Alkaline Oxidation Process and Device for Treating Refractory Sulfide Ore, in Particular Refractory Gold Ore

Abstract

Alkaline oxidation process for treating refractory sulfide ore or concentrate particles enriched in a metal to be recovered comprising stages in which refractory ore or concentrate particles are surface-oxidized in an alkaline oxidation step in alkaline liquid phase with calcium hydroxide forming an alkaline slurry, which slurry is thereafter mechanically activated to remove passivating coatings from the surface oxidized refractory ore particles.

Claims

1: Alkaline oxidation process for treating refractory sulfide ore particles enriched in a metal to be recovered wherein refractory sulfide ore particles are submitted to at least 3 stages in each of which said refractory sulfide ore particles are surface-oxidized by an oxidizing agent in an alkaline oxidation step (CSTR) in alkaline liquid phase and form an alkaline slurry containing surface oxidized refractory sulfide ore particles and in each of which said alkaline slurry is thereafter submitted to a mechanical activation step (MA) for at least partly removing a surface layer containing oxidized matter from the surface-oxidized refractory sulfide ore particles, said mechanical activation forming a mechanically activated slurry containing oxidized matter, refractory sulfide ore particles from which oxidized matter has been removed, alkaline liquid phase and liberated metal to be recovered by further processing or returned to a next or a previous alkaline oxidation step, said alkaline liquid phase containing calcium hydroxide as an alkaline agent and said refractory sulfide ore being a refractory sulfide ore or concentrate enriched in a metal selected from the group consisting of gold, silver, platinum, palladium, coper, nickel, zinc, cobalt and combinations thereof, said at least 3 stages comprising: a series of n alkaline oxidation steps in a series of n reactors (CSTR), each n.sup.th alkaline oxidation forming a n.sup.th alkaline slurry, where n is an integer comprised between 3 and 10, a series of x mechanical activation steps (MR), each mechanical activation step being a mechanical activation of said n.sup.th alkaline slurry, where x is an integer, equal or lower than n, and comprised between 3 and 10, wherein said series of n alkaline oxidation steps comprises at least: a first alkaline oxidation step wherein said refractory ore particles enriched in a metal to be recovered are fed into an agitated reactor and form a first alkaline slurry, at least one intermediate alkaline oxidation step fed with one alkaline slurry from a previous alkaline oxidation step, a last alkaline oxidation step being said n.sup.th alkaline oxidation step fed with one alkaline slurry from a previous alkaline oxidation step, said series of x mechanical activation steps comprising at least: a first mechanical activation step of said first alkaline slurry in a first mechanical activation means to form a first mechanically activated slurry, at least one intermediate mechanical activation step of a n.sup.th alkaline slurry with said n.sup.th alkaline slurry being comprised between a 2.sup.nd alkaline slurry and a (x−1).sup.th alkaline slurry in a y.sup.th mechanical activation means to form a y.sup.th mechanically activated slurry, a last mechanical activation step of said last alkaline slurry being said x.sup.th mechanical activation step in a x.sup.th mechanical activation means to form an x.sup.th mechanically activated slurry.

2-3. (canceled)

4: Alkaline oxidation process according to claim 1, wherein said oxidizing agent is an oxidizing liquid, an oxidizing powder or an oxidizing gas.

5: Alkaline oxidation process according to claim 1, wherein said calcium hydroxide of said alkaline liquid phase is obtained by one or more addition of dry quicklime CaO, hydrated lime Ca(OH).sub.2, or milk of lime, said one or more addition being selected from the group consisting of an addition upstream one n.sup.th reactor, in one n.sup.th reactor, upstream a x.sup.th mechanical activation means, in a x.sup.th mechanical activation means and a combination thereof.

6: Alkaline oxidation process according to claim 1, wherein each intermediate alkaline oxidation step is fed by a previous alkaline oxidation step, via a mechanical activation step.

7: Alkaline oxidation process according to claim 1, wherein at least one alkaline oxidation step of said series of n alkaline oxidation steps is performed at atmospheric pressure.

8: Alkaline oxidation process according to claim 1, wherein at least one alkaline oxidation step of said series of n alkaline oxidation steps is performed at a temperature comprised between 70 and 100° C.

