OXIDATIVE HEAP LEACHING OF BASE METALS

Abstract

A method of leaching copper from a heap of ore which includes an ore agglomeration step, an ore stacking step wherein agglomerated ore is stacked to form a heap, a curing step, a leach step, and a rinse step, wherein, during the ore agglomeration step the ore is contacted with an acidified solution, nitrates or nitrites, and chloride, to create an oxidative environment prior to the leach step.

Claims

1. A method of leaching copper from a heap of ore, the method comprising: agglomerating ore to form agglomerated ore, stacking the agglomerated ore form a heap; curing the agglomerated ore in the heap; leaching the agglomerated ore in the heap; and rinsing the agglomerated ore in the heap; wherein, during the agglomerating step, the ore is contacted with an acidified solution, nitrate or nitrite, and chloride, to create an oxidative environment prior to the leaching step, wherein: a. the ore is selected from a primary copper ore comprising chalcopyrite or secondary copper sulfide minerals; b. wherein during the agglomerating and curing steps, the oxidation potential of the solution in contact with the ore is in the range of >780 mV <1250 mV SHE, thereby increasing the heap temperature as a result of pyrite and copper sulfide mineral oxidation; c. wherein during the leaching step, the heap is irrigated with an irrigation solution which is supplemented with nitrate salts or nitrite salts to maintain the concentration of nitrate or nitrite in the range of up to 20 g/L and wherein the addition of nitrate salts or nitrite salts in the irrigation solution maintains oxidation potentials in the range of >780 mV<1250 mV SHE to enhance the rate of copper dissolution.

2. The method of claim 1, wherein: a. the chloride is added as a solid, in solution with a raffinate solution or as a separate concentrated salt solution in the range of 1-100 kg/t ore; b. the acidified solution comprises sulfuric acid and is added as an acid raffinate solution formed by premixing the sulfuric acid with a raffinate leach solution, as a concentrated acid or as a combination thereof in the range of 1-100 kg/t ore; c. the nitrate or nitrite is added as a solid salt, in solution by dissolving in a raffinate solution or water, as a separate concentrated nitric acid solution or as a combination thereof in the range of 1-50 kg/t ore; and d. wherein the acid raffinate solution is added at a rate determined to achieve an agglomerate moisture content of 3% - 20% moisture.

3. The method of claim 2, wherein the acid raffinate solution contains dissolved iron, copper and chloride.

4. The method of claim 3, wherein the chloride is present at a concentration of 40-160 g/L for primary sulfide ores.

5. The method of claim 1 wherein prior to the agglomerating step, the ore is crushed to a size in the range of P80 of 6 mm to P80 of 50 mm.

6. The method of claim 1, further comprising scrubbing and recovering gases produced in the agglomerating step by passing the gases through a process raffinate solution in the presence of air or oxygen enriched air.

7. The method of claim 1, wherein in the curing step, the agglomerated ore is cured for a period of 5-50 days.

8. The method of claim 1, wherein the curing step is carried out without aeration.

9. The method of claim 1, wherein during the curing step, aeration is applied from the bottom of the heap at a rate of up to 0.02 Nm.sup.3/hr.t of ore treated.

10. The method of claim 1, wherein during the leaching step, the heap is irrigated with an irrigation solution which a. has a chloride concentration of 10 - 230 g/L; and b. has an acid concentration of 2-20 g/L with a pH is less than pH 3.

11. The method of claim 1, wherein the leaching step is carried out without aeration.

12. The method of claim 1, wherein the leaching step is carried out with aeration at a rate of up to 0.02 Nm.sup.3/hr.t of ore treated.

13. The method of claim 1, wherein a resultant pregnant leach solution from the leaching step is treated in a solvent extraction step to recover the leached copper and wherein the solvent extraction step includes at least one copper-loaded organic washing stage to maintain a chloride ion concentration below 50 ppm.

14. The method of claim 1, wherein in the rinsing step, the ore is rinsed with process water to displace and recover acid, chloride, residual copper and residual nitrates or nitrites present in the heap.

