Electrolytic Aluminum Industry Research Articles

Selective preparation of lithium carbonate from overhaul slag by high temperature sulfuric acid roasting – Water leaching

An efficient recycling process is developed to recover valuable materials from overhaul slag and reduce its harm to the ecological environment. The high temperature sulfuric acid roasting - water leaching technology is innovatively proposed to prepare Li2CO3 from overhaul slag. Under roasting conditions, fluorine volatilizes into the flue gas with HF, lithium is transformed into NaLi(SO4), aluminum is firstly transformed into NaAl(SO4)2, and then decomposed into Al2O3, so as to selective extraction of lithium. Under the optimal roasting – leaching conditions, the leaching rate of lithium and aluminum are 95.6% and 0.9%, respectively. Then the processes of impurity removal, and settling lithium are carried out. The Li2CO3 with recovery rate of 72.6% and purity of 98.6% could be obtained under the best settling lithium conditions. Compared with the traditional process, this work has short flow, high controllability, remarkable technical, economic, and environmental benefits. This comprehensive recycling technology is suitable for overhaul slag, and has great practical application potential for the disposal of other hazardous wastes in electrolytic aluminum industry.

Study on the recovery of graphite from spent carbon cathode via ultrasound-assisted foam flotation

Spent carbon cathode is a kind of solid waste produced by the electrolytic aluminum industry, and it poses a high environmental risk. However, it also contains valuable graphite resources that can be recycled and used as raw materials in industry. In this work, we aimed to enhance the efficient recovery of graphite by introducing ultrasonic waves into the flotation process. Particle size test, inductively coupled plasma (ICP), BET surface area, and other analytical techniques confirmed that ultrasound could promote particle dispersion, clean the surface of minerals, increase the porosity of the graphite surface, and thus strengthen the flotation performance. The results showed that after ultrasonic treatment for 30 min, the recovery rate of graphite increased from 86.22% to 96.24%, the fixed carbon content of tailings decreased from 26.99% to 9.68%, and the fixed carbon content (84.95%) was close to that of conventional flotation (85.32%). Compared with the traditional purification method, this strategy reduces the follow-up cleaning steps of tailings, simplifies the process, reduces the use of chemicals, and features strong market competitiveness and green environmental protection advantages. Additionally, the process is simple and low-cost and can be applied to the recycling of other waste graphite, with potential industrial application prospects.

The effect of crystallographic orientation of α-Al2O3 on the wetting behavior and adhesion characteristics of aluminum droplets

To solve the problem of adhesion of aluminum fluid to the inner wall of the vacuum ladle in the aluminum electrolysis industry, molecular dynamics simulation is performed to research the wetting behavior of Al droplets on the surfaces of the α-Al2O3 substrates C (0001), M ( 11ˉ00 ), and R ( 11ˉ02 ) at 1073 K. Meanwhile, the adhesion characteristics of the Al droplet are evaluated by the potential of the mean force (PMF) for the separation of the Al droplets from different surfaces of the α-Al2O3 substrate. The results show that the wetting behavior of Al droplets on the α-Al2O3 substrate is influenced by the different crystallographic orientations. The diffusion of Al droplets in the x-o-y plane of the substrate exhibits isotropic. The PMF and the interfacial potential energy reveal that the magnitude of the adhesion work in the solid-liquid separation of Al droplets from α-Al2O3 substrates follows the order C (0001) > R ( 11ˉ02 ) > M ( 11ˉ00 ). These findings characterize the wetting properties and adhesion behavior of Al droplets on an atomic scale and provide a theoretical basis for the selection of materials for the inner wall of the vacuum ladle.

