BACKGROUND OF THE INVENTION

[0001] This invention relates to a hydrometallurgical method suited for the recovery of copper from sulphide minerals. The invention is described hereinafter with particular reference to the recovery of copper from copper sulphide minerals such as chalcocite, covellite and bornite. This is illustrative only and non-limiting for copper may be recovered from a mixture of a copper sulphide and oxide minerals.

[0002] Heap and dump bioleaching processes for recovering copper from sulphide and mixed ores are widely used due to low capital and operational costs and a reduced environmental impact.

[0003] Publications such as Biomining [Rawlings and Johnson, 2007] and Microbial Processing of Metal Sulfides [Donati and Sand, 2007] disclose the importance of the use of microorganisms as catalysts in the oxidation of ferrous (Fe ²⁺ ) to ferric (Fe ³⁺ ) iron (extent mostly expressed as Eh or mV vs. SHE) acting as an oxidant of sulphide minerals, as well as operating conditions (e.g. inoculation, pH, ionic strength, entrained organic and oxygen supply) that improve the microbial function (cell growth and activity) and in turn enhance the iron oxidation rate:

3Fe ²⁺ + 0.75C-2 + 3H ^{+ Bac,eria} 3Fe ³⁺ + 1.5H ₂ 0

►

[0004] A two-stage ferric iron-dependent mechanism for the leaching of chalcocite (CU2S) via a secondary covellite like intermediate species (CU1.2S), is known in a sulphate system;

Cu ₂ S + 1.6Fe ³⁺ → 1.6Fe ²⁺ + 0.8Cu ²⁺ + Cu _{1 2} S (rapid) (1) Cu _Λ .2 S + 2.4Fe ³⁺ → 2.4Fe ²⁺ + 1.2Cu ²⁺ + S° (slower than 1 ) (2)

The first stage chalcocite leaching is fast in relation to the secondary covellite leaching stage, and in relation to the leaching of naturally occurring covellite at ambient temperatures. Reaction 2 proceeds more rapidly at Eh values exceeding 700 mV vs. SHE.

[0005] The dissolution of chalcocite in aerated diluted sulphuric acid is slower in the absence of ferric iron at ambient temperatures.

Cu ₂ S + 0.5O ₂ + 2H ⁺ → Cu ²⁺ + H ₂ 0 + Cu _2-x S (slower than 1) (3)

[0006] Several methods for heap and dump leaching of copper sulphide minerals or mixed ores have been proposed, using solutions containing chloride ions, in the absence of micro-organisms, as an alternative to bioleaching.

[0007] Chilean patent No. 40,891 describes a process of agglomerating fine crushed copper ore with the addition of calcium chloride and sulphuric acid to improve heap structural stability and air/liquid permeability via formation of calcium sulphate (gypsum) as a binding agent. The leaching is conducted at a high irrigation rate (typically 19 l/(hm ² ) with an acidic chloride (80 - 130 g/l), copper (0.5 -10 g/l) and iron (15 - 40 g/l) containing solution to promote rapid leach kinetics in the absence of microorganisms. The leaching stage is followed by a washing step with a highly acidic solution to re-dissolve precipitated copper. Reactions, based on the chloride in the system stabilising the Cu ²⁺ /Cu ⁺ redox couple, are:

2Cu ⁺ + / ₂ 0 ₂ + 2 H ⁺ → 2Cu ⁺² + H ₂ O (cuprous oxidation in air) (4) Fe ⁺² + Cu ⁺² → Fe ⁺³ + Cu ⁺ (ferrous oxidation by cupric) (5) CuS + 2 Fe ⁺³ → Cu ⁺² + 2Fe ⁺² + S ₀ (sulphide mineral oxidation by ferric) (6) The main disadvantages of the process, for example, are the high net consumption of chloride, the requirement of a specific reagent such as calcium chloride, high acidity, and acid accumulation in the circuit that could compromise solvent extraction performance. Elevated impurities associated with the high acid level, and the requirement for constructional materials which can handle a chloride concentration in excess of 80 g/l and for an additional acidified solution to recover precipitated copper at the end of the leach cycle, are further disadvantages.

