Aluminum Electrolysis Research Articles

China’s carbon market: Development, evaluation, coordination of local and national carbon markets, and common prosperity

Launching national carbon market with the power sector is a good start to achieve the commitments to both carbon peaking and carbon neutrality at the lowest cost. Since its operation, the carbon prices fall CNY40∼60 per ton. The block agreement transaction dominates trading, but with an average discount rate of 9.6% in block agreement, the aforementioned carbon prices overestimate the overall carbon prices. While the compliance rate measured against entities reached 94.4%, it differs significantly across provinces, ranging from 82.9% to 100%. Entities engaging in trading are mainly for compliance, and therefore transaction is driven by compliance. This article argues that the development of carbon market requires further reform of the electricity pricing mechanism and coordinated development of various related markets. The article argues that iron and steel, cement industry, and electrolytic aluminum should be given priority for inclusion of the national carbon market in the second batch, suggests necessity to establish a national carbon trading legislation and a scientific long-term mechanism for quota allocation, and emphasizes the importance of small entities in achieving the full compliance. Given the co-existence of national carbon market and regional carbon pilots, the article suggests the specific areas for the regional carbon markets to take the initiative to strengthen the synergistic effects of national carbon market. Furthermore, the article strongly recommends to continuously increase the proportion of carbon allowances auctions, and to set up a transformation fund from the proceeds of paid allocation of allowances to support regions at low levels of development and technology.

Co-treatment of spent carbon anode and copper slag for reuse and the solidification of the constituent fluorine and heavy metals

Spent carbon anode (SCA) is a typical global hazardous solid with high risk of environmental pollution. We therefore report for the first time a novel co-treatment technology for SCA and copper slag involving clearly defined transformation paths for F, Cu, Pb and Zn. The results of the investigation showed that the F and C contents in the SCA decreased the viscosity of the slag, thereby promoting the flotation recovery of Cu, whereas the F was transformed into CaF2. The PbSiO3 and ZnO contents in the slag were reduced to Pb(g) and Zn(g), respectively. Under optimal conditions, the solidification ratio of F reached 95.71%, and 81.50% Cu, 55.84% Pb and 36.48% Zn were recovered. The residual Cu content in the tailings decreased to 0.15 wt% after flotation. Toxicity characteristic leaching procedure test revealed that the F, Cu, Pb and Zn leaching contents were 46 mg/L, 4.58 mg/L, 2.17 mg/L and 10.71 mg/L, respectively, which are below their standard limits. The tailings can be reused as high-quality building materials with minimal environmental risk. The process herein reported presents a new and effective waste disposal approach, thus contributing to the sustainability of electrolytic aluminium and copper smelting industries.

Design of Soft-Sensing Model for Alumina Concentration Based on Improved Deep Belief Network

Alumina concentration is an important parameter in the production process of aluminum electrolysis. Due to the complex production environment in the industrial field and the complex physical and chemical reactions in the aluminum reduction cell, nowadays it is still unable to carry out online measurement and real-time monitoring. For solving this problem, a soft-sensing model of alumina concentration based on a deep belief network (DBN) is proposed. However, the soft-sensing model may have some limitations for different cells and different periodic working conditions such as local anode effect, pole changing, and bus lifting in the same cell. The empirical mode decomposition (EMD) and particle swarm optimization (PSO) with the DBN are combined, and an EMD–PSO–DBN method that can denoize and optimize the model structure is proposed. The simulation results show that the improved soft-sensing model improves the accuracy and universality of prediction.

A self-supervised temporal temperature prediction method based on dilated contrastive learning

Due to the scarcity of the labeled data, traditional supervised learning methods have a limited application scope, which caused the supervised-based model performance will greatly be decreased. In this paper, we propose a promising model based on self-supervised learning. To update the weight and the contrastive relation in the features, a new self-supervised loss, is introduced. First, the convolution neural network is used in the proposed network to extract the deep feature in the first processing. Second, the self-supervised long–short time memory (LSTM) sequential is constructed for further deal. At last, the teacher net and student net have coordinately fine-tuned the credibility of the temperature prediction. By the experimental comparison, our proposed CNN-SSDLSTM is competitive with other supervised and semi-supervised methods. The evaluation experiments achieve state-of-the-art performance in aluminum electrolysis temperature prediction applications.

