Recovery of Platinum-group Metals from Stillwater Complex, Montana, Flotation Concentrates by Matte Smelting and Leaching.
As part of an effort to decrease U.S. dependence on imports of critical and strategic metals, the Bureau of Mines investigated three leaching methods for extracting the base metals and recovering platinum-group metals (PGM) from matte prepared from Stillwater Complex flotation concentrates. Because of a limited supply of natural material, synthetic matte of composition similar to matte prepared from natural concentrates was used to devise the extraction sequences. The validity of the sequences was verified with matte prepared from Stillwater concentrates. In two-stage matte leaching, first-stage h2so4 leaching extracted 99 pct of the nickel and iron from the matte but negligible amounts of copper and PGM. Second- stage oxidative leaching with fe2(so4)3 or fecl3 solubilized more than 98 pct of the copper and no precious metals. Sequential leaching, in which acid and oxidizing lixiviants were added sequentially to the matte without an intervening filtration step, extracted 99 pct of the fe, ni, and cu. A roasting-leaching technique, in which the residue for first-stage leaching was roasted between 360 deg and 500 deg c before a second leaching with h2so4, solubilized 98 pct of the copper. The three leaching methods yielded high-grade PGM residues containing more than 98 pct of the PGM in the matte.