Alumina Extraction Efficiency Research Articles

Preparation of high-purity crystalline aluminum chloride based on aluminum separation from circulating fluidized bed fly ash

Improving the purity of aluminum products can significantly enhance the comprehensive utilization value of fly ash. In the present paper, a novel process of preparing high-purity aluminum chloride of 99.99% from the aluminum-rich circulating fluidized bed fly ash by acid leaching-recrystallization method was developed, and the leaching kinetics and mechanism were further investigated. The alumina extraction efficiency was 87.9% and the filtration rate of the leached slurry reached 2.5 m3·m−2·h−1 under the optimal conditions of 150 °C, 2 h, hydrochloric acid with a concentration of 20% and the liquid-to-solid ratio of 5. The alumina leaching process followed the shrinking unreacted core model, and the process was controlled by a chemical reaction with an activation energy of 34.90 kJ/mol. Finally, high-purity aluminum chloride was prepared by introducing HCl gas into the leaching solution during three cycles of recrystallization.

Kinetics of alumina extraction from coal gangue by hydrochloric acid leaching

The leaching kinetics of alumina extraction from coal gangue with hydrochloric acid was investigated. The effects of calcination activation temperature and leaching conditions on the alumina extraction efficiency were studied. The results revealed that the alumina extraction efficiency can reach 91.56% under the following optimum conditions: a calcination temperature of 700 °C, dissolution at 150 °C for 2 h in hydrochloric acid with a concentration of 20%, and a liquid-to-solid ratio of 3:1. Notably, lithium was leached simultaneously with alumina, and its leaching rate reached 80.00 %. The leaching of alumina followed the shrinking unreacted core model, and the process was controlled by a chemical reaction, with activation energy of 45.64 kJ/mol.

Two-stage process for the safe utilization of secondary aluminum dross in combination with the Bayer process

Secondary aluminum dross (SAD) is a hazardous waste produced from Al electrolysis, casting, and regeneration. A two-stage process for the safe utilization of SAD was proposed by combining with the Bayer process liquor. In the first stage, approximately 32%, 100%, and 27% of Al, Cl, and F, respectively, from the reactive substances were dissolved into the saturated sodium aluminate solution. A kinetic synergy with a considerable amount of SAD at high temperatures led to an uncontainable reaction, and the frequent addition of SAD at a low dosage was determined for the separation and utilization of H2 and NH3. The inert alumina-bearing compounds and CaF2, which accounted for 50% of the mass of SAD, remained in the residue after the first stage. Efficient conversion of the inert alumina-bearing compounds and elimination of impurities from the Bayer process were both achieved by sintering at 1000 °C for 1 h in the second stage. An alumina extraction efficiency of 99% was obtained at 85 °C after the as-prepared sinter was leached into the saturated sodium aluminate solution. The saturated sodium aluminate solution obtained in the two-stage process was incorporated into the Bayer liquor to produce alumina. Furthermore, cl− and F− were removed by forming sodalite in the desilication process and precipitating CaF2 in the dilute sodium aluminate solution, respectively. Using this novel approach, an estimated 30,000 t/a of SAD has been processed in an alumina refinery in China since 2019. These results provide a novel short process for the safe and efficient utilization of SAD

Removal of Aluminosilicate Bond and Process Optimization on Recovery of Sol Gel Alumina Abrasive Grain from Abrasive Industry Waste

Sol-gel alumina abrasive grains are used in high cutting speed and high material removal process. The need of sol gel alumina abrasive grain is huge. So, the waste generated from the grinding process is also high. In grinding operation, flange portion of the grinding wheel is unused and dumped as waste. Recovery of abrasive grains from this unspent portion adds wealth. This study deals with recovery and re-use of sol gel alumina abrasive grains. Chemical method is opted to recover the sol gel abrasive grains. Based on previous studies, the process parameter such as solid liquid ratio, lump size, immersion time and stirring speed were selected as the control factors and extraction efficiency of sol gel alumina abrasive is considered as response parameter. The experimental results show that major influencing control factor sequence were lump size> solid liquid ratio > stirring speed and > immersion time. The optimal combination for the maximum sol gel alumina grain recovery will be smaller lump size, maximum solid liquid ratio, maximum stirring speed and intermediate immersion time. Apart from that, the surface features of the recovered abrasive grains are studied by scanning electron microscopy. The elements during chemical washing process are observed using energy-dispersive spectroscopy (EDS). Recovered grains from this process can also be reused in the grinding wheel production. This is the feasible and economical method for recovering the sol gel alumina grains from the spent grinding wheel waste.

