CA2191243C

CA2191243C - Fluid extraction

Info

Publication number: CA2191243C
Application number: CA002191243A
Authority: CA
Inventors: Chien Wai; Neil Smart; Yuehe Lin
Original assignee: Idaho Research Foundation Inc
Current assignee: Idaho Research Foundation Inc
Priority date: 1994-06-02
Filing date: 1995-06-02
Publication date: 2002-04-02
Anticipated expiration: 2015-06-02
Also published as: EP0762918A1; KR970703184A; JPH10501732A; CN1152884A; EP0762918B1; CA2191243A1; DE69528831D1; AU3269895A; EP0764042A4; CA2191227A1; WO1995033542A1; JP2002508697A; CN1077803C; CA2191227C; EP0764042A1; EP0764042B1; US5730874A; CN1082825C; CN1152260A; DE69431260D1

Abstract

A method for extracting metalloid and metal species from a solid or liquid m aterial by exposing the material to a fluid solvent, particularly supercritical carbon dioxide, containing a chelating agent is d escribed. The chelating agent forms chelates that are so luble in the fluid to allow removal of the species from the material. In preferred embodiments, the extraction solvent is supercritical ca rbon dioxide and the chelating agent comprises a trialkyl phosphate, a trialkyl p hosphate, an alkyl-aryl phosphate, a trialkylphosphine o xide, a triarylphosphine oxide, an alkyl-arylphosphine oxide, and mixtures thereof. The method provides an environmentally benign process fo r removing contaminants from industrial waste. The method is particularly usef ul for extracting actinides from acidic solutions, and t he process can be aided by the addition of salts, such as nitrate salts, and mo difiers, such as water and the lower alkyl alcohols and esters. The chelate and supercritical fluid can be regenerated, and the contaminant species recovered, to provide an economic, efficient pro cess.

Description

2 1 9 1 243 P~ J., 5 938 FLUID EXTRACTION
F3iF~D OF THE INVENTION
This invention conrP~nc extraction of metalloids and metals from solids and liquids, and is more particularly directed to a treatment process in which metals are efficiently extracted from acidic waste material .
ur~l~ OF 'rT~ vr ~
A particular environmental problem is the removal of toxic metals and radioisotopes from solid or liquid industrial wastes. Such contaminants can be removed from soils, for example, by treating the soil with an acid that dissolves the metals. Acid dissolution is followed by selective precipitation, electrowinning, or solvent extraction. Acid dissolution is unfortunately very noncpPrific, and often produces many by-products that can create serious environmental problems in their own right.
An alternative detoxification process is to PncArS-llAte contaminants in a container or insoluble matrix that prevents their entry into the environment.
This approach still requires storage of the bulky matrix, and does not allow regeneration or reuse of the contaminants. Hence there is a need for a biologically compatible waste treatment process that efficiently and effectively separates metals from contaminated materials. There also is a need for such a process that is biologically compatible and permits selective regeneration and reuse of the contaminants.
Fluid extraction and supercritical fluid extraction are methods known to be useful for selectively removing materials from a matrix. A
supercritical fluid is typically one that is gaseous at ambient conditions, but which is maintained at a temperature and ~L.3!5aUL~ above its critical temperature and pressure. Although materials may perform as solvents at sub-critical temperatures and pressures, Wo 9s/33s42 2 1 9 1 ? 4 3 , ~ C 938 fluids often perform better as solvents at supercritical conditions .
However, direct extraction of metal ions by supercritical fluids, such as carbon dioxide, generally 5 is inefficient. One reason for this may be because of the weak van der Waals lnteraction between metal ions and carbon dioxide. This weak interaction has apparently discouraged ef f orts to perf orm supercritical fluid extraction of metals from environmental wa6te6.
S~I~RY OF l'HE lr v~,~lD~
The pre6ent invention provides a method f or extracting a metal or metallQid specie6 (including lan~hAni~lPS and actinide6) from a media, particularly acidic madia, by expo6ing the media to a fluid 601vent, 15 particularly a 6upercritical f luid solvent, that contains a chelating agent . The f luid or 6upercritical f luid solvent and chelating agent are exposed to the solid or liquid for a sufficient period o~ time to form a chelate between the species and chelating agent. The 20 fluid or supercritical fluid is then removed from the media with the solllhi 1~ 7Pd metal chelate dissolved therein. The metal chelate6 subsequently can be precipitated from the fluid. For example, if the fluid is supercritical, then the metal chelates can be 2S precipitated by reducing the pressure of the supercritical f luid. The chelating agent can al60 be regenerated for reuse. This process i6 an efficient, cost-effective method for removing metals from the environment without using environmentally harmful 30 extraction solvent6.
The chelating agents can be any agent that f orms a chelate with the metal being extracted, wherein the chelate is soluble in the f luid or 6upercritical f luid 601vent. Examples of suitable chelating agent6, without 35 limitation, include trialkyl phosphates, triaryl phosphates, alkyl-aryl phosphates, trialkylrh- 5ph; nP
oxides, triaryl rhocrh; nl~ oxides, alkyl-ary1 rhosrh; ne oxides, and mixtures thereof.

