SOLVENT EXTRACTION PROCESS

Abstract: A process for the extraction of palladium from an aqueous medium containing palladium and halide or pseudo-halide ions using as extractant a nitrogen-containing heterocyclic carbonamide or ester characterised in that the total concentration of free halide and pseudo-halide in the aqueous medium is less than 1 molar.

Description: <Desc/Clms Page number 1> Solvent Extraction Process This specification describes an invention relating to a solvent extraction process and especially a process for the extraction of palladium from aqueous solutions in which it may exist together with other noble metals, especially platinum, using a ligand which complexes with the metal in order to extract the metal into an organic medium. It is known from various publications, e. g. EP 57,797, EP 112,617, GB 2,150,133 and US 4,675,172 that copper can be extracted from aqueous solutions containing halide or pseudo-halide ions using, as extraction ligand, a heterocyclic carboxy amide or ester containing at least one nitrogen atom in the heterocycle (hereinafter referred to as the 'Extractant'), especially a pyridinecarboxyester of the type described in EP 57,797. Fundamental studies of the kinetics and mechanism of this extraction process have shown that it proceeds by way of an Extractantmetal complex, Ex-Cu-Hd, in which Ex is the Extractant, Cu the metal (copper) and Hd the halide or pseudo-halide ion, and that the equilibrium between the aqueous and organic phases is represented by Equation (1), Cu++ + 2Ex + 2Hd' Ex2-Cu-Hd2 (1) It is also known from EP 332314, that palladium is strongly extracted from aqueous halide or pseudo-halide solutions using the same ligands and from WO 92/01819 that the same ligands can be used to extract palladium from aqueous nitrate solutions. Subsequent studies of the extraction of palladium from nitrate solution have shown that this is favoured by high nitrate concentrations and thus behaves analogously to copper in halide solution. EP 332,314 teaches that the concentration of the halide anions should typically be at least 1 molar (1M) and may be as high as 10M. The strong extraction of palladium in conjunction with copper from solutions with halide anion concentrations in the region from 3M to 6M shown in EP 332,314 and the relationship between extraction efficiency and nitrate concentration for the extraction of palladium from nitrate solutions, suggests that, like copper, palladium conforms to a positive relationship between extraction efficiency and anion concentration and, thus, that its extraction with the Extractant would be favoured by high anion concentration. However, it has now been unexpectedly found that the mechanism for the extraction of palladium (Pd) from an aqueous halide or pseudo-halide solution is not as shown above for copper, because palladium is associated with four anions when it forms a complex with <Desc/Clms Page number 2> the Extractant and the equilibrium between the aqueous and organic phases in the extraction of palladium from halide or pseudo-halide solutions is actually represented by Equation (2), Pd-Hd4-- + 2Ex @ Ex-Pd-Hd2 + 2Hd' (2) Thus, contrary to the position with copper, extraction of palladium from aqueous halide solutions is not favoured by high halide concentration, provided there is sufficient free halide ion in the medium to form the Ex-Pd-Hd complex, i.e. at least two moles of halide per mole of metal. However, it is often desirable to recover palladium from a solution containing a very low concentration of palladium, well below 1 molar and frequently below 0.1 molar, so it is a commercially important discovery that the metal is readily extracted from solutions containing a low halide concentration. As the formation of the Extractant-metal complex requires the presence of halide ions, it is important for efficient recovery of the palladium that the aqueous medium contains at least two moles of free halide ion per mole of palladium. According to the present invention there is provided a process for the extraction of palladium from an aqueous medium containing palladium and halide or pseudo-halide ions using as extractant a heterocyclic carboxy amide or ester containing at least one fatty hydrocarbon chain characterised in that the total concentration of free halide and pseudo-halide in the aqueous medium is less than 1 molar. The fatty hydrocarbon chain or chains preferably contain in total from 5 to 50 carbon atoms, more preferably from 10 to 35 carbon atoms in one or two chains attached to the nitrogen or oxygen atom of each amide or ester group The terms 'halide' and 'pseudo-halide' have the same meaning as in EP 332,314, the content of which is incorporated herein by reference, and will be