Abstract
With the effects of climate change becoming more apparent, countries around the globe are taking unprecedented actions towards lowering their CO2 emissions in accordance with Paris Agreement and United Nations Sustainable Development Goals. Clean energy technologies, however, are more material intensive than its conventional counterparts and will therefore require steadfast supply of metals (e.g. copper, nickel, cobalt). This implies a need of more extensive mining operations for these metals over the next decades to successfully support a rapid clean energy transition scenario.
An increase in mining activities would also result to an elevated generation of mining-related wastes such as acid mine drainage (AMD). AMD is a serious environmental concern that can pose detrimental effects to ecosystems and human health if not properly managed. This stream resulting from oxidation of sulfide minerals, such as in pyritic rocks, can be highly acidic (pH 1-3) and thus can naturally leach out potentially toxic elements such as heavy metals. A common technique used to treat AMD is by the addition of limestone (CaCO3) as neutraliser. This can raise the pH of AMD to alkaline levels to trigger precipitation of contaminants. However, such approach is limited by challenges associated with high neutraliser consumption and formation of massive amounts of hazardous precipitate sludge. While AMD can be viewed as an environmental challenge, it can also be considered as potential secondary resource of valuable metals. Applying conventional metal recovery techniques, however, can be ineffective due to the complex nature of AMD that may contain multiple co-existing metal ions present in low concentrations.
In this study, an electrochemical-based approach using aluminum-iron (Al-Fe) bimetallic particles was developed and tested to recover copper, nickel, and cobalt from dilute sulfate solutions. Bench-scale experimental results suggest that the synthesised Al-Fe bimetallic particles can be an effective tool to recover redox-active metals even in low concentrations. High metal recoveries (>90% for nickel and >97% for cobalt and copper) was achieved by 5 g/L dosage of Al-Fe bimetallic particles from dilute solutions containing 10 ppm of metal ions. Thermodynamic considerations suggest that copper was recovered thru electrochemical reduction to Cu0 and oxide formation. While nickel and cobalt were likely recovered via reduction or cementation to Ni0 and Co0 and via adsorption by Fe-Al(oxy)hydroxides. A substantial increase in pH (from initial pH ~2.5 to >5.5) during the metal recovery tests was also observed – demonstrating the ability of the Al-Fe bimetallic material to neutralise highly acidic stream and to lower alkaline requirements for further contaminant precipitation. Moreover, the synthesis of the Al-Fe bimetallic material was further optimised by using aluminum scraps/waste shavings to increase the overall economics and sustainability of the technique.
These results demonstrated the ability of the technique to potentially valorise AMD as secondary metal resource, and at the same time partially treating its heavy metal content and acidic nature. Other potential applications of the technique such as resource recovery from low-grade resources will also be explored.