METHOD OF SELECTIVE EXTRACTION OF PLATINOIDS, FROM A SUPPORT CONTAINING SAME, WITH AN EXTRACTION MEDIUM CONSISTING OF A SUPERCRITICAL FLUID AND AN ORGANIC LIGAND

Abstract

Method of selective extraction of a metal of the family of platinoids, from a ceramic support containing said metal, comprising the following successive steps: a) said ceramic support containing said metal is brought into contact, in an extraction chamber, with an extraction medium consisting of a pressurized dense fluid containing an organic ligand that is selective for the metal and that is capable of forming a complex with said metal in the 0 state; whereby are obtained, on the one hand, a ceramic support depleted in said metal, or even free of said metal, and, on the other hand, a medium consisting of the pressurized dense fluid containing the complex of the organic ligand with the metal in the 0 state; b) said pressurized dense fluid containing the complex of the organic ligand with the metal in the 0 state is brought back to atmospheric pressure and to ambient temperature, whereby the complex of the organic ligand with the metal in the 0 state separates from the fluid; c) the ceramic support depleted in said metal, or even free of said metal, and the complex of the organic ligand with the metal in the 0 state, are recovered.

Claims

1. A method of selective extraction of a metal of the family of platinoids, from a ceramic support containing said metal, comprising the following successive steps: a) said ceramic support containing said metal is brought into contact, in an extraction chamber, with an extraction medium consisting of a pressurized dense fluid and of an organic ligand that is selective for the metal and that is capable of forming a complex with said metal in the 0 state; whereby are obtained, on the one hand, a ceramic support depleted in said metal, or, optionally, free of said metal, and, on the other hand, a medium consisting of the pressurized dense fluid containing the complex of the organic ligand with the metal in the 0 state; b) said pressurized dense fluid containing the complex of the organic ligand with the metal in the 0 state is brought back to atmospheric pressure and to ambient temperature, whereby the complex of the organic ligand with the metal in the 0 state separates from the fluid; c) the ceramic support depleted in said metal, or, optionally free of said metal, and the complex of the organic ligand with the metal in the 0 state, are recovered.

2. A method according to claim 1, wherein the metal of the family of platinoids is selected from the group consisting of platinum, palladium, rhodium, ruthenium, gold, silver, and alloys and mixtures thereof.

3. A method according to claim 1, wherein the metal is in the form of nanoparticles dispersed on at least one external surface of the ceramic support and the metal is extracted in the form of nanoparticles of the metal in the (0) metal state.

4. A method according to claim 1, wherein the fluid is selected from the group consisting of carbon dioxide; helium; xenon; nitrogen; nitrous oxide; sulphur hexafluoride; gaseous alkanes of 1 to 5 carbon atoms; gaseous fluorinated hydrocarbons; gaseous chlorinated and/or fluorinated hydrocarbons; ammonia; and mixtures thereof.

5. A method according to claim 1, wherein the pressurized dense fluid is at a pressure of 50 to 700 bars, and at a temperature from 15° C. to 200° C.

6. A method according to claim 1, wherein the pressurized dense fluid is in the liquid state, dense gas state, or in the supercritical state.

7. A method according to claim 1, wherein the organic ligand that is selective for the metal is selected from thiols of formula RSH, in which R represents a linear or branched alkyl group of 1 to 20 C.

8. A method according to claim 1, wherein the organic ligand that is selective for the metal represents from 1 to 20% by weight of the extraction medium.

9. A method according to claim 1, wherein, simultaneously to being brought into contact, the support and/or the extraction medium are subjected to a mechanical action.

10. A method according to claim 9, wherein the mechanical action is selected from the group consisting of one or more among an agitation, a turbulence, a shear, an electromechanical action and an ultrasonic action.

11. A method according to claim 1, which further comprises, at the end of step c), a step d) during which the metal is recovered from the complex.

12. A method according to claim 1, which is carried out in static mode, and which further comprises, prior to step a), a step a1): a1) the ceramic support containing the metal and the selective organic ligand 0 are placed in the extraction chamber, then the fluid is introduced into the extraction chamber, and a temperature and a pressure are established in the extraction chamber such that the fluid is dense and pressurized; and wherein, during step b), the pressure in the extraction chamber is brought back to atmospheric pressure, the temperature is brought back to ambient temperature, the fluid is evacuated from the extraction chamber in the form of a stream of gas, the complex is carried away with the stream of gas, and optionally the complex is recovered in a separator placed at the outlet of the extraction chamber.

