CA2821421A1

CA2821421A1 - Electrorecovery of gold and silver from leaching solutions by simultaneous cathodic and anodic deposits

Info

Publication number: CA2821421A1
Application number: CA2821421A
Authority: CA
Inventors: Gretchen Terri Lapidus Lavine; Alejandro Rafael Alonso Gomez; Ricardo Benavides Perez; Carlos Lara Valenzuela; Javier Alejandro Silva Alonso
Original assignee: Universidad Autonoma Metropolitana (UAM); Servicios Administrativos Penoles SA de CV
Current assignee: Universidad Autonoma Metropolitana (UAM); Servicios Administrativos Penoles SA de CV
Priority date: 2010-12-13
Filing date: 2011-12-09
Publication date: 2012-06-21
Also published as: WO2012081952A3; PE20140494A1; US20140076735A1; WO2012081952A2; AU2011341844A1; BR112013014874A2; CN103380234A; EP2653590A2; CO6801793A2; WO2012081952A4; AU2011341844A2; RU2013132451A; MX2010013717A; JP2014505788A

Abstract

The invention relates to the mining industry, for the treatment of minerals and materials containing gold and silver. The invention specifically relates to a method for recovering gold and silver from thiosulfate and thiourea solutions, by means of an electrolysis method with simultaneous metal deposition on the cathode and anode. The advantages of the invention compared to prior art are that the method is faster and the energy consumption is lower than that observed in conventional cells. The electrolysis is carried out in the potential zones that enable the reduction of the silver and gold on the cathode and the oxidation of the ligand on the anode.

Description

, , ELECTRORECOVERY OF GOLD AND SILVER FROM LEACHING
SOLUTIONS BY SIMULTANEOUS CATHODIC AND ANODIC
DEPOSITS
FIELD OF THE INVENTION
The present invention is related to the mining industry for treat-ment of minerals and materials which contain gold and silver.
Specifically, it is related to a process to recover gold and silver, from leaching solutions with a simultaneous anodic and cathodic electrodeposition process, after which the poor solution is recy-cled back to the leaching stage.
BACKGROUND OF THE INVENTION
The recovery of gold and silver from their minerals has been performed by various methods; among the most employed are pyrometallurgical treatments, in which upon the addition of a considerable amount of energy, part of the mineral is oxidized, in this manner liberating the precious metals. This great amount of energy is the principal inconvenience of the process, which in the end reflects on the operation costs.
On the other hand, the hydrometallurgical methods are charac-terized for their high selectivity and relatively low reagent and energy costs. Gold and silver has been obtained by one such method for over 100 years, using cyanide and oxygen as a complexing agent and an oxidant, respectively. Despite the high efficiency of this system, the treatment of complex minerals, as well as environmental restrictions, has encouraged research on other leaching systems that could compete with cyanide, without its disadvantages.
Thiosulfate, in the presence of copper, and the combination of thiourea with formamidine disulfide (Poisot-Diaz, M.E., Gonza-lez, I. and Lapidus, G.T. (2008), " Effect of Copper, Iron and Zinc Ions on the Selective Electrodeposition of Doree from Acid-ic thiourea Solutions", Hydrometallurgy 2008, Eds. C.A. Young, P.R. Taylor, C.G. Anderson y Y. Choi, Society for Mining, Metal-lurgy and Exploration, Inc. (SME), Littleton, Colorado, U.S.A., ISBN: 978-0-87335-266-6, pp. 843-848 and Alonso-Gomez, A.R.
and Lapidus, G.T. (2008), "Pretreatment for Refractory Gold and Silver Minerals before Leaching with Ammoniacal Copper Thio-sulfate", Hydrometallurgy 2008, Eds. C.A. Young, P.R. Taylor, C.G. Anderson y Y. Choi, Society for Mining, Metallurgy and Ex-ploration, Inc. (SME), Littleton, Colorado, U.S.A., ISBN: 978-0-87335-266-6, pp. 817-822.) are two chemical systems that leach gold and silver from minerals for which cyanidation has proved to be inefficient. In this same manner, it was shown possible to = CA 02821421 2013-06-12 recover gold and silver metals in both systems using direct electrodeposition (A. Alonso. G.T. Lapidus and I. Gonzalez, A
strategy to determine the potential interval for selective silver electrodeposition from ammoniacal thiosulfate solutions Hydro-metallurgy, Volume 85, Issues 2-4, March 2007, Pages 144-153);
However, this recovery was accomplished in geometrically com-plex reactors (F.C. Walsh, C. Ponce de Leon and C.T. Low, The rotating cylinder electrode (RCE) an its application to the electrodeposition of metals, Australian Journal of Chemistry, 58, (4), 246-262 and A. Alonso, G.T. Lapidus and I. Gonzalez, Se-lective silver electroseparation from ammoniacal thiosulfate so-lutions using a rotating cylinder electrode reactor (RCE), Hy-drometallurgy, Volume 92, Issues 3-4, June 2008, Pages 115-123), with an energy consumption that renders un attractive from an economic and financial standpoint.
At this point, it is important to mention a characteristic of the thiourea and thiosulfate systems: both complexing agents can oxidize at potentials near the reduction potential of silver (Fig-ures 1 and 2). The diagrams of both ligands with gold are simi-lar. This originates the formation of a narrow potential region where Ag(I) and Au(I) ions are soluble and because of this, both the leaching as well as the electroseparation conditions should be controlled with precision. This could imply a great disad-vantage with respect to other systems and has motivated the use of membrane reactors, in order to avoid contact of these solu-tions with the anode.
OBJECTIVES OF THE INVENTION
One objective of the present invention is to provide a method to separate gold and silver from thiosulfate or thiourea solutions by simultaneous anodic and cathodic electrodeposition, increasing in this manner the velocity of the process. Another is to accom-plish this with a minimum affectation of the solution composi-tion, so that it may be recirculated back to the leaching stage.
Yet another is to promote efficient energy use.
Other objectives and advantages that apply the principles and are derived from the present invention may be apparent from the study of the following description and diagrams that are included here for illustrative and not limitative purposes.
BRIEF DESCRIPTION OF THE INVENTION
The present invention is intended to solve the problem of gold , =

