Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B11/00—Obtaining noble metals
  • C22B11/04—Obtaining noble metals by wet processes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C1/00—Electrolytic production, recovery or refining of metals by electrolysis of solutions
  • C25C1/20—Electrolytic production, recovery or refining of metals by electrolysis of solutions of noble metals

Definitions

• the present invention relates to a method of recovering gold from a gold-bearing solution, such as a gold-bearing concentrated copper chloride solution.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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);
• FIGS. 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:
• the present invention relates to a method of recovering gold from a gold-bearing concentrated copper chloride solution comprising:
• 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
• 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.
• 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).
• WEEE waste electrical and electronic equipment
• 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.
• 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.
• 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 method comprises as step a) an electrodeposition step, wherein an external potential, or a reducing current, is applied to the electrode.
• 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 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.
• 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.
• 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.
• 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).
• typically predominant part of the metal deposited is copper, and optionally lesser part is gold and impurity metal(s).
• 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.
• 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.
• step b) 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.
• 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.
• 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.
• the reducing current is cut-off completely.
• 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 .
• 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.
• 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).
• 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.
• 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.
• 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.
• 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".
• WEEE waste electrical and electronic equipment
• 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.
• 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.
• 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.
• a solution capable of dissolving gold such as chloride, cyanide, thiosulphate, thiourea, glycine
• 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.
• the electrodes were removed from HydroCopper solution and rinsed with water.
• 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.
• 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.
• 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
• Electrodeposition -0.27 V vs. SCE, 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.

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