CRITICAL MINERAL ENRICHMENT IN SLAG FROM THE KENNECOTT CU SMELTER AND IMPLICATIONS FOR SECONDARY RECOVERY

Ore deposits typically are mined for only a few commodities, but also can contain other critical minerals and valuable elements at elevated concentrations relative to the unmineralized country rock. Understanding the deportment of these critical minerals among value and waste streams during the smelting process of the primary ore can provide insights into targets for additional resource recovery. Furthermore, determining the mineralogical partitioning of critical minerals within the streams is essential for developing efficient metallurgical approaches for their recovery. In this study, we evaluate critical mineral enrichment of a suite of Cu slag samples from the Bingham Canyon porphyry Cu-Mo mine in Utah using bulk geochemistry, optical and electron microscopy, and EDS-based mineral chemistry.

Relative to the Cu-Mo ore, the slag is highly enriched in critical minerals and other commodities including As, Cd, In, Mo, Pb, Sb, Sn, and Zn, with enrichment factors generally greater than 20 (enrichment factor = concentration in slag/concentration in head sample). The slag is also enriched, albeit to a lesser extent, in Bi, Cu, Co, Ge, and Tl (enrichment factors usually less than 10).

The slags are mainly composed of spinifex fayalite laths, euhedral magnetite, skeletal clinopyroxene, subhedral quartz, and a silica-rich interstitial glass. Less abundant phases include immiscible blebs of chalcocite and Cu-arsenide minerals such as domeykite, ranging in size from less than one µm to as large as 300 µm in diameter. Metallic and sulfidic µm-sized inclusions containing Mo, Pb, Bi, and Ag were also observed. These preliminary observations suggest that many of the critical minerals enriched in the slag are associated with the discrete metal, sulfide, or arsenide phases or in trace to minor amounts in the silicate glass. Documenting the mineralogical distribution of critical minerals within the slag is the first step in determining approaches to concentrate them for recovery. Understanding trace element behavior during smelting is therefore important for developing slag as a potential secondary source of critical minerals, with the added benefit of removing from mine waste elements that may be environmentally harmful.