Title of Invention	ELECTROLYSIS PROCESS AND CELL FOR USE IN SAME.
Abstract	An electrolysis process for the recovery of metal from an aqueous solution is defined. On electrolysing the solution metal is caused to deposit on a deposition surface of a cathode. The process includes the step of inducing a non-uniform current density across the deposition surface so as to form areas of high current density interspaced by areas of low current density. The difference between the areas of high current density and low current density is sufficient to cause metal deposition to be concentrated on the areas of high current density so as to promote non-uniform deposition of metal across the deposition surface. An electrolysis cell for the electro-recovery of metal from an aqueous solution is also defined. The cell includes a cathode which includes a deposition surface on which metal is deposited on electrolysing of the aqueous solution. In operation of the cell, the deposition surface has a non-uniform electrical field having areas of strong electrical field interspaced by areas of weak electrical field. The difference between the areas of strong electrical field and weak electrical field is sufficient to cause metal deposition to be concentrated on the areas of high electrical field so as to promote non-uniform deposition of metal on the surface.

Title of Invention

ELECTROLYSIS PROCESS AND CELL FOR USE IN SAME.

Abstract

An electrolysis process for the recovery of metal from an aqueous solution is defined. On electrolysing the solution metal is caused to deposit on a deposition surface of a cathode. The process includes the step of inducing a non-uniform current density across the deposition surface so as to form areas of high current density interspaced by areas of low current density. The difference between the areas of high current density and low current density is sufficient to cause metal deposition to be concentrated on the areas of high current density so as to promote non-uniform deposition of metal across the deposition surface. An electrolysis cell for the electro-recovery of metal from an aqueous solution is also defined. The cell includes a cathode which includes a deposition surface on which metal is deposited on electrolysing of the aqueous solution. In operation of the cell, the deposition surface has a non-uniform electrical field having areas of strong electrical field interspaced by areas of weak electrical field. The difference between the areas of strong electrical field and weak electrical field is sufficient to cause metal deposition to be concentrated on the areas of high electrical field so as to promote non-uniform deposition of metal on the surface.

