Device for Anti-dendrite electrodeposition

Abstract

The present invention relates to a device for reducing or even eliminating edge dendrites that form on a metal deposition plate (6) in a metal extraction process using electrolytic reduction, said device comprising a piece (1) that is non-conducting, at least along a first surface (2) or, respectively, a second surface (3), proximal to the anode (4) and to the cathode (5) at their respective edge (8', 8') in use, said piece (1) further having a groove (9) connecting the first and second surfaces (2, 3), said groove (9) being positioned in the direction of flow of the electrolyte to create an increase in the width of the electrolytic channel (7) between the anode (4) and the cathode (5).

Description

DEVICE FOR ANTI-DENDRITE ELECTRODEPOSITION Subject of the invention

[0001] The present invention relates to new improvements to an installation for producing iron or other pure metals or alloys by electrolytic means, in the form of metal plates deposited at the cathode of an electrolytic cell. Technological background and state of the art

[0002] It is well known that electroplating, electroplating or electroextraction systems used on an industrial scale are techniques allowing the economical and simple purification of non-ferrous metals by electrolytic means.

[0003] Furthermore, the electrodeposition of iron, in particular, accompanied by a simultaneous electrochemical reduction of its oxides such as hematite and magnetite, constitutes a very promising avenue currently in a context of need for drastic reduction of CO2 emissions in industrial steel production processes. Indeed, the reduction of iron ore by electrodeposition does not use the reducing capacities of carbon.

[0004] The production process consists of reducing iron ore to metallic iron by an electrolysis reaction, the iron ore preferably being suspended in finely divided form in an alkaline solution promoting the reduction of its oxidized state.

[0005] In particular, the process already implemented by the inventors uses caustic soda as an alkaline electrolyte in which fine particles of iron ore, in particular hematite, are suspended, as well as than other iron oxides and hydroxides. The anode is inert, for example in the form of nickel, to avoid the emission of CCh which occurs with carbon anodes. The reaction generates deposits of pure iron at the cathode in solid form, generally in plate form, and a release of gaseous oxygen at the anode, as well as parasitic hydrogen at the cathode since the electrolytic efficiency is not 100%.

[0006] Typically, the electrolytic cell for implementing the above method comprises a planar anode and cathode separated by a gap in which the electrolytic fluid circulates. The anode should preferably be perforated or gas-permeable to prevent contact of the cathode with the gas and therefore reoxidation of the iron deposited on it. Extraction of the gas also helps to ensure the passage of a uniform current between the two electrodes. Indeed, if the gas remained "stuck" in the electrolyte, there could be gas pockets between the electrodes which would generate a change in electrical resistance, with an impact on the flow of current. The cell also includes means for circulating the electrolyte, means for injecting the oxide powder according to a hydraulic loop and means for extracting the gases.

[0007] Electrolytic iron deposition requires specific conditions to be energy efficient. Among the constraints to be taken into account are the use of high-concentration sodium hydroxide and a temperature that can exceed 100°C. Hence the imperative need for a closed cell to prevent evaporation of the electrolyte and the escape of gases generated by electrolysis. A closed cell is therefore a sealed or airtight chamber that can be opened at the end of production to extract the electrolysis products.

[0008] In addition, to have the lowest possible energy consumption, the cell must have a limited distance between the electrodes, typically of the order of 10mm and of course less as the deposit grows. The difficulty with closed cells is then, given the narrowness of this inter-electrode separation, the extraction of the metal deposited on the cathode, especially for generally large electrodes (>1 m 2 ), because the cathode must remain static in the cell.

[0009] Studies carried out particularly in the field of lithium metal batteries have suggested that the distribution of the deposited metal depends on certain factors such as current density, temperature or the concentration of ions in the electrolyte.

[0010] In electrodeposition, it is thus known and common to observe an irregular distribution of deposits and in particular overloads on the edges of the electrodes which can take, for example, long dendritic forms, i.e. having the appearance of protruding needles, the growth of which can go so far as to cause the anode and the cathode to come into contact with the formation of short circuits between them.