Alumina Extraction from Coal Fly Ash via Low-Temperature Potassium Bisulfate Calcination

Owing to the depletion of bauxite and increasing demand for alumina, calcination methods for extracting alumina from coal fly ash (CFA) were developed. However, these methods have disadvantages such as the need for high temperatures and the emission of toxic gases. Hence, in this study, Al2O3 was extracted from CFA via low-temperature potassium bisulfate calcination technology. Effects of the potassium bisulfate amount, calcination temperature, and calcination time on the alumina extraction efficiency were investigated using X-ray diffraction, thermal gravimetry, scanning electron microscopy, differential scanning calorimetry, and energy-dispersive spectroscopy. It was found that this technique could recover alumina efficiently, and potassium bisulfate significantly contributed to the degradation of mullite and corundum phases. Al2O3 in CFA was converted into soluble K3Al(SO4)3. With a KHSO4/Al2O3 molar ratio of 7:1, calcining temperature of 230 °C, and calcining time of 3 h, the alumina extraction efficiency reached a maximum of 92.8%. The Avrami–Erofeev equation showed the best fit with the kinetic data for the low-temperature calcination of CFA with KHSO4. The activation energy was 28.36 kJ/mol.

Mineral transformation in treating low-grade bauxite using the calcification–carbonization process and preparing cement clinker with the obtained residue

During the Bayer process for alumina production, the molecular formula of the bauxite residue is approximately equivalent to Na2O·Al2O3·1.7SiO2·nH2O. This means that 1 kg of SiO2 in bauxite will produce 1 kg of Al2O3 and 0.608 kg of Na2O in bauxite residue. Thus, the extraction efficiency of alumina is low when processing low-grade bauxite. In addition, it is difficult to utilize bauxite tailings with solid alkali in the structure. In this study, low-grade bauxite was treated using a calcification–carbonization process. A new type of residue (C-C residue) with a different structure from that of traditional bauxite residue was obtained. A cement clinker was prepared with C-C residue as the main ingredient at a dosage of 50%. The experimental results show the alumina extraction efficiency reached 75.82% when processing low-grade bauxite of A/S 3.06 using the calcification–carbonization process. This is higher than the 61.44% of the Bayer process. The main phases of C-C residue [CaCO3, amorphous Ca(4−2x)SixO4, and undecomposed Ca3Al2(SiO4)(OH)4] convert into Ca2SiO4, Ca3SiO5, and Ca2FexAl(2−x)O5 after calcination. The calcification–carbonization process combined with cement production helped low-grade bauxite achieve highly efficient and clean alumina production, which is conducive to the sustainable development of alumina industry.

Kinetics of extracting alumina by leaching coal fly ash with ammonium hydrogen sulfate solution

A newly developed technology for extracting alumina from coal fly ash with ammonium hydrogen sulfate was studied in this paper. Kinetics of extracting alumina was determined using a reduced timescale method. Coal fly ash and leaching residue were characterized by X-ray diffraction and field emission scanning electron microscopy. The results show that the extraction efficiency of alumina increases with the increase of leaching temperature and time. Compared with 240 min of leaching time, the extraction efficiency of alumina at 180 °C decreases by 1.4% for 300 min. Aluminum ions, sulfate and ammonium ions dissolve in solution and interact with water to form ammonioalunite (NH4Al3(SO4)2(OH)6) precipitate, which is the key factor causing the decrease of extraction efficiency of alumina. The apparent activation energy is 40.95 kJ/mol. The chemical reaction is a rate-controlling step for extracting alumina from coal fly ash with ammonium hydrogen sulfate solution.

Formation mechanism of an undesirable by-product in the mild hydro-chemical process for the extraction of alumina from fly ash and its mitigation

The mild hydro-chemical process has been commercially demonstrated as an effective method to extract alumina from high alumina fly ash. This study investigated the formation mechanism of an undesirable aluminum-containing by-product, hydrated sodium calcium aluminosilicate (HSCA, 1.2Na2O·0.8CaO·Al2O3·2SiO2·H2O), produced during the mild hydro-chemical process and its mitigation strategy. The effects of reaction temperature, residence time and mole ratio of calcium to silicon on the alumina extraction and phase transformation of the leaching residues have been investigated, confirming the formation of HSCA phase being responsible for the decrease of alumina extraction efficiency. At the low Ca(OH)2 addition, the HSCA phase could be easily transformed from Na8Al6Si6O24(OH)2(H2O)2 combining with Ca(OH)2. On the other hand, when the addition of Ca(OH)2 was abundant, the existence of the NaCaHSiO4 product layer would insulate most of Ca(OH)2 from the Na8Al6Si6O24(OH)2(H2O)2 causing formation of HSCA. Based on the reaction mechanism and coating effect analysis, the mitigation strategy was also proposed. By increasing stirring speed to break the NaCaHSiO4 coating layer, HSCA phase could be completely avoided and a qualified alumina extraction efficiency of 90.84% could be achieved.

