Separating the Platinum Group Metals by Liquid-Liquid Extraction

NEW PROCESS HAS POTENTIAL ECONOMIC ADVANTAGES OVER CONVENTIONAL SELECTIVE PRECIPITATION By P. Charlesworth Marthc.! Kuqtenhurg Refiners (I’K) Limited, Ro>ston, England

To extract the platinum group metals from the ore, and to refine them to the very high purity required for their many applications, requires a multitude of complex operations. At present the final re.fining stage that produces the individual platinum group metals is carried out by selective precipitation from a solution of mixed platinum group metals, but this is inefficient as far as the degree of separation is concerned. An improved process which makes use of liquid-liquid extraction has been developed to a pilot plant stage, and this paper highlights some of the chemical and process principles that underlie this method of separation.

The platinum group metals platinum, these are often inefficient in terms of the palladium, rhodium, iridium, osmium and separation efficiency achieved. Interfering ele- ruthenium together with silver and gold ments can be co-precipitated, and filter cakes generally occur in nature associated with the often contain entrained filtrate. Thorough major base metals iron, copper, nickel and washing of these cakes is difficult to achieve cobalt and a wide range of minor elements such owing to their nature, and their structure, and as lead, tellurium, selenium and arsenic, and they often have poor filtration characteristics. both technical and commercial considerations Processing is therefore complex, and repeated demand that the individual platinum group washing and filtration cycles are required, as metals be separated from the other metals and each stage generates recycles and residues from each other to high purity, with high yield requiring recovery. and with a high percentage recovery. The large and complex recycles that are Refining of platinum group metals consists of necessary result in low primary yields. The several stages: nature of the process, and the problems I. Ore concentration by physical techniques associated with corrosion and the engineering such as flotation of these filtration and cake handling stages, 2. I’vrometallurgical concentration producing makes plant unreliable, complicated and labour copper-nickel sulphide matte, and hydro- intensive. metallurgical concentration Recognition of the problems associated with 3. Final refining to produce the individual current technology led Johnson Matthey and platinum group metals. Matthey Rustenburg Refiners to embark on a This article concentrates on the last stage, research and development programme during the final refining stage, of this overall process. the I 970s to examine potential alternative Current refining processes are based on refining technology. Several alternatives were complex selective precipitation techniques, and examined and liquid-liquid extraction was

Platirzum Metals Reu., 1981, 25, (31, 106-112 106 identified as a technique capable of giving the is defined by a distribution coefficient, D, where desired separation characteristics, and satisfy- D(l -Concentration of element in the solvent phase ing the process constraints. A-Concentration of element in the aqueous phase This applies at a given set of conditions for the Liquid-Liquid Extraction system and is an equilibrium relationship, Generally several requirements exist for a which is usually a constant for dilute solutions. refining process, and major criteria include the However, in commercial systems solutions are avoidance of precipitates, high separation not dilute and solvents have only a limited efficiency of the desired element, and good capacity for extracting metal. A typical plot is selectivity for the desired element. shown in Figure I. Liquid-liquid extraction supplies these; as a technique it has long been recognised in the Chemical Considerations sphere of analytical chemistry. Industrial The process chemistry is the key to separa- applications are more recent and are increas- tion (I). It must allow separation of the ingly employed in the non-precious metals platinum group metals from the base metals, industry, for example during the extraction where the major difference is in complex forma- of uranium, plutonium, zirconium, hafnium, tion, and also separation of individual platinum the rare earths, copper, cobalt and nickel. group metals from each other, to a high degree Currently liquid-liquid extraction is employed of purity. commercially by Matthey Rustenburg Refiners The chloride system provides the most effec- for the separation of copper, cobalt and gold. tive operating medium for platinum group The separation relies on the desired metal metals and is widely used. The separation being selectively extracted from the aqueous process chemistry considered here is therefore phase by an immiscible organic solvent. It is based on this system. The normal platinum often forgotten, but equally important, that the group metals species encountered are shown in metal must also be capable of back-extraction Table I. These species can aquate in weak with another suitable aqueous phase. The chloride solutions and water, but this is organic and aqueous phases used must be com- inhibited in stronger chloride media. Platinum patible with process, health and safety, and cost group metal complexes are generally much considerations. more stable than the equivalent base metal The ability of the solvent to extract the metal complexes and this allows platinum group

