Ind. Eng. Chem. Res. 1993,32, 931-942
937
Precipitation Stripping of Neodymium from Carboxylate Extractant with Aqueous Oxalic Acid Solutions Yasuhiro Konishi,' Satoru Asai, and Tetuya Murai Department of Chemical Engineering, University of Osaka Prefecture, 1-1, Gakuen-cho, Sakai, Osaka 593, Japan
This paper describes a precipitation stripping method in which neodymium ions are stripped from carboxylate extractant in organic solvent and simultaneously precipitated with aqueous oxalic acid solution. For the single-stage process, a quantitative criterion for precipitating oxalate powders was derived theoretically, and stripping experimenta were done under the precipitation conditions. The resultant precipitates were neodymium oxalate, which is completely free from contamination by the carboxylate extractant and the organic solvent. The overall rate of stripping was controlled by the transfer of neodymium carboxylate in the organic solution, indicating that the presence of oxalic acid in the aqueous phase has no effect on the stripping rate. These findings demonstrate the feasibility of combining the conventional stripping and precipitation stages in a solvent extraction process for separation and purification of rare earths.
Introduction
Experimental Section
Development of advanced technologies has created a great demand for rare earths as excellent functional materials. In general, rare earths are chemically leached from minerals such as bastnasite, monazite, and xenotime, and the mixtures of rare earths leached into solution are separated into individual species by a series of solvent extraction processes. In commercial processes, tertiary carboxylic acids and bis(2-ethylhexy1)phosphoric acid (DBEHPA) are widely used as solvent extractants, and rare earths extracted in the organic solutions are stripped with strongly acidic aqueous solutions. After adjustment of the pH of the aqueous solution, the dissolved rare earths are precipitated as oxalates or carbonates, which can be readily calcined to oxides for industrial applications. Such conventional processes require very large amounts of chemicals: a strong acid is used to complete the stripping of rare earth from the organic solution and a neutralizing agent is consumed to precipitate the rare-earth compounds. It is quite likely that a combined process of the conventional stripping and precipitation stages offers much savings, at least in the chemicals. Doyle and co-workers (1989, 1992) recently proposed the single-stageprecipitation stripping method, in which rare earth ions are simultaneously stripped from a solvent extractant and precipitated. These investigators demonstrated that lanthanum and yttrium can be directly precipitated as oxalates from carboxylate or D2EHPA solutions. To assess the feasibility of the precipitation stripping process, further, it is of importance to characterize the resultant precipitates in detail and to derive quantitative information on the equilibrium and kinetics of the precipitation stripping process. The purpose of this paper is to derive a quantitative criterion for the precipitation of neodymium oxalate powders from organic solution of carboxylic acid, to compare the characteristics of precipitates obtained by the single-stage stripping method with those of powders precipitated by the conventionalmethod, and to determine the rate-controlling step in the precipitation stripping process.
Materials. The solvent extractant used in this work was Versatic 10, a synthetic tertiary monocarboxylic acid produced by Shell Chemical Company. The materials contained 98.5% CIOacid, and the acid value was 318. The Versatic 10 was diluted to desired concentration levels using Exxsol D80, aliphatic hydrocarbons produced by Exxon Chemical Company. These organic materials were used without further purification. The rare earth used was neodymium, which was considered to be representative of lanthanum series. Aqueous solutions of neodymium were prepared by dissolving pure neodymium oxide in hydrochloric acid. The excess hydrochloricacid in the aqueous solution was removed by boiling. Neodymium-loadedVersatic 10solutionswere prepared by solvent extraction of neodymium from aqueous solutions into Versatic 10-Exxsol D8O solutions. During the extraction operation, the pH of the aqueous phase was adjusted around neutrality by using dilute NaOH solution. The neodymium-loaded organic solution was washed with deionized water to remove residual anions and then passed through phase-separatingpaper to remove entrained water. The initial concentration of neodymium in the organic solution was varied from 0.01 to 0.3 kmol/m3,and the initial concentration of free Versatic 10 was from 0.04 to 1.10 kmol/m3. The aqueous strip solutions were 0.02-0.5 km0l/m3 oxalic acid solutions. In some runs, hydrochloric acid was added to the oxalic acid solution. Stripping Studies. A 20-cm3volume of neodymiumloaded organic solution was shaken vigorously with an equal volume of aqueous oxalic acid solution for about 5 min in a screw-cap bottle of 250 cm3. The experimental temperature was 40 OC. The precipitate was filtered, washed with deionized water and acetone, dried at room temperature, and stored in a desiccator. For comparison of precipitate characteristics, neodymium oxalate was prepared according to the conventional stripping procedure. To strip neodymium ions into the aqueous phase, 20 cm3 of neodymium-loaded organic solution was mixed with 100 cm3 of 1.0 kmol/m3 hydrochloric acid solution. The aqueous solution containing neodymium ions was adjusted to pH 2.0by adding 10cm3of 5 kmol/m3ammonia solution, and then 20 cm3 of 0.2 kmol/m3 oxalic acid solution was added to precipitate neodymium oxalate.
