Background

The present invention relates to a process and reactor for the leaching of primary sulphide minerals, with removal of the passivation layer and the possibility of selective metal deposition on cathode.

The process is designed to leach primary sulphide minerals. Primary minerals tend to be formed through geologic processes (as opposed to secondary minerals, which tend to form under atmospheric conditions from primary minerals).

Examples of primary sulphide minerals are chalcopyrite (iron and copper ore), galena (lead ore), sphalerite (zinc ore), millerite (nickel ore) and pyrite (iron ore). This invention is relevant for primary sulphide minerals of all metals, in particular but not exclusively for the aforementioned metal ores, covering the primary mineral sulphide ores of all metals. The refractory nature of these minerals hinders the industrial viability of hydrometallurgy leaching, as it is too slow, thereby imposing pyrometallurgy as the conventional method (Haver and Wang, 1971). Therefore, the industrial use of hydrometallurgy In copper mineral processing is currently restricted to secondary copper minerals such as copper oxide.

Pyrometallurgy is very costly, principally due to the high temperatures required and toxic waste generation. Conversely, hydrometallurgy entails significantly lower operating costs and is cleaner (Dresner, 2001 ). Therefore, as hydrometallurgy is the preferred option, a hydrometallurgy process with an industrially viable rate of recovery would be preferable to pyrometallurgy. Many hydrometallurgy processes have been devised, in the hope of achieving faster recovery rates than the traditional process. Examples of these processes for chalcopyrite are outlined thus:

• The Brisa Process (Dresher, 2004)

-Firstly, Ferric iron oxidises disulphide, being in turn reduced to Ferrous iron.

-Secondly, Ferric iron is regenerated from Ferrous iron: As the latter reaction is slow, bacteria are used as catalysts to biologically oxidise Ferrous iron (indirect bioleaching).

• The Biocop process (Dresher, 2004) Bioteaching in a stirred tank reactor with hypertherrnophiles {such as Sutfolobus metallicus) in aerated sulfuric acid at up to 90oC.

• The Ga!vanox process (Dixon et al. 2011 )

Uses pyrite to galvanieally assist tha leaching of chaicopyrita.

• Dynatec process (Collins, 1 98)

Oxidative leaching with coal at 150°C.

• Total pressure oxidation {King et al. 994)

Autoclave at 220°C and 700 kPa, where water reacts with chalcopyrite to form copper sulphate and sulphuric acid.

These processes are restricted to laboratory research or demonstration plants, as they a re too slow to be i rid ustrially viable.

Another option is the electro-assisted reduction of primary copper sulphide minerals, whereby the mineral is reduced to form less refractory compounds, which can than be chemically leached (Fuentes-Aceitu o, 2008).

K is a known fact that refractory primary sulphide minerals such as chalcopyrite can be corroded at very high rates when subjected to transpassive potentials, compared to active or passive potentials (Gomez et al, 1996). However, despite this finding, transpassive electrochemical leaching of these minerals has not made the leap from laboratory Studies to industry.

The main reason for transpassive leaching not being conventionally used in industry is the problem of the passivation layer. At transpassive potentials, a passivation layer containing sulphur and other sulphur-based compounds rapidly forms, coating the mineral and adsorbing to the electrode, quickly diminishing the efficiency of the leaching process. Firstly, the passivation layer coating around the mineral particles stops leaching, as the layer is electrically isolating and may therefore prevent electric flow at the electrode* mineral and mineral-electrolyte interfaces, Secondly, the passivation layer adsorbs to the non-mineral electrode, decreasing its electrical conductivity, rendering it inefficient or completely unable to perform leaching. Moreover, the passivation layer traps a significant proportion of the leached copper metal, preventing its recovery even after it has been leached. Therefore, without a solution for this problem, It is not possible to design a reactor for industrial operation.

