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.