* To whom all correspondence should be addressed.
0 8 8 8 - 5 8 8 5 / 9 3 / 2 6 3 2 - 0 9 3 1 ~ 0 ~ . ~ /0 0 1993 American Chemical Society
938 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
The precipitates were characterized by X-ray diffraction analysis (XRD), infrared (IR)absorption spectrophotometry, thermogravimetry (TG), and differential thermal analysis (DTA). The particle size and morphology were observed by using scanning electron microscopy (SEM). Kinetic Studies. The apparatus used to perform the kinetic experiments was an agitated vessel with a flat interface. The vessel made of glass was 80-mm i.d. and was the same as that used in a previous work (Asai et al., 1983). The upper phase in the vessel consisted of organic solution,and the lower phase consisted of aqueous solution. The liquid stirrer placed at the center of each phase was a paddle agitator of 32-mm diameter. The agitated vessel was operated batchwise with respect to the organic and aqueous phases. A volume of 380 cm3 of 0.049 kmol/m3 aqueous oxalic acid solution was first charged into the vessel and then held at 25 "C. The acidity of the oxalic acid solution was adjusted to 1.2 kmol/m3 HC1, by using hydrochloric acid. A volume of 380 cm3 of neodymium-loaded organic solution was adjusted separately to 25 "C and then introduced into the vessel. The organic composition was 0.03 kmol/m3 neodymium and 0.97 kmol/m3 Versatic acid. The stirring of each phase was started immediately. The liquid stirrers were driven at three different speeds of 50,100, and 200 rpm. At the stirring speeds, the organic and aqueous solutions were completely mixed, and most of the precipitates were incorporated into the bulk of aqueous phase. Organic samples of 5 cm3 were periodically withdrawn from the vessel for analysis. The organic samples were mixed with 6 kmol/m3 hydrochloric acid solution to strip the neodymium, and the strip solutions were analyzed for neodymium by EDTA titration to determine the organic-phase concentration.
Results and Discussion Quantitative Criterion for Oxalate Precipitation. The precipitation stripping of rare earth with oxalic acid can be represented as LnR3-3HR+ 3H+ Ln3++ 3H&, (a)
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where overbars denote speciespresent in the organicphase, HzR2 represents dimers of carboxylic acid, and Ln3+ indicates trivalent rare earth (lanthanide) ions. The rare earth ions are exchanged with hydrogen ions present in the aqueous phase and subsequently precipitated as oxalate. Since the precipitation reaction, eq b, permits a marked decrease in the aqueous-phase concentration of rare earth ions, the presence of oxalate anions shifts the position of chemical equilibrium in eq a to the right and promotes the stripping of rare earth from the organicphase. Consequently, in the case of precipitation stripping, rare earth is stripped at rather lower acid concentrations than for the conventional stripping. The precipitation condition of rare earth oxalate is where KSis the solubility product constant. The aqueousphase concentration [Ln3+l of rare earth ions can be expressed in terms of the equilibrium constant Kex for the extraction, i.e., the reverse process of eq a:
Kex= [LnR3~3HRl[H+13/[Ln3+l[~13 (2) To define the oxalate anion concentration [CZO~~-I in eq
1, the dissociation equilibria of oxalic acid must be considered:
HCZO;
-
H+ + CZOt-;
KA2
= [H'] [C,0,2-1/ [HC,O;l
(4)
The experimentally measurable concentration of oxalate is the total oxalate concentration Cy:
(5) Cy = [H,CzO,l+ [HC,O;l + [C,0,2-1 Combining eqs 3-5 gives the oxalate anion concentration: [c,O,2-1 = cy/{1 + [H+]/KA,+ [H'12/(K~1K~2)l(6) Substituting eqs 2 and 6 into eq 1and rearranging give a criterion for the precipitation of rare earth oxalate: CY
'
JP
(7)
where J p is defined by
