METHOD AND PROCESS FOR THE ENHANCED LEACHING OF COPPER SULFIDE MINERALS CONTAINING CHALCOPYRITE
A method of leaching a copper bearing sulfide mineral slurry containing chalcopyrite is described. The method comprises the steps of providing a slurry having chalcopyrite particles therein, exposing the slurry to an acidic leach solution, and chemically leaching copper from the slurry into the acidic leach solution in the presence of microwave irradiation. The microwave irradiation of the slurry takes place under process conditions whereby crystalline pyrite may be formed in-situ on surfaces of the chalcopyrite particles. Crystalline pyrite may be formed on surfaces of the chalcopyrite particles from amorphous phase pyrite. Leached copper is recovered from said acidic leach solution. A device for more efficiently leaching a copper bearing sulfide mineral slurry containing chalcopyrite is also described herein.
This international application claims the benefit of U.S. Provisional Patent Application No. 61/725,206 filed on Nov. 12, 2012.
FIELD OF THE INVENTIONThis invention relates to methods and systems for leaching metals from metal sulfide ores and concentrates and more particularly to methods and systems for microwave controlled formation of iron sulfides during leaching of metal values from sulfide ores and concentrates.
BACKGROUND OF THE INVENTIONThis invention relates to the hydrometallurgical processing of sulfide ores for metals recovery. Chalcopyrite (CuFeS2) is the primary copper-containing mineral found in the majority of the copper sulfide ores of commercial interest. Other copper-containing ore minerals of commercial interest include chalcocite (Cu2S), bornite (Cu5FeS4), covellite (CuS), digenite (Cu2S), enargite (Cu3AsS4), tennantite (Cu12As4S13), and tetrahedrite (Cu12Sb4S13). Copper sulfide ores, aside from containing a variety of copper-containing minerals, will also contain a wide variety of gangue minerals including, but not limited to, silicates, pyrite (FeS2) and pyrrhotite (FeS).
In the processing of metal sulfide ores, flotation is commonly and successfully used to effect a separation of the metal values from the gangue. However, separation of individual metal sulfides from each other can be a challenge, and the separation of copper-bearing sulfide minerals from pyrite by flotation remains a technical problem. It would be desirable to recover the copper by hydrometallurgical processes, such as leaching, without effecting a chemical or physical change in gangue minerals like pyrite.
It is also known from prior art that the dissolution of chalcopyrite is a slow process and that copper recoveries from chalcopyrite can be limited by surface passivation reactions that prevent complete mineral dissolution. The nature of the surface reactions that lead to passivation are not completely understood, but many researchers ascribe the hindered dissolution, at least partly, to the formation of a relatively impermeable layer of elemental sulfur on the chalcopyrite surface.
In U.S. Pat. No. 7,846,233 B2, a method for selectively leaching chalcopyrite includes using pyrite as a catalyst for improving copper recoveries and increasing chalcopyrite dissolution rates. The leaching is carried out in acidic ferric/ferrous sulfate solutions containing dissolved oxygen under redox conditions whereby the pyrite is not significantly oxidized. This enhancement in copper leaching rates has been attributed to galvanic interactions between pyrite and chalcopyrite particles present in the leach process. The prior art leach conditions lead to the formation of a porous sulfur layer, which facilitates rapid mass transport of copper from the mineral surface to the leach solution. The overall mineral dissolution reaction, which is the sum of the anodic and cathodic half-cell reactions, can be described as:
CuFeS2+2Fe2(SO4)3→CuSO4+5FeSO4+2So
The oxidative dissolution process is optimally carried out at temperatures which are below the melting point of elemental sulfur (So), which is about 110 to 120° C. and at redox potentials which minimize the degree of sulfide oxidation to sulfate. However, it will be recognized by one skilled in the art that the reaction temperatures must be sufficiently high to produce rapid leaching of copper. Thus, prior art methods specify an optimum leach temperature of about 70-90° C.
The kinetics of ferric ion reduction to ferrous ion has been shown by Majuste, et. al., in Hydrometallurgy, 113-114, 167-176 (2012) to be faster on pyrite surfaces than on chalcopyrite surfaces, when the leach conditions are such that there are no anodic reactions at the galvanically coupled pyrite surfaces. Thus, the dissolution of chalcopyrite will proceed faster when pyrite is galvanically coupled to chalcopyrite.
To maintain the galvanic couple as the leach reaction progresses, electron conduction must occur across the porous layer of elemental sulfur formed on the surfaces of the chalcopyrite particles. As the electrical resistivity of elemental sulfur is the highest of all known materials (i.e., approximately 1015 Ω-m), no explanation as to the mechanism by which the galvanic couple is maintained during the leaching reaction is provided in the method described in U.S. Pat. No. 7,846,233 B2.
