ASSESSMENT AND RECOVERY OF CRITICAL ELEMENTS IN WASTE FROM PORPHYRY COPPER MINING

Worldwide, the annual mass of rocks moved in porphyry copper mining is comparable to the mass flux of global terrestrial volcanism. Rudimentary geochemical estimates show this mining represents a huge volume of elements, many of them US-designated critical elements. However, current (pyro)metallurgical practices largely recover only Cu and Mo minerals, ~1% by mass of the rock mined. Byproducts, if recovered at all, are thus restricted to what these minerals contain (S, Re, Ag, Au, etc.). Hydrometallurgy typically recovers only Cu. Others either do not leach and remain in residues (Si, Be, Li, W, Ti, semimetals) or circulate indefinitely in leaching solutions (Al, Mg, Zn, REEs). The behavior of still others during leaching is unknown (Co, Ni), although they are known to be enriched in porphyry systems.

With either method, current Cu processing recovers < 10 of the elements available from the rocks mined. The vast majority of the critical elements end up in tailings, the voluminous residues left after processing. These amount to 200M metric tons each year in Arizona alone, adding to the estimated 17.5B tons already existing. Reprocessing them is an old idea, now receiving new impetus from recent concerns about minerals security and sustainability. Preliminary experiments show that some critical elements, such as REEs, can be extracted from Cu raffinates and other waste. Given the huge annual fluxes involved, recovering Ti, Al, Be, V, and REEs from the world’s porphyry Cu mining could supply, indeed overwhelm, global demand for these commodities. Recovering Li, Co, F, and U would approximate global demand.

However, the mineral hosts, element movement in process streams, fraction recovered, and variation across deposits and alteration types remain unknown. Such a lack of critical information is the main obstacle to recovery, as trace minerals and elements are often not analyzed in feeds, let alone tailings. Element movement therefore has to be extrapolated from comparisons between ore feeds and flotation concentrates, anodes, raffinates, and other process products which (unlike tailings) are more often analyzed. Though preliminary, such comparisons and ongoing research suggest that current processing methods send substantial proportions of the deposits’ contained Co, Ni, Be, Li, Te, REEs, semimetals, and other critical elements to waste, though some may be recoverable.