Equations 7 and 8 indicate that the occurrence of precipitation is strongly dependent on the aqueous and organic compositions. The concentration of hydrogen ions in aqueous oxalic acid solution in the absence of hydrochloric acid can be expressed through the electroneutrality relation: [H'I = [HC,O;l + 2[CzO,2-l (9) Combining eqs 3,4,5, and 9 gives the following equation: [H+13+ KA~[H+]' + (KA~KAzcyK~l)[H+l2cYKAlKAz = 0 (10) For a given concentration Cy of total oxalate, the hydrogen ion concentration can be computed from eq 10. Figure 1 is a nomogram relating the total oxalate concentrations Cy to the J p values in eq 8 as a function of the organic-phase concentration ratio [=I2/ [LnR3-3HR12/3. To calculate the JPvalue, the thermodynamic data were obtained from previous literature. The extraction equilibrium constant K,, for the neodymiumVersatic 10 system was estimated as 1.99 X 10-'6 at 20 "C from the data of Preston (1985). The dissociation constants K Aand ~ K Awere ~ 5.60 X lo-, and 5.42 X m3/ kmol, respectively, at 25 "C and infinite dilution (Martell and Smith, 1977). The solubility product constant KSof neodymium oxalate was 1.54 X (krnol/m3l5at 25 "C (Sarver and Brinton, 1927). The diagonal lines in Figure 1are constant oxalate concentrations Cy that are related to the aqueous-phase hydrogen ion concentrations [H+] through eq 10. The critical precipitation curve is obtained by connecting each of the points J p = Cy on the diagonal line. The curve indicates the boundary of two regions: P represents the region where precipitation can occur and N the region where precipitation cannot occur. It is evident that, for a typical solvent extraction system, precipitation of neodymium oxalate from Versatic 10 solution is easily realized by using aqueous oxalic acid solution. In the present work, the stripping experiments were performed under conditions such that the quantitative criterion for precipitation, eq 7, is satisfied. Moreover, the total amount of oxalic acid was 1.5 times or above the
:nd. Eng. Chem. Res., Vol. 32,No. 5, 1993 939 102
I
/ /
-
I 1 OB
N
Cy=O.OOl kmollrn3
I
10'0
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1
1
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\\
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//
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10'2
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1
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4600
PzRz] I [LnR 3' 3 H R y , krn~I'~/ rn4
Figure 1. Criterion of precipitation of neodymium oxalate from Versatic 10 solutions.
stoichiometricrequirement for the amount of neodymium present in the organic solution. From the experimental studies it was confirmed that the neodymium in the organic phase was entirely stripped and precipitated within the first 5 min of vigorous shaking with the aqueous oxalic acid solution in the screw-cap bottle. After that, the organic and aqueous phases were completely separated from each other, and the precipitates easily settled at the bottom of an aqueous phase. Precipitate Characterization. The precipitated powders were identified by XRD analysis. There was no difference in the crystalline phase between precipitates obtained by the two different stripping methods. Main XRD peaks for both powders precipitated in this work completely agreed with a standard pattern of neodymium oxalate decahydrate. The IR spectra of neodymium oxalates precipitated by the different stripping methods are shown in Figure 2. These precipitates have the identical spectra. The strong absorption bands in the region from 1700 to 1300 cm-l provide the evidence for an oxalate. The strong, broad absorption band at 3000-3600 cm-l indicates the presence of the water of hydration in the oxalate. It should be noted that the absorption band around 2900 cm-', which is assigned to the saturated C-Hstretching, is not detected even for the oxalate precipitated directly from the organic solution. This demonstrates that the oxalate product by the precipitation stripping method is free from contamination by the extractant and the organic solvent. The thermal decomposition transformation of the neodymium oxalate decahydrate to oxide was examined by using T G D T A analysis. As shown in Figure 3, the data for the precipitation stripping method are identical to those for the conventional stripping method. The TG curves show distinct stages, corresponding to dehydration and decomposition. The dehydration reaction appears on the DTA curves as an endothermic peak centered a t 150 O C . It is completed by 200 OC,being followed by a horizontal weight level on the TG curves. The observed weight loss a t 200 "C corresponds exactly to the conversion of decahydrate into anhydrous oxalate. Both exothermic
