Separation and recovery of cobalt and aluminium from spent gas-to-liquid catalysts using CYANEX® 272

In the field of Fischer-Tropsch synthesis catalysts, cobalt, platinum, and aluminium are the primary metal constituents of the conventionally used cobalt catalyst. While platinum recovery remains a focal point in this field, a recent endeavour by a South African company emerged, which aimed to recover cobalt and aluminium from spent Fischer-Tropsch synthesis catalysts for potential use in the agricultural sector. This study aimed to optimally separate cobalt and aluminium from spent Fischer-Tropsch synthesis catalyst for this purpose. It further leverages the OLI® database to predict metal speciation during solvent extraction to aid with experimental planning. Shake-out tests were conducted to determine optimal separation conditions, which were validated using three distinct methods: (i) evaluating the experimental error, (ii) comparing results with similar research, and (iii) validating results against the OLI® database. The investigation revealed the pivotal role of the aqueous pH yielding effective separation at pH 3.13 ([H2SO4] = 2.5 x 10-4 M) when using 20 vol.% CYANEX® 272 and 50 mol.% pre-neutralisation. Effective cobalt scrubbing was achieved with 50 g/L Al at pH 2.8, while successful stripping required 1 M H2SO4. Competitive interactions between complexing aluminium and cobalt species were observed when contacting with CYANEX® 272 at a pH < 4. Equipment design analysis for a targeted separation efficiency of 87% dictated the necessity of two mixer stages and a settler measuring 1.5 m x 1.5 m x 6.0 m (height x width x length). This settler would ensure sufficient residence time for gravity separation at a flow rate of 10 m3/h.