EP1051541B1 - Method for the electrolytic regeneration of contaminated rhodium solutions - Google Patents

Abstract

The invention relates to a method for electrically regenerating rhodium solutions, designed especially for regenerating rhodium solutions containing sulphuric acid and/or phosphoric acid or rhodium chloride solutions. The solution to be regenerated is introduced in the anode area (12) of an electrolytic cell (10), which is separated from the associated cathode area (14) by an ion exchange membrane (16), said cathode area being filled with a dilute acid having a good conductivity. The rhodium solution pH is increased so as to reach a value greater than 10, preferably in the range from 10 to 14, by adding an alkaline solution, i.e. a concentrated hydroxide potassium solution. Electrolysis is carried out at electric densities that are so high that the trivalent rhodium of the rhodium solution is oxidised to form an hexavalent rhodium, and possible impurities are depleted by the ion exchange membrane (16) in the catholytes. Hexavalent rhodium is transferred in the cathode area at a speed slightly smaller than trivalent rhodium, and therefore no noticeable rhodium depletion can be observed. This is particularly true when pH during electrolysis is maintained at a value permanently greater than 1.5 by means of proper addition of alkaline solution.

Description

The present invention relates to a method for electrolytic regeneration of contaminated rhodium solutions, especially for the regeneration of sulfur and / or phosphoric acid rhodium solutions or of rhodium chloride solutions suitable is.

Rhodium solutions or rhodium baths of the type mentioned are used, for example, in the jewelry and silverware industry for the electroplating of jewelry. The solutions usually contain 1-3 g / l rhodium, 40-80 ml / l concentrated phosphoric acid and / or 25-80 ml / l sulfuric acid. Their temperature is between 40 ° C and 50 ° C. The work is carried out with platinum anodes or platinized anodes, the current densities being 0.5-10 A / dm ² .

In the electrical and electronics industry, however, mainly rhodium sulfate solutions are used, which contain 4 - 20 g / l rhodium and 25 - 50 ml / l concentrated sulfuric acid and have a temperature between 30 ° C and 50 ° C. Platinum or platinized titanium is used as the anode material, the current densities being 0.5-3 A / dm ² .

It comes with solutions in the strongly acidic range worked that are very sensitive to organic and are inorganic contaminants.

Most organic contaminants are caused by dust, Buildup of masking tapes, masking lacquer, Circuit board material and organics from unsuitable Plastic tubs or plastic tanks entered. organic Contamination cause voltages in the galvanic deposited precipitation. The layer becomes fragile and looks cloudy. Organic compounds can be obtained through a Activated carbon treatment can be separated. The solutions are allowed don't be too acidic. For the separation of short-chain Another treatment with a hydrocarbon suitable activated carbon may be necessary.

In addition to the organic components, the activated carbon also takes Precious metal components that can only be obtained by ashing the Have activated carbon recovered as precious metal.

Malfunctions can occur if the pH value becomes higher than 2, because then basic rhodium salts precipitate and form in the Store precipitation. This error can be solved by a easy control of the pH or a determination and Correct the acidity setting.

When overheating rhodium solutions or rhodium baths can anionic rhodium complexes form. Rhodium baths that such complexes also tend to separate of layers with increasing proportion of anionic Rhodium complexes show an increased voltage rise.

Anionic rhodium complexes can be oxidized with chlorine in an alkaline medium and subsequent reduction with Hydroquinone can be converted into the trivalent cation. To this treatment is the rhodium bath by the entry of Foreign ions can no longer be used. Inorganic Contamination is usually caused by the base metal brought in. The warm phosphoric acid or phosphoric acid and sulfuric acid or sulfuric acid solution is extremely aggressive. Parts that are hung into the bathroom without electricity are picked up by the Electrolyte immediately. This also applies to parts that accidentally fall into the bathroom. The Major impurities found in rhodium baths were Cu, Fe, Sn, Pb, Ni, Au, Ag. This Contaminants also cause dark and dirty ones Precipitation that can burst from the surface. For The separation of the metallic impurities becomes the precipitation with potassium ferrocyanide recommended (metal finishing, guidebook Directory 1986, p. 280). An effectiveness of this Precipitation method must be questioned, however, because the Solubility of the precipitates in the strongly acidic medium too high and an exact dosage of the precipitant is not possible is, so that in addition potassium and ferrocyanide ions be introduced.