[0008] Chilean provisional patent application No. 1572-01 describes a heap or dump leach process to recover metals such as copper from run-of-mine (ROM), coarse or fine-crushed (agglomerated if required) sulphide or mixed material with chloride ions without the use of microorganisms. Chloride salt addition is not required during agglomeration and is carried out with sulphuric acid, water and leach circuit solutions. The process is therefore also applicable to heap or dump leaching where agglomeration is not practical. The leaching is carried out with acidic solutions containing ferric. The cupric ions act as mineral oxidants at a minimum of 0.5 g/l total iron. Both oxidants are spontaneously regenerated by oxygen (naturally ventilated or forced heap aerated) and protons in the heap or dump, to produce an Eh potential range from 600 to 800 mV, which is described as thermodynamically favourable for leaching sulphide minerals. It was determined that the presence of chloride ions at a minimum concentration of 5 g/l promoted an increase in potential of the leach solution, stabilised the cuprous ions (cupric to act as mineral oxidant) and enhanced proton activity to lessen the acid required to maintain the same pH, without chloride.

[0009] International patent application No. PCT/ZA2007/000025 "Chloride Heap Leaching" involves the use of acidic chloride solutions subject to a controlled environment to promote mineral surface potentials, preferably below 600 mV vs. SHE (in the presence of dissolved oxygen), for the heap leaching of chalcopyrite ores in particular.

[0010] Chilean patent application No. 1474-09 refers to a method of bioleaching a sulphide mineral or a mixed sulphide and oxide mineral, particularly for copper recovery, in a chloride ion solution with a chloride-resistant mixed microbial culture consortium, which is characterised in that the chloride ion concentration is in the range of 1.5 - 30 g/l and the pH 1 - 3. The disclosure exemplifies microbial mediated oxidation of ferrous to ferric iron as a mineral oxidant in the presence of chloride and cupric ions, but with no reference to cupric as an oxidant.

[0011] US patent application No. 2008/0026450 describes a method of recovering copper from primary sulphide ores containing chalcopyrite by means of a chloride ion resistant sulphur oxidising bacterium in a leach solution with a chloride ion concentration of 6 to 18 g/l, a pH range of 1.6 to 2.5 and a copper concentration of 0.5 to 5 g/l. Elemental sulphur produced in the proposed leach reactions (below) was shown to cause a mineral coating phenomenon, which decelerated leaching - an effect which was alleviated via microbial mediated sulphur oxidation (reaction 9).

Fe ⁺² + Cu ⁺² Fe ⁺³ + Cu ⁺ (7) CuFeS ₂ + 2Cu ⁺ + Fe ⁺³ → Cu ₂ S+2Fe ²⁺ + S (8) Cu ₂ S + 4Fe ³⁺ → 2Cu ₂₊ + 4Fe ₂₊ + S (9)

[0012] An object of the present invention is to provide an innovative "dual" method for operating a heap bioleach process with the aim of significantly decreasing leach cycles and enhancing the rate of copper extraction by including at least one chloride mediated leach stage or cycle within a bioleach cycle. SUMMARY OF THE INVENTION

[0013] The invention is particularly concerned with addressing copper extraction efficiency during an initial irrigation period of a "bioheap" leach cycle, stipulated herein as the first 50 -100 days of heap operation.

[0014] In applying the method of the invention to the heap bioleaching of copper secondary/mixed sulphides a leach solution may exhibit a low Eh or low ferric iron (< 700 mV SHE) concentration during an initial irrigation period, for example due to any of the following: a) the period of time required to complete the process of microbial colonization (naturally occurring or from inoculation) thereby to establish an adequate microbial population within the heap to promote a higher Eh environment by microbial oxidation of ferrous iron in solution,

b) a high solution pH environment e.g. due to gangue acid consumption and/or lack of acid addition during agglomeration or irrigation, such that the dissolved ferric iron is not stable, and

c) a fast reduction of ferric iron in solution due to rapid oxidation of relatively easily leached sulphide minerals e.g. chalcocite.

