GB1575025A - Process for the separation of metals - Google Patents

Description

(54) PROCESS FOR THE SEPARATION OF METALS (71) We, PGP INDUSTRIES, INC., a corporation of the State of Delaware, having a place of business at 13429 Alondra Blvd., Santa Fe Springs, California 90670, U.S.A., do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the separation of rhodium from ruthenium and/or iridium in aqueous acidic solutions containing rhodium and at least one of ruthenium and iridium.

The separation of rhodium and iridium has long been considered a difficult aspect of platinum group metal separation. The traditional methods for separating rhodium-iridiumruthenium from one another are well known in the art, but involve long and tedious processing operations.

More recently, somewhat faster methods have been evolved for separating iridiumrhodium-ruthenium from aqueous solution by ion exchange. There are, however, several drawbacks and disadvantages involved in such processes. Ruthenium may be reduced on ion exchange resins and the IrCl62~ ion is difficult to elute. Also, the nature of the rhodium-containing species is very sensitive to solution conditions on the resin column and may change when the metal bearing solution is on the column to prevent the separation.

Tertipis et al describe the solvent extraction of iridium from hydrochloric acid solutions containing rhodium through the use of tributyl phosphate in Analytical Chemistry 33 (1961), No. 12, pages 1650 to 1652. However, this technique is undesirable since it involves reaction conditions which significantly restrict its general usefulness.

The problem of successfully separating rhodium from iridium by solvent extraction with tributyl phosphate is further complicated when the pregnant aqueous solution in which the metals are dissolved also contains ruthenium. In such instances substantial difficulty is encountered in obtaining a pure rhodium product as the ruthenium contaminates both the iridium and rhodium thereby frustrating the isolation of a pure form of either metal.

According to the present invention there is provided a process for the separation of rhodium values from ruthenium and/or iridium values in an aqueous acidic solution containing rhodium and at least one of ruthenium and iridium, which process comprises contacting the aqueous acid solution with a water-immiscible organic solvent having dissolved therein a quaternary ammonium salt the cation of which has the structure

(wherein each of Rl, R2, R3 and R4 represents a hydrocarbon group), the salt having a solubility of at least 1% in the solvent, the contacting step being carried out under oxidizing conditions and the rhodium being in cationic form separating the aqueous rhodium-containing raffinate phase and organic ruthenium and/or iridium-containing phase so formed, contacting the separated organic phase sequentially with a sufficient quantity of an aqueous alkaline stripping agent solution to neutralize the organic phase and with at least the stoichiometric amount, based on the alkaline solution, of an acidified aqueous reducing agent, and separating the aqueous ruthenium and/or iridium-containing aqueous phase and stripped organic phase so formed.

Rhodium may then be recovered from the aqueous raffinate phase and if desired ruthenium and/or iridium recovered from the aqueous phase containing them.

This process enables rhodium values to be separated quickly and easily from iridium ånd/or ruthenium in an aqueous acid solution containing rhodium and at least one of iridium and ruthenium with an organic solvent containing an organically substituted quaternary ammonium salt. In this process the iridium and/or ruthenium are extracted into the solvent phase, and the rhodium is left in the aqueous phase from which it can be recovered.

The starting aqueous acidic solution used in the process is usually an aqueous mineral acid solution, preferably an aqueous hydrochloric acid solution. This aqueous solution is contacted with the solution of quaternary ammonium salt in organic solvent under oxidizing conditions. The rhodium is maintained in the form of a cationic complex. Under the oxidizing conditions the quaternary ammonium salt is believed to form a complex with iridium and/or ruthenium present in the solution in anionic state, the complex being preferentially soluble in and therefore extracted into the solvent phase leaving rhodium (present in the acid solution as a cation) in the aqueous raffinate phase.

As used in the present specification, the term "raffinate" refers to an aqueous solution (or phase) after solvent extraction, i.e. a solution that has been depleted of all or part of its valuable meal content by transfer to an organic phase.

The rhodium, present in the acid solution in its cationic state, is not extracted into the organic phase and can be recovered from the aqueous raffinate by conventional processes such as cementation with copper powder.