In WO 2012/000090 A1, it is disclosed that pyrite from different sources could adversely affect the dissolution of chalcopyrite, and that the presence of silver is required to facilitate the prior methods of catalytic dissolution of chalcopyrite described in U.S. Pat. No. 7,846,233 B2. It was postulated by Nazari, Dixon and Dreisinger in Hydrometallurgy, 113-114, 122-130, (2012) that minute amounts of silver ions may act to increase the electrical conductivity of both the pyrite surface and the elemental sulfur product layer. While the incorporation of silver ions may provide for improvements in copper dissolution kinetics and copper recoveries from chalcopyrite, the commercial viability of this approach is limited by the high cost of adding silver to the leaching system. Thus, there is a need for lower-cost approaches to initiating and maintaining high copper dissolution rates and copper recoveries during atmospheric leaching of copper-bearing sulfide minerals.
It is known from prior art that catalysts such as Cuo, Auo, MnO2, carbon, silver sulfide (Ag2S), and pyrite can enhance the rate of copper dissolution from chalcopyrite and that the enhancement is attributable to galvanic coupling between the chalcopyrite surface and the catalyst. However, to maintain a galvanic couple with the catalyst, it is necessary that electron conduction occur across the elemental sulfur layer which forms around the chalcopyrite particles.
The resistivity of elemental sulfur is about 1015 Ω-m, while that of FeS2 is about 3×10−2 Ω-m, which amounts to a difference of about 1017 Ω-m. In the method of the present invention, microwave irradiation is used to promote the formation of a crystalline FeS2 coating on the surface of the elemental sulfur layer to maintain galvanic contact between the catalyst and the sulfide mineral. The presence of a pyrite coating on the elemental sulfur layer thereby decreases the surface electrical resistivity of the sulfur coating by a factor of about 1017 Ω-m.
In the method of the present invention, the leaching of chalcopyrite, and other copper sulfides, is done under dissolution conditions wherein pyrite is thermodynamically stable. The redox potentials which satisfy this condition are readily obtained from Pourbaix diagrams, often referred to as Eh-pH diagrams. Referring to
In prior methods of leaching chalcopyrite, and other copper sulfides, the production of pyrite as a reaction product has not been observed, particularly under conditions where FeS2 is the thermodynamically stable iron-sulfur phase. This is because slow kinetics prevents any significant formation of the more thermodynamically preferred phase within the time scale of the leaching process. Under prior art conditions, the precursors required for pyrite formation (i.e., Fe2+, HS−, So, and polysulfides such as Sn2−) are produced during the course of the copper sulfide dissolution, but kinetic factors prevent conversion to pyrite at timescales commensurate with the leaching processes. For example, it is well-known that the formation of pyrite is relatively slow at temperatures below 100° C.
Elemental sulfur, polysulfides, and Fe2+/Fe3+ are all known constituents resulting from chalcopyrite dissolution in acidic ferric sulfate solutions. Prior art teaches that elemental sulfur can be produced by the oxidation of sulfide by ferric ion in the presence of oxygen:
HS−+2Fe3+→So+2Fe2++H+
In a similar manner, a companion reaction takes place between ferrous ion and hydrogen sulfide:
Fe2++HS−→FeS+H+
In this case FeS is an amorphous phase and would not be electrically conductive, and hence would not be expected to aid in the dissolution of chalcopyrite through galvanic effects.
Without being limited to any particular theory, it is believed that with microwave irradiation, the formation of pyrite on the surface of elemental sulfur can be achieved in timescales commensurate with the copper sulfide dissolution reactions via the following reaction:
Fe2++Sn2−+HS−→FeS2+S(n-1)2−+H+
The formation of nano-scale and micro-scale FeS2 crystallites on the surface of the elemental sulfur layers provides a mechanism for maintaining electron conduction across the elemental sulfur surface. Thus, by the inventive methods discussed herein it is possible to eliminate the need for silver addition as means for increasing the surface conductivity of elemental sulfur.
The present invention provides for the rapid dissolution of chalcopyrite by subjecting a leach slurry to microwave irradiation under typical leach conditions (i.e., temperature, pH and Eh), wherein pyrite is in the thermodynamically stable iron-sulfur phase. Without being held to any specific theory, it may be reasonably expected that the exposure of the leach slurry to microwave irradiation of 2.45 GHz reduces the time for pyrite formation through the crystallization of amorphous iron sulfides. Accordingly, microwave irradiation advantageously reduces the timescale of formation from days or weeks to minutes. It will be understood by those skilled in the art, that any microwave frequency and/or field intensity which produces the best intended results is within the scope of the present invention, and only routine experimentation is necessary to determine optimum levels of irradiation for a given slurry/process.
