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CHARLES A. BLAKE, Jr., CHARLES F. BAES, Jr., and KEITH B. BROWN Oak Ridge National Laboratory, Oak Ridge, Tenn.
Solvent Extraction with
Alkyl Phosphoric Compounds
N e w solvent extraction reagents, unlike tributyl phosphate, have the advantage of recovering uranium from nearly any solution nation power of the cation are important. ORGANOPHOSPHORUS For dialkylphosphinic acids (R2POzH)
COMPOUNDS, both acid and neutral, provide a versatile range of solvent extraction reagents. The acid hldrolysis products of tributyl phosphate (TBP), mono- and dibutyl phosphoric acids, were recognized as effective uranium extractants in their own right in 1949 (70, 77, 75). Other types of organophosphorus reagents, neutral as well as acidic, have proved useful extractants, primarily for uranium. These extraction reagents must be compatible with diluents, insoluble in the aqueous phase; stable; selective in the extraction of the desired elements; pure, to prevent undesirable side reactions and poor phase separations; and, in the case of commercial applications, available at low cost. Aqueous insolubility is readily controlled by variation in the chain lengths of the substituent alkyl groups. Until recently, compounds satisfying these criteria have not been commercially available a t low cost. As a result of interest in uranium extraction, several new reagents are now available-several dialkylphosphoric acids, monoalkylphosphoric acids, some new trialkyl phosphates, some dialkyl alkylphosphonates and, in limited quantities, one phosphine oxide.
Chemistry of Extraction Extraction Reactions. Acidic organophosphorus reagents extract uranium and other metals primarily by cation exchange between the extracted metal ion and acidic hydrogen of the reagent. The individual extraction reactions, however, can be rather complex. For example, in di(2-ethylhexy1)phosphoric acid (DZEHPA) extraction of uranium a t low levels the following reaction occurs :
+
UOzaq++ 2(HX)zOrg= UOzX4Hzora 2H+,,
+
where dialkylphosphoric acid
--e.g., di(2-ethylhexy1)phosphinic acid, DZEHPINA-mono a1k y 1p h o s p h o r i c acids (ROPOzHz), such as mono(Z,6,8trimethyl- 4 - n o n y l ) p h o s p h o r i c acid. DDPA, and phosphonic acids (RPOaH2), similar complications can be expected. The first seem dimeric in solution and the latter two appear even more highly associated. Combination of dialkylphosphoric acids with neutral organophosphorus reagents enhances the uranium extraction coefficient beyond the sum of the coefficients obtainable by the separate reagents. The synergistic effect is caused by addition of the neutral reagent to the uranyl dialkylphosphate complex. UOzXdHz
+ RBPO = (UO&Hz.)(RsPO)
(2)
The neutral alkyl phosphine oxides RaPO, such as tributylphosphine oxide, TBPO, and tri-n-octylphosphine oxide, TOPO, phosphinates R2(RO)PO, such as butyl dibutyl phosphinate, BDBP. and butyl dihexylphosphinate, BDHP, and phosphonates R(RO),PO, such as dibutyl butylphosphonate, DBBP. dihexyl hexylphosphonate, DHHP, may be compared to the familiar alkyl phosphate, tributyl phosphate, in their extraction behavior (3, 8, 70). Thus, in the extraction of uranyl nitrate by trioctylphosphine oxide, the equlibrium UOz++
+ 2N03- + 2RsPO = UOz(NOs)z(RsPO)z ( 3 )
analogous to that proposed for tributyl phosphate. describes the extraction behavior fairly well. Acidic Reagents. The aqueous pcid-
ity and extractant concentration are variables which appear explicitly in extraction reactions such as that in Equation 1. Less apparent is the influence of aqueous anions. As the extracted species is the metal cation, aqueous phase complexing anions will compete with the extracting reagent for the metal ion, and its extraction coefficient will decrease (Figure 1). For the di- and monoalkylphosphoric acids and the synergistic combination, the uranium extraction coefficient, E: = U,,, concn./ U,, concn., decreases with increasing sulfate concentration. The decrease is even more rapid in the case of phosphate. As an indication of the great magnitude of this anion effect, uranium extraction coefficients from noncomplexing perchlorate solutions are over 100 times greater than those shown from the sulfate solutions. Acidic phosphate solutions are perhaps the most unfavorable media from which to extract uranium. Yet, usable extraction coefficients can be obtained particularly with synergistic reagents. An extraction variable of particular pertinence to countercurrent extraction processes is the concentration of the extractable metal. Figure 2 shows several uranium extraction isotherms wherein the equilibrium concentration of organic uranium and aqueous uranium are plotted. The other composition variables are held constant. Initially these curves are linear, the corresponding slopes giving the extraction coefficient. With increasing organic uranium concentration, the extractant i s consumed, the extraction coefficients decrease. and the curves level off. The organic uranium concentration corresponding to
Table 1. (1)
(RO) 2
Both Reagent Type and Choice of Diluent Influence Uranium Extraction (Aqueous phase initially 0.004M U(V1); pH 1 ; 0.1M extractant in indicated diluent; phase ratio 1 to 1. 25' C.)
