I
JULIUS BOCHINSKI, MORTON SMUTZ, and F. H. SPEDDING Ames Laboratory for Atomic Research, Ames, Iowa
Separation of Monazite Rare Earths Solvent Extraction Equilibrium data for the rare earth nitrate water tributyl phosphate system are presented, with a simple algebraic method of calculating the proper operating conditions needed to effect a given separation. Predicted results agree with those obtained in a simulated continuous countercurrent extractor
-
-
TI,,
expected increase in mixed rare earths from monazite has intensified research in all methods of separating the mixture into purer fractions (7-5, 7, 8 ) . Aqueous solutions of mixtures of rare earth nitrates were brought in contact
with undiluted tributyl phosphate in batch extraction studies. The total concentration of rare earths in the tributyl phosphate phase is dependent on the nitric acid and total rare earth nitrate concentration in the aqueous *phase (Figure 1). Thus, a t low acidities at least, one equilibrium curve can represent equilibrium data for all possible mixtures of monazite rare earth nitrates. Curves for the individual monazite rare earths differ only slightly from this average curve. A separation factor is a aeasure of the relative ease of separating the component; of a mixture. Figure 2 shows how the separation factors vary with total rare earth concentration at low acidities, and points out the desirability of maintaining high rare earth nitrate concentrations in the extractor, if the acidity is low or no salting-out agents are used. Solvent Extraction Theory
5m Ce LO iO%Nd, 41%Pi 4 5 % LO
60 *A Nd , 2 0 % Gd , 2 0 Y. 5 m UONlZfTE R A R E E A R T H 5
0
100 700 300 400 500 CONCENTRATION OF AQUEOUS PHASE ( x T 1 ( G R A M S OF O X I D E S I L I T E R )
Figure 1. Equilibrium data on rare earth nitrate Figure 2. Separation factors for rare earth nitrates in undiluted tributyl phosphate, no excess acid
-
40
I
I O
-
Figure 3. Mixer-settler extractor. Each box reprqsents one stage
LN(YT)N
=
R N + ~ ( X T )-NRI(XT)R +I (1)
The operating line for the scrub section is determined in a similar manner (Equation 2). These relationships consider all the rare earths as a single solute and can be used only to determine the total concentration of rare earths in the streams in the extractor.
Figure 4 shows the equilibrium curve at low acidity, with possible operating lines for the two sections of the extractor. As the location of the operating lines depends upon the relative flow rates of the immiscible streams and the concentration of solutes in the streams leaving the extractor, their positions can be varied, although not independently. To obtain high rare earth concentrations in many stages of the extractor, thus taking advantage of high separnation factors, many stages must be pinched in between the operating lines and the equilibrium curve at high rare earth nitrate concentrations (Figure 4). Solute Reflux Ratio Figure 4 (left) shows that the concentration of rare earths in the raffinate stream, ‘(XT)R,is much lower than the concentration of the aqueous streams from the pinched-in stages of the extraction section. AS the rates of flow are nearly constant at steady state, the stages not pinched in in the extraction section act as a refluxing device by transferring solutes from the aqueous to the tributyl phosphate phase. These stages perform the same function as if some solutes were recovered from the aqueous stream leav-
5 O r
w
1 (L
li
A rare earth extractor consisting of a series of mixer-settler stages is shown in Figure 3. The relationship between the composition of the liquids leaving a stage is determined by equilibrium relationships, if the stage is an ideal one. The relationship between the composition of the tributyl phosphate phase entering a stage and the aqueous phase leaving that stage is given by the operating line, which is determined by material balances at steady-state conditions. If all the rare earths in a mixture are considered as one component, a single equilibrium curve can be drawn, representing all possible combinations of these rare earths, ifno free nitric acid is present. The operating line for the extraction section can be obtained by making a
material balance about the extraction section. Equation 1 assumes no solutes in the incoming solvent.
FEED
y
, . L-. I I -
< b’
0
100
200
CONCENTRATION OF GRAMS
300 AQUEOUS
400 PHASE
500 1x7
1
OF OXIDES/LITER
VOL. 50, NO. 2
FEBRUARY 1958
157
intersection of the operating lines, shows the relationship between RE and Rs.