An added advantage of the present invention is the increased deportment of at least a portion of the iron and sulfur generated from chalcopyrite dissolution as FeS2. This catalyzed reaction thereby reduces the amount of catalytic pyrite that must be added from external sources. An additional advantage of the present invention is that the pyrite which is produced during the microwave irradiation comprises a very high surface area. This high surface area makes the pyrite a more efficient catalyst for the reduction of Fe3+ during chalcopyrite dissolution than catalysts taught in the prior art.
In the inventive method, the microwave irradiation is applied in such a way as to prevent excessive heating of the leach slurry and thereby prevent the generation of temperatures that would result in the melting of the So layers surrounding the leaching chalcopyrite particles. Preferably, the power of the microwave device should be limited so as to prevent temperatures of the slurry from approaching 110 degrees C. The intensity of the incident irradiation energy and/or the frequency of the microwaves may be varied as a function of time or applied intermittently as indicated in
Alternatively, cooling of the slurry can be provided to maintain the slurry temperature below the melting point of elemental sulfur. Pre-cooling of slurry (prior to leaching and/or microwave irradiation) may be done using one or more heat exchangers. Alternatively, cooling of slurry 115, 125, 135, 145 may be facilitated during leaching and/or microwave irradiation process through the use of internal cooling rods, fins, pipes, or other forms of heat exchangers. The aforementioned cooling devices may be positioned within, around, or adjacent to a tank 114, 124, 134, 144, or may otherwise be operatively connected to a leaching device 110, 120, 130, 140.
In some embodiments, a microwave frequency between about 1.6 and about 30 GHz (wavelengths between about 187 mm and 10 mm) may be utilized, wherein the intensity of the microwave is selected to be high enough to transform amorphous FeS2 to stable crystalline phase FeS2, but low enough to avoid generating a sulfur plasma phase in the elemental sulfur layer which forms around the chalcopyrite particles during dissolution. In other words, any microwave frequency and intensity may be used to irradiate a copper-sulfate containing slurry alone or in combination with other microwave frequencies and intensities, so long as the following is satisfied:
FeS2(amorphous)→FeS2(crystalline)
-
- wherein the following reactions do not occur:
FeS2→FeS+S(plasma)
So→S(plasma)
For instance, microwaves used may incorporate intensities between 0.2 to 200 kW-s per gram of slurry, for example, between 2 and 30 kW-s per gram, so long as the above is satisfied. Higher and lower intensities are also envisaged. X-ray diffraction patterns can be used to differentiate between the amorphousFeS2 and the crystal phases of FeS2. Electrical conductivity measurements show that crystalline FeS2 has much better electrical conductivity properties than amorphous FeS2 or elemental sulfur (So). By converting amorphous FeS2 to a stable crystalline phase, electron transfer between the pyrite and chalcopyrite is improved, which aids in the dissolution of chalcopyrite through the galvanic effect.
Turning back to
All references disclosed herein are specifically incorporated by reference thereto.
While preferred embodiments of this invention have been described, it should be understood that changes and modifications can be made therein in accordance with ordinary skill in the art without departing from the invention in its broader aspects as defined in the following claims.
Claims
1. A method of leaching a copper-bearing sulfide mineral slurry, comprising the steps of:
- (a) providing a slurry having copper sulfide particles therein;
- (b) exposing the slurry to an acidic leach solution;
- (c) chemically leaching copper from the slurry into the acidic leach solution in the presence of microwave irradiation, under conditions whereby crystalline pyrite can be formed in-situ, on surfaces of the copper sulfide particles;
- (d) generating microwaves at a predetermined frequency or intensity that is configured for forming crystalline pyrite on surfaces of the copper sulfide particles in-situ; and
- (d) recovering leached copper from said acidic leach solution.
2. The method of claim 1, wherein step (c) is further performed under conditions whereby elemental sulfur (which may form surface passivation layer(s) on the copper sulfide particles as a result of exposure to the acidic leach solution), is prevented from entering a plasma phase.
3. The method of claim 2, further comprising forming crystalline pyrite on surfaces of the copper sulfide particles in-situ and/or converting amorphous pyrite to crystalline pyrite.
4. The method of claim 1, wherein step (b) comprises mixing the acidic leach solution with the slurry in a tank.
5. A method according to claim 4, wherein the tank comprises an agitation mechanism, a fluidization mechanism, or a mechanism for changing a residence time of the slurry in the tank.