P02H (HX) is dimeric in the organic diluent and the uranyl-extractant complex contains unreplaced acidic hydrogens (7). At higher uranium levels, increasingly long-chain polymers of the composition ( U O Z ) . X ~ , + ~ H are ~ evidently formed. Thus, although the extraction is by cation exchange, the products appear rather specialized structures in which the charge and coordi-
DZEHPA DZEHPINA DDPA pKu = 3.2" pKa = 5.6" pKa = 3.1' Diluent 0.5MSO;- 0.4M PO;-- 0.4M PO;-- 0.4M PO;-Kerosine 135 25 14 200 Carbontetrachloride 17 3 6 240 2 6 Benzene 13 Chloroform 3 0.5 1.4 2-Ethylhexan01 0.1 0.02 pKu = relative acidity. Apparent pH at half neutralization in 70% ethyl alcohol. Dielectric Constant 2 2.2 2.3 5.1
..
..
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saturation is one half the total acid reagent concentration. The steeper initial slope given by the synergistic combination corresponds to a higher extraction coefficient a t low uranium levels. In a countercurrent process, this means that fewer stages are required to obtain a given separation. The leveling off of this isotherm at higher levels corresponds to the stabilization of the 1 to 4 uranyl-dialkyl phosphate complex (Equation 2). Within a given class of acidic reagents, uranium extraction power generally decreases with decreasing acidity of the reagent. Increased branching of the alkyl chains near the phosphate is also associated with decreased acidity and extraction power. Extractants of different types show no simple correlation of extraction power with acidity (Table I). Thus, the mono- and dialkylphosphoric acids listed have roughly the same pKa values. Table II. Combinations of Dialkylphosphoric Acids and Neutral Compounds Show a Synergistic Enhancement of Extraction (Aqueous phase initially 0.004V U(T'I), 0.5M SO;-, pH 1, reagents in kerosine; phase ratio 1 to 1, 25' C.) E: In synergistic combination Concn., Reagent with 0.lM Reagent M alone DSEHP.4
DZEHPA TBP DBBP BDBP TBPO
135 0.0002 0.002 0.002 0.0025
0.1
0.1 0.1
0.1 0.05
.. 500 1700 3500 7000
yet the former generally gives higher extraction coefficients. The dialkylphosphoric and -phosphinic acids give similar extraction coefficients, at least in kerosine and carbon tetrachloride, yet pKa values differ considerably. The organic diluent is an important extractio 1 variable (Table I). For the dialkylphosphoric and -phosphinic acids, the extraction coefficient generally decreases as the dielectric, constant of the solvent increases, though the magnitude of this decrease is less in the case of the dialkylphosphinic acid. Chloroform and 2-ethylhexanol, which give the lowest extractions, should form hydrogen bonds with the extractants. Effects of diluent, reagent type, and reagent structure on extraction power are interrelated, particularly, with these acidic reagents as these factors influence their degree of association through hydrogen bonding. Synergistic Combinations. The order of synergistic enhancement of uranium extraction (Equation 2). when a neutral organophosphate is added to a dialkylphosphoric acid, is (R0)qPO < R(RO)2PO < Rz(R0)PO
< R3PO
which is also the order of increasing base strength of the phosphoryl oxygen. The corresponding factors by which the dialkylphosphoric acid extraction coefficient is increased are, respectively, around 4. 15, 25, and 50 (Table 11). With the synergistic combinations, E$ rises to a maximum with increasing neutral reagent concentration and then decreases (Figure 3). The positions of these maxima are relatively independent of the acidic reagent concentra-
