A heap leach process

Abstract

A method of leaching a base metal from a heap of a base metal-containing sulfidic material from a mine including: processing a leach solution obtained from leaching the heap or another heap of the base metal-containing sulfidic material to remove at least some sulfates in the leach solution and forming a reduced sulfate concentration leach solution, with the leach solution being produced after a base metal recovery step; and supplying the reduced sulfate concentration leach solution to the heap or to another heap of a base metal-containing sulfidic material and acid leaching a base metal from the base metal-containing sulfidic material.

Description

[0001] A HEAP LEACH PROCESS

[0002] TECHNICAL FIELD

[0003] The present invention relates to heap leaching of a base metal, such as copper or nickel or zinc or cobalt, from a base metal-containing sulfidic material.

[0004] The present invention relates particularly, although not exclusively, to microbial-assisted heap leaching of a base metal, such as copper or nickel or zinc or cobalt, from a base metal-containing sulfidic material.

[0005] The present invention relates particularly, although not exclusively, to microbial-assisted heap leaching of copper-containing sulfidic material.

[0006] The invention also relates particularly, although not exclusively, to microbially-assisted heap leaching of agglomerates of abase metal -containing sulfidic material.

[0007] The invention also relates particularly, although not exclusively, to recovering a base metal, such as copper, from a base metal-containing sulfidic material that has been leached.

[0008] The invention also relates particularly, although not exclusively, to end use products made from a recovered base metal, such as copper.

[0009] BACKGROUND ART

[0010] The following description focuses on copper as one example of a base metal in a base metalcontaining sulfidic material.

[0011] In conventional heap leaching methods (which is understood herein to comprise percolation and dump leaching) of copper-containing sulfide material, a copper-containing sulfide material is stacked in heaps, aerated through direct injection of air via aeration pipes extending into the heap and / or by natural convection through exposed areas of the heap, and irrigated with an acidic leach solution for extraction of copper into solution. The leaching method requires an acid and an oxidant to dissolve copper into solution. Copper is subsequently recovered from a copper-containing solution collected from the heap, i.e., a pregnant leach solution, by a range of recovery options including for example solvent extraction and electrowinning (SX / EW), cementation onto more active metals such as iron, hydrogen reduction, sulfidization via H2S or NaHS addition, crystallization of sulfate salts, and direct electrowinning.

[0012] Generally, heap leaching is resource intensive and requires significant amounts of acid and water.

[0013] This demand for acid and water is reduced by regenerating a copper-depleted solution produced after recovering copper from a pregnant leach solution into an acidic leach solution and recycling the solution through the heap to leach more copper from the copper-containing material in the heap.The use of the copper-depleted solution is also useful in terms of minimizing waste generation and / or water treatment cost.

[0014] The copper-depleted solution is a raffinate in the case of SX processing of a pregnant leach solution.

[0015] However, the presence of residual substances such as metal sulfate in recycled acidic leach solutions can impact on the efficacy of heap leaching steps, particularly when the process involves microorganisms which are often sensitive to their living environment.

[0016] Specifically, the Applicant has found in trials of microbial-assisted heap leaching of copper-containing sulfidic material with an acidic leach solution that elevated metal sulfate levels in a raffinate resulting from (1) addition of make-up acid to recycled raffinate to lower the pH to a desired value for leaching, (2) microbial oxidation of certain copper sulfide minerals in the heap, (3) microbial oxidation of pyrite that occurs naturally in copper-containing sulfidic material, (4) microbial oxidation of any added pyrite concentrate can negatively affect microbial growth in a heap when the raffinate is recycled back onto the heap and (5) dissolution of gangue minerals by the acidic leach solution and acid derived from pyrite in, or added to, the heap material.

[0017] The Applicant believes that this is a problem that is not limited to heap leaching copper-containing sulfidic material and is also encountered when processing other base metals such as nickel or zinc or cobalt.

[0018] The Applicant has developed a method of controlling levels of sulfate in an acidic leach solution for supply to a microbial-assisted heap leach of a base metal -containing sulfidic material.

[0019] The above description is not to be taken as an admission of common general knowledge in Australia or elsewhere.

[0020] SUMMARY OF THE DISCLOSURE

[0021] The basis of the invention is to process a pregnant leach solution that is produced by heap leaching a base metal -containing sulfidic material, often but not always after a base-metal recovery step, to form a processed leach solution with a reduced concentration of sulfates (also termed “reduced sulfate concentration leach solution” in this specification).

[0022] It is believed that the invention may also apply to other impurities in addition to sulfates such as chlorides (>5 g / L), fluorides (e.g. HF), nitrates and nitrites.

[0023] The invention is supported by trials conducted by the Applicant of a microbial-assisted heap leach method of a copper-containing sulfidic material. The method evaluated in the trials included extracting copper from a pregnant leach solution using an organic extractant in solvent extraction and supplying a copper-depleted leach solution, i.e. a raffinate, to the same or a different copper leaching process to be used as an acidic leach solution.It was found in the trials that the presence of copper sulfate and / or other metal sulfates increased the osmotic pressure between cell walls of microbes and the leach solution in the raffinate and that the increase was material to microbial activity in the copper-leaching process.

[0024] Particularly, it was found that elevated copper sulfate and / or other metal\ sulfate levels in the raffinate negatively affected microbial growth in the heap and resultant heap leach performance.

[0025] In one experiment in the trials, it was observed that microbial activity started to be significantly impacted at a sulfate concentration of around 120 g / L. The observed sulfate concentration was much higher than the concentration of chloride of around 5 g / L at which a similar decrease in microbial activity was observed

[0026] In broad terms, the present invention provides a method of leaching a base metal from a heap of a base metal -containing sulfidic material from a mine including:

[0027] processing a leach solution obtained from leaching the heap or another heap of the base metal-containing sulfidic material to remove at least some sulfates in the leach solution and forming a reduced sulfate concentration leach solution, with the leach solution to be processed being suitably a base metal-depleted solution (e.g. raffinate) produced after a base metal recovery step; and supplying the reduced sulfate concentration leach solution to the heap or to another heap of a base metal-containing sulfidic material and acid leaching a base metal from the base metal-containing sulfidic material.

[0028] The base metal-containing sulfidic material may comprise ores, concentrates of ores and waste material such as tailings and mineralized waste that are uneconomic to process using current processes.

[0029] A reason for forming the reduced sulfate concentration leach solution which is supplied to the heap or another heap and not in the heap (or elsewhere) is to minimize complications within the heap such as the formation of precipitates that likely would lead to decreased rate and extent of leaching of the base metal-containing sulfidic material, in particular, chalcopyrite in situations where the base metal is copper. Controlling the presence of sulfates external to the heap allows much better control of the sulfate concentration and location of precipitates without having to alter the heap operating parameters such as adding additives to the heap (e.g. to control pH and precipitation of sulfates). Controlling sulfate concentration in the leach solution also minimizes any lag time required for the changed heap operating parameters to take effect. Furthermore, it provides an opportunity to recover and process sulfates into downstream products such as sulfuric acid which can be monetized or returned to the process.