13. A method according to claim 12, wherein during step a) the ceramic support containing the metal is brought into contact with the extraction medium for a duration of 1 to 24 hours.

14. A method according to claim 12, wherein the proportion by weight of the ceramic support containing the metal compared to the volume of selective organic ligand is 0.01 g/mL to 0.5 g/mL.

15. A method according to claim 12, wherein during step a1) the ceramic support containing said metal and the selective organic ligand are placed in the extraction chamber in such a way that the ceramic support containing the metal and the selective organic ligand are in direct contact.

16. A method according to claim 12, wherein during step a1) the ceramic support containing said metal and the selective organic ligand are placed in the chamber in such a way that the ceramic support containing the metal and the selective organic ligand are not in direct contact.

17. A method according to claim 1, which is carried out in dynamic mode, and which further comprises, prior to step a), steps a2), a3) and a4): a2) the selective organic ligand is placed in a solubilisation chamber in fluidic communication with the extraction chamber, and the ceramic support containing the metal is placed in the extraction chamber; then a3) a stream of pressurized dense fluid is sent continuously into the solubilisation chamber, whereby is obtained the extraction medium consisting of the pressurized dense fluid and of the selective organic ligand; a4) a stream of the extraction medium is sent continuously into the extraction chamber; and wherein, at the end of step a), a stream of the pressurized dense fluid containing the complex of the organic ligand with the metal in the 0 state is drawn off from the extraction chamber and sent into a reservoir in which steps b) then c) are carried out.

18. A method according to claim 17, wherein, during step b), the pressure in the reservoir is brought back to atmospheric pressure, the temperature is brought back to ambient temperature and the fluid is evacuated from the reservoir in the form of a gas whereas the complex of the organic ligand with the metal in the 0 state remains in the reservoir.

19. A method according to claim 7, wherein R represents a linear or branched alkyl group of 1 to 12 C.

20. A method according to claim 11, wherein the metal is recovered from the complex by thermal treatment of the complex.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0094] FIG. 1 is a schematic view of an installation for the implementation of the method according to the invention according to a first embodiment thereof, carried out in static mode;

[0095] FIG. 2 is a schematic view of an installation for the implementation of the method according to the invention according to a second embodiment thereof, carried out in dynamic mode;

[0096] FIG. 3 is a graph that gives the extraction yields of palladium (in %), obtained with the ligands Cyanex® 272, Cyanex® 302, and dodecanethiol and the extraction yield of platinum (in %), obtained with dodecanethiol during extraction tests. The extraction tests were carried out by bringing into contact, in a beaker, 1 g of catalyst supported on alumina (1 g represents the total mass of alumina and metal) and 20 mL of ligand at 60° C. for 72 hours.

[0097] FIG. 4 is a graph that gives the extraction yields of palladium (in %) and platinum (in %), obtained with the dodecanethiol ligand for different contact durations, namely 5, 24, 42, and 72 hours for palladium, and 5 and 72 hours for platinum, during extraction tests. The extraction tests were carried out by bringing into contact, in a beaker, 0.25 g of catalyst supported on alumina (0.25 g represents the total mass of alumina and metal) and 5 mL of ligand at 60° C. for the desired duration.

[0098] FIG. 5 is a graph that gives the extraction yields of palladium (in %) and platinum (in %), obtained with the dodecanethiol ligand for different contact temperatures, namely 25° C., 60° C., 80° C., 110° C., and 150° C. for palladium, and 60° C. and 150° C. for platinum, during extraction tests. The extraction tests were carried out by bringing into contact, in a beaker, for 5 hours, 0.25 g of catalyst supported on alumina (0.25 g represents the total mass of alumina and metal) and 5 mL of ligand at the desired temperature.

[0099] FIG. 6 is a graph that gives the absorbance as a function of wavelength (in nm) of the solutions in dodecanethiol obtained during palladium extraction tests carried out at different contact temperatures (FIG. 5).

[0100] The tests are carried out by bringing into contact, in a beaker, for 5 hours, 0.25 g of catalyst supported on alumina (0.25 g represents the total mass of alumina and metal) and 5 mL of ligand at the desired temperature.

[0101] Curve A is the curve for the solution obtained at a contact temperature of 150° C.

[0102] Curve B is the curve for the solution obtained at a contact temperature of 110° C.

[0103] Curve C is the curve for the solution obtained at a contact temperature of 80° C.

