I
C. F. COLEMAN,
K.
B. BROWN, J. G. MOORE, and
D. J.
CROUSE
Oak Ridge National Laboratory, Oak Ridge, Tenn.
Solvent Extraction with Alkyl Amines Although the processes are similar, the liquid-form extractants make amine extraction more versatile than resin sorption
SIXCE
1952, a wide range of amines and related organonitrogen compounds have been studied a t Oak Ridge National Laboratory, primarily for use in recovering uranium and related values from ore-leach liquors (5-74, 78, 27). The Amex process developed in this work is now being used or installed for uranium production in several mills, and wider applications ranging from hydrometallurgy to radiochemical processing are being developed in several laboratories. This article describes the chemistry of extraction by organic solutions of amines and their salts.
Literature Background
Ref.
The molecular or free base forms of these amines are soluble in many organic diluents, but some of their salts show limited organic solubilities, typically decreasing in the order of sulfate, bisulfate. chloride, and nitrate. Class and structure of the amine and tke type of diluent are both important in determining the solubilities. Salts of all the longstraight-chain, primary amines tested showed insuficient solubility in practiTable 1. Simple Amines of Molecular cable diluents. Salts of the long-straightWeights in the Range of 250 to 600 chain, secondary and tertiary amines are Generally Performed Best sufficiently soluble-e.g., >0 lhl-for use in aromatic hydrocarbons a t ordi(The structural formulas given are believed t o represent typical forms) nary temperatures. Their normal sulfates are also soluble in aliphatic hydroPrimene J M , trialkylmethyl, , , H amine, homologous mix':':':':'!" carbons like kerosine, but the bisulfates ture, 18-24 carbon atoms of the straight-chain secondaries sepaAmine 21F81, 1-(3-ethyl'..' H rate out of kerosine solution, and those of pentyl)-4-ethyloctylamine ;.''." the straight-chain tertiaries are borderOctadecylamine H line. The solubility of the bisulfates (and .................. " the chlorides and nitrates) in kerosine c2,;,,,(.:: NBHA, A'-benzyl-l-(3-ethylincreases on addition of alcohols, and is pentyl) -4-ethyloctylamine sufficient for process use in kerosine modAmine S-24, bis(1-isobutyl,,....:,; :,;,:, ified with 3 to 5% of a long-chain alcohol 3,5-dimethylhexyl) amine like Oxo-process tridecanol. When the Amine 9D-178, dodecenyl,. ,.,. alkyl groups are considerably branched, , . . trialkylmethylamine, the salts of all three amine classes are homologous mixture, 24satisfactorily soluble in plain kerosine. 27 carbon atoms The variation of solubility with temDi(tridecyl)amine, mixed '213 !i perature is slight in some cases but conalkyls from tetrapropylene 8 :. by the Oxo process siderable in others (74); for instance, the solubility of dilaurylamine sulfate in -4msco D-95 (a commercial aromatic Triiso-octylamine, mixed CS ;,;.' naphtha) at 2', 20°, and 50' C. is 0.02, alkyls from the Oxo proc- : :..N .:.: 0.10, and >1.5M, respectively. ess For process use, where diluents must Tris(tridecyl)amine, mixed ..... be inexpensive, nontoxic, and not too C13 alkyls from tetrapro.: :.,.,,i ::,: volatile. the first choice is kerosine, keropylene by the Oxo process sine modified with an alcohol, or in some Trifatty amine RC-3749, trispecial cases a heavy aromatic naphtha. alkylamine with mixed n- .......i...... ... There is less limitation in analytical and octyl and n-decyl alkyls physicochemical applications, for which Trilaurylamine such diluents as benzene, hexane, and chloroform may be preferred. ''
Anion extraction from aqueous solution by organic solutions of high molecular amine5 first reported Amine extraction in analytical separations
(20,23,26, $6)
Amex processes developed
(f6-f'7, 19)
(W
Out of several hundred organonitrogen compounds tested, favorable extraction performance was almost entirely limited to simple amines-i.e., compounds with a single amine group and without other functional groups or heterocyclic structures. Strong-base quaternary ammonium compounds are also potentially important for extractions at high pH, but the problem of organic phase solubility is more severe than with the salts of most of the simple amines. Of the simple amines, compounds studied have been largely limited to those having molecular weights from about 250 to 600 (Table I). The lower limits of molecular weight were set primarily by aqueous solubility of the amine salts, because loss of extractant to the aqueous phase should be low, particularly for recoveries from low-assay liquors. The upper limits are not so definite, and reflect various minor limitations such as availability, organic phase solubility,
1756
effects on phase separation, and ratio of ionic capacity to weight. Some of the reagents are commercial mixtures of compounds in isomeric or homologous series. This includes the iso-octyl-, tridecyl-, and trialkylmethyl groups in Table I, and the particular structures listed for these represent forms believed to be typical.
