RECYCLING METHODS FOR INDUSTRIAL METALS AND MINERALS Leaching Gold and Silver with an Alternative System, Glycine: Thiosulfate from Mineral Tailings GUILLERMO T. MUNIVE, 1,2 MARTIN A. ENCINAS, 1 MARIA M. SALAZAR CAMPOY, 1 VICTOR E. A ´ LVAREZ, 1 VICTOR M. VAZQUEZ , 1 and DANDY C. CHOQUE 1 1.—Departament of Chemical Engineering and Metallurgy, University of Sonora, Rosales and Encinas Blvd, 83000 Hermosillo, Sonora, Mexico. 2.—e-mail: gtmunive@iq.uson.mx An alternative system to cyanide as a leaching agent is presented for the recovery of gold and silver from cyanicide minerals. These minerals are characterized by the presence of copper and carbonaceous minerals in their composition, therefore increasing both the consumption of reagents and en- ergy requirements for the detoxification and destruction of cyanide. The characterization was carried out with a mesh of + 80 to À 325; gold and silver were determined by a fire assay. The determination of mineralogic species was made by scanning electron microscopy and x-ray diffraction. Thiosulfate and glycine are environmentally friendly agents because of their low toxicity and easy degradation; both agents can form stable complexes with gold and silver in a basic media. These features were studied for the treatment of mine tail- ings from a Mexican mine. Both systems were evaluated by 48-h leaching tests at different liquid–solid ratios (in the range of 1:1, 2:1 and 3:1) and showed significant recoveries of gold, > 80%, with thiosulfate as well as considerable consumption of thiosulfate. These consumptions were, however, lower with the presence of glycine in the system. The recovery of gold and silver through the non-ammoniacal thiosulfate system adds an important step in the devel- opment of a sustainable mining process. INTRODUCTION Because of the limited recovery of precious metals from some mineral deposits containing cyanicide elements, there is a constant search for alternative leaching agents to improve the rate of recovery of gold and silver through hydrometallurgical pro- cesses. 1–3 Some efforts were made to reduce the concentration of cyanicide elements such as copper. 4 The use of thiosulfate has proven to be an important alternative to consider, one that can replace cyanidation for gold ores, which then opens up the possibility of recovering metallic values in an envi- ronmentally friendly way. 5,6 When leaching silver and gold with thiosulfate, it is necessary to use an oxidizing agent (cupric, oxygen, ozone, persulfate, etc.), as the cupric ions and ammonia used in this kind of system can oxidize and catalyze precious metals. However, cupric ions can form cupric tetraamine, and these can be reduced to cuprous thiosulfate, as expressed in Eqs. 1 and 2, respectively. 7 This behavior can lead to high thiosulfate consumption if the copper concentration is not controlled, as shown in Eq. 3. Au þ 2S 2 O À2 3 ! Au S 2 O À2 3   À3 2 þ e À ð1Þ Cu NH 3 ð Þ À2 4 þ 3S 2 O À2 3 ! Cu S 2 O 3 ð Þ À5 3 þ 4NH 3 ð2Þ Cu NH 3 ð Þ À2 4 þ 8S 2 O 2À 3 ! 2Cu S 2 O 3 ð Þ À5 3 þ8NH 3 þ S 4 O À2 6 ð3Þ The use of ammonium shows severe restrictions, and this is caused by the toxicity and the ability to form stable complexes with hazardous elements; therefore, non-ammoniacal thiosulfate systems have more potential through the use of new organic agents such as glycine to promote the recovery of metallic values and reduce the consumption of thiosulfate by degradation. Those systems that have JOM https://doi.org/10.1007/s11837-019-03652-z Ó 2019 The Minerals, Metals & Materials Society