Heap leaching method

Abstract

A process of extracting copper from copper sulphide minerals which is enhanced at solution potentials exceeding 700 mV SHE, in the absence of any microorganism, by contacting the minerals in a pre-treatment phase using an acid solution at a high chloride content containing dissolved copper.

Claims

1. A method of recovering a base metal from a crushed or run-of-mine (ROM) ore comprising: constructing the ore in a heap; subjecting the ore to a pre-treatment phase, under ambient conditions, wherein the pre-treatment phase comprises a step of contacting the ore with a solution by irrigating the heap with the solution to achieve a final ore moisture content in a range about 2 to about 25 wt. %; and wherein the solution contacting the ore is characterized by the following a potential that exceeds 700 mV vs Standard Hydrogen Electrode (SHE), in the absence of microorganisms; a total iron concentration greater than about 0.1 g/L; a pH that does not exceed pH 3.0; a Cl.sup.− ion concentration between 130 and 230 g/L; and a dissolved oxygen level below 1 mg/L; thereafter, leaching the pre-treated ore in an active leach cycle.

2. The method according to claim 1 further comprising agglomerating the ore and contacting the ore with the solution during the agglomeration.

3. The method according to claim 2 wherein the pH level of the solution in contact with the ore is maintained by adding sulphuric acid directly to the ore during the agglomeration.

4. The method according to claim 2 further comprising generating heat on surfaces of the ore particles during the agglomeration by contacting the ore with a solution that has the Cl.sup.− ion concentration and to which concentrated sulphuric acid has been added.

5. The method according to claim 2 wherein the solution has a copper to iron ratio of 1 which is obtained by adding copper sulphate directly to the ore during the agglomeration.

6. The method according to claim 1 wherein the final ore moisture content is in the range of about 5 to about 8 wt %.

7. The method according to claim 1 wherein the pH of the solution is below pH 2.5.

8. The method according to claim 1 wherein chloride ions are introduced directly to the ore by at least one of adding NaCl to the ore; adding MgCl.sub.2 to the ore; adding KCl to the ore; adding AlCl.sub.3 to the ore; adding salt water to the ore or adding brine to the ore.

9. The method according to claim 1 wherein chloride ions are introduced to the solution by least one of adding NaCl to the solution; adding MgCl.sub.2 to the solution; adding KCl to the solution; adding AlCl.sub.3 to the solution; adding salt water to the solution, or adding brine to the solution.

10. The method according to claim 1 further comprising: introducing chloride ions to a pond to form the solution by at least one of adding NaCl to the pond; adding MgCl.sub.2 to the pond; adding KCl to the pond; adding AlCl.sub.3 to the pond; adding salt water to the pond, or adding brine to the pond; and then drawing the solution from the pond.

11. The method according to claim 1 wherein the pH level of the solution in contact with the ore is maintained by adding sulphuric acid to the solution.

12. The method according to claim 1 wherein the solution has a copper to iron ratio >1, which is obtained by at least one step selected from the following processes: (a) adding copper sulphate to the solution; (b) adding an electrolyte, containing copper, mixed with the solution; (c) adding a leach solution, containing copper, directly to the ore; or (d) adding copper sulphide or copper oxide minerals.

13. The method according to claim 1 further comprising actively leaching the ore, by irrigating with a leach solution which contains sulphuric acid and which has a pH<2.5.

14. A method of recovering a base metal from a crushed or run-of-mine (ROM) ore comprising: introducing chloride ions to a pond to form a solution by at least one of adding NaCl to the pond; adding MgCl.sub.2 to the pond; adding KCl to the pond; adding AlCl.sub.3 to the pond; adding salt water to the pond, or adding brine to the pond; drawing the solution from the pond; subjecting the ore to a pre-treatment phase, under ambient conditions, wherein the pre-treatment phase comprises a step of contacting the ore with the solution to achieve a final ore moisture content in a range about 2 to about 25 wt. %; and wherein the solution contacting the ore is characterized by the following a potential that exceeds 700 mV vs Standard Hydrogen Electrode (SHE), in the absence of microorganisms; a total iron concentration greater than about 0.1 g/L; a pH that does not exceed pH 3.0; a C1.sup.− ion concentration between 130 and 230 g/L; and a dissolved oxygen level below 1 mg/L; thereafter, leaching the pre-treated ore in an active leach cycle.