The starting aqueous phase from which iridium and/or ruthenium are extracted in the present invention is ordinarily a mineral acid leach solution of the type normally resulting from the fusion and leaching of platinum metal ore concentrates. The invention will be further described by reference to separation and recovery of rhodium, iridium and ruthenium from hydrochloric acid solutions such as generally occur in platinum group metal recovery. However, the invention may be operated to separate metals from solutions of other, generally mineral, acids which are used in the recovery or assay of platinum group metal values provided the acid does not attack or degrade the organic extractant and will afford the formation of organic soluble complexes of the desired metals with the quaternary amine extractant.In aqueous chloride solutions, the soluble rhodium-iridium-ruthenium compounds are generally present as complex chloro salts or in a form of their corresponding hydrochloric acid complexes. Typically, such leach solutions result from crude ore processing operations and range between 0.1 to 5 N HC1 and up to 250 grams per litre Cl and in some instances higher. In addition to the platinum group metals, the solutions may contain other impurities such as silica, silver and base metal impurities such as lead, copper, bismuth. nickel, aluminium. and barium.

It has been discovered that in order to achieve an effective separation of iridium and ruthenium from rhodium in the preferred hydrochloric acid solutions, rhodium should be present in the form of a cationic chloro complex of rhodium and for the iridium and ruthenium to exist as oxidised iridium and ruthenium chloro complexes respectively. It is believed that the oxidised iridium and ruthenium chloro complexes behave as an anion toward the organic extraction mixture and are extracted, whereas the rhodium chloro complex behaves as a cation and is not extractable with the organically substituted quaternary ammonium extraction agent.The foregoing differences in ionic condition are maintained throughout the extraction so that the organic phase containing the quaternary ammonium salt does not become fouled with rhodium chloro complexes which would behave as anionic species and be extracted. The aqueous acid solutions from which the aforementioned metals are extracted are preferably substantially free of gold, iron, platinum, and palladium which may be removed beforehand by conventional techniques well known in the art.

Suitable quaternary ammonium salts, which may be used in the present process, have a cation of the following basic structure:

wherein each of R. R2, 1:3 and R4 represents a straight or branched alkyl or aromatic hydrocarbon group. Generally at least one of Rl, R2. R3 and R4 is a straight or branched alkyl group.Aliquat 33tri. methyl tricaprylyl ammonium chloride, manufactured by General Mills, is an effective such extractant and has the following cation:

wherein Rl, R2 and R3 are hydrocarbon chains having 8 to 10 carbon atoms, with 8 carbon atoms prevailing. [ALIQUAT is a registered Trade Mark.] Also useful as the quaternary ammonium extractant are Adogen 468 methyltri-n-alkylammonium chloride (average Cut0), and Adogen 464 methyltri-n-alkylammonium chloride (Cs-Clo) (both made by Ashland Chemical Co.). The organically substituted quaternary ammonium salt used in the present invention must be sufficiently soluble in the solvent used to make at least a 1% solution.

Also, the ammonium compound should provide for ready phase disengagement following extraction. The preferred organic extractant in the present invention is Aliquat 336. Prior to use in the extraction the quaternary ammonium extractant is usually conditioned to the form of the acid solution to be contacted. Thus in the preferred embodiment in which iridium and ruthenium are extracted from hydrochloric acid solution, the extractant is conditioned to chloride form e.g. by treatment with NaCl in 1N HC1.

The major constituent of the extraction liquid is a water-immiscible organic solvent in which the quaternary ammonium extractant is dissolved. Conventional organic solvents including, for example, aliphatic hydrocarbons such as petroleum derived liquid hydrocarbons, either straight chain or branched, kerosene, and fuel oil, are useful in the invention.

Various aromatic solvents or chlorinated aliphatic solvents may also be employed such as benzene, toluene, xylene, carbon tetrachloride and perchloroethylene. The organic solvents must be substantially water-immiscible and capable of dissolving the organically substituted quaternary ammonium extractant. In addition, the solvent should be inert and not interfere with the extraction of iridium and ruthenium metal values from acid solution by the organically substituted quaternary ammonium compound. Kerosene available as AMSCO 175 is preferably employed because of its ready availability and as a matter of economy. {AMSCO is a registered Trade Mark.] A phase modifier is preferably mixed with the organic solvent and extractant to prevent the formation of a third phase in stripping the pregnant organic.Water-insoluble straight or branched chain aliphatic alcohols containing at least 6 carbon atoms are generally used as phase modifiers. Examples of suitable phase modifiers include isodecanol, 2-ethyl-hexanol and tridecanol. Isodecanol is preferred for use in the present invention.