15. The method of claim 1, wherein the chloride is selected from one or more of NaCl, KCl, MgCl, AlCL.sub.3, CaCl.sub.2.

16. The method of claim 1, wherein the nitrite is selected from NaNO.sub.2, KNO.sub.2, HNO.sub.2.

17. The method of claim 1, wherein the nitrate is selected from one or more of NaNO.sub.3, KNO.sub.3, HNO.sub.3.

18. The method of claim 1, wherein the acidified solution in contact with the ore during the agglomerating step has a pH of less than 3.

19. The method of claim 1, wherein secondary sulfide materials comprise enargite, bornite, chalcocite, or covellite.

20. The method of claim 2, wherein prior to the agglomerating step, the ore is crushed to a size in the range of P80 of 6 mm to P80 of 50 mm.

Description

DESCRIPTION OF THE DRAWINGS

[0064] The invention is further described by way of example with reference to the accompanying drawings in which:

[0065] FIG. 1 illustrates the mineralogical composition of a sample used in test columns showing the main copper source;

[0066] FIG. 2 is a comparison plot of solution potentials of a pregnant leach solution for the present invention and chloride leach processes;

[0067] FIG. 3 is a comparison plot solution of pH profiles in a pregnant leach solution for the present invention and chloride leach processes;

[0068] FIG. 4 is a comparison plot solution of iron concentration in a pregnant leach solution for the present invention and chloride leach processes;

[0069] FIG. 5 is a comparison plot solution of acid concentration in the pregnant leach solution for the present invention and chloride leach processes;

[0070] FIG. 6 is a comparison plot of net acid consumption between the method of the present invention and chloride processes for the same period of operation;

[0071] FIG. 7 is a comparison plot of copper concentration in the pregnant leach solution for the present invention and chloride leach processes;

[0072] FIG. 8 is a comparison plot of copper extractions showing the effect of different curing times for columns of the present invention;

[0073] FIG. 9 is a comparison plot solution of copper extractions showing the effect of different nitrate or nitrite additions in the present invention;

[0074] FIG. 10 is a comparison plot of copper extractions for the present invention and chloride process columns;

[0075] FIG. 11 shows the dissolution of the different copper species under leach conditions of the invention.

[0076] FIG. 12 illustrates a solution colour change after the addition of pyrite with time using the method of the present invention;

[0077] FIG. 13 illustrates a solution colour change after the addition of pyrite with time using a chloride leach process; and

[0078] FIG. 14 is a flow diagram of the method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0079] FIG. 14 shows a simplified and generalised dynamic copper heap leach circuit 10 according to the invention which can be used for the leaching of primary sulfides, secondary sulfides, oxides and mixed copper ores.

[0080] 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.

[0081] In an agglomeration step 12, ore 14 to be treated is added to an agglomeration drum together with sulfuric acid 16, a leach raffinate solution 18A, chloride 20, nitrate or nitrite 22, and, optionally, sea water 24, to produce an agglomerated ore 28.

[0082] Sulfuric acid 16 is added as a concentrated acid to the ore during the agglomeration step 12 as required to supplement the acid in the solution. The concentrated acid is added in in the range of 1 -100 kg/t, preferably 5 - 20 kg acid/t ore treated. Optionally, sulfuric acid 16 is premixed with the leach raffinate solution 18A to form a high acid raffinate solution that is then added to the agglomeration drum.

[0083] The leach raffinate solution 18A contains a chloride concentration in the range of 10 -230 g/L, preferably 120-160 g/L chloride for primary sulfides.

[0084] The chloride 20 is added in the agglomeration step 12 in the range of 1 - 50 kg/t of the ore 14, preferably 5 - 20 kg chloride salt /t of ore treated.

[0085] The chloride 20 is added to the ore 14 either before the agglomeration step 12 as a solid, in solution in the agglomerating drum as contained in a contact solution, as a solid after the agglomeration step 12 or in both forms (solid and liquid) before the agglomerated ore 28 is stacked (30A) on a heap 30.

[0086] The nitrate or nitrite 22 is added to the agglomeration step 12 in the range of 1 -50 kg/t of the ore 14, preferably 3-20 kg nitrate or nitrite salt/t of ore treated.