[0104] Curve D is the curve for the solution obtained at a contact temperature of 60° C.

[0105] Curve E is the curve for the solution obtained at a contact temperature of 25° C.

[0106] FIG. 6 also shows curve F, which is the curve obtained for a solution of palladium of degree of oxidation II.

[0107] FIGS. 7A and 7B show transmission electron microscopy (TEM) images of nanoparticles of palladium (A) and nanoparticles of platinum (B) in dodecanethiol obtained by bringing into contact 0.25 g of catalyst and 5 mL of ligand at 150° C. for 5 hours.

[0108] The scale given in FIGS. 7A and 7B represents 50 nm.

[0109] FIG. 8 is a graph that gives the extraction yield (in %) after 5 hours of being in contact during palladium extraction tests carried out in static mode in supercritical CO.sub.2 medium comprising the dodecanethiol ligand at 250 bars.

[0110] The tests were carried out with 0.25 g of catalyst supported on alumina (0.25 g represents the total mass of alumina and metal) and 5 mL of ligand. The tests were carried out either with direct contact of the ligand and the supported catalyst (bars A and B) at 60° C. (bar A grey) and 110° C. (bar B white); or without direct contact of the ligand and the supported catalyst, this being placed in a basket (bars C and D) at 60° C. (bar C grey) and 110° C. (bar D white). As a reference, FIG. 8 also shows the extraction yields obtained during tests obtained by bringing into contact 0.25 g of supported catalyst and 5 mL of dodecanethiol, without CO.sub.2, at atmospheric pressure, for 5 hours respectively at 60° C. (curve E) and at 110° C. (curve F).

[0111] FIG. 9 is a graph that gives the extraction yield (in %) as a function of the flow rate of CO.sub.2 (in mL/min) during tests of extraction of palladium by dodecanethiol in supercritical medium carried out in dynamic mode for 3 hours at 250 bars and 60° C.

[0112] The tests were carried out with 0.25 g of catalyst supported on alumina and at three different flow rates, namely from left to right: 0.5; 1; and 1.5 mL/min.

[0113] FIG. 10 is a graph that gives the extraction yield (in %) as a function of the duration of the extraction (in minutes) during tests of extraction of palladium by dodecanethiol in supercritical CO.sub.2 medium carried out in dynamic mode at 250 bars, 60° C., and at a flow rate of CO.sub.2 of 0.5 m L/min.

[0114] The tests were carried out with 0.25 g of catalyst supported on alumina (0.25 g represents the total mass of alumina and metal).

[0115] FIG. 11 is a graph that gives the extraction yield (in % on the left) and the weight of palladium extracted (in g, on the right) as a function of the quantity of catalyst (in g), during tests of extraction of palladium by dodecanethiol in supercritical CO.sub.2 medium carried out in dynamic mode for 5 hours at 250 bars, 60° C., and with a flow rate of CO.sub.2 of 0.5 mL/min.

[0116] The tests were carried out either with 2 g (bars on left), or with 10 g (bar on right) of catalyst supported on alumina (2 g or 10 g represent the total mass of alumina and metal).

[0117] The hatched grey bars represent the extraction yield of Pd and the black bars represent the weight of Pd extracted.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0118] Particular embodiments of the method of the invention are described hereafter, namely a first embodiment of the method according to the invention wherein it is carried out in so-called “static” mode and a second embodiment of the method according to the invention, wherein it is carried out in so-called “dynamic” mode.

[0119] This description of particular embodiments of the method of the invention is rather made in relation with installations used for the implementation of the method but the teachings that are provided therein obviously apply to the methods as such.

[0120] An installation for the implementation of the method according to the invention according to a first embodiment thereof, in static mode, is described in FIG. 1.

[0121] This installation comprises a reservoir of fluid (1), for example liquid CO.sub.2. This reservoir (1) is connected to a pipe (2), which makes it possible to convey the fluid to the extraction chamber or reactor (3) going successively through a cold exchanger (4) and a high pressure pump (5). The high pressure pump (5) makes it possible to reach the desired fluid pressure, for example up to 300 bars.

[0122] The pipe (2) may also comprise a manometer (6) and a valve (7), after the pump (5) and before the chamber or reactor (3).

[0123] The reactor (3) may have the shape of a straight vertical cylinder and its volume is such that it makes it possible to receive supports capable of being treated by the method according to the invention.

[0124] The material of the reactor (3) is chosen in such a way as to be compatible with the pressurized dense fluid and the thickness of the walls of the reactor is chosen to withstand the pressure that reigns inside the reactor.

[0125] The reactor may be a stirred reactor. Thus, the reactor (3) represented in FIG. 1, includes a stirrer (8) fixed to the lower end of a rotating stirring rod (9).

[0126] The reactor (3) is provided with heating means, for example of a ceramic heater band (10).

[0127] The installation further comprises an evacuation pipe or vent line (11), which makes it possible to evacuate the fluid from the reactor in the form of a gas and thus to bring back the pressure inside the chamber to atmospheric pressure. In other words, the depressurisation of the reactor takes place through the vent line (11).

[0128] The pipe (11) may also comprise a manometer (12) and a valve (13).

[0129] The installation may further comprise a temperature regulator/programmer (14) connected to a thermocouple (15) placed in the heating means and to a thermocouple (16) placed in the reactor in contact with the fluid. More exactly, the thermocouple is placed in a sleeve that is immersed in the fluid.

[0130] The support containing a metal of the family of platinoids (for example a supported catalyst) and the extractant, ligand are firstly introduced into the reactor.

[0131] In FIG. 1, the embodiment is represented where the support containing a metal of the family of platinoids (for example a supported catalyst) and the extractant, ligand are in direct contact. In FIG. 1, the support, for example in the form of beads (17), is immersed in the liquid ligand (18) placed at the bottom of the reactor.

[0132] If it is wished to avoid direct contact between the support containing the metal and the ligand, then means will be provided in the reactor to receive the support and to maintain it at a distance from the ligand.

[0133] The ligand being generally placed in the lower part of the reactor, at the bottom of the reactor, the means for receiving the support and maintaining it at a distance from the ligand may consist of a receptacle such as a basket placed in the reactor at a height such that the support is maintained above the upper level of the ligand at the bottom of the reactor.

[0134] An installation for the implementation of the method according to the invention according to a second embodiment thereof, in dynamic mode, is described in FIG. 2.

[0135] This installation comprises a reservoir of fluid (21), for example a cylinder of liquid CO.sub.2. This reservoir (21) is connected to a pipe (22), which makes it possible to convey the fluid up to a first reactor (23) going through a cold exchanger (24) and a high pressure pump (25). The high pressure pump (25) makes it possible to reach the desired fluid pressure, for example up to 300 bars.

[0136] The pipe (22) generally comprises a first valve (26) upstream of the pump (25), and a second valve (27) between the pump (25) and the exchanger (24).

[0137] The pipe (22) arrives generally at the top of the first reactor (23).

[0138] The first reactor (23) is called solubilisation reactor or chamber.

[0139] In fact, in this reactor the ligand is solubilised in the pressurized dense fluid.

[0140] The first reactor (23) is connected to a second reactor (28) by a pipe (29), generally provided with a valve (30).

[0141] This pipe (29) generally connects the bottom of the first reactor (23) to the top of the second reactor (28). But the pipe (29) could conversely connect the top of the first reactor (23) to the bottom of the second reactor (28) or connect the two flanges of each reactor (23), (28).

[0142] The second reactor (28) is called extraction reactor or chamber.

[0143] In fact, in this second reactor (28) is placed the support containing a metal of the family of platinoids and the extraction of the metal contained in the support is carried out by the extraction medium comprising the pressurized dense fluid, notably supercritical, and the ligand.

[0144] The second reactor (28) is connected via a pipe (31) to a vessel, reservoir, designated collection vessel (32).

[0145] Indeed, this vessel (32) is intended to collect the pressurized dense fluid loaded with the complex of the ligand and the metal in the (0) oxidation state coming from the extraction operation carried out in the second reactor.

[0146] The pipe (31) generally comprises, from the second reactor (28), an outlet valve (33) then an overflow valve (34).

[0147] The pipe (31) generally connects the bottom of the second reactor (28) to the top of the collection vessel (32). But the pipe (31) could conversely connect the top of the second reactor (28) to the bottom of the collection vessel (32), or connect the flange of the second reactor (28) and the flange of the collection vessel (32).

[0148] The installation further comprises an evacuation pipe or vent line (35), which makes it possible to evacuate the fluid from the collection vessel in the form of a gas and thus to bring back the pressure inside the collection vessel to atmospheric pressure. In other words, the depressurisation of the collection vessel takes place through the vent line (35).

[0149] In fact, the depressurisation generally begins from the valve (33).

[0150] There is generally practically no more pressure in the collection vessel (32). The fluid, such as CO.sub.2, is then in gaseous form and goes to the vent (35) whereas the ligand/complex mixture is recovered in the form of a liquid, or in the form of a suspension of metal particles in the ligand.

[0151] The exchanger (24), the reactors (23) and (28) and the collection vessel (32) are generally placed in a temperature regulated chamber (36) such as an oven.

[0152] The invention will now be described with reference to the following examples, given by way of illustration and non-limiting.

EXAMPLES

Example 1

[0153] In this example, the capacity of different ligands, extracting molecules, complexing agents, to extract selectively platinum or palladium from catalytic supports on which they are supported is studied.

[0154] To this end, tests were carried out in a beaker on commercially available supported catalysts of palladium on alumina and platinum on alumina type with different ligands, extracting, complexing molecules, or extractants.

[0155] Tests were thus carried out with dodecanethiol (CH.sub.3(CH.sub.2).sub.10CH.sub.2—SH) as extracting molecule. Dodecanethiol is in fact capable of playing the role of soft ligand to complex platinoid type metals in metal (0) form.

[0156] Other comparative tests were carried out with Cyanex® 272 and Cyanex® 302 as extracting molecules.

[0157] Cyanex® 272 is an oxygenated ligand of phosphonate type, whereas Cyanex® 302 is a ligand of thiophosphonate type.

[0158] Cyanex® 272 and Cyanex® 302 meet the formulas below:

##STR00001##

[0159] The efficiency of extraction of palladium and platinum is determined by direct contact of the extractant—generally a volume of extractant of 5 mL or 20 mL is used—with the supported catalyst—generally a weight of catalyst of 0.25 g or 1 g (0.25 g or 1 g represent the total mass of alumina and metal) is used at atmospheric pressure.

[0160] After the extractant and the supported catalyst have been brought into contact, the solid phase, which essentially consists of the residual supported catalyst, is separated from the liquid organic phase consisting of the extractant and of the complexed metal.

[0161] The solid phase is then dried and calcinated at 200° C. to remove any trace of extractant remaining in the porosity of the support.

[0162] The quantity of metal remaining in the solid is then determined by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) carried out using an iCAP® 6300 Duo apparatus from Thermo Scientific®.

[0163] This analysis is carried out after grinding and digestion of the catalyst with nitric acid. To do so, 0.20 g of solid are dissolved in 5 mL of 68% HNO.sub.3 placed in a Teflon coated autoclave Model 4744 from Parr® at 150° C. for 2 h.

[0164] The extraction efficiency is defined by determining the extraction yield R %, which is given by the following formula:

[00001]$R.Math..Math.\%=\left(1-\frac{c_f}{c_i}\right)\times 100$

where c.sub.i is the initial concentration of metal in the catalyst before extraction, and c.sub.f is the final concentration of this same metal in the catalyst after extraction.

[0165] FIG. 3 gives the extraction yields obtained during tests of extraction of palladium or platinum supported on alumina, carried out with different ligands, namely dodecanethiol, Cyanex® 272 and Cyanex® 302.

[0166] The operating procedure of these tests is that described above: during each of the tests, 20 mL of each ligand is brought into contact, in a beaker, with 1 g of metal catalyst supported on alumina, at 60° C. for 72 h.

[0167] The results of these tests, shown in FIG. 3, demonstrate good affinity of dodecanethiol for palladium, compared to that observed for Cyanex® described in the literature.

[0168] Indeed, the extraction yield of Pd obtained with dodecanethiol lies around 50% after 72 h of being in contact at 60° C. (FIG. 3).

[0169] The extraction of platinum by dodecanethiol—with an extraction yield R % around 20% after 72 h of being in contact at 60° C.—although less good than that observed for palladium, was also demonstrated (FIG. 3).

Example 2

[0170] In this example, tests are carried out to study the influence of the parameters of duration of contact between the ligand and the catalyst, and extraction temperature, on the extraction yield of Pd or Pt by dodecanethiol.

[0171] The operating procedure of these tests is that described above in the example 1.

[0172] To study the effect of the duration of contact between the ligand and the catalyst on the extraction yields of Pd and Pt by dodecanethiol, 0.25 g of catalyst and 5 mL of ligand were brought into contact at 60° C. for durations of 5, 24, 42 and 75 hours for palladium and for durations of 5 hours and 72 hours for platinum.

[0173] The results of these tests are given in FIG. 4.

[0174] It may be noted that, for palladium, the extraction yield increases with the duration of contact and reaches a maximum for a contact duration of 42 hours.

[0175] To study the effect of the extraction temperature on the extraction yields of Pd and Pt by dodecanethiol, 0.25 g of catalyst and 5 mL of ligand were brought into contact for 5 h at temperatures of 25° C., 60° C., 80° C., 110° C. and 150° C. for palladium and at temperatures of 60° C. and 150° C. for platinum.

[0176] The results of these tests are given in FIG. 5.

[0177] It may be noted that, both for palladium and for platinum, the extraction yields increase with contact duration. Thus, for palladium, the extraction yield may reach 75% at 150° C. whereas the increase in the extraction yield of platinum by dodecanethiol at high temperature (150° C.) is also demonstrated in FIG. 5.

[0178] It may be concluded therefrom that parameters such as the contact time and the extraction temperature seem to greatly influence the extraction of Pd, but also notably as regards the temperature, and more particularly at high temperature (150° C.), that of Pt, by dodecanethiol.

Example 3

[0179] In this example, it is shown that the extraction of the metal by the specific ligands used in the method according to the invention takes place in reduced form, that is to say in the form of metal (0) nanoparticles.

[0180] The ligands having thiol groups used in the method according to the invention, such as dodecanethiol, are very soft ligands that easily complex nanoparticles of noble metals.

[0181] The high selectivity of these ligands vis-à-vis platinoids and notably vis-à-vis palladium compared to harder and more ionic ligands such as oxygenated ligands of phosphonate type such as Cyanex® 272 is a first hint of the extraction of a reduced form of these platinoids. Reference could be made in this respect to example 1 and to FIG. 3.

[0182] On the other hand, particles in suspension are clearly visible in the fractions extracted with dodecanethiol at temperatures above 110° C.

[0183] Then, FIGS. 7A and 7B, which are respectively images taken by transmission electron microscopy (TEM) of the suspensions of nanoparticles of Pd (FIG. 7A) and Pt (FIG. 7B) in dodecanethiol obtained by bringing into contact 0.25 g of catalyst and 5 mL of ligand at 150° C. for 5 h, indeed confirm the presence in these suspensions of nanoparticles of Pt or Pd of a size of around 3 nm, namely a size of nanoparticles equivalent to the size of the nanoparticles of Pd or Pt measured in the supported catalyst.

[0184] For the suspensions obtained by extraction at 60° C., metal nanoparticles were not able to be observed by transmission electron microscopy.

[0185] It may be thought that, either the particles are finer, or less concentrated and thus difficult to see, or the metal is solubilised in dodecanethiol as could be shown by the different colour of the solution obtained by extraction at this temperature.

[0186] However, the UV/Visible spectra of the solutions/suspensions of Pd (FIG. 6) obtained in dodecanethiol at different temperatures compared to the spectrum of a solution of Pd with a degree of oxidation II seems to demonstrate the presence of Pd metal (0) and especially the absence of Pd II.

Example 4

[0187] In this example, extraction tests are carried out in which the ligand, extractant is placed in a supercritical CO.sub.2 medium.

[0188] In this example, the tests are carried out in static mode, in order to check the feasibility of the extraction in this supercritical medium and to study potential limitations due to the solubility of the ligand in CO.sub.2.

[0189] The pilot plant used, which is that represented in FIG. 1 already described above, consists of a liquid CO.sub.2 reserve, of a high pressure pump making it possible to work up to 300 bars, and of a 0.5 L stainless steel reactor.

[0190] The reactor is a reactor stirred by means of a stirrer fixed to the lower end of a rotating stirring rod.

[0191] The reactor is heated using a ceramic heating band and may reach a temperature of 350° C. The depressurisation of the reactor takes place via the vent line.

[0192] The catalyst and the extractant, ligand are firstly introduced into the reactor.

[0193] Two embodiments are, in this respect, possible:

[0194] (1) The catalyst and the ligand are both in direct contact at the bottom of the reactor, or instead

[0195] (2) The ligand is at the bottom of the reactor, and the catalyst is in a basket fixed to the stirring rod. This embodiment thus avoids direct contact between the ligand and the catalyst and demonstrates the solubilisation of the ligand in supercritical CO.sub.2.

