WO2012175803A2

WO2012175803A2 - Permanent cathode and a method for treating the surface of a permanent cathode

Info

Publication number: WO2012175803A2
Application number: PCT/FI2012/050637
Authority: WO
Inventors: Mari Lindgren; Henri Virtanen; Ville Nieminen
Original assignee: Outotec Oyj
Priority date: 2011-06-23
Filing date: 2012-06-19
Publication date: 2012-12-27
Also published as: AT15652U1; EP2723920A4; KR20140024470A; JP2014517159A; PE20142001A1; FI20110210A0; BR112013033245A2; FI20110210L; EP2723920A2; WO2012175803A4; ES1186663U; AU2012273906B2; RU2566119C2; CA2838877A1; RU2013155581A; BG2816U1; KR20160005798A; AU2012273906A1; MX2013014910A; CN103748267A

Abstract

The invention relates to a permanent cathode (1) to be used as an electrode in the electrowinning of metals, including a permanent cathode plate (4) at least partially made of steel and providing the possibility of electrochemically depositing metal from an electrolytic solution onto its surface (6), in which case the dimensions of the grain boundaries (7) on the surface (6) of the permanent cathode plate (4) are arranged to be suitable for the adhesion of deposited metal on the surface and the stripping of metal from the surface at least in a part of the surface that is in contact with the electrolyte. The invention also relates to a method for treating the surface of a permanent cathode.

Description

PERMANENT CATHODE AND A METHOD FOR TREATING THE SURFACE OF A PERMANENT CATHODE

FIELD OF THE INVENTION

The invention relates to a permanent cathode defined in the independent claims, for use in the electrolytic recovery and electrowinning of metals. Furthermore, the invention relates to a method for treating the surface of a permanent cathode plate.

BACKGROUND OF THE INVENTION

When the intention is to manufacture pure metal such as copper, hydrometallurgical methods such as electrolytic refining or recovery are used. In electrolytic refining, impure copper anodes are dissolved electrochemically, and the copper dissolved from them is reduced onto the cathode. In electrolytic recovery, the copper is reduced directly from the electrolytic solution, which is typically a copper sulphate solution. The rate of deposit of the metal, such as copper, on the cathode surfaces depends mostly on the current density used. The cathodes used in the process can be starter sheets made of the metal to be reduced, or permanent cathodes made of steel, for example. A transition to the use of permanent cathodes has been the prevailing trend at electrolytic plants for a long time, and in practice, all new copper electrolysis processes are based on this technology. A permanent cathode by itself is formed of a cathode plate and an attached suspension bar using which the cathode is suspended in the electrolytic bath. The copper can be mechanically stripped from the permanent cathode's cathode plate, and the permanent cathodes can be reused. Permanent cathodes can be used in both electrolytic refining and recovery of metals. The mere corrosion resistance of the steel grade used as a permanent cathode plate in the electrolyte is not enough to guarantee that the properties required of the cathode are fulfilled. Substantial attention must be paid to the adhesion properties of the cathode plate surface. The surface properties of a permanent cathode plate must be appropriate so that the depositing metal does not spontaneously strip off from the surface during the electrolytic process but adheres sufficiently, however not preventing the deposited metal from being removed using a stripping machine, for example. The most important properties required of a permanent cathode plate include corrosion resistance, straightness and surface properties with regard to the adhesion and removability of the deposit. A prior art method is the manufacture of permanent cathode plates from stainless steel. Stainless steel is an iron-based alloy containing more than 10.5% chromium and less than 1 .2% carbon. The chromium forms a thin oxide layer on the steel surface, known as the passive film, which substantially improves the corrosion resistance of the steel. Other alloying elements can also be used to influence the properties of the passive film and thus corrosion resistance. For example, molybdenum improves the endurance of the passive film against pitting caused by chlorides, in which the protective passive film is damaged locally. Alloying elements are also used to influence other properties, for example mechanical properties and manufacturing properties such as weldability.