The organic extractant solution used in the present invention will usually contain 5 to 15 volume percent of the organic quaternary ammonium extractant, between 85 and 95 volume percent of the organic solvent, and from 1 to 5 volume percent of the phase modifier. Since only a limited amount of the active extracting ingredient is present in the solvent phase, it can only hold a limited amount of any given metallic element at saturation.

Once the concentration of metal in the solvent has reached the saturation level, no additional metal will go into the solvent regardless of its concentration in the aqueous phase. The quantity of metal which a given solvent extractant will hold is termed "the maximum loading' and governs the total quantity of solution required to do a given amount of extraction. Based upon the maximum loading characteristics of the-particular extractant, the metal-bearing characteristics of the liquor to be extracted and the number of extraction stages to be employed, the concentration of extractant and phase modifier in the solvent may be adjusted, or the Organic/Aqueous (O/A) ratio for any particular extractant concentration may be varied to achieve a desired loading.In one effective version of the extraction process the organic liquid mixture used to extract iridium and ruthenium from an aqueous hydrochloric acid solution comprises 10 volume percent Aliquat 336, 87 volume percent kerosene and 3 volume percent isodecanol. As a measure of economy, it is preferred to employ the lowest organic/aqueous ratio that will provide efficient separation of the desired metal values from a given aqueous acid solution.

As mentioned above the contacting step is carried out under oxidizing conditions.

Preferably the metal bearing acid solution is conditioned (oxidized) to an e.m.f. or redox potential as measured by means of platinum-calomel electrodes of between -500 and -1000 millivolts (optimally about -900 mv) prior to the organic extraction in order to maintain high extraction efficiencies and promote the production of rhodium solutions essentially barren of iridium and ruthenium. It should be noted that while the extraction process will operate at emf values less than -500 mv, extraction efficiencies tend to become correspondingly lower. The conditioning treatment is continued through the extraction to ensure that the aqueous phase is maintained in the oxidized state.The conditioning operation can be accomplished by the addition of sodium hypochlorite (NaOCl) solution at a controlled rate to the aqueous acid solution to be extracted to maintain the solution in an oxidized condition (indicated by obtaining an emf reading between -500 and -1000 millivolts and preferably about -900 mv). Alternatively, chlorine gas (Cl2) or other oxidants (e.g., peroxide) can be employed to accomplish the same results as sodium hypochlorite.

The liquid-liquid extraction may be carried out by continuous countercurrent or batch processing procedures.

Typical apparatus for use in the present invention could include a multiple stage countercurrent mixer-settler system in which the organic solvent-containing extractant and the starting aqueous acidic solution stream are mixed together for a predetermined time period following which they are permitted to separate in a settling reservoir. The solvent and aqueous then flow in opposite directions to the next stage of contact.

As mentioned above in the present process rhodium can be recovered from the aqueous raffinate phase. The rhodium may be recovered in conventional manner e.g. by cementation with copper powder.

The separated organic phase containing the ruthenium and/or iridium complex is contacted with at least the stoichiometric quantity of an aqueous alkaline solution required for neutralisation of the separated organic phase. The treatment results in the breakup of the quaternary ammonium ruthenium and/or iridium complex to produce a precipitate containing ruthenium and/or iridium values. The alkaline treated organic phase, and preferably the whole mixture obtained after treatment with the aqueous alkaline solution, is treated with at least a stoichiometric amount, based on the stoichiometric value of the alkaline solution, of an acidified aqueous reducing agent. This subsequent treatment results in dissolution of the iridium- and/or ruthenium-containing precipitate into the aqueous acid phase.The organic phase accordingly is stripped and the organic and (loaded) aqueous phases are separated and ruthenium and/or iridium may be recovered from the aqueous phase using conventional techniques known in the art.

It has been unexpectedly discovered that a consistently higher percentage of the iridium and ruthenium metals present in the organic solvent are stripped using sequential treatment with alkaline solution and an acidic reducing agent.

The iridium-ruthenium extraction and stripping operations are preferably carried out at about 25"C although satisfactory performance has been achieved at temperatures in the range 20"C - 40"C and up to 50"C and higher. At temperatures below about 20"C the phase disengagement is slow, while operation above 40"C is hazardous due to the danger of fire.