Wo95/33542 2~ 91243 r~ 8 With reference to particular P-nhotl;~-nts, the invention provides a method for extracting metal or metalloid species from an acidic media comprising exposing the media to carbon dioxide containing a 5 chelating agent selected from the group consisting of trialkyl phosphates, triaryl phosphates, alkyl-aryl phosphates, trialkyl rh--Crh; ne oxides, triaryl rho5rh i n~
oxides, alkyl-aryl rhl~5rh i n~ oxides, and mixtures thereof. At least one of the chelating agents forms lO chelates with the species. The chelated material is then soluble in the carbon dioxide. The carbon dioxide may be subcritical, but pref erred embodiments use supercritical carbon dioxide.
The phosphate chelating agents generally are 15 represented by the formula o Il R30 ¦ OR5 wherein R3 - Rs are ; nfl~r~nrl~ntly selected from the group consisting of lower alkyl groups, aryl groups, and 25 mixtures thereof. As used herein, "lower alkyl" refers to ~ having 10 or fewer carbon atoms, both straight and branched chains. The term "aryl" refers generally to aromatic ~ '-, including but not limited to, benzene and the alkyl benzenes, such as 30 toluene, as well as other aromatic uu--ds such as naphthalene. The rho~:~h i n/~ oxide chelating agents generally are represented by the formula /

Wo95/33542 21 91 243 r~ . . s~r wherein R~; - R8 are independently selected from the group consisting of lower alkyl groups, aryl groups, and mixtures thereof. The method is best applied to acidic solutions, such as solutions comprising nitric acid.
The solutions also may include a metal 5alt, such as a nitrate salt. Optimal chelating agents currently are selected from the group consisting of tributyl phosphate, triphenyl phosphate, tributylphosphine oxide, tri-n-octylrhnsrh;ne oxide, triphenylrhnsrh;ne oxide, and mixtures thereof.
Thus, a pref erred method according to the invention comprises extracting metal or metalloid species from a nitric acid solution that contains a nitrate salt. The method comprises exposing the acidic solution to supercritical carbon dioxide containing a chelating agent selected from the group consisting of tributyl phosphate, triphenyl phosphate, tributylrhosrh;ne oxide, tri-n-octylphosphine oxide, triphenylrhnsrh;nP oxide, and mixtures thereof.
Still another Pmho~ of the method involves treating acidic waste material that includes metalloid and metal waste species in a container. The waste material in the container is exposed to the supercritical carbon dioxide which contains a chelating agent selected from the group consisting of trialkyl phosphates, triaryl phosphates, alkyl-aryl phosphates, trialkylphn~:rh;nP oxides, triarylphosphine oxides, alkyl-arylrhnsrh;nP oxides, and mixtures thereof. As with the previous Pmho~ 5 ~ at least one of the chelating agents forms chelates with the species, the chelates being soluble in the carbon dioxide. The carbon dioxide and solubilized waste species are then removed from the container.
Each ~-~o~;r-nt of the method may further comprise adding a modifier to the carbon dioxide. The ~; f; Qr may enhance the ef f iciency of the extraction method by increasing the solubility of the metal chelate in the supercritical fluid. Carbon dioxide, for ... , , . . _ . ... . _ _ _ W095/33542 2 t ~ ~ 243 ~ 9.~