I
t=
I
tion level. The decrease in E: beyond the maximum is due to increasing interaction between the acidic and neutral extractant through H-bonding (2). The phosphinic acids showed no synergistic enhancement. With manoalkylphosphoric acids, an immediate antagonistic decrease in Et is found upon addition of a neutral reagent. Neutral Reagents. The neutrdl reagents offer a greatly increased range of extraction power over that of tributyl phosphate (Figure 4). Trioctylphosphine oxide, the strongest extractant, gives uranium extraction coefficients more than 100,000 times greater than does tributyl phosphate from the same aqueous medium and permits extraction of uranium from solutions in which the latter is completely ineffective. The phosphinates and phosphonates are intermediate between these extremes. Nitrate is the most favorable aqueous medium. with rapidly decreasing extraction from chloride and sulfate. An important consequence of the high extractability from nitrate solutions is that by the introduction of relatively small amounts of nitrate into much less favorable aqueous solutions-e.g., sulfate and phosphate-uranium may be extracred by a phosphine oxide. Variation of uranium extraction with reagent type and with aqueous anion roughly parallels the extraction of mineral acids by these reagents. Thus, all these neutral reagent types extract phosphoric, sulfuric, hydrochloric, and nitric acids from aqueous solutions, extraction increasing roughly in that order. The strength of extraction of a given mineral acid increases in the order phosphate, phosphonate, phosphinate, and phosphine oxide. Selective Extraction. Selective separation among several elements can be achieved by proper control of extraction I
I
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2
D2EHPA+ TBP
1
O.I_M DDPA
a a I-
x
w
I I 0.5
O
I .o
M
SOS,pH I
U 1.5
0
1.0,
0.5
M
PO;,
1.5
pH I
I 0
I 0.I
I
I
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0.2 0.3 0.4 0.5 URANIUM I N AQUEOUS PHASE, gm./litsr
0.6
Figure 1. Uranium extraction is reduced by com- Figure 2. The extraction isotherms reflect the strength of extraction a t low uranium loading and the formation of complex species a t 4 plexing aqueous anions to 1 and 2 to 1, D2EHPA to U(VI) Organic phase, 0.1M reagents in kerosine Organic phase, O.lM reagents in kerosine Aqueous phase initially, 0.004M U(Vl); phase ratio 1 to 1 at Aqueous phase, 0.5M Sod--, p H 1 a t 25' C 25' C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
NUCLEAR TECHNOLOGY io'ooo
'
1,000
Rp ,O ,
L TO PO
IO0
IO
r
I.o
+ OBBP
D2EHPA
-
on
/
EDHP
0.1
0.01
0.001 0.000I
'\
t I
D2EHPlNA
'\\\
\.DOPA -
0.00001
+ TBP + TBP '
.
l
A
1
0. I
0.2 N E U T R A L A D D I T I V E , molrllitrr
Figure 3. Neutral organophosphate additives enhance uranium extraction b y dialkylphosphoric acid, but impair extraction by other organophosphoric acids Organic phase, 0.1M acid reagent in kerosine modified with indicated neutral reagent. Aqueous phase, initially 0.004M U(VI) in 1.5M H&04;
' j
l,SM H,SO,
I 4.OM H,SO,
I
I
0.5M PO:
O.5M
0.5y
0.3MNO;
HCI
CI-
pH 1.0
Figure 4. power
0.5MSO:
0.3MN05
p H 1.0
pH 1.0
O.IM
NO; pH 1.0
The neutral compounds offer a wide range of extraction
Organic phase, 0.1M reagent in kerosine Aqueous phase, initially 0.004M U(VI); phase ratio 1 to 1 at 25' C.
phase ratio 1 to 1 at 25" C.