Alkaline (stripping) reagents for use in the present invention must be water-soluble compounds which will convert the extracted metal values contained in the organic solvent into reaction products that are readily soluble when contacted with the acidic reduction solution. Stripping efficiency (i.e. the ability to remove a large quantity of metal salt per unit volume of strippant) is also an important criterion for selection of an alkaline stripping agent. Suitable alkaline stripping agents include water-soluble alkali and alkaline earth carbonates, bicarbonates and hydroxides, e.g., sodium and potassium hydroxide, carbonate or bicarbonate, although sodium hydroxide is preferably employed.The amount of alkaline strippant required is at least the quantity which will neutralise the acid complex (usually the chloride) form of the quaternary ammonium complex and desirably includes an excess of the stoichiometric amount (preferably about 50-100%) of the alkaline agent to ensure efficient stripping within the shortest possible contact times. By contacting the loaded organic phase with the alkaline stripping solution, the organic solvent-soluble iridium and ruthenium complexes are converted to insoluble metal compounds.

The acidified reducing (stripping) agents are selected based upon their capacity to contribute additional stripping action to the alkaline treated loaded quaternary ammonium organic as well as for their ability to maintain a reducing environment in the strip solution.

The latter criterion is most important to prevent inadvertent re-extraction of the platinum group metal values from the strip solution. Also, this strippant should not contribute any foreign metals to the organic phase which might cause eventual fouling or a reduction in loading capacity. Satisfactory reducing stripping agents include acidic solutions of hydrazine salts, hydroxylamine salts, SO2, and conventional organic reducing agents (i.e., organic acids, preferably dicarboxylic acids e.g. oxalic acid). The reducing stripping solutions are generally acidified to between 0.5 - 2.5 N (preferably 2.0 N) to solubilise the iridium and ruthenium salts. One suitable reducing solution is hydrazine dihydrochloride (N2H4.2HCl), most suitably acidified to 2.0 N HCI.

The quantity of acidified reducing agent utilised is generally at least the stoichiometric amount based upon the 100-150% alkaline strippant previously added, and desirably includes an excess of the stoichiometric amount (preferably about 100-150%) to ensure complete dissolution of the precipitated iridium and ruthenium values in the aqueous phase. Additionally. some further stripping action is realised during the contact period with the acidified reducing strip solution.

Concentrations of strippant solutions may be varied depending upon the organic volumes to be treated, and the stripping efficiency of a particular strippant to adjust the quantity and concentration of strip to yield strip solutions containing significant quantities of dissolved iridium and ruthenium values to avoid dilution and handling of weak and/or large volumes of solution.

Preferably the mixture obtained after treatment with alkaline reagent is contacted with sufficient acidified reducing agent solution to maintain the emf of the loaded aqueous phase below -600 millivolts.

The time required for stripping contact will vary from one loaded organic to another depending upon the particular solvent system, the quantity of iridium and ruthenium sought to be stripped and the temperature at which the stripping operation is conducted. In most instances strip contact times between 1 and 10 minutes can be utilised to provide satisfactory results.

In particular, the invention provides a continuous process for the separation and recovery of rhodium from iridium in an aqueous hydrochloric acid solution containing rhodium and iridium which process comprises: contacting the aqueous hydrochloric acid solution with a water-immiscible organic solvent having dissolved therein at least 1% by weight of quaternary ammonium chloride of the following structure::

(wherein each of R1, R2, R3 and R4 represents a hydrocarbon group), the contacting step being carried out under oxidizing conditions and the rhodium being in cationic form; separating the organic phase and aqueous raffinate phase so formed; recovering rhodium from the aqueous raffinate phase; contacting the separated organic phase with at least the stoichiometric amount of aqueous sodium hydroxide solution required to neutralise the chloride form of the quaternary ammonium complex and form an iridium containing precipitate; contacting the mixture obtained with at least a stoichiometric amount based on the stoichiometric value of the sodium hydroxide solution of an acidified aqueous reducing agent; separating the resulting iridium-containing aqueous phase and the stripped organic phase, and recovering iridium from the aqueous phase.

The invention is further illustrated by the following Examples.

The Examples presented in Table 1 were performed to illustrate the method of effecting a more complete separation of Rhodium from Iridium and Ruthenium by maintaining a high oxidation state of the aqueous feed liquor.

