# Patent

## Title
Weldable alloy

## Abstract

A process for the recovery or purification of rhodium wherein: precipitate insoluble pentammino-chloro rhodium dichloride; complex cation; pentammino-nitro rhodium complex; and,

## Background
THIS INVENTION relates to the recovery and purification of Rhodium.

Rhodium is a member of the `platinum-group` of transition metals and is usually found in nature in close association with other metals of the group. It is recovered together with the other platinum group metals by various methods usually to produce a `precious metal concentrate` which contains, besides the six platinum-group metals, (viz. platinum, palladium, ruthenium, iridium, rhodium and osmium) substantial concentrations of gold, silver, and usually a number of other metals of lesser value.

Such concentrates are then treated by various processes to separate the constituents and to produce the individual metals in as pure a form as possible. Examples of such processes include those based on selective precipitation techniques and new processes employing solvent extraction, ion-exchange methods or both of the latter.

Although the processes used in the separation of the platinum metals are widely varied it is remarkable that there is usually little variation in the order in which the metals are separated. This order is usually: Ag, Au, Pt, Pd, Ru and Os, Ir and finally Rh but in some cases the order in which platinum and palladium are removed is reversed. Almost always, however, rhodium is the last element to be removed.

The reason for this is related to the chemistry of the elements and also to their relative abundance in the ores; usually the major constituents are removed first. Of all the platinum group metals, rhodium has the chemistry least suited to easy and quantitive separation from the others, while the other elements exhibit chemistry which allows their separation in efficient fashion.

Examples of the chemistry which makes for easy separation of the other platinum group metals are as follows: solution separation by precipitation or solvent extraction techniques. precipitation through ligand exchange reactions to be achieved. Solvent extraction techniques can also be used. the volatility of the tetroxide which enables it to be distilled off from solution. the unique stability of nitrosylruthenium complexes can be employed. similar to that used for platinum.

Because of these factors the other metals are usually separated away from rhodium. In consequence rhodium is gradually concentrated in solution together with the base metals present in the concentrate, and any other platinum group metal which is not quantitatively removed by the prior separation processes.

Because of this, rhodium is thus finally present in solution together with major amounts of base metals, and, almost inevitably in practice, minor amounts of all of the other platinum group metals.

The separation and purification of rhodium contained in such solutions has always presented one of the most intractable problems in the platinum group metals refining industry.

Various techniques have been used which rely on a number of different effects. Such techniques include: selectively dissolved. rhodium solution has undergone extensive prior purification, especially with respect to platinum group metal and heavy base metal content. hexanitritorhodium precipitation which generally exhibits rather poor selectivity.

Because of the lack of really effective separation techniques for rhodium, pure metal can only be produced after a number of stages in which recoveries are generally low. This means that extensive recycling of solutions is necessary and that the process time involved is very long.

It is the object of this invention to provide a process having both high selectivity and good recovery.

In accordance with this invention there is provided a process for the recovery or purification of rhodium comprising the steps of: and in the presence of ammonia, a rhodium containing solution to precipitate rhodium as the insoluble complex pentammino-chloro rhodium (III) dichloride (Rh(NH.sub.3).sub.5 Cl)Cl.sub.2, the equivalent hydroxo complex, (Rh(NH.sub.3).sub.5 OH).sup.2+, form the nitro complex (Rh(NH.sub.3).sub.5 NO.sub.2).sup.2+ and acidifying the solution with nitric acid to precipitate the nitrate salt (Rh(NH.sub.3).sub.5 NO.sub.2)(NO.sub.3).sub.2, rhodium (III) dichloride by boiling with HCl, and,

If required the step (d) above may be followed by a recrystallization step in which the nitrate salt is recrystallized using hot water to achieve further purification thereof.

Also, step (c) may be followed by a lowering of the pH of the solution formed in step (c) to between 8,5 and 9,5 and removing any precipitated base metal hydroxides by filtration prior to commencing with step (d).

The above defined steps are preferably carried out in the following manner:

The rhodium bearing solution is treated by suitable means to produce a soluble chloride complex anion in weak hydrochloric acid solution which is preferably less than 1 Molar. To this solution is added a small quantity of commercial ethanol to catalyse the formation of the desired complex and the solution is then treated with ammonia liquor or gas until a pH value greater than 9 is achieved. Formaldehyde, formic acid or any other suitable organic compound capable of generating hydride ions may be used instead of ethanol as the catalyst. The resultant solution is allowed to boil under reflux for a few hours (generally at least 2 hours) and is then acidified to a pH value less than 0 or until the free acid concentration is at least 0,5 Moles HCl per liter and the rhodium salt is filtered off.

Selectivity against base metal precipitation in this first stage is high but not absolute as the salt will include base metal hydroxides formed during the alkaline stage and which are slow to dissolve during acid treatment. Selectivity against other platinum group metals is not high in this stage.

The recovery of rhodium in the precipitate should be extremely good, exceeding 99% in some cases, the limiting concentration of rhodium in the barren solution being about 250 ppm Rh.

The salt is then slurried with demineralised or otherwise purified water and a concentrated caustic soda solution is added while boiling. Sufficient caustic soda is added to dissolve the precipitate. The solubility of the salt is at least 200 g/l Rh.

Some base metals are soluble at this high pH value (of the order of 13,5) and base metals may be removed almost entirely by lowering the pH value of the solution to between 8.5 and 9.5 using a neutralising agent. A convenient reagent is sodium bicarbonate. Filtration of the rhodium solution at this stage results in a virtually quantitive separation of rhodium from any base metals remaining after the first stage.