[0087] The nitrate or nitrite 22 can be added to the ore 14 either before the agglomeration step 12 as a solid, in solution form in the agglomeration step 12, as a solid after the agglomeration step 12 or in both forms (solid and liquid) before the ore 28 is stacked (30A) on the heap 30.

[0088] The oxidation reaction of the nitrate or nitrite 22, sulfuric acid 16 and the leach solution 18 with the ore 14 is exothermic and thus increases the temperature of the agglomerated ore 28 once it is stacked in a heap. 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.

[0089] In a standard operation, the leach solution 18 contains a copper concentration of 0.5 g/L, Fe(T) of 2 g/L with other dissolved species from the circuit. However, these concentrations can be varied based on the plant conditions and on the nature of the ore 14 being treated.

[0090] If required, and based on availability, an intermediate leach solution (ILS) 26 containing 1- 5 g/L copper can be used instead of the raffinate leach solution 18A during the agglomeration step 12. The slightly higher copper concentration in the intermediate leach solution 26 is beneficial as the copper ions are expected to increase the oxidation capacity in the agglomerated ore 28 during the subsequent curing step 30B in the heap 30.

[0091] During the agglomeration step 12, a combination of gasses 64 (HCI and NO.sub.x) are produced. These gasses 64 are passed through a process water or raffinate solution 32 in the presence of air or oxygen enriched air 36 in a gas scrubbing step 34. NO gas is oxidised to NO.sub.2.

[0092] The resulting scrubber raffinate 38 can be recycled 38A to the agglomeration step 12 to be used as a contact solution for agglomeration or can be used as a heap irrigation solution (38B) for leaching. The use of the recycled scrubbing raffinate 38A for agglomeration leads to a decrease in the amount of fresh nitrate to be added to the ore, thus optimising reagent usage. Tests conducted show that the gasses produced, HCI and NO.sub.x, are highly soluble in the standard contact solution used resulting in an increase in solution potential, >750 mV<1250 mV, due to dissolved oxidants. NO gas evolved is oxidised to NO.sub.2 by the action of oxygen in the gas scrubbing step.

[0093] The generation of NOx and HCl gases during agglomeration demands that the reactants be added in a manner that should inhibit the spontaneous reaction to occur and generate the gases. It is therefore recommended that optimal amounts of nitrate or nitrite should be added to the ore when it exits the agglomeration drum, just before the ore is stacked.

[0094] It is important that adequate Personal Protective Equipment (PPE) is worn by an operator to avoid exposure to the gases.

[0095] in the agglomeration step 12, the reactants (16, 18, 20, 22, 24, 26, 38A) are mixed with the ore 14. The moisture content of the final agglomerated ore 28 is in the range of 3% -20% moisture, more preferably 6% - 12% moisture. The final moisture content of the agglomerated ore 28 should be such that all the important reactions continue taking place during the curing step 30B. If the moisture content is not sufficient, the ore will dry and the curing step 30B will not be effective.

[0096] The agglomerated ore 28 is transferred to a heap 30 for stacking 30A. A system of belts (not shown) can be used to transfer the agglomerated ore 28 to the heap 30.

[0097] The stacked ore 30A in the heap 30 is then subjected to a curing step 30B and is allowed to cure for a period of 2 - 100 days, preferably 30 - 45 days. During the curing step 30B, forced aeration 42 may be applied from the bottom of the heap 30 at a rate determined based on the mineral being treated, preferably 0 - 0.01 Nm.sup.3/hr.t of ore treated.

[0098] Once curing is completed, the leach or raffinate solution 18B from a raffinate pond 44 is applied on top of the heap 30 in a leach step 30C to displace copper leached during the cure step 30B and to continue copper leaching of the agglomerated ore 28 in the heap 30.

[0099] The leach solution 18B percolates through the heap 30 and is collected at the bottom of the heap 30 as a pregnant leach solution (PLS) 46 which contains a high copper concentration and other cationic and anionic species from the leaching of the agglomerated ore 28 in the heap 30.

