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Separation of tungsten and molybdenum in dilute hydrochloric acid solution by extraction with sulfoxides
Hydrometallurgy, 13 (1984) 137--153 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
137
SEPARATION OF TUNGSTEN AND MOLYBDENUM IN DILUTE HYDROCHLORIC ACID SOLUTION BY EXTRACTION WITH SULFOXIDES
G.S. DAI, B.Y. XUAN and Y.F. SU East China Institute of Chemical Technology, Shanghai (China) (Accepted May 8, 1984)
ABSTRACT Dai, G.S., Xuan, B.Y. and Su, Y.F., 1984. Separation of tungsten and molybdenum in dilute hydrochloric acid solution by extraction with sulfoxides. Hydrometallurgy, 13: 137--153. By using di-n-butylsulfoxide and petroleum sulfoxide as extractants, the extraction behavior of tungsten and molybdenum in hydrochloric acid solutions was studied under different acidities. When acidity is low, molybdenum is preferentially extracted, the selectivity can be very high, and the separation of these two metals is considered favorable. The separation was confirmed by dissolving in dilute hydrochloric acid the combined oxides of both metals, which were obtained from the acid leaching liquor of scheelite. From the experimental curves the molecular ratios of the complexing reaction were analyzed and the neutral molecular complex mechanism was discussed.
INTRODUCTION T h e solvent e x t r a c t i o n t e c h n i q u e has b e c o m e increasingly i m p o r t a n t in t h e h y d r o m e t a l l u r g y o f t u n g s t e n and m o l y b d e n u m . M a n y r e s e a r c h w o r k e r s h a v e b e e n engaged in finding e f f e c t i v e e x t r a c t a n t s a n d d e v e l o p i n g n e w processes to p r o d u c e t u n g s t e n and m o l y b d e n u m c o m p o u n d s b y solvent e x t r a c t i o n . A m o n g various e x t r a c t a n t s such as A l a m i n e 336, M I B K , D 2 E H P A and TBP [1] b e i n g used in t h e r e c o v e r y and s e p a r a t i o n o f t u n g s t e n a n d m o l y b d e n u m , t h e t e r t i a r y a m i n e s have b e e n studied m o s t e x t e n s i v e l y . B e f o r e 1 9 6 6 t h e r e w e r e a l r e a d y several p l a n t s in t h e U.S.A. using a m i n e s f o r t u n g s t e n r e c o v e r y . In C h i n a a few p l a n t s have also c h a n g e d t h e i r f o r m e r p r o c e s s t o e x t r a c t i o n . H o w e v e r , t h e o n l y e x t r a c t a n t in use is a t e r t i a r y a m i n e , and t h e r e m o v a l o f d e t r i m e n t a l e l e m e n t s , such as Mo, P, As a n d Si, is b y c h e m i c a l m e a n s . In general, t h e p r e c i p i t a t i o n o f t h e s e e l e m e n t s m a y cause an a p p r e c i a b l e loss o f t u n g s t e n . T h e m e c h a n i s m o f e x t r a c t i n g t u n g s t e n b y a m i n e s f r o m sulfuric acid m e d i u m has b e e n p u t f o r w a r d b y Z e l i k m a n [2] a n d similar studies have b e e n m a d e b y Yu Shu-qiu a n d C h e n J i a y o n g [3]. H o w e v e r , t h e s e p a r a t i o n o f
0304-386X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
138 tungsten from m o l y b d e n u m by using amines as extractants is difficult because of the similar behavior of these two elements and their compounds. In the Proceedings of ISEC 74, Esnault [4] proposed a new process for separating tungsten from m o l y b d e n u m by using D2EHPA as extractant, by which the m o l y b d e n u m is first preferentially extracted; subsequently, tungsten is extracted by amines. Large amounts of tungsten {>40 g/1 WO3) present in solution will seriously affect the extraction of m o l y b d e n u m ; therefore the application of this process is limited. Another m e t h o d of processing revealed by a U.S. Patent [5] in 1976 is by using TBP as extractant for separating tungsten from molybdenum. In order to increase the selectivity of m o l y b d e n u m the solution has to be pretreated with hydrogen peroxide. Up to now, research directed towards finding a more efficient extractant and developing an alternative process for the separation of these two metals seems to be worthwhile. The present authors have suggested the use of sulfoxides as extractant to separate tungsten from m o l y b d e n u m in dilute hydrochloric acid. The experimental work was done in a laboratory and the best separating conditions were found. EXPERIMENTAL
Apparatus As shown in Fig. 1, an all-glass apparatus has been designed and made for the equilibration of two liquids of partial miscibility. The main part of the apparatus, 1, consists of a mixing chamber and a calibrated burette of nearly equal volumes. When it is placed in an upright position, all liquids remain in the mixing chamber and can be mixed by the magnetic stirrer, 2, at the
1
4>-Z~
I-0
0
0
o
o]
Fig. 1. Apparatus: 1, equilibrator; 2, magnetic stirrer; 3, graduated feeder; 4, infrared lamp.
139 b o t t o m as shown. After the magnetic stirrer is stopped, the apparatus is turned upside down until phase disengagement occurs. The volume of both phases can be measured accurately. A drying tube in connection with the graduated feeder, 3, and an infrared lamp, 4~ are used if the extractant is hygroscopic and/or m a y solidify at ambient temperature such as is the case with pure dibutyl or dioctyl sulfoxide.
Aqueous feed solutions When pure or synthetic solutions of tungsten and m o l y b d e n u m were used, the feeds were prepared by dissolving sodium tungstate or/and a m m o n i u m molybdate of A.R. grade in water, and adjusted to proper pH values by adding hydrochloric acid or sodium hydroxide. A solution of scheelite was prepared by decomposing a sample of scheelite concentrate using hydrochloric acid; the crude tungstic acid was then dissolved in sodium hydroxide and filtered. The resulting clear solution was adjusted to the proper pH value by adding acid or alkali.
Extractants and diluents Dibutyl sulfoxide (DBSO) was synthesized by a conventional method, and the crude product was purified by repeated crystallization in benzene. The physical properties of the dibutyl sulfoxide product were measured and are shown in Table 1. The infrared spectra were also examined to confirm its purity. Dioctyl sulfoxide (DOSO) was purchased from Tienjin Reagent Co., Branch III and used w i t h o u t further purification. All diluents were of technical grade. Other reagents were A.R. chemicals. Another extractant used was petroleum sulfoxide and its physical properties are shown in Table 2.
