EP0190088B1 - Device for the separation and extraction of metals in solution by electrolysis - Google Patents

Abstract

A cathode compartment is defined by a pair of filter walls between which a cathode or electrolyzing ionic solution of a metal is received. The electrolysis is forced through this compartment and the current density against an anode outside the compartment is sufficiently high to cause at least some of the metal in solution to deposit as a powder which accumulates in the cathode compartment and augments the effective surface area of the cathode.

Description

The present invention relates to a device for isolating and extracting metals in solution, by electrolytic means.

The possibility of isolating and extracting metals in solution by electrolytic means on a cathode has been known for a long time. This technique is commonly used for the recovery of precious metals in solution in ionic form, and for the decontamination of effluents polluted by heavy metals, such as cadmium, nickel, copper and mercury, for example.

However, known techniques require the use of electrolysis cells of a particular design, and do not allow, given the high cost of financial investment, the recovery of small amounts of metals.

Document GB-A-1 598 306 describes a device for recovering metals in solution by electrolytic means, comprising a cylindrical cathode arranged coaxially inside an anode itself cylindrical, the cathode being made from fibers of carbon sandwiched between two layers of synthetic material. The solution is circulated from the inside of the cathode to the outside.

Document US-A-4 367 127 relates to a device for the recovery of metals in solution by electrolytic means, constituted by a cell comprising several elements arranged concentrically and in particular, from inside to outside, a diffuser constituted by a cylinder. pierced, a dielectric tube having openings and supporting a winding of conductive elements forming the cathode, which is separated from the anode by a dielectric spacer. The solution is circulated from the cathode to the anode.

The present invention aims to remedy these drawbacks.

To this end, in the device that it relates to, in which the cathode, made of copper or another material that is good conductor of electricity, is disposed between two filtering walls made of a material that is not conductive of electricity and passed through continuously by the solution containing the metal, the two filters define a completely closed enclosure for recovering the metal adhering to the cathode or detached from it, means for circulating the solution are provided so that the solution flows from the anode to the cathode, and the filtering wall, located between the cathode and the anode, is not in contact with the cathode.

During the electrolysis operation, metal will deposit on the surface of the cathode, while another part of the metal in powder form will be retained around the cathode by the filtering surfaces located on either side of this, or collected in the enclosure delimited by the two filtering walls.

The presence of the two filter walls is very interesting because it allows the use of high current densities which accelerate the electrolytic process, but which tend to produce deposits of powdery metal, which detach from the cathode and are subsequently found in suspension in the solution. However, during the implementation of the device according to the invention, the presence of such particles is not a problem since they are retained by the filtering walls, or collected in the enclosure delimited by the two filtering walls. It should be noted that the volume of this enclosure is not critical and only depends on the amount of metal to be recovered.

According to another characteristic of the invention, the means for circulating the solution are such that the pressure and the flow rate of the solution ensure the renewal of the volume of the enclosure in 1 to 10 seconds.

The flow rate of the liquid flow is chosen at a large value of the order of 10 to 40 l / min and per dm ² of cathode surface, which provides very advantageous performances allowing a reduction of the surface of the cathode very substantial compared to on the surface of the cathodes of traditional electrolysis cells. The circulation of the solution is obtained using a pump, and produced in such a way that the electrolyte, once injected under pressure into the filtration chamber, passes under pressure through a filtering wall in the cathode enclosure, where it is subjected to strong turbulence around the cathode. The pressure and the flow rate of the electrolyte are such that the volume of the enclosure is renewed in 1 to 10 seconds. Given the rapid formation of a deposit on a reduced cathode surface, subjected to high current densities, the cathode is well protected and resists well to the chemical aggressiveness of certain electrolytes.

• - l'électrolyte traverse la cellule d'électrolyse délimitée par les deux parois filtrantes, de telle sorte que le renouvellement de la pellicule liquide en contact avec la cathode est très rapide et se produit au même moment sur toute la surface de celle-ci et;
• - la cathode est une cathode à surface croissante du fait que la fraction de dépôt métallique non adhérant à celle-ci et prisonnière des parois filtrantes, devient cathodique à son tour. En fin d'électrolyse, la surface acitve est notablement accrue par rapport à sa surface initiale. Donc, pour une même intensité de courant, la densité du courant cathodique tend à diminuer. De plus, cette densité est dégressive de l'extérieur vers l'intérieur, ce qui favorise l'extraction sélective d'une impureté métallique d'une solution galvanique et, de façon générale, l'isolement complet du métal contenu dans une solution.