Efficient lithium recovery from electrolytic aluminum slag via an environmentally friendly process: Leaching behavior and mechanism

The electrolytic aluminum industry generates a large amount lithium (Li)-containing electrolytic aluminum slag (EAS) annually, and this can result in severe environmental pollution and wasting of Li resources. This paper proposes a method for recovering Li from the slag using AlCl3 as the leaching agent. The effects of the leaching temperature, AlCl3 concentration, pH, and solid-to-liquid ratio (S/L) on the leaching efficiency of Li+ were systematically examined. The structural transformation during the Li leaching process was revealed by SEM, XRD, 27Al NMR, and XPS analyses. The results indicate that up to 88.3% of Li + can be leached out using 0.85 M of AlCl3 with an S/L of 1:3 at a leaching temperature of 95 °C and pH of 0.5. The addition of AlCl3 played an essential role in the decomposition of cryolite/cryolithionite, ultimately resulting in Al–F fracture and Li+ release in the leaching solution. Moreover, Al3+ enters the interior of the slag to participate in the reconstruction of the chiolite with a low NaF/AlF3 ratio, and reacts with OH− ionized from H2O to form hydroxyl aluminum fluoride species. Fluorine ions were solidified in the leaching residues for subsequent cryolite regeneration. This study not only provides an environmentally friendly process for resource utilization from EAS but also reduces the environmental threat caused by the storage of the hazardous waste.

Structure characteristics and adsorption performance of graphene oxide prepared by spent carbon cathode-an ultra-low temperature graphitized carbon material.

Spent carbon cathode (SCC) is a hazardous waste from the aluminum electrolysis industry. It is commonly used as a carbon source in the current disposal and recovery strategies, such as combustion, or as a reductant for smelting. The novelty of this study is to propose a strategy for recycling SCC as a graphite resource and to investigate the unique structural characteristics and adsorption properties of graphene oxide produced from this low temperature graphitized carbon. The adsorption kinetics and isotherms of SCC-GO on methylene blue (MB) were studied and compared with the GO prepared from natural flake graphite (NFG) and artificial graphite (AG). The results show that SCC-GO exhibits the highest adsorption rate and adsorption capacity (647.83 mg/g) for MB, which is much higher than NFG-GO (451.22 mg/g) and AG-GO (533.12 mg/g). The analysis of the spectroscopy and morphology confirmed that SCC-GO has a high degree of crystal defects, oxidation, and surface wrinkle. Overall, this study reveals the unique structure of SCC-GO and highlights its significant scientific and application potential as an ultra-low temperature graphitized carbon. This research is also significant for recycling aluminum electrolytic cathode solid waste in the form of a graphite source.

Study on preparation of aluminum fluoride from cryolite

As China's aluminum electrolysis industry expanded its production capacity, it produced a large amount of bath materials– cryolite based molten salt, which is not utilized fast enough and piled up in aluminum factories. It is of great significance to find a proper way to process the accumulated bath materials. Through the method introduced in this paper, aluminum fluoride can be produced from cryolite-based electrolyte and fed back into the aluminum electrolysis cell as a necessary additive. This method includes four steps: Calcination of crystalline aluminum nitrate with cryolite. Cryolite and crystalline aluminum nitrate are mixed and preheated. The main chemical reaction that occurs in this step will produce sodium nitrate (NaNO3), β-aluminum fluoride (β-AlF3), chiolite (Na5Al3F14) and the aluminum hydroxyfluoride hydrate (AHF). After calcination, the products are washed with water to separate soluble NaNO3 from the other products. The insoluble products obtained after washing are calcined with ammonium bifluoride to convert AHF into AlF3. The mixture obtained after preheating is then washed with an aqueous solution of aluminum salt (such as Al(NO3)3) to remove Na5Al3F14. Aluminum fluoride of high purity is obtained in the end. Calcination step is the most important among the four steps. The effects of calcination temperature (85 – 135 ℃) and molar ratio of crystalline aluminum nitrate to cryolite (0.5 – 2) on the conversion rate of cryolite were studied. The experimental results show that the highest conversion rate of cryolite can reach 97.03%. Through SEM, EDS, ICP and other analysis methods, it was found the final products contained more than 98.8 wt% aluminum fluoride and less than 0.4 wt% sodium, and the average particle size of aluminum fluoride is approximately 15 – 20 µm.