Stainless steels are widely used in applications requiring good corrosion resistance, such as the process industry, the chemical industry and the pulp and paper industry. Due to the large volume of use, stainless steels are usually manufactured by hot rolling. After this, the rolling scale is removed from the steel surface. When making thinner plates with tighter thickness tolerances, cold rolling is used. Processing after cold rolling depends on the desired surface quality. Standard SFS-EN 10088-2 defines, for example, that a surface of type 2B shall be cold rolled, heat treated, pickled and skinpass- rolled. 2B thus describes the manufacturing route of the material and therefore only specifies the surface properties at a very general level, with the basic parameters being surface smoothness and brightness. Surface roughness is typically used to describe surfaces. Surface roughness can be defined in a myriad of different ways but, for example, the widely used R_a index refers to the mean deviation of surface roughness. However, it does not address the surface profile at all - whether the surface roughness is formed by peaks or valleys. In other words, surfaces of very different qualities may have exactly the same R_a index. This is illustrated in Figures 1 a, 1 b and 1 c. According to patent publication US 7807028 B2, it is proposed that a permanent cathode plate be made of an alloy at least partially comprised of duplex steel. A duplex grade of steel refers to a steel containing 30% to 70% austenite with the remainder being of ferritic structure. The desired structure can be created through appropriate alloying. According to the publication, the roughness of the cathode plate surface is an essential factor for the adherence of the metal deposit. The publication also presents structures to be made on the cathode plate surface for ensuring the adherence of the metal deposit. Such structures include, for example, various types of holes, grooves and ledges.

According to patent publication US 7807029 B2, it is proposed that a permanent cathode plate be made of Grade 304 steel. This grade is a universal stainless steel having a composition very close to the grade known as acid-proof steel and an austenitic structure. According to this publication, the roughness of the cathode plate surface is an essential factor for the adherence of the metal deposit, and also this publication presents structures to be made on the cathode plate surface for ensuring the adherence of the metal deposit. It is further proposed that the steel be manufactured with 2B finish in order to achieve appropriate adhesion of the metal deposit.

An optimal surface is typically defined using parameters such as the surface roughness parameter R_a. A way of describing a surface with a certain finish is AISI 316 2B, describing a certain grade of steel that has been skinpass- rolled. The characteristic manufacturing route produces a smooth, semi- bright but not mirroring surface. The publication US 7807028 B2 proposes the parameter 2B for the cathode surface finish, meaning that the surface has been processed by methods including cold rolling, heat treatment and pickling. Material processing and the processing parameters are used to influence the properties of the final surface. However, merely the above- mentioned ways of defining the surface cannot be considered sufficient for determining an optimal surface for a permanent cathode.

In electrodeposition of stiff metals such as nickel on a permanent cathode several problems are faced. Adherence to the cathode plate should be very strong, because the metal deposit easily starts to peel off from the plate. On the other hand, if the adhesion is too strong, it is hard to strip the deposit off, because it is almost impossible to slip a knife between the deposit and the cathode plate.

OBJECT OF THE INVENTION It is an object of the invention to present a new type of permanent cathode for electrolytic purification and recovery of metal, with usable properties and preference over prior art. It is a further object of the invention to define the surface finish parameters for an optimal permanent cathode plate, taking into account the above problems with the use of permanent cathodes.

A further object of the invention is to provide an improved permanent cathode for electrodeposition of stiff metals. SUMMARY OF THE INVENTION

The essential characteristics of the invention are evident from the attached claims.

The invention relates to a permanent cathode to be used as an electrode in the electrowinning of metals, including a permanent cathode plate at least partially made of steel and providing the possibility of electrochemically depositing metal from an electrolytic solution onto its surface. The grain boundary dimensions of the permanent cathode plate surface have been arranged to be suitable for the adhesion of deposited metal on the surface and the stripping of metal from the surface at least in a part of the surface that is in contact with the electrolyte. According to an embodiment of the invention, the size of the grains in the permanent cathode plate is 1 to 40 micrometres measured by the linear intercept method. According to an embodiment of the invention, the average grain boundary width W in the permanent cathode plate is 1 to 3 micrometres. The average grain boundary depth d in the permanent cathode plate is less than 1 micrometre. According to the invention, an optimal permanent cathode can be created by influencing the grain boundary properties of the permanent cathode plate surface.

