OF BASE METAL

FIELD OF THE INVENTION

[001 ] The present invention relates to a method of leaching of a metal from minerals. More specifically, the method utilizes an integrated chemical and bacterial process to leach base metals from minerals.

BACKGROUND OF THE INVENTION

[002] Conventional chemical leaching of base metals typically through the application of sulfuric acid is largely limited to the recovery of copper from oxide minerals in ore heaps and to the recovery of zinc and nickel in metallurgical vessels. Bacteria are not used to leach oxide minerals because there is no source of energy for the bacteria.

[003] Existing bacterial leaching of base metals is limited to the recovery of copper from ore heaps and waste heaps through the application of solutions carrying Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans, and to a limited extent other bacteria such as Acidithiobacillus thioparus. Such bacterial leaching is carried out under acidic pH conditions (pH 1-3) at ambient temperature. In large part, existing bacterial leaching is essentially limited to recovery of copper from secondary copper sulfides including chalcocite and covellite, with limited recovery from bornite and chalcopyrite, both of which are considered to be primary copper sulfide minerals and both of which contain iron. The existing processes rely on the oxidation of ferrous iron to ferric iron by Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans and the secondary benefit of the oxidation effect of ferric sulfate formed following the oxidation of ferrous iron, in addition to the direct oxidation of sulfur in ores.

[004] Since chalcopyrite dominates copper mineral reserves in the world, there has been significant effort in the mining industry to design processes using bacteria that would recover copper from chalcopyrite economically. Recently, research and development have moved in the direction of elevated temperature and the use of bacteria that thrive under high temperature conditions. This type approach relies on iron oxidation as the fundamental reaction, with pyrite as the source of iron, and uses the pyrite oxidation to raise the temperature of the heap. However, the elevated temperature conditions result in higher cost and more complex process management. These approaches have not been implemented commercially as yet.

[005] In part, the move to higher temperatures to increase recovery of copper is necessitated by the formation of a "passivation" layer on the surface of minerals, particularly chalcopyrite, at ambient temperature under conventional bacterial leaching conditions. The passivation layer is believed to be composed of jarosite, a potassium or ammonium iron hydroxy-oxide mineral, and\or modified elemental sulfur (S°). In general, this passivation layer stops or substantially slows further bacterial actions. This technical difficulty may have been exacerbated by the fact that the existing bacterial leaching process is based on the oxidation of Fe (II) to Fe (III), with Fe (III) oxidizing the mineral and releasing the copper. However, the increase in iron content in the leachate results in a rise in the redox potential (Eh) which in turn contributes to the formation of jarosite. Therefore, the existing bacterial leaching processes are often described as self-limiting.

[006] Bacterial leaching used in the existing technology is a slow process. A typical operation needs a leaching time of about 8 to 12 months in processing chalcocite and covellite minerals. The process is substantially slower in the leaching of bornite and chalcopyrite minerals.

[007] Therefore, there is a strong need in mining industry for an improved method that overcomes the above discussed major technical obstacles and enhances the base metal recovery efficiency in bacterial leaching processes.

SUMMARY OF THE INVENTION

[008] In one embodiment, the present invention is directed to a method of integrated chemical and bacterial leaching of a base metal from a mineral. The method comprises leaching a base metal from a mineral with an ammoniacal thiosulfate chemical leaching solution having a pH from about 8.5 to about 10.5, thereby forming a first leachate having a first concentration of the base metal; subsequently adjusting the first leachate to a neutral pH in a range from about 6.0 to about 8.0; inoculating neutralized first leachate with one or more neutrophilic bacteria; and bioleaching the base metal from the mineral with inoculated first leachate containing one or more neutrophilic bacteria, thereby forming a second leachate having a second concentration of the base metal.

[009] In a further embodiment, the method further comprises inoculating the second leachate with one or more chloride-tolerant acidophilic bacteria, when the second leachate reaches an acidic pH of 4.5 or less after the bioleaching with one or more neutrophilic bacteria, and further bioleaching the base metal from the mineral with inoculated second leachate containing one or more chloride-tolerant acidophilic bacteria at the acidic pH.

[010] The advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings showing exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[011 ] Fig. 1 is a schematic flow chart illustrating the method of integrated chemical and bacterial leaching in a heap leaching process in some embodiments of the present invention.

[012] Fig. 2 shows pH and redox potential (Eh) of the leachate during the integrated chemical and bacterial leaching process illustrated in Example 1.

[013] Fig. 3 shows the cumulative copper recovery at the end of each phase of the integrated chemical and bacterial leaching process illustrated in Example 1.

