WO2000053663A1

WO2000053663A1 - A polymeric or oligomeric compound and its uses in absorption processes

Info

Publication number: WO2000053663A1
Application number: PCT/IB2000/000249
Authority: WO
Inventors: Klaus Robert Koch; Jörn Derrick Schuster MILLER; Claire Joanna Lawrence
Original assignee: Klaus Robert Koch; Miller Joern Derrick Schuster; Claire Joanna Lawrence
Priority date: 1999-03-11
Filing date: 2000-03-09
Publication date: 2000-09-14
Also published as: AU2821300A

Abstract

The invention provides a polymeric or oligomeric compound of formula (I), wherein R1 is selected from linear and branched alkyl and alkenyl groups, C6 aromatic rings, substituted C6 aromatic rings, fused aromatic rings, substituted fused aromatic rings, and aralkyl groups, or is absent; R2 is selected from linear and branched alkyl and alkenyl groups, C6 aromatic rings, substituted C6 aromatic rings, linked aromatic rings, fused aromatic rings, substituted fused aromatic rings and aralkyl groups; each X is independently H, alkyl or phenyl or X-N-R2-N-X form a ring in which R2 is alkyl or substituted alkyl and the two X groups together are alkyl or substituted alkyl; and m is an integer greater than or equal to 2. The invention extends to a method of preparing the compound, and to a method of recovering a transition metal from an aqueous solution containing the transition metal.

Description

A POLYMERIC OR OLIGOMERIC COMPOUND AND ITS USES IN ABSORPTION PROCESSES

THIS INVENTION relates to a polymeric or oiigomeric compound and its use in absorption processes. In particular, it relates to a polymeric or oiigomeric compound, to a method of preparing the polymeric or oiigomeric compound and to a method of recovering a transition metal from an aqueous solution containing the transition metal.

According to a first aspect of the invention, there is provided a polymeric or oiigomeric compound of formula I

w /rherein R-_j is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, fused aromatic rings, substituted fused aromatic rings, and aralkyl groups, or is absent; R2 is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, linked aromatic rings, fused aromatic rings, substituted fused aromatic rings and aralkyl groups; each X is independently H, alkyl or phenyl or X-N-R₂-N-X form a ring in which R₂ is alkyl or substituted alkyl and the two X groups together are alkyl or substituted alkyl; and m is an integer greater than or equal to 2.

According to a second aspect of the invention, there is provided a method of preparing a compound of formula I

wherein m is an integer greater than or equal to 2, the method including the steps of reacting, in an anhydrous solvent, a bi-functional organic acyl halide of formula II

0 0

Y- C- R- C- Y ff

wherein Y is Cl, Br or I; with a thiocyanate salt to form a reaction mixture; and adding, to the reaction mixture, a diamine of formula III or formula IV or an ammo or alkylamino substituted heterocyc c amine

wherein R-j is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, fused aromatic rings, substituted fused aromatic rings, and aralkyl groups, or is absent;

R₂ is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, linked aromatic rings, fused aromatic rings, substituted fused aromatic rings and aralkyl groups; and each X is independently H, alkyl or phenyl or X-N-R₂-N-X form a ring in which R₂ is alkyl or substituted alkyl and the two X groups together are alkyl or substituted alkyl.

In one embodiment of the invention, a diamine of formula IV is added to the reaction mixture. The diamine may be a heterocyclic amine e.g . a bi-functional or multi-functional heterocyclic diamine, such as piperazine or a substituted piperazine.

Preferably, the compound of formula III or IV is added to the reaction mixture in the same anhydrous solvent as the acyl halide, e.g . anhydrous acetone.