Kinetics of decomposition of mullite and corundum in coal fly ash under highly alkaline condition

Abstract Decomposition kinetics of mullite and corundum in coal fly ash with highly alkaline solution was studied. The effects of the reaction temperature and reaction time on decomposition rates of mullite and corundum and alumina extraction efficiency were investigated. The results show that increasing reaction temperature and reaction time increases the decomposition rates of mullite and corundum and alumina extraction efficiency, with the decomposition temperature of mullite lower than that of corundum. After 90 min reaction at 220 °C, more than 100 g alumina was extracted when recycling 1 L of alkaline solution. The decomposition processes of mullite and corundum corresponded with the shrinking unreacted core model, and the reaction rate was under chemical reaction control, with the activation energies of mullite and corundum being 67.46 and 161.82 kJ/mol, respectively.

Improved catalyst recovery combined with extracting alumina from Na2CO3-catalyzed gasification ash of a high-aluminium coal char

The catalyst recovery from Na2CO3-catalyzed steam gasification ash of Sunjiahao (SJH) high-aluminium coal char was investigated using water leaching method combined with extracting alumina by means of Ca(OH)2 thermal activation. The recovery efficiency of sodium catalyst from Na2CO3-catalyzed gasification ash (NCGA) with single water leaching method was as low as 39.5 wt%, which resulted from the formation of water insoluble sodium aluminosilicate during Na2CO3-catalyzed gasification. In order to improve sodium recovery, the water-washed NCGA was activated by means of Ca(OH)2 thermal activation for simultaneous recovery of sodium and alumina. The recovery efficiency of sodium from the water insoluble sodium aluminosilicate is 63.6 wt%, thus the total sodium recovery can be remarkably improved and reach 72 wt%. Meanwhile, the extraction efficiency of alumina from NCGA can reach 88.46 wt%. The low sodium recovery with Ca(OH)2 thermal activation is attributed to the formation of Na2CaSiO4 during Ca(OH)2 calcination. Extracting alumina process from NCGA of SJH high-aluminium coal can effectively promote the recovery of sodium catalyst and the process economy.

A promising green process for synthesis of high purity activated-alumina nanopowder from secondary aluminum dross

Abstract More than one million tons of secondary aluminum dross as a hazardous waste are produced by aluminum manufacturers per year throughout the world, which imposes high disposal costs and serious environmental problems. As secondary aluminum dross is considered as an alumina-rich inorganic waste, this study aims to propose a promising approach for clean production of activated-alumina as a widely-used adsorbent, catalyst and catalyst support. In this paper, synthesis and characterization of high crystalline activated-alumina nanopowder from secondary aluminum dross using a novel five-step leaching-based process are presented. To optimize the process, various parameters of dross particle size, leaching time, leaching temperature, acid concentration, and liquid-to-solid ratio were studied using One-Factor-At-a Time approach (OFAT). For the following optimum leaching conditions: particle sizes between 38 and 75 μm, leaching time of 120 min, leaching temperature of 85 °C, acid concentration of 5 M, and the liquid-to-solid ratio of 20 ml/g, the extraction efficiency of alumina was around 83%. The leach liquor was purified by adding 2 N NaOH resulting in sodium aluminate-rich solution. The purified liquor was treated by adding HCl to precipitate Al(OH)3, and the Al(OH)3 was then washed and calcined at 700 °C for 2 h to obtain alumina. Chemical composition, mineralogical phases, chemical bonds, morphology, and particle size distribution of the as-synthesized activated-alumina were determined using X-ray fluorescence (XRF), X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), and dynamic light scattering (DLS), respectively. The as-synthesized activated-alumina exhibited 97.61% purity. Its XRD pattern showed mainly characteristic peaks of gamma phase of alumina. Morphological study by FESEM revealed that the as-synthesized alumina particles have rounded-corner shape. The crystal and average particle size values were about 18.47 and 513 nm, respectively.