Full Text	ELECTROLYSIS PROCESS AND CELL FOR USE IN SAME Field of the Invention The present invention relates generally to an electrolysis process for the recovery of metals from an aqueous solution and to an improved cathode for use in such a process. The primary application of the invention disclosed herein is in relation to the recovery of copper, although the invention finds equal application with electro-recovery of other metals such as nickel, lead, zinc etc. Background Art Processes for leaching base metals from ores and concentrates with subsequent recovery of the base metal in electrolytic cells are known in the field of the hydrometallurgy. One example is disclosed in Indian Patent Application No. 656/Del/93 (187010). This process is multi-stage and produces a pregnant liquor stream following leaching of the mineral in a chloride medium. The pregnant liquor stream is electrolysed in an electrolysis cell to recover the metal from the solution which deposits on a cathode of the cell. Under high current densities, dendritic copper of high purity is produced on the cathode . In the past it has been necessary to regularly remove the cathodes to strip the plates of the metal deposits so as to maintain current efficiency in the cell. Optimisation of the electrowinning operation is a function of the purity of the pregnant liquor stream, and general cell parameters such as current density, stripping cycle, cell configuration and the degree of agitation. Accordingly, an aim of the present invention is to improve the efficiency of the electrowinning operation. In particular, an aim is to provide an electrolysis process and cell configuration which is able to better control current density across the deposition surface of the cathode so as to assist in both formation and removal of the metal deposit. Summary of the Invention In a first aspect the present invention provides an electrolysis process for the recovery of metal from an aqueous solution wherein on electrolysing the solution metal is caused to deposit on a deposition surface of a cathode, the process including the step of inducing a non-uniform current density across the deposition surface so as to form areas of high current density interspaced by areas of flow current density, the difference between the areas of high current density and low current density being sufficient to cause metal deposition to be concentrated on the areas of high current density so as to promote non-uniform deposition of metal across the deposition surface. In the context of the invention, the deposition surface may be of unitary structure or alternatively may be formed from discrete elements which may be spaced apart or in direct contact with one another. Providing a non-uniform current density across the deposition surface provides a mechanism by which the deposition of metal on that surface can be controlled. In particular, it allows the metal deposition to be concentrated in certain areas (i.e. the areas of high current density) so as to promote non-uniform deposition across the surface. Non-uniform deposition of the metal is beneficial as it is easier to remove from the cathode which assists in the metal recovery process. ,. Preferably, the metal deposition is heavily concentrated on the areas of hi\|;h current density so that the metal deposition is effectively discontinuous across the deposition surface. Preferably, the concentration of metal deposition in operation of the cell is greater than 80% in the areas of high current density and more preferably greater than 95%. Preferably, the areas of high current density and low current density extend along the surface in one direction and alternate across the surface in an opposite direction. With this arrangement, the metal deposits in a series of generally linear bands which is ideally suited for removal using a wiping action as will be described in more detail below. Preferably, the electrolysis process induces a non-uniform current density across the deposition surface by providing a cathode which in operation of the cell, creates a non-uniform electrical field having areas of strong electrical field and weak electrical field. With this arrangement, the areas of strong electrical field induce the areas of the high current density and the areas of weak electrical field induce the areas of low current density. The non-uniform electrical field can be created through numerous mechanisms, including the geometry of the surface, and by varying the electrical resistance between the cathode and anode along the deposition surface, or by a combination of both these mechanisms. The geometry of the surface influences the electrical field and is related to its surface curvature. Electrical fields are always perpendicular to the surface so that, sharp edges, or peaks at the deposition surface induce areas of high electrical field as compared to areas of flat surface, or valleys. The electrical resistance can be varied by using different materials along the deposition surface (e.g. providing sections with insulating material) or by changing the current path length between the cathode and the anode. In a preferred form, the non-uniform electrical field is induced at the deposition surface by the geometry of the surface and in particular by forming a series of alternate ridges and valleys across the surface. By virtue of this geometry, in operation of the cell, there is a higher electrical field along the ridges as compared to the valleys. In addition, the current path length at the ridges is shorter as compared to the valleys thereby creating a situation where there is less resistance at the ridges as compared to the valleys. The variation in current density across the deposition surface maybe such that there is a sharp demarcation between the areas of high current density and low current density, or alternatively there may be a more gradual transition between the areas of highest current density and lowest current density. The applicant has found that inducing a gradual transition between the areas of highest and lowest current density still provides good deposition patterns so as to promote substantially discontinuous growth across the deposition surface. In particular, the applicant has found that using a cathode which includes a deposition surface having ridges and valleys which do not include a sharp transition between the ridge and valleys so that there is a more gradual change between the highest current density and the lowest current density provides excellent performance. This arrangement induces secondary effects which assist in concentration of the metal deposition at the ridges as described in more detail below and also provides for easier removal of the metal as it allows easier access to the entire deposition surface in contrast to a sharp transition between the ridge and the valley may provide areas which are difficult to access! In a preferred form, where the cell is operative to remove copper from an aqueous solution current density in the areas of high current density is in the range of 500 to 2,500A/m2 and more preferable 1,000 AM/2. Preferably, the areas of low current density is in the range of 0 to 2,050A/m2 and more preferably 0 to 500A/m2. Where there is a gradual transition between the areas of highest current density and lowest current density, the demarcation between an area of "high current density" and "low current density" is somewhat arbitrary. In this arrangement, the transition region may be regarded as an area of moderate current which in turn is located between areas of adjacent "high current density" and areas of "low current density". Preferably, the process further includes the step of removing deposited metal from the deposition surface by passing an element over the surface. Preferably, in the arrangement where the areas of high current density and low current density extend along the surface in one direction and alternate across the surface in an opposite direction, the element is moved in the direction in which Hie areas of high and low current density extend. Preferably, the deposited jngtal is removed by the element whilst maintaining current flow in the aqueous solution, In this way, the process can be substantially continuous. In yet a further aspect, the present invention relates to an electrolysis cell for the electrorecovery of metal from an aqueous solution, the cell including a cathode which includes a deposition surface on which metal is deposited on electrolysing of the aqueous solution,-wherein in operation of the cell, the deposition surface has a non- uniform electrical field so as to have areas of strong electrical field interspaced by areas of low electrical field, the difference between the areas of high electrical field and low electrical field being sufficient to cause metal deposition to be concentrated on the areas of high electrical field so as to promote non-uniform deposition of metal on the surface. Preferably, the areas of high electrical field and low electrical field extend along the surface in one direction and alternate across the surface in an opposite direction. In a particularly preferred form, the deposition surface of the cathode includes an array of alternate ridges and valleys, with the ridges forming areas of high electrical field and the valleys forming the areas of low electricalfield. Profiling the deposition surface to have an array of alternate ridges and valleys has significant benefit in promoting substantially discontinuous metal deposition on the cathode. Typically such profiling promotes metal deposition as a dendrite growth on each of the ridges. Advantageously, the resulting dendrites are easy to remove (as described below). Not only does the profile provide, in the initial operation of the cell, the appropriate non-uniform current density to concentrate metal deposition as dendrites on the ridges, but it also assists in maintning discontinuous growth as the process continues. As will be appreciated, once metal deposits on the deposition surface, the deposited metal forms an extension of a deposition surface. An advantage of having an arrangement of ridges and valleys is that as the dendrites grow on the ridges, they tend to "shadow" the valleys which further inhibits metal deposition in the valleys. In addition, the aqueous solution tends to stagnate in the valleys which further inhibits deposition of metal in the valleys. In tests conducted by the applicant, using a profile of alternate ndges and valleys, more than 98.8% of metal was deposited on the ridges of the deposition surface. Whilst the beneficial effects of including the ridges and valleys may be achieved over a range of profiles, the applicants have found that a regular profile where the surfaces between the top of the ridge and the base of the valley is substantially linear and have an internal angle of approximately 60o between adacent surfaces provides good results. Furthermore preferably the pitch distance between adjacent ridges is in the order of 1 0-40mm, and more preferably 15-25mm, and the depth between the ridge and the valley is in the order of 8-32mm and more preferably in the range of 12-2Qmm. A deposition surface having these characteristics has been found to produce substantially discontinuous metal deposits. A further advantage is that this profile enables the surface to be substantially cleaned without creating "hot spots" of current density which would lead to impure metal deposits. If the current density at a site is too high, as the deposition progresses, it leads to concentration polarisation (which takes place around the growing deposit). When mis phenomenon occurs, a relative increase in impurity inclusions in the depositing metal (e.g. in copper) can occur. Thus it is important to control the current density at the site, The advantage of the profile mentioned above is that the areas of high current density where metal deposits still takes up a substantial part of the total area of the cathode(i.c. in the vicinity of 25-35% of the total area of the deposition surface). With this arrangement, the current is able to be maintained at a substantially constant rate regardless of whether the surface is clean of metal deposits or whether deposition has already occurred. As such, there is no need to ramp up the current on initiating the cell as the profile itself does not tend to induce strong "hot spots" of current density which is likely to cause problems in initial metal deposition. In a particularly preferred form, the cathode includes a sheet having at least one major surface which forms the deposition surface of the cathode, the sheet being preformed so as to incorporate the j}tematejridges_and valleys. The sheet may thus define a corrugated profile. Preferably, this preforming operation is achieved by folding of the sheet but it could be made by any other appropriate process such as a stamping, milling, swaging, casting process or combinations thereof. In a particularly preferred form, the sheet is formed from titanium or similar oxidation resistant material. Whilst other oxidation resistant materials may be used, such asjplatitmm, stainless steel, corrosion resistant metal alloys, titanium is most preferred because of its excellent oxidation resistance, its capcity to resist forming a metallurgical bond with metals such as copper, and because of its relative availability. A further advantage of using a corrugated profile is that it assists in maintaining dimensional stability for the sheet Such an arrangement can assist in overcoming the disadvantages of prior art arrangements where sheet cathodes had a tendency to flex and buckle. Further, when metal deposits on the sheet as a dendriticor crystalling growth the dimensional stability of the sheet enables wiping methods to be used to easily remove the deposit from the sheet The applicants have found that titamium sheets in the order of 1,6mm thickness provide sufficient dimensional stability for this process. Preferably, the sheet is adapted in use for attachment to a conductive header bar. This