OXIDATIVE BIOLEACHING OF BASE METALS

Abstract

An oxidative bioleaching process for leaching a base metal from an ore that includes an ore agglomeration step, an ore stacking step wherein agglomerated ore is stacked to form a heap, a curing step, a rinse step, an inoculation step and a leach step, and wherein, during the ore agglomeration step, the ore is contacted with an acid solution containing nitrate or nitrite thereby to accelerate tthe leaching rate in the leach step.

Claims

1. An oxidative bioleaching process for leaching a base metal from a heap of ore comprising: agglomerating ore to form agglomerated ore; stacking the agglomerated ore to form a heap; curing the agglomerated ore in the heap; rinsing the agglomerated ore in the agglomerated heap; and inoculating the agglomerated ore in the agglomerated heap; and leaching the agglomerated ore in the agglomerated heap; wherein, during the agglomerating step, the ore is contacted with an acidified solution and nitrate or nitrite, to create an oxidative environment prior to the inoculating and leaching steps.

2. The oxidative bioleaching process of claim 1 wherein, during the ore agglomeration step, the solution in contact with the ore has oxidation potentials in the range of >750 mV<1250 mV versus SHE (standard hydrogen electrode) thereby to accelerate a leaching rate in the leach step.

3. The oxidative bioleaching process of claim 1 wherein prior to the agglomerating step, the ore is crushed to a crush size in the range of P80 of 6 mm to P80 of 50 mm.

4. The oxidative bioleaching process of claim 1, wherein the nitrate or nitrite is added in solution or as a solid salt to the ore in the agglomerating step in a range of 1-50 kg/t of ore treated.

5. The oxidative bioleaching process of claim 1, wherein the nitrate is selected from one of more of NaNO.sub.3, KNO.sub.3, HNO.sub.3.

6. The oxidative bioleaching process of claim 1, wherein the nitrite is selected from one of more of NaNO.sub.2, KNO.sub.2, HNO.sub.2.

7. The oxidative bioleaching process of claim 1, wherein the acidified solution in contact with the ore during the agglomerating step has a pH lower than pH 3.

8. The oxidative bioleaching process of claim 1, wherein the acid concentration in the acidified solution is in the range of 2-250 g/L.

9. The oxidative bioleaching process of claim 1, wherein concentrated acid is added to the ore during the agglomerating step to supplement the acid in the acidified solution in a range of 1-100 kg/t of ore treated.

10. The oxidative bioleaching process of claim 1, wherein during the agglomerating step, the acidified solution in contact with the ore contains iron, copper and other dissolved cations and anions as leach product species.

11. The oxidative bioleaching process of claim 1, wherein recycled process solution containing nitrate or nitrite is used in the agglomerating step.

12. The oxidative bioleaching process of claim 1, wherein the moisture content of the agglomerated ore is in the range of 3-20%.

13. The oxidative bioleaching process of claim 1, further comprising scrubbing nitrous gases (NOx) produced in the agglomerating step by passing the nitrous gases through a raffinate solution in the presence of air or oxygen enriched air.

14. The oxidative bioleaching process of claim 1, wherein during the curing step, the ore is cured for a period of 2-50 days.

15. The oxidative bioleaching process of claim 1, wherein the curing step is carried out without aeration.

16. The oxidative bioleaching process of claim 1, wherein the curing step is carried out with aeration at a rate of up to 0.05 Nm.sup.3/hr.Math.t of ore treated.

17. The oxidative bioleaching process of claim 1, wherein during the rinsing step, a raffinate solution is applied to the heap in the range 0.1-2 m.sup.3 solution per ton of ore to displace leached copper and residual nitrate or nitrite salts.

18. The oxidative bioleaching process of claim 1, wherein the rinsing step is carried out without aeration.

19. The oxidative bioleaching process of claim 1, wherein the rinsing step is carried out with aeration at a rate of up to 0.05 Nm.sup.3/hr.Math.t of ore treated.

20. The oxidative bioleaching process of claim 17, wherein a resulting high nitrate or nitrite pregnant leach solution from the rinsing step is treated separately by solvent extraction to recover copper.