,
'
"
:,
'.
,,
Butyldilaurylamine Methyldilaurylamine Tris(2-ethylhexyl)amine Tribenzylamine
INDUSTRIAL AND ENGINEERING CHEMISTRY
............N ............ ...........i., ........
. ....... .
....:.N .:....
.o
CW.0
General Extraction Behavior The extractions by all three classes of amines are similar in general aspects, and also parallel to sorptions by the amine or alkylammonium groups in anion exchange resins. Organic solutions of the free bases extraci acids from aqueous
NUCLEAR TECHNOLOGY solutions to form the akylammonium salts. R3N ...
+ H X aRINHX .....
I
(The dotted underlines mark species in the organic phase.) With sulfuric acid, the usual acid in the hydrometallurgy of uranium and thorium, the normal sulfate is formed first, and then additional acid may be extracted to form the bisulfate.
0. VANADIUM (VI
C Aqueous U Concn
One anion is readily exchanged for another from the aqueous pKase and the RINHX .....
. . . . . + X-
+ Y-
R3NHY
(3)
order of preference in the amine solutions is similar to that in anion exchange resins-C104- > NOS- > C1- > HS04> F-. As these are weak bases, the acid extraction is reversed by contact with basic solutions, regenerating the free base. RjNHX .....
+ OH-+
2R3NHX ......
R3N ...
+ X - + HzO (4) 2R3N ....+
+ Na2C03 2NaX + CO, -t -.f
H2O
(5)
Even neutral solutions can hydrolyzk the amine salt and strip out the extracted acid, in direct reversal of Equation 1. Metal ions are extracted from acidic aqueous solutions when they exist as anions-e.g., dichromate, vanadate-or form anionic complexes-e.g., uranyl and ferric sulfates. Uranyl sulfate extraction can be described either as the extraction of a neutral uranyl sulfate complex forming a further complex with the amine sulfate,
+
UO,++
sod--
@uozso4
1
or as an anion exchange of sulfate for an anionic uranyl sulfate complex already formed in the aqueous phase, '
UOz++
+ 2so4--
i
2 moles Amine :I mole Fe
0.2
(1)
2(R3NH)HS04 . . . . . . . . . (2)
---------
I
I
mole Amine:2.3molesV
,
I
_M
0.1 02 Aqueous V Concn
03
, M
Aqueous Fe Concn
,M
Figure 1. Nature of metal ion and limiting amine-metal ratio determine shape of extraction isotherm curve (0.1M amine extractants) A. Amine-uranium ratio i s typically lower with tertiary than with secondary amines (I. Tri-n-octylamine-98% kerosine-2% tridecanol. II. Di-n-decylamine-Amsco D-95) 6.
Extraction of polyvanadate ions gives S-shaped curve and amine-vanadium ratio less than unity (Amine 9D-178-kerosine) C. Extraction of hydrolyzed ferric sulfate gives amine-iron ratio approaching two (di-n-decylamine-benzene)
tration of the metal ion itself in those cases where it forms polymeric species or polynuclear complexes. Uranium extraction from sulfate solutions more acid than pH 3 is not affected by the uranium concentration-the extraction coefficient E i ( U ) = [U],,,/[U],, is not changed by changing uranium concentration, except indirectly through changing the free extractant concentration, as shown by the normal shape of the uranium extraction isotherms, Figure 1. In contrast, the extraction coefficients of some metals like vanadium(V) and molybdenum(V1) increase as their concentrations increase, because polymeric forms are important in the extraction.
The extracted uranium is readily stripped by displacement with nitrate or chloride, or by hydrolysis of the amine salt.
+ 2NOa- e
N
(RINH)2U02(SO4)2 ............
+ UOz+++ 2S04--
2R3NHN03 ........
-
+
(8)
(R3NH)zU02(S04)2 . . . . . . . . . . . . 4NazC0~
+ Na4UO2(C0d3+ 2Na2SO4+ H,O + C 0 2 2(RaNH)oUOz(S04)z . . . . . . . . . . . . . + 5Mg0 4RjN . . . . + MgU20.r + 4MgS0, + 2R3N ....
(9)
.-L
2 H 2 0 (10)
lo
The principal parameters controlling the extraction of metal ions from sulfate solution include the aqueous sulfate concentration, concentrations of other complexers that compete for the metal ion, concentrations of other anions that compete for the amine, the pH, the temperature, the effective free amine sulfate extractant concentration, and the concen-
c
z
v
00
W
01
I
Aqueous S u l f a t e
Figure 2. Excess sulfate ion competes with metal ion extraction. Asmall excess of sulfate is required for maximum uranyl sulfate extraction, less for ferric sulfate extraction, none for vanadate extraction (0.1M tri-n-octylamine-Amsco D-95, Amine 9D-178-kerosinet di-n-decylamine-benzene)
Concn.