ans silver separation from thiosulfate and thiourea leaching so-lutions, providing an improvement over the traditional electro-chemical reactors now in use. This improvement is characterizes by a novel process to simultaneously deposit metals in on the 5 anode and cathode in a one compartment reactor, using a com-mercial copper sheet as the anode and a titanium sheet as the cathode.
The conditions which permit this technique to operate were cho-sen from the analysis of Figure 1, where a region of the soluble complex Ag(S203)23- is observed within the metallic silver stabil-ity zone. When the potential is decreased below -110 mV, the Ag(I) species is reduced to Ago, in a typical electrolytic process.
However, the most interesting aspect of this diagram is when the potential is less negative than -50 mV, where part of the thiosul-fate oxidizes, destabilizing the soluble complex and forming me-tallic silver. The present invention takes advantage of this phe-nomenon and has not been previously reported for this or other ligands.
The application of the simultaneous anodic-cathodic electrodeposition of gold and silver allows more efficient use of the electrical energy in electrochemical reactors of simple ge-ometry without a membrane; additionally, the separation process occurs in less time than that required in conventional electro-chemical reactors.

In order to better understand the characteristics of the inven-tion, the following description is accompanied by diagrams and figures, which form an integral part of the same and are meant to be illustrative but not !imitative and are described in the fol-lowing section.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a Pourbaix-type diagram in which the predominance zones for the soluble species Ag(S203)23- (thiosulfate-silver complex) and metallic silver Ago are shown.
Figure 2 is a Pourbaix-type diagram in which the predominance zones for the soluble species AgTu3+ (thiourea-silver complex) and metallic silver Ago are shown.
Figure 3 shows a leaching-electrodeposition scheme for obtain-ing gold and silver which utilizes the present invention.
Figure 4 is a diagram showing a recirculation system which in-cludes the electrochemical reactor.

Figure 5 is a schematic diagram of the electrochemical cell in which the simultaneous anodic and cathodic deposits are achieved.
Figure 6 is a graphic representation of the change in silver con-centration with leaching time.
Figure 7 is a graphic representation of the change in silver con-centration with electrolysis time where there is simultaneous an-odic and cathodic electrodeposition.
Figure 8 is a graph that compares the change in silver concen-tration for leaches 1, 2 and 3 with the same solution.
Figure 9 shows the comparison of the silver concentration during electrolysis 1, 2 and 3 with the same solution.
Figure 10 shows a comparison of XRD spectra for the anodic de-posit obtained after the electrolysis and for pure metallic silver.
DETAILED DESCRIPTION OF THE INVENTION
The simultaneous electrodeposition process, referred to in the present invention, is illustrated in Figure 3.
= A thiosulfate or thiourea solution, rich in gold and silver ions, originating from the leaching stage (100) and after having been filtered (200), is introduced into the electrochemical reac-tor (300).
= Once the electrodeposition has finalized, the cathode (312, Figure 5) and the anode (313, Figure 5) are removed from the reactor and mechanically abraded to remove the gold and silver metals. The solution is then recirculated back to the leaching stage (301).
The electrodeposition is performed in a recirculation scheme, illustrated in Figure 4, in which the solution is charged to the reservoir (320) from which it is pumped (330) to the electro-chemical reactor (310) and then returned by gravity to the reser-voir.
EXAMPLES