hereinafter collectively referred to as 'halide'. The concentration of free halide in the aqueous medium is preferably at or below 0.5 molar and more especially at or below 0.3 molar. The first aqueous medium may be, and preferably is, acidic, that is the pH is below 7, in order to avoid hydrolysis of the palladium complexes and precipitation of metal oxides. However, the pH is more preferably at or below 2, which may be achieved by the addition of acid, especially a mineral acid, such as HC1 or HSO. The present extraction process preferably comprises, (1) contacting a first aqueous medium containing palladium ions and halide ions, in which the concentration of halide ions is less than 1 molar, with a water-immiscible medium containing <Desc/Clms Page number 3> the extractant whereby a complex may be formed between the palladium and the extractant; (2) separating the first aqueous medium from the water-immiscible medium containing the complex; and (3) contacting the water-immiscible medium containing the complex with a second aqueous medium under conditions permitting the dissociation of the complex and transfer of the palladium ions into the second aqueous medium. The Extractant is preferably selected from the class of compounds represented by the Formula (1), K- [ (A) a (COX) y] (1) wherein K a heteroaromatic group comprising a single heterocyclic ring, a fused heterocyclic ring system or a pair of directly linked heterocyclic rings, each heterocyclic ring containing from 1 to 3 ring nitrogen atoms and each group -[(A)a(COX)y] being linked to a ring atom of a heterocyclic ring; each A independently is a linking group selected from methylene, vinylene and phenylene each of which is substituted or unsubstituted; each X independently is OR1 or NR2R3; R1 is C5-36-hydrocarbyl or substituted C5-36-hydrocarbyl; and R2 & R3 each independently is H, hydrocarbyl or substituted hydrocarbyl provided R2 and R3 together contain from 5 to 36 carbon atoms; a is 0 or 1; y is from 1 to 3; n is from 1 to m; and m is the number of free valencies on the heterocyclic ring in K or a composition of such compounds, especially a composition in which the components differ in the structure of the hydrocarbyl groups attached to the heteroaromatic group, K. The group K may be a single 5- or 6-membered heterocyclic ring or a fused ring system in which the heterocyclic ring is fused to another ring, especially a benzene ring or a pair of such rings or fused rings directly linked through carbon atoms, as in biimidazole and bibenzimidazole. Examples of such single, linked and fused ring systems are pyridine, pyrimidine, pyrazine, pyridazine, 1,2,4-triazole, triazolopyrimidine, quinoline, benzimidazole and bibenzimidazole. Extractants of this type, except for bibenzimidazoles are fully described in EP 332,414, the content of which is incorporated herein by reference. Bibenzimidazole extractants are more fully described in <Desc/Clms Page number 4> EP 196,153 and EP 513,966 the contents of which are also incorporated herein by reference. Any substitutable ring atom in K which is not occupied by a group -[(A)a(COX)y] group, including a ring atom in a non-heterocyclic fused ring, may be unsubstituted, i.e. occupied by a H, or may be substituted by a group Y, preferably selected from halogen, alkyl, aryl, alkoxy, aryloxy, aralkyl, alkaryl, cyano, nitro and carboxylic acid. Preferred substituents are chlorine, bromine, fluorine, C1-4-alkyl, C1-4-alkoxy, phenyl, phenoxy, benzyloxy, cresyl, cyano, nitro and carboxylic acid. If K carries two or more COX groups these may be the same or different and may differ in respect of the definition of the group X as -OR' or -NR2R3 or in respect of the definition of the groups R1, R2 and/or R3. Thus, if K carries two -COX groups these may be different -COOR1 groups or one may be -COORl and the other -CONR2R3. When a is other than zero, A is preferably vinylene which is unsubstituted apart from one or two -COX groups and where it carries two -COX groups these are preferably on different carbon atoms. Examples of specific extractants are 3,5-di-(isodecyloxycarbonyl)pyridine, 2-(octadecyloxycarbonyl)pyrazine, 1-(octadecyloxycarbonyl)benzimidazole, 3-(2-[hexyl]decyloxycarbonyl)pyridine, 3-(N,N-dioctylaminocarbonyl)pyridine, EMI4.1 2,2'-bis-[1-(tridecyloxycarbonyl)-benzimidazole], 2,2'-bis-[1- (siooctyldecyloxycarbonyl)-benzimidazole], 6-(2-[hexyl]decyloxycarbonyl)-7-methyl-1,2,4-triazolo[2,3-a]pyrimidine, 1-(2-[hexyl]decyloxycarbonyl)-1,2,4-triazole, 5-(isooctyldecyloxycarbonyl) -2,4-dimethylpyrimidine, 4- (isodecyloxycarbonyl)pyridazine, 4,5-di(isodecyloxycarbonyl)pyridazine and 2,2' -bis- [1- (tridecyloxy- carbonyl)-5(6)-methylbenzimidazole). The water-immiscible medium is preferably a non-polar