example, is a relatively non-polar solvent. Its polarity can be increased by adding a more polar solvent to the supercritical carbon dioxide. Disclosed examples of more polar solvents include, without limitation, 5 water and lower alkyl alcohols and esters, particularly low to medium boiling point alcohols or esters, such as methanol, ethanol and ethyl acetate. The water, alcohol or ester appears to increase the polarity of the supercritical fluid, enhance the solubility of the metal chelate in the fluid, and further improve the extraction efficiency of the method.
The invention also provides a composition for extracting metal or metalloid species from acidic media.
The composition comprises supercritical carbon dioxide and a chelating agent as described above, and also may contain a salt, such as, without limitation, a nitrate salt. The composition also may include a modifier selected from the group consisting of water, lower alkyl alcohols, lower alkyl esters, and mixtures thereof.
Accordingly, it is an object of this invention to provide an; uv~d method for extracting metal or metalloid species from liquids or solids, including complex matrices.
Another object of the invention to provide such an; uv~d method that allows efficient and biologically compatible extraction of metal or metalloid species from the environment.
Another object is to provide such an; uv~d method that allows selectivity as to the type of metal or metalloid species extracted by the system.
Another object is to provide such an improved method that can selectively extract lanthanides and actinides .
Another object of this invention is to provide such an improved method that is efficient and economical compared to many other extraction processes.

WO 95133~42 r~ x _ 0~38 Another object of this invention is to provide a process for the selective removal of ions from acidic media .
These and other objects of the invention will be understood more clearly by reference to the following drawings and ~i~t~ description.
R~T~!l;' DESCRIPTION OF T~E DRAWIN68 FIG . 1 is a phase diagram f or carbon dioxide.
FIG. 2 is a schematic drawing of a waste treatment system in accordance with the present invention .
DET~D DE8CRIPTION OF SEVERAL ~n~ nR~iL/ ~M~ODTMP~'TS
The present invention is suitable for removing many different types of metalloids or metals from liguids or solids. Metalloids are elements with both metallic and non-metallic properties, and include arsenic, selenium and tellurlum. A metal is an element that forms positive ions in solutions, and produces oxides that form hydroxides rather than acids with water. ~etals include alkali metals, alkali-earth metals, transition metals, noble metals (including the precious metals gold, platinum and silver~, rare metals, rare-earth metals (lanth;~nid~s), actinides (including the transuranic metals), light metals, heavy metals, - 25 synthetic metals and radioactive metals. Specific examples are given herein of extraction metho~s f or removing lanthanides and actinides (collectively referred to as the f-group elements from the filling of their 4f and 5f orbitals). The present invention also provides sp~ f i c examples of extraction methods for radioactive metals, such as uranium, particularly the extraction of such metals from acidic solutions. This provides an attractive alternative to the PUREX process for recovering uranyl ions from acidic solutions.
Suitable fluids and/or supercritical fluids for use in the present invention include carbon dioxide, nitrogen, nitrous oxide, methane, ethylene, propane and propylene. Carbon dioxide is a currently optimal fluid _ _ . . . .. _ . ... _ _ _ ... . .. _ . _ _ _ _ W0 95/33542 2 1 q l 2 ~ 3 r~ n~g3~
for both subcritical and supercritical fluid extractions because of its moderate ~hPm~ constants (TC=31C, Pc73 atm) and its inertness ( i . e . it is non-explosive and thoroughly safe for extractions, even extraetions 5 performed at supercritical conditions). Carbon dioxide also is a preferred solvent because it i5 abundantly available and relatively i n~Yp~n~ive.
The conditions n~eDc~Ary to produce subcritieal or supercritical carbon dioxide can be det~rm; ned using 10 a phase diagram for carbon dioxide as shown in FIG. 1.
Although all conditions above the triple point (Tp) produce a carbon dioxide f luid solvent ef f ective f or practicing the present invention, the preferred carbon dioxide solvent is supercritical. Therefore the 15 eonditions typically must be above the critieal temperature and pLes~uL~ for carbon dioxide. However, virtually any eonditions that are above the eritieal point are acceptable for producing a supercritical carbon dioxide fluid solvent useful for practicing the 2 o extraction process of the present invention .
The f luids may be used either individually or in combinations, as mixed fluids or supercritical fluid solvents. Examples of other fluids, and their critical temperature and pressure, are shown in the following 25 Table I:

