BASSON, Petrus (200 Hans Strijdom Drive, 2194 Randburg, ZA)
NICOL, Michael, J (200 Hans Strijdom Drive, 2194 Randburg, ZA)
MULLER, Elmar, L (200 Hans Strijdom Drive, 2194 Randburg, ZA)
BASSON, Petrus (200 Hans Strijdom Drive, 2194 Randburg, ZA)
NICOL, Michael, J (200 Hans Strijdom Drive, 2194 Randburg, ZA)
CLAIMS
1. A method of recovering copper from a copper sulphide mineral, in a reactor, which includes the steps of leaching the mineral in an acidic chloride or a mixed chloride/sulphate slurry in the presence of dissolved oxygen, maintaining the mineral's surface potential below 600 mV (vs. SHE) to cause the dissolution of the copper sulphide, and recovering copper from the slurry.
2. A method according to claim 1 wherein the potential is maintained within the range of 550 mV (vs. SHE) to 600 mV (vs. SHE).
3. A method according to claim 1 or 2 wherein the potential is maintained at a value which is dependent on the concentration of chloride.
4. A method according to any one of claims 1 to 3 wherein the copper sulphide mineral includes at least one of the following: bornite, chalcocite, chalcopyrite, covellite or enargite.
5. A method according to any one of claims 1 to 4 which is applied to the leaching of a concentrate of the copper sulphide mineral.
6. A method according to any one of claims 1 to 5 wherein the pH of the slurry is less than 3.
7. A method according to claim 6 wherein the pH of the slurry is between pH1 and pH2.
8. A method according to any one of claims 1 to 7 which includes the step of controlling the pH of the slurry by the addition of H2SO 4 , HCI or HNO3.
9. A method according to any one of claims 1 to 8 wherein the dissolved oxygen level of the slurry is in excess of 1 ppm.
10. A method according to any one of claims 1 to 9 wherein the chloride concentration is controlled at a level of 5 to 100 g/L.
11. A method according to any one of claims 1 to 10 wherein the chloride concentration is controlled by the addition of at least one of the following, HCI, NaCI, MgCI 2 , saline water, sea water, and chloride-containing process water.
12. A method according to any one of claims 1 to 11 which includes the step of controlling the slurry temperature at a value below 100 0 C.
13. A method according to claim 12 wherein the slurry temperature is controlled at a value between 5O 0 C and 100 0 C.
14. A method according to any one of claims 1 to 13 which includes the step of controlling the ratio of Cu(II) to Cu(I).
15. A method of recovering copper from a copper sulphide mineral which includes the steps of forming a slurry, in a reactor, from a concentrate of the copper sulphide mineral and an acidic aqueous solution, maintaining the pH of the slurry below 3, maintaining the temperature of the slurry below 100 0 C, maintaining the dissolved oxygen concentration of the slurry above 1 ppm, maintaining the slurry potential within the range of 550 mV (vs. SHE), to 600 mV (vs. SHE) agitating the slurry, -and subjecting a pregnant leach solution residue from the slurry to a solid/liquid separation step to produce a pregnant leach solution from which copper is recovered.
16. A method according to claim 15 in which the pH of the slurry is 1 or lower.
17. A method according to claim 15 or 16 wherein the temperature of the slurry is in the range of 60 0 C to 8O 0 C.
18. A method according to any one of claims 15 to 17 wherein the pH is controlled by the addition of H 2 SO 4 , HCI, or HNO 3 .
19. A method according to any one of claims 15 to 18 wherein the slurry contains chloride at a level of 5 to 100 g/L.
20. A method according to claim 19 wherein the chloride level is controlled by the addition of at least one of the following: HCI, NaCI, MgCI 2 , saline water, sea water and chloride-containing process water.
21. A method according to any one of claims 15 to 20 wherein the slurry contains copper at a level of 0 to 10 g/L.
22. A method according to any one of claims 15 to 21 wherein the slurry contains iron at a level of 0 to 20 g/L.
23. A method according to any one of claims 15 to 22 wherein the pulp density of the slurry is in the range of 5% to 40%.
24. A method of recovering copper which includes the steps of leaching a slurry of a copper sulphide mineral concentrate and an acidic or mixed chloride/sulphate solution in the presence of dissolved oxygen while maintaining the surface potential of the mineral between 550 mV (vs. SHE) and 600 mV (vs. SHE), separating a pregnant leach residue into a leach residue and a pregnant leach solution, and recovering copper from the pregnant leach solution. |
CHLORIDE TANK LEACHING
BACKGROUND OF THE INVENTION
[0001] This invention relates to a hydrometallurgical method for the recovery of copper from copper sulphide minerals, such as bornite, chalcocite, chalcopyrite, covellite and enargite.