header bar supports the cathode in use andsupplies electrons to it In one form, the opposite major surfaces of the folded sheet axe used as deposition surfaces in operation of the cathode. m an alternative form, me cathode is made from a composite structure and further includes a conducting element which extends along the sheet The conducting element is in electrocommunication with the sheet so as in use to supply the deposition surface with electrons in the electrolysis process. One advantage of using a conducting element which extends along the sheet is that it minimises ohmic drop which occurs when the electrons are supplied solely from one edge of the sheet A second advantage of using a conducting element is mat is may be of sufficient size to provide rigidity to the sheet to further assist in maintaining dimensional stability of the cathode. With the composite arrangement it may thus be possible to use thinner sheet structures for the deposition surfaces). m a preferred form of this latter arrangement, the cathode includes a second sheet which is connected to the first sheet and which has a major surface which forms a second deposition surface of the cathode, the second sheet being preformed so as to incorporate the alternate ridges and valleys along that deposition surface. Preferably, the second sheet is connected to the first sheet of the cathode so as to form a plurality of pockets which extend in the direction of the alternate ridges and valleys. At least some of these pockets are operative to receive the conducting element of the cathode. In a preferred form, the wiping device is operative to pass over a deposition surface of the cathode so as to remove deposited material from the deposition surface. In a particularly preferred form, where the cathode includes the ridge and valley profile, the wiping device includes a plurality of projections which are operative to locate within respective valleys of the deposition surface. In a preferred form, these projections are made from a ceramic material but can be made of any other corrosive resistant material. In a preferred form, the projections are movable between a first and a second position and are operative to pass over the surface in either of these positions, m a first position, the element is in contact or in close proximity to the deposition surface so as to remove substantially all of the deposition material from mat surface. In the second position, preferably the element is spaced from the deposition surface and is operative to remove deposited material which extends a predetermined distance from the deposition surface. In yet a further aspect, the present invention relates to a cathode for use in a process or electrolysis cell as defined in any form above. In yet a further aspect, the present invention relates to a wiping system for use in an electrolysis cell in any form as defined above. In yet a further aspect, the present invention relates to a cathode for use in an electrolysis cell for the electrorecovery of metal from an aqueous solution, trie cathodes including a deposition surface having a plurality of ridges which are interspaced by a plurality of valleys, the profile of the cathode being operative on operation of the cell to cause metal deposition to be concentrated on the ridges so as to promote non-uniform deposition of metal on that surface. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Notwithstanding any forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a generalised flowchart for processing and recovery of copper; Figure 2 is a sectional elevation of an electrolysis cell in accordance with one embodiment of the invention with wiper sets of the cell in a closed position; Figure 3 is a sectional side view of the cell of Figure 2; Figure 4 is a sectional elevation of the cell of Figure 2 with the wipers in an open position; Figure 5 is a detailed view of the linkage assembly in the cell of Figure 2; Figure 6 is a cut-away perspective view of the cell of Figure 2; Figure 7 is a schematic view to an enlarged scale showing the wipers located in an open position at the top of the cathode plates; Figure 8 is a detailed view to an enlarged scale of the wipers in a closed position; Figure 9 is a front elevation of a cathode panel used in the cell of Figure 2; Figure 10 is an end view of the panel of Figure 9; Figure 11 is a schematic perspective view of a wiper engaging a cathode in the cell of Figure 2; Figure 12 is a sectional view along section line XII-XII in Figurel 1; Figure 13 is a detailed view of the blade construction of the wipers used in the cell of Figure 2; Figures 14 and 15 are variations of the blade instruction shown in Figurel3; Figure 16 is a schematic perspective view of an alternative cathode designed for use in the cell of Figure 2; and Figure 17 is a cross sectional view along section line XVH-XVH of the cathode of Figure 16. Modes for Carrying out the Invetion la Figure 1, a schematic representation of a combined process 100 including leach and electrorecovery 104 of metal is depicted. In a preferred form of this process, ground copper sulfide 106 is fed to a multistage counter current leaching process in which the metals are solubilised through oxidation by a lixiviant. La a preferred form, the IMviant includes a complex halide species which is formed in the anode of the subsequent electrolysis stage and is fed back into the leach stage as part of the electrolyte recycle 108. Dissolved metals in desirable oxidation states are removed at various stages from the leach process in the leachate. The leachate is passed through filtration 110 to remove unwanted solids such as sulfur and ferric oxide. The leachate is then passed to purification 112 to remove metals which may otherwise contaminate subsequent electrolysis (such as silver and mercury). The contaminant metals may be precipitated as the metal oxide or carbonate form. The purified leachate is then fed to the electrolysis stage 104 which may include a plurality of electrolysis cell groups in series and/or in parallel Jn each group, a different metal may be produced, with typically copper metal being electrorecovered in a first cell group and metals such as zinc, lead and nickel being recovered in subsequent or parallel cell groups. The electrolysis process is typically operated such that a highly oxidising lixiviant (such as a complex halide species) is produced at the anode. The spent electrolyte (anolyte) is then recycled to the leaching stage and includes the highly oxidising lixiviant which participates in further counter current leaching. Thus, the process can operate continuously. The present invention is concerned with optimising the electrorecovery of metals and relates to significant design improvements in the electrolysis process, including improved cathode design and geometry. Referring now to Figures 2-5 the electrolysis cell 10 for use in the process 100 includes a series of cathode plates 11 which are disposed within the electrolysis cell tank SO and interspaced by anodes 12. Electrolyte fed to the cell enables current flow between the anodes and the cathodes. The outer surfaces 13,14 of the respective cathodes form a deposition surface for the cell on which the metal to be recovered deposits in operation of the cell 10. As will be described in more detail below, the cathode plates are formed from a generally corrugated profile having alternate ridges and valleys so as to influence the mode of deposition of the metal on the respective deposition surfaces 13 and 14. The cell 10 includes a wiper system 15 which includes a plurality of wiper sets 16 operative to interfit between respective cathodes and anodes with the wipers 17 of respective wiper sets 16 being operative to move across the deposition surfaces 13 and 14 of respective cathodes 11 so as to remove metal deposits from those surfaces. The wipers 17 are arranged to be wiped down the respective deposition surfaces 13 and 14 at predetermined intervals to cause the dislodged metal to drop to the bottom of the cell 10 wherein it is transferred to a conveyor 18 for removal from the cell. To achieve this wiping action, the wiping system 15 includes two principle movements; the first being a vertical movement to allow the wiper sets 16 to move between the top and the bottom of the respective cathodes 11, the second being to allow the wipers 17 in each set 16 to move from an open position (as best illustrated in Figueg" 7) to a closed position (as best illustrated in Figure 8). The wiper sets 16 are mounted on a frame 32 which is secured at its upper end to four supporting struts 19, 20, 21 and 22. Each of the struts include a helical track 23 which cooperates with a worm gear 24 connected to the frame 32. In this way, the frame 32 moves relative to the struts. An electric motor 25 mounted on a cross beam 26 is operative to drive the worm gears 24 so as to achieve the vertical movement of the wiper sets relative to the deposition surfaces 13 and 14. Under this action, the wipers are able to move between a lower position as disclosed in Figure 2 to an upper position as disclosed in Figure 4. The frame 32 supports a linkage assembly 27 which in turn is connected to the wiper sets 16. The linkage assembly 27 includes a pair of link plates 28 at each end of the wiper sets 16 which are connected to respective link arms 29. A crank 30 is pivotally connected to respective pairs of the link plates 28 through pivot points 31. Crank arms 40 extend from the crank 30 to the wiper sets 16 so as to support each end of the wiper sets. The link arms 29 are capable of vertical movement through a second actuator 41. In the illustrated form, the second actuator is in the form of worm gears which cooperate with helical tracks formed on the respective link arms. The worm gears rotate which impacts the rotation to the the link arms 29 to cause vertical displacement of those arms relative to the frame 32 which in turn drives the crank 30 so as to move the wipers between their open and closed positions. The second activator can be damped to prevent over-tightening and jamming of the wipers against the cathode. Damping can be provided by a spring-loaded coupling or by using a pneumatic cylinder in place of the worm gear. As best illustrated in Figure 6 each line of cathodes in the cell 10 is formed from a plurality of cathode plates 11 which are connected to a header bar 34 so that the individual plates are suspended in the tank SO. The header bar 34 is conductive and connected to a power source so as to supply electrons to the cathode. Typically the electrolyte is highly corrosive, resulting from typically a 5 molar or greater concentration of alkali or alkaline-earth metal halides. To enable the components to be able to operate in this environment, the wiper system IS is made from a corrosion resistant material which is preferably titanium. Other suitable materials include platinum stanless steel corresion resists metal allows (such as hastalloy C. 22), or even some plastics. Also, titanium is most preferred for the cathode because of its excellent corrosion resistance and its capacity to resist forming a metallurgical bond with metal such as copper, and because of its relative availability (hence cost benefit). Its resistance to forming a metallurgic bond improves the ability of the plates to be stripped using the wiper system described above. Figures 9 and 10 illustrate the construction of the individual cathode plates 11. In the illustrated form, the cathode plate 11 is formed from a titanium sheet having a thickness which is preferably about 1.6mm. Sheets of this thickness have been found by the applicant to give adequate rigidity to the cathode plate to prevent buckling in use. The titanium sheet is folded to form a generally corrugated profile so as to provide on each deposition surface 13,14 alternate valleys and ridges 35,36 respectively. These corrugations run along the entire length of the cathode between its upper and lower edges 37,38. In the illustrated form, the distance between adjacent ridges 36 is 20mm, whereas the depth between the top of the ridges 36 and the bottom of the valleys 35 is approximately 16mm. The wall surfaces 43 formed on the corrugated sheet are generally linear and have an internal angle at the top at the ridges and bottom of the valleys of approximately 60°. A primary purpose for incorporating the corrugations in the cathode is to influence the current density on the deposition surfaces 13,14 under operation of the cell. In particular, the corrugations on the deposition surface cause a non-uniform electrical field across that surface in operation of the cell. The corrugated deposition surface on the cathode creates bands of high current density along the ridges of the cathode due to its corresponding high electrical field at those areas and relatively low current densities in the valleys. This causes metal deposition to be concentrated in the areas of high current density and promotes non- uniform deposition across the surface so that the vast majority of the deposition includes in the ridge regions 35 of the deposition surface. Creating substantially discontinuous deposition improves the ability to be able to remove recovered metal from the cathode using the wiping system 15. The profile of the deposition surface with the valleys and ridges causes the non-uniform electric field by two mechanisms. Firstly, in view of the geometry of the profile, the electrical field will be stronger at the ridges than the valleys because of its surface curvature. In general, the electric field lines are always perpendicular to the surface. Therefore, at each ridge there will be a concentration of a field along those points. Secondly, the current flow path at the ridges is less than the current flow path at the valleys. As a result, there is less resistance at the ridges than there is at the valleys. In addition, the use of the corrugated profile of the cathode allows better control at the main sites of deposition (i.e. along the ridges). If the current density at a site is