AU2009286832A1

AU2009286832A1 - Cathode for the electrolytic deposition of zinc or the like from an electrolyte bath

Info

Publication number: AU2009286832A1
Application number: AU2009286832A
Authority: AU
Inventors: Wolfgang Reiser
Original assignee: EMS ELEKTRO METALL SCHWANENMUHLE GmbH
Current assignee: EMS ELEKTRO METALL SCHWANENMUHLE GmbH
Priority date: 2008-08-29
Filing date: 2009-08-11
Publication date: 2010-03-04
Also published as: DE102008041698A1; WO2010023101A1; CA2738135A1; EP2324143A1

Description

PCT/EP2009/060391 EMS Elektro Metall Schwanenmdhle GmbH, D-66851 SchwanenmOhle Cathode for the electrolytic deposition of zinc or the like from an electrolyte 5 bath The invention relates to a cathode for the electrolytic deposition of zinc or the like from an electrolyte bath, comprising a cathode sheet, a support and contact rail which is attached to the upper edge of the plate-shaped 10 cathode sheet and has a higher stability than the cathode sheet, and edge strips attached to the lateral edges and, optionally, the lower edge of the cathode sheet. The known electrolytic deposition of metal ions on a cathode is suitable for I5 obtaining largely pure zinc. In the process, metal ions are electrolytically dissolved in an acid bath of recycled material, such as ore, and deposited on a cathode. The transport of the metal ions through the acid electrolyte is caused by direct current flowing between the cathode and the anode. 20 For this purpose, suitable cathodes can generally be divided into two main components, namely the cathode plate or cathode sheet on which the metal to be obtained is deposited, and the part referred to as contact, support or also head rail, which serves for introducing the electrical current into the plate-shaped cathode sheet and for mechanically stabilizing and 25 moving the cathode sheet. The electrolytic deposition on the surfaces of cathodes of the zinc to be obtained follows the current density distribution between the anode and the cathode and, at the same time, protects the cathode sheet against 30 corrosion in the acid bath. In order to obtain a uniform deposition or a metal build-up that is as uniform as possible on the cathode, the goal is as constant a current density 2 distribution over the cathode surface as possible. Normally, no metal deposition occurs at the upper liquid level of the acid bath, the so-called liquid zone, so that consequently, no protection of the cathode sheet against the acid electrolyte is created here by the deposited zinc. 5 Depending on the material used, the cathode sheet can therefore corrode in the liquid zone and wear down prematurely. The metal deposited and built-up on the cathode during the electrolytic deposition is peeled off both sides of the cathode outside the acid bath 10 (stripping). In this context, it is an impediment if the material to be obtained grows over the edges of the cathode sheet during deposition in the acid bath or the electrolyte, thus connecting the two sides of the cathode sheet mechanically. There is, therefore, a requirement to keep the cathode edges free from the zinc to be obtained during the electrolytic deposition. For this 15 purpose, it is known to attach to or on the cathode sheet so-called edge strips, which up till now consist of an electrically insulating material. The edge strips enclose the areas of the edges or rims of the cathode, so that the direct current flowing through the cathode is not introduced into the electrolyte bath at the edges, so that no metal ions are deposited and 20 accumulated in those areas. The edge strips enclose either only the two vertically extending edges of the cathode sheet, or also the lower, horizontally extending edge. A variety of solutions are known with regard to the manner of attachment of the edge 25 strips to the plate-shaped cathode sheet. However, what all of these solutions have in common is the use of an electrically insulating material such as plastic, rubber or wax for the edge strips. These known cathodes with edge strips have some drawbacks which in 30 practice lead to additional operating costs. The known edge strips are unable to cope with large mechanical stresses and can be damaged when the cathode is processed outside the 3 electrolyte bath, This can lead to impedimental metal depositions being produced in the area of the zone of the edge if the cathode is used again. Moreover, damages to the cathode sheet itself may also result. 5 The known edge strips are not connected sufficiently securely to the cathode sheet. Thus, infiltration by the electrolyte occurs. This leads to metal build-up in the area of the edge zones of the cathode sheet underneath the edge strips, or even to the dissolution of the cathode material in the electrolyte. In both cases, the area of the edge zones of the cathode sheet 10 underneath the edge strips is destroyed. The period of use of the cathode sheet is thus reduced. The edge strips may possibly also lose their purchase on the cathode sheet and remain in the electrolyte bath. This can cause considerable disturbances in operation. 15 Due to being used in the acid electrolyte, the edge strips change their chemical and physical properties, As a consequence, the electrolyte is contaminated with foreign chemicals or other foreign substances. There is also the danger of the edge strips wearing down prematurely. 20 The invention is based on the object of avoiding the drawbacks described above in the electrolytic deposition of metals from an electrolyte bath, and accordingly, of providing a cathode for the deposition of zinc and similar metals that can be used longer than known cathodes and thus has a longer life span. 25 According to the invention, this object is achieved with a cathode comprising the features of claim 1. Advantageous embodiments and developments of the invention are the subject matter of the sub-claims dependent on claim 1, 30 In contrast to the prior art described above, the edge strips, according to the present invention, do not consist of an electrically insulating material, but from a metal which is resistant against the electrolyte, but which has a 4 significantly and substantially lower electrical conductivity than the cathode sheet. The deposition of the metal ions on the metallic edge strips of the cathode is reduced in accordance with the ratio of the conductance values of the metal used for the edge strips to the metal of the cathode sheet. The 5 thickness of the metal layer thus formed in the edge strips is so small that the subsequent peeling process of the obtained zinc or similar metal is not affected by the surfaces of the cathode sheet. According to the invention, the edge strips consist, for example, of stainless 0 steel or titanium, whereas the cathode sheet can be an aluminum sheet. These metallic edge strips can be securely and permanently attached to the cathode sheet in a suitable manner, for example by means of rivets, bolts, clamps or welding, so that the above-mentioned drawbacks of the 5 known non-metallic edge strips are avoided. Due to the substantially higher mechanical stability of the edge strips to be provided according to the invention, the danger of damage and loss is significantly reduced. 20 The metallic edge strips provided according to the invention can be connected to the cathode sheet with a much stronger support. The metallic edge strips have an inert behavior with regard to the 25 electrolyte. There is no change of the chemical or physical structure of the edge strips if the metallic material is suitably selected. In order to protect the area of the cathode sheet located at the surface or the level of the electrolyte bath, a plastic coating process, for example, is 30 carried out. The plastic is applied to the cathode sheet in a suitable manner and prevents corrosion so long as the plastic coating rests against the cathode sheet firmly and without a parting line, If this is not the case, an infiltration behind the plastic may occur and the corrosion will progress 5 correspondingly fast. Cathodes protected in the liquid zone are known. What these known cathodes have in common is the use of a chemically resistant plastic which, 5 however, detaches relatively easily from the cathode sheet, thus becoming incapable of fulfilling its function. Moreover, the area of the cathode sheet provided with a plastic coating is subjected to large mechanical stresses by the peeling process during the removal of the obtained metal, which leads to a premature wear of the plastic coating. 10 According to the invention, a chemically resistant metal, for example stainless steel or titanium, is used as a protection for the cathode sheet instead of plastic. Such a metal can be bonded to the cathode sheet very intimately, so that an infiltration of the electrolyte under the coating of the I5 cathode sheet is prevented. Since metals generally have a higher mechanical strength than plastic, the wear when the obtained zinc or other metal is peeled off is reduced to a fraction as compared with plastic. The durability and the life span of the protection of the cathode are therefore significantly increased also in the area of the liquid zone. 20 In order to facilitate the process of peeling off the zinc deposited on the cathode by the electrolysis, the use of a so-called peeling blade is known. From above, this blade pushes between the cathode sheet and the metal layer deposited thereon. In many cases, contact disks of plastic, which are 25 provided on one or also on both sides of the cathode sheet at the level of the liquid zone or electrolyte surface, serve as the contact for the peeling blade. The known contact disks consist of plastic, since that is not electrically conductive and therefore prevents the build-up of a metal layer in this area. However, the contact disks are subjected to strong mechanical 30 stress due to the impact of the peeling blade during stripping, Therefore, the contact disks are often damaged or even knocked out. Then, their functional capability is not provided anymore.

6 According to the invention, the contact disks also consist of metals resistant to the acid bath or the electrolyte, such as, for example, stainless steel or titanium. The build-up of the metal layer produced on the cathode during the electrolytic deposition occurs in accordance with the ratio of the 5 conductance values of the metals used for the contact disks and the cathode sheet. The thickness of the metal layer forming on the contact disk can be influenced by the electrical transition between the cathode sheet and the contact disks. A metal layer deposited appropriately thinly on the contact disks does not impede the function of the peeling blade. The 10 impact of the peeling blade on the contact disks consisting of metal is incapable of damaging or destroying the contact disks. Therefore, the metallic contact disks have a significantly higher life span than known contact disks of plastic, 15 Because the contact disks are disposed on the cathode in the area of the electrolyte surface or liquid zone, they can be combined with the metallic liquid zone protection described above and connected therewith to form a unit. Liquid zone protection with an integrated contact disk also extends the life span of the cathode sheet. 20 For further explanation of the invention, an exemplary embodiment of the cathode according to the invention is represented schematically in an illustration in the drawing. 25 The cathode 1 comprises a cathode sheet 2 consisting of aluminum sheet, for example, which is attached to a support rail 3. At the position shown in the drawing, the cathode sheet 2 can be hung into an electrolyte bath whose surface is indicated by a dashed line 4. 30 The support rail 3 is provided with hooks 5 for handling and with contacts for introducing current 6. To the vertical edges 7 of the cathode sheet, edge strips 8 are permanently 7 attached which consist of metal whose electrical conductivity is significantly lower than that of the cathode sheet. In the exemplary embodiment shown, another edge strip 9 consisting of the same metal as the edge strip 8 is attached to the lower edge of the cathode sheet 2. However, this edge strip 5 9 may also be omitted. Contact disks 10 of the some material as the edge strips 8 and 9, which serve for facilitating the contact of peeling blades, are attached - on one side or both sides - to the cathode sheet 2 at the level of the surface of the 10 electrolyte both (dashed line 4). A protective layer 11 that can consist of the same material as the edge strips 8 and 9 and the contact disks 10 is disposed on the front and the rear of the cathode sheet 2 in the area of the liquid zone of the electrolyte bath 15 or the surface thereof (dashed line 4). An electrical insulation, for example, an insulating coating or an insulating intermediate layer, can be provided between the cathode sheet 2 and the edge strips 8 and 9 which are applied thereto and consist of poorly 20 conductive metal and the also poorly conductive contact disks 10, in order to keep the deposition of zinc or other metal on these additional components as small as possible. The ratio of the electrical conductivity between the cathode sheet 2 25 consisting of aluminum and the edge strips 8 and 9 consisting of stainless steel is about 50:1. If titanium is used for the edge strips and the contact pieces, this ratio is much larger since titanium is a very poor conductor.

Claims

1. Cathode for the electrolytic deposition of zinc or the like from an electrolyte bath, with an electrically conductive cathode sheet to 5 whose upper edge a supporting and contact rail having a higher stability than the cathode sheet is attached, and with edge strips attached to the other edges of the cathode sheet, characterized in that the edge strips (8; 9; 10) of the cathode sheet (2) consist of metal 10 which is resistant to the electrolyte used and whose electrical conductivity is significantly lower than that of the cathode sheet,

2. Cathode according to claim 1, characterized in that the cathode sheet consists of aluminum. I5

3. Cathode according to claim 1 or 2, characterized in that the edge strips (8; 9) consist of stainless steel.

4. Cathode according to claim 1 or 2, characterized in that the edge 20 strips (8; 9) consist of titanium.

5. Cathode according to any one of the claims 1 to 4, characterized in that the cathode sheet (2), in the region of the liquidity zone (4) of the electrolyte bath, is provided with a cover or a coating or a protection 25 of stainless steel or titanium.

6. Cathode according to any one of the claims 1 to 5, characterized in that the cathode sheet (2) is provided with contact disks (10) for contacting a peeling blade, which consist of a metal with a 30 significantly lower electrical conductivity than the cathode sheet.

7. Cathode according to any one of the claims 1 to 6, characterized in that an electrical insulation is provided between the cathode sheet (2) 2 and the edge strips (8; 9) and the contact disks (10).