[0196] The reactor is then closed, then CO.sub.2 is added and the temperature adjusted to reach the desired experimental conditions.

[0197] In this example, the tests were carried out at 250 bars and two temperatures were studied, 60° C. and 110° C. for each of the embodiments. The operating conditions were maintained for 5 h before depressurising the reactor, recovering the catalyst and analysing the quantity of metal that it contains by digestion in nitric acid medium and ICP-AES analysis as has already been described above in example 1.

[0198] The tests carried out according to embodiment (1) by immersing the beads of catalyst in dodecanethiol in the presence of CO.sub.2 lead to extraction yields equivalent to those obtained in a beaker in the same operating conditions, namely 0.25 g of catalyst for 5 mL of ligand, for 5 h at the same temperature. These extraction yields R % are in fact around 35% at 60° C. and around 65% at 110° C. The presence of CO.sub.2 does not prevent the extraction (FIG. 8, left bars A and B).

[0199] The same tests carried out according to embodiment (2), in supercritical CO.sub.2 medium, without direct contact of the beads of catalyst with the ligand, also lead to the extraction of the metal particles, thus proving the solubility of dodecanethiol in CO.sub.2.

[0200] However, the extraction yield is reduced with respect to that obtained when the catalyst and the ligand are in direct contact according to embodiment (1). Indeed, R % then lies at around 10% at 60° C. and around 35% at 110° C. (right bars C and D). This reduction in the extraction yield is due to the low exchange surface in embodiment (2).

Example 5

[0201] In this example, just as in example 4, extraction tests are carried out in which the ligand, extractant is placed in a supercritical CO.sub.2 medium.

[0202] But in this example the tests are carried out in dynamic mode, in dynamic conditions, in order to favour contact between the ligand and the catalyst and thus improve the extraction yields.

[0203] The experimental installation used for these tests is analogous to that represented in FIG. 2 already described above. It consists of the following main elements: [0204] a CO.sub.2 pump with cooling by Peltier effect, flow rate: 0.001 to 10 mL.Math.min.sup.−1; [0205] a 35 mL reactor for the solubilisation of the ligand, maximum acceptable pressure:

[0206] 50 MPa, maximum temperature 300° C.; [0207] a 60 mL extraction reactor enabling the ligand and the catalyst to be brought into contact up to a pressure of 35 MPa—150° C.; [0208] a 50 mL collection vessel enabling the recovery of the metal complexes.

[0209] The ligand is introduced into the solubilisation reactor and the supported catalyst consisting of metal particles dispersed on a porous support is introduced into the extraction reactor.

[0210] The flow of CO.sub.2 passes through the first reactor, carrying away (sweeping along) the ligand which is then brought into contact with the catalyst in the second reactor. The complex formed is then swept along and recovered in the collection vessel. The tests in dynamic mode were carried out at 60° C. and 250 bars.

[0211] FIG. 9 shows the effect of the flow rate of CO.sub.2 on the extraction yield of Pd supported on an alumina support for an extraction carried out with dodecanethiol in supercritical CO.sub.2 medium in dynamic mode for a duration of 3 h at 250 bars and 60° C.

[0212] This FIG. 9 shows the necessity of working at low flow rates to favour solubilisation of the ligand in CO.sub.2 and ligand/catalyst contact by increasing the residence time of the ligand is retained in the extraction reactor.

[0213] All the following tests were thus carried out with a flow rate of CO.sub.2 of 0.5 mL/min.

[0214] The kinetic of extraction of Pd by dodecanethiol, at 60° C. and 250 bars and a flow rate of 0.5 L/min, was then determined (FIG. 10).

[0215] The extraction yield of Pd at the end of 5 h of treatment in these conditions is equivalent to that obtained for an extraction duration of 5 h at 60° C. in a beaker, and lies around 35%.

[0216] For Pt, the extraction yield by dodecanethiol in supercritical CO.sub.2 is also equivalent to that determined for an extraction duration of 5 h at 60° C. in a beaker, and lies around 20%.

[0217] To illustrate the effect of the operating conditions, different filling rates of the extraction reactor with beads of supported catalyst were tested.

[0218] Thus, either 2 g, or 10 g of supported catalyst were introduced into the extraction reactor, for the same quantity of ligand implemented.

[0219] By increasing the quantity of material to treat, the yield drops but the weight of metal extracted is for its part greater, as shown in FIG. 11.

[0220] This result seems to show that the ligand is not saturated with metal during its passage through the column of catalyst beads.