Cascading electrolyte bleed streams

Abstract

A system of multiple extraction in which each of the metal elements extracted is bonded to the extractant with a different degree of intensity. To reduce impurities in each metal recovery stage, a bleed stream from each recovery stage is cascaded backwards. In general, the backward cascade disposes of the bleed stream from the most tightly bonded species by recycling it to any preceeding stage of a less tightly bonded species. An example of a process for recovering copper and nickel from an aqueous leach solution containing these values is given. In this type of process, ammonia, copper and nickel are first coextracted onto an organic extractant (oxime). Ammonia is stripped first. In a nickel recovery circuit, nickel values are selectively stripped from the oxime with acid, pure nickel is electrowon, and the acidic, nickel-barren raffinate from the electrowinning cell is recirculated. Downstream of the nickel recovery circuit, in a copper recovery circuit, copper is stripped from the oxime with acid, metal is electrowon from the acid, and metal-barren acid is recirculated. Nickel and other metallic impurities build up in the copper recovery circuit, and impurities build up in the nickel recovery circuit. To overcome this problem, bleed streams are taken from both circuits and fed to any preceding stage including the metal extraction stage. Because of the property of the extractant to extract both copper and nickel, the metal values in this directed stream eventually get re-extracted.