[014] Fig. 4 shows pH and redox potential (Eh) of the leachate during the integrated chemical and bacterial leaching process illustrated in Example 2.

[015] Fig. 5 shows the cumulative copper recovery at the end of each phase of the integrated chemical and bacterial leaching process illustrated in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[016] Embodiments of the present invention generally relate to a method of leaching a metal from minerals. Embodiments of the invention are described more fully hereinafter with reference to the accompanying drawings. The various embodiments of the invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Elements that are identified using the same or similar reference characters refer to the same or similar elements.

[017] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another.

[018] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. [019] In some embodiments, the method provides an integrated chemical and bacterial leaching process to leach base metals from minerals. In one embodiment, the method comprises the following steps:

(a) leaching a base metal from a mineral with an aqueous ammoniacal thiosulfate chemical leaching solution having a pH from about 8.5 to about 10.5 to obtain a first leachate having a first concentration of the base metal;

(b) subsequently adjusting the first leachate to a neutral pH ranging from about 6.0 to about 8.0;

(c) then inoculating the neutralized first leachate with one or more neutrophilic bacteria; and

(d) bioleaching the base metal from the mineral with inoculated first leachate containing the neutrophilic bacteria, thereby forming a second leachate having a second concentration of the base metal.

[020] Herein, the base metal includes, but is not limited to, copper, nickel, zinc, and lead. The method of the present invention can be used for leaching the base metal from various minerals. Herein, mineral includes ores, concentrates, process intermediates, and metallurgical products. The method can be further used for leaching precious metal contained in the minerals, as described hereinafter. The precious metal includes, but not limited to, gold, silver and platinum. Some examples of the mineral are copper oxides, primary sulfides and secondary sulfides, such as atacamite (Cu ₂ CI(OH) ₃ ), malachite (Cu ₂ C03(OH) ₂ ), copper wad (CuO Mn0 ₂ -7H ₂ 0), copper pitch (MnO(OH)CuSi0 ₂ nH ₂ 0), chrysocola (CuSi03.2H ₂ 0), covellite (CuS), chalcocite (Cu ₂ S), bornite (CusFeS- , chalcopyrite (CuFeS ₂ ); minerals containing nickel, such as pentlandite ((Fe,Ni)9S8); minerals containing zinc, such as sphalerite ((Zn,Fe)S); minerals containing gold, minerals containing silver, and minerals containing platinum, or combinations thereof.

[021 ] In some embodiments, the integrated chemical and bacterial leaching process described above includes two phases: phase 1 , including step (a) above, is a chemical leaching process using an ammoniacal thiosulfate chemical leaching solution; and phase 2, including steps (b) to (d) above, is a bacterial leaching process, which is also referred to as bioleaching. [022] Herein, ammoniacal thiosulfate chemical leaching solution refers to an aqueous solution containing ammonia under the alkaline pH described above and thiosulfate anion. The ammonia may be formed from ammonium cation in situ under the alkaline pH condition. Hereinafter, the ammoniacal thiosulfate chemical leaching solution is also referred to as chemical leaching solution interchangeably.

[023] In some embodiments, the ammoniacal thiosulfate chemical leaching solution used in phase 1 comprises one or more ammonium compounds. The ammonium compound may include ammonium halide, ammonium hydroxide, ammonium thiosulfate, or a combination thereof. In an exemplary embodiment, a combination of ammonium chloride and ammonium hydroxide is used. Furthermore, the chemical leaching solution comprises a thiosulfate salt. The thiosulfate salt may include alkali metal thiosulfate, ammonium thiosulfate, or a combination thereof. In an exemplary embodiment, sodium thiosulfate is used.

[024] In some embodiments, the chemical leaching solution comprises sodium thiosulfate, ammonium chloride and ammonium hydroxide. The concentration of sodium thiosulfate in the chemical leaching solution can be from about 0.05 to about 0.5 molar (M), and preferably from about 0.1 to about 0.3 M. The concentration of ammonium chloride can be from about 0.05 to about 1.0 M, and preferably from about 0.1 to about 0.5 M. The concentration of ammonium hydroxide can be from about 0.05 to about 1.0 M, and preferably from about 0.1 to about 0.5 M. The chemical leaching solution has an alkaline pH in a range from about 8.5 to about 10.5, preferably from about 9.0 to about 10.0.