Temperature of extraction Preliminary tests indicated that with dibutyl sulfoxide the extraction of both metals is favoured by a low temperature. But as the melting points of pure DBSO and dioctyl sulfoxide are 32.6°C and 71°C, respectively, all experiments using these extractants were carried out at temperatures several degrees above their melting points in order to avoid the possibility of the separation of any solid.
Time of equilibration As the mixing was done by magnetic stirrer and hence the agitation was not vigorous, the time required to attain phase equilibrium was found to be less than a quarter of an hour. Therefore, 30 min of equilibration were used to obtain all the experimental data.
140 TABLE 1 Physical properties of DBSO Crystal form
m.p. (°c)
Density, d~ °
Refractive index, .~
Solubility in %, 40°c
Source
needle needle
32.5 32.6
0.9242 0.9216
1.4600 1.4606
7.42 slight
measured Ref. [6 ]
TABLE 2 RI"--~ Physical properties of petroleum sulfoxide, R 2 j b = O- , where R~ R 2 = alkyl group with 4 to 5 carbon atoms Average molecular weight Density, d~ ° 20 Refractive index, n D Viscosity at 10°C 20°C 30°C Solubility in H:O n-hexane CC14 alcohol benzene
176.2 1.0253 1.4853 22.1 15.2 7.59
mPa s (22.1 cP) mPa s (15.2 cP) mPa s (7.59 cP) slight 10 vol. % miscible miscible miscible
Method of analysis Both tungsten and molybdenum were analysed by the thiocyanate photom e t r i c m e t h o d . T h e p h o t o m e t r i c m e a s u r e m e n t f o r t u n g s t e n was m a d e at 400 n m a n d f o r m o l y b d e n u m at 460 n m . Within t h e e x p e r i m e n t a l c o n c e n t r a t i o n range it was f o u n d t h a t t h e t w o m e t a l s do n o t i n t e r f e r e w i t h e a c h o t h e r in analysis. H o w e v e r , in t h e p r e s e n c e o f s u l f o x i d e , t h e a q u e o u s m e t a l s o l u t i o n has to be e x t r a c t e d first b y c a r b o n t e t r a c h l o r i d e to free it f r o m t h e e x t r a c t a n t in o r d e r to a v o i d its i n t e r f e r e n c e . RESULTS AND DISCUSSION D i b u t y l s u l f o x i d e (DBSO) is similar in e x t r a c t i n g p r o p e r t y to T B P a n d c o n s i d e r e d a n e u t r a l e x t r a c t a n t . It is well k n o w n t h a t in s t r o n g h y d r o c h l o r i c acid, say 6 M or m o r e , t h e e x t r a c t i o n o f t u n g s t e n (or m o l y b d e n u m ) b y T B P is in t h e f o r m o f a c a t i o n i c c o o r d i n a t i o n c o m p o u n d , as f o l l o w s [7] : WO2C12 + n T B P ~ WO2C12" n T B P A c c o r d i n g to De a n d R a h a m a n [8] n was f o u n d t o be 2. A l t h o u g h the e x t r a c t i o n o f t h e c o m p l e x is a l m o s t q u a n t i t a t i v e , it c a n n o t be used in p r a c t i c e d u e to t h e limited solubility o f WO2 C12 (or MoO2 C12) in s t r o n g acid
141 solutions. It is also reported that 4--5 M HC1 is the best medium for complete extraction of Mo(VI) by TOPO and the extracted species is MoOzC12" 2TOPO [9]. On the other hand, in weak hydrochloric acid solutions, it is most probable that these two metals exist in the form of isopolymetallic acids which will precipitate when the pH value of the aqueous solution is about 2 or less; however, the metal, W or Mo, will be in the form of sodium or a m m o n i u m tungstate or molybdate when the aqueous solution approaches the neutral point, e.g. at pH > 6. It has been found that between these two pHs, DBSO can extract W or Mo from HC1 solutions. The present authors prepared the aqueous solutions containing W and/or Mo by adding hydrochloric acid to sodium tungstate and/or molybdate until a proper pH was attained. After equilibration with DBSO (or D B S O diluent), the organic phase was separated and tested for C1- and Na*. Experiments indicated t h a t the a m o u n t of C1- in the organic phase is < 1 0 -s mol. Similarly, Na + was not detected in the organic phase by the emission spectra, although there was a large a m o u n t of Na ÷ in the aqueous phase which was in equilibrium with the organic phase. Therefore, it seems apparent that the extractable species are complexes formed by isopolymetallic acid with DBSO instead of either MeO: C12 on one hand, or Na2 MeO4 on the other (where Me is W or Mo).
Effect o f acidity o f the aqueous solution In weak hydrochloric acid solutions the species of isopolymetallic acids of both tungsten and m o l y b d e n u m are very complex and change markedly with a slight variation of hydrogen ion and metal concentrations. It was reported [10] t h a t when the ratio of H ÷ to WO42- equals 2:1, 6:6, 7:6, 14:12 or 18:12, different species will form and the distribution of the different species changes accordingly. The general formula of the anion is HzWxOy q- [10]. In the case of isopolymolybdate ions, similar species were confirmed to exist [11]. When acid is added to a solution of sodium tungstate or molybdate, the following reaction will occur: p H ÷ + q MeO42- ~ (Hp_2rMeqO4q_r) (2q-p)- + r H 2 0 where Me denotes tungsten or m o l y b d e n u m . In the presence of an extractant, the anions will combine with protons to form isopolymetallic acid--extractant complexes and pass to the organic phase. In consequence, H ÷ is consumed so that the pH of the aqueous solution changes as the reaction proceeds and the equilibrium pH is always higher than the initial value. A part of the experimental data for the extraction of W and Mo by DBSO at 40°C is shown in Tables 3 and 4. In Table 3, the pH of the feeds was adjusted to 2.0 and changes to 2.5 at equilibrium, at the same time, the
142 TABLE 3 Equilibrium data for extraction of W by DBSO (40°C) Run no.
301 302 304 305 307 308
Before equilibration pH = 2.0
At equilibrium pH = 2.5
D(O/A) (WO3)o
vol. (ml)
cone. (g/l)
vol. (ml)
cone. (g/l)
org.
aq.
