Thiosulfate leaching combined with ion-exchange resins is an innovative alternative for gold recovery. According to the properties of activated carbon, it could replace resins in the gold recovery process, improve efficiency, and reduce operating cost. In this research, the adsorption process of gold thiosulfate complex on thiol-modified activated carbon was studied. Thioglycolic acid (ATG) was impregnated in activated carbon, and its adsorption ability was tested with synthetic solutions of gold and sodium thiosulfate (Au 10 mg·L−1, Na2S2O3 0.1 mol·L−1, pH = 10.0). Carbon was characterized by infrared spectroscopy, SEM-EDS, PZC titration, hardness number measures, and proximal analysis. Synthetic solutions were also characterized by UV-vis spectroscopy and cyclic voltammetry. The percentage of volatile material increased from 10.0 to 13.9% due to the impregnation process of ATG. Infrared spectra show characteristic bands of C-H, S-H, and C-S bonds. In the adsorption tests, the ATG-impregnated carbon achieved 91% of gold recovery, while the same amount of ATG in the liquid phase stirred with unmodified activated carbon reached 90% of gold recovery. The 44.9% of gold recovered with activated carbon impregnated with ATG was eluted with sodium cyanide ([NaCN] = 0.2 mol·L−1; [NaOH] = 0.25 mol·L−1; [CH3CH2OH] = 30% V/V; pH = 12.0; t = 24 h). These results suggest the gold transferred from the thiosulfate complex to a new gold thiolate complex.

Carbon reactivation is a strategy to reduce waste and cost in many industrial processes, for example, effluent treatment, food industry, and hydrometallurgy. In this work, the effect of physical and chemical reactivation of granular activated carbon (AC) was studied. Spent activated carbon (SAC) was obtained from a carbon in pulp (CIP) leaching process for gold extraction. Chemical and physical reactivations were evaluated using several acid-wash procedures (HCl, HNO 3 , H 2 SO 4 ) and thermal treatment (650-950 • C) methods, respectively. The effect of the reactivation processes on the mechanical properties was evaluated determining ball pan hardness and normal abrasion in pulp resistance. The effect on the adsorptive properties was evaluated via the iodine number, the gold adsorption value (k expressed in mg Au/g AC), and Brunauer-Emmett-Teller (BET) surface area. Initial characterization of the SAC showed an iodine number of 734 mg I 2 /g AC, a k value of 1.37 mg Au/g AC, and a BET surface area of 869 m 2 /g. The best reactivation results of the SAC were achieved via acid washing with HNO 3 at 20% v/v and 50 • C over 30 min, and a subsequent thermal reactivation at 850 • C over 1 h. The final reactivated carbon had an iodine number of 1199 mg I 2 /g AC, a k value of 14.9 mg Au/g AC, and a BET surface area of 1079 m 2 /g. Acid wash prior to thermal treatment was critical to reactivate the SAC. The reactivation process had a minor impact (<1% change) on the mechanical properties of the AC.Molecules 2020, 25, 1681 2 of 18 refining [10,11]. Leaching is achieved typically with a solution of NaCN; the CN − ion complexes gold to form an aurocyanide complex, which remains dissolved in the solution. Subsequently, AC in the carbon in pulp process (CIP) absorbs and concentrates the aurocyanide complex. The gold is desorbed from the loaded AC by an elution process, and it is recovered as metal via electrolysis. Once the elution is over, AC goes to a reactivation process that regenerates its adsorptive capabilities, and this regenerated AC is reused in the CIP process [1,10,11].The gold extraction industry is a very profitable business worldwide. It involves the use of large amounts of activated carbon, especially in medium and large capacity operations. AC is used owing to the high gold recovery reached during the CIP process. However, the gold adsorption capacity of AC decreases after several batch cycles owing to the blockage of the carbon pores caused by the progressive deposition of clays, oxides, and hydroxides of calcium and magnesium on the AC surface. Consequently, the reactivation process is not only essential to restore the adsorption capacities of the carbon, but also it could improve the economy of the process by reducing the amount of virgin carbon needed [10,[12][13][14].Some of the most used reactivation procedures include the following: acid-wash, treatment with supercritical fluids, microwave reactivation, electrochemical process, and thermal reactivation. Previous studies concluded that, in order to achi...