organic liquid, such as a liquid hydrocarbon. Alternatively, it may be a solid water-insoluble and immiscible material on which the Extractant is supported, such as an organic (e.g. polymeric) or an inorganic (e.g. silicacious) support, and over which the first aqueous medium may be passed in order to bring the palladium into contact with the Extractant so that it can be complexed and retained on the solid support. The Extractant may comprise a composition of these esters and/or amides which differ in the number, position and identity of the substituents represented by Y and -COX compounds. It is especially preferred to use a composition in which the components differ in the nature of the group X in the group -COX, especially where these are isomers of hydrocarbyl groups with different degrees of branching. The main purpose of the alkyl group or groups in the group(s) -COX is to <Desc/Clms Page number 5> impart good compatibility with, or solubility in, the water-immiscible medium, particularly where this is an organic liquid in which the extractant is dissolved or solid organic matrix on which the Extractant is supported. It has been found that compatibility with the organic medium, especially solubility in the organic liquid (e. g. kerosene), is generally improved by the use of mixtures of similar or isomeric compounds of Formula (1) and/or single compounds in which the alkyl group(s) in the group(s) -COX is branched, especially multiply branched. The hydrocarbyl groups represented by R1, R2 and R3 are preferably substantially aliphatic and more preferably alkyl or alkenyl, especially iso-alkyl, such as octyl, nonyl, decyl, iso-decyl, dodecyl, tridecyl, hexadecyl and octadecyl, though they may comprise or contain cycloalkyl and/or aryl groups as in benzyl, cyclohexyl, 4-nonylphenyl and 4-dodecylphenyl. R1 and R2 (when R3 is H) preferably contain from 9 to 24 carbon atoms and are preferably branched alkyl, especially branched alkyl, such as the alkyl groups on alcohols prepared by the Guerbet and Aldol condensations. Alkyl groups of this type conform to Formula (2), - CH2 - CH(RS) - R4 (2) in which R4 and R5 may be the same or different though one generally contains 2 less carbon atoms than the other and each may be, and preferably is itself branched in one or more positions. Examples of such p-branched alkyl groups are P-hexyl-decyl, p-octyl-dodecyl and pheptyl-undecyl. The Guerbet alcohols are a good source of the mixture of various branched alkyl groups, some of which are isomeric, for the preparation of an Extractant, in the form of an ester, which is a composition of compounds differing in the identity of the alkyl groups attached to the heterocyclic group. An especially preferred p-branched alcohol containing a branched alkyl group of this type (preparable by the dimerisation of 3,5,5-trimethylhexanol) is of Formula (3), EMI5.1 CH3-C(CH3)z-CHz-CH(CH3)-CH(CHZOH)-CHz-CHz-CH(CH3)-CHz-C(CH3)z-CH3 (3) Other useful alkyl groups of this type are the isomeric nonyl groups derived from the hydroformylation of mixed octenes, the isomeric decyl groups derived from commercially available isodecanol and the isomeric tridecyl isomers derived from commercially available tridecanol. Where the Extractant is a diester or a di-sec-amide (n is 2), the alkyl groups on the two substituents preferably contain in total from 16 to 36 carbon atoms. <Desc/Clms Page number 6> Where X is a tert-amide group, R2 and R3 may be the same or different and taken together preferably contain from 5 to 36, and more especially from 16 to 36, carbon atoms. If the two alkyl groups both contain more than 4 or 5 carbon atoms the Extractant generally has reasonable solubility in.non-polar solvent even with straight alkyl chains but, if one alkyl group is short (i.e. #4 carbon atoms), the other chain is preferably branched. If the Extractant is a di-tertamide (n is 2), there are preferably from 20 to 70 carbon atoms in the 4 alkyl groups. A preferred Extractant is a pyridine derivative or composition of pyridine derivatives of Formula (4), EMI6.1 wherein X is a group -OR1 or -NR2R3; n is from 1 to 3; each R1 independently is an alkyl group provided the group or groups represented by R1 contain a total of from 16 to 36 C atoms; R2 & R3 each independently is an alkyl group provided that the groups represented by R2 and R3 contain a total of from 20 to 70 C atoms; z is from 0 to (5-n); and each Y independently is selected from halogen, alkyl, aryl, alkoxy, aryloxy, aralkyl, cyano, nitro and carboxylic acid. When z has a value other than zero the pyridine ring carries one or more substituents corresponding to the value of z in addition to the n COX groups. Where the substituent is a carboxylic acid group, the Extractant of Formula (4) is a partial ester or amide of a pyridine polycarboxylic acid. It is preferred, however, that z. is zero so that the compound is an unsubstituted pyridine carrying from 1 to 3, and more especially 2 or 3 -COX groups. Examples of Extractants of Formula (4) are esters and amides of pyridine mono- di- and tri-carboxylic acids and mixtures thereof derived from different pyridine carboxylic acids such as nicotinic acid, iso-nicotinic acid, picolinic acid, pyridine-2,4-dicarboxylic acid, pyridine- 2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid and pyridine-2,4,6-tricarboxylic acid and one or more different alcohols and/or amines as hereinbefore described. An especially preferred Extractant of Formula (4) is a composition of isomeric 3,5(di-iso-decyloxycarbonyl)-pyridines derived from commercially available isodecanol. <Desc/Clms Page number 7> Further information on suitable pyridine esters and amides is given EP 57,797, the content of which is incorporated herein by reference. Details of extractants based upon other heterocyclic systems such as the pyrimidines, pyrazines and pyridazines are described in EP 112,617, the triazoles in GB 2,150,133, and the imidazoles and benzimidazoles in EP 193,307, the contents of which are incorporated herein by reference. The present extractants are characterised by good selectivity for palladium over other metals normally associated therewith (except for copper) especially other noble metals and iron. They also have a reduced tendency to protonation, which is advantageous as protonation tends to encourage migration of inorganic ions into the second aqueous medium and also reduce selectivity. The present process may be effected by contacting the first aqueous medium with a solution of the Extractant in a water-immiscible organic liquid, whereby a complex, soluble only in the organic liquid, is formed between the Extractant and any palladium and/or copper present in the aqueous medium. Contact may be effected by intimate vigorous mixing of the two phases and then allowing them to disengage into two layers which are readily separated in a known manner. The metal or metals can be subsequently recovered from the organic liquid by selective stripping into aqueous media as hereinafter described. Examples of suitable water-immiscible organic solvents are aliphatic, aromatic and alicyclic hydrocarbons, chlorinated hydrocarbons such as perchloroethylene, trichloroethane and trichloroethylene. Mixtures of solvents may be used. Especially preferred are mixed hydrocarbon solvents such as high boiling, high flash point, petroleum fractions (e.g. kerosene) with varying aromatic content. The Extractant and its metal complexes are generally more soluble in hydrocarbon solvents having a high aromatic content, such as AROMASOL H, (essentially a mixture of trimethylbenzenes commercially available from ICI; AROMASOL is a registered trade mark). On the other hand kerosenes having a relatively low aromatic content (such as ESCAID 100 a petroleum distillate containing 20% aromatics commercially available from EXXON; ESCAID is a registered trade mark) can enhance the hydrometallurgical performance of the extractant. The concentration of the Extractant in the water-immiscible organic solvent may be chosen to suit the particular aqueous solution to be treated. Typical values of Extractant concentration in the organic phase are between about 0.1 to 2 molar, and an especially convenient range is from 0.2 to 0.8 molar in the organic solvent. Alternatively, the present process may be effected by contacting the first aqueous medium with the Extractant supported on a <Desc/Clms Page number 8> water-immiscible support, conveniently by passing the aqueous medium through a column containing the supported Extractant. The metal or metals may be subsequently stripped from the column by passage of an appropriate aqueous strip medium, as hereinafter described, through the column. The present process is useful for the extraction of palladium from a first aqueous medium as an Extractant-metal complex into a waterimmiscible medium. The palladium may subsequently be stripped into a second aqueous medium by contacting the water-immiscible medium with an aqueous alkaline medium, preferably a dilute aqueous solution of ammonia. The aqueous solution of ammonia preferably contains from 2% to 20%, more especially from 5% to 15%, (wt/vol) ammonia. By use of the present process it is possible to reduce the amount of palladium in the first aqueous medium to levels of around 1 ppm and to leave only about 1 ppm of palladium in the water-immiscible medium after stripping. The present process provides a very efficient means for removing palladium from relatively dilute aqueous solutions of the metal