Alkali Leaching Research Articles

Investigation on red mud and fly ash-based geopolymer: Quantification of reactive aluminosilicate and derivation of effective Si/Al molar ratio

The utilization of red mud (RM) as an aluminosilicate precursor (AP) for geopolymer production is attracting increasing research interest. This use of RM offers a desirable outlet for the increasing RM inventory and provides a sustainable solution for the immobilization of heavy metals and the valorization of waste as eco-friendly building materials. However, the effect of RM and its caustic residue on geopolymerization remains uncertain because of the extremely varied characteristics of RM. This study investigates the potential of using RM supplied from a local alumina refinery as an AP. First, RM was used to partially replace fly ash (FA) and the strength activity index (SAI) of the RM was evaluated. Second, to gain a deeper insight into the effect of caustic residue on geopolymerization, performances of traditional FA-based geopolymer and red-mud–fly-ash (RMFA)–based geopolymer under different NaOH concentrations were investigated and compared. Third, based on alkali leaching tests, the effective molar ratios considering the reactive content of RM and FA were developed, compared with conventional nominal molar ratio, and verified intensively using scanning electron microscopy–energy-dispersive spectroscopy (SEM-EDS).

Microwave low-temperature treatment – Step leaching process for recovering black mass from spent lithium-ion batteries

Cathode materials in the black mass from spent lithium-ion batteries can be efficiently reduced using microwave treatment. However, the interaction of different components in the recovery process has not been fully understood. The study aims to reveal the influence of impurities on the recovery of valuable metals using microwave low-temperature roasting and step leaching, realizing the separation of impurities and valuable metals. The sample black mass containing impurities was obtained by industrial dismantling process. The mechanisms of microwave-enhanced reduction were revealed by studying the dielectric properties and thermal evolution of different elements. Dielectric loss tangent increased with temperature and reached a peak value of 0.12 at 550 °C, facilitating microwave absorption and reduction reactions. The cathode materials were reduced efficiently in a semi-closed microwave system without inert atmosphere. The impurities graphite, Al and organics (electrolyte and PVDF) were potential in-situ reducing agents. The reduction products consists of Li2CO3, CoO(Co) and a small amount of LiAlO2, LiF and CoF2. The co-leaching of Al and Co was observed during the carbonated water leaching for recovering Li2CO3, which can be precipitated as Co6Al2CO3(OH)16·4H2O by aging of the leachate. Al in the water leaching residue was effectively removed using alkali leaching of 6 M NaOH. Finally, Co was selectively leached without dissolution of Cu by 2 M H2SO4. The optimized process parameters were obtained. The research results may provide technical guidance for the efficient recovery of industrial black mass.

Regeneration of raw materials for aluminum electrolysis from spent carbon anodes via a closed-loop environmentally-friendly process based on aluminum–fluorine complexation

Spent carbon anode (SCA) discharged from the aluminum electrolysis industry is an unavoidable solid waste with an estimated production of 700 kilotons in 2021, which has been widely identified as the hazardous solid waste. A new two-stage leaching scheme comprising an aluminum salt leaching and an alkali leaching has been investigated in this study, by which valuable elements have been extracted from SCA. Effect of reaction conditions on the first-stage aluminum salt leaching process was investigated in detail. Under the optimal leaching conditions, the leaching rates of F, Na, and Al reached 98.00%, 94.94%, and 93.94%, respectively, and the recovered carbon purity was 85.84%. The fluorine recovery route was followed by the preparation of aluminum hydroxyfluoride precursor, namely Al2F3.24(OH)2.76 1.65H2O, through the mixing of two filtrates. Then thermogravimetry-differential scanning calorimetry (TG-DSC) curves were used to explore its calcination temperature point in detail. The final product was obtained as a mixture of AlF3 and Al2O3 with 95.94% purity. The characterization results showed that the calcined product was very similar to the commercial aluminum fluoride. Finally, the neutralized filtrate will be prepared by freezing and crystallizing Na2SO4, and the crystallized solution will be returned to the aluminum salt leaching process for recycling, which is the promise of green, recyclable, and high-value technology for realizing the detoxification of SCA and its subsequent utilization.

The solvent-free hydrogenation of butyl levulinate to γ-valerolactone and 1,4-pentanediol over skeletal Cu-Al-Zn catalyst

γ-valerolactone (GVL) is a sustainable chemical, which can be obtained by hydrogenation of levulinic acid (LA) and levulinate. Herein, we reported a solvent-free hydrogenation of butyl levulinate (BL) to GVL and 1,4-pentanediol (1,4-PDO) over a skeletal Cu-Al-Zn catalyst. The skeletal Cu-Al-Zn catalyst was prepared from Devarda's alloy by alkali leaching and the catalyst was characterized by different analysis methods. The aluminum in the Devarda's alloy was dissolved by sodium hydroxide in the leaching process and many active Cu atoms were generated on the catalyst surface and the skeletal Cu-Al-Zn exhibited many micrometer-scale irregular particles with rough surface. The skeletal Cu-Al-Zn catalyst showed the excellent performance in the solvent-free hydrogenation of BL to GVL under constant hydrogen pressure (5 wt% catalyst loading, 180°C, 6 MPa of H2, 6 h): the LA conversion was 100%, the GVL yield was 74.75% and the 1,4-PDO yield was 25.63%. Moreover, the catalyst showed stable performance and it was reusable up to 5 times without apparent loss of activity. A stable hydrogen pressure with Zn would suppressed the dehydration of 1,4-PDO.

Recovery of molybdenum from (Co – Mo / - Al2O3) spent catalyst

In the present research, hydro – desulfurization spent catalyst with an alumina base has been found to be as a source for molybdenum recovery. It was compose of 19 - 30 % of molybdenum as MoO3. Two methods were used for the recovery of molybdenum. First method was using H2S gas for precipitation of Co as CoS, and the precipitation of Mo has been done by HCl acid solution addition. The second method was the alkali leaching of spent catalyst and recovery of molybdenum using dilute NH4OH solution and then the precipitation of molybdenum as trioxide using HCl solution. The optimum conditions were predicated for the recovery of molybdenum, and different parameters were studied such as the alkali concentration and leaching time.

Mitigation effect of glass powder on external sulfate attack and its relation to alkalinity of pore solution

Due to the presence of high alkali content, it is inferred that the improvement of sulfate attack resistance for blend with glass powder should not be solely ascribed to the refinement of microstructure. This study aims to explore the effect of glass powder on the pore solution and its relationship with the chemical sulfate attack process. Paste samples blended with 30 % wt. glass powder and quartz powder were prepared and subjected to a simulated external sulfate attack (ESA) test. To eliminate the effect of microstructure on sulfate ingress and alkali leaching, the pressed paste grains (after pore solution extraction) were immersed in test solution consisting of 5 % Na2SO4 and extracted pore solutions during the ESA test. The evolution of the chemical composition of test solution and phase assemblage of pressed paste grains during the ESA test were quantitatively characterized. Results indicate that the improvement of sulfate attack resistance is related to the increase in alkalinity of pore solution. The high alkalinity of pore solution leads to the dissolution of monosulfate, which suppresses the formation of ettringite. The maintained high alkalinity reduces the saturation degree of ettringite, monosulfate and gypsum with respect to the pore solution, preventing it from precipitating even at a high sulfate ions concentration up to 0.5 M. This study highlights the effect of glass powder on alkalinity of pore solution and its relationship with stability of ESA products.

Experimental Study on the Dressing and Smelting of a Pyrrhotite-bearing Primary Gold Ore

The content of pyrrhotite in a primary gold mine containing pyrrhotite is 0.68%. In the process of alkaline leaching pretreatment, there are problems such as large ore quantity and high alkali consumption. In the process of cyanide leaching, there are problems such as high cyanide consumption and slow gold leaching rate. In order to solve these problems, combined with production practice, experimental studies on magnetic separation, magnetic separation tailings leaching, magnetic separation concentrate alkali leaching pretreatment - leaching, magnetic separation concentrate flotation - cyanide leaching, etc. were carried out. A new process of "magnetic separation-magnetic flotation flotation flotation flotation flotation alkali leaching pretreatment mixed cyanide leaching" has been developed to reduce the pretreated minerals to less than 2% of the original ore, which is higher than the gold leaching rate of the conventional "alkali leaching pretreatment cyanide leaching" process, and the alkali (sodium hydroxide) consumption and cyanide (sodium cyanide) consumption are reduced by 80.83% and 32% respectively, and the annual reagent cost is saved by more than 600000 yuan.

Facile Gram-Scale Synthesis of Co3O4 Nanocrystal from Spent Lithium Ion Batteries and Its Electrocatalytic Application toward Oxygen Evolution Reaction

In this study, we demonstrate a new approach to easily prepare spinel Co3O4 nanoparticles (s-Co3O4 NPs) in the gram-scale from the cathode of spent lithium ion batteries (SLIBs) by the alkali leaching of hexaamminecobalt(III) complex ions. As-obtained intermediate and final products were characterized with powder X-ray diffraction (PXRD), Ultraviolet-Visible (UV-Vis), Fourier transform infrared (FTIR), and Transmission electron microscopy (TEM). Additionally, the synthesized s-Co3O4 NPs showed better electrocatalytic properties toward the oxygen evolution reaction (OER) in comparison to previously reported Co3O4 NPs and nanowires, which could be due to the more exposed electrocatalytic active sites on the s-Co3O4 NPs. Moreover, the electrocatalytic activity of the s-Co3O4 NPs was comparable to the previously reported RuO2 catalysts. By taking advantage of the proposed recycling route, we would expect that various valuable transition metal oxide NPs could be prepared from SLIBs.

Study on Precipitation Kinetics of Calcium Pyro-Vanadate and Thermodynamics of Vanadium Water System

Selective catalytic reduction (SCR) is a technology widely used in large coal-fired units to remove nitrogen oxides from flue gas, but it also generates a large number of waste catalysts every year. At present, the recovery of V from discarded SCR catalysts has good application prospects and environmental significance. In this paper, the kinetics and thermodynamics of vanadium precipitation process are described with the vanadium-containing liquid of waste denitration catalyst recovered by alkali leaching as raw material and CaCl2 as precipitant in order to further explore the mechanism of vanadium precipitation. The kinetics study showed that the crystallization process of calcium pyrovanadate can be well-described by Avrami kinetic model when the precipitation time is 95–130 min, and the vanadium precipitation temperature is 60–80 °C. After that, the Arrhenius equation was used to analyze the fitted kinetic data, and the apparent activation energy Ea of vanadium precipitation reaction was calculated to be 98.196 kJ/mol, and the pre exponential factor A = 8.59 × 1039 min−1. Thermodynamic study showed that when the pH of the vanadium water system is low, the +5 valence vanadium in the solution mainly exists in the form of VO2+ cation. When the pH is between 0–1, the solubility of vanadium reaches the minimum and then increases the solution pH again, and various polymerized anions are formed in the vanadium water system. When the temperature is 25 °C, the activity of vanadium in vanadium-containing solution is 10−1, the pH of solution is 8–12, and the existence form of +5 valence vanadium in solution is mainly HV2O73−. By analyzing the existing forms of V with different activities in a vanadium water system at 25 °C, it can be seen that with the decrease of V activity in liquid, the dominant region of polymerized vanadium-containing species in the potential pH diagram will disappear, indicating that vanadium mainly exists in the form of mononuclear ions in low-concentration vanadium-containing solutions, which is not conducive to precipitation. Therefore, in the process of precipitation of vanadium in solution, the concentration of V should be increased as much as possible.

Extraction of tungsten and molybdenum from waste alloy assisted by a recyclable roasting additive: β-MnO2

Extraction of molybdenum (Mo) and tungsten (W) from waste alloy significantly impacts the alleviation of resource shortage and ecological environment. In this study, Mn additive was utilized as a recyclable roasting aid to convert Mo and W for facilitating extraction, by considering thermodynamic calculations. The effect of β-MnO2 as a roasting additive on the phase evolution of Mo, W, and Mn elements and the optimal process conditions were evaluated by XRD, SEM-EDS, thermal analysis, and alkali leaching experiments. Notably, Mo and W in the alloy waste got oxidized and converted into soluble MnMoO4 and MnWO4, respectively, under the conditions of roasting temperature of 750 °C, β-MnO2 amount of 75% of waste alloy mass, and time of 1 h. During alkaline leaching, 100.00% Mo and 99.59% W in the roasted product were extracted, which was accompanied with the precipitation of leached residue of α-MnO2, Mn3O4, and Mn2O3 from Mn present in the leach liquor (NaOH concentration 2 mol⋅L−1, solid–liquid ratio 10% (w/v), time 5 h, temperature 90 °C). When the recycled leached residue was employed in the next roasting process, the leaching efficiency of Mo and W was maintained above 98.56%. Moreover, the reaction mechanisms of roasting and leaching were investigated by kinetics study. Roasting with β-MnO2 fundamentally reduced the production cost with the high extraction efficiency of Mo and W, and the flue gas recovery process was no longer required, which is beneficial to industrial application.

Study on the properties of bauxite modified by acid leaching and calcination for improving fluorine removal

AbstractIn order to improve the fluorine removal performance of bauxite, the effects of different calcination temperatures and several acids (HCl, H2SO4, H2C2O4) on the fluorine removal performance of bauxite were investigated. It was found that the calcination temperature of 500°C and the modification of H2SO4 had the best fluorine removal performance. Under the same experimental conditions, the fluoride removal rate of the modified bauxite could reach 99.54%, which was much higher than that of unmodified bauxite (52.69%). The adsorbent was characterized by XRD, FTIR, BET, TG/DSC, and XRF. The influence of the addition amount of adsorbent, initial concentration of fluorine, adsorption time, and temperature on fluorine removal performance was studied. The adsorption process reached equilibrium in 32 min, and the inhibition order of coexisting anions was H2PO4− < HCO3− < CO32− < Cl−. The pseudo‐second‐order kinetic model was more suitable for the fluoride removal process of modified bauxite. Thermal regeneration had better regeneration performance than alkali leaching regeneration.

Effects of sample crumbling and particle size on accelerated carbonation of alkaline construction sludge treated with paper-sludge ash-based stabilizers

Construction sludge generated from underground and pile construction works frequently appears in a liquid state. For its utilization as a construction geomaterial, it can be improved by mixing in cement or lime. The target strength can be achieved relatively easily by adjusting the amount of cement or lime added to the sludge. However, this type of chemical treatment is associated with high alkalinity, which causes environmental concerns associated with alkali leaching. In this study, therefore, the accelerated carbonation of alkaline construction sludge through treatment with a paper-sludge ash-based stabilizer (PSAS) was experimentally attempted to neutralize the pH. The alkaline sludge treated with the PSAS was exposed to pure CO2 gas to accelerate the carbonation. The effects of granulation via crumbling prior to the accelerated carbonation on the pH neutralization period were analyzed from the test results. It was observed that crumbling the sludge prior to the CO2 gas exposure accelerated the pH neutralization. These results suggest that a short pH neutralization period can be achieved by reducing the particle size. Therefore, the effect of the particle size of the PSAS-treated sludge on the pH neutralization period was also investigated. The results showed that the pH neutralization period decreased with an increasing particle size. However, it was also observed that, when the addition ratio of the PSAS was increased, the difference in the pH neutralization period induced by the change in particle size became less significant.

Correlating alkali-silica reaction (ASR) induced expansion from short-term laboratory testings to long-term field performance: A semi-empirical model

Correlating short-term expansion of concrete specimens in the laboratory and long-term expansion of concrete in the field is crucial to evaluate the reliability of laboratory test methods and essential for the prognosis of alkali-silica reaction (ASR) in concrete infrastructures. In this study, a novel semi-empirical approach is proposed for forecasting ASR-induced expansion of unrestrained concrete in the field using laboratory measurements data. In addition to the use of short-term laboratory expansion data, the model accounts for the effects of alkali leaching, alkali contribution from aggregates, and environmental conditions (i.e., temperature and relative humidity). A comprehensive database from the literature was gathered for the development and calibration of the proposed model. Finally, the model was used for various concrete blocks incorporating different reactive aggregates and exposed to three outdoor conditions in Canada and the USA. Model outcomes show that it is highly promising for forecasting the induced expansion of concrete in the field from the accelerated laboratory tests data. Analysing the modelling results also highlights the importance of alkali leaching and environmental conditions on the correlation between laboratory and field performance.

Novel Technology for Synergistic Extraction of Li and Rb from a Complex Lithium Concentrate

Li, known as a crucial new energy metal, has received explosive attention. Spodumene and lepidolite are abundant in Li and are usually associated with Rb, and their strategic position becomes increasingly visible. Herein, a novel and economical technology, combining thermal activation with alkali leaching, was proposed to synergistically extract Li and Rb from a complex lithium concentrate composed of spodumene and lepidolite. First, the comparative cooling experiment showed that the chemical reactivity of the water quenching slag was higher than that of the furnace cooling slag. Furthermore, the phase conversion of main minerals during thermal activation was explored. In addition, inert α-spodumene was converted to active β-spodumene after activation. Subsequently, the leaching behavior of each component in the water quenching slag was investigated. In addition, the results demonstrated that 93.0% of Li, 88.1% of Rb, 41.3% of Si, and 2.8% of Al were extracted at the optimal conditions. Ultimately, the adsorption studies confirmed that the leach residue was a feasible material for removal of heavy metal ions. In contrast to the existing extraction technologies, the technology proposed in this study had a significant advantage of low alkali consumption. Additionally, the utilization of leach residue could reduce the generation of solid wastes as well as improve the ecological and economic value of the proposed process.

Crystallization and pyrolysis of nitric acid pressure leach liquor of limonitic laterite and separation of valuable metals

Based on the advantages of nitric acid pressure process (NAPL) for laterite nickel ores, this study innovatively proposes nitric acid pressure leach liquor crystallization pyrolysis-mixed metal oxides separation process. Firstly, the feasibility and the possible reactions of mixed nitrate pyrolysis were confirmed by thermodynamic analysis. Subsequently, the optimal pyrolysis temperature was experimentally investigated. Then, the pyrolysis process of combined nitrate was thermodynamically analyzed, and the phase composition of mixed metal oxides was determined by X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS). The Pourbaix diagram shows that using NaOH as leaching agent can selectively separate aluminum from mixed metal oxides, and the variables influencing aluminum's leaching rate are investigated. The experimental results are as follows: the optimal pyrolysis temperature of mixed nitrate crystals is 600 °C; the pyrolysis products are mainly oxides with the formation of NiAl2O4 and MgAl2O4 spinel phases; the aluminum leaching rate is 81.9% under the optimal alkali leaching conditions. The presence of spinel phase is not favorable for aluminum leaching. After leaching, an enriched residue of 15.8% Ni, 1.9% Co and 0.19% Sc was obtained.

Efficient Recovery of V, W, and Regeneration of TiO2 Photocatalysts from Waste-SCR Catalysts

Discarded Waste-SCR (selective catalytic reduction) catalysts not only harm the natural environment, but also hinder the recovery of valuable metals such as V, W, and Ti, thus resulting in a waste of resources. Therefore, it has become imperative to dispose of Waste-SCR catalysts in large amounts. This study aims to develop a complete, eco-friendly, and economical process of dealing with Waste-SCR catalysts. In this process, the recovery of V and W from Waste-SCR catalysts was achieved in several steps. In addition, the leaching conditions of the acid-base process were optimized, while the structure and composition of the leaching residual were studied in depth. Sodium titanates generated by alkali leaching were then applied for regeneration of the TiO2 photocatalyst. It is worth noting that Fe-doped TiO2 photocatalysts were also prepared, while the required iron source was obtained through acid leaching. Regenerated photocatalyst achieved an excellent performance in the photocatalytic degradation of dye pollutants, which is attributable to small particle size and low forbidden band width. This contributes a new idea to the recycling of solid waste, and the economical treatment of wastewater.

Paleo-lateritization and metalliferous ore formation over the metadioritic rock in the Poli group – Northern Cameroon

This study reports on the geochemical characterization of paleo-lateritic covers cropping out in the Poli area, Northern Cameroon. This study aimed at obtaining a better understanding of their formation processes in regard to climate changes and implication in the occurrence of metalliferous ores. Hence, an analytical approach was adopted and consisted of inductively coupled plasma (ICP) atomic emission for major element geochemistry and mass spectrometry for trace and REEs (ICP-AES and ICP-MS respectively). The Poli study area belongs to the geomorphic unit of the Benoue plain and displays two main geomorphic sub-units: the upper and lower landsurfaces. Macroscopically, paleo-lateritic surfaces in this area rest on a Neoproterozoic basement mostly composed of metavolcanic and metasedimentary rocks passing upwards to a soil which consists of three main horizons (bottom to top): saprolitic horizon, spotted clayey horizon and iron-rich duricrust. These horizons result to an intense chemical weathering and moderate lateritization of metadiorite (CIA: 96.6–99.1%; IOL: 41.2–61.7%), related to Cenozoic era known as age of paleo-lateritic surface formation in Africa and during which the Northern Cameroon was located in the humid tropical zone of the Earth's surface. The paleo-lateritization process in the Poli study area is characterized by a moderate loss of silica (RR: 3.2 to 1.7) and a near-complete leaching of alkali, alkaline earth elements (TRB: 742.3 to 7.1 mg kg−1; WIP: 44.1 to 0.9%). Fe2O3 (9.6–43.5 wt %) showed significant variation with increasing trend from the bedrock to the iron duricrust. Among trace elements, Ba, U, Pb, Zr, Hf, Cr, Co, Cu, Ni, Sc and V are significantly enriched in the iron rich duricrust and constitute the mineralization indices with maximum enrichment factor in the lower landsurface. However, only V with an average content of 740 mg kg−1 in the upland and 667 mg kg−1 in the lowland indicates an economic level. Therefore, according mostly to the Fe2O3 and V contents, the iron duricrust cover of Poli displays a considerable mining potential.

Geochemical fingerprinting and magnetic susceptibility to unravel the heterogeneous composition of urban soils

The typically high heterogeneity of urban soil properties challenges their characterization and interpretation. The objective of this study was to investigate if proximally sensed volume-specific magnetic susceptibility and/or geochemical soil properties can uncover differences in anthropogenic, lithogenic and pedological contributions in, and between, urban soils. We also tested if volume-specific magnetic susceptibility can predict heavy metal enrichment. Data on 29 soil properties of 103 soil horizons from 16 soils from Ghent, Belgium, were analyzed by factor analysis. A correlation analysis, and in-depth analysis of five contrasting urban soils supplemented insights gained from the factor analysis.The factor analysis extracted four factors: 29.2 % of the soil property variability was attributed to fossil fuel combustion and industrial processes, with high (>0.80) loadings for S, organic carbon, magnetic susceptibility, and Zn. Furthermore, 26.0 % of the variability was linked to parent material differences, with high loadings (>0.80) for K, Rb and Ti. In absence of geogenic carbonates, increased soil alkalinity due to anthropogenic input of CaCO3 explained 17.0 % of the variability. Lastly, 4.7 % of the variability resulted from variable Zr contents by local geology. Elemental analysis by XRF, possibly combined with magnetic susceptibility measurements, helped to explain lateral or vertical differences related to (1) the nature of anthropogenic influence, for instance burning (e.g., by the S and Zn content) or the incorporation of building rubble (e.g., by the Ca content); (2) the particle size distribution (e.g., by the K, Rb or Ti content); (3) lithology (e.g., by the Zr content); or (4) pedology, such as organic matter build-up (e.g., by the S content) or leaching of alkalis (e.g., by the Ca content). Even though artifacts and soil translocation were common in the studied soils, volume-specific soil magnetic susceptibility measured on fine earth predicted the total heavy metal pollution loading index well (Pearson correlation = 0.85).

Extraction of molybdenum from a spent HDS catalyst using alkali leaching reagent

This research investigated extraction of molybdenum from spent hydrodesulfurization (HDS) catalyst used in petroleum refinery. The spent HDS catalyst still however contains significant amounts of valuable metals such as molybdenum and nickel for example, thereby recovery of such metals are of great interest. Pyro-hydrometallurgical process was utilized in this research to selectively extract molybdenum from the spent HDS catalyst using alkali leaching reagent. The process start from calcination of spent HDS catalyst, alkali leaching using sodium carbonate, purification by carbon adsorption-desorption, precipitation of ammonium molybdate and finally calcination to give molybdenum trioxide (MoO3) as the final recycling product. Effects of calcination temperature (450℃ to 650℃) together with alkali concentration (1 M to 3 M Na2CO3), solid to liquid ratio (100 g×L-1 to 300 g×L-1) and leaching time (30 min to180 min) have been investigated. Calcination at 450℃ has shown to give the highest leaching efficiency. The optimum leaching condition, giving 97% leaching efficiency was determined to be 40 g×L-1 sodium carbonate concentration, 2 h-leaching time, 100 g×L-1 solid to liquid ratio and 90℃ leaching temperature, by controlling the temperature at 90℃ and the pH at 2, ammonium molybdate then precipitated. Calcination at 450℃ finally converted the obtained precipitate to molybdenum trioxide as the final recycling product of 99.5% purity.

Coal fly ash cleaner utilization by ferric oxide assisted roasting – leaching silica: Recycling lixiviant by seeded precipitation of leachate

Recycling lixiviant from sodium silicate solution is a key to the cleaner utilization of aluminosilicate solid wastes such as coal fly ash and coal gangue via ferric oxide assisted roasting - alkali leaching silica process. This work focuses on seeded precipitation of higher-modulus sodium silicate solution to obtain recyclable spent solution with lower-modulus. The results show that adding silica hydrate seeds can induce the decomposition of sodium silicate solution to obtain amorphous silicate hydrate and low-modulus spent solution. The SiO2 precipitation yield, precipitation rate and the spent solution modulus can respectively reach up to 108.26 g/L, 72.17 % and 0.86, under the appropriate conditions: initial modulus 3.09, total Na2O concentration 50 g/L, seed amount 350 g/L-liquor, temperature 50 ℃, and duration 6 h. Meanwhile, the average particle size of precipitation product is ~ 56.20 µm. Moreover, the structural evolution of silicates during the precipitation implies that silica hydrate mainly precipitates from high-polymerized silicates. The precipitation product was further characterized as opal-A by XRD, chemical composition, TG-DSC and FT-IR analyses. This work may be beneficial either to improving the ferric oxide assisted roasting - alkali leaching silica process for treating aluminosilicate solid wastes or to developing a new technology for preparing opaline silica hydrate.