Another issue with the Electro-assisted transpassive oxidation of primary sulphide minerals is that the leached metal ions such as copper and iron can simultaneously deposit at the cathode. This might be undesirable, as cathodes with high purity of a metal tend to be the goal.

The here proposed process and reactor provide an electrochemical hydro metallurgy method for the leaching of refractory primary sulphide minerals, which is fast enough to be of industrial viability. The proposed process solves the problem of the passivation layer, thereby opening the possibility of long-term operation at an industrial scale. In addition, the process provides ways to prevent the mixed metal deposition at the cathode, by avoiding any deposition or by selectively depositing the metal/chemical of interest to a high purity. What is more, the process provides a mechanism to bypass solvent exchange, by concentrating selectively passing through the metaC of interest directly to/at the cathode compartment The metal of interest may be copper, yet the method enables the selection of other metal ions/chemicals present in the solution. In this way, the conventional four- step hydrometailurgy process {leaching, solvent exchange extraction, solvent exchange stripping and electrowinning) is reduced to a one-step process in which leaching, selection concentration and electro-deposition are simultaneously carried out in the reactor. Finally, the process can be user-friendly, as ft may be undertaken at room temperature relatively low temperatures and atmospheric pressure/relatively low pressures, and it is cleaner and cheaper than pyrometallurgy.

According to this invention, in a first aspect there is provided a process for leaching and optionally concentration of and/or deposition of metal(s) from material comprising metal sulp ide minerals} wherein the sulphide minerals) is (are) chemically leached using a chemical species with a reduction potential as high as or higher than that of the onset of transpassive or pseudo-transpassive oxidation of said mineral(s) and/or said chemical species having the capability of driving such a chemical leaching / electrochemical reaction..

Chemicals suitable as said chemical s ades with such transpose ive potential include for example Cobalt, Silver, Cerium, Thallium, Chlorine, Manganese, Fluorine, Lead and a number of Larrthanide ions. Such chemicals may optionally be added as salts such as Silver Sulphate, Cobalt Sulphate, and other sulphates. The chemicals may optionally be regenerated chemically (for example by subjecting them to oxidizing conditions e.g. with oxygen, fluorine or other oxidizing chemical) and/or ©lectrochemical!y (such as via oxidation at an electrode and/or via a mediator that is itself oxidized at an electrode) and/or via other direct or indirect mechanisms such as microbial oxidation. The chemical species used in the present process may be reused by in situ regeneration, direct recirculation and/or recovery and reintroduction e.g. by precipitation/cementation. Chemicals may also be discarded as waste after one-off use or at some stage in the process.

The process may be an electrochemical process optionally using an electrode of or in contact, directly and/or indirectly via a chemical mediator with said minerals), wherein at least during said leaching step the formation of any electrically isolating passivation layer upon such electrode and/or minerals) is avoided, altered, limited or minimised and/or wherein at least part of any such passivation layer that forms is removed. Further preferred and optional features of the process are to be ίονηϋ in the sub claims.

The present invention further provides, in another aspect, electrochemical ceH reactor apparatus adapted to carry into effect a process according to the first aspect; the apparatus comprising a receptacle or a number of receptacles such as a tank(s) containing liquid electrolyte, at least one anode or anode chamber, at least one cathode or cathode chamber, an electrical connection between anode and cathode cathode chambers) or between anode chamber and cathode/cathode chambers), direct and/or via additional circuitry apparatus and/or one or various source(s) of controiled electrical energy and/or supporting circuitry such as diodes and/or condensers and/or correctors and/or rectifiers and/or fuses to be input into the cell.

Oxidative leaching may arise from the half-cell potential of the anode electrode (in a chemicaOy mediated or mediator-less fashion) while removat eiteretion of the passivation layer may result from the half-cell potential of the cathode electrode (in a chemicaOy mediated or mediator-less fashion). The leaching and passivation layer removal ste s may occur simultaneously and/or alternately- The steps may take place in the same compartment and/ or in separate compartments / sub-compartments and/or connected compartments with regulated inlet and/or outlet flow rates. Compartments may be subdivided by combinations of membrane and membrane-less arrangements, e.g. anode and cathode may be isolated by cation exchange membrane and/or anion exchange membrane and/or specific selective membrane and/or other membrane/ dialysis membrane filter/ other separator/ no separator.

