EP1050607B1

EP1050607B1 - Equipment for the electrolytic deposition of gold or gold alloys

Info

Publication number: EP1050607B1
Application number: EP00500086A
Authority: EP
Inventors: Josep Ferre Torres
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-05-06
Filing date: 2000-05-03
Publication date: 2005-10-05
Anticipated expiration: 2020-05-03
Also published as: DE60022940D1; EP1050607A2; ES2166660A1; EP1050607A3; ATE305988T1; ES2250092T3; ES2166660B1

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

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

BACKGROUND OF THE INVENTION

Methods for the electrodeposition of gold or gold alloys are known since long time ago, and are essentially based in the deposition of a layer of metal or alloy on cathodes immersed in an electrolyte of adequate composition. These methods can serve either to produce alloyed or pure gold hollow items (electroforming), and in this case the gold or alloy layer is plated on a model that is subsequently removed, or to clad objects with a layer of a certain thickness of gold or alloy.
In these processes some of the products contained in the electrolytes are consumed, for example gold and other metals codeposited in the alloy, as well as brighteners, wetting agents, complexing agents, and others, that are transformed or destroyed during the electrolytic process.
Generally gold is replenished to the electrolytes by means of a cyano-complex solution, mainly potassium dicyanoaurate K(Au(CN)₂), dissolved in deionized or distilled water to a specified concentration.
It is known that in the alkaline-cyanide electrolytes the cyanurated complex species that react and plate out on the cathode vary with the free cyanide concentration of the electrolyte. Therefore, to maintain this concentration as invariable as possible is of capital importance when codepositing gold, copper and cadmium, or gold and silver, from its respective cyano complexes.
Free cyanide plays a very important roll in cathode kinetics. Gold, copper and cadmium in one electrolyte and gold and silver in another, all codeposit from cyano complexes, so that under the conditions in which the process is carried out, every atom-gram of metal plated out releases the following amounts of free potassium cyanide:
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
The larger amount of free KCN released when electroplating the most frequent alloys, corresponds to gold that is the main element present with a higher content in the alloy. This free KCN released accumulates in the electrolyte, and is eliminated very slowly only through anodic oxidation, through hydrolysis and by the effect of the electrolyte temperature.
The KCN is released in the cathodic boundary layer to a rate that depends on the applied current density, and therefore its local concentration in this layer is higher than in the bulk electrolyte. Owing to this concentration gradient, the KCN released in the cathodic boundary layer is slowly transported to the bulk electrolyte where it accumulates. As the concentration gradient between the cathode boundary layer and the bulk electrolyte decreases, diffusion decreases too, and KCN tends to accumulate in the same place where it is produced. Since copper is, of all metals present in the electrolyte, the more sensitive to free KCN concentration, and by adding KCN forms higher complexes, thus it deposits to a lower rate, so that an increase in the partial currents of other metals is produced, and as a consequence leading to a change in the electroplated alloy composition.
In existing electrolytic systems there is a permanent increase in the free KCN concentration, equivalent to some grams per litre at every working run, and this makes difficult the work and the electrolyte maintenance.
On the other hand, in the known electrodeposition systems, differences in coating thickness and composition are always present between different parts of the cathode, owing to the tertiary current distribution.
Indeed, all existing equipments contain an anode composed by two concentric cylinders of equal mesh, with the cathode turning concentrically and equidistant between the two cylindrical parts of the anode. With this geometry, some cathode parts are reached by higher currents and other are reached by lower currents. This leads to differences in deposited thickness both with pure gold and with alloys.
In case of electrodepositing binary alloys, for instance gold and silver, current distribution leads to differences in both electroplate thickness and composition, since areas reached by a lower current are richer in silver because in the alkali-cyanide electrolytes this metal behaves as more noble than gold.
To electroplate tertiary alloys, for instance gold, copper and cadmium, the situation is even more complicate: in this case in the cathode areas reached by a lower current, a higher gold content alloy than foreseen will be electrodeposited, while the opposite happens in areas reached by a higher current. Metals codeposit at ratios according to the local cathocic polarisation.
On one hand, objects made out of gold or gold alloys are submitted to a legislation that imposes a minimum gold content, and on the other hand it is not convenient to exceed this content in order to avoid raising uselessly the price of the products; moreover in electroplating technical applications it is important to have in mind that electroplates have physical properties that vary with its composition and therefore they behave in a different manner with respect to the technical requirements to which the objects are submitted.
It is therefore obvious that the control of the alloys composition is of capital importance, and that it would be desirable to have a method and an equipment that guarantee an even thickness and composition all along the cathode.
Finally, it has to be pointed out that all existing and known methods and equipments present several common defects, related with the aforementioned drawbacks and with other aspects that will be stated below, and that do not allow to profit fully of the advantages of this fabrication method.

