Abstract
For gold extraction, the most used extraction technique is the Merrill-Crow process, which uses lixiviants as sodium or potassium cyanide for gold leaching at alkaline conditions. The cyanide ion has an affinity not only for gold and silver, but for other metals in the ores, such as Al, Fe, Cu, Ni, Zn, and other toxic metals like Hg, As, Cr, Co, Pb, Sn, and Mn. After the extraction stage, the resulting wastewater is concentrated at alkaline conditions with concentrations up to 1000 ppm of metals. Photocatalysis is an advanced oxidation process (AOP) able to generate a photoreaction in the solid surface of a semiconductor activated by light. Although it is well known that photocatalytic processes can remove metals in solution, there are no compilations about the researches on photocatalytic removal of metals in wastewater with cyanide. Hence, this review comprises the existing applications of photocatalytic processes to remove metal and in some cases recover cyanide from recalcitrant wastewater from gold extraction. The use of this process, in general, requires the addition of several scavengers in order to force the mechanism to a pathway where the electrons can be transferred to the metal-cyanide matrices, or elsewhere the entire metallic cyanocomplex can be degraded by an oxidative pathway.
Highlights
Gold has always had a high value since prehistoric times as ornaments in rituals, and it occupies an essential role in the world economy
Mining wastewaters can show problems of metal mobility and local cyanide release where they are stored; mining wastewaters are directly discharged to tailings ponds for periods of three to six months where degradation is expected by the sun [13]
These photocatalytic treatments were used for the degradation of free cyanide, all applications were limited to synthetic solutions
Summary
Gold has always had a high value since prehistoric times as ornaments in rituals, and it occupies an essential role in the world economy. In large-scale operations, wastewater treatment is carried out with highly oxidizing processes, such as chlorination, sulfur dioxide, hypochlorite oxidation, electrolytic oxidation, ozonation, use of hydrogen peroxide, high thermal transformation, biological treatments, adsorption on activated carbon, among others, usually at high oxidation conditions and operation cost [14].On the other hand, advanced oxidation processes (AOPs) have the advantage of removing liquid and recalcitrant gaseous matrices by non-selective chemical species. Among these processes, heterogeneous photocatalysis (HPC) has received considerable attention as a promising technology, able to use renewable energy from the sun. Different photocatalytic processes used for the elimination of synthetic and real cyanide matrices of gold extraction are explained and described
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