochemical extraction process was investigated for recovery of alumina. An alumina extraction efficiency of 92.31% was attained using a 45% NaOH solution, an original caustic ratio (molar ratio of Na2O to Al2O3 in the sodium aluminate solution) of 25, a molar ratio of CaO to SiO2 in the fly ash of 1.1, a liquid volume to solid mass ratio of 9, a reaction temperature of 280 °C, and a residence time of 1 h when treating fly ash with an alumina to silica mass ratio (A/S) of 0.78 and an alumina content of 32.43%. Additionally, the alumina leaching mechanism was explored via structural and chemical analysis, which revealed that after alkaline digestion, the main solid phase containing silica was NaCaHSiO4 with a theoretical A/S of zero.

Effect of urea on decomposition of sodium aluminate solution

Aluminium hydroxide particles were synthesized from sodium aluminate liquor by urea decomposition method at elevated temperatures in presence and absence of seed. Different parameters such as alumina to caustic (A:C) ratio, temperature, time period of precipitation, urea concentration, etc. were studied. Characterisations of the products were also carried out using X-ray diffraction, scanning electron microscopy, TG-DTA, etc. In general, higher precipitation ratios were observed with the addition of urea in sodium aluminate solution. Temperature was found to have a significant role in precipitating aluminium hydroxide with higher yield enhancement ratio compared to that obtained at lower temperatures. Higher precipitation ratios were also obtained with urea at elevated temperatures under available supersaturation. In the presence of urea, the precipitation ratios on yield enhancement front were observed to increase with decrease in A:C ratio. It has been observed that the liquor concentration reached below the equilibrium solubility limit after 30h of precipitation when 460mmol/L urea is added. The synthesized aluminium hydroxide particles showed gibbsitic nature and globular morphology, with agglomerates of mostly hexagonal platelets, as revealed through XRD and SEM studies, respectively. Calcination of the product at 1000°C results in weight loss similar to that observed with gibbsite. Endothermic peaks at 325°C and 550°C revealed through DTA study indicated boehmite and chi-alumina transformations, respectively. The possible mechanism of urea decomposition method is also discussed.

Relationship between Al(OH)3 solubility and particle size in synthetic Bayer liquors

Surface tension of sodium aluminate solution and the contact angle between Al(OH)3 particles and aluminate solution were measured, then the dependence of Al(OH)3 solubility on its particle size was calculated and thus the variation of the critical nucleus sizes was determined based on the Ostwald ripening formula. The results show that the Al(OH)3 solubility in sodium aluminate solution decreases with the increment of particle size, and the critical nucleus sizes increase with the rise of alkali concentration, caustic ratio and precipitation temperature. The results also imply that the presence of small particles in seeded precipitation system is an important factor to limit the depth of precipitation.

Recovery of alumina and alkali in Bayer red mud by the formation of andradite-grossular hydrogarnet in hydrothermal process

Bayer red mud (RM) is an alumina refinery waste product rich in aluminum oxides and alkalis which are present primarily in the form of sodium hydro-aluminosilicate desilication product (DSP). A hydrothermal process was employed to recover alumina and alkali from “Fe-rich” and “Fe-lean” RM, the two representative species of RM produced in China. The hydrothermal process objective phase is andradite-grossular hydrogarnet characterized by the isomorphic substitution of Al and Fe. Batch experiments were used to evaluate the main factors influencing the recovery process, namely reaction temperature, caustic ratio (molar ratio of Na 2O to Al 2O 3 in sodium solution), sodium concentration and residence time. The results revealed that the Na 2O content of 0.5 wt% and A/S of 0.3 (mass ratio of Al 2O 3 to SiO 2) in leached residue could be achieved with Fe-rich RM under optimal conditions. However, the hydrothermal treatment of Fe-lean RM proved less successful unless the reaction system was enriched with iron. Subsequent experiments examined the effects of the ferric compound's content and type on the substitution ratio.