Intelligent Online Inspection of the Paste Quality of Prebaked Carbon Anodes Using an Anomaly Detection Algorithm

Prebaked carbon anodes are a critical consumable in the aluminum electrolysis industry. Prebaked carbon anode paste is the intermediate product of the prebaked carbon anode, and its quality significantly impacts the prebaked carbon anode. Therefore, inspecting the quality of the prebaked carbon anode paste is essential. Currently, the quality inspection of the paste still relies on laboratory analysis or manual experience. A laboratory inspection cannot obtain results in real time, while manual inspection poses potential risks. To address these issues, an online intelligent inspection method for prebaked carbon anode paste based on an anomaly detection algorithm was proposed. Firstly, we acquired the temperature of the paste and the power of the kneading motor. Secondly, we transformed these time-series data into images using the Gramian Angular Field (GAF) technique and joined them to create the paste anomaly detection dataset. Thirdly, we trained a matched anomaly detection model based on the PatchCore algorithm. Finally, we compared two advanced models: HaloAE and TSRD. PatchCore performs best on our dataset with an AUC-ROC score of 0.9943, followed by HaloAE (0.9906) and TSRD (0.9811). Our proposed method enables on-time intelligent inspection of prebaked carbon anode paste quality. This eliminates the need for manual inspection, reduces labor requirements, and ensures worker safety.

Melting and Crystallization Behaviors of Mold Flux with the Addition of Waste Electrolyte

The influence of waste fluoride electrolyte addition on key properties of mold flux is investigated, with the aim to convert the hazardous waste in electrolytic aluminum industry into treasure in steel industry. The results show that the initial melting temperature of mold fluxes decreases from 1614.40 K (1341.40 °C) to 1564.67 K (1291.67 °C), and the complete melting temperature decreases from 1704.45 K (1431.45 °C) to 1674.34 K (1401.34 °C) with the addition of waste electrolyte from 10.08 to 24.89 wt%. The crystallization of mold flux is enhanced, and its crystallization mechanism transforms from “constant nucleation rate and 2D growth” into “constant nucleation number and 3D growth” with the addition of waste electrolyte. The main precipitated crystals in mold fluxes are grossular (Ca3Al2(SiO4)3) and CaF2, but the phase transforms into gehlenite (Ca2(Al(AlSi)O7) at higher temperature. In addition, the crystallization activation energy is 13.80, 12.40, 12.01, 11.03, and 10.27 kJ mol−1, respectively, which further confirms that the addition of waste electrolyte can reduce the energy barrier of crystallization and promote the crystallization ability of mold flux.

Extraction of ferronickel concentrate from laterite nickel ore by reduction roasting-magnetic separation using spent cathode carbon

Laterite nickel ores are the most important raw materials in nickel production, but processing laterite nickel ores with a low nickel content is difficult. Annually, the aluminum electrolysis industry produces a large amount of spent cathode carbon containing toxic substances, such as fluorine and cyanide, but a considerable amount of it cannot be effectively utilized. The purpose of the present study is to prepare ferronickel concentrates by reduction roasting–magnetic separation using spent cathode carbon as a reducing agent. Different factors affecting the agglomeration and growth of metal particles are studied. The migration of fluorine and the aggregation behavior of iron and nickel after the addition of spent cathode carbon and CaO are studied. Results show that the nickel grade in the ferronickel concentrate is 5.4 wt%, the nickel recovery rate is 84.80%, the iron grade is 67.93 wt%, and the iron recovery rate is 89.91%. Efficient recovery and utilization of nickel and iron in laterite nickel ores are realized. Waste utilization of spent cathode carbon thus provides a new method for the treatment of spent cathode carbon.

