Electrochemically enhanced leaching of metal sulfides

Abstract

The invention reduces the formation of a passive film on a metal sulfide during hydrometallurgical leaching, which would hinder the extraction of the metal into solution. The invention provides a method and apparatus for leaching a metal from a metal sulfide, such as leaching copper from chalcopyrite. The method comprises contacting the metal sulfide with an acidic leaching solution, applying a transient electrical current to the metal sulfide at a current frequency that reduces passivation of the metal sulfide during leaching, and collecting the leaching solution containing the leached metal. The apparatus comprises means for applying a leach solution to the metal sulfide, a pair of electrodes for passing a transient electrical current through the metal sulfide, means for applying the transient electrical current to the electrodes at a current frequency sufficient to reduce passivation of the metal sulfide during leaching, and means for collecting the leach solution containing the leached metal.

Description

[0001] ELECTROCHEMICALLY ENHANCED LEACHING OF METAL SULFIDES

[0002] Field of the Invention

[0003] This invention pertains to hydrometallurgical processes for recovering metal from metal sulfides, such as copper from chalcopyrite. More particularly, it pertains to metal leaching processes in which a transient electrical current is used to reduce passivation of the metal sulfide.

[0004] Background of the Invention

[0005] The global push for sustainability influences the demand and supply trends for certain metals. For example, in the case of copper, on the demand side, new technologies in electrification have created increased demand. The predicted worldwide copper demand for 2018 to 2035 indicates that sustainable applications such as energy efficiency and electric vehicles contribute to higher cumulative annual growth rates (CAGR) for copper of 4.1 % and 14.2% respectively, compared to 1.7% for traditional applications such as electrical equipment, buildings, and infrastructure, for a total copper CAGR of 2.6%.

[0006] On the supply side, increasing impurity levels in mined ore coupled with stricter environmental regulations will constrain future supply of copper. For example, the average arsenic levels in world copper concentrates have steadily increased from 0.17% in 2010 to 0.23% in 2017.

[0007] The increase in impurity levels in copper concentrate has required smelters to upgrade processes to treat or dispose of hazardous residues to meet stricter environmental regulations. This limits the number of impurities in copper concentrate that smelters can process. In order to provide a suitable copper concentrate feed for their operations, smelters will source copper concentrates of varying concentration of impurities and then blend the cleaner copper concentrates with impure concentrates to prepare a desirable feed. In addition, smelters will charge penalties beyond threshold limits of certain impurities. While smelters are required to meet stricter environmental regulations, air emissions are unavoidable in these large-scale operations.

[0008] To meet the increasing demand for copper while alleviating impending supply constraints of viable copper resources, there is a desire to develop sustainable hydrometallurgical methods for copper recovery.

[0009] Chalcopyrite (CuFeS2) is the most abundant copper sulfide mineral, accounting for approximately 50% of the world’s copper production. As such, developing hydrometallurgical processes to treat chalcopyrite minerals will have the greatest potential impact on future sustainable copper production. Hydrometallurgical treatment of chalcopyrite is a known challenge due to the formation of a passive layer during leaching which hinders the extraction of the copper into solution.

[0010] The present invention addresses existing problems by an electrometallurgical approach that alleviates known technical challenges with respect to passivation during leaching of metals from ores that has precluded large scale adaptation of hydrometallurgical processes as an alternative to smelting, for example as an alternative to copper smelting.

[0011] Summary of the Invention

[0012] The invention is based on surprising discovery that the application of a transient electrical current at a sufficiently high frequency reduces the passivation of the metal sulfide during acidic leaching of the metal from the metal sulfide.

[0013] One aspect of the invention provides a method for leaching a metal from a metal sulfide comprising the steps of:(a) contacting the metal sulfide with an acidic leaching solution; (b) applying a transient electrical current to the metal sulfide at a current frequency that reduces passivation of the metal sulfide during leaching; and (c) collecting the leaching solution containing the leached metal. In some embodiments, the metal is copper and the metal sulfide is chalcopyrite.

[0014] Another aspect of the invention provides an apparatus for leaching a metal from a metal sulfide comprising : (a) means for applying a leach solution to the metal sulfide; (b) at least one pair of electrodes for passing a transient electrical current through the metal sulfide; (c) means for applying the transient electrical current to the electrodes at a current frequency sufficient to reduce passivation of the metal sulfide during leaching; and (d) means for collecting the leach solution containing the leached metal.

[0015] Further aspects of the invention and features of specific embodiments of the invention are described below.

[0016] Brief Description of the Drawings

[0017] Figure 1 is a schematic depiction of the formation of a passive film on chalcopyrite during leaching.

[0018] Figure 2 is a schematic depiction of an electrical circuit analogue of a passive film on chalcopyrite.