and back-extraction (or stripping) variables. In the treatment of ore leach liquors, for example, uranium may be extracted alone or in combination with vanadium by utilization of their different acidity dependencies and adjusting the reagent concentration. I n this respect, solvent extraction of cations is to be contrasted with cation exchange resin adsorption. Ability to adjust reagent concentration and select a diluting solvent greatly increases the potential for selective solvent extraction. Reagents which are relatively strong extractants for uranium are also relatively strong extractants for other metal ions. Thus, di-(2-ethylhexyl)phosphoric acid is not only a stronger extractant for uranium but also for iron(II1) when compared with bis(di-isobutylmethy1)phosphoric acid. As selectivity is the ratio of coefficients under a particular set of conditions, extraction of less iron does not necessarily mean a higher uranium selectivity. I n fact, the two dialkylphosphoric acids named show nearly the same selectivity in iron-uranium separations. The type and branching of the reagents has exerted a profound influence on the behavior during extraction. As few detailed extractions have been made with reagents other than those developed for the raw materials work, the importance of reagent type and chain brancjing in determining selectivity properties of organophosphorus acids has not been clearly established. In any event, structural effects may be
I
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small when compared with the large differences which can be obtained by controlling the extraction variables. Neutral reagents, however, show selectivity properties relating to structure, probably due to being able to arrange three alkyl groups of varying degrees of branching closely around the central phosphorus atom. Also, the neutral reagents require the extracted metal ion to be capable of forming a neutral complex in the organic phase, and this in turn provides additional possibility fox selective extraction. With both neutral and acidic reagents, additional selectivity may be gained by reduction or oxidation to convert metal ions to less extractable forms-e.g., reduction of iron(II1) to iron(I1) or chromium(V1) to chromium(II1). A special case in which uranium selectivity over iron, aluminum, thorium, and vanadium is increased is by use of the synergistic reagents. Here, uranium coefficients were increased by addition of neutral organophosphorus reagents to dialkylphosphoric acids, while extraction of other metals was either not affected or was decreased. Uranium Extraction Processes Ability of reagents to meet the criteria mentioned above can vary with different reagents of the same series. Although other reagents with proper chemical structure would be suitable, di(2-ethylhexy1)phosphoric acid has been the reagent of choice of this type as it is commercially available and is
generally above the average with respect to performance desired. In the series of monoalkylphosphoric acids, greatest process use has been made of a dodecylphosphoric acid (DDPA) prepared from 2,6,8,-trimethyl-4-nonanol.Some consideration has been given to a heptadecylphosphoric acid (HDPA). In many respects, reagent behavior is similar: Both types can be used in kerosine, and extraction power with moderate reagent concentrations in these solvents is sufficiently high for application to liquors over a wide p H and sulfate concentration range. Solubility losses to the aqueous acidic liquors with these longer chain reagents are low. Typical losses for di(2-ethylhexy1)phosphoric acid amount to about 5 mg. per liter of aqueous phase contacted and for dodecylphosphoric acid, about 60 mg. per liter. Losses to the strip solutions are somewhat greater, so that the amounts of reagent involved per amount of uranium treated are small. The extraction abilities of dodecylphosphoric acid and heptadecylphosphoric acid are qualitatively similar to di(2-ethylhexy1)phosphoric acid in that most metals are not appreciably extracted, whereas significant extractions are shown for rare earths, molybdenum, vanadium(IV), and more extraction power for iron(III), titanium. thorium, and other quadrivalent metals. The extractability of some of these metals with both types of reagents-cg., thorium and vanadium-gives a clue to possible additional applications. While vanadium(1V) can be extracted and recovered from high pH liquors using comparatively high extractant concentrations, a t the pH, vanadium concentration. and extraction conVOL. 50, NO. 12
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water. When formed from a kerosine solution of di(2-ethylhexy1)phosphoric acid, the salt tends to separate as a liquid phase containing small amounts of both water and kerosine. The salt remains miscible with the total organic phase if the acid is dissolved in solvents like carbon tetrachloride. ethers, or kerosine modified with small amounts of long-chain alcohols or neutral organophosphorus compounds-e.g., tributyl LRAFFINATE phosphate. Other salts such as lithium remain miscible even when the diluting Figure 5. The Dapex process has been used commercially on the Colorado solvent is unmodified kerosine. The Plateau for 2 years alkali metal salts of the phosphinic acids behave similarly. The salts of centration levels ordinarily existing system. Uranium is recovered from the monoalkylphosphoric acids and phosin a uranium recovery circuit, the the strip solutions by standard chemical phonic acids are either completely interference from this source is not procedures. The process has been opersoluble