A period of operation at low solution Eh (with correspondingly low ferric concentrations in solution) and a relatively low acidity (high pH) may be expected to result in lower rates of copper extraction, according to reactions 1 , 2 and 3.

[0015] Most bioleaching or acidophilic microbial strains do not show adequate metabolic activity at copper concentrations exceeding 20 g/l [Dopson et al., 2003] and may not survive at such copper concentrations in combination with chloride levels exceeding 8 g/l. [0016] Another negative factor is the problem of low air and solution permeability observed in prolonged heap bioleach cycles due to gangue mineral disintegration and condensing.

[0017] The method of the invention may be used for the leaching of copper from uncrushed run-of-mine (ROM) ore, from a crushed, or a crushed and agglomerated, secondary sulphide mineral, optionally including oxide or halide minerals.

[0018] The ore may be in an ore column, dump, heap or vat, collectively referred to herein as "a heap".

[0019] Non-limiting examples of copper minerals to which the method of the invention can be applied include chalcocite, covellite, bornite, energite, atacamite, chrysocolla, brochantite and copper bearing clays.

[0020] The method may be carried out at ambient or elevated temperature.

[0021] The invention differs from the disclosures in US patent application No. 2008/0026450 and Chilean application No. 1474-09 in that impurities (e.g. chloride ions, protons and related cations) associated with the aforementioned chloride leach cycle are managed separately from the bioleach circuit so that there exists no microbial related constraint to the impurity level of the leach solution in the chloride leach cycle.

[0022] The chloride leach stage may be executed at any time during a heap leach cycle, but preferably during the first 100 days of heap operation, and may or may not be prevalent during an agglomeration process.

[0023] The chloride leach cycle may be continued for up to 200 days of the total heap bioleach cycle. [0024] The chloride leach cycle may include periods in which there is no irrigation or curing, followed by irrigation, within the bioleach circuit, with acidic solutions containing chloride ion levels controlled at 7g/l to 80 g/l by means of the addition of: a) NaCI and/or MgC^ salt directly to the leach solution, for example by means of a specially designed salt addition pond;

b) halide or chloride containing mineral e.g. atacamite. The halide mineral may occur in nature as a "mixed" mineral, or may be intentionally blended with the sulphide mineral, for example during agglomeration or heap construction;

c) natural salt-containing water e.g. sea water and water from salt lakes or reservoirs; and

d) brine as a byproduct from a desalination process.

[0025] The method of the invention requires no solution potential management of the irrigation solution within the chloride leach cycle, which may be between 550 - 700 mV (mV vs. SHE), depending on the chloride concentration, but preferably is between 600 and 700 mV.

[0026] The irrigation solution from the chloride leach cycle may be a raffinate solution, an intermediate leach solution (ILS), or a pregnant leach solution (PLS), containing 3 to 20 g/l sulphuric acid.

[0027] Agglomeration is not a prerequisite. However heap construction may be preceded by agglomeration of crushed material with sulphuric acid and water, a leach solution from the bioleaching circuit, or a solution from the chloride leach cycle.

[0028] The chloride leach cycle may be managed so that the chloride ion concentration in the bioleach circuit under extreme cases does not exceed 6 g/l and preferably is not above 2 g/l, with the sulphate levels not exceeding 100 g/l. [0029] The present invention differs from the disclosures in Chilean patent No. 40,891 , Chilean patent application No. 1572-01 and International patent application No. PCT/ZA2007/000025 in that, in the bioleach cycle, bioleaching microorganisms are required to maintain an Eh environment in the heap exceeding 700 mV vs. SHE.

[0030] Heap inoculation may be carried out after a chloride leach cycle in any appropriate way e.g. via irrigation with bioleach circuit solutions or through the use of an inoculation plant of any suitable kind.

[0031] The bioleach circuit may have a bleed, rinse or purge system in order to manage impurity levels thereby to ensure sound microbial functions.

[0032] The high impurity-containing solution from the bioleach circuit may be diverted to a separate leach circuit e.g. a high oxide material leach circuit, which may have a high acid requirement and may not require any microbial function.