too high, as deposition progresses, it leads to concentration polarisation (which „ takes place around the growing deposit). When this phenomenon occurs, a relative increase in impurity inclusion in the depositing metal (e.g. in copper) can occur. With the corrugated profile, the main sites of deposition account for approximately 25-35% of the total surface area of the cathode. As a function of the mass transfer, ideally the current at the deposition surface should be in the vicinity of l,000A/m2 or less. As the dendrites grow on the surface, the actual area of deposition surface increases as metal is deposited on the previously deposited metal. If the initial deposition sites on the cathode are too small, then there is a tendency that once the dendrite is removed from the cathode the current density at that site becomes too high. Through trials conducted by the applicant, using the corrugated profile, it is found that the current density at the deposition sites both on initial operation of the cell and after dendritic growth has occurred, it is able to be maintained in the vicinity of l,000A/m2 so as to provide high quality metal deposition. As such, there is no need to vary the current during the process. A further advantage of using the corrugated profile on the cathode is that it improves the rigidity of the cathode plate, as the corrugated profile is inherently stiffer than a flat plate along the direction of the ridges and valleys. In addition, the corrugated profile is ideally suited to be cleaned using the wiping blade system as is described in more detail below. With reference to Figures 11 to 15, the wipers 17 include fingers 39 which are mounted between a pair of rails 42. In the illustrated form, each of the individual fingers are formed from a ceramic material with the rails being made of titanium. Each of the fingers is spaced along the rail 42 so that the wipers 17 conform generally to the shape of the corrugated cathode plate 11, with the individual fingers locating within the valleys 35 of the deposition surface and over the associated ridges 36. As best illustrated in Figure 12, the wiping system 15 is designed so that when the wiper sets 16 are in their closed position, the wipers 17 are angled to the cathode plate 11 so that the individual fingers 39 are in a trailing position relative to the line of movement of the wiper 17 down the cathode plate 11. This arrangement is preferred as. it inhibits jamming of the fingers in the valleys as may occur if the fingers 39 v&ere in a leading position relative to the direction of movement of the wipers down the cathode plate. As described above, in view of the configuration of the cathode plates 11, the metal recovered from the electrolysis cell is concentrated on the ridges of the respective deposition surfaces of the cell. As such, when the wiper 17 is moved across the deposition surface the dislodged material from the ridges tends to move into the adjacent valleys of the deposition surface. This causes an accumulation of the metal within the valleys which tends to envelop the fingers 39 thereby protecting the ceramic fingers 39 from wear. In addition, there is a build up in frictional force as the mass of material is moved down the deposition surface thereby aiding removal of material as the material is dragged from the surface under this frictional force. It is not necessary that the fingers 39 are in direct contact with the deposition surface to ensure adequate cleaning of that surface. Another advantage of the design of the wiping system 15 is that it enables different stages of cleaning of the cathodes. In particular, as described above, the wipers 17 can be operated to remove the bulk of the deposited material on the deposition surfaces by dragging across those surfaces when in their closed position. The wipers can also be moved across the deposition when in their open position. This is used not to fully clean the deposition surface but rather to ensure that there is no extended dendritic growth on part of the deposition surface which could otherwise grow to an extent that it contacts the anode and thereby causes short circuiting of the electrolysis cell. Also, this allows for more consistent growth across the ridges of the cathode, which aids in control of the current density along the deposition surface. Figures 14 and 15 illustrate some variations in the design of the wipers 17. As in the arrangement of Figure 13, each of the wipers 17 include ceramic fingers 39. However, rather than using the rail arrangement 42 as disclosed in Figure 13, the fingers 39 are interconnected by an internal connecting bar 44. In the embodiment of Figure 14 the bar 44 is formed as a square section, whereas in Figure 15 the connecting bar is made up of two cylindrical bars 45. Referring now to Figures 16 and 17, an alternative cathode construction is depicted. In this embodiment the cathode is formed as a composite structure wherein the outer deposition surfaces 13,14 are defined by separate sheets which are fastened together along their respective lateral edges 60,61, and which may optionally be fastened together at intermittent regions 62. In this embodiment, a plurality of conducting bars 63 form part of the construction and extend downwardly from the header bar 34, the conducting bars typically also being formed from titanium (or a titanium coated copper bar to further improve conductively). Typically the conducting bars extend for the full length the plates 13,14 through each of the passages defined between a plate and are fastened thereto. Such an arrangement provides enhanced distribution of electrons through the assembly, thereby minimising ohmic drop which can occur when the elections are supplied solely to one edge of the sheet In addition, it has been found that the composite arrangement, including the arrangement of the conducting bars in the passages, enhances the dimensional stability of the sheet so that thin plate structures (e.g. as small as lmm) or alternatively wide plate structures can be employed for the cathode. Otherwise, the principles of operation of the cathode of Figures 16 and 17 are described above. Whilst the invention has been described with reference to a number of preferred embodiments, it should be appreciated that the invention can be embodied in many other forms. WE CLAIM: 1. An electrolysis process for the recovery of metal from an aqueous solution wherein on electrolysing the solution metal is caused to deposit on a deposition surface of a cathode, characterised in that the process comprises the step of - inducing a non-uniform current density across the deposition surface-so as to form areas of high current density interspaced by areas of low current density, the difference between the areas of high current density and low current density being sufficient to cause metal deposition to be concentrated on the areas of high current density so as to promote non- uniform deposition of metal across the deposition surface. 2 . A process as claimed in claim 1, wherein the areas of high current density and low current density extend along the surface in one direction and alternate across the surface in an opposite direction. 3 . A process as claimed in either claim 1 or claim 2, that is operative to recover copper from the aqueous solution and the current density in the areas of high current density is in the range of 500-2500 A/m2and is more preferably 1000 A/m2. 4 . A process as claimed in claim 1 or claim 2, that is operative to recover copper from the aqueous solution and the current density in the areas of lower current density is in the range of 0-1250 A/m2and is more preferably 0-500 A/m2. 5 . A process as claimed in claim 1 or claim 2, further including the step of removing deposited metal from the deposition surface by passing an element over said surface. 6. A process as claimed in claim 5, when dependent on claim 2, wherein the element is moved in the direction in which the areas of high and low current density extend. 7 . A process as claimed in claim 5, wherein deposited metal is removed by the element whilst maintaining current flow in the aqueous solution. 8. A process as claimed in claim 5, wherein the element is moveable between first and second positions, and is operative to be passed over the deposition surface in either of the first and second positions. 9. A process as claimed in claim 8, wherein when in its first position, the element is in contact with, or in close proximity to, the deposition surface so as to remove substantially all of the deposition material from that surface. 10. A process as claimed in either claim 7 or 8, wherein when in its second position, the element is spaced from the deposition surface and is operative to engage and remove deposited material which extends a predetermined distance from the deposition surface. 11. An electrolysis cell for the electro-recovery of metal from an aqueous solution, the cell including a cathode which includes a deposition surface on which metal is deposited on electrolysing of the aqueous solution, wherein in operation of the cell, the deposition surface has a non-uniform electrical field having areas of strong electrical field interspaced by areas of weak electrical field, the difference between the areas of strong electrical field and weak electrical field being sufficient to cause metal deposition to be concentrated on the areas of high electrical field so as to promote non- uniform deposition of metal on the surface. 12. An electrolysis cell as claimed in claim 11, wherein the areas of strong electrical field and weak electrical field extend along the surface in one direction and alternate across the surface in an opposite direction. 13 . An electrolysis cell as claimed in either claim 11 or 12, wherein the deposition surface of the cathode includes an array of alternate ridges and valleys, with the ridges forming the areas of strong electrical field and the valleys forming the areas of weak electrical field. 14. An electrolysis cell according to claim 13, wherein the cathode includes a sheet having at least one major surface which forms the deposition surface of the cathode, the sheet being preformed so as to incorporate the alternate ridges and valleys. 15 . An electrolysis cell according to claim 14, wherein the sheet has opposite major surfaces, each of which forms a deposition surface of the cathode. 16. An electrolysis cell as claimed in claim 15, wherein the sheet is folded so as to form the valleys and ridges on the opposite deposition surfaces with the ridges on one deposition surface being directly opposite the valleys on the opposite deposition surface and vice versa. 17 . An electrolysis cell as claimed in claim 14, wherein the sheet is of generally uniform thickness. 18. An electrolysis cell as claimed in any claim 14, 15, 16 or 17, wherein the sheet is formed from titanium. 19 . An electrolysis cell as claimed in claim 14, further including at least one conducting element which extends along the sheet, the conducting element being in electrocommunication with the sheet so as in use to supply the deposition surface with electrons in the electrolysis process. 2 0 . An electrolysis cell as claimed in claim 19, wherein the conducting element is of sufficient size to add rigidity to the sheet. 21. An electrolysis cell as claimed in either claim 19 or 20, wherein the cathode includes a second sheet which is connected to the first sheet and which has a major surface which forms a second deposition surface of the cathode, the second sheet being preformed so as to incorporate the alternate ridges and valleys along the deposition surface. 22. An electrolysis cell as claimed in claim 21, wherein the second sheet is connected to the first sheet of the cathode so as to form a plurality of pockets which extend in the direction of the alternate ridges and valleys, the pockets being operative to receive a conducting element of the cathode. 23 . An electrolysis cell as claimed in claim 11, 12, 14, 15, 16, 17,19, 20 or 22, further including a wiping device which is operative to pass over the deposition surface of the cathode so as to remove deposited material from that deposition surface. 2 4 . An electrolysis cell as claimed in claim 23, when dependent on claim 13, wherein the wiping device includes a plurality of projections which are operative to locate within respective valleys of the deposition surface. 25. A cathode for use in an electrolysis cell for the electrorecovery of metal from an aqueous solution, the cathode having a deposition surface including an array of alternate ridges and valleys. 2 6. A mechanism for removing metal deposited onto the deposition surface of the cathode of claim 25, the mechanism including a plurality of elements arranged to project into respective valleys and be moved therealong so as to dislodge deposited metal from the ridges and valleys. 27. A mechanism as claimed in claim 26, wherein the elements have a shape generally corresponding to the valleys. 28. A mechanism as claimed in claim 26 or claim 27, wherein the elements are formed from a ceramic material. 29. A mechanism as claimed in claim 26 or claim 27, wherein the elements are pivotally operable between a first position in which the elements protrude into the valleys and a second position in which the elements do not so protrude. The present invention relates to the field of fiber-reactive dyes. Black dyeing mixtures of fiber-reactive dyes are known from U.S. Patent Nos 5,445,654 and 5,611,821 as well as from Korean Patent Application Publication No. 94-2560. Deep balck dye mixtures are known, for example, from Japanese Patent Application Publication Sho-58-160 362 which are based on a navy-bule disazo dye and an orange monoazo dye. However these dye mixtures have some deficiencies. With the present invention, deepblack-dyeing dye mixtures of improved properties, for example wash fastness have unexpectedly been found, comprising a disazo dye conforming to the general formula (1), and ore or more disazo dyes conforming to the general formula (2).