in the form of any appropriate salt, such as sulphate, by the addition of at least 2 moles of halide ion per mole of palladium and extraction with the Extractant as hereinbefore defined. The present process is useful for the separation of palladium from aqueous media containing palladium in the presence of other metals, especially other noble metals and those found in association with palladium in metal containing ores, because the extraction into the water-immiscible medium is selective for palladium over most metals with which it is normally associated, except copper. If the first aqueous medium contains copper, this will be extracted to some extent by the Extractants together with the palladium, but the copper and palladium can be subsequently separated by selective stripping into the second aqueous medium (copper is readily stripped into water or dilute aqueous acid. If efficient extraction of both copper and palladium is required the halide concentration in the first aqueous medium should preferably be above 1M as described in EP 332,314. According to a further feature of the present invention there is provided a process for the separation of palladium and copper contained in an aqueous medium in the presence of halide at a concentration below 1 molar, optionally in the presence of other metals, wherein, in step (3) as hereinbefore defined the water-immiscible medium containing the complex is firstly contacted with water or a dilute aqueous acid to strip copper from the water-immiscible medium and subsequently contacted with aqueous ammonia to strip palladium from the water-immiscible medium. <Desc/Clms Page number 9> The aqueous strip solution containing palladium alone can be treated in any appropriate manner to recover a palladium salt for processing to metallic palladium. The extraction stage and the strip stage of the solvent extraction process may conveniently take place at ambient temperature for example in the range from 10 C to 30 C. The invention is illustrated by the following Examples in which all parts and percentages are by weight unless otherwise stated. Example 1 A series of aqueous solutions was prepared containing 1000 mg/l palladium (added as palladium chloride, PdCl2), 0.08 moles/1 hydrochloric acid, and various amounts of NaCl to give total Clconcentrations ranging from 0.1 to 5.0 molar. An organic Extractant solution (Solution A) containing 0.1 moles (44.7g) of 3,5-di(isodecyloxycarbonyl)-pyridine (Extractant A) per litre of ESCAID 100 (ESCAID 100 is a commercial kerosene sold by Exxon for use as an organic carrier in solvent extraction processes) was prepared. Portions of Solution A (15 ml) were contacted by vigorous stirring for 2 hours at 25 C with 15 ml portions of the aqueous palladium solutions having different chloride contents. In each case the organic and aqueous phases were then separated and filtered and the aqueous phases were analysed for palladium content, by atomic absorption spectrophotometry. The results are shown in Table 1: Table 1 EMI9.1 <tb> <tb> Experiment <SEP> No <SEP> 1 <SEP> 2 <SEP> 3 <SEP> 4 <SEP> 5 <SEP> 6 <SEP> <tb> Chloride <SEP> in <SEP> feed <tb> (moles/litre) <SEP> 0.1 <SEP> 0.3 <SEP> 0.6 <SEP> 1.0 <SEP> 2.0 <SEP> 5.0 <tb> Palladium <SEP> remaining <tb> after <SEP> extraction <SEP> 0.02 <SEP> 0.03 <SEP> 0.11 <SEP> 0.56 <SEP> 10.5 <SEP> 192 <tb> (mg/1) <tb> These results clearly show the ability of the extractant to reduce the concentration of palladium in the aqueous solution to very low levels ( < 0.1 mg/1) giving a very high recovery of palladium. They also show the inverse relationship between palladium recovery and chloride concentration. Sample 3 To demonstrate the effect of the complete absence of chloride ion on the extraction of palladium, an aqueous solution was prepared containing approximately 1000 mg per litre of palladium added as palladium sulfate in a dilute sulfuric acid medium. Equal portions of this solution and Solution A described in Example 1 were mixed vigorously for 2 hours at 25 C. The phases were then separated, <Desc/Clms Page number 10> filtered and analysed for palladium by atomic absorption spectrophotometry. Analysis indicated the extraction of 568 mg/l palladium into the organic phase, with 350 mg/l remaining in the aqueous phase, in marked contrast to Example 1 (extraction in the presence of chloride) where very little palladium was left in the aqueous phase after extraction. Example 3 In this Example, the utility of the invention was demonstrated by the recovery of palladium from a simulated aqueous (sulphate) leach solution of an ore, based on the composition of an actual commercial aqueous leach solution. The aqueous leach solution was of the following composition Copper 40 g/l Nickel 60 g/l