21-30. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The invention is further described by way of example with reference to the accompanying drawings wherein;

[0070] FIG. 1 is a flow diagram of a method according to the invention;

[0071] FIG. 2 is illustrates a colour change in a bioleach flask arising during the execution of the method of the invention, a defined period after pyrite addition.

[0072] FIG. 3 is illustrates showing a colour change in a standard bioleach flask a defined period after pyrite addition.

[0073] FIG. 4 is a graph showing the ore composition used in a column leach test using the method of invention.

[0074] FIG. 5 is a graph showing the copper dissolution achieved for leaching of crushed agglomerated ore in a column using the method of invention compared to a standard bioleach.

DESCRIPTION OF PREFERRED EMBODIMENT

[0075] FIG. 1 shows a simplified and generalised dynamic copper heap leaching circuit 10 which can be used for the leaching of primary sulfides, secondary sulfides, oxides and mixed copper ores.

[0076] The dynamic copper heap leach is not limiting and the invention applies to any chosen method of leaching agglomerated ore in a heap, which may be a permanent copper heap leach circuit.

[0077] An ore pre-treatment agglomeration step 12 is conducted in an agglomeration drum where an ore 14 is contacted with sulfuric acid 16, a high nitrate or nitrite raffinate solution 18A and nitrate/nitrite 20 to form an agglomerated ore 22.

[0078] The high nitrate or nitrite raffinate solution 18A has a low chloride concentration as higher chloride levels are detrimental to a microbial inoculum 54 used in a subsequent inoculation step 36D and chloride concentrations of zero, or 0-10 g/L, are required.

[0079] Nitrate or nitrite 20 is added in the agglomeration step in the range of 1-50 kg/t of ore, preferably 1-10 kg/t of ore treated.

[0080] The nitrate or nitrite 20 can be added as a solution or as a solid, to the ore 14 in the agglomeration step 12, before the ore 14 is added in the agglomeration step 12, or after the ore agglomeration step 12 and before the agglomerated ore 22 is stacked in a heap 24.

[0081] The oxidation reaction of the nitrate or nitrite 20, sulfuric acid 16 and the high nitrate or nitrite raffinate solution 18A with the ore 14 is exothermic and thus increases the temperature of the agglomerated ore 22 once it is stacked in the heap 24. Oxidation of 14.5 kg/t of pyrite (1.4% pyrite content in the ore) generates 21 watts per ton of ore and will increase heap temperatures above ambient by 10-30 C., depending on heat losses by convection and radiation from the heap surfaces, and heat loss from moist air leaving from the top of the heap and drainage solution leaving from the bottom of the heap.

[0082] In a standard operation, the contact leach solution (high nitrate or nitrite raffinate 18A) contains a copper concentration of 0.5 g/L, Fe(T) of 2 g/L with other dissolved species from the circuit. These concentrations are not limiting and depend on the plant conditions and composition of the ore being treated.

[0083] During agglomeration 12, nitrogen oxide (NO.sub.x) 26 may be produced and therefore the ore agglomeration step 12 includes provision for the capture of evolved acid fumes of the volatile oxide gases of nitrogen through scrubbing. The NOx gasses 26 are contacted with raffinate or process water 28 with air or oxygen enriched air 30 in a gas scrubbing step 32 to form a scrubber raffinate solution 34. The NO gas evolved is oxidised to NO.sub.2 gas. The NO.sub.2 gas produced is highly soluble in the raffinate solution or process water 28 used in the gas scrubbing step 32.

[0084] The scrubber raffinate solution 32 is combined with a high nitrate raffinate from a subsequent rinse step 36C to produce a high nitrate or nitrite raffinate 18 which is then recycled to the agglomeration step 12 as the high nitrate or nitrite raffinate solution 18A, or to the heap 24, as high nitrate or nitrite raffinate 18B, to be used in the rinse step 36C. The recycling of high nitrate raffinate solution 18A for ore agglomeration decreases the consumption of nitrate and nitrite salts.

[0085] It is necessary that adequate Personal Protective Equipment (PPE) should be worn by the operators to avoid excessive exposure and health risks.

[0086] During agglomeration 12 the reactants are mixed with the ore 14 and the moisture content of the agglomerated ore 22 is in the range of 3-20% moisture, preferably 6-12% moisture.