WO 9~/33542 ~ ~ 9 1 ~ 4 3 ~ o~g38 T~BLE I
PHYSICAL pARANlF~FR!:: OF SELECTED SUPERCRITICAL FLUIDS

Molecular Fluid For_ula T (C) p (atm) p (g/mL) 10P400~tm~ C c c Carbon dioxide CO2 31.1 72.9 0.47 0.96 Nitrous oxide N2O 36.5 71.7 0.45 0.94 15Am.monia NH3 132.5 112.5 0.24 0.40 rl-Pentane C5H12 196.6 33,3 0.23 0.51 20~l--ButAne C~,H10 152.0 37,5 0.23 0.50 71-Propane C3H6 96 . 8 42 . O O . 22 --Sulfur hexafluoride SF6 45.5 37.1 0.74 1.61 25Xenon Xe 16.6 58.4 1.10 2.30 Dichlorodifl~ hAnr~ CC12F2 111.8 40,7 0.56 1.12 Trifl~JL~ h~ o CHF 25.9 46.9 0.52 ----MethAnol CH30H 240.5 78.9 0.27 ~~
F:thanol C2H50H 243 . 4 63 . O O . 28 __ 35Isopropanol ~ C3H70H 235.3 47.0 0.27 Diethyl ether (C2H25~2O 193.6 36.3 0.27 --Water H2O i74.1 218.3 data from l~athe~on Gas Data Book (1980) and CRC Handbook of rh~ trv and PhY~!iics (CRC Press, Boca Raton, Florida 1984).
~5Tr 1.03 In addition, a modif ier may be added to the f luid, including supercritical f luids, to improve the solvent characteristics thereof. The mo5t useful 50 modifiers are water, and organic solvents such as the low to medium boiling point alcohols and esters, particularly the lower alkyl alcohols and the lower alkyl esters . Typical organic modif iers include methanol, ethanol, ethyl acetate and the like. The 55 modifiers are~ added in sufficient concentrations to enhance the solubility of the metal or metalloid species in the extracting material. With more specificity, but WO 95/33542 2 1 9 1 2 ~ 3 r~ c sg38 g without limitation, the modifiers typically are added to the fluids at proportions of between about 0.1% and 20.0% by weight. The modifiers contemplated for use herein are most typically not supercritical f luids at 5 the disclosed operating conditions. Rather, the modifiers are simply dissolved in the fluid solvents, including the supercritical fluid solvents, to improve their solvent properties.
In one ~mho~l; L the chosen ~nh~nr-~r is 10 combined with a supercritical f luid at the described proportions prior to feeding the supercritical fluid to the extraction vessel. Alternatively, the :,uye~Litical fluid is fed to the extraction vessel without the ~nhAn~r. The F~nh~nr~r is then introduced into the 15 extraction vessel and thereby combined with the _u~e~:Litical fluid.
I. Fluia Extraction Apparatus ~nd Nethod One proposed ' 'i--nt for a continuous selective-chelation supercritical fluid extraction 20 process is illustrated in FIG. 2. This process is suitable for chelating metals that are contained in solid or liquid waste held in a container 50. A
supercritical fluid, such as carbon dioxide gas, is supplied from a reservoir 52 which is connected by a 25 conduit 54 containing a valve 55 to a pressurization unit 56. If the gas is carbon dioxide, unit 56 increases the pressure on the gas to greater than 73 a - ,~^res at a temperature of greater than 32C to form ~u~e~ ., itical carbon dioxide. The supercritical CO2 30 then travels through a valve 57 and conduit 58 to a reservoir 60 that holds a solid or liquid chelating agent, such as any of the agents described in the examples of this specif ication . The CO2 is there passed through a column containing solid or liquid chelating 35 reagent to extract the chelating agent into the ~iuye~iLitical fluid CO2 stream. The supercritical fluid and chelating agent leave reservoir 60 through a conduit 62 and are introduced into container 50.

WO 9~/33542 1~

The supercritical fluid/chelating agent is intimately mixed with the solid or liquid waste in container 50 using either a batch or continuous process.
In a batch process, simple mixing would occur through 5 stirring or sonif ication . Alternatively, mixing could occur by allowing C02 to f low through a column of solid waste. In a continuous mixing: ` -';-^nt, C02 would flow through a column of solid waste material. Continuous mixing with a liquid waste could be achieved with 10 counter current f low .
After mixing, the metal chelate and C02 are removed through a conduit 64. A dt:~Les~iu~ izer valve 66 is present in line 64 to reduce the pressure to below seventy-two atmospheres such that the metal chelate 15 precipitates in container 67. The C02 gas is then recycled by pump 68 through line 70 to gas reservoir 52.
Metal chelates can be removed $rom the bottom of container 67 through line 72 such that the chelating agent can be regenerated from the metal chelate. When 20 regeneration of the chelating agent is desired, metal ions can be stripped from the chelate using a nitric acid solution having a pH less than one.
The extraction system should be thPr`n~] Iy controlled, either by known electrical means or 25 immersion in a constant temperature bath. Thermal control allows the carbon dioxide or other supercritical f luid to be maintained above its supercritical temperature .
II . C0z 8FF of Met~l~ /Net~lloia~ ~rom Acidic Nedi~
one specific ~rhorl;r-nt of the present invention is useful for extracting radioactive ions from solid and liquid materials. For instance, the described SFE
process can be used to extract Act;nidP~ in acid solutions such as those produced by the PUREX process 35 (Plutonium Uranium Recovery by Extraction). In the PUREX process, nuclear fuel material is f irst dissolved in hot nitric acid followed by extraction of the dissolved uranium and plutonium with an organic solvent , . . . ~

