FIELD OF THE INVENTION

The present invention relates to a method of recovering gold from a gold-bearing solution, such as a gold-bearing concentrated copper chloride solution.

BACKGROUND OF THE INVENTION

WO 2010/121317 discloses a method for recovering gold from a gold concentrate comprising: dissolving gold from the concentrate in an aqueous liq- uor to provide a gold liquor; subjecting the gold liquor to electrolysis in an elec- trowinning cell to provide cathode-associated gold-material; leaching the cathode associated gold material in an aqueous liquor under reducing conditions to provide a treated solid residue; and smelting the treated solid residue to recover gold.

WO 2012/081952 discloses a method for recovering gold and silver from thiosulphate and thiourea solutions, by means of an electrolysis method with simultaneous metal deposition on the cathode and anode.

Various publications relating to optimising the formation of nanothin films, nanoparticles or functional surfaces have been published. Examples of these are

Kirsi Yliniemi, D. Wragg, B. P. Wilson, H. Neil McMurray, D. A. Worsley, P. Schmuki, K. Kontturi, Electrochimica Acta 88 (2013) p. 278. In this publication, Pt nanoparticles are prepared and Pb is used as a sacrificial metal; the purpose is to create Pt nanoparticle surfaces from synthetic solutions, to be used as a cata- lysts in dye-sensitized solar cells.

S. Cherevko, N. Kulyk, C.-H. Chung, Nanoporous Pt@AuxCul00-x by hydrogen evolution assisted electrodeposition of AuxCul00-x and galvanic replacement of Cu with Pt: electrocatalytic properties Langmuir, 28 (2012), p. 3306. In this publication, Pt nanoporous materials are prepared for catalysis of hydro- gen evolution.

S.R. Brankovic, J.X. Wang, R.R. Adzic, Metal monolayer deposition by replacement of metal adlayers on electrode surfaces, Surface Science, 474 (2001), p. L173. This is a seminal paper of surface-limited redox replacement. The paper shows that using Cu as a sacrificial metal, a submonolayer of Pt, bilayer of Ag and textured monolayer of Pd were prepared. M. Fayette, Y. Liu, D. Bertrand, J. Nutariya, N. Vasiljevic, N. Dimitrov, From Au to Pt via surface limited redox replacement of Pb UPD in one-cell configuration, Langmuir, 27 (2011), p. 5650. In this publication, a one-pot method for SLRR is introduced.

However, none of these publications aims to recover gold, only for forming nanostructures/thin films. Moreover, these publications use only pure synthetic solutions; whereas this invention provides a method so as to alleviate the challenge of gold recovery from impure industrial process solutions, where the amount of gold is low compared to impurities. BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a method so as to alleviate disadvantages relating to traditional gold recovery methods. The objects of the invention are achieved by a method which is characterized by what is stated in the independent claims. Further embodiments of the invention are disclosed in the dependent claims.

The invention is based on the idea of first electrodepositing copper and optionally gold on the surface of an electrode from a solution containing gold, copper and chlorides by applying suitable external potential or reducing current to the electrode. After a suitable time period, external applied potential or reduc- ing current is cut off or reduced after which spontaneous redox replacement step takes place. During the redox replacement step, the electrode is let to stand in the solution thereby allowing the less noble copper deposited on the surface of the electrode to be spontaneously replaced by more noble gold ions or complexes contained in the solution. Thus an electrode enriched with gold is obtained.

Traditionally, the more the solution from which gold is desired to be recovered contains copper, the more challenging the recovery of gold is. Traditionally also, the less gold is in the solution, the more challenging the recovery of the gold is. However, the present invention is especially suitable for recovering gold from solutions which contain higher amounts of copper and chloride and very low amounts of gold, as the presence of copper in the solution is taken advantage of in the present invention. Increasing the amount of copper on the electrode surface gradually creates improved possibility for redox replacement to take place. The present method is especially suitable for recovering gold from an industrial solution with low gold content and high copper and chloride content. Furthermore, the present invention is suitable for recovering gold from solutions which contain significant amounts of impurities, such as Na, Ca, K, Pb, Fe, as these impurities do not remarkably decrease the amount of gold recovered.

