Process for Gold and/or Platinum Group Metals Heap Leaching with Lime

Abstract

Process for gold and/or platinum group metals heap leaching comprising irrigating a heap with an irrigation solution containing sodium cyanide for leaching gold and/or platinum group metals from a gold and/or platinum group metals containing ore. A lime reagent is added by feeding a fine particle lime suspension containing lime particles in an aqueous phase in an irrigation solution.

Claims

1. Process for gold, and platinum group metal, heap leaching comprising the steps of: (i) Stacking gold and/or platinum group metals containing ore on a collection apparatus provided for retaining solid material and collecting a liquor, said stacking of gold and/or platinum group metals containing ore being provided to form a heap, said ore optionally containing dry quicklime added prior to or during heap stacking, (ii) Irrigating said heap with an irrigation solution containing sodium cyanide by means of an irrigation apparatus containing an irrigation pipe and irrigation emitters for irrigating said heap and leaching gold and/or platinum group metals from said gold and/or platinum group metals containing ore, forming a gold and/or platinum group metals pregnant liquor containing complexed gold and/or platinum group metals-cyanide dissolved in a liquid phase, (iii) Collecting said gold and/or platinum group metals pregnant liquor, (iv) Feeding a purification step where purification apparatus is fed with said gold and/or platinum group metals pregnant liquor and separating said complexed gold and/or platinum group metals-cyanide from a barren solution containing said liquid phase substantially depleted in gold and/or platinum group metals-cyanide and (v) Feeding said irrigation apparatus with said barren solution, wherein said process further comprises a lime reagent addition by forming a dispersed fine particle lime suspension containing Ca(OH).sub.2 particles in an aqueous phase where the Ca(OH).sub.2 particles have a d.sub.97 smaller than or equal to 50 μm and have a d.sub.50 lower than or equal to 10 μm, said lime reagent addition being fed upstream or at the irrigation apparatus and downstream of the collecting step of said gold and/or platinum group metals pregnant liquor and is dosed to achieve a Ca(OH).sub.2 concentration in the irrigation solution in the range of 0.1 to 3.5 g/dm.sup.3 of irrigation solution, expressed as grams of Ca(OH).sub.2 per litre.

2. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein said lime reagent in said lime reagent addition is a Ca(OH).sub.2 suspension having a concentration comprised between 10 and 550 g CaO equivalent/dm.sup.3, said lime reagent addition being made directly or via dilution tanks.

3. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein said lime reagent is a powdery hydrated lime added into an irrigation circuit of the irrigation apparatus, forming, directly or via dilution tanks, when reaching the irrigation emitters, said dispersed fine particle lime suspension.

4. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein said lime reagent is a powdery dry quicklime added into an irrigation circuit of the irrigation apparatus, directly or via dilution tanks, or after passing through a hydrocyclone to achieve a suitable particle size cut-off, forming when reaching the irrigation emitters, said dispersed fine particle lime suspension.

5. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein the said fine particle lime suspension contains a dispersant and/or a viscosity modifier.

6. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein said purification step comprises at least a step of passing the gold and/or platinum group metals pregnant liquor through at least one carbon adsorption column during which gold and/or platinum group metals-cyanide adsorb on a carbon-based adsorption material and the barren solution is collected with a flow-through fraction.

7. Process for gold and/or platinum group metals heap leaching according to claim 6, wherein gold and/or platinum group metals-cyanide adsorbed on the carbon-based adsorption material is further eluted by a solution containing sodium/potassium hydroxide and cyanide to recover a gold and/or platinum group metals-containing fraction for further gold and/or platinum group metals refining steps.

8. Process for gold and/or platinum group metals heap leaching according to claim 7, further comprising a regeneration step of the carbon-based adsorption material.

9. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein said barren solution circulating in said irrigation apparatus is further enriched by a sodium cyanide solution.

10. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein the irrigation solution feeds the heap by exiting the irrigation emitters, said irrigation emitters being selected from the group consisting of sprinkling emitters or dripper emitters.

11. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein the lime reagent addition is fed at an irrigation pipe in which the barren solution circulates before reaching the irrigation emitters.

11. Process for gold and/or platinum group metals heap leaching according to claim 1, wherein the lime reagent addition is fed upstream or at the irrigation apparatus and downstream of the collecting step of said gold and/or platinum group metals pregnant liquor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] FIG. 1 is a schematic chart showing the gold and/or platinum group metals heap leaching principle using the fine particle lime suspension containing lime particles in an aqueous phase according to the present invention.