Photoluminescent and Photocatalytic Properties of Eu3+-Doped MgAl Oxide Coatings Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy

The synthesis of Eu3+-doped MgAl oxide coatings containing MgO and MgAl2O4 was accomplished through plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate electrolyte with the addition of Eu2O3 particles in various concentrations. Their morphological, structural, and above all, photoluminescent (PL) and photocatalytic activity (PA) were thoroughly investigated. PL emission spectra of MgAl oxide coatings feature characteristic emission peaks, which are ascribed to f–f transitions of Eu3+ ions from excited level 5D0 to lower levels 7FJ. The charge transfer state of Eu3+ or direct excitation of the Eu3+ ground state 7F0 into higher levels of the 4f-manifold are both attributed to the PL peaks that appear in the excitation PL spectra of the obtained coatings. The fact that the transition 5D0 → 7F2 (electrical dipole transition) in Eu3+-doped MgAl oxide coatings is considerably stronger than the transition 5D0 → 7F1 (magnetic dipole transition) indicates that Eu3+ ions occupied sites with non-inversion symmetry. Because of oxygen vacancy formation, the Eu3+-doped MgAl oxide coatings had a higher PA in the degradation of methyl orange than the pure MgAl oxide coating. The highest PA was found in Eu3+-doped MgAl oxide coating formed in an aluminate electrolyte with 4 g/L of Eu2O3 particles. The PA, morphology, and phase of Eu3+-doped MgAl oxide coatings did not change after several consecutive runs, indicating outstanding stability of these photocatalysts.

ZnO Particles Modified MgAl Coatings with Improved Photocatalytic Activity Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy in Aluminate Electrolyte

MgAl and MgAl/ZnO coatings were prepared by plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy in aluminate electrolyte (5 g/L NaAlO2) without and with addition of ZnO particles in various concentrations. The MgAl coating was partially crystallized and contained MgO and MgAl2O4 phases. The addition of ZnO particles to aluminate electrolyte had no significant effect on the surface morphology of formed coatings, while the Zn content increased with ZnO particle concentrations. X-ray diffraction confirmed the incorporation of ZnO particles in the coatings. The photodegradation of methyl orange (10 cm3 of 8 mg/L) was used to measure the photocatalytic activity (PA) of MgAl and MgAl/ZnO coatings. The PA of MgAl coating after 8 h of irradiation was around 58%, while the PA of MgAl/ZnO coatings formed in aluminate electrolyte with the addition of ZnO particles in concentrations of 4 g/L, 8 g/L, and 12 g/L were around 69%, 86%, and 97%, respectively.

SIMULATION OF THE PROCESS DYNAMICS OF ALUMINA CALCINATION IN ROTATING DRUM FURNACES

The issues of modeling processes occurred in drum-type rotary kilns in steady-state (static) modes are considered in details in the literature, unlike the processes occurred in dynamic conditions (transient modes). However, management and optimization of technological regimes require gaining information about the dynamics of those processes. The goal of the study is analysis of the process of alumina calcination. The quality indicators of the process have a significant impact on the process of aluminum electrolysis. Calcination of aluminum hydroxide is a final stage in all technologies of alumina production. We present the results of theoretical construction of mathematical models of the dynamics of heat and mass transfer processes upon heat treatment of the initial aluminum hydroxide. Three types of models are constructed under different initial conditions which describe the main regularities of the process with different degree of accuracy. Proceeding from the data of the theoretical study a methodology has been developed for experimental study of the dynamics of the process along the main control channels: "hydrate loading - flue gas temperature," "natural gas consumption - temperature in the calcination zone," and "natural gas consumption - flue gas temperature." Transfer functions and differential equations of the process were thus obtained for the studied channels. The revealed good agreement between the structures of theoretical and experimental models made it possible to formulate recommendations for the construction of a process control system for alumina calcination. The temperature in the calcination zone affects the fuel (gas) consumption and determines the quality of the resulting alumina. The flue gas temperature must be maintained at the desired level using a two-circuit system which stabilizes the aluminum hydroxide charge with correction for the flue gas temperature. The temperature in the lower head of the furnace is an integral indicator of the interaction of two flows: the flow of burning gas and the counter-moving flow of the material. This temperature in the steady state operation of the furnace is a control parameter associated with the quality of the resulting alumina. When managing gas consumption, it is also necessary to maintain an optimal fuel-air ratio to ensure complete fuel combustion with maximum efficiency.