[025] As can be understood, the chemical leaching solution in the embodiment described above contains ammonium thiosulfate, which is formed in situ by reaction between sodium thiosulfate and ammonium chloride. Ammonia/ammonium and thiosulfate anion are active agents in the chemical leaching of the base metal. In alternative embodiments, ammonium thiosulfate can be added directly in the chemical leaching solution. The concentration of ammonium thiosulfate can be from about 0.02 to about 1.0 M, and preferably from about 0.05 to about 0.5 M.

[026] Fig. 1 illustrates the integrated chemical and bacterial leaching process in some embodiments of the present invention. As shown, during the chemical leaching process in phase 1 the chemical leaching solution is recirculated through the mineral for a first predetermined period of time, or until the concentration of a base metal in the leachate reaches a first target concentration. The first predetermined period of time, or chemical leaching time, may be from 1 to about 60 days. In one exemplary embodiment, the chemical leaching time is about 7 days. The first target concentration can be determined empirically depending on the minerals to be processed. Herein, the leachate obtained from the chemical leaching in phase 1 is referred to as the first leachate, and the concentration of the base metal in the first leachate is referred to as the first concentration.

[027] The chemical leaching is carried out at an ambient temperature, and no heating is required. The reaction temperature for chemical leaching in phase 1 is from about 5°C to about 40°C, preferably from about 15°C to about 35°C. Herein, the reaction temperature refers to a temperature that the minerals are exposed to during the chemical leaching or bioleaching process inside heaps, dumps, or inside a vat or a tank.

[028] The chemical leaching in phase 1 of the integrated process has two important functions. Firstly, chemical leaching in phase 1 partially releases the base metal from the mineral. Using the copper oxide and copper sulfide minerals described above as an example, the chemical leaching involves reactions of ammonia with the oxide and sulfide minerals to form copper-ammine complexes, as well as subsequent reactions of copper-ammine complexes with thiosulfate, resulting in various soluble copper compounds in the first leachate under the reaction condition described above. Moreover, during the chemical leaching process the minerals become more porous because of dissolution of the base metal. The porous minerals have a substantially larger surface area, which is beneficial for the subsequent bioleaching process using bacteria. Secondly, the chemical reactions of the chemical leaching solution with the minerals produce reduced intermediate sulfur compounds (RISCs) in the first leachate, which become the energy source to the bacteria used in the bioleaching in phase 2. The reduced intermediate sulfur compounds (RISCs) include, but are not limited to, thiosulfates, tetrathionates, trithionates, and polysulfides as well as elemental sulfur (S°). Therefore, in the integrated process, in addition to chemical leaching, phase 1 functions as an indispensable preconditioning step for the subsequent bacterial leaching process.

[029] After chemical leaching, in phase 2 the first leachate is adjusted to a neutral pH in a range from about 6.0 to about 8.0, preferably from about 6.5 to about 7.5. In an exemplary embodiment, the pH of the first leachate is adjusted to about 7.0 using an acid. Various known acids compatible with the bacteria used for bioleaching can be used for adjusting the pH of the first leachate. In some exemplary embodiments, sulfuric acid and hydrochloric acid are used.

[030] Then, the neutralized first leachate is inoculated with one or more neutrophilic bacteria. The neutrophilic bacteria are mesophilic and chemolithotrophic. Suitable bacteria include, but are not limited to, Alpha-Proteobacteria, such as Starkeya novella and Paracoccus pantotrophus, Beta Proteobacteria such as Thiobacillus denitrificans, various sulfur oxidizing Bacilli species, and other similar neutrophilic chemolithotrophic bacteria that possess the metabolic pathways which allow them to metabolize the reduced intermediate sulfur compounds, or combinations of the above mentioned bacteria. In some exemplary embodiments, a combination of Thiobacillus denitrificans, Starkeya novella, Paracoccus pantotrophus, and Bacillus subtilus is used for bioleaching in phase 2.

[031 ] The above described neutrophilic bacteria are known sulfur oxidizers, which possess metabolic pathways that allow the bacteria to oxidize the reduced intermediate sulfur compounds (RISCs) formed in phase 1 to sulfates, and therefore, release copper and other base metals in a soluble form into the leachate.