Detailed description

The present inventio relates to a device to perform the process of leaching of primary sulphide minerals, with prevention, altering and/or removal of the passivation layer.

The first reactor eel-tip is based on the chemically assisted leaching at oxidative transpassive potentials. This set-up consists of a tank in which the mineral and a chemical species (mediator/catalyst) with a reduction potential as high as or higher than that of the onset of transpassive leaching are mixed, providing there is sufficient reactivity. As a result chalcopyrite is oxidised and the leaching agent is reduced. Typically, providing reactivity, a chemical species with a reduction potential above 1.2 V vs. HSE could be suitable. The mineral may be oxidized directly or via the formation of other sulphur-based compounds with a lower redox potential, hence easier to leach. These compounds may be formed exclusively with the elements present in the mineral initially (i.e. mai ly copper and iron sulphides) or they may incorporate the transpassive chemical (e.g. cobalt sulphides, silver sulphides, etc.).

The second reactor set-up is based on the electro-assisted teaching at oxidative transpassive potentials. For this purpose, an electrochemical cell arrangement is proposed. In simple terms, the electrochemical cell comprises a first electrode, a second electrode and a fluid in contact with the electrodes. As the metal cations are more stable at acidic H, said fluid may be typically based oh one or more acids such as sulphuric add or hydrochloric acid.

The first purpose of said electrochemical cell reactor is to host the electro-assisted anodic transpassive oxidative leaching of the primary sulphide mineral. The onset of the transpassive behaviour depends mainly on the mineral, electrolyte composition, temperature and pH. Therefore, the choice of anodic half-cell potential will depend on these factors. Such a potential may be achieved with a galvanic set-up (by having a reduction reaction or reactions at the cathode, with higher reduction potentigl/s than that of transpassive leaching) or with an electrolytic set-up (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpassive leaching). Anodic, cathodic and cell potentials may be controlled with a poterrttostat or another source of electrical energy, which may be aided by a potentiometer-like device as well as a number of circuitry elements such as diodes, fuses, rectifiers, condensers, etc.

Oxidative leaching may arise from the half-cell potential of the anode electrode (in a chemically mediated or mediator-lees fash ton) while removal alteration of the passivation layer may result from the half-cell potential of the cathode electrode (in a chemically mediated or mediator-less fashion). The leaching and passivation layer removal steps may occur simultaneously and/or alternately. The steps may take place in the same compartment and/ or in separate compartments / sub-compartments and/or connected compartments with regulated inlet arid/or outlet flow rates. Compartments may be subdivided by combinations of membrane and membrane-less arrangements, e.g. anode and cathode may be isolated by cation exchange membrane and/or anion exchange membrane and/or specific selective . membrane and/or other membrane/ dialysis membrane filter/ other separator/ no separator.

The anode electrode may be made of the mineral being leached, or of an electrically conductive or semi-conductive material other than the mineral being leached, which may be immune to the transpassive potential used (does not get corroded and does not need to be replaced, e g. platinum) or may be a sacrificial electrode (gets corroded and needs 1» be replaced/replenished e.g. copper). Ideally, the electrode material presents an overpotentiai for the oxidation of water or the supporting electrolyte and the half-cell potential is chosen such that the main oxidation taking place is the mediated or mad is tor- less leaching in detriment of the oxidation of water or the supporting electrolyte. In the case of the electrode being made of the mineral, the aim is to corrode the electrode itself. In the case of the electrode being made of a material other than the mineral being leached, the mineral must be deposited onto the electrode and contact between the electrode and the mineral particles is necessary for electric current to flow. Therefore, the electrode may for example be pieced at the bottom of the anode chamber, such that if the particles have adequate dimensions to sink (fast enough), these may do so in order to touch the electrode. Conversely, if the particles are too small to sink or to do so fast enough, contact between the mineral and anode electrode may be ensured by mechanically pressing them together with a press filter type arrangement. A number of arrangements may be put in place to ensure contact In addition, alternative methods of mineral-electrode contact may be employed, such as hydrophobic hydrophilic or magnetic interactions. As well, suspended mineral may be subjected to leaching via chemical mediators. These mediators may have a transpassive redox potential and recirculate between the electrode, where they are oxidized, to the mineral, where they are reduced.