W0 9~/33S42 2 1 q 1 2 4 3 E~~ 938 .

containing 20-30% tributyl phosphate (TBP) in kerosene or in n-flocl~c~n~ Supercritical fluid extraction of metals and metalloids in acid solutions using carbon dioxide also has proven useful.
EXaMPLE I
This example concerns the extraction of U(VI), Th(IV) and Nd(III) from 6 molar nitric acid (HNO3). The ligands used for this example included TBP, thenoyltrif louroa~eton~ ( TTA), tri-n-octy l phosrh ; n~
oxide (TOPO) and TTA. For the TBP experiments, the ligand (5ml TBP) was placed in a stainless steel vessel with supercritical carbon dioxide introduced from the bottom of the vessel. In this arrangement, the fluid phase is saturated with TBP. For the extractions with a binary mixture of TBP and TTA, supercritical carbon dioxide was f irst saturated with TBP . The f luid phase then passed through a second ligand cell containing 100 mg of TTA. After this, the supercritical fluid, saturated with both TBP and TTA, was introduced into a liquid extraction cell from the bottom. The extraction conditions were 15 minutes static extraction followed by 15 minutes of dynamic extraction at 60C and 150 atm.
The supercritical conditions were 60C and 150 atmospheres. The results of these extractions are presented below in Table II, which shows that the mixed ligand composition provides a novel and efficient means for removing radioactive ions from acidic solutions, without using toxic or flammable organic chemicals for the extraction process. Thus, the present invention provides an attractive alternative for the PUREX
process .

2 ~ 9 ~ 243 Wo 95/33s42 r~ 938 TABL~ II
Extraction of U(VI), Th(IV), and Nd(III) from 6 M Nitric Acid with Supercritical Co2 and Mixed Ligands %Extraction Ligands U Th Nd TBP+TTA 95 82 75 TBP+TTA 97 91 77 TOPO+TTA 9 9 9 9 7 3 a. Sample composition: 50 ~g/mL each of U, Th, and Nd in 6M HNO3+3M LiNo3 b. Extraction conditions: 15 min static plus 15 min dynamic extraction at 60C and 150 atm.
Table II shows that TBP alone can extract uranyl, Th(IV) and Nd(III) from 6 M HNO3 with a reasonably high efficiency at 60C and 150 atm. without limiting the extraction of metal and/or metalloid ion~
25 from acidic solutions to one theory of operation, it appears that these ions are extracted as the neutral nitrates UO2(NO3)2, Th(NO3)4 and Nd(NO3)3 in supercritical _ carbon dioxide because of the high nitrate concentration in 6 M r~NO3 . Binary mixtures of TBP and f luorinated ~B-30 diketones, such as TTA, also extract uranyl, Th(IV) and Nd(III) from the acid solution using supercritical carbon dioxide. These results show that the actinides and lanth~n~ [Nd(III) is a typical lanthanide~ can be extr2cted from acid solutions, particularly nitric acid 35 solutions, on an industrial scale using supercritical carbon dioxide as a solvent.
The triple point of carbon dioxide is 5.1 atm and -56.3 C. Therefore, at room t~ clI_uLe carbon dioxide becomes a liquid above 5 .1 atm. Dep~nrl; n~ on 40 the pressure, liguid carbon dioxide has a density comparable or slightly greater than supercritical carbon dioxide, thus the solvation power of liguid carbon W095/33542 2~ ~1243 r~ 938 dioxide is comparable to that of supercritical carbon dioxide. This means liquid carbon dioxide should also - be able to dissolve the metal complexes described above.
However, liquid carbon dioxide does not have the "gas-5 like" properties of the supercritical carbon dioxide.
This means liquid carbon dioxide has large a viscosity, small diffusivity, and consequently poor penetration power compared with Lu~e~L:Litical carbon dioxide. Thus, it is expected that liquid carbon dioxide should also be able to extract lanthanides and actinides from acid solutions with TBP or a mixture of TBP and a f luorinated ~-diketone as the extractant, but with lower efficiencies. The extraction efficiency of liquid carbon dioxide is expected to depend on the applied pressure. It is also expected that the extraction efficiency of liquid carbon dioxide can be improved with mechanical stirring and agitation.
The following Examples II-V concern the extraction of metal or metalloid species from acidic solutions using primarily or~AnrrhnsphAte or organoph~srh; n~ oxides as the extracting agents . The preceding examples indicated that TBP alone, and TBP or other extracting agents in combination with ,l~-diketones, efficiently extract metal or metalloid species from acidic environments. The following examples demonstrate that mixtures of extracting agents that include ~-diketones are not nPcpcsArily required for the efficient extraction of metal or metalloid c:pecies from acidic mixtures. The results obtained from Examples II-V are summarized below in Table III.
EXaMPLE I I
This example describes the extraction of U (VI ) and Th~IV) from an acidified aqueous media using ~U~ ;L itical carbon dioxide and tributylrhr,srh i n.o oxide (TBP0) as a chelating agent. UtVI) and Th(IV) were prepared from their nitrate salts obtained commercially from such distributors as Baker rh~miCAl Co. and MAllinrlrodt Inc. All acidic solutions, regardless of WO95/33542 21 91243 r~ c~3~