Iron (III) 25 g/l Palladium 200 mg/l To this was added 2 g/l NaCl, to give a test solution (Solution B) having a total Cl- concentration of 1.2 g/l. Equal portions of this solution and Solution A described in Example 1 were stirred vigorously for 2 hours at 25 C, the phases separated and analysed. The aqueous phase was found to contain less than 0.1 mg/l of palladium while the organic phase contained 191 mg/l of palladium, representing > 99.9% removal of palladium from the aqueous phase. Example 4 The procedure of Example 3 was repeated except that one volume of Solution A (10 ml) was contacted with ten times its volume (100 ml) of Solution B. Following vigorous stirring for 2 hours and separation, the aqueous phase was found to contain < 0.1 mg/l palladium and the organic phase was found to contain 1995 mg/l. This represents > 99.9% removal of palladium from the aqueous leach solution and a tenfold increase (compared with Example 3) in the concentration of palladium in the organic phase. Example 5 To demonstrate further the potential commercial utility of extractants of the type described in Example 1 for recovery of palladium, in conjunction with low concentrations of chloride ion, a study was made of the rate of extraction of palladium in the process, and hence the time taken to achieve the high recovery of palladium even from quite dilute palladium solutions. Equal portions (100 ml) of Solution A and Solution B were stirred vigorously at 25 C in a 1 litre cylindrical vessel fitted with baffles and mixed by a vaned impeller of diameter 50 mm which was rotated at a speed of 1000 rpm. <Desc/Clms Page number 11> A small sample of the dispersion was withdrawn from the reactor after various intervals of time and allowed to separate into aqueous and organic phases. Each aqueous and organic phase was analysed for palladium content with the results shown in Table 2. Table 2 EMI11.1 <tb> <tb> Time <SEP> Palladium <SEP> in <SEP> Aqueous <SEP> phase <SEP> Palladium <SEP> in <SEP> Organic <SEP> Phase <SEP> <tb> (min) <SEP> (mg/1) <SEP> (mg/l) <tb> 0 <SEP> 174 <SEP> 0 <tb> 0.5 <SEP> 60.2 <SEP> 127 <tb> 1 <SEP> 37.2 <SEP> 144 <tb> 2 <SEP> 8.7 <SEP> 174 <tb> 5 <SEP> < 1 <SEP> 178 <tb> This rate of extraction for palladium (Pd concentration reduced to (1 mg/l within 5 minutes) is extremely fast compared with the rate of extraction using other known palladium extractants. Example 6 In this Example, the utility is demonstrated of the present Extractants when physically supported on a polymer substrate in the recovery of palladium from solutions containing low concentrations of chloride ion. Beads of a highly porous, high surface area copolymer of styrene-divinylbenzene (commercially available under the name XAD1180, Rohm and Haas) were impregnated with a 50% solution in ESCAID 100 of Extractant A described in Example 1 to form a supported Extractant (Extractant B) . Extractant B (1.52 g) was added to 100 ml of an aqueous solution containing 100 mg/l palladium and 1 g/l of Cl- ion (0.03M). After 16 hours a sample of the aqueous phase was removed and analysed. It was found to contain < 0.1 mg/l of palladium, indicating an extraction efficiency of > 99.9%. le 7 The procedure of Example 6 was repeated using a supported Extractant (Extractant C) prepared by impregnating beads of high porosity, high surface area polyacrylate (commercially available under the name XAD7, Rohm and Haas) with a 50% solution in ESCAID 100 of Extractant A in place of Extractant B. After a period of 16 hours the palladium in the aqueous phase was found to have been reduced to < 0.1 mg/1. <Desc/Clms Page number 12> Examples 8 to 13 The procedure of Example 1 was repeated at a chloride concentration of 0.1M with each of the Extractants identified below in place of Extractant A: Ex Extractant EMI12.1 <tb> <tb> 8 <SEP> C: <SEP> 2-(octadecyloxycarbonyl)pyrazine <tb> 9 <SEP> D: <SEP> 1-(octadecyloxycarbonyl)benzimidazole <tb> 10 <SEP> E: <SEP> 3-(2-[hexyl]decyloxycarbonyl)pyridine <tb> 11 <SEP> F: <SEP> 3-(N,N-dioctylaminocarbonyl)pyridine <tb> 12 <SEP> G: <SEP> 2,2' <SEP> -bis <SEP> [1- <SEP> (tridecyloxycarbonyl) <SEP> benzimidazole] <SEP> <tb> 13 <SEP> H: <SEP> 2,2' <SEP> -bis <SEP> [1- <SEP> (tridecyloxycarbonyl) <SEP> -5 <SEP> (6) <SEP> - <SEP> <tb> methylbenzimidazole] <tb> The results are as follows: Ex Extractant Pd remaining after extraction (mg/1) EMI12.2 <tb> <tb> 8 <SEP> C <SEP> 1.1 <tb> 9 <SEP> D <SEP> < 0.5 <tb> 10 <SEP> E <SEP> < 0.5 <tb> 11 <SEP> F <SEP> < 0.5 <tb> 12 <SEP> G <SEP> 0.5 <tb> 13 <SEP> H <SEP> < 2.0 <tb> These results demonstrate the efficacy of palladium removal from aqueous solutions at low chloride concentrations using a variety of different extracants representative of those within the present invention.