Preferably, the reaction of the acyl halide with the thiocyanate salt is effected at elevated temperature or with heating at reflux. Similarly, the reaction mixture is preferably heated under reflux after the bi-functional amine or heterocyclic amine has been added . The thiocyanate salt is typically a metal thiocyanate salt, e g KSCN, or ammonium thiocyanate

Typically, the acyl halide is an acyl chloride

According to a third aspect of the invention, broadly, there is provided a method of recovering a transition metal from an aqueous solution containing the transition metal, the method including contacting the solution with a sorbent comprising a polymeric or oiigomeric compound having at least one thiourea moiety in its repeating unit thereby to absorb and retain the transition metal on the sorbent

Preferably, the polymeric or oiigomeric compound has two thiourea moieties in its repeating unit

More particularly according to the third aspect of the invention, there is provided a method of recovering a transition metal from an aqueous solution containing the transition metal, the method including contacting the solution with a sorbent comprising a polymeric or oiigomeric compound of formula I

wherein R^ is selected from linear and branched alkyl and alkenyl groups, C aromatic rings, substituted Cg aromatic rings, fused aromatic rings, substituted fused aromatic rings, and aralkyl groups, or is absent;

R₂ is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, linked aromatic rings, fused aromatic rings, substituted fused aromatic rings and aralkyl groups; each X is independently H, alkyl or phenyl or X-N-R -N-X form a ring in which R₂ is alkyl or substituted alkyl and the two X groups together are alkyl or substituted alkyl; and m is an integer greater than or equal to 2, thereby to absorb and retain the transition metal on the sorbent.

The solution may be strongly acidic, having a pH of 2 or less. The transition metal may then be one or more of the Platinum

Group Metals, i.e. Pt, Pd, Rh, Ir, Os, or Ru in any of their oxidation states, e.g . Pt(IV) , Pt(ll) , Pd(ll) , Ir(IV) , Rh(ll l) , Rh(l) , Ru(IV) , Os(IV) , or the like. The transition metal may also be Au (l/lll) .

The solution may be an effluent stream from a metal separation process for the production of Platinum Group Metals and may thus include anions such as chlorides, nitrates and sulphates, and transition metals, other than Platinum Group Metals, such as Cu, Ni, Fe and Zn.

Instead, the solution may have a pH of greater than 4. The transition metal may then be one or more of Ni(ll) , Cu(ll) , Fe(ll) , Fe(lll) , Zn(ll) , Co(ll) , Co(lll), Hg(ll) , Ag(l) or the like. The sorbent may absorb the transition metal, when it is a Platinum Group Metal, up to at least 20 mass % of the dry mass of the sorbent. Preferably, the sorbent absorbs the transition metal up to at least 25 mass % of the dry mass of the sorbent. Most preferably, the sorbent absorbs the transition metal up to at least 30 mass % of the dry mass of the sorbent.

The solution may be at elevated temperature when being contacted with the sorbent, e.g . at a temperature of about 60°C to 80 °C.

The method according to the third aspect of the invention may include recovering the transition metal from the sorbent, e.g . by treating the sorbent with a strong acid, or by ashing the sorbent.

R._| may be -(CH₂)_n- where n is an integer greater than or equal to one, -(CH₃) ₂C-CH₂-, -(CH₃)CH-(CH₃)CH-, -CH = CH-, -CgH₄-, - C₆H₃(COOH)- or

-(CH₂) CgH₄(CH₂) - where p and q are integers greater than or equal to one.

When R-_j is -(CH₂) _n-, n is typically between 2 and 8.

Preferably R-] is the aromatic ring -CgH^-.

R₂ may be -(CH₂) _|<- and k may be an integer greater than or equal to 1 , e.g. 6. Instead, R₂ may be the aromatic ring -CgH₄- or the linked aromatic rings -CgH^-CgH^-.

When X-N-R₂-N-X form a ring, which is preferred, the ring may be a piperazine or substituted piperazine, e.g . 2, 5 dimethyl piperazine.

Preferably, m is between 2 and 1 000. More preferably, m is between 1 0 and 20.

Typically, the compound of formula I comprises 50 mass % to 55 mass % C, e.g. 53.3 mass%, 4 mass % to 5 mass % H, e.g. 4.7 mass %, 1 5 mass % to 1 7 mass % N, e.g . 1 5.4 mass %, and 1 4 mass

% to 1 8 mass % S, e.g . 1 4.9 mass % .

The compound of formula I may be selected from the group consisting of poly (N-terephthaloylthiourea)-N',N'-piperazineand poly -(N- isophthaloylthiourea)-N' ,N'-piperazine.