[0019] Figure 3 is a graph of the waveform of applied DC current during leaching of chalcopyrite.

[0020] Figure 4 is a graph of the waveform of applied square wave current during leaching of chalcopyrite.

[0021] Figure 5 is a graph of chalcopyrite reference potential versus time with application of DC dissolution current.

[0022] Figure 6 is a graph of chalcopyrite reference potential versus time with application of square wave dissolution current.

[0023] Detailed Description

[0024] The present inventors have determined, based on calculations of an electrical circuit analogue, that the application of a transient current at a sufficient frequency can reduce the electrical impedance of a passive layer on a metal sulfide during leaching due to its capacitive properties. The invention pertains to the application of a transient electrochemical current (such as AC or pulsed DC) to enhance the dissolution kinetics of metal sulfides, such as chalcopyrite, by reducing the electrical impedance of the passive film.

[0025] Hydrometallurgical extraction of copper from chalcopyrite, described in part by the simplified anodic half-cell reaction in Equation 1 , is known to be challenging due to the formation of a passive film that inhibits dissolution of the CuFeS2 mineral. This film formation is shown in Figure 1.

[0026] Anode: CuFeS2 = Cu2++ Fe2++ 2S° + 4e’ (1)

[0027] The inventors have determined that the passive film on chalcopyrite behaves similarly to an electrical circuit consisting of a capacitor and resistor in parallel (representing passive film capacitance and resistance) and the passive film in series with the electrical resistance of the solution. This is shown in Figure 2. With an applied transient potential, such as alternating current (AC) or pulsed direct current (DC), the electrical impedance of the capacitor decreases with increasing transient frequency according to Equation 2 below, where IZcl is the total electrical impedance of a capacitor (Ohms), f is the frequency of the applied transient potential (Hz) and C is a capacitance (Farads). It has been determined that applying a transient current at increasing frequency minimizes the effects of passivation due to the inverse relationship with electrical impedance.

[0028] Calculations for magnitude of impedance for an electrical analogue of a passive film on chalcopyrite clearly indicate the potential to reduce electrical impedance of a passive film with a transient current of increasing frequency.

[0029] The invention provides a method for leaching a metal from a metal sulfide. The metal sulfides include copper sulfides, such as chalcopyrite, from which the process leaches copper. The metal sulfides also include zinc sulfides, such as sphalerite, from which the process leaches zinc. The metal sulfides also include nickel sulfides, such as pentlandite, from which the process leaches nickel. The metal sulfide may be subjected to the leaching process in various ways. For example, the leaching may be done where the metal sulfide is in an ore bed, or in mine tailings, or in a heap, or in a concentrate; or the leaching may be done in a reactor such as a stirred tank reactor or a fluidized bed reactor.

[0030] The metal sulfide is contacted with an acidic leaching solution. Examples of suitable acidic leaching solutions include sulfuric acid solutions, hydrochloric acid solutions, and combinations of such acids. The acidic leaching solutions may contain a dissolved chloride salt, such as NaCI or KCI, for example at a concentration of up to 200 g / L. In some embodiments, the acidic leaching solution has a pH in the range of 0 to 3, alternatively in the range of -1 to 0, alternatively in the range of 3 to 4.

[0031] One example of an acidic leaching solution is a 0.5M sulfuric acid solution with a pH of 0.

[0032] In some embodiments, the temperature of the acidic leaching solution during the process may be in the range of 5 to 50°C, alternatively in the range of 15 to 25°C, alternatively in the range of 25 to 50°C, alternatively at any temperature above the freezing point of the acidic leaching solution.

[0033] A transient electrical current is applied to the metal sulfide during the leaching process. The transient current may be an alternating current (AC) or a pulsed direct current (DC). The pulsed DC current may be a square wave current. The transient current may be an anodic current.

[0034] The frequency of the transient current is selected to be high enough to reduce passivation of the metal sulfide, i.e., to minimize or eliminate the formation of a passive film on the metal sulfide during leaching. The frequency of the transient current may be in the range of 10 to 100 Hz, alternatively in the range of 100 to 1000 Hz, alternatively in the range of 1000 to 10000 Hz, alternatively a frequency of greater than 10000 Hz.

[0035] The transient current may be applied to the metal sulfide in various ways. For example, it may be applied by means of electrodes embedded in the metal sulfide mass, or by means of a pair of electrode plates placed on opposite sides of an ore body of the metal sulfide, or by means of electrodes arranged in a stirred tank, fluidized bed or other reactor.

[0036] The leaching solution containing leached metal is collected after the leaching is done. The metal can then be recovered from the leaching solution by means of a downstream or subsequent process, such as electrowinning.