DESCRIPTION OF THE INVENTION

It is the aim of the present invention to give a solution to the above drawbacks and to provide an electrodeposition equipment to take the maximum profit of the advantages of this technique.
In an aspect the invention relates to an equipment for the electrodeposition of gold and gold alloys as defined in claim 1.
In one embodiment of the invention, the auxiliary electrolytic cell comprises an ancde made of a metallic framework intended to hold gold sheets and a metallic mesh disc arranged horizontal at the bottom of the cell and connected to said framework, and a cathode made of a metallic mesh cylinder arranged in the middle of the cell, inside a semipermeable cylindrical diaphragm, and in that the inside of said semipermeable diaphragm is filled with a conductive solution.
The diaphragm avoids that any part of gold be lost by electrodepositing on the metallic mesh cylindrical cathode; the metallic mesh disc in the cell's bottom collects the gold fragments loosened from the gold sheets and transmits the anodic potential thereto, so that the fragments are also profited.
Advantageously, the electrolytic cell where the electrodeposition process takes place comprises an anode formed by two concentric pieces of metallic mesh, of generally cylindrical configuration, between which the cathode is rotatably arranged.
According to a preferred embodiment of the equipment, the inner piece of the anode has the shape of an elongate truncated revolution ellipsoid, convex towards the outside of the cell, and the outer anode piece has the shape of an elongate one-sheet revolution hyperboloid convex towards the inside of the cell, such that the distance between both anodes is shorter in the central area than at the upper and lower ends.
In this way, an outstanding improvement in the current distribution in the vertical sense is achieved, thus avoiding differences in the composition, thickness and properties of the coatings electrodeposited on objects placed at different levels in the cathodic rack.
Alternatively, the inner piece of the anode has the shape of two truncated cones superimposed with their larger bases in contact, and the outer piece of the anode has the shape of two truncated cones superimposed with their smaller bases in contact, in such a way that the distance between both anodes is shorter in the central area of the cell than at the upper and lower ends.
This alternative form of the anode pieces optimises to a lower degree the currents, but in exchange it is easier and cheaper to fabricate.
Advantageously, the inner piece of the anode has an area factor higher than the outer piece. In this way the difference between the areas of both parts of the anode owing to its different diameters is compensated, and the currents reaching the inner and outer parts of the cathode are equal, and so are the characteristics of the coatings on both areas.
Another optional equipment characteristic that improves the current distribution on the objects mounted on the cathode, and therefore the coating's homogeneity, is that the cathode is placed closer to the inner piece of the anode than to the outer piece.
Preferably the cathode is suspended from a supporting and rotation driving device, that can be arranged an upper loading position, in which the cathode is out of the electrolyte, an intermediate operation position in which the part of the cathode that holds the objects is completely immersed in the electrolyte and the cathode engages the rotation driving means, and a lower control position in which the cathode is disengaged from the driving means and rests on a vertical projection of a scale placed in the middle of the electrolytic cell, in an are lacking in electrolyte.
The main advantage of this cathode and scale arrangement is that it allows to weigh the cathode without extracting it from the electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

For a greater understanding of what has been stated above, some drawings are enclosed where schematically and only by way of non-limiting example, a practical case of embodiment is presented.
In said drawings,
figure 1 represents a diagram of the electrodeposition equipment according to the present invention,
figures 2 and 3 show two embodiments of the shape of the anode for the electrodeposition cell of figure 1;
figure 4 is a diagram of the development and control of the electrodeposition process; and
figures 5a, 5b and 5c show the electrodeposition cell cathode in three different positions, corresponding to three method stages.