[0097] FIG. 2 is a schematic representation of alternative irrigation emitters.

[0098] FIG. 3 is a graph showing the metal ion solubility depending on pH.

[0099] FIG. 4 is a series of 3 graphs showing the gradient in the heap of respectively Ca(OH).sub.2, pH and cyanide fraction with respect to the depth from surface.

[0100] In the drawings, the same reference numbers have been allocated to the same or analogue element.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0101] FIG. 1 is a schematic chart showing the gold and/or platinum group metals heap leaching principle in a gold and/or platinum group metals heap leaching plant using the fine particle lime suspension containing lime particles in an aqueous phase according to the present invention.

[0102] The process for gold, or platinum group metals, heap leaching according to the present invention comprises a first step of stacking gold and/or platinum group metals containing ore on a collection means 2 provided for retaining solid material and collecting a liquor, said stacking of gold and/or platinum group metals containing ore being provided to form a heap 1, said ore optionally containing dry quicklime added prior to or during heap stacking

[0103] Upstream the gold and/or platinum group metals heap leaching plant, an ore processing and handling systems that may include crushing, conveying and ore stacking equipment or dump trucks can be present. Further the ore processing and handling systems can include ore agglomeration means (if needed) to stabilise the finer size fractions of the gold and/or platinum group metals containing ore.

[0104] The collection means 2 is preferably a lined heap leach pad and impermeable drainage collection apron to provide containment and recovery of the pregnant leach solution (liquor). In this case, the collection means 2 includes preferably a coarse drainage layer with network of solution collection pipes (not shown in FIG. 1) installed on the impermeable draining collection apron and an external solution collection system (not shown in the diagram)to route pregnant solution to the pregnant solution pond 4.

[0105] The gold and/or platinum group metals heap leaching plant comprises a solution irrigation means or system (9, 10, 11 & 12) to apply the cyanide leach solution to the surface of the heap 1 including pumps 10, irrigation pipe(s) 9, 11 and irrigation emitters, i.e. solution distribution systems and emitters 12.

[0106] The heap 1 is irrigated with an irrigation solution containing sodium cyanide for leaching gold and/or platinum group metals from said gold and/or platinum group metals containing ore, forming a gold and/or platinum group metals pregnant liquor 3 containing gold and/or platinum group metals-cyanide dissolved in a liquid phase.

[0107] The gold and/or platinum group metals pregnant liquor 3 is collected in a pregnant solution pond 4. The pregnant solution pond 4 is connected by means of piping 5 & 6, typically with a pump 13 to purification means 7. The gold and/or platinum group metals pregnant liquor 3 is fed to a purification step. The purification means are fed with said gold and/or platinum group metals pregnant liquor and separate said complexed gold and/or platinum group metals-cyanide from a barren solution containing said liquid phase substantially depleted in gold and/or platinum group metals-cyanide. The barren solution is fed to a barren pond 8. The irrigation pipe(s) 9, 11 are connected to the irrigation emitters 12, eventually through a pump 10. The barren solution therefore feeds the irrigation means.

[0108] As already noted, the cyanide leach solution flows by percolation through the heap ore bed. Cyanide is mainly added to the solution in the form of sodium cyanide and at concentrations ranging from 200-600 mg/dm.sup.3 (see addition point A). The cyanide addition can be performed anywhere between the barren pond 8 and the irrigation emitters 12 to enrich the barren solution from the barren solution pond 8. The purification step comprises at least a step of passing the gold and/or platinum group metals pregnant liquor 3 from the pregnant liquor barren 4 through piping 5 & 6, typically by means of a pump 13 on the purification means 7. The purification means 7 comprise at least one carbon adsorption column onto which gold and/or platinum group metals-cyanide adsorb on a carbon-based adsorption material and the barren solution (from which gold-cyanide complex has been removed) is sent to the barren solution pond 8. Gold and/or platinum group metals-cyanide adsorbed on the carbon-based adsorption material is further eluted by a solution containing sodium/potassium hydroxide and cyanide, as is known in the art, to recover a gold and/or platinum group metals-containing fraction for further gold and/or platinum group metals refining steps, such as gold and/or platinum group metals electrowinning (not illustrated).