[0037] The apparatus for leaching a metal from a metal sulfide can take various forms. In general terms, its comprises: (a) means for applying a leach solution to the metal sulfide; (b) at least one pair of electrodes for passing a transient electrical current through the metal sulfide; (c) means for applying the transient electrical current to the electrodes at a current frequency sufficient to reduce passivation of the metal sulfide during leaching; and (d) means for collecting the leach solution containing the leached metal. The means for applying the leach solution may, for example, be a sprayer that sprays the solution on an ore body or a heap of metal sulfide. The means for applying the transient electrical current may, for example, be a source of AC or pulsed DC current with transformer and function generator to produce the selected frequencies and waveforms. The means for collecting the leach solution may be a collection vessel positioned to receive the leach solution containing the leached metal.

[0038] Examples

[0039] Experiments were conducted at a 1 to 10 mA / cm2average current density. The chalcopyrite electrode surface area was approximately 2 cm2. An applied frequency for a transient AC or pulsed current of at least 100 Hz enabled experiments to be conducted at frequencies where there is convergence of electrochemical impedance for enhanced and non-enhanced leaching. The ability of running current control permitted the comparison of leach extraction tests with the same applied Coulombs.

[0040] The experiments were conducted at up to 20 mA current controlled output with 1 % resolution, frequencies of 0 to 1000 Hz, and with the ability to monitor reference potential versus an electrochemical reference electrode. The potentiostat used a Howland current pump amplifier design providing for internal voltage control to maintain current setpoint with changing impedance of the electrochemical cell. The amplifier was integrated with a function generator to provide the desired range of signal waveforms. An auxiliary terminal was added to the circuit to monitor reference potential versus an electrochemical reference electrode. A PLC was integrated into the potentiostat system to provide computer control of the experimental set-points and log current, cell voltage and reference potential voltage versus time. The potentiostat was connected to an electrochemical cell consisting of a chalcopyrite working electrode (mounted in epoxy and finished to 600 grit), a graphite rod counter electrode and a Hg / HgSC>4 reference electrode in 700 mL of 0.5M sulfuric acid, mixed with a magnetic stir bar.

[0041] Two experiments were conducted to compare electrochemical dissolution of chalcopyrite with a constant DC current versus a square wave transient current. The test conditions of these two experiments are summarized in Table 1 below. Figures 3 and 4 depict the applied electrical current waveforms for DC and square wave respectively.

[0042] Table 1. Test Conditions for Experiments Comparing DC versus Square Wave Current for Electrochemical Dissolution of Chalcopyrite

[0043] In order to determine total copper (and iron) dissolution from the chalcopyrite electrodes, for experimental purposes some of the dissolved copper was electroplated onto the graphite counter electrode and ICP analysis was conducted on the final solutions and aqua regia digests of the metal electroplated onto the graphite counter electrodes. The ICP results are provided in Table 2.

[0044] Table 2. ICP Analysis of Final Electrolyte Solutions and Metal Digests from Counter

[0045] Electrodes

[0046] The total copper and iron extracted from each experiment was calculated by multiplying the experimental solution volume (0.7L) with the concentration of metals in the final electrolyte solutions and adding the weight of metals extracted from the counter electrodes by aqua regia digestions. Also, using the total coulombs applied in each experiment (222 coulombs), the theoretical copper and iron was calculated, and current efficiency determined by dividing actual metal extracted by the theoretical amounts. Table 3 summarizes these results.

[0047] Table 3. Copper and Iron Extraction and Current Efficiencies for DC vs 1000 Hz Pulsed Electrochemical Dissolution Tests.

[0048] The significant improvement of current efficiency for dissolution of copper and iron from chalcopyrite by applying a square wave versus DC current, demonstrates that the transient current minimized the effect of the passivation. Data logs of the reference potentials of the chalcopyrite working electrodes (VSHE = VHg / Hgso4 + 0.615) for both the DC and the square wave experiments are graphed in Figures 5 and 6 respectively. The chalcopyrite reference potential during the DC experiment decreased and then increased and leveled off after approximately 100 minutes. This indicates that passivation was occurring. In the experiment with the applied square wave current, the chalcopyrite reference potential continued to drop over time, indicating that passivation was avoided or minimized.

[0049] In the foregoing description, where a component (e.g. an assembly, device, etc.) is referred to, reference to that component (including reference to a means) should be interpreted as including as equivalents of that component any component which performs the same function as the described component, including components which are not structurally equivalent to the disclosed structures which perform the function in the illustrated exemplary embodiments of the invention.

[0050] Throughout the foregoing description and the drawings, in which corresponding and like parts are identified by the same reference characters, specific details have been set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

[0051] As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the following claims.

Claims

Claims1 . A method for leaching a metal from a metal sulfide comprising the steps of:(a) contacting the metal sulfide with an acidic leaching solution;(b) applying a transient electrical current to the metal sulfide at a current frequency that reduces passivation of the metal sulfide during leaching; and(c) collecting the leaching solution containing the leached metal.