DESCRIPTION OF A PREFERRED EMBODIMENT

Figure 1 shows schematically an equipment according to the invention. It basically consists of one main electrolytic cell 1, in which the gold or gold alloys electrodeposition takes place on objects to clad or on models that will be subsequently removed from inside the hollow electroformed pieces, and one auxiliary electrolytic cell 2 used to continuously replenish the gold consumed in the electrodeposition process.
Even though cells 1 and 2 have been represented in the figure as being similar in size, in practice the auxiliary cell is much smaller.
Electrodeposition cell 1 includes a process tank 3, for instance a vessel with the shape of a cube made of a non current-conductive plastic material, that contains an electrolyte of the alkaline-cyanide type, suitable for the electrodeposition of gold or gold alloys.
Inside the tank 3 are provided a first cylindrical wall of larger diameter 4 and a second cylindrical wall of smaller diameter 5, both made out of the same material than the tank, defining: a central area 6 without electrolyte whose usefulness will be described later; an inner or main annular compartment 7, that contains the anode and the cathode and where the electrodeposition is carried out; and an outer or secondary compartment 8, which is used for the operations of products replenishment, heating, circulation, level control, etc.
The electrolyte level in the main compartment 7 is higher than in the secondary compartment 8, and electrolyte can overflow from one to the other over the cylindrical wall 4. A filtration and circulation pump B1 propels the electrolyte again towards the main compartment 7 through two pipes 9 (for the sake of clarity only one has been represented in the figure) arranged at the bottom of the main compartment 7 and provided with a plurality of output holes.
In the main compartment are arranged two metallic mesh pieces 10 and 11, essentially cylindrical and concentric, that are connected to the positive polarity of a power source, and that constitute the electrolytic cell anode. The construction details of these mesh pieces are very important, and will be discussed later on.
Between the two pieces 10, 11 is arranged a cathodic rack 12, also cylindrical, on which the objects to be electroplated are mounted. The rack is made of titanium, and coated with a layer of isolating material on all its parts that remain immersed and should not be coated by gold or gold alloy.
The cathodic rack 12 is attached to a support 13 through which it receives both the rotation movement and the corresponding negative electric potential by means of a conventional system with brushes. The support 13 has also ascent and descent means, and the rack can engage and disengage the rotation driving means, as will be explained later on.
In its lower part, the cathodic rack 12 presents a housing 14 suitable to rest on a projection 15 of a scale 16, that is located in the central area 6 of the process tank, to perform a weighing of the cathode during the process, as will be explained later on.
In the secondary compartment 8 of the process tank 1 are arranged the following components:

thermostatic probe for the temperature control;
heating elements for the heating of the electrolyte;
liquid level control;
deionized water feeding means, according to the level existing in each moment;
continuous input of electrolyte replenishing products consumed during the electrochemical process;
suction conduit for fumes generated on the electrolyte.

These components are of conventional type, and for the sake of clarity have not been represented in the drawing. Neither are represented nor mentioned other conventional elements of the equipment such as power sources, electrovalves, switches, control computer, etc.
The operation of cell 1 is analogous to other electrodeposition electrolytic cells: a current is applied, pure direct or pulsating, and the electrodeposition of gold and other alloying metals if they are present, is produced on the cathode.