Modelling the process of Al(OH)3 crystallization from industrial sodium aluminate solutions using artificial neural networks

This paper presents an attempt to define the non-linear correlation dependence between the degree of decomposition of the aluminate solution, the average diameter of the crystallized gibbsite, the total Na2O content in the obtained alumina and the specific utilization level of the process on the one hand and important input parameters of the process on the other. As input parameters having an influence on the process, the concentration of Na2O (caustic), the caustic ratio and the crystallization ratio, the starting and final temperature of the process, the average diameter of the crystallization seed and the duration of the decomposition process were considered. As the result of measurements of these process parameters and the acquisition of the resulting output parameters of the process, a database with 500 data lines was obtained. To define the correlation dependence, with the aim of predicting the process parameters of the decomposition process of the sodium aluminate solution, the artificial neural network (ANN) methodology was applied.

Statistical Modeling of the Industrial Sodium Aluminate Solutions Decomposition Process

This article presents the results of the statistical modeling of industrial sodium aluminate solution decomposition as part of the Bayer alumina production process. The aim of this study was to define the correlation dependence of degree of the aluminate solution decomposition on the following parameters of technological processes: concentration of the Na2O (caustic), caustic ratio and crystallization ratio, starting temperature, final temperature, average diameter of crystallization seed, and duration of decomposition process. Multiple linear regression analysis (MLRA) and artificial neural networks (ANNs) were used as the tools for the mathematical analysis of the indicated problem. On the one hand, the attempt of process modeling, using MLRA, resulted in a linear model whose correlation coefficient was equal to R2 = 0.731. On the other hand, ANNs enabled, to some extent, better process modeling, with a correlation coefficient equal to R2 = 0.895. Both models obtained using MLRA and ANNs can be used for the efficient prediction of the degree of sodium aluminate solution decomposition, as the function of the input parameters, under industrial conditions of the Bayer alumina production process.

Alumina recovery from spent Bayer liquor by methanol

Decomposition of sodium aluminate solution is the key step for alumina production; however, approximately only 55% alumina yield is obtained by seeded precipitation. So, it is difficult to obtain sodium aluminate solution with high caustic ratio. In this study, a method was explored to recover alumina from spent Bayer liquor by deep decomposition with methanol. A variety of conditions, including reaction temperature, reaction time, methanol amount and seed coefficient were elaborately investigated. The results showed that the appropriate conditions were 1:1 in volume ratio of methanol to spent Bayer liquor, more than 1.0 seed coefficient and in a 40 °C water bath for 24 h. By characterizing through XRD, the crystal products were found to be Al(OH) 3. With this method, the molar ratio of Na 2O to Al 2O 3 of the spent liquor can be increased from about 3.0 to 10.0 due to the recovery of alumina, which is beneficial for the treatment of red mud.

Alkali desilicated coal fly ash as substitute of bauxite in lime-soda sintering process for aluminum production

By desilication treatment, the Al 2O 3/SiO 2 molar ratio of coal fly ash could be improved to the range of 1.63–2.0. The desilicated coal fly ash (DSCFA) was enriched in alumina extraction. A processing technology was developed for alumina extraction from the DSCFA with the lime sintering process. Ca/(SiO 2+TiO 2) molar ratio, and NaO/Al 2O 3 molar ratio, sintering time, and temperature were the most significant parameters impacting on the aluminum extraction efficiency. The optima aluminum extraction efficiency was obtained under conditions of Ca/(SiO 2+TiO 2) molar ratio of 2.0, NaO/Al 2O 3 molar ratio of 0.98, and sintering at 1 200 °C for 60 min. A standard industrial dissolution method was used under conditions of caustic ratio ( α k= n(NaO)/ n(Al 2O 3) of 2.0, Al 2O 3 concentration of 50 g/L, sodium hydroxide concentration( N k) of 60.78 g/L, Na 2CO 3 concentration of 10 g/L, temperature of 85 °C, and dissolution duration of 10 min. The final aluminum extraction efficiency was 90%.

Preparation of Al(OH) 3 by ion membrane electrolysis and precipitation of sodium aluminate solution with seeds

The preparation of Al(OH) 3 by the ion exchange membrane electrolysis followed by the precipitation of sodium aluminate solution with seeds was made. During the process of ion membrane electrolysis, the sodium aluminate solution is rapidly acidified and the caustic ratio ( a K) is decreased due to oxygen evolution in the anodic region. And the causticity of solution is increased due to hydrogen evolution in the cathode region, producing the high concentration of caustic soda solution. Regulating the acidity of the anodic solution by controlling the electric quantity in the electrolysis and subsequent decomposing the solution, Al(OH) 3 could yield with very large rate and high efficiency. The experiments also indicate that the quality of aluminum hydroxide product is greatly affected by the impurity silicon.