the acid concentration, contained about 50 ~Lg/ml each of U(VI) and Th(IV). For this particular example, the acidic solution was 6M HNo3 and 3M LiNo3. TBP0 was obtained from Aldrich Chemical Company. All other tl~PmiC~lR used were analytical reagent grade.
All experiments were performed with a lab-built SFE extraction apparatus as described above and shown in FIG. 2. TBP0 is a solid extraction reagent, and was placed in a 3 . 5 mL high pressure cylinder connected upstream of the liquid extraction vessel. A fused-6ilica tubing (Dionex, 50 um i.d. and 20 cm in length) was used as the pres6ure restriction f or the exit gas .
Supercritical C0z was introduced through the upstream cylinder containing TBP0 and then introduced through the extraction vessel containing the acidic solution of UtVI) and Th(IV). The ions were complexed and extracted under static conditions for about 15 minutes, and then under dynamic conditions for 15 minutes. The E;upercritical conditions were 60OC and 200 atm.
The results of this extraction procedure are provided below in Table III. More specifically, TBP0 extracted U(VI) from the acidic medium with greater than 999~ efficiency. TBP0 extracted Th(IV) from the acidic medium with an average efficiency of about 97 . 5 percent.
Thus, these results show that actinides can be extr~cted efficiently from an acidic medium using or~nc~hosrhinP
oxides in supercritical carbon dioxide.
In a manner similar to that described in Example II, TBP0 has been used to extract U(VI) and Th(IV) ions from various acidic solutions. These results also are summari2ed in Table III. TBP0 in supercritical carbon dioxide efficiently extracted U(VI) from 6M HN03 without added nitrate salts at an average efficiency of about 98 . 5% . TBP0 in supercritical carbon dioxide extracted Th(IV) from 6M HN03 without added nitrate salts with an average efficienCy of about 98%. When the ~ nce~ Lation of the acid was reduced to lM HN03, U (VI ) was extracted with an av~rage efficiency of about 99%, and Th(IV) was . _ . _ . _ . _, _ WO9~/33~42 2~912~3 P~ g~8 .