In traditional electrowinning, copper is always unavoidably also depositing in the product if the electrolyte solution has a high copper and low gold content. However, the portion of copper in the product can be remarkably reduced with the method of the present invention, wherein electrodepositing is alternated with redox replacement step and copper once deposited on the electrode is replaced with gold.

A further advantage of the present invention is that the consumption of energy and chemicals can be reduced. The spontaneous redox replacement step consumes no or very low amount of electricity, even if gold is majorly recovered during this step when the applied external potential or current is cut off or remarkably reduced. Furthermore, use of extraction chemicals can be avoided as well as use of ion exchange resins or precipitation chemicals, which are typically required in traditional methods for recovering gold.

BRIEF DESCRIPTION OF THE DRAWINGS

Results of the Examples are presented with reference to the attached drawings, in which

Figure 1 illustrates comparison of gold stripping peak (in cyclic volt- ammetry) after 10 cycles of electrodeposition-redox replacement (ED+RR) steps with different parameters (Lines 1-3) and a stripping peak after a single electro- depostion (ED) step (single ED step indicates the behaviour in traditional electrowinning type of process) (Line 4);

Figures 2A and 2B illustrate detection of gold recovery from Hydro- Copper solution (47.3 g/1 copper, 4-5 M chloride and impurities such as 1.4 g/1 Zn, 0.5 g/1 Pb, 20 mg/1 Fe,) with 10 ppm or 100 ppm gold: (A) the presence of gold enrichment confirmed by cyclic voltammograms measured in 20 mM CuCl ₂ + 3 M NaCl + 100 ppm Au solution after 10 ED+RR cycles in HydroCopper solution (ED: -0.27 V, 10s; RR: potential cut-off, until open circuit potential reaches 0 V vs. SCE, after which a new ED step can commence), (B) magnification to Au stripping peak in the cyclic voltammograms of (A);

Figure 3 illustrates the purity of deposits after 10 electrodeposition + redox replacement (ED+RR) cycles (Sample 1) and after a single electrodeposition (ED) step (single ED step indicates the behaviour in traditional electro- winning type of process) (Sample 2): the total deposition time and applied poten- tial are the same in both cases (Sample 1: -0.27 V vs. SCE, 10 x 10 s = 100 s, Sample 2: -0.27 V vs. SCE, 100 s).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of recovering gold from a gold-bearing concentrated copper chloride solution comprising:

a) an electrodeposition step, wherein an external potential or reducing current is applied to an electrode in the gold-bearing concentrated copper chloride solution thereby depositing copper and optionally gold on the electrode, b) a redox replacement step, wherein the in step a) applied external potential or reducing current is cut-off or reduced thereby allowing copper deposited on the electrode to be spontaneously replaced by gold contained in the solution thereby obtaining an electrode, which contains gold.

The present method is especially suitable for recovering gold from industrial solutions with low gold content and high copper and chloride content. The gold-bearing concentrated copper chloride solution used as a starting material in the present method typically originates from chloride leaching, more typically from cyanide-free chloride leaching, of gold mineral(s), gold ore(s), gold concentrate(s), gold containing tailing(s), gold-copper mineral(s), gold-copper ore(s), gold-copper concentrate(s), waste electrical and electronic equipment (WEEE) and/or other gold containing primary or secondary raw material(s). The method of the present invention may be performed after a leaching of the fore mentioned material(s) or during the actual leaching process of the fore mentioned materials. In other words, the gold-bearing concentrated copper chloride solution may also be the leaching solution in the actual leaching process of the chloride leaching, more typically from cyanide-free chloride leaching, of gold mineral(s), gold ore(s), gold concentrate(s), gold containing tailing(s), gold- copper mineral(s), gold-copper ore(s), gold-copper concentrate(s), waste electrical and electronic equipment (WEEE) and/or other gold containing primary or secondary raw material(s).