I
c 3
1800 lo00 Wavenurnbers, crn-'
3000
Figure 2. IR spectraof Ndz(Cz04)3particles. (a)Prepared bysinglestage precipitation stripping: 0.5 kmol/m3 oxalic acid solution, 0.3 kmol/m3 Nd, 0.2 kmol/m3 carboxylate solution. (b) Prepared by conventional stripping method. I
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1
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'
I1
Ila I
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200
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800 "C Figure 3. TGDTA curves of Ndz(CzO& particles. (a) Prepared by single-stage precipitation stripping: 0.5 kmol/m3 oxalic acid solution, 0.3 kmol/m3 Nd, 0.2 kmol/m3 carboxylate solution. (b) Prepared by conventional stripping method. 0
a0
600
Twnpsrature,
and endothermic peaks between 200 and 700 O C are assigned to the decomposition of the anhydrous oxalate to the oxide. Above 700 "C,a horizontal weight level is obtained on the TG curves, and the weight loss from 200 to 700 "Cis exactly equal to that expected for neodymium oxide. Scanning electron micrographs of the neodymium oxalates are shown in Figure 4 for each of the particles obtained by the two different sripping methods. In both cases, the particles are tabular and elongated. Although the particles are irregular in size, the precipitation stripping gives particles having a size range from 1to 5 pm, smaller than those from the conventional method. Figure 5 shows neodymium oxalate powders from the single-stage precipitation stripping method, which were
940 Ind. Eng. Chem. Res.. Vol. 32, No. 5, 1993 suggests thatmanyprimaryparticles(nuclei)are generated
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at higher organic-phaae neodymium concentration. Rate-Controlling Step. During precipitation strip ping, three separate steps take place: (i) mass transfer of neodymium carboxylate from the bulk of organic solution to the liquid-liquid interface, (ii) stripping (backward extraction) reaction of neodymium ions, and (iii) mass transfer of neodymium ions in the aqueous phase accompanied by chemicalreaction withoxalateions. Tocalculate the overall rate of the stripping, it is necessary to evaluate the rate of each of the three steps. A t steady state the overall stripping rate Rs per unit interfacial area is equal to the diffusion fluxes in the organic and aqueous phases:
R, = ko([Lnh.3HR1 - [Lnh.3HRli)
l"Ul
F i r e 4. SEM of NddC2033 particles. (a) Prepared hy single stage precipitation Stripping method: 0.06 kmol/m3 oxalic acid solution, 0.01 kmol/m3 Nd, 0.04 kmol/m3carboxylate solution. (b) Prepared hy conventional stripping method.
= 4kw([Ln3+li- [Ln3+I)
(11)
where ko and kw are the mass-transfer coefficients in the organic and aqueous phases and 4 is the reaction factor (ratio of mass-transfer rate in the presence of aqueousphase reaction to rate without reaction). The stripping reaction, eq a, occurs at the interface, and the reaction rate is assumed to be expressed as
R, = kR[H+l:[LnR3.3HRli - kRTwl:[Ln3+li (12) where kR and kR' are the forward and reverse reaction rate constants. When [H2R21 >> [LnRs3HRl and [H+l >> [LnR3*3HR], as in the present experimental runs, the interfacial concentrations [H~Rzliand [H+li can be regarded to equal the bulk concentrations. In this case, eq 12 can be rewritten as
R, = k,[Lnh3HRli = k,([LnQ3HRli
- k,'[Ln"], - KD[Ln3+li)
(13)
with k,
= kR[H+13
w
Figure 5. SEM of Nd2(C201)3 particles prepared hy single-stage precipitation stripping method: (a)0.02 kmol/m?oxalicacid solution, 0.01 kmoUm3Nd, 0.04 kmol/m3carboxylate solution; (b) 0.23 kmoll m3oxalic acid solution, 0.01 kmol/m3Nd, 0.04 kmol/m3carboxylate solution; (e) 0.02 kmoUm3oxalic acid solution with 0.1 kmol/m3HCI, 0.01 kmollm' Nd. 0.04 kmol/m3carboxylate solution; (d) 0.5 kmoU m3 oxalic acid solution, 0.31 kmol/m3 Nd, 1.1 kmol/m3 carboxylate solution.