[0033] The leach solution used within the chloride leach cycle may be part of a separate leach circuit for example used for treating material containing high oxide and halide minerals.

[0034] The dissolved oxygen level in solution at all times is preferably in excess of 1 ppm.

[0035] During the initial heap irrigation period, the chloride leach conditions described herein may achieve superior copper extraction from secondary copper sulphide minerals (e.g. chalcocite and covellite) compared to conventional bioleaching at solution potentials below 700 mV vs. SHE. [0036] The extent of copper extraction within the chloride leach cycle, confined to conditions described herein, may be enhanced with an increase in the chloride concentration.

[0037] In the case of chalcocite, secondary covellite and native covellite, dissolution within the chloride cycle may occur according to the following reactions:

Cu ₂ S + 2Fe ³⁺ → 2Fe ²⁺ + CuS + Cu ²⁺ (1 ^st stage chalcocite via Fe ³⁺ ) (10)

Cu ₂ S + 2Cu ⁺ → CuS + 2Cu ⁺ (1 ^st stage chalcocite via Cu ²⁺ ) (11)

CuS + 2Cu ²⁺ → 3Cu ⁺ + S (secondary or primary covellite via Cu ²⁺ ) (12)

[0038] Reaction 12 is slower than reaction 10 and reaction 11.

[0039] In a commercial plant, an increase in the chloride concentration within the chloride cycle may increase the ferric iron concentration or the Eh of the irrigation solution by way of the rapid equilibrium reaction:

Fe ²⁺ + Cu ²⁺ Cu ⁺ + Fe ³⁺ (enhance rate with increase chloride ions) (13)

This is followed by the re-oxidation of cuprous in the presence of oxygen:

Cu ⁺ + 0 ₂ + 4H+→ Cu ²⁺ + 2H ₂ 0 (this reaction is rapid in the presence of chloride ions) (14)

[0040] The leach solution within the chloride leach cycle may contain a minimum of 0.5 g/l and 0.2 g/l soluble iron and copper, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The invention is further described by way of examples with reference to the accompanying drawings in which: Figure 1 presents an example of a simplified flowsheet of one way in which the invention can be implemented;

Figure 2 shows PLS copper concentration curves of some heaps within a commercial bioleach circuit containing > 70 % CSR(Copper Source Ratio) of a secondary sulphide mineral;

Figure 3 demonstrates iron oxidation activity vs. copper concentration of a bioleach mesophile culture at 0 g/l CI;

Figure 4 shows initial PLS copper concentration graphs of six 1 meter columns containing a secondary sulphide material with ~1 wt. % total copper, a CSR of 75 % chalcocite and 0.18 wt. % atacamite;

Figure 5 shows initial PLS chloride concentration graphs of the six columns presented in Figure 4;

Figure 6 presents the PLS Eh profiles of the six columns with the column irrigation solution Eh maintained at approximately 700 mV;

Figure 7 shows the PLS pH values of the six columns with the column irrigation solution pH maintained at approximately 1.3;

Figure 8 shows the percentage copper extracted from the six columns;

Figure 9 illustrates chloride concentration vs. Eh for a commercial chloride heap leach circuit;

Figure 10 demonstrates secondary covellite conversion from sequential 1 meter column residues, analysed via Qem-Scan, as a function of irrigation solution Eh at 15 g/l CI - the iron and copper concentrations in the irrigation solution were approximately 1 g/l and 0.5 g/l, respectively and the columns were loaded with 80% passing 1/2" secondary sulphide material containing -0.8 wt. % total copper and a chalcocite CSR of 83 %; Figure 11 shows copper extraction graphs from 6 meter columns (designated K30, K31 , K33, K34, K35 and K36) irrigated with different chloride concentrations in the raffinate and a narrow Eh range of 620 - 640 mV - the columns were loaded with 80% passing 1/2" secondary sulphide material containing ~0.8 wt. % total copper and chalcocite CSR of 83 %;

Figure 12 shows raffinate Eh graphs for the columns presented in Figure 11 - the iron and copper concentrations in the raffinate were approximately 7 g/l and 0.5 g/l, respectively;

Figure 13 shows PLS Eh profiles from the columns K34, K30 and K33 for a first period of rapid leaching;

Figure 14 is a graphical illustration of the effect of chloride ions on solution potential (ferrous iron oxidation) in a controlled batch reactor system at 25 °C, pH of 1.2, 1 g/l Fe (all ferrous initially) and dissolved oxygen > 1ppm - this experimental work was conducted in the absence of micro-organisms using the preservative sodium benzoate;

Figure 15 presents PLS and irrigation solution (1 g/l Fe, 0 g/l CI and pH 1.3) Eh curves of a number of 1 meter bioleach columns at 25 °C, which were inoculated via agglomeration and irrigation and which were loaded with 80% passing 1/2" secondary sulphide material containing between 0.8 and 1.4 wt. % total copper and chalcocite CSR of above 85 % - no halide minerals were detected in the material; Figure 16 depicts PLS and irrigation solution (1 g/l Fe, 25 g/l CI and pH 1.3) Eh curves of a number of 1 meter chloride leach columns at 25 °C which were loaded with the same material as referred to in connection with Figure 15; and

Figure 17 presents copper extraction graphs from the chloride and bioleach columns referred to in Figures 15 and 16. DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] Figure 1 is an example of a simplified flowsheet of the manner in which the invention may be applied. General operational aspects of a heap leach circuit, including solvent extraction, are not described herein.

[0043] In Figure 1 the following components are identified as follows: a first heap circuit 10; a second heap circuit 12; an agglomerator 14; a first raffinate pond 16; a first pregnant leach solution (PLS) pond 18; a second raffinate pond 20; a second PLS pond 22; a bioleach culture supply source 24; and a salt addition pond 26.

[0044] The first heap circuit 10 primarily contains copper oxide and/or copper halide minerals which may be mixed with copper sulphide (< 70 % copper source ratio or CSR), and is operated with a leach solution which is high in chloride (stipulated herein from 7 g/l to 80 g/l CI), and has insignificant bioleach related microbial activity.

[0045] A chloride salt may be added to the first heap circuit 10 using a specially designed salt addition pond 26 thereby to maintain or increase the chloride concentration in the first heap circuit. The chloride addition pond 26 may be linked to the first raffinate pond 16 which is connected to the agglomerator 14.

[0046] The second heap circuit 12 has a high copper source ratio (> 70%) copper secondary sulphide material and is operated with low chloride solutions (stipulated herein < 6 g/l CI) to promote microbial activity. [0047] Material for the construction of the heaps in the circuits 10 and 12 may be agglomerated with raffinate from the circuit 10. The heaps in the circuit 10 are only irrigated with leach solutions from the circuit.

[0048] Heaps in the second circuit 12 (the so-called "sulphide heaps") are irrigated with leach solutions from the first circuit or with "high chloride" solutions, during initial heap operation, followed by solutions from the second circuit 12 or "bioleach" solutions.

[0049] If a heap inside the second circuit is irrigated with solution from the first circuit then the drainage of the heap is directed to the PLS pond 18 which is associated with the first heap leach circuit.

[0050] A heap in the second circuit 12 is irrigated with solution from the second circuit, for example from the second raffinate pond 20, directly after irrigation with solutions from the first circuit. The initial "bleed" solution or chloride "rinse" is directed to the first PLS pond 18 until a desired chloride concentration is achieved. Thereafter the drainage is directed to the second PLS pond 22.

[0051] Bioleach-related cultures or an inoculum may be added to the second heap leach circuit 12 from the source 24.

[0052] Figure 2 presents initial high copper concentrations (20 to 50 g/l) observed in PLS from commercial bioleach heaps containing mostly secondary sulphide material and, in this case, predominantly chalcocite. The iron oxidation activity of a bioleach mesophile culture at various copper concentrations is presented in Figure 3, with no microbial activity observed at copper concentrations exceeding 20 g/l.