In Test No. 1 a predetermined amount of Iridium-Rhodium-Ruthenium aqueous acid solution analyzing 4.6 g/l Rhodium, 1.25 g/l Iridium and 4.5 g/l Ruthenium, 265 g g/l Cl- at 1 normal HC1 and having a measured emf of -520 millivolts was contacted four times in succession with fresh organic extractant at an organic to aqueous ratio of 2 to 1. Each contact was for a period of 2 minutes. Following each contact the phases were separated and the amount of Iridium and Ruthenium extracted determined by analysis. In Test No. 2 the identical Rhodium-Iridium-Ruthenium solution was adjusted to emf -900 millivolts by gaseous Cl oxidation and then extracted 4 times in succession with fresh solvent in a like manner described above.The extraction organic in both of the above examples contained 10 volume % Aliquat-336, 3 volume % isodecanol, and 87 volume % kerosene (AMSCO 175) and was conditioned to the chloride form of the organic by contacting with a solution of 100 g/l NaCl in 1 normal HC1 followed by washing using a solution 20 g/l NaCI adjusted to pH 1.5. with HCI.

TABLE 1 Rh-Ir-Ru Extraction by Aliquat-336 at Various Solution EMFs Assay g/l Test Contact Aqueous Feed Aqueous R@ffinate Loaded Organic % Extracted No. No. EMF. mv Rh Ir Ru Rh Ir Ru Rh Ir Ru Rh Ir Ru 1 -520 4.6 1.25 4.5 1 4.3 0.43 1.8 0.06 0.30 1.4 2 4.1 0.34 1.3 0.08 0.05 0.23 3 3.9 0.31 1.1 0.09 0.03 0.09 4 3.7 0.27 0.8 0.09 0.03 0.08 20 78 82 2 -900 4.6 1.25 4.5 1 4.4 0.02 0.7 0.04 0.54 2.0 2 4.1 0.03 0.1 0.07 0.02 0.27 3 3.9 0.03 0.02 0.08 0.01 0.04 4 3.8 0.03 0.008 0.06 < 0.01 0.009 17 97 99 It will be seen from the above results that maintenance of a high oxidation state during extraction results in a more complete separation of Iridium-Ruthenium from Rhodium and produces a lower Iridium/Ruthenium raffinate for recovery of Rhodium by cementation.

The Examples presented in Table 2 were performed to show the increase in stripping efficiency when using the combination alkaline plus acidified reducing solution strip treatment. For both examples in Table 2 a 10% by volume Aliquat-336 solution in kerosene containing 3% by volume isodecanol was loaded with Iridium and Ruthenium by contacting with an acidic Rhodium-Iridium-Ruthenium chloride solution oxidized to emf -900 mv by addition of 50 g/l NaOCl solution. A 100 ml portion of the loaded organic was agitated for 10 minutes with 50 g/l N2H4.2HCl in 2 N HCI at an organic to aqueous ratio of 2 to 1 at room temperature. The percentage of Iridium and Ruthenium stripped was determined by analysis of the separated phases.A second 100 ml portion of the same loaded solvent was agitated with 8 ml of 200 g/l NaOH (2X stoichiometric based on the normality of the prepared quaternary ammonium compound for 5 minutes at room temperature. Following the caustic reaction period, 42 ml of 50 g/l N2H4.2HCl in 2 N HC1 was added (2.1 X stoichiometric based on the amount of caustic solution added) and the mixture stirred for an additional 10 minutes at room temperature. The final stripped volumes so obtained had an organic to aqueous ratio of 2/1. As in the first test, the percentage of Iridium and Ruthenium stripped was determined by analysis of the separated phases.

TABLE 2 % of Iridium-Ruthenium Stripped Organic Assay g/l Assay g/l Test Loaded Stripped Strip Solution % Stripped No. Strippant Rh Ir Ru Rh Ir Ru Rh Ir Ru Rh Ir Ru 1 N2H4.2HCl 0.030 0.32 0.59 0.020 0.18 0.49 0.020 0.28 0.21 33 44 17 in 2N HCl 2 NaOH plus 0.030 0.32 0.59 0.008 0.04 0.08 0.044 0.56 0.97 73 88 86 N2H4.2 HCl in 2N HCl It can be seen from the above table that the alkaline plus acidified reductant strip system significantly increases the percentage recovery of Iridium and Ruthenium from the loaded solvent. The beneficial result obtained from the two-step stripping system is the production of good barren organic for recycle back to the extraction stages of the Rhodium separation circuit.