15. The method according to claim 14 further comprising agglomerating the ore and contacting the ore with the solution during the agglomeration.

16. The method according to claim 15 wherein the pH level of the solution in contact with the ore is maintained by adding sulphuric acid directly to the ore during the agglomeration.

17. The method according to claim 15 further comprising generating heat on surfaces of the ore particles during the agglomeration by contacting the ore with a solution that has the Cl.sup.− ion concentration and to which concentrated sulphuric acid has been added.

18. The method according to claim 14 wherein the final ore moisture content is in the range of 5 to 8 wt %.

19. The method according to claim 14 wherein the pH of the solution is below pH 2.5.

20. The method according to claim 14 wherein the pH level of the solution in contact with the ore is maintained by adding sulphuric acid to the solution.

21. The method according to claim 14 wherein the solution has a copper to iron ratio >1, which is obtained by at least one step selected from the following processes: (a) adding copper sulphate to the solution; (b) adding an electrolyte, containing copper, mixed with the solution; (c) adding a leach solution, containing copper, directly to the ore; or (d) adding copper sulphide or copper oxide minerals.

22. The method according to claim 14 further comprising actively leaching the ore, by irrigating with a leach solution which contains sulphuric acid and which has a pH<2.5.

23. A method of recovering a base metal from a crushed or run-of-mine (ROM) ore comprising: subjecting the ore to a pre-treatment phase, under ambient conditions, wherein the pre-treatment phase comprises: agglomerating the ore and contacting the ore with a solution during the agglomeration: constructing the agglomerated ore in a heap and contacting the ore in the heap with the solution by irrigating the heap with the solution to achieve a final ore moisture content in a range about 2 to about 25 wt. %; wherein the solution contacting the ore is characterized by the following a potential that exceeds 700 mV vs Standard Hydrogen Electrode (SHE), in the absence of microorganisms; a total iron concentration greater than about 0.1 g/L; a pH that does not exceed pH 3.0; a Cl.sup.− ion concentration between 130 and 230 g/L; and a dissolved oxygen level below 1 mg/L, wherein the solution has a copper to iron ratio of 1 which is obtained by adding copper sulphate directly to the ore during the agglomeration; thereafter, leaching the pre-treated ore in an active leach cycle.

24. The method according to claim 23 wherein the final ore moisture content is in the range of 5 to 8 wt %.

25. The method according to claim 23 wherein the pH of the solution is below pH 2.5.

26. The method according to claim 23 wherein chloride ions are introduced directly to the ore by at least one of adding NaCl to the ore; adding MgCl.sub.2 to the ore; adding KCl to the ore; adding AlCl.sub.3 to the ore; adding salt water to the ore or adding brine to the ore.

27. The method according to claim 23 wherein chloride ions are introduced to the solution by least one of adding NaCl to the solution; adding MgCl.sub.2 to the solution; adding KCl to the solution; adding AlCl.sub.3 to the solution; adding salt water to the solution, or adding brine to the solution.

28. The method according to claim 23 further comprising introducing chloride ions to a pond to form the solution by at least one of adding NaCl to the pond; adding MgCl.sub.2 to the pond; adding KCl to the pond; adding AlCl.sub.3 to the pond; adding salt water to the pond, or adding brine to the pond; and then drawing the solution from the pond.

29. The method according to claim 23 wherein the pH level of the solution in contact with the ore is maintained by adding sulphuric acid directly to the ore during the agglomeration.

30. The method according to claim 23 wherein the pH level of the solution in contact with the ore is maintained by adding sulphuric acid to the solution.

31. The method according to claim 23 further comprising generating heat on surfaces of the ore particles during the agglomeration by contacting the ore with a solution that has the Cl.sup.− ion concentration and to which concentrated sulphuric acid has been added.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 illustrates in block diagram form aspects of a hydrometallurgical method for the heap leaching of copper,

(3) FIG. 2 illustrates in further detail a pre-treatment phase in the heap leaching process shown in FIG. 1, and aspects of a subsequent active leach cycle, for the recovery of copper, and

(4) FIGS. 3 to 16 are graphical depictions of different characteristics related to the heap leaching method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(5) The invention is described herein with reference to the use of a high chloride mediated, high solution potential, pre-treatment phase, prior to an active leach cycle, for crushed or run-of-mine (ROM) ore, heap leaching. The invention is based on the surprising discovery that, in respect of copper recovery, the conditions referred to herein enhance copper extraction from copper sulphide minerals and from copper sulphide/copper oxide minerals in a shorter leach cycle at solution potentials exceeding 700 mV SHE, in the absence of any sort of microorganism.