According to an embodiment of the invention, the permanent cathode plate is at least partially ferritic steel. According to another embodiment of the invention, the permanent cathode plate is at least partially austenitic steel. According to an embodiment of the invention, the permanent cathode plate is at least partially duplex steel. The permanent cathode plate material surface properties according to the invention make it possible to use different grades of steel for the electrowinning of metals. According to an embodiment of the invention, the permanent cathode plate comprises a surface area provided with strong adhesion properties and a surface area provided with weak adhesion properties, said adhesion properties being dependent on the dimensions of the grain boundaries in said surface area. Preferably, the surface area with weak adhesion properties forms a part of the surface that is in contact with the electrolyte and said surface area is located at a point where the stripping of metal deposit is meant to start. The invention also relates to an arrangement to be used for the electrowinning of metals, said arrangement containing an electrolytic bath of an electrolytic solution in which anodes and permanent cathodes are alternately arranged, and said permanent cathodes being supported in the bath by a support element, the permanent cathode according to the invention thus being a part of the arrangement.

The invention also relates to a method for treating the surface of a permanent cathode plate, in which the permanent cathode plate is formed at least partially of steel plate. According to the method, the grain boundaries of the permanent cathode plate surface at least on a part of the surface that is in contact with the electrolyte are treated chemically or electrochemically to achieve the desired surface properties for the adhesion of deposited metal on the surface and the stripping of metal from the surface. According to a characteristic feature of the invention, the permanent cathode plate surface is treated until the desired separating force is achieved, for example by etching the surface of the permanent cathode plate.

According to an embodiment of the invention, different areas of the permanent cathode plate surface that are in contact with the electrolyte are treated differently to produce an area with strong adhesion and an area with weak adhesion. Preferably, the area with weak adhesion is produced on a part of the cathode plate surface where the stripping of metal deposit is meant to start.

LIST OF FIGURES

The invention is described in more detail by reference to the drawings, where

Figures 1 a, 1 b and 1 c illustrate the roughness of a permanent cathode plate surface,

Figure 2 illustrates an arrangement according to the invention,

Figure 3a illustrates a permanent cathode,

Figure 3b illustrates the surface of the permanent cathode,

Figure 4 illustrates the surface of a sample piece from a permanent cathode plate,

Figures 5a and 5b illustrate permanent cathodes with areas of different adhesion properties,

Figure 6 illustrates stripping of a deposit from the permanent cathode,

Figure 7 illustrates the preferred fracture path between a deposit and the cathode plate.

DETAILED DESCRIPTION OF THE INVENTION

Figures 1 a, 1 b and 1 c illustrate different versions of the surface roughness of a cathode plate 4 in a permanent cathode 1 . Figures 1 a, 1 b and 1 c have the same R_a index describing surface roughness even though they look different in closer view, as schematically illustrated by the Figures. According to the invention, the mere surface roughness index is not enough to achieve a sufficiently optimal permanent cathode surface. The permanent cathode 1 according to the invention is illustrated in its operating environment in Figure 2. The permanent cathode is intended to be used for the electrowinning of metals. In this case the permanent cathode is placed in an electrolytic solution in the electrolytic bath 3 alternately with anodes 2 over the entire length of the bath, and the desired metal is deposited from the electrolytic solution onto the surface of the cathode plate 4 in the permanent cathodes 1 . The permanent cathode plate 4 is supported in the bath using a support element 5. Prior art has described permanent cathodes in which the surface roughness constitutes a crucial factor for the adhesion of the metal deposit. However, in addition to surface roughness caused by the manufacturing process, the metal surface also has grain boundaries that play an essential role in the adhesion of copper onto the surface. Solid metal has a crystalline structure, which means that the atoms are tightly packed in a regular array, and the same array extends over a long distance compared to the interatomic distance. These crystals are collectively referred to as grains. The grains form irregular volume areas because their growth is limited by adjacent grains growing at the same time. In multigranular metal, each grain is joined with its neighbouring grains tightly across its surface at the grain boundary. The grain boundary is an area of high surface energy in which the depositing copper primarily nucleates. Therefore special attention must be paid to the number and properties of grain boundaries. Grain boundaries can be seen with an optical microscope or a scanning electron microscope but examination of the dimensions of grain boundaries requires an atomic force microscope (AFM). An AFM has a sharp probe connected to a flexible support arm. When the probe is moved on the surface of the sample under examination, interactions between the surface and the probe are registered as bending of the support arm. The bending can be measured with a laser beam, allowing the generation of a three-dimensional image of the surface profile of the sample. An AFM can be used to measure the dimensions, depth and width, of the grain boundary. The width and depth of grain boundaries naturally vary to a certain extent. This variation can be represented as a normal deviation, allowing statistical processing of the dimensions.

