Selective Recovery of Graphite from Spent Potlining (SPL) by Froth Flotation

Spent potlining (SPL) consists of the carbon cathodes (1st cut), aluminosilicate refractories, and the insulation lining of Hall-Heroult cells (2nd cut) used in aluminum smelting. SPL is classified as hazardous material due to its high content in soluble fluoride salts and the presence of cyanides. The 1st cut contains considerable amount of carbon in graphitic form that can exceed 65 wt%; however, during cell's dismantling process inevitable mixing with the 2nd cut occurs causing its contamination with undesirable aluminosilicates. The study investigates the selective recovery of graphite from SPL sample and its separation from mullite and quartz through flotation. Prior to this, SPL has been subjected to special chemical treatment to render it safe for use by minimizing cyanides and free fluorides content through leaching with H2O2 and NaOH, respectively. Flotation tests were carried out aiming at the maximization of carbon grade and recovery in the concentrate, while minimizing silicon content. The effect of pH, collector, and sodium silicate (depressant) dosage was investigated on two feeds of different granulometry (- 90 mu m, 90-420 mu m) and optimum conditions were determined. For pH 8 and kerosene dose of 500 g/t, carbon grade and recovery in the concentrate reached 85.6 wt% and 85.1% for the fine sample (- 90 mu m), and 84 wt% and 94.9% for the sample with particle size 90-420 mu m, respectively. The results clearly show that mullite and quartz were effectively separated since the silicon content was less than 1 wt% in both concentrates. Further grade improvement is possible through further size reduction of the concentrate and subsequent alkaline leaching towards elimination of fluoride salts and cryolite.