(WO~) O
(WO3) A
org.
aq.
(WO3)o
(WO3)A
7.95 8.20 3.90 4.10 4.10 3.60
10.0 15.0 15.0 15.0 10.0 10.0
0 0 0 0 0 0
1.07 2.72 10.4 19.7 49.8 58.7
9.10 8.93 3.80 4.15 4.45 3.85
8.55 13.9 14.8 14.6 9.35 9.55
1.15 4.14 29.6 37.3 44.1 47.3
0.025 0.280 2.84 9.68 32.3 42.4
(WO3)A
46.0 14.8 10.4 3.85 1.37 1.12
TABLE 4 Equilibrium data for extraction of Mo by DBSO (40°C) Run Before equilibration no. pH = 5.0
401 402 403 404 405
vol. (ml)
cone. (g/t)
org.
aq.
(MoO~)o
5.0 5.0 5.0 5.0 5.0
10.0 10.0 10.0 10.0 10.0
0 0 0 0 0
At equilibrium pH = 5.5
D(O/A) (MoO~) °
vol. (ml)
cone. (g/l)
(MoO3)A
(MoO 3)A
org.
aq.
(MoO~)o
(MoO3)A
2.40 4.83 9.35 19.7 39.0
5.45 5.50 5.45 5.60 5.45
9.40 9.44 9.40 9.39 9.39
2.16 4.23 8.35 13.4 13.8
1.31 2.65 5.09 13.0 33.6
1.65 1.60 1.64 1.03 0.411
volumes of the aqueous phase always decrease and those of the organic phase i n c r e a s e d u r i n g e q u i l i b r a t i o n . S i m i l a r c h a n g e s in p H a n d v o l u m e a r e a l s o o b s e r v e d in t h e c a s e o f M o { T a b l e 4). The changes of distribution coefficient of individual metals with pH v a l u e o f t h e a q u e o u s s o l u t i o n a t e q u i l i b r i u m a r e s h o w n in F i g . 2. F r o m t h e f i g u r e it is a p p a r e n t t h a t m o l y b d e n u m may be easily extracted at pH v a l u e s b e t w e e n 4 . 6 a n d 6, w h e r e a s t h e e x t r a c t i o n o f t u n g s t e n is i n s i g n i f i c a n t . T h e r e f o r e , it i n d i c a t e s a p o s s i b i l i t y t o d e s i g n a p r o c e s s t o s e p a r a t e t h e s e t w o m e t a l s . T h e p H o f t h e a q u e o u s s o l u t i o n is f i r s t a d j u s t e d t o a h i g h e r p H v a l u e t o r e m o v e t h e M o f i r s t , a n d t h e n m o r e a c i d is a d d e d t o l o w e r t h e p H , t o s a y a b o u t 2 . 5 , in o r d e r t o e x t r a c t a l l t h e t u n g s t e n f r o m t h e f e e d t o t h e o r g a n i c solvent. A s s h o w n in T a b l e 2, p e t r o l e u m s u l f o x i d e h a s a d e n s i t y v e r y n e a r t o t h a t of water and possesses a high viscosity. Therefore, a diluent has to be added
143
t o i m p r o v e its p h y s i c a l p r o p e r t i e s . In t h e case o f using P S O - - B u O H as a n e x t r a c t a n t - - d i l u e n t s y s t e m in t h e e x t r a c t i o n o f t u n g s t e n , it has b e e n f o u n d t h a t , c o n t r a r y t o t h e case o f using u n d i l u t e d D B S O , t h e o p t i m a l c o n d i t i o n f o r e f f i c i e n t e x t r a c t i o n shifts t o t h e high a c i d i t y range, as s h o w n in Fig. 3.
7.5" ..5< O n 5.0-
WO3:20 G/L 40 °
MOO3:20 G/L
2.5
--'1i ~~ 2.0
4.0
6.0
pN
Fig. 2. Effect of equilibrium pH on distribution coefficients of W and Mo using undiluted DBSO and initial aqueous concentrations as indicated; O/A = 1.
500 40 °
t O c5
INITIAL AQ. CONCN.: / (wO3)A 26.7 G / L ~ ' / 250/_/ 41.0
]
o
0. 0.3 0.5 0.7 0.9 I.I ACIDITY AT EQUILIBRIUM, (H+)A, N Fig. 3. Effect of (H~)~ on D(O/A) at different initial concentrations of (WO3)A; solvent: PSO :BuOH = 1:1; O/A = 1.
Extraction isotherms T h e e q u i l i b r i u m d i s t r i b u t i o n s o f t h e t w o m e t a l s b e t w e e n s u l f o x i d e and a q u e o u s p h a s e s w e r e d e t e r m i n e d . T h e results o b t a i n e d f o r t u n g s t e n a n d m o l y b d e n u m , w h e n p r e s e n t alone, are s h o w n in Figs. 4 a n d 5. T h e s e results w e r e o b t a i n e d u n d e r t h e e x p e r i m e n t a l c o n d i t i o n s s h o w n in Tables 3 a n d 4.
144 T h e curves in Figs. 4 a n d 5 (also t h e n u m e r i c a l values in Tables 3 a n d 4) s h o w t h a t at a p p r o p r i a t e p H c o n d i t i o n s p u r e d i b u t y l s u l f o x i d e c a n efficiently e x t r a c t t u n g s t e n a n d m o l y b d e n u m f r o m dilute h y d r o c h l o r i c acid s o l u t i o n ; t h e d i s t r i b u t i o n c o e f f i c i e n t s h a v e b e e n f o u n d to increase w i t h decreasing c o n c e n t r a t i o n s o f b o t h metals. F r o m this f a c t o n e m a y a n t i c i p a t e t h e p o s s i b i l i t y o f a high r e c o v e r y o f m e t a l values even at low c o n c e n t r a t i o n s . T h e d a t a also reveal t h a t t h e loading c a p a c i t y o f t h e organic p h a s e is high, a n d c o n s e q u e n t l y t h e c o n c e n t r a t i o n o f t u n g s t e n or m o l y b d e n u m in t h e
40.
X -
Y
- 40
\
d
[
2
pH =2"5
,:,
>..: 20 [o
20
o
2b 4'0 X, (WO a)A, G/L
Fig. 4. Distribution coefficient of tungsten, D(O/A), and equilibrium curve at 40°C (experimental conditions and data are shown in Table 3).
