Bacteria facilitate selective recovery of critical raw materials
Europe is almost entirely dependent on a foreign supply of rare earth elements (REEs), magnesium and platinum group metals (PGMs). The EU is accelerating efforts to increase independence while improving mining sustainability and supporting a circular economy. To support these goals, the EU-funded BIORECOVER project developed biotechnology-based solutions to recover critical raw materials (CRMs) from unexploited primary and secondary sources.
Target minerals and recovery process methodology
According to Cristina Martínez of CETIM, project scientific leader: "BIORECOVER focused on the recovery of three CRMs: REEs from bauxite residue (a waste product of aluminium processing), magnesium (Mg) from low-grade minerals, and PGMs from process residues and low-grade PGM ores.” The team’s multi-step process at semi-pilot scale was tailored to the CRM target metals and sources. BIORECOVER isolated and screened a plethora of bacteria from the natural population inhabiting mine residues and investigated their detailed mechanisms of action, including studies of their DNA. The bacteria were then used to remove impurities in the sources. The pre-processing was followed by CRM mobilisation via bioleaching –extraction and solubilisation of CRMs using biologically produced mineral and organic acids – and immobilisation via microorganisms to produce a bioleachate enriched in target CRMs. Innovative methodologies supported selective recovery of CRMs from the multi-metal bioleachate. A combination of modelling and testing in synthetic and natural solutions containing CRMs enabled optimisation.
Laying the foundation for selective CRM recovery
Following the bioleaching process, a series of advanced technologies were implemented, each addressing critical steps specific to the three distinct groups of CRMs studied in the project. These tailored approaches ensured optimal recovery and processing for each CRM group, highlighting the importance of precise technological application at key stages of the process. Novel microcapsules to selectively recover yttrium and scandium from REE-enriched bioleachate achieved a maximum recovery rate of approximately 80 % for yttrium and scandium. The maximum recovery rate for neodymium was approximately 40 % and for lanthanum and caesium about 20 %. Using commercial polymeric microcapsules and synthetic solutions, BIORECOVER achieved a selectivity of 95 % for yttrium. A milestone in the purification of bioleachates has been achieved using electroprecipitation technologies for the highly successful specific recovery of Mg from Mg bioleachates, resulting in a maximum recovery rate of 100 % and selectivity of 92 % in the post-treatment process. The final purity of magnesium hydroxide (brucite) was almost 90 %. Finally, a biopolymer modified with bacterial siderophores facilitated selective recovery of PGMs. The biopolymer enabled a maximum recovery of about 78 % and 58 % of platinum (Pt) and palladium, respectively, from PGM by-products and approximately 98 % and 92 % selectivity for Pt and iridium from PGM synthetic leachate. “Mobilisation of PGMs from low-grade PGM ore was challenging due to the refractory nature of the PGM hosting minerals. However, BIORECOVER achieved more than 90 % mobilisation of gold,” adds Martínez.
Sustainable future in focus
The life-cycle assessment (LCA), based on laboratory-scale processes and equipment, provided important insight for scale-up. “In all recovery pathways studied, reduced energy consumption is key for large-scale viability – autoclave electricity and energy for reactor heating should be reduced,” explains Martínez. The latter could be done by optimising bioleaching time to balance bioreactor energy consumption relative to increased CRM mobilisation. The LCA identified the culture medium and chemicals to produce it as the main environmental bottleneck in REE recovery from bauxite residue. Using by-products such as sugarcane bagasse, vinasse or molasses could reduce this impact. The need to sterilise bauxite residue prior to processing leads to the main environmental impact regarding Mg waste. BIORECOVER’s biotechnology-based pathways to CRM recovery from unexploited sources with less energy, water and waste pave the way to a more sustainable and secure supply of CRMs and a brighter future for the environment.
Keywords
BIORECOVER, CRM, critical raw materials, metals, bioleachate, biotechnology, bioleaching, platinum group metals, rare earth elements, biopolymer
