In this work the dissolutive behavior of gold in alkaline medium using thiourea (TU), under different variables, was studied in a theoretical and experimental way, in order to determine the conditions under which it is feasible to dissolve gold in thiourea-alkaline medium. A thermodynamic study was conducted by chemical speciation using the method of Ro-jas-Hernández, together with an electrochemical study where the electric potential was swept in the anodic direction. The main results of the thermodynamic study were that formamidine disulfide (FDS) and sulfinic compounds (S.C.) prevail at alkaline pH; by increasing the initial concen-tration of thiourea at alkaline pH, the presence of the gold complex is al-most zero for any initial concentration of thiourea. By including sodium sulfite in the gold-thiourea system, it was possible to obtain the Au(I)-TU complex at alkaline pH, with a presence of 95.13%. Electrochemical tests allowed verifying that in the absence of sodium sulfite the dissolution of gold in an alkaline medium is very slow but adding sodium sulfite im-provements become evident in the dissolution of the metal. Therefore, sodium sulfite catalyzes the gold dissolution process and stabilizes the thiourea. With this study it was possible to establish the feasibility of using thiourea in an alkaline medium for the dissolution of gold, and the conditions under which it is possible to dissolve the gold in that medium. With these fundamentals and conditions, it is now possible to move forward to test this system for minerals and/or concentrates containing gold.
The use of thiourea (SC(NH2)2) (TU) has been studied extensively, mainly because it is not harmful to the environment, nontoxic to human beings, and the dissolution of gold in thiourea solutions is much faster than in cyanide solutions. The dissolution of gold in thiourea solutions was reported for the first time in 1941, and it was continued in 1960 [
In the leaching of gold with acid TU solutions there are several problems, such as a greater consumption of TU during the redissolution of Au, equipment corrosion in acid solution, low selectivity of the Au leaching reaction, and complicated reagent regeneration and purification procedures. To overcome the above inconveniences, the leaching of Au with an alkaline TU solution has been proposed, and some research work has been carried out [
The chemical speciation diagrams are constructed to show the relation of the different chemical species (thiourea and gold-thiourea) either as a function of pH or of ligand concentration and establishing their predominant behavior in solution. The construction of those diagrams in this study is based on the method of Rojas-Hernández [
Linear potential scans were made, from the rest potential to 1000 mV in the anodic direction, at a scanning rate of 10 mV/s. To determine the electrochemical behavior of the gold, tests were run under different pH, thiourea concentration, and additive presence and content.
The solutions used were prepared with distilled and deionized water, with analytical grade reagents.
Metal | Ligand | Equilibrium reaction | Log K |
---|---|---|---|
Au(I) | TU (SC(NH2)2 TU (SC(NH2)2 | 2SC(NH2)2= [SC(NH)NH2]2 + 2H+ + 2e Au + + 2 SC(NH 2 ) 2 = Au[SC(NH 2 ) 2 ] 2 + | −4.3 23.3 |
All the electrochemical tests were performed with a conventional three-electrode system consisting of a gold working electrode, a platinum wire as counter electrode, and a Ag/AgCl reference electrode.
The electrochemical measurements were made with a model PGP-201 Radiometer Potentiosat/Galvanostat controlled by the Voltamaster 4.0 software. All the potential values are referred to the standard hydrogen electrode (SHE).
Experimental evidence has been found that TU is degraded irreversibly to formamidine disulfide (FDS) and sulfinic compounds (S.C.), reaching elemental sulfur as final product. To confirm the above and determine its stability under different conditions, the speciation diagrams for this ligand were constructed.
It is seen that at pH above 10 there is no more TU in the system, DSF starts degrading, and sulfinic compounds become predominant at pH greater than 12.5. In
Gold can form only one complex with thiourea, namely Au [ SC ( NH 2 ) 2 ] 2 + . The behavior of the Au-TU system as a function of pH and of the TU concentration is shown in
The behavior of this system in the presence of sodium sulfite was also studied. This compound stabilizes TU over the whole pH range, so it is very important in the stability of the Au-TU complex.
It is seen that the Au-TU complex predominates up to pH 13.7. At higher pH TU starts predominating; its presence is due to the stabilizing action of sulfite, preventing its irreversible oxidation.
It is seen that in the Au-TU system, at acid pH (1.4) two peaks or singularities appear, pointing to the occurrence of some transformation in the system. The
first peak (0.52 V) would correspond to the formation of the Au-TU complex, while the second (0.95 V) would indicate the transformation of TU into DSF. These results are like those obtained by Bolzán et al. [
Au + 2 SC ( NH 2 ) 2 ⇔ Au [ SC ( NH 2 ) 2 ] 2 + + e , E ° = 0.38 V NHE (1)
2 SC ( NH 2 ) 2 ⇔ NH 2 ( NH ) CSSC ( NH ) NH 2 + 2 H + + 2 e, E ° = 0.42 V NHE (2)
At pH 9.5 no singularity is seen, but not so at pH 12.5, when the peak of the DSF formation reaction (0.76 V) appears. Furthermore, a greater electrochemical activity under acid conditions (pH = 1.4) is seen than at more alkaline pH
(9.5 and 12.5), and at pH 12.5 it is not possible to see the typical singularity that indicates the formation of the Au-TU complex, confirming what had been shown previously for chemical speciation. Comparing the curves shown in
The effect of the presence of additives on the electrochemical behavior of gold in an alkaline medium with thiourea was also determined.
The speciation studies made allowed establishing that regardless of the TU concentration and in an alkaline medium, the TU is not predominant in the system because it decomposes irreversibly until it forms sulfinic compounds. This means that there is no thiourea available for complexing the gold, and that is why under alkaline conditions and for any TU concentration it is not possible to obtain the Au-TU complex as the predominant species. However, the addition of sodium sulfite stabilizes the thiourea, allowing the predominance of the Au-TU complex in an alkaline medium.
The electrochemical studies showed different current peaks at given potential values, which represent the transformations undergone by the species present in the system. In an alkaline medium there was no evidence of the formation of the Au-TU complex. The concentration of thiourea does not affect the potential at which the singularities occur, however, a greater TU concentration does not imply a higher rate of dissolution. The addition of sodium sulfite to the thiourea solution showed clearly the formation peak of the Au-TU complex, greatly increasing the anodic current of the gold electrode, speeding up its dissolution.
The results obtained allow establishing certain conditions of use of thiourea in alkaline medium to dissolve gold. The application of this system for minerals and/or concentrates required additional study regarding the effect and interaction of some impurities, as well as the determination of kinetic mechanisms and parameters of interest for a better understanding and optimization of the process.
Support by the Dirección de Investigaciones Científicas y Tecnológicas of the Universidad de Santiago de Chile (DICYT) through project 051314VR is gratefully acknowledged.
The authors declare no conflicts of interest regarding the publication of this paper.
Vargas, C., Navarro, P., Espinoza, D., Manríquez, J. and Mejía, E. (2019) Dissolution Behavior of Gold in Alkaline Media Using Thiourea. International Journal of Nonferrous Metallurgy, 8, 1-8. https://doi.org/10.4236/ijnm.2019.81001