Alcan has identified the LCL&L hydrometallurgical process as the most efficient means of recuperating the chemical values contained in spent potlining. The SPL leachate of the LCL&L process will yield valuable crystalline NaF and caustic liquor for recycling into the Bayer plant, but it was felt necessary to also demonstrate that the solid residues meet environmental impact criteria. 19 samples of SPL, each approximately 10 tons in weight, representative of Alcan's pre-bake and Soderberg reduction technologies were crushed, ground, homogenised and characterised. They were used to evaluate the efficiency of the two-stage extraction and leaching unit operation of the LCL&L process. Both bench scale and pilot scale tests for a minimum of five hours of steady-state operation allowed a mass balance to be carried out. An initial water leach was followed by a dilute caustic extraction and washing steps. Lithium containing Soderberg SPL were the most difficult to treat, particularly those from pots with long cell life that tend to contain higher fluoride concentrations as the less soluble cryolite. Cyanides were readily leached in the initial water slurry from SPL originating from freshly decommissioned pots but with more difficulty from old stored SPL and where the initial NaCN content had been converted into solid ferrocyanide compounds. The resulting solid residues were tested for leachable fluorides and cyanides and all met the EPA regulatory limit for TLCP leachable fluorides and the limits for total and amenable cyanides.
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