[0030] Suitably, the method includes microbially-assisted leaching of the heap or another heap of the base metal-containing sulfidic material with an acidic leach solution.

[0031] It is not always necessary that the reduced sulfate concentration leach solution be produced from a microbially-assisted leach.Suitably, the method includes returning the reduced sulfate concentration leach solution to the same heap or another heap of the base metal-containing sulfidic material.

[0032] The method may include:

[0033] carrying out a microbially-assisted leach of the base metal-containing sulfidic material in the heap with the reduced sulfate concentration leach solution;

[0034] collecting the base metal-containing pregnant leach solution from the heap; and recovering the base metal from the pregnant leach solution.

[0035] The method may be a multi-stage leaching process.

[0036] The multi-stage leaching method may include:

[0037] a first stage of non-microbially assisted leaching of the base metal-containing sulfidic material and producing a first pregnant leach solution (which may also be referred to as an intermediate leach solution); and

[0038] a second stage of microbially-assisted leaching of the base metal-containing sulfidic material using the first pregnant leach solution and producing a second pregnant leach solution.

[0039] Alternatively, the multi-stage leaching method may include:

[0040] a first stage of microbially assisted leaching of the base metal-containing sulfidic material and producing a first pregnant leach solution; and

[0041] a second stage of non-microbially-assisted leaching of the base metal-containing sulfidic material using the first pregnant leach solution and producing a second pregnant leach solution.

[0042] In both embodiments, a base metal-depleted solution from the base metal recovery step may be processed to form the reduced sulfate concentration leach solution.

[0043] The leaching step may include controlling the pH of any of the previously mentioned leach solutions, suitably for microbially-assisted leaching, to be less than 4.0, typically less than 3.2, typically less than 3.0, typically less than 2.5, typically less than 2.0, typically less than 1.8, typically less than 1.5, typically less than 1.2, and typically less than 1.0.

[0044] The leaching step may include controlling the pH of any one of the previously mentioned leach solutions, suitably for microbially-assisted leaching, to be greater than 0.3, typically greater than 0.5.

[0045] Suitably, the base metal is copper.

[0046] However, it can be appreciated that the method is equally applicable to other base metals, suitably base metals such as cobalt, nickel and zinc.

[0047] The processing step may include reducing the sulfate concentration by removing a predetermined amount of sulfates from the leach solution formed after recovering the base metal from the pregnant leach solution (i.e. the leach solution produced after a base metal recovery step).The processing step may include reducing the sulfate concentration by removing the sulfates in solution or as a precipitate from the leach solution formed after recovering the base metal from the pregnant leach solution.

[0048] The method may include recovering the base metal, for example copper, from the pregnant leach solution by any suitable recovery method.

[0049] The base metal recovery step may include producing a concentrated base metal-containing leach solution (from the pregnant leach solution) and a base metal-depleted leach solution. Suitably, the base metal recovery step includes solvent extraction and electrowinning (SX / EW) the base metal.

[0050] SX / EW includes solvent extraction that comprises extracting the base metal from the pregnant leach solution with an organic extractant in a solvent extraction step and stripping the base metal from the organic extractant and forming the concentrated base-metal containing leach solution.

[0051] Solvent extraction also forms the base metal -depleted leach solution in the form of a raffinate. SX / EW also includes electrowinning the base metal from the concentrated base-metal containing solution to produce a solid product.

[0052] The processing step may include controlling the sulfate concentration in the leach solution so that it does not exceed a threshold concentration.

[0053] The processing step may include monitoring the sulfate concentration in the leach solution and controlling the sulfate concentration in the leach solution so that it does not exceed the threshold concentration.

[0054] The processing step may include controlling the sulfate concentration in the leach solution to not exceed 200 g / L, suitably not exceed 170 g / L sulfate in a leach liquor collected from the heap, more suitably not exceed 120 g / L sulfate in a leach liquor collected from the heap, more suitably not exceed 100 g / L sulfate in a leach liquor collected from the heap, more suitably not exceed 80 g / L sulfate in a leach liquor collected from the heap, more suitably not exceed 50 g / L sulfate in a leach liquor collected from the heap.

[0055] The processing step may include maintaining the sulfate concentration of the leach solution in a range from 20-120 g / L sulfate in a leach liquor collected from the heap.

[0056] The processing step may include reducing the sulfate concentration by electrochemical precipitation of base metals associated with the sulfate in an electrolytic cell and producing sulfuric acid.

[0057] The processing step may include reducing the sulfate concentration by reacting the leach solution with limestone (calcium carbonate) and / or burnt lime (calcium oxide) and / or hydrated lime (calcium hydroxide) to precipitate calcium sulfate and associated metals as hydroxides (oxides, oxyhydroxides).

[0058] The processing step may include reducing the sulfate concentration by precipitating jarosite ((AEe3(SO4)2(OH)g)) where A is typically but not limited to H3O, K, Na or NH4.Suitably, the leach solution in the processing step may be a base metal-depleted leach solution from the base metal recovery step.

[0059] The leach solution may have a pH ranging from 1.5-2.0.

[0060] The method may include adjusting the pH of the leach solution to precipitate substances such as jarosite and magnesium hydroxide.

[0061] The method includes adjusted the pH up to 11. The control of pH is one of a number of suitable physical and / or chemical techniques for controlling the sulfate concentration. Suitably, the method includes using a suitable base to increase the pH of the leach solution, suitably the raffinate.

[0062] In one example, the pH is adjusted up to 2.8, suitably up to 3.5, suitably up to 3.8.

[0063] Adjusting the pH of the leach solution up to a pH ranging from 1.8 - 2.8 may induce precipitation of jarosite.

[0064] In another example, adjusting the pH of the leach solution up to 3.0-3.8 may induce precipitation of ferric iron as ferric hydroxide / oxyhydroxide. The processing step may include reducing the sulfate concentration by precipitating alunite (K

[0065]

[0066] SO.;} (( iH)-). Alunite may precipitate in a scenario involving the precipitation of jarosite in an aluniinium-containing leach solution. In this scenario, the aluminium replaces Fe to form alunite. Suitably, the method includes adjusting the pH of the leach solution, suitably the raffinate, to range from pH 3-3.5 to precipitate alunite.

[0067] In another example, adjusting the pH of the leach solution to range from 10-11 for example by adding lime precipitates magnesium as magnesium hydroxide.

[0068] The processing step may include reducing the sulfate concentration by adding an organic additive into the leach solution. The organic additive may chemically reduce the sulfate concentration in the processing step, for example, by converting the sulfate into another functional group such as sulfide.