[0011] During electrodeposition, stochastic processes thus lead to the formation of localized surface imperfections or roughness on the deposited surface. These local imperfections correspond to areas of high current density, promoting a strengthening of the deposits and leading to the growth of dendrites. This is accelerated when the electrodeposition speed approaches the limiting (faradaic) current density.

[0012] Several systems have been devised in the state of the art to counteract the problem of dendrite formation in industrial electrolysers designed for metal harvesting on a flat cathode, including: - the reduction of the anode surface area compared to the cathode surface area; - the use of pulsed currents; - the use of screens of given geometry (called masks in jargon) and interposed between the cathode and the anode; and - the use of a device with a mobile and continuously moving electrode.

[0013] It should be noted that, although several patents have been filed in this field, the majority of these relate to the electrolytic deposition of copper or non-ferrous metals in acid solution, none of which refer to the deposition of iron, nor as in the present application to a metallic deposit by reduction of the oxidized particles on the cathode.

[0014] Document RU2763699C1 relates to an electrolytic cell for extracting a metal in acid solution, which can be used in electroplating devices for electrochemical production. The electrolyzer contains a chamber, a means for injecting a solution with an extractable metal and anodes and cathodes immersed in the bath and arranged alternately. The inventors propose to use a larger cathode of the anode with a cathode / anode surface area ratio of between 1.5 and 2.5. Such an approach induces a very large loss of active surface of the cathode, a very high non-uniformity of the deposition and a very low deposition rate in weight per unit of cathode surface area.

[0015] In US2004079642A1, it is aimed to achieve efficient recovery of metals from process solutions and effluents by means of pulsed cathode currents, preferably with coupled anodic processes. To precipitate metals by means of direct current in undivided or separator-divided electrolysis cells, the pulsed cathode currents are generated by dividing the anodes into stationary strips past which the undivided cathode surface is guided. The shape and frequency of the current pulses thus formed on the cathode surface can be modified by the arrangement of the anode strips and by current diaphragms. This method cannot be applied in a closed cell as in the application which is envisaged according to the present application.

[0016] Patent US11411258B2 is similar to the previous one but differs in the nature of the current pulse.

[0017] In AU2015295324B2, reference is also made to an electroplating apparatus suitable for the electrolytic production of copper and other non-ferrous metals from ion solutions, with complete immersion of a series of anodes and cathodes separated by an ion-porous wall. The inventors propose a method for controlling the electric current delivered to each anode and cathode, thus avoiding the creation of dendrites on a particular cathode. Such a system cannot be applied in the present case where only one cathode at a time is subject to deposition.

[0018] Documents CA2907410C, as well as EP3325693B1 and TH163148A, again relating to completely submerged electrodes, propose the use of special screens positioned between the cathode and the anode, which makes it possible to reduce / control the formation of dendrites, in particular to prevent them from reaching the surface of the anode. The use of a closed cell as in the present application prohibits the presence of screens between the anode and the cathode, for reasons of overall design and in particular given the reduced distance between the anode and the cathode.

[0019] Patent US7335289B2 proposes an invention dedicated to the deposition of copper in a halide bath, where the cathode which is totally immersed in the bath has a convex and insulated shape on its edges, each of these convex sections being 3 mm or smaller in width and having angles of 80 to 110 degrees. This solution is also not applicable to a closed cell.

[0020] Document BRPI0407972B1 discloses a process for the production of metallic copper in a crystalline form substantially free of dendrites, comprising electrowinning from a cuprous and / or cupric chloride solution carried out in a fluidized bed cell comprising a cathode consisting of a descending bed of metallic beads. Aims of the invention

[0021] The present invention aims to provide a solution making it possible to overcome the drawbacks of the state of the art.