(6) An object of the invention is to significantly increase the oxidation rate of sulphide minerals or mixed sulphide and oxide minerals during a pre-treatment step prior to active heap irrigation, and thereby improve metal recovery in a shorter leach cycle and, additionally, to lower, at least to some extent, the operational cost of a heap leach by reducing or eliminating the requirement of forced aeration during a pre-treatment step. This type of aeration is normally implemented through the use of blowers, compressors and similar devices. The reduction of aeration is possible since in the method of the invention the pre-treatment reaction proceeds at lower dissolved oxygen levels, and at a lower oxygen demand, compared to conventional heap leach practice. Consequently, forced aeration of the heap at the high aeration rates which are normally required to maintain solution oxidation potentials at 700 mV vs. SHE or greater, is not necessary.

(7) The pre-treatment phase can be carried out, at least partly, using agglomeration techniques, while a heap is being constructed. Alternatively or additionally the pre-treatment phase can be implemented after the heap has been constructed. During the pre-treatment phase which may be from 1 to 200 days in duration but which is preferably less than 100 days in duration, and prior to active heap irrigation, the invention aims to achieve rapid oxidation of sulphide minerals or mixed sulphide and oxide minerals. The duration of the period of pre-treatment is determined by the time to complete mineral oxidation and is dependent, at least, on the method used to contact the ore with the solution e.g. by using an agglomeration technique, by direct solution contact with the ore, or by irrigation of the heap.

(8) FIG. 1 of the accompanying drawings illustrates a basic ROM heap leach flowchart for the process of the invention. FIG. 2 illustrates in further detail steps involved in a pre-treatment phase of the ore which is being leached, and aspects of a subsequent active leach cycle.

(9) FIGS. 1 and 2 show a heap 10. It is to be understood that the heap can be constructed from ore 12 before the pre-treatment phase is carried out. Alternatively the pre-treatment phase is carried out, at least partly, using an agglomeration process while the heap is being constructed. Thus both techniques can be used. The pre-treatment phase can commence using agglomeration techniques and can then be continued after the heap has been constructed. The heap 10 is shown to be a conventional heap. This is illustrative only for the heap may in fact comprise an ore column, a dump, a vat or a similar collection of ore which contains a mineral to be recovered.

(10) An irrigation network 14 is positioned to irrigate the heap with a solution 16 prepared with water drawn from a source 18. The irrigation network includes a reticulated system built from irrigation pipes 20 dispersed above and within the heap, and sprays or outlet nozzles 22 of any suitable kind. Moisture sensors 24 and related instruments are positioned on or in the heap, as appropriate, in order to obtain a measure of the moisture content of the ore in the heap.

(11) The pre-treatment phase is implemented under the control of a control system 26. This system, in response to various parameters and variables, controls the addition of chloride ions 28, sulphuric acid 30 and copper-containing material 32, to the water 18 to produce the solution 16 with desired characteristics. The system is also responsive to characteristics of a solution 34 draining from the heap.

(12) The ore 12 may, as noted, be placed in the heap using an agglomeration process i.e. the ore particles are treated with the solution 16 while the heap is being constructed. Alternatively the heap is constructed from the ore particles and thereafter the solution 16 is applied to the heap. Both approaches can however be used, in succession.

(13) In the pre-treatment phase the solution 16 is used to irrigate the ore for a period of up to 200 days. The solution, which contacts the ore, complies with the following characteristics: a) a solution potential that exceeds 700 mV SHE in the absence of microorganisms; b) a copper to iron ratio greater than 1; c) a total iron concentration above 0.1 g/L; d) a pH of less than pH3.0—preferably the pH is lower than pH2.5; e) a chloride ion concentration of between 130 g/L and 230 g/L; f) a dissolved oxygen content of less than 1 mg/L.

(14) The solution is applied to the heap to achieve an ore moisture content of between 2 wt. % and 25 wt. %. Preferably the ore moisture content lies in the range of from 5 wt. % to 8 wt. %.

