EP0094308A2 - Process and apparatus for the electrolytic preparation of metal, especially lead - Google Patents

Abstract

The present invention is directed to an apparatus for preparing a lead or copper metal by electrolysis in a cell having a cathode and an anode which are separated by a diaphragm. The surface of the cathode is arranged in a substantially vertical direction and has a sufficiently low density of sites of nucleation so that metallic particles which are formed from the sites keep their individuality in relation to the adjacent particles, until the particles reach a dimension of at least 100 micrometers. An electrolyte is circulated between the cathode and an anode parallel to the surface of the cathode at a flow rate in a laminar to weakly turbulent manner so that under the action of the weight of the particles and the force exercised by the flow of the electrolyte the metallic particles are detached and fall into the electrolyte and are collected and are removed from the bottom of the cell.

Description

The present invention relates to the preparation of a metal by electrolysis, and in particular the preparation of lead from metal chloride. It relates in particular to the electrolysis of very pure solutions of lead chloride.

Current metallurgical treatment processes for lead ores allow the preparation of lead chloride solutions which are very pure, for example after purification by a solvent or crystallization. The invention relates to the preparation of lead from such solutions.

French Patent No. 73-30.657 describes a process for depositing metallic lead from aqueous solutions of lead chloride. More precisely, this patent describes the electrolysis of such a solution in a diaphragm cell, in the presence of ferrous chloride which oxidizes to ferric chloride during the operation; in example 1 of this patent, the concentration of lead in the electrolyte is reduced to a value between 25 and 11 grams per liter, in a 3M solution of ferrous chloride, with a current density of 323 A / m ² and a faradic yield of 70%. The patent does not indicate the properties of metallic lead deposition such as its density, its adhesion to the cathodic support (lead sheet), its compact or pulverulent nature or its purity, nor the mode of extraction of lead.

The book "Electrometallurgy of Chloride Solutions" by VV Stender, Consultants Bureau, New York, (1965) indicates that a deposit of non-compact lead, coarse crystallization and having a metallic luster, can be obtained from brine concentrated sodium chloride containing lead. The lead concentration decreases from 40 to 10 grams per liter during an electrolysis carried out with a current density of between 500 and 1000 A / m ² . The work does not give details on the properties of lead such as its purity, its density, its adhesion to the cathode which consists of a lead sheet, nor on the mode of extraction of the thimble. pot. The faradaic yield obtained would be between 85 and 90%. The same book also indicates that a lead powder can be obtained from solutions containing 300 grams per liter of NaCl and 10 grams per liter of lead in the form of chloride, with a faradaic yield of about 80%.

The report "Aqueous electrolysis of lead chloride" by FP Haver, DL Bixby and MM Wong, USBM Report of Investigations, 8276 (1978) describes the electrolysis of lead chloride crystallized on a horizontal cathode placed at the bottom of the cell, so that the lead concentration in solution remains constant. This document indicates that, as soon as the last lead chloride crystal disappears, the deposit becomes spongy, non-metallic in appearance and adherent. In the presence of crystals, the faradic yield obtained is 96% for a current density of 150 A / m ² , in a 20% solution of HCl at 25 ° C.

French patent n ° 79-12.867 describes a process for recovering lead from sulphide ores. This process ensures the regeneration of the reagent, ferric chloride, at the anode of an electrolyser comprising neither a diaphragm nor a membrane. Lead is deposited on a cathode formed by an assembly of rods mounted in special supports so that shocks can be applied to the rods due to their rotation or their mounting device. The lead formed is detached under the action of shocks and falls to the bottom of the tank. It is then evacuated. This patent does not describe the effects of the electrolysis current in the vicinity of the electrode and neither describes the recovery of the lead fragments, nor the treatment of the latter before fusion.

French Patent No. 2,386,349 describes a process and an apparatus for recovering metallic particles by electrolysis. This patent essentially concerns copper and, incidentally, transition metals, those which are indicated as preferable being two of groups VIII, 1b and 2b of the Periodic Table of Elements. This patent therefore does not concern the treatment of lead. According to this patent, the metal, essentially copper, forms, on the cathodes, particles which are torn off by using vigorous stirring ensured by mechanical agitators moved in front of the cathodes. According to an essential characteristic of the process described in this patent, the powders must be washed before being extracted from the electrolysis cell.

Thus, the methods described have drawbacks and the aforementioned documents have shortcomings. In particular, it is not known what is the quality of the powder obtained, in particular its purity, its density, its air oxidation properties, all essential properties in the industrial exploitation of such a powder.

These documents do not indicate any process for extracting the lead formed, which can be used in an industrial electrolyser.