[0002] Chalcopyrite is one of the most refractory copper sulphide minerals in relation to leaching in acidic ferric chloride and ferric sulphate systems at low temperature. This is exemplified by the mineral's slow leaching kinetics, which level off with time. This has been attributed to a process of "passivation", but uncertainty in regard to the mechanism still remains.
[0003] It has been shown that the oxidative dissolution of chalcopyrite is a potential- dependent process and that the onset of "passivation" seems to occur at a surface potential (mixed potential) in excess of about 0.6 V (vs. SHE). Studies have also shown that under typical ferric leaching conditions, such as bioleaching and atmospheric leaching in ferric chloride and ferric sulphate systems, the mixed potential of the mineral is normally fixed in the so-called "passive region" of the anodic oxidation process, at convential solution potentials in the region of 800 mV (vs. SHE) to 900 mV (vs, SHE), as measured against an inert platinum electrode. In this potential region, the mineral is subjected to the process of "passivation", which is typified by the leveling-off of the leaching kinetics. This defines the fundamental problem of oxidative dissolution of chalcopyrite in such systems.
[0004]ι Many* methods have been suggested to alleviate the problem of "passivation", one of which is thermophile bioleaching at elevated temperatures. In one approach, thermophile bioleaching is carried out on a chalcopyrite-bearing concentrate in a stirred tank, where the slurry temperature is typically 65 0 C or 75°C. However, one of the major draw-backs of this process is its low solids throughput due to sensitivity of the thermophile microorganisms sensitivity to operational conditions at high pulp density and metal tenor.
[0005] A number of prior art techniques have been proposed for the recovery of copper from chalcopyrite. These include the methods in: (a) US6277341 , wherein ferric sulphate is used as an oxidant and the surface potential of the chalcopyrite is controlled in the region of 350-450 mV (vs. SCE); (b) WO03038137A, which describes a reductive process followed by an oxidative process, using at least ferric and oxygen to oxidize sulphur in chalcopyrite; ,(c) a patent to UBC, which describes a chalcopyrite-concentrate leaching process with pyrite as a catalyst in a sulphate lixiviant, at a temperature in excess of
5O 0 C;
(d) a patent to CYPRUS, which describes the reaction of copper sulphate with chalcopyrite concentrate at elevated temperatures to form insoluble copper sulphide, soluble iron sulphate and sulphuric acid, and the leaching of copper sulphide with oxygen in an acid medium, or with ferric or cupric chloride or in an ammoniacal solution; and
(e) CL 40891 which relates to an agglomeration process, suited to supergene ores, with the addition of calcium chloride and stoichiometric quantities of acid. The chloride level is high and the solution is highly acidic.
[0006] ■• The recovery of copper from a high-grade, chalcopyrite-bearing concentrate via thermophile tank leaching at larger solids throughputs remains problematic.
[0007] The aforegoing review is principally in the context of chalcopyrite but similar considerations, to a greater or lesser extent, can be applicable to other copper sulphide minerals.
[0008] The invention aims to address, at least partly, this situation. The use of the invention is however not confined to these circumstances and may be extended to the leaching of low-grade supergene, transitional and hypogene ores at low temperatures.
SUMMARY OF THE INVENTION
[0009] The invention provides a method of recovering copper from a copper sulphide mineral in a reactor, which includes the steps of leaching the mineral in an acidic chloride or a mixed chloride/sulphate slurry in the presence of dissolved oxygen, maintaining the mineral's surface potential below 600 mV (vs. SHE) to cause the dissolution of the copper sulphide, and recovering copper from the slurry.
[0010] Preferably, the potential is maintained within the range of 550 mV (vs. SHE) to
600 mV (vs. SHE) for optimum chalcopyrite leaching. The optimum potential value depends on the concentration of chloride.
[0011] Depending on the application, the method may be carried out at ambient or at an elevated temperature.
[0012] <■ The Copper sulphide mineral may include bornite, chalcocite, chalcopyrite, coveliite or enargite. These are non-limiting examples.
[0013] The method of the invention may be applied to the leaching of a copper sulphide concentrate.