SCRUB OPERATING LINE
-RE Rs 1
+
80
I:__ 1:
(‘T’R
0
I
I “T’PE I
1
I l l 1
4.
W P S
“T’PE
0
100 200 300 400 500 CONCENTRATION OF AQUEOUS
Figure lines
100 200 300 400 500
PHASE, GRAMS OF OXIDES/LITER
Stagewise concentrations from equilibrium d a t a and operating
ing the pinched-in region and returned to the solvent entering the pinched-in region. The solute reflux ratio for this section is defined as the ratio between the net amount of solutes transferred to the solvent in the nonpinched-in stages and the amount of solute leaving in the raffinate (Equation 3).
ratio and pinch-in stages at high rare earth nitrate concentration. Adding acid to the scrub solution does not alter conditions in the extraction section, as nitric acid is transferred to the tributyl phosphate phase before the aqueous phase reaches the feed stage. Calculation of Stream Compositions
(3)
As the solutes transferred from the aqueous phase in the nonpinched-in stages must equal the solutes picked u p by the tributyl phosphate, L(YT)PE = ~ ( X T ) , ,- ~ I ( X T ) ~Neglecting . the difference between H and E f l and substituting this relationship in Equation 3 give Equation 4.
(x,)F -
(Y%)E
(7)
Equation 8, the defining equation of p, can be used to calculate rare earth composition in one phase at equilibrium, if composition and concentration of the rare earths in the other phase are known.
[L EQU ILI BRI UM
(‘T’R
(XOR- (X%)F
T o calculate the composition of the rare earth mixtures in the streams in the extractor, equilibrium and operating line relationships must be used for each rare earth. The operating lines for each component can be obtained by multiplying each concentration term of Equations l and 2 by the proper mass fraction of each component.
As the summation of x,’s in the aqueous stream and of y2’s in the organic stream must both equal unity, the summation of PJ, will equal l/kc8 and the summation of yt/Pz will equal k,, for that stage. The stages not pinched in the extraction section do not cause as much spparation of the rare earths as the pinched-in stages, because the concentration and the separation factors are low. If it is assumed that all the stages not pinched in here are equivalent to one ideal pinchedin stage, the composition of the pinchedin and product streams can be calculated simply. This approximation holds reasonably well. At total reflux ( X T ) = ~ 0, F = 0, and = H. If pure solvent were used, (YT)~ would equal zero. A material balance at steady state for component -4 in stage 1 (the fictitious stage representing all nonpinched-in stages of the extraction section) is given in Equation 9. L(Ya)i(ya)i= H ( X T ) ( ~x A ) ~
(9)
A similar expression for solute B is given by Equation 10. L(YT)l(YB)I
H(XT)Z(XB)Z
(lo)
Dividing Equation 9 by Equation 10 gives Equdtion 11. The stages not pinched in in the scrub section cause a similar refluxing action by transferring solutes from the organic phase to the incoming scrub solution.
Again, as the solutes transferred from the organic phase must equal the gain in solutes by the aqueous phase a t steady state, H ( X T ) P S= L(YT)Ps- LI’(YT)E. Neglecting the difference between L and L1’ and substituting this relationship in Equation 5 give Equation 6.
The shape of the low acid equilibrium curve of Figure 1 prevents simultaneously obtaining a high solute reflux ratio in the scrub section and pinching in many stages at a high rare earth nitrate concentration. Its shape can be altered by adding nitric acid to the incoming scrub solution and obtaining a high reflux
158
Equation 8 can be written for this system in terms of A and B. When Equations 1A and 2A are solved simultaneously (equating values for yz) an over-all material balance for that component results, showing that the operating lines intersect at (x,)F. This assumes 1 = H , H M = H, and LN = L” = L. The operating lines may be expressed as functions of the solute reflux ratios by substituting Equation 3 in Equation 1A and Equation 5 in Equation 2A, and noting that ( X T ) N + = ~ (XT)PE and (YT).w+I= (YT)Ps.