The mineral may be unrefined or it may be a concentrate. In the former case, the process may stili work, yet may not be profitable for low-grade ores. In addition, it may require continuous cleaning of the anode electrode or mixing dhamber(for transpassive leaching), as associated minerals such as quartz may accumulate on the surface of the electrode or in the mixing chamber, decreasing the electrode surface available for leaching of the mineral or the volume of the mixing tank. If the mineral extracted is a mixture of mainly chalcopyrrte and pyrite, the preliminary flotation process may be replaced (or aided) by pyrite-consuming microorganism cultures, such as mixed consortia based on Addithiobacillus thiooxidans and Leptospirillum ferrooxidans. In the case of concentrate, the process may be more efficient and the mineral may be delivered in a fluid as pulp, for instance from flotation, or as a drier paste.

The cathode electrode may be made of the metal of interest (if the aim is to deposit it on the cathode) or it may ba made of a different conductive or semi- conductive material. The cathode electrode is immersed in the elecfrolyte, maximising the surface area exposed to the fluid. A suitable shape for the cathode electrode would be a flat shape.

The electrolyte may also at least partly come from waste effluent for example from other processing plant processes and/or from acid mine drainage locations (e.g. natural or artificial reservotrs streams rivers). This may enable making use of the acidity and/or chemical/ion content (e.g. copper already in solution to be extracted tike the leached ions for example by electrodeposition and/or concentration and/or precipitation).

The receptacles may include tanks designated for electrodes and/or tanka designated for mixing/stirring/reaction of the mineral to be treated alone (without electrodes) optionally buffer tanks. The restriction of the mineral to certain receptacles or compartments may be optionally undertaken by the use of filters and/or other separators or separating step(s).

There are different possibilities for the design of tie electrochemical cell set-up reactor, Some are outined thus:

1) Single chamber reactor illustrated in figure 1. The reactor consists of a single chamber (2) in which the anode (1) and cathode (3) are immersed in the electrolyte, in a membrane-less electrochemical cell set-u The deposition on the cathode may be controlled by the choice οΓ cathode half-cell potential, which determines whether no metal is deposited on the cathode, whether the metal of interest is selectively deposited on the cathode, or whether a mixture of metals is indiscriminately deposited on the cathode. The electrodes are in electrical connection by a wire (4). This set-up can be operated galvanicaliy (by having a reduction reaction or reactions at the cathode, with higher reduction potential/s than that of transpassive leaching) or with an electrolytic set-up (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpassive leaching). Anodic, cathodic and cell potentials may be controlled with a potentiostat, or simply with any form of electrical input should it be required (battery, plug, circuitry elements, etc. which may be aided by a potentiometer) (5).

2) Two chamber reactor illustrated in figure 2. The reactor consists of two

chambers (2 and 4), separated by a cation exchange membrane (3), in which the anode (1) and cathode (5) are respectively immersed in the electrolyte. Again, the electrodes are in electrical connection by a wire (6). and this set-up can be operated galvanically (by having a reduction reaction or reactions at the cathode, with higher reduction potential/s than that of transpassive leaching) or with an electrolytic set-u (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpassive teaching). Anodic, cathodic and cell potentials may be controlled with a potentiostat or simply with any form of electrical input should it be required (battery, plug, circuitry elements etc. which may be aided by a potentiometer) (7).