To better understand the invention, one of the many experiments is detailed as an example, which employs a system such as that schematized in Figures 3 to 5. A 60 cm2 (exposed geometrical area) titanium plate was used as the cathode and a copper plate with the same exposed area was the anode.

As shown in Figure 3, the first stage is gold and silver leaching from the mineral or concentrate, using a thiosulfate solution, in this case, whose composition is presented in Table 1. The pH
was adjusted to 10.0 with NH4OH.
Table 1. Composition of the leaching solution Component Composition (mol/L) (NH4)2S203 0.2 CuSO4 0.05 EDTA 0.025 (NH4)2HPO4 0.1 The solutions were prepared with reagent grade chemicals using deionized water (1x1010 MOcm-1). 500 mL of this solution was placed in contact with 3.75 g of a flotation concentrate, with a particle size less than 10 pm, containing 21 kg/ton of silver. Af-ter six hours in continuous agitation, the solution was separated from the solid by filtration and placed in a reactor such as that represented in Figures 4 and 5.
During the electrodeposition, a flow of 1.1 L/min was used with a cell voltage of 100 mV; with this voltage, the potential at the cathode was -260 mV versus the normal hydrogen electrode (NHE), which is adequate to obtain a selective silver deposit on " CA 02821421 2013-06-12 this electrode Figure 6 shows a graphic representation of the silver concentra-tion with respect to the leaching time. A maximum value was at-5 tamed in 120 minutes, after which time the concentration re-mained relatively constant.
The change in silver concentration during the electrolysis is shown in Figure 7. Within the first 15 minutes a sharp descent is 10 observed, which then gradually decreases to values below 10 mg/L. The current registered throughout the experiment was 0.01 A, which together with the cell voltage translates to 0.004 W-h.
Considering that the deposited mass of silver was 0.065 g, the energy consumption was 0.062 W-h per g of deposited silver.
After finalizing the electrodeposition, the solution was recycled back to the leaching stage, where it was contacted with fresh unleached concentrate, under the same conditions as described previously. The entire procedure was repeated until three full cycles were completed.
Figure 8 shows a graphic representation of the leaching results for all three cycles; an increase in the leaching velocity and the maximum silver concentration may be observed in the second and third leach, relative to the first, possibly due to the stabili-zation of the equilibria between the thiosulfate and the Cu(II) and Cu(I) ions.
On the other hand, the second and third electrolyses (the dashed and dotted lines of Figure 9) show similar tendencies to that of the first (solid line), only differentiable by the initial val-ue, which depends on the previous leaching stage. In all three cases, the values reached below 10 mg/L in approximately 4 hours.
These results clearly show that the thiosulfate solution can be recirculated after the electrodeposition stage, back to the leach-ing stage, at least three times without reconditioning or make-up. Additionally, during the three electrolyses, the current main-tained a constant value of 0.01 A, conserving the same energy expenditure as the first cycle. Anode consumption was negligible after three electrodeposition cycles.
Finally, it is important to mention that X-ray diffraction analysis of both the anodic and the cathodic deposits showed that they consisted exclusively of metallic silver. Figure 10 compares the XRD spectra for the deposit obtained from the anode, at the end of the electrolysis and the corresponding spectra for pure metal-lic silver. As can be observed, the anodic deposit corresponds to metallic silver; indicating that oxidation of thiosulfate is forming only soluble species, such as tetrathionate, dithionate or even sulfate, and is not contaminating the deposit.

Claims

Having described the invention, being considered a novelty and, because of this is demanded as property the content of the fol-lowing claim.

1. Electrolysis for the silver recovery from thiosulfate or thiourea leaching solutions characterized by accomplishing me-tallic deposits simultaneously on the anode and cathode surfac-es, by operating in the potential zones that permit silver or gold reduction at the cathode and ligand oxidation at the anode.