Anode characteristics and cathode location

As has already been mentioned with relation to the background art, current distribution has a great influence on the uniformity of the thickness and of the chemical composition of the electrodeposited layer.
In the present invention a number of anode characteristics in the electrolytic cell 1 have been studied, in order to provide an optimal current distribution.
In figure 2 is schematically represented an anode according with an embodiment of the invention. Every piece 10, 11 of the anode is made out of metallic mesh, advantageously of titanium coated with platinum or a mixture of ruthenium and iridium oxides, at least on the active parts.
Instead of being cylindrical, the inner piece 10 and the outer piece 11 of the anode have a curved shape, so that the distance between both pieces in the central part is smaller than at the ends; more specifically, for an optimal current distribution in the vertical sense, the inner piece 10 is an elongate truncated revolution ellipsoid, while the outer piece 11 is an elongate one-sheet revolution hyperboloid.
The equations of the generatrix ellipse or generatrix hyperbola and of the eccentricities of pieces 10, 11 are functions of geometrical parameters of the system, such as cathode diameter and cathode working height, distances between cathode and each one of the anode pieces, and the electrolyte level above the cathode working part.
This anode configuration would be ideally the most satisfactory from the current distribution point of view; however, it can represent obstacles given the fabrication complexity. As a consequence in figure 3 an alternative embodiment of easier fabrication is shown.
In this case, the pieces 10 and 11 are formed each one by two truncated cones; in the case of the outer piece 10, both truncated cones are in contact by their smaller bases, while in the inner piece 11 are in contact by their larger bases, so that the distance between both pieces is smaller in the central part.
The pieces 10, 11 can also be cylindrical, if appropriately dimensioned in height.
On the other hand, to compensate the difference between the areas of both pieces, that causes the current reaching the cathode from the inner piece to be lower than the current coming from the outer piece, two equipment construction characteristics have been provided.
On one side, the mesh of the inner piece 11 is much thicker than that of the outer piece 10, that is, has a lower mesh size (higher area factor). The optimal ratio between the area factors f and F of the pieces 10 and 11 is: F/f = A(D-a)/a(A-D) where A and a are respectively the inner diameter of the cylindrical wall 4 and the outer diameter of the cylindrical wall 5, with which the pieces 10 and 11 are in contact, and D is the cathode diameter.
On the other side, it has been foreseen that the distance between the cathode and the pieces 10, 11 be not the same, but that the cathode remain arranged closer to the inner part of the anode, also with the aim of compensate the difference of areas and thus of current. The distance L from the cathode to the outer piece 10 and its distance l to the inner piece 12 are preferably: L = (A-D)/2 l = (D-a)/2
All these geometric characteristics contribute to improve the coating thickness and composition.

Replenishing of products

In the electrodeposition process, on one hand, the deposited metals are consumed, giving rise to the free KCN formation and, on the other hand a number of products such as brightener compounds, wetting agents, complexing agents, etc. are also consumed.
Replenishing the alloying metals is done in a conventional way, and therefore will not be described in detail: copper, for instance, is added to the electrolyte at the end of every process, under the form of a solid mixture that includes also other consumed additives, since maintaining copper concentration in the electrolyte is not a critical factor, the deposition of this metal not being much influenced by mass transport. Other metals such as silver or cadmium are added in an intermittent form by means of a high precision peristaltic pump, under the form of replenishing solutions that contain exactly known concentrations of the metals and other compounds consumed during the process.
On the other hand, gold replenishing is done according to the invention, in a new and original way. As has been mentioned in relation with the background art, gold replenishing has a great importance because most of the KCN released during the process is generated by the gold deposition.

Electrolytic cell for gold replenishing

Next is described, referring again to the figure 1, the part of the equipment related to the gold replenishing in the electrolyte.
The secondary compartment 8 of the process tank 1 is connected through two conduits 17, 18 with the auxiliary electrolytic cell 2 for gold replenishing. In the conduit 17 is interposed a pump B2 that sucks in electrolyte from the electrodeposition cell 1 at a speed determined by the process being realised, and transport it to cell 2; the electrolyte that reaches a certain level in cell 2 goes back to cell 1 by gravity, through the conduit 18.
Cell 2 is made up by a vessel 20 of plastic material, for instance methyl methacrylate, polypropylene, or others, and contains a metallic framework 21, preferably of titanium, to which are fastened sheets 22 of 999,9 pure gold in a quantity adequate to the cyano-complex generation.
At the lower part of the framework is fastened a metallic mesh disc 23, that remains near the bottom of the cell, and that collects the gold fragments that can fall off from the sheets. The whole assembly of framework 21, disc 23 and gold sheets 22 make up the anode of cell 2 and is connected to the positive pole of a current power supply.
The cell cathode, connected to the negative pole, is made up of a stainless steel mesh cylinder 24 placed in the central part of the cell. Around the cathode 24 is arranged a semipermeable diaphragm 25 that prevents ions of gold and other heavy metals present in the electrolyte from penetrating inside it, and that is filled with a solution to allow conductivity, preferably KCN in deionized water.
The diaphragm 25 is portaole and rests on the vessel 20 by means of three or more radial arms; the cathode 24 can have at least one radial arm resting on the diaphragm.
The dimensions of cell 2 depend on the needs of cyano-complex generation.
The electrolyte that enters cell 2 has a relatively high free KCN concentration, the one 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 KCN reacts with the gold to form the cyano-complex K(Au(CN)₂) according to the global equation: 2Au + 4KCN + 2H₂O → 2K(Au(CN)₂) + 2KOH + H₂
The gold anodic oxidation reaction takes place according to:
As a consequence, the electrolyte returns to cell 1 enriched with K(Au(CN)₂); the continuous or intermittent recirculation of electrolyte between cells 1 and 2, along with a strict step by step control of the operating parameters of both cells, guarantees the maintenance of a constant gold concentration in the electrolyte in the electrodeposition cell 1, so that the gold or gold alloy layer characteristics can be controlled with high precision.

