U.S. Critical Minerals for the Energy Transition

Abstract:

Elements and minerals that are important for renewable energy production, storage, and usage include Ag, Al, As, Cd, Co, Ga, Ge, In, Li, Mn, Ni, rare earth elements (REE), Sb, Se, Te, and graphite. Of these, Al, As, Co, Ga, Ge, In, Li, Mn, REE, Sb, Te, and graphite are currently classified as critical minerals in the U.S. Some of these are products of mining operations, and the distribution of conventional resources of these commodities—Al, Li, Mn, REE, and graphite—are relatively well-known and well-documented. However, unconventional sources are less well documented, but may bring significant resources to market in the future. Examples include Li in clay minerals, oilfield brines, geothermal fluids, and saline lakes; and REE in phosphate deposits that are mined to produce fertilizer. The distribution, resource endowment, and recoverability of byproduct critical minerals—As, Co, Ga, Ge, In, Sb, Te—is less well known. For some of these, such as Ge and Te, publicly available resource and production figures exist for only one deposit (each), even though it is known that these commodities have been, and continue to be, recovered from other deposits in the U.S. For others, such as Co, proposed recovery from unmined deposits depends on extracting Co from pyrite, which otherwise has no value, and therefore the economics of extraction rely on developing an appropriate recovery circuit, and recovery of the commodity at an economic cost. For Te, recovery is only from a very specific part of only one metallurgical pathway—from cathode slimes in Cu electrowinning plants—and recovery from other metallurgical circuits, such as cyanide recovery of Au from gold-telluride ores, yields no Te. Although recovery of critical minerals from waste piles of former mining operations is an attractive option, metallurgical considerations are paramount, in part because post-mining mineralogical changes due to weathering may significantly impact the recoverability of critical minerals. The endowment of critical minerals in waste piles is very poorly known. Here we document the presence and availability of critical minerals in conventional and unconventional resources, and briefly discuss metallurgical challenges for their recovery.