The galvanic stripping technique uses a heterogeneous system composed of an organic solution containing Au, in contact with a solid metal used as the reducer where electrochemical reactions may occur at the interface between reducing agent and organic solution. The product of these reactions consists of a metal deposit on the surface of the reducer sample and ionic/complex species from the reducing agent adsorbed onto the organic phase. The galvanic stripping reaction can be written as an aqueous displacement reaction (Flores and O'Keefe, 1994; Escobar, 1994; Belew et al., 1996; Martins, 1998). The generic reaction can be written as

where M₁ represents the metal ion to be removed, M₂ is the reducing metal and R-M_n is the organic phase containing ion M_n.

Sufficient organic solution should be used in the galvanic stripping to form complex with both M₁ and M₂. In the case of Au recovery (Riveros, 1990), the use of strong-base anionic extractants of a quaternary amine salts, such as ALIQUAT336® (Henkel Co.-USA), allows attainment of higher values than 90% Au extraction from the metal initially present in the alkaline cyanide solution.

The electrochemical reactions occurring during the galvanic stripping start with the partial dissolution of the reducing agent in the organic solution. This works in a way similar to that of the process of cementation in aqueous solutions, reducing that species with lower oxidation potential. It is important to point out that the electrochemical reactions are controlled by the availability of reducing agents in the system and the concentration of the species to be reduced in the organic phase, as well as by continuous covering by the reducing agent surface with the reduced species desorbed from the organic solution diminishes the surface available for the electrochemical reactions. Theoretically, the surface of the reducing agent is fully covered, the galvanic stripping reaction ceases completely.

It is possible to identify anodic areas on the surface of the reducing metal where oxidation reactions occur, i.e., ions are released and electrons become available on the reducer surface. These electrons take part in the reduction reactions of the metal species adsorbed in the organic solution. The areas where reduction reactions occur are called cathodic areas.

During the galvanic stripping process (Flores and O'Keefe, 1994; Escobar, 1994; Belew et al., 1996; Martins, 1998), the dample of reducing metal loses mass due to surface oxidation that occurred at anodic areas generating metal ions which are adsorbed onto the organic phase. These adsorbed metal ions replace the Au cyanocomplex in the organic solution, which is reduced at cathodic areas on the surface of the reducer.

This work presents the results for Au recovery by galvanic stripping of strong-base anionic extractants of a quaternary amine salt, ALIQUAT336®, dissolved in xylene using zinc as the reducing metal. The parameters studied were contact time for the organic phase containing Au and the samples of reducing zinc metal, temperature of the system, Au concentration in the organic phase and type of stirring used in the galvanic stripping system. The experimental results obtained in this work were helpful in the evaluation of the technological feasibility of galvanic stripping in the area of mineral technology.

Gold aqueous solutions where used for the metal loading of the organic phases for galvanic stripping experiments. These solutions were prepared by the dissolution of 4.0 grams of Au metal powder (99.9% pure; Morro Velho Mining Co., Brazil) in alkaline sodium cyanide solution (0.2 M NaOH and 0.5 M NaCN).

The organic solutions used in the tests (Riveros, 1990) were 50 mL of organic phase containing 20 vol.% ALIQUAT336® (Henkel Co., USA) dissolved in 90% pure xylene (Fisher Scientific, USA).

Zinc disks (99% pure metal) were used as solid metal reducing agents in all the galvanic stripping experiments. They were 19 mm in diameter 2.0 mm in thickness and their surfaces (1133.5 mm²) were polished with sandpaper number 600 (Buehler-USA) to obtain homogeneous surfaces suitable for galvanic stripping tests. The polished samples were washed several times with distilled water, dried with compressed air and weighed on an analytic balance.

The galvanic stripping reactors used in the experiments were 100-mL borossilicate glass cylindrical beakers (Kimax-USA). Some experiments were carried out using a heating plate with magnetic stirring (Fisher Scientific-USA), while other tests were conducted using an ultrasonic thermostatic bath (Cole-Parmer, USA, model 8845-50).