Other published methods are for the known ones Rhodium solutions or rhodium baths with the above Impurities are also not transferable. According to the teaching of British patent 662,299, the contaminants are removed separated from a cation exchanger. This procedure continues the presence of an anionic rhodium complex. The conversion of the rhodium into an anionic complex would be however, for further use in electrolysis bath very much disadvantageous.

In order to obtain pure rhodium, one can do the sulfide precipitation coupled with nitrite precipitation (Brauer, Handbuch der preparative inorganic chemistry, 3rd volume, Ferdinand Enke Verlag, Stuttgart, 3rd ed., 1981, p. 1737; DE-OS-2835 159; GB-PS-1491521). The impurities are precipitated as sulfide. This precipitation is only possible if Rhodium is bound in the anionic complex because Rhodium cations can also be precipitated as sulfide. The Refining rhodium as rhodium nitrite sets a multiple Precipitation of the rhodium nitrite in advance. Then the Rhodium nitrite can be converted into the rhodium sulfate. This Process works with low yields, high ones Precious metal losses and an unacceptable Workload.

According to the teaching of British patent 11 44716, the Rhodium solution over a loaded with hydroxyl groups Given anion exchangers. The precipitated rhodium hydride is again dissolved in acid. With the rhodium, too Impurities precipitated as hydroxides. Dissolving in acid you get back the starting solution with the same Impurities.

In the electrolysis described in EP 0 288 344 B1 together with the impurities a large proportion Rhodium deposited.

As there is currently no economically viable process for Cleaning of rhodium solutions or rhodium baths exists, these are usually delivered to refineries in which the rhodium contained in the solution according to one of the the above-mentioned process is recovered.

The object of the present invention is therefore in Creation of an economical process for regeneration contaminated rhodium solutions with which a costly metallurgical refurbishment, a new one Rhodium salt production and a new approach to the solutions can be avoided so that the cleaned solutions immediately returned to the industrial use mentioned above can be.

This object is achieved by the electrolysis process according to the invention. The rhodium solution to be regenerated is fed into the anode compartment of an electrolytic cell, which is separated from the associated cathode compartment filled with a dilute, highly conductive acid by a cation exchange membrane. H ₂ SO ₄ , H ₃ PO ₄ , HCl or HNO ₃ is preferably used as the acid, with 10-20% H ₂ SO ₄ being currently preferred. According to the invention, the pH of the rhodium solution is increased to a value of more than 10 by adding a suitable alkali solution, a pH between 12 and 14 preferably being set. Concentrated potassium hydroxide solution can in particular be used as the alkali solution.

The electrolysis now takes place, the current density being chosen so high that trivalent rhodium is oxidized to hexavalent rhodium in the rhodium solution. The current densities are preferably 1-20 A / dm ² , which corresponds to a maximum current load of the membrane of 4 kA / m ² (40 A / dm ² ). A further increase in the current strength could lead to the destruction of the membrane.

With the oxidation, with which the light yellow solution dark purple colors, parts of the organics become by oxidation destroyed. Entered chloride impurities become chlorine converted and discharged as gas. The chlorine can for example in sodium hydroxide solution by conversion into Sodium hypochlorite rendered harmless and one become a salable product.

In the case of electrolysis, there may be existing ones Impurities through the cation exchange membrane in the Depleted catholyte. The hexavalent rhodium, however is in the at a much slower rate Cathode compartment converts as trivalent rhodium, so that it too there is no significant depletion of the rhodium content.