Full Text

ELECTROLYSIS PROCESS AND CELL FOR USE IN SAME
Field of the Invention
The present invention relates generally to an electrolysis process for the
recovery of metals from an aqueous solution and to an improved cathode for use in such
a process. The primary application of the invention disclosed herein is in relation to the
recovery of copper, although the invention finds equal application with electro-recovery
of other metals such as nickel, lead, zinc etc.
Background Art
Processes for leaching base metals from ores and concentrates with subsequent
recovery of the base metal in electrolytic cells are known in the field of the
hydrometallurgy. One example is disclosed in Indian Patent Application No.
656/Del/93 (187010). This process is multi-stage and produces a pregnant liquor
stream following leaching of the mineral in a chloride medium. The pregnant liquor
stream is electrolysed in an electrolysis cell to recover the metal from the solution
which deposits on a cathode of the cell. Under high current densities, dendritic copper
of high purity is produced on the cathode . In the past it has been necessary to regularly
remove the cathodes to strip the plates of the metal deposits so as to maintain current
efficiency in the cell.
Optimisation of the electrowinning operation is a function of the purity of the
pregnant liquor stream, and general cell parameters such as current density, stripping
cycle, cell configuration and the degree of agitation. Accordingly, an aim of the present
invention is to improve the efficiency of the electrowinning operation. In particular, an
aim is to provide an electrolysis process and cell configuration which is able to better
control current density across the deposition surface of the cathode so as to assist in
both formation and removal of the metal deposit.
Summary of the Invention
In a first aspect the present invention provides an electrolysis process for the
recovery of metal from an aqueous solution wherein on electrolysing the solution metal
is caused to deposit on a deposition surface of a cathode, the process including the step
of
inducing a non-uniform current density across the deposition surface so as
to form areas of high current density interspaced by areas of flow current
density, the difference between the areas of high current density and low
current density being sufficient to cause metal deposition to be concentrated on
the areas of high current density so as to promote non-uniform deposition of
metal across the deposition surface.
In the context of the invention, the deposition surface may be of unitary
structure or alternatively may be formed from discrete elements which may be spaced
apart or in direct contact with one another.
Providing a non-uniform current density across the deposition surface provides
a mechanism by which the deposition of metal on that surface can be controlled. In
particular, it allows the metal deposition to be concentrated in certain areas (i.e. the
areas of high current density) so as to promote non-uniform deposition across the
surface. Non-uniform deposition of the metal is beneficial as it is easier to remove from
the cathode which assists in the metal recovery process. ,.
Preferably, the metal deposition is heavily concentrated on the areas of hi|;h
current density so that the metal deposition is effectively discontinuous across the
deposition surface. Preferably, the concentration of metal deposition in operation of the
cell is greater than 80% in the areas of high current density and more preferably greater
than 95%.
Preferably, the areas of high current density and low current density extend
along the surface in one direction and alternate across the surface in an opposite
direction. With this arrangement, the metal deposits in a series of generally linear bands
which is ideally suited for removal using a wiping action as will be described in more
detail below.
Preferably, the electrolysis process induces a non-uniform current density
across the deposition surface by providing a cathode which in operation of the cell,
creates a non-uniform electrical field having areas of strong electrical field and weak
electrical field. With this arrangement, the areas of strong electrical field induce the
areas of the high current density and the areas of weak electrical field induce the areas
of low current density.
The non-uniform electrical field can be created through numerous mechanisms,
including the geometry of the surface, and by varying the electrical resistance between
the cathode and anode along the deposition surface, or by a combination of both these
mechanisms.
The geometry of the surface influences the electrical field and is related to its
surface curvature. Electrical fields are always perpendicular to the surface so that,
sharp edges, or peaks at the deposition surface induce areas of high electrical field as
compared to areas of flat surface, or valleys. The electrical resistance can be varied by
using different materials along the deposition surface (e.g. providing sections with
insulating material) or by changing the current path length between the cathode and the
anode.
In a preferred form, the non-uniform electrical field is induced at the deposition
surface by the geometry of the surface and in particular by forming a series of alternate
ridges and valleys across the surface. By virtue of this geometry, in operation of the
cell, there is a higher electrical field along the ridges as compared to the valleys. In
addition, the current path length at the ridges is shorter as compared to the valleys
thereby creating a situation where there is less resistance at the ridges as compared to
the valleys.
The variation in current density across the deposition surface maybe such that
there is a sharp demarcation between the areas of high current density and low current
density, or alternatively there may be a more gradual transition between the areas of
highest current density and lowest current density.
The applicant has found that inducing a gradual transition between the areas of
highest and lowest current density still provides good deposition patterns so as to
promote substantially discontinuous growth across the deposition surface. In particular,
the applicant has found that using a cathode which includes a deposition surface having
ridges and valleys which do not include a sharp transition between the ridge and valleys
so that there is a more gradual change between the highest current density and the
lowest current density provides excellent performance. This arrangement induces
secondary effects which assist in concentration of the metal deposition at the ridges as
described in more detail below and also provides for easier removal of the metal as it
allows easier access to the entire deposition surface in contrast to a sharp transition
between the ridge and the valley may provide areas which are difficult to access!
In a preferred form, where the cell is operative to remove copper from an
aqueous solution current density in the areas of high current density is in the range
of 500 to 2,500A/m2 and more preferable 1,000 AM/2. Preferably, the areas of low
current density is in the range of 0 to 2,050A/m2 and more preferably 0 to 500A/m2.
Where there is a gradual transition between the areas of highest current density
and lowest current density, the demarcation between an area of "high current density"
and "low current density" is somewhat arbitrary. In this arrangement, the transition
region may be regarded as an area of moderate current which in turn is located between
areas of adjacent "high current density" and areas of "low current density".
Preferably, the process further includes the step of removing deposited metal
from the deposition surface by passing an element over the surface.
Preferably, in the arrangement where the areas of high current density and low
current density extend along the surface in one direction and alternate across the surface
in an opposite direction, the element is moved in the direction in which Hie areas of high
and low current density extend.
Preferably, the deposited jngtal is removed by the element whilst maintaining
current flow in the aqueous solution, In this way, the process can be substantially
continuous.
In yet a further aspect, the present invention relates to an electrolysis cell for
the electrorecovery of metal from an aqueous solution, the cell including a cathode
which includes a deposition surface on which metal is deposited on electrolysing of the
aqueous solution,-wherein in operation of the cell, the deposition surface has a non-
uniform electrical field so as to have areas of strong electrical field interspaced by areas
of low electrical field, the difference between the areas of high electrical field and low
electrical field being sufficient to cause metal deposition to be concentrated on the areas
of high electrical field so as to promote non-uniform deposition of metal on the surface.
Preferably, the areas of high electrical field and low electrical field extend
along the surface in one direction and alternate across the surface in an opposite
direction. In a particularly preferred form, the deposition surface of the cathode
includes an array of alternate ridges and valleys, with the ridges forming areas of high
electrical field and the valleys forming the areas of low electricalfield.
Profiling the deposition surface to have an array of alternate ridges and valleys
has significant benefit in promoting substantially discontinuous metal deposition on the
cathode. Typically such profiling promotes metal deposition as a dendrite growth on
each of the ridges. Advantageously, the resulting dendrites are easy to remove (as
described below). Not only does the profile provide, in the initial operation of the cell,
the appropriate non-uniform current density to concentrate metal deposition as dendrites
on the ridges, but it also assists in maintning discontinuous growth as the process
continues. As will be appreciated, once metal deposits on the deposition surface, the
deposited metal forms an extension of a deposition surface. An advantage of having an
arrangement of ridges and valleys is that as the dendrites grow on the ridges, they tend
to "shadow" the valleys which further inhibits metal deposition in the valleys. In
addition, the aqueous solution tends to stagnate in the valleys which further inhibits
deposition of metal in the valleys. In tests conducted by the applicant, using a profile of
alternate ndges and valleys, more than 98.8% of metal was deposited on the ridges of
the deposition surface.
Whilst the beneficial effects of including the ridges and valleys may be
achieved over a range of profiles, the applicants have found that a regular profile where
the surfaces between the top of the ridge and the base of the valley is substantially linear
and have an internal angle of approximately 60o between adacent surfaces provides
good results. Furthermore preferably the pitch distance between adjacent ridges is in
the order of 1 0-40mm, and more preferably 15-25mm, and the depth between the ridge
and the valley is in the order of 8-32mm and more preferably in the range of 12-2Qmm.
A deposition surface having these characteristics has been found to produce
substantially discontinuous metal deposits. A further advantage is that this profile
enables the surface to be substantially cleaned without creating "hot spots" of current
density which would lead to impure metal deposits. If the current density at a site is too
high, as the deposition progresses, it leads to concentration polarisation (which takes
place around the growing deposit). When mis phenomenon occurs, a relative increase
in impurity inclusions in the depositing metal (e.g. in copper) can occur. Thus it is
important to control the current density at the site, The advantage of the profile
mentioned above is that the areas of high current density where metal deposits still
takes up a substantial part of the total area of the cathode(i.c. in the vicinity of 25-35%
of the total area of the deposition surface). With this arrangement, the current is able to
be maintained at a substantially constant rate regardless of whether the surface is clean
of metal deposits or whether deposition has already occurred. As such, there is no need
to ramp up the current on initiating the cell as the profile itself does not tend to induce
strong "hot spots" of current density which is likely to cause problems in initial metal
deposition.
In a particularly preferred form, the cathode includes a sheet having at least
one major surface which forms the deposition surface of the cathode, the sheet being
preformed so as to incorporate the j}tematejridges_and valleys. The sheet may thus
define a corrugated profile. Preferably, this preforming operation is achieved by folding
of the sheet but it could be made by any other appropriate process such as a stamping,
milling, swaging, casting process or combinations thereof.
In a particularly preferred form, the sheet is formed from titanium or similar
oxidation resistant material. Whilst other oxidation resistant materials may be used,
such asjplatitmm, stainless steel, corrosion resistant metal alloys, titanium is most
preferred because of its excellent oxidation resistance, its capcity to resist forming a
metallurgical bond with metals such as copper, and because of its relative availability.
A further advantage of using a corrugated profile is that it assists in
maintaining dimensional stability for the sheet Such an arrangement can assist in
overcoming the disadvantages of prior art arrangements where sheet cathodes had a
tendency to flex and buckle. Further, when metal deposits on the sheet as a dendriticor
crystalling growth the dimensional stability of the sheet enables wiping methods to be
used to easily remove the deposit from the sheet The applicants have found that
titamium sheets in the order of 1,6mm thickness provide sufficient dimensional stability
for this process.
Preferably, the sheet is adapted in use for attachment to a conductive header
bar. This header bar supports the cathode in use andsupplies electrons to it