Substituting for ( y l ~ ) l / ( y B ) l in Equation 11 results in Equation 12.
n~i.
By extending this process to the entire extractor, the familiar Fenske equation results, where is the total number of ideal pinched-in stages.
Equation 13B is the same equation written in terms of the total mass of A and B rather than mass fractions. A simultaneous solution of Equations 1B and 2B, noting that xi is ( x i ) ~at the
INDUSTRIAL AND ENGINEERING CHEMISTRY
NUCLEAR T~ECHNOLOGY If one chooses the fraction of one of the components fed that is to be in a stream leaving the extractor-i.e., 80% of the cerium in the feed is to be in the raffinate product-and the number of theoretical stages at total reflux, it is possible to calculate the composition of all streams in the extractor, the number of stages needed at a finite reflux ratio, and the proper flow rates. A series of such calculations will permit selection of the desired operating conditions.
Sample Calculation The utility and simplicity of these relationships may be shown by determining the operating conditions for separating a feed into two fractions (Table I). If 80% of the cerium fed is wanted in the raffinate product, the extract will contain 20% of the cerium fed.
-=-= ( M P ~
a ~ ~ l )0.15608 ~
o.13501
1.15608 ( M P ~=~0.13501 ~ ~ ~ X) 0.059 ~ = 0.00796 ( M P ~= ~0.05900 ~ ~ ~ 0.007966 ) ~ = 0.05103
(M~reOI1)p
If the aqueous streams are to be pinched in at a concentration of 430 grams of rare earth oxides per liter, the separation factors shown in Table I will apply. Choosing the total number of ideal pinched-in stages at total reflux to be eight, the mass of each component in the raffinate and extract streams can be calculated by Equation 13B. At total reflux no actual solutes would be leaving in either stream. This calculation determines only composition of exit streams as total reflux is approached. Selection of Operating Conditions By using the terminal compositions calculated above, the number .of ideal pinched-in stages required at various finite solute reflux ratios can be determined. Stage-by-stage calculations are
Table I.
made from each end of the column, using Equation 8 and the proper operating line Equation 1B or 2B. The number of ideal pinched-in stages in each section of the column can be determined by plotting the stagewise compositions and noting that similar streams must intersect at the feed stage. Figure 5 shows possible extract solute reflux ratios, and thus stream flow rates, that can be used to obtain desired separation. This type of figure can be a starting point in determining optimum operating conditions by a cost analysis. If the concentration of the aqueous streams in the pinched-in region is 430 grams of oxides per liter, and water is the scrub solvent, the maximum value of the scrub solute reflux ratio is about 0.45. Above Rs = 0.45, the operating line would be tangent to or intersect the equilibrium curve and result in an inoperable column. If Rs = 0.45, RE must be about 0.83 (Equation 7). More than 30 ideal pinched-in stages would be required if no nitric acid were used in the scrub solution (Figure 5). By using the proper concentration of nitric acid in the incoming scrub solution, it is possible to alter the shape of the equilibrium curve and obtain extremely high scrub solute reflux ratios. The extraction section is not influenced, because the nitric acid is extracted into the tributyl phosphate stream before the scrub solution reaches the feed stage. The proper nitric acid concentration can be determined by a simulated countercurrent extraction run simulating the scrub section. The calculation method used for determining the proper number of ideal pinched-in stages is essentially correct, because the nitric acid concentration of the streams in the pinched-in region is low. If an extract solute reflux ratio of 3.92 is used to recover 80% of cerium in the r a f i a t e , the value of the scrub solute reflux ratio would be 5.8 (Equation 7 ) . 3.92
R
T
=
0.6057 0.4800
about 87.5 grams of oxides per liter of aqueous phase, as shown by Equation 4.
