EP1051541A2

EP1051541A2 - Method for electrically regenerating contaminated rhodium solutions

Info

Publication number: EP1051541A2
Application number: EP99910109A
Authority: EP
Inventors: Sigrid Herrmann; Uwe Landau
Original assignee: OTB Oberflaechentechnik in Berlin GmbH and Co
Current assignee: OTB Oberflaechentechnik in Berlin GmbH and Co
Priority date: 1998-02-05
Filing date: 1999-02-05
Publication date: 2000-11-15
Anticipated expiration: 2019-02-05
Also published as: DE59900461D1; EP1051541B1; WO1999040238A3; AU2920599A; HK1034544A1; DE19804534C1; ATE209709T1; WO1999040238A2

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

Process for the electrolytic regeneration of contaminated

Rhodium solutions

description

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

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 / 1 rhodium, 40-80 ml / 1 concentrated phosphoric acid and / or 25-80 ml / 1 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 / 1 rhodium and 25 - 50 ml / 1 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 .

In each case, solutions are used in the strongly acidic range, which are very sensitive to organic and inorganic contaminants.

Most organic contaminants are introduced by dust, masking tape, masking tape, circuit board material and organics from unsuitable plastic tubs or plastic tanks. Organic contaminants cause tension in the electrodeposited precipitation. The layer becomes brittle and looks cloudy. Organic compounds can be separated by an activated carbon treatment. The solutions must not be too acidic. For the separation of short-chain Hydrocarbons may require further treatment with a suitable activated carbon.

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

Disruptions can occur if the pH value becomes higher than 2, since then basic rhodium salts precipitate out and become deposited in the precipitation. This error can be remedied by simply checking the pH or by determining and adjusting the acidity.

When rhodium solutions or rhodium baths are overheated, anionic rhodium complexes can form. Rhodium baths containing such complexes also tend to deposit layers which show an increased voltage increase with an increasing proportion of anionic rhodium complexes.

Anionic rhodium complexes can be converted into the trivalent cation by oxidation with chlorine in an alkaline medium and subsequent reduction with hydroquinone. After this treatment, the rhodium bath can no longer be used due to the introduction of foreign ions. Inorganic contaminants are usually carried in through the base metal. 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 immediately attacked by the electrolyte. This also applies to parts that accidentally fall into the bathroom. The main contaminants found in rhodium baths are Cu, Fe, Sn, Pb, Ni, Au, Ag. These contaminants also cause dark and dirty precipitates that can burst from the surface. Precipitation with potassium ferrocyanide is recommended to remove the metallic contaminants (Metal Finishing, Guidebook Directory 1986, p. 280). An effectiveness of this Precipitation method must be questioned, however, since the solubility of the precipitates in the strongly acidic medium is too high and an exact dosage of the precipitant is not possible, so that potassium and ferrocyanide ions are also introduced.

Other published methods are also not transferable for the known rhodium solutions or rhodium baths with the above-mentioned impurities. According to the teaching of British patent 662 299, the contaminants are separated off with a cathode exchanger. This procedure presupposes the presence of an anionic rhodium complex. However, the conversion of the rhodium into an anionic complex would be very disadvantageous for further use in the electrolytic bath.

In order to obtain pure rhodium, sulfide precipitation can be used coupled with nitrite precipitation (3rauer, Handbook of Preparative Inorganic Chemistry, Volume 3, Ferdinand Enke Verlag, Stuttgart,

3rd ed., 19.81, Ξ. 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, since rhodium cations are also precipitated as sulfide. The refining of rhodium as rhodium nitrite requires repeated precipitation of the rhodium nitrite. The rhodium nitrite must then be converted into the rhodium sulfate. This process works with low yields, high precious metal losses and an unacceptable amount of work.

According to the teaching of British patent 11 44716, the rhodium solution is passed through an anion exchanger loaded with hydroxyl groups. The precipitated rhodium hydroxide is redissolved in acid. The impurities are precipitated as hydroxides with the rhodium. When dissolved in acid, the starting solution with the same impurities is obtained again. In the electrolysis described in EP 0 288 344 B1, a large proportion of rhodium is deposited together with the impurities.

Since there is currently no economically viable method for cleaning rhodium solutions or rhodium baths, these are usually supplied to separating plants in which the rhodium contained in the solution is recovered by one of the methods mentioned above.

The object of the present invention is therefore to provide an economical process for the regeneration of contaminated rhodium solutions, with which an expensive metallurgical workup, a renewed production of rhodium salts and a new preparation of the solutions can be avoided, so that the cleaned solutions are immediately returned to the industrial use mentioned above can.

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. As the acid is in this case preferably H ₂ S0 _4, H ₃ P0 _4, HCI or HN0 ₃ is used, with a presently 10 - 20% H ₂ S0 ₄ is 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 added to the rhodium solution hexavalent rhodium is oxidized. The current densities here are preferably 1-20 A / dm ² , which corresponds to a maximum current load on 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.

During the oxidation, when the light yellow solution turns dark purple, parts of the organic matter are destroyed by oxidation. Entered chloride impurities are converted to chlorine and discharged as a gas. For example, the chlorine in sodium hydroxide solution can be rendered harmless by conversion into sodium hypochlorite and can become a salable product.

