FIELD OF THE INVENTION The invention relates to an electrolyser comprising a plurality of elementary cells suitable for the electrowinning of metals - in particular for the electrolytic production of copper and other non-ferrous metals starting from ionic solutions - and to the

electrowinning plant in which such electrolyser is installed. BACKGROUND OF THE INVENTION

Electrochemical plants for non-ferrous metal deposition, such as for example plants for electrolytic extraction and refining of metals, also known respectively as electrowinning and electrorefining plants, typically make use of one or more electrolysers comprising a plurality of elementary cells, each containing an anode and a cathode.

In electrolysers intended for the above mentioned plants, the anodes and the cathodes are generally arranged in the electrolytic bath in alternate positions and mutually parallel. Each electrode is mechanically and electrically connected to a hanger bar and is supplied with electricity through the contact of its respective hanger bar with a busbar. In an electrolyser, the same bus-bar is shared between electrodes of same polarity, mutually connected in parallel.

In the case for example of plants for the production of non-ferrous metals such as copper, cobalt, zinc or nickel, the metal produced by the electrochemical reaction is deposited under the action of the passage of electrical current onto the cathode of each elementary cell. The deposited product is harvested at periodic intervals, typically of some days, upon extraction of the cathodes from the relevant electrolyser. The deposition of metal onto the cathode surface may take place in a non-uniform fashion giving rise to localised deposits, also known as dendrites or dendritic formations, growing towards the facing anode at increasing speed under the effect of the electrical current passage, until the onset of an electrical short circuit. In such case, the

temperature increase of the anodic surface in correspondence of the contact with the dendritic formation may cause severe damages; in some cases, the dendritic formation gets locally welded to the anode surface, hindering the subsequent cathode extraction and consequently the whole metal harvesting operation. The above mentioned problems related to the formation of dendrites are particularly relevant in the anodes of modern conception manufactured out of titanium substrates, such as meshes or expanded sheets, provided with a coating of catalytic material.

Although such anodes are preferred due to their increased efficiency compared to conventional lead anodes, short circuit conditions can often create extensive and irreparable damages thereto. Such problem remains unsolved also with the advanced anode type disclosed in patent application WO2013060786, wherein a catalyst-coated titanium mesh is inserted inside an envelope consisting of a permeable material, such as a porous polymeric separator or an ion-exchange membrane. The damaging of the anodes entails higher plant maintenance costs, loss of metal production and further damages associated to the possible forced shutdown of the plant.

It would therefore be desirable to provide a form of protection of the anodes of the elementary cells of an electrolyser in the case of growth of dendritic formations. It is also desirable to promptly identify and report the individual anodes of an electrolyser in which the growth of one or more dendritic formations constitutes a potential threat. Since metal electrowinning plants are unhealthy environments due to the high temperatures and the possible presence of acid mists in the vicinity of the electrolysers, the detection and in particular the signalling of the individual anodes affected by dendritic formations also has the purpose of allowing a quicker intervention by the plant maintenance personnel, reducing their residence time inside the electrolysis room.

SUMMARY OF THE INVENTION

Various aspects of the invention are set out in the accompanying claims.

Under one aspect, the invention relates to a an electrolyser for the electrowinning of metals made of a plurality of elementary cells, each cell comprising an anode and a cathode with an electrically conductive porous screen interposed therebetween, the anode being provided with a catalytic surface towards oxygen evolution reaction and the cathode being suitable for the deposition of metal from an electrolytic bath. The cell further comprises an electrically conductive anodic hanger bar, electrically and mechanically connected to the anode, and an electrically conductive cathodic hanger bar, electrically and mechanically connected to the cathode. Each elementary cell also comprises a device suitable for direct or indirect detection of the electric current flowing across the corresponding anodic hanger bar. The electrolyser is also equipped with an anodic bus-bar, electrically connected to the anodic hanger bars of each cell and a cathodic bus-bar electrically connected to the cathodic hanger bars of each cell. The electrolyser may also comprise a cathodic balancing bar placed parallel and in proximity to the anodic bus-bar.

The conductive porous screen, interposed between the anode and the cathode of the elementary cell is a structure that can present different degrees of compactness and is made in such a way as to allow the passage of the electrolytic solution without interrupting the ionic conduction between cathode and anode.