1541~ N~
40 °
pH=S.S
-I.5
8 O -I.0 v C5
2 5
0
.0.5
-Y
0
2'0
40
X, ( MoO3)A, G/L Fig. 5. Distribution coefficient of molybdenum, D(OZA), and equilibrium curve at 40°C (experimental conditions and data are shown in Table 4).
145
s t r i p p i n g l i q u o r c a n a l s o b e h i g h . T h i s is d e s i r a b l e in v i e w o f e n e r g y c o n s u m p t i o n if it is n e c e s s a r y t o e v a p o r a t e t h e s o l u t i o n f o r t h e r e c o v e r y o f t h e m e t a l v a l u e in t h e f o r m o f s a l t s . F r o m t h e f o r m o f t h e e x t r a c t i o n i s o t h e r m s it can be seen that only a few theoretical stages are required to attain a high percentage of recovery. Figure 6 indicates that with petroleum sulfoxide and butanol as extracting
50-
X
d
X" +
\ t_9
6
>: 25
:¢ /
40 °
I
f o
Xo (W03) A, G/L Fig. 6. Equilibrium curves for the extraction of tungsten b y PSO--BuOH at different acidities; equilibrium HC1 concentrations as indicated; solvent: PSO :BuOH = 1:1. 1.0
<0 . 8 S ~ 40 °
pH=3.2
0
0.6-
×
~ 0.4-
o.I
S'" 0
, 1.0 X, ( MoO3)A, G/L
2.0
Fig. 7. Equilibrium curves for the extraction of molybdenum by petroleum sulfoxide with different diluents; o, PSO :BuOH = 1:1; x, PSO :CsH17OH = 1:1.
146 s o l v e n t f o r t u n g s t e n , t h e p o w e r o f e x t r a c t i o n increases w i t h increasing acidity. In c o m p a r i s o n w i t h Fig. 4, t h e e x t r a c t i o n o f t u n g s t e n is f a v o u r e d b y a s t r o n g e r acid w i t h P S O - - b u t a n o l r a t h e r t h a n w i t h DBSO. Figure 7 s h o w s t h e e q u i l i b r i u m curves o f m o l y b d e n u m o b t a i n e d w h e n using PSO as e x t r a c t a n t a n d a l c o h o l as d i l u e n t . It s e e m s t h a t m o l y b d e n u m can still be e x t r a c t e d at m u c h l o w e r a c i d i t y t h a n t u n g s t e n ; t h e s e p a r a t i o n o f t h e t w o m e t a l s is also possible. Figure 7 f u r t h e r indicates t h a t b o t h b u t a n o l a n d o c t a n o l m a y be used as diluent; h o w e v e r , t h e f o r m e r s h o w s higher e x t r a c t i n g p o w e r t h a n t h e latter.
Separation of tungsten from molybdenum Synthetic solution When t h e feed s o l u t i o n was m a d e o f m i x e d p u r e t u n g s t a t e and m o l y b d a t e o n l y , t h e p l o t o f s e p a r a t i o n f a c t o r vs. e q u i l i b r i u m a q u e o u s p H value is s h o w n in Fig. 8 ( u p p e r curve). B e t w e e n p H values 3 a n d 5, t h e s e p a r a t i o n f a c t o r s are fairly high and t h e curve s h o w s a m a x i m u m at p H 4. In c o n t r a s t to t h e result o b t a i n e d b y E s n a u l t [4] e m p l o y i n g D 2 E H P A as e x t r a c t a n t , t h e use o f D B S O enables t h e s e p a r a t i o n o f t h e t w o metals even if t h e feed s o l u t i o n bears a high WO3/MOO3 ratio. T h e e f f e c t o f an even larger ratio (WO3/MOO3) in the feed s o l u t i o n o n t h e d i s t r i b u t i o n c o e f f i c i e n t s a n d h e n c e t h e s e p a r a t i o n f a c t o r s was also e x a m i n e d . T h e e x p e r i m e n t a l results are illustrated b y t h e e x a m p l e s in T a b l e 5. E x p e r i m e n t a l d a t a w e r e also o b t a i n e d w i t h p e t r o l e u m s u l f o x i d e - - b u t a n o l as e x t r a c t a n t (see T a b l e 6). TABLE 5 Separation factors when DBSO is used as extractant Aqueous feed (g/l)
pH
WO 3
MoO 3
WOflMoO~
61.6 93.2
2.42 0.48
25.5 194
4.8 4.4
D(O/A)
~(Mo/W)
WO3
MoO 3
0.537 0.178
5.63 6.35
10 36
TABLE 6 Separation factors when petroleum sulfoxide (PSO) is used as extractant; solvent used: PSO :BuOH = 1:1 Aqueous feed (g/l)
pH
WO3
MoO 3
WOflMoO3
7.74 2.26
2.16 2.54
3.!58 0.890
2.2 2.2
D(O/A)
2(Mo/W)
WO3
MoO~
0.0581 0.126
4.20 3.47
72.3 27.5
147 By close examination of the data in Tables 5 and 6, and the trend of the distribution of W and Mo in Figs. 4 and 5 (irrespective of different pHs) the following generalizations can be made: (1) At a proper pH value, the distribution of either metal in the organic phase will be favoured by a low concentration, whether the aqueous solution contains a single metal or both. (2) With undiluted DBSO as extractant, the optimal pH for the separation of the two metals is around 4, whereas this optimal point will shift to higher acidity, if PSO--BuOH is used as solvent. (3) A comparison of DBSO-- and PSO--diluent indicates that PSO is not only a cheaper, but may also be a better extractant than the former. Solution from scheelite
The extraction behavior of a solution obtained by decomposition of scheelite by hydrochloric acid is also studied. The solution usually contains 50--100 g/1 WO3 and 1--5 g/1 MOO3. The distribution coefficients of both metals between DBSO and aqueous phases at different pH values were determined, and the resulting plot of separation factor vs. equilibrium aqueous pH is also shown (lower curve of Fig. 8). Although the curve is slightly lower than that of the synthetic solution, the maximum separation factor is still about 12, which is found at the same pH as the latter. The slight lowering of separation factor curve may be explained by the influence of other impurities originally present in scheelite.
40 °
~15C~
SYNTH.