The flow ratio, RlIL, would be 0.493, as shown by Equation 3. L 3.92 = -
R1
169 87.5
If was estimated to be about 5% greater than BI, as the concentration of aqueous pinched-in stages was to be about 430 grams of oxides per liter and the concentration of the aqueous stream leaving the extractor was about 87 grams of oxides per liter. Therefore,
=
1.05 X 0.493 = 0.517
A simulated continuous countercurrent extraction run for the scrub section showed that the scrub stages could be pinched in a t a high rare earth concentration with a scrub solute reflux ratio of about 5.8, if the entering scrub solution was 8M in nitric acid and H / L was 0.382. NIL is virtually the same as H I L I , because the tributyl pho%phate phase takes on nitric acid as it gives up its rare earths in the nonpinched-in stages of the scrub section. The value of ( Y T ) Eis about 25 grams of oxides per liter (Equation 6).
A material balance about the feed stage shows that the ratio of the flow of feed to the tributyl phosphate phase in the pinched-in region is the difference between g / L and H/L. m
4
2
I
RE
I
II
/
- 0.4800
- 0.2623
The concentration of the raffinate stream leaving the extractor would be
Composition of Feed and Product Streams
Basis: Unit mass of feed (Lindsay Chemical Go. Code 36O).O Eight ideal pinched-in stages. SOTo cerium recovery in raffinate. Aqueous phase concentrations~430g. /I. (as oxides)
Pim La203
CeOn PreOll NdzOs Others
0.544 1.000 1.50 2.18 4.20
Feed Mass 0.2410 0.4800 0.0590 0.1900 0.0300 1.0000
Raffinate Mass Mass fraction 0,2405 0.3840 0.0080 0.0015 0.0600 0.6340
0.3793 0.6057 0.0126 0.0024 0.0000 1.0000
Extract Mass 0.0005 0.0960 0.0510 0.1885 0.0300 0.3660
Mass fraction 0.0014 0.2623 0.1393 0.5150 0.0820 1.0000
While the rare earths are actually present as nitrates in extractor, they are determined analytically as oxides and usually reported as such.
0
01
0 2 03 0 4 0 5 0 6 07 08 09
IC
-
RE
R E C I P R O C A L OF E X T R A C T SOLUTE REFLUX RATIO
Figure 5. Effect of solute reflux ratio on the number of ideal pinched-in stages required ( X T ) P ~= 430, 80% of cerium in raffinate VOL. 50, NO. 2
e
FEBRUARY 1958
159
',"t
W v)
2
a
R8 = solute reflux ratio in scrub section X T = grams of rare earth nitrates, as
0.90
0.90 0.80
0
4
0.60 COMPOSITIONS
x
z- 0.40
8
0
Lato,
cc oz ''6 '11
0.30
d
a
P
0
Nde03
0.20
0 IO
I 2 3 4 5 67
I 2 3 4 5 6 6'5'4'3'2' I' STAGE NUMBERS
Figure 6.
F/L
=
As the tributyl phosphate entering the extractor has no solutes and in the pinched-in region contains about 170 grams of oxides per liter, the volumetric flow rate increases by about 5% as it flows through the extractor. If the organic flow rate in the pinched-in region is 200 ml. per minute, only 190 ml. per minute need be fed as fresh solvent because of volume increase as it picks u p solutes. As F / L and H / L have been calcan be determined. culated, F, R, and F = 0.125 L = 0.135 X 200 = 27.0 ml. per H = 0.382 L = 0.382 X 200 = 76.4 ml. per R = 0.517 L = 0.517 X 200 = 103.4 ml. per R1= 103.4 X 0.95 = 98.0 ml. per
minute minute minute minute
The feed concentration is 503 grams of oxides per liter, as calculated by an over-all solute balance.