Claims: Claims 1. A process for the extraction of palladium from an aqueous medium containing palladium and halide or pseudo-halide ions using as extractant a heterocyclic carbonamide or ester carrying at least one fatty hydrocarbon chain characterised in that the total concentration of free halide and pseudo-halide in the aqueous medium is less than 1 molar. 2. A process according to Claim 1 wherein the concentration of free halide in the aqueous medium is at or below 0.3 molar. 3. A process according to Claim 1 or Claim 2 comprising, (1) contacting a first aqueous medium containing palladium ions and halide ions, in which the concentration of halide ions is less than 1 molar, with a water-immiscible medium containing the extractant whereby a complex may be formed between the palladium and the extractant; (2) separating the first aqueous medium from the water-immiscible medium containing the complex; and (3) contacting the water-immiscible medium containing the complex with a second aqueous medium under conditions permitting the dissociation of the complex and transfer of the palladium ions into the second aqueous medium. 4. A process according to any one of Claims 1 to 3 wherein the extractant is selected from the class of compounds represented by the Formula (1), K- [(A)a(COX)y]n (1) wherein K is a heteroaromatic group comprising a single heterocyclic ring, a fused heterocyclic ring system or a pair of directly linked heterocyclic rings, each heterocyclic ring containing from 1 to 3 ring nitrogen atoms and each group [ (A) a (COX) y] being linked to a ring atom of a heterocyclic ring; each A independently is a linking group selected from methylene, vinylene and phenylene each of which is substituted or unsubstituted; each X independently is OR1 or NR2R3; R1 is C5-36-hydrocarbyl or substituted C5-36-hydrocarbyl; and R2 & R3 each independently is H, hydrocarbyl or substituted hydrocarbyl provided R2 and R3 together contain from 5 to 36 carbon atoms; a is 0 or 1; y is from 1 to 3; n is from 1 to m; and m is the number of free valencies on the heterocyclic ring in K <Desc/Clms Page number 14> or a composition of such compounds. 5. A composition of compounds according to Claim 4 in which the components differ in the structure of the hydrocarbyl groups attached to the heteroaromatic group, K. 6. A process according to any one of Claims 1 to 4 wherein the extractant comprises mixed isomers of 3,5-(di-iso-decyloxycarbonyl)pyridine. 7. A process according to any one of Claims 1 to 6 wherein the extractant is supported on an inert, water-immiscible substrate. 8. A process according to Claim 7 wherein the water-immiscible substrate is a polymeric material. 9. A process according to any one of Claims 3 to 8 wherein the second aqueous medium is an aqueous alkaline medium. 10. A process according to Claim 9 wherein the aqueous alkaline medium is aqueous ammonia 11. A process according to Claim 9 or Claim 10 wherein in Step 3 the water-immiscible medium is contacted with water or a dilute aqueous acid to remove any copper complexed with the extractant before it is contacted with the aqueous alkaline medium.

Inventors: COSTA MARIA-CLARA, DALTON RAYMOND FREDERICK, COSTA, MARIA-CLARA, DALTON, RAYMOND, FREDERICK
Applicants: COSTA MARIA CLARA, DALTON RAYMOND FREDERICK, ZENECA LTD, COSTA, MARIA-CLARA, DALTON, RAYMOND, FREDERICK, ZENECA LIMITED
Application year: 1995
Main classification: C22B3
IPC codes: C22B3, C22B11