The Au concentration in the aqueous solutions before and after the addition of the organic phase and in samples of the aqueous solution from the dissolution of the metal deposit obtained in the galvanic stripping experiments was determined with an atomic absorption espectrophotometric equipment (Perkin-Elmer, USA, model 560).

The pH of the feed aqueous solution containing Au was adjusted to 10.5 by addition of sulfuric acid. The Au solution was added to the organic phase in a 250 mL glass reactor and stirred for at least 15 minutes. Then, the phases were placed in a 500 mL borossilicate glass separation funnel for at least 10 minutes in order to allow separation of the aqueous and organic phase. A sample of the aqueous solution was collected and analyzed by atomic absorption spectrophotometry to determine Au concentration determination, which allowed calculation of the Au concentration in the organic phase. The organic phase removed more than 99% of the gold initially present in the aqueous solution.

The galvanic stripping experiments were conducted by adding 50 mL of the organic phase to the test reactor where a polished zinc disk was also introduced. The zinc disk was fixed on a glass plate kept in a vertical position inside the organic phase. After some time, the zinc disk was removed from the reactor, washed with distilled water and then acetone in order to evaporate the adsorbed water on the sample surface, dried with compressed air and weighed on an analytic balance. The zinc sample with Au deposited on its surface was analyzed by observation under an optic magnifier. The mass of Au deposited on the zinc surface was determined by solubilization of the Au deposit in a sodium cyanide alkaline solution under intense forced aeration for at least 48 hours. The solution produced was analyzed by atomic absorption spectrophotometry for determination of the Au.

Table 1 shows the experimental results for the influence of stirring on the gold recovery by galvanic stripping from an organic phase (20 vol.% ALIQUAT336® solubilized in xylene) containing 16.0 g/L of Au and zinc disk. The values for Au recovery are expressed as the percent of Au extracted from the organic phase and deposited on the surface of the zinc metal.

The experimental results show the highest Au recovery using magnetic stirring, when compared to other stirring methods for the same stirring time. On the other hand, for the unstirred system, the increase in time of experiment allowed to be obtained a higher value those compared to for the stirring types.

Table 2 shows the experimental results obtained for the influence of the time of contact between the organic phase containing Au and the sample of zinc metal reductant sample on the recovery of Au by galvanic stripping. The increase in the time of contact increases gold recovery by galvanic stripping for the system with magnetic stirring and at room temperature. This was expected because the electrochemical reactions that occur in the process are time dependent. Time allows ionic or complex species to migrate to the anodic and cathodic areas on the surface of the reducing zinc metal.

Another important aspect shown in Table 2 is the initial time of galvanic stripping, i.e., most of the Au recovered was obtained during the first 1.5 hours, during the initial steps of the experiment. This might be explained by the fact that one side of the surface was coated with Au in the initial moments of the experiment, reducing the zinc surface available for the electrochemical reactions between the organic phase and the surface of the reducing zinc metal.

Table 3 shows the experimental results obtained for the influence of temperature on Au recovery by galvanic stripping. The results show an increase in Au recovery when the temperature rises from 20ºC to 50ºC for 90 minutes of magnetic stirring time. Raising temperature increases the migration of reagent species towards the interface between the organic phase and the sample of reducing zinc metal improving the Au deposition on the surface of the zinc disk. On the other hand, the Au deposition may reduce the effective zinc surface area, causing the rate of Au deposition to decrease.

Table 4 shows the experimental results obtained for the influence of Au concentration in the organic phase on the recovery of gold by galvanic stripping. High Au recoveries are achieved when organic phases with higher gold concentrations are used in the galvanic stripping experiments. This can be explained by the increase in the number of organic molecules with functional groups bound to Au cyanocomplexes reaching the interface between the organic phase and the sample of zinc metal. Therefore, zinc solubilization and adsorption onto the organic phase make more electrons on the zinc surface available to take part in the cathodic reactions for reduction of Au ions followed by metal deposition on the reducer surface.

