FIELD OF THE INVENTION

[0001] The present invention relates to mineral processing.

[0002] A particular application of the present invention relates to processing of refractory ore, such as refractory gold ore.

BACKGROUND TO THE INVENTION

[0003] “Refractory” gold is a term used to describe particular gold deposits that do not respond to the conventional and widely used gold cyanidation process.

[0004] These ores are naturally resistant to recovery by standard cyanidation processes generally because they contain sulphide minerals, organic carbon, or both. Sulphide minerals are impermeable minerals that occlude gold particles, making it difficult for the leach solution to form a complex with the gold.

[0005] Typically, such refractory ores require pre-treatment in order for cyanidation to be effective in recovery of the gold.

[0006] Historically, existing processes are generally high in capital and operating costs and place conditions on the volume and quality of the ore/concentrates presented for treatment. High costs and the process requirements can lead to smaller and lower grade deposits being deemed too uneconomical to recover.

[0007] To overcome such past problems of refractory ores, pressure oxidation, bacterial oxidation, ultrafine grinding (comminution) followed by atmospheric oxidation (atmospheric leaching - Albion process) and roasting of refractory gold ores have all been utilised.

[0008] Iron III (Fe ³⁺ ) has been proposed as an oxidant for thiocyanate in a chemical reaction process for the dissolution of gold as an alternative to conventional cyanide leaching processes. However, disadvantageously, the iron is not recovered and the resulting SCN compounds not very stable. Also the addition of iron tends to make more stable the thiocyanate.

[0009] It is with such problems in mind that the present invention has been realised.

SUMMARY OF THE INVENTION

[0010] The present invention proposes at least an alternative mineral recovery process.

[0011 ] One or more forms of the present invention is particularly suited to gold recovery from refractory ores or other gold bearing ores.

[0012] One or more forms of the present invention is applicable to refractory gold ores that suit processing using existing process options as well as such ores that are of too small a volume and/or of too low a grade to justify the expense of the conventional processes.

[0013] It will be appreciated that one or more forms of the present invention suit low capital cost and low operating cost applications, and does not require the characteristics of the material to be treated (specifically a minimum level of sulphur content) for the process to be effective. [0014] In conventional sulphide mineral treatment (where the aims to oxidise the sulphur from the mineral to liberate the contained value), the sulphur content of the material being processed acts as a “fuel” for the reaction, providing sufficient heat to keep the process reaction rates at desired levels (or to achieve and maintain the reaction activation energy) when reacting with the oxygen being added to the process. Without the generated heat, there is a need for the process to be provided with an additional fuel or external heat source. This is evident in the roasting process, pressure oxidation and bacterial oxidation processes.

[0015] With the aforementioned in mind, an aspect of the present invention provides a process for treatment of an amount of mineral bearing material wherein the process includes reacting an oxidant and the mineral bearing material thereby oxidising at least part of the mineral bearing material, recovering at least a portion of the oxidant and subsequently introducing at least part of the at least a portion of the oxidant to a further amount of the mineral bearing material.

[0016] Generation of the oxidant can be achieved through introduction of a said oxidant, such as at significantly lower addition rates than would be required stoichiometrically to complete the oxidation reaction, to a comminution or grinding stage of the process.

[0017] The oxidant can include or can be ferric iron.

[0018] The process can include generation and reuse of the oxidant ferric iron within the process.

[0019] The ferric iron can include Fe ²⁺ and/or Fe ³⁺ [0020] Preferably, gold is liberated from the mineral bearing material. The mineral bearing material may be or include refractory gold ore.

[0021] The process may be used for the treatment/liberation of other value metal minerals, such as silver, copper, zinc or lead

[0022] The following examples are applicable to the present invention:

• The mineral bearing material (e.g. an ore or slurry containing an ore) may be a refractory ore.

• The mineral bearing material may include at least one of pyrite (FeS2), arsenopyrite (FeAsS) (and their analogues) such as cattierite (CoS2), vaesite (NiS2), clinosafflorite ((Co,Fe,Ni)AsS), gudmundite (FeSbS), glaucodot or alloclasite ((Fe,Co)AsS) or ((Co,Fe)AsS), iridarsenite ((lr,Ru)AsS), osarsite or ruarsite ((Os,Ru)AsS) and ((Ru,Os)AsS).