[032] In general, the above described neutrophilic bacteria do not oxidize iron, because these bacteria are obligatory sulfur oxidizers. Among the bacteria described above, Thiobacillus denitrificans has the ability to oxidize iron in pyrite, however, this oxidation process is sufficiently slow such that the effect is minor under the leaching condition and for the leaching time used in the process of the present invention. As demonstrated in the instant integrated chemical and bacterial leaching process as described hereinafter, the above described neutrophilic bacteria do not contribute to the dissolution of iron, and consequently do not contribute to the formation of jarosite. [033] As further shown in Fig. 1 , during the bioleaching process in phase 2, the neutralized first leachate inoculated with the above described neutrophilic bacteria is recirculated through the mineral for a second predetermined period of time, or until the concentration of the base metal reaches a second target concentration. In some embodiments, the second predetermined period of time, or bioleaching time, may be from 1 day to about 60 days. In one exemplary embodiment the bioleaching leaching time is about 7 days. Similarly, the second target concentration can be determined empirically depending on the minerals to be processed. Herein, the leachate obtained from the bioleaching in phase 2 is referred to as the second leachate, and the concentration of the base metal in the second leachate is referred to as the second concentration. In the integrated process, the second concentration of the base metal is a cumulative concentration resulted from phase 1 and phase 2, which is higher than the first concentration resulted from chemical leaching in phase 1.

[034] The bioleaching described above in phase 2 is carried out at an ambient temperature. No heating is required for the integrated chemical and bacterial leaching process. The reaction temperature for bioleaching in phase 2 is from about 12°C to about 40°C, preferably from about 15°C to about 35°C, more preferably from about 20°C to about 30°C.

[035] It has been discovered surprisingly by the present inventors that the above described integrated chemical and bacterial leaching process has achieved an unexpectedly high yield of recovery for base metal, such as copper. As illustrated by examples hereinafter, the cumulative copper recovery from the minerals in the integrated chemical and bacterial leaching process was above 50% in a period of a few weeks.

[036] As described above, in phase 1 the reactions of the chemical leaching solution with the minerals generate reduced intermediate sulfur compounds (RISCs) in the leachate, which serves as the energy source to the bacteria used in the bioleaching in phase 2, as such phase 1 functions as an indispensable preconditioning step to the bioleaching of phase 2. It has been found by the present inventors that contrary to the significant recovery of copper achieved using the integrated chemical and bacterial leaching process, in the absence of chemical leaching of phase 1 the same combination of bacteria at the same neutral pH condition could not produce a meaningful recovery of copper.

[037] Moreover, it has been found unexpectedly by the present inventors that in the integrated chemical and bacterial leaching process, the chemical leaching solution containing a combination of sodium thiosulfate, ammonium chloride and ammonium hydroxide provides a more effective leaching, which results in a higher cumulative recovery of copper in the integrated process than a chemical leaching solution made of ammonium thiosulfate in the absence of ammonium chloride. Without being bound by any theoretical explanation and based on observations by the inventors, it is believed that chloride assists in dissolving copper oxides, and it may also play a role in preventing precipitation of released copper in the leachates.

[038] Furthermore, as a distinct feature of the present invention, the present inventors discovered that in the above integrated chemical and bacterial leaching process iron concentration in the leachate remained extremely low in the context of metal leaching in the mining industry. In the examples described below, the iron concentration in the leachate at the end of phase 2 was at ppm level. At this low concentration, iron does not interfere with leaching of the base metal in the instant integrated process. In contrast, the iron concentration in the leachate of existing copper bioleaching processes is significantly higher, at a level of grams per liter. At this higher level, jarosite formation is a known major obstacle of the existing copper bioleaching processes, because it can render the copper bioleaching processes self- limiting. Similar to iron, aluminum and magnesium contents in the leachate of the instant integrated process also remain similarly low, such that these ions do not interfere with leaching of the base metal in the process.

[039] The integrated chemical and bacterial leaching process is particularly advantageous for recovering base metals in mixed ores including both oxide and sulfide minerals. In phase 1 , the chemical leaching recovers most copper, or other base metals, in oxide minerals and a portion of copper in the sulfide minerals. In the subsequent phase 2, a portion or most of the remaining copper in the sulfide minerals are recovered by the bioleaching. As demonstrated by examples hereinafter, the integrated chemical and bacterial leaching process effectively recovers copper in ores with a complex composition. [040] On the other hand, it has been found that when a precious metal is present in the mineral, the process described above further leaches the precious metal from the mineral through chemical reactions of the mineral with the chemical leaching solution in phase 1. Under this circumstance, the first leachate further includes released precious metal. The precious metal may be recovered at the same time of recovering the base metal as described hereinafter, or alternatively recovered at the end of phase 1.

