ES2250092T3 - EQUIPMENT FOR THE ELECTROLYTIC DEPOSITION OF GOLD OR GOLD ALLOYS. - Google Patents

Abstract

Replenishing of gold is carried out by continuous recirculation of the cyanide-alkaline electrolyte between the electrodeposition electrolytic cell (1) and an auxiliary electrolytic cell (2) in whose anode (21) are placed gold sheets (22) so that free potassium cyanide formed in the electrolyte as a consequence of electrodeposition process, combines with gold in said auxiliary electrolytic cell (2) to produce the cyano-complex (K(Au(CN)2). The equipment comprises an electrodeposition electrolytic cell (1) and an auxiliary electrolytic cell (2) that allows the continuous replenishing of gold. The features of the invention allow to produce an electroplate of uniform thickness and composition, keeping the costs, especially those of replenishing gold, at a low level. <IMAGE>

Description

Equipment for electrolytic deposition of gold or gold alloys

The present invention relates to equipment for the electrodeposition of gold or gold alloys, for example gold with Silver or gold with copper and cadmium.

Background of the invention

Processes have been known for a long time electrodeposition of gold or gold alloys, which consist essentially in the deposition of a layer of metal or alloy on cathodes immersed in an electrolyte of suitable composition. These processes can be used for the manufacture of hollow articles of pure or alloyed gold (electroconformation), in which case the layer of gold or alloy is deposited on a model that is subsequently extract, and also to coat items with a thick layer Determined gold or alloy.

In these processes some of the products contained in electrolytes, such as gold and metals that are co-deposited in the alloy, and the other compounds brighteners, surfactants, complexers, and others, which transform or destroy in the electrolytic process.

Generally gold is replaced by electrolytes by dissolving a cyanocomplex, fundamentally potassium dicyanoaurate K (Au (CN) 2), dissolved in deionized or distilled water at a certain concentration.

It is known that in electrolytes alkaline-cyanide complex species cyanides that react and discharge into the cathode vary with the concentration of free cyanide. Therefore keep that least variable concentration is of the utmost importance to the time to co-deposit gold, copper and cadmium or gold and silver, from of their respective cyanocomplexes.

Free cyanide plays a very important role in the cathodic kinetics. Gold, copper and cadmium in a electrolyte, and gold and silver in another, are co-positioned from cyanocomplexes, so that in the conditions in which it is performed the process, the following amounts of KCN are released for each Gram atom of deposited metal:

Au: 2 mol gram of free KCN

Cu: 3 or 4 mol gram of free KCN

Cd: 4 mol gram of free KCN

Ag: 1 mol gram of free KCN

For electrodeposited alloys currently, the greatest release of free KCN corresponds to gold, which is the one that participates in greater proportion in them. This KCN released builds up in the electrolyte and is only removed very slowly by anodic oxidation, by hydrolysis and by the effect of electrolyte temperature

The KCN is released in the cathodic layer at a speed that depends on the applied current density, and by both its local concentration in this layer is higher than the rest of the electrolyte. As a consequence of this gradient of concentration, the cyanide released in the cathodic layer is slowly transported to the bosom of the electrolyte, where it goes accumulating. As the concentration gradient decreases between the cathodic layer and the interior of the electrolyte, the diffusion It decreases and the KCN tends to accumulate in the place where it originates. Since copper is the most sensitive of the metals present to the concentration of free KCN, and is able to form complexes higher when added, it is deposited in smaller proportion, of so that there is an increase in the partial currents of other metals, ultimately producing a change in the Composition of the deposited alloy.

In existing electrolytic systems, produces a constant increase in the concentration of free KCN, equivalent to a few grams per liter for each work process, which makes the work and maintenance of the electrolytes

On the other hand, in the processes of known electrodeposition differences in thickness occur and coating composition deposited between one point and another of the cathode, due to the tertiary distribution of the current.

Indeed, known teams have a anode formed by two concentric cylinders of equal meshes, among which the cathode rotates concentrically and equidistant from the two cylindrical parts of the anode. With this geometry, there are zones of the cathode that receive more current and others that receive less stream. This affects differences in the deposited thickness, both in the case of gold deposition and in the case of alloys

In the case of electrodeposition of alloys binary, for example gold and silver, the current distribution causes differences in thickness and also differences in composition, since the areas reached by a lower current they will be richer in silver, since in electrolytes alkaline-cyanide this behaves like more noble.

