Recovery of precious metals from evaporite sediments

Abstract

A process is disclosed for the recovery and assay of gold, silver (PMs) and the 6 platinum group elements (PGEs) from playa-evaporite sediments. Fine sediments are subjected to a primary leach with a highly caustic sodium thiosulfate leach liquor to produce a metal-pregnant solution. After filtration the pregnant solution is subjected to a two-stage precipitation of gold, silver and PGE values. In a first stage precious metals and some PGE elements are precipitated by lead acetate or sodium borohydride reagents. The precipitate may be subjected further to desulfurization and metallic footing to produce a lead-precious metal sponge. The sponge is de-leaded by firing and cupelling, selective cementation or electrolysis and reduction to dore is best effected by a carburizing distillation. The final recovery of the metal values is produced by controlled acid parting into individual precious metals and cupellation to the metallic state. In a second stage, the first stage precipitation liquor is scavenged by metal-exchange at reduced pH for additional metal values and cupellation to the metallic state. The second stage scavenger precipitate is an impure metallic powder and values are recovered by standard reduction and separation methods. Assay of the product (heads and precipitates) may be effected by high temperature methods developed for these types of deposits. Recovered flower sulfur and sulfite liquors are used to regenerate recycled thiosulfate. Mud leach tails are treated for agricultural use and land reclamation.

Dependencies are inferred from the claim text.