Separation and recovery of valuable elements from acid leachate of spent carbon cathode by fractional precipitation method

Spent carbon cathode (SCC), a hazardous waste from aluminum electrolysis industry, has been confirmed that it can be direct treated and purified by hydrothermal acid leaching in the previous study. This work focuses on a hydrometallurgical process to separate and recover the valuable elements from the acid leachate of SCC by fractional precipitation method. Thermodynamic calculation, single factor experiment and characterization of the products are utilized to obtain the ion-transformation mechanism, optimize the conditions of process and confirm the physicochemical properties of the products. The results indicate that 92.6 % Si, 93.07 % Al and more than 99 % Fe, Ca are recovered from the acid leachate in the form of Na2SiF6, Na3AlF6, Fe(OH)3 and CaF2 respectively and concentration of F- in acid leachate decrease from 3115 mg/L to 6.6 mg/L when the conditions attach the optimal. Finally, Na and Cl in acid leachate is recovered in the form of NaCl via evaporation crystallization. The whole process completes the separation and recovery of all elements in acid leachate of SCC, which has no wastewater discharge and is a closed-cycle route.

Influence of B2O3 on viscosity and structure of the CaO–Al2O3-waste electrolyte (WE) based melt

The fluorides contained waste electrolyte (WE) from the electrolytic aluminum industry can be used as a substitution of fluorite (CaF2) in the newly designed mold flux. In this study, the influence of B2O3 on viscosity and structure of CaO–Al2O3-WE based melt was investigated. Results show that the viscosity of mold flux melt decreases with both increasing temperature and B2O3 content. The apparent activation energy (Ea) also reduces from 78.96 ± 1.75 to 55.26 ± 2.79 kJ/mol with the addition of B2O3 from 0 to 7 wt%. The analyses of fourier transform infrared (FTIR) and Raman spectroscopies suggest that the lower symmetry of the original aluminate and silicate structure due to the insertion of [BO4]-tetrahedral and [BO3]-triangular, and the formation of more non-bridging oxygen (O−) and 2D structural units in the network with the addition of B2O3, deceases the viscosity and Ea of the CaO–Al2O3-WE Based Melt.

Pilot experimental study on pollutant emission characteristics from co-combustion of coal and spent cathode carbon block

Spent cathode carbon block (SCCB) is a hazardous waste produced by the electrolytic aluminum industry. Collaborative treatment of the SCCB in pulverized coal boilers is a potentially valuable technology. Pilot experiments were carried out on a 240 t/h pulverized coal boiler and its supporting flue gas treatment measures as a function of the mixing ratio of SCCB to verify the feasibility of coal collaborative disposal of SCCB. The thermal conversion characteristics of fluoride after mixing SCCB with coal were studied. A continuous emission monitoring system was used to monitor the impact on SO2, NOx and particulate matter in real time. The effectiveness of existing flue gas control measures was evaluated, and optimization suggestions of the flue gas control measures were postulated. The content of heavy metals (Hg, Cd, Ni, Pb, As, Se) in solid waste from co-combustion and the physical phase of solid waste were investigated. In addition, the transformation of heavy metals such as Hg, Cd, Ni, Pb, Cr, As and fluoride in defluorination wastewater during flue gas treatment were analyzed. The results show that it is feasible to use a pulverized coal boiler to dispose SCCB. The existing flue gas treatment measures proved to be appropriate, and desulfurization wastewater can achieve the discharge standards. Defluorination gypsum, fly ash and slag are not hazardous waste. When the mixing ratio of SCCB is increased, the existing neutralization and flocculation precipitation processes of the desulfurization wastewater are affected. It is suggested to add Al2O3 for the defluorination process before the desulfurization tower.

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.

Recovery of Carbon and Cryolite from Spent Carbon Anode Slag Using a Grinding Flotation Process Based on Mineralogical Characteristics

The aluminum electrolysis industry continually and unavoidably produces hazardous solid waste in the form of carbon anode slag. Carbon anode slag poses a serious environmental pollution risk, and it must be disposed of in a harmless manner. On the other hand, it contains a few valuable resources, as well. In order for the aluminum electrolysis industry to develop in an environmentally friendly and high-quality manner, the harmless disposal of carbon anode slag and its resourceful utilization are of considerable importance. The selective comminution of carbon and cryolite particles in carbon anode slag can be effectively achieved with grinding pretreatment. However, the optimization study of grinding process parameters has yet to be investigated. Therefore, firstly, the mineralogical characteristics and existing mode of carbon anode slag from the perspective of mineralogical properties are analyzed in this study. Then, the effects of grinding time, grinding concentration, and steel ball diameter on the particle size of the ground product (γ−0.074 mm) are investigated using response surface analysis. The results showed that the effect of grinding time was the most significant, followed by grinding concentration and steel ball diameter. In addition, the performance of the multi-stage flotation process for separating the −0.074 mm ground product was analyzed. Cryolite with a purity of 93.12% and a carbon product with an ash content of 10.67% could be simultaneously obtained through multi-stage flotation. It should be pointed out that the deep dissociation and efficient recovery of fine undissociated particles still need to be further explored.