Control of the electrodeposition process

The equipment includes control means (not shown), for instance a PC computer with an appropriate program, that carry out the control of all the electrodeposition process and products replenishing, and govern the operation of electrovalves, pumps, etc.
The process control method is represented in the essential in figure 4. In a first phase A the required data are inputted to the computer: the area of cathodes or models on which the metal is going to be deposited, the layer thickness, the desired gold content of the alloy that is going to be deposited, the cathode weight, the initial concentrations of metals in the electrolyte, the current density, and others.
In phase B, a first weighing of the cathode immersed inside the electrolyte is performed, in the way that will be explained forward, and after this the equipment calculates, in phase C, the total weight to be electrodeposited on the base of the alloy composition, the thickness, the cathode area, etc.
Next, in phase D, are calculated the parameters of an initial electrodeposition stage or, in other words, 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 be applied to each cell, and the total charge that has to be consumed. For this calculation assumption is made that the coating will be performed in five stages, and therefore the intensity and the charge to be applied in cell 1 will be the appropriate to deposit roughly a fifth of the weight of material (although for the calculations the increase of cathode area between one stage and the following has to be borne in mind).
In the following phases, represented by blocks E and F in the diagram of figure 4, the calculated parameters are applied to cells 1 and 2, respectively. The control means stop the stage in each cell once they check that the foreseen electric charge quantity has circulated through the electrolyte. To this end the function i x t, that is, the circulating current intensity by the intensity circulation time, is integrated.
Once the first stage is finished, a control weighing of the cathode is performed (phase G), to verify the deposited weight and to compare it with the foreseen result; if any discrepancy exists, this result is used to adjust the parameters in the following stages.
Owing to legal and economic reasons, it is also desirable that the method allows to control the composition of the electrodeposited alloy.
In the present invention, the composition control is based in the determination of the electrochemical equivalent of the alloy, since to each composition corresponds a well defined electrochemical equivalent.
For each stage, in the control phase G and on the base of the consumed electric charge and on the deposited weight, the control means determine the electrochemical equivalent of the deposited alloy, and, as a consequence, deduce its composition.
On the base of this result, if necessary a new current intensity and a new charge are calculated, in order to correct possible differences between the deposited alloy composition and that theoretically foreseen.
An alternative possibility to control the deposited alloy composition is to use its density instead of its electrochemical equivalent.
Next, the control means check if the five stages have already been completed, and, if it is not the case, return to calculation phase D and a new electrodeposition stage is started.
When the control means verify that the process has been completed, they go on to phase H, to make a final check balance, and the results are outputted to the user, the possibility existing of an additional stage, for instance if it is wished to deposit a greater gold or gold alloy quantity.
This control method guarantees optimal results of the electrodeposition process.

Cathode assembly and weighing

The part of the equipment related to the cathode rotation driving and to the cathode weighing during the electrodeposition process will be described in the following, with reference to figures 5a to 5c.
Figure 5a shows the cathode 12 hanging over the process tank, in a loading position. The support 13 includes a motor 30 that allows to put the cathode 12 in rotation clock and anticlockwise, alternatively, during time periods fixed in advance and with a selected speed. The motor 30 rotates a driving element 31 that, by friction, transfers the rotation to a driven element 32 integral with the rod 33 of the cathodic rack 12. The negative polarity is also transmitted to cathode 12 through the support 13, for instance by a conventional system of brushes (not shown).
The support 13 is attached to a jamb 34 in such a way that it can slide vertically along it, propelled, for example, by a pneumatic piston (not shown).
Once the cathodic rack is loaded with the items or models to be coated, the support 13 is lowered to the working position of figure 5b. In this case, elements 31, 32 are engaged and the rotation of motor 30 is transmitted to the cathode 12; it should be noticed that, in this position, the seat 14 of the cathodic rack stays separate from the projection 15 of the scale 16.
When an electrodeposition stage finishes, the rotation of the motor 30 stops and the support 13 is lowered to the position of figure 5c. As can be seen, this position is calculated so that the cathodic rack rests through its seat 14 on the projection 15 of the scale, while the driven element 32 is disengaged from the driving element 31 of the support 13, and therefore the cathode 12 has no contact point with the latter.
As a consequence, in this position the cathode can be weighed while immersed in the electrolyte.
All the elements related with the movement of cathodic rack and with the weighing operations are commanded by the equipment control means.
Finally, it is important to point out that in the equipment according to the invention it is foreseen that all the dimensions of the essential elements (tank, anodes, cathode) be given in a generic form, so that fixing some of them, the rest become defined. This allows to tailor the equipment at will.
In spite that a specific embodiment of the present invention has been described and represented, it is obvious that the man skilled in the art can introduce variants and alterations, or replace the details by others technically equivalent, without departing from the scope of protection of the enclosed claims.