[0022] In particular, the invention aims to prevent the formation of edge dendrites during the production of iron or other pure metals or alloys by electrolytic means, in the form of metal plates deposited at the cathode of an electrolytic cell. Main characteristic elements of the invention

[0023] A first aspect of the invention relates to a closed cell for the extraction of a metal plate deposited electrolytically on a cathode, said cell comprising respectively a planar anode and a planar cathode, separated by an electrolytic channel, characterized in that the cell comprises, at each of its edges, a device for reducing, or even eliminating, the edge dendrites which form on a metal deposition plate in a process for extracting metal by electrolytic reduction, said device comprising a non-conductive part at least along a first surface, respectively a second surface, proximal to the anode and the cathode at their respective edge in use, said part further having a groove connecting the first and second surfaces, said groove being positioned in the direction of flow of the electrolyte to create an increase in the width of the electrolytic channel between the anode and the cathode, the groove of the non-conductive part having a depth, a shape and a surface roughness in contact with an electrolytic fluid of given density and viscosity, said depth,shape and roughness being adapted to provide a speed of the electrolytic fluid at the edge of the cathode greater than 40% of the average speed of the fluid, preferably between 70% and 90% and ideally equal to 80% of the average speed of the fluid, the average speed of the fluid being determined or calculated over the entire width of the intra-electrode channel and considering the point along its length where it is the lowest.,

[0024] According to preferred embodiments, the device further comprises one of the following features or a suitable combination of several of them: - the throat has a parabolic, asymmetrical, elliptical or triangular shape; - the non-conductive part is made of a solid insulating material or of a metallic material coated with an electrically insulating layer, for example a polymer layer; - the non-conductive part is made longitudinally in a single block or is multi-element to cover the length of the electrolytic cell; - the non-conductive part is designed to allow the use of an anode that is narrower than the cathode, i.e. it has a greater width on the anode side than on the cathode side. - the cell comprises an anode narrower than the cathode on each bank, the width reduction being between 10mm and 30mm, preferably between 10mm and 15mm; - the non-conductive part is positioned at a distance from the edge of the anode or cathode which is less than 20% of the distance between the anode and the cathode; - electrolytes are composed of hematite particles and possibly other oxides, such as magnetite, and iron hydroxides, suspended in an alkaline electrolytic medium; - said suspended particles have a minimum size of 200nm and a maximum size of 100pm, and preferably between 500nm and 40pm; - the cell has turbulence promoters.

[0025] Another aspect of the invention relates to the use of an electrolytic cell as described above, characterized in that either the fluid flow regime is locally laminar and corresponds to a Reynolds number Re<2000, or the fluid flow regime is transitional turbulent or purely turbulent and corresponds to a Reynolds number Re>2000 or 3000. Brief description of the figures

[0026] Figure 1 schematically represents an embodiment of the invention with an anode narrower than the cathode and an example of an anti-dendrite edge piece. Figure 1 also shows an embodiment with a parabolic groove in the anti-dendrite piece.

[0027] Figure 2 shows a graph with the distance to the electrode on the abscissa and the concentration of the solution on the ordinate for two different flow velocities with the corresponding thicknesses of the boundary layer. The curve C_V1 corresponds to the limiting density because the concentration is zero at the wall.

[0028] Figure 3 also schematically shows an alternative embodiment with a triangular groove for the anti-dendrite part.

[0029] Figure 4 shows a realistic three-dimensional view of a closed electrolytic cell for iron production having an anti-dendrite system according to the invention.

[0030] Figure 5 shows an example of an iron plate produced using the installation and method of the invention and characterized by the absence of dendritic formations at the edges. Description of preferred embodiments of the invention

[0031] The inventors observed that, in the case of electrolytic iron production from oxide particles such as hematite, by deposition on a cathode, it was possible in particular to reduce edge deposits on the cathode and therefore the possible growth of dendrites by reducing the current density in this area as well as by modifying the boundary layer. This thus makes it possible to have a sufficient fluid velocity to prevent or reduce the formation of non-compact deposits. In the present case of the reduction of iron oxides with electrodeposition on a cathode, the formation of dendrites is linked to a high current density but also to the fact that, as soon as an outgrowth appears, it causes an amalgamation of particles which reduce locally and therefore adhere and build the deposit of reduced iron. Therefore, increasing the fluid velocity also makes it possible to avoid accumulations by sweeping around the discontinuities.

[0032] According to the invention, the reduction of the current density on the cathode bank is achieved by using an anode slightly narrower than the cathode. The anode must be between 10 and 30 mm narrower than the cathode on each bank, preferably between 10 mm and 15 mm narrower than the cathode.

[0033] On the other hand, still according to the invention, a so-called "anti-dendrite" part will be used which must be made of a solid insulating material or alternatively of a metallic material coated with an electrically insulating layer. With respect to each of the two electrodes, the anti-dendrite part must be positioned at a distance corresponding to less than 20% of the distance between the cathode and the anode.

[0034] Figure 1 shows schematically an example of an anti-dendrite part 1 positioned in the apparatus in such a way that the distance 8', 8” with the edge of the anode 4, with the edge of the cathode 5 respectively, is maximum 2mm, i.e. maximum 20% of the anode-cathode distance which is 10mm.

[0035] It is known in fluid mechanics that, in the case of mass transfer at an interface such as a plate, the thickness of the diffusion (or mass) boundary layer is related to the flow velocity over the plate. More precisely, an increase in flow velocity leads to a reduction in the thickness of the boundary layer, with the consequent increase in the potential intensity of the transfer. The increase in concentration then results from the higher flow velocity as well as the shear of the flow in the vicinity of the cathode.

[0036] Thus, according to the invention, the diffusion boundary layer will be advantageously modified by widening the electrolytic channel by means of an insert 1 having a groove 9 designed to have a fluid velocity at the edge of the cathode greater than 40% of the average fluid velocity, preferably between 70% and 90% and ideally equal to 80% of this average velocity. The increase in velocity promotes, as already mentioned above, the non-accumulation of particles at existing or emerging obstacles. The average fluid velocity is calculated over the entire width and length of the intra-electrode channel. The parameters which make it possible to obtain this characteristic are the depth of the groove 9, the shape thereof as well as its surface roughness in contact with the fluid. A turbulence promoter can also reduce the minimum groove depth required.A turbulence promoter is any type of insert that disrupts the flow by generating recirculation and vortices.

[0037] The turbulent phenomena possibly caused by the presence and characteristics of the anti-dendrite part 1 and / or by turbulence promoters favor the increase in the concentration of metal oxide particles in contact with the cathode at the edges. This therefore has a beneficial impact on the quality of the iron deposit and prevents the formation of dendrites.

[0038] More precisely, the widening of the channel where the electrolyte circulates allows an increase in the speed of the flow of the latter at the edge of the cathode. The increase in the flow speed accentuates its either the shear of the flow and therefore the supply of particles, or possibly the turbulent phenomena of the flow, which reduces the thickness of the diffusion boundary layer and therefore promotes the increase in the concentration of oxide particles in contact with the cathode at the edges. The device can operate with a low flow velocity and therefore without turbulence but also with a higher flow velocity and there is then the contribution of turbulence.

[0039] This has a beneficial effect on the quality of the iron deposit and prevents the formation of dendrites. Figure 2 shows the decrease in boundary layer thickness (e2, concentration C_v2 vs e1, C_v1) for a flow velocity v2>v1.

[0040] According to embodiments of the invention, this groove 9 can have several shapes: parabolic (Figure 1), asymmetrical, elliptical, triangular (Figure 3) or any other shape.

[0041] In terms of design, it is a matter of finding a compromise between all the factors that influence the boundary layer, besides the throat depth and surface roughness, such as the shape of the throat, the flow velocity, the length of the cell, the viscosity and the density of the electrolyte.

[0042] In particular, a shorter cell, the presence of turbulence promoters or higher surface roughness require a smaller throat depth.

[0043] According to an example of execution, the surface roughness of the anti-dendrite part is approximately 2pm and the depth of the groove is 8mm.

[0044] The anti-dendrite system according to the invention was tested in the case of a closed cell for the electrolytic production of iron, the cell having an inclination of 50°C with respect to the horizontal. In this respect, a realistic embodiment is shown in Figure 4. But the anti-dendrite system could also be effective in the case of vertical cells, such as those used in electro-refining processes (this variant is not claimed).

[0045] According to one embodiment, the two parts with anti-dendrite design 1 can be manufactured as a single element, but solutions multi-elements are also possible if a significant length of the electrodes must be taken into account (see figures 4).

[0046] The iron plate produced by the above-mentioned cell is well characterized by the absence of dendritic formations at the edges (see figure 5). Reference symbols 1 piece anti-dendrites 2 end of the anti-dendrite part facing the anode 3 end of the anti-dendrite piece facing the cathode 4 anode 5 cathode 6 electrolytically deposited iron plate at the cathode 7 inter-electrode channels (circulation for electrolyte) 8', 8” distance anode (resp. cathode) - anti-dendrite part 9 throat 10 throat depth 11 direction of electrolyte flow 12 deposited iron plate edge with absence of dendrites 20 axis of symmetry of the cell

Claims

CLAIMS 1. A closed cell for the extraction of a metal plate deposited electrolytically on a cathode, said cell comprising respectively a planar anode (4) and a planar cathode (5), separated by an electrolytic channel (7), characterized in that the cell comprises, at each of its edges, a device (1) for reducing, or even eliminating, the edge dendrites which form on a metal deposit plate (6) in a process for extracting metal by electrolytic reduction, said device comprising a non-conductive part (1) at least along a first surface (2), respectively a second surface (3), proximal to the anode (4) and the cathode (5) at their respective edge (8', 8”) in use, said part (1) further having a groove (9) connecting the first and second surfaces (2, 3),said groove (9) being positioned in the direction of flow of the electrolyte to create an increase in the width of the electrolytic channel (7) between the anode (4) and the cathode (5), the groove (9) of the non-conductive part (1) having a depth (10), a shape and a surface roughness in contact with an electrolytic fluid of given density and viscosity, said depth, shape and roughness being adapted to provide a speed of the electrolytic fluid at the edge of the cathode greater than 40% of the average speed of the fluid, preferably between 70% and 90% and ideally equal to 80% of the average speed of the fluid, the average speed of the fluid being determined or calculated over the entire width of the intra-electrode channel (7) and considering the point along the length thereof where it is the lowest.

2. The electrolytic cell according to claim 1, characterized in that the groove (9) has a parabolic, asymmetrical, elliptical or triangular shape.

3. The electrolytic cell according to claim 1, characterized in that the non-conductive part (1) is made of a solid insulating material or of a metallic material coated with an electrically insulating layer, for example a polymer layer.

4. The electrolytic cell according to claim 1, characterized in that the non-conductive part (1) is produced longitudinally. in a single block or is multi-element to cover the length of the electrolytic cell.

5. The electrolytic cell according to claim 1, characterized in that the non-conductive part (1) is designed to allow the use of an anode (4) narrower than the cathode (5), namely that it has a greater width on the anode side (4) than on the cathode side (5).

6. The electrolytic cell according to claim 1, characterized in that it comprises an anode (4) narrower than the cathode (5) on each bank, the reduction in width being between 10mm and 30mm, preferably between 10mm and 15mm.

7. The electrolytic cell according to claim 1 or 6, characterized in that the non-conductive part (1) is positioned at a distance from the edge of the anode (4) or the cathode which is less than 20% of the distance between the anode (4) and the cathode (5).

8. The electrolytic cell according to claim 1, characterized in that the electrolytes are composed of hematite particles and possibly other oxides, such as magnetite, and iron hydroxides, suspended in an alkaline electrolytic medium.

9. The electrolytic cell according to claim 8, characterized in that said suspended particles have a minimum size of 200nm and a maximum size of 100pm, and preferably between 500nm and 40pm.

10. The electrolytic cell according to claim 1, characterized in that it comprises turbulence promoters.

11. Use of an electrolytic cell according to any one of the preceding claims, characterized in that either the fluid flow regime is locally laminar and corresponds to a Reynolds number Re<2000, or the fluid flow regime is transitional turbulent or purely turbulent and corresponds to a Reynolds number Re>2000 or 3000.

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