[041 ] In a further embodiment, optionally the method may further include addition of a nitrate compound in the neutralized leachate in phase 2 for the above described bioleaching. Among the neutrophilic bacteria described above, Thiobacillus denitrificans metabolizes aerobically (with oxygen) and anaerobically (in an oxygen deficit) with different metabolic capabilities. Under an anaerobic condition, Thiobacillus denitrificans may oxidize reduced intermediate sulfur compounds (RISCs) and pyrite using nitrate as an electron receiver. In dump leaching, no artificial aeration is provided and anaerobic condition could be present in dumps. Also, in heap leaching, anaerobic condition may potentially be micro-localized in certain parts of the heaps. Therefore, presence of nitrate may support metabolism of the bacteria under such a condition, and hence may further enhance efficiency of the bioleaching.

[042] The nitrate compound may include sodium nitrate, ammonium nitrate, or other nitrate salts compatible with the neutrophilic bacteria used in the bioleaching. The choice of a specific nitrate compound may also depend on the type of ore being leached. The amount of nitrate may vary depending on how many reduced intermediate sulfur compounds need to be oxidized and may also depend on the content of iron in the mineral. In some embodiments, sodium nitrate is used. The concentration of sodium nitrate may be about 0.05 to about 1.0 M, and preferably from about 0.1 to about 0.5 M.

[043] In some further embodiments, the method of the integrated chemical and bioleaching may optionally include a further phase, phase 3, which involves an additional bioleaching at an acidic pH using acidophilic bacteria. As further illustrated in Fig. 1 , in one embodiment the method further comprises the following steps: inoculating the second leachate from phase 2 with one or more chloride- tolerant acidophilic bacteria, when the second leachate reaches an acidic pH of 4.5 or less during the bioleaching with the neutrophilic bacteria described above; and then further bioleaching the base metal from the mineral with the second leachate inoculated with the chloride-tolerant acidophilic bacteria at the acidic pH.

[044] Various acidophilic bacteria are known. For the purpose of the present invention, the acidophilic bacteria used are chloride-tolerant because of the chloride contained in the second leachate. In one exemplary embodiment, Acidithiobacillus thiooxidans is used for the additional bioleaching at an acidic pH.

[045] It should be understood that during the bioleaching using neutrophilic bacteria in phase 2 described above, these bacteria oxidize the reduced intermediate sulfur compounds formed in phase 1 to sulfates, which leads to a decrease of pH of the second leachate. It has been found that the pH of the second leachate can be reduced to 4.5 or less naturally by the neutrophilic bacteria described above. As such, in phase 3 the additional bioleaching with acidophilic bacteria that requires an acidic environment can be conducted. Moreover, optionally the pH of the second leachate may also be adjusted using an acid to the extent required by specific acidophilic bacteria if needed.

[046] Similar to phases 1 and 2, during the additional bioleaching process in phase 3, the second leachate inoculated with the above described acidophilic bacteria is recirculated through the mineral for a third predetermined period of time, or until the concentration of the base metal reaches a third target concentration. Herein, the leachate obtained from the additional bioleaching in phase 3 is referred to as the third leachate, and the concentration of the base metal in the third leachate is referred to as the third concentration. The additional bioleaching process in phase 3 is also carried out at ambient temperature. No heating is required. Same as in phase 2, the reaction temperature in phase 3 is from about 12°C to about 40°C, preferably from about 15°C to about 35°C, more preferably from about 20°C to about 30°C.

[047] It has been found that the additional bioleaching at an acidic pH using acidophilic bacteria can be used to further leach the base metal from the mineral if the chemical leaching and bioleaching with the neutrophilic bacteria described above have not sufficiently leached the base metal from the mineral to a desired extent. This is a situation as illustrated in Example 1 described hereinafter. However, as further shown in Example 2, with an extended leaching time applied to chemical leaching and bioleaching with the neutrophilic bacteria described above, the additional bioleaching at an acidic pH using acidophilic bacteria may not be needed. The additional bioleaching with acidophilic bacteria functions as a safe guard, which can be implemented optionally to further improve an overall recovery of the base metal in the integrated leaching process.

[048] In the instant method, the metals in the leachate may be recovered at the end of the entire integrated chemical and bacterial leaching process, or may be recovered during or at the end of a certain phase. For example, as illustrated in Fig.

1 , the second leachate can be withdrawn at the end of phase 2, and copper or other base metals can be recovered by solvent extraction and electrowinning. As further illustrated in Fig. 1 , when the additional phase 3 is used in the integrated process, the leachate can be withdrawn at the end of phase 3, instead of at the end of phase

2. The base metal can then be recovered from the leachate as shown. In an alternative embodiment, partial recovery during or at the end of phase 1 may also be contemplated. Reducing the overall concentration of a metal in the leachate may drive the chemical reaction toward the direction of further dissolving the metal, and may also decrease metal precipitation in the leachate. If a partial recovery occurs at phase 1 , the second concentration of the base metal at the end of phase 2 will be a partial cumulative concentration.