From the foregoing it will be seen that the present invention combines a rapid technique for separation of Iridium and/or Ruthenium fom Rhodium with an efficient extraction and stripping system. The separation and recovery procedure are quite specific and will function in solutions containing widely varying quantities of the respective metals. The economy and speed of operation of the present process make it ideal for incorporation as part of a continuous processing system for separation of Rhodium essentially free of Iridium and/or Ruthenium from aqueous acid solutions of such metals.

WHAT WE CLAIM IS: l. A process for the separation of rhodium values from ruthenium and/or iridium values in an aqueous acidic solution containing rhodium and at least one of ruthenium and iridium, which process comprises contacting the aqueous acid solution with a water-immiscible organic solvent having dissolved therein a quaternary ammonium salt the cation of which has the structure

(wherein each of R1, R2, R3 and R4 represents a hydrocarbon group), the salt having a solubility of at least 1% in the solvent, the contacting step being carried out under oxidizing conditions and the rhodium being in cationic form separating the aqueous rhodium-containing raffinate phase and organic ruthenium and/or iridium-containing phase so formed, contacting the separated organic phase sequentially with a sufficient quantity of an aqueous alkaline stripping agent solution to neutralize the organic phase and with at least the stoichiometric amount based on the alkaline solution, of an acidified aqueous reducing agent, and separating the aqueous ruthenium and/or iridium-containing aqueous phase and stripped organic phase so formed.

2. A process according to claim 1 wherein the starting aqueous acidic solution is an aqueous hydrochloric acid solution.

3. A process according to claim 2 wherein the anion of the quaternary ammonium salt is chloride.

4. A process according to any one of claims 1 to 3 wherein, in the quaternary ammonium cation. each of RI, R2, R3 and R4 represents an alkyl or aromatic hydrocarbon group.

5. A process according to claim 4 wherein each of R1, R2, R3 and R4 represents an alkyl group.

6. A process according to any one of the preceding claims wherein, during the first said contacting step, the medium is maintained at an e.m.f. between -500 and -1000 mv measured by means of platinum-calomel electrodes.

7. A process according to claim 6 wherein oxidant is introduced into the aqueous medium to maintain the e.m.f.

8. A process according to claim 7 wherein the oxidant is sodium hypochlorite.

9. A process according to any one of the preceding claims wherein rhodium is recovered from the aqueous raffinate phase.

10. A process according to claim 9 wherein copper powder is added to the aqueous raffinate phase to recover rhodium therefrom.

ll. A process according to any one of the preceding claims wherein the alkaline stripping agent solution contains a water-soluble carbonate, bicarbonate or hydroxide of an alkali metal or alkaline earth metal.

12. A process according to any one. of the preceding claims wherein the mixture obtained after contacting with the aqueous alkaline stripping agent solution is itself contacted with the acidified aqueous reducing agent.

13. A process according to claim 12 wherein the mixture is contacted with sufficient acidified reducing agent solution to maintain the e.m.f. of the aqueous phase below -600 millivolts measured on platinum-calomel electrodes.

14. A process according to any one of the preceding claims wherein the reducing agent is an acidified solution of a hydrazine or hydroxylamine salt, SO2 or an organic dicarboxylic

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (19)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   It can be seen from the above table that the alkaline plus acidified reductant strip system significantly increases the percentage recovery of Iridium and Ruthenium from the loaded solvent. The beneficial result obtained from the two-step stripping system is the production of good barren organic for recycle back to the extraction stages of the Rhodium separation circuit.

   From the foregoing it will be seen that the present invention combines a rapid technique for separation of Iridium and/or Ruthenium fom Rhodium with an efficient extraction and stripping system. The separation and recovery procedure are quite specific and will function in solutions containing widely varying quantities of the respective metals. The economy and speed of operation of the present process make it ideal for incorporation as part of a continuous processing system for separation of Rhodium essentially free of Iridium and/or Ruthenium from aqueous acid solutions of such metals.

