This is an international PCT application claiming priority to United States Provisional Patent Application Serial No. 61/746,377 filed on 27 December 2012.

FIELD OF THE INVENTION

This invention relates to methods and systems for leaching metals from metal sulfide ores and concentrates, and more particularly to methods and systems for the improved recovery of base and precious metals during leaching of metal values from ores and concentrates. In particular, the present invention is utilized in the process known as bioleaching.

BACKGROUND OF THE INVENTION

Bioleaching is the extraction of metals from their ores through the use of living organisms. Bioleaching is known practice in mineral processing, especially in the area of copper leaching. Bacteria are bioleach agents that are naturally present or independently added to the heap leaching system. Bacteria oxidize iron, produce sulfuric acid, and thus enhance the kinetics of copper leaching. While other microbes. Including bacteria, are naturally present in the heap, adding cultures of specifically chosen microbes to the heap has been touted as an improvement over prior art leaching methods. However, bioleaching is too slow kinetically to be completely accepted by the industry. This is because there are inherent inefficiencies associated with bacterial-assisted leaching. The bacteria first and foremost utilize reactions to aid their own growth and metabolism. Much energy is expended by the bacteria to produce the enzymes and reagents necessary for essential life processes that may have no bearing on the reactions relevant for mineral leaching, and as a result bioleaching is inherently inefficient. Furthermore, the health of the microbial culture within the heap is dependent not only on a sufficient supply of nutrients but also on the temperature of the heap, the redox potential of the heap lixivant, the presence of absence of oxygen, and the absence of adventitious microbes that may compete with the desired microbes. It is anticipated that the optimum growth conditions can be supplied through proper engineering of the heap and process control during heap operation. It is inherent that all microbes have a chemical speed limit which is their own life processes. Bioleaching also often requires the input of nutrients into the leach pad or agitated reactor, which is an expense and can have downstream ramifications.

Thus, there is a need for improved bio-leaching processes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a conventional heap leaching operation; and

FIG. 2 is a schematic diagram of a conventional agitated tank leaching operation.

SUMMARY OF THE INVENTION

According to the invention there is an improved bioleaching process in which industrial enzymes that are cupredoxins such as amicyanin, plastocyanin, pseudoazurin, plantacyanin, azurin, auracyanin, rusticyanin, stellacyanin, and mavicyanin are utilized as a bioleach agent in a leaching operation such as a heap leaching process or an agitated tank leaching process.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a specified industrial enzyme is utilized as a bioleach agent in a bioleaching process. The industrial enzyme can either completely or partially replace bacteria that are typically utilized as the bioleach agents in a bioleaching process. The term "industrial enzyme(s)" as used herein refers to specified oxidizing enzymes that are produced by industrial processes such as submerged fermentation or solid state fermentation. The thus-produced enzymes are subsequently subject to isolation and optional purification. The enzymes for use in the present invention will typically be in the form of an aqueous dispersion or alternatively a lyophilized powder. The industrial enzymes suitable for use in the present invention are cupredoxins such as amicyanin, plastocyanin, pseudoazurin, plantacyanin, azurin, auracyanin, rusticyanin, stellacyanin, and mavicyanin. The most preferred cupredoxin for use in the present invention is rusticyanin.

The invention can be utilized to promote metal recovery, such as copper and gold recovery, from metal producing ores such as conventional sulfide ores, refractory gold ores, and sulfidic/carbonaceous double refractory gold ores.

For example, chalcopyrite (CuFeS ₂ ), the primary copper-containing mineral found in the majority of the copper sulfide ores, is suitable for processing per the present invention. Other suitable copper-containing ore minerals include chalcocite (Cu ₂ S), bornite (Cu ₅ FeS ₄ ), covellite (CuS), digenite (Cu ₂ S), enargite (Cu ₃ AsS ₄ ), tennantite (Cu ₁₂ As ₄ S ₁₃ ), and tetrahedrite

(Cu ₁₂ Sb ₄ S ₁₃ ). Suitable low grade gold ores may include, for example, Calaverite, Sylanite, Nagyagite, Petzite , Krennerite, and other alluvial or oxide-type deposits.