Extremely high corrosion resistance of ZnxNi1–xCr2O4 spinel as sidewalls in the aluminum electrolyte

The sidewall material is a key component in new electrolytic cell with an inert electrode for the aluminum electrolysis industry. The continuous development of novel sidewall materials with excellent corrosion resistance in molten salts electrolyte is an important topic. Herein, a new system of sidewall material, spinel structured ZnxNi1–xCr2O4 (x = 0 – 1), is prepared by solid-phase reaction and the corrosion-resistance enhancement is investigated. The results prove that Zn2+ plays two roles in the ZnxNi1–xCr2O4 spinels. Firstly, Zn2+ tunes the surface energies of spinels resulting in the octahedral grains, which suppresses the cation diffusion in the corrosion process. Secondly, Zn2+ stabilizes the Cr3+ in the spinels. As a result, the Zn0.5Ni0.5Cr2O4 spinel displays an extremely low corrosion rate ∼0.007 cm·a–1 in NaF-KF-AlF3 bath at 800 °C comparing with other sidewall materials. The as-obtained spinel shows great potential as a novel sidewall material for the new electrolytic cell.

Ionic liquids as electrolytes in aluminum electrolysis.

Herein, the characteristics, research progress, and application prospects of ionic liquid-based electrolytic aluminum deposition are reviewed and analyzed in comparison with the Hall-Héroult method. The reaction conditions and production procedures of this material are discussed alongside the problems ionic liquids face in the electrolytic aluminum industry. Ionic liquid-based electrolytic aluminum deposition realizes the electrolytic aluminum reaction at low temperatures, achieving a reaction energy consumption close to the theoretical minimum value. The reaction also avoids harmful CO2 or HF emissions, demonstrating a green and environmental-friendly approach to the production of electrolytic aluminum. In the future, in-depth work on the implementation of ionic liquid electrolytes should be carried out, establishing the necessary technical criteria and laying the foundation for the integration of this approach.

Comprehensive performance study of aluminum ash and calcium carbide slag for brick making under ultra-high pressure

Aluminum ash is a large amount of industrial solid waste produced by the electrolytic aluminum industry, and calcium carbide slag is an alkaline industrial solid waste produced by the energy industry. Both of them will cause serious pollution risks to the environment. Its harmless and resource-based treatment has become one of the main research directions. In this paper, the ultra-high pressure contact molding technology is used for the first time to prepare high-performance building materials with aluminum ash and calcium carbide slag as raw materials. The changes of mechanical properties and waterproof properties of bricks under different proportion, pressing pressure, curing time and curing conditions were investigated. Finally, the leaching toxicity was investigated. The experimental results show that the best ratio of aluminum ash and calcium carbide slag is 1:9, the best pressure is 300mpa, the best curing time is 5d, and the best curing condition is natural curing. The compressive strength and flexural strength of the brick are about 75 MPa and 3.6 MPa respectively, and the softening coefficient is about 0.99. This shows that the bricks formed by ultra-high pressure contact have high mechanical properties and waterproof properties, and the leaching toxicity meets the class V water quality standard in the environmental quality standard for surface water (GB3838-2002). The brick was characterized by SEM, EDS, XRD and particle size analysis. By further exploring its comprehensive properties, it is proved that ultra-high pressure contact molding is feasible in the resource utilization of aluminum ash and calcium carbide slag. This study provides a feasible idea for the harmless and resource treatment of bulk solid waste, simulate the change of minerals under surface pressure in the earth’s crust, providing a research path for the solidification and stabilization of pollutants.