9: Alkaline oxidation process according to claim 1, wherein at least one alkaline oxidation step of said series of n alkaline oxidation steps is performed with a dissolved oxygen content comprised between 1 and 30 mg/dm.sup.3 of liquid phase.

10: Alkaline oxidation process according to claim 9, wherein at least one alkaline oxidation step of said series of n alkaline oxidation steps is performed with a dissolved oxygen content comprised between 1 and 5 mg/dm.sup.3 of liquid phase.

11: Alkaline oxidation process according to claim 1, wherein at least one alkaline oxidation step of said series of n alkaline oxidation steps is performed at a pH comprised between 10 and 12.5, said pH being controlled with a controlled addition of said alkaline agent.

12: Alkaline oxidation process according to claim 1, wherein at least one alkaline oxidation step of said series of n alkaline oxidation steps is performed with solid content in the agitated reactor comprised between 10 and 70 wt % with respect to the total weight contained in said agitated reactor.

13: Alkaline oxidation process according to claim 1, wherein at least one of said series of x mechanical activation steps is performed in one of a vertical mill, a vertical stirred mill, a horizontal mill, an attritor, a stirred ball mill or a horizontal stirred mill.

14: Alkaline oxidation process according to claim 1, wherein said refractory sulfide ore particles fed to said first alkaline oxidation step are obtained by a previous crushing and grinding, down to having 80% of the particles with a diameter in the range between 25 and 200 μm.

15: Alkaline oxidation process according to claim 14, wherein said crushed and ground refractory sulfide ore particles are subjected to mineral flotation to produce a concentrate of refractory sulfide ore particles fed to said first alkaline oxidation step.

16-22. (canceled)

Description

[0084] Other characteristics and advantages of the present invention will be derived from the non-limitative following description, and by making reference to the drawings.

[0085] FIG. 1 is a schematic view of the status of a refractory ore material particle during one stage of alkaline oxidation and mechanical activation.

[0086] FIG. 2 is a flow chart of one embodiment of the process according to the present invention, showing the device to carry out such process.

[0087] FIG. 3 is a flow chart of another embodiment of the process according to the present invention, showing the device to carry out such process.

[0088] FIG. 4 is a flow chart of yet another embodiment of the process according to the present invention, showing the device to carry out such process.

[0089] FIG. 5 is a flow chart of a further embodiment of the process according to the present invention, showing the device to carry out such process.

[0090] FIG. 6 is a flow chart of a variant embodiment of the process according to the present invention, showing the device to carry out such process.

[0091] FIG. 7 is a graph showing the extent of gold extraction by cyanidation leaching (or other target method) as a function of extent of pyrite oxidation.

[0092] In the drawings, the same reference numbers have been allocated to the same or analogue element.

[0093] The present invention relates to an alkaline oxidation process for treating refractory sulfide ore particles enriched in a metal to be recovered.

[0094] The refractory ore particles are submitted to at least 3 stages, preferably 4, 5, 6, 7 or even 8 or up to 10 stages, in which refractory ore particles are oxidized in surface in an alkaline oxidation step in alkaline liquid phase containing calcium hydroxide for forming an alkaline slurry containing surface oxidized refractory ore particles. Thereafter the slurry is mechanically activated to remove at least partly a surface layer from the surface oxidized refractory ore particles. The surface layer contains oxidized matter. The mechanical activation forms a slurry which is called a mechanically activated slurry and which contains oxidized matter, refractory sulfide ore or concentrate particles from which said surface layer has been removed, alkaline liquid phase and liberated metal to be recovered for further processing.

[0095] The process according to the present invention is contrary to the conventional industrial practice of using lime-based reagents, where it is generally used to retard sulfide mineral oxidation. The present invention is based on the use of a lime-based reagent to induce alkaline conditions for sulfide mineral oxidation by explicitly overcoming the gypsum and iron oxide coating effect, thereby facilitating high rates of sulfide mineral oxidation.