The migration of potassium ions from the anode to the cathode space simultaneously reduces the pH of the anolyte, which at pH values of less than 1.5 leads to an increased conversion of Rh _VI ions into Rh _III ions and a related one Depletion of the rhodium would result. If the impurities contained in the rhodium solution are not yet completely depleted when a pH of 1.5 is reached, the pH is therefore preferably kept constant by adding an alkali solution, such as concentrated potassium hydroxide solution.

Optimal depletion of impurities results then when anolyte and / or catholyte are in circulation continuously led past the cation exchange membrane the flow rate of the anolyte 1-4 l / min. The movement of the catholyte takes place through the Development of hydrogen in electrolysis.

The temperature of the anolyte here is preferably 20 - 50 ° C, so that industrially used rhodium solutions or Rhodium baths of the type mentioned above directly one Regeneration or cleaning supplied and then also are industrially usable again without this additional heat treatment of the solution would be required.

Accordingly, the preferred acidity is Rhodium solution with 40 - 80 ml / l concentrated phosphoric acid and / or 25 - 80 ml / l sulfuric acid to the above adapted industrial use of the rhodium solution.

Rhodium solutions with a higher acidity become Reduction in acidity and / or depletion any anionic components present are preferred first in an upstream electrolysis cell Subjected to electrolysis. Appropriate solutions are here passed into the cathode compartment of the electrolytic cell, which is in front of the with an aqueous sodium sulfate solution or the like associated anode space filled by a Anion exchange membrane is separated so that the Acid residues and anionic impurities in the Anode compartment to be transferred.

Fig. 1

ein Flußdiagramm eines erfindungsgemäßen Elektrolyseverfahrens zum Regenerieren verunreinigter Rhodiumlösungen zeigt.

Further features and advantages of the present invention result not only from the associated claims - individually and / or in combination - but also from the following detailed description of specific exemplary embodiments which takes place with reference to FIG

Fig. 1

shows a flow diagram of an electrolysis process according to the invention for regenerating contaminated rhodium solutions.

The rhodium solution to be regenerated is used according to the invention in the anode compartment (12) of an electrolysis cell (10) passed through the associated cathode compartment (14) a cation exchange membrane (16) is separated.

In the anode compartment (12) of the electrolytic cell (10) is also concentrated from a storage container (18) Potassium hydroxide solution fed to adjust the pH of the Increase rhodium solution to 12-14. This process will monitored by means of a measuring device (20) through which the Infeed is controlled accordingly. If necessary, you can however, other suitable alkali solutions can also be used. In addition, the feed can also be located at a point outside of the Electrolysis cell (10) take place.

The cathode compartment (14) of the electrolytic cell (10) is filled with 20% H ₂ SO ₄ , which is circulated through a storage container (22). Depending on the intended use, however, a different concentration, such as 10% H ₂ SO ₄ , or another suitable acid can also be used, H ₃ PO ₄ , HCl or HNO ₃ being particularly worth mentioning. The pH of the catholyte is adjusted to pH ≤ 0.5 in order to enable the transferred noble metals to be separated. The base metals are then removed with the catholyte when enriched to approx. 10 g / l.

At current densities of 1 - 20 A / dm ² , electrolysis is now carried out, with trivalent rhodium being oxidized to hexavalent rhodium in the rhodium solution, which can be recognized by a color change in the solution from light yellow to dark purple. Since hexavalent rhodium is transferred through the cation exchange membrane into the cathode compartment at a much slower rate than trivalent rhodium, there is no appreciable depletion of the rhodium in the specified conditions.

As already mentioned above, during the oxidation process Parts of the organic system destroyed by oxidation. Registered Chloride contaminants are converted to chlorine and as Discharged gas. The chlorine can, for example, in sodium hydroxide solution rendered harmless by conversion to sodium hypochlorite and become a salable product.