3) Two chamber reactor illustrated in figure 3. The reactor consists of two chambers (2 and 4), separated by a proton exchange membrane (3), in which the anode <1) and cathode <S) are respectively immersed in the electrolyte- The membrane stops the metal cations resulting from leaching to migrate from one chamber to the other, thereby preventing their deposition on the cathode during leaching. Again, the electrodes are connected by a wire (6), and this set-up can be operated galvanicaily (by having a reduction reaction or reactions at the cathode, with higher reduction potentiai/s than that of transpassive leaching) or with an electrolytic set-up (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpessrve leaching). Anodic cathodic and cell potentials may be controlled with a potenftostat, or simply with any form of electrical input should it be required battery, plug, circuitry elements etc. which may be aided by a potentiometer) (7).

The cathode chamber may serve for metal deposition if the metal of interest is retrieved from the leachate solution at the anode and introduced into the cathode chamber, far example by solvent extraction.

4) Two chamber reactor illustrated in figure 4. The reactor consists of two chambers (2 and 4), separated by an anion exchange membrane <3), in which the anode (1) and cathode (S) are respectively immersed in the elBclroiyte. The membrane stops the metal cations resulting from leaching to migrate from one chamber to the other, thereby preventing their deposition on the cathode during leaching. Again, the electrodes are connected by a wire (6). and this set-up can be operated galvanicaily (by having a reduction reaction or reactions at the cathode, with higher reduction potentiai/s than that of trenspessive leaching) or with an electrolytic set-u (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpassive leaching). Anodic, cathodic and cell potentials may be controlled with a potentiostat, or simply with any form of electrical input should it be required (battery, plug, circuitry elements, etc. which may be aided by a potentiometer) (7).

The cathode chamber ma serve for metal deposition if the metal of interest is retrieved from the leachate solution at the anode and introduced into the cat ode chamber, e.g. by solvent extraction.

5) Two chamber reactor illustrated in figure 5. The reactor consists of two chambers (2 and 4), separated by an ion-selective membrane specific for specific lon s of a metal of interest (3), in which the anode (1) and cathode (5) are respectively immersed in the electrolyte. The membrane only (predominantly) allows the migration of ions of trie metal of interest from one chamber to ti other, which enables to selectively deposit the metal of interest on the cathode, as long as the cathode potential is adequate for such a reaction to occur. Again, the electrodes are connected by a wire (6>, and this set-up can be operated ga!vanically {by having a reduction reaction or reactions at the cathode, with higher reduction potential/B than that of transpassive leaching) or with an electrolytic setup (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpassive leaching). Anodic, cathodic and cell potentials may be controlled with a pcrtentiastat, or simply with any form of electrical input should it be required (battery, plug, circuitry elements etc. which may be aided by a potentiometer) (7).

6) Two-chamber reactor illustrated in figure 6. The reactor consists of two chambers (2 and 4), separated by a combination of ion selective membranes (3 and 8). Of particular relevance is the configuration in which the anode (2) and cathode (4) are connected via a first, ion-selective membrane (3) , which blocks metal torts, and via a second, ion-selective membrane with high selectivity for a particular metal of interest (8), which allows its migration from one chamber to the other, thereby enabling its selective deposition on the cathode, providing the cathode potential is adequate. Again, the electrodes are connected by a wire (6), and this set-up can be operated galvanically (by having a reduction reaction or reactions at the cathode, with higher reduction potential/s than that of transpassive leaching) or with an electrolytic set-up (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpassive leaching). Anodic, cathodic and cell potentials may be controlled with a potentiostat. or simply with any form of electrical input should it be required (battery, plug, circuitry elements etc. which may be aided by a potentiometer) (7).