[049] The integrated chemical leaching and bioleaching process of the present invention can be carried out in a form of heap leaching, tank leaching, vat leaching, or dump leaching. Heap leaching is the most commonly utilized bacterial process for recovering base metal, such as copper. The process involves stacking crushed ore onto a specially prepared impermeable pad. The pad is designed so that the leachate draining from the heap collects at a point from which it is drained to a collection pond. Air may be injected into the heap using various methods, such as with appropriate air blowing systems. The base metal is recovered from the leachate, also commonly referred to as pregnant liquor solution, via precipitation, or solvent extraction and electrowinning.

[050] Industrially, the ore is crushed to proper sizes, typically from about 2 inches to about 0.25 inch. Furthermore, the crushed ore can be agglomerated with binders, acid or other reagent, and water prior to stacking, which results in a more uniform particle size. Heaps are irrigated with the chemical leaching solution and the leachate inoculated with the bacteria, respectively, as described above.

[051 ] Dump leaching is similar to heap leaching and is generally reserved for lower grade ores. Typically, little or no crushing will be performed prior to stacking. Only a minimum pad preparation will be performed, and there will be no forced aeration. The chemical leaching solution, the first leachate inoculated with the neutrophilic bacteria and the second leachate inoculated with the acidophilic bacteria, respectively, are recirculated through the stacked ore similarly as that carried out in heap leaching.

[052] In vat leaching, the mineral to be treated is fully immersed in the chemical leaching solution without extensive agitation. The process has the advantage over heap or dump leaching in that complete wetting of the mineral surfaces is achieved. Finer crush sizes can also be handled better in a vat. Aeration may be provided by submerged pipe, or could be accomplished by intermittently draining the vat and allowing air to be drawn into the ore by recirculating the leachate.

[053] Tank leaching is carried out with aerated mineral slurries in agitated tanks. The leaching solution, or inoculated leachate is circulated in the tanks in different phases.

[054] The method of the present invention is further illustrated by examples in Examples 1 and 2. In both examples, crushed ore containing atacamite, chalcocite, covellite, bornite and chalcopyrite was used, which had a total copper content of 0.5%. In both examples, a chemical leaching solution containing 0.20M sodium thiosulfate, 0.40M ammonium chloride and 0.18M ammonium hydroxide, and having a pH of 9.5 was used in phase 1 for chemical leaching of copper. A consortium of four neutrophilic bacteria, namely, Thiobacillus denitrificans, Starkeya novella, Paracoccus pantotrophus, and Bacillus subtilus, was used to inoculate the first leachate at a neutral pH after phase 1. The leachate inoculated by these neutrophilic bacteria was used for bioleaching copper from the minerals. After phase 2, an acidophilic bacterium, Acidithiobacillus thiooxidans, was used to inoculate the second leachate at pH below 4.5. The second leachate inoculated by the acidophilic bacterium was used for further bioleaching of copper. The entire leaching process was carried out at room temperature.

[055] In Example 1 , the leaching time in both phases 1 and 2 was relatively short, and they were 2 days and 5 days, respectively. As shown in Fig. 3, the cumulative copper recovery at the end of each phase was 21.8%, 30%, and 39%, respectively. In this example, each phase of the integrated chemical and bacterial leaching process contributed to the overall recovery of copper.

[056] In Example 2, a substantially higher copper recovery was obtained by increasing leaching time in phase 1 and phase 2, which were 7 days in both phases. As shown in Fig. 5, the cumulative copper recovery at the end of phase 1 and phase 2 was 37.5% and 53.3%, respectively.

[057] In this example, the integrated chemical and bioleaching process of phase 1 and phase 2 achieved a significant recovery of above 50% copper content in the ore in merely about 15 days. Such a high yield copper recovery in a short time is a major breakthrough in mining industry. This substantially exceeds the current copper recovery of about 30% to 40% over a period of about one year leaching time using the existing bioleaching technologies.

[058] As described above, in the integrated chemical leaching and bioleaching process of the present invention the concentrations of aluminum, magnesium and iron remain low in the leachate in both phase 1 and phase 2. As shown in Example 2, at the end of phase 2, the content of iron in the second leachate was about 130 ppm, the content of aluminum and magnesium was less than 80 ppm. In contrast, the typical concentrations of these metals in the leachate of the existing bioleaching processes are from 7 to 18 g/L for aluminum, from 1 to 12 g/L for magnesium, and from 1 to 12 g/L for iron, respectively. At these concentrations, interferences from these metals have been major obstacles for bioleaching copper in the existing methods. On the contrary, in the instant process, at the substantially lower concentration described above, these metals do not hinder the integrated chemical leaching and bioleaching process. More particularly, at an iron concentration of about 130 ppm, no appreciable amount of jarosite formation was observed. Therefore, the integrated process of the present invention has successfully overcome a long time technical problem in the mining industry wherein various known bacterial base metal leaching processes have been rendered self-limiting by jarosite formed in these processes.