[0100] The PLS 46 is collected in a pond 48 which feeds a solvent extraction (SX) plant 50 where copper is concentrated to produce an advance solution 52 that is sent to a tank-house 54 for copper electrowinning. The copper stripped PLS solution is a raffinate solution 18 and is collected in the pond 44. The raffinate 18 is recycled (18B) to the heap 30 or is recycled (18A) to the agglomeration step 12.

[0101] In the raffinate pond 44, fresh water 60 may be added as make-up water in order to compensate for volume loss due to moisture retained in residue ore, spillages, leakages and evaporation. Fresh concentrated sulphuric acid 58 is added, as required, in order to replace acid that is consumed in the leach step 30C and to maintain a required acid concentration in the raffinate 18.

[0102] As leaching proceeds in the step 30C, the copper concentration decreases in the PLS 46 and the resulting intermediate leach solution (ILS) 26, is routed to the ILS pond 62 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 26, from the pond 62, can be pumped to the heap 30 where it can be used as a leach solution in step 30C. The resulting solution from the heap 30 is sent to the PLS pond 48 or to the ILS pond 62, depending on the copper concentration. Typically the copper concentration in the PLS 46 is >5 g/L Cu, however this is not limiting. The ILS pond 62 also provides capacity for additional storage in the event of disruption to downstream treatment of the PLS 46 in the SX step 50.

[0103] On completion of the leach step 30C the ore is rinsed in a rinse step 30D using process water 66 to displace and recover acid, chloride, and residual copper and residual nitrates or nitrites present in the heap 30. The rinse solution 68 is directed to the raffinate pond 44.

[0104] On completion of solution drainage from the heap 30 after the rinse step 30D, in a dynamic heap leach circuit, the ore is unloaded from the leach pad and sent to a residue dump.

[0105] The performance of the method of the invention (hereinafter referred to as the “NitrosaL 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. 1. Two versions of the method were tested referred to respectively as NitrosaL 1 and NitrosaL 2, for distinguishing purposes.

[0106] The sample, received as Run of Mine (ROM), was prepared in order to produce a final sample of P.sub.80∼¾″ through a series of crushing and blending in order get a representative sample.

[0107] The prepared sample was agglomerated with 15 kg/t sodium chloride, and 20 kg/t sodium nitrate or nitrite and 12 kg/t sulfuric acid, producing agglomerates with an average moisture content of 8% for the NitrosaL process. For the standard high chloride process, similar conditions were used except that no sodium nitrate or nitrite was added -see Table 1.

TABLE-US-00001 Conditions of Agglomeration NitrosaL 1 NitrosaL 2 Chloride 1 Particle Size P80 0.75″ 0.75″ 0.75″ Acid dosage kg/t 12 12 12 Sodium Chloride dosage* kg/t 15 15 15 Sodium Nitrate or nitrite dosage** ko/t 20 20 0 Temperature °C Ambient ∼14 Aeration rate Nm.sup.3/hr.t 0.01 0.01 0.01 Irrigation rate L/m2.hr 2 2 2 Irrigation Time hrs/day 12 12 12 Cure Period days 30 45 45 Column Height m 1 1 1 Column Diameter cm 16 16 16

TABLE-US-00002 Agglomeration Raffinate Solution Compositions Column CuT (g/l) FeT (g/l) Fe +2 (g/l) H+ (g/l) Cl (g/l) Density Nitrate or nitrite (kg/t) SO4 (g/l) NitrosaL 0.56 1.21 0.52 8.2 151* 1.26 20** 90 Chloride 0.56 1.21 0.52 8.2 151* 1.26 0 90 * Chloride required: 15 kg/t NaCl — mostly added in raffinate at 150 g/l Cl and if the desired value is not achieved (based on the agglomerate moisture), solid NaCl was added to the ore directly during agglomeration to make-up. ** Nitrate or nitrite required: 20 kg/t NaNO.sub.3— added as solid to the drum during agglomeration or dissolved in known volume of raffinate solution that was used for ore agglomeration.