R1 ( X T ) R
+ L(YT)E
( X T ) =27.0 = 87.5 X 98.0 f 200 X 25 (XT).P = 503 grams of oxides per liter
Experimental Work
A simulated continuous countercurrent extraction run was carried out under conditions similar to the sample calculation, using modified separatory funnels according to the double withdrawal system shown by Perry (6). A side stream of aluminum nitrate was used to obtain a solute reflux ratio of 3.92 in the extraction section. Eight actual stages were used in the extraction section and six in the scrub section, because the end stages were known to be less effective than the pinched-in stages. The flow ratios calculated above were approximated and the extractor was operated 1 60
8 6'5'4'3'2' I '
NUMBERS
Comparison of calculated and simulated column data
0.517 - 0.382 = 0.135
F(XT)F =
STAGE
until the unit was near steady state. Stagewise samples were analyzed. Agreement between experimental and calculated results is shown in Figure 6 . For comparison purposes, the first three stages of the extraction section, and the first two stages of the scrub section, were considered equivalent to one pinched-in stage. The agreement is very good, in view of the numerous assumptions, limited accuracy of the separation factor data, and analytical problems involved.
oxides, per liter of aqueous phase YT = grams of rare earth nitrates, as oxides, per liter of tributyl phosphate phase kt = distribution coefficient for a rare earth, mass fraction of a rare earth in organic phase, yi, divided by mass fraction of rare earth in aqueous phase, xi, a t equilibrium k,, = distribution coefficient for cerium, reference component x i = mass fraction of individual rare earth in rare earth mixture, as oxides, in aqueous phase y a = mass fraction of individual rare earth in rare earth mixture, as oxides, in tributyl phosphate phase 0 = separation factor. Ratio of distribution coefficient for one rare earth to distribution coefficient of reference component, cerium FIRSTSUBSCRIPT (nature of solute) T = total rare earths, considering all rare earths present as one solute i = any individual rare earth-La, Ce, Pr, Nd, etc.
SECOND SUBSCRIPT (identity of sample) E = extract, organic phase leaving scrub section
R
extraction section
PE = stages pinched in in extraction section stages vinched in in scrub section N = typical pinched-in stage in extraction section M = typical pinched-in stage in scrub section 1, 2, 3 = stages in extraction section, counting toward feed stage l', 2' = stages in scrub section, counting toward feed stage
PS Discussivn The calculation method is very useful for selecting the proper operating conditions for separating a monazite rare earth mixture. I t is a simple algebraic method making use of terms defined as the solute reflux ratios. At the high rare earth nitrate concentrations discussed, the solutions are very viscous. To separate the rare earths at these high concentrations, a continuous extractor capable of pumping the streams must be used. Diluted tributyl phosphate or lower concentrations can be used, but more stages are required. Acidified solvent necessitates auxiliary equipment for recovering nitric acid from the raffinate stream. Nomenclature
F
.E
= flow rate of aqueous feed stream,
ml. per minute rate of heavy stream (aqueous) in scrub section, ml. per minute H = flow rate of aqueous stream in extraction section, ml. per minute L = flow rate of light stream (tributyl phosphate), ml. per minute RE = solute reflux ratio in extraction section
H
INDUSTRIAL AND ENGINEERING CHEMISTRY
= flow
= raffinate, aqueous phase leaving
=
"
I
Literature Cited (1) Appleton, D. B., Selwood, P. W., J . Am. Chem. Sac. 6 3 , 2029 (1949). (2) Asselin, G. F., Audrieth, L. F., Comings, E. W., J. Phys. Chem. 54, 640 (19501. (3) Bock, R., Angew. Chem. 6 2 A , 375-82 (1950). (4) Fischer, W., Dietz, W., Jubermann, O., A'aturwisscnschaften 2 5 , 348 (1937). (5) Peppard, D. F., Faris, J. R., Gray, P. R., Mason, G. W., J. Phys. Chem. 5 7 , 294 (1953). (6) Perry, J. H., "Chemical Engineers' Handbook," 3rd ed., p. 718, McGraw-Hill, New York, 1950. (7) Templeton, C. C., Peterson, J. A., J , Am. Chem. Sac. 7 0 , 3976 (1948). (8) Weaver, B., Kappelman, F. A., Topp, A. C., Ibid., 75, 3943 (1953). RECEIVED for review April 1 9 , 1957 ACCEPTEDDecember 7, 1957 Division of Industrial and Engineering Chemistry, Symposium on Nuclear Technology in the Petroleum and Chemical Industries, 131st Meeting, ACS, Miami, Fla., April 1957.