A closed-circuit cycle process for recovery of carbon and valuable components from spent carbon cathode by hydrothermal acid-leaching method.

Spent carbon cathode (SCC) as a hazardous solid waste produced in aluminum electrolysis industry, contains plenty valuable components but generate a seriously threat to the environment. This paper focus on a closed-circuit cycle process for direct treatment of SCC based on the hydrothermal acid-leaching method. Thermodynamic calculation, single factor experiment, orthogonal experiment and kinetic study are utilized to obtain the leaching properties of impurities, optimize the leaching conditions, study the influence of conditions on leaching, and capture the restriction factors of leaching. The results indicate that the carbon content of the treated SCC can reach 97.3% when the leaching condition attach the optimal (liquid-solid ratio of 25mL/g, temperature of 413K, time of 270min and acid concentration of 4mol/L), and liquid-solid ratio is regarded as the crucial factor influencing on that. In addition, the activation energy of impurities reaches 6.25kJ/mol and the whole leaching process is controlled by the diffusion extent. Finally, the filtrate after the hydrothermal acid leaching is treated, and calcium fluoride, cryolite and sodium chloride are successfully separated. The proposed process eliminates the harm of SCC to the environment, and completes a closed-circuit cycle for the treatment of SCC and recovery of valuable components. It enriches the hydrometallurgical processes of SCC, and provides an attractive scheme for the treatment of SCC.

Hazard identification & risk control in aluminum production

With the rapid development of China's economy, the safe production of electrolytic aluminum is becoming increasingly prominent. It is particularly important for the production safety of the electrolytic aluminum industry to identify the hazards of electrolytic aluminum operation and find a practical and effective operational risk control method. A two-dimension hazard classification method and an evidence - based hazard identification framework are put forward. A process oriented hazard evolution and risk control method system is constructed, which provides a theoretical method for systematically investigating hazards, reducing risks and preventing and controlling accidents. Taking the molten aluminum explosion accident in an electrolytic workshop section as an example, the hazards were identified, the process from hazards to accident was analyzed by means of a fault tree, event tree and bow tie diagram, and a hazard evolution path was selected to set up safety measures to explain the feasibility of risk control methods.

Fluoride-immobilized co-processing and resource utilization of aluminum-electrolyzed spent cathode carbon in brick-fired kiln

Spent cathode carbon (SCC) is hazardous waste from the electrolytic aluminum industry due to its high levels of soluble fluoride, while brick-fired kiln provides the clay and heating conditions needed to immobilize fluoride. However, SCC reusing is still understudied, meanwhile co-processing and resource utilization of SCC in brick-fired kiln were still not reported in the literatures in addition to a Chinese patent of the authors. Here, the effects of firing temperatures, firing time, clay doses andcalcium doses on the fluoride-immobilized performance of SCC co-processing were explored in a simulated brick-firing kiln, and their mechanisms were analyzed by SEM and XRD. The results indicated that clay-added co-processing in brick-fired kiln was a preferred choice without required additional additives or operations. The leached fluoride met Chinese standards by clay-added co-processing at ≥ 800°C/ ≥ 40g clay/ ≥ 120min. Clay and calcium-added co-processing in brick-fired kiln was another alternative choice with higher fluoride-immobilization rates. The leached fluoride met Chinese standard (GB5085.3-2007) by clay and calcium-added co-processing at ≥ 500°C/ ≥ 30min/ ≥ 5g clay/ ≥ 0.5g CaCO3. SEM and XRD indicated that SiO2 in clay reacted with sodium in SCC and formed vitreous analog (Na1.55Al1.55Si0.45O4) to prevent fluoride ion migration and the newly-formed k-Feldspar (K2O.Al2O3.6SiO2) may adsorb fluoride ions in clay-added co-processing. Soluble fluoride NaF in SCC were converted into water-insoluble cuspidine in clay and calcium-added co-processing, in addition to the crystalline phase conversion in clay-added co-processing. Therefore, the risks of finished bricks to human health and the environment were greatly reduced after clay-added or clay and calcium-added treatments.