Photocatalytic gold recovery from spent cyanide plating bath solutions

Abstract

Gold-cyanide compounds represent the gold-containing chemicals most widely used in mining and electroplating industries. Despite many efforts focused on the substitution of cyanide salts by less toxic compounds, no successful alternatives have been found. The unique complexing capabilities of cyanide ions in aqueous solutions make these compounds the best choice for the heap leaching process in the gold mining sites or for gold plating applications (1).Wastewater effluents of these processes show significant concentrations of gold-cyanide complexes (2) that, due to its high stability, are very difficult to remove using the conventional oxidation technologies for cyanide degradation (3). Recovery of the gold present in the spent plating bath solutions, a desirable process from an economical standpoint, is currently carried out mainly by electrolysis and ion exchange processes.Some research groups (4-8) have shown the viability of the photochemical deposition and recovery of metallic gold from aqueous solutions of chloride species like AuCl 3 and AuCl 4 -in the presence of titania. These species are considered easy to reduce, due to their reduction potential of E 0 = +1.0 V NHE (NHE, normal hydrogen electrode), whereas the value of the reduction potential in oxygenated systems for Au(CN) 2 -is E 0 = -0.6 V NHE. However, to the best of our knowledge, there are no references in the literature about the TiO 2 -assisted photocatalytic decomposition of gold-cyanide complexes. Serpone et al. (6) reported the only work in which gold was deposited from air-equilibrated solutions prepared by mixture of NaCN and AuCl 3 ·3H 2 O. No trials for characterization of the gold-cyanide complexes that could be formed in the system were done. The goal of their work was to investigate the disposal of free CN -by oxidation with peroxides prior to irradiation of the solutions in the presence of TiO 2 . The fast oxidation of free cyanides avoided the formation of gold-cyanide complexes thus allowing Au(III) reduction on irradiated TiO 2 .On the other hand, the photocatalytic processes have shown a high efficiency for the oxidation of free cyanides (9,10), iron cyanocomplexes (11,12) and copper cyanocomplexes (13).In this work, a new procedure for gold recovery from solutions containing gold-cyanide complexes by using TiO 2 -based semiconductor materials is reported. The photocatalytic treatment performed in the absence of oxygen leads to the reduction of gold to the metallic state and increases the availability of cyanide for subsequent oxidation treatment (14).

Formation of TiO2 photoanodes by simultaneous electrophoretic deposition of anatase and rutile particles for photoassisted electrolytic copper ions removal

“…[29][30][31] Photocatalysis has been shown to be an attractive alternative, and using TiO 2 particles in suspension has achieved promising results. [32][33][34] The metal ions present in effluents are usually found complexed by substances such as EDTA or Cyanide, species widely used in industrial applications.…”

“…Normally, in order to add a hole scavenger (generally an alcohol), it is necessary to enhance the reduction of the metal complex, thereby inhibiting the oxidation of CN -, resulting in an undesirable accumulation of toxic species in the solution. 29 Although the results obtained with particulate materials have shown that the heterogeneous photocatalysis is a promising alternative of remediation for this type of effluent, findings have also revealed disadvantages related with the separation, recovery and recycling of the catalyst and metallic deposits from the treated effluents. [29][30][31][32][33][34] An alternative and effective methodology to overcome these inconveniences is photoassisted electrolysis.…”

“…29 Although the results obtained with particulate materials have shown that the heterogeneous photocatalysis is a promising alternative of remediation for this type of effluent, findings have also revealed disadvantages related with the separation, recovery and recycling of the catalyst and metallic deposits from the treated effluents. [29][30][31][32][33][34] An alternative and effective methodology to overcome these inconveniences is photoassisted electrolysis. 35 With this technique, the TiO 2 is supported on a conductive substrate to be used as the anode and coupled with a conductive material acting as a cathode.…”

Photo-assisted electrochemical copper removal from cyanide solutions using porous TiO2 thin film photo-anodes

“…Specially, the metal ions removal has received certain attention due to the feasibility of valuable metals recuperation 7,9,12 . The metal ions present in effluents are usually found to be complexed by substances like EDTA or Cyanide, species widely employed for industrial applications.…”

“…For example, whereas the EDTA oxidation is achieved, the metal ions reduction is not attained 9 . On the other hand, when cyanide complexes are present, normally it is required to add a hole scavenger (generally an alcohol) to enhance the metal complex reduction, inhibiting the oxidation of CN -, resulting in an undesirable accumulation of such toxic species in the solution 7 . Although the results obtained with particulate materials have shown that the heterogeneous photo-catalysis is a promising alternative for remediation of that type of effluents, they have also revealed disadvantages related with the separation, recovery and recycling of the catalyst and metallic deposits from the treated effluents 4,5,14 .…”