Carbon dross is a hazardous waste generated during the production of electrolytic aluminum. Recovering resources such as carbon from carbon dross can effectively reduce the amount of hazardous waste. This study systematically investigated the flotation process and mechanism for recovering carbon and electrolyte products from carbon dross. Utilizing a super-depth microscope system to scrutinize the post-mechanical liberation of carbon and electrolyte, it was determined that at a mineral ball ratio, liberation concentration, and time of 4.2 %, 70 %, and 8 min, respectively, the liberation degree of carbon particles achieved 80.56 %, with an the liberation degree of carbon particles degree of 83.4 %. Employing the Box-Behnken design (BBD), the optimal flotation conditions were predicted as 35 % pulp concentration, 600 g/t collector dosage, and 42 r/s agitation speed. Under these optimal conditions, the carbon grade of the floated carbon product was 85.4 %, 2.3 % higher than predicted, and the carbon recovery rate was 91.1 %, 0.7 % lower than predicted. The flotation of carbon dross conformed to the classical first-order flotation kinetics model. Meanwhile, contact angle, zeta potential, and FTIR analyses showed that tailings were most wettable, concentrates least, with the collector enhancing adhesion and kerosene improving hydrophobicity, aiding flotation separation. Based on the findings from liberation-flotation processes, liberation analysis, and flotation mechanisms, key guidance was provided for equipment selection and process optimization in industrial production line design. This enabled successful implementation of our technical prototype at a Chinese electrolytic aluminum plant, transforming carbon dross from hazardous waste into valuable products.

In this study, sludge biochars (SBCs) were prepared from aluminum electrolytic sludges in electrolytic aluminum plants and used for the treatment of carbon dross flotation wastewater (CDFW). Under the optimum condition, the KOH-modified SBC showed a high adsorption capacity of 58.3 mg·g−1 for fluoride removal, which is higher than that of most biochars from natural wastes. Brunauer–Emmett–Teller (BET) analysis revealed abundant meso/microporous structures, enabling efficient adsorption of fluoride ions. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) confirm that the SBC-KOH surface contains M–OH2/M–OH (M=Al/Fe) groups. Fluoride ions form Al–F/Fe–F bonds via ion exchange and complexation with these groups. Adsorption kinetics and isotherms were well described by the pseudo-first-order (PFO), pseudo-second-order (PSO), and Langmuir models, suggesting chemisorption as the dominant mechanism with ion diffusion involvement. Thermodynamic analysis indicated a spontaneous, endothermic process driven by entropy. Thus, its adsorption mechanism involves synergistic pore diffusion filling, ion exchange, and surface complexation. Density functional theory (DFT) calculations on plate models of Fe3O4(100) and KAlO2(002) show negative adsorption energy, indicating a spontaneous exothermic process: Fe3O4 adsorbs F− through Fe–F complexation, while KAlO2 relies on Al–F complexation. Interestingly, the F− adsorbed by SBC-KOH achieved the highest capacity in alkaline conditions, unlike previously activated carbon in acidic solutions. SBC-KOH with good fluoride ion adsorption performance provides a new way for the resource utilization of sludge.

After grinding, the carbon component in hazardous carbon dross is liberated from the electrolytes, and the liberation of the grinded carbon dross is crucial for the carbon recovery in the subsequent flotation. This study proposed a novel method using UTMS (Ultra-deep Three-dimensional Microscope System), ISS (Image Series Software), and BFS (BiDoseResp function in the S-function) to evaluate the liberation. UTMS captured particle images before and after grinding, ISS performed particle recognition and counting, and BFS analyzed liberation through S-shaped function fitting. The results showed post-grinding, unliberated particle proportions declined across sizes, with a 37.70 % drop at 0.054-0.063 mm. Large particles decreased, while medium-to-small ones increased, meeting flotation standards at 0.054-0.075 mm. The accuracy of the results was verified through a combination of manual counting, Bland-Altman plots, and residual distribution analysis. This method quantifies liberation based on color differences, offering potential for online monitoring and enhancing flotation efficiency for resource recovery.

This review focuses on the long-overlooked carbon dross from aluminum electrolysis, a hazardous waste enriched in carbon and high-fluoride salts, despite its relatively low generation volume. With the continuous expansion of global aluminum production, the output of carbon dross has increased accordingly, yet its environmental risks and resource potential have not received sufficient attention. For the first time, this article provides an examination of carbon dross following a "generation-hazards-prevention-control" framework. It elucidates the formation mechanism via corrosion, spalling, and entrainment of carbon anodes in electrolytic cells, and analyzes the release behavior and ecotoxicological effects of its toxic components. The limitations of existing end-of-pipe treatment technologies are critically assessed. Furthermore, based on the 3C (clean-cycle-control) green development strategy, a management framework is proposed: clean first, through the adoption of inert anodes, energy-efficient electrolysis processes, and intelligent optimization to minimize dross generation at the source; for unavoidable dross, the cycle is employed to achieve high-value recovery of aluminum fluoride and functional carbon materials; finally, by integrating cross-scale environmental risk assessment and policy instruments, a science-technology-management integrated control decision-making system is established, offering a paradigm for low-carbon, high-value, and safe management of carbon dross.