In the course of the electrolysis, the migration of potassium ions from the anode to the cathode space reduces the pH of the anolyte, and at the same time the impurities present in the rhodium solution are depleted in the catholyte. The pH values at which the various impurities are depleted are summarized in Table I below. pH hydroxide 9 Ag (OH) 7-8 Co (OH) ₂ Ni (OH) ₂ 6 Pb (OH) ₂ Fe (OH) ₂ Zn (OH) ₂ 5 Cu (OH) ₂ Cr (OH) ₃ 4th Al (OH) ₃ 2-3 Sn (OH) ₄ Fe (OH) ₃

If the impurities in the rhodium solution are not yet depleted when the pH reaches 1.5, it is advisable to prevent the pH from falling further by adding potassium hydroxide or another alkali solution and to keep the pH constant hold. In this way, a conversion of Rh _VI ions into Rh _III ions and an associated depletion of the rhodium in the cathode space can be avoided.

To improve the exchange of materials, the anolyte in the Cycle and continuously at the Pumped by cation exchange membrane, its Flow rate is 1 - 4 l / min.

The temperature of the anolyte is 20 - 50 ° C, so that the in industrial sulfuric and / or phosphoric acids Rhodium solutions are directly regenerable.

The catholyte is transported through the resulting H ₂ .

The current load on the membrane is 4 kA / m ² (40 A / dm ² ). A further increase in the current strength can destroy the membrane.

The acidity of the rhodium solution is 40-80 ml / l concentrated phosphoric acid and / or 25-80 ml / l sulfuric acid. At higher acid contents, the rhodium solution to be cleaned is passed to reduce the acid content and / or to deplete any anionic components present in the cathode compartment of an additional electrolysis cell (not shown) in the anolyte circuit, which passes from the associated anode compartment filled with an aqueous potassium sulfate solution through an anion exchange membrane is separated. The acid residue anions SO ₄ ^- and / or PO ₄ ^- are transferred into the anode compartment, the voltage being chosen to be lower than the voltage required for Rh deposition.

This electrolysis process according to the present invention is as follows with the help of a few special exemplary embodiments explained.

Example 1:

50 ml of a rhodium sulfate solution containing 3.1 g / l rhodium were adjusted to a pH of 13.5 with potassium hydroxide and electrolyzed at 20 A / dm ² according to the electrolysis process described above. After 10 minutes the solution turned deep purple, indicating the oxidation of trivalent rhodium to hexavalent rhodium. After 6 hours, the pH in the anode compartment had reached 1.0. Subsequent analysis of the anolyte showed that there was no appreciable depletion of the rhodium content.

Comparative Example 2:

50 ml of a rhodium sulfate solution according to Example 1 were without Add potassium hydroxide under the same conditions as electrolyzed in Example 1. After 6 hours the contained Anolyte solution only 1.5 g / l rhodium, so that about 50% of Rhodium had been transferred to the cathode compartment.

This is due to the fact that - in contrast to Example 1 - no oxidation of trivalent rhodium to hexavalent rhodium takes place in the acidic medium without the addition of potassium hydroxide, and that the trivalent rhodium, as already mentioned above, is depleted into the cathode compartment at a much higher rate as hexavalent rhodium. This depletion takes place together with the hydronium ions (H ₃ O ⁺ ).

In the experiments given in the examples, the current load on the membrane was 4 kA / m ² (40 A / dm ² ), with a further increase in the current strength leading to destruction.

Another negative effect is the strong heating of the rhodium solution with increased power supply. At temperatures above 50 ° C the electrolysis of an alkaline rhodium solution promotes the formation of coffee-brown RhO ₂ . The precipitation of rhodium oxide and the metallic impurities present as hydroxide clog the membrane.

As can be seen in Example 1, during the electrolysis of the alkaline solution containing rhodium _VI ion in the membrane electrolysis cell, the pH value in the anolyte is reduced from pH = 12 to pH = 1 by the migration of potassium ions from the anode to the cathode space , whereby the pH value can also shift into the negative range if the electrolysis continues. There is a conversion of Rh _VI ions into Rh _III ions and the rhodium is depleted from the rhodium solution to be refined.