Spectra of sodium aluminate solutions

The UV spectra of sodium aluminate solutions were obtained in the sodium oxide concentration range from 59 to 409 g/L and the caustic ratio range from 1.5 to 4.0 to reveal the structure characteristics of them, It is found that a new peak appears at about 370 nm besides peaks at about 220 and 266 nm in all solutions. The new peak is strongly favored by high hydroxide concentration and high caustic ratio. And it only appears when the solutions are prepared by dissolving sodium hydroxide and aluminum hydroxide. In addition, the IR and Raman spectra of sodium aluminate solutions with high alkali concentration and high caustic ratio were measured, and the UV spectra of aqueous solutions of Al(H 2O) 3+ 6 and AlF 3- 6 were measured as well. According to the crystal field theory in coordination chemistry as well as the above spectra characteristics, this new peak at about 370 nm is determined as the evidence of a new species of aluminate ion with a coordination number of 6.

Phenomena in late period of seeded precipitation of sodium aluminate solution

Aiming at seeded precipitation of aluminate solution with high caustic ratio(α k>2.4), corresponding to the late period of seeded precipitation, the influence of different types of seed on precipitation ratio was explained with respect to solution structure in the interface of seed and the evolution of Al(OH) 3 growth units in this layer. The effects of solid content and seed size on agglomeration were determined by calculating the particle number of product. The results imply that the solution structure in the interface of seed imposes a notable significance on the process in the late period of seeded precipitation. Agglomeration still exists in this period. However, the agglomeration bodies break in the case of prolonging precipitation due to the mechanical effect, which results in the increase of particle number.

A novel synthesis of high‐area alumina via H 2 O 2 ‐precipitated boehmite from sodium aluminate solutions

A novel procedure was devised, whereby crystalline boehmite, γ-AlO(OH), was efficiently precipitated from sodium aluminate solutions, NaAlO 2 (aq), following controlled addition of H 2 O 2 aqueous solution. The effects of the aluminate solution variables (viz. concentration, volume, caustic ratio, pH and temperature), as well as the H 2 O 2 /Al 2 O 3 molar ratio, on the amount and composition of the yield, were examined. Accordingly, optimal conditions for 98% recovery of the aluminium content of the parent aluminate solution were realized. The remaining NaOH can be recycled and used to extract further amounts of aluminium from bauxite (Bayer process). Thermal stability of the resulting boehmite was explored by thermogravimetry and differential thermal analysis. Accordingly, various calcination products of the boehmite were obtained at some selected temperatures in the range 400-1100°C and characterized by chemical analysis, X-ray powder diffractometry, and surface charge and specific area measurements. The results obtained indicated the synthesis of relatively high surface area transitional aluminas of industrial and catalytic merits.

Filtration characteristics of aluminium hydroxide mixtures (alumina hydrates)

AbstractThe specific cake resistance of the alumina hydrates obtained from sodium aluminate solutions was investigated against the caustic ratio, ageing and filtration pressure using a laboratory test filter at constant pressure filtration conditions. The alumina hydrate product was produced from sodium aluminate solutions by seeding the aluminium hydroxide crystals of definite size distribution at constant solution temperature.The specific cake resistance was found to increase with the caustic ratio and filtration pressure and to decrease with the ageing of alumina hydrates in aluminate solutions.

Electrochemical method for the solubility and dissolution of solid compounds. Some thermodynamic properties of the system Al(OH)3-NaOH-H2O

An electrochemical method is described enabling the investigation of dissolution processes and of solubility of solid substances. It is based on a galvanic cell system consisting of an electrode of the second kind and an electrode of the third kind in only one electrolyte solution.We have investigated by this method the dissolution of Al(OH)3 (hydrargillite) in NaOH solution. From the experimental results we conclude that polymerization of the Al(OH)−4 anions does not occur in the course of the dissolution. The temperature dependence of both the apparent and thermodynamic equilibrium constants and the standard free energy change of the equilibrium process are calculated. A relation is established between the precipitation efficiency, the apparent equilibrium constant and the “caustic ratio.” This enables us to determine the dependence of the theoretical precipitation efficiency on the NaOH concentration, on the mole ratio of the supersaturated solutions used for precipitation, and on the end temperature of the precipitation.