[0087] The agglomerated ore 22 is transferred and stacked 36A to form a heap 24. The stacked ore 36A is allowed to cure in a curing step 36B for 2-50 days, preferably 5-20 days, or more preferably 2-10 days. During the curing step 36B, forced aeration 40 is applied from the bottom of the heap 24 at the rate of 0-0.04 Nm.sup.3/hr.Math.t.

[0088] In a further embodiment the curing step 36B is carried out without aeration.

[0089] Once curing is completed, a high nitrate or nitrite raffinate solution 18B from a high nitrate raffinate pond 42 is applied on top of the heap 24 in the rinse step 36C. An irrigation volume of high nitrate or nitrite raffinate solution 18B within the range 0.1-2 m.sup.3 per ton of ore is applied in the rinse step 36C. The high nitrate or nitrite raffinate solution 18B displaces residual nitrate, or nitrite salts, and copper leached in the cure step 36B. A resulting high nitrate or nitrite drainage solution, in the form of a pregnant leach solution 1 (PLS1) 44, from the rinse step 36C, is collected in PLS 1 pond 46. The PLS1 44 is high in copper, nitrate or nitrite salts and other cationic and anionic species from the leached ore. The PLS 1 pond 46 feeds a solvent extraction (SX1), plant 48A where copper is concentrated to produce an advance electrolyte solution 50 which is treated to recover copper by means of electrowinning in the electrowinning tankhouse 52. The copper stripped PLS1 44 constitutes a high nitrate or nitrite raffinate solution and is mixed with the saturated scrubber raffinate 34 to produce a high nitrate or nitrite raffinate 18 which is then collected in the high nitrate or nitrite raffinate pond 42. The high nitrate or nitrite raffinate solution 18 is recycled to the ore agglomeration step 12 as a high nitrate or nitrite raffinate solution 18A, and to the rinse step 36C as a high nitrate or nitrite raffinate solution 18B.

[0090] During the rinse step 36C, forced aeration 40 is applied from the bottom of the heap 24 at the rate of 0-0.05 Nm.sup.3/hr.Math.t.

[0091] In a further embodiment the rinse step 36C is carried out without aeration.

[0092] Following the rinse step 36C to displace leached copper and recover excess residual nitrate or nitrite salts in the heap 24 an inoculation solution 54 is applied to the heap 24 in an inoculation step 36D. Inoculation is necessary in order to achieve a microbial cell population in the heap in the range of 110.sup.9 to 510.sup.11 cells/t of ore.

[0093] During the inoculation step 36D, forced aeration 40 is applied from the bottom of the heap 24 at the rate of 0-0.05 Nm.sup.3/hr.Math.t.

[0094] By way of example, the microbial strains that can be used in the inoculation step are shown in the following table listed broadly into temperature ranges and whether bacteria or archaea, the latter dominant at temperatures above 60 C.; reference: HIGH TEMPERATURE HEAP LEACHING OF CHALCOPYRITE: METHOD OF EVALUATION AND PROCESS MODEL VALIDATION; D. W. Dew, G. F. Rautenbach, I. J. Harvey, J. S. Truelove, and R. P. van Hille; Conference Proceedings; The Southern African Institute of Mining and Metallurgy, Percolation Leaching: The status globally and in Southern Africa 2011.

TABLE-US-00002 Tem- Opti- per- Opti- mum ature mum pH Temper- Growth Growth Operating ature Range pH Range ( C.) ( C.) BACTERIA Acidithiobacillus ferrooxidans 1.7-2.5 1.3-4.5 30-35 10-37 Acidithiobacillus thiooxidans 2.0-3.0 0.5-5.5 28-30 10-37 Leptospirillum ferrooxidans 1.6-2.1 1.5-4.0 30 20-35 Leptospirillum ferriphilum 1.4-2.1 1.4-4.0 37 30-45 Sulfobacillus disulfidooxidans 1.5-2.5 SG1 0.5-5.0 35 4-40 Acidithiobacillus caldus 2.0-2.5 1.0-3.5 45 32-52 Acidithiomicrobium SAR 1.7-2.5 1.0-3.5 50 40-55 Acidimicrobium ferrooxidans 1.7-2.5 1.0-3.5 50 40-55 Sulfobacillus 1.7-2.4 1.0-3.0 50 28-60 thermosulfidooxidans Sulfobacillus MAD 2.0-3.0 1.0-4.0 50 38-55 ARCHAEA Ferroplasma acidiphilum 1.7 1.3-2.2 37 15-45 Sulfolobus metallicus 1.7-2.0 1.0-5.5 70 65-78 Metallosphaera sedula 2.0 1.0-4.5 75 50-80 Metallosphaera hakonensis Closely related to Metallosphaera sedula