The above pH range, at which there is a depletion of the rhodium can be avoided as far as possible Example 3.

Example 3:

A rhodium sulfate solution containing 3.4 g / l rhodium was adjusted to a pH of 12.5 with potassium hydroxide solution. The rhodium solution was diluted to 3.0 g / l by the addition of the potassium hydroxide solution. The solution was then subjected to membrane electrolysis, working with a current density of 1 A / dm ² and a current load on the membrane of 4 kA / m ² . The transfer of potassium ions reduced the pH from pH = 12.0 to pH = 1.5.

The rhodium concentration was limited after the electrolysis by the one done with the potassium ion transfer Water transfer 3.6 g / l. With a further decrease in pH Rhodium depletion occurs. After a Electrolysis time of 2 hours was the rhodium concentration in of the analytical solution dropped to 2.4 g / l.

Example 4:

A rhodium sulfate solution containing 5.3 g / l rhodium and 400 ppm impurities in iron and nickel was adjusted to a pH of 13 with potassium hydroxide. With a current density of 2 A / dm ² , a current load on the membrane of 4 kA / m ² and a flow rate of the electrolyte of 3 l / min, the Fe / Ni impurities could be reduced to a value of 50 ppm within 5 hours. The rhodium depletion was 0.3 g / l.

Example 5:

A rhodium sulfate solution with a content of 4.5 g / l rhodium and a total impurity of 1 g / l Cu, Fe, Sn, Pb, Ni was at a current density of 2.5 A / dm ² , a current load of the membrane of 4 kA / m ² and a flow rate of the anolyte of 4 l / min electrolyzed. Under these conditions, the contaminants were reduced to 50 ppm within 8 hours.

As can be seen from the following example, traces of gold and Remove silver.

Example 6:

From a rhodium sulfate solution with 300 ppm impurities Silver and gold could be achieved using the above The inventive method after an electrolysis time of 5 Hours depleted to 50 ppm silver and gold become.

A further depletion of the impurities to a value less than 50 ppm is possible. However, it is very time consuming.

Contains the rhodium sulfate solution to be regenerated high acidity, then the acid can, as above was mentioned by electrolysis in a separate Deplete the electrolytic cell, leaving the in the rhodium solution contained anionic components also separated become.

An embodiment of this method variant is in given in Example 7 below.

Example 7:

A rhodium solution containing 100 ml / l of sulfuric acid was placed in the cathode compartment of an electrolytic cell provided with an anion exchange membrane. An aqueous sodium sulfate solution was used as the anolyte. With a current density of 0.2 A / dm ² and a current load on the membrane of 0.2 kA / m ² , the acid content in the catholyte could be reduced to 80 ml / l.

Embodiments 1-7 relate to regeneration contaminated rhodium sulfate solutions. However, it was on it pointed out that in parallel measurements comparable results with phosphoric acid rhodium solutions surrendered.

For example, a phosphoric acid rhodium solution containing 5 g / l rhodium potassium hydroxide solution, contaminated with 200 ppm tin, 200 ppm lead, 150 ppm iron and 100 ppm copper, was added in order to raise the pH to 14. This solution was then placed in the anode compartment of an electrolysis cell, the anode and cathode compartments of which were separated by a cation exchange membrane, and electrolyzed at 4 A / dm ² . During the electrolysis, the rhodium solution to be regenerated was directed past the cation exchange membrane at a rate of 3 l / min. The movement of the catholyte, which comprised 5% sulfuric acid, was caused by the evolution of hydrogen during the electrolysis. Under the specified conditions, potassium ions and most of the contaminants were transferred from the anode compartment to the cathode compartment during the electrolysis, so that the content of impurities in the rhodium solution was only 20 ppm after 6 hours. The contaminants transferred into the cathode compartment separated out on the cathode.