[0096] Refractory ore is preferably crushed and milled in order to typically obtain at least 80% of particles having a diameter between 25 and 150 μm. This ore can either be fed directly into the first alkaline oxidation reactor, or can first be subjected to a mineral flotation process and thereby upgraded to a concentrate. The upgraded concentrate A, containing the metal and sulfide mineral of interest, is then be fed into the first alkaline agitated oxidation reactor. The present invention is herein illustrated with gold refractory ore, without being limited thereto. Other precious metal refractory ore can also be treated in the process according to the present invention, for example refractory ores containing silver, palladium or platinum, but also base metal sulfide ore, such as zinc sulfide ore, copper sulfide ore, cobalt sulfide ore, nickel sulfide ore or combined metal sulfide ore such as low grade copper-gold sulfide ores.

[0097] In refractory sulfide gold ore, the main sulfide minerals in the concentrate are most typically pyrite or arsenopyrite. Lime as dry quicklime, dry hydrated lime (also known as slaked lime or dry slaked lime), or a milk of lime (a slurry of Ca(OH).sub.2 particles and water) or a paste of lime may be added to at least one agitated reactor and/or to at least one mechanical activation means, preferably to at least 3 agitated reactors and/or to at least 3 mechanical activation means, more preferably to each agitated reactors and/or to at least each mechanical activation means. Lime (in any form) is added to as many of the process units (agitated reactors and/or mechanical activation means) as is required to maintain the pH throughout the processing circuit at the target pH setpoint. Practically, if the lime is added to each oxidation reactor, to maintain the pH at the setpoint, the lime consumption can be used as an indicator of the extent of mineral oxidation. Therefore it could be a very important parameter to monitor the performance of the process circuit.

[0098] It is also possible that the lime is added to each stage of the process by adding it to the attritioner feed. In this way the mechanical activation means acts to mill and disperse added lime.

[0099] The sulfide mineral concentrate (1) is subjected to a succession of alkaline oxidation ((CSTR).sub.n) and surface attritioning ((MA).sub.n) in order to reduce the impact of surface coating and passivation effects, illustrated in FIG. 1. The surface layer (2) or passivation layer (2) contains ferric iron hydroxide (Fe(OH).sub.3) and gypsum (CaSO.sub.4.2H.sub.2O). The surface attritioning forms a slurry which is called a mechanically activated slurry and which contains oxidized matter (4), refractory sulfide ore or concentrate particles from which oxidized matter has been removed (3), alkaline liquid phase and liberated metal to be recovered (5).

[0100] The sulfide mineral concentrate is subjected to an elevated temperature alkaline oxidation process into which oxygen gas (preferably with an oxygen content greater than 95% v/v) is sparged and lime is added.

[0101] A combination of surface attritioning (mechanical activation) and alkaline oxidation, prior to gold leaching, is thereby induced. The rationale for this approach is to limit milling/grinding to achieve mineral surface attritioning (also called herein mechanical activation and encompassing both mechanical activation or attritioning) in a stirred media reactor sufficient to disrupt/alter or remove gypsum and iron oxide surface coating and passivation. In effect, the alkaline oxidation process is applied to mineral surfaces and milling (mechanical activation) is only used to expose fresh mineral surfaces (“active” layer) to sustain elevated mineral oxidation rates, as illustrated in FIG. 1. The process of mechanically activation also results in a high degree of strain being introduced into the sulphide mineral lattice, increasing the number of grain boundary fractures and lattice defects in the mineral. The introduction of strain lowers the activation energy for the oxidation of the sulfides and enables oxidation under atmospheric conditions. The rate of oxidation is also enhanced, due to the increased mineral surface area.

[0102] The process according to the present invention can occur in a number of different circuit configurations, with the basic feature of alternating alkaline oxidation (shown in CSTR reactors below) with inter-stage surface attritioning (also called mechanical activation, MA). The number of stages can range from 3-10, preferably from 3 to 8. More preferably from 4 to 7.

[0103] As it can be seen from the FIGS. 2 to 6, alkaline sulfide oxidation process for treating refractory ore particles enriched in said metal to be collected comprises said at least 3 stages comprising: [0104] a series of n alkaline oxidation steps in a series of n agitated reactors (CSTRs) forming n alkaline slurries, where n is an integer comprised between 3 and 10, preferably between 4 and 8, more preferably between 5 and 7, [0105] a series of x mechanical activation steps in a series of x mechanical activation means (MA), each mechanical activation step being a mechanical activation of a n.sup.th alkaline slurry, where x is an integer, equal to or lower than n, and comprised between 3 and 10, preferably between 4 and 8, more preferably between 5 and 7.

