The cobalt-nickel pertraction refinery to process recycled spent catalysts leach solutions
Over the last two years, the cobalt (Co) price has increased considerably. The observed increase can be attributed to the increase in demand of lithium-ion batteries (LIBs) of which Co forms an integral part. As a consequence of the envisaged steady rise in the demand for LIBs, a steady increase in the demand for Co is expected. Currently, more than 50 % of the world’s produced Co is obtained from the Democratic Republic of the Congo. This supply chain is, however, threatened by political instability, geopolitics, corruption, child labour and artisanal mining. An alternative source could be, for example, Co-rich spent hydro-treatment catalysts, which would however, require new cobalt-nickel refining capacities. This creates the perfect opportunity for the industrial introduction of a novel solvent extraction (SX) based technology, i.e. pertraction (PX), also known as membrane-based solvent extraction. During this study, liquid-liquid extraction (LLE) data were obtained and used to identify optimum conditions for the extraction, scrubbing and stripping of Co from a pregnant leach solution (PLS) obtained from spent hydro-treatment catalysts. Accordingly, a solvent containing 22 wt% of the extractant Cyanex272 (C272), of which 50 % was pre-neutralized with NH4OH, was able to extract 96 % of the Co (with a 5 % co-extraction of Ni) from the PLS provided by Minemet PTY(LTD) resulting in a raffinate containing a Ni purity of 97 %. When scrubbing the loaded organic phase in an organic to aqueous ratio (O/A) of 30:1 with a 50 g/L Co solution at a pH of 5.0, the Co purity was increased by 12 %. Simultaneously, virtually all the co-extracted Ni was scrubbed from the organic phase. Finally, using a 0.1 M H2SO4 striping liquor, 98 % of Co was stripped from the scrubbed organic, resulting in a scrub liquor containing 5.7 g/L Co at a purity of 97 %. Alternatively, when using a 0.2 M H2SO4 stripping liquor, 100 % of the Co was recovered from the scrubbed organic, resulting in a scrub liquor containing 5.8 g/L Co at a purity of 96 %. The optimised conditions from the LLE data were subsequently used when optimising the mass transfer during Co PX processing. According to the resistance in series model, three PX design parameters are of importance: mass transfer coefficient at aqueous interphase (kAq), mass transfer coefficient of the solvent in the pores and lumen combined (kMO) and the distribution (D). It was firstly shown that an increase in the distribution coefficient lead to enhanced mass transfer kinetics of Co, where a 50 % increase in kov was observed when increasing the C272 from 9 to 22 wt%. It was then found that an increase in extractant concentration also led to increased mass transfer kinetics. Lastly, it was shown that the largest mass transfer resistance was due to the transport and diffusion in the module and not due to reaction kinetics. Applying the data (feed composition analysis, loading isotherms, mass transfer kinetics and mass balance) to the process models, a conceptual design of the extraction section of a PX refinery was proposed. Using the optimised conditions, a membrane area of 2125 m2 would be required to obtain a Ni purity of 99.9 % in the raffinate. Since the Liqui-Cel™ 14x40 XF modules delivers a contact area of 373 m2 each, 5.7 of these modules in series would attain the required stream of 200 L/h. Using an integer of six modules, a raffinate purity of 99.99 % would be attainable.