The faradic yields obtained are most often less than 90%. The anodic reaction is not described in general and it is not indicated if the chlorine is released at the anode or if on the contrary this release is avoided; and how. Î

The invention relates to the preparation by electrolysis of a very pure metal, preferably lead.

The subject of the invention is the preparation by electrolysis of a metal present in the electrolyte in a non-cationic and in particular anionic form.

It relates to such a method which implements a continuous detachment of the metal which forms on the cathodes.

It relates to such a process which allows the preparation of a particulate metal, in particular particulate lead which is not pyrophoric and which can be easily formed into a useful semi-finished product.

It also relates to such a process which makes it possible to obtain a very high faradic yield.

According to the invention, the electrolyte circulates parallel to the cathodes which are placed vertically with a speed such that its flow is of the laminar or slightly turbulent type, so that this current, in cooperation with the apparent weight of the particles, ensures the detachment of these from the cathodes and, simultaneously, the renewal of the nearby electrolyte of the electrode surfaces.

More specifically, the invention relates to a process for the preparation of a metal, preferably lead, by electrolysis in a diaphragm cell, of the type which comprises the formation of an electrolyte containing a chloride of the metal to be prepared and a chloride of an alkali or alkaline earth metal, and the circulation of the electrolyte between the electrodes and parallel to the surface of the cathodes. According to the invention, the cathode surface is arranged in a substantially vertical direction and has a sufficiently low density of nucleation sites so that the metallic particles which form from these sites keep their individuality with respect to the adjacent particles, up to 'that they reach a dimension of at least 100 micrometers; the flow of electrolyte along the cathode surface is of the laminar type or slightly turbulent, so that, under the action of their weight and the drag forces exerted by the electrolyte current, the metal particles get detach and fall into the electrolyte; the method further comprises removing the metallic particles collected at the bottom of the cell.

When the metal of the electrolysis is lead, it is present, in the form of chloride, in an amount of between approximately 5 and 50 grams per liter, preferably between 15 and 25 grams per liter in the electrolyte.

The alkali or alkaline earth metal chloride is preferably sodium chloride. Its concentration in the electrolyte is preferably between 230 and 300 grams per liter.

During the electrolysis, the density of the electrolysis current is between 500 and 1500 A / m ² , preferably between 700 and 1000 A / m ² . It is better that this current density gradually increases since the start of electrolysis.

During the electrolysis, the temperature of the electrolyte is advantageously between 70 and 95 ° C.

The cathode surface having a low density of nucleation sites is preferably formed of titanium, stainless steel or graphite.

It is advantageous that the electrolyte also contains iron in the form of chloride. The concentration of iron is then advantageously greater than 10 grams per liter and preferably between 20 and 60 grams per liter.

The laminar or slightly turbulent flow of the electrolyte along the cathode surface is obtained when the electrolyte current flows near the cathodes at a speed of between 0.01 and 0.15 meters per second.

The removal of the particles collected at the bottom of the cell is advantageously carried out by transporting the particles out of the cell, then by densification of the particles by compression. In addition, the densified particles can undergo a rolling intended to drive out the electrolyte inclusions. The densified particles can also undergo fusion in the presence of soda.

The invention also relates to an apparatus for preparing a metal by electrolysis, in particular lead, in which the cathodes and anodes are arranged vertically. According to the invention, the cathodes are formed from a material chosen from the group which comprises titanium, stainless steel and graphite, and the apparatus comprises at least one pump intended to circulate a current of electrolyte of the type laminar or weakly turbulent along the cathodes, and a transport device for removing divided solids which may fall to the bottom of the cell.

In an advantageous embodiment, the electrodes are bipolar.

The device advantageously includes a recovery hood when chlorine gas is released at the anodes.

The anodes are advantageously formed from a metal which cannot be attacked by the electrolyte and in deployed form.

The transport device can advantageously be a worm, a bucket elevator or a conveyor belt and preferably the gooseneck system described below.

The apparatus may also include an extruder for receiving the particles and for densifying them.

The invention also relates to a semi-finished product of lead, prepared by the above-mentioned process and containing less than 0.2% of electrolyte in the form of inclusions.

The method and the apparatus according to the invention have all the advantages of apparatuses in which the metal detaches automatically from the cathodes. The main advantage is the almost total elimination of manipulations of the electrodes. This reduction in handling increases the useful service time of the electrolysers so that the number of electrolysis cells can be reduced, with corresponding reduction in investments.

In addition, thanks to the very high faradaic efficiency ensured by the invention, greater than 90% and often 95%, the energy losses are reduced.

We now consider in more detail the various parameters which influence the implementation of the process, first in the case where the starting solution contains iron in chloride form, in addition to lead.

The solution which constitutes the electrolyte contains chlorides of lead, alkali or alkaline-earth metals, iron and possibly other metals, for example zinc.