[0014] The pH of the slurry may be less than 3 and preferably is between pH 1 and pH
2. The pH may be controlled in any appropriate way, for example by the addition of H 2 SO 41 HCl Or HNO 3 .
[0015] The dissolved oxygen level of the slurry is preferably in excess of 1 ppm.
[0016] The chloride concentration may be controlled at a level of 5 to 100 g/L added via HCI or any suitable chloride salt including NaCI, MgCb, saline water ("salares"), sea water or chloride-containing process water.
[0017] The slurry temperature may be controlled at a temperature below 100 0 C, preferably between 50 0 C and 100 0 C.
[0018] In general terms, the mineral's surface potential can be controlled by manipulating variables within the leaching system. In one approach, the ratio of Cu(II) to Cu(I) is controlled. When the method of the invention is applied to the leaching of a concentrate then the aforementioned ratio of Cu(II) and Cu(I) in solution, may be controlled.
[0019] t It has' been observed that the leaching kinetics remain remarkably linear under the conditions defined by the method of the invention and that there is little or no indication of "passivation". Thus, the rate of dissolution of the copper-bearing mineral remains constant and, over time, results in substantially complete dissolution.
[0020] It has also been observed that under the defined conditions copper and iron dissolve in near stoichiometric quantities from a chalcopyrite mineral.
[0021] In the case of chalcopyrite, dissolution may possibly occur in accordance with the following scheme of reactions, which comprises a sequential non-oxidative / oxidative process:
Non-Oxidative Process
CuFeS 2 + 4H + → Cu(II) + Fe(II) + 2H 2 S (aq) (1) or
CuFeS 2 + 2H + → CuS + Fe(II) + H 2 S (aq ) (2)
[0022] Although Equation 2 is slightly more thermodynamically favourable than Equation 1 in chloride or chloride / sulphate mixed solutions, both equations are used as a starting point in a proposed reaction mechanism and result in the same intermediate oxidation reaction, as reflected in Equation 6, and in the same overall reaction, as set out in Equation 9.
Cu(II) + H 2 S ( aq) → CuS + 2H + (3) Equations 2 and 3 can be written as:
CuFeS 2 + Cu(II) → 2CuS + Fe(II) (4)
Oxidative Processes
CuS + ViO 2 + 2H + → Cu(II) + S" + H 2 O (5)
From Equations 3 and 5:
H 2 S( a q, + Y 2 O 2 → S 0 + H 2 O (6)
[0023] This constitutes the copper-catalyzed oxidation of soluble hydrogen sulphide by dissolved oxygen.
Following from Equation 1 :
4Cu(II) + 2H 2 S(aq) → 4Cu(I) + 2S° + 4H + (7)
4Cu(I) + O 2 + 4H + → 4Cu(II) + 2H 2 O (8) Equations 7 and 8 can be written as:
2H 2 S ( aq) + O 2 → 2S° + 2H 2 O (6)
[0024] This constitutes the oxidation of soluble hydrogen sulphide by cupric ion, with regeneration of the oxidant (cupric ion) by oxidation of cuprous to cupric ion by dissolved oxygen.
[0025] Equation 6 is the net result of each reaction route. It is believed that, under these conditions, the oxidation of hydrogen sulphide perturbs the equilibrium portrayed in Equation 1 or 2, and results in an overall dissolution process which is given by Equation 9.
Overall Reaction CuFeS 2 + O 2 + 4H + → Cu(II) + Fe(II) + 2S° + 2H 2 O (9)
[0026]' Although the applicant is not bound thereby, the preceding proposed reaction mechanism for chalcopyrite dissolution, at conditions of low mixed potential [0.6 V (vs. SHE)], in the presence of dissolved oxygen in chloride or chloride/sulphate mixed systems, is consistent with experimental observations.
[0027] In contrast to what is believed to happen at a mixed potential below 0.6 V (vs.
SHE), the applicant believes that at potentials above this value and fixed within the "passive region" of the anodic oxidation process, chalcopyrite undergoes direct anodic oxidation according to the following half-cell reaction:
CuFeS 2 → Cu(II) + Fe(II) + 2S° + 4e " (10)
[0028] The overall reactions generally accepted, depending whether cupric
(Equation 11) or ferric (Equation 12) ions are employed as oxidants within chloride systems, are:
CuFeS 2 +3Cu(II) → 4Cu(I) + Fe(II) +2S° (11)
CuFeS 2 +4Fe(III) → Cu(II) + 5Fe(II) + 2S° (12)
[0029] There are indications from experimental observations reported in the literature that the elemental sulphur (S 0 ) formed by direct anodic oxidation of the chalcopyrite mineral (Equation 10) and that formed via the proposed non-oxidative / oxidative route (Equations 2, 3 and 5) may not be the same.
