US3049478A - Process for the production of pure indium - Google Patents

Description

Aug. 14, 1962 w. MORAWIETZ PROCESS FOR THE PRODUCTION OF PURE INDIUM Filed June 26, 1961 INVENTOR.

' Wilhelm Morawiezz nited States atent 3,049,478 PROCESS FOR THE PRODUCTION OF PURE INDIUM Wilhelm Morawietz, Duisburg, Germany, assignor t Duisburger Kupt'erhiitte, Duisburg, Germany, a German corporation Filed June 26, 1961, Ser. No. 119,434 Claims priority, application Germany July 12, 1960 22 Claims. (Cl. 204-64) The present invention rel-ates to a process for the continuous electrolytic production of metallic indium of the highest purity from indium salt solutions, and more particularly to such a process wherein a reflux is employed to enhance the purity of yield of indium produced by the overall process.

Present-day requirements demand metallic indium of the highest purity. Highest purity indium is especially desired for the production of semiconductors. As is known, indium is capable of being electrolytically deposited from its aqueous salt solutions at the cathode of an electrolysis cell. Such electrolytic deposition, as a rule, is coupled with a purifying effect with respect to the cathodic indium, the impurities remaining in the electrolyte. This purifying effect is further aided by the fact that impure indium can be freed from a part of its impurities by subjecting the impure metal to anodic dissolution in an electrolytic cell, whereby the impurities either remain on the anode or pass into the electrolyte, such that upon reversing the polarity of the system subsequent cathodic deposition of indium may take place without contamination of the impurities present on the anode or remaining in the electrolyte.

It has been recognized that the effect of the purifying process of indium by means of re-electrolysis can be improved by carrying out the purifying process in several successive stages, whereby the cathodic indium obtained in a prior stage is subjected to anodic dissolution in the next stage. As a consequence thereof, an electrolytic step-by-step improvement in the purity of indium is achieved.

One of the major difliculties which occurs in the electrolytic deposition of indium from aqueous solutions where comparatively greater thicknesses of sediment are to be deposited, usually on solid electrodes, is that the indium is deposited at the cathode unevenly. The indium deposit builds up in the form of so-called dendrites, such that the cathode metal extends toward the anode over comparatively short periods of electrolytic operation. Consequently, a short circuit between the opposing anode and cathode is soon caused and the electrolysis is interrupted.

The accompanying diificulties which occur on account of the dendritic deposition of indium, at the cathode, can be overcome in the intermediate stages of a multi-stage electrolysis by not depositing the indium in its pure form in the first and in all the subsequent stages save the last and by first placing mercury at the cathode as an alloying element. The reason for employing mercury at the cathode is due to the fact that indium is soluble in mercury to a high degree such that the indium can be later dissolved out or separated anodically and selectively from the indium amalgam, obtained, cathodical-ly, in view of the great difference in the normal potential between indium and mercury being subjected to electrolytic conditions.

While the use of mercury may avoid the formation of dendrites in all but the last stage of electrolysis, the formation of dendrites at the cathode of the final deposition stage, however, and the difiiculties caused thereby remain quite undirninished since mercury is, of course, not used in the final stage. Attempts have been made to overcome such difiiculties by choosing a suitable electrolyte'in the final stage. The avoidance of these difliculties, nevertheless, cannot be. achieved successfully where his sought to deposit comparatively larger amounts of pure'indium in the final stage.

One of the usual disadvantages attending the conventional processes for the electrolytic production of purest indium is the fact that the impurities, which are primarily introduced into the "electrolysis with the impure indium, become enriched in the electrolyte, even in the electrode amalgam of the intermediate stage, and hence increasingly contaminate the final product during the course of operation. As a result, after a more or less short period of time, electrolysis has to be terminated and the electrolyte in the final stage replaced. Therefore, indium salt or indium metal of the highest purity must be used for the production of the electrolyte in the finalstage in order that contamination from this source will not aggravate the condition caused by the enrichment of impurities from the impure indium starting material used. Understandably, because of the foregoing disadvantages, purest indium has only been produced by conventional techniques in small laboratory amounts with poor yields regarding the amount of crude metal introduced as starting material in the electrolysis.

In a special arrangement for the electrolytic purification of metals with the aid of intermediate amalgam stages, in order to produce primarily pure indium, static amalgam electrodes have been used which serve the func tion of a cathode in the first stage and simultaneously the function of an anode in the next stage in succession. The indium must pass from the cathode surface of one stage to the anode surface of the next stage by diifusion through the amalgam. Disadvantageously, in the course of this diffusion, undesirable polarization phenomena occur which lessen the purifying effect of the electrolysis, unless the arrangement is operated with only a very low current density and correspondingly low rate of yield of the metal to be obtained.

In another arrangement, also using amalgam as an intermediate stage, the indium is deposited at a fixed cathode in the final stage.

None of the foregoing conventional processes have actually solved the problem of purification of the electrolyte in the final stage and of the avoidance of ,formation of dendrites at the cathode in such a way that the continuous production of purest indium material can be carried out effectively and in large yields.

It is an object of the present invention to overcome the foregoing disadvantages and to provide a process for the continuous electrolytic production of metallic indium of the highest purity from indium salt solutions in amounts significantly higher than the laboratory amounts heretofore obtained, and more economically than was heretofore the case.

Other and further objects of the invention will become apparent from a study of the within specification and accompanying drawing, in which the FIGURE represents .a schematic view of an electrolytic cell arrangement, including a series of successively connected cells for carrying out the electrolytic purification of indium metal against a reflux of electrolyte and mercury,

It has been found in accordance with the present invention that a process for the continuous electrolytic production of metallic indium of the highest purity from indium salt solutions may be provided economically and with comparatively high yields of pure indium metal. The process comprises electrolyzing an impure indium salt solution in a first-stage electrolysis using mercury as cathode whereby to form a first amalgam of mercury and electrolytically separated metallic indium, passing said amalgam to an intermediate stage electrolysis for 3 electrolysis therein, using an indium salt solution as electrolyte, said amalgam as anode and additional mercury as cathode, whereby to dissolve electrolytically said metallic indium from said amalgam into said electrolyte and in turn electrolytically deposit said indium in said additional mercury as cathode to form further amalgam, passing electrolyte from said intermediate stage electrolysis to said first-stage electrolysis for combining with said impure salt solution for further electrolysis therein, passing mercury remaining upon electrolytic removal of metallic indium from said first amalgam as anode from said intermediate stage to said first stage for use therein as cathode, passing said further amalgam to a last-stage electrolysis using said further amalgam as anode, molten indium salt as electrolyte and molten pure indium metal as cathode for dissolving electrolytically said metallic indium from said further amalgam into said molten indium salt and in turn electrolytically depositing said indium in said molten indium metal as cathode, and thereafter recovering pure indium metal from said last stage.

Preferably, the electrolyte in the first and intermediate stages is indium halide solution and that in the last stage is molten water-free indium halide. The indium halide may be indium chloride, indium bromide, indium iodide, or indium fluoride. Advantageously, the last stage electrolyte is maintained at a temperature above its own fusion point and above the fusion point of indium. The last stage electrolyte may comprise a low-melting mixture of indium halides including one or more of indium chloride, indium bromide, indium iodide, and indium fluoride. The last-stage electrolyte may additionally include an alkali halide, an alkali earth halide, or a mixture of one or more of these halides.

The electrolyte is preferably agitated in the intermediate stage in order to more effectively transfer indium passing from the anode amalgam to the cathode mercury for electrolytically effecting the flow through the succession of stages. In the same way, the amalgam is pumped from one stage to the other in a lively manner for increasing the rate of exchange.

The electrolyte of the first stage may be produced in accordance with one feature of the invention by anodic dissolution of impure indium metal. In accordance with a further feature of the invention, electrolyte is added to the intermediate stage in the form of aqueous hydrohalic acid, such as hydrochloric acid, hydrobromic acid, hydroiodic acid, and hydrofluoric acid. Such acid preferably has a concentration of 3-8 normal. In this connection, mercury may be added to the intermediate stage substantially free from indium for passage against the electrolytic flow of indium being purified.

In the case of the electrolyte in the last stage, the same may the produced, if desired, by the action of a halogen gas or a hydrogen halide gas on the further amalgam passed to the last stage. In this way, the indium halide salt solution is formed in molten water-free condition. The particular gas used may be rarefied by an inert gas or hydrogen, if desired.

In accordance with a particular feature of the invention, the first-stage electrolysis includes at least two individual electrolysis cells connected in series, the electrolyte and amalgam being passed through the series of cells in this stage in opposite directions. In this manner, a more effective utilization of the mercury for cathodically removing indium from the electrolyte may be carried out.

In accordance with a preferred embodiment of the invention, therefore, a process is provided for the continuous electrolytic production of metallic indium of highest purity from indium salt solutions, which comprises electrolyzing an impure indium salt solution being passed to a first-stage electrolysis using mercury as cathode whereby to form a first amalgam of mercury and electrolytically separated metallic indium, passing said amalgam to an intermediate stage electrolysis for electrolysis therein using an indium salt solution as electrolyte, said first amalgam as anode and additional mercury as cathode whereby to dissolve electrolytically said metallic indium from said first amalgam into said electrolyte and in turn electrolytically deposit at least a portion of said indium in said additional mercury as cathode forming further amalgam, passing a portion of said indium salt solution enriched by a portion of said metallic indium dissolved electrolytically therein from said intermediate stage electrolysis back to said first stage electrolysis for combining with said impure indium salt solution for further electrolysis therein, passing mercury remaining upon electrolytic removal in said intermediate stage electrolysis of metallic indium from said first amalgam as anode from said intermediate stage back to said first stage for use therein as cathode, passing said further amalgam to a laststage electrolysis using said further amalgam as anode, molten indium salt as electrolyte, and pure molten indium metal as cathode for dissolving electrolytically said metallic indium from said further amalgam into said molten indium salt and in turn electrolytically depositing said indium in said molten indium metal as cathode, recovering a portion of pure indium metal corresponding to that produced in said electrolysis from said last stage, recovering a portion of electrolyte from said first stage, adding fresh electrolyte to said intermediate stage to replace that passed to said first stage and corresponding to that recovered from said first stage, recovering a portion of mercury being passed from said intermediate stage to said first stage, and adding fresh mercury to said intermediate stage, corresponding to that recovered from said first stage.

Accordingly, the present invention provides for the removal of impurities from an impure indium starting material by mechanically directing a flux or flow of electrolyte and electrode mercury against the conveyance of indium to the cathode of the final deposition stage by the electrolytic passage of indium successively through the various stages. The flux or flow carries back the impurities along with a portion of the indium being subjected to electrolytic action and eventually removes these impurities from the process. The dissolution and deposition steps occurring at the mercury and amalgam electrodes are effected with small phase passage potentials and, hence, with a high selectivity for the Nernst potentials of the metals present in the electrode material as well as in the electrolyte.

It will be appreciated that where only one stage of electrolysis is employed, it is necessary to remove from the process a considerable minimum amount of indium in reflux in order to attain desirably a certain degree of purity of the metal finally recovered. Where several stages of electrolysis are used in succession, on the other hand, so that in each stage the indium is anodically dissolved and cathodically deposited again, such that the metal cathodically obtained is used for the anodic dissolution in the next stage, the amount of indium which must be recirculated by means of the reflux flow and finally removed from the electrolysis may be reduced yet the same purifying effect achieved if the following requirement is met. The reflux of the mercury and electrolyte must successively pass the electrode sumps and the electrolytes, respectively, of the individual cells in that order of succession which is opposite to the direction in which the indium is conveyed to the final stage through the succession of stages by means of the electrolytic action or specifically the electric field adjoining the electrolysis stages.

In the case of a large number of stages, connected in series, laws of distribution prevail which are similar to those in the case of fractional distillation in a distilling column or in the case of counter-current extraction in an exchange column. Hence, the less precious metals are selectively or preferably dissolved out of the anode mercury and deposited into the next cathode mercury to a lesser extent than the indium, in consequence of which these less precious metals become concentrated in the reflux flowing electrolyte. It is just the reverse in the with reflux mercury.

case of the more precious impurities which selectively or preferably remain deposited in the anode mercury in the case of anodic dissolution of indium out of such metal, while in the case of cathodic deposition of indium, they too selectively or preferably enter or dissolve into the cathode mercury. Consequently, the mercury becomes enriched with the more precious impurities in the same way as the electrolyte becomes enriched with the less precious impurities.

For the purpose of continuously separating both the more precious and less precious impurities for an unlimited length of time, in accordance with the process of the invention, parts of both the electrolyte and the electrode metal are continuously recirculated from the final stage of the succession of electrolyses to the initial stage, and removed from the process at that point. A continuous reflux of the electrode metal can be effected with particular ease using liquid metal. It is on account of the ready solubility of indium in mercury, that mercury is chosen as the metallic solvent electrode for indium.

The magnitude of the counter-currents of electrolyte and liquid electrode materials which are necessary in order to remove the less precious and the more precious impurities, depends upon the number of stages of electrolysis and upon the desired degree of purity of the finally recovered indium metal. By increasing the number of stages, or by enlarging the counter-current, it will be appreciated that any such improvement in the purifying effect may be achieved. In so doing, it is not necessary for the purpose of feeding the counter-current to feed into the final stage pure indium for the electrode metal or pure indium salts for the electrolytes. Instead, it will suffice to add to a prior stage mercury as solvent for the indium metal and a pure acid, such as, for example hydrochloric acid, as electrolyte. Both of these substances can be readily and economically produced by conventional methods with suflicient purity for utilization in accordance with the invention, i.e. mercury with impurities less than 0.01 ppm. and hydrochloric acid distilled in a quartz apparatus with a still higher degree of purity. Due to the side reactions which normally occur at the electrodes, especially in the intermediate stages, for example by cathodic evolution of hydrogen, the electrolyte advantageously forms in the process itself.

It is convenient for the purpose of obtaining a good yield of metallic indium with an optimum degree of purity to adjust with respect to one another the power-loads in the individual stages of electrolysis with due regard to the side reactions at the electrode, the feeding of the first stage with primary electrolyte containing indium in impure form, the feeding of the last stage but one with reflux electrolyte, and that of the last amalgam circulation In this way the concentrations in the electrolytes of the individual stages and in the amalgam electrode are maintained at a convenient and suitable level.

It has been found to be convenient for the purposes of obtaining a high electrolytic efficiency in the cathodic deposition of metals to use high indium concentrations in the electrolyte, i.e. concentrations not below about 50 grams of indium per litre of solution. However, as high a proportion as possible of indium primarily introduced into the electrolysis is still capable of being cathodically deposited where the initial electrolyte fed to the electrolysis possesses a very high indium concentration, i.e. from 400500 grams per litre, and where the primary stage is divided into several individual cells. In this manner, the indium content of the electrolyte may be reduced in stages to the final concentration of about 50 grams per litre, so that, even allowing for the losses due to the countercurrent reflux and considering the decrease in volume which is caused by the removal of the metal and of the halogen from the electrolyte, about 80-90% of the indium, primarily fed to the electrolysis with the initial elec- 6 trolyte, can be obtained in the final stage as purest metal. Of course, the electrolyte and the cathode amalgam will flow through the series of cells in this primary stage in opposite directions in order to achieve a more efiicient exchange in the manner of a conventional extraction technique.

In the final deposition of the pure indium in the last stage of electrolysis, the difficulties which were encountered by the continuous carrying out of the process due to the formation of dendrites at the cathode as well as through the necessity of frequently replacing the cathode are overcome in consequence of the present invention by operating this stage at a temperature above the melting point of indium. Such operation offers as well all the advantages of a molten cathode whereby the metal obtained electrolytically can be continuously separated from the electrolysis, as for example via a Syphon. At such an electrolytic temperature above the melting point of indium, of course, an aqueous electrolyte can no longer be used.

Surprisingly, however, it was discovered that in the case of indium, the anhydrous halides can readily be used in the molten stage as an effective electrolyte in the last stage, this feature being contrary to expected performance based upon the analogy of the behavior of aluminum, 2. metal also recovered by electrolytic purification. In the presence of molten metallic indium, the metal present in the molten anhydrous indium halide has an average valence of 1.0 to 1.5. Therefore, a smaller amount of current is required for the deposition of a certain amount of indium than would be required were the electrolysis to be carried out in aqueous solution.

Tne mercury used for the production of the amalgam reflux, in accordance with the over-all process of the invention, is conveyed to the mercury cathode of the penultimate stage of electrolysis or to the amalgam anode of the last stage of electrolysis, or to the circulation between these two stages whereas the electrolyte used for the production of the electrolytic reflux is conveyed to the electrolyte of the penultimate stage only.

The melting point of low indium halide amounts to about 250 degrees C. and is sufficiently low in itself for use in the final stage in accordance with the present invention. The melting point can be further lowered, nevertheless, by admixing several indium halides as well as by admixing with the particular indium halide or mixture of halides one or more alkali halides and/or alkali earth halides such as sodium, potassium, and lithium halides and/or calcium and barium halides. Consequently, the electrolysis in the final stage can be carried out at a temperature approximating 200 degrees C. The iowerin g of the temperature of the final stage electrolysi might seem negligible except that its advantage lies in the fact that in the final stage less mercury is conveyed with the indium from the anode through the electrolyte to the cathode, whereby less mercury will be found present as an impurity in the pure indium recovered from the last stage. The mercury alloying constituent in the deposited indium in the last stage can be reduced by employing in the last or final stage a large anode surface and a small cathode surface.

Any small amount of residual mercury, !which.rernains in the finally deposited indium, can be readily removed by heating the tapped metal to about 800 degrees C. in a vacuum, in a current of hydrogen, or in a current of another inert gas. In this manner, the indium metal obtained will be of highest purity and will be subject to no restrictions on its use for semi-conductor purposes.

'Indium halides are hygroscopic and at increased temperatures are sensitive to air. Thus, it is necessary for carrying out the electrolysis in the last or final electrolytic cell to protect such cell from any access to air. This may be achieved by filling the cell with an inert gas. The simplest way of producing the halide which serves as aoaaavs electrolyte in the final stage, in this connection, is to prepare it in the cell itself by means of the action of halogen or hydrogen halide, which can be rarefied, if desired, with hydrogen or with an inert gas, on the amalgam obtained in the course of electrolysis.

Where, in the final stage, smelting electrolysis is carried out using a molten indium salt as electrolyte and molten indium metal as cathode, the electrolytic countercurrent or reflux is carried out by introducing an electrolyte into the penultimate stage of electrolysis. It will be appreciated, in this connection, that for the purpose of preventing aqueous electrolytic traces of materials from penetrating into the final stage from the aqueous electrolysis of the penultimate stage, separator devices are employed, such as those of non-wettable synthetic material, particularly those of Teflon, a chemically inert synthetic material. Separators of the foregoing type may be conveniently interposed in the amalgam circulation stream in order to prevent short circuiting between mercury material used as anode and mercury material used as cathode.

The primary object of the final or last stage operated with a smelting electrolyte is to eflect the substantially complete separation of indium from mercury added as an auxiliary material for conveying the indium through the succession of cells. The separation of indium from the impurities present in the starting material is already sufticiently complete at this point and is of no great concern. Nevertheless, the final stage may also be used for this purpose if fresh electrolyte is continuously produced from the anode amalgam in the final stage by the action of halogen or halogen compounds, such as, for example, hydrogen halides free from detrimental impurities, and if a corresponding portion of the electrolyte is conveyed into the penultimate stage, where it is dissolved partially in the electrolyte, partially in the amalgam, with disproportionation occurring, i.e. In(l)halide is decomposed into In(III)halide solving in the electrolyte and metallic indium, solving in the mercury.

The phase passage processes at the electrodes, in accordance with the present invention, take place sufliciently selectively up to very high current densities. In order to avoid harmful concentration polarizations, detrimental to the purifying effect of the electrolysis, it is convenient to repump the electrolyte between the anode space and the cathode space of each stage in a turbulent manner. The same holds true for passing the amalgam between the cathode space of one cell and the anode space of the cell of the next adjacent stage. In this manner, current densities of up to 1,000 a./m. can be employed without being noticeably detrimental to the selectivity of the deposition of impurities.

In the primary stage, the anodic current density must be less if the cell is fed with an electrolyte of indium chloride which has been produced outside of the electrolysis. In this anodic evolution of chlorine in the first electrolysis stage, it is convenient to use electrodes of graph ite. Current density and anode wear amount to an order of magnitude similar to that of the aqueous chlorinealkali hydrolysis of the conventional type. In using insoluble anodes with anodic evolution of chlorine in the first stage, it is convenient to renew continuously only the surface of the cathode amalgam by means of repumping while only little motion should be effected in the electrolyte in order not to decrease the electrolytic efficiency at this point by reducing the chlorine at the cathode which ha been anodically evolved and dissolved in the electrolyte. Impure indium metal itself can also be used as an anode in the first stage if desired. In such case, the feeding of a foreign electrolyte may be partially or even completely omitted since the indium metal will produce the required electrolyte concentrations necessary for carrying out the over-all process of the invention.

Where an electrolyte is employed in the intermediate stages having a comparatively high indium concentration, i.e. of more than 100 grams per litre of indium, ad-

vantageously part of the indium is present in such aqueous solution with a valence of less than 3 due to the reaction with the cathode metal. Accordingly, with regard to trivalent indium, electrolytic etficiencies higher than are reached. The acid added in the penultimate stage, in order to produce the reflux is only partially decomposed by cathodic separation of hydrogen such that the electrolyte remains suificiently acid that hydrolysis will not occur even if there are high indium concentrations. Thus, for the purpose of obtaining high indium concentration in the electrolyte of the intermediate stages, it is convenient to add the acid which is introduced for the purpose of effecting the electrolytic reflux with a sulficiently high concentration preferably from 3 to 8 normal. If hydrochloric acid is used, the simplest way of attaining the required purity is by distillation or by introducing gaseou HCl and adding water to the electrolyte of the penultimate stage.

It will be appreciated that Where amalgam is used, it is normally exposed to a certain danger of slagging in any process. In accordance with the present invention, however, there is little such danger in the counter-current electrolysis since the electrolyte is sufficiently acid. Nevertheless, if need be, such danger may be avoided by shutting off the electrolytic cell from the atmosphere and filling such cell with an inert gas. Conveniently, the inert gas to be used may be the hydrogen evolved by means of the decomposition of the acid added for the purpose of producing the counter-current, if such hydrogen successively passes through the gas volumes or spaces of the individual cells. For convenience and effectiveness, such inert gas should be conveyed in counter-current flow to the direction of indium being conveyed through the electric field along the series of successive electrolysis stages.

The invention will be further illustrated in conjunction with the accompanying example taken in connection with the accompanying drawing, but it will be appreciated that the invention is not to be limited thereby.

Example The electrolytic deposition and redissolution of indium is effected in four stages: 1, 2, 3, and 4. The first stage is divided into three individual cells: 1a, 1b, and 1c. The impure initial electrolyte, an indium chloride solution having an indium content of 400 grams per litre and a content of free hydrochloric acid of 10 grams per litre, is fed into the cell 1a at 5 and successively flows through the cells 1a, 1b, and 1c such that the indium content is reduced to 50 grams per litre, due to the deposition of indium from such solution at the mercury cathodes 6, 7 and 8 of these cells and the evolution of chlorine at the graphite anodes 9, 10, and 11 disposed in opposing relation to the cathodes in the respective cells. The indium chloride electrolyte having a concentration of 50 grams per litre leaves cell 10 at 12.

The cathode amalgam is successively drawn through cells 1c, 1b, and 1a in countercurrent direction to the flow of electrolyte through the cells. The cathode amalgam is conveyed by means of a pump 13 through an overflow device 14, preventing a short circuit with the anode 15 of stage 2, to which the cathode amalgam is circulated for again dissolving anodically the indium from such cathode amalgam now maintained as an anode in stage 2. By means of a further overflow device 35, used for insulation of the anode amalgam of stage 2, from the cathode amalgam of cell 1c, the mercury is returned to the cathode of cell 10. A level controller device 34 is interposed in the circulation of mercury between the anode of stage 2 and cell 10 for purpose of maintaining constant the amount of amalgam in circulation.

The indium, dissolved in stage 2 from the anode 15 is conveyed via the electrolyte 16 into the cathode amalgam 17. The stirrer device 18 repumps in a turbulent manner the electrolyte of stage 2 between the cathode space and the anode space thereof, the partition Wall 19 situated in stage 2 preventing the mixing of cathode mercury 17 with anode mercury 15.

However, there is an exchange between cathode mercury or amalgam 17 on the one hand and anode amalgam or mercury 21 of stage 3 on the other hand by means of the pump 20. The cathode amalgam 22 of stage 3 is exchanged in turn for the anode amalgam 24 of final stage 4 by means of a circulating pump 23.

The electrolyte 25 of stage 4 consists of molten waterjfree indium halide and is kept at a temperature of 200-300 degrees C. The overflow 36 of the amalgam circulation between stages 3 and 4 consists of a synthetic material,.non-wettable with water, and prevents the penetraction of leaking traces of the electrolyte of stage 3 from passing into stage 4. The cathode metal 26 in stage 4 consists of purest molten indium, the amount of which is maintained constant by means of the outlet discharge at 27. The pure metal is discharged at 27 in proportion to the amount of increase of the cathode metal 26 caused ,by the electrolysis. The pure metal discharged at 27 may be cast into bars at this point. The electrolytic countercurrent or reflux is produced by means of continuously adding hydrochloric acid, forexample, of about normal to the electrolyte of stage 3 at 28. The electrolytes of stages 3, 2, and 1b are in communication with one another via the pipes 29 and 30. As a result of the arrangement .shown, no redifiusion of electrolyte is possible from cell 1b to stage 2 or from stage 2 to stage 3. Consequently, there flows through pipes 29 and 30 an electrolytic reflux which corresponds to the amount of hydrochloric acid .added at 28.

In stage 3, and to a lesser extent, in stage 2, an evolution of hydrogen occurs at the amalgam electrode in addition to the cathodic deposition of indium. Therefore, the hydrochloric acid added at 28 for producing the electrolytic reflux or counter-current, is converted on its way through stages 3 and 2 to an acidic indium chloride solution having an indium content of about 100 grams per litre. With regard to its concentration, the same roughly corresponds to that obtained by the feed electrolyte, added at inlet 5, on its Way through tne first stage in cell 1]). The reflux electrolyte leaves the apparatus at 12 along with the feed electrolyte which has at this point been completely electrolyzed. The mercury serving to produce the amalgam counter-current or reflux is added at 31 to the circulation between the anode amalgam 24 of stage .4 and the cathode amalgam 22 of stage 3. The amount of such amalgam is thereby increased, so that via the level controller and pump device 32 a portion of amalgam is introduced into the circulation between stages 2 and 3. In the same way, a portion of amalgam corresponding theretois introduced via the level controller device and pump .33-into the circulation between stages 1 and 2. Such amalgam corresponding to the portion introduced at 31is eventually completely removed from the process during the circulation of amalgam between stages 2 and .1 by means of the level controller device 34. It will be appreciated, in this connection that a similar level controller device effecting electric insulation as that of device 35 between anode and cathode 8 is provided as well between cathode 17 of stage 2 and anode 21 of stage 3 and between cathode 22 of stage 3 and anode 24 of stage 4.

For the same purpose, device 36 is interposed in the circulation of pump 23 between anode 24 of stage 4 and cathode 22 of stage 3, device 14 is interposed in the circulation of pump 13 between anode 15 of stage 2 and cathode 6 of cell In, and device is interposed between anode 21 of stage 3 and cathode 17 of stage 2. Moreover, in the conveying of mercury in counter-current reflux, the similar devices 32and 33 are provided, so that in the entire arrangement no short-circuiting is possible during the passage of mercury from one cell to the next or from one electrode to the next.

.Inthe amalgam which is discharged at 34, the proportion of silver, copper, and lead impurities to indium has aoaatws 10 changed to about 20 times the value of that which is present in the initial electrolyte feed added at 5. On the other hand, Zinc and iron are concentrated in the final electrolyte discharged at 12, such metals being originally present in the impure indium feed added at 5 to cell 1 1. The only impurities detectable in the purest indium discharged at 27 are small amounts of mercury. The indium is freed from such mercury prior to casting, by means of heating the same to 800 degrees C. in a current of hydrogen whereby the mercury is separated.

Accordingly, the present invention relates to an improvement in the process for the continuous electrolytic production of metallic indium of highest purity from indium salt solutions'by means of a succession of stages of individual electrolyses in which the indium cathodically deposited from the electrolyte salt solution into a mercury electrode in a previous stage is anodically dissolved from the mercury into a further electrolyte salt solution and in turn cathodically deposited from said solution into a further mercury electrode inthe next stage. Such improvement essentially contemplates continuously introducing mercury and electrolyte into the penultimate stage of a succession of stages of individual electrolyses, passing said electrolyte successively forward therefrom to the beginning stage of said succession against the electrolytic flow of indium through said stages and continuously withdrawing electrolyte from the beginning stage corresponding to that introduced into the penultimate stage, with respect to the penultimate stage and previous stages excluding the beginning stage circulating mercury between the anode of one stage and the cathode of the next previous stage and passing from said penultimate stage successive- 1y forward against the electrolytic flow of indium through said stages, at least a portion of said mercury corresponding to that introduced into the penultimate stagefrom the circulating mercury between the anode of one stage and the cathode of the next previous stage to the circulating mercury between the anode of said next previous stage and the cathode of the stage before that, with respect to said beginning stage circulating the mercury between the cathode of said beginning stage and the anode of the next successive stage after the beginning stage and passing .a portion of mercury, corresponding to that portion introduced into said penultimate stage, from the mercury circulating between the cathode of the next successivestage after the beginning stage and the anode of the next stage after that to the mercury circulating between the anode of said next successive stage after the beginning stage and the cathode of the beginning stage, continuously withdrawing mercury from the beginning stage corresponding to that introduced into the penultimate'stage, maintaining a molten indium salt as electrolyte and molten pure indium metal as cathode in the end stage of said succession, circulating mercury between the cathode of said penultimate stage and the anode of said end stage, said mercury introduced into said penultimate stage, being introduced into said last-mentioned circulating mercury, adding impure indium salt solution to the said beginning stage and recovering from said molten cathode in pure molten form indium electrolytically deposited from said molten electrolyte into said molten indium pure cathode.

What is claimed is:

1. Process for the continuous electrolytic production of metallic indium of the highest purity from indium salt solution which comprises electrolyzing an impure indium salt solution in a first stage electrolysis using mercury as cathode whereby to form a first amalgam of mercury and electrolytically separated metallic indium, passingsaid amalgam to an intermediate stage electrolysis for electrolysis therein using an indium salt solution as electrolyte, said amalgam as anode and additional mercury as cathode whereby to dissolve electrolytically said metallic indium from said amalgam into said electrolyteand in turn electrolytically deposit said indium in said additional mercury as cathode to form further amalgam, passing electrolyte from said intermediate stage electrolysis to said first stage electrolysis for combining with said impure salt solution for further electrolysis therein, passing mercury remaining upon electrolytic removal of metallic in dium from said first amalgam as anode from said intermediate stage to said first stage for use therein as cathode, passing said further amalgam to a last stage electrolysis using said further amalgam as anode, molten indium salt as electrolyte and molten pure indium metal as cathode for dissolving electrolytically said metallic indium from said further amalgam into said molten indium salt and in turn electrolytically depositing said indium in said molten indium metal as cathode, and recovering pure indium metal from said last stage.

2. Process according to claim 1 wherein the electrolyte in the first and intermediate stages is indium halide solution and that in the last stage is molten water-free indium halide.

3. Process according to claim 2 wherein the last stage electrolyte is maintained at a temperature above its own fusion point and above the fusion point of the indium.

4. Process according to claim 3 wherein the last stage electrolyte is a low-melting mixture of indium halides.

5. Process according to claim 4 wherein said low-melting mixture additionally includes a member selected from the group consisting of alkali halide, alkaline earth halide, and mixtures thereof.

6. Process according to claim 1 wherein the electrolyte is agitated in said intermediate stage.

7. Process according to claim 1 wherein the passing of amalgam from one stage to the other is carried out by pumping in a lively manner.

8. Process according to claim 1 wherein the electrolyte of the first stage is produced by anodic dissolution of impure indium metal.

9. Process according to claim 1 wherein the electrolyte is added to the intermediate stage in the form of pure aqueous hydrohalic acid having a concentration of 3 to 8 normal and mercury substantially free from impurities is also added to said intermediate stage.

10. Process according to claim 1 wherein the electrolyte in the last stage is produced by the action of a member selected from the group consisting of halogen and hydrogen halide gas on said further amalgam passed to said last stage.

11. Process according to claim 10 wherein said gas is rarefied by an inert gas.

12. Process according to claim 10 wherein said gas is rarefied by hydrogen.

13. Process according to claim 1 wherein said first stage includes at least two individual electrolysis cells in series, the electrolyte and amalgam being passed through the series of cells in this stage in opposite directions.

14. Process for the continuous electrolytic production of metallic indium of the highest purity from indium salt solutions which comprises electrolyzing an impure indium salt solution being passed to a first stage electrolysis using mercury as cathode whereby to form a first amalgam of mercury and electrolytically separated metallic indium, passing said amalgam to an intermediate stage electrolysis for electrolysis therein using an indium salt solution as electrolyte, said first amalgam as anode and additional mercury as cathode whereby to dissolve electrolytically said metallic indium from said first amalgam into said electrolyte and in turn electrolytically deposit at least a portion of said indium in said additional mercury as cathode to form further amalgam, passing a portion of said indium salt solution enriched by a portion of said metallic indium dissolved electrolytically therein from said intermediate stage electrolysis back to said first stage electrolysis for combining with said impure indium salt solution for further electrolysis therein, passing mercury remaining upon electrolytic removal in said intermediate stage electrolysis of metallic indium from said first amalgam as anode from said intermediate stage back to said first stage for use therein as cathode, passing said further amalgam to a last stage electrolysis using said further amalgam as anode, molten indium salt as electrolyte and molten pure indium metal as cathode for dissolving electrolytically said metallic indium from said further amalgam into said molten indium salt and in turn electrolytically depositing said indium in said molten indium metal as cathode, recovering a portion of pure indium metal corresponding to that produced in said electrolysis from said last stage, recovering a portion of electrolyte from said first stage, adding fresh electrolyte to said intermediate stage to replace that passed to said first stage and corresponding to that recovered from said first stage, recovering a portion of mercury being passed from said intermediate stage to said first stage, and adding fresh mercury to said intermediate stage corresponding to that recovered from said first stage.

15. Process according to to claim 14 wherein the fresh electrolyte added to said intermediate stage is in the form of a pure aqueous hydrohalic acid having a concentration of 3 to 8 normal.

16. Process according to claim 14 wherein the fresh mercury free from impurities is added to the additional mercury used as cathode in said intermediate stage.

17. Process according to claim 16 wherein a portion of the mercury being passed from the cathode of said intermediate stage to the anode of said last stage is withdrawn and passed to the anode of said intermediate stage.

18. Process according to claim 14 wherein a series of individual cells is used in said first stage and in said intermediate stage, said mercury passed to said first stage from said intermediate stage passing inversely successively through the cells of said first stage in opposite direction to the flow of electrolyte from said intermediate stage and the flow of impure indium salt solution through the cells of said first stage, said fresh electrolyte passing into and through the cells of said intermediate stage in the same direction as the flow through the cells of said first stage, the electrolytic passage of indium from the electrolyte in the cells of said first stage by said first amalgam to the electrolyte in the first cell of said intermediate stage and from the electrolyte of said first cell successively through the remaining cells of said intermediate stage by means of said additional mercury and further amalgam being in opposite direction to the flow of said fresh electrolyte, whereby a portion of pure indium will be continuously deposited at the cathode of the last stage, and a portion of indium present in the oppositely flowing electrolyte will be continuously passed back therewith through the successive cells in said intermediate stage and through at least one cell of said first stage and will be withdrawn with the electrolyte recovered from said first stage.

19. In the process for the continuous electrolytic production of metallic indium of highest purity form indium salt solutions by means of a succession of stages of individual electrolyses, in which the indium cathodically deposited from the electrolyte salt solution into a mercury electrode in a previous stage is anodically dissolved from the mercury into a further electrolyte salt solution and in turn cathodically deposited from said solution into a further mercury electrode in the next stage, the improvement which comprises continuously introducing mercury and electrolyte into the end stage of a succession of stages of individual electrolyses, passing said electrolyte successively forward to the beginning stage of said succession and continuously withdrawing electrolyte from one of the earlier previous stages corresponding to that added to the end stage, in the case of a portion of the later successive stages including the end stage circulating mercury between the anode of one stage and the cathode of the next previous stage, in the case of said portion of later successive stages passing at least a portion of said mercury, corresponding to that introduced into the end stage, from the circulating mercury between the anode of one stage and the cathode of the next previous stage to the circulating mercury between the anode of said next previous stage and the cathode of the stage before that, in the case of the remaining portion of earlier successive stages including the beginning stage circulating mercury successively through the cathodes of said stages in opposite direction to the flow of electrolyte therethrough and in turn through the anode of the next successive stage corresponding to the first of said portion of later successive stages including the end stage, passing a portion of mercury, corresponding to that portion introduced into the end stage, from the mercury circulating between the cathode of the first and the anode of the next successive stage of said portion of later successive stages including the end stage to the last of said remaining portion of earlier successive stages including the beginning stage, and continuously withdrawing mercury from one of the remaining portion of earlier successive stages corresponding to that added to the end stage.

20. Improvement according to claim 19 wherein a further recovery stage is provided having molten indium salt as electrolyte and molten pure indium metal as cathode, and amalgam as anode, the anode amalgam being circulated between the anode of said further recovery stage and the cathode of the end stage of said succession of stages, whereby indium electrolytically deposited from said molten electrolyte into said molten pure indium cathode is recovered from said molten cathode in pure molten form.

211. In the process for the continuous electrolytic production of metallic indium of highest purity from indium salt solutions by means of a succession of stages of individual electrolyses, in which the indium cathodically deposited from the electrolyte salt solution into a mercury electrode in a previous stage is anodically dissolved from the mercury into a further electrolyte salt solution and in turn cathodically deposited from said solution into a further mercury electrode in the next stage, the improvement which comprises continuously introducing mercury and electrolyte both free from detrimental impurities into the penultimate stage of a succession of stages of individual electrolyses, passing said electrolyte successively forward therefrom to the beginning stage of said succession against the electrolytic flow of indium through said stages and continuously Withdrawing electrolyte from the beginning stage corresponding to that introduced into the penultimate stage, with respect to the penultimate stage and previous stages excluding the beginning stage circulating mercury between the anode of one stage and the cathode of the next previous stage and passing from said penultimate stage successively forward against the electrolytic flow of indium through said stages at least a portion of said mercury corresponding to that introduced into the penultimate stage from the circulating mercury between the anode of one stage and the cathode of the next previous stage to the circulating mercury between the anode of said next previous stage and the cathode of the stage before that, with respect to said beginning stage circulating mercury between the cathode of said beginning stage and the anode of the next successive stage after the beginning stage and passing a portion of mercury, corresponding to that portion introduced into said penultimate stage, from the mercury circulating between the cathode of the next successive stage after the beginning stage and the anode of the next stage after that to the mercury circulating between the anode of said next successive stage after the beginning stage and the cathode of the beginning stage, continuously withdrawing mercury from the beginning stage corresponding to that introduced into the penultimate stage, maintaining a molten indium salt as electrolyte and molten pure indium metal as cathode in the end stage of said succession, circulating mercury between the cathode of said penultimate stage and the anode of said end stage, said mercury introduced into said penultimate stage being introduced into said lastmentioned circulating mercury, adding impure indium salt solution to said beginning stage, and recovering from said molten cathode in pure molten form indium electrolytically deposited from said molten electrolyte into said molten pure indium cathode.

22. Improvement according to claim 21 where said impure indium salt and said molten indium salt is indium halide and said electrolyte continuously introduced into said penultimate stage is pure aqueous hydrohalic acid having a concentration of 3 to 8 normal.

References Cited in the file of this patent UNITED STATES PATENTS 2,382,434 McNitt Aug. 14, 1945 FOREIGN PATENTS 800,153 Great Britain Aug. 20, 1958 581,668 Canada Aug. 18, 1959 OTHER REFERENCES Iahresberichte uber die Fortschritte der Chemie, Theil 1 (1888), pages 388-389.

Claims (1)

1. PROCESS FOR THE CONTINUOUS ELECTROLYTIC PRODUCTION OF METALLIC INDIUM OF THE HIGHEST PURITY FROM INDIUM SALT SOLUTION WHICH COMPRISES ELECTROLYZING AN IMPURE INDIUM SALT SOLUTION IN A FIRST STAGE ELECTROLYSIS USING MERCURY AS CATHODE WHEREBY TO FORM A FIRST AMALGAM OF MERCURY AND ELECTROLYTICALLY SEPARATED METALLIC INDIUM, PASSING SAID AMALGEN TO AN INTERMEDIATE STAGE ELECTROLYSIS FOR ELECTROLYSIS THEREIN USING AN INDIUM SALT SOLUTION AS ELECTROLYTE, SAID AMALGAM AS ANODE AND ADDITIONAL MERCURY AS CATHODE WHEREBY TO DISSOLVE ELECTROLYTICALLY SAID METALLIC INDIUM FROM SAID AMALGAM INTO SAID ELECTROLYTE AND IN TURN ELECTROLYTICALLY DEPOSITED SAID INDIUM IN SAID ADDITIONAL MERCURY AS CATHODE TO FORM FURTHER AMALGAM, PASSING ELECTROLYTE FROM SAID INTERMEDIATE STAGE ELECTROLYSIS TO SAID FIRST STAGE ELECTROLYSIS FOR COMIBINING WITH SAID IMPURE SALT SOLUTION FOR FURTHER ELECTROLYSIS THEREIN, PASSING MERCURY REMAINING UPON ELECTROLYTIC REMOVAL OF METALLIC IN-

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