The lead chloride solution is advantageously formed from a concentrate of lead sulphide ore which, in addition to lead, contains small amounts of zinc, copper, iron, calcium and magnesium. sium. After purification, for example according to the techniques described in French patents No. 2,323,766, 2,359,211 and 2,387,293 and in European patent application No. 0024987, the solution contains practically only lead and iron, the other metals being in negligible quantity. Said patents or patent applications are deemed to form part of this description.

The amount of lead in chloride form, present in the electrolyte, is preferably greater than 5 grams per liter but it does not preferably exceed 50 grams per liter. These two values are determined from the current densities used during the electrolysis and the circulation speeds of the electrolyte in the vicinity of the electrodes, so that the faradaic yield and the production capacity are optimal.

The electrolyte also contains in high concentration an alkaline or alkaline-earth chloride. The most advantageous is sodium chloride, for reasons of cost and availability. The amount of this chloride in solution is advantageously chosen so that the concentration of chloride ion is greater than 3 equivalent-grams (that is to say in the case of sodium chloride at 200 grams per liter approximately), preferably between 4 and 5 gram equivalent (i.e. for sodium chloride between 230 and 300 grams per liter). The role of this chloride is to increase the concentration of chloride ions in the electrolyte, which makes it possible to dissolve the metals whose chlorinated complexes are soluble, and to reduce the losses by Joule effect.

In the embodiment considered, the electrolyte also contains iron in the chloride form. In the absence of iron, the chloride ions oxidize at the anode to chlorine gas at an electrode potential of 1.2 V compared to the saturated calomel electrode (DHW). The apparatus must then include a suitable collecting system. When chlorine is not desirable, the electrolyte advantageously contains iron so that, at the anode, the ferrous ion oxidizes to ferric ion at a po potential close to 0.6 V / DHW. It is therefore necessary that the electrolyte contains iron in the form of ferrous iron. Not only is the chlorine no longer released at the anode, but also the energy efficiency is clearly increased. In addition, the ferric chloride formed at the anode is recovered and can be used again for the treatment of lead sulphide ores and the transformation of galena into elemental sulfur and lead chloride.

The concentration of iron in the electrolyte, in the chloride form, is preferably between 20 and 60 grams per liter, and advantageously it is of the order of 40 grams per liter. It is important that this concentration is at least equal to 20 grams per liter in the anolyte, that is to say near the anodes.

Table I which follows indicates the main characteristics of the composition of the electrolyte.

The nature of the electrodes used and in particular of the cathodes is important for the implementation of the invention. It is found in fact that many materials are too "active", that is to say they form too many nucleation sites. As a result, lead particles begin to form at too many locations on the surface of the cathodes and can not not gain weight individually. For this reason, it is essential according to the invention that the density of the nucleation sites, under the electrolysis conditions used, is sufficiently low so that the particles can reach a dimension of at least 100 micrometers without joining with the particles. adjacent. Preferably, the particles retain their individuality until they reach a dimension of at least 600 micrometers and preferably one millimeter. Under these conditions, the particles individually have a large enough surface so that, when the electrolyte moves along the surface of the cathode, it exerts a tearing force which, in combination with the force of gravity, is sufficient for detachment particles when they have a dimension of a few hundred micrometers.

This density of sites is important according to the invention because, if the number of sites is too large, the particles formed are small and numerous and, when they are subsequently placed in the air, they oxidize easily because they form a pyrophoric powder. On the contrary, if the density of nucleation sites is too low, the production capacity is reduced.

It is found that a suitable density of nucleation sites is obtained by the use of cathodes whose surface is formed of smooth titanium. Stainless steel or graphite surfaces can also be used. Of course, other materials can also be used, when these have the appropriate density of nucleation sites. This density can be obtained by an activation treatment or, more often, deactivation according to techniques known to those skilled in the art.

The anodes can be formed from graphite. However, as it is desirable that the transport of material is favored, it is preferable that the anodes be formed from an expanded metal, for example ruthenized titanium. However, the nature of the anode is much less important for the implementation of the method of the invention as that of the cathode.

Obtaining high faradaic yields requires control of the transport of ferric iron formed at the anode to the catholyte. The choice of a suitable diaphragm having a low permeability, the use of a high current density and the maintenance of a difference in hydrostatic pressures between the catholyte and the anolyte, this difference in pressure being at least 20 millimeters of liquid column, make it possible to avoid the passage of Fe III towards the catholyte. In this way, the entire practical feed rate of the cell passes through the diaphragm. It is advantageously formed from chemically inert textile fibers in the electrolyte. Suitable materials are polyester coated with silicone, teflon-coated glass fibers, and preferably synthetic fibers based on fluorinated polymers.