Platinum Metals Rev., 1981, 25, (3) 107 copper. A generalised form of this reaction is: Table I Common Platinum Group Metal [MCIJ- + y S + (M C1,-,SJy-" + y C1- Chloro Species Solvating Systems Oxidation state Element Major chloro Solvating extractants are either carbon or species phosphorus bonded oxygen bearing extractants, Gold Au(lll) d8 (AuCI,) for example ketones and ethers, and react as Platinum Pt(l1) d8 (PtCI,)* follows: Pt(lV) de (PtC1,)2 - (MC1,)"- + y S * (M CIJ"-.y S Palladium Pd(ll) de (PdC1,)2 The basicity of such solvents is low, with Pd(lV) de (Pd equilibrium lying more to the left. Solvating Iridium Ir(lV) d6 (1 r CI,) *- Idlll) d6 (lrCle)3- systems particularly are effective for extraction Rhodium Rh(lll) de (Rh of gold as (AuCld-. Ruthenium Ru(IV) d4 (RuCI,)~ Ion Pair Formation Ru(lV) d4 (RU,OCI,,,)~ Rullll) d6 ( R u CI ,) Ion pair formation is of wide interest, and Ru(ll1) d6 (RuCI,H,O)* includes particularly the high molecular weight Osmium OdlV) d4 (oscl,)2- amines and quaternary ammonium compounds. In general the reaction is: [MClJ"- + nRtX- + (Ri)JMCl,l"- + nX metaldbase metals separation. Complexes con- and equilibrium depends on the basicity of R. taining the heavier donor atoms are more stable Amine solvents can be used for the extraction of and the following overall order applies: platinum and iridium. The structure of some commercially available S-C > I > Br > C1> N > 0 extractants in each class is given in Table 11. Although sulphur bonded systems have been used, and they give excellent distribution Platinum Group Metal coefficients, the kinetics of the reaction are slow Separation Schemes and the reverse reaction, stripping, is usually Various schemes can be postulated for difficult. The chloride system, while having less extracting both primary and secondary favourable, but nonetheless acceptable, extrac- materials, depending on starting feedstock and tion characteristics, gives the good overall process constraints. Following dissolution, gold extract-strip balance required for a commercial is usually removed at an early process stage, process. and is therefore considered first. The reaction mechanisms employed for the Solvent extraction for gold is well known (2) separation process fall into the following three and has been commercially operated for a categories: compound formation, solvation and number of years. Extraction as the [AuClJ ion ion pair formation. with solvating reagents such as methyl is0 butyl ketone (MIBK) or dibutyl carbitol (Butex) is Compound Formation rapid and efficient. The gold is recovered as With compound formation extractants can be metal by direct reduction of the organic phase, chelating agents, carboxylic or sulphonic following scrubbing to remove co-extracted acids, or acidic organophosphorus compounds. impurities. Important in this class are the oxime reagents. Platinum can be removed, in the absence of Substitution kinetics for the platinum group palladium and gold, by ion exchange if iridium metals, for example platinum or palladium, are is in the Ir(II1) oxidation state. This is relatively slow compared to base metals such as illustrated in Figure 2 which shows the

Platinum Metals Rev., 1981, 25, (3) 108 Table II Structure of Extractants

Reagent class Structure Commercial examples

Compound: Oximes R3 Lix 63 I rl -hydroxy R, - C - C - R, X18A I I1 OH NOH R,,Rz.R3 = Alkyl or H

p -hydroxy Lix 65 N Lix 70 P 17 P 5000 SME 529 NOH OH R, = Alkyl R, = Alkyl, Aryl, Alkaryl

Solvating: Oxygenated

Methyl is0 butyl ketone (MIBK) cT,CH - CHZ2=O

CH, - CH, - 0 - C4Hg I Di butyl carbitol 0 (Butex) I CH, - CH, - 0 - C4Hg

Ion Pair: Amines R, >N - R, Alamine 336 R2

distribution of a range of platinum group metal from leach liquors. These oximes have high chloroanions with tri-n-octyl amine. The distribution coefficients for palladium but reaction below lies well to the right. suffer from slow reaction kinetics. To overcome this, new accelerating additives based on 2RH+ + [PtClJ” = (RH),(FtCl,) organic amines and other organic compounds and consequently in the reverse, stripping, reac- containing sulphur, phosphorus and arsenic are tion it is difficult to break the ion pair unless used. When these are coupled with a novel strong acid or alkali are used. design of extractor they permit continuous Palladium extraction systems based on both commercial operation. long chain alkyl sulphides (3) and hydroxy- Copper, which may under certain conditions oximes have been reported in recent years. be co-extracted with palladium, can be washed Oximes are produced commercially and widely out prior to stripping of palladium with acid, available, particularly for copper extraction thus separating copper as an effluent stream.

Platinum Metals Rev., 1981, 25, (3) 109 tion, or extraction as [RuNOClJ- with anion exchangers (4) have been reported. Iridium can be oxidised to Ir(1V) and then removed with platinum as illustrated in Figure 2, once palladium and gold have been removed. Equally if iridium remains as Ir(III), platinum can be extracted leaving iridium in solution. It can then be oxidised to Ir(1V) and extracted. This ability to select iridium extraction by control of the iridium oxidation state gives great flexibility to the separation process. Amine solvents are preferred and extraction is similar to that for platinum. A high C1- concentration suppresses extraction of rhodium and co-extracted impurities are removed by an acid scrub stream. [IrC16]3- is less stable than [IrCl,]’- and stripping can be readily accomplished following reduction of the organic phase. Rhodium recovery can be effected from the resulting solution by conventional precipitation, cation exchange removal of base metals, or anion exchange removal. Once all of the platinum group metals have been removed, the solution, containing base metals only, can be discharged for effluent treatment.