[0030], In addition, mineralogical investigations on leach residue samples have indicated that very little of the sulphur formed during the dissolution process, under conditions of low mixed potential (< 0.6 V vs, SHE) and in the presence of dissolved oxygen, is associated with chalcopyrite particles, but occurs largely as (i) larger globules, and (ii) around smaller size particles of other mineral suphides such as pyrite (FeS 2 ). In other words, it seems that sulphur is formed away from the chalcopyrite mineral's surface. However, under conditions where the mixed potential is in excess of 0.6 V (vs. SHE) and fixed in the "passive region" of the anodic oxidation process, it seems that sulphur is formed directly on the mineral's surface, This would mean that a potential-dependent route determines the type (morphology) of sulphur, and also the deportment thereof. This is schematically illustrated in Figure 1, where the following terminology is used to describe the different types of sulphur believed to form (Table 1).
Table 1 : Sulphur Formation
[0031] The roles of the parameters and constituents of the method of the invention can be summarized as follows:
Chloride
• affects type and morphology of sulphur and deportment thereof;
• » stabilizes Cu(I) species which enables Cu(II) / Cu(I) couple to control Eh;
• enhances the thermodynamics of non-oxidative reaction;
• may enhance the rate of the non-oxidative reaction;
• increases formal potential of Cu(Ii) / Cu(I) couple; • results in a reduction of the acid required to achieve the same pH;
• affects the rate of oxidation of Cu(I) to Cu(II); and
• affects DO (dissolved oxygen) level.
Copper • Cu(II) is an oxidant;
• Cu ions catalyze the oxidation of H 2 S;
• the Cu(ll)/Cu(l) couple controls solution potential;
• affects Cu(I) to Cu(II) oxidation;
• the concentration of Cu(II) affects the rate of oxidation of Cu(I) to Cu(Il); and • the rate of oxidation of Cu(I) to Cu(II) is dependent on the concentration of Cu(I).
Dissolved oxygen
• is an oxidant for oxidation of Cu(I) to Cu(II); and
• allows for Eh control. Eh • Eh determines the mixed potential at mineral's surface that controls the mechanism of chalcopyrite dissolution. Iron
• no direct role in the mechanism.
Acid , i
• a leaching agent to drive the non-oxidative reaction;
• provides for pH control;
• affects kinetics of chalcopyrite dissolution; and • affects the rate of oxidation of Cu(I) to Cu(II).
• an intermediate product formed in the non-oxidative reaction;
• soluble H 2 S diffuses away from chalcopyrite surface; and
• H2S oxidation gives predominantly secondary elemental S 0 . Sulphate
• affects pH control; and
• affects DO level. Altitude
• affects DO. Agents that may enhance the kinetics of oxidation of H2S
• pyite;
• magnetite, hematite;
• activated carbon or coal;
• zeolites; and
• silver, bismuth, cadmium and mercury.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The invention is further described by way of examples with reference to the accompanying drawings in which:
Figure <1 is a 1 schematic diagram, which conceptualizes experimental observations in regard to the route of elemental sulphur formation, type (morphology) of sulphur formed, as well as the deportment of the sulphur;
Figure 2 is a graph which depicts the effect of chloride concentration on the formal potential of the Cu(Il) / Cu(I) couple;
Figure 3(a) shows graphs of copper dissolution versus time at low solution potential
(Test I) and at high solution potential (Test II);
Figure 3(b) shows graphs of the corresponding solution potential versus time profiles of
Tests I and II; Figure 4 shows the results of a series of tests, in which the effect of dissolved oxygen on the rate of chalcopyrite dissolution is demonstrated at low solution potential (in the absence of ferric ions) and high solution potential (in the presence of ferric ions);
Figure 5 is a graph of copper dissolution versus time, to illustrate the defined solution potential range for optimum chalcopyrite leaching, and to highlight the results achieved when leaching under conditions in accordance with the invention;
Figure 6 is a comparative graph, which depicts copper dissolution versus time, to illustrate the importance of the presence of dissolved oxygen when leaching within a defined optimum solution potential range;
Figure 7(a) shows a graph of copper dissolution versus time, to illustrate the feasibility of a two-stage (reductive / oxidative) leach, in a variation of the invention;
Figure 7(b) shows graphs of the corresponding solution potential versus time profile of the two-stage (reductive / oxidative) leach;
Figure 8 graphically depicts the effect of temperature on the rate of leaching chalcopyrite; and
Figures 9(a) and 9(b) are schematic diagrams which illustrate a process for the tank- leaching of copper concentrates.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] Various tests were conducted, to define certain key concepts of the method of the invention, on a variety of chalcopyrite-bearing samples which are summarized in Table 1. All potential values hereafter are reported against the Standard Hydrogen Electrode (SHE).