During electrolysis, any impurities that may be present are depleted by the cation exchange membrane in the catholyte. The hexavalent rhodium, on the other hand, is transferred into the cathode compartment at a much lower rate than trivalent rhodium, so that there is no appreciable 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 _πι 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 is obtained when the anolyte and / or catholyte are continuously circulated past the cation exchange membrane, the flow rate of the anolyte being 1-4 l / min. The movement of the catholyte takes place through the Development of hydrogen in electrolysis.

The temperature of the anolyte is preferably 20 ° -50 ° C., so that industrially used rhodium solutions or rhodium baths of the above-mentioned type are fed directly to regeneration or cleaning and can then also be used industrially again without additional heat treatment of the solution being necessary for this.

Accordingly, the preferred acid content of the rhodium solution with 40-80 ml / 1 concentrated phosphoric acid and / or 25-80 ml / 1 sulfuric acid is adapted to the industrial use of the rhodium solution mentioned above.

Rhodium solutions with a higher acid content are preferably first subjected to electrolysis in a preceding electrolysis cell in order to reduce the acid content and / or to deplete any anionic components present. Appropriate solutions are passed into the cathode compartment of the electrolytic cell, which is separated by an anion exchange membrane in front of the associated anode compartment filled with an aqueous sodium sulfate solution or the like, so that the residual acid anions and anionic impurities are transferred to the anode compartment.

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

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 passed according to the invention into the anode compartment (12) of an electrolytic cell (IC), which is separated from the associated cathode compartment (14) by a cation exchange membrane (16).

Concentrated potassium hydroxide solution is also fed into the anode compartment (12) of the electrolytic cell (10) from a storage container (18) in order to increase the pH of the rhodium solution to 12-14. This process is monitored by means of a measuring device (20) by means of which the feed is controlled accordingly. If necessary, other suitable alkali solutions can also be used. In addition, the feed can also take place at a point outside the electrolytic cell (10).

The cathode compartment (14) of the electrolytic cell (10) is filled with 20% H _∑ S0 ₄ , which is circulated through a storage container (22). Depending on the application, however, a different concentration, such as 10% H ₂ S0 ₄ , or another suitable acid can also be used, H ₃ P0 ₄ , HCl or HN0 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 about 10 g / l.

At current densities of 1 - 20 A / dm ¹ , an 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. Because hexavalent rhodium passes through the

Cation exchange membrane is transferred into the cathode space as trivalent rhodium, under the specified conditions there is no significant depletion of the rhodium. As already mentioned above, parts of the organic matter are destroyed by oxidation during the oxidation. Entered chloride impurities are converted to chlorine and discharged as a gas. For example, the chlorine in sodium hydroxide solution can be rendered harmless by conversion into sodium hypochlorite and can 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 deplete in the catholyte. The pH values at which the various impurities are depleted are summarized in Table I below.

Table I

pH hydroxide

Ag (OH)

7 - 8 Co (OH) ₂ Ni (OH) ₂

Pb (OH) ₂ Fe (OH) ₂ Zn (OH) ₂

Cu (OH) ₂ Cr (OH) ₃

AI (OH). pH hydroxide

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 _v , ions into Rh _HI ions and an associated depletion of the rhodium in the cathode space can be avoided.

To improve the exchange of materials, the anolyte is circulated and continuously pumped past the cation exchange membrane, its flow rate being 1 - 4 1 / min.

The temperature of the anolyte is 20 - 50 ° C, so that the sulfuric and / or phosphoric acid rhodium solutions obtained in industry can be regenerated directly.

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 / 1 concentrated phosphoric acid and / or 25 - 80 ml / 1 sulfuric acid. At higher acid levels, the rhodium solution to be purified becomes less acidic to reduce the acidity and / or to deplete any anionic that is present Parts passed into the cathode compartment of an additional electrolysis cell (not shown) in the anolyr circuit, which is separated from the associated anode compartment filled with an aqueous potassium sulfate solution by an anion exchange membrane. The acid residue anions S0 ₄ ^'" and / or PO. ^"" 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 invention is explained in more detail below with the aid of some special exemplary embodiments.

Example 1:

50 ml of a rhodium sulfate solution with a content of 3.1 g / 1 rhodium were adjusted to a pH of 13.5 with potassium hydroxide and, according to the electrolysis process described above, at 20 A / dm ² electrolysers. After 10 minutes, the solution turned deep purple, indicating the oxidation of trivalent rhodium to hexavalent rhodium. After 5 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 electrolyzed without the addition of potassium hydroxide under the same conditions as in Example 1. After 6 hours, the anolyte solution contained only 1.5 g / 1 rhodium, so that about 50% of the 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 than hexavalent rhodium. This depletion takes place together with the hydronium ions (H ₃ 0 ⁺ ).

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 formation of coffee-brown Rh0 _{2 is} favored in the electrolysis of an alkaline rhodium solution. 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 _v , ions in the membrane electrolysis cell, the pH in the anolyte is reduced from pH = 12 to pH = by the migration of potassium ions from the anode to the cathode compartment 1, 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, _π ions and the rhodium is depleted from the rhodium solution to be refined.