The phenomena of transport of materials during electrolysis are of primary importance on the morphology of the particles formed and on the faradaic yield obtained. It has already been noted that it was advantageous for the anodes to be formed from an expanded metal, allowing good transport of the materials, by an effect analogous to that of the turbulence promoters. However, this phenomenon of increased turbulence is advantageously used only at the anodes. It is preferable, for obtaining particles of suitable morphology, that the electrolyte current along the cathodes is of the laminar type or at least only slightly turbulent, while ensuring sufficient renewal of the electrolyte at the cathodes. It is indeed important that the concentration of lead varies only slightly throughout the electrolyte. Obtaining a laminar or slightly turbulent flow at the cathode depends not only on the nature of the surface of the cathodes but also on the speed of the liquid along the cathodes. It is thus desirable, according to the invention, for the linear speed of the catholyte, parallel to the cathodes, to be at least 0.01 meter per second and preferably between 0.01 and 0.15 meter per second. At the anodes, the speed of circulation of the solution, which may be zero, is preferably at least 0.01 meter per second; the maximum value can be moderate, for example 0.05 meters per second, given the shape of the anodes which promote the creation of turbulence.

The temperature of the electrolyte is advantageously between 70 and 95 ° C, preferably between 70 and 90 ° C. No heating is necessary because the normal losses by Joule effect are sufficient to maintain the temperature in the aforementioned range.

The implementation of the method of the invention allows the use of very high current densities. They can be between 500 and 1,500 A / m ² . Preferably, they are between 700 and 1000 A / m ² .

Table II summarizes the various conditions mentioned above.

It is desirable, during the implementation of the method according to the invention, that the electrolysis begins at a low current density, lower than the values indicated above, and gradually increases to the chosen value, included in the aforementioned range . In fact, when the solution contains ferrous chloride, iron can deposit at the same time as lead, on the cathodes, when the current density is initially very high. The metal then adheres to the entire surface of the cathodes, so that the latter no longer have a suitable density of nucleation sites.

When the electrolyte contains practically no iron chloride, the initial use of a high current density can cause the evolution of hydrogen whose bubbles tend to cling to the metal particles so that these, instead of falling to the bottom of the electrolysis cell, tend to float.

The period during which the current density increases progressively or in stages, up to the desired final value, is advantageously a few hours.

As indicated above, it has been seen that it was desirable for the product obtained to be in the form of individual particles having a dimension of a few hundred micrometers, for example from 300 to 600 micrometers. Their shape can be branched and relatively flat, but their surface is relatively small for their volume. It is this characteristic which gives the particles formed their non-pyrophoric nature.

The particles are formed from very pure lead. For example, metals such as zinc, copper, cadmium, magnesium, etc. are present in an amount less than 1 ppm by weight. The amount of iron is less than a few ppm by weight. It is therefore lead which does not subsequently require any refining for most applications. The examples which follow give the purity of the lead obtained under different conditions.

An important parameter for carrying out an electrolysis is the faradaic yield obtained since this indicates the importance of the electrical losses. According to the invention, this faradaic yield is at least equal to 90% and it generally reaches and exceeds 95%. Of course, these yields are only obtained when the different parameters have the desired values, corresponding for example to the abovementioned tables I and II.

The lead particles which deposit at the bottom of the cell are then extracted, using a suitable mechanism, as indicated in the remainder of this specification with reference to an apparatus intended for carrying out the method according to the invention. The lead particles, when removed, are associated with occluded electrolyte, present in an amount between about 20 and 30% by weight. It is therefore desirable that the material undergoes compaction or rolling. It is in particular desirable that the particles be densified by extrusion, in a piston or roller press, exerting pressures greater than about 70 MPa. Filtration of the product is undesirable since the smallest particles may partially oxidize in air.

When the particles are taken from the bottom of the tank, they have an apparent density of the order of 1.5 to 2.0. After extrusion, this density exceeds 10.5. The semi-finished product of lead obtained, for example in the form of a strip, is stable with respect to oxidation in air. It can be used as is in some applications.

Alternatively, lead can undergo fusion in the presence of sodium hydroxide, according to a well known technique.

The foregoing description relates to the electrolysis of a concentant solution of ferrous chloride. This feature is not essential. When the process is carried out without ferrous chloride in the electrolyte, chlorine is released at the anodes. The device used must therefore include a chlorine collection system. Such systems are well known in the electrochemical industries and are therefore not described in detail.

The current density can then have an increased value, between 800 and 2,000 A / m ² , preferably between 800 and 1,200 A / m ² .