If the alloy to be electrodeposited is tertiary, for example gold, copper and cadmium, the situation is even more complicated: in this case, in the cathode areas that receive a lower current will be electrodeposited an alloy of higher content in gold than desired, while the opposite occurs at points reached by a higher current. The metals are co-deposited in adequate proportions to the local cathodic polarization.

On the one hand, gold objects or alloys of gold are subject to legislation that requires content minimum in gold, and, on the other hand, should not exceed so significant these contents so as not to unnecessarily increase the product; in addition, in the technical applications of the electrodeposition, keep in mind that electrodeposits they have physical properties that vary with their composition, and They therefore have a different response to the technical requirements to which the objects are subjected.

Therefore, it is clear that the control of Composition of the alloys is crucial, and it would be desirable to arrange of a procedure and equipment that would guarantee a constant thickness and composition throughout the cathode.

Finally, it should be noted that known procedures and equipment present a series of defects common, related to the aforementioned inconveniences and with others aspects that will be revealed later, and not allow full use of the advantages of this method of manufacturing.

Description of the invention

The present invention aims to give solution to the above mentioned inconveniences and provide a procedure and an electrodeposition equipment that take advantage of the maximum all the advantages of this technique.

According to a first aspect, the invention is refers to equipment for the electrodeposition of gold and alloys of gold as defined in claim 1.

In one embodiment of the invention, the cell Auxiliary electrolytic comprises an anode, formed by a reinforcement metal designed to hold gold foil and a mesh disk metal attached to said reinforcement and arranged horizontally next to the cell bottom, and a cathode formed by a mesh cylinder metal housed in the center of the cell, inside a semipermeable cylindrical membrane, and by the fact that the interior of the semipermeable membrane is filled with a solution conductive

The semipermeable membrane prevents it from being able to Missing a part of the gold by electrodepositing on the cathode cylindrical metal mesh; the metal mesh disk in the background from the cell collects the gold fragments that can be detached from the sheets and transmits them the anodic potential, so that they are also taken advantage of.

Advantageously, the electrolytic cell in which the electrodeposition process is performed comprises an anode formed by two pieces of concentric metal mesh, of general cylindrical configuration, between which it is arranged rotating cathode.

According to a preferred embodiment of the equipment, the inner part of the anode has an elongated ellipsoid shape of revolution, truncated, convex towards the outside of the cell, and the outer part of the anode has an elongated hyperboloid shape of revolution of a leaf, convex towards the inside of the cell, of so that the distance between the two anodes is smaller in the area center of the cell than at the upper and lower ends.

In this way, the vertical current distribution, avoiding differences in composition, thickness and properties of coatings deposited on objects placed at different cathode frame height.

Alternatively, the inner part of the anode It has the shape of two overlapping truncated cones, with the bases larger in contact, and the outer part of the anode has the form of two overlapped truncated cones, with the minor bases in contact, so that the distance between the two anodes is shorter in the area center of the cell than at the upper and lower ends.

This alternative form of anode parts optimizes currents to a lesser extent, but in return it is more Simple and economical to manufacture.

Advantageously, the inner piece of anode has greater area factor than the outer piece. This compensates the difference between the areas of the two parts of the anode, due to its different diameters, and the currents that reach the inner and outer parts of the cathode, and therefore the coating characteristics in both zones.

Another optional feature of the equipment that improves the distribution of current on objects mounted on the cathode, and therefore the homogeneity of the coating, is that the cathode is located closer to the inner part of the anode than to The outer piece.

Preferably, the cathode is suspended from a support and rotation mechanism, which can adopt a higher loading position, in which the cathode is outside the electrolyte, an intermediate operating position, in which part of the cathode that holds the objects is totally immersed in the electrolyte and the cathode is coupled with the rotating drive means, and a lower position of control in which the cathode is decoupled from the means of drive and is supported on a vertical projection of a balance placed in the center of the electrolytic cell, in an area devoid of electrolyte.

The main advantage of this provision of cathode and balance is that it allows the cathode to be weighed without removing it from the electrolyte.

Brief description of the drawings

For greater understanding of how much has been exposed Some drawings are accompanied in which, schematically and only to non-limiting example title, a case study of realization.

In these drawings,

Figure 1 represents a diagram of the equipment of electrodeposition according to the present invention;

Figures 2 and 3 show two embodiments of the shape of the anode for the electrodeposition cell of the figure one;

Figure 4 is a development scheme and electrodeposition process control; Y

Figures 5a, 5b and 5c show the cathode of the electrodeposition cell in three different positions, corresponding to three stages of the procedure.

Description of a preferred embodiment

Figure 1 schematically shows an equipment of according to the invention. It basically consists of a cell main electrolytic 1, in which the electrodeposition of gold or gold alloys on objects to coat or on models that will later be removed from the inside of the hollow pieces formed, and an electrolytic cell auxiliary 2 that is used to continually replenish gold consumed in the electrodeposition process.

Although the figure has represented the cells 1 and 2 of similar sizes, in practice the auxiliary cell 2 is much smaller.

The electrodeposition cell 1 comprises a process tank 3, for example a bucket in the shape of a bucket of a non-conductive plastic material of the current, which contains an alkali-cyanide type electrolyte, suitable for the electrodeposition of gold or gold alloys.

Inside the tank 3 are provided a first cylindrical wall of greater diameter 4 and a second wall cylindrical of smaller diameter 5, both made of the same material as the tank, which defines: a central zone 6 without electrolyte, whose utility will be explained later; an annular compartment inner or main 7, which contains the anode and cathode and in the which electrodeposition is carried out; and a compartment exterior or secondary 8, which is used for the operations of product replacement, heating, circulation, control of level, etc.

The electrolyte level in the compartment main 7 is higher than in the secondary compartment 8, and the electrolyte can overflow from one to the other above the wall cylindrical 4. A filtration and circulation pump B1 drives from again the electrolyte to the main compartment 7 through one or two tubes 9 (for clarity, only one has been represented in the figure) arranged at the bottom of the main compartment 7 and provided with a plurality of outlet holes.

In the main compartment are arranged two pieces of metal mesh 10 and 11, substantially cylindrical and concentric, which connect to the positive polarity of a source feeding and constituting the anode of the cell electrolytic The constructive details of these mesh pieces They are very important, and are discussed later.

Between the two pieces 10, 11 a cathode frame 12, also cylindrical, in which the objects on which electrodeposition should be performed. He Frame is built in titanium, and coated with a layer of insulating material in all parts that are submerged and not They must be coated by gold or gold alloy.

The cathode frame 12 is coupled to a support 13, through which it receives both the rotational movement as the corresponding negative electrical potential, through a conventional brush system. The support 13 also has lifting and lowering means, and the frame can be coupled and decouple from rotating drive means, as will explain later.

In its lower part, the cathode frame 12 it has a suitable housing 14 to rest on a projection 15 of a balance 16, which is arranged in the central zone 6 of the process tank, in order to weigh the cathode during the process, as will be explained later.

In the secondary compartment 8 of the tank process 1 the following components are located:

- thermostatic probe for the control of temperature;

- heating elements for heating of the electrolyte;

- liquid level control;

- means of feeding deionized water, of agreement with the level existing at each moment;

- continuous feeding of electrolyte maintenance products, consumed during the electro-
chemical

- Vapor suction channel generated on The electrolyte

These components are of the conventional type, and For clarity they have not been represented in the drawing. I do not know represent or mention other conventional elements of equipment, such as power supplies, solenoid valves, controls, control computer, etc.

The operation of cell 1 is analogous to that of other electrolytic cells for electrodeposition: a current, continuous pure or pulsating, and the electrodeposition of gold and other alloy metals, if any, On the cathode

Characteristics of the anode and position of the cathode

As explained in the background, the current distribution has a great influence on the uniformity of the thickness and chemical composition of the layer electrodeposited

In the present invention a series of characteristics of the electrolytic cell anode 1 for facilitate optimal current distribution.

Figure 2 schematically depicts a anode according to an embodiment of the invention. Each piece 10, 11 of the anode is metal mesh, preferably titanium, which is coated with platinum or a mixture of ruthenium oxides and iridium, at least in the active parts.

Instead of being cylindrical, the inner part 10 and the outer part 11 of the anode have a curved shape, so that the distance between both pieces in the central part is smaller that in the extremes; more specifically, for optimal distribution of the current vertically, the inner part 10 is a elongated ellipsoid of revolution, truncated by both ends, while the outer part 11 is an elongated hyperboloid of One leaf revolution.

The equations of the generating ellipse or generatrix hyperbola and the eccentricities of the pieces 10, 11 they are functions of geometric parameters of the system, such as the useful diameter and height of the cathode, the distances between the cathode and each of the pieces of the anode, and the height of the electrolyte on the active part of the cathode.

This anode configuration would ideally be the more satisfactory from the point of view of the distribution of the stream; however, it may have problems given the complexity of its manufacture. Consequently, it is presented in the figure 3 an alternative embodiment of manufacturing more simple.

In this case, pieces 10 and 11 are formed each for two truncated cones; in the case of the outer part 10, the two cone trunks are in contact by the smaller bases, while in the inner part 11 they are in contact by their bases greater, so that the distance between both pieces is less in the central part

The pieces 10, 11 can also be cylindrical, adequately dimensioned in height.

Moreover, to compensate for the difference between the areas of both pieces, which makes the current that reaches the cathode since the inner part is lower than the one comes from the outer part, two features are planned construction of the team.

On the one hand, the mesh of the inner part 11 is much busier than that of the outer piece 10, that is, it has less mesh light (greater area factor). The optimal relationship between The area factors f and F of pieces 10 and 11 is:

F / f = A (D-a) / a (A-D)

where A and a are the diameter inside of the cylindrical wall 4 and the outside diameter of the wall cylindrical 5, respectively, with which the pieces 10 and 11, and D is the diameter of the cathode.

Moreover, it is expected that the distance between the cathode and the pieces 10, 11 it is not the same, but it is arranged closer to the inside of the anode, also in order to compensate for the difference of areas and therefore of current. The distance L of the cathode to the outer part 10 and its distance l to the Inner piece 11 are preferably:

\hskip1cm

l = (D-a)/2L = (AD) / 2

 \ hskip1cm 

l = (Da) / 2

All these geometric features they contribute to improve the thickness and composition of coating.

Product Replenishment

In the electrodeposition process, on the one hand, deposited metals are consumed, leading to the formation of free potassium cyanide and, on the other hand, a series of products such as brightener compounds, surfactants, complexes, etc.

The replacement of alloy metals is performed in a conventional manner, so it will not be described with detail: copper, for example, is added to the electrolyte in the form of a solid mixture that also includes other additives consumed, at the end of each process, since the maintenance of the Copper concentration in the electrolyte is not a critical factor because the deposition of this metal is not very influenced by the mass transport Other metals, such as silver or cadmium, are intermittently added by a peristaltic pump of high precision, in the form of replacement solutions containing exactly known concentrations of metals and others compounds consumed in the process.

On the other hand, the gold replenishment is made of according to the invention in a new and original way. How has it mentioned in the background, gold replenishment has a great importance because most of the potassium cyanide released in The electrolyte during the process is generated by the deposition of the gold.

Gold replacement electrolytic cell

Described below, with reference to new to figure 1, the part of the team related to the Gold replacement in the electrolyte.

The secondary compartment 8 of the tank process 1 is connected through two conduits 17, 18 with the Auxiliary electrolytic cell 2 for gold replacement. At conduit 17 is interposed a pump B2 that sucks electrolyte from the electrodeposition cell 1 with a speed determined by the process that is carried out and transported to cell 2; the electrolyte which reaches a certain height in cell 2 returns to cell 1 by gravity, through the duct 18.

The cell 2 is constituted by a container 20 of plastic material, for example methyl methacrylate, polypropylene or others, and contains a metal armor 21, preferably of titanium, to which gold sheets 22 are fixed pure 999.9 in adequate quantity to the generation of cyanocomplex

To the bottom of the armor is fixed a metal mesh disk 23, which is next to the bottom of the cell, and which collects the gold fragments that can be detached from the sheets. The whole set of armor 21, disc 23 and gold plates 22 constitutes the anode of cell 2 and is connected to the pole positive from a power supply.

The cathode of the cell, connected to the negative pole, is constituted by a stainless steel 24 mesh cylinder arranged in the central part of the cell. Around the cathode 24 is located a semipermeable membrane 25, which prevents the penetration of gold ions and other heavy metals present in the electrolyte inside, and which is filled with a solution to allow conductivity, preferably potassium cyanide in deionized water. -
zada

The semipermeable membrane 25 is portable and is resting on the container 20 through three or more arms radials; the cathode 24 may have at least one radial arm supported on said membrane.

The dimensions of cell 2 depend on the cyanocomplex generation needs.

The electrolyte that enters cell 2 has a relatively high concentration of free potassium cyanide, the which is initially present in the electrolyte plus the one generated in cell 1 because of the deposition of gold and other metals. In cell 2, this free potassium cyanide reacts with gold to form the cyanocomplex K (Au (CN) 2) of according to the global equation:

2Au + 4KCN + 2H 2 O → 2K (Au (CN) 2) + 2KOH + H2

       \ newpage
    

The oxidation reaction of gold at the anode is Produces according to the equation:

100

Consequently, the electrolyte returns to the cell 1 enriched with K (Au (CN) 2); the continuous or intermittent electrolyte recirculation between cells 1 and 2, along with strict step-by-step control of the operating parameters of both cells, guarantees the maintaining a constant concentration of gold in the electrolyte in electrodeposition cell 1, so that the Features of the gold layer or deposited gold alloy will They can control very accurately.

Electrodeposition process control

The equipment includes control means (no represented), for example a PC-type computer with a program adequate, they carry out control of the entire process of electrodeposition and replacement of products and govern the operation of pumps, solenoid valves, etc.

The process control procedure, as far as essential, it is represented in figure 4. In a first phase A enter the necessary data into the computer: the area of cathodes or models on which the metal is to be deposited, the layer thickness, the gold content of the alloy you want electrodeposit, cathode weight, initial concentrations of metals in the electrolyte, current density and others.

In phase B, a first weighing of the cathode inside the electrolyte, so that more will be explained forward, after which the team calculates, in phase C, the weight total to deposit based on the composition of the alloy, the thickness, cathode area, etc.

Then, in phase D, the parameters for an initial stage of electrodeposition, or, said otherwise, the parameters that will be applied to cells 1 and 2 to deposit a first layer on the cathode. These parameters are the current intensity to apply in each cell, and the load total that has to be consumed. For this calculation, we start from the base that the coating will be done in five stages, so the intensity and load to be applied in cell 1 will be appropriate to deposit approximately one fifth of the material weight (although the increase of cathode area between one stage and the next).

In the next phase, represented by the Tables E and F of the scheme in Figure 4, the parameters apply calculated to cells 1 and 2, respectively. The means of control stop the stage in each cell when they verify that the expected amount of electric charge has circulated through the electrolyte. To do this, the function i x t is integrated, that is, circulating current intensity per circulation time of the intensity.

Once the first stage is finished, it is done a control weighing (phase G) of the cathode, in which the weight deposited and compared with the expected result; if there is one discrepancy, this result is used to adjust the parameters in the successive stages.

For economic and legal reasons, it is also convenient that the procedure allows to control the composition of the alloy electrodeposited

In the present invention, the control of composition is based on the determination of the equivalent electrochemical alloy, since each composition a well-defined electrochemical equivalent corresponds.

For each stage, in the control phase G and in based on the electric charge consumed and the weight deposited, the means of control determine the electrochemical equivalent of the alloy deposited, and, consequently, deduct its composition.

Based on this result, if necessary, calculates a new current intensity and a new load for correct possible differences between alloy composition deposited and planned theoretically.

An alternative possibility, to control the composition of the deposited alloy, is to use its density in instead of its electrochemical equivalent.

Then the control means check if the five stages of the process have already been completed, and, in case otherwise, they return to the calculation phase D and a new one starts electrodeposition stage.

When the control means verify that it has been Once the process is completed, they go to phase H, in which a final check balance and the results are supplied to user, there being the possibility of an additional stage, by example if you want to deposit a larger amount of gold or alloy of gold.

This control procedure guarantees some optimal results of the electrodeposition process.

Assembly and weighing of the cathode

It will be described below, with reference to Figures 5a to 5c, the part of the equipment related to the cathode rotation drive and cathode weighing during the electrodeposition process.

Figure 5a shows the suspended cathode 12 over the process tank, in a loading position. Support 13 includes a motor 30 that allows the cathode 12 to rotate in either way, alternatively, for a period of time preset and with a selected speed. Motor 30 spins a conductive element 31 which, by friction, transmits the rotation to a driven element 32 integral with the rod 33 of the frame cathode 12. The negative polarity is also transmitted to cathode 12 through the support 13, for example by means of a system Conventional brush (not shown).

The support 13 is coupled to an upright 34 of so that you can slide vertically along it, driven, for example, by a pneumatic piston (not represented).

Once the cathode frame is loaded with the objects or models to be coated, the support 13 is lowered to the working position of figure 5b. In this case, the elements 31, 32 are coupled and the rotation of the motor 30 is transmitted to the cathode 12; it should be noted that, in this position, accommodation 14 of the cathode frame is separated from the projection 15 of the balance 16.

When an electrodeposition stage ends, it stops the rotation of the motor 30 and the support 13 descends to the position of figure 5c. As you can see, this position is calculated so that the cathode frame is supported by its housing 14 on the projection 15 of the balance, while the conducted element 32 is separated from the conductive element 31 of the support 13, so the cathode 12 has no point of Contact with the latter.

Consequently, in this position you can weigh the cathode while immersed in the electrolyte.

All the elements related to the cathode and heavy frame movement is governed by The equipment control means.

Finally, it should be noted that in the team of according to the invention, it is envisaged that all dimensions of the essential elements (tank, anodes, cathode) are given so generic or replacing the details with others technically equivalent, without departing from the scope of protection of claims included.

Although it has been described and represented a concrete embodiment of the present invention, it is evident that the subject matter expert may introduce variants and modifications, or replace the details with technically equivalent ones, without depart from the scope of protection of the claims attached.

Claims (8)

1. Equipment for the electrodeposition of gold and gold alloys, comprising an electrolytic cell (1) with an anode (10, 11), a cathode (12) in which the objects on which the gold should be deposited are placed or the alloy, and an alkaline-cyanide electrolyte, and means for replenishing the gold consumed in the electrodeposition process, said means for replenishing the gold comprise an auxiliary electrolytic cell (2), connected with the electrolytic cell (1) in which the electrodeposition process is carried out by two conduits (17, 18) that allow the electrolyte to be recirculated from one to the other, said auxiliary electrolytic cell (2) comprising an anode (21) in which gold sheets (22) characterized due to the fact that the distance between the outer part and the inner part of the anode (10, 11) of the electrolytic cell (1) is shorter in the central area than in the upper and lower ends, due to the fact that the inner part (11) of the year It has a greater area factor than the outer part (10) and due to the fact that the cathode (12) is placed closer to the inner part (11) of the anode than to the outer part (10). 2. Equipment according to claim 1, characterized in that the auxiliary electrolytic cell (2) comprises an anode formed by a metal armor (21) intended to hold gold foils (22) and a metal mesh disk (23) attached to said reinforcement and arranged horizontally next to the bottom of the cell (2), and a cathode (24) formed by a metal mesh cylinder housed in the center of the cell, inside a semipermeable cylindrical membrane (25), and by the fact that the interior of the semipermeable membrane (25) is filled with a conductive solution. 3. Equipment according to claims 1 or 2, characterized in that the electrolytic cell (1) in which the electrodeposition process is performed comprises an anode formed by two concentric metal mesh pieces (10, 11), of configuration cylindrical general, between which the cathode (12) is rotatably arranged. Equipment according to claim 1, characterized in that the inner part (11) of the anode has an elongated shape of elongated revolution, truncated, convex towards the outside of the cell (1), and the outer part (10 ) of the anode has an elongated hyperboloid shape of revolution of a leaf, convex towards the inside of the cell. 5. Equipment according to claim 1, characterized in that the inner part (11) of the anode has the shape of two overlapped truncated cones, with the major bases in contact, and the outer part (10) of the anode has the shape of two overlapping truncated cones, with the minor bases in contact. Equipment according to any one of claims 1 to 5, characterized in that the cathode (12) is suspended from a rotating support and actuation mechanism (13), which can adopt a higher loading position, in the which cathode (12) is outside the electrolyte, an intermediate operating position, in which the part of the cathode (12) that holds the objects is fully submerged in the electrolyte and the cathode (12) is coupled with the actuation means in rotation, and a lower control position in which the cathode (12) is decoupled from the actuation means and is supported on a vertical projection (15) of a balance (16) placed in the center of the electrolytic cell (1) ), in an area (6) devoid of electrolyte. 7. Equipment according to claim 1 characterized in that the optimum relationship between the area factors of parts 10 and 11 is: F / f = A (D-a) / a (A-D) where F is the area factor of the piece 11, f is the area factor of piece 10, A and a are the inner diameter of the cylindrical wall 4 and the outer diameter of the cylindrical wall 5, respectively, with which they are in contact pieces 10 and 11, and D is the diameter of the cathode. 8. Equipment according to claim 1 characterized in that the distance between the cathode and the parts (10, 11) is: L = (AD) / 2

 \ hskip2cm 

l = (Da) / 2 where L is the distance of cathode to outer part 10, l is the distance from the cathode to the internal part 11 and A, a and D are as defined in the claim 7.