This results in two specific characteristics of the device:

• the electrolyte passes through the electrolysis cell delimited by the two filtering walls, so that the renewal of the liquid film in contact with the cathode is very rapid and occurs at the same time over the entire surface thereof and ;
• - The cathode is a cathode with a growing surface owing to the fact that the fraction of metal deposit not adhering to it and trapped in the filtering walls, becomes cathodic in turn. At the end of electrolysis, the acitve surface is notably increased compared to its initial surface. Therefore, for the same current intensity, the density of the cathode current tends to decrease. In addition, this density decreases from the outside to the inside, which favors the selective extraction of a metallic impurity from a galvanic solution and, in general, the complete isolation of the metal contained in a solution.

Advantageously, and in order to facilitate subsequent recovery of the metal in a simple, practical, inexpensive manner and without loss of metal, the filtering walls are made of fully combustible dielectric material, have a thickness of between 2 and 20 mm and have pores with a diameter between 1 to 50 J.1m and the cathode surface is between 1 and 10 dm ² .

Depending on the chemical nature of the electrolyte and the metal concentration of the solution, the cathodic current density is between 0.1 and 20 A / dm ² .

According to one embodiment, the two filter walls are cylindrical in shape, arranged concentrically.

tric to each other, and joined by two end pieces made of an electrically insulating material, one of which has a diameter equal to the outside diameter of the outer wall, and consists of a solid disc and the other of which same outer diameter, is in the form of an annular crown whose central opening has a diameter substantially equal to the inner diameter of the inner wall, the cathode being housed in the annular space formed between the two walls and being connected to a wire feed through one of the two end pieces.

In this case, the direction of circulation of the solution is oriented from the outside towards the inside of the cell delimited by the filtering walls. This results, taking into account the increase in the speed of the solution in the external filtering wall and the convergence of the flow of the solution, an improvement in the plating of the metal particles against the cathode.

The assembly, constituted by the filtering walls, the cathode and the nozzles, can advantageously be shaped to have the dimensions of a commercial cartridge.

This cartridge can thus be mounted inside a traditional cylindrical filtration chamber in which is mounted an anode, preferably insoluble, placed in the space provided between the cartridge and the outer wall of the chamber and whose cover is modified to allow the passage of the electrical connections of the anode and the cathode.

According to one embodiment of this device, the cathode consists of two rods made of a conductive material such as copper, arranged longitudinally to the inner wall, and held in contact with the latter by a conductive wire such as copper, helically wound, one of the rods having a length at most equal to that of the filtering walls, and the other rod having a longer length, so as to pass through one of the end pieces to make the electrical connection.

This interesting device by its simplicity, its moderate cost and its performances, makes it possible economically to recover metal, for example, when it is a question of recovering 20 to 50 g of gold during an operation.

In addition to the recovery of precious metals in ionic solution, such as gold, silver or platinum, this device can be used for the decontamination of effluents polluted by dissolved heavy metals such as cadmium, zinc, lead, nickel, copper, etc. ..., where the regeneration of pickling baths of copper and copper alloy saturated with metal.

• Fig. 1 est une vue très schématique des principaux composants du dispositif en vue d'illustrer le fonctionnement de celui-ci;
• Fig. 2 est une vue en coupe longitudinale d'une cellule d'électrolyse;
• Fig. 3 est une vue en coupe transversale de cette cellule selon la ligne III-III de fig. 2;
• Fig. 4 est une vue d'une installation équipée de la cellule de figures 2 et 3.

In any case, the invention will be clearly understood with the aid of the description which follows, with reference to the appended diagrammatic drawing representing, by way of nonlimiting example, an embodiment of this device:

• Fig. 1 is a very schematic view of the main components of the device in order to illustrate the operation thereof;
• Fig. 2 is a view in longitudinal section of an electrolysis cell;
• Fig. 3 is a cross-sectional view of this cell along line III-III of FIG. 2;
• Fig. 4 is a view of an installation equipped with the cell in FIGS. 2 and 3.

