Search Results (38)

Regarding the erosion–corrosion problem of 304 stainless steel, which is commonly used in the production of alumina, in high-temperature, high-pressure, and strongly alkaline aluminum ammonium solutions, a detailed study was conducted on the erosion–corrosion behavior and damage mechanism of 304 stainless steel in a sodium aluminate solution with varying S²⁻ concentrations at 65 °C and pH = 14 under the influence of key factors such as erosion speed. This study quantitatively revealed, for the first time, the flow rate threshold effect (critical point at 2 m/s) of 304 stainless steel during scouring corrosion in a strongly alkaline aluminum ammonium solution, identified its peak weight loss rate (1.892 × 10⁻³ g/m²·d), and innovatively elucidated the mechanism reversal phenomenon: below the threshold, passive film destruction and corrosion synergistically dominate, while above the threshold, high oxygen mass transfer promotes film regeneration. These findings provide a critical theoretical basis for precise flow rate control and equipment life prediction in alumina production processes. Full article

In the alumina production process, potassium from bauxite transfers to the alumina product through digestion and precipitation, posing significant risks to the subsequent aluminum electrolysis. The current understanding of this transfer behavior, especially the distribution patterns of potassium, remains inadequate. This research examines the influence of key parameters—including K₂O concentration, seed addition amount, final precipitation temperature, and the molar ratio of the sodium aluminate solution—on the precipitation of potassium. The findings demonstrate that increasing the seed addition, the initial K₂O concentration in the solution, the final temperature, and the solution molar ratio all promote the precipitation efficiency of potassium. Moreover, it was observed that raising the seed addition, solution molar ratio, and final temperature effectively increases the proportion of coarse particles in the precipitated product. Conversely, a higher K₂O concentration leads to a greater fraction of fine particles. Under typical industrial conditions (C_OH⁻ = 4.8 mol/L, K₂O = 70 g/L, α_k = 1.53, initial temperature of 75 °C, final temperature of 37 °C, seed addition of 700 g/L, and a precipitation duration of 40 h), only about 0.19% of the potassium from the initial solution transferred into the aluminum hydroxide product. Full article

This article focuses on studying the phase transformation of bauxite iron minerals during electrolytic reduction processes in alkaline solutions (400 g/L Na₂O), with the aim of improving aluminum extraction in the subsequent Bayer process. The research employs electrolytic reduction to convert the refractory minerals in unmilled bauxite (alumogoethite (Fe,Al)OOH, alumohematite (Fe,Al)₂O₃, chamosite (Fe²⁺,Mg,Al,Fe³⁺)₆(Si,Al)₄O₁₀(OH,O)₈) into magnetite, elemental iron (Fe) and to minimize aluminum (Al) extraction during electrolysis. Preliminary thermodynamic research suggests that the presence of hematite (α-Fe₂O₃) and chamosite in boehmitic bauxite increases the iron concentration in the solution. Cyclic voltammetry revealed that, in the initial stage of electrolysis, overvoltage at the cathode decreases as metallic iron deposited and conductive magnetite form on the surface of the particles. After 60 min, the reduction efficiency begins to decrease. The proportion of the current used for magnetization and iron deposition on the cathode decreased from 89.5% after 30 min to 67.5% after 120 min. After 120 min of electrolytic reduction, the magnetization rate exceeded 65%; however, more than 60% of the Al was extracted simultaneously. Al extraction after electrolysis and subsequent Bayer leaching exceeded 91.5%. Studying the electrolysis product using SEM-EDS revealed the formation of a dense, iron-containing reaction product on the particles’ surface, preventing diffusion of the reaction products (sodium ferrite and sodium aluminate). Mössbauer spectroscopy of the high-pressure leaching product revealed that the primary iron-containing phases of bauxite residue are maghemite (γ-Fe₂O₃), formed during the hydrolysis of sodium ferrite. Full article