(15) The system 26, using information from the sensors 24, and information relating to the characteristics of the drainage solution 34 (derived using suitable monitoring methods and sensors), monitors the moisture content in the ore and controls the addition of water to the solution and hence to the irrigation network 14. The rate at which the solution is applied to the heap may be varied. Alternatively the solution may be applied to the heap in a “batch” mode i.e. application of the solution to the heap during one period is followed by a period in which no solution is applied to the heap, and so on.

(16) The chloride ions (block 28) are introduced into the solution 16 using at least one of the following: a) the addition of one or more of the following: NaCl, MgCl.sub.2, KCl and AlCl.sub.3: 1. directly to the ore 12 during an agglomeration process; or 2. directly to the solution 16 during an agglomeration process by drawing salt from a salt addition pond 40, designed for the purpose; or b) the addition of one or more of the following: NaCl, MgCl.sub.2, KCl and AlCl.sub.3, (42) to a solution held in a specially designed salt addition pond (40) and by applying solution drawn from the pond to the heap 10 via the irrigation network 14; or c) brine 46 which is produced during a desalination process can be added to the ore 12 during an agglomeration process or via the irrigation network 14; or d) water 48 which naturally contains salt e.g. sea water, salt lakes or reservoir water, can be used as a chloride ion source applied to the ore 12 during an agglomeration process or by use of the irrigation network 14.

(17) The sulphuric acid (block 30) may be added directly to the ore during a process of agglomeration or may be added to the solution 16 which in turn is applied to the ore 12 during an agglomeration phase or via the irrigation network 14.

(18) In order to achieve the desired copper to iron ratio, which should prevail during the pre-treatment phase, various techniques may be employed. These include one or more of the following: a) the addition of copper sulphate 50 directly to the ore 12 during an agglomeration process, or to the solution 16 which in turn is used during an agglomeration process or which is applied to the ore via the irrigation network 14; b) the addition of an electrolyte 52 which contains copper ore which is mixed with the solution 16; c) the addition of a leach solution 54, containing copper, which is drawn from any part of the leach circuit; and d) the dissolution of copper sulphide and/or copper oxide minerals into the solution 16.

(19) During the pre-treatment phase, with the establishment of a solution potential exceeding 700 mV SHE, oxidation of secondary covellite (a product of first step chalcocite oxidation), and of native covellite, by ferric iron, is enhanced: a) first stage chalcocite leaching is initiated by the oxidation with some ferric iron in the solution contacting the ore. This reaction proceeds at a solution potential exceeding 500 mV SHE;
Cu.sub.2S+2Fe.sup.3+.fwdarw.2Fe.sup.2++CuS+Cu.sup.2+; b) under the aforementioned conditions, ferrous iron is oxidised by cupric copper to solution potential values exceeding 700 mV according to the following equilibrium reaction:
Fe.sup.2++Cu.sup.2+Cu.sup.++Fe.sup.3+; c) cuprous is more effective than ferrous iron in utilising dissolved oxygen, stipulated as being below 1 mg/L, and is oxidised according to the following reaction:
4Cu.sup.++O.sub.2+4H.sup.+.fwdarw.4Cu.sup.2++2H.sub.2O; and d) oxidation of secondary and/or primary covellite is enhanced, at stipulated solution potentials exceeding 700 mV SHE, thereby contributing to the extent of copper extraction from secondary sulphides:
CuS+2Fe.sup.3+.fwdarw.2Fe.sup.2++S+Cu.sup.2+.

(20) FIG. 2 also illustrates aspects of an active leach cycle which follows the pre-treatment phase.

(21) During the active leach cycle the ore in the heap is irrigated with a leach solution 60. Copper leached from the heap is recovered from the draining solution 34 via a solvent extraction process 64. The leach solution 60 has a pH less than pH 2.5, achieved, as appropriate, by the addition of sulphuric acid 30. Optionally, the leach solution includes hydrochloric acid 66. The leach solution may also contain copper, iron and other anion and cation species 70 originating from processed water employed in the leaching cycle or dissolved from the ore which is being treated.

(22) During the solvent extraction process 64 use may be made of one or more copper-loaded, organic washing stages 72 to promote an electrolyte chloride ion concentration below 50 ppm. The recovered copper is designated 74.