[059] The following examples are illustrative of the invention and are in no way to be interpreted as limiting the scope of the invention, as defined in the claims. It will be understood that various other ingredients and proportions may be employed, in accordance with the preceding disclosure.

Example 1

[060] Ore containing 0.5% copper present in the form of oxide and sulfide minerals was crushed to less than 0.04 inch (or 10 mm). The ore copper content was distributed in atacamite (5.8%), covellite (1.3%), chalcocite (15.9%), bornite (52.9%) and chalcopyrite (24.1 %), wherein the percentage in the parenthesis represents the portion of total copper of the ore contained in a specific mineral.

[061 ] In phase 1 , three individual samples of 150 grams of the crushed ore were each placed in a 2 liter open-mouth bottle together with 450 ml of a chemical leaching solution. The chemical leaching solution contained 0.20M sodium thiosulfate, 0.40M ammonium chloride and 0.18M ammonium hydroxide, and had a pH of 9.5.

[062] The three open-mouth bottles were placed on a rotating roller table for 48 hours (2 days as shown in Fig. 2). Then the mixture of ore and solution in each bottle was filtered with a vacuum funnel to obtain the first leachate. A 20 ml sample of the first leachate from each bottle was taken for copper analysis.

[063] After the sampling, phase 2 was started. The pH of the first leachate in each bottle was adjusted to pH 7 using sulfuric acid, and the amount of the leachate was adjusted with deionized water to the volume before sampling. The filtered ore was returned to each bottle. The first leachate in each bottle was then inoculated with an inoculum containing Thiobacillus denitrificans, Starkeya novella, Paracoccus pantotrophus, and Bacillus subtilus, and the inoculated leachate had a bacteria concentration of about 5 x 10 ⁷ per gram of mineral. The bottles were placed back on the rotating roller table.

[064] After 72 hours of rotation (i.e., the 5 ^th day from the beginning of the experiment), the mixture of ore and solution in each bottle was filtered again to obtain the second leachate. A 20 ml sample of the second leachate from each bottle was taken for copper analysis.

[065] After the sampling, phase 3 was started. The filtered ore and the second leachate were returned to each bottle, and the solution volume was adjusted with deionized water to compensate for the sample removed. Then, an inoculum containing Acidithiobacillus thiooxidans was added to the second leachate in each bottle with a concentration of this bacterium of about 5 x 10 ⁷ per gram of mineral. The bottles were placed back on the rotating roller table.

[066] A 20 ml sample of the third leachate from each bottle was taken for copper analysis on 7, 15, 18, 20 and 32 days from the beginning of the experiment. For each sampling, the mixture of ore and solution in each bottle was filtered before sampling. After adjusting the solution volume with deionized water to compensate for the sample removed, the filtered ore and the leachate were recombined and returned to each bottle. The bottles were placed back on the rotating roller table.

[067] During the experiment, temperature of the solution and ore remained at room temperature about 22°C. Redox potential (Eh) and pH of the solution in each bottle were measured throughout the experiment. Fig. 2 shows the average Eh and pH of the solution in the three bottles during the experiment. As shown, at the beginning of phase 2 the initial pH was adjusted to 7. Then, the pH decreased during the bioleaching with the consortium of neutrophilic bacteria. On day 5, the pH of the solution in the bottles was below 4 at which Acidithiobacillus thiooxidans was inoculated for the additional bioleaching in phase 3. [068] Fig. 3 shows the recovery of copper at the end of each phase. As shown, at the end of phase 1 , the copper recovery was 21.8%; at the end of phase 2, the cumulative copper recovery was 30%; and at the end of phase 3, the cumulative copper recovery was 39%.

Example 2

[069] Ore containing 0.5% copper present in the form of oxide and sulfide minerals was crushed to less than 0.04 inch (or 10 mm). The ore copper content was distributed in atacamite (5.8%), covellite (1.3%), chalcocite (15.9%), bornite (52.9%) and chalcopyrite (24.1 %). Similarly, the percentage in the parenthesis represents the portion of total copper of the ore contained in a specific mineral.