Table 1 : Test Samples
Notes:
1 ) chalcopyrite copper source ratio (CSR);
2) wet screened; and
3) dry screened.
Example 1 : The Effect of Solution Potential
[0034] Two batch leaching tests were conducted on a fine-milled, chalcopyrite-bearing concentrate (Cone. A) in 1 L glass beakers. The reaction mixtures or slurries of these each contained a solids concentration of 10 g/L and mixing of the slurry was achieved
by magnetic "stirring. Furthermore, the slurries were exposed to atmospheric air and regulated at 2O 0 C. The acidity was controlled at pH 0.5 by the addition of 98% concentrated sulphuric acid (H2SO4) when required. The solution potential was not controlled.
[0035] The objective of these tests was to investigate the effect of solution potential, more specifically the effect of high against low potential, on the rate of chalcopyrite dissolution. For this purpose, Test Cs solution contained only acid (98% H 2 SO 4 ), distilled water and sodium chloride (NaCI) to render a low potential, whereas Test ll's solution also included ferric sulphate (Fe 2 (SO 4 ) 3 ) to render a relatively high potential. The details of the test conditions are summarized in Table 2.
Table 2 : Test Conditions (Example 1'
Notes: 1) the subscript, 0, refers to initial conditions (t = 0 h);
2) the slurry pH was controlled by the addition of 98% H 2 SO 4 ;
3) the stirrer speed was controlled;
4) the slurry temperature was controlled; and
5) the solution 1 potential was not controlled.
[0036] Figure 3(a) shows the copper dissolution transients of Tests I and II, and Figure 3(b) shows the corresponding solution potential values. The results show a copper dissolution of 70.4% achieved at low potential (in the absence of ferric ions) against only 6.87% at high potential (in the presence of ferric ions), over a period of 912 h.
Example 2: The Effect of Dissolved Oxygen
[0037] A series of batch leaching experiments were executed to test the effect of dissolved oxygen on the rate of chalcopyrite dissolution at low solution potential (in absence of ferric ions) and high solution potential (in presence of ferric ions). These were conducted on a fine-milled, chalcopyrite-bearing concentrate (Cone. B) in 100 m L shake flasks. The slurries each contained a solids concentration of 1 g/L and mixing was achieved by shaking the flasks at 200 rpm on an orbital shaker, in a temperature- controlled incubator.
[0038] Gaseous nitrogen, saturated with water vapour, was sparged into those slurries, which needed to be purged of dissolved oxygen. This was accomplished by sparging the gas into the slurry via two injection needles, which were inserted through a rubber septum that sealed off the flask. The slurries that did not need to be purged of dissolved oxygen were closed with a cotton wool stopper.
[0039] The details of the test conditions, which include the various solution make-ups, are summarized in Table 3. Figure 4 depicts the results achieved after a period of 24 h, at 35 0 C and 5O 0 C.
Table 3 : Test Conditions (Example 2)
Notes:
1) the dissolved oxygen concentration was not controlled;
2) the shaking speed was controlled;
3) the slurry pH was not controlled;
3) the slurry temperature was controlled; and
4) the solution potential was not controlled.
[0040] The low solution potential (0.2 M HCI) results show a marked difference in overall copper dissolution achieved in the presence of dissolved oxygen (air) against dissolution in the absence of dissolved oxygen (N2). For example, 22.2% (air) vs. 11.0% (N 2 ) at 35 0 C, and 43.5% (air) vs. 15.4% (N 2 ) at 5O 0 C, were achieved after 24 h.
[0041] The high solution potential (0.2 M HCI, 0.1 M FeCI 3 & 0 M NaCI - 1.5 M NaCI) results show no difference in overall copper dissolution, whether the tests were conducted in the presence or absence of dissolved oxygen, at both 35 0 C and 50 0 C.
Since this is the case, it can be concluded that under these conditions chalcopyrite is not oxidized by dissolved oxygen according to Equation 9.
