Interseparation of platinum group metals

Abstract: This invention relates to a method of interseparation of platinum group metals, and in particular to the use of glycol-methacrylate chromatographic media for interseparation of the PGMs.

Description: This invention relates to a method of interseparation of platinum group metals, (PGMs) and in particular to the use of glycol methacrylate chromatographic media for interseparation of the PGMs. It is known to use hydrophilic gels as chromatographic media for the separation of simple inorganic anions and the interseparation of PGMs. These gels are usually used in the separation of biomolecules by size exclusion. There have been a number of papers published on the use of gel permeation chromatography to extract precious metals: Journal of Chromatography 135 (1977) 173-182, (auth. Limoni, Schmuckler) concerning Pt and Pd and Analytica Chimica Acta, 61 (1972) 277-283, (auth. Kitayevitch, Rona, Schmuckler) concerning Ru, for example. US Patent No 4,885,143 (Schmuckler) describes a method in which the interseparation of the PGMs from an oxidised, gold-free halide solution is achieved using a chromatographic medium, such as a polysaccharide gel (Sephadex) or a polyacrylamide gel (Biogel). Sephadex is a trademark of Pharmacia Biotech and Biogel is a trademark of Bio-Rad Laboratories. The PGMs, when dissolved in a chloride solution, form complexes, wherein at least the iridium and ruthenium ions are in the tetravalent oxidation state, which are adsorbed onto the chromatographic medium and are claimed to be eluted selectively in the order ruthenium, rhodium, palladium, platinum, iridium and caesium, although it is clear from the rest of her patent that Schumuckler meant osmium. There are disadvantages to this method; in particular we have found there is no clean separation of the PGMs, and the chromatographic media denature over a period of time which results in a steadily decreasing separation of the PGMs and consequently the media have a limited effective lifetime. The present invention has sought to overcome this problem by providing an improved method for the interseparation of platinum group metals. The present invention provides a method for the interseparation of PGMs from a PGM-containing halide feed solution comprising the steps of passing the solution through a glycol methacrylate chromatographic medium and adsorbing the PGMs onto the medium, eluting each adsorbed PGM using an acidic solution to obtain fractions, each fraction containing at least one PGM, provided that the halide solution does not contain iridium and ruthenium in the trivalent oxidation state and the order of elution is not Ru, Rh, Pd, Pt, Ir and Os. The glycol methacrylate chromatographic medium is preferably a co-polymer of ethylene glycol and methacrylic acid (for example a medium from the Macro-Prep (trademark of Bio-Rad Laboratories) range of chromatographic media) or a co-polymer of oligoethyleneglycol, glycidylmethacrylate and pentaerythrol-dimethacrylate (for example a medium from the Toyopearl (trademark of TosoHaas and previously known as Fractogel) range of chromatographic media). The presence of ether linkages in the polymer and hydroxyl groups confer a highly hydrophilic nature to both the outer and internal surfaces of the gel particles. Most preferably the medium is from the Toyopearl range of chromatographic media. This medium has been known for use in the separation of biomolecules but the advantages for use in the interseparation of PGMs had not been previously realised. Advantages of this medium over Sephadex and Biogel include an improved lifetime in acidic media and the fact that a greater pressure can be applied which permits the use of higher flow rates. The last mentioned property is of considerable benefit in the scaling up of the chromatographic process because high pressure can be applied to a column containing the medium to achieve high flow rates. This requires less material which is reflected in a reduction in cost of the process. The eluent is an acidic solution, such as hydrochloric acid. The strength of the hydrochloric acid is not important and down to very dilute concentrations can be used. However, the inventors have found that an approximately 6M solution gives beneficial results. 6M hydrochloric acid is the composition of the HCl/H 2 O azeotrope, and consequently the eluent can be readily separated and recycled. Another advantage of using a strong hydrochloric acid eluent is that the retention times of [PdCl 4 ] 2- and the tetravalent hexachloro PGM complexes are increased and this improves the interseparation. The interseparation may be carried out using a chromatographic medium using polymer beads of any particle size. However, suitably, the medium has beads of particle size of from 32 to 300µm, and preferably of from 50 to 180µm, most preferably from 50 to 100µm. The interseparation process may be carried out using known chromatographic techniques, for example a batch column, a simulated moving bed chromatograph, a continuous annular chromatograph or a "Gatling gun" chromatograph as described in more detail below. In a batch column chromatograph, the chromatographic media is contained in a single column. An aliquot of feed is loaded onto the column and then eluted. A valve arrangement is employed to switch the output so that the various products are collected in separate fractions. This is by far the most commonly used preparative chromatographic technique. With a simulated moving bed chromatograph, the chromatographic media is contained in a number of columns connected in series via an arrangement of valves. Eluent is fed in at one end and flows counter-current through the columns until it emerges at the last but one column. The last column is isolated from the rest and is eluted separately using a purge stream. The feed joins the eluent as it flows into one of the columns in the middle. At regular intervals the valves are switched so that the positions of the eluent, feed and purge are all shifted along one column. By this technique a solute which is less strongly retained will flow with the eluent along the columns and in due course emerge in the eluate stream. A more strongly retained solute will remain with the column and move in the opposite direction until it is removed from the column by the purge. The most common example is UOP's Sorbex Chromatograph. With a continuous annular chromatograph, the chromatographic media is enclosed in the annulus between two cylinders. The feed is fed into the top of the bed at a fixed point on the circumference whilst eluent flows in all the way round the rest of the annulus. The annulus is rotated with the result that the separated solutes emerge from the bottom of the annulus at different angular displacements relative to the feed point. Most of the work on continuous annular chromatographs has been carried out at Oak Ridge National Laboratories. A "Gatling gun" chromatograph is similar to the continous annular chromatograph except the chromatographic media is contained in a number of columns arranged in a circle instead of an annulus. The most common unit is the ISEP system from Advanced Separation Technologies Incorporated. The invention will be described with reference to examples which are illustrative of but are not limiting of the invention. EXAMPLE 1 Lifetime in Acidic Media Toyopearl HW40-EC (which is a hydrophilic polymer, water compatible, grade 40 - extra coarse grade) media was contained in a column having an internal diameter of 10mm, and a bed depth of an approximately 200mm. The column remained unchanged after 4 months exposure to flowing 6M HCl, flowing at a rate of 1ml min -1 . Furthermore, the back pressure across the column, for a given flowrate, remained unchanged indicating that the integrity of the gel particles was retained. Similarly the platinum retention time remained unchanged. In static equilibration experiments, the capacity of Toyopearl HW40-F (fine grade) for platinum in 6M HCl remained unchanged after being left to stand for 4 months. Results for the long term testing of Sephadex G10 using 1M HCI in addition to 6M follow. The testing conditions were effectively the same as that for Toyopearl. Two columns 10mm id were packed with a 200mm bed of Sephadex G10. Hydrochloric acid, 1 and 6M respectively, was then pumped through continuously at a flowrate of 1ml min -1 . At periodic intervals the retention time of platinum was checked using a test solution containing 20gl -1 platinum (IV) as [PtCl 6 ] 2- and 2gl -1 iridium(III) as [IrCl 6 ] 3- . The column exposed to 6M hydrochloric acid showed the greater change and after 46 days the retention time for platinum had decreased from 52.9 to 40.7 minutes (Figure 1) which equates to a 28.5% degradation (see Table 1 below). The degradation of the column exposed to 1M HCl was slower but had still decreased by 10% after 48 days. The Change in Platinum Retention Time and Calculated Percentage Degradation with Period Exposure to 1 and 6M Hydrochloric Acid for Sephadex G-10 Day Pt(IV) Retention Time (mins) Ir(III) Retention Time (mins) Percentage Degradation 1M HCl column, 0.25ml sample 20gl -1 Pt, 2gl -1 Ir 0 35.7 7.2 0 7 35.2 7.4 1.7 25 34.2 7.4 5.9 48 32.9 7.2 10 6M HCl column, 0.25ml sample 20gl -1 Pt, 2gl -1 Ir 0 52.9 6.9 0 7 50.7 7.6 6.2 21 44.6 7.2 19.1 46 40.7 7.8 28.5 Throughout the trial the gel which was exposed to 6M hydrochloric acid steadily expanded and after 21 days the column showed a 5% increase in volume. The degradation also affected the pressure drop across the column and the pressure required to give a flowrate of 1.5ml min -1 had risen from ≤3psi at the start to 11psi after 21 days. The 1M acid column had only expanded slightly and there was no measurable change in the pressure drop which remained at less than 3psi. Tests on Biogel P2 show its volume decreasing by 10% over a week, which we believe to be evidence of degradation. We have also found that Biogel turns into a gelatinous mess after several weeks' exposure to 6M HCI. EXAMPLE 2 Back Pressure Toyopearl offers advantages over Sephadex and Biogel in terms of pressure. The flowrate through a gel and hence the volume of material which can be processed depends on the pressure applied. Both Sephadex and Biogel are relatively weak and as the pressure applied increases the gel particles deform and then collapse with the result that the back pressure goes up exponentially. For Biogel P2 the maximum recommended pressure is 15psi. Toyopearl on the other hand is much more robust and can be operated at pressures of up to 7 bar ( ca 100psi) without any problems. Furthermore, Toyopearl is available as an extra coarse grade and therefore the pressure drop is consequently lower than for the smaller particles of Sephadex or Biogel.