[0069] The processing step may include reducing the sulfate concentration by directing a raffinate bleed stream, suitably into an evaporation pond or a natural sulfate reduction / dilution source, such as a tailings dam or a run-of-mine (ROM) heap. Suitably, the discharged raffinate bleed solution is replaced with fresh or process water.

[0070] The method may include heating the leach solution to at least 70 °C, suitably at least 80 °C, more suitably at least 90 °C.

[0071] Suitably, the method includes heating the leach solution using an electric heater. Alternative heating sources include direct fired propane, natural gas, solar, and diesel heaters, and indirect heating sources including steam heating. Beneficially, reaction rates increase with temperature. This allows the use of smaller tanks.

[0072] The method may include processing the leach solution in an electrolysis cell and forming sulfuric acid.

[0073] The method may include separating a part of the leach solution as a bleed stream.

[0074] The method may include processing the bleed stream to reduce the concentration of sulfates.Processing of the bleed stream may include removing sulfates from the stream to reduce its sulfate concentration and form a processed bleed stream.

[0075] The method may include reintroducing the processed bleed stream into the leach solution to achieve a desired overall sulfate concentration.

[0076] The method may include discharging the bleed stream into a sink or processed to reduce its sulfate concentration before the processed raffinate bleed stream is returned into the leach solution.

[0077] A suitable sink for the bleed stream is a waste or residue storage unit.

[0078] One benefit of processing the raffinate bleed stream rather than the entire raffinate is the reduced volume of fluid required to be processed.

[0079] The method may include transferring at least part of the leach solution to an evaporation pond.

[0080] The method may include transferring the leach solution to a high acid consuming stockpile. In some embodiments, the method may include transferring the bleed stream to the high acid consuming stockpile.

[0081] Suitably, the stockpile contains acid consuming gangue minerals that release calcium ions into solution, such as calcite. This enables removal of sulfate as calcium sulfate (gypsum). Otherwise, only free acid will be consumed by other, non-calcium containing gangue minerals, raising the pH which could precipitate ferric iron as jarosite, removing some sulfate but not all the sulfate associated with ferric, because jarosite precipitation produces acid.

[0082] The stockpile may contain copper oxide minerals.

[0083] Suitably, the stockpile contains secondary copper sulfide minerals.

[0084] The method may include controlling precipitation of sulfates in the high acid consuming stockpile.

[0085] The method may include irrigating the heap with a reduced sulfate concentration leach solution at a rate ranging from 1-50 L / h / m2, suitably ranging from 1-20 L / h / m2. Deploying sprinklers or wobblers typically results in an irrigation rate at the higher end of the range whereas deploying drippers typically results in an irrigation rate at the lower end of the range.

[0086] The base metal -containing sulfidic material may be in any suitable size for the heap.

[0087] The rock size may range from coarse to fine depending on other operational considerations. For example, the base metal-containing sulfidic material may have a rock size in a range between a Pxo of 500 mm and a Pxo of 9 mm, typically in a range between a Pxo of 400 mm and a Pxo of 30 mm, and typically in a range between a Pxo of 100 mm and a Pxo of 5 mm, suitably 9 mm.

[0088] It is noted that the size of the base metal -containing sulfidic material may be larger or smaller than the above-described size ranges.

[0089] The base metal-containing sulfidic material may be any suitable shape, noting that size ranges described in the preceding paragraph are based on one dimension only.The method may include selecting a mining method for the base metal-containing sulfidic material in a suitable form, including size distribution and / or shape, for heap leaching.

[0090] The present invention also provides a base-metal heap leach circuit that is configured to perform the previously described method.

[0091] In broad terms, the present invention provides a base-metal heap leach circuit that includes a leach solution treatment unit configured to:

[0092] process a leach solution from a base metal recovery unit, and form a reduced sulfate concentration leach solution; and

[0093] supply the reduced sulfate concentration leach solution onto a heap of a base metalcontaining sulfidic material from amine.

[0094] The base-metal heap leach circuit may also include a pregnant leach solution storage unit configured to collect and store pregnant leach solution from leaching of the base metal-containing sulfidic material.

[0095] The base-metal heap leach circuit may also include a base metal recovery unit to process pregnant leach solution to recover the base metal and produce the reduced sulfate concentration leach solution.

[0096] The base metal recovery unit may be a solvent extraction unit that is configured to produce a base metal containing electrolyte and an electrowinning unit that is configured to win base metal from the electrolyte.

[0097] The pregnant leach solution or the base metal-depleted leach solution may be obtained from the heap of base metal-containing sulfidic material receiving the reduced sulfate concentration leach solution or may be from another heap of base metal-containing sulfidic material.

[0098] The leach solution treatment unit may be configured to monitor the sulfate concentration of the leach solution.

[0099] Suitably, the leach solution treatment unit includes a sensor to monitor the sulfate concentration.

[0100] More suitably, the leach solution treatment unit issues an alert when the leach solution exceeds a threshold sulfate concentration.

[0101] The leach solution treatment unit may issue an alert when the sulfate concentration exceeds 200 g / L, suitably 170 g / L sulfate in a leach liquor collected from the heap, more suitably 120 g / L sulfate in a leach liquor collected from the heap, more suitably 100 g / L sulfate in a leach liquor collected from the heap, more suitably 80 g / L sulfate in a leach liquor collected from the heap, more suitably 50 g / L sulfate in a leach liquor collected from the heap.

[0102] The leach solution treatment unit may include a controller that is configured to receive a signal from the sensor to measure the sulfate concentration in the leach solution.

[0103] The controller may be configured to issue an alert or take rectification action if the sulfate concentration exceeds a predetermined value. For example, the controller may activate a bleed valveto purge part of the raffinate exiting the solvent extraction step into a waste storage unit such as a tailings dam.

[0104] The controller may be configured to maintain the sulfate concentration at a predetermined range, suitably in a range from 50-100 g / L sulfate in a leach liquor collected from the heap.

[0105] The leach solution treatment unit may include a precipitation unit configured to precipitate sulfates from the leach solution, suitably after the leach solution has been depleted of base metal.

[0106] Suitably, the precipitation unit is configured to precipitate one or more of calcium sulfate, alunite and jarosite.

[0107] More suitably, the precipitation unit is configured to dose the leach solution with calcium carbonate and precipitate calcium sulfate. It is understood that calcium carbonate will first neutralise free acid to precipitate calcium sulfate (gypsum). As the pH increases, ferric will start to precipitate first. followed by aluminium.

[0108] The leach solution treatment unit may include a heater configured to increase the temperature of the leach solution.

[0109] The heater may be an electric heater.

[0110] Alternative heaters include direct fired propane, natural gas, solar and diesel heaters.