Concentrated solution means in this context that chloride concentration is high enough to complex copper in the solution, resulting in the increase of redox potential to leach gold typically present in the gold minerals, gold ores, gold concentrates, gold containing tailings, gold-copper minerals, gold-copper ores, gold-copper concentrates, waste electrical and electronic equipment (WEEE) and/or other gold containing primary or secondary raw material(s). The chloride concentration is typically more than 0.5 M, more typically in the range of 0.5 M to 12 M, more typically 1 to 6 M, even more typically 2 to 5 M. In the gold-bearing concentrated copper chloride solution, the copper is typically in the form of cu- pric or cuprous ions or chloride complexes in the solution. Gold is typically in the form of aurous or auric chloride complex. The gold-bearing concentrated copper chloride solution has typically a high chloride concentration, typically more than 0.5 M, more typically in the range of 0.5 M to 12 M, more typically 1 to 6 M, even more typically 2 to 5 M. The copper concentration in the solution is typically high, typically more than 1 g/1, typically in the range of 1 - 100 g/1, more typically 5 - 90 g/1, even more typically 10 - 80 g/1, even more typically 20 - 70 g/1. The gold concentration in the solution is typically low, typically in the range of 0.1 - 100 ppm, even more typically 0.1 - 20 ppm, even more typically 0.5 ppm - 5 ppm. The present invention is especially suitable for recovering gold from starting materials containing as low as 0.1 - 20 ppm gold. The solution may also contain bro- mides in the range of 0 - 20 g/1, more typically in the range of 1 - 10 g/1, more typically 2 - 6 g/1. In addition to those the gold-bearing concentrated copper chloride solution typically contains impurity metals, such as Fe, Zn, Pb typically below 2 g/1, while Ca and Na amounts can be typically relatively high, depending on the source of the chlorides (even over 80g/l).

The fact that gold can be recovered from the gold-bearing concentrated copper chloride solution with the present method is remarkable. From the starting material recovery of gold can be very challenging and economically unfeasible due to the very low Au concentration. Furthermore, copper in the chloride leaching solution, i.e. cupric ion or cupric chloride complex is known to be able to dissolve Au, and gold is dissolved as gold-chloride complexes. During the electrodeposition step, however, copper precipitates and during the redox replacement step deposited copper is replaced by gold originating from dissolved gold such as the gold-chloride complex.

The method comprises as step a) an electrodeposition step, wherein an external potential, or a reducing current, is applied to the electrode. Typically the potential of the electrode is in the range of capable of depositing copper, i.e. less than +0.2 V vs. SCE (saturated calomel electrode), typically in the range of +0.2 - -1.2 V vs. SCE, more typically +0.2 V - -0.9 V vs. SCE and even more typically -0.1 V - -0.5 V vs. SCE. The absolute value of the current density (reducing current in the electrodeposition step) is typically in the range of 0.1-1000 mA/cm ² , more typically in the range of 10-300 mA/cm ² , more typically 20-200 mA/cm ² , even more typically 50-100 mA/cm ² . The residence time of the electrodes in the solution under the applied external potential or applied reducing current in the elec- trodeposition step is typically less than 1 h, typically in the range of 1 s - 1 h, more typically 1 s - 1 min and even more typically 1 s - 20 s. It is also possible to interrupt the electrodeposition step unfinished. The electrodeposition step is performed galvanostatically or potentiostatically. Galvanostatically means that a constant current is applied. Potentiostatically means that a constant potential is applied. It is possible - but unconventional- to deposit copper using potentiody- namic or galvanodynamic deposition. Potentiodynamic means that potential is varied in a limited potential range in which copper deposits on the electrode and galvanodynamic means that current is varied in a limited current range in which copper deposits on the electrode surface.