The grain size of a material can be defined in several different ways. One of the methods is the linear intercept method (Metals Handbook, Desk Edition, ASM International, Metals Park, Ohio, USA, 1998 pp. 1405-1409), in which the grain size I is

I = 1 /NL in which N_L is the number of grain boundaries divided by the measurement distance. According to the formula, grain size is inversely proportional to the number of grain boundaries per unit length.

Figures 3a and 3b illustrate the surface 6 of a permanent cathode plate 4 in a permanent cathode 1 according to the invention, and the schematic drawing presents the width W and depth d of the grain boundary between grains 8 in the surface. The grain boundary width can be estimated from an image taken using an optical microscope or a scanning electron microscope, or it can be measured from AFM results. In accordance with the invention, at least a part of the surface of the permanent cathode plate 6 that it is in contact with the electrolyte is treated. The grain boundaries 7 between grains 8 in the permanent cathode plate surface 6 are treated so as to be suitable for the adhesion of deposited metal onto the surface and the stripping of metal from it. An optimal surface for the growth of metal can be achieved in accordance with the invention. In accordance with the invention, the dimensions of grain boundaries 7 in the surface 6 are modified in order to achieve an optimal permanent cathode surface. The grain size of grains 8 in the surface 6 of an optimal permanent cathode plate 4, measured by the linear intercept method, is 1 to 40 micrometres, the average grain boundary width W is 1 to 3 micrometres, and the grain boundary depth d is less than 1 micrometre. A permanent cathode plate according to the invention can be manufactured of austenitic steel, for example. In accordance with the invention, the permanent cathode plate surface is treated by electroetching, for example, until the desired separating force is achieved. The separating force represents the separability of the deposited material from the surface. If the separating force is too small, the metal deposit will be prematurely self- stripped from the permanent cathode plate surface, while an excessively great separating force makes it difficult to strip the metal deposit from the permanent cathode plate surface.

Since the full deposit of stiff metal needs a strong adhesion on the cathode surface in order to avoid peeling or self-stripping, it also makes the start of stripping more difficult. It may be difficult to insert a knife between the cathode plate and deposit to strip the deposit from the plate. Flexing the plate may be impossible because of the stiffness of the metal deposit. This problem can be solved by arranging an area with less adhesion close to the electrolyte level, that is, close to the level where deposition starts. This area of weak adhesion strips off easily and gives a good starting point for stripping of the deposit. Two or more areas of different adhesion properties can be easily manufactured, for instance, by etching one area and by not etching another area.

Figure 5a illustrates a permanent cathode provided with three surface areas 6a, 6b and 6c with different adhesion properties. Line L indicates the level of electrolytic solution when the permanent cathode plate 4 is immersed in an electrolytic bath. The main part of the cathode plate surface, area 6a, is etched in such a way that the desired relative dimensions of the grain boundaries are achieved to improve the adhesion of metal deposit onto the permanent cathode plate 4. The part of the permanent cathode plate 4 above the electrolyte level L, area 6c, may be non-etched or gently etched. Between the more strongly etched area 6a and the non-etched or gently etched area 6c, below the electrolyte level L, there is a third area 6b which is non-etched or etched in such a way that grain boundary dimensions cause only weak adhesion. The adhesion properties of the two non-etched or gently etched areas 6b and 6c may be similar or different. It is important that the permanent cathode plate 4 contains at least one area 6a with strong adhesion and at least one area 6b with weak adhesion, the area 6b with weak adhesion at least partly lying below the electrolyte level L.