[0111] The controller may be configured to control heating the leach solution to a temperature of at least 70 °C, suitably at least 80 °C, more suitably at least 90 °C.

[0112] The leach solution treatment unit may include an electrolytic cell configured to form sulfuric acid from the leach solution.

[0113] The leach solution treatment unit may include a bleed valve to remove at least part of the leach solution as a bleed stream.

[0114] Suitably, the bleed stream is in fluid communication with a waste storage unit.

[0115] The waste storage unit may be in the form of a tank or a pond.

[0116] Hie circuit may include an evaporation pond to receive at least part of the leach solution. The circuit may include a high acid consuming stockpile.

[0117] Suitably, the stockpile comprises sulfide-rich material from a mine. More suitably, the stockpile comprises acid consuming gangue minerals. Acid consuming, calcium containing gangue minerals release calcium ions into solution, suitably from calcite, to remove sulfate as calcium sulfate (gypsum).

[0118] Either the solvent extraction unit or the leach solution treatment unit may be in fluid communication with the stockpile. In use, the leach solution may be delivered to the stockpile and allowed to percolate through the stockpile. Doing so allows precipitation of metals and sulfate from the leach solution to form a reduced sulfate concentration leach solution that can be recycled back to the heap.The circuit may include a SX unit for extracting copper from the pregnant leach solution using an organic reagent and stripping the copper from the organic phase using a concentrated acid, typically sulfuric acid, to produce an electrolyte, typically a concentrated copper sulfate solution.

[0119] The circuit may include an electrowinning unit for processing the concentrated base metalcontaining solution to form a base-metal containing product.

[0120] Suitably, the base-metal containing product is in the form of a cathode.

[0121] The circuit may include an end-product manufacturing unit to form end-use products such as billets, ingots, bars, and tubes, each of which may be on-sold to downstream manufacturers of other end-use products.

[0122] The heap may be equipped with sprinklers, wobblers or drippers to supply the leach solution onto the heap of the base metal-containing sulfidic material or onto another heap of a base metalcontaining sulfidic material.

[0123] The pregnant leach solution storage unit may be in the form of a tank or a pond.

[0124] The circuit may include a reduced sulfate concentration leach liquor storage unit such as a tank or a pond to hold the reduced sulfate concentration leach solution.

[0125] The invention also relates to end use products made from the base metal recovered from the pregnant leach solution.

[0126] The end use products may be any suitable end use products.

[0127] As noted above, electrowinning a base metal produces a product described as a “cathode”. The cathode is a form of an end-use product in that it is a marketable product that can be sold to downstream manufacturers to be processed into other end-use products.

[0128] For example, the cathode may be processed in any suitable way to produce other end-use products, including billets, ingots, bars, and tubes.

[0129] The other end-use products may comprise the following categories of products:

[0130] Semi-fabricated products: including copper wire rods for the wire and cable industry. Power Generation: including electrical conductors, transformers, wires, and cables.

[0131] Construction: including plumbing, roofing, and architectural elements.

[0132] Electronics: including printed circuit boards (PCBs), wiring, and semiconductors.

[0133] Automotive Industry: radiators, connectors, and wiring.

[0134] Telecommunications: including communication networks.

[0135] Copper alloys, such as brass and bronze, that are widely used in various applications due to their unique properties such as high strength, corrosion resistance, and aesthetic appeal. Decorative Items: including coins, medals, and other decorative items.

[0136] The end use products may be manufactured by any suitable method.

[0137] EXPLANATION OF TECHNICAL TERMS RELEVANT TO THE DISCLOSUREThe base metal-containing sulfidic material may be (a) run-of-mine (“ROM”) material or (b) ROM material that has been subjected to intermediate processing. ROM material is typically mined material that has not been graded according to size or quality.

[0138] The base metal-containing sulfidic material may be material that is in a stockpile of a ROM material or a ROM material that has undergone intermediate processing

[0139] The base metal-containing sulfidic material may be (a) ROM material or (b) ROM material that has been subjected to intermediate processing.

[0140] The term “intermediate processing” relates to any type of processing of ROM material including processing that falls under the general description of “ore dressing” including but not limited to any one or more of comminution, size separation into different size fractions, sorting by grade of a target base metal (e.g., concentration of the base metal) into different grade fractions, sorting by other chemical or mineralogical composition of the ROM material (such as a contaminant), sorting by other property of the ROM material, and agglomeration.

[0141] The ROM material may be obtained from any mining operation in a mine.

[0142] The mining operation may be above ground.

[0143] The mining operation may be underground.

[0144] For example, the mining operation may be a drilling and blasting operation in an open pit mine, with the ROM material being rocks that form when a mine bench is drilled and blasted and slumps into a pit and is then transported by haul trucks or other suitable vehicles or conveyors from the pit.

[0145] By way of further example, the mining operation may be an operation involving the use of a continuous miner, with the ROM material being rocks that are produced from the continuous miner.

[0146] By way of further example, the mining operation may be an underground mining operation including block cave mining, sub-level cave mining, or any other suitable underground mining method, with material being removed from extraction points, such as draw points in block cave mines, as metal sulfide -containing material and being transported by haul trucks or other suitable vehicles or conveyors to above-ground.

[0147] By way of further example, the mining operation may be a block caving operation in an underground mine, with the ROM material being rocks in rill piles at draw points of a block cave.

[0148] The term “mine” is understood herein to be a broad term that covers, by way of example only, a site in which there is (a) an area (above and / or below ground) being mined, with material being removed from the area, (b) an area (above and / or below ground) that has been mined, with material already removed from the area, (c) stockpiles of mined material from areas (a) and (b), (d) downstream processing units including, for example, a comminution circuit for comminuting mined material (such as crushers and grinding mills) and size separation units for separating mined and comminuted mined material based on size, a heap leach circuit for producing a pregnant leach liquor, the heap leach circuit including for example heap leach pads, equipment for forming heaps on thepads, circuits for supplying leach liquor to heaps and recovering pregnant leach liquor from heaps, a recovery circuit for recovering the metal from the pregnant leach liquor including solvent extraction and electrowinning units, with solvent extraction producing a raffinate, and a unit for regenerating and recycling the raffinate to the heap leach circuit, and (e) storage units such as for reagents, water, pregnant leach solution, concentrates of valuable metals, and tailings.

[0149] The base metal-containing sulfidic material may be an ore.

[0150] The term “ore” is understood herein to mean natural rock or sediment that contains one or more valuable metals that can be mined, reclaimed, treated and sold at a profit. It is noted that the term “ore” is a relative term in that a material may be regarded as an ore, i.e., profitable at one point in time and a waste material at another point in time. It is also noted that an assessment of whether a material is an “ore”, i.e., profitable, can also be dependent on the mine from which the material is mined and the capital and operating costs in the mine, including whether the mine is a brownfield or greenfield mine and the metal value being extracted or produced.