The invention will now be described, by way of example, with reference to the following examples and the accompanying drawings, in which

Figure I shows a graph of the absorbtion of Pt(ll) over time by poly

(N-terephthaloylthiourea)-N' , N'-piperazine; Figure II shows a graph of the absorbtion of Pt(ll) over time by poly (N-terephthaloylthiourea)-N' , N'-piperazine and poly -(N- isophthaloylthiourea)-N', N'-piperazine; Figure III shows a graph of the absorbtion of Pd(ll) over time by poly (N-terephthaloylthiourea)-N', N'-piperazine and poly -(N- isophthaloylthiourea)-N', N'-piperazine;

Figure IV shows a graph of the absorbtion of Pt(ll) in the presence of Ni(ll) and Cu(ll) over time by poly (N-terephthaloylthiourea)-N', N'- piperazine;

Figure V shows a graph of the absorbtion of Pt(ll) in the presence of Pd (ll) over time by poly (N-terephthaloylthiourea)-N' , N'-piperazine;

Figure VI shows a graph of percentage precious metal left in solution, as a function of time, when a precious metal containing solution is contacted with poly -(N-terephthaloylthiourea)-N', N'-piperazine; and

Figure VII shows a graph of percentage platinum and selenium left in solution, as a function of time, when a platinum and selenium containing solution is contacted with poly-(N-terephthaloylthiourea)-N', N'-piperazine.

EXAMPLE 1 Synthesis of poly(l\l-terephthaloylthiourea)-IM\ N'-piperazine

To a stirred solution of KSCN (7.7973 g, 0.0802 mol) in dry acetone ( 1 00 ml) was added a solution of terephthaloyl chloride (8. 1 278 g, 0.0400 mol) in acetone (21 0 ml) under an inert atmosphere. The mixture was heated under reflux for 40 min, after which a solution of piperazine (3.4452 g, 0.0400 mol) in acetone ( 1 30 ml) was added dropwise. The reaction mixture was heated under reflux for a further 40 min. The mixture was then poured into ice water (400 ml) to crystallise the product, hereinafter referred to as B1 a. Once the acetone had evaporated off, the product was collected by centrifugation and was washed three times with water and then twice with acetone. The crude product was then dried in vacuo. The yield of B1 a was 1 0.1 907 g (76.1 8 mass %) .

1 .0022 g of B1 a was redissoived in and recrystallised from a mixture of dimethyl sulphoxide and water to yield the product hereinafter referred to as B1 b. The yield of B1 b was 0.3324 g (33.23 mass %) .

A further portion of B1 a was again redissoived in and recrystallised from a mixture of dimethyl sulphoxide and water to yield the product hereinafter referred to as B1 c.

The procedure for the synthesis of poly (N- terephthaloylthiourea)-N', N'-piperazine (Product B1 a) as set out above was repeated to give the product hereinafter referred to as B7a.

Elemental analysis of the products B1 a, B 1 b, B1 c and B7a provided the results as set out in Table 1 .

Table 1 : Elemental analysis of products Bl a, Bi b, B1 c and B7a:

* Calculated from C₂-₇H₂-₇0₄S₃N-₇ As can be seen from Table 1 , B1 b can be deemed to be an unsuccessful attempt at recrystallisation of B1 a, assuming that the calculated mass composition accurately reflects the composition of poly(N-terephthaloylthιourea)-N',N'-pιperazιne.

EXAMPLE 2

Synthesis of poly-(N-isophthaloylthiourea)-N\ N'-piperazine

To a stirred solution of KSCN (3.9827 g, 0.041 0 mol) in dry acetone (55 ml) was added a solution of isophthaloyl chloride (4 061 2 g, 0.0200 mol) in acetone ( 1 00 ml) under an inert atmosphere. The mixture was heated under reflux for 60 mm, after which a solution of piperazine ( 1 .7227 g, 0.0200 mol) in acetone (75 ml) was added dropwise The reaction mixture was heated under reflux for a further 60 mm The mixture was then poured over ice water (200 ml) to crystallise the product hereinafter referred to as B4a. Once the acetone had evaporated off, the product was collected by centπfugation and was washed three times with water and then twice with acetone The product was dried in vacuo. The yield of B4a was 5.31 80 g (79.51 mass %) .

