METAL RECOVERY FROM WASTE STREAMS

Waste streams can contain a variety of metals and chemical elements that may represent untapped resources for recovery. Moreover, there is a growing interest in many elements that, until recently, have had little economic significance, but that are now being used in high-technology defense and green-energy applications (e.g., solar panels, wind turbines). Historically discarded mining wastes (e.g., rock piles, tailings, slag, treatment sludge) may be untapped resources for some of these technologically important elements. We are currently reanalyzing historical mining wastes for a variety of elements to determine the degree to which they are enriched and how their enrichments may vary as a function of mineral deposit type. For example, elements that may be present at relatively high concentrations in porphyry copper deposits include Cd, Co, Ga, In, Ni, and Te (Yano et al., 2013). Municipal waste streams illustrate another potential untapped resource for metals. Biosolid samples from the Denver Metro Wastewater Reclamation District have been collected and analyzed by the USGS for a variety of elements since 1999. The biosolids contain approximately 600-800 ppm (0.06-0.08%) Cu and Zn, and some samples exceed 50 ppm Ag and 0.5 ppm Au. By comparison, porphyry Cu ore typically contains 0.5 - 1% Cu and economically viable Au deposits can contain as little as 0.3 ppm Au. We are characterizing additional biosolids for their metal content and examining the possibility of using mineral processing fluids for metal recovery. For any type of waste stream it is important to note that the economic and technical feasibility of metal recovery needs to be evaluated on a case-by-case basis. Although recovery may not be economically viable, it may be used to offset treatment and disposal costs as well as to reduce liability.

Yano R, Price J, Thompson T, Emsbo P, Koenig A, 2013, Trace elements in porphyry copper systems as strategic minerals. SME Proc.