TABLE-US-00003 Irrigation Raffinate Composition* Raffinate CuT (g/l) FeT (g/l) Fe +2 (g/l) H+ (g/l) Cl (g/l) Density Chloride 0.56 1.21 0.52 8.2 151* 1.26 * Same raffinate was used for both processes

[0108] The agglomerated ores were loaded into their respective 1 m × 160 mm ID columns and allowed to cure for 30 and 45 days respectively for the NitrosaL process. For the high saline chloride process (referred to as Chloride 1), the samples were cured for 45 days. Aeration of 0.01 Nm.sup.3/hr.t was used during curing and irrigation for both processes.

[0109] Upon the completion of the curing step, a contact solution with a solution composition as presented in Table 2 above was used as irrigation solution for ore loaded in the columns which were irrigated. The PLS was analysed for Cu.sup.2+, Fe(T), Fe.sup.2+ and acid respectively.

[0110] The results show a much higher solution potential of up to 1100 mV for the NitrosaL columns at solution break through compared to 700 mV from the columns using the standard chloride process (see FIG. 2). The solution potential then decreased for both processes with the NitrosaL columns dropping to below 900 mV but increased again to stabilise just above 950 mV while the chloride column stabilised around 650 mV. All potential measurements are against the standard hydrogen electrode (SHE).

[0111] The PLS obtained contained a much higher Fe(T) concentration compared to what was measured in the chloride process column (see FIG. 4). Above 80 g/L Fe(T) was obtained for NitrosaL 1 and 40 g/L Fe(T) for NitrosaL 2. The Fe(T) existed mainly as ferric which was more than double the Fe(T) in the standard chloride process column.

[0112] Similarly, the NitrosaL columns produced three times more acid compared to the chloride process before the concentration decreased and stabilised to comparable values by day 30 (see FIG. 5). The higher acid generation is confirmation of the pyrite oxidation that was taking place in the columns due to the high oxidising environment. In a standard chloride process, pyrite oxidation is limited due to the lower solution potentials of below 750 mV.

[0113] Due to the high acid generation in the first few days of operation, the net acid consumption (NAC) was about 45% lower in the NitrosaL columns compared to the chloride process for the same period of operation (see FIG. 6). The high acid generation is shown by the low pH values of the solution drainage from the NitrosaL columns (shown in FIG. 3 as NitrosaL 1 and NitrosaL 2).

[0114] A maximum of 74% copper dissolution was achieved in less than 75 days in the NitrosaL columns compared to 30% copper dissolution in the chloride process for the same period of operation (see FIG. 10). The copper dissolution results also show that increasing the nitrate or nitrite content during the agglomeration step improves the copper dissolution rate as this makes the leach environment more aggressive (see FIG. 9). Mineralogy results of the ore show that the sample contained 3% pyrite and after leaching almost 50% of the pyrite was oxidized during the period of operation. Pyrite oxidation is critical to heat generation in heaps. Such heat generation increases the leach rate. The results show that 75% chalcopyrite dissolution was achieved during the same period of operation (See FIG. 10).

[0115] In order to achieve 70% copper extraction for the same sample, it took more than 540 days of operation in the chloride process compared to 75 days for the NitrosaL process (see FIG. 9). The sample treated under chloride conditions also had to be crushed finer (P.sub.80 ½”) and operated at a fixed temperature of 20° C. The sample loaded in columns under NitrosaL conditions was crushed to P.sub.80 ¾” and operated at ambient temperature (~14° C.).

[0116] There was no observed difference in copper dissolution when the results of the sample that was cured for 45 days were compared with those for sample cured for 30 days under NitrosaL conditions (see FIG. 8). This therefore provides an opportunity for a shorter leach cycle time. The rapid dissolution of copper from the ore in the NitrosaL columns is shown in FIG. 7.

[0117] The results obtained demonstrate that the NitrosaL process creates a higher oxidising environment in the heap which facilitates the dissolution of metals of interest at a faster rate and also reduces the cycle time.