An electrode is in electrical contact with an external potential or current source. The electrodes may be made of any suitable material, which is con- ductive and resistive for chlorides in order to avoid corrosion. Typically, the copper and optionally gold are deposited on a conductive electrode such as a metal oxide, titanium, stainless steel, duplex steel or Pt cathode during the electrodeposition step. The electrodes may have any suitable shape, such as plate, ring, sheet, mesh, stick or any other applicable form.

The gold-bearing concentrated copper chloride solution can be stagnant, stirred or pumped.

In the electrodeposition step the electrodes are immersed in the gold- bearing concentrated copper chloride solution contained in a suitable vessel, pool, tank etc. and external potential or reducing current is applied. The gold- bearing concentrated copper chloride solution may also be within a leaching process. This allows copper and possibly gold to deposit on the cathode, in other words, during this constant potential or current feed a deposit rich in copper, and optionally lower in gold forms on the cathode. If the electrodeposition step is repeated after the redox replacement step, in that case typically a layer comprising copper, gold, and possible impurity metals is formed on top of the previous layer (s). During the electrodeposition step typically predominant part of the metal deposited is copper, and optionally lesser part is gold and impurity metal(s). Typically the thickness of the final product can be measured in micrometres, more typically however in millimetres. Copper in the chloride solution can oxidise gold from the raw material. Furthermore, copper is capable of depositing in electrodeposition step and the redox potential of the solution is such that a redox re- placement of copper with gold takes spontaneously place after the electrodeposi- tion step.

After step a) the method comprises a step b), which is a redox replacement step, wherein applied external potential or reducing current of the step a) is cut-off or reduced thereby allowing copper deposited on the electrode to be spontaneously replaced by gold contained in the solution thereby obtaining an electrode, which contains gold, in other words an electrode enriched with gold. If the external potential or current is cut-off, redox replacement takes place until a pre-determined open circuit potential of the copper and gold containing electrode is reached. This pre-determined open circuit potential value is selected to be below gold stripping potential value. Open circuit potential means the potential of the electrode when no external potential or current is applied: open circuit potential is dictated by the solution composition, surface composition of the electrode and possible reactions taking place in the electrode. The pre-determined open circuit potential value at which the redox replacement step is finished, is typically below 0.8 V vs. SCE, more typically 0.8 - 0 V vs. SCE, even more typically 0.6 V - 0.1 vs. SCE, even more typically 0.3 - 0.2 V. vs. SCE.

Also, redox replacement step can be finished before this pre-determined open circuit potential is reached and typically such cut-off times are less than 24 hours, more typically 3 s -12 hour, even more typically 3 s - 1 h, even more typically 3 s - 30 min. This time is dependent also on the mass transfer in the solution and can be altered e.g. by stirring or pumping the solution.

Typically the external potential is cut-off. If the external applied potential is reduced but not cut-off, it is typically altered to the value in the range of 0.4 - 0.7 V vs. SCE, even more typically 0.4 - 0.6 V vs. SCE, even more typically 0.5 - 0.6 V vs. SCE.

Typically the reducing current is cut-off completely. However, it is also possible to reduce the reducing current to a value of where spontaneous redox replacement takes place, typically the absolute value of the current density is less than 50 mA/cm ² , more typically 0 - 30 mA/cm ² , even more typically 0 - 10 mA/cm ² , even more typically 0 - 0.5 mA/cm ² .

During the redox replacement step copper, or possible impurity met- al(s) which were deposited on the surface of the electrode during the electrodep- osition step, donates its electron(s) to gold ions still contained in the solution and copper is dissolved from the electrode back to the solution and gold is deposited on the electrode instead. Thus gold can deposit on the electrode both during the electrodeposition step and the redox replacement step. The spontaneous redox replacement step consumes no or very low amount of electricity, even if gold is majorly recovered during this step when the applied external potential or current is cut off or remarkably reduced. Surprisingly it was found out that with this method, majority of gold can be recovered during redox replacement step without any applied external energy (Example 3).