In one form, the opposite major surfaces of the folded sheet axe used as
deposition surfaces in operation of the cathode.
m an alternative form, me cathode is made from a composite structure and
further includes a conducting element which extends along the sheet The conducting
element is in electrocommunication with the sheet so as in use to supply the deposition
surface with electrons in the electrolysis process. One advantage of using a conducting
element which extends along the sheet is that it minimises ohmic drop which occurs
when the electrons are supplied solely from one edge of the sheet A second advantage
of using a conducting element is mat is may be of sufficient size to provide rigidity to
the sheet to further assist in maintaining dimensional stability of the cathode. With the
composite arrangement it may thus be possible to use thinner sheet structures for the
deposition surfaces).
m a preferred form of this latter arrangement, the cathode includes a second
sheet which is connected to the first sheet and which has a major surface which forms a
second deposition surface of the cathode, the second sheet being preformed so as to
incorporate the alternate ridges and valleys along that deposition surface. Preferably,
the second sheet is connected to the first sheet of the cathode so as to form a plurality of
pockets which extend in the direction of the alternate ridges and valleys. At least some
of these pockets are operative to receive the conducting element of the cathode.
In a preferred form, the wiping device is operative to pass over a deposition
surface of the cathode so as to remove deposited material from the deposition surface.
In a particularly preferred form, where the cathode includes the ridge and valley profile,
the wiping device includes a plurality of projections which are operative to locate
within respective valleys of the deposition surface. In a preferred form, these
projections are made from a ceramic material but can be made of any other corrosive
resistant material.
In a preferred form, the projections are movable between a first and a second
position and are operative to pass over the surface in either of these positions, m a first
position, the element is in contact or in close proximity to the deposition surface so as to
remove substantially all of the deposition material from mat surface. In the second
position, preferably the element is spaced from the deposition surface and is operative
to remove deposited material which extends a predetermined distance from the
deposition surface.
In yet a further aspect, the present invention relates to a cathode for use in a
process or electrolysis cell as defined in any form above.
In yet a further aspect, the present invention relates to a wiping system for use
in an electrolysis cell in any form as defined above.
In yet a further aspect, the present invention relates to a cathode for use in an
electrolysis cell for the electrorecovery of metal from an aqueous solution, trie cathodes
including a deposition surface having a plurality of ridges which are interspaced by a
plurality of valleys, the profile of the cathode being operative on operation of the cell to
cause metal deposition to be concentrated on the ridges so as to promote non-uniform
deposition of metal on that surface.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Notwithstanding any forms which may fall within the scope of the present
invention, preferred forms of the invention will now be described, by way of example
only, with reference to the accompanying drawings in which:
Figure 1 is a generalised flowchart for processing and recovery of copper;
Figure 2 is a sectional elevation of an electrolysis cell in accordance with one
embodiment of the invention with wiper sets of the cell in a closed position;
Figure 3 is a sectional side view of the cell of Figure 2;
Figure 4 is a sectional elevation of the cell of Figure 2 with the wipers in an
open position;
Figure 5 is a detailed view of the linkage assembly in the cell of Figure 2;
Figure 6 is a cut-away perspective view of the cell of Figure 2;
Figure 7 is a schematic view to an enlarged scale showing the wipers located in
an open position at the top of the cathode plates;
Figure 8 is a detailed view to an enlarged scale of the wipers in a closed
position;
Figure 9 is a front elevation of a cathode panel used in the cell of Figure 2;
Figure 10 is an end view of the panel of Figure 9;
Figure 11 is a schematic perspective view of a wiper engaging a cathode in the
cell of Figure 2;
Figure 12 is a sectional view along section line XII-XII in Figurel 1;
Figure 13 is a detailed view of the blade construction of the wipers used in the
cell of Figure 2;
Figures 14 and 15 are variations of the blade instruction shown in Figurel3;
Figure 16 is a schematic perspective view of an alternative cathode designed
for use in the cell of Figure 2; and
Figure 17 is a cross sectional view along section line XVH-XVH of the cathode
of Figure 16.
Modes for Carrying out the Invetion
la Figure 1, a schematic representation of a combined process 100 including
leach and electrorecovery 104 of metal is depicted. In a preferred form of this process,
ground copper sulfide 106 is fed to a multistage counter current leaching process in
which the metals are solubilised through oxidation by a lixiviant. La a preferred form,
the IMviant includes a complex halide species which is formed in the anode of the
subsequent electrolysis stage and is fed back into the leach stage as part of the
electrolyte recycle 108.
Dissolved metals in desirable oxidation states are removed at various stages
from the leach process in the leachate. The leachate is passed through filtration 110 to
remove unwanted solids such as sulfur and ferric oxide. The leachate is then passed to
purification 112 to remove metals which may otherwise contaminate subsequent
electrolysis (such as silver and mercury). The contaminant metals may be precipitated
as the metal oxide or carbonate form.
The purified leachate is then fed to the electrolysis stage 104 which may
include a plurality of electrolysis cell groups in series and/or in parallel Jn each group, a
different metal may be produced, with typically copper metal being electrorecovered in
a first cell group and metals such as zinc, lead and nickel being recovered in subsequent
or parallel cell groups. The electrolysis process is typically operated such that a highly
oxidising lixiviant (such as a complex halide species) is produced at the anode. The
spent electrolyte (anolyte) is then recycled to the leaching stage and includes the highly
oxidising lixiviant which participates in further counter current leaching. Thus, the
process can operate continuously.
The present invention is concerned with optimising the electrorecovery of
metals and relates to significant design improvements in the electrolysis process,
including improved cathode design and geometry.
Referring now to Figures 2-5 the electrolysis cell 10 for use in the process 100
includes a series of cathode plates 11 which are disposed within the electrolysis cell
tank SO and interspaced by anodes 12. Electrolyte fed to the cell enables current flow
between the anodes and the cathodes. The outer surfaces 13,14 of the respective
cathodes form a deposition surface for the cell on which the metal to be recovered
deposits in operation of the cell 10. As will be described in more detail below, the
cathode plates are formed from a generally corrugated profile having alternate ridges
and valleys so as to influence the mode of deposition of the metal on the respective
deposition surfaces 13 and 14.
The cell 10 includes a wiper system 15 which includes a plurality of wiper sets
16 operative to interfit between respective cathodes and anodes with the wipers 17 of
respective wiper sets 16 being operative to move across the deposition surfaces 13 and
14 of respective cathodes 11 so as to remove metal deposits from those surfaces. The
wipers 17 are arranged to be wiped down the respective deposition surfaces 13 and 14
at predetermined intervals to cause the dislodged metal to drop to the bottom of the cell
10 wherein it is transferred to a conveyor 18 for removal from the cell.
To achieve this wiping action, the wiping system 15 includes two principle
movements; the first being a vertical movement to allow the wiper sets 16 to move
between the top and the bottom of the respective cathodes 11, the second being to allow
the wipers 17 in each set 16 to move from an open position (as best illustrated in Figueg"
7) to a closed position (as best illustrated in Figure 8).
The wiper sets 16 are mounted on a frame 32 which is secured at its upper end
to four supporting struts 19, 20, 21 and 22. Each of the struts include a helical track 23
which cooperates with a worm gear 24 connected to the frame 32. In this way, the
frame 32 moves relative to the struts. An electric motor 25 mounted on a cross beam 26
is operative to drive the worm gears 24 so as to achieve the vertical movement of the
wiper sets relative to the deposition surfaces 13 and 14. Under this action, the wipers
are able to move between a lower position as disclosed in Figure 2 to an upper position
as disclosed in Figure 4.
The frame 32 supports a linkage assembly 27 which in turn is connected to the
wiper sets 16. The linkage assembly 27 includes a pair of link plates 28 at each end of
the wiper sets 16 which are connected to respective link arms 29. A crank 30 is
pivotally connected to respective pairs of the link plates 28 through pivot points 31.
Crank arms 40 extend from the crank 30 to the wiper sets 16 so as to support each end
of the wiper sets. The link arms 29 are capable of vertical movement through a second
actuator 41. In the illustrated form, the second actuator is in the form of worm gears
which cooperate with helical tracks formed on the respective link arms. The worm
gears rotate which impacts the rotation to the the link arms 29 to cause vertical
displacement of those arms relative to the frame 32 which in turn drives the crank 30 so
as to move the wipers between their open and closed positions. The second activator
can be damped to prevent over-tightening and jamming of the wipers against the
cathode. Damping can be provided by a spring-loaded coupling or by using a pneumatic
cylinder in place of the worm gear.
As best illustrated in Figure 6 each line of cathodes in the cell 10 is formed
from a plurality of cathode plates 11 which are connected to a header bar 34 so that the
individual plates are suspended in the tank SO. The header bar 34 is conductive and
connected to a power source so as to supply electrons to the cathode.
Typically the electrolyte is highly corrosive, resulting from typically a 5 molar
or greater concentration of alkali or alkaline-earth metal halides. To enable the
components to be able to operate in this environment, the wiper system IS is made from
a corrosion resistant material which is preferably titanium. Other suitable materials
include platinum stanless steel corresion resists metal allows (such as hastalloy C.
22), or even some plastics. Also, titanium is most preferred for the cathode because of
its excellent corrosion resistance and its capacity to resist forming a metallurgical bond
with metal such as copper, and because of its relative availability (hence cost benefit).
Its resistance to forming a metallurgic bond improves the ability of the plates to be
stripped using the wiper system described above.
Figures 9 and 10 illustrate the construction of the individual cathode plates 11.
In the illustrated form, the cathode plate 11 is formed from a titanium sheet having a
thickness which is preferably about 1.6mm. Sheets of this thickness have been found
by the applicant to give adequate rigidity to the cathode plate to prevent buckling in use.
The titanium sheet is folded to form a generally corrugated profile so as to provide on
each deposition surface 13,14 alternate valleys and ridges 35,36 respectively. These
corrugations run along the entire length of the cathode between its upper and lower
edges 37,38.
In the illustrated form, the distance between adjacent ridges 36 is 20mm,
whereas the depth between the top of the ridges 36 and the bottom of the valleys 35 is
approximately 16mm. The wall surfaces 43 formed on the corrugated sheet are
generally linear and have an internal angle at the top at the ridges and bottom of the
valleys of approximately 60°.
A primary purpose for incorporating the corrugations in the cathode is to
influence the current density on the deposition surfaces 13,14 under operation of the
cell. In particular, the corrugations on the deposition surface cause a non-uniform
electrical field across that surface in operation of the cell.
The corrugated deposition surface on the cathode creates bands of high current
density along the ridges of the cathode due to its corresponding high electrical field at
those areas and relatively low current densities in the valleys. This causes metal
deposition to be concentrated in the areas of high current density and promotes non-
uniform deposition across the surface so that the vast majority of the deposition
includes in the ridge regions 35 of the deposition surface. Creating substantially
discontinuous deposition improves the ability to be able to remove recovered metal
from the cathode using the wiping system 15.
The profile of the deposition surface with the valleys and ridges causes the
non-uniform electric field by two mechanisms. Firstly, in view of the geometry of the
profile, the electrical field will be stronger at the ridges than the valleys because of its
surface curvature. In general, the electric field lines are always perpendicular to the
surface. Therefore, at each ridge there will be a concentration of a field along those
points. Secondly, the current flow path at the ridges is less than the current flow path at
the valleys. As a result, there is less resistance at the ridges than there is at the valleys.
In addition, the use of the corrugated profile of the cathode allows better
control at the main sites of deposition (i.e. along the ridges). If the current density at a