Figure 1 shows the scanning electron microscopy (SEM) spectrogram of an area of a zinc disk surface where Au is deposited. The deposited Au was obtained by galvanic stripping from 50 mL of 20 vol. % ALIQUAT336® in xylene, a concentration of 24 g/L Au in the organic phase and 90 minutes of experiment with magnetic stirring and at room temperature. Au peaks in the spectrogram confirm that solid metal deposition occurred by galvanic stripping.

The Arrhenius plot for Au recovery from the organic extractant by galvanic stripping is shown in Figure 2. Under the conditions studied, two experimental activation energies (Ea) were calculated. For temperatures between 20ºC and 35ºC Ea was 2.5 kcal/mol but it increased to 14.2 kcal/mol at higher temperatures between 35 and 50ºC. Experimental results at temperatures higher than 50ºC are necessary to reach an evaluation more conclusive concerning the rate controlling step of the galvanic stripping reaction. However, it is not recommended to use the organic extractant ALIQUAT336® at temperatures higher than 60ºC because it causes a chemical change that affects its ion exchange performance. At any rate, these experimental results obtained can suggest a change from the rate-controlling step from diffusion control to chemical reaction control with increasing temperature.

Experimental results showed that highest Au recovery when magnetic stirring was used as compared to the ultrasonic and unstirred systems for the same experimental time. On the other hand, gold recovery by galvanic stripping is enhanced when the temperature and the time of contact between reducer sample and loaded organic phase are increased. High Au recoveries are also achieved when organic phases with higher gold concentrations are used in the galvanic stripping experiments.

The Arrhenius plot showing experimental results for Au recovery from organic solvent suggests a change in the rate controlling step from diffusion control to chemical reaction control) with increasing temperature in the range of 20 to 50ºC.

The galvanic stripping technique for metal recovery from typical hydrometallurgical commercial organic extractants is still in its initial stage of technological study. Results obtained for the samples of zinc metal showed Au recoveries higher than 28.0% reflecting a potential for technological development. This justifies investing in the further study of its fundamentals, aiming at developing applications for the mineral processing industry.

The next stage of study should concentrate efforts on the use of other metal powder reducers or an increase in surface area of the disks. A higher specific surface area should favor an increase in the percentage of Au recovered from the organic phase as a metal deposit on the reducer metal.

The authors are grateful to Fundação de Amparo à Pesquisa do Estado de Minas Gerais-FAPEMIG (Brazil) and Programa de Apoio a Núcleos de Excelência-PRONEX/MCT, Brazil for their financial support and to Dr. Thomas J. O'Keefe (Graduate Center for Materials Research, University of Missouri-Rolla, USA) for his technical support.

Flores, C. and O'Keefe , T.J., Gold Recovery from Organic Solvents Using Galvanic Stripping. In Separation Processes: Heavy Metals, Ions and Minerals, edited by M. Misra, The Minerals, Metals & Materials Society, USA, P. 187-201 (1994).        [ Links ]

Escobar, C.F., Recovery of Noble Metals from Organic Solvents Using Galvanic Stripping. Ph.D. diss., University of Missouri-Rolla (USA), 93 pp. (1994).        [ Links ]

Belew, B., O'Keefe, T.J. and Watson, J.L., Reductive Stripping of Iron (III) from Di(2-ethylhexyl) Phosphoric Acid, TMS AIME Wadsworth Symposium, Salt Lake City (USA), August, 13 pp. (1996).        [ Links ]

Martins, A.H., Recuperação de Ouro por Stripping Galvanico de Extratante Organico Anionico(in Portuguese), Research Project Report, Fundação de Amparo à Pesquisa do Estado de Minas Gerais-FAPEMIG/Brazil, February, 32 pp. (1998).        [ Links ]

Riveros, P.A., Studies on the Solvent Extraction of Gold from Cyanide Media, Hydrometallurgy, 24, pp.135-156 (1990).        [ Links ]