~lOv
c~ 5
SOLN. F R O M SCHEELI TE
3:o
4'.o
s.o
6:o
pH
Fig. 8. Effect of equilibrium pH on ~ = D(Mo)/D(W) during the extraction of tungsten and molybdenum by undiluted DBSO; initial aqueous concentration: o, synthetic solution: (WO3)A, 20 g/l, (MoO~)A, 2.5 g/l; A, solution from scheelite: (WO3)A, 53.3 g/l, (MoO3)A, 2.3 g/1.
148
Effect of diluen ts The addition of any small amount of water-immiscible organic diluent w i l l s e r i o u s l y i m p a i r t h e e x t r a c t i o n o f b o t h m e t a l s b y D B S O , as s h o w n in F i g . 9 a n d T a b l e 7.
40 °
40 °
~4 7 c~
v
C5 3
I.O2
0
I00
5'0
I O0 5'O DBSO °,o (V)
0
Fig. 9. Effect o f addition of diluent to DBSO on the distribution of W and Mo: for W, the diluent is dibutyl ether; for Mo, the diluent is toluene; initial aqueous concentration: (WO3) A 25.7 g/l, (MoO~) A 8.51 g/l; O/A = 0.5. TABLE 7 Effect of diluents on the extraction of W and Mo by DBSO and DOSO Extractant
DBSO 1 M DBSO 1 M DBSO DOSO 1 M DOSO 1 M DOSO 1MDOSO 1 M DOSO
Diluent
None BuOH Bu~O None Xylene Bu:O BuOH BuOH+EtOH
Temp. (°C)
40 13 25 80 25 25 25 25
Miscibility (%) Solvent in water
Water in solvent
6.46 8 0.5 1 0.013 insol. 0.06 1
29.5 16 3 1 0.04 0.06 10 20
Extractability
strong medium weak medium nil nil nil weak
H o w e v e r , in c a s e o f t h e a d d e d d i l u e n t , s u c h as B u O H w h i c h is p a r t i a l miscible with water, the reduction of the percentage extraction of the metals w i l l b e less. T h e u s e o f u n d i l u t e d d i o c t y l s u l f o x i d e , w h i c h h a s a h i g h e r melting point and much smaller mutual solubilities with water than DBSO ( s e e T a b l e 7), g r e a t l y r e d u c e d t h e p e r c e n t a g e e x t r a c t i o n o f t h e m e t a l s . H o w -
149
ever, the addition of BuOH and EtOH to increase its miscibility with water did improve its extracting power somewhat. This fact leads us to suspect that the species extracted into the solvent may carry water molecules in solvated form. A preliminary test was done and the results obtained are illustrated in Table 7 also. This may give a clue to induce further study in order to gain a clear insight into the mechanism of the extraction of both metals. The effect of diluent on petroleum sulfoxide is quite similar to that on DBSO, see Fig. 10 and Table 8. In this table, the petroleum sulfoxide is pre150 / X
~100-
70
o cl
/0
503 0 O/o P S O
0
0.2
0.4.
0.6
0.8
(H+)A '
1.0 N
Fig. 10. E f f e c t s o f p e r c e n t P S O in s o l v e n t a n d e q u i l i b r i u m a c i d i t y , (H+),~, o n D ( O / A ) ; initial a q u e o u s c o n c e n t r a t i o n : (WO3) A 26.7 g/l; d i l u e n t is b u t a n o l ; O / A = 1. TABLE 8 Effect of diluents on the extraction of tungsten; solvent: PSO :diluent = 1:1 (by vol.), shaken with 3.5 N HC1 before use Diluent
Before equilibration acidity [ H+] 0
MiBK Bu20 BuOH
At equilibrium
vol (ml) aq.pH
0.01 M 1.4 0 . 0 3 M 1.4 0 . 7 1 M 2.0
WO 3 (g/l)
org.
aq.
org.
aq.
9.90 9.55 10.85
10.0 10.0 10.0
0 0 0
52.9 52.9 54.4
acidity aq. pH
D (O/A)
vol (ml)
WO 3 (g/l)
org.
aq.
org.
aq.
10.15 10.10 11.45
14.9 10.8 50.7
37.6 42.2 4.99
0,57 9.90 0.62 9.55 0.60 N 9.60
TABLE 9 Mutual solubilities of diluents and water (25°C) Diluent
Aqueous phase wt. % diluent
Xylene Bu20 MiBK BuOH EtOH
1.7 7.31 ~
Organic phase w t . % H 2O
Source
1.9 20.4
Ref. [ 1 2 ] Ref. [ 1 2 ]
0.40 0.26 10.2
150
equilibrated with hydr ochl or i c acid of m o d e r a t e c o n c e n t r a t i o n so that a controlled equilibrium acidity can be maintained w i t h o u t the precipitation o f isopolymetallic acid prior to the extraction. The mutual solubilities of diluent--water systems are shown in Table 9. It indicates that there exists a certain degree of parallellism between mutual solubility and extractability. Synergistic behavior When a mixture of extractants, such as PSO and D2EHPA, is used, the separation factor for one metal with respect to t he ot her is greatly enhanced. The e n h a n c e m e n t of ~ by using a mixed e xt ract ant is due to an increase of DMo on the one hand and a decrease of D w on the ot her hand under proper extraction conditions. An example is given in Table 10. TABLE 10 Separation of tungsten from m o l y b d e n u m Extractant
pH
PSO only D2EHPA only PSO--D2EHPA a
1.5 1.4 1.4
Aqueous feed (g/l)
WO~
MoO~
70 70 70
0.774 0.316 0.142
DMo
Dw
~(Mo/W)
0.655 2.25 3.15
0.0192 0.156 0.0117
34.1 14.4 269
al:l
Extraction mechanism In weak acid solutions the species of bot h tungsten and m o l y b d e n u m are very c o m p l e x and their ionic states have been studied by m any researchers. Based on the data presented by T y t k o [10] and under the present extracting conditions, the tungsten m a y be considered to exist mainly as paratungstate and meta-tungstate, and the m o l y b d e n u m as h e p t a m o l y b d a t e . The main ionic equilibrium and extraction equations m ay be as follows: n H ÷ + m MeO42- -~ M e m O yH q - + w H 2 0
(1)
(Mere Oy Hq - )