• The mineral bearing material may include at least one of chalcopyrite (CuFeS2), stannite (Cu2FeSnS4), kesterite (Cu2ZnSnS4), talnakhite (Cu9Fe8S16), mooihoekite (Cu9Fe9S16), haycockite (Cu4Fe5S8), cubanite (CuFe2S3), argentopyrite (AgFe2S3), enargite (Cu3AsS4), proustite (Ag3AsS3), calaverite (AuTe2), stibnite (Sb2S3), sphalerite (ZnS), hawleyite (CdS), wurtzite (a-ZnS), greenockite (CdS), linnaeite (Co3S4), violarite (FeNi2S4), carrollite (CuCo2S4), greigite (Fe3S4), molybdenite (MoS2), tungstenite (WS2), and/or pentlandite ((Ni,Fe)9S8).

[0023] Ferric/ferrous iron coupling with the mineral bearing material in the process is preferably for direct oxidation of sulphide minerals within or mixed with the mineral bearing material. Metal mineral(s) within the mineral bearing material can subsequently be effectively treated by existing processes (e.g. cyanidation in the case of gold). [0024] The process can include mixing the mineral bearing material (such as an ore) with a liquid, e.g. to form a slurry, before feeding the combined liquid and mineral bearing material (e.g. the slurry) to a mill, such as a vertical grinding mill or ball mill, to grind/comminute the mineral bearing material and promote said oxidation.

[0025] It will be appreciated that said oxidation can be achieved through the ferric/ferrous coupling replacing other processes, such as bacterial oxidation, pressure oxidation (autoclave) and roasting.

[0026] The process may include pre-oxidation of incoming feed of the mineral bearing material to initiate, augment or accelerate the oxidation process.

[0027] The process may further include post-grinding/comminution oxidation of the mineral bearing material.

[0028] The process may include adding additional oxidant to the post grinding/comminuted mineral bearing material.

[0029] The process may include providing additional/extended residence time of the oxidant and the mineral bearing material together (additional or extended over an amount of time considered sufficient to oxidise a certain level/proportion/percentage of the mineral bearing material).

[0030] Residence time can be variable. Residence time can be from 15 minutes to 10 hours, preferably up to 5 hours, more preferably up to 3 hours and yet more preferably up to 2 hours.

[0031 ] Residual heat from the process and addition of further said oxidant may be used to sustain/continue the oxidation process. [0032] The residual heat and added said oxidant preferably oxidises residual sulphides.

[0033] The oxidant and the mineral bearing material may together be agitated in a tank.

[0034] One or more additions, variations or alternatives to the present process may include at least one of:

[0035] Pre-oxidation of incoming feed of the mineral bearing material to “kick start” the process

[0036] Post-grinding “polishing” oxidation to add additional oxidation to the mineral bearing material. This can include adding additional oxidant to the post grinding mineral bearing material or by allowing additional residence time.

[0037] Such polishing can include the slurry, after being treated in the comminution process, being placed/flowing into an agitated tank into which oxygen (or other suitable oxidant) as added - the residual heat from the earlier process and the fresh levels of oxidant helps the oxidation process continue and oxidise residual sulphides from the main process.

[0038] Other methods/arrangement that can be used are oxygen/air agitated Pachuca, or Brown, tanks (see Figure 4), pipe reactor style reactors or high shear in-line mixing systems.

[0039] Residence time can be adjusted by selection of appropriate tank numbers or size or pipe reactor length and pressure, necessary to achieve the desired final oxidation level of the sulphides. [0040] 100% oxidation may not be achieved in the process of one or more embodiments of the present invention. Sufficient oxidation may be provided to liberate economical levels of value from the mineral being processed - for example - pressure oxidation may yield 95% oxidation of the mineral to provide a 90% recovery of the value mineral but it comes at a high processing cost. It may be more economical in this case to aim for a lower oxidation, accept a lower value recovery but a significantly lower operating cost so that the financial benefit is greater.