[0106] The series of n alkaline oxidation steps comprises at least: [0107] a first alkaline oxidation step where said refractory ore particles enriched in a metal to be recovered are fed into an agitated reactor and form a first alkaline slurry, [0108] at least one intermediate alkaline oxidation step fed with one alkaline slurry from a previous alkaline oxidation step, [0109] a last alkaline oxidation step being said n.sup.th alkaline oxidation step fed with one alkaline slurry from a previous alkaline oxidation step.

[0110] Said series of x mechanical activation steps comprises at least: [0111] a first mechanical activation step of said first alkaline slurry in a first mechanical activation means to form a first mechanically activated slurry. [0112] at least one intermediate mechanical activation step of a n.sup.th alkaline slurry with said n.sup.th alkaline slurry being comprised between a 2.sup.nd alkaline slurry and a (x−1).sup.th alkaline slurry in an intermediate (y.sup.th) mechanical activation means to form an intermediate (y.sup.th) mechanically activated slurry, [0113] a last mechanical activation step of said last alkaline slurry being said x.sup.th mechanical activation step in a x.sup.th mechanical activation means to form a last mechanically activated slurry.

[0114] A first embodiment is illustrated in FIG. 2. As it can be seen, the series of n reactor comprises 6 agitated reactors. Each agitated reactor (CSTR 1 to CSTR 6) is followed by one mechanical activation means (MA 1 to MA 6). In this embodiment of FIG. 2, x=n=6. The first agitated reactor is fed with refractory ore particles, having preferably a P.sub.80 lower than 150 μm. Each alkaline oxidation step of said series of n alkaline oxidation step is followed by a mechanical activation step of said series of x mechanical activation steps. Each n.sup.th alkaline oxidation step forms a n.sup.th slurry of surface oxidized refractory ore particles. Each n.sup.th alkaline oxidation step from n=2 is fed by a x.sup.th mechanically activated slurry where x=n−1. Each x.sup.th mechanical activation step forms a x.sup.th mechanically activated slurry.

[0115] In other words, the first alkaline oxidation step in agitated reactor CSTR 1 forms a first slurry of surface oxidized refractory ore particles and is followed by a first mechanical activation step in a mechanical activation means MA1. The first mechanical activation forms a first mechanically activated slurry. The first alkaline oxidation step is fed by refractory ore particles.

[0116] The second alkaline oxidation step in agitated reactor CSTR 2 forms a second slurry of surface oxidized refractory ore particles and is followed by a second mechanical activation step in a mechanical activation means MA 2. The second mechanical activation forms a second mechanically activated slurry. The second alkaline oxidation step is fed by the first mechanically activated slurry.

[0117] The third alkaline oxidation step in agitated reactor CSTR 3 forms a third slurry of surface oxidized refractory ore particles and is followed by a third mechanical activation step in a mechanical activation means MA 3. The third mechanical activation forms a third mechanically activated slurry. The third alkaline oxidation step is fed by the second mechanically activated slurry.

[0118] The fourth alkaline oxidation step in agitated reactor CSTR 4 forms a fourth slurry of surface oxidized refractory ore particles and is followed by a fourth mechanical activation step in a mechanical activation means MA 4. The fourth mechanical activation forms a fourth mechanically activated slurry. The fourth alkaline oxidation step is fed by the third mechanically activated slurry.

[0119] The fifth alkaline oxidation step in agitated reactor CSTR 5 forms a fifth slurry of surface oxidized refractory ore particles and is followed by a fifth mechanical activation step in a mechanical activation means MA 5. The fifth mechanical activation forms a fifth mechanically activated slurry. The fifth alkaline oxidation step is fed by the fourth mechanically activated slurry.

[0120] The sixth alkaline oxidation step in agitated reactor CSTR 6 forms a sixth slurry of surface oxidized refractory ore particles and is followed by a sixth mechanical activation step in a mechanical activation means MA 6. The sixth mechanical activation forms a sixth mechanically activated slurry. The sixth alkaline oxidation step is fed by the fifth mechanically activated slurry.

[0121] In another alternative arrangement as illustrated in FIG. 3, the atritioners or mechanical activation means (MA 1 to MA 6) will not be between the alkaline oxidation stages (CSTR 1 to CSTR 6), but rather in parallel with the oxidation stages, forming an internal loop between the alkaline oxidation reactor and the mechanical activation reactor.