2. The method according to claim 1 , wherein the metal sulfide comprises chalcopyrite.

3. The method according to claim 1 , wherein the metal sulfide comprises sphalerite.

4. The method according to claim 1 , wherein the metal sulfide comprises pentlandite.

5. The method according to claim 1 or 2, wherein the metal comprises copper.

6. The method according to claim 1 or 3, wherein the metal comprises zinc.

7. The method according to claim 1 or 4, wherein the metal comprises nickel.

8. The method according to any one of claims 1-7, wherein the current frequency is in the range of 10 to 100 Hz.

9. The method according to any one of claims 1-7, wherein the current frequency is in the range of 100 to 1000 Hz.

10. The method according to any one of claims 1-7, wherein the current frequency is greater than 1000 Hz.11 . The method according to any one of claims 1-7, wherein the current frequency is in the range of 1000 to 10000 Hz.

12. The method according to any one of claims 1-7, wherein the current frequency is greater than 10000 Hz.

13. The method according to any one of claims 1-12, wherein the transient electrical current is an anodic current.

14. The method according to any one of claims 1-12, wherein the transient electrical current is a pulsed direct current.

15. The method according to any one of claims 1-12, wherein the transient electrical current is a square wave current.

16. The method according to any one of claims 1-12, wherein the transient electrical current is an alternating current.

17. The method according to any one of claims 1-16, wherein the acidic leaching solution has a pH in the range of 0 to 3.

18. The method according to any one of claims 1-16, wherein the acidic leaching solution has a pH in the range of -1 to 0.

19. The method according to any one of claims 1-16, wherein the acidic leaching solution has a pH in the range of 3 to 4.

20. The method according to any one of claims 1-19, wherein the acidic leaching solution has a pH less than 7.21 . The method according to any one of claims 1-20, wherein the acidic leaching solution comprises sulfuric acid.

22. The method according to claim 21 , wherein the sulfuric acid is 0.5M.

23. The method according to any one of claims 1-20, wherein the acidic leaching solution comprises sulfuric acid and hydrochloric acid.

24. The method according to any one of claims 1-23, wherein the acidic leaching solution contains Fe or an Fe derivative.

25. The method according to any one of claims 1-24, wherein the acidic leaching solution contains a dissolved chloride salt.

26. The method according to claim 25, wherein the dissolved chloride salt comprises NaCI or KCI.

27. The method according to claim 25 or 26, wherein the dissolved chloride salt has a concentration up to 200 g / L.

28. The method according to any one of claims 1-27, wherein the metal is leached from the metal sulfide at a temperature in the range of 5 to 50 °C.

29. The method according to any one of claims 1-27, wherein the metal is leached from the metal sulfide at a temperature in the range of 15 to 25 °C.

30. The method according to any one of claims 1-27, wherein the metal is leached from the metal sulfide at a temperature in the range of 25 to 50 °C.31 . The method according to any one of claims 1-27, wherein the metal is leached from the metal sulfide at a temperature above the freezing point of the acidic leaching solution.

32. The method according to any one of claims 1-31 , wherein the metal sulfide is contained in an ore bed.

33. The method according to any one of claims 1-31 , wherein the metal sulfide is contained in an ore heap.

34. The method according to any one of claims 1-31 , wherein the metal sulfide in contained in a concentrate.

35. The method according to any one of claims 1-31 , wherein the metal is leached from the metal sulfide in a stirred tank reactor.

36. The method according to any one of claims 1-31 , wherein the metal is leached from the metal sulfide in a fluidized bed reactor.

37. An apparatus for leaching a metal from a metal sulfide comprising :(a) means for applying a leach solution to the metal sulfide;(b) at least one pair of electrodes for passing a transient electrical current through the metal sulfide;(c) means for applying the transient electrical current to the electrodes at a current frequency sufficient to reduce passivation of the metal sulfide during leaching; and(d) means for collecting the leach solution containing the leached metal.

38. The apparatus according to claim 37, wherein the means for applying a leach solution to the metal sulfide comprises a sprayer.

39. The apparatus according to claim 37 or 38, wherein the means for applying the transient electrical current to the electrodes comprises a source of AC or pulsed DC current with a transformer and function generator to produce selected frequencies and waveforms.

40. The apparatus according to any one of claims 37-39, wherein the means for collecting the leach solution containing the leached metal comprises a collection vessel positioned to receive the leach solution containing the leached metal.41 . The apparatus according to any one of claims 37-40, wherein the apparatus comprises a stirred tank reactor.

42. The apparatus according to any one of claims 37-40, wherein the apparatus comprises a fluidized bed reactor.

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