Ruthenium and osmium extraction from Process Considerations chloro species is difficult owing to the complex Having defined the chemical basis of the series of equilibria that exist in solution. The process this must be translated into the process removal of ruthenium and osmium as tetroxides equipment capable of achieving the desired using distillation or carbon tetrachloride extrac- separation.

Platinum Metals Rev., 1981, 25, (3) 110 mined by consideration of the reaction kinetics of the process. A typical unit is illustrated in Figure 3. The unit is fed continuously and the dispersed phase flows into the settling chamber. This must be sized such that the settler is not flooded with dispersion. The dispersion-band thickness depends on several parameters such as mixing conditions, flowrates, phase continuity, and organic and aqueous phase composition. A typical curve showing the variation of dispersion band thickness with flowrate is shown in Figure 4. Not all of the desired metal is extracted in a single equilibrium contact, and several stages are usually needed. Also a practical stage is usually only about 90 per cent efficient. The number of actual stages required is derived from knowledge of the equilibrium curve as shown in Figure I, and of the efficiency and flowrate conditions required. This is shown gra- phically in Figure 5. It can be demonstrated that the operating line has a slope equal to the Equipment falls into two classes, with sub- phase flow ratio in the cascade. Mixer settlers divisions, as follows: can be readily banked into multi-stage units to Mixer Settlers-Pump Mix or Hydraulic Flow; give the desired flowsheet configuration. Columns-Packed, Pulsed and Multiple Mixer The cascade is operated with countercurrent (agitated or disc). aqueous and solvent flow. Both pump mix units, Other types of equipment exist, but these are generally favoured by the copper industry, and either variants of the above or not of normal hydraulic flow units favoured by the nuclear commercial interest. industry are used at appropriate locations. Mixer settlers are widely used as discrete Columns can also be used for operation. stage contactors. The two phases are first They are less flexible than mixer settlers and mixed in a chamber, the size of which is deter- are limited to systems with fast kinetics and

Platinum Metals Rev., 1981, 25, (3) 111 phase separation characteristics, needing programme has shown that solvent extraction relatively few theoretical stages. processes can give: (i) Operational safety due to enclosure of Process Development platinum solutions, which have some allergenic properties. Separation schemes for platinum group metal (ii) Reduction of wet solids handling. refining have been examined at bench and pilot (iii) Improved primary yields and operating plant scale for several feedstocks of interest. efficiencies. The development pilot plant programme had (iv) Shortened pipeline times. several objectives: (v) Improved intermediate products. (a) To prove that the process technology (iv) Versatility. would work on real solutions and could The net result should, once capital invest- operate for an extended period. ment has been made, give improvements to (b) To generate design and scale-up data for refinery operation and costs. the eventual production refinery. Following the research by Johnson Matthey (c) To examine long term effects such as and development with pilot plant operation of solvent poisoning and circuit accumula- this process at the Matthey Rustenburg tion. Refiners (UK) Limited, Royston Refinery, (d) To examine process yields, efficiencies, Matthey Rustenburg Refiners has recently refining time and operating costs. announced its decision to erect a Solvex produc- (e) To determine process flexibility with tion refinery on the Royston site (5). respect to feed-stocks, impurities, throughput, etc. References Significant quantities of platinum group I M. J. Cleare, P. Charlesworth and D. J. Bryson, 3. Chem. Technol. Biotechnol., 1979,29, (4), 210 metals were successfully processed during the 2 Y. Marcus and A. S. Kertes, “Ion Exchange and pilot plant phase of the process. Solvent Extraction of Metal Complexes”, Interscience, London, I 969 3 R. I. Edwards, Proc. Int. Solvent Extr. Conf., Conclusions ISEC 77, Toronto, September gth-16th, 1977, CIM Special Volume 21, Canadian Institute of Solvent extraction has a number of Mining and Metallurgy, 1979, p. 24 advantages over conventional precipitation 4 US.Patent 4,105,442; 1978 processes. The research and development 5 Platinum Metals Rev., 1981,25,(I), 31

Catalyst Systems for Exhaust Emission Control from Motor Vehicles, Past, Present and Future

The MacRobert Award lecture, with the above mixed platinum group metals could enable the title, was given in London last month by Dr. G. J. necessary major advances in technology to be made. K. Acres, who together with Dr. B. J. Cooper, Many of the improvements in activity, Mr. B. S. Cooper, Dr. W. D. J. Evans and Dr. D. selectivity and durability that were made in E. Webster, won the 1980 MacRobert Award for platinum group metal catalysts for exhaust emission their contribution to the development of automo- control, during the work that resulted in the tive catalyst systems by the Johnson Matthey MacRobert Award, have been reported here Group (Platinum Metals Rev., 1981, 21, (1). 22). already. However Dr. Acres also considered topics In the 1960s the Inter-Industries Emission that may be of great future use to the car industry. Control team, led by Ford and Mobil, defined the Two of these, namely the recent development of catalyst systems that would be required to meet lead-tolerant catalyst systems and the greater proposed legislation in the U.S.A., and Johnson control of diesel emissions, are planned to appear in Matthey considered that catalysts based upon future issues of this Journal.

Platinum Metals Rev., 1981, 25, (3) 112