[0122] The equipment arrangement would have preferably 4 to 6 reactors for alkaline oxidation, with 4 to 6 atritioners “next to” the reactors.

[0123] In this case, the slurry is drawn from a n.sup.th agitated reactor, sent through the attritioner, which then returns the slurry to the same n.sup.th reactor.

[0124] This arrangement offers the added benefit that the circuit may continue to operate even if an atritioner needs to be taken offline for maintenance or replacement. It also provides more flexibility to alter the relative extent of attritioning vs oxidation, by adjusting the return (to the same oxidation reactor) versus the forwarding (to the next oxidation reactor in the series) flow rate. Also, not all alkaline oxidation stages may require a mechanical activation step, therefore some mechanical activation steps can be omitted (see FIG. 5).

[0125] Indeed, as illustrated in FIG. 3, in this arrangement, the number of agitated reactors in said series is preferably 6 and the number of mechanical activation means in said series is also preferably 6.

[0126] Accordingly, x=n=6. Each alkaline oxidation step of said series of n alkaline oxidation step is followed by a mechanical activation step of said series of x mechanical activation steps. Each n.sup.th alkaline oxidation step forms a n.sup.th slurry of surface oxidized refractory ore particles. The n.sup.th alkaline oxidation step is fed by a (n−1).sup.th slurry from a (n−1).sup.th agitated reactor to which a (n−1).sup.th mechanically activated slurry is returned.

[0127] In other words, the first alkaline oxidation step in agitated reactor CSTR 1 forms a first slurry of surface oxidized refractory ore particles and is followed by a first mechanical activation step in a mechanical activation means MA1. The first mechanical activation forms a first mechanically activated slurry. The first alkaline oxidation step is fed by refractory ore particles and by said first mechanically activated slurry.

[0128] The second alkaline oxidation step in agitated reactor CSTR 2 forms a second slurry of surface oxidized refractory ore particles and is followed by a second mechanical activation step in a mechanical activation means MA 2. The second mechanical activation forms a second mechanically activated slurry. The second alkaline oxidation step is fed by the first slurry from the first agitated reactor and by the second mechanically activated slurry.

[0129] The third alkaline oxidation step in agitated reactor CSTR 3 forms a third slurry of surface oxidized refractory ore particles and is followed by a third mechanical activation step in a mechanical activation means MA 3. The third mechanical activation forms a third mechanically activated slurry. The third alkaline oxidation step is fed by the second slurry from the second agitated reactor and by the third mechanically activated slurry.

[0130] The fourth alkaline oxidation step in agitated reactor CSTR 4 forms a fourth slurry of surface oxidized refractory ore particles and is followed by a fourth mechanical activation step in a mechanical activation means MA 4. The fourth mechanical activation forms a fourth mechanically activated slurry. The fourth alkaline oxidation step is fed by the third slurry from the third agitated reactor and by the fourth mechanically activated slurry.

[0131] The fifth alkaline oxidation step in agitated reactor CSTR 5 forms a fifth slurry of surface oxidized refractory ore particles and is followed by a fifth mechanical activation step in a mechanical activation means MA 5. The fifth mechanical activation forms a fifth mechanically activated slurry. The fifth alkaline oxidation step is fed by the fourth slurry from the fourth agitated reactor and by the fifth mechanically activated slurry.

[0132] The sixth alkaline oxidation step in agitated reactor CSTR 6 forms a sixth slurry of surface oxidized refractory ore particles and is followed by a sixth mechanical activation step in a mechanical activation means MA 6. The sixth mechanical activation forms a sixth mechanically activated slurry. The sixth alkaline oxidation step is fed by the fifth slurry from the fifth agitated reactor and by the sixth mechanically activated slurry.

[0133] FIG. 4 illustrates a variant embodiment compared to FIG. 2, where the number of agitated reactors in said series of n agitated reactor is not the same as the number of mechanical activation means of said series of x mechanical activation means. This arrangement can be carried out as such or at any location in the series and can be carried out as illustrated by construction or because one mechanical activation means needs to be maintained. In this case, a by-pass is realized between two consecutive agitated reactors.