~n,m
(H+)n (MeO42_)m
M e m O y H q - + q H ÷ + e R2SO ~ MemOyHp+ q" (R2SO)e
(la)
(2)
Combining eqns. (1) and (2) gives h m H ÷ + m MeO42- + e (R2SO) ~- M e m O y H z ( R 2 S O ) e + w H 2 0 Kex, rn,e =
(MemOyHz(R2SO)e) (H+)hm (MeO42_)m (R 2 s o ) e
(3)
(3a)
151 where Me is tungsten or m o l y b d e n u m , and R2 SO is dialkyl sulfoxide. Based on the above equations the expression for the distribution coefficient for the metals is as follows:
(H+)hmKex,m,e(Me042-) m-'
(R2SO) e
D = m 1 + E(MeO42-) rn-' ~ n , m (H+) h m
(4)
n
According to the expression derived from the assumed mechanism, D depends on the pH, and the concentrations of R2SO and the metallic ions. The change of experimental values of D with these parameters agrees well with the prediction. In order to determine the value of e in eqn. (4), slope analysis was used, and the resulting data are plotted in Fig. 11. In the case of dilute aqueous solutions, when the acidity of the solution and the initial concentration are kept constant, D may be considered to be affected only by extractant concentration. At low extractant concentrations, the activity coefficients can be conveniently taken as near enough to unity to be ignored. From the experimental straight line fitted by least squares, the molecular ratios of the extracted complexes were estimated. The slope of the line for tungsten is 2.09 and that for m o l y b d e n u m is 1.96. The data of linear dependent analysis are shown in Table 11. 0
--0.5
SLOPE ° / / ° 2.09 -0.5
--I.0 £3
£3
© -I.0 0_J
W~o /
SLOPE
-I .5
-2.0
-os
o --1.5 0-J -2.0
b
5s
LOG [DBSO'I
i
1.0
-2.4
Fig. 11. Slope analysis; initial aqueous concentrations: ©, (WO3)A, 25.6 g/l; z~, ( M o O a ) A 8.51 g/l; solvent: DBSO in M in diluent.
The heat effect of extraction The extracting equilibrium process involves dissolution, the complexing reaction and phase equilibration, and is very complicated. From eqn. (4),
152 T A B L E 11 L i n e a r d e p e n d e n t analysis System
Numbers of d a t a
Degree of freedom
Incidence coefficients
D e g r e e of reliance
Critical correlation coefficient
Significance
Slope I n t e r c e p t C o r r e l a t i o n coefficient Mo--DBSO
6 6
4 4
1.91 1.91
-1.92 -1.92
0.995 0.995
0.05 0.01
0.811 0.917
*** ***
W--DBSO
9 9
7 7
2.09 2.09
-0.895 -0.895
0.987 0.987
0.05 0.01
0.666 0.798
*** ***
it is clear that D is related to Kex,m,e , (MeO42-)A, pH and (R2SO). When the temperature changes, (MeO42-)A, pH and (R2SO) will change only very slightly. It m a y be considered that D is linear with respect to Kex, m,e. By making such assumptions and considering all above processes as being pseudomacroscopic, the relation between distribution coefficient and temperature may be written as follows: lnD = lnD °
AHp ( 1 ) R "T
(5)
where T is temperature, AHp the total heat of extraction, and R the gas constant. From a plot of ln D vs. 1 / T (Fig. 12), AHp values for extraction of tungsten and m o l y b d e n u m from dilute hydrochloric acid solution were estimated from experimental data to be AHp, w = - 1 9 kJ/mol (-4.5 kcal/ mol), and AHp,Mo = - 1 5 kJ/mol (-3.5 kcal/mol).
2.0
v
W03
SLOPE
2260
pH= 2.5
SLOPE
1770
pH=5.5
Z d
1.0
0 3.0xlO-3
3.1xlO-3 l / T , °K-1
3.2x10 -3
Fig. 12. Plot of In D vs. 1/T; initial aqueous concentrations: o, ( W O 3 ) A 20 g/l; A, ( M o O 3 ) A 8.5 g/l; initial phase ratio: O / A = 1 .
153 CONCLUSIONS 1. D i b u t y l s u l f o x i d e a n d p e t r o l e u m s u l f o x i d e have b e e n f o u n d to be useful in t h e e x t r a c t i o n a n d s e p a r a t i o n o f t u n g s t e n a n d m o l y b d e n u m . T h e loading c a p a c i t i e s o f b o t h e x t r a c t a n t s are s u f f i c i e n t l y high to be used in practice. 2. T h e d i s t r i b u t i o n c o e f f i c i e n t s o f b o t h m e t a l s d e p e n d s t r o n g l y on t h e a c i d i t y o f t h e a q u e o u s phase. By v a r y i n g t h e a c i d i t y (or p H ) it is possible t o e x t r a c t o n e m e t a l a n d leave t h e o t h e r in t h e r a f f i n a t e in c o u n t e r c u r r e n t o p e r a t i o n . D u e t o t h e s a m e d i s t r i b u t i o n b e h a v i o r , either m e t a l can be easily s t r i p p e d . 3. U n d e r t h e p r e s e n t e x p e r i m e n t a l c o n d i t i o n s , t h e e x t r a c t e d species m a y be H z M e m O y " (R2SO)e, w h e r e e = 2 f o r D B S O and Me = W or Mo. F r o m t h e p h e n o m e n a e x p e r i e n c e d in l a b o r a t o r y , it s e e m s t h a t t h e e x t r a c t i n g p o w e r o f t h e solvent c o n t a i n i n g d i a l k y l s u l f o x i d e as e x t r a c t a n t is s o m e w h a t r e l a t e d t o t h e m u t u a l s o l u b i l i t y o f t h e solvent a n d water. 4. With t h e a d d i t i o n o f D 2 E H P A to p e t r o l e u m s u l f o x i d e , a synergistic e f f e c t a p p e a r s t o exist. A t t h e p r o p e r p H c o n d i t i o n s t h e s e p a r a t i o n f a c t o r ~ ( m o / W ) m a y be increased m a n y f o l d .