Pyrite Oxidation Tests

Example 1

[0118] Two flat-bottomed flasks were used to determine pyrite oxidation under NitrosaL conditions and chloride conditions. The composition of the sample used for the test-work is presented in Table 4 below which shows that the sample contained 97% pyrite with minor impurities of sulfates and copper sulfides.

TABLE-US-00004 Pyrite sample composition Mineral Mass (%) Formula Composition Cu Sulfides 0.30 Pyrite FeS.sub.2 97.26 Other Sulfides 0.14 Silicates 1.68 Sulfides 0.28 Others --- 0.35 Total 100.00

Procedure:

[0119] In the experiment, 2 × 2 L of flat-bottomed flasks were filled with 400 mL of chloride raffinate of a known chemical composition as presented in Tables 6 and 7. Known amounts of nitrate and sulfuric acid were added to the NitrosaL flask and only acid was added to the standard chloride flask as presented in Table 5 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 chloride raffinate. The flask mixture was neither aerated nor agitated during the test period.

TABLE-US-00005 Quantities of reagents used during the test NitrosaL Chloride Raffinate type Chloride Chloride Volume Raffinate (ml) 400 400 Nitrate Sodium Added (g) 24 0 Acid Added (ml) 38 38 Pyrite (g) 50 50

TABLE-US-00006 Composition of Chloride Raffinate Raffinate CuT (g/l) Fe(T) (g/l) Fe +2 (g/l) H+ (g/l) Cl (g/l) Density Nitrate (kg/t) SO4 (g/l) Chloride 0.56 1.40 0.19 8.1 151* 1.26 0 101

[0120] The sampling was repeated after 3 hours and then once a day for the next 4 days. The results, tabulated in Tables 7 and 8 below, show a slight decrease in all the measured species for both flask contents once the pyrite was added.

[0121] However, after a day, there was a sharp increase in both Fe(T) and SO.sub.4.sup.2- content in the flask under NitrosaL conditions while in the flask under standard chloride conditions, there was only a slight increase in both Fe(T) and Fe.sup.3+ respectively. The potential increased from 703 mV to 1023 mV in the NitrosaL flask within the first three hours of the test. After 15 minutes of adding pyrite, the color of the raffinate changed in the NitrosaL flask from dark brown as presented in the flask shown on the left hand side in FIG. 12 to a much clearer yellowish brown solution as shown in the flask on the right hand side in FIG. 11. During the days that followed, the solution potential dropped slightly from 1023 mV and stabilized just below 1000 mV by 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 remained constant but dropped on the last day from 152 g/L to 115 g/L. The sulfate also increased from 238 g/L to a maximum of 283 g/L before decreasing to 273 g/L on the last day.

[0122] In the flask under chloride conditions, shown in FIG. 13, no significant change was observed, and the solution concentration remained almost constant with minor changes in the species (see Table 8).

TABLE-US-00007 Chemical analysis results in NitrosaL flask Time (hrs) Comments FeT(g/L) Fe2+(g/L) Fe3+(g/L) H2SO4 (g/L) SO4(g/L) Eh (mV) Initial solution 1.4 0.19 1.2 8.1 101 712 Initial solution + acid 1.3 0.16 1.1 150 261 710 Initial solution + acid + Nitrate 1.2 0.12 1.1 148 239 712 Initial solution + acid + Nitrate, 10 min Later 1.3 0.13 1.1 146 251 697 0.0 Initial solution + acid + Nitrate + Pyrite 1.3 0.13 1.1 146 251 703 2.85 1023 23.6 7.1 0.03 7.0 146 260 1001 50.0 10.8 0.03 10.7 148 283 995 70.4 10.3 0.06 10.3 152 271 997 98.1 13.1 0.04 13.0 115 273 1004

TABLE-US-00008 Chemical analysis results in chloride flask Time(hrs) comments FeT(g/L) Fe2+(g/L) Fe3+(g/L) H2SO4 (g/L) SO4(g/L) Eh(mV) Initial solution 1.4 0.19 1.18 8.0 102 712 Initial solution + acid 1.3 0.16 1.18 146 242 710 0.0 Initial solution + acid + Pyrite 1.3 0.16 1.18 146 242 711 24.1 1.4 0.20 1.23 144 253 673 50.6 1.5 0.16 1.34 147 230 687 71.0 1.5 0.23 1.30 149 233 696 98.7 1.4 0.2 1.23 148 243 698

[0123] The invention significantly improves the initial rate of metal dissolution and reduces the leach period, or leach cycle time, required to achieve the maximum metal recovery for a particular ore. The process of the invention achieves enhanced leach rates, and metal recovery for heap leaching, and constitutes a significant improvement on the established chloride heap leach methods for treatment of base metal ores. The process competes favourably with conventional processing producing a flotation concentrate.