Claims

An equipment for the electrodeposition of gold and gold alloys, comprising an electrolytic cell (1) with an anode (10,11), a cathode (12) where are arranged the objects on which the gold or the alloy have to be deposited, and an alkali-cyanide electrolyte, and means to replenish the gold consumed in the electrodeposition process, said means to replenish gold comprise an auxiliary electrolytic cell (2), connected with the electrolytic cell (1) in which the electrodeposition process takes place through respective conduits (17,18) that allow the recirculation of the electrolyte from one cell to another, said auxiliary electrolytic cell (2) comprising an anode (21) where the gold sheets (22) are arranged characterised in that the distance between the outer piece and the inner piece of the anode (10,11) of the electrolytic cell (1) is shorter in the central area than at the upper and lower ends, in that the inner piece (11) of the anode has a higher area factor than the outer piece (10) and in that the cathode (12) is placed closer to the inner piece (11) of the anode than to the outer piece (10).
An equipment according to claim 1, characterised in that the auxiliary electrolytic cell (2) comprises an anode made of a metallic framework (21) intended to hold gold sheets (22) and a metallic mesh disc attached to said framework and arranged horizontal at the bottom of the cell (2), and a cathode (24) made of a metallic mesh cylinder arranged in the middle of the cell inside a cylindrical semipermeable diaphragm (25), and in that the inside of said semipermeable diaphragm (25) is filled with a conductive solution.
An equipment according to claims 1 or 2, characterised in that the electrolytic cell (1) where the electrodeposition process takes place comprises an anode formed by two concentric pieces of metallic mesh (10,11) of generally cylindrical configuration, between which the cathode (12) is rotatably arranged.
An equipment according to claim 1, characterised in that the inner piece (11) of the anode has the shape of an elongate truncated revolution ellipsoid, convex toward the outside of the cell (1), and the outer piece (10) of the anode has the shape of an elongate one-sheet revolution hyperboloid, convex toward the inside of the cell.
An equipment according to claim 1, characterised in that the inner piece (11) of the anode has the shape of two truncated cones superimposed with their larger bases in contact, and the outer piece (10) of the anode has the shape of two truncated cones superimposed with their smaller bases in contact.
An equipment according to any of the claims 1 to 5, characterised in that the cathode (12) is suspended from a supporting and rotation driving device (13), that can be arranged an upper loading position, in which the cathode (12) is out of the electrolyte, an intermediate operation position in which the part of the cathode (12) that holds the objects is completely immersed in the electrolyte and the cathode (12) engages the rotation driving means, and a lower control position in which the cathode (12) is disengaged from the driving means and rests on a vertical projection (15) of a scale (16) placed in the middle of the electrolytic cell (1), in an area (6) lacking in electrolyte.
An equipment according to claim 1 characterized in that the optimal ratio between the area factors of the pieces 10 and 11 is: F/f= A(D-a)/a(A-d) where F is the area factor of piece 11, f is the area factor of piece 10, A and a are respectively the inner diameter of a cylindrical wall 4 and the outer diameter of the cylindrical wall 5, with which the pieces 10 and 11 are in contact, and D is the cathode diameter.
An equipment according to claim 1 characterised in that the distance between the cathode and pieces (10, 11) is: L= (A-D)/2 l = (D-a)/2 where L is the distance from the cathode to the outer piece 10, l is the distance from the cathode to the inner piece 11 and A, a and D are as defined in claim 7.