As can be seen from the following example, the electrolysis process according to the invention is suitable not only for the regeneration of rhodium solutions containing sulfuric and / or phosphoric acid, but also for the regeneration of rhodium solutions containing chloride. Potassium hydroxide solution was added to a rhodium chloride solution with a rhodium content of 10 g / l and with copper, nickel and iron impurities of 100 ppm in each case in order to raise the pH to 13.5. This solution was placed in the anode compartment of an electrolytic cell, the anode compartment and cathode compartment of which were separated by a cathode exchange membrane and electrolyzed at a current density of 3.5 A / dm ² . The anolyte was moved past the membrane at a rate of 3 l / min, while the catholyte, which comprised 20% hydrochloric acid, was moved through the hydrogen produced during the electrolysis. During the electrolysis, Cl ^{- was converted} into Cl ₂ , with the chlorine released acting as an oxidizing agent. Again, potassium ions and most of the contaminants were transferred from the anode compartment to the cathode compartment, so that the contaminants were depleted to a value of 20 20 ppm after 5 hours. Hydrochloric acid was carefully added to the anolyte during depletion.

Claims (13)

1. Method of electrolytically regenerating contaminated rhodium solutions,
   characterized in that
   the rhodium solution to be regenerated is passed into the anode compartment (12) of an electrolytic cell (10), said anode compartment being separated from the corresponding cathode compartment (14), which is filled with a dilute, highly conductive acid, by a cation exchange membrane (16), wherein the pH of the rhodium solution is raised to a value greater than 10 by the addition of an alkali solution and the electrolysis is carried out at current densities sufficient to oxidize trivalent rhodium in the rhodium solution to sexivalent rhodium and to deplete any impurities present through the cation exchange membrane (16) into the catholyte.
2. Method according to Claim 1,
   characterized in that
   a pH of between 12 and 14 is set.
3. Method according to Claim 1 or 2,
   characterized in that
   the pH of the anolyte during the electrolysis is at all times greater than or equal to 1.5 and, if required, is suitably adjusted by the addition of alkali solution.
4. Method according to one of Claims 1 to 3,
   characterized in that
   the alkali solution used is concentrated potassium hydroxide solution.
5. Method according to one of Claims 1 to 4,
   characterized in that
   the current density during the electrolysis is 1 - 20 A/dm².
6. Method according to any one of Claims 1 to 5,
   characterized in that
   the temperature of the anolyte is 20 - 50°C.
7. Method according to any one of Claims 1 to 6,
   characterized in that
   the anolyte and/or the catholyte is continuously recirculated past the cation exchange membrane (16).
8. Method according to Claim 7,
   characterized in that
   the flow rate of the anolyte is 1 - 4 l/min.
9. Method according to any one of Claims 1 to 8,
   characterized in that
   the acid content of the rhodium solution is 40 - 80 ml/l of concentrated phosphoric acid and/or 25 - 80 ml/l of sulphuric acid.
10. Method according to any one of Claims 1 to 8,
    characterized in that
    in order to reduce the acid content and/or to deplete any anionic fractions present, rhodium solutions having an acid content of more than 80 ml/l are passed into the cathode compartment of an additional electrolytic cell, said cathode compartment being separated from the associated anode compartment filled with an aqueous sodium sulphate solution by an anion exchange membrane.
11. Method according to any one of Claims 1 to 10,
    characterized in that
    the dilute, highly conductive acid comprises H₂SO₄, H₃PO₄, HCl or HNO₃.
12. Method according to Claim 11,
    characterized in that
    the dilute, highly conductive acid comprises 10 - 20% strength H₂SO₄.
13. Method according to any one of Claims 1 to 12,
    characterized in that
    the pH of the dilute, highly conductive acid is set to pH ≤ 0.5, to enable precipitation of the transferred noble metals.

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