[0042] Under these conditions the oxidative dissolution of chalcopyrite is mainly due to ferric ions and according to Equation 12:
CuFeS 2 + 4Fe(III) → Cu(II) + 5Fe(II) + 2S° (12)
[0043] The results provide strong evidence of different reaction mechanisms present under conditions of high solution potential (in presence of ferric ions) against low solution potential (in absence of ferric ions). This is exemplified by the different effects that the presence of dissolved oxygen within the system has on the dissolution rate, e.g.:
• the rate increases at low potential (in absence of ferric ions); and
• the rate is not affected at high potential (in presence of ferric ions).
[0044] In addition,
• the rate is higher at low potential and in the presence of dissolved oxygen than at high potential (whether in the absence or presence of dissolved oxygen); and
• the rate is lower at low potential and in the absence of dissolved oxygen than at high potential (whether in the absence or presence of dissolved oxygen).
[0045] These results emphasize the importance of the role of dissolved oxygen when leaching is conducted under conditions of low solution potential (in absence of ferric ions).
Example 3: The Effect of Controlled Solution Potential in the Presence of Dissolved Oxygen
[0046] The effect of controlled solution potential on the rate of chalcopyrite dissolution, in the presence of dissolved oxygen, was investigated: • to confirm whether potential is a key driver in leaching chalcopyrite successfully in chloride or chloride/sulphate mixed systems; and, if so • to define the potential range for optimum chalcopyrite leaching
[0047] For this purpose, batch leaching experiments were initiated on a fine-milled, chalcopyrite-bearing concentrate (Cone. A) in 1 L glass reactors. Each reactor was fitted with baffles and sealed with a multi-port, poly-vinyl chloride lid, which supported a variable-speed, stirrer motor to drive an impeller to mix the slurry. Each reactor was also equipped with a redox sensor, glass sparger and inlets for sparging air, nitrogen or oxygen. The redox sensor measured the solution potential of the slurry, which was controlled to a set point value by means of a control loop from a Labview™ data acquisition system. The control loop caused the opening or closing of a solenoid valve, to allow for appropriate gas sparging. Each reactor was also enclosed in a temperature- controlled surround.
[0048] The batch leaching experiments were all conducted at 35°C, and controlled at the following respective solution potentials: 540 mV, 550 mV, 580 mV, 600 mV and 620 mV, The test at 540 mV was controlled at set point by sparging gaseous nitrogen, saturated with water vapour, into the slurry from t = 100 h. The tests at 550 mV, 580 mV and 600 mV were all controlled by air injection. The test at 620 mV was initially
operated by air injection; however, gaseous oxygen, saturated with water vapour, was sparged into the slurry from t = 328 h. The details of the test conditions are summarized in Table 4.
Table 4 : Test Conditions (Example 3)
Notes:
1 ) the dissolved oxygen concentration was not controlled;
2) the slurry pH was not controlled;
3) the slurry temperature was controlled;
4) the solution potential was controlled; and
5) the stirrer speed was controlled.
[0049]- Figure 5 shows the copper dissolution transients of the five leaching tests, with 30.3% (540 mV), 73,1 % (550 mV), 79.2% (580 mV), 76.2% (600 mV) and 22.5% (620 mV) copper dissolution achieved after 1000 h. The 550 mV test achieved 88.0% after 124O h.
[0050] Two boundary conditions can be established, viz a lower boundary at 550 mV and an upper boundary at 600 mV, which define the solution potential range to achieve optimum chalcopyrite dissolution rates within the systems under investigation. This is depicted in Equation 13:
550 mV < Ehoptimum ≤ 600 mV (13)
where Ehoptimum is the solution potential for optimum rate of chalcopyrite dissolution, in mV.
[0051] The 540 mV results were achieved under gaseous nitrogen sparging, i.e. in the absence of dissolved oxygen. It is possible that the optimum solution potential range is bordered on the lower end by potentials lower than 550 mV. However, it is considered very difficult to achieve and maintain such low potentials in the presence of dissolved oxygen concentrations considered sufficient for optimum leaching purposes, within the systems under investigation.
[0052] The potentials pertain to bulk solution or slurry potential measurements against a platinum (Pt) electrode. However, diffusion effects can be ignored, because of the fact that a fine-milled (+25-38 μm), high-grade (+80%) and liberated chalcopyrite-bearing concentrate (Cone. A) was used in these experiments, and the fact that the reaction
mixtures were all well stirred. Therefore, under these conditions of low potential, the bulk solution potential (Eh) and the potential at the chalcopyrite mineral's surface or mixed potential (E m i Xe d) are very much the same (Equation 14):
Eh * E mixed (14) where Eh is the bulk solution potential, in mV; and
Emixed is the mixed potential (at the chalcopyrite mineral's surface), in mV.
