Process and mechanism of Zn and Co leaching from cobalt xanthate slag enhanced by oxidative roasting

Cobalt xanthate slag is the waste slag produced by the use of xanthate cementation and decontamination before the wet refining and electrowinning of Zn, in which Zn, Co, Cd, etc., and butyl xanthate exist in the form of hydrophobic complexes, which are poorly soluble with water. In addition, most of the cobalt xanthate in the slag exists in the form of high-valent Co, which lowers the extraction rate when directly leaching the metal elements. In this study, the atmosphere of the roasting process is regulated to destroy the hydrophobic complexes and change the mineral phase structure of the cobalt xanthate slag. In the cobalt xanthate slag, the Co and Fe valence states change during the roasting and leaching steps; specifically, during the roasting process, the complexes first decompose into CS₂, reducing part of the Co³⁺. Following the disappearance of this sulfur-rich environment, the oxygen in the flowing air oxidises Fe²⁺ to Fe³⁺, but the valence state of Co remains unchanged. During the subsequent leaching process, Co³⁺ and Fe²⁺ further react to realise the benign transformation of Co³⁺ to Co²⁺ and Fe²⁺ to Fe³⁺ without an added redox agent. The rapid redox reaction between Co³⁺ and Fe²⁺ controls Co₃O₄ dissolution; thus, the reaction rate is controlled by the slower process of internal diffusion. Under the optimal roasting conditions, the Co³⁺ content in cobalt xanthate slag decreases to 21.10 % and that of Fe²⁺ to 48.78 %. Consequently, the optimum Zn and Co leaching rates reach 99.92 % and 99.57 %, respectively, and only 0.86 % of Fe in the final leaching solution exists in the form of Fe²⁺.