REFERENCES 1 De, A.K., Khophar, S.M. and Chalmers, R.A., Solvent Extraction of Metals, VNR Company, London, 1970. 2 Zelikman, A.N., Vol'dman, G.M., Rakova, N.N. and Stenyushkina, T.P., Tsvet. Metal, 45 (1972) 38. 3 Yu Shuqiu and Chen Jiayong, Acta Metall. Sin., 18 (1982) 187. 4 Esnault, F., Robaylia, M., Latard, J.M. and Demarthe, J.M., in: Proceedings International Solvent Extraction Conference ISEC '74, Vol. 3, Soc. Chem. Ind., London, 1974, p. 2765. 5 Zelikman, A.N., Vol'dman, G.M., Rumyantsev, V.K., Ziberov, G.N. and Kagermanian, V.S., U.S. Patent 3,969,478, 1976. 6 Weast, R.C. (Ed.), Handbook of Chemistry and Physics, 59th edn., C.R.C. Press, Boca Raton, 1979. 7 Dhara, S.C. and Khopkar, S.M., Ind. J. Chem., 5 (1967) 12. 8 De, A.K. and Rahaman, M.S., Talanta, 11 (1964) 601. 9 White, J.C. and Ross, W.J., Atomic Energy Commission, Rept. NAS-NS 3102, 1961. 10 Tytko, K.H. and Glemser, O., in: Emeleus, H.J. and Sharpe, A.G. (Eds.), Advances in Inorganic Chemistry and Radiochemistry, Vol. 19, Academic Press, New York, 1976, pp. 271--285. 11 Sasaki, Y. and Sillen, L.G., Acta Chem. Scand., 18 (1964) 1014. 12 Marcus, Y. and Kertes, A.S., Ion Exchange and Solvent Extraction of Metal Complexes, Wiley-Interscience, London, 1969.
137
SEPARATION OF TUNGSTEN AND MOLYBDENUM IN DILUTE HYDROCHLORIC ACID SOLUTION BY EXTRACTION WITH SULFOXIDES
G.S. DAI, B.Y. XUAN and Y.F. SU East China Institute of Chemical Technology, Shanghai (China) (Accepted May 8, 1984)
ABSTRACT Dai, G.S., Xuan, B.Y. and Su, Y.F., 1984. Separation of tungsten and molybdenum in dilute hydrochloric acid solution by extraction with sulfoxides. Hydrometallurgy, 13: 137--153. By using di-n-butylsulfoxide and petroleum sulfoxide as extractants, the extraction behavior of tungsten and molybdenum in hydrochloric acid solutions was studied under different acidities. When acidity is low, molybdenum is preferentially extracted, the selectivity can be very high, and the separation of these two metals is considered favorable. The separation was confirmed by dissolving in dilute hydrochloric acid the combined oxides of both metals, which were obtained from the acid leaching liquor of scheelite. From the experimental curves the molecular ratios of the complexing reaction were analyzed and the neutral molecular complex mechanism was discussed.
INTRODUCTION T h e solvent e x t r a c t i o n t e c h n i q u e has b e c o m e increasingly i m p o r t a n t in t h e h y d r o m e t a l l u r g y o f t u n g s t e n and m o l y b d e n u m . M a n y r e s e a r c h w o r k e r s h a v e b e e n engaged in finding e f f e c t i v e e x t r a c t a n t s a n d d e v e l o p i n g n e w processes to p r o d u c e t u n g s t e n and m o l y b d e n u m c o m p o u n d s b y solvent e x t r a c t i o n . A m o n g various e x t r a c t a n t s such as A l a m i n e 336, M I B K , D 2 E H P A and TBP [1] b e i n g used in t h e r e c o v e r y and s e p a r a t i o n o f t u n g s t e n a n d m o l y b d e n u m , t h e t e r t i a r y a m i n e s have b e e n studied m o s t e x t e n s i v e l y . B e f o r e 1 9 6 6 t h e r e w e r e a l r e a d y several p l a n t s in t h e U.S.A. using a m i n e s f o r t u n g s t e n r e c o v e r y . In C h i n a a few p l a n t s have also c h a n g e d t h e i r f o r m e r p r o c e s s t o e x t r a c t i o n . H o w e v e r , t h e o n l y e x t r a c t a n t in use is a t e r t i a r y a m i n e , and t h e r e m o v a l o f d e t r i m e n t a l e l e m e n t s , such as Mo, P, As a n d Si, is b y c h e m i c a l m e a n s . In general, t h e p r e c i p i t a t i o n o f t h e s e e l e m e n t s m a y cause an a p p r e c i a b l e loss o f t u n g s t e n . T h e m e c h a n i s m o f e x t r a c t i n g t u n g s t e n b y a m i n e s f r o m sulfuric acid m e d i u m has b e e n p u t f o r w a r d b y Z e l i k m a n [2] a n d similar studies have b e e n m a d e b y Yu Shu-qiu a n d C h e n J i a y o n g [3]. H o w e v e r , t h e s e p a r a t i o n o f
0304-386X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
138 tungsten from m o l y b d e n u m by using amines as extractants is difficult because of the similar behavior of these two elements and their compounds. In the Proceedings of ISEC 74, Esnault [4] proposed a new process for separating tungsten from m o l y b d e n u m by using D2EHPA as extractant, by which the m o l y b d e n u m is first preferentially extracted; subsequently, tungsten is extracted by amines. Large amounts of tungsten {>40 g/1 WO3) present in solution will seriously affect the extraction of m o l y b d e n u m ; therefore the application of this process is limited. Another m e t h o d of processing revealed by a U.S. Patent [5] in 1976 is by using TBP as extractant for separating tungsten from molybdenum. In order to increase the selectivity of m o l y b d e n u m the solution has to be pretreated with hydrogen peroxide. Up to now, research directed towards finding a more efficient extractant and developing an alternative process for the separation of these two metals seems to be worthwhile. The present authors have suggested the use of sulfoxides as extractant to separate tungsten from m o l y b d e n u m in dilute hydrochloric acid. The experimental work was done in a laboratory and the best separating conditions were found. EXPERIMENTAL
Apparatus As shown in Fig. 1, an all-glass apparatus has been designed and made for the equilibration of two liquids of partial miscibility. The main part of the apparatus, 1, consists of a mixing chamber and a calibrated burette of nearly equal volumes. When it is placed in an upright position, all liquids remain in the mixing chamber and can be mixed by the magnetic stirrer, 2, at the
1
4>-Z~
I-0
0
0
o
o]
Fig. 1. Apparatus: 1, equilibrator; 2, magnetic stirrer; 3, graduated feeder; 4, infrared lamp.