By carrying out the electrolysis with an electrolyser design as hereinbefore described, the dendrites that may be formed on one or more cathodes of the elementary cells come into contact with the facing porous screen before they can reach the anodic surface aft so that their growth is stopped, or in any case slowed down. It was observed that in case a dendritic formation comes into contact with the porous screen, part of the metal produced in the cell is deposited directly on the screen as a coating, provided the screen has some electrical conductivity. In this case, the assembly consisting of cathode, dendrite and porous screen, by virtue of the existing electrical connection between these elements, additionally performs the function of new cathode of the elementary cell, furthermore being placed closer to the anode than the original one. In this situation, the lower ohmic drop in the electrolyte associated to the reduced gap between the new cathode and the anode causes an increase in the electric current flowing across the relevant anodic hanger bar. It was found that the extent of this current increase can be used as an indication of dendrite growth.

In one embodiment, the direct or indirect detection of the electric current flowing in each anodic hanger bar can be effected on the bar itself, or on elements electrically connected thereto, by means of a detection device capable of measuring voltage or temperature variations.

In one embodiment, the measurement of voltage variation is effected through the connection of the detection device to the relevant anodic bus-bar on one side and to the cathodic hanger bar on the other side by means of an electrically conductive and optionally flexible pressure contact. This configuration can give the advantage of avoiding fixed electrical connections on the anodic hanger bar, facilitating subsequent maintenance operations of the cell.

In a further embodiment, the measurement of voltage variation is effected through the connection of the detection device to the corresponding anodic hanger bar in two points, located at a certain distance along the major axis thereof. In one embodiment, the measurement of the temperature variation can be effected by means of a thermosensitive device, for example a thermocouple. This measurement may be done, for example, with the thermosensitive device installed on each anodic hanger bar, preferably in correspondence of the terminal portion thereof, or alternatively on the anodic bus-bar in correspondence with each point of contact with the anodic hanger bars. The thermosensitive device may be equipped with a chemically resistant lining, suitable to protect and/or to increase its thermal insulation from the surrounding environment.

In another embodiment, the measurement of the temperature variation can be effected through the use of thermochromic paints which change their colour whenever the temperature exceeds a predetermined threshold. Such paints are applied either on the anodic hanger bar or on the anodic bus-bar in correspondence of the point of contact with the anodic hanger bar. This embodiment can have the advantage of making potentially critical situations determined by the growth of dendritic formations

immediately evident to the personnel working inside the electrochemical plant, also enabling a quickly identification of the electrolyser and of the anode or anodes in such electrolyser where these situations occur. In one embodiment, each detection device can be connected to its own microprocessor configured for the comparison between the measurement made by the device and a predetermined reference range; if the measure does not fall within the reference range, the microprocessor can activate one or more signalling systems acting sequentially or simultaneously. The microprocessor and/or the signalling system can be turned off during the operation of cathode extraction, e.g. in view of product harvesting. The microprocessor can be integrated with the signalling system and/or the detection device within a single unit. In one embodiment the microprocessor is powered by the process electrical voltage, so as to avoid the use of batteries which would require a periodic replacement. In particular, the microprocessor can be connected directly to the anodic bus-bar and to the cathodic balancing bar in case the electrolyser is equipped therewith. If the electrolyser does not include a cathodic balancing bar and one wishes to avoid fixed wirings which would interfere with the plant operations, the microprocessor directed to monitor a certain anodic hanger bar can be connected to the anodic bus-bar and to the hanger bar of the adjacent cathode via a preferably flexible pressure contact.

In one embodiment, the microprocessor actuates at least one signalling system consisting of a light-emitting diode which can be coupled to an optical fibre, either directly or through an optical coupling device. The optical fibre, optionally lined with a polymeric material, allows transferring the light signal to the terminal portions of each anodic hanger bar or even better to the outside of the electrolyser, thereby facilitating its identification by the plant operating personnel and allowing to quickly spot the electrolyser and the relevant anode or anodes presenting direct or indirect current values outside the range of predetermined values.