[0134] As it can be seen, in this arrangement, x is different from n, hence lower than n. x=5 while n=6. Some alkaline oxidation step of said series of n alkaline oxidation step are followed by a mechanical activation step of said series of x mechanical activation steps, but not each alkaline oxidation steps. Each n.sup.th alkaline oxidation step forms a n.sup.th slurry of surface oxidized refractory ore particles and is fed by a (n−1).sup.th slurry from the (n−1).sup.th agitated reactor, optionally subsequently mechanically activated.

[0135] In other words, the first alkaline oxidation step in agitated reactor CSTR 1 forms a first slurry of surface oxidized refractory ore particles and is followed by a first mechanical activation step in a mechanical activation means MA1. The first mechanical activation forms a first mechanically activated slurry. The first alkaline oxidation is fed by refractory ore particles.

[0136] The second alkaline oxidation step in agitated reactor CSTR 2 forms a second slurry of surface oxidized refractory ore particles. The second alkaline oxidation step is fed by the first mechanically activated slurry.

[0137] The third alkaline oxidation step in agitated reactor CSTR 3 forms a third slurry of surface oxidized refractory ore particles and is followed by a second mechanical activation step in a mechanical activation means MA 2. The second mechanical activation forms a second mechanically activated slurry. The third alkaline oxidation step is fed by the second slurry from the second alkaline oxidation step.

[0138] The fourth alkaline oxidation step in agitated reactor CSTR 4 forms a fourth slurry of surface oxidized refractory ore particles and is followed by a third mechanical activation step in a mechanical activation means MA 3. The third mechanical activation forms a third mechanically activated slurry. The fourth alkaline oxidation step is fed by the second mechanically activated slurry.

[0139] The fifth alkaline oxidation step in agitated reactor CSTR 5 forms a fifth slurry of surface oxidized refractory ore particles and is followed by a fourth mechanical activation step in a mechanical activation means MA 4. The fourth mechanical activation forms a fourth mechanically activated slurry. The fifth alkaline oxidation step is fed by the third mechanically activated slurry.

[0140] The sixth alkaline oxidation step in agitated reactor CSTR 6 forms a sixth slurry of surface oxidized refractory ore particles and is followed by a fifth mechanical activation step in a mechanical activation means MA 5. The fifth mechanical activation forms a fifth mechanically activated slurry. The sixth alkaline oxidation step is fed by the fourth mechanically activated slurry.

[0141] FIG. 5 illustrates a variant embodiment compared to FIG. 3, where the number of agitated reactor in said series of n agitated reactor is not the same as the number of mechanical activation means of said series of x mechanical activation means. This arrangement can be carried out as such or at any location in the series and can be carried out as illustrated by construction or because one mechanical activation means needs to be maintained, as afore explained.

[0142] As it can be seen, in this arrangement, x is different from n, and lower than n. x=5 while n=6. Some alkaline oxidation step of said series of n alkaline oxidation steps (CSTR 1 to CSTR 6) being followed by a mechanical activation step of said series of x mechanical activation steps (MA 1 to MA 5). Each nth alkaline oxidation step forming a nth slurry of surface oxidized refractory ore particles and being fed from a (n−1)th agitated reactor optionally to which a xth mechanically activated slurry with x<(n−1) is returned.

[0143] In other words, the first alkaline oxidation step in agitated reactor CSTR 1 forms a first slurry of surface oxidized refractory ore particles and is followed by a first mechanical activation step in a mechanical activation means MA1. The first mechanical activation forms a first mechanically activated slurry. The first alkaline oxidation is fed by refractory ore particles and by said first mechanically activated slurry.

[0144] The second alkaline oxidation step in agitated reactor CSTR 2 forms a second slurry of surface oxidized refractory ore particles and is followed by a second mechanical activation step in a mechanical activation means MA 2. The second mechanical activation forms a second mechanically activated slurry. The second alkaline oxidation step is fed by the first slurry from the first agitated reactor and by the second mechanically activated slurry.

[0145] The third alkaline oxidation step in agitated reactor CSTR 3 forms a third slurry of surface oxidized refractory ore particles. The third alkaline oxidation step is fed by the second slurry from the second agitated reactor.