[0151] Examples of ores include, but are not limited to, (a) ROM and (b) ROM ore that has been subjected to at least primary crushing or similar or further size reduction after the material has been mined and prior to being sorted. The ore may also include agglomerates of crushed ores. Suitably, the agglomerates include non-ore material.

[0152] The following summary of the invention focuses on copper as one example of a base metal in a base metal-containing sulfidic material. The copper sulfide -containing material may comprise copper containing material that contains primary and secondary copper sulfide-containing minerals such as chalcopyrite (C’uFcSj). enargite (CU3ASS4), tetrahedrite ((Cu,Fe,Zn,Ag)i2Sb4Si3), tennantite (CU12AS4S13), bornite (Cu5FeS4), chalcocite ( 12S), covellite (CuS), emplectite (CuBiS2), or any combination thereof. Suitably, the copper sulfide-containing material is chalcopyrite.

[0153] The base metal-containing sulfidic material may be a waste material, such as tailings or mineralised waste that is uneconomic to process using current processes.

[0154] The base metal-containing sulfidic material may have any base-metal grade, i.e., concentration of copper in the material.

[0155] By way of example, the copper-containing sulfidic material may have an average copper concentration of < 1.5% by weight (wt.%), typically < 1.2 wt.%, and more typically < 1.0 wt.%, and more typically < 0.6 wt.%.

[0156] The microbes may be any suitable microbes.

[0157] The microbes may be any microbes that can oxidise ferrous iron and / or sulfur compounds and comprise, but are not limited to, members of the bacterial gcnc ^Acidiihiobacilliis. Leptospirillum, Sulfobacillus and Ferrimicrobium, and the archaeal genera Acidianus, Acidiplasma, Ferroplasma, Metallosphaera and Thermoplasma.

[0158] The microbes may be selected from mesophiles, moderate thermophiles and psychrotolerant or mesophilic or thermophilic (moderate or extreme) bacteria or archaea. The microorganisms maybe acidophilic bacteria or archaea. The microorganisms may be thermophilic acidophiles. A diverse population allows activity across a range of operating conditions, including low pH conditions, high sulfate concentrations, and a wide temperature range of 5 - 80 °C.

[0159] Typically, the heap leach method includes using an acidic leach solution.

[0160] The term “acidic leach solution” is understood herein to mean any solution that contains acid that is capable of leaching a base metal from a base metal -containing sulfidic material.

[0161] The solution may be obtained from any suitable source or combination of sources. One source may comprise water from other sources on site such as mine site run-off, tailings dam solutions and reclaim, acid mine drainage liquors, mine water, bleed streams and other water sources both associated and not associated with the mining operations. Another source may be a raffinate from a solvent extraction circuit for recovering copper from a pregnant leach solution.

[0162] An example of a suitable acid is H2SO4.

[0163] The acid may be any other suitable mineral acid or a suitable organic acid.

[0164] From a terminology perspective, “pregnant leach solution” is a generally well-understood term for a base metal -containing leach solution that is collected from a heap and transferred to base metal recovery. The base metal-containing leach solution may be transferred directly to base metal recovery or stored temporarily in one or more storage units. Other process solutions and make-up chemicals may be added to these storage units. When a base metal-containing leach solution is not transferred to base metal recovery but is instead stored and / or recycled back to the leach step and / or utilised in another part of a leaching operation, the base metal-containing leach solution may also be referred to as an “intermediate leach solution”.

[0165] References to “leach solution” in the specification include situations where the leach solution can also be described as a “pregnant leach solution”, a “raffinate”, a “base-metal depleted leach solution”, a “processed leach solution” and “reduced sulfate concentration leach solution”.

[0166] The term “base-metal depleted leach solution” may be used to describe a solution that is discharged from base metal recovery. In situations where base metal recovery is solvent extraction, the term “raffinate” may be used instead of “base-metal depleted leach solution” to describe a solution that is discharged from solvent extraction. Typically, “base-metal depleted leach solution” and “raffinate” are supplied to the heap as an acidic leach solution, noting that other process solutions and make-up acid and water, etc. may be added. The pregnant leach solution may be processed to control the concentration of sulfate to be within certain levels after recovering the base metal from the pregnant leach solution to ensure that microbial growth in the heap is not overly affected.

[0167] As noted above, copper sulfide-containing material is oxidized in a heap leach process. Oxidants, such as ferric ions, and acid, such as sulfuric acid, are consumed during oxidation in a heap leach process, and metal dissolution rates will decrease unless they are replenished.Under aerobic conditions, microbes regenerate ferric ions and acid and generate heat through biological oxidation of ferrous ions (such as from pyrite FcS2or chalcopyrite CuFcSj) and sulfur compounds (including elemental sulfur), as follows:

[0168] 2Fe2++ 2H++ 0.5O2- 2Fe3++ H2O

[0169] 2S + 3O2+ 2H2O 2H2SO4

[0170] The sulfur compounds may be derived from oxidation of sulfide minerals (such as pyrite or copper sulfide minerals) or as an addition (such as elemental sulfur, purchased or derived from a biological sulfate reduction process) from any source, such as cleaner scavenger tailings from a concentrator circuit.

[0171] Additional material (additives), some of which may include sulfur, may be added to enhance copper dissolution.

[0172] Examples of such additives include sulfur-containing inorganic compounds such as thiosulfate or polythionates or polysulfides, or sulfur-containing organic compounds such as thiourea or other thiocarbamides.

[0173] The additional material (additives) may be added to the base metal-containing sulfidic material before the heap is formed, as the heap is being formed, or after the heap has been formed, or to the leach solution.

[0174] The additional material (additives) may be sulfide containing additives - such as pyrite, with the sulfide containing additives obtained from any suitable source, such as cleaner scavenger tailings from a concentrator circuit, and noting that typically the copper sulfide -containing material in the copper-containing material contains pyrite. The material from the concentrator circuit may also contain residual flotation reagents.

[0175] The additional material (additives) may be elemental sulfur containing additives - such as elemental sulfur derived from a biological sulfate reduction process.

[0176] Another additive may be silver. Silver may be mixed with the metal-containing sulfidic material within a defined concentration range. Typically, the defined concentration range is less than 2 g Ag / kg Cu.

[0177] The silver may be dispersed in a form on the surface of the base metal-containing sulfidic material.

[0178] The silver may be added during an agglomeration step to form agglomerates.

[0179] The added silver concentration in the agglomerates may be less than 2 g silver per kg copper in the ore in agglomerates.

[0180] The added silver concentration in the agglomerates may be less than 1 g silver per kg copper in the ore in agglomerates.

[0181] The added silver concentration in the agglomerates may be less than 0.5 g silver per kg copper in the ore in the agglomerates.The added silver concentration in the agglomerates may be greater than 0.02 g silver per kg copper in the ore in agglomerates.