Example 2

[0124] 2 × 1 m columns loaded with the sample containing 2.98% pyrite, based on its mineralogy composition as presented in Table 1, were leached under NitrosaL and chloride conditions respectively. After completion of the leach cycle, 100 days for the NitrosaL process and 500 days for the chloride process, the leach residues were submitted for mineralogy analysis. The results, presented in Table 1 below, show that very little pyrite oxidation, if at all, occurred during the 500 days of leaching under chloride conditions while almost half of the pyrite loaded in the column was oxidised in 100 days under NitrosaL conditions. This was evident from the comparatively high iron (mostly ferric) and acid concentrations content in the first PLS obtained from the NitrosaL columns.

[0125] Based on the above pyrite oxidation rate, the heat generation rate is 21.6 watts per ton ore and the heap temperature is expected to increase by 10 - 30° C. above the ambient temperature under NitrosaL conditions as compared to less than 3° C. under hypersaline process. The exact temperature increase of the heap is dependent on the rate of heat loss from the heap, that is, loss by convection and radiation from the heap surface and principally, by moist air leaving the top of the heap and drainage solution leaving from the bottom of the heap. This means that the leach kinetics are expected to more than double under NitrosaL conditions compared to hypersaline conditions and thus reduce the leach cycle time as demonstrated in the copper extraction rate presented above.

TABLE-US-00009 Mineralogy results for the sample used Mineral Mass (%) Formula Head Residue NitrosaL Residue Chloride Chalcocite/Digenite Cu.sub.2S - Cu.sub.1.4S 0.07 0.02 0.01 Covellite Cu.sub.1.4S - CuS 0.06 0.01 0.02 Chalcopyrite CuFeS.sub.2 1.20 0.31 0.48 Bornite/Idaite Cu.sub.5FeS.sub.4 0.01 0.00 0.00 Other Cu Mins Cu.x 0.07 0.01 0.01 Pyrite FeS.sub.2 2.98 1.53 2.91 Other Sulphides PbS / ZnS / MoS.sub.2 0.13 0.15 0.12 Fe Oxides/Hydroxides FeO.OH / Fe.sub.2O.sub.3 / Fe.sub.3O.sub.4 0.08 0.20 0.18 Quartz SiO.sub.2 48.90 48.77 49.17 Feldspars (K,Na)AISi.sub.3O.sub.8 / (Ca,Na)Al.sub.2Si.sub.2O.sub.8 6.94 9.82 8.48 Muscovite/Illite KAl.sub.2(Si.sub.3Al)O.sub.10(OH,F).sub.2 35.40 34.97 33.92 Kaolinite/Clays Al.sub.2Si.sub.2O.sub.5(OH).sub.4 / Al.sub.2Si.sub.4O.sub.10(OH).sub.2 2.77 2.40 2.20 Biotite/Phlogopite K(Mg,Fe.sup.2+).sub.3[AlSi.sub.3O.sub.10(OH,F).sub.2 0.32 0.13 0.20 Chlorite (clinochlore group) (Mg,Fe.sup.2+).sub.5Al[(OH).sub.8AlSi.sub.3O.sub.10] 0.31 0.13 0.11 Jarosite/Hydronio jarosite (K,Na,H.sub.3O)Fe.sup.3+.sub.3(SO.sub.4).sub.2(OH).sub.6 0.02 0.26 0.14 Other Sulphates x.SO4.nH.sub.2O 0.25 0.38 0.58 Others - - - 0.51 0.91 1.50