[0109] In the process according to the present invention, a regeneration step of the carbon-based adsorption material is provided.

[0110] Lime reagent addition is performed in this preferred embodiment in location B, by feeding a lime reagent resulting in, or forming, a fine particle lime suspension by the time the particles reach the irrigation emitters containing lime particles in an aqueous phase where the lime particles have a d.sub.97 smaller than or equal to 50 μm, preferably smaller than or equal to 30 μm and have a d.sub.50 lower than or equal to 10 μm, preferably lower than or equal to 5 μm, and more preferably lower than or equal to 3 μm. The lime reagent addition being fed upstream or at the irrigation means (9, 10, 11 & 12) and downstream of the collecting step of said gold and/or platinum group metals pregnant liquor 3 and is dosed to achieve a Ca(OH).sub.2 concentration in the irrigation solution in the range of 0.1 to 3.5 g/dm.sup.3, more particularly in the range of 0.3 to 3.0 g/dm.sup.3, most preferably between 0.35 and 2.5 g/dm.sup.3 of irrigation solution.

[0111] Preferably the fine particle lime suspension contains a dispersant.

[0112] To determine the Ca(OH).sub.2 concentration in the irrigation solution, the first step is to determine the lime demand of the ore, using methods known in the art, including bottle roll tests, supplemented with laboratory column testing if required and determining the proportion of the total lime demand to be added as dry quicklime added to the ore during heap stacking and the proportion of lime demand to be added in the irrigation solution over the leaching period.

[0113] Pilot scale (6 m tall, 30 cm diameter) column test work should preferably be conducted in advance of heap leaching operation to determine the efficacy of the selected lime addition proportions added during stacking relative to that added over time via the irrigation solution.

[0114] The combination of dry quicklime added during stacking and Ca(OH).sub.2 added via the irrigation solution can be used as method of providing a portion of the lime consumption demand (and thus coarse pH control) via dry quicklime addition during stacking and the remaining portion of the lime consumption demand as make-up lime (and thus finetune pH control) as Ca(OH).sub.2 via the irrigation solution.

[0115] A large proportion of the lime demand should be satisfied by lime reagent addition via the irrigation solution, thereby minimizing, or in eliminating, the amount of dry lime reagent co-added with the ore during heap stacking.

[0116] Lime addition is provided via the cyanide-containing irrigation solution to enable self-regulating proximity of lime to cyanide thereby ensuring adequately high pH at the point of cyanide lixiviant contact with gold and/or platinum group metals.

[0117] Based on the expected leaching time (to be determined by test work as is known in the art) required to achieve economically acceptable gold and/or platinum group metals recovery, heap height (i.e. ore mass per square metre of heap surface), irrigation rate, and pH of the pregnant leach solution, the optimal Ca(OH).sub.2 dose in the irrigation solution can be determined.

[0118] As said previously, the Ca(OH).sub.2 concentration in the irrigation solution can be applied in the range of 0.1 to 3.5 g/dm.sup.3, (expressed as grams of Ca(OH).sub.2 per litre) more particularly in the range of 0.3 to 3.0 g/dm.sup.3, most preferably between 0.35 and 2.5 g/dm.sup.3 of irrigation solution. The concentration should be adjusted to maintain the pH of the leach solution, emanating from the heap, above a pH setpoint, preferably 9.5. A process control feedback mechanism can be used for this purpose, as is known in the art, to link the Ca(OH).sub.2 concentration in the irrigation solution to the pH of the pregnant leach solution, while taking into account the lag time represented by the percolation time. The process control mechanism may either operate for an entire heap, sections of the heap to improve the resolution of control.

[0119] Maintaining the heap pH in this manner significantly reduces the negative impacts caused by the pH gradient effects referred to earlier. Cyanidation speciation is, for example, more efficient. This is partly due to improved cyanidation speciation and reduction in copper mineral dissolution in the heap.

[0120] Gangue mineral dissolution is significantly reduced by managing the pH gradient in this manner, thereby inhibiting the precipitation of soluble elements, such as magnesium, at or after the lime addition point (see FIG. 3). Iron sulfide mineral dissolution (as from pyrite) is reduced at thus reducing the iron-cyanide complexation. Silica dissolution is also significantly reduced using the method according to the invention. While silica dissolution is conventionally believed to increase with hydroxyl concentration, the presence and preferably supersaturation of calcium, significantly reduces this effect due to the formation of calcium-silica-hydrates plombierite and tobermorite on gangue minerals, thereby reducing silica dissolution into the solution circuit. Importantly, this does not occur with the use of sodium hydroxide and in the absence of calcium.