139 b o t t o m as shown. After the magnetic stirrer is stopped, the apparatus is turned upside down until phase disengagement occurs. The volume of both phases can be measured accurately. A drying tube in connection with the graduated feeder, 3, and an infrared lamp, 4~ are used if the extractant is hygroscopic and/or m a y solidify at ambient temperature such as is the case with pure dibutyl or dioctyl sulfoxide.
Aqueous feed solutions When pure or synthetic solutions of tungsten and m o l y b d e n u m were used, the feeds were prepared by dissolving sodium tungstate or/and a m m o n i u m molybdate of A.R. grade in water, and adjusted to proper pH values by adding hydrochloric acid or sodium hydroxide. A solution of scheelite was prepared by decomposing a sample of scheelite concentrate using hydrochloric acid; the crude tungstic acid was then dissolved in sodium hydroxide and filtered. The resulting clear solution was adjusted to the proper pH value by adding acid or alkali.
Extractants and diluents Dibutyl sulfoxide (DBSO) was synthesized by a conventional method, and the crude product was purified by repeated crystallization in benzene. The physical properties of the dibutyl sulfoxide product were measured and are shown in Table 1. The infrared spectra were also examined to confirm its purity. Dioctyl sulfoxide (DOSO) was purchased from Tienjin Reagent Co., Branch III and used w i t h o u t further purification. All diluents were of technical grade. Other reagents were A.R. chemicals. Another extractant used was petroleum sulfoxide and its physical properties are shown in Table 2.
Temperature of extraction Preliminary tests indicated that with dibutyl sulfoxide the extraction of both metals is favoured by a low temperature. But as the melting points of pure DBSO and dioctyl sulfoxide are 32.6°C and 71°C, respectively, all experiments using these extractants were carried out at temperatures several degrees above their melting points in order to avoid the possibility of the separation of any solid.
Time of equilibration As the mixing was done by magnetic stirrer and hence the agitation was not vigorous, the time required to attain phase equilibrium was found to be less than a quarter of an hour. Therefore, 30 min of equilibration were used to obtain all the experimental data.
140 TABLE 1 Physical properties of DBSO Crystal form
m.p. (°c)
Density, d~ °
Refractive index, .~
Solubility in %, 40°c
Source
needle needle
32.5 32.6
0.9242 0.9216
1.4600 1.4606
7.42 slight
measured Ref. [6 ]
TABLE 2 RI"--~ Physical properties of petroleum sulfoxide, R 2 j b = O- , where R~ R 2 = alkyl group with 4 to 5 carbon atoms Average molecular weight Density, d~ ° 20 Refractive index, n D Viscosity at 10°C 20°C 30°C Solubility in H:O n-hexane CC14 alcohol benzene
176.2 1.0253 1.4853 22.1 15.2 7.59
mPa s (22.1 cP) mPa s (15.2 cP) mPa s (7.59 cP) slight 10 vol. % miscible miscible miscible
Method of analysis Both tungsten and molybdenum were analysed by the thiocyanate photom e t r i c m e t h o d . T h e p h o t o m e t r i c m e a s u r e m e n t f o r t u n g s t e n was m a d e at 400 n m a n d f o r m o l y b d e n u m at 460 n m . Within t h e e x p e r i m e n t a l c o n c e n t r a t i o n range it was f o u n d t h a t t h e t w o m e t a l s do n o t i n t e r f e r e w i t h e a c h o t h e r in analysis. H o w e v e r , in t h e p r e s e n c e o f s u l f o x i d e , t h e a q u e o u s m e t a l s o l u t i o n has to be e x t r a c t e d first b y c a r b o n t e t r a c h l o r i d e to free it f r o m t h e e x t r a c t a n t in o r d e r to a v o i d its i n t e r f e r e n c e . RESULTS AND DISCUSSION D i b u t y l s u l f o x i d e (DBSO) is similar in e x t r a c t i n g p r o p e r t y to T B P a n d c o n s i d e r e d a n e u t r a l e x t r a c t a n t . It is well k n o w n t h a t in s t r o n g h y d r o c h l o r i c acid, say 6 M or m o r e , t h e e x t r a c t i o n o f t u n g s t e n (or m o l y b d e n u m ) b y T B P is in t h e f o r m o f a c a t i o n i c c o o r d i n a t i o n c o m p o u n d , as f o l l o w s [7] : WO2C12 + n T B P ~ WO2C12" n T B P A c c o r d i n g to De a n d R a h a m a n [8] n was f o u n d t o be 2. A l t h o u g h the e x t r a c t i o n o f t h e c o m p l e x is a l m o s t q u a n t i t a t i v e , it c a n n o t be used in p r a c t i c e d u e to t h e limited solubility o f WO2 C12 (or MoO2 C12) in s t r o n g acid
141 solutions. It is also reported that 4--5 M HC1 is the best medium for complete extraction of Mo(VI) by TOPO and the extracted species is MoOzC12" 2TOPO [9]. On the other hand, in weak hydrochloric acid solutions, it is most probable that these two metals exist in the form of isopolymetallic acids which will precipitate when the pH value of the aqueous solution is about 2 or less; however, the metal, W or Mo, will be in the form of sodium or a m m o n i u m tungstate or molybdate when the aqueous solution approaches the neutral point, e.g. at pH > 6. It has been found that between these two pHs, DBSO can extract W or Mo from HC1 solutions. The present authors prepared the aqueous solutions containing W and/or Mo by adding hydrochloric acid to sodium tungstate and/or molybdate until a proper pH was attained. After equilibration with DBSO (or D B S O diluent), the organic phase was separated and tested for C1- and Na*. Experiments indicated t h a t the a m o u n t of C1- in the organic phase is < 1 0 -s mol. Similarly, Na + was not detected in the organic phase by the emission spectra, although there was a large a m o u n t of Na ÷ in the aqueous phase which was in equilibrium with the organic phase. Therefore, it seems apparent that the extractable species are complexes formed by isopolymetallic acid with DBSO instead of either MeO: C12 on one hand, or Na2 MeO4 on the other (where Me is W or Mo).
Effect o f acidity o f the aqueous solution In weak hydrochloric acid solutions the species of isopolymetallic acids of both tungsten and m o l y b d e n u m are very complex and change markedly with a slