The present invention is related to the mining industry and treatment of mineral and materials that contain gold and silver. Specifically, it is related to a process to recover gold and silver, from copper thiosulfate solutions with a autogenerated electrolysis process, in which the metallic values are recovered from the rich solution in the cathodic compartment. The barren solution is then used as the anolyte, re-establishing the copper concentration needed to be recycled back to the leaching stage.
At present, gold and silver are obtained from their minerals, concentrates and other materials, using different processes. These processes are in function of the nature of the gold and silver containing material, as well as their grade. Accordingly, if it is a high grade material, smelting is employed. On the other hand, if the material contains only small amounts of gold and silver, a hydrometallurgical treatment is usually selected (leaching).
Since the end of the XIX century, the process based on cyanide solutions, has been successfully used for leaching gold and silver from low grade materials. However, cyanide solutions are highly toxic. Additionally, some materials are refractory towards this process or contain a high copper concentration, which consumes large amounts of cyanide during leaching, as the following article teaches [G. Senanayake, Gold leaching in non-cyanide lixiviant systems: critical issues on fundamentals and applications, Mineral Engineering 2004(17)785-201].
Several alternatives to cyanidation have been proposed, among them is the method based on thiosulfate. This chemical system has been utilized, on a pre-industrial scale, since the 1920's [Fathi Habashi. A Textbook of Hydrometallurgy, 2nd edition (Second ed). Quebec City, Canada: Métallurgie Extractive Québec, 1999]. However, the elevated reagent consumption, caused by its oxidation to tetrathionate and even sulfate by the cupric ions (Cu(II)), has hindered its large scale implementation.
Recently, this inconvenience has been solved with additives that modify the oxidative properties of the cupric ions, [Gretchen Lapidus-Lavine, Alejandro Rafael Alonso-Gómez, José Angel Cervantes-Escamilla, Patricia Mendoza-Münoz and Mario Francisco Ortiz-Garcia, “Mejora al Proceso de Lixiviación de Plata de Soluciones de Tiosulfato de Cobre (Improvement to the Silver Leaching Process with Copper Thiosulfate Solutions)”], Mexican patent granted the 26 Feb. 2008, MX 257151], by limiting thiosulfate consumption to less than 5% of its initial value [Alonso-Gómez, A. R. and Lapidus, G. T. (2009), “Inhibition of Lead Solubilization during the Leaching of Gold and Silver in Ammoniacal Thiosulfate Solutions (effect of phosphate addition)”, Hydrometallurgy, 99(1-2), 89-96].
On the other hand, the recovery of values from the thiosulfate baths has been performed principally by cementation, in which a reducing agent, usually a metal, is added to generate a redox reaction which produces gold and silver in their metallic state. A disadvantage of this technique is that it is not possible to adequately control the reductive capacity of the agent, which causes a poor separation efficiency, obtaining gold and silver contaminated with copper.
Direct electrodeposition, used as a separation method, is a viable option, including from solutions with low concentrations of gold and silver, even when the copper ion concentration is more than 50 times greater than that of silver and over 100 times that of gold [Alonso-Gómez, A. R., Lapidus, G. T. and González, I., “Proceso de Lixiviación y Recuperación de Plata y Oro con Soluciones de Tiosulfato Amoniacales de Cobre, solicitud PCT/MX2009/000022, fecha 14 Mar. 2008 (WO20097113842, publicada 17 Sep. 2009)]. To attain efficiencies greater than 50%, a rotating cylinder electrode was employed in a reactor with separate anodic and cathodic compartments in order prevent the oxidation of the thiosulfate and the re-oxidation of the deposited gold and silver. In this type of cell, deposits were obtained with less than 2% impurities [Alonso, A. R., Lapidus, G. T. and González, I. (2008), “Selective silver electroseparation from ammoniacal thiosulfate leaching solutions using a Rotating Cylinder Electrode reactor (RCE)”, Hydrometallurgy, 92 (3-4), 115-123].
Despite the excellent results obtained with this type of reactor, the relatively low current efficiencies can be considered a disadvantage due to the high cost of electricity.
Recently, autogenerated electrolyses have been explored. These consist of a two electrode cell, in which metal ions are reduced and deposited on the cathode, differing from a traditional current-driven electrodeposition, in the fact that the anode is made of a material whose oxidation potential is less than the reductive potential of the metal ions and therefore does not require addition electricity to drive the process. Upon anode oxidation, an electron flow travels through an electrical conductor to the cathode, where the electrodeposit occurs. For this reason, the anodic and cathodic compartment must be separated by an ion exchange membrane.
Autogenerated electrolysis shares with cementation the principle that the oxidation of a metal is used to reduce another more noble.
However, in autogenerated electrolysis, the separated anodic and cathodic compartments allow, on one hand, the election of the substrate upon which the metal is deposited (similar to a conventional electrolysis), eliminated the contamination of the deposit. On the other, because the anode is in contact with a solution which is different from the one that contains the metallic ions to be deposited, it is also possible to tailor the anolyte composition according to the requirement of the process and in this manner modulate the reductive power of the system.
