Aluminum Electrolysis Research Articles

(Digital Presentation) Influence of Pulse Electrolytic Conditions on Deposition Morphology of Electrolytic Aluminum Foil Using Chloroaluminate Ionic Liquids

We have investigated the influence of additive agent on the deposition morphology of the electrolytic Al foils using the AlCl3–1-ethyl-3-methylimidazolium chloride (EMIC) (molar ratio 2 : 1) bath. The electrolytic Al foils were prepared by the pulse electrolysis. The morphology and the surface roughness of the electrolytic Al foils were characterized by FE-SEM, AFM, and XRD. The surface roughness and the crystallite size of the electrolytic Al foils were decreased by increasing the pulse frequency and the addition of 1,10-phenanthroline (OP). It was found that the OP addition had more greatly influence on the surface roughness of the electrolytic Al foil than the pulse frequency.

Sintering kinetics and properties of NiFe2O4‐based ceramics inert anodes doped with TiN nanoparticles

AbstractSintering kinetics of NiFe2O4‐based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe2O4‐based ceramics possessed high‐temperature semiconductor essential characteristics. The electrical conductivity of NiFe2O4‐based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe2O4‐based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.

Purification of spent cathode carbon from aluminum electrolysis cells by alkali fusion-acid leaching process

Purification of spent cathode carbon from aluminum electrolysis cells by alkali fusion-acid leaching process

Distributed Robust Dictionary Pair Learning and Its Application to Aluminum Electrolysis Industrial Process

In modern industrial systems, high-dimensional process data provide rich information for process monitoring. To make full use of local information of industrial process, a distributed robust dictionary pair learning (DRDPL) is proposed for refined process monitoring. Firstly, the global system is divided into several sub-blocks based on the reliable prior knowledge of industrial processes, which achieves dimensionality reduction and reduces process complexity. Secondly, a robust dictionary pair learning (RDPL) method is developed to build a local monitoring model for each sub-block. The sparse constraint with l2,1 norm is added to the analytical dictionary, and a low rank constraint is applied to the synthetical dictionary, so as to obtain robust dictionary pairs. Then, Bayesian inference method is introduced to fuse local monitoring information to global anomaly detection, and the block contribution index and variable contribution index are used to realize anomaly isolation. Finally, the effectiveness of the proposed method is verified by a numerical simulation experiment and Tennessee Eastman benchmark tests, and the proposed method is then successfully applied to a real-world aluminum electrolysis process.

High-efficiency and rapid cyanide removal in SPL derived from aluminum electrolysis production under microwave and sensitizer synergistic action

Spent potliner (SPL) has been listed as a hazardous waste derived from aluminum electrolysis production with high cyanide content. In this study, a novel microwave-sensitizer synergistic technique was proposed to rapidly remove cyanides in SPL. The occurrence states and removal behavior of cyanide in SPL were studied systematically. The results show that cyanide in SPL mainly exists in the form of sodium cyanide and ferricyanide . The maximum cyanide removal rate of 96.67% was successfully achieved under the experimental conditions. Meanwhile, the synergistic removal mechanisms were comprehensively investigated by XRD , XRF , TG-DTA, TG-MS, and FactSage thermal simulation software. The sensitizer Fe 2 O 3 can accelerate the decyanation reaction by decreasing activation energy and acting as an oxidant. This study not only develops a high-efficiency and rapid cyanide removal technique, but also provides theoretical guidance for non-hazardous treatment of other cyanogens-containing wastes.

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.

The Development Scenarios and Environmental Impacts of China's Aluminum Industry: Implications of Import and Export Transition.