As shown in fig. 1, the solution 2 to be treated is located in a reservoir 3. A centrifugal pump 4 brings the solution from this reservoir to a filtration chamber 5. The chamber 5 is divided into two compartments, respectively, lower 6 and upper 7, by two horizontal filter walls parallel to each other, respectively, a lower filter wall 8 and an upper filter wall 9, the wall 9.

Between the two walls 8 and 9 delimiting an enclosure, is disposed, parallel to these, a cathode 10. The cathode 10 is connected to the negative pole of a direct current source, which is located inside the chamber. filtration, by a conductive wire 12 housed inside a sealed sheath 13 in the part located inside the filtration chamber 5.

In the upper compartment, there is an anode 14 connected to the positive pole of the direct current source by a wire 15 housed inside a sheath 16, in the part which passes through the upper compartment of the chamber.

The filtration chamber 5 is equipped, at its upper end, with an overflow (17) returning the liquid having passed through the filtering walls to the reservoir 3.

It should be noted that all the constituent elements of the device are made of an electrically insulating material, with the exception of the cathode which is made of copper, and the anode of the “insoluble” type which is, as the case may be. , stainless steel, titanium, graphite, etc ...

The filter walls 8 and 9 are made of a porous material, such as a porous plastic or felt material, stiffened by a perforated plastic wall.

The thicknesses of the filtering walls vary from 2 to 20 mm approximately, and the diameter of the pores varies between 1 and 50 microns.

In practice, the electrodes are put under a voltage of 5 volts for example. During the operation of the pump 4, the solution 2 to be treated passes through the filtering walls 8 and 9 and passes into contact with the cathode.

The flow of the liquid is of the order of 10 to 40 l / min per dm ² of cathodic surface, the cathodic current density being of the order of 0.1 to 20 A / dm ² depending on the chemical nature of the electrolyte and according to its metal concentration.

During this operation, the insulated metal will be, partly retained on the cathode and, in part, retained around the cathode in a pulverulent form, these latter particles being trapped in the porosities of the wall 9, or remaining in the space between the two walls 8 and 9.

Figures 2 and 3 show an embodiment of an electrolysis cell constituted by the two filter walls and the cathode. This cell comprises two cylindrical and concentric walls 18 and 19, each having a length of 250 mm, the diameter of the external wall 18 being 70 mm and the diameter of the internal wall 19 being 30 mm. The thickness of the wall 18 is 2 mm, while the thickness of the wall 19 is of the order of 5 to 7 mm.

The two walls 18 and 19 are made integral with one another by two end pieces 20 and 22 fixed at their ends, the end piece 20 being in the form of a solid disc, while the end piece 22 is under the shape of a crown having a central opening 23 of diameter substantially equal to the internal diameter of the internal filtering wall 19.

On the inner wall 19 are arranged longitudinally two copper rods 24 and 25 each having a diameter of the order of 4 mm. The length of the rod 25 is of the order of 200 mm, while that of the rod 24 is of the order of 300 mm, so that it crosses the end piece 20 for making the electrical connection.

The rods 24, 25 are fixed to the tube 19 forming the internal filtering wall, by means of a copper wire 26 wound helically. The length of the copper wire is calculated, so that the surface of the rods 24, 25 increased by the surface of the wire is of the order of 1 dm ² .

The gap delimited by the walls 18, 19 and by the end pieces 20, 22, in which the cathode 24, 25, 26 is trapped, forms an electrolysis cell which has exactly the dimensions of a filter cartridge used in the conventional filtration devices. This cell is advantageously made from combustible materials.

The cell, shown in Figures 2 and 3, can be mounted inside a cylindrical filtration chamber 27 of standard dimensions, in which is arranged an insoluble anode 28, disposed between the cartridge and the interior wall of the chamber.

It suffices simply to modify the cover 29 of the chamber to allow a connection between the anode and the cathode on the one hand, and an adjustable direct current source on the other hand.

The installation also comprises, as shown in FIG. 4, a prefiltration chamber 30 and a centrifugal pump 32, the liquid discharged from the center of the filtering wall 19 being recycled inside the installation by the centrifugal pump 32 .