6. The method of claim 4, wherein the tank comprises at least one microwave generating device which provides said microwave irradiation.
7. The method of claim 6, wherein the tank comprises multiple microwave generating devices.
8. The method of claim 1, wherein the microwave irradiation is provided intermittently.
9. The method of claim 1, wherein the microwave irradiation changes in intensity as a function of time.
9. The method of claim 1, wherein the microwave irradiation changes in frequency as a function of time.
10. The method of claim 1, wherein step (b) and/or step (c) is performed above atmospheric pressure.
11. The method of claim 1, further comprising adding catalytic pyrite to the slurry and acidic leach solution to instigate galvanic reactions.
12. The method of claim 1, wherein said copper sulfide particles comprises at least one of chalcopyrite (CuFeS2), chalcocite (Cu2S), bomite (Cu5FeS4), covellite (CuS), digenite (Cu2S), enargite (Cu3AsS4), tennantite (Cu12As4S13), or tetrahedrite (Cu12Sb4S13).
13. The method of claim 1, wherein the crystalline pyrite formed on the surfaces of the copper sulfide particles comprises micro- or nano-scale particles.
14. The method of claim 1, wherein the crystalline pyrite formed on the surfaces of the copper sulfide particles in-situ, is capable of catalyzing a reduction of Fe3+ (Ferrous iron) to Fe2+ (Ferric iron).
15. The method of claim 1, further comprising providing microwaves which have been optimized through the process of measuring microwave absorption for FeS2(Amorphous) to maximize the conversion efficiency to FeS2(Crystalline).
16. The method of claim 1, further comprising tuning one of a frequency or an intensity of said generated microwaves during leaching to maintain optimized formation of crystalline pyrite on surfaces of the copper sulfide particles in-situ and/or to maintain optimized copper leaching kinetics.
17. A device for leaching a copper bearing sulfide mineral slurry, comprising:
- (a) a tank for providing a slurry having copper sulfide particles therein; and,
- (b) at least one microwave generating device configured to irradiate the slurry during leaching: wherein the at least one microwave generating device is capable of producing microwaves at a predetermined frequency and intensity which are configured to facilitate in-situ crystalline pyrite formation on surfaces of the copper sulfide particles in the slurry.
18. The device of claim 17, wherein the at least one microwave generating device is further configured to produce microwaves which are configured to prevent elemental sulfur (which may form surface passivation layer(s) on the copper sulfide particles), from entering a plasma phase.
19. The device of claim 17, wherein the tank is configured to withstand a mixture of acidic leach solution with the slurry in a tank.
20. The device of claim 17, wherein the tank comprises a window comprising a microwave-permeable material.
21. The device of claim 17, wherein the tank comprises an agitation mechanism, a fluidization mechanism, or a mechanism for changing residence time of the slurry in the tank.
22. The device of claim 21, wherein the agitation mechanism comprises an impeller, the fluidization mechanism comprises a fluidized bed, and the mechanism for changing residence time comprises a false bottom, interior chamber, dividing wall, baffle, lamella, screen, slip stream area, or tortuous path.
23. The device of claim 17, wherein the at least one microwave generating device is configured to irradiate slurry in the tank intermittently.
24. The device of claim 17, wherein at least one microwave generating device is configured to irradiate slurry in the tank with different frequencies.
25. The device of claim 17, wherein the at least one microwave generating device is configured to irradiate slurry in the tank at various intensities.
26. The device of claim 17, wherein the at least one microwave generating device comprises a plurality of microwave generating devices.
27. The device of claim 26, wherein a plurality of microwave generating devices are configured to emit different microwave signals in any one of intensity, frequency, or continuity.
28. The device of claim 17, wherein the tank comprises a cooling system to cool the slurry prior to or during leaching, particularly during microwave irradiation.
29. The device of claim 28 wherein said cooling system comprises a heat exchanger, a cooling fin, a cooling pipe, or a cooling rod protruding into the tank.
30. The device of claim 17, wherein the device comprises at least one reflector within the tank, which may be provided on a wall, a baffle, or an impellor.
31. The device of claim 17, wherein said at least one microwave generating device further comprises means for adjusting a frequency or intensity of said generated microwaves during leaching to maintain optimized formation of crystalline pyrite on surfaces of the copper sulfide particles in-situ and/or optimized copper leaching.
Type: Application
Filed: Nov 11, 2013
Publication Date: Jul 30, 2015
Inventor: David J. Chaiko (South Jordan, UT)
Application Number: 14/417,850