Figure 5b shows an alternative embodiment where the area 6b with low adhesion is located in the central area of the width of the cathode plate 4 and the edges of the area below the electrolyte line L form a part of the more strongly etched area 6a.

The embodiments of Figures 5a and 5b make it easy to start stripping when the main part 6a of the permanent cathode plate 4 has strong deposit adhesion. In the case of copper, stripping can be easily started by flexing the permanent cathode plate 4 in order to loosen the adhesion of the deposition on the plate. However, if nickel is deposited as a thick deposit using so called full-deposit permanent cathodes, bending of the permanent cathode plate 4 may be difficult, because nickel is a stiff metal which does not deform easily.

Good adhesion properties are preferably achieved by etching at least a part of the cathode plate 4. In the embodiments of Figures 5a and 5b, the part 6b of the cathode plate 4 located just below the electrolyte level L is kept non- etched or it is etched just gently to obtain an area 6b with much weaker adhesion properties than the major part 6a of the cathode plate 4. Manufacturing of this kind of permanent cathode plate 4 is in principle easy. The areas 6b, 6c that are not to be etched are, for instance, covered by a tape, or even more simply, the plate is just immersed to a certain depth into an etching solvent. Figure 6 illustrates the operation of a permanent cathode plate 4 according to Figure 5a. In practice, metal deposits 1 1 are on both sides of the cathode plate 4, but for the sake of simplicity, only one metal deposit 1 1 is shown in Figure 6.

Stripping the metal deposit 1 1 off the permanent cathode plate 4 is started by pushing a knife 10, or wedge, of a stripping machine between the permanent cathode plate 4 and the metal deposit 1 1 . The major part of the metal deposit 1 1 is strongly adhered to the surface 6a of the cathode plate 4 with strong adhesion. In the upper part of the metal deposit 1 1 , there is a deposit 1 1 b having only a weak adhesion to the surface 6b of the cathode plate 4. Consequently, in that area it is easy to push the knife 10 between the metal deposit 10b and the plate 4. This makes a good starting point for stripping off the metal deposit 1 1 .

The principle behind the functioning of the starting point of stripping can be theoretically explained with basic fracture mechanics. The force required to generate a fracture, that is, to remove the deposited metal 1 1 from the permanent cathode surface 6a, 6b, can be approximated by the following formula:

1.12 πα where F is the required force, A is the area to be stripped, K is the stress intensity factor, and a is the initial crack size. If the initial crack size a is very small, the force F required will consequently be very high. In contrast, when the value of a is increased, for instance by generating an above described starting point for stripping, the force F can be reduced substantially. Figure 7 illustrates the self-alignment of the preferred fracture path 13 into the interface between the metal deposit 1 1 and the permanent cathode plate 4 when stripping in the presence of imperfections 12 in the upper end of the deposit 1 1 . Because the interface between the metal deposit 1 1 and the cathode plate 4 is the weakest point, the fracture will preferentially occur along the interface, even though the edges 12 of the metal deposit 1 1 were "feather-like", as depicted in Figure 7.

In the following, the invention is illustrated with the help of examples.

Example 1

The permanent cathode plates used having materials with different grain boundary properties. The materials were: AISI 316L (EN 1 .4404) in delivery state 2B (sample 1 ), AISI 316L (EN 1 .4404) heavily etched (sample 2), LDX 2101 (EN 1 .4162) in delivery state 2E (sample 3) ja AISI 444 (EN 1 .4521 ) 2B with two different degrees of etching (samples 4 and 5). Material AISI 316L was etched to enlarge the grain boundaries, and material AISI 444 was etched to open the grain boundaries. The etching method used was electrolytic etching. Small samples were cut off the permanent cathode plate materials and subjected to AFM inspection for determining the grain boundary dimensions of the materials. The measured dimensions are presented in Table 1 . In the table, W refers to grain boundary width and d refers to grain boundary depth.

Table 1 . Average grain boundary dimensions in the permanent cathode plate materials.