[070] In phase 1 , three individual samples of 150 grams of the crushed ore were each placed in a 2 liter open-mouth bottle together with 450 ml of the same chemical leaching solution described above in Example 1. The three open-mouth bottles were placed on a rotating roller table.

[071 ] On day 2, 4, and 7 from the beginning of the experiment, a 20 ml sample of the first leachate from each bottle was taken for copper analysis. For each sampling, the mixture of ore and solution in each bottle was filtered with a vacuum funnel, and the sample was taken from the filtered leachate. After each sampling, the volume of the solution was adjusted with deionized water to the volume before sampling, and the filtered ore and solution were returned to each bottle.

[072] On day 7, after the sampling, phase 2 was started. The pH of the first leachate in each bottle was adjusted to pH 7.1 using sulfuric acid. Then, a 20 ml inoculum containing Thiobacillus denitrificans, Starkeya novella, Paracoccus pantotrophus, and Bacillus subtilus was added into the first leachate in each bottle, and the inoculated leachate had a bacteria concentration of about 5 x 10 ⁷ per gram of mineral. The bottles were placed back on the rotating roller table.

[073] On day 11 and 14 from the beginning of the experiment, the solution in each bottle was sampled, and copper analysis of the second leachate was carried out. Same as before, for each sampling the mixture of ore and solution in each bottle was filtered with a vacuum funnel, and 20 ml sample was taken from the filtered leachate. After each sampling, the volume of the solution was adjusted with deionized water to compensate for the sample taken, and the filtered ore and solution were returned to each bottle.

[074] On day 14, the pH of the solution reached 2.62. After the sampling, phase 3 was started. A 20 ml of an inoculum containing Acidithiobacillus thiooxidans was added into the solution in each bottle with a bacteria concentration of about 5 x 10 ⁷ per gram of mineral. The bottles were placed on the rotating roller table again.

[075] On day 22, 26 and 30 from the beginning of the experiment, the solution was sampled, and copper analysis of the third leachate was carried out. The sampling was carried out in the same manner described above in phase 1 and phase 2.

[076] During the experiment, temperature of the solution and ore remained at room temperature about 22°C. Redox potential (Eh) and pH of the solution in each bottle were measured daily during the experiment. The average pH and Eh of the solution in the three bottles during the experiment are shown in Fig. 4. As shown, during 7 days chemical leaching of phase 1 , the solution remained alkaline with a pH between 9 and 10. In phase 2, the pH of the solution decreased gradually from 7.1 on day 7 to below 4 on day 11 due to formation of sulfates. In phase 3, the pH of the solution decreased slightly from 2.6 to 2.1 at the end of phase 3.

[077] As further shown in Fig. 4, redox potential (Eh) increased gradually during phase 1 and phase 2. In phase 2, Eh increased from 407 to 512 mV. In phase 3, Eh further increased slightly to 532 mV. This Eh level remained below the range that would normally support the formation of jarosite.

[078] At the end of phase 2, concentrations of iron, aluminum and magnesium in the second leachate of each bottle were also measured. It was found that iron concentration reached a maximum of 130 ppm at the end of phase 2, aluminum was about 74 ppm, and magnesium was about 72 ppm.

[079] Cumulative copper recovery at the end of each phase is shown in Fig. 5. As shown, at the end of phase 1 , the copper recovery was 37.5%; at the end of phase 2, the cumulative copper recovery was 53.3%; and at the end of phase 3, the cumulative copper recovery was 52.1 %.

[080] In this experiment, with an increase of both chemical leaching time and bioleaching time in phase 1 and phase 2, the additional bioleaching with acidophilic bacteria may not be needed. No further recovery was observed in phase 3. The cumulative copper recovery at the end of phase 3 was essentially the same as that at the end of phase 2. However, it was observed that during phase 3, pH of the solution decreased slightly from 2.6 to 2.1 , indicating further formation of sulfates. Thus, it was also possible that more copper was released into solution, but the copper might have precipitated and became unavailable to the analysis.

[081 ] The invention has been described with reference to particularly preferred embodiments. It will be appreciated, however, that various changes can be made without departing from the spirit of the invention, and such changes are intended to fall within the scope of the appended claims. While the present invention has been described in detail and pictorially shown in the accompanying drawings, these should not be construed as limitations on the scope of the present invention, but rather as an exemplification of preferred embodiments thereof. It will be apparent, however, that various modifications and changes can be made within the spirit and the scope of this invention as described in the above specification and defined in the appended claims and their legal equivalents. All patents and other publications cited herein are expressly incorporated by reference.