Oxidative exfoliation of spent cathode carbon: A two-in-one strategy for its decontamination and high-valued application

Spent cathode carbon (SCC) from aluminum electrolysis is a potential graphite resource. However, full use of the SCC remains a challenge, since it contains many hazardous substances (e.g., fluoride salts, cyanides), encapsulated within the thick carbon layers and thus posing serious environmental concerns. This work presents a chemical oxidative exfoliation route to achieve the recycling of SCC and the decontaminated SCC with high-valued graphene oxide (GO)-like carbon structures (SCC-GO) is applied as an excellent adsorbent for organic pollutants. Specifically, after the oxidative exfoliation, the embedded hazardous constituents are fully exposed, facilitating their subsequent removal by aqueous leaching. Moreover, benefiting from the enhanced specific surface areas along with abundant O-containing functional groups, the as-produced SCC-GO, shows an adsorption capacity as high as 347 mg·g−1 when considering methylene blue as a pollutant model, which exceeds most of the recently reported carbon-based adsorbents. Our study provides a feasible solution for the efficient recycling of hazardous carbonaceous wastes.

An improved TOPSIS-based multi-criteria decision-making approach for evaluating the working condition of the aluminum reduction cell

The working condition evaluation of the aluminum reduction cell is the basis of formulating operation strategy, ensuring production safety and realizing stable and optimized operation of the aluminum electrolysis process. In this study, we deal with the cell working condition evaluation problem from a new perspective, and an improved Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) based multi-criteria decision-making approach is proposed. This approach starts from the process data and working condition standard. In order to obtain the appropriate weights to the process indicators, Criteria Importance Through Inter-criteria Correlation (CRITIC) approach is used in the proposed approach. Then the TOPSIS is used to calculate the relative closeness of the cell working condition to the working condition standard. Finally, a crisp level determination approach is used to provide a delicate level associated with the working condition standard. Two real cases demonstrated that the proposed approach is useful and effective in the working condition evaluation of the aluminum reduction cell.

Copper hexacyanoferrate as a long-life cathode for aqueous aluminum ion batteries

Aqueous aluminum ion batteries (AAIBs) have emerged as promising low-cost and energy-dense alternatives to conventional lithium-ion batteries. However, AAIBs exhibit a poor rate capability and limited cycle life owing to the rapid degradation. In this study, a copper hexacyanoferrate (CuHCFe) electrode was introduced into AAIBs with an aluminum trifluoromethanesulfonate aqueous electrolyte. Common aluminum electrolytes were compared to highlight the effects of anions and pure acid solutions were examined to eliminate the concerns on proton. A reversible capacity of 69.35 mAh/g was obtained at 70 mA/g, with 73% capacity maintained at 50 C, exhibiting excellent rate performance. Furthermore, the types of anions played an important role in the electrochemical performance. With triflate anions, the CuHCFe electrode exhibited remarkable cycling stability with a neglectable capacity loss in 3000 cycles (eight months of continuous testing) at 70 mA/g and 70% retention after 2300 cycles at 350 mA/g. Ex situ characterization revealed that both transition metals in CuHCFe were simultaneously involved in the redox reaction with minimal lattice change upon ion insertion/extraction. The results demonstrate the compatibility and potential of CuHCFe cathodes and aluminum triflate electrolytes for AAIBs, which can promote the development of such batteries for electrochemical energy storage.

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.

Time series clustering method with cluster validation to identify unknown local cell conditions in the aluminum reduction cell

It is important to identify local cell conditions in the aluminum reduction cell for cell control. However, in the actual aluminum electrolysis process, local cell conditions are unknown based on traditional measurements. Fortunately, the appearance of anode current signals as a new process variable makes it possible to identify local cell conditions. In this paper, without any prior knowledge on local cell conditions, a new time series clustering method with cluster validation is proposed to identify unknown local cell conditions. First, the nature of anode current signals is analyzed and a new process variable named the pseudo-resistance of an anode–cathode path (Rpseudo) is transformed from the anode current signals, which can provide more insights to the aluminum electrolysis process. Then, a time series clustering algorithm that combines fuzzy c-means with the dynamic time warping distance is used to cluster the anode current signals and Rpseudo. Finally, a new clustering validity index is designed and a cluster validation process is proposed to determine the optimal number of clusters. Moreover, different anode current signals for different local cell conditions such as normal working, anode deformation, anode effect and anode slippage are deliberately selected to verify the proposed method. The experimental results show that the proposed method can effectively identify local cell conditions.