[0095] The inoculation step 36D is followed by the leach step 36E. A low nitrate or nitrite raffinate solution 58 is applied to the heap 24 and is collected in the drainage at the bottom of the heap 24 as a low nitrate or nitrite pregnant leach solution 2 (PLS2) 60 which contains a high copper concentration and other cationic and anionic species from the leached ore.

[0096] During the leach step 36E, forced aeration 40 is applied from the bottom of the heap 24 at the rate of 0-0.05 Nm.sup.3/hr.Math.t.

[0097] The PLS2 60 is collected in a PLS 2 pond 62 which feeds the solvent extraction (SX2) plant 48B where copper is concentrated to produce an advance electrolyte solution 50 which is treated to recover copper by means of electrowinning in the electrowinning tankhouse 52. The resulting PLS 2 60 stripped of copper and enriched with acid constitutes the low nitrate or nitrite raffinate solution 58 and is collected in a low nitrate or nitrite raffinate pond 64. The raffinate 58 is recycled to the leach step 36E in the heap 24.

[0098] In the low nitrate or nitrite raffinate pond 64, fresh water 66 may be added as make-up water in order to compensate for water loss in the heap leach circuit due to moisture in the leached residue ore, spillages, leakages and evaporation. Sulfuric acid 68 is added in order to replace the acid that is consumed by the ore in the leach step 36E.

[0099] As leaching proceeds, the copper concentration decreases in the PLS2 solution 60 and the resulting intermediate leach solution (ILS) 70, is routed to an ILS pond 72 where the concentration is normally in the range of 1-5 g/L copper. Besides being used as a contact solution in the agglomeration step 12, the intermediate leach solution 70, from the pond 72, can be pumped to the heap 24 where it can be used as an irrigation leach solution in leach step 36E. The resulting drainage solution from the heap 24 is sent to the PLS2 pond 60 or to the ILS pond 72, depending on the copper concentration. Typically the copper concentration in the PLS 2 is >5 g/L Cu, however this is not limiting. The ILS pond 72 also provides capacity for additional storage in the event of disruption to downstream treatment of the PLS2 solution 60 in the SX step 48.

[0100] The ore agglomeration step 12 ensures that the solution in contact with the ore 14 achieves high oxidation potentials of 750-1200 mV versus standard hydrogen electrode (SHE) resulting in the rapid oxidation of copper sulfide minerals and iron sulfide minerals including pyrite once the ore is stacked 36A in a heap 24 and during the cure step 36B. The high solution oxidation potentials achieved (750-1200 mV vs. SHE) increase the sulfide mineral oxidation rate increasing the rate of metal dissolution and reducing the heap leach cycle time compared to conventional heap bioleaching methods for the treatment of base metal ores.

[0101] The present invention is particularly suitable for the treatment of ores containing chalcopyrite and secondary copper sulfide minerals (for example enargite, bornite, chalcocite and covellite). The pre-treatment step activates the mineral sulfides prior to the acid leaching step, thereby increasing the rate of metal dissolution compared to conventional acid bioleaching. Copper or other base metals such as nickel and cobalt can be recovered from the pre-treated ore by bioleaching according to the method of the invention.

[0102] The tests conducted confirm the oxidation of pyrite occurring with or without aeration. The oxidation of sulfide minerals without aeration is a unique feature of the proven method of the invention resulting in heat generation and increased heap temperatures without the need for high rates of aeration limited by the heap permeability to fluid flow. Process operators can take advantage of the high oxidising environment to reduce the overall air supply to the process thereby simplifying operation and lowering operating costs.