[0182] The added silver concentration in the agglomerates may be greater than 0.05 g silver per kg copper in the ore in agglomerates.

[0183] The added silver concentration in the agglomerates may be greater than 0.1 g silver per kg copper in the ore in agglomerates.

[0184] The added silver concentration in the agglomerates may be greater than 0.2 g silver per kg copper in the ore in agglomerates.

[0185] Typically, the heap leach method is a microbially-assisted method.

[0186] The additional material (additives) may be added in any suitable way to the heap.

[0187] For example, the additional material (additives) may be added to the base metal-containing sulfidic material in or at any one or more of:

[0188] (a) a location at which the base metal-containing sulfidic material forms in a mining operation (for example slumped material that forms after a mine bench is drilled and blasted);

[0189] (b) a location where base metal-containing sulfidic material is loaded onto haul vehicles (such as haul trucks or load-haul-dump vehicles) or conveyors or any other transport options;

[0190] (c) as the base metal-containing sulfidic material is being transported from a loading location(s) in the mine to a heap, a stockpile, or an intermediary station, or from the stockpile or the intermediary station to the heap;

[0191] (d) during agglomeration of the base metal-containing sulfidic material;

[0192] (e) as the base metal-containing sulfidic material is being added to the heap;

[0193] (f) at an intermediary station located between the loading location(s) and the heap;

[0194] (g) at an intermediary station located between the stockpile and the heap;

[0195] (h) in a blending operation including blending together the base metal-containing sulfidic material and additional pyrite and then adding the blend to the heap;

[0196] (i) in the stockpile; and

[0197] (j) in the heap, for example, in a leach solution or directly as a separate additive as the heap is being formed or after the heap has been formed, such as to a top of the heap during the heap leaching step.

[0198] For copper sulfide -containing materials:

[0199] (a) during mineral dissolution that releases copper into solution, ferric ions are reduced to ferrous ions, and elemental sulfur and sulfuric acid is produced; and

[0200] (b) microbes, when present under aerobic conditions, oxidise ferrous ions to ferric ions, and oxidise available solid and soluble sulfur compounds to generate sulfuric acid. In summary, sulfur oxidation generates acid and reactions that convert ferrous ions to ferric ions consume acid.The term “solvent extraction and electrowinning (SX / EW)” in the context of copper is understood herein to be a hydrometallurgical process that comprises a solvent extraction step and an electro winning step. The SX step includes an extraction step and a stripping step. The extraction step includes extracting copper from the pregnant leach solution using an organic reagent and the stripping step includes stripping copper from the organic phase using a concentrated acid, typically sulfuric acid, and producing an electrolyte, typically a concentrated copper sulfate solution, which is directed to the EW step.

[0201] The term “electrowinning” is understood herein to mean a process that results in the base metal depositing on a cathode from a base-metal bearing solution by the passage of an electric current using an insoluble anode. The base metal-containing product is typically described as “copper cathode”.

[0202] BRIEF DESCRIPTION OF THE DRAWINGS

[0203] The invention is described below by way of example only with reference to the following figures:

[0204] Figure 1 is a flowsheet of one embodiment of a method and a heap leach circuit for heap leaching a copper-containing sulfidic material in accordance with the invention;

[0205] Figure 2 is a flowsheet of another embodiment of a method and a heap leach circuit for heap leaching a copper-containing sulfidic material in accordance with the invention;

[0206] Figure 3 is a graph of microbe count versus sulfate concentration for microbes in pregnant leach liquor and on solids which shows the effect of solution sulfate concentration on the cell population in solution and on ore solids for a 50 °C moderate thermophile microbes;

[0207] Figure 4 is a graph of microbe count versus sulfate concentration for microbes which shows the maximum bacterial cell concentration versus sulfate concentration for a 60 °C extreme thermophile microbes;

[0208] Figure 5 is a graph of copper extraction versus time with leach solutions having different sulfate concentrations which shows the effect of starting solution sulfate concentrations (first value in the legend) and maximum sulfate operating level (second value) on copper extraction from a primary copper sulfide dominant ore in a column test operated at 60 °C;

[0209] Figure 6 is a graph of % sulfate removal vs pH which shows the percentage change of sulfate removed from a raffinate bleed stream subjected to a pH increase to about 3.8, and

[0210] Figure 7 is a graph of % sulfate removal as a function of process conditions (at a constant pH of3.68).

[0211] DETAILED DESCRIPTION OF FIGURE

[0212] An embodiment of a method and a heap leach circuit for heap leaching a copper-containing sulfidic material in accordance with the present invention is shown in Figure 1.The method includes microbial -assisted heap leaching agglomerates of low-grade copper-containing sulfide material in a heap using an acidic leach solution and producing a pregnant leach solution and recovering copper from the pregnant leach solution.

[0213] A portion of the copper-depleted leach solution from the copper recovery step is processed to reduce the concentration of sulfate ions in the copper-depleted leach solution before being returned to the heap or to another heap to extract more copper.

[0214] In the flow sheet shown in Figure 1, the copper sulfide-containing material is in the form of waste rock having low grades of copper that is in a stockpile 1.

[0215] Currently, these stockpiles are considered too low-grade to be economically processed in flotation and other ore processing systems for recovering copper from copper sulfide-containing ores and concentrates.

[0216] The invention is equally applicable to other base metal -containing sulfidic material, for example, ores and waste materials such as tailings and mineralised waste.

[0217] The copper sulfide-containing material may be material that is considered too low grade to be economically processed for recovering copper by known conventional methods in test work carried out on a section of a mine before being mined (for example by drilling and blasting) and then, after mining, is transferred directly from the mine (without being stockpiled) for processing in primary, secondary and tertiary comminution units 2 and 3.

[0218] The stockpiled waste rock is transported in suitable vehicles, such as haul trucks or front-end loaders, or on conveyor belts to comminution circuits and crushed and milled in primary, secondary and tertiary comminution circuits in steps 2, 3 to the extent required to produce a suitable particle size distribution for the agglomeration unit 4.

[0219] The primary, secondary and tertiary comminution circuits may include single or multiple crushing steps delivering crushed copper-containing sulfide material to single or multiple milling and sizing steps to produce the comminution product stream having a desired particle size distribution for the agglomeration step 4.

[0220] Units 2, 3 may be any suitable combination of gyratory, cone and high-pressure grinding roll (HPGR) crushers for a given material.

[0221] The resultant comminuted copper sulfide-containing material is transferred to the agglomeration unit 4.

[0222] It is noted that the copper sulfide-containing material may be any suitable copper sulfide-containing material having regard to the characteristics, such as particle size distribution, of the tailings-derived concentrate and the requirements for downstream processing of the agglomerates.