Evaluation of Electrochemical Properties of Aluminum Electrolytes with Ammonium Salt Using an Ultramicro Disk Electrode

Cyclic voltammetry of a DMSO2–AlCl3 electrolyte was performed using an ultramicro disk electrode to evaluate the effects of ammonium salts on the Al electrodeposition reaction. A side reaction in the electrolyte at potentials more negative than −0.8 V (vs Al-wire) was found to suppress the Al electrodeposition reaction. The addition of even a small amount of NH4Cl promoted an Al electrodeposition reaction, which proceeded at more positive potentials than the side reaction. With the excessive addition of tetramethylammonium chloride (TMAC) electrolyte, the Al electrodeposition reaction was also suppressed, proceeding in the negative potential region where a side reaction occurs. In the electrolyte containing 0.02-mol NH4Cl and 1.0-mol TMAC, a decrease in the deposition overpotential on the reducing side and an increase in the dissolution current on the oxidizing side were observed simultaneously.

Material innovation and performance optimization of multi-solid waste-based composite grouting materials for semi-flexible pavements

With the increasing power generation capacity of circulating fluidized bed boilers and electrolytic aluminum production worldwide, the emissions of fluidized bed coal combustion fly ash (FBCF) and bayer red mud (BRM) have increased dramatically, resulting in extremely severe environmental issues as it is difficult to utilize these as resources. To address this issue, this study uses industrial solid wastes such as BRM, FBCF, and silica fume as mineral admixtures to develop a green and low-carbon early strength multi-solid waste composite grouting material (ESMSWCGM) for semi-flexible pavements (SFPs). According to the findings of this study, 18 % of the cement in the ESMSWCGM could be replaced with the three solid waste mineral admixtures. With the optimal ratio, the developed ESMSWCGM exhibits initial and 20-min flow-cone fluidities of 12 s and 14 s, respectively, and initial and final setting times of 66 min and 69 min, respectively. The compressive strengths at 3 h, 1 d, 7 d, and 28 d are 17.05 MPa, 22.95 MPa, 29.55 MPa, and 30.72 MPa, respectively, and the dry shrinkage rate at 28 d is 0.12 %. The performance of the SFP material obtained through grouting is excellent. When the grouting saturation is 95 %, the SFP has a dynamic stability of 28911, a rutting depth of 1.19 mm, and a splitting strength of 1.91 MPa.

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.

Pyrolysis kinetics, thermodynamics, and interaction analysis of co-pyrolysis of bituminous coal and electrolytic aluminum solid waste

The thermal degradation behavior, kinetics and thermodynamic characteristics were investigated during the co-pyrolysis of Tangkou bituminous coal (TK) and spent cathode carbon block (SCCB) derived from electrolytic aluminum solid waste. In the N2 atmosphere, thermogravimetric analysis was carried out at four heating rates (5, 10, 20 and 40 K/min). The novelty was that the deconvolution technology with bi-Gaussian function was used to separate the complex thermogravimetric curves. The devolatilization of organic matter, the secondary depolymerization of macromolecules, the secondary repolymerization of macromolecules in coal and the thermal decomposition of fluoride in SCCB were found as different components of the whole co-pyrolysis process of TK and SCCB. Kinetic parameters were estimated by the model-free method and model-fitting method. The results showed that the average activation energy for four stages was 211.30, 222.10, 253.31 and 268.12 kJ/mol, and four stages of co-pyrolysis process fit well with chemical reaction model with reaction order n = 2.5, 3.0, 3.0 and 1.5, respectively. The thermodynamic parameters, including enthalpy (ΔH), Gibbs free energy (ΔG) and entropy (ΔS) were calculated to determine the feasibility and reactivity of the co-pyrolysis process. Eventually, the interaction effect was found in the fourth stage of co-pyrolysis of TK/SCCB.