[0053] Figures 4, 5 and 6 show PLS copper concentrations, chloride concentrations and Eh values respectively from six columns containing secondary sulphide material and operated under similar conditions, but inoculated differently. Columns 1 and 2 were inoculated during agglomeration only and not during column irrigation, with a cell concentration approximately ten times the current commercial bioleach target. Columns 3 to 6 were not inoculated during agglomeration, but only during irrigation with different cell dilutions within the feed solutions, representing cell concentrations typically observed in heap bioleach operations.

[0054] The PLS Eh profiles (see Figure 6), which are an indication of microbial iron oxidation activity within the columns, showed no microbial activity within columns 1 and 2 after 65 days, compared to the columns inoculated via irrigation, which showed microbial activity after 30 days of operation. It is believed that the initial high copper and chloride concentrations (Figures 4 and 5) of the first leach solution in contact with material diminished the microbial population added via agglomeration, for Figure 7 demonstrates that pH is not a limiting factor for microbial growth.

[0055] It is important to note the low Eh (<600 mV vs. SHE) observed within the initial 30 days for columns 3 to 6. The low Eh is linked to a difference of approximately 15% in the copper extraction values of columns 1 and 2 on the one hand and columns 3 to 6 on the other hand, see Figure 8.

[0056] Figure 9 demonstrates that an increase in the chloride concentration within a commercial chloride heap leach circuit, from approximately 14 to 22 g/l CI, caused an increase in the Eh of the "bulk" raffinate and PLS from approximately 630 mV to 660 mV (vs. SHE).

[0057] The conversion of secondary covellite to soluble copper is demonstrated in Figure 10 as a function of Eh in the irrigation solution, with a slight benefit observed between 550 mV and 600 mV (vs. SHE), but with an increasing conversion rate from 650 mV to 700 mV (vs. SHE) at 15 g/L chloride.

[0058] Figure 11 shows an enhanced copper extraction from 6 meter columns (identified as K30, K31 , K33, K34, K35 and K36) with an increasing raffinate chloride concentration within a narrow Eh range of 620 to 640 mV vs. SHE (Figure 12). The rapid copper extraction observed as a function of chloride was confined within the initial 20 to 30 days of irrigation, achieving approximately 30% copper extraction at raffinate chloride concentrations below 2 g/l, compared to 55% at 55 g/l chloride. It is suggested in this invention that the fast initial copper extraction rate is due to the rapid kinetics of ferrous iron oxidation according to reactions 13 and 14, and chalcocite oxidation as depicted in reactions 10, 11 and 12. The regeneration of ferric iron after the initial rapid reduction (mineral oxidation) as a function of chloride concentration, is illustrated in Figure 13, in that the PLS Eh recovers more rapidly to the 620 - 640 mV (vs. SHE) feed Eh, at increasing chloride concentrations.

[0059] The rate of ferrous iron oxidation by cuprous as a function of chloride concentration, as depicted in the reactions 13 and 14, is illustrated in Figure 14. It is noted that the rapid initial increase in Eh is followed by a drastic decrease over time as the thermodynamics become more unfavorable, approaching 700 mV vs. SHE.

[0060] The slower rate observed after 30 days in Figure 11 may be attributable to a lower mineral surface Eh (rate limiting) compared to the bulk solution Eh, which may be strongly dependent on intra-particle diffusivity as a function of shrinking core radius.

[0061] Increasing the bulk solution Eh above 700 mV (vs. SHE), using bioleaching related micro-organisms may, at least partly, alleviate the effect of lower surface Eh as a function of intra-particle diffusion. The PLS and irrigation Eh profiles of bioleach and chloride leach secondary sulphide columns are shown in Figure 15 and 16, respectively. The bioleach columns exhibited a characteristically low Eh period for an initial 25 to 40 days, during which period the chloride columns showed enhanced copper recovery (Figure 17) even though the irrigation Eh was approximately 100 mV lower than in the case of the bioleach columns. Conversely, once the bioleach columns show sufficient microbial iron oxidation activity to increase the Eh, the rate considerably increases (inflection points on the graphs) such as to achieve slightly higher final copper extractions. The extent of the benefit of the higher Eh may depend on the rock characteristics.