[0041] Addition of iron based or iron bearing grinding media to the grinding stage to provide additional iron for the coupling. Such iron based or iron bearing media may replace, or augment, all or part of traditional grinding media (such as ceramic media) used in contemporary fine grinding circuits.

[0042] Addition of an amount of iron based/iron bearing grinding media would serve the purpose of providing some grinding media but also breaking down intimately with the mineral bearing material and providing a strong (iron) oxidant at the mineral surface.

[0043] Recycling oversize mineral bearing particles into/within the grinding mill.

[0044] Separating the iron rich solution (liquid) from the ground slurry (solids) and returning this to the mill feed as slurry make up water, taking advantage of the contained iron and acidity to enhance the reaction in the grinding mill

[0045] Stage grinding with the first stage providing iron through a high iron media mix and the second stage using ceramic media and recycled, oxidised/oxidant rich solution for enhanced oxidation in the second stage of the process. Such additional processing is particularly beneficial where the initial slurry (concentrate) was coarse and a cheaper initial grinding system could be employed to get efficient size reduction to provide feed to the second fine grinding mill - e.g. taking a slurry from ~150 micron particles to ~ 10 micron in a single stage is inefficient and not best use for a mill designed to produce 10 micron material. Using a cheaper and simpler grinding mill to reduce the feed from -150 micron to -50-60 micron and then an ultrafine grinding mill to take it from -50 to -10 micron would result in significantly less capital in two smaller mills and an improved operating cost through more efficient energy transfer and reduced wear.

[0046] One or more forms of the present invention may include adding activated carbon to the mineral bearing material (such as when as a mineral slurry (mineral plus water)) to adsorb (attach) dissolved metals and remove them from the mineral slurry. The metals are stripped from the carbon by acid washing and circulation of a caustic cyanide solution. Such a process is not otherwise deemed economical with low gold content (refractory) ores. Such carbon-in-leach (CIL) or carbon-in-slurry (CIP) process follows on from cyanidation of the ore.

One or more forms of the present invention may utilise such processes downstream of the aforementioned process of oxidation of the mineral bearing material.

[0047] A further aspect of the present invention provides a mineral recovered from a process according to any one of the preceding claims. The recovered mineral may be gold recovered from, at least in part, refractory ore bearing the gold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] One or more embodiments of the present invention will hereinafter be described with reference to the accompanying Figure(s), in which:

[0049] Figure 1 shows a schematic representation of a system and process according to an embodiment of the present invention. [0050] Figure 2 shows a schematic representation of a system and process according to a further embodiment of the present invention with separation of oversize particulates fed back into the grinding mill.

[0051 ] Figure 3 shows a schematic representation of a system and process according to an embodiment of the present invention utilizing a coarse primary grinding mill with iron bearing grinding media and a fine secondary grinding mill.

[0052] Figure 4 shows a sectional representation of a Pachuca tank arrangement.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0053] In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

[0054] With reference to Figure 1 , a system/process 10 according to an embodiment of the present invention has a feed 12 of ore/ore concentrate to a mixing reservoir 14 (e.g. including mixing device/means 16). The mixing device/means 16 can include a rotatable stirrer/agitator. The ore/concentrate can be de-watered before adding to the mixing reservoir. [0055] In this process, the solid to liquid ratio can be adjusted so that a desired circuit residence time is achieved and that there is sufficient slurry fluidity to allow the solids to flow through the circuit and so that the solution does not become “saturated” with reactants, resulting in slower reaction kinetics.

[0056] Preferably, in one or more embodiments of the present invention, the slurry % solids will be between 25-45%, although other ratios may be applicable for certain concentrate types(high sulphur may require lower % solids).

[0057] Prior to the mill, agitation of the slurry can be applied sufficient to maintain the solids in suspension and this will vary from process to process depending on the nature of the concentrate.

[0058] Agitation within the mill can be achieved by the action of the mill and downstream of the mill, agitation is sufficient to keep the solids in suspension so that process lines and tankage do not become blocked or “sanded”.