Combined treatment of red mud and coal fly ash by a hydro-chemical process

Red mud (RM) is a solid waste produced in the process of alumina extraction from bauxite and causes serious environmental problem. The recycle of alumina and alkali from RM by hydro-chemical process is believed to be an effective method for the utilization of RM. However, this industrial application has been limited for its difficulty in the solid-liquid separation of leached slurry and the low alumina extraction efficiency. In this paper, a novel process for the combined treatment of RM and coal fly ash (CFA) was proposed, and the effect of the mass ratio of CFA to RM, the Fe2O3 content and A/S (mass ratio of Al2O3 to SiO2) of the CFA on the extraction alumina efficiency and the filtration rate of leached slurry were investigated. When the CFA with A/S of 1.35 was used and the mass ratio CFA to RM was 1:1, the alumina extraction efficiency can be elevated from 72.14% to 91.70%. Especially, the filtration rate of leached slurry increased from 0.308m3/(m2·h) to 1.264m3/(m2·h). In addition, by decreasing the Fe2O3 content and increasing the A/S of CFA, the filtration rate of leached slurry could be enhanced obviously and the alumina in the mixture of RM and CFA could be extracted effectively. The results of phases and morphology of the residues showed that the increase of alumina extraction efficiency is attributed to two chemical processes, one is the complete transformation from Na8(Al6Si6O24)(OH)2.04(H2O)2.66 to NaCaHSiO4, and the other is the reduction of the generation of Ca3(Fe0.87Al0.13)2(SiO4)1.65(OH)5.4 with the increase of mass ratio CFA to RM. Furthermore, the growing number and length-diameter ratio of the rod-like structure of NaCaHSiO4 particles resulted in the increase in the filtration rate of leached slurry.

Mineral phase transition of desilicated high alumina fly ash with alumina extraction in mixed alkali solution

High alumina fly ash (HAFA), a main industrial solid waste produced in increasing amounts in western China, is a type of alumina-rich resource that can replace bauxite in the preparation of alumina. A new process of alumina extraction from HAFA is proposed in this paper. This method involved the raw HAFA being treated first by a low concentration alkali solution to form desilicated HAFA and then alumina was effectively extracted from desilicated HAFA in mixed alkali solution (NaOH+Ca(OH)2). The effects of the reaction conditions on alumina leaching from desilicated HAFA were investigated. The extraction efficiency of alumina could reach 92.0% under the following optimal conditions: alkali to ash ratio of 6, calcium to silicate ratio of 1.0, and reaction temperature of 280°C. After alumina extracted from desilicated HAFA, the solid residues were collected and characterized to better understand the mechanism of mineral phase transition of desilicated HAFA with alumina extraction in mixed alkali solution. By changing the calcium to silica ratio or alkali to ash ratio, the mineral phases could be transformed from 1.2Na2O·0.8CaO·Al2O3·2SiO2·H2O to NaCaHSiO4. A relatively high alkali to ash ratio or calcium to silicate ratio was preferred for the extraction of alumina from desilicated HAFA.

Study of extracting alumina from high-alumina PC fly ash by a hydro-chemical process

Alumina extraction from high-alumina pulverized coal (PC) fly ash by a mild hydro-chemical process characterized by a high alkaline sodium aluminate solution was studied. A high alumina extraction efficiency of 96.03% has been achieved. The effects of temperature and reaction time on alumina extraction ratio, the phase transformation and the morphology change of the extraction residues have been investigated. The alumina extracting process involves two steps: transformation from mullite and corundum to an intermediate product aluminasilicate, Na8Al6Si6O24(OH)2(H2O)2, and then from the aluminasilicate to calcium sodium hydrate silicate (NaCaHSiO4). The reaction temperature has been found to be critical for the alumina extraction and especially for the decomposition of the intermediate product Na8Al6Si6O24(OH)2(H2O)2 at high temperatures (>250°C). With the aid of a specially designed rapid real-time sampling device, the kinetics of the decomposition of Na8Al6Si6O24(OH)2(H2O)2 at high temperatures has also been studied and fitted by the Avrami–Erofeev equation. Furthermore, some engineering suggestions about the reactors design and prevention of pipe scaling have been proposed based on the kinetic results.

A novel process for synthesis of tobermorite fiber from high-alumina fly ash

A new two-step process was developed for the synthesis of tobermorite fiber from high alumina fly ash. The results reveal that high alumina extraction efficiency from the fly ash can be achieved in the first step and tobermorite fiber with excellent properties can be produced from the dealumination slag in the second step. FETEM images show that typical single crystal fibers with a length of 5–10 μm and a length-to-diameter ratio of 50–100 have been synthesized successfully. The strength tests after compression molding demonstrate that the compressive and flexural strengths of the products are greater than 0.34 MPa and 0.85 MPa, respectively, with a low bulk density of 218 kg/m3 and a thermal conductivity of 0.059 W/(m·k). It fully complies with China's national standards for calcium silicate insulation materials. Therefore, the fibers production is very promising to be used in the building external wall thermal insulation application.