! i Uorg
0 01
O
o
, F
0
i
uoz(~o4)z--
+ ~OZ(SO~)Z-(R3NH)2UOz(SO4)2 . . . . . . . . . . . .+
(RINWZSOI ........
sod--
(7)
1
As far as measurable net results are concerned, these two sets of equations are exactly equivalent. The actual course of the extraction may follow either or both.
t
1
0.001-0.002 M_ FeOrg
I
1
1
0.0I Aqueous S u l f a t e
1
1
1
0.1
0.I Concn.,
E
I
Aqueous S u l f a t e
VOL. 50, NO. 12
Concn.,
DECEMBER 1958
M_
1757
This results in S-shaped extraction isotherms (Figure 1). Polymerization of the extracted vanadium is also demonstrated by the loading reached, which is more than 2.3 moles of vanadium per mole of amine in the case shown. Essentially no sulfate remains in the organic phase at high loading, confirming that the extracted species is a polyvanadate rather than a sulfate complex. The complex species of iron(II1) extracted under the conditions noted in Figure 1 is a hydrolyzed and a t least partially dimerized sulfate complex, perhaps [FeOHS04]2.2(Rz"d$04 (3). As indicated by Equations 6 and 7, excess aqueous sulfate competes with uranium extraction. The extent of this is illustrated in Figure 2. Equations 6 and 7 do not indicate any direct effect of p H on uranium extraction, but p H does have an important indirect effect through the aqueous sulfate-bisulfate equilibrium. I n the usual concentration ranges, bisulfate competes more severely than sulfate for the amine; hence the uranium extraction decreases with increasing acidity (Figure 3). In the extraction of many other metal ions, there is a n additional indirect effect of pH, through the hydrolysis equilibria of the metal ion itself-the extraction coefficients of iron, vanadium, and molybdenum increase rapidly with rising pH jn the region around pH 2. Uranium sulfate extraction coefficients decrease with rising temperature, to varying extents with different amines, typically by about 20 to 30% per 10' C. ( 74). T h e effect of the amine sulfate extractant concentration on the uranium extraction coefficient has not yet been explained theoretically. Fortunately, how-
1000
c
I
-I
IO
-
1
-
-
VANADIUM ( V I
0.5 M SO4
y
0.04
Vorg
0 0
W
I 0.01 y Vorg
IO0
/
0.5 M
I
so,
B
0.3
2
I
PH 3
Figure 3. pH affects extraction indirectly through hydrolysis equilibria of the metal ions and of the complexing anions (0.1M amine extractants)
I
A.
Uranyl sulfate extraction decreases at low pH because of increased bisulfate competition [Amine 9D-178-kerosine) B. Vanadate extraction increases sharply on approaching the vanadium(\/) isoelectric point at around pH 2.3 (Amine 9D178-kerosine) C. Both bisulfate competition a t lower pH and hydrolysis at higher pH influence ferric sulfate extraction (di-n-decylaminebenzene)
ever, the effect is well defined and can be described satisfactorily for empirical use. The extraction isotherms indicate a limiting " association of each uranium with four to six amines, consistently averaging close to six with straight-chain or moderately branched secondary amines, and between four and five with symmetrical tertiary and highly branched secondary amines (74, 78). Precise measurement of the hydrogen ion transfer when uranium was extracted by known mixtures of amine sulfate and bisulfate indicated a n average association number of 4.7 with tri-n-octylamine ( 7 ) and of 6.0 with di-n-decylamine (24). The uranium extraction coefficient varies
0 0
W
0.1 I
2
PH
with approximately the first power of the extractant concentration when the uranium loading is low enough to neglect the fraction of the extractant complexed with the uranium, and also at higher loadings if the concentration remaining uncomplexed is calculated on basis of the observed limiting association numbers. This is expressed by the empirical equation E,O(U) = k[iM(Zamine) - nM(U),,,]
(11)
in which n has a characteristic value be-
~~
Table 11.
One Group of Metals Is Extracted Best b y Primary Amines, Another Group about Equally b y All Classes
(1 M Sod, pH 1, -1
Metal Ion Tested
gram metal ion per liter: phase ratio a 1 :1; 0.1M amine" in aromatic hydrocarbon diluent) Metal Ion Extraction Coefficient, E 3 M ) = [MI,,,/ [MIaq Primary Secondary Tertiary H N
H
NH
I
,...
Mg,Ca, Al, V(IV),
Cr(III), M n ( I I ) , Fe(II), Co(II), N i ( I I ) , Cu(II), Z n
40
1000 > 1000
> 1000 > 1000
300
150
100 H N
Kerosine Benzene Chloroform
a)!
H O N
H N
O
H
H N
N
25 100 3
130 25
H
O
N
NlOOO
