Review of trace toxic elements (Pb, Cd, Hg, As, Sb, Bi, Se, Te) and their deportment in gold processing: Part II: Deportment in gold ore processing by cyanidation

Abstract

The second part of a literature review on the deportment of trace toxic elements in gold processing by cyanidation is presented. Part 1 of this review collated the relevant information on trace element mineralogy, aqueous solution chemistry and toxicity for lead, cadmium, mercury, arsenic, antimony, bismuth, selenium, and tellurium. This paper reviews the deportment of these elements during gold ore cyanidation (including carbon adsorption and elution) and tailings disposal. It also captures related information from the treatment of industrial wastes and wastewaters, including fixation and encapsulation processes that may be applicable to tailings generated from gold ore processing. The main findings were that only lead, cadmium and mercury form cyano complexes ranging from very weak (Pb) through moderate (Cd) to the strong mercuro complexes. The neutral cyano-complexes of cadmium and mercury strongly adsorb onto activated carbon and thus potentially deport to the gold recovery circuit and carbon regeneration kiln. The strong mercury cyano complexes will persist in tailings storage environments. Arsenic, antimony and bismuth do not form cyanide complexes. The less metallic of these (As and Sb) may form oxyanions and/or thioanions that consume oxygen and retard or prevent gold dissolution. They can be present in solution, including tailings solutions, at relatively high concentrations. Bismuth, being more metallic, remains in the solid phase as a sulfide or oxide during cyanidation. Depending on the redox potential in solution, selenium and tellurium form the oxyanions selenite and tellurite or selenate and tellurate in cyanide solutions. Selenium may form selenocyanate (SeCN -) in cyanide solutions, which is thermodynamically more stable than the oxyanions selenite and selenate. Tellurocyanate is unstable and does not form. The potential for adsorption of selenium or tellurium onto activated carbon is unknown, as is their persistence in tailings storage facilities. Lead, cadmium and mercury are best removed from wastewaters by precipitation as sulfides or adsorption onto activated carbon or iron or aluminium oxides. Arsenic and antimony in acidic tailings and mine drainage waters are also attenuated by precipitation onto iron oxides, or by precipitation as scorodite (FeAsO 4.2H 2O) or senarmontite (Sb 2O 3). Selenite in wastewater efficiently adsorbs on to both iron and manganese oxides but selenate does not. Tellurium is less soluble in the aqueous phase than selenium and adsorbs more strongly onto iron oxides.