Laboratory-scale electrolysis experiments were conducted to deposit copper onto these selected permanent cathode surfaces. The permanent cathode surface was covered with a perforated plastic sheet so that it was possible to deposit a total of four copper discs of 20 mm diameter onto each permanent cathode during one electrolysis experiment. The anode used in the experiments was a plate cut from copper cathode sheet. The distance between the cathode and anode surfaces was 30 mm. After deposition, the copper discs were separated from the permanent cathode plate using a special stripping device that could measure the force required for separation. The electrolysis equipment consisted of a 3-litre electrolytic cell and a 5-litre circulation tank. The electrolyte was pumped from the circulation tank into the electrolytic cell, from which it returned back to the circulation tank by overflow at a solution circulation rate of 7 litres per minute. The circulation tank was fitted with heating equipment and an agitator.

The electrolyte used for the experiments was made of copper sulphate and sulphuric acid and contained 50 g/l of copper and 150 g/l of sulphuric acid. Hydrochloric acid was also added to the electrolyte so that the electrolyte had a chloride content of 50 mg/l. Bone glue and thiourea were used as additives and were continuously fed into the circulation tank as an aqueous solution. The electrolyte temperature in the electrolytic cell was maintained at 65 ^QC by regulating the electrolyte temperature in the circulation tank. The cathodic current density in the experiments was 30 mA/cm², which corresponds well to the current density used in production-scale electrolysis. The electrolysis duration in each experiment was 20 hours. After electrolysis, the mask plate was removed from the permanent cathode, and the copper discs were separated from the permanent cathode after a fixed period of time from the end of the experiment. The force required for separation was measured, and the forces are presented in Table 2 as relative forces where the reference is AISI 316L in delivery condition 2B. The choice of reference is based on the fact that such permanent cathode material is generally used at copper electrolysis plants.

On the basis of the experimental results, the magnitude of the separating force is clearly dependent on the grain boundary dimensions of the permanent cathode material. Etching can be used to further open the grain boundaries of the materials in both the width and the depth dimension. The duplex material LDX 2101 was not treated in any way before the experiments, and also the separating force measured on that material is greater than the separating force measured on the reference material.

Table 2. Separating forces measured on different permanent cathode materials

Sample Permanent cathode plate

number material Relative separating force

1 AISI 316L 2B 1.0

2 AISI 316L 2B, etched 3.9

3 LDX 2101 2E 1.8

4 444 2B, etched 0.8

5 444 2B, etched 2.5 A comparison of the measured separating forces with the grain boundary dimensions measured in AFM analysis (Table 1 ) shows that the wider and deeper the grain boundaries, the greater separating force is required. Particularly the relation between the width and depth of the grain boundaries has a substantial effect on the required separating force.

The surface roughnesses (R_a indices) of the permanent cathode materials chosen for the separation experiments were also measured, and the measured values are presented in Table 3. It can be noted that etching treatment, among other things, has changed the surface roughness to some extent. However, no clear correlation can be found in a comparison of surface roughnesses and the measurement results from the separation experiments. The surface roughness index does not measure the grain boundary dimensions. Therefore the roughness index alone cannot be considered a sufficient criterion for achieving the desired adhesion and separating force.

Table 3. R_a indices of the permanent cathode plate materials.

Furthermore, average grain sizes of the different permanent cathode materials were measured using a microscope and the linear intercept method. The measurement results are presented in Table 4. Table 4. Grain sizes of the permanent cathode plate materials.

Example 2

When the permanent cathodes were tested in production-scale copper electrolysis, a phenomenon called self-stripping occurred immediately after start-up. This means that the copper deposited on the permanent cathode surface spontaneously strips off from the permanent cathode plate surface either during the deposition process or when the permanent cathode is lifted from the electrolytic bath. The phenomenon naturally causes problems at an electrolytic plant, and such permanent cathodes cannot be used. A sample piece was cut off the self-stripping permanent cathode (material AISI 316L) for analysing its surface. The surface structure of the permanent cathode plate is illustrated in Figure 4 as a scanning electronic microscopic image.