precipitated by varying the aqueous- and organic-phase compositions. In all cases, agglomeration of fine particles appears. It is evident from Figure 5a.b that the aqueous oxalic acid concentration has only a slight effect on the particle size. Comparing Figure 5a with 5c, it is found that the addition of HCI to the aqueous phase gives an increase in particle size. The presence of HC1 in the aqueous oxalic acid solution appears to promote particle growth or agglomeration. Comparing Figure 5a with 5d, it is also evident that the higher concentration of neodymium in the organic phase gives smaller particles. This
(14)
= kR'[H&13 (15) KO = k,'/k, (16) The distribution ratio Kn of neodymium between the organic and aqueous phases can be written in t e r n of the extraction equilibrium constant K., defined by eq 2 k',
1 "rn
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KD = K e J ~ 1 3 / [ H + 1 3
(17)
Eliminating the interfacial concentrations [L&3HRli and [Ln3+li from eqs 11and 13gives the stripping rate Rs:
Rs = K([LnR3.3HRl -Kn[Ln"l) (18) where the overall reaction rate constant K is given by K = l/[l/ko + l/k,
+ Kd(4kw)l
(19) The overall resistance, 1/K, is a sum of three resistancan in series. To eximine the ratecontrolling step in the precipitation stripping process, several kinetic studieswere carried out using the agitated vessel with a flat interface. Kinetic data obtained at different liquid-phase stirring speedsareshown inFigure6, wherethe bulkconcentration [LnR,-3HRl of neodymium carboxylate is plotted against treatment time. It is evident that the presence of oxalic acid in the aqueous phase has only a very slight effect on
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 941
i
0.015
* 0
60
120 T i m e , rnin
180
240
Figure 6. Stripping kinetics of neodymium from 0.03 kmol/m3Nd, 1.0 kmol/m3 carboxylate solution in an agitated vessel with a flat interface: (A)n~ = 50 rpm, 0.05 kmol/m3 oxalic acid solution with 1.2 kmol/m3HCl; ( 0 )n~ = 100rpm, 0.05 kmol/m3oxalicacid solution with 1.2 kmol/m3 HC1; (0) n~ = 200 rpm; 0.05 kmol/m3 oxalic acid solution with 1.2 kmol/m3 HC1; ( 0 )n~ = 200 rpm, 1.2 kmol/m3 HCl solution in the absence of oxalic acid.
the stripping rate. In the presence of oxalate anions, the bulk of the aqueous phase was found to be practically free from trivalent neodymium ions. This indicates that neodymium ions stripped from the organic phase are immediately consumed in the bulk of the aqueous solution and precipitated with oxalate ions. Figure 6 also demonstrates that the stripping rate is significantly affected . result indicates by the liquid-phase stirring speed n ~ This that the resistance to interfacial reaction, l / k u , is unlikely to be dominant. Under the present experimental conditions, the distribution ratio KD,which is given by eq 17, becomes much less than unity. Since the mass-transfer coefficient in the organicphase is similar in magnitude to that in the aqueous phase, the resistance to aqueous-phase mass transfer, KD/ +kw, becomes relatively small. Thus, it is assumed that the rate of stripping is controlled by the organic-phase mass transfer of neodymium carboxylate, namely, that K = ko. When, in addition, the bulk concentration [Ln3+l of neodymium ions in the aqueous phase is practically zero, then eq 18 can be simplified to
I
0
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60
120
I 150
I 240
Time, rnin
Figure 7. Plot of log([LnR3.3HR1/[LnR3.3HRIini,)versus time. Solid lines are predicted by eq 21. The symbols are the same as in Figure 6.
assuming that the organic-phase mass transfer is rate controlling.
Conclusions The stripping of neodymium from carboxylic acid extractant was combined with the precipitation with aqueous oxalicacid solution. For this single-stageprocess, a quantitative criterion for precipitating oxalate powders was derived theoretically, and the stripping experiments were done under conditions such that the criterion, eq 7, is satisfied. The powder characterization showed that the neodymium oxalate powders precipitated from carboxylate solutions are unaffected by organic contamination. The overall rate of the precipitation stripping was found to be controlled by the organic-phase mass transfer, indicating that the presence of oxalic acid in the aqueous phase has no effect on the stripping rate. These findings demonstrate the feasibility of the single-stage precipitation stripping process.