[0103] The presence of nitrate or nitrite in copper solutions can have a detrimental effect on the SX plant operation as it degenerates the extractants through nitrification. It is therefore necessary to add a proprietary reagent modifier to the organic phase to prevent excessive reagent degradation.

Pyrite Oxidation Tests

[0104] Two flat-bottomed flasks were used to determine pyrite oxidation under bioleach conditions according to the method of the invention, hereinafter referred to as Nitro-bioleach and standard bioleach conditions. The composition of the sample used for the test-work is presented in Table 1 below. The sample contained 97% pyrite with minor impurities of sulfates and copper sulfides.

TABLE-US-00003 TABLE 1 Pyrite sample composition Mineral Mass (%) Formula Composition Cu Sulphides 0.30 Pyrite FeS.sub.2 97.26 Other Sulphides 0.14 Silicates 1.68 Sulphates 0.28 Others 0.35 Total 100.00

Procedure:

[0105] 22 L flat-bottomed flasks were filled with 400 mL of chloride raffinate of known chemical composition. Known amounts of sodium nitrate and sulfuric acid were added to the Nitro-bioleach flask. Only acid was added to the standard bioleach flask as presented in the table below. At time T=0, pyrite was added to the mixture which was cautiously mixed and allowed to settle. A 15 ml volume of the solution was immediately taken from both flasks and analysed for Fe(T), Fe.sup.2+, H.sup.+, SO.sub.4.sup.2 and Eh respectively. The solution volume taken out was replaced by an equivalent amount of bioleach raffinate. The flask mixture was neither aerated nor agitated during the test period.

TABLE-US-00004 TABLE 2 Quantities of reagents used during the test Nitro-bioleach Bioleach Raffinate type Bioleach Bioleach Volume Raffinate (ml) 400 400 Nitrate Sodium Added (g) 24 0 Acid Added (ml) 48 48 Pyrite (g) 50 50

TABLE-US-00005 TABLE 3 Composition of Bioleach Raffinate CuT Fe(T) Fe+2 H+ CL Nitrate SO4 (g/l) (g/l) (g/l) (g/l) (g/l) Density (kg/t) (g/l) Raffinate 0.56 1.21 0.52 8.2 1.5 1.10 0 90 Bioleach

[0106] The sampling was repeated once a day for the next 4 days that followed. The results, tabulated in Table 3 below, show a slight decrease in Eh, Fe.sup.3+ and H.sup.+ while the Fe(T) and Fe.sup.2+ both increased for both flask contents once the pyrite was added.

[0107] However, after a day, there was a sharp increase in both Fe(T) and SO4.sup.2 content in the flask under Nitro-bioleach conditions while in the flask under standard bioleach conditions, there was only a slight increase in both Fe(T) and Fe3+ respectively. The potential increased from 692 mV to 1017 mV in the Nitro-bioleach flask within the first 10 minutes of nitrate addition. After 15 minutes of adding pyrite, the color of the raffinate in the Nitro-bioleach flask changed from dark brown as presented in FIG. 2 (left) to a clear yellowish brown solution as shown in FIG. 2 (right). The solution potential dropped slightly from 1017 mV during the days that followed and stabilized just above 950 mV until the end of the test. A steady increase in the concentration of Fe(T) was recorded in the days that followed, which existed mainly as Fe.sup.3+ indicating that the environment was highly oxidizing even in the absence of forced aeration. The acid concentration increased from 147 g/L to 277 g/L and then to a maximum of 308 g/L at the end of the test.

[0108] In the flask under standard bioleach conditions (FIG. 3), no significant change was observed, and the solution concentration remained almost constant with minor changes in the species, see Table 5. No color change was observed in this flask as compared to the FIG. 2 flask.