FIG. 1 illustrates a conventional heap leach circuit 1 suitable for use in the present invention. The circuit incorporates a feed stream 2 of ore, which is crushed via a cone-crusher 3 forming a crushed ore 4. The particles are moved via first conveying means 5 to an

agglomerator 13. A polymeric binder may be added to the agglomerator via inlet 15. The agglomerator 13 lumps the various particle size distributions within the crushed ore 4 into larger agglomerated balls 19 which are typically coin-sized. The agglomerated balls 19 are moved (via secondary conveying means 17) as agglomerated feed 14 to a heap leach pad 16 having an impermeable pad liner 9 thereunder. In a typical prior art bioleach process, bioleach solution 7 (containing an appropriate biological leaching agent such as Acidithiobacillus microorganisms)is delivered via delivery system 6 comprising drip/spray irrigation nozzles 12. As leach solution 7 trickles through top 8a, middle 8b, and bottom 8c layers of the heap leach pad 16, it passes between the spaces and interstices created by the larger stacked agglomerated balls 19. During this percolation, target metals dissolve into the leach solution 7 thereby forming a pregnant leach solution 10, which may be recycled to the nozzles 12 or removed for further downstream processing. In the instance shown, pregnant metal bearing leach solution 10 from the heap 16 is moved to a conventional solvent extraction process. The heap may be engineered to improve production efficiency and speed metal recovery by providing a means to introduce the leaching agent(s) throughout the heap at intermediate levels during its construction.

The bacterial bioleach agents operate at low pH conditions, and produce sulfuric acid. The ore in the heap is in contact with extremely acidic water of high ferric iron concentration during the percolation period. The pH of the percolating solution will generally range from about 0.3 to 4, and often within the range of from about 1.2 to about 2.6. This percolation period can be extensive, lasting even for months until the desired degree of breakdown of the sulfides is achieved.

According to the present invention, the specified industrial enzymes may be added as a bioleach agent, typically in an aqueous solution or an acidic solution such as dilute sulfuric acid, via inlet 15 or another inlet to agglomerator 13. The industrial enzymes will typically have a concentration in the solution of about 1 ppm to about 1000 ppm. By adding the industrial enzymes during agglomeration, the leaching process will begin immediately prior to the material reaching the leach pad as opposed to prior art methods where an induction period is required before the bacteria added into the heap begin to thrive.

In another embodiment of the invention, the industrial enzymes may completely replace bacteria as the bioleach agent in bioleach solution 7 that is added to the heap. In such an embodiment, the disadvantages of using a living organism as the bioleach agent are no longer a consideration.

FIG. 2 illustrates a conventional agitated tank leaching operation 21 used in a bioleach process. Slurried feed ore 22 is finely ground in a fine grinding mill 23, such as a stirred media detritor, or attrition mill. The slurried fine ore 24 that exits the mill 23 is very fine - generally much finer than the particle size that could be expected from a cone crusher 3 in the heap leach process of FIG 1. Alternatively, the feed ore to be added as a slurry can be flotation concentrate. Lixiviant (sulfuric acid solution for copper) is added to the stirred tank to get the appropriate solids concentration. A typical weight solids is 10%. The slurried fine ore 24 is mixed with a bioleach solution delivered via inlet 27 and aerated with oxygen gas or air 26 in a stirred reactor/agitated leach vessel 28. A rotor 29, which may comprise an axial or radial impeller, stirs the mixture and helps accelerate mass transfer and reduce residence time. Pregnant metal bearing slurry 30 leaves the stirred reactor 28 and may be separated and further processed in a conventional downstream SX/EW circuit or other conventional recovery process. In prior art agitated tank bioleaching processes bacteria was the bioleach agent in the bioleach solution.

According to the present invention at least some— and up to all— of the bacteria typically utilized in the prior art bioleach solution are replaced by the specified industrial enzymes. The enzyme concentrations in the bioleach solution preferably range between 1-1000 ppm. The leaching conditions in the tank involve a pH between 0-3, a high ionic strength, and temperatures up to 80 deg C, although for effectiveness at higher temperatures precautions may be taken to stabilize the enzymes, such as through chemical stabilization on a polymer surface.

If used to replace some or all of the bacteria normally added to the tank in the bioleach delivery system, the industrial enzymes will be added in an amount so that the enzymes will produce an equivalent or greater biological activity than the replaced bacteria. When utilized in addition to bacteria in the agitated tank the enzymes can be in the same delivery system as the bacteria, or they may be added to the tank in a separate stream, such as with water, dilute sulfuric acid, and/or raffinates.

Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.