This study presents an approach to synthesizing LTA-type zeolite from spodumene residue generated during a lithium extraction process. A residue was obtained after leaching β-spodumene with 2 mol/L phosphoric acid. After solid–liquid separation, the delithiated residue was first treated with 2 mol/L sodium hydroxide and then subjected to hydrothermal synthesis using sodium aluminate as an additional aluminum source. The resulting material was characterized by XRD, SEM-EDS, XPS, and FTIR, which collectively confirmed the formation of a crystalline material exhibiting the structural features, elemental composition, and morphological characteristics consistent with LTA-type zeolite. Additional analyses, including BET surface area, particle size distribution, and zeta potential measurements, were performed to further evaluate the physicochemical properties of the synthesized zeolite. The spodumene leach residue (SLR)-derived zeolite was further tested for its adsorption performance in heavy metal ions removal from a mixed ion solution containing Pb²⁺, Cu²⁺, Zn²⁺, and Ni²⁺ ions. The zeolite demonstrated a high selectivity for Pb²⁺, followed by moderate uptake of Cu²⁺, while Zn²⁺ and Ni²⁺ adsorption was minimal. These findings demonstrate that spodumene residue, a waste by-product of lithium processing, can be effectively upcycled into LTA zeolite suitable for heavy metal remediation in water treatment applications. Full article

Municipal solid waste incineration (MSWI) fly ash, classified as hazardous waste (HW18) due to the presence of heavy metals and dioxins, necessitates both harmless treatment and resource utilization. In this study, a Na-P1 zeolite adsorbent was synthesised from MSW incineration fly ash using its intrinsic Si and Al sources, supplemented by silica sol and sodium aluminate solution. The synthesised zeolite was employed for the adsorption removal of tetracycline hydrochloride (TCH) from wastewater. Under the optimised conditions (initial TCH concentration of 10 mg·L⁻¹, adsorbent dosage of 0.4 g·L⁻¹, pH 5.0, temperature 45 °C, and contact time 60 min), a maximum adsorption capacity of 14.8 mg·g⁻¹ and a removal efficiency of 59.1% were achieved. Kinetic analysis revealed that the adsorption process followed the pseudo-first-order model (R² = 0.975). The Langmuir isotherm provided a better fit than the Freundlich model (R² = 0.988), indicating monolayer adsorption on homogeneous sites. Thermodynamic parameters (ΔG < 0, ΔH > 0) confirmed that the adsorption was spontaneous and endothermic, with higher temperatures favoring enhanced TCH adsorption. This work demonstrates the feasibility of converting hazardous MSW incineration fly ash into a value-added Na-P1 zeolite adsorbent with excellent performance for antibiotic wastewater treatment, thereby offering a sustainable strategy for fly ash resource recovery and environmental remediation. Full article

In this study, we investigate the effects of adding varying proportions of fulvic acid during the digestion of pyrite on the iron concentration in both dissolved and diluted sodium aluminate solutions. Based on the occurrence characteristics of iron in the solutions, oxygen was introduced into the diluted solution to examine its iron removal efficiency, and the influence of organic compounds in the solution on iron removal through oxidation was investigated. The results indicate that, during high-pressure digestion, organic compounds forms complexes with iron, disrupting the hydrophilic iron (or ferrous) hydroxide film formed on the pyrite surface, thereby accelerating its dissolution and leading to a sharp increase in sulfur and iron content in the leachate. After cooling and dilution (100 °C, Na₂O_k 170 g/L), the iron content in the sodium aluminate solution continued to be influenced by organic compounds, showing a significant positive correlation. Oxygenation experiments for iron removal were performed using the diluted solution. Under conditions of an oxygen flow rate of 60 mL/min and an oxidation duration of 2 h (95 °C, oxygen partial pressure was 0.05 Mpa), the iron content (calculated as Fe₂O₃) decreased from 0.078 g/L to 0.021 g/L. Characterization and analysis of the iron removal precipitates revealed that the iron-containing minerals were primarily trivalent iron phases, such as goethite and hematite, with minimal ferrous iron content. Additionally, organic carbon also precipitated together with iron, which confirms the synergistic removal of iron and organic compounds. These findings demonstrate that the oxidation of reducing sodium aluminate solutions containing organic compounds, sulfur, and iron with atmospheric oxygen during the Bayer process sedimentation stage can effectively oxidize predominantly ferrous iron into less soluble ferric iron, thereby achieving iron removal. Full article

The durability of construction and building materials under sulphate environments is an important indicator to evaluate their service life. In this study, the physical and mechanical behaviours of wood-ash-based alkali-activated materials (AAMs) incorporating coal fly ash, metakaolin, natural zeolite, and calcined phosphogypsum were assessed before and after being subjected to sodium sulphate corrosion cycles via the compressive strength, mass, and volume changes. The microstructure, elemental composition, and phase identification were further analysed using X-Ray Diffraction(XRD) and scanning electron microscope(SEM). The results show that the exposure to sulphate solution caused decalcification and dealumination of hydrates, releasing calcium and aluminium to react with sulphate and forming expansive erosion products, ettringite and gypsum. This contributed to the microstructural damage, leading to mass change, volume expansion, and compressive strength loss of 7.33, 1.29, and 60.42%. The introduction of binary aluminosilicate precursors enhanced the sulphate resistance by forming a well-bonded microstructure consisting of calcium (aluminate) silicate hydrate and sodium aluminate silicate hydrate, with the compressive strength loss decreasing up to 18.60%. The co-usage of calcined phosphogypsum deteriorated the mechanical properties of AAMs but significantly improved the sulphate resistance. The sodium sulphate environment facilitated anhydrate hydration, generating more sulphate hydrates and hemigypsums that co-existed with erosion products, forming a compact microstructure and improving the compressive strength by twofold. Full article

This study presents the results of research on the use of Portland cement as a binder for producing semi-permanent molds intended for large-scale castings made from complex alloyed steels. Based on the conducted experiments, the optimal composition of a molding mixture based on Portland cement was determined to manufacture large molds with high operational performance. The technological properties of the mixtures were investigated, focusing on the flowability, sedimentation stability, and strength after curing. The recommended mixture composition is as follows: Portland cement—18.75%; sand—56.5%; quartz powder—25%; water—25%. To accelerate the hardening process, the use of curing accelerators is advised. The most effective additives are a 9% aluminum nitrate solution at 0.6–1.5% by weight or sodium aluminate at 3–4%. This composition ensures the required strength within a short curing time. A specific thermal treatment regime is also recommended to further stabilize the mold structure: heating to 450 °C at a rate of 75 °C per hour, holding for 2 h, followed by controlled cooling together with the furnace. Full article

An advanced alkali-acid (NaOH-HCl) chemical method was used to deash aluminum-rich coals (ARCs) with a high ash content of 27.47 wt% to achieve a low ash content of 0.46 wt%. In the deashing process, aluminum in the coal ashes was dissolved in both alkali solutions and acid solutions. The deashing alkali solutions with dissolved coal ashes were regenerated by adding CaO, and the resulting precipitates were added with sodium bicarbonate for aluminum extraction. High temperatures increased aluminum extraction, and excessive sodium bicarbonate addition decreased aluminum extraction. The deashing acid solutions were concentrated by evaporation, and silica gels formed during the process. The obtained mixtures were calcinated at 350 °C for the decomposition of aluminum chlorides, and soaked with water at 60 °C to remove the soluble chlorides. For the insoluble oxides after soaking, diluted alkali solutions were used to extract the aluminum at 90 °C, and aluminum extraction failed due to the formation of albite in the presence of sodium, aluminum and silicon elements as proved by XRD and SEM/EDS. When silica gels were separated by pressure filtering, aluminum extraction greatly increased. Aluminum extractions were accordingly made in the form of sodium aluminate from the deashing solutions of coals, which could be advantageous for sandy alumina production. Full article

The utilization of steel slag (SS), fly ash (FA), and ground granulated blast furnace slag (GGBFS) as soil additives in construction represents a critical approach to achieving resource recycling of these industrial by-products. This study aims to activate the SS-FA-GGBFS composite with a NaOH solution and Na₂CO₃ and employ the activated solid waste blend as an admixture for lateritic clay modification. By varying the concentration of the NaOH solution and the dosage of Na₂CO₃ relative to the SS-FA-GGBFS composite, the effects of these parameters on the activation efficiency of the composite as a lateritic clay additive were investigated. Results indicate that the NaOH solution activates the SS-FA-GGBFS composite more effectively than Na₂CO₃. The NaOH solution significantly promotes the depolymerization of aluminosilicates in the solid waste materials and the generation of Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate gels. In contrast, Na₂CO₃ relies on its carbonate ions to react with calcium ions in the materials, forming calcium carbonate precipitates. As a rigid cementing phase, calcium carbonate exhibits a weaker cementing effect on soil compared to Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate gels. However, excessive NaOH leads to inefficient dissolution of the solid waste and induces a transformation of hydration products in the modified lateritic clay from Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate to Sodium-Silicate-Hydrate and Sodium-Aluminate-Hydrate, which negatively impacts the strength and microstructural compactness of the alkali-activated solid waste composite-modified lateritic clay. Full article

Xinjiang is one of China’s most significant energy bases, and the generated fly ash (FA) contains a high concentration of metallic elements that can be used as a valuable resource. In this study, we looked into a roasting-acid leaching process technique for efficiently extracting gallium metal (Ga) from FA, employing sodium fluoride (NaF) as the roasting auxiliary and citric acid inzter (C₆H₈O₇) acid leaching. After high-temperature activation by NaF, the glassy phase of FA was converted into silica aluminate with excellent acid solubility, and Ga was extracted from FA via acid leaching. The effects of optimal roasting and acid leaching process conditions on the Ga leaching rate were investigated. The results showed that it exhibited 83.71% Ga extraction under the conditions of a roasting temperature of 850 °C, FA-NaF coordination ratio of 1:0.5, roasting time of 10 min, C₆H₈O₇ solution concentration of 1.75 mol/L, ratio (S/L) of 1:15, acid leaching temperature of 100 °C, and acid leaching time of 1 h. The results also indicated that it was possible to obtain a higher extraction efficiency for the Ga extracts under the conditions of roasting temperature of 850 °C and FA-NaF coordination ratio of 1:0.5. Full article

The diaspore is a typical representative of bauxite resources in China, which is the primary raw material for the Bayer process in alumina production, particularly in regions such as Shanxi, Guangxi, Guizhou, and Henan. Clarifying the phase transformations and reaction mechanisms of the silicon-containing minerals during the Bayer leaching process of diaspore is essential for improving the efficiency of alumina production. This article focuses on montmorillonite, which is one of the silicon-containing minerals of diaspore-type bauxite, investigating the reaction mechanisms and phase changes of montmorillonite under the high-calcium sodium aluminate solution system by using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Magic Angle Spinning Nuclear Magnetic Resonance (MAS–NMR) and Fourier Transform Infrared Spectroscopy (FTIR). The results show that montmorillonite dissolved and transformed into Na₆(AlSiO₄)₆ (hydrated sodium aluminosilicate) under the high-calcium sodium aluminate solution system, and calcium oxide and sodium aluminate in the solution reacted to form (CaO)₃Al₂O₃(H₂O)₆ (hydrated calcium aluminate). With the increase of reaction temperature, caustic alkali concentration (N_k), and reaction time, hydrated calcium aluminate and hydrated sodium aluminosilicate react and transform into Ca₃Al₂SiO₄(OH)₈ (hydrogarnet). Under the optimal reaction conditions of a 120 min reaction time, a temperature of 240 °C, an N_k of 240 g/L, and a CaO–to–SiO₂ mass ratio (C/S) of 3.5:1, the montmorillonite reaction degree can reach a maximum of 93.71%. Full article