0.5024 g of B4a was redissoived in and recrystallised from a mixture of dimethylformamide and water to yield the product hereinafter referred to as B4b. The yield of B4b was 0.2563 g (51 .02 mass %) .

Elemental analysis of the products B4a and B4b provided the results as set out in Table 2. Table 2 : Elemental Analysis of the products B4a and B4b:

Calculated from C₂₇ H₂₇ 0₄ S N-

EXAMPLE 3

A number of absorption rate studies were carried out for the products B1 a and B4a. In each of the absorbtion rate studies, a tenfold molar excess of the product (B1 a or B4a) being investigated was added to an acid solution (pH < 2) containing at least one Platinum Group Metal, and the solution was stirred vigorously. Samples (approximately 5 ml) were withdrawn with a pastuer pipette at predetermined times and were filtered through filter paper (Whatman No 1 ) into pill vials for analysis. Each absorption rate study was conducted in quadruplet, with the starting time of each solution being staggered by exactly 1 minute, so that it was possible to take samples from each solution at exactly the right time. In each case, samples were taken within 20 seconds of the time required . Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used as the detection method for the Platinum Group Metal or metals in the samples.

The following absorption rate studies were carried out: Absorption Rate Study A:

0.170 g of B1a was added to a 0,05M HCI solution containing lOOppm Pt(ll). The results for two separate studies (A1 and A2) are set out in Table 3 and is shown in Figure I.

Table 3:

Absorption Rate Study B:

0,314 g of B1a was added to a 0,05M HCI solution containing lOOppm Pd (II). The results for two separate studies (B1 and B2) are set out in Table 4.

Table 4:

Absorption Rate Study C:

0,17 g of B4a was added to a 0,05M HCI solution containing lOOppm Pt (II). The results are set out in Table 5 and in Figure 2, compared to the same study for B1a (Study A2).

Table 5:

Absorption Rate Study D:

0,314 g of B4a was added to a 0,05M HCI solution containing lOOppm Pd(ll). The results are set out in Table 6 and in Figure 3, compared to the same study for B1a (Study B2).

Table 6:

Absorption Rate Study E:

0,0943 g of B1 a was added to a 0,05M HCI solution containing 60ppm Pt (II) 500ppm Cu(ll) and 500ppm Ni(ll) . The results are set out in Table 7 and in Figure 4, compared to the previous study A2 for B1 a in the absence of Ni(ll) and Cu(ll) set out above.

Table 7:

Absorption Rate Study F:

0, 1 57 g of B1 a was added to a 0,05M HCI solution containing 50ppm P (II) , 500ppm Cu(ll) and 500ppm Ni(ll) . The results are set out in Table 8, compared to the previous study B2 for B1 a in the abence of Ni(ll) and Cu(ll) set out above.

Table 8:

Absorption Rate Study G:

0, 1 7 g of B1 a was added to a 0,05M HCI solution containing δOppm Pt (II) and 27ppm P (II) . The results are set out in Table 9 and in Figure δ . In Figure δ, the Pt(ll) absorption is compared to Study A1 set out above.

Table 9:

EXAMPLE 4

A number of capacity studies were conducted by adding differing amounts (in duplicate or triplicate) of the product under investigation (B1 a, B 1 c, B4a) to an acidic solution containing at least one Platinum Group Metal such that the metal was in excess. The solutions were stirred vigorously for 3 days for palladium solutions and 7 days for platinum solutions, at which time a 5 ml sample was withdrawn and filtered into a pill vial for ICP-AES analysis.

The result for the capacity study for B1 a in a P (ll) solution is shown in Table 1 0.

Table 10 : Results for capacity of B1a for Pt(ll)

An average of 0.3206 g Pt was absorbed per gram of B1 a.

The results for the capacity study for B1 a in a Pd (II) solution is shown in Table 1 1 .

Table 1 1 : Results for capacity of Bla for Pd(ll)

The results obtained for the palladium capacity of B1 a seem more consistent than the platinum capacity of B1 a, with an average capacity of 0.221 6 g Pd adsorbed per gram of B1 a. The results for the capacity study for B1 c in a Pt(ll) solution is shown in Table 1 2.

Table 12 : Results for capacity studies of Bl c for Pt(ll)

The results for the capacity study for B4a in a Pt(ll) solution is shown in Table 1 3.

Table 13 : Results for capacity of B4a for Pt(ll)

Clearly, the capacity of B4a for Pt(ll) , despite being inconsistent, is substantially less than the capacity of B1 a for Pt (It) , as shown in Table 1 1 above.

The results for the capacity study for B4a in a Pd ( II) solution is shown in Table 14. Table 14 : Results for capacity of B4a for Pd(ll)

Clearly, the capacity of B4a for Pd (II), despite being inconsistent, is substantially less than the capacity of B1 a for Pd(ll) , as shown in Table 1 2 above.

The Applicants believe that, in order for more precise and consistent results for these capacity studies, either these solutions must be allowed to stir for longer period of time, or the study should be performed at elevated temperature in order to ensure that any kinetic problems are avoided.

EXAMPLE 5

The kinetics of Platinum Group Metal and Au absorption by poly-(N-terephthaloylthιourea)-N , N -piperazine were tested using a typical primary feed solution from a PGM refinery.

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) were used to determine the individual PGM, Au, base metal (Cu, Ni, Fe, Pb) and amphoteric metal (Sb, Sn, As, Se, Te, Bi, Zn) concentrations of the feed solution. The individual PGM concentrations and the base metal concentrations of the solution were all above 1 .9 g/litre. The amphoteric metal concentrations were all above 60 ppm. Two 25 ml aliquots of the feed solution were then diluted to 500 ml volumes and 1 .5 M HCI concentration. Each solution was heated to 70 °C. Then 1 5 g of the polymer was added to each solution and the mixture continuously stirred . At regular time intervals, a 1 0 ml sample of each solution was filtered to remove any polymer. The filtered sample solution was then analysed by ICP-AES and ICP-MS for its PGM, base metal and amphoteric metal content. After correcting for the dilution factor, these concentration values were recalculated as a percentage of the metal concentrations in the original feed .

The results are shown in Table 1 6 and in Figure 6 and indicate that all the PGMs were absorbed by the polymer within 200 minutes - this despite the initial high concentrations of base and amphoteric metals in solution.

Table 15 : Results of polymer absorption of precious metals from real process solutions in 1 .5 hydrochloric acid: 15 q poly-(IM-terephthaloylthiourea)-IM', N'- piperazine polymer per 500 ml solution at 60-70° Celcius

EXAMPLE 6

The kinetics of platinum (Pt) absorption by poly-(N- terephthaloylthiourea)-N',N'- piperazine was also tested using the effluent generated by a Pt recovery stage in a PGM refinery. The mother liquor of this effluent is a relatively pure Pt solution. For recovery, the

Pt is precipitated as ammonium hexachloroplatinate. The solution remaining is regarded as an effluent. However, it still contains a significant amount of Pt, and is thus usually recycled in the refining process. Excepting for Se, few other metals are present in the solution.

The specific batch of effluent used for the absorption tests with the polymer was found to contain 40 ppm Pt and 1 20 ppm Se. For the tests, 500 ml aliquots of effluent with 2.0 g of polymer were stirred at room temperature. At regular intervals, a 1 0 ml sample of each solution was filtered to remove any polymer. The filtered sample solution was then analysed by ICP-MS for its Pt and Se content. These concentration values were recalculated as a percentage of the metal concentrations in the original effluent.

The results are shown Figure 7. It is clear that Pt is selectively and quantitatively absorbed by the polymer, while the Se remains dissolved in the effluent.

It is a further advantage of some of the polymeric or oiigomeric compounds of the invention, as exemplified, when used as sorbents, that they can absorb the Platinum Group Metal ions to up to

30% of the dry mass of the sorbent, and that it is relatively easy to recover the valuable Platinum Group Metal ions from these sorbents. It is yet a further advantage of the polymeric or oiigomeric compounds of the invention as exemplified that they can be prepared easily from inexpensive starting materials.

It is another advantage of the polymeric or oiigomeric compounds of the invention, as exemplified, that under appropriate conditions and in higher pH solutions, they can also serve as sorbents for transition metals other than Platinum Group Metals.

Claims

CLAIMS:

1 . A polymeric or oiigomeric compound of formula

wherein R-_| is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, fused aromatic rings, substituted fused aromatic rings, and aralkyl groups, or is absent;

R₂ is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, linked aromatic rings, fused aromatic rings, substituted fused aromatic rings and aralkyl groups; each X is independently H, alkyl or phenyl or X-N-R -N-X form a ring in which R₂ is alkyl or substituted alkyl and the two X groups together are alkyl or substituted alkyl; and m is an integer greater than or equal to 2.

2. A compound as claimed in claim 1 , wherein R-_j is -(CH₂) _n- where n is an integer greater than or equal to one, -(CH₃) ₂C-CH₂-, - (CH₃)CH-(CH₃)CH-, -CH = CH-, -CgH₄-, -CgH₃(COOH)- or - (CH₂) _DCgH₄(CH₂) _Q- where p and q are integers greater than or equal to one.

3. A compound as claimed in claim 1 or claim 2, wherein R-, is -(CH )_n- and n is between 2 and 8.

4. A compound as claimed in claim 1 or claim 2, wherein R₁ is the aromatic ring -CgH^-.

δ 5. A compound as claimed in any one of the preceding claims, wherein R₂ is -(CH )^- and k is an integer greater than or equal to 1 .

6. A coumpound as claimed in any one of claims 1 to 4 inclusive, wherein R₂ is the aromatic ring -CgH^- or the linked aromatic rings -CgH₄-CgH₄-.

0 7. A compound as claimed in any one of claims 1 to 4 inclusive, wherein X-N-R -N-X form a piperazine or substituted piperazine ring .

8. A compound as claimed in any one of the preceding claims, wherein m is between 2 and 1 000.

6 9. A compound as claimed in claim 8, wherein m is between

10 and 20.

1 0. A compound as claimed in any one of the preceding claims, which comprises 60 to 65% by mass C, 4 to 5% by mass H, 1 δ to 1 7% by mass N, and 1 4 to 1 8% by mass S. 2δ 1 1 . A compound is claimed in claim 1 , which is selected from the group consisting of poly (N-terephthaloylthiourea)-N',N'-piperazine and poly -(N-ιsophthaloylthιourea)-N',N'-pιperazιne.

1 2. A method of preparing a compound of formula I

0 o

Y- C- Rr C- Y IT

wherein Y is Cl, Br or I with a thiocyanate salt to form a reaction mixture; and adding, to the reaction mixture, a diamine of formula III or formula IV or an ammo or alkylamino substituted heterocyclic amine

H \ / H

^Hx /^H

N-R -N N-R -N

\ / ² \ w

H H X X wherein R^ is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, fused aromatic rings, substituted fused aromatic rings, and aralkyl groups, or is absent; R₂ is selected from linear and branched alkyl and alkenyl groups, C aromatic rings, substituted C aromatic rings, linked aromatic rings, fused aromatic rings, substituted fused aromatic rings and aralkyl groups; and each X is independently H, alkyl or phenyl or X-N-R₂-N-X form a ring in which R₂ is alkyl or substituted alkyl and the two X groups together are alkyl or substituted alkyl.

1 3. A method as claimed in claim 1 2, in which a diamine of formula IV is added to the reaction mixture, the diamine being a heterocyclic amine.

1 4 A method as claimed in claim 1 2 or claim 1 3, in which the compound of formula III or IV is added to the reaction mixture in the same anhydrous solvent as the acyl halide.

1 δ. A method as claimed in any one of claims 1 2 to 1 4 inclusive, in which the thiocyanate salt is a metal thiocyanate salt.

1 6. A method as claimed in any one of claims 1 2 to 1 5 inclusive, in which the acyl halide is an acyl chloride.

1 7. A method of recovering a transition metal from an aqueous solution containing the transition metal, the method including contacting the solution with a sorbent comprising a polymeric or oiigomeric compound having at least one thiourea moiety in its repeating unit thereby to absorb and retain the transition metal on the sorbent.

1 8. A method as claimed in claim 1 7, in which the polymeric or oiigomeric compound has two thiourea moieties in its repeating unit.

1 9. A method of recovering a transition metal from an aqueous solution containing the transition metal, the method including contacting the solution with a sorbent comprising a polymeric or oiigomeric compound of formula I

wherein R₁ is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, fused aromatic rings, substituted fused aromatic rings, and aralkyl groups, or is absent;

R₂ is selected from linear and branched alkyl and alkenyl groups, Cg aromatic rings, substituted Cg aromatic rings, linked aromatic rings, fused aromatic rings, substituted fused aromatic rings and aralkyl groups; each X is independently H, alkyl or phenyl or X-N-R₂-N-X form a ring in which R₂ is alkyl or substituted alkyl and the two X groups together are alkyl or substituted alkyl; and m is an integer greater than or equal to 2, thereby to absorb and retain the transition metal on the sorbent.

20. A method as claimed in claim 1 9, in which the aqueous solution has a pH of 2 or less, and the transition metal is at least one of the platinum group metals in any of the oxidation states of the metal.

21 . A method as claimed in claim 20, in which the aqueous solution includes anions and transition metals other than platinum group metals.

22. A method as claimed in claim 1 9, in which the aqueous solution has a pH of 4 or greater, and the transition metal is at least one of Ni(ll) , Cu (ll) , Fe(lll) , Zπ(ll) , Co(ll), Co(lll) , Hg (ll) or Ag (l) .

23. A method as claimed in any one of claims 1 9 to 22 inclusive, in which R-_j is -(CH₂) _π- where n is an integer greater than or equal to one, -(CH₃) ₂C-CH₂-, -(CH₃)CH-(CH₃)CH-, -CH = CH-, -CgH₄-, - CgH₃(COOH)- or -(CH₂) _pCgH₄(CH ) - where p and q are integers greater than or equal to one.

24. A method as claimed in any one of claims 1 9 to 23 inclusive, in which R-| is -(CH₂)_n- and n is between 2 and 8.

25. A method as claimed in any one of claims 1 9 to 23 inclusive, in which R-_| is the aromatic ring -CgH₄-.

26. A method as claimed in any one of claims 1 9 to 25 inclusive, in which R₂ is -(CH₂)_|<- and k is an integer greater than or equal to 1 .

27. A method as claimed in any one of claims 1 9 to 25 inclusive, in which R₂ is the aromatic ring -CgH₄- or the linked aromatic rings -CgH₄-CgH₄-.

28. A method as claimed in any one of claims 1 9 to 26 inclusive, in which X-N-R₂-N-X form a piperazine or substituted piperazine ring .

29. A method as claimed in any one of claims 1 9 to 28 δ inclusive, in which m is between 2 and 1000.

30. A method as claimed in claim 29, in which m is between 1 0 and 20.

31 . A method as claimed in any one of claims 1 9 to 30 inclusive, in which the compound of formula I comprises 60 to 65 % by 0 mass C, 4 to 5 % by mass H, 1 δ to 1 7% by mass N, and 1 4 to 1 8% by mass S.

32. A method as claimed in claim 1 9, in which the compound of formula I is selected from the group consisting of poly (N- terephthaloylthiourea)-N', N'-piperazine and poly -(N-isophthaloylthiourea)- 5 N' , N'-piperazine.

33. A compound as claimed in claim 1 , substantially as herein described and illustrated .

34. A method of preparing a compound as claimed in claim 1 2 substantially as herein described and illustrated.

0 35. A method of recovering a transition metal as claimed in claim 1 8 or claim 1 9, substantially as herein described and illustrated .

36. A new compound, a new method of preparing a compound, or a new method of recovering a transition metal, substantially as herein described.