The above-mentioned pH range, in which depletion of the rhodium can be largely avoided, is obtained from Example 3.

Example 3;

A rhodium sulfate solution containing 3.4 g / 1 rhodium was adjusted to a pH of 12.5 with potassium hydroxide solution. By adding the potassium hydroxide solution, the Rhodium solution diluted to 3.0 g / 1. 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 after the electrolysis was 3.6 g / l due to the water transfer carried out with the potassium ion transfer. If the pH value drops further, rhodium depletion occurs. After an electrolysis time of 2 hours, the rhodium concentration in the analyte solution had 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 pH 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 1 / min, the Fe / Ni impurities could be reduced to a value of 50 ppm within 5 hours. The rhodium depletion was 0.3 g / 1.

Example 5;

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

As can be seen from the following example, also remove traces of gold and silver with the method according to the invention.

Example 6;

A depletion to 50 ppm silver and gold could be achieved from a rhodium sulfate solution with 300 ppm impurities in silver and gold by means of the inventive method described above after an electrolysis time of 5 hours.

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

If the rhodium sulfate solution to be regenerated contains an excessively high acid content, then, as already mentioned above, the acid can be depleted by electrolysis in a separate electrolysis cell, the anionic constituents contained in the rhodium solution also being separated off.

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

Example 7;

A rhodium solution containing 100 ml / 1 sulfuric acid was placed in the cathode compartment with a

Given anion exchange membrane provided electrolysis cell. 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 / 1.

Embodiments 1-7 relate to the regeneration of contaminated rhodium sulfate solutions. However, it was on it noted that comparable results were obtained when measurements were carried out in parallel on phosphoric acid rhodium solutions.

For example, a rhodium solution containing 5 g / l rhodium potassium hydroxide and contaminated with 200 ppm tin, 200 ppm lead, 150 ppm iron and 100 ppm copper was added to increase 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 led past the cation exchange membrane at a rate of 3 l / min. The movement of the catholyte, which comprised 5% sulfuric acid, was due to the evolution of hydrogen during electrolysis. Under the specified conditions, potassium ions and most of the impurities 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 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 sulfuric and / or phosphoric acid rhodium solutions but also for the regeneration of chloride-containing rhodium solutions. 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 each, 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 at a rate of 3 1 / min on the membrane passed while the catholyte, which comprised 20% hydrochloric acid, was moved by the hydrogen produced during the electrolysis. During the electrolysis, Cl- was converted into Cl ₂ , 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 <20 ppm after 5 hours. Hydrochloric acid was carefully added to the anolyte during depletion.

Claims

claims

1. A method for the electrolytic regeneration of contaminated rhodium solutions, characterized in that the rhodium solution to be regenerated is passed into the anode compartment (12) of an electrolytic cell (10), which from the associated cathode compartment (14) filled with a dilute, highly conductive acid, through a cation exchange membrane (16) is separated, the pH of the rhodium solution being increased to a value of more than 10 by adding an alkali solution and the electrolysis being carried out at such high current densities that trivalent rhodium is oxidized to hexavalent rhodium in the rhodium solution and any impurities present are depleted in the catholyte by the cation exchange membrane (16).

2. The method according to claim 1, characterized in that a pH value between 12 and 14 is set.

3. The method according to claim 1 or 2, characterized in that the pH of the anolyte in the electrolysis is always greater than or equal to 1.5, and is optionally adjusted accordingly by adding an alkali solution.

4. The method according to any one of claims 1 to 3, characterized in that concentrated potassium hydroxide solution is used as the alkali solution.

5. The method according to any one of claims 1 to 4, characterized in that the current density in the electrolysis is 1 - 20 A / dm ² .

6. The method according to any one of claims 1 to 5, characterized in that the temperature of the anolyte is 20 - 50 ° C.

7. The method according to any one of claims 1 to 6, characterized in that the anolyte and / or the catholyte in the circuit are continuously guided past the cation exchange membrane (16).

8. The method according to claim 7, characterized in that the flow rate of the anolyte is 1-4 1 / min.

9. The method according to any one of claims 1 to 8, characterized in that the acid content of the rhodium solution is 40-80 ml / 1 concentrated phosphoric acid and / or 25-80 ml / 1 sulfuric acid.

10. The method according to any one of claims 1 to 8, characterized in that rhodium solutions with an acid content of more than 80 ml / 1 to reduce the acid content and / or to deplete any anionic components present in the cathode compartment of an additional electrolysis cell, which are led by the associated anode space filled with an aqueous sodium sulfate solution is separated by an anion exchange membrane.

11. The method according to any one of claims 1 to 10, characterized in that the dilute, highly conductive acid comprises H ₂ S0 ₄ , H ₃ P0 ₄ , HCl or HN0 ₃ .

12. The method according to claim 11, characterized in that the dilute, highly conductive acid comprises 10-20% H ₂ S0 ₄ .

13. The method according to any one of claims 1 to 12, characterized in that the pH of the dilute, highly conductive acid is adjusted to pH <0.5 in order to enable a separation of the transferred noble metals.