This procedure is adequate for gold and silver recovery from thiosulfate solutions, eliminating the need for electrical energy through the oxidation of a metallic anode. It is important to mention that the election of the anode material will depend on the difference between the redox potentials of the anode and cathode, as well as the advantages that the dissolution of a certain material might offer to the process. This should result in lower process costs.
One objective of the present invention is to provide a selective separation process for gold and silver from thiosulfate solutions, at an increased velocity compared with copper cementation, without the use of electrical current.
Another objective is to accomplish the aforementioned task using the barren solution as the anolyte, conserving in this manner the level of soluble copper, in order to maintain the composition of the thiosulfate solution so that it can be recycled back to the leaching stage.
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.
The present invention is designed to solve the problem of gold and silver separation from copper thiosulfate leaching solutions, providing an improvement over the traditional separation methods (cementation and external current-driven electrolysis). This improvement is characterized by the use of a novel autogenerated electrolysis process, employing a commercial copper sheet as the anode and a titanium cathode, in a reactor with anodic and cathodic compartments separated by ion exchange membrane which prevents the contamination of the thiosulfate solution.
The membrane achieves the purpose of separating the anodic and cathodic sections, to prevent the solutions used in each compartimentsto from mixing. This is important to avoid cementation of gold and silver on the copper surface, which slows the process and contaminates the product. On the other hand, it is important to consider that the rich (pregnant) solution (located in the cathodic compartment) is poor in copper due to the nature of the leaching process, and its oxidation power is limited; this allows an efficient gold and silver deposition because there is little re-dissolution. By preventing contact of the pregnant solution with the copper anode, the copper concentration in this solution is kept low and for this reason the membrane plays a double role.
In order to better understand the characteristics of the invention, 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 limitative and are described in the following section.
The process referred to in the present is performed according to the illustration in
The operation of the electrochemical autogeneration reactor is represented in
To better understand the invention, one of the many experiments is detailed as an example, which employs a reactor such as that schematized in
A synthetic solution, poor in copper ions, whose composition is detailed in Table 2, was placed in the anodic compartment (430).
The solutions were prepared with analytical grade reagents and deionized water (1×1010 MΩcm−1). Once the solutions were placed in their respective compartments, the electrodes were connected in short circuit. Stirring in both compartments was maintained during the electrodeposition process. Samples of the solution were taken every 20 minutes for four hours, after which time the test was detained. The samples were analyzed for silver and copper with atomic absorption spectrometry.
In
On the other hand, the copper concentration in the cathodic compartment remained constant during the electrolysis (data not shown), indicative of a selective silver deposit.
In order to determine the leaching power of the recycled solution, after having stripped the silver ions in the autogeneration process, experiments were performed with real leaching solutions, whose results are shown in the following example.
As was shown in
To better understand the process, a block diagram is shown (
Stream A3, stripped of its values, is placed in the anodic compartment of the reactor (An2), where the first electrodeposit from the pregnant solution lot B (Ca2) occurs.
Stream A4 is sent back to a new leaching stage (LA2), where it is contacted with fresh mineral. The pregnant solution (A5) is sent to the electrochemical reactor for silver recovery in the cathodic compartment (Ca3). In this case, the anodic compartment is occupied by the solution of lot B originating from Ca2.
Subsequently, the process is repeated, passing the stream A6 to the anodic compartment (An4) during the electrodeposition of B5 (Ca4).
Finally, stream A7 is again introduced into the leaching stage with fresh mineral, obtaining a pregnant solution in stream A8.
The route that lot B follows is practically the same as lot A. Table 3 shows the initial composition used in the leaching solutions for both lots; the volume of each one was 250 mL. Each leach used 2.5 g of a lead concentrate from Fresnillo mine, whose silver content is 24 kg/ton with approximately 25% of lead.
In
In the subsequent leaches performed with lot A, within the recirculation scheme, extractions similar to that observed in LA1 were achieved (approximately 200 ppm silver ions). The results obtained in leaches with lot B are very similar, again observing the solubility limitation of 200 ppm Ag(I). These results are significant since they show that the thiosulfate solution maintains its leaching power after three cycles of leaching-electrodeposition, in which no additional reagent was added to make-up the solutions.
Finally, a comparison of the change in silver concentration during the autogenerated electrolyses for lot A is shown in
It is important to remember that the mineral leached was a lead concentrate, the reason for which an important quantity of this metal dissolved, despite the use of phosphate to inhibit this process. In
In any event, these examples are evidence that the use of a autogenerated electrolysis reactor is viable within a leaching-electroseparation scheme, maintaining the leaching capacity of the thiosulfate solution.
Number | Date | Country | Kind |
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MX/A/2010/013510 | Dec 2010 | MX | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/MX11/00150 | 12/9/2011 | WO | 00 | 11/28/2013 |