The pH of the electrolyte is at an equi free between 1.2 and 1.7, at a temperature of 70 to 80 ° C. This pH depends on the concentration of sulfate ions and the current density. However, it may be advantageous to work at a pH of between 2 and 3 so as to avoid the proton electrolysis reactions to give hydrogen. In this case, a pH regulation system must then be provided by adding a base which is preferably chosen so as not to add foreign ions to the electrolytes. Preferably, the basic sodium compounds (sodium hydroxide, soda ash, or even basic lead compounds, lead hydroxide, litharge, basic lead carbonate, etc.) would be used.

Furthermore, the different parameters considered previously must have substantially the same values. They are therefore not described again in detail.

Although the process has been described with reference to the deposition of lead, it is not limited to this single metal. Indeed, the process also allows the formation of copper particles, under similar conditions and this in particular from cuprous chloride.

Another object of the present invention is to provide a device comprising a very large number of anode-cathode pairs placed in a non-isopotential manner. In fact, the process which has just been explained above involves the use of numerous pumps for recirculation. These numerous pumps lead to investment costs which can be significant. This is why we have sought to develop an electrolysis device which makes it possible to reduce the number of electrolyte recirculation pumps and which generally reduces the investment costs of the devices 'electrolysis.

These are non-isopotential devices derived from those whose implementation has already been carried out for the electro-refining of copper in a sulphate medium. We can refer to "Mining Annual Review" 1982, page 282 as well as to the article "Technoloqically advanced smelter incorporates latest design concepts", Journal of Metals, July 1978, pages 16 - 26.

The above electrolytic cell, commonly called "pool cell", is equipped with a series of substantially parallel rows of electrolytic cells between them. Each cell is made up of the couple formed by an anode surface and a cathode surface. Each anode and each cathode, except at the end of each row, belong to two cells. In the same row all the cells are mounted in parallel, in a comb (or rake), that is to say that all the anodes in the same row have the same potential, however, as all the cathodes in the same row are also at the same potential. The rows of said electrolytic-pool tank are mounted in electrical series; there is therefore a potential gradient in the tank-pool.

However, such a technology can only be used for electro-refining copper because of the very small potential difference between anode and cathode, of the order of 0.25 volts, and even so, it is necessary that the distance separating two rows of electrodes is approximately 0.5 meters, otherwise the energy losses due to stray current and / or cell-to-cell leakage become considerable.

This is why these devices described in the above articles had to be deeply modified to be adapted to the method according to the present invention in order to minimize the leakage currents and the various energy losses caused by these leakage currents.

According to the invention, it has been observed that when the cathodes of two different rows located in the same plane are connected by insulating partitions made of non-conductive material and when "anode channels" are formed by connecting the anode boxes between them by means of non-conductive material so as to isolate the anolyte from the catholyte, it is possible to produce these non-equipotential devices without the energy losses becoming too great as soon as the distance separating two rows of electrodes was included in be 0.8 and 2 meters and preferably between 1 and 1.5 meters. These insulating partitions have substantially the same height as the electrodes.

It should be noted here that the high current densities make it possible to significantly reduce the proportion of stray or leakage currents. However, during the implementation of such devices, phenomena liable to deteriorate the quality of the electrodes have been observed since the current density ceased to be homogeneous over the entire surface of the electrodes. This inhomogeneity leads to overvoltage phenomena on the edges of the cathodes, which results in parasitic reactions such as for example release of hydrogen and deposit of iron, which leads to modify the structures of the cathodes of which it was wrote above how important they were for the process according to the present invention.

This is why a study was necessary to remedy these harmful phenomena during the production of this non-isopotential device. This study showed that it was possible to eliminate or at least mitigate the phenomenon by shifting the anodes relative to the cathodes by a distance of between 5 and 20 centimeters, preferably around 20 centimeters, this shift being carried out in the direction of the decreasing potentials in the electrolysis tank.

These conditions, as described in Examples 6 and 7, allow excellent results to be obtained.

Thus, the present device, which can be called "tank-channel", in order to implement the invention is constituted by a series of rows of anodes and cathodes mounted in parallel, the anodes of each row being then offset in parallel themselves by a value of between 5 and 20 centimeters in the direction of the decreasing potentials in the electrolysis cell, the distance between two rows being between 0.8 and 2 meters.

Cathodes of different rows and located on the same plane are joined together by partitions made of insulating material so as to limit stray or leakage currents. Although this is less important, the anodes of different rows and located in the same anode channel can be joined together by partitions made of insulating material so as to limit parasitic and / or leakage currents.

The assembly forms a juxtaposition of channels parallel to each other and perpendicular to the rows of electrodes. Pump systems similar to those described in the present application require a circulation of the catholyte and the anolyte, while between each cathode and each anode there is the diaphragm which was mentioned during the description of the process.

It goes without saying that such a device can be used not only for the implementation of the present invention but also in any electrolysis device in which it is desired to carry out a forced circulation of the electrolytes.

We now consider more precisely the characteristics of an apparatus intended for implementing the method of the invention. The characteristics of the cathodes and anodes which have already been specified are not considered again. The device can be of the monopolar or bipolar type. Bipolar mounting has advantages because it reduces energy losses (by reducing ohmic drops in the electrodes and related structures), it reduces the cost of the electrodes since they have a double role, and it simplifies the mounting of sets of electrodes, while allowing better energy efficiency. This advantageous assembly, however, poses certain configuration problems at the ends of the electrodes, in particular to avoid leakage currents, as those skilled in the art know.

FIG. 1 is a diagram of an example of an apparatus for implementing the method according to the invention.

In the figures, the reference 1 designates a tank of electrolysis containing an anode box 2. The diaphragm is schematically identified by the reference 3.

The catholyte circulation circuit comprises a reservoir 4 and a circulation pump 5. The catholyte circulates parallel to the plane of the cathodes which are mounted in the tank 1.

The anolyte circuit includes a reservoir 6 and a pump 7 which circulates the anolyte.

Reference 8 designates a pump intended to extract part of the anolyte which has concentrated in ferric chloride and is suitable for the treatment of lead sulphide ores. Reference 9 designates the feed solution which restores the catholyte to the appropriate composition in the reservoir 4. The particles which detach from the cathodes fall to the bottom of the cell and are taken up by an endless screw 10 mounted on a shaft 11 driven in rotation by a motor 12. The particles arriving at the end of the screw arrive at a recipe 13 and are then treated as described above.

In the device schematically shown in the single figure, the nature of the electrodes and their mounting are as described above. The diaphragm and anodes also have the properties indicated above. When the solution does not contain ferrous chloride, a collecting hood must be mounted above the anodes so that it collects the chlorine which is released.

The adjustment of the weir makes it possible to maintain a difference in level between the catholyte and the anolyte, as indicated previously. The flow rates of pumps 5 and 7 are adjusted so that the speeds of the anolyte and the catholyte, along the anodes and cathodes, have the values indicated above, that is to say at least equal to 0.01 meter per second. The flow through the diaphragm is almost equal to the flow of the feed solution. In this way, the ferric iron can hardly pass into the catholyte. The additional feed solution flow rates pass by overflow from the catholyte tank 4 to the tank anolyte 6.

The cell preferably has a trapezoidal or rounded bottom so that the falling particles are guided towards the worm.

Although a worm driven by a motor has been shown, other mechanisms are suitable. For example, bucket elevators or conveyor belts can also be advantageously used. The product can also pass through an extruder which makes it undergo a prior densification, up to an apparent density of 3 to 6. The extruder can be provided with a die long enough for it to seal the liquid.

According to a preferred implementation of the invention, the metallic particles formed are recovered using a gooseneck operating in batch mode. In this case, the bottom of the cell is given a pyramid-like shape in order to direct the lead particles towards a swan neck which rises vertically along the cell. The liquid level in the gooseneck is in hydrostatic equilibrium with that of the electrolysis cell, that is to say that the point of rejection of the swan neck is located from 2 to 20 centimeters above the level of the surface of the catholyte: lead aggregates accumulate in the lower part of the swan neck, constituting a real plug; intermittently one or more ejectors, which can be produced by nozzles, are supplied by catholyte without solid at a rate sufficient to create a suction effect at the bottom of the cell and to reach a linear speed of flow of the liquid in the. gooseneck of at least 0.5 meters per second. The lead is entrained and recovered after separation of the liquid in a suitable system which is hydraulically disconnected from the electrolysis cell.

Lead agglomerates can also be entrained by air entrainment (air-lift). The ejector (s) are disposed under the swan neck at the appropriate locations known to those skilled in the art to obtain a good "suction" or "air-lift".

The following nonlimiting exemplary embodiments of the present invention are intended to enable specialists to easily determine the operating conditions which should be used in each particular case.

Example 1

A sulfurized raw material consisting of a galena concentrate, containing 75.5% lead, 0.70% zinc, 0.85% copper, 1.40% iron, is treated with a solution of ferric chloride and sodium. , 1.0% calcium and 0.6% magnesium.

After purification, the electrolyzer supply solution and the electrolyte have the following compositions:

The electrolysis is carried out in an installation of the type shown in the figure; the circulation speed of the catholyte is 0.06 meters per second and that of the anolyte is 0.01 meters per second. The cathodes are made of smooth titanium. The current density, in steady state, is 550 A / m ² . The distance between the electrodes is 70 millimeters.

It is found that the lead obtained is in the form of particles having a length of the order of 300 to 600 micrometers and does not adhere to the cathodes. The faradaic efficiency observed is 95%, and the energy yield is 0.57 kWh per kilo of lead.

The purity of the lead obtained on the one hand in simply laminated form and on the other hand in the form of an ingot is as follows:

Example 2

The same installation is used and an electrolyte of the same composition as in Example 1. The circulation speed of the catholyte is 0.10 meters per second and that of the anolyte of 0.02 meters per second. The current density used is 850 A / m ² and the distance between the electrodes is the same as in Example 1.

The lead produced is similar to that described in Example 1. The energy efficiency of electrolysis is 0.74 kWh per kilo.

Example 3

An installation similar to that of Example 1 is used. The cathodes are formed from smooth titanium and the anodes from expanded titanium covered with ruthenium oxide. The distance between them is 70 millimeters. The anodes are placed in an anode box in which the anolyte does not circulate. The pressure difference between the anolyte and the catholyte is 20 millimeters of water column. The installation is intended to allow the recovery of chlorine.

In this example, the lead content of the electrolyte is maintained by continuous introduction of crystallized lead chloride. The crystals contain the following impurities, expressed in grams per tonne:

• densité de courant : 1 000 A/m²
• température : 75°C
• vitesse linéaire du catholyte : 0,04 mètre par seconde.

The electrolysis conditions are as follows:

• current density: 1000 A / m ²
• temperature: 75 ° C
• linear speed of the catholyte: 0.04 meters per second.

The energy efficiency of electrolysis is 1 kWh per kilo of lead. The lead particles form a powder with an apparent density of between 1.5 and 2.5 and contain 20 to 30% by weight of occluded electrolyte. After densification with a rolling mill, this electrolyte is extracted from the powder.

The following table indicates not only the composition of the electrolyte but also the purity of the products obtained, on the one hand after rolling and on the other hand after shaping an ingot.

Example 4

The operating conditions are identical to those of Example 3, but the electrolyte contains 10 grams per liter of sulfate. At this concentration, the electrolysis is not disturbed by the sulfate ions and the energy yield remains substantially equal to 1 kWh per kilo of lead deposited.

The lead particles obtained have the same purity and the same level of occluded electrolyte as in Example 3.

Example 5

Identical operating conditions are used to that of Example 4, but the current density is brought to 1500 A / m ²

The energy efficiency reaches 1.24 kWh per kilo of lead deposited. The purity of the lead particles obtained and the characteristics before densification remain the same as in the previous example.

Example 6: Assembly of electrodes of the same type as standard electric in the same tank

In a laboratory electrolysis cell 2 meters long, 0.15 meters high and 0.03 meters wide, the magnitude of the leakage currents between two cells was evaluated. Each cell consists of an anode and a cathode. The electrolysis of copper sulphate was chosen to facilitate the measurements which essentially relate to the evolution of the leakage currents and the distribution of the current density at the surface of the cathodes. In fact, in a sulphate medium, the copper deposits are compact and the faradaic yield of the deposits very close to unity in a current density range of 200 to 300 amperes per square meter. Under these conditions, it is possible, by cutting the deposit into strips of equal width, to determine from the weight of each, the average current density of electrolysis on each surface element and thus to know the distribution profile of the average current density at the surface of the cathodes.

FIG. 2 represents the experimental device used. The copper sulphate solution is kept in circulation between the tank 4, heated 5 and the channel type electrolysis tank 1 by the centrifugal pump 6. Each cell 2 consists of a lead anode and a steel cathode stainless, spaced 1.6 cm apart. In the tank 1, one, two or three cells 2 can be mounted in electrical series and the spacing L between each cell can vary.

In FIG. 3, the diagram mainly represents the electrical connections between the anodes 7 and the cathodes 8. Each cell 2 is connected externally by a conductor 9. Between each cell 2 there is a leakage current I _F which decreases the overall energy efficiency of the electrolyser and which disturbs the distribution of the current density on the edges of the electrodes, mainly between the anode of one cell and the cathode of the neighboring cell.

The following table shows the main results obtained with an electrolyte containing 40 grams per liter of copper and 165 grams per liter of sulfuric acid. All the experiments were carried out at a temperature of 40 ° C for 15 to 20 hours.

On a small scale, the leakage currents are an important relative value with respect to the intensity of the current supplied by the rectifier 3. This relative importance will be considerably attenuated on a larger scale.

The graph below gives by way of example the average density profile obtained on the cathodes 8 for test 2.

Example 7

The overcurrent on the edges of the cathodes is not acceptable due to the increase in the local electrode overvoltage which may cause the appearance of parasitic reactions.

This major drawback has been overcome by shifting the vertical axes of the anodes and cathodes of each cell and by imposing a calculated intensity taking into account the selected current density and the facing electrode surfaces. The objective is to be able to ensure a true current density at the surface of the cathodes, less than the chosen current density.