ln([LnR,~3HRl/[LnR,~3HRlinit) = -(koS/ v)t (21)
Acknowledgment We wish to thank Mr. Takashi Goto and Mr. Michiya Ohashi, Santoku Metal Industry Co., Ltd., Kobe, Japan, for providing pure neodymium oxide and for their assistance in characterizing the precipitates. We also acknowledge Mr. Yasushi Namekata, Shell Chemical Co., Ltd., Tokyo, Japan, for supplying Versatic 10for this study.
where [LnR3.3HRlinit is the initial concentration of neodymium carboxylate. The organic-phasemass-transfer coefficient ko in the agitated vessel was predicted from a previous correlation (Asai et al., 1983). For this prediction, the densities and viscosities of the present system were measured by conventional methods, and the interfacial tension was determined by a capillary-rise method. The diffusion coefficient of neodymium carboxylate in the organic phase was estimated by using the Wilke-Chang equation. Under the liquid-phase stirring speeds n~ covered in this work, the value of ko was predicted as 0.774 X 10-6 m/s for n~ = 50 rpm and 2.16 X 10-6 m/s for n~ = 200 rpm. The kinetic data from Figure 6 were replotted to test the correspondence between the experimental data and eq 21. Figure 7 shows that a plot of log([LnR3-3HR]/[LnR3-3HRIinit) versus time results in a straight line for each experimental run. The solid lines in the figure represent the theoretical predictions calculated from eq 21 using the predicted values of ko. The kinetic data are consistent with the predictions made
Nomenclature CY = total concentration of oxalate in aqueous phase, kmol/ m3 [H,R,l = concentration of carboxylic acid dimer in the bulk of organic phase, kmol/m3 J p = function defined by eq 8, kmol/m3 ko = mass-transfer coefficient in organic phase, m/s k ~kR’ , = reaction rate constants for stripping, m3/(kmol s) kRA, kRA’ = reaction rate constants for stripping, 5-1 kw = mass-transfer coefficient in aqueous phase, m/s K = overall reaction rate constant, m/s K A ~K, A=~ dissociation constants of oxalic acid, km0l/m3 KD = distribution ratio, dimensionless K,, = extraction equilibrium constant, dimensionless Ks = solubility product constant, kmol5/m15 [Ln3+] = concentration of trivalent rare earth ions in the bulk of aqueous phase, kmol/m3 [LnR3.3HRl = concentration of rare earth in the bulk of organic phase, km0l/m3
R, = -(V/S)d[LnR,BHRl/dt = k,[LnR3+3HRl (20) Integration of eq 20 gives the time variation of the organicphase concentration of neodymium:
942 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
[LnR3+3HR],= concentration of rare earth at the liquidliquid interface, kmol/m3 [LnR3.3HRIini,= initial concentration of rare earth in the bulk of organic phase, km0l/m3 n~ = liquid-phase stirring speed, mi+ Rs = stripping rate of rare earth, kmol/(m2min) S = liquid-liquid interfacial area, m2 t = time, s V = volume of organic solution, m3 Greek Symbol 4 = reaction factor, dimensionless
Literature Cited Asai, S.; Hatanaka, J.; Uekawa, Y. Liquid-Liquid Mass Transfer in an Agitated Vessel with a Flat Interface. J. Chem. Eng. Jpn. 1983,16,463-469.
Lee, J.-C.; Doyle, F. M. Precipitation of Yttrium Oxalate from Di2-Ethylhexyl Phosphoric Acid Solution. In Rare Earths: Resources, Science, Technology and Applications; Bautista, R. G., Jackson, N., Eds.; T M S Warrendale, PA, 1992;pp 139-150. Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1977;Vol. 3, p 92. Preston, J. S. Solvent Extraction of Metals by Carboxylic Acids. Hydrometallurgy 1985,14,171-188. Sarver, L. A.; Brinton, H. M.-P. The Solubilitiesof Some Rare-Earth Oxalates. J. Am. Chem. SOC.1927,49,943-958. Yoon, J. H.; Doyle, F. M. Precipitation of Rare-Earth Powders from Aqueous solutions and Emulsions. In Innovations in Materiak Processing Using Aqueous, Colloid,and Surface Chemistry;Doyle, F. M . , Raghavan, S., Somasundaran, P., Warren, G. W., Eds.; T M S Warrendale, PA, 1989; pp 195-211. Received for review August 24, 1992 Revised manuscript received December 25, 1992 Accepted January 15,1993