site is too high, as deposition progresses, it leads to concentration polarisation (which „
takes place around the growing deposit). When this phenomenon occurs, a relative
increase in impurity inclusion in the depositing metal (e.g. in copper) can occur. With
the corrugated profile, the main sites of deposition account for approximately 25-35%
of the total surface area of the cathode. As a function of the mass transfer, ideally the
current at the deposition surface should be in the vicinity of l,000A/m2 or less. As the
dendrites grow on the surface, the actual area of deposition surface increases as metal is
deposited on the previously deposited metal. If the initial deposition sites on the
cathode are too small, then there is a tendency that once the dendrite is removed from
the cathode the current density at that site becomes too high. Through trials conducted
by the applicant, using the corrugated profile, it is found that the current density at the
deposition sites both on initial operation of the cell and after dendritic growth has
occurred, it is able to be maintained in the vicinity of l,000A/m2 so as to provide high
quality metal deposition. As such, there is no need to vary the current during the
process.
A further advantage of using the corrugated profile on the cathode is that it
improves the rigidity of the cathode plate, as the corrugated profile is inherently stiffer
than a flat plate along the direction of the ridges and valleys. In addition, the corrugated
profile is ideally suited to be cleaned using the wiping blade system as is described in
more detail below.
With reference to Figures 11 to 15, the wipers 17 include fingers 39 which are
mounted between a pair of rails 42. In the illustrated form, each of the individual
fingers are formed from a ceramic material with the rails being made of titanium. Each
of the fingers is spaced along the rail 42 so that the wipers 17 conform generally to the
shape of the corrugated cathode plate 11, with the individual fingers locating within the
valleys 35 of the deposition surface and over the associated ridges 36.
As best illustrated in Figure 12, the wiping system 15 is designed so that when
the wiper sets 16 are in their closed position, the wipers 17 are angled to the cathode
plate 11 so that the individual fingers 39 are in a trailing position relative to the line of
movement of the wiper 17 down the cathode plate 11. This arrangement is preferred as.
it inhibits jamming of the fingers in the valleys as may occur if the fingers 39 v&ere in a
leading position relative to the direction of movement of the wipers down the cathode
plate.
As described above, in view of the configuration of the cathode plates 11, the
metal recovered from the electrolysis cell is concentrated on the ridges of the respective
deposition surfaces of the cell. As such, when the wiper 17 is moved across the
deposition surface the dislodged material from the ridges tends to move into the
adjacent valleys of the deposition surface. This causes an accumulation of the metal
within the valleys which tends to envelop the fingers 39 thereby protecting the ceramic
fingers 39 from wear. In addition, there is a build up in frictional force as the mass of
material is moved down the deposition surface thereby aiding removal of material as the
material is dragged from the surface under this frictional force. It is not necessary that
the fingers 39 are in direct contact with the deposition surface to ensure adequate
cleaning of that surface.
Another advantage of the design of the wiping system 15 is that it enables
different stages of cleaning of the cathodes. In particular, as described above, the
wipers 17 can be operated to remove the bulk of the deposited material on the
deposition surfaces by dragging across those surfaces when in their closed position.
The wipers can also be moved across the deposition when in their open position. This
is used not to fully clean the deposition surface but rather to ensure that there is no
extended dendritic growth on part of the deposition surface which could otherwise grow
to an extent that it contacts the anode and thereby causes short circuiting of the
electrolysis cell. Also, this allows for more consistent growth across the ridges of the
cathode, which aids in control of the current density along the deposition surface.
Figures 14 and 15 illustrate some variations in the design of the wipers 17. As
in the arrangement of Figure 13, each of the wipers 17 include ceramic fingers 39.
However, rather than using the rail arrangement 42 as disclosed in Figure 13, the fingers
39 are interconnected by an internal connecting bar 44. In the embodiment of Figure 14
the bar 44 is formed as a square section, whereas in Figure 15 the connecting bar is
made up of two cylindrical bars 45.
Referring now to Figures 16 and 17, an alternative cathode construction is
depicted. In this embodiment the cathode is formed as a composite structure wherein
the outer deposition surfaces 13,14 are defined by separate sheets which are fastened
together along their respective lateral edges 60,61, and which may optionally be
fastened together at intermittent regions 62.
In this embodiment, a plurality of conducting bars 63 form part of the
construction and extend downwardly from the header bar 34, the conducting bars
typically also being formed from titanium (or a titanium coated copper bar to further
improve conductively). Typically the conducting bars extend for the full length the
plates 13,14 through each of the passages defined between a plate and are fastened
thereto. Such an arrangement provides enhanced distribution of electrons through the
assembly, thereby minimising ohmic drop which can occur when the elections are
supplied solely to one edge of the sheet In addition, it has been found that the
composite arrangement, including the arrangement of the conducting bars in the
passages, enhances the dimensional stability of the sheet so that thin plate structures
(e.g. as small as lmm) or alternatively wide plate structures can be employed for the
cathode. Otherwise, the principles of operation of the cathode of Figures 16 and 17 are
described above.
Whilst the invention has been described with reference to a number of
preferred embodiments, it should be appreciated that the invention can be embodied in
many other forms.
WE CLAIM:
1. An electrolysis process for the recovery of metal from an aqueous solution
wherein on electrolysing the solution metal is caused to deposit on a deposition surface
of a cathode, characterised in that the process comprises the step of
- inducing a non-uniform current density across the deposition surface-so as
to form areas of high current density interspaced by areas of low current
density, the difference between the areas of high current density and low
current density being sufficient to cause metal deposition to be
concentrated on the areas of high current density so as to promote non-
uniform deposition of metal across the deposition surface.
2 . A process as claimed in claim 1, wherein the areas of high current density and
low current density extend along the surface in one direction and alternate across the
surface in an opposite direction.
3 . A process as claimed in either claim 1 or claim 2, that is operative to recover
copper from the aqueous solution and the current density in the areas of high current
density is in the range of 500-2500 A/m2and is more preferably 1000 A/m2.
4 . A process as claimed in claim 1 or claim 2, that is operative to recover copper
from the aqueous solution and the current density in the areas of lower current density is
in the range of 0-1250 A/m2and is more preferably 0-500 A/m2.
5 . A process as claimed in claim 1 or claim 2, further including the step of
removing deposited metal from the deposition surface by passing an element over said
surface.
6. A process as claimed in claim 5, when dependent on claim 2, wherein the
element is moved in the direction in which the areas of high and low current density
extend.
7 . A process as claimed in claim 5, wherein deposited metal is removed by the
element whilst maintaining current flow in the aqueous solution.
8. A process as claimed in claim 5, wherein the element is moveable between first
and second positions, and is operative to be passed over the deposition surface in either
of the first and second positions.
9. A process as claimed in claim 8, wherein when in its first position, the element
is in contact with, or in close proximity to, the deposition surface so as to remove
substantially all of the deposition material from that surface.
10. A process as claimed in either claim 7 or 8, wherein when in its second
position, the element is spaced from the deposition surface and is operative to engage
and remove deposited material which extends a predetermined distance from the
deposition surface.
11. An electrolysis cell for the electro-recovery of metal from an aqueous solution,
the cell including a cathode which includes a deposition surface on which metal is
deposited on electrolysing of the aqueous solution, wherein in operation of the cell, the
deposition surface has a non-uniform electrical field having areas of strong electrical
field interspaced by areas of weak electrical field, the difference between the areas of
strong electrical field and weak electrical field being sufficient to cause metal
deposition to be concentrated on the areas of high electrical field so as to promote non-
uniform deposition of metal on the surface.
12. An electrolysis cell as claimed in claim 11, wherein the areas of strong
electrical field and weak electrical field extend along the surface in one direction and
alternate across the surface in an opposite direction.
13 . An electrolysis cell as claimed in either claim 11 or 12, wherein the deposition
surface of the cathode includes an array of alternate ridges and valleys, with the ridges
forming the areas of strong electrical field and the valleys forming the areas of weak
electrical field.
14. An electrolysis cell according to claim 13, wherein the cathode includes a sheet
having at least one major surface which forms the deposition surface of the cathode, the
sheet being preformed so as to incorporate the alternate ridges and valleys.
15 . An electrolysis cell according to claim 14, wherein the sheet has opposite
major surfaces, each of which forms a deposition surface of the cathode.
16. An electrolysis cell as claimed in claim 15, wherein the sheet is folded so as to
form the valleys and ridges on the opposite deposition surfaces with the ridges on one
deposition surface being directly opposite the valleys on the opposite deposition surface
and vice versa.
17 . An electrolysis cell as claimed in claim 14, wherein the sheet is of generally
uniform thickness.
18. An electrolysis cell as claimed in any claim 14, 15, 16 or 17, wherein the sheet
is formed from titanium.
19 . An electrolysis cell as claimed in claim 14, further including at least one
conducting element which extends along the sheet, the conducting element being in
electrocommunication with the sheet so as in use to supply the deposition surface with
electrons in the electrolysis process.
2 0 . An electrolysis cell as claimed in claim 19, wherein the conducting element is
of sufficient size to add rigidity to the sheet.
21. An electrolysis cell as claimed in either claim 19 or 20, wherein the cathode
includes a second sheet which is connected to the first sheet and which has a major
surface which forms a second deposition surface of the cathode, the second sheet being
preformed so as to incorporate the alternate ridges and valleys along the deposition
surface.
22. An electrolysis cell as claimed in claim 21, wherein the second sheet is
connected to the first sheet of the cathode so as to form a plurality of pockets which
extend in the direction of the alternate ridges and valleys, the pockets being operative to
receive a conducting element of the cathode.
23 . An electrolysis cell as claimed in claim 11, 12, 14, 15, 16, 17,19, 20 or 22,
further including a wiping device which is operative to pass over the deposition surface
of the cathode so as to remove deposited material from that deposition surface.
2 4 . An electrolysis cell as claimed in claim 23, when dependent on claim 13,
wherein the wiping device includes a plurality of projections which are operative to
locate within respective valleys of the deposition surface.
25. A cathode for use in an electrolysis cell for the electrorecovery of metal from
an aqueous solution, the cathode having a deposition surface including an array of
alternate ridges and valleys.
2 6. A mechanism for removing metal deposited onto the deposition surface of the
cathode of claim 25, the mechanism including a plurality of elements arranged to
project into respective valleys and be moved therealong so as to dislodge deposited
metal from the ridges and valleys.
27. A mechanism as claimed in claim 26, wherein the elements have a shape
generally corresponding to the valleys.
28. A mechanism as claimed in claim 26 or claim 27, wherein the elements are
formed from a ceramic material.
29. A mechanism as claimed in claim 26 or claim 27, wherein the elements are
pivotally operable between a first position in which the elements protrude into the
valleys and a second position in which the elements do not so protrude.
The present invention relates to the field of fiber-reactive dyes. Black dyeing mixtures
of fiber-reactive dyes are known from U.S. Patent Nos 5,445,654 and 5,611,821 as well
as from Korean Patent Application Publication No. 94-2560. Deep balck dye mixtures
are known, for example, from Japanese Patent Application Publication Sho-58-160 362
which are based on a navy-bule disazo dye and an orange monoazo dye. However these
dye mixtures have some deficiencies. With the present invention, deepblack-dyeing dye
mixtures of improved properties, for example wash fastness have unexpectedly been
found, comprising a disazo dye conforming to the general formula (1), and ore or more
disazo dyes conforming to the general formula (2).