[0146] The fourth alkaline oxidation step in agitated reactor CSTR 4 forms a fourth slurry of surface oxidized refractory ore particles and is followed by a third mechanical activation step in a mechanical activation means MA 3. The third mechanical activation forms a third mechanically activated slurry. The fourth alkaline oxidation step is fed by the third slurry from the third agitated reactor and by the third mechanically activated slurry.

[0147] The fifth alkaline oxidation step in agitated reactor CSTR 5 forms a fifth slurry of surface oxidized refractory ore particles and is followed by a fourth mechanical activation step in a mechanical activation means MA 4. The fourth mechanical activation forms a fourth mechanically activated slurry. The fifth alkaline oxidation step is fed by the fourth slurry from the fourth agitated reactor and by the fourth mechanically activated slurry.

[0148] The sixth alkaline oxidation step in agitated reactor CSTR 6 forms a sixth slurry of surface oxidized refractory ore particles and is followed by a fifth mechanical activation step in a mechanical activation means MA 5. The fifth mechanical activation forms a fifth mechanically activated slurry. The sixth alkaline oxidation step is fed by the fifth slurry from the fifth agitated reactor and by the fifth mechanically activated slurry.

[0149] FIG. 6 shows another arrangement in which a combination of several variants is illustrated.

[0150] In this embodiment, the first alkaline oxidation step in agitated reactor CSTR 1 forms a first slurry of surface oxidized refractory ore particles and is followed by a first mechanical activation step in a mechanical activation means MA 1. The first mechanical activation forms a first mechanically activated slurry. The first alkaline oxidation is fed by refractory ore particles and the first mechanically activated slurry.

[0151] The second alkaline oxidation step in agitated reactor CSTR 2 forms a second slurry of surface oxidized refractory ore particles and is followed by a second mechanical activation step in a mechanical activation means MA 2. The second mechanical activation forms a second mechanically activated slurry. The second alkaline oxidation step is fed by the first slurry from the first agitated reactor and by the second mechanically activated slurry.

[0152] The third alkaline oxidation step in agitated reactor CSTR 3 forms a third slurry of surface oxidized refractory ore particles. The third alkaline oxidation step is fed by the second slurry from the second agitated reactor.

[0153] The fourth alkaline oxidation step in agitated reactor CSTR 4 forms a fourth slurry of surface oxidized refractory ore particles and is followed by a third mechanical activation step in a mechanical activation means MA 3. The third mechanical activation forms a third mechanically activated slurry. The fourth alkaline oxidation step is fed by the third slurry from the third agitated reactor.

[0154] The fifth alkaline oxidation step in agitated reactor CSTR 5 forms a fifth slurry of surface oxidized refractory ore particles. The fifth alkaline oxidation step is fed by the third mechanically activated slurry from the third mechanical activation means.

[0155] The sixth alkaline oxidation step in agitated reactor CSTR 6 forms a sixth slurry of surface oxidized refractory ore particles and is followed by a fourth mechanical activation step in a mechanical activation means MA 4. The fourth mechanical activation forms a fourth mechanically activated slurry. The sixth alkaline oxidation step is fed by the fifth slurry from the fifth agitated reactor.

[0156] The seventh alkaline oxidation step in agitated reactor CSTR 7 forms a seventh slurry of surface oxidized refractory ore particles and is followed by a fifth mechanical activation step in a mechanical activation means MA 5. The fifth mechanical activation forms a fifth mechanically activated slurry. The seventh alkaline oxidation step is fed by the fourth mechanically activated slurry from the fourth mechanical activation means.

[0157] According to the present invention, further possible separation steps can be foreseen between two agitated oxidation reactors or between one mechanical activation means and one agitated reactor, such as for example to remove a portion of said liquid phase and keeping a higher concentration in solid matter in a further alkaline oxidation step.

[0158] In the process according to the present invention, the conditions for alkaline oxidation process are preferably as follows: [0159] Pressure: atmospheric [0160] Solids concentration: 40-65% wt [0161] Temperature: 80-90° C. [0162] Dissolved oxygen: 2-30 mg/dm.sup.3 (preferably >10 mg/dm.sup.3) [0163] pH control level: 11 with controlled lime addition

[0164] Lime reagent addition into the oxidation reactors (CSTR) and stirred media (attritioning or mechanical activation) reactors can be achieved either by addition of dry quicklime (CaO—in powder form), lime hydrate (Ca(OH).sub.2—in powder form), or milk of lime also known as slaked lime slurry (a suspension of Ca(OH).sub.2). The lime addition rate would be dependent upon reaction demand and maintaining the pH at 11 in all alkaline oxidation reactors. Said lime addition in the oxidation reactors and mechanical activation means is performed in an usual known manner which is not illustrated on the Figures.