The filtered solution is then cooled and sodium nitrite is added in twice the stoichiometric amount. Concentrated nitric acid is then added to the solution until a pH value less than one and preferably less than zero is obtained. The solution must be kept cool during this operation.

This results in the formation of a heavy white precipitate of (Rh(NH.sub.3).sub.5 NO.sub.2)(NO.sub.3).sub.2. Selectivity against the other platinum group metals present in the solution is very high at this stage, but some impurities may be entrained in the crystals.

Recovery of rhodium in this stage should be even higher than in the first stage.

Final purification is achieved by recrystallisation of the nitropentammine salt. This salt has the useful property of being practically insoluble in cold water whereas it has a high solubility near the boiling point of water. This means that recrystallisation can be achieved with high recoveries and in fact the procedure may be repeated a number of times without incurring significant loss of rhodium.

This purified salt cannot be directly reduced to rhodium metal sponge as there is a danger of an explosion occurring. For this reason the salt is converted back to the chloropentammine by boiling it with concentrated HCl whereupon the nitrite and nitrate radicals are destroyed and the chloropentammine rhodium (III) dichloride is reprecipitated.

The operation of the procedure described above is illustrated by the following two examples:

A rhodium solution in weak HCl contained: Rh            50g/l       Ru      100 ppm Al             7g/l       Pt       20 ppm Ni             2g/l       Pd       5 ppm Fe,Cu,Pb,etc. 5-20 ppm Ir            500 ppm ______________________________________

This was treated as outlined above to give the following yields of rhodium in the salt form. 1.      initial chloropentammine dichloride 98.5% 2.      initial nitropentammine dinitrate 97.0% 3.      recrystallised         94.0% 4.      pure chloropentammine dichloride 91.5% ______________________________________

i.e. less than 10% of the rhodium was lost in effluent solutions which would require recycling.

The final pure salt was calcined to metal and analysed for impurities with the following results: Impurity          Concentration ppm ______________________________________ Ir                100 Ru                5 Pb                5 Al                25 Cu                5 Pt                5 ______________________________________

i.e. the purity was better than 99.95% which is acceptable as pure metal.

The rhodium solution treated in this case contained a different array of impurity elements as follows: Rh                  60 g/l Pt                  250 ppm Pd                  120 ppm Ir                  65 ppm Ru                  300 ppm Ag                  1200 ppm Pb                  2000 ppm Te                  150 ppm Fe                  500 ppm Cu                  750 ppm Ni                  250 ppm ______________________________________

This was treated in the same manner as in Example 1. Yields during all stages were similar, the overall yield being slightly lower at 89%.

The metal produced from the process was analysed and found to contain. Pt                   45 ppm Pd                   N.D. Ir                    5 ppm Ru                   25 ppm Ag                   10 ppm Fe                    5 ppm Cu                   10 ppm Ni                   10 ppm Pb                   15 ppm ______________________________________

i.e. a purity just below 99.99% was thus achieved.

## Claims

1. A process for the recovery or purification of rhodium comprising the steps of: and in the presence of ammonia, a rhodium containing solution to precipitate rhodium as the insoluble complex pentammino-chloro rhodium (III) dichloride (Rh(NH.sub.3).sub.5 Cl)Cl.sub.2, the equivalent hydroxo complex, (Rh(NH.sub.3).sub.5 OH).sup.2+, form the nitro complex Rh(NH.sub.3).sub.5 NO.sub.2).sup.2+ and acidifying the solution with nitric acid to precipitate the nitrate salt (Rh(NH.sub.3).sub.5 NO.sub.2)(NO.sub.3).sub.2 rhodium (III) dichloride by boiling with HCl, and,

2. A process as claimed in claim 1 in which step (d) is followed by at least one recrystallisation step in which the nitrate salt formed in step (d) is dissolved in hot water and recrystallized by cooling the resultant solution.

3. A process as claimed in claim 2 in which the recrystallization step is repeated at least one further time.

4. A process as claimed in claim 1 in which step (c) is followed by a lowering of the pH of the solution by addition of a neutralising agent to a pH of between 8,5 and 9,5 and any precipitate formed is separated from the solution prior to commencing with step (d).

5. A process as claimed in claim 4 in which sodium bicarbonate is utilized as the neutralizing agent.

6. A process as claimed in claim 1 in which the rhodium, if not already present in such form, is converted to a soluble chloride complex anion in weak hydrochloric acid solution prior to commencing step (a) of the process.

7. A process as claimed in claim 1 in which ethanol is utilized as the catalyst in step (a) of the process.

8. A process as claimed in claim 1 in which, in step (a) of the process, ammonia is added until a pH of at least 9 is achieved.

9. A process as claimed in claim 1 in which in step (a) of the process, the solution from which the stated insoluble complex is precipitated is boiled under reflux and then acidified to a free acid content of at least 0.5 M HCl prior to step (b) being carried out.

10. A process as claimed in claim 1 in which step (c) is carried out by forming a slurry of the precipitate from step (b) in purified water and adding caustic soda thereto whilst boiling the slurry.

11. A process as claimed in claim 1 in which in step (d) of the process sodium nitrite is utilized as the alkali metal nitrite and it is added in about twice the stoichiometric amount.

12. A process as claimed in claim 1 in which, in step (d) of the process, acidification with nitric acid is effected using concentrated nitric acid until the pH of the solution is less than 1.

13. A process as claimed in claim 12 in which acidification is carried out until the pH of the solution is less than zero.