TABLE-US-00006 TABLE 4 Chemical analysis results in Nitro-bioleach flask Time FeT Fe2+ Fe3+ H2SO4 SO4 Eh (hrs) Comments (g/L) (g/L) (g/L) (g/L) (g/L) (mV) Initial solution 1.6 0.28 1.3 8.6 109 691.9 Initial solution + acid 1.5 0.26 1.2 147 252 673.7 Initial solution + acid + 1.5 0.26 1.2 147 252 708.4 Nitrate Initial solution + acid + 1.4 0.07 1.3 277 163 1017 Nitrate, 10 min Later 0.0 Initial solution + acid + 3.5 0.22 3.3 278 167 977 Nitrate + Pyrite 2.85 7.2 0 7.2 283 166 980 23.6 12.2 0.00 12.2 305 160 969 50.0 12.2 0 12.2 305 160 978 70.4 13.0 0 13.0 300 159 978 98.1 14.4 0 14.4 308 158 976

TABLE-US-00007 TABLE 5 Chemical analysis results in standard bioleach flask Time FeT Fe2+ Fe3+ H2SO4 SO4 Eh (hrs) Comments (g/L) (g/L) (g/L) (g/L) (g/L) (mV) Initial solution 1.58 0.28 1.29 8.6 109.0 692 Initial solution + acid 1.51 0.26 1.25 147.0 251.9 674 0.0 Initial solution + acid + Pyrite 1.51 0.26 1.25 147.0 251.9 708 24.5 1.61 0.28 1.32 151.0 255.0 696 50.9 1.62 0.28 1.34 151.7 257.7 707 71.4 1.73 0.52 1.21 151.3 230.2 709 99.1 1.57 0.29 1.28 146.9 710

[0109] The performance of the method of the invention (herein referred to as the Nitrobioleach process) was tested by the applicant on a primary copper sample containing 80% chalcopyrite as the main copper source with a copper grade of 0.49% as presented in FIG. 4.

[0110] The sample, received as Run of Mine (ROM), was prepared in order to produce a final sample of P.sub.80 through a series of crushing and blending steps in order get a representative sample.

[0111] The prepared sample was agglomerated with 20 kg/t sodium nitrate (sodium nitrite may be used instead of sodium nitrate) and 12 kg/t sulfuric acid, producing agglomerates with an average moisture content of 8% for the Nitrobioleach process. For the standard bioleach process, similar conditions were used except that no sodium nitrate or nitrite was addedsee Table 6.

TABLE-US-00008 TABLE 6 Conditions of Ore Agglomeration Nitrobioleach Bioleach Particle Size P80 0.75 0.75 Acid dosage kg/t 12 12 Sodium Nitrate or kg/t 20 0 nitrite dosage** Temperature C. 25 25 Aeration rate Nm3/hr .Math. t 0.04 0.04 Irrigation rate L/m2 .Math. hr 2 2 Irrigation Time hrs/day 24 24 Cure Period days 10 10 Column Height m 1 1 Column Diameter cm 16 16 **Nitrate dose: 20 kg/t NaNO.sub.3 added as solid during ore agglomeration or dissolved in known volume of raffinate solution that was used for ore agglomeration. Sodium nitrite may be used instead of sodium nitrate.

TABLE-US-00009 TABLE 7 1. Ore agglomeration and irrigation raffinate solution composition Nitrate CuT FeT Fe +2 H+ Cl or nitrite SO4 (g/l) (g/l) (g/l) (g/l) (g/l) Density (kg/t) (g/l) Solution 0.56 1.21 0.52 8.2 2 1.26 0 90 Raffinate

[0112] The agglomerated ore was loaded into respective 1 m160 mm ID duplicate columns and allowed to cure for 10 days. Aeration of 0.04 Nm.sup.3/hr.Math.t was used during agglomerated ore curing and ore irrigation for both processes. The column tests were completed in duplicate.

[0113] Upon the completion of the curing step, a contact solution with a solution composition as presented in Table 7 above was used as irrigation solution for irrigation of ore loaded in the columns. The PLS was analysed for Cu.sup.2+, Fe(T), Fe.sup.2+ and acid respectively.

[0114] A maximum of 50% copper dissolution was achieved in the Nitrobioleach columns compared to 15% copper dissolution in the standard bioleach process for the same period of operation (see FIG. 5).

[0115] The results obtained demonstrate that the Nitrobioleach process creates a higher oxidising environment in the heap in the ore agglomeration and cure steps which facilitates the dissolution of metals of interest at a faster rate and thereby reduces the leach cycle time.