[0053] Equation 14 can be corroborated with potential measurements made with massive chalcopyrite electrodes during these and other tests. In addition, the following observations are made when the system is operated within the optimum solution potential range and in the presence of sufficient dissolved oxygen (more than 1 ppm), under the above conditions:
• continued linear kinetics, i.e. no leveling-off of the dissolution rate ("passivation");
• moles of copper leach to moles of iron leached indicate an almost 1 : 1 ratio over the whole leaching period;
• nearly complete chalcopyrite dissolution;
• rate of dissolution is largely independent of potential; and
• rate of dissolution is constant at ± 3 x 10 "12 mol Cu / cm 2 .s.
Example 4: The Effect of Controlled Solution Potential in the Absence of Dissolved Oxygen
[0054] The importance of the presence of dissolved oxygen on the rate of chalcopyrite dissolution under conditions of low solution potential (in the absence of ferric ions) has
already been" illustrated in Example 2. In order to confirm this under conditions of controlled potential, more specifically within the optimum potential range of 550 mV to 600 mV, some batch leaching experiments were performed in the absence of dissolved oxygen.
[0055] The tests were executed on a fine-milled, chalcopyrite-bearing concentrate (Cone. A) in the same 1 L glass reactors as described in Example 3. The tests were all conducted at 35 0 C under gaseous nitrogen (saturated with water vapour), and at 550 mV, 580 mV and 600 mV. The solution potential was controlled at the desired set point by controlling the Cu(II) / Cu(I) ratio by means of electrical current. The test condition details are summarized in Table 5.
Table 5 : Test Conditions (Example 4)
Notes:
1) the slurry pH was not controlled;
2) the slurry temperature was controlled;
3) the solution potential was controlled; and
4) the stirrer speed was controlled.
[0056] Figure 6 shows the copper dissolution transients of these tests in comparison with those achieved at corresponding solution potentials, in the presence of dissolved oxygen (in Example 3), The overall copper dissolutions after a period of 1000 h are as follows:
550 mV: 73.1 % (air) vs. 14.8% (N 2 ); 580 mV: 79.2% (air) vs. 14.2% (N 2 ); and 60O mV: 76.2% (air) vs. 15.0% (N 2 ). [0057] The results show that, in order to achieve optimum chalcopyrite dissolution rates, it is essential to have dissolved oxygen present in the system, even when the solution potential is controlled within the optimum range of 550 mV to 600 mV.
Example 5: The Feasibility of a Two-Stage (Reductive / Oxidative) Leach
[0058] A batch leaching experiment was conducted to test whether chalcopyrite could also be leached successfully by the use of a variation of the aforementioned techniques of the invention. This constituted a two-stage leach, which included a period of initial leaching under reducing conditions of low solution potential (Stage 1), followed by leaching under oxidative conditions (Stage 2).
[0059] The test was conducted on a fine-milled, dry screened (+25-38 μm) sample of a chalcopyrite-bearing concentrate (Cone. A) in a 1 L glass reactor (as described in
Example 3). The slurry contained a solids concentration of 10 g/L and the temperature was controlled at 35°C. In order to achieve low solution potentials, the slurry was
maintained deaerated by continuous sparging with gaseous nitrogen (saturated with water vapour) for the first 139 h (Stage 1 ); thereafter, a higher potential was affected by means of gaseous oxygen (saturated with water vapour) sparging (Stage 2). The details of the test conditions are summarized in Table 6.
Table 6 : Test Conditions (Example 5)
Notes:
1 ) the slurry pH was not controlled; 2) the slurry temperature was controlled;
3) the solution potential was not controlled; and
4) the stirrer speed was controlled.
[0060] Figures 7 (a) and 7(b) show the copper dissolution and solution potential profiles for this test. Initially, very little copper dissolved during the period of nitrogen sparging, with only 1 1.1 % dissolution achieved after 139 h. The potential was as low as 500 mV over this period (Stage 1 ). The rate of copper dissolution increased significantly on introduction of oxygen, with an overall dissolution of 95.5% achieved after 787 h. The potential ranged from 570 mV to 591 mV over this period (Stage 2).