This type of assembly was tested with the experimental device described above. The following table shows the comparative results between two electrical assemblies of three cells spaced 0.63 meters apart, one with the electrodes offset and the other with the electrodes not offset in each cell.

The following figures show the distribution profiles of the true current density for the cathodes with or without offset.

Claims (24)

1. Process for the preparation of a metal by electrolysis in a diaphragm cell, of the type which comprises: the formation of an electrolyte containing a chloride of the metal to be prepared and at least one chloride of an alkali or alkaline-earth metal, and - the circulation of the electrolyte between the electrodes, parallel to the surface of a cathode,
characterized in that: - the cathode surface is arranged in a substantially vertical direction and has a sufficiently low density of nucleation sites so that the metallic particles which form from these sites keep their individuality vis-à-vis adjacent particles, until they reach a dimension of at least about 100 micrometers, the flow of the electrolyte along the cathode surface is of the laminar or slightly turbulent type, - so that, under the action of their weight and the drag forces exerted by the electrolyte current, the metal particles detach and fall into the electrolyte, and - The process includes the removal of metal particles gathered at the bottom of the cell. 2. Method according to claim 1, characterized in that the metal to be prepared is lead. 3. Method according to claim 2, characterized in that the lead is present in the electrolyte in the form of chloride in an amount between about 5 and 50 grams per liter, preferably between about 15 and 25 grams per liter. 4. Method according to one of claims 2 and 3, characterized in that the surface of the cathode is formed of titanium, stainless steel or graphite. 5. Method according to any one of claims 2 to 4, characterized in that the electrolyte also contains iron in the form of chloride, essentially of ferrous chloride. 6. Method according to claim 5, characterized in that the concentration of iron in the form of chloride in the electrolyte is between 20 and 60 grams per liter. 7. Method according to any one of the preceding claims, characterized in that the concentration of at least one chloride of alkali or alkaline-earth metal is between 4 and 5 equivalent grams per liter. 8. Method according to any one of the preceding claims, characterized in that said alkali or alkaline-earth metal chloride is sodium chloride. 9. Method according to any one of the preceding claims, characterized in that the density of the electrolyte electric current is between 500 and 1500 A / m ² and preferably between 700 and 1000 A / m ² . 10. Method according to claim 9, characterized in that it comprises the progressive or stepwise increase in the current density up to a steady state value. 11. Method according to any one of the preceding claims, characterized in that the electrolyte circulates along the cathode surface with a speed of between 0.01 and 0.15 meters per second. 12. Method according to any one of the preceding claims, characterized in that the removal of the particles collected at the bottom of the cell comprises the transport of the particles out of the cell and their densification by compression. 13. The method of claim 12, characterized in that it further comprises, after densification, the rolling of the particles so that most of the electrolyte inclusions are removed. 14. Method according to one of claims 12 and 13, characterized in that it comprises the fusion of the densified particles, in the presence of soda. 15. Apparatus for the preparation of metal by electrolysis, of the type which comprises a diaphragm cell, characterized in that: - the cathodes and the anodes are arranged vertically, the cathodes are formed from a material chosen from the group which comprises titanium, stainless steel and graphite, and the device comprises at least one pump intended to circulate a current of electrolyte of laminar or slightly turbulent type along the cathodes, and - a transport device intended to remove divided solid matter which may fall to the bottom of the cell. 16. Apparatus according to claim 15, characterized in that it comprises bipolar electrodes. 17. Apparatus according to claim 15, characterized in that the anodes are formed of expanded metal. 18. Apparatus according to any one of claims 15 to 17, characterized in that it further comprises a chlorine gas recovery hood. 19. Apparatus according to any one of claims 15 to 18, characterized in that the transport device is chosen from the group which comprises a gooseneck, a worm screw, a bucket elevator and a conveyor belt. 20. Apparatus according to claim 19, characterized in that it further comprises an extruder intended to densify the divided solid matter displaced by the transport device. 21. Semi-finished product of lead, characterized in that it is prepared by a process according to any one of claims 1 to 14. 22. Lead semi-product according to claim 21, characterized in that it contains at most 0.2% by weight of inclusions of an electrolyte containing chloride ions. 23. Non-isopotential device constituted by a series of rows of anodes and cathodes connected to the same current supply, characterized in that the electrodes of two different rows located in the same plane are connected by partitions of non-conductive material, the distance separating two rows of electrodes being between 0.8 and 2 meters. 24. Device according to claim 23, characterized in that the anodes are offset with respect to the cathodes by a distance of between 5 and 20 centimeters, preferably around 20 centimeters, this offset being made in the direction of the decreasing potentials in the electrolysis tank.

Images