in the aqueous phase. or form large. If necessary, vanadium can be ating successfully for 2 years a t two precipitates which cannot be made removed from the solvent prior to western mills; it is currently installed soluble in the kerosine solvent with uranium stripping by scrubbing with in a total of four plants, two of which reasonable amounts of added modifier. dilute sulfuric acid. I n treating uranium include vanadium recovery cycles. VaWhile the organophosphorus acids ore leach liquors, iron is reduced to the nadium recovery plants are being conform their alkali metal salts readily and ferrous state and other metals are ordisidered for two additional mills. completely in alkaline solutions, it is narily limited to low concentrations. Monoalkylphosphoric Acid Process, interesting that the monoalkyl acids Dialkylphosphoric Acid Process. I n The mono(2,6,8-trimethyl-4-nonyl) also have the ability to do so partially dialkylphosphoric acid extraction phosphoric acid process (Figure 6 ) , as in acidic solutions. Using mono(2,9(Dapex) (3-6) (Figure 5), by appropriate developed by Dow Chemical Co. and the diethyl-6-tridec)phosphoricacid in keroadjustment of the extractant concentraBureau of Mines, is in commercial sine, cesium has been preferentially tion and of flow rates, uranium is essenoperation (76). extracted from 0.5M sodium nitrate tially completely extracted from the Ey appropriate adjustment of flow solutions a t pH 1.7 with an indicated liquor in three to six countercurrent rates, the uranium is extracted with cesium-sodium separation factor (E:Cs/ stages. Uranium loading of the solvent 0.1.44 mono(2,6,8-trimethyl-4-nonyl)- EgNa) of greater than 20, the cesium will depend upon selection of process phosphoric acid in about four to five extraction coefficient at the 0.5’44 reagent conditions but, when using 0.litl dicountercurrent stages. The loaded level being about 3. Neither the dialkyl(2-ethylhexy1)phosphoric acid, it will organic usually contains 7 to 8 grams phosphoric acids nor the neutral reagents usually be around 4.5 to 7 grams of of uranium oxide per liter. Uranium have this ability. uranium oxide per liter. is stripped with hydrochloric acid to Both di- and monoalkylphosphoric The salts of the dialkylphosphoric reverse the extraction reaction. To acids extract the alkaline earth elements acids formed during stripping tend counteract the high affinity between the from mildly acid solution. Di(2-ethyleither to precipitate from the kerosine alkyl phosphate and uranyl ion, high hexy1)phosphoric acid. 0.1M in kerosine, diluent or to form a third liquid phase acid strengths, 10M, are used to obtain for example. extracts strontium from containing the salt, some diluent, and good stripping coefficients. Small 0.5M sodium nitrate solutions with some water. However, this third phase amounts of iron, aluminum, vanadium, coefficients of about 200 at a pH level can be avoided by adding neutral and molybdenum extracted with the of about 5. The coefficient decreases organophosphorus compounds such as uranium are also removed from the markedly a t both higher and lower pH the trialkyl phosphates, phosphonates, solvent by the acid strip. Some metals, levels. The strontium-sodium separation phosphinates, and phosphine oxides or held more tenaciously, are not removed factor at the maximum was over 1000. long-chain aqueous insoluble alcohols. -e.g., titanium and thorium. T o preThe monoalkyl acids show no such The critical requirements a t a given vent build-up and eventual intersharp maximum in strontium extraction temperature of an additive for this ference with extraction, such metals in the pH range 1.8 to 6.0. The purpose can be related to the extractant are periodically stripped with dilute coefficient and strontium-sodium separaconcentration. The quantities involved hydrofluoric acid; sodium carbonate tion factor increase to about 20 and 140, are sma!l--e.g., 0.1M di(2-ethylhexy1)cannot be used because of the large solurespectively, at pH 5 with 0.1.44 heptaphosphoric acid in kerosine requires adbili t y of sodium salt in the aqueous phase. decylphosphoric acid. dition of 2.2 w./v. % tributyl phosphate Yttrium, Lanthanum, and the to prevent third phase formation. Lanthanides. Extractions of the lanExtraction of Other Metals Organophosphorous additives provide thanide elements with di(2-ethylhexy1)a synergistic enhancement of the uraAlkali Metals and Alkaline Earths. phosphoric acid has been reported by nium extraction coefficient. The alcoAlkaline solutions such as sodium carPeppard and others (72). They find a hols impair uranium extraction somebonate will strip extracted metals from gradual increase in extraction coefficient what, but the coefficients are still high. organic phases containing acidic rewith an increase in atomic weight. Sepagents, converting the reagent to its In the carbonate stripping cycle, aration factors between adjacent elements sodium salt. The distribution of the the uranium and any small amounts were indicated to be 2.5. Independent organic salt between the phases is a of other extracted metals are removed observations a t Oak Ridge National Laboratory confirm these trends and function of the base and its concentrafrom the solvent in two to three countertion, the reagent and the diluting current stages, giving a solution conadditional tests have been made with solvent. Sodium di (2-ethylhexyl) other dialkylphosphoric acids, some taining 50 to 65 grams of uranium monoalkylphosphoric acids, and some phosphate, for example, is insoluble oxide per liter. The extractant, comneutral organiphosphorus compounds. (