(23) FIGS. 3 to 15 illustrate graphically certain aspects of the method of the invention and benefits which accrue from its use.

(24) FIG. 3 shows curves of the extraction of copper on a percentage basis versus time in days from whole ore containing secondary copper sulphide as a function of solution potential at 550 mV, 600 mV, 650 mV and 700 mV, respectively at ambient conditions. It is apparent that copper recovery rate is enhanced at 700 mV.

(25) FIG. 4 illustrates, on a comparative basis, percentage copper recovery rates at ambient conditions, from whole ore containing secondary copper sulphide using the pre-treatment step of the invention (A) compared to the use of a chemical leach at a chloride ion concentration below 130 g/L (B) and a conventional bioleach using microorganisms (C).

(26) FIG. 5 shows the solution potential of the pregnant leach solutions from the systems referred to in connection with FIG. 4. After the pre-treatment process the solution potential exceeded 700 mV (SHE). The chloride iron concentration at this stage was 180 g/L.

(27) FIG. 6 is a graphical comparison showing an enhanced rate of copper extraction at ambient conditions from whole ore containing secondary copper sulphide during the pre-treatment step of the invention, compared to that achieved using a bioleaching process. The tests were done in 10m high column leach systems with only natural air diffusion into the ore bed. There was no forced aeration.

(28) FIG. 7 depicts the difference in copper extraction from whole ore containing secondary copper sulphides after one day of pre-treatment, (curve F) and after 30 days of pre-treatment (curve G), according to the invention, respectively.

(29) Tests were conducted in batch reactors at 25° C. containing various concentrations of copper (as shown in FIG. 8), 1 g/L starting ferrous iron, 80 g/L chloride ions and 4 g/L sulphuric acid. Solution potentials were measured over time. These conditions represent potentials that can be expected during the greater part of an active heap irrigation cycle. A pseudo-equilibrium condition pertains at a solution potential below 700 mV SHE.

(30) Solutions potentials were measured over time in hours in batch reactors with natural air diffusion into an ore bed i.e. there was no forced aeration. This was at 25° C. The reactors contained various concentrations of copper, indicated in FIG. 9, 1 g/L starting ferrous iron, 180 g/L chloride ions and 1 g/L sulphuric acid. These conditions are not easily reproducible during an active heap irrigation cycle but are readily obtained by using the “lower moisture resting” pre-treatment step, of the present invention, prior to active irrigation. With a copper to iron ratio greater than 1, solution potentials of more than 700 mV were obtained.

(31) FIG. 10 shows solution potentials measured over time in batch reactors, with natural aeration only, at 25° C. for various concentrations of chloride ions (as identified in FIG. 10) ranging from 130 to 180 g/L, 1 g/L starting ferrous iron, 16 g/L copper ions and 1 g/L sulphuric acid. A pseudo-equilibrium condition prevailed above 700 mV, under chloride concentrations referred to herein.

(32) FIG. 11 shows the dissolved oxygen measured and extrapolated as a function of chloride concentrations in commercial plant steady state mature leach solution at atmospheric system pressure. The dissolved oxygen concentration was below 1 mg/L at chloride concentrations exceeding 100 g/L chloride ions.

(33) FIG. 12 is a curve of ore temperature vs time in minutes after agglomeration with concentrated sulphuric acid and a solution containing 4 g/L sulphuric acid, 1 g/L iron, 5 g/L copper and 180 g/L chloride ions.

(34) FIG. 13 is included for comparative purposes and illustrates a rise in temperature, above ambient temperature, of a cast metallic block which is contacted with concentrated sulphuric acid mixed with water (H), acidic water containing 180 g/L chloride ions (J), and acidic water containing 180 g/L chloride ions and 5 g/L copper ions (K).

(35) FIG. 14 shows the difference in copper extraction achieved at ambient conditions from whole ore containing secondary copper sulphide, at two different sulphuric acid concentrations, within the pre-treatment phase.

(36) FIG. 15 is related to FIG. 14 and shows the pregnant leach solution pH of the copper extraction values as depicted in FIG. 14.

(37) FIG. 16 shows the copper extracted from whole ore containing copper sulphide as a function of an increased pre-treatment period, stipulated herein as less than 100 days.