[0059] Residence time for pre mill agitation can vary from seconds to minutes depending on the purpose - if the agitation is just to keep the solids in suspension during the slurry being fed into the mill, the holding tank can be sufficiently sized to allow for up-stream and down-stream process variations such that the feed tank does not become empty or overflow. If the mixing /agitation also serves the purpose of pre-mill oxidation / slurry conditioning, a holding time of 5-15 minutes could generally be considered but the decision will be based on metallurgical testing results.

[0060] A pre-mix tank can be used for the pulping of the concentrate with the recycled oxidant solution from the mill discharge and fresh oxidant. The oxidant solution could make up all or a small portion of the solution portion of the slurry - likewise, dependant of the selected oxidising material to be added to the slurry, the fresh oxidant could be added in concentrations of ppm or %. An example would be the addition of oxygen as a gaseous oxidant at a flow rate of 20-50 m ³ /h for every tonne of concentrate. Oxidant solution recycle could make up 50% of the solution content of the slurry and fresh oxidant (say hydrogen peroxide) could be added at the rate of 30-40kg/t of concentrate.

[0061] pH and temperature ranges can be dictated by the reaction chemistry, residual contaminant gangue components in the concentrate, mill residence time, energy input and particle size reduction in the mill and the level of reactants in the slurry. pH ranges can be from 1 -6 for the conditioned mill feed and the temperature from ambient to slightly elevated due to the temperature of the recycled oxidant solution and preliminary oxidation occurring in the pre-mix/mill feed tank but will generally be below the boiling point of water as the process is conducted under atmospheric pressure.

[0062] The mixture 18 can be supplied via an output 20 from the mixing reservoir 14 (e.g. via a pump 22) to an infeed 24 for grinding mill 30 (or similar). The grinding mill contains grinding media 32 to aid comminution of the mixture containing the ore. Whilst a vertical grinding/comminution mill is shown, it will be appreciated that other configurations of mill can be used. An oxidant 26 can be supplied via an input 28 into the flow of mixture 18.

[0063] Overflow of comminuted mixture 34 from the grinding mill can be fed to at least one separation tank 36 to separate solids and liquids (S-L separation). Solids 38 settle to the bottom of the separation tank(s) with the liquid 40 above.

[0064] High solid content can be supplied via an output 42 from the separation tank(s) 36 (e.g. via a pump 44) to an output 46 for further processing of the comminuted mixture. [0065] The liquid 40 containing low volume of solid particulates can be supplied via an output 48 (e.g. using a pump 50) to a feed conduit 52 and returned to the mixing reservoir 14.

[0066] It will be appreciated that the returned liquid 40 contains iron (Fe ²⁺ and Fe ³⁺ ) recovered for re-introduction into the system and process.

[0067] Mineral reaction chemistry: [0068] Ferric / ferrous reaction:

• The ferrous is re-oxidized by dissolved oxygen as follows:

4 Fe2+ + 02 + 4 H+ ® 4 Fe3+ + 2 H20

• The ferric ion then reacts with the sulphide mineral as follows:

CuFeS2 + 4 Fe3+ ® Cu2+ + 5 Fe2+ + 2 SO

FeAsS + 5 Fe3+ ® As3+ + 6 Fe2+ + SO

[0069] An alternative process and system is represented in Figure 2. The process and system is similar to that shown in Figure 2, additionally with oversize material separated (such as by a filter, screen or cyclone 54) and returned 56 to the mill 30. [0070] As shown in Figure 3, pre-grinding of the feed of mineral bearing material (such as in the slurry) can occur in a coarse primary grinding mill 60, which may contain iron bearing grinding media 62 whereby iron from such media contributes to oxidation.

[0071 ] As shown by way of example in Figure 4, a Pachuca tank is a hydrometallurgical reactor used for the leaching of nonferrous minerals. They are cylindrical vessels that normally have a conical bottom and include a central draft tube rising through it. Typically, air is injected at the base in the form of bubbles to create a circulation flow within the tank to agitate the slurry therein.