An example of the use of this device is given below.

An electrolyte containing 1.25 g of gold in acid solution is pumped under exactly the same conditions as if this liquid had to be filtered to rid it of solid particles in suspension.

The volume of the solution is approximately 100 liters, the pump flow rate of 2000 I / h and the amperage of 20 Amps on average.

The residual gold content in the solution being analyzed every ten hours, the results are as follows:

It appears from this table that after 60 hours, the solution can be considered as exhausted in precious metal. After stopping the electrolysis the cartridge is calcined and the molten metal, to recover the gold it contains, allowing the obtaining of an ingot very rich in gold, since containing more than 95% gold.

It is interesting to note that no significant loss of gold has been observed and that a cartridge of this size can isolate and recover more than one kg of gold, the profitability of the operation being ensured in the since the cartridge is used to recover at least 20 to 50 grams of gold.

As is apparent from the above, the invention brings a great improvement to the existing technique by providing a device of simple design, of moderate cost price, and very efficient, which can be implemented for numerous metal recovery applications. in ionic solution.

It goes without saying that the invention is not limited to the sole embodiments of this device, nor to the sole conditions of implementation described above by way of examples; on the contrary, it embraces all of its variant embodiments; Thus, in particular, in the case of the device described with reference to Figures 2 to 4, the cathode could be constituted by a metal cylinder having regularly distributed openings to achieve a transverse passage of the solution, without the this goes beyond the scope of the invention.

Claims (7)

1. Device for isolating and extracting metals in solution by an electrolytic process in which the cathode (24, 25, 26), consisting of copper or some other material which is a good conductor of electricity, is disposed between two filtering walls (18, 19) of a material which does not conduct electricity and which are traversed continuously by the solution containing the metal, characterised in that the two filtering walls (18, 19) define a totally enclosed space for the recovery of the metal adhering to the cathode or detached therefrom, and in that means are provided which are adapted to produce circulation of the solution so that the solution circulates from the anode towards the cathode, and in that the filtering wall situated between the cathode and the anode is not in contact with the cathode.

2. Device according to claim 1, characterised in that the means of causing the solution to circulate are such that the pressure and rate of flow of the solution provide for the contents of the enclosed space to be renewed in 1 to 10 seconds.

3. Device according to any one of claims 1 and 2, characterised in that the two filtering walls (18, 19) are of cylindrical shape and are disposed concentrically of each other and are joined by two sockets which consist of an electrically insulating material and of which one (20) has a diameter equal to the diameter of the outer wall (18) and consists of a solid disc while the other (22), of the same outside diameter, is in the form of an annular crown of which the central opening (23) has a diameter substantially equal to the inside diameter of the inner wall (19), the cathode being housed in the annular space provided between the two walls and being connected to a feed wire which passes through one of the two walls.

4. Device according to claim 3, characterised in that the assembly consisting of the filtering walls (18, 19), the cathode (24, 25, 26) and the sockets (20, 22) is shaped in such a way that it has the dimensions of a filtering cartridge.

5. Device according to any one of claims 3 to 4, characterised in that the cathode consists of two rods (24, 25) consisting of a conductive material such as copper, disposed longitudinally in relation to the inner wall, and maintained in contact therewith by a conductor wire (25) such as a copper wire, wound helically, one (25) of the rods having a length at most equal to that of the filtering walls, the other rod (24) having a greater length so that it passes through one (20) of the sockets to establish the electrical connection.

6. Device according to any one of claims 1 to 5, characterised in that the filtering walls (18, 19) are of a dielectric material, having a thickness of between 2 and 20 mm and having pores of a diameter between 1 and 50 microns, and in that the cathode surface is comprised between 1 and 10 square decimetres and in that the rate of flow of liquid is around 10 to 40 litres per minute and per square decimetre of cathode surface area and in that the means of causing the solution to circulate are provided in such a way that the rate of flow of the liquid is around 10 to 40 litres per minute and per square decimetre of cathode surface.

7. Device according to any one of claims 1 to 6, characterised in that the electrical supply means are such that the density of the cathode current is between 0.1 and 20 amps per square decimetre.

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