In one embodiment, the porous screen can be made of carbon fabrics of suitable thickness. In another embodiment, the porous screen can consist of a mesh or punched sheet made of a corrosion-resistant metal, for instance titanium, provided with a coating catalytically inert towards oxygen evolution reaction. In one embodiment, the

catalytically inert coating can be based on tin, tantalum, niobium or titanium, for example in the form of oxides. In one embodiment, the anodes are obtained from titanium meshes or expanded sheets coated with a catalytic material. In yet another embodiment, the catalyst-coated titanium mesh is inserted inside an envelope consisting of a permeable separator, for example a porous sheet of polymeric material or a cation-exchange membrane, fixed to a frame and surmounted by a demister.

The optimum porous screen-to-anode surface gap depends on the process

characteristics and the overall size of the plant. In plants used to verify the invention, the best performances were obtained with cells employing anodes and cathodes spaced apart by 25 to 100 mm and porous screens positioned at a distance of 1 -20 mm from the facing anodes.

Under another aspect, the invention relates to an anodic element for elementary cells of electrolysers for metal electrowinning comprising an anode having a catalytic surface towards oxygen evolution reaction, a porous screen, an anodic hanger bar mechanically and electrically connected to the anode and a device suitable for direct or indirect detection of the electric current flowing across the anode hanger bar. The device suitable for direct or indirect detection of the electric current can be made as

hereinbefore described and can be optionally connected to a microprocessor suitable for comparing the detected value with a predetermined range of values and actuating one or more alert signals in the event that the detected value is not comprised in the preset range. The alert signal can be acoustic, visual, electromagnetic or of any other nature, and can consist a combination of multiple signals.

Under another aspect, the invention relates to a elementary cell of an electrolyser for metal electrowinning, comprising:

- an anode with a catalytic surface towards oxygen evolution reaction;

- a cathode suitable for the deposition of metal from an electrolytic bath arranged parallel to said anode;

- a porous screen interposed between said anode and said cathode;

- an electrically conductive anodic hanger bar, integral with the anode and

electrically connected thereto;

- a device suitable for direct or indirect detection of the electric current flowing

across the anodic hanger bar; - an anodic bus-bar, electrically conductive and electrically connected to the anodic hanger bar.

Under another aspect, the invention relates to a process for obtaining copper from a solution containing cuprous and/or cupric ions comprising electrolysing the solution inside an electrolyser as hereinbefore described.

Some implementations exemplifying the invention will now be described with reference to the attached drawings, which have the sole purpose of illustrating the reciprocal arrangement of the different elements relatively to said particular implementations of the invention; in particular, drawings are not necessarily drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows an elementary cell of an electrolyser for metal

electrowinning representing an embodiment of the invention.

Figure 2 schematically shows a possible system for signalling the growth of dendritic formations in an elementary cell of an electrolyser for metal electrowinning representing another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 schematically shows an elementary cell of an electrolyser for metal

electrowinning comprising an anode (100) and a cathode (200) suitable for the deposition of metal from an electrolytic bath arranged parallel to the anode, a porous screen (300) interposed between anode and cathode, an anodic hanger bar (400) integral with the anode and electrically connected thereto, a cathodic hanger bar (450), a device (500) suitable for direct or indirect detection of the electric current flowing across anodic hanger bar (400), an electrically conductive anodic bus-bar (600) in electrical connection with anodic hanger bar (400). Device (500) suitable for direct or indirect detection of the electric current can be connected to a microprocessor (700) configured for comparing the quantity detected by device (500) with a predetermined range of values. Microprocessor (700) is connected to a signalling system (800) actuated in case the detection provides a value outside the reference range.

Figure 2 shows a device (500) suitable for direct or indirect detection of the electric current connected to a microprocessor (700) for comparing said detection with a predetermined range of values. Microprocessor (700) is configured to actuate signalling system (800) in case the detection provides a value outside the reference range.

Signalling system (800) can consist of a light-emitting diode (801 ) which emits a light signal in case of actuation by microprocessor (800). The signal of diode (801 ) is transported by an optical fibre (803), optionally coupled to diode (800) via an optical coupling system (802).

The extremity of the fibre from which light signal (803) is output is placed in an appropriate position, possibly easy to identify by staff (900) operating in the plant.

The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

Throughout the description and claims of the present application, the term "comprise" and variations thereof such as "comprising" and "comprises" are not intended to exclude the presence of other elements, components or additional process steps.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.