[0121] The lower concentration of soluble elements in the solution circuit, also improves the operation of the activated carbon gold and/or platinum group metals-cyanide adsorption recovery process and reduces the formation of precipitates and coating effects on activated carbon.

[0122] Even at low concentration, i.e. below the 1.5 g/dm.sup.3 solubility limit of Ca(OH).sub.2, not all Ca(OH).sub.2 will be dissolved in the irrigation solution, due to slow dissolution kinetics. The Ca(OH).sub.2 reagent is explicitly dosed in fine dispersed particle suspension form, where the particle size of the colloids are smaller than 50 μm and preferably smaller than 20 μm in diameter, to avoid clogging of irrigation emitters.

[0123] It is beneficial to the process that fine Ca(OH).sub.2 particles are present and that Ca(OH).sub.2 is not fully dissolved, as undissolved Ca(OH).sub.2 particles do not directly participate in lime consuming reactions and are carried by solution flow to deeper levels within the heap. Instead it is dissolved Ca(OH).sub.2 that reacts with lime consuming reaction. The unreacted particles, carried downward by percolation flow, will then continuously supplement the dissolved Ca(OH).sub.2 concentration as lime consuming reactions occur and ensure a sufficient pH buffer in order to optimize cyanide speciation and gold and/or platinum group metals recovery.

[0124] The preferable location for adding the fine particle lime suspension into the circuit, is into the solution pumped from the barren pond prior to discharge through the network of irrigations pipes delivering the solution to the heap surface, as illustrated in the diagram. Addition can occur either before, with, or after cyanide addition. A part of the Ca(OH).sub.2 may also be dosed into the pregnant solution pond, to optimize pH conditions for carbon adsorption, although the concentration of Ca(OH).sub.2 to this part of the circuit is limited due to the fact that too high pH conditions (>11) may negatively impact adsorption performance. In addition, particles of Ca(OH).sub.2 may accumulate in the activated carbon columns that may causes excessive carbonation reactions and increase acid consumption during the acid wash cycle applied during activated carbon regeneration.

[0125] FIG. 2 is a schematical representation of an alternative outlay of irrigation emitters, installed in layers within the heap in addition to the surface of the heap.

[0126] The method according to the present invention may also be deployed at multiple irrigation layers of heap lifts by means of several irrigation emitters 12, as illustrated in FIG. 2, rather than at a single location at the top of a heap 1.

[0127] FIG. 4 is a series of 3 graphs showing the gradient in the heap of respectively Ca(OH).sub.2, pH and cyanide fraction with respect to the depth from surface.

[0128] A general phenomenon of heap leaching is that the pH of the pregnant leach solution emanating from the bottom of a heap, has a lower pH than the irrigation solution applied to the surface. The reason for the pH gradient is because of the alkali consuming reactions within the heap. As solution percolates it encounters the cumulative alkali consuming demand of the ore. The further down the heap from the surface, the larger the ore mass it encounters and thus the larger the cumulative alkali consuming demand, thereby resulting in a pH gradient. The series of diagram of FIG. 4 shows an indicative scenario (often encountered at industrial operations) with a decreasing concentration of soluble Ca(OH).sub.2 down the heap, resulting in a pH gradient. This pH gradient also has an impact on the speciation of cyanide, resulting in a significant reduction in the fraction of cyanide available in the desirable, i.e. CN.sup.−, species. The increasingly unfavourable speciation of cyanide, down the heap, is in addition to the overall consumption of cyanide by cyanide consumption reactions. The net result is a decrease in cyanide efficiency and gold and/or platinum group metals recovery.

[0129] As explained earlier, according to the present invention, the concentration of Ca(OH).sub.2 is adjusted to maintain the pH of the leach solution, emanating from the heap, above a pH setpoint, preferably 9.5. Maintaining the heap pH in this manner significantly reduces the negative impacts caused by the pH gradient effects referred to earlier. Cyanidation speciation is, for example, more efficient. This is partly due to improved cyanidation speciation and reduction in copper mineral dissolution in the heap.

[0130] It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the claims.