   WHAT WE CLAIM IS: l. A process for the separation of rhodium values from ruthenium and/or iridium values in an aqueous acidic solution containing rhodium and at least one of ruthenium and iridium, which process comprises contacting the aqueous acid solution with a water-immiscible organic solvent having dissolved therein a quaternary ammonium salt the cation of which has the structure

   (wherein each of R1, R2, R3 and R4 represents a hydrocarbon group), the salt having a solubility of at least 1% in the solvent, the contacting step being carried out under oxidizing conditions and the rhodium being in cationic form separating the aqueous rhodium-containing raffinate phase and organic ruthenium and/or iridium-containing phase so formed, contacting the separated organic phase sequentially with a sufficient quantity of an aqueous alkaline stripping agent solution to neutralize the organic phase and with at least the stoichiometric amount based on the alkaline solution, of an acidified aqueous reducing agent, and separating the aqueous ruthenium and/or iridium-containing aqueous phase and stripped organic phase so formed.
2. 2. A process according to claim 1 wherein the starting aqueous acidic solution is an aqueous hydrochloric acid solution.
3. 3. A process according to claim 2 wherein the anion of the quaternary ammonium salt is chloride.
4. 4. A process according to any one of claims 1 to 3 wherein, in the quaternary ammonium cation. each of RI, R2, R3 and R4 represents an alkyl or aromatic hydrocarbon group.
5. 5. A process according to claim 4 wherein each of R1, R2, R3 and R4 represents an alkyl group.
6. 6. A process according to any one of the preceding claims wherein, during the first said contacting step, the medium is maintained at an e.m.f. between -500 and -1000 mv measured by means of platinum-calomel electrodes.
7. 7. A process according to claim 6 wherein oxidant is introduced into the aqueous medium to maintain the e.m.f.
8. 8. A process according to claim 7 wherein the oxidant is sodium hypochlorite.
9. 9. A process according to any one of the preceding claims wherein rhodium is recovered from the aqueous raffinate phase.
10. 10. A process according to claim 9 wherein copper powder is added to the aqueous raffinate phase to recover rhodium therefrom.
11. ll. A process according to any one of the preceding claims wherein the alkaline stripping agent solution contains a water-soluble carbonate, bicarbonate or hydroxide of an alkali metal or alkaline earth metal.
12. 12. A process according to any one. of the preceding claims wherein the mixture obtained after contacting with the aqueous alkaline stripping agent solution is itself contacted with the acidified aqueous reducing agent.
13. 13. A process according to claim 12 wherein the mixture is contacted with sufficient acidified reducing agent solution to maintain the e.m.f. of the aqueous phase below -600 millivolts measured on platinum-calomel electrodes.
14. 14. A process according to any one of the preceding claims wherein the reducing agent is an acidified solution of a hydrazine or hydroxylamine salt, SO2 or an organic dicarboxylic

    acid.
15. 15. A process according to claim 14 wherein the acidified reducing agent is hydrazine dihydrochloride of the formula N2H2.2HCl.
16. 16. A process according to any one of the preceding claims wherein ruthenium and/or iridium is recovered from the separated aqueous phase containing them.
17. 17. A process according to any one of the preceding claims wherein, after separation of the organic and aqueous phases, the organic phase is recycled to extract fresh aqueous metal-containing acidic solution.
18. 18. A continuous process for the separation and recovery of rhodium from iridium in an aqueous hydrochloric acid solution containing rhodium and iridium which process comprises: contacting the aqueous hydrochloric acid solution with a water-immiscible organic solvent having dissolved therein at least 1% by weight of quaternary ammonium chloride of the following structure::

    (wherein each of R1, R2, R3 and R4 represents a hydrocarbon group), the contacting step being carried out under oxidizing conditions and the rhodium being in cationic form; separating the organic phase and aqueous raffinate phase so formed; recovering rhodium from the aqueous raffinate phase; contacting the separated organic phase with at least the stoichiometric amount of aqueous sodium hydroxide solution required to neutralise the chloride form of the quaternary ammonium complex and form an iridium-containing precipitate; contacting the mixture obtained with at least a stoichiometric amount based on the stoichiometric value of the sodium hydroxide solution of an acidified aqueous reducing agent separating the resulting iridium-containing aqueous phase and the stripped organic phase, and recovering iridium from the aqueous phase.
19. 19. Rhodium, ruthenium or iridium values separated by the process claimed in any one of the preceding claims.