After the redox replacement step gold has enriched to the electrode. It was surprisingly found out that when cycling between electrodeposition and redox replacement steps, gold was effectively enriched from very impure industrial concentrated copper chloride solution. For example only ten times was enough, as can be seen for example from Example 2, where the solution contained Cu 47.3 g/1, 4-5 M chlorides (and impurities such as Zn 1.4 g/1, Pb 0.5 g/1, Fe 20 mg/1,) and Au 10 or 100 ppm.

Typically steps a) and b) are repeated consecutively several times (al- so called cycling between the steps), typically 1 to 100 000 times, more typically 10 to 50 000 times, even more typically 100 to 50 000 times, even more typically 500 to 5000 until the desired gold-containing product is achieved. Thus, after the redox replacement step b) a new step a) is performed, wherein the external potential or reducing current is applied again and after that a new step b) is pre- sented by cutting-off the potential or reducing the applied external potential or the applied reducing current. The parameters used in the second or further round may be the same or different from those of the first or previous round falling in the ranges presented.

Typically steps a) and b) are performed for gold-bearing concentrated copper chloride solution after leaching of gold minerals, gold ores, gold concentrates, gold containing tailings, gold-copper minerals, gold-copper ores or gold- copper concentrates, waste electrical and electronic equipment (WEEE) or other gold containing primary or secondary raw material. Additionally steps a) and b) can be performed simultaneously when leaching takes place i.e. "ED+RR in leach".

The present method may also contain an optional gold recovery step, wherein the in step b) obtained electrode or electrode obtained after step a) or after step b) after cycling of steps a) and b), which contains gold is subjected to hydrometallurgical method, pyrometallurgical method, chemical stripping, physical stripping or electrochemical stripping for recovering gold from the electrode. Thus, the method of the invention may be interrupted at any point during the cycling between steps a) and b) and the electrode may be taken after step a) or after step b) to a further gold recovery step. Typically the gold contained in the electrode is recovered by leaching the deposited gold from the electrode to a solution capable of dissolving gold, such as chloride, cyanide, thiosulphate, thiourea, glycine and recovering the dissolved gold from the solution by precipitation or elec- trowinning, or by any other suitable method known for a person skilled in the art.

EXAMPLES Example 1

Recovery of gold was studied from synthetic gold bearing copper chloride solution (20 mM CuCl ₂ + 3 M NaCl + 100 ppm Au) using 10 cycles of electro- deposition (ED) and redox replacement (RR) steps (ED+RR). Pt sheet (0.32 cm ² ) was used as the electrode collecting gold (cathode), another Pt sheet as a counter electrode (anode) and a standard calomel electrode (SCE) as a reference electrode.

ED step was performed either at potential of -0.4 V vs. SCE or -0.27 V vs. SCE and the ED time was either 10 s or 5 s. Redox replacement step - during which external applied potential was cut off and deposited copper was spontaneously replaced by gold - was let to take place until open circuit potential of the electrode (cathode) reached 0 V vs. SCE.

As a comparison, one gold recovery was performed with a single elec- trodeposition step at deposition potential -0.27 V vs. SCE and time 100 s.

Immediately after 10 cycles of ED-RR steps, a cyclic voltammogram was measured in the same solution (25 mV/s, -0.2 V -> 1.1 V ->-0.7 V -> -0.2 V vs. SCE) and the sizes of Au stripping peak around 0.75-0.80 V vs. SCE were compared to each other; the size of stripping peak indicates the amount of gold on the electrode. These stripping peaks are illustrated in Figure 1.

Confirmation of recovery from gold from synthetic solutions (20 mM CuC12+3 M NaCl + 100 ppm Au) is detected after ED+RR steps (Lines 1-3) and single ED step (Line 4). The observations are:

- Line 1 vs. Line 2: effect of electrodeposition potential

o more reducing ED potential (lower potential, -0.4 V vs. -0.27 V vs.