extracted with an average efficiency of about 99.5%.
Finally, when the concentration of the acid was reduced to O.lM, TBPO extracted U(VI) with an average efficiency of about 93%, and Th(IV) with an average efficiency of 5 about 929c. Thus, Table III clearly d -- ~L~tes that TBPO efficiently extracts metal and/or metalloid species from acidic solutions, without the need f or using additional extracting agents such as ~5-diketones.
EXAMPLE III
In a manner similar to that described in Example II, a number of org InorhosrhAte and organorhosrhin,~
oxide extracting agents have been tested, including TBP, TOPO, and triphenylphosphine oxide (TPPO). Again, the metal ions were U(VI) and Th(IV). These ions were extracted from 6M HNO3 and 3M LINO3, 6M HNO3, lM HNo3 and `
O.lM HNO3 using supercritical carbon dioxide. U(VI) and Th(IV) were prepared from their nitrate salts obtained commercially from such distributors as Baker Chemical Co. and M~ 1 1; nrkrodt Inc. All acidic solutions, regardless of the acid concentration, contained about 50 ~g/ml each of U(VI) and Th(IV). TBP, TOPO and TPPO were obtained from Aldrich ~'h~mi ral Company . All other chemicals used were analytical reagent grade. The ions were complexed and extracted under static conditions from each of the acidic test environments for about 15 minutes, and then under dynamic conditions for about 15 minutes. The ~u~.Litical conditions were 60C and 200 atm. The amount of uranium and thorium ions extracted were determined as described above.
The results provided in Table III show that actinides were efficiently extracted from acidic solutions using both or~J~nc~rhocrh;te and organophosrh;n~
oxides in supercritical carbon dioxide. Extraction utilizing the lower alkylphosphate r ,_ 'c alone, such as TBP, is especially convenient because TBP is a relatively inexpensive chelating agent. TBP is a liquid and is highly soluble in ~u~l.;.itical carbon dioxide, WO9~/33542 2t 91243 r~llu~ Sg38 and therefore is easily mixed with samples requiring extraction .
TABLE: III
5 Sample Matrix ~xtraction Reagent Percent Extraction ( % ) U (VI ) Th ( IV ) 6M HN03 + 3M LiNo3 lM HN03 TBP 52 26 TBP+TTA 7 0 8 5 3 5 TBP+TTA 7 0 8 6 TPPO+TTA 99 86 TPPO+TTA 9 9 9 4 0 . lM HN03 TBP 12 20 TBP+TTA 87 96 TBP+TTA 88 96 =
W0 95l33542 l ~ 1 / IJ ~ ~
2~

*TBP: Tributyl phosphate TBPO: Tributylrhosrhin~ oxide TOPO: Tri-n-octyl rhn5rh i n~ oxide TPPO: Triphenylrh~srhin-~ oxide TTA: Thenoyltrifluoroacetone ~Yr P~E IV
This example describes the possible extraction of U(VI) and Th(IV~ ions from acidified aqueous media (;nrlll~;nq, but not limited to, 6M HNO3, with or without the addition of a nitrate salt, lM HNO3 and 0. lM HNO3) using supercritical carbon dioxide and di-n-butyl-phenylrh~srhin~ oxide. U(VI) and Th(IV) may be prepared from their nitrate salts obtained commercially. All acidic solutions of different acid concentration should contain about 50 yg/ml each of the U(VI) and Th(IV~
ions .
This example would show that actinides may be extracted from acidic solutions using supercritical carbon dioxide and orqAnnrhncrhinP oxides having mixed ligands, that is both alkyl and aryl ligands attached to the same phosphorous atom, with high efficiency. The results 6hould be similar to that shown in Table III.
The àddition of a nitrate salt would be expected to enhance the extraction efficiency when orqAnophosrh; n~
oxides having mixed ligands are used as the extracting agent .
EXAMPLE V
This example describes the possible extraction of U(VI) and Th(IV) ions from acidified aqueous matrices (such as 6M HNO3, with or without the addition of a nitrate salt, lN HNO3 and 0. lM HNO3) using supercritical carbon dioxide and di-n-butyl-phenyl phosphate. U(VI~
and Th(IV~ may be prepared from their nitrate salts obtained commercially. All acidic solutions of different acid concentration should contain about 50 ~g/ml each of the U(VI~ and Th(IV~ ions.
This example would show that actinides may be extracted from acidic solutions with an efficiency Wo 95/33542 2 ~ 9 ~ 2 ~ 3 F~lll C ~6938 comparable to that shown in Table III using supercritical carbon dioxide and organophosphates having mixed ligands, that is both alkyl and aryl ligands attached to the same ~hosrhorous atom. The addition of 5 a nitrate salt would be expected to enhance the extraction ef f iciency when organophosphates having mixed ligands are used as the extracting agent.
Having illustrated and described the principles of the invention in several preferred G ~ ; Ls~ it 10 should be apparent to those skilled in the art that the invention can be modif ied in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method for extracting metal and/or metalloid species from an acidic media comprising exposing the media to supercritical carbon dioxide containing a chelating agent selected from the group consisting of trialkyl phosphates, triaryl phosphates, alkyl-aryl phosphates, trialkylphosph-ine oxides, triarylphosphine oxides, alkyl-arylphosphine oxides, and mixtures thereof, at least one of the chelating agents forming chelates with the species, the chelates being soluble in the supercritical carbon dioxide.