Reduction of alkalinity in bauxite residue during Bayer digestion in high-ferrite diasporic bauxite

Bauxite residue is a hazardous solid waste generated during the Bayer process of extracting alumina from bauxite ore, the high alkalinity of which limits its large-scale industrial applications. In order to reduce the alkalinity of bauxite residue, rich lime additions from 0 to 30wt.% and high digestion temperatures from 250°C to 300°C were performed on a high-ferrite diasporic bauxite during the Bayer digestion. The alumina extraction efficiency from bauxite and the Na2O to SiO2 weight ratio of the bauxite residue were systemically investigated. The alkalinity in the bauxite residue decreases considerably with the increasing lime addition, digestion temperature and duration. The bauxite residues after digestion are mainly comprised of hematite, zeolite, grossular hydrogarnet, andradite–grossular hydrogarnet and perovskite, but their proportions vary greatly. A new method to accurately calculate the mineralogical compositions is proposed according to both the chemical compositions and XRD analyses of the bauxite residues. The mechanisms of alkalinity reduction in the bauxite residue and performance change in bauxite digestion are discussed.

Effect of Temperature on Phase and Alumina Extraction Efficiency of the Product from Sintering Coal Fly Ash with Ammonium Sulfate

A new developed technology for extracting alumina from coal fly ash was studied in this paper. In this technology, coal fly ash is first sintered with ammonium sulfate, forming ammonium aluminum sulfate in the resultant product, where alumina can be easily leached without using any strong acid or alkali. The products obtained under different sintering conditions were characterized by X-ray diffractometry. Alumina extraction efficiency of these products was also investigated. The results show that the sintering temperature and time substantially influence the phase composition and alumina extraction efficiency of sintered products, while the heating rate has little influence. The optimal sintering condition is 400°C for 3h in air with a heating rate of 6°C·min−1. Under the optimal sintering condition, the alumina extraction efficiency from as-sintered coal fly ash can reach 85% or more.

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 Temperature on Phase Composition of Sintering Coal Fly Ash with Ammonium Sulfate

The new processing technology of extraction alumina by sintering coal fly ash with ammonium sulfate was studied. The phase evolutions of the sintered coal fly ash in different temperature schemes were characterized by X-ray diffractometry. The alumina extraction efficiency of sintered coal fly ash obtained from different sintering temperature schemes had also been investigated. The results show that the sintering temperature and holding time markedly influence the phase composition and alumina extraction efficiency of sintered coal fly ash, while the temperature rising time has little influence on it. The optimal temperature scheme for sintering coal fly ash with ammonium sulfate has been eventually confirmed that increasing temperature from ambient temperature to 400 °C for 1 h and heating for 3 h in air and the alumina extraction efficiency from as-sintered coal fly ash can reach 85.6%.

Effects of MgO and Na2CO3 Additives, Sintering Temperature and Leaching Conditions for Extraction of Alumina from Bayer’s Process Waste Residue (Red Mud)

The objective of this study was to explore the sintering and leaching provision of Bayer's process waste residue (Red Mud). The chemical composition of red mud was ~16.07% Al 2O3, ~53.75% Fe 2O3, ~4.24% TiO 2, ~8.25% SiO 2, ~3.82% Na 2O, ~0.148% V 2O5, ~0.02% MgO, ~0.157% MnO, ~0.085% Ga 2O3, ~0.007% ZnO, 0.099% K 2O, ~11.83% LOI (loss on ignition) and rare earth elements in ppm label. The effect of Na 2CO 3 additive, MgO and Na 2CO 3 additives, sintering temperature, sintering time and leaching time on the efficiency of alumina extraction from red mud was investigated. Studies show that it was possible to extract alumina from red mud with highest extraction efficiency of 98.70%. The formation of Na 2MgSiO 4, Mg 2SiO 4, MgTiO 3 and Mg 2Fe2O5 phases during sintering of red mud with MgO and Na 2CO 3, enhance alumina extraction efficiency. X-ray diffractometer and Scanning Electron Microscope studied the formation of Na 2MgSiO 4, Mg 2SiO 4, MgTiO 3 and Mg 2Fe 2O5 phases.