Aluminum is widely used in buildings, transportation, and home appliances. However, primary aluminum production is a resource, energy, and emission-intensive industrial process. As the world's largest aluminum producer, the aluminum industry (ALD) in China faces tremendous pressure on environmental protection. This study combines material flow analysis and scenario analysis to investigate the potential of resource conservation, energy saving, and emission reduction for China's ALD under the import and export trade transition. The results show China's per capita aluminum stock will follow a logistic curve to reach 415 kg/capita by 2030. However, unlike the continued build-up of stocks, domestic demand for aluminum will peak at 44 million tons (MT) in 2025 and fall to 36 MT in 2030. The scenario analysis reveals that China's primary aluminum output could peak in 2025 at around 52 MT if the restrictions are not implemented (Scenario A). Compared to Scenario A, demand for primary aluminum is effectively limited in Scenarios B and C where exports of aluminum products are reduced. Correspondingly, both scenarios also have obvious benefits in reducing the environmental load of China's ALD. Besides, if hydropower used in aluminum electrolysis increases to 25% by 2030, the total GHG emissions in 2030 will be reduced by 12%. Therefore, promoting import/export and energy mix transformation can become an essential means for the sustainable development of China's ALD.Graphical Supplementary InformationThe online version contains supplementary material available at 10.1007/s40831-022-00582-0.

Effective Extraction of the Al Element from Secondary Aluminum Dross Using a Combined Dry Pressing and Alkaline Roasting Process.

Secondary aluminum dross (SAD) is a hazardous solid waste discharged from aluminum electrolysis and processing and the secondary aluminum industries, which causes severe environmental pollution and public health disasters. The stable presence of the α-Al2O3 and MgAl2O4 phases in SAD makes it difficult for it to be efficiently utilized. A combined dry pressing and alkaline roasting process was proposed for extracting the valuable Al element from SAD. Two alkaline additives (NaOH and Na2CO3) were selected as a sodium source for extracting the aluminum source from SAD in order to perform the thermodynamic analysis and roasting experiments. The phase transition behavior and the leaching performance tests were conducted using X-ray diffraction, scanning electron microscopy, X-ray fluorescence, leaching kinetics and thermal analysis. The recovery of Al and Na reached the values of 90.79% and 92.03%, respectively, under the optimal conditions (roasting temperature of 1150 °C, Na2CO3/Al2O3 molar ratio of 1.3, roasting time of 1 h, leaching temperature of 90 °C, L/S ratio of 10 mL·g−1 and leaching time of 30 min). Meanwhile, the removal efficiency of N and Cl reached 98.93% and 97.14%, respectively. The leaching kinetics indicated that the dissolution of NaAlO2 clinkers was a first-order reaction and controlled by layer diffusion process. The green detoxification and effective extraction of the Al element from SAD were simultaneously achieved without any pretreatments.

Characteristics of generated aerosol suspensions-complexes at traditional and modernized aluminum electrolysis technologies

Introduction. Assessment of occupational risk and mechanisms of workers' health disorders due to exposure to complex aerosol suspensions determines the need for in-depth study of the physicochemical properties of dust particles in the air. The study aim was the assessment of the dispersion and chemical composition of complex aerosol suspensions affecting workers in traditional and modernized aluminum production technologies. Materials and methods. The monitoring of air pollution of the working area with soluble and insoluble fluorides, aluminum trioxide aerosols was carried out using standard analysis methods, the dispersed and chemical composition of aerosol suspensions was studied using scanning electron microscopy and energy dispersive X-ray microanalysis. Results. The highest average shift concentration of fluorides, exceeding the occupational exposure limit by 4.7-12.5 times, are observed in the working area of professions serving electrolyzers and anodes, with a predominance of insoluble fluorides over soluble ones. Cases of exceeding the occupational exposure limit of aluminum trioxide by 1.9-2.6 times were noted. Dust suspended in the air of the working area consists of highly and ultradispersed aerosol mixtures of various chemical nature, including particles of the nanoscale range, impurities of heavy metals and toxic compounds. Highly dispersed dust particles, mainly alumina and fluorocarbon compounds, dominated in the air of workshops with traditional aluminum electrolysis technology, while micro- and nanoparticles, consisting mainly of cryolite and a mixture of aluminum fluoride with alumina, dominated in the modernized one. Conclusion. The exposure of complex multicomponent aerosol mixtures of aluminum production can pose a danger to the health of workers, which requires an in-depth analysis of the chemical and dispersed composition of aerosols when assessing the exposure of the dust factor and improving the complexes of preventive measures to prevent the development of diseases.

Comparative analysis of hydrogen production technologies: Hydrothermal oxidation of the "carbonless" aluminum and water electrolysis

The paper deals with the technical and economic comparison of methods of hydrogen production by means of water electrolysis and hydrothermal oxidation of aluminum. Technological chains required for the realization of these methods are analyzed, costs of equipment and maintenance are evaluated. It is demonstrated that for the large-scale production of hydrogen aluminum-water technology is competitive with electrolysis option under the cost of electricity up to 0.02 USD/kWh. The conditions of the maximal feasibility of hydrogen generation using aluminum are specified. Metal hydride thermal sorption compressor is proved to be the best device to commercialize the generated hydrogen. Its application lowers the cost of compression 5–10 times.

Calcific stabilization and clay solidification of fluorides in graphitized sidewall blocks from aluminum electrolysis

Few studies have reported the hazard-free treatment of graphitized sidewall block (GSB), which is a numerous-fluoride-containing hazardous waste from aluminum electrolysis. The performance, parameters, and mechanism of the hazard-free treatment of GSB via calcific stabilization (Calci-S), clay solidification (Clay-S), and their co-processing (Co–S/S) were investigated. The single-variable experimental results showed that Calci-S and Clay-S were both favorable hazard-free choices for treating GSB, whereas Co–S/S was not a reasonable choice. Using Calci-S, the leached fluoride concentrations met the standards of China, Brazil, and Singapore at temperatures above 700 °C for over 30 min with more than 3 g CaCO3, and using Clay-S, leached fluoride concentrations met the standards at temperatures above 850 °C for over 90 min with more than 30 g clay. Response surface methodology results indicated that the decreasing order of impact on the fluoride immobilization percentages of Calci-S and Clay-S was doses, heating temperatures, and heating time. The optimum parameters of Calci-S were at 582 °C for 94 min with 4 g CaCO3, and those of Clay-S were at 838 °C for 175 min with 61 g clay. The mechanism analysis of fluoride immobilization using x-rays diffraction and scanning electron microscope showed that the soluble fluorides, NaF and AlF3, in GSB was converted into water-insoluble CaF2 and cuspidine in Calci-S, whereas SiO2 in clay reacted with sodium in GSB and formed a glass-like viscous diffusion barrier (Na23.55Al24.00Si24.00O96) encapsulating the fluoride ions in Clay-S. To summarize, this study presents the two innovative and effective hazard-free approaches of Calci-S and Clay-S for GSB and provides valuable insights into the fluoride immobilization mechanism, thereby providing more options for GSB disposal.

Effect of Pre-Anodized Film on Micro-Arc Oxidation Process of 6063 Aluminum Alloy.

In the current investigation, micro-arc oxidation (MAO) ceramic coatings on aluminum are galvanostatically synthesized at various processing stages in an alkaline silicate system. The resultant coatings are systematically investigated in terms of the following respects: The working voltage and surface sparking evolution over the studied course of MAO are recorded by the signal acquisition system and the real-time imaging, respectively; the phase composition, the surface morphology, and the polished cross-section of the coatings are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) assisted with an energy-dispersive X-ray spectrometer (EDS), respectively. In particular, with the help of a low-rate increase in working voltage, the evolution of the sparks, the energy consumption, and the microstructure development of aluminum in alkaline silicate electrolyte by pre-anodizing are systematically investigated. The results show that the pre-anodized film can accelerate the evolution process of MAO spark and shorten the reaction process in the early stage of MAO reaction, reducing energy consumption and improving the corrosion resistance of the MAO coating. The γ-Al2O3 phase content after pre-anodized is significantly increased in MAO coatings. In particular, the thicker the pre-anodized film (beyond 8 μm) was broken down and fragmentation thinning in the early stage of the MAO process with the presence of micro discharges. This is due to the fact that the electron transition will be released by the emission of radiative recombination and reveals obvious galvanoluminescence (GL) behavior on the surface of the pre-anodized film. Further, based on the present MAO coating microstructure, a model of coating growth after pre-anodized that evolves over time is proposed.

Resource utilization strategies for spent pot lining: A review of the current state

During a long-term operation of the aluminum electrolysis cell, the molten salts continuously infiltrate and corrode the pot lining, resulting in a huge amount of hazardous waste after the overhaul. The first cut of spent pot linings (SPL) contains several carbon-rich materials with potential economic value and hazardous matters such as soluble fluorite and cyanide. The continuous accumulation and disposal of SPL in depots or landfills have created a severe challenge to the aluminum industry. Nowadays, the technologies of harmless disposal and resource utilization of SPL have been paid more attention. This work has explored and presented the properties of SPL (including chemical, physical, and thermodynamic properties, etc.) and methods for its detoxification and purification. In this regard, the resource utilization strategies of SPL are systematically sorted out from three aspects: traditional, co-processing, and high-value technologies. In the end, the current challenges and future perspectives for the environmental recycling of SPL have been analyzed and summarized.

Thermal Behavior of the early Life of an Aluminum Electrolysis Cell

The electrolytic production of aluminium starts after the completion of the cathode lining and the baking process. After that, the cell start-up period is followed by the early operating period. During the early operating period, the following parameters (cell voltage, metal height, electrolyte height, cryolite ratio, electrolyte temperature, and ledge formation) were measured and investigated. The required times for these parameters to reach the steady-state have been investigated. The cell voltage, metal height, electrolyte height, cryolite ratio, and electrolyte temperature were stabilized after 35, 25, 24, 86, and 45 days, respectively from cell start-up. These cells took four months to form a stable ledge at a thickness of 10 cm. Also, the thermal behavior of the sidewall carbon blocks was studied during the early operating period by inserting twenty thermocouples at these locations in three prebaked cells. The cell instability during the early operation period for these cells was illustrated.

Hydrometallurgical stepwise separation of alumina and recovery of aluminum fluoride from waste anode cover material of aluminum electrolysis

Anode cover material holds marked importance owing to its effect of maintaining the thermal balance of aluminum reduction cells, and is categorized as solid waste because of its continuous accumulation. However, cover materials contain large amounts of alumina and fluoride; thus, can be considered for strategic resource recovery. This research has developed a process for separating alumina and recovering aluminum fluoride from anode cover materials by aluminum salt solution leaching. In the present work, the effects of different factors on the leaching behaviors of sodium and fluoride during aluminum salt solution leaching were investigated. The results showed that 98.1 % sodium and 95.1 % fluoride were extracted when the Al3+ concentration was 2.5 mol/L, the liquid to solid ratio was 20 mL/g, and the stirring speed was 300 rpm at 90 °C for 2 h. Aluminum and fluoride were then precipitated as aluminum hydroxyfluoride hydrate (AHF) from the leachate. The aluminum fluoride (AlF3) product with high purity was obtained by calcining AHF precipitation, and the average particle size was 66.1 μm. This work demonstrates significant application potential on green recycling of anode cover materials and realize a high-value utilization of aluminum and fluoride resources. This plays a crucial role in the life cycle assessment (LCA) of the anode cover material.

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.

Investigation of phase transformation related electrical conductivity of long-term heat treated aluminium electrolysis cathodes

This study presents an investigation on the specific electrical conductivity of the cathode materials used in an aluminium electrolysis cell over a temperature range between room temperature and 950 °C. Those materials are subjected to a diffusion related aging process due to the high operating temperature of the cell, leading to a change in chemical composition and microstructure. The materials were investigated both in the initial state before use in an aluminium electrolysis cell and after an operating period of 5 years. It is shown that the changes in chemical composition and thus also in microstructure over the service life at elevated operating temperature exert an effect on the electrical conductivity. In addition, calculations based on thermodynamic data were used to relate phase transformations to the changes in electrical conductivity. On the one hand, the electrical conductivity of the collector bar at 950 °C is reduced by about 11% after 5 years of service. On the other hand, the ageing process has a positive influence on the cast iron with an increased conductivity by about 41% at 950 °C. The results provide an understanding how diffusion related processes in the cathode materials affect energy efficiency of the aluminium electrolysis cell.

Solidification/stabilization of spent cathode carbon from aluminum electrolysis by vitric, kaolin and calcification agent: fluorides immobilization and cyanides decomposition.

Spent cathode carbon (SCC) is a hazardous waste containing fluorides and cyanides from aluminum electrolysis. Many literatures have focused on SCC leaching; however, SCC hazard-free treatment remains understudied. This article used 10.0 g raw SCC sample to explore the vitric/kaolin solidification and calcium stabilization of SCC, and analyze their hazard-free mechanisms by the methods of XRD and SEM. The leached fluorides were all below the Chinese identification standard for hazardous wastes (GB5085.3-2007), whether at 750/950 °C for 60 min above 8.0 g vitric, or at 1200 °C for 120 min with above 8.0 g kaolin, or above 700 °C for more than 30 min with above 0.5 g CaCO3. Kaolin/vitric solidification relied on the massive addition of vitric and kaolin to produce glassy or glass-like material (K2O·Al2O3·6SiO2) which may retain fluoride. Calcium stabilization converted soluble fluoride NaF in raw SCC sample into insoluble CaF2. Heating 60 min at 500-1200 °C at oxygen atmosphere decomposed almost of cyanides, with leached cyanides meeting Chinese standard GB5085.3-2007. Mass-loss rates of kaolin addition came from a large amount of adsorbed water and structural water in kaolinite and illite wai lost, and that of CaCO3/CaSO4 addition was attributed to their decomposition into volatile CO2/SO2, while that of CaO was a little negative due to its absorption of water vapor and CO2. In brief, as the effective hazard-free manner of SCC, both kaolin/vitric solidification and calcium stabilization successfully have achieved fluoride immobilization and cyanide decomposition.

A Sobering Examination of the Feasibility of Aqueous Aluminum Batteries

Aqueous aluminum (Al) batteries are posited to be a cheap and energy dense alternative to conventional Li-ion chemistries, but an aqueous electrolyte mediating trivalent aluminum cations (Al3+) warrants greater scrutiny. This study provides a rigorous examination of aqueous Al electrolytes, with the first compelling evidence for a dynamic octahedral solvation structure around the Al3+, without Al-OTf contact ion pairs, even at high concentrations. This solvation behavior and the concomitant, transient electrostatic hydrolysis of Al-OH2 ligands contrasts strongly with previously reported water-in-salt electrolytes, and occurs due to the high charge density of the Lewis acidic Al3+. Nuclear magnetic resonance spectroscopy and other physicochemical measurements quantitatively reveal how species activity evolves with concentration and temperature. This new understanding exposes practical concerns related to the corrosiveness of the acidic aqueous solutions, the degree of hydration of aluminum trifluoromethanesulfonate (Al(OTf)3) salt, and the grossly insufficient reductive stability of the proposed electrolytes (>1 V between HER onset and Al3+/Al). Collectively, these factors constitute multiple fundamental barriers to the feasibility of rechargeable aqueous Al batteries.Table of Contents: We assess the challenges associated with aqueous aluminum electrolyte by investigating the solvation structure of the Al3+ Lewis acid in Al(OTf)3-H2O solutions. The deprotonation of water ligands in the first solvation sheath leads to early onset of hydrogen evolution in super-concentrated aqueous aluminum electrolyte. Figure 1