Documents:

00843-kolnp-2005-abstract.pdf

00843-kolnp-2005-assignment.pdf

00843-kolnp-2005-claims.pdf

00843-kolnp-2005-correspondence.pdf

00843-kolnp-2005-description (complete).pdf

00843-kolnp-2005-drawings.pdf

00843-kolnp-2005-form 1.pdf

00843-kolnp-2005-form 18.pdf

00843-kolnp-2005-form 3.pdf

00843-kolnp-2005-form 5.pdf

00843-kolnp-2005-gpa.pdf

00843-kolnp-2005-letter patent.pdf

00843-kolnp-2005-reply first examination report.pdf

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Patent Number

216899

Indian Patent Application Number

843/KOLNP/2005

PG Journal Number

12/2008

Publication Date

21-Mar-2008

Grant Date

19-Mar-2008

Date of Filing

09-May-2005

Name of Patentee

INTEC LTD.

Applicant Address

GORDON CHIU BUILDING, J01, DEPARTMENT OF CHEMICAL ENGINEERING, UNIVERSITY OF SYDNEY, NEW SOUTH WALES 2006

Inventors:

#	Inventor's Name	Inventor's Address
1	LAM CHUNG HO	9/144 EDWIN STREET, CROYDON, NEW SOUTH WALES 2132

PCT International Classification Number

C25C 1/00

PCT International Application Number

PCT/AU2003/001393

PCT International Filing date

2003-10-21

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2002952181	2002-10-21	Australia