Recovery of Carbon and Valuable Components from Spent Pot Lining by Leaching with Acidic Aluminum Anodizing Wastewaters
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DURING the process of aluminum electrolysis, the carbon cathode block degrades due to electrolyte erosion and electrochemical infiltration, which affects cell performance and requires the cathode to be replaced every three to 8 years.[1] The used carbon cathode block is known as spent pot lining (SPL). Some 7 to 50 kg SPL are generated per tonne of industrially produced molten electrolytic aluminum.[2,3] In 2017, global electrolytic aluminum production was 69 Mt,[4] so the primary aluminum industry currently generates more than 1.5 Mt SPL every year.[5] The main components of SPL usually include inorganic compounds, such as Na3AlF6, NaF, CaF2, Al2O3, and NaAl11O17.[6] SPL is classified as a hazardous waste
XIAOMING LI is with the School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an, 710055, P.R. China and also with the Research Center of Metallurgical Engineering and Technology of Shaanxi Province, Xi’an University of Architecture and Technology, Xi’an, 710055, P.R. China. Contact e-mail: [email protected] WEIDONG YIN, ZHAO FANG, QIHANG LIU, JUNXUE ZHAO and HAO JIA are with the School of Metallurgical Engineering, Xi’an University of Architecture and Technology. Contact e-mail: [email protected] Manuscript submitted August 10, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
due to its highly leachable fluoride (up to 20 mass pct) and cyanide (up to 1 pct, but typically 0.3 to 0.4 mass pct) contents.[7] Fluoride is toxic to aquatic and terrestrial organisms, because high levels in water are the main cause of dental and skeletal fluorosis in vertebrates,[8] so this species is not permitted to be discarded into the environment. Some comprehensive processes for SPL use in various industries have been proposed. In the cement industry, SPL can be added into the cement clinker kiln for cement manufacturing, where the waste carbon replaces part of the fuel and fluoride, as a mineralizer, improves kiln firing conditions.[9] However, the SPL has to be transported in a closed container and the maximum allowable limits for sodium (< 0.6 pct) and fluoride restrict the addition up to a few percent of the feed stock. SPL can be added into steelmaking processes, where the fluoride improves slag formation, but the amount that can be added is limited due to the hazardous waste produced.[10] SPL also has the possibility to be employed as a component of the burden of a submerged-arc furnace producing ferrosilicon manganese alloy,[11] as a flux for the recovery of metals from molten nickel or copper converter slags,[12] and as a raw material to manufacture SiC bricks.[13] SPL can also be used in the production of cathode blocks, prebaked anodes, rock wool, and pig iron.[5] However, more than 50 pct of SPL generated is stored in lined or unlined sites or buildings, awaiting further treatment.[14]
Processes for the recovery of carbon, electrolyte, and fluoride from SPL have been developed.[5,15–17] The processes can be classified into two categories: thermal treatment,[18,19] in which the harmful su
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