[0223] The agglomeration unit 4 may be any suitable agglomeration step using any suitable apparatus, such as agglomeration drums.

[0224] By way of example, required ratios of additives such as pyrite -containing concentrate and the copper sulfide-containing material are added to a mixing device and are mixed together, with orwithout a binder, with or without an acid, and with or without added water, and with or without recycled leach solution.

[0225] The selection of the binder (when used) and the acid and the addition of water and / or recycled leaching solution are a function of several factors, including the copper sulfide-containing feed materials and the required mechanical properties of the agglomerates.

[0226] The applicant has found that it is not essential to use a binder. Agglomerates are formed via surface tension effects and having the correct moisture content is crucial. With some copper sulfide-containing materials, the reaction of concentrated sulfuric acid with silicate gangue minerals in the copper sulfide-containing material may have a physical binding effect.

[0227] The agglomeration unit 4 may include any suitable protocol for adding and mixing the copper sulfide-containing solid feed materials and the binder and water, if required.

[0228] The agglomerates are conveyed and stacked in a heap 5 in preparation for leaching in a leach circuit 6.

[0229] The leach circuit 6 may be any suitable heap circuit that includes, for example:

[0230] (a) a leach solution storage and delivery system to supply acidic leach solution to an upper surface of the heap;

[0231] (b) a pregnant leach liquor collection system for collecting leach liquor containing copper in solution that is extracted from copper sulfide-containing materials in agglomerates in the heap; and

[0232] (c) microbes (such as bacteria or archaea) or other suitable oxidants supply system to oxidise ferrous iron to ferric iron, with the ferric iron being an oxidant in the leaching process. Pregnant leach liquor produced in the heap circuit 6 is processed by solvent extraction unit 9 that extracts copper from the liquor in an organic medium and then strips copper from the organic medium and produces a copper-rich solution (analogous to the previously described concentrated base metal -containing leach solution).

[0233] The copper-rich solution is transferred to an electrowinning unit 10 to recover copper from solution. In some embodiments, the copper-rich solution is directed into a solvent stripping system to further concentrate the solution before being subjected to electrowinning.

[0234] If necessary, part of the electrolyte from the electrowinning step may be purged or recycled to the solvent extraction unit 9.

[0235] The copper cathodes formed in the electrowinning step may be further processed in the circuit or in a separate downstream process to form end products such as billets, ingots, bars, and tubes.

[0236] The copper-depleted leach solution, i.e. a raffinate, from the solvent extraction unit 9 is regenerated and returned to the heap circuit 6 as a reduced sulfate concentration leach solution 12 (via a raffinate storage unit). The regeneration is performed in a leach solution treatment unit 11 in the form of a raffinate treatment unit.The main purpose of the raffinate treatment unit 11 is to ensure that the regenerated leach solution has a copper sulfate concentration that does not significantly impact on the microbial activity during the heap leaching step. In one embodiment, the treatment involves one or more of the following steps:

[0237] (i) controlling the pH of the raffinate to range from 10-11 for example by adding lime to precipitate magnesium as magnesium hydroxide;

[0238] (ii) controlling the pH of the raffinate to range from 3 to around 3.8 for example by mixing calcium carbonate with the raffinate to precipitate metals and sulfate in the form of calcium sulfate and metal hydroxides (oxides / oxyhydroxides); and

[0239] (iii) controlling the pH of the raffinate to range from 1.8 to around 2.8 to induce precipitation of sulfate-containing compounds including jarosite and / or alunite.

[0240] The raffinate treatment unit can include a heater to control the temperature of the raffinate. The precipitated sulfide-containing solids are separated from the raffinate and impounded in step 14 into a waste storage unit such as a tailings dam. These solids may be separated using any known dewatering techniques such as thickening and filtration.

[0241] A bleed 15 may be used to reduce the total amount of sulfate entering the raffinate treatment unit 11. The bleed discharges a part of the copper-depleted leach solution from the solvent extraction unit 9 to waste storage 14.

[0242] The raffinate stream transferred to the raffinate treatment unit 11 may be supplemented in with a make-up fluid stream of water or acid, suitably sulfuric acid, from source 15 A.

[0243] Figure 2 illustrates a second process flowsheet in which the stream from bleed 15 is processed by the raffinate treatment unit 15 to reduce its sulfate concentration instead of the main raffinate stream. The unprocessed main raffinate stream would be stored in a raffinate storage unit before being returned to the heap 6 the reduced sulfate concentration leach solution 12. The bleed stream can be processed according to previously described step (i)-(iii) to reduce the amount of sulfate in this stream. This provides an alternative approach to reducing the overall concentration of sulfate in the leach solution being returned to the process. In this example, the processed bleed stream is reintroduced into the main raffinate stream to reduce its sulfate concentration to a predetermined threshold limit, which in this example was determined to be around 120 g / L. This avoids the need for the main raffinate stream to be continuously processed.

[0244] In another embodiment, the bleed 15 was treated by increasing its pH from 1.5-2.0 to about 3.5-3.8 to reduce its sulfate concentration. Figure 6 reveals that the increase in pH of the bleed 15 from 3.2 to 3.7 has an associated increase in % sulfate removal from 55% to 85%.

[0245] It can be appreciated that the same pH treatment can be applied to the main raffinate stream. Beneficially, the bleed stream also provides an alternative means to control the iron content in the raffinate.The trials performed by the Applicant has shown that sulfate control can be readily achieved by deploying the described pH adjustment (neutralisation) technique. Table 1 below exemplifies performances achievable under non-optimised conditions (i.e. pH of 3.68 with a 2:1 recycle sourced from a thickener underflow).

[0246] Table 1: Results obtained from non-optimised neutralisation pilot test

[0247]

[0248] The raffinate bleed containing 120 g / L SO4 was evaluated via a High Density Sludge neutralisation process, and it was able to remove 83% of sulfate from the feed raffinate, producing thickener underflow slurry with up to ~ 57 % w / w solids. This slurry was easily pumpable (< 7 Pa yield stress) and filterable.

[0249] It was also observed that operating the heap at as high a sulfate tenor as possible promotes slightly greater relative sulfate removal, thereby reducing the bleed volume requirement. Figure 7 compares the efficiency of the base case (at pH 3.8, with feed at a ‘normal’ 90 g / L sulfate level) and upset conditions in removing sulfate from the feed raffinate. The middle graph in Figure 7 is a tank bypass upset condition and the right side graph in Figure 7 is a high sulfate upset condition.

[0250] The result suggests that the high sulfate upset condition leads to a notable (~6 % absolute) increase in % sulfate removal. This is a higher result than that obtained even at pH 3.8, with feed at the ‘normal’ 90 g / L sulfate level (the left side graph in Figure 7). This is thought to occur because the high sulfate provides a higher driving force for the precipitation process, resulting in a greater extent of reaction.