. Example 6: The Effect of Increased Temperature on the Rate of Leaching
[0061] In order to test the effect of increased temperature on the rate of leaching of chalcopyrite, a batch leaching experiment was conducted under conditions of low solution potential and in the presence of dissolved oxygen, at elevated temperature.
[0062] The test was executed on a fine-milled, chalcopyrite-bearing concentrate (Cone. A) in a 1 L glass reactor. The reactor was fitted with baffles and sealed with a multi-port, glass lid, which allowed, amongst others, the fitting of a condenser to reduce water loss via evaporation. A variable-speed, stirrer motor drove an impeller to mix the slurry. The reactor was fitted with a gas sparger to allow for aeration with compressed air. The test was conducted at a temperature of 6O 0 C. The details of the test conditions are summarized in Table 7.
Table 7 : Test Conditions (Example 6)
Notes:
1 ) the slurry pH was controlled;
2) the slurry temperature was controlled;
3) the solution potential was not controlled; and
4) the stirrer speed was controlled.
[0063] Figure 8 shows the copper dissolution transient at 60 0 C in comparison to results achieved at 35°C. A copper dissolution of about 88% was achieved after only 144 h at . 6O 0 C as opposed to the same dissolution after 1240 h at 35 0 C.
[0064] The preceding tests were conducted primarily on chalcopyrite (for which the optimum solution potential range applies) but are deemed to be equally applicable to bornite, chalcocite, covellite, enargite and, more generally, to copper sulphide minerals.
[0065] Figure 9(a) shows steps in a tank leaching process based on the method of the invention, and Figure 9(b) depicts hardware aspects thereof. A milled copper sulphide concentrate 40 is mixed with an acidic aqueous solution 42 to form a pulp 44 which is charged to a batch reactor 46, open or closed, for leaching. The reactor includes an electrically driven agitator 48. Leaching is performed under oxidative conditions at a pulp pH of below 3.0, preferably about 1.0, and at a temperature in the range from ambient temperature to 10O 0 C, preferably at an elevated temperature of 6O 0 C to 8O 0 C.
[0066] The pH of the aqueous leach solution is controlled (50) in any appropriate manner, for example by the addition of H 2 SO 4 , HCI, or HNO 3 .The leach solution may contain the following: chloride at a level of 5 to 100 g/L added via HCI or any suitable chloride salt including NaCI, MgCb, saline water or sea water; copper at a level of 0 to 10 g/L added via the corresponding chloride or sulphate salts; and iron at a level of 0 to 20 g/L added via the corresponding chloride or sulphate salts.
[0067] ' The plilp density inside the reactor 46 may range from 5 to 40%. The residence time in the reactor is determined by the leach rate and is sufficient to achieve at least 80% copper dissolution. The dissolved oxygen concentration of the leach solution is measured by a sensor 52 and is maintained above 1 ppm. This is achieved via a controller 54 by sparging air or oxygen (56) at a controlled rate into the tank. The leach solution potential is maintained by the controller 54 within the range of 550 mV (vs. SHE) to 600 mV (vs. SHE). This is achieved via electric means, manipulating the composition of the leach solution, control of the slurry feed and or gas flow control.
[0068] A heat exchanger 58 is used to control the temperature of the slurry in the tank.
[0069] The pregnant leach solution residue 60 is subjected to solid/liquid separation 62 to produce a leach residue 64 and a pregnant leach solution 66. The residue 64 is discarded although elemental sulphur 68 can be recovered there from. The pregnant leach solution 66 is treated by solvent extraction 70 and electrowinning 72, according to standard industry practice, to recover copper 74. The spent leach solution can be recycled to be used in another leach cycle.
References '
1. Majima, H. Awakura, Y., Hirato, T. and Tanaka, T., The Leaching of Chalcopyrite in Ferric Chloride and Ferric Sulphate Solutions, Can. Metall. Q., 24(4), 1985, pp.283 - 291.
2. Hirato, T., Kinoshita, M., Awakura, Y. and Majima, H., The Leaching of
Chalcopyrite in Ferric Chloride, Metal. Trans., 17B, 1986, pp. 19 -28.
3. Dutrizac, J., The Leaching of Sulphide Minerals in Chloride Media, Hydrometallurgy, 29, 1992, pp 1 - 45.
4. Nicol, M. J., Kinetics of the Oxidation of Copper (I) by Oxygen in Acidic Chloride Solutions, S. Afr. J. Chem., 37, 1984, pp. 77 - 80.