2. The method according to claim 1 wherein the trialkyl, triaryl, or alkyl-arylphosphate is represented by the formula wherein R3 - R5 are independently selected from the group consisting of lower alkyl groups and aryl groups, and wherein the trialkylphosphine oxide, triarylphosphine oxide and alkyl-arylphosphine oxide are represented by the formula wherein R6 - R8 are independently selected from the group consisting of lower alkyl groups and aryl groups.

3. The method according to claim 2 wherein R3=R4=R5.

4. The method according to claim 2 wherein R6=R7=R8.

5. The method according to claim 2 wherein R3 - R5 are independently selected from the group consisting of butyl, octyl, and mixtures thereof, and wherein R6 - R8 are inde-pendently selected from the group consisting of butyl, octyl, phenyl, and mixtures thereof.

6. The method according to claim 5 wherein R3=R4=R5 and wherein R6=R7=R8.

7. The method according to claim 1 wherein the exposing step comprises continuously flowing supercritical carbon dioxide through the media.

8. The method according to claim 1 wherein the media is an acidic solution.

9. The method according to claim 8 wherein the acidic solution also includes a metal salt.

10. The method according to claim 9 wherein the solution is a nitric acid solution, and the salt is a nitrate salt.

11. The method according to claim 1 wherein the chelating agent is selected from the group consisting of tributyl phosphate, triphenyl phosphate, tributylphosphine oxide, tri-n-octylphosphine oxide, triphenylphosphine oxide, and mixtures thereof.

12. A method for extracting metal or metalloid species from an acidic solution that contains a salt, comprising exposing the acidic solution to supercritical carbon dioxide containing a chelating agent selected from the group consisting of phosphate chelating agents and phosphine oxide chelating agents, wherein the phosphate chelating agents are represented by the formula wherein R3 - R5 are selected from the group consisting of lower alkyl groups and aryl groups, and wherein the phosphine oxide chelating agents are represented by the formula wherein R6 - R8 are selected from the group consisting of lower alkyl groups and aryl groups.

13. The method according to claim 12 wherein the chelating agent is selected from the group consisting of tributyl phosphate, triphenyl phosphate, tributylphosphine oxide, tri-n-octylphosphine oxide, triphenylphosphine oxide, and mixtures thereof.

14. A method for extracting metal or metalloid species from a nitric acid solution that contains a nitrate salt, comprising exposing the acidic solution to supercritical carbon dioxide containing a chelating agent selected from the group consisting of tributyl phosphate, triphenyl phosphate, tributylphosphine oxide, tri-n-octylphosphine oxide, triphenylphosphine oxide, and mixtures thereof.

15. A method for treating acidic waste material containing metalloid and metal waste species, comprising the steps of:
providing a container of the acidic waste material;

exposing the waste material in the container to supercritical carbon dioxide containing a chelating agent selected from the group consisting of trialkyl phosphates, triaryl phosphates, alkyl-aryl phosphates, trialkylphosphi-ne oxides, triarylphosphine oxides, alkyl-arylphosphine oxides, and mixtures thereof, at least one of the chelating agents forming chelates with the species, the chelates being soluble in the carbon dioxide; and removing the carbon dioxide and solubilized waste species from the container.

16. The method according to claim 15 further comprising adding a salt to the waste material, and wherein the chelating agent is selected from the group consisting of tributyl phosphate, triphenyl phosphate, tributylphosphine oxide, tri-n-octylphosphine oxide, triphenylphosphine oxide, and mixtures thereof.

17. The method according to any one of claims 1 through 16, further comprising adding a modifying solvent to the supercritical carbon dioxide, wherein the modifying solvent is selected from the group consisting of water, lower alkyl alcohols, and mixtures thereof.

18: A composition for extracting metal or metalloid species from acidic media, comprising:
supercritical carbon dioxide; and a chelating agent selected from the group consisting of trialkyl phosphates, triaryl phosphates, alkyl-aryl phosphates, trialkylphosphine oxides, triarylphosphine oxides, alkyl-arylphosphine oxides, and mixtures thereof.

19. The composition according to claim 18 wherein the media is an acidic solution, and wherein the solution contains a salt.

20. The method according to claim 19 wherein the solution comprises nitric acid and the salt is a nitrate salt.

21. The composition according to claim 20 wherein the chelating agent is selected from the group consisting of tributyl phosphate, triphenyl phosphate, tributylphosphine oxide, tri-n-octylphosphine oxide, triphenylphosphine oxide, and mixtures thereof.