Carbon dross, a hazardous waste from aluminum electrolysis, poses a challenge to the global aluminum industry. This paper proposes a combined flotation-leaching (CFL) method to recover valuable resources from carbon dross, and achieved a total recovery yield of 95.9 %. This included 17.35 % carbon powder (95.67 wt% purity) and 78.55 % electrolytes (99.65 wt% purity). Kinetics results showed that the carbon dross flotation process followed the classical firstorder model, while fluoride ion leaching followed the shrinking core model, controlled by internal diffusion. Leaching thermodynamic calculations indicated that Na3AlF6 was the main impurity removed. DFT calculations showed that the higher surface energy of the Al exposed surface and the lower binding energy with NaOH resulted in a higher reaction rate for alkaline leaching compared to acid leaching, but with poorer leaching efficiency. Treating one ton of carbon dross with CFL reduces at least 539 kg of CO2 emissions, aiding carbon reduction in the aluminum industry.

Discharging waste water from the bauxite desilication process will bring potential environmental risk from the residual ions and organic compounds, especially hydrolyzed polyacrylamide. Characterization of the microbial community diversity in waste water plays an important role in the biological treatment of waste water. In this study, eight waste water samples from five flotation plants in China were investigated. The microbial community and functional profiles within the waste water were analyzed by a metagenomic sequencing method and associated with geochemical properties. The results revealed that Proteobacteria and Firmicutes were the dominant bacterial phyla. Both phylogenetical and clusters of orthologous groups’ analyses indicated that Tepidicella , Paracoccus , Pseudomonas , and Exiguobacterium could be the dominant bacterial genera in the waste water from bauxite desilication process for their abilities to biodegrade complex organic compounds. The results of the microbial community diversity and functional gene compositions analyses provided a beneficial orientation for the biotreatment of waste water, as well as regenerative using of water resources. Besides, this study revealed that waste water from bauxite desilication process was an ideal ecosystem to find novel microorganisms, such as efficient strains for bio-desilication and bio-desulfurization of bauxite.

During bauxite flotation, flotation indexes decrease with increasing water circulation time, resulting in the discharge of bauxite flotation wastewater as well as a waste of resources and great environmental safety risks. To determine how to return the wastewater back to the flotation process, this study aimed to identify the main components affecting the flotation indexes of flotation wastewater, and coagulation, biological and combined biological-coagulation methods were used to remove the components that affected the production index of wastewater. The results revealed that the accumulation of hydrolyzed polyacrylamide (HPAM) in circulating water reduced the flotation indexes. The combined biological-coagulation method was efficient and consisted of two steps: two-stage biological pretreatment to reduce the stability of HPAM in wastewater followed by coagulation treatment to remove HPAM. At an initial pH of 8, the biological coagulation treatment system stabilized after 4 days of continuous operation, and the treated water was subjected to a flotation test and attained good indexes: the proportion of alumina recovered increased from 62.80% to 68.52%, and the mass ratio of Al2O3 to SiO2 in the tailings decreased from 1.73 to 1.36. These indexes proved the feasibility of reusing the treated water for bauxite flotation. Microbial diversity analysis showed that Labrenzia played an important role in the biological pretreatment, and Gemmobacter, Stappia, Exiguobacterium, Pseudofulvimonas, and Acetoanaerobium played supporting roles. Based on the research results, a prototype reuse technology based on bauxite flotation wastewater treatment was established, and an industrial engineering application was developed through engineering design.

Flotation indexes gradually decrease with the increase of cycle time of the backwater in bauxite floatation, and discharge of backwater brings environmental risk. In this study, methods such as Fenton-oxidation, adsorption and coagulation were used in the treatment of backwater, the flotation indexes were checked after backwater treatments, and Box-Behnken design (BBD) was used in the optimization of the main operating parameters. The results reveal that flotation indexes are effectively improved after coagulation treatment by polyaluminum ferric chloride (PAFC). The optimum parameters predicted by BBD are pH 7.55, 1.09 g/L PAFC dosage and temperature of 25 °C. Under these optimum conditions, a maximum recovery of Al2O3 of 82.83% and a minimum A/S of 1.30 of tailings are gained, while the deviations are less than 3% from the predicted values. These findings encourage the application of BBD for the optimization of critical parameters in backwater treatment.

Recycling effluent has become a bottleneck and an environmental risk associated with the regular production of bauxite via flotation and the sustainable development of the aluminum industry in China. To find a practical direction for biotreatment, the bacterial and archaeal communities in recycling effluents containing concentrate and tailings from bauxite flotation plants were investigated by a metagenomic sequencing method in association with the evaluated geochemical properties. The results showed that Paracoccus , Desulfomicrobium , Exiguobacterium , Tindallia , Ercella and Anoxynatronum were the primary bacterial genera and Methanothrix , Methanobacterium , Nitrososphaera and Methanosarcina were the dominant archaeal genera. Upon combining the microbial diversity and the geochemical properties of the two sample types, the microbial community containing Desulfomicrobium , Paracoccus , Tindallia , Methanobacterium , Methanothrix and Nitrososphaera was better adapted to the biodegradation of flotation collectors, and the microbial community consisting of Paracoccus , Exiguobacterium , Methanothrix and Methanobacterium was more efficient at hydrolyzed polyacrylamide (HPAM) biodegradation. In addition, a large proportion of unclassified OTUs has indicated that recycling effluent is a worthy resource for isolating new strains from the Firmicutes phylum.