[0165] The heat for maintaining the temperature at or above 80° C. is derived from the stirred media reactor as well as the exothermic chemical reactions and is sustained by managing the various heat balance factors of the processing circuit. These heat balance factors are specific to each application scenario and should be evaluated for each individual scenario as key inputs into the process design and selection of lime reagent type. The mass feed and subsequent rate of sulfide oxidation, as well the type of lime reagent used are particularly relevant to the heat balance.

[0166] The alkaline oxidation reaction, summarized by the overall reaction below, is exothermic (ΔH −1732 kJ/mol @ 30° C.) thereby contributing to heat generation in the agitated reactor.

4FeS.sub.2+8Ca(OH).sub.2+15O.sub.2+14H.sub.2O=4Fe(OH).sub.3+8CaSO.sub.4.2H.sub.2O

[0167] The hydration reaction of quicklime is also exothermic (ΔH−65 kJ/mol @ 30° C.).

CaO+H.sub.2O═Ca(OH).sub.2

[0168] The choice of lime reagent in the form of CaO, compared to already-hydrated Ca(OH).sub.2 may impact the heat balance, and may be used to manage the amount of heat in the process.

[0169] Oxygen is supplied preferably in each oxidation reactor in a known manner which is not illustrated on the Figures. Oxygen utilization efficiency may also be improved by making use of a cascade of multiple oxidation reactors where the oxygen gas from one reactor is captured and introduced to a next reactor vessel in the cascade.

[0170] A key design element of the processing circuit is the extent of pyrite (or other sulfide mineral) oxidation to be targeted, as this is the key driver for determining the extent surface attrition, the extent of oxidation and therefore also the extent of lime and oxygen reagent requirement. In turn, these factors influence the process residence time and thereby the reactor size. These factors thus also influence the operational and capital costs of the processing circuit.

[0171] The optimal solids content is determined by optimal viscosity of the solids slurry. A too viscous slurry will reduce the efficiency of oxygen mass transfer and would negatively impact the efficiency of attritioning. The design of the process circuit should take into account that the that the solids content, and thus viscosity, of the slurry will increase through the circuit as a result of increased gypsum generation. The process design should take into consideration this increased viscosity and the optimal viscosity for oxygen transfer and attritioning or mechanical activation. Dilution process water may also be introduced along the process circuit to reduce the solids concentration and viscosity if required to maintain optimal conditions

[0172] The extent of gold extraction by cyanidation leaching (or other target method) should be determined as a function of extent of pyrite oxidation, as is known in the art and shown by way of example in the graph in FIG. 7.

[0173] The process circuit design, and in particular the extent of surface attritioning and oxidation is based on obtaining key process design parameters experimentally. Lime consumption (to maintain the pH at a setpoint of 11), can be used as an indicator or proxy of the extent of pyrite oxidation because of the correlation via the reaction mechanism:

4FeS.sub.2+8Ca(OH).sub.2+15O.sub.2+14H.sub.2O=4Fe(OH).sub.3+8CaSO.sub.4.2H.sub.2O

[0174] Once initial oxidation is commenced, using the concentrate feed A at particle size as obtained from the mineral flotation process, lime and/or oxygen consumption is monitored (according to the above reaction) until a plateau is reached indicating the onset of the particle coating effect caused by gypsum and iron oxides. The mineral slurry is then transferred to a surface attritioning step to induce surface cleaning for a monitored period of time, that can be adjusted as required. Oxidation is again resumed in the same way until a lime/oxygen consumption plateau is reached. The stage of said series of at least 3 stages is repeated until no further oxidation occurs.

[0175] The lime consumption is correlated with the oxygen consumption via the reaction mechanism, which allows for the oxygen demand for the system to be calculated and the oxygen supply system to be designed for the oxidation reactors.

[0176] It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the claims.