This study investigates how different sodium silicate SiO₂/Na₂O MS ratios (0.75, 0.9, and 1.2) affect the hydration behavior of amorphous wollastonitic hydraulic (AWH) binders containing various amounts of Al₂O₃ content (4, 7, 10, and 12%wt). The effects of and interaction between the MS ratio of the activator and the Al₂O₃ content of the sample on the hydration reaction and paste performance were investigated. The reaction was followed by calorimetry, and the pastes’ compressive strength performances were tested at different curing times (2, 7, and 28 days). The hydrated pastes were characterized by FTIR, thermogravimetry analysis, and X-ray diffraction. The calorimetric results show that a higher Al₂O₃ cContent and a higher MS ratio result in a longer induction period. In terms of paste performance, an increase of the Al₂O₃ coupled with an activation with a 1.2 MS ratio results in a lower compressive strength after 28 days of hydration; the results range from 76 to 52 MPa. A decrease of the MS ratio to 0.9 allowed the obtention of a narrower range of results, from 76 to 69 MPa. Even though a decrease of the MS ratio to 0.75 led to higher hydration kinetics and high compressive strength results at early ages, at 28 days of curing, a decrease in compressive strength was observed. This may be a consequence of the fast kinetic of the mixture, since the rapid growth of hydration products may inhibit the dissolution at later ages and increase the porosity of the paste. Moreover, the high Al intake in the hydration product, facilitated by the high sodium content of the activator, promotes the formation of a higher number of calcium aluminate silicate hydrate structures (C-A-S-H) to the detriment of calcium silicate hydrate structures (C-S-H), decreasing the compressive strength of the samples. The TGA results indicate that the samples hydrated with the MS075 solution resulted in a higher number of hydrated products at early ages, while the samples hydrated with the MS09 and MS1.2 solutions exhibit a steady increase with curing time. Hence, an equilibrium in the hydration kinetic promoted by Si saturation–undersaturation appears to be fundamental in this system, which is influenced by both the MS ratio and the Al(OH)⁴⁻ content in solution. The results of this study suggest that for this type of binder, optimal performance can be achieved by decreasing the MS ratio to 0.9. This composition allows for a controlled kinetic and overall higher compressive strength results in pastes produced with this AWH precursor. Full article

Hard ceramic coatings were successfully prepared on the surface of ZM5 magnesium alloy by micro-arc oxidation (MAO) technology in silicate and aluminate electrolytes, respectively. The optimization of hard ceramic coatings prepared in these electrolyte systems was investigated through an orthogonal experimental design. The microstructure, elemental composition, phase composition, and tribological properties of the coatings were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and tribological testing equipment. The results show that the growth of the hard ceramic coatings is significantly influenced by the different electrolyte systems. Coatings prepared from both systems have shown good wear resistance, with the aluminate electrolyte system being superior to the silicate system in performance. The optimized formulation for the silicate electrolyte solution has been determined to be sodium silicate at 8 g/L, sodium dihydrogen phosphate at 0.2 g/L, sodium tetraborate at 2 g/L, and potassium hydroxide at 1 g/L. The optimized formulation for the aluminate electrolyte solution consists of sodium aluminate at 5 g/L, sodium fluoride at 3 g/L, sodium citrate at 3 g/L, and sodium hydroxide at 0.5 g/L. Full article

Organic montmorillonite (OMMT) was prepared from Na-montmorillonite (MMT) by Hexadecylamine (HDA) modification. The composite material has good smoothness, acidity, and salt resistance. OMMT was characterized using small-angle X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and a video optical contact angle measuring instrument. The results showed that the layer spacing was enlarged from 1.44 nm to 2.87 nm after the modification, and the hydrophobicity performance was greatly improved. The organic modification of MMT was successful. The surface morphology, roughness, and anticorrosion properties of the organic montmorillonite/epoxy (OMMT/EP) composite coating were investigated and compared with those of the epoxy (EP) coating. The OMMT/EP composite coating had a flatter surface than the EP coating. The roughness was reduced from 65.5 nm to 10.3 nm. The electrochemical impedance spectroscopy showed that the composite coating’s thickness positively affected its anticorrosion performance, the corrosion current density (I_corr) decreased with the increase in thickness, and its maximum impedance was much larger than that of EP coating. The protection efficiency of the OMMT/EP composite coating was 77.90%, which is a significant improvement over the EP’s 31.27%. In addition, the corrosion resistance of the composite coating gradually decreased with increasing immersion time, but the change was insignificant. Full article