7) Multi-chamber reactor illustrated in figure 7. The reactor consists of an anode chamber (2) containing the anode (1), connected to multiple cathode chambers (4) containing the cathodes (5), where the cathode-anode separation is a particular cation-selective membrane (3) and the cathode-cathode separation is an electrolyte-impermeable or cation-impermeable material or may be separate compartments in alternate positions Hh various anode electrodes. In this way. the metals of interest present in the solution may be selectively deposited on different cathodes, using efectrowinning and/or electrorefirring processes during electrochemical leaching at he anode or in a separate step. Again, the electrodes are connected by a wire (6), and this set-up can be operated galvanically (by having a reduction reaction or reactions at the cathode, with higher reduction potent ta 1/8 than that of transpasstve leaching) or with an electrolytic set-up (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that oftranspassive leaching). Anodic, cathodic and eel potentials may be controlled with a potentiostat or simply with any form of electrical input should it be required (battery, plug, circuitry elements etc. which may be aided by a potentiometer) (7).

The use of ton-selective membranes selective for various metals in addition to the principal metal of interest may be particularly useful to recover metals present in low proportions. 8) Multi-chamber reactor illustrated in figure 8. The reactor consists of an anode chamber (2) containing the anode (1). connectad to muitiple chambers (4 and β) via particular cation-selective membranes (3), where the non- anode chambers are separated from one another by an electrolyte-impermeable or cation-impermeable material or may be separate compartments in alternate positions with various anode electrodes. Certain chambers (4) with an electrode (5) may have the role of cathode, whereas other chambers (8) without electrode may act as deposits, for specific metal ions for which the delimiting membranes are selective. Again, the electrodes are connected by a wire <6), and this setup can be operated galvanically (by having a reduction reaction or reactions at the cathode, with higher reduction potentials than that of transpassive leaching) or with an electrolytic set-up (by applying an additional potential difference when the reduction potential of the desired cathode reaction is tower than that of transpassive leaching). Anodic, cathodic and cell potentials may be controlled with a potentiostat, or simply with any form of electrical input should it be required (battery, plug. etc. which may be aided by a potentiometer) (7).

The use of ion -selective membranes selective for various metals in addition to the principal metal of interest may be particularly useful to recover metals present in low proportions. 9) Mufti-chamber reactor illustrated in figure 9. The reactor consists of an anode chamber (2) containing the anode (1 ), connected to multiple chambers (4 and Θ) via combinations of ion-selective membranes (3 and 8). where the non-anode cham bers are separated from one another by an electrolyte- Impermeable or cation-impermeable material or may be separate compartments in alternate positions with various anode electrodes, Certain chambers <4) with an electrode (5) may have the rale of cathode, whereas other chambers (9) without electrode may act as deposits, for specific metal ions or chemicals for which the delimiting membranes ere selective. Again, t e electrodes are connected by a wire (6), and this set-up can be operated gaivanically (by having a reduction reaction or reactions at the cathode, with higher reduction potent tai s than that of transpassive leaching) or with an electrolytic set-up (by applying an additional potential difference when the reduction potential of the desired cathode reaction is lower than that of transpassive leaching). Anodic, cathodic and cell potentials- may be controlled with a potentiostat. or simply with any form of electrical input should H be required (battery, plug, circuitry elements etc. which may be aided by a potentiometer) (7). The membrane divisions between the anode and cathode compartments may be a combination of a first, ion- selective membrane (3), which blocks metal ions, and via a second, ion-seiecttve membrane with high selectivity for a particular metal of interest {8}, which allows its migration from one chamber to the other, thereby enabling its selective deposition on the cathode, providing the cathode potential is adequate. This is the preferred embodiment for the reactor.

The use of ion-selective membranes selective for various metals and chemicals in addition to the principal metal of interest may be particularly useful to recover meta!e and chemicals present in low proportions.

An example for figure 9 would be the oxidation of chalcopyrite at the anode, whereby copper ions and iron ions are released into the solution. One cathode chamber couid be selective for copper and the other for iron, resulting in the deposition of copper on one cathode and of iron on the other cathode. The remaining chamber could be a deposit for a trace element such as lead.

The anode and cathode chambers need not be connected directly via a single membrane; there can be spaces filled with electrolyte separating the various compartmentB. Moreover, the chambers and electrodes may be arranged in various configurations, including the juxtaposition of various cathodes next to an anode Or various anodes next to a cathode, to the alternation of parallel plates such as in a typical industrial electrowinning eel! (Bnode-cathode-ano de-cathode, etc.).

Another example lor figure 9 would be the oxidation of a chemical rnediator/cataly&t at the anode, such that the oxidized chemical has a transpassive redox potential, therefore being able to, in turn, oxidise the mineral in suspension. The mineral can be in the anode compartment in any compartment in the electrochemical reactor tank, or in a separate compartment. The preferred configuration is for the chemical leaching agent to be oxidized at the electrode such that the solution in the electrochemical reactor contains a high concentration of its oxidized form. This solution then flows into a separate mixing/buffer tank into which the mineral is fed (in a batch, semi-batch or continuous fashion); here, the mineral is oxidized by the leaching agent, releasing the metals of interest into solution. In this process, the leaching agent is reduced, and hence the solution in this reaction lank contains a lower concentration of its oxidized form than in the electrochemical tank solution. This solution flows into the electrochemical reactor again, in order for the leaching agent to be reoxidised-

The mixing tank may contain mechanisms for floating and collecting of elemental sulphur resulting from the reacfon(s). (These mechanisms may be undertaken in situ, ax situ, simultaneously or at different times, e.g. if more than one mixing tank is used, hence reaction can reach completion, after which collection of the elemental sulphur may take place. The tank may be in room temperature and atmospheric pressure conditions, or may be heated/cool ed/at equilibrium temperature and at lower or higher than atmospheric pressure (e.g. autoclave conditions).

It might be necessary to change the conditions of the mixing tank, e.g. redox potential, pH, temperature, mixing stirring speed, etc or to have a series of different mixing tanks Such that the mineral is subjected to a series of different conditions (this may be of particular importance in order to effectively remove the passivating layer). in one form of apparatus in which contact between the mineral and the andde electrode is ensured by mechanically pressing them together, this may be effected by forming cakes by a fitter press mechanism, optionally by ensuring exposure of the cake to the electrolyte e.g. the filter press enables electrolyte-mineral contact via channels/perforations/other orifices in the mechanism pressing the filter to the cake. Such mechanism may optionally be automated and/or operated continuously and/cr as semi-batch and/or as batch and/or alternated with a cleaning flushing step(s) and/or in situ and/or ex situ. Such mechanism may make use of for example one or more of the following: filter pressing mechanism (hydraulic, with screws, springs and/or other resorts), one or more ports per electrode/electrode face electrode chamber situated adjacent to the electrode or within the electrode itself optionally with the function of supplying mineral (optionally pulp) and/or letting out reacted/un reacted mineral and/or cleaning/flushing/rinsing/replenishing the electrolyte.

The problem of ttte passivation layer can be tackled in different ways. One option is to operate the reactor above the melting point of the passivation layer (typically at least above 115°C>. An electrolyte with a higher boiling point may be chosen in order to operate at atmospheric pressure. Conversely, if the electrolyte has a lower boiling point, the reactor may be operated at above-atmosphere pressure. Another option is to chemically or electrically assist the oxidation or reduction of the passivation layer. This may be done in a step simultaneous to or alternating with transpassive leaching. In the case of removi ng the passivation layer by reducing it, the polarity of the cell can be inverted with respect to the leac ing step arrangemet; the electrodes with the role of anode during transpassive leaching becomes the cathode in the passivation layer removal step, and the electrode with the role of cathode during transpassive leaching becomes the anode in the passivation layer removal step. If, during reductive passivation layer removal, the anode potential is high enough to corrode the electrode and this wants to be avoided, a different cathode electrode may be used for this step. Another way of avoiding corrosion of the anode after potential reversal is to subdivide the initial anode electrode into distinct sections that can then be used as anodes and cathodes, without the need to use the initial cathode as an anode in the reversal step, There are a nuber of ways of undertaking this step. Another possibility would be to undertake the reduction step in a different tank, either reacting the mineral in situ or flowing this solution with a reducing potential into the initial or subsequent mixing buffer tanks. As this step might generate hydrogen sulphide, hydrogen sulphide oxidising microorganisms could be used for rts consumption, either in situ in the reactor itserf or ex situ in a separate tank. Possible microorganisms could for example be ThiobaeiUus denitrificans for room temperature operation, or Suifolobus tokodaii for higher temperature operation. Other mechanisms for the removal of hydrogen sulphide can also be employed, such as ferric oxidation (again, the iron can be maintained in its ferric form by oxidation via an electrode, chemical or indirect such as biological mechanism). This step may also generate colloidal sulphur in the leachate, resulting from the reaction of hydrogen sulphide or other forms of sulphur such as polysulphide with sulphuric acid and ferric iron in solution. This sulphur could be for example floated, distilled, filtered, centrifuged, liquid/solid separated, phase separated, Dr further biologically consumed by sulphur-oxidising microorganisms such as Addithiobacilius thiooxtdans for room temperature operation and Suifolobus acidocaldarius for higher temperature operation. The reaction of hydrogen sulphide with ferric iron in solution may be sided by ferrous iron-oxidising microorganisms, such as Leptospirillumferrooxtdans for room temperature operation and Sulfotobus metal licus for higher temperature operation. Another possibility is the biological removal of the passivation layer with a culture of sulphur-oxidising microorganisms such as Acidithiobaciilus thiooxidans for room temperature operation and Suifolobus acidocaldarius for higher temperature operation, optionally used in conjunction with ferric iron-oxidising microorganisms such as LeptospiriUum ferrooxidans for room temperature operation and Suifolobus metallicus for higher temperature operation. The passivation sulphur layer may also be removed by mechanical mechanisms such as polishing, scraping or hosing, in order to physically remove the passivation layer from the mineral and electrode surface. Alternatively, the passivation sulphur layer may be chemically removed, by dissolution in a solvent such as carbon disulphide or acetone.

The ion-selective membranes are commercially available in the case of proton exchange membranes, general cation exchange membranes and anion exchange membranes. However, cation exchange membranes specific for particular ions of interest do not tend to be commercially available. They may be synthesised using a combination of all or a few of the following: electroactive material (such as an ionophore), plasSciser (such as OOP), anion discriminator (such as NaTPB) and PVC (Gupta et al. 2003). Other types of membranes that could be used are (commercially available) glass membranes and crystalline membranes. The reactor may be fitted with a system to recycle the electrolyte, by recirculating K upon adjustment of pH and other parameters. In the case of a sulphuric acid based electrolyte, sulphuric acid may be generated from sulphur and hydrogen sulphide in the solution, for example chemically or by sulphur oxidising and hydrogen sulphide oxidising microorganisms such as those aforementioned. The may be done in situ in the reactor or ex situ in a separate tank from which it is the electrolyte is recirculated.

Finally, an industry set-up could require maximising the electrode surface area or adjusting parameters such as currant density, plate distance and faradaic efficiency. For this purpose, the reactor would Ideally be scaled up by having a stacked arrangement with repeating sections consisting of many layers of the reactor morphological unit. Intelligent control systems for physical, electrical and chemical parameters of the system may monitor and adjust the reactor. In addition, automated transport systems may be put in place, for processes such as mineral delivery to the electrode or reaction compartment or mixing chamber, electrolyte recirculation, leachate transport and cathode extraction. In addition, the reactor design could enable changing the membrane and electrodes as well as other component parts without the need for disassembly.

References

■ Collins. . J„ and Kofluk. O. ., 1998, U.S. Patent 5. 730 - 776.

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