[0251] Development and design of the method of heap leaching a copper-containing material according to the present invention was motivated by the results of experimental work summarised in Figures 3 to 7.Figure 3 is a graph of microbe count versus sulfate concentration for microbes in pregnant leach liquor and on solids which shows the effect of solution sulfate concentration on the cell population in solution and on ore solids for a 50 °C moderate thermophile microbe. The Figure shows that the cell count for this microbe declined only marginally with increasing sulfate concentration for microbes on solids. There was a steeper decline in cell count for microbes in pregnant leach liquor. This indicates that attachment of microbes on solids are more resistant to sulfate and important to maintain cell count at higher sulfate concentrations.

[0252] Figure 4 is a graph of microbe count versus sulfate concentration for microbes which shows the maximum bacterial cell concentration versus sulfate concentration for a 60 °C extreme thermophile microbe. The Figure shows that this culture was more affected by sulfate concentration than the 50 °C moderate thermophile microbe culture. Nevertheless, the Figure shows that there are still significant cell populations at higher sulfate concentrations.

[0253] Figure 5 is a graph of copper extraction versus time with leach solutions having different sulfate concentrations. The Figure shows the effect of different starting solution sulfate concentrations (first value in the legend) and maximum sulfate operating level (second value) on copper extraction from a primary copper sulfide dominant ore in a column test operated at 60 °C.

[0254] Particularly, the present invention seeks to optimise copper extraction by conditioning the raffinate to have a sulfate concentration that balances copper extraction and microbial cell population. From the experiments performed by the Applicant, the upper limit of this sulfate concentration was assessed to be about 100 g / L (see Figures 3 and 5) which maintains a microbial cell count of 0.8x 107. Advantageously, this approach avoids having to adjust the heap conditions to counteract the effects of sulfate concentration on the microbial population.

[0255] Many modifications may be made to the flow sheets of Figures 1-3 without departing from the spirit and scope of the invention.

[0256] By way of example, whilst the embodiments include steps 1-3 to process waste rock to form the copper sulfide-containing material that is one feed for the agglomeration step 4, the invention is not confined to this combination of steps and the waste rock may be processed in any suitable steps to produce a suitable feed material for the agglomeration step 4.

[0257] In addition, whilst the embodiments are described in the context of recovering copper, it is noted that the invention is not confined to copper and extends to recovering metals such as nickel or zinc or cobalt from waste rock containing at least one of these metals in a metal-sulfide containing material.

[0258] In addition, whilst the embodiments focus on tailings from wet processing plants for copper sulfide-containing ores, the invention also extends to tailings derived from processing ores containing other metals, such as cobalt, nickel and zinc.

[0259] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” orvariations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS1. A method of leaching a base metal from a heap of a base metal-containing sulfidic material from a mine including:processing a leach solution obtained from leaching the heap or another heap of the base metal-containing sulfidic material to remove at least some sulfates in the leach solution and forming a reduced sulfate concentration leach solution, with the leach solution being produced after a base metal recovery step; andsupplying the reduced sulfate concentration leach solution to the heap or to another heap of a base metal-containing sulfidic material and acid leaching a base metal from the base metal-containing sulfidic material.

2. The method defined in claim 1 includes:carrying out a microbial-assisted leach of the base metal-containing sulfidic material in the heap with the reduced sulfate concentration leach solution,collecting a base metal-containing pregnant leach solution from the heap, andrecovering the base metal from the pregnant leach solution.

3. The method defined in claim 1 or claim 2 includes a multi-stage leaching process that comprises:a first stage of non-microbially assisted leaching of the base metal-containing sulfidic material and producing a first pregnant leach solution, anda second stage of microbially-assisted leaching of the base metal-containing sulfidic material using the first pregnant leach solution and producing a second pregnant leach solution.

4. The method defined in claim 1 wherein the processing step includes reducing the sulfate concentration by removing a predetermined amount of sulfates fromthe leach solution produced after a base metal recovery step.

5. The method defined in claim 1 wherein the processing step includes reducing the sulfate concentration by removing the sulfates in the leach solution or as a precipitate from the leach solution produced after a base metal recovery step.

6. The method defined in any one of the preceding claims wherein the processing step includes controlling the sulfate concentration in the leach solution produced after a base metal recovery step so that it does not exceed a threshold concentration.

7. The method defined in claim 6 wherein the processing step includes monitoring the sulfate concentration in the leach solution and controlling the sulfate concentration in the leach solution so that it does not exceed the threshold concentration.

8. The method defined in either claim 6 or 7 wherein the processing step includes controlling the sulfate concentration in the leach solution to not exceed 200 g / L sulfate in a leach liquor collected from the heap.

9. The method defined in claim 8 wherein the processing step includes maintaining the sulfate concentration of the leach solution in a range from 20-120 g / L sulfate in a leach liquor collected from the heap.

10. The method defined in any one of the preceding claims wherein the processing step includes reducing the sulfate concentration by electrochemical precipitation of the base metal in an electrolytic cell and producing sulfuric acid.

11. The method defined in any one of claims 1 to 9 wherein the processing step includes reducing the sulfate concentration by reacting the leach solution with limestone and / or burnt lime and / or hydrated lime to precipitate calcium sulfate.

12. The method defined in any one of claims 1 to 9 wherein the processing step includes reducing the sulfate concentration by precipitating jarositeV13. The method defined in any one of the preceding claims, including adjusting the pH of the leach solution to induce precipitation of ferric iron as ferric hydroxide / oxyhydroxide.

14. The method defined in any one of the preceding claims, including controlling the pH of the leach solution to precipitate magnesium as magnesium hydroxide.

15. The method defined in any one of the preceding claims, including separating a part of the leach solution as a bleed stream, and processing the bleed stream to reduce sulfate concentration.

16. The method defined in any one of the preceding claims, including irrigating the heap with the reduced sulfate concentration leach solution at a rate ranging from 1-50 L / h / m2.

17. A base-metal heap leach circuit comprising:a leach solution treatment unit configured to:process a leach solution from a base metal recovery unit, and form a reduced sulfate concentration leach solution; andsupply the reduced sulfate concentration leach solution onto a heap of a base metalcontaining sulfidic material from amine.

18. The base-metal heap leach circuit defined in claim 17 includes a pregnant leach solution storage unit configured to collect and store a pregnant leach solution from leaching of the base metalcontaining sulfidic material.

19. The base-metal heap leach circuit defined in claim 18 includes a base metal recovery unit to process the pregnant leach solution to recover the base metal and produce the reduced sulfate concentration leach solution.

20. The base-metal heap leach circuit defined in claim 19 wherein the base metal recovery unit includes a solvent extraction unit that is configured to produce a base metal-containing electrolyte and an electrowinning unit that is configured to win copper from the electrolyte.

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