The surface structure of the permanent cathode plate immediately reveals that the grain boundaries of the material have opened too much during pickling, and no appropriate adhesion surface for copper can be found any longer. The delivery state of the permanent cathode plate was 2B, and according to measurements, the R_a index of its surface varied between 0.4 and 0.5 μιτι. The grain boundary width of the sample, measured from a scanning electronic microscopic image, was 8 to 10 μιτι. The occurrence of self-stripping on the cathode shows that the delivery state and surface roughness indices of a permanent cathode plate are not sufficient criteria for proper operation of the plate in copper electrolysis but that the grain boundary dimensions have to be managed.

It is obvious to a person skilled in the art that with the progress of technology, the basic idea of the invention can be implemented in several different ways. Thus the invention and its embodiments are not restricted to the examples described above but may vary within the scope of the claims.

Claims

1 . A permanent cathode (1 ) to be used as an electrode in the electrowinning of metals, including a permanent cathode plate (4) at least partially made of steel and providing the possibility of electrochemically depositing metal from an electrolytic solution onto its surface (6, 6a, 6b), characterised in that the dimensions of the grain boundaries (7) on the surface (6) of the permanent cathode plate (4) are arranged to be suitable for the adhesion of deposited metal on the surface and the stripping of metal from the surface at least in a part of the surface (6, 6a) that is in contact with the electrolyte.

2. A permanent cathode according to Claim 1 , characterised in that the grain size of the grains (8) in the permanent cathode plate (4) measured by the linear intercept method is 1 to 40 micrometres.

3. A permanent cathode according to Claim 1 or 2, characterised in that the average grain boundary width W of the permanent cathode plate is 1 to 3 micrometres.

4. A permanent cathode according to Claim 1 , 2 or 3, characterised in that the average grain boundary depth d of the permanent cathode plate is less than 1 micrometre.

5. A permanent cathode according to any one of Claims 1 to 4, characterised in that the permanent cathode plate (4) comprises a surface area (6a) with strong adhesion properties and a surface area (6b) with weak adhesion properties, said adhesion properties being dependent on the dimensions of the grain boundaries (7) in said surface area (6a, 6b).

6. A permanent cathode according to Claim 5, characterized in that the surface area (6b) with weak adhesion properties forms a part of the surface that is in contact with the electrolyte and that said surface area (6b) is located at a point where the stripping of metal deposit (1 1 ) is meant to start.

7. A permanent cathode according to any one of Claims 1 to 6, characterised in that the permanent cathode plate (4) is at least partially ferritic steel.

8. A permanent cathode according to any one of Claims 1 to 6, characterised in that the permanent cathode plate (4) is at least partially austenitic steel.

9. A permanent cathode according to any one of Claims 1 to 6, characterised in that the permanent cathode plate (4) is at least partially duplex steel.

10. An arrangement (9) to be used for the electrowinning of metals, including an electrolytic bath (3) containing an electrolytic solution in which anodes (2) and permanent cathodes (1 ) are alternately arranged, said permanent cathodes being supported in the bath by a support element (5), characterised in that the arrangement includes a permanent cathode (1 ) according to any one of Claims 1 to 9.

1 1 . A method for treating the surface (6) of a permanent cathode plate (4), in which the permanent cathode plate (4) is formed at least partially of steel plate, characterised in that the grain boundaries (7) of the permanent cathode plate surface (6) on at least a part of the surface (6, 6a, 6b) that is in contact with the electrolyte are treated chemically or electrochemically to achieve the desired surface properties for the adhesion of deposited metal on the surface and the stripping of metal from the surface.

12. A method according to Claim 1 1 , characterised in that the surface (6) of the permanent cathode plate (4) is treated until the separating force desired for the surface is achieved.

13. A method according to Claim 1 1 or 12, characterised in that the surface of the permanent cathode plate is treated by etching.

14. A method according to Claim 1 1 , 12 or 13, characterized in that different areas (6a, 6b) of the permanent cathode plate surface that are in contact with the electrolyte are treated differently to produce an area (6a) with strong adhesion and an area (6b) with weak adhesion.

15. A method according to Claim 14, characterized in that the area (6b) with weak adhesion is produced on a part of the cathode plate surface where the stripping of metal deposit (1 1 ) is meant to start.