Thermo-mechanical Studies of Thimble Geometry and Stub Materials for Voltage Drop Across the Anode-Rod Connection in Aluminum Electrolysis Cell

Thermo-mechanical Studies of Thimble Geometry and Stub Materials for Voltage Drop Across the Anode-Rod Connection in Aluminum Electrolysis Cell

Recovery of carbon and cryolite from spent carbon anode slag of electrolytic aluminum by flotation based on the evaluation of selectivity index.

One of the main electrolytic aluminum production costs is the consumption of carbon anodes, and carbon anode slag is a common hazardous waste in the aluminum industry. In this work, electrolytic aluminum carbon anode slag was separated by flotation. Using the selectivity index (SI) as an indicator, the influencing factors of the carbon slag flotation process were optimized, and the separation performance of carbon and cryolite in the carbon anode slag was investigated. The raw carbon anode slag was ground for 40 min to achieve dissociation of the cryolite from the carbon, the optimized SI value was then used to determine the optimal flotation test conditions. The test results showed that the SI value under the optimal grinding flotation was approximately four times larger than the value of direct flotation. This indicated that carbon anode slag had a better flotation selectivity under the grinding flotation, which significantly improved the flotation performance.

Anodic Behaviour of Ni-Fe-Cu electrode In Molten Fluoride Salts

The industrial process for Aluminium production is the Hall-Héroult process, an electrolysis process where the alumina raw material is dissolved in a cryolite based electrolyte, the aluminium is deposited at the cathode while the oxygen from the alumina reacts anodically with the carbon anodes, which are consumed in the process. The chemical energy in the carbon anodes provides part of the process energy, resulting in CO2 emission of about 1.6 kg/kg aluminium produced. In industrial cells, the electrolyte is modified from the cryolite, Na3AlF6, composition by adding AlF3 to lower the process temperature and improve current efficiency. This modification can be characterized by the Cryolite Ratio, CR, which is the molar ratio of NaF/AlF3. In modern cells, this ratio is often as low as 2.2, as compared to 3 in pure cryolite, which enables an operating temperature of around 960°C, as compared to more than 1009°C for Cryolite.One way to eliminate CO2 process emissions from aluminium electrolysis is to switch from consumable carbon anodes to inert anodes, resulting in O2 emissions instead of CO2. Significant effort has been put into achieving this, but the challenge is to find anode materials that can withstand the harsh conditions in the high temperature cryolite based electrolyte. It would help anode life to reduce the electrolysis temperature from 960 °C to 800 °C or lower. Lowering the CR to 1.3 or lower would achieve this, but for traditional electrolyte components this would lower the alumina solubility to unacceptably low levels. By replacing some of the NaF in the electrolyte with KF, it is possible to operate at this very low CR with acceptable alumina solubility.In this paper, we present a study on anode performance in potassium-rich low CR electrolyte. Both electrodes are vertical, the anodes made of homogenised CuFeNi alloy and cathodes of hot-pressed TiB2. The paper describes the effect of varying the anode composition, anode surface treatment as well as current density on anode stability. Also, the effect of varying the ratio of KR = KF/(NaF+KF), process temperature as well as alumina saturation is addressed.In general, pre-oxidation of the metallic anodes as well as the Cu content greatly affects anode stability. It is possible to operate at low temperature at lower KR, improving the industrial viability of the process, as NaO is added to the cells with the alumina, this reduces the alumina solubility which puts constraints on the process control. The aluminium obtained by electrolysis in this system had less than 0.2% impurities from the electrodes.

Primary Production of Aluminium and Its Alloys in Molten Salts with Oxygen Evolving Anodes: Overview

Due to environmental and economic concerns, carbon-free aluminium production has been an ultimate target for aluminium industries. For the last few decades, a considerable amount of research has been conducted to find a suitable inert anode that could replace the consumable carbon anodes for aluminium electrolysis. Material types such as metals, ceramics and cermets have been studied extensively. All the anode materials have their advantages and disadvantages. However, metallic alloys have been considered attractive anode material candidates due to their high electrical conductivity, thermal stability and easy fabrication and machining properties. For a successful adaptation of metallic anodes into the aluminium electrolysis cell, an electrolyte with a low-operating temperature and high alumina solubility is required. Another significant component of the carbon-free aluminium electrolysis cell is a wettable cathode. This overview discusses the research progress on inert anodes, wettable cathodes and electrolytes.