Claims: A method for the interseparation of (platinum group metals) (PGMs) from a halide solution comprising the steps of passing the solution through a glycol methacrylate chromatographic medium and adsorbing the PGMs onto the medium, eluting each adsorbed PGM using an acidic solution to obtain fractions, each fraction containing at least one PGM, provided that the halide solution does not contain iridium and ruthenium in the trivalent oxidation state and the order of elution is not Ru, Rh, Pd, Pt, Ir and Os.. A method according to claim 1, wherein the glycol methacrylate is a co-polymer of ethylene glycol and methacrylic acid. A method according to claim 2, wherein the co-polymer of ethylene glycol and methacrylic acid is from the Macro-Prep range of chromatographic media. A method according to claim 1, wherein the glycol methacrylate is a co-polymer of oligo ethylene glycol, glycidylmethacrylate and pentaerythrol-dimethacrylate. A method according to claim 4, wherein the co-polymer of oligo ethylene glycol, glycidylmethacrylate and pentaerythroldimethacrylate is from the Toyopearl range of chromatographic media. A method according to claim 1, wherein the acidic solution is hydrochloric acid. A method according to claim 6, wherein the hydrochloric acid is of approximately 6M strength. A method according to any one of claims 1 to 7, wherein the chromatographic medium has beads of particle size of from 32 to 300 µm. A method according to claim 8, wherein the beads are of particle size of from 50 to 180µm. A method according to any preceding claim, wherein the interseparation is carried out using a batch column, a simulated moving bed chromatograph, a continuous annular chromatograph or a "Gatling gun" chromatograph.

Inventors: GRANT RICHARD ALASDHAIR, TAYLOR YVONNE, GRANT, RICHARD ALASDHAIR, TAYLOR, YVONNE
Applicants: MATTHEY RUSTENBURG REFINERS
Application year: 1996
Main classification: C22B3
IPC codes: C22B3, C22B11