SCE) increased the recovered gold amount

- Line 1 vs. Line 3: effect of electrodeposition time

o longer deposition time increased recovered gold amount

- Line 2 vs. Line 4: effect of RR step

o more gold was recovered with ED+RR than with single ED step, even when the applied energy was the same in both cases (same deposition time, i.e. 10 x 10 s (ED+RR) or 100 s (single ED step), and same deposition potential)

o During RR step no external energy was required for redox replacement to take place (the external applied potential was cut off during redox re- placement step and RR step was finished when open circuit potential reached 0 V vs. SCE, after which new ED step starts)

Example 2

Recovery of gold was verified also from industrial process solution (HydroCopper process) with 47.3 g/L copper,chloride 4-5 M (and impurities such as Zn 1.4 g/1, Pb 0.5 g/1, Fe 20 mg/1,) and 100 ppm or 10 ppm gold. 10 cycles of electrodeposition (ED) and redox replacement (RR) steps (ED+RR) were performed. Pt sheet (0.32 cm ² ) was used as the electrode collecting gold (cathode), another Pt sheet as a counter electrode (anode) and standard calomel electrode (SCE) as a reference electrode. ED step was performed in -0.27 V vs. SCE and each ED step was 10 s. Redox replacement step - during which external applied potential was cut off and deposited copper is spontaneously replaced by gold was let to take place until open circuit potential reached the potential 0.2 V vs. SCE.

Immediately after 10 cycles ED+RR steps, the electrodes were removed from HydroCopper solution and rinsed with water. After this, the elec- trodes were placed in 20 mM CuCl ₂ + 3 M NaCl + 100 ppm Au solution and a cyclic voltammogram was measured (25 mV/s, -0.2 V -> 1.1 V ->-0.7 V -> -0.2 V vs. SCE). The sizes of Au stripping peak around 0.75-0.80 V vs. SCE were detected and they verified the enrichment of gold to the electrode surface. These cyclic voltammograms are illustrated in Fig. 2a and a magnification to the gold stripping peak potential range is seen in Fig. 2b.

Figure 2 shows that using ED+RR method a good gold recovery is possible from industrial hydrometallurgical process solutions which contain a high concentration of copper (« 47.3 g/1), high concentration of chloride (4-5 M), low concentration of gold (100 ppm or 10 ppm) and impurities. This is remarkable especially as majority of gold was recovered during redox replacement step without any applied external energy.

Example 3

The recovery of Au was compared between samples prepared by 10 electrodeposition + redox replacement steps (Sample 1) and a single electrodepo- sition step (Sample 2) performed in 20 mM CuCl ₂ + 3 M NaCl + 10 ppm Au. The mass-% of Au (vs. Cu) was determined with SEM-EDS analysis (Scanning Electron Microscope, LEO 1450 VP, Germany - Energy Dispersive X-ray Spectroscopy, INCA software, UK), using the average value of 20 points EDS spectra (each spectrum was measured at different location at the electrode surface). Before analysis, the samples were first rinsed with distilled water and dried in air at room temperature. The analysis results are shown in Figure 3.

The parameters for gold recovery are presented for both samples below.

Sample 1:

Electrodeposition step: -0.27 V vs. SCE, 10 s

Redox replacement step: applied external potential is cut-off, RR step takes place until open circuit potential is 0 V vs. SCE

Number of cycles: 10

Total time for applied external potential: 10 x 10 s = 100 s

Sample 2:

Electrodeposition: -0.27 V vs. SCE, 100 s

No redox replacement step

Number of steps: 1

Total time of applied potential: 100 s

Figure 3 shows that using the invented method, surprisingly the gold content in the deposit is clearly higher than when using the same total deposition time and applied potential with a single electrodeposition step.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.