ELECTRODE FOR ELECTROREFINING

Abstract: A carbon block acts as a cell electrode. Channels are formed in its face which is to face the cell diaphragm. The channels provide an interconnected network including retention pools arranged to hold, release, break up and mix a liquid stream passing through them.

Description: This invention relates to an electrode for use in electro-refining of metals, to a sub-assembly including the electrode, to a cell including the sub-assembly and to an electrorefining and an electrowinning method using the cell. A known packed bed cell for electrorefining metals is described in UK Patent Specification 1515216, and comprises an anode compartment containing a bed of conductive particles, such as carbon or a refractory hard metal such as TiB₂, in a salt which is molten or in a conductive solution, means for passing a stream of molten metal or molten salt or salt in a conductive solution into the bed, a diaphragm of which one side (at least in part) bounds the anode compartment, a cathode compartment containing a bed of conductive particles in a salt which is molten or in a conductive solution on the other side of the diaphragm, which is pervious to the salt(s) but not to the molten metal. The cathode compartment may have means for passing a stream of molten metal through the bed. The anode compartment may have means for recirculating the liquid passed into and through it. The diaphragm is saturated with the salt and, although preventing mixing of molten metal from opposite sides thereof, it is pervious to the salt and thus does allow metal ions to move through freely. The conductive particles may for example be granules of carbon or of titanium diboride; even metal particles can be used if unattacked by the salt(s) or the metal being refined and its contaminant(s). The salt is preferably a halide, (usually these are cheaper), e.g. zinc chloride or aluminium chloride, either possibly including as impurities or diluents up to 95% of sodium chloride and/or potassium chloride and/or lithium chloride. The salt advantageously is or includes a salt of the metal to be refined. Although the salt at the anode most conveniently has the same composition as that at the cathode, this is not essential. The metal may be zinc including as impurities for example aluminium, lead, cadmium, copper, tin and/or iron. Such a combination of impurities may arise when recovering zinc from scrap diecastings. The metal may alternatively be aluminium, which may include as impurities such metals as zinc, tin, lead, copper and/or gold. Another cell for electrolytic refining of molten metal is disclosed in European Patent Application 49600. Here, the impure metal flows over slightly inclined graphite trays having passages for flow of the molten metal, which follows a serpentine path from tray to tray. The trays constitute an electrical connection between the anode layer of one refining stage and the cathode layer of the next refining stage, each thus representing a successive and consecutive refining step. In an alternative arrangement, EP-A-49600 discloses a cell with many refining sections arranged in parallel, several such cells being intended to be connected in series. According to the present invention, an electrode for use in electrowinning or electrorefining of metals comprises an electronically conductive block having a generally planar face, the face having formed therein channels of varying cross section and direction which form a network of interconnecting channels which provide a plurality of continuous routes for molten metal to flow overall downwardly across said face when the electrode is in its use orientation, said network comprising a plurality of downward channels interconnected by generally horizontal distribution-and-mixing channel leading to further downward channels, thereby promoting mixing and break-up of liquid flowing along the routes. Retention pools may be provided with exits constricted to break up the flow of liquid, and leading to further similar retention pools. The retention pools may be interconnected via the generally horizontal distribution-and-mixing channels. The block may be of any inert electronically conducting material, such as carbon. The invention extends to a sub-assembly comprising the electrode with a diaphragm impervious to molten metal but pervious to metal ions placed facing said one face with an optional electrically insulating sheet or sheets e.g. of mica interposed and shaped to expose the channels to the diaphragm. The invention extends to a cell comprising the sub-assembly set forth above with a second electrode as set forth above sandwiching the diaphragm. An alternative sub-assembly comprises the electrode with such an insulating sheet and/or the electrode mounted in a slot-in frame adapted to receive electrodes, sheets if any and diaphragms. The invention extends to a cell comprising the sub-assembly fitted with a second electrode as set forth above and with a diaphragm interposed between the electrodes. In both cases the second electrode's channels may be substantially a mirror-image of, and in registry with, the first electrode, or the second electrode could have a plane surface facing the diaphragm, in which case some second-electrode/diaphragm separation is advisable, so that any material electrodeposited on the second electrode will not pierce the diaphragm. The diaphragm may be a fibrous ceramic fabric impervious to molten metal. It should be mounted either touching the electrode or may be spaced slightly from the electrode face; in the latter case, molten metal will not enter the space if it is kept small enough for surface tension to restrain it. As the diaphragm cannot, as a practical matter, be relied upon to remain so taut that this spacing is always accurately assured, the face of the electrode may be insulated and hence the diaphragm protected by a mica sheet cut out to fit the face of the electrode, i.e. reveal its channels. A plurality of cells as set forth above may be arranged contiguously, that is with the anode of a first cell serving also as the cathode of a contiguous second cell, with the anode of the second cell optionally serving as the cathode of a contiguous third cell, and so on as often as desired. The invention extends to a method of refining using the cell set forth above, comprising passing a stream of molten metal through the channels of the first electrode in the presence of a conductive salt saturating the diaphragm, and making the first electrode anodic with respect to the second electrode, and recovering the refined metal(s) which appear at the cathode. In place of the molten metal, a conductive salt of the metal to be recovered may be used, so that the cell is effecting a primary metal-electrowinning from salt. By "conductive" in relation to salt, we mean that the salt is molten or in a conductive solution. The invention will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is an end elevation of an electrode according to the invention, and Figure 2 is a schematic plan of a cell according to the invention used in a possible refining scheme. Turning to Figure 1, a cuboidal graphite block 150 mm high x 100 mm wide x 30 mm thick has a network of channels machined out to a depth of 3 mm on one face. Alternatively, the channels could have been formed by pressing carbon in a shaped and pre-profiled mould to make the channelled electrode, or otherwise. The channels consist of narrow straight elements running between wider retention pools. The channels are at 20 mm centres, the horizontal straight sections being about 5 mm wide, the vertical sections being narrower and the pools being 15 mm across. The arrangement is intended to cause the metal stream to change direction many times and to be well stirred and mixed while also ensuring its retention in pools for reasonable periods. It is possible for the electrode to be grooved such that some 80-90% of its surface area is molten metal. The arrangement of grooves further seeks to restrain the downward flow of molten metal in such a way that the body of liquid is broken up such as to impose a hydrostatic head nowhere exceeding about 1 cm. (If the block 1 had a plain uniform serpentine channel conveying a continuous body of molten metal, the hydrostatic head of metal imposed on the base of any adjoining diaphragm would be equivalent to the full 150 mm.) In Figure 2, as seen in plan, the block of Figure 1 acts as an anode 1. A mirror-image block of graphite acting as a cathode 3 is mounted in registry with the anode, the two electrodes sandwiching a diaphragm 2. The elements 1, 2 and 3 are mounted with slight clearance (too small to be illustrated) into a prefabricated slot-in frame (not shown). The diaphragm 2 is a fibrous ceramic fabric consisting of aluminosilicate or silica fibres felted or spun and woven to form a material e,g. Fiberfrax PH (Carborundum Co.) or Triton Kaowool (available from Morganite) in 1cm (half-inch) or 2½cm (one-inch) thickness, or Refrasil (Chemical & Insulating Co. of Darlington (Darchem Group)) 2½mm (one-tenth of an inch) thick. Fiberfrax, Triton, Kaowool and Refrasil are trade marks. An alternative diaphragm material is carbon felt, which is more resistant to puncturing by dendrites, but to avoid short-circuiting care must be taken to keep it from actually touching the electrodes (for example by using spacers). The diaphragm is normally an insulator but when saturated with electrolyte (as will be described) can transport current in the form of ions. In use, in one application, bismuth-manganese alloy is to be separated, the manganese being recovered in the form of aluminium-manganese master alloy. The molten bismuth-manganese alloy is supplied to the top of the anode 1 and is allowed to trickle down the channels. The clearance between the anode 1 and the diaphragm 2 is sufficiently fine to restrict the metal to the channels. The diaphragm 2 is impervious to the molten alloy, but is saturated with molten sodium chloride - potassium chloride - manganous chloride electrolyte. The labyrinthine configuration of the channels allows the metal to flow through the pools of alloy and molten salt held in the electrode surface. The cathode 3 contains molten electrolyte including sodium chloride in its channels and molten aluminium is trickled through its channels. The electrolytic action of anode and cathode selectively oxidises the manganese contained in the BiMn alloy at the anode, and this manganese is ionically transported across the diaphragm 2 to the cathode 3, where it is reduced to elemental manganese, which is collected by dissolution in the aluminium as it trickles down the cathode channels. The aluminium supplied directly to the cathode assists physically the collection of the cathodically deposited manganese, whose melting point without the presence of the aluminium would be impracticably high. The shallowness of the channels and their labyrinthine course have the advantage that no large head of liquid metal builds up anywhere to stress the diaphragm 2. The diaphragm, traditionally a troublesome component of any cell, should therefore have a better chance of a long reliable service life. The short anode-cathode distance keeps cell resistive losses to a minimum and also allows closer control over the actual voltage applied, local variations due to the thickness of the cell being kept relatively minor by the geometry and construction (especially the narrow anode/cathode spacing) of the cell according to the invention. This close control over the voltage allows a user to differentiate between say elements of close electrode potentials such as tin and lead (Sn²/Sn⁰ = -1.04V, pb²/Pb⁰ = -1.11V); thus it might be possible to select an applied voltage which would transport lead across the diaphragm while leaving the tin behind. The individual constituents of alloys such as solder could thus be recovered separately whereas this would be impossible in a conventional cell, where the large cathode/anode spacings necessary to prevent back-reaction of products would introduce the very voltage irregularities which would swamp any distinction between tin and lead. At the cathode, as a further application, other metals than manganese such as titanium can be recovered from molten solution in bismuth, or metals such as magnesium from molten solution in antimony. The cell can also be used to deposit elemental metal from an aqueous or molten salt running through the channels of the anode 1 onto the cathode 3. In the case of refining a zinc-lead alloy in eutectic molten chloride in a cell as set forth above, a current of 6 kAm⁻² was observed at ¼V, the cathode product containing two to three orders of magnitude less lead than the anode feedstock. A plurality of cells as set forth above may be arranged contiguously, that is with the anode of a first cell serving also as the cathode of a contiguous second cell, with the anode of the second cell optionally serving as the cathode of a contiguous third cell, and so on as often as desired.

Claims: An electrode for use in electrowinning or electrorefining metals, comprising an electronically conductive block having a generally planar face, the face having formed therein channels of varying cross section and direction which form a network of interconnecting channels which provide a plurality of continuous routes for molten metal to flow overall downwardly across said face when the electrode is in its use orientation, said network comprising a plurality of downward channels interconnected by generally horizontal distribution-and-mixing channels leading to further downward channels, thereby promoting mixing and break-up of liquid flowing along the routes. An electrode according to Claim 1, wherein the routes include retention pools with exits constricted to break up the flow of a liquid and leading to further like retention pools. An electrode according to Claim 2, wherein the retention pools are interconnected via the generally horizontal distribution-and-mixing channels. An electrode according to any preceding claim, wherein the block is of carbon. A sub-assembly comprising an electrode according to any preceding claim and a diaphragm impervious to molten metal but pervious to metal ions, the diaphragm being placed facing said one face of said block. A sub-assembly according to Claim 5, further comprising a second electrode according to any of Claims 1 to 4 sandwiching the diaphragm. A sub-assembly according to Claim 5 or 6, further comprising an electrically insulating sheet or sheets interposed between the diaphragm and the electrode, and shaped to expose the channels to the diaphragm. A sub-assembly comprising an electrode according to any of Claims 1 to 4 and an electrically insulating sheet or sheets on said face but shaped to expose the channels. A sub-assembly according to any of Claims 5 to 8, comprising the electrode mounted in a slot-in frame adapted to receive electrodes, diaphragms and optionally sheets. A cell comprising a sub-assembly according to any of Claims 5 to 9 fitted at least with two electrodes and an interposed diaphragm. A cell according to Claim 10, wherein the electrodes' channels are substantially a mirror-image of, and in registry with, each other. A plurality of cells according to Claim 10 or 11, wherein the cells are arranged contiguously, with the anode of a first cell serving as the cathode of a contiguous second cell. A method of electrowinning a metal, using a cell according to Claim 10 or 11, comprising passing a conductive stream of a salt of the metal through the channels of the first electrode in the presence of a conductive salt saturating the diaphragm, and making the first electrode anodic with respect to the second electrode, and recovering the refined metal(s) which appear(s) at the cathode. A method of refining, using a cell according to Claim 10 or 11, comprising passing a stream of molten metal through the channels of the first electrode in the presence of a conductive salt saturating the diaphragm, and making the first electrode anodic with respect to the second electrode, and recovering the refined metal(s) which appear(s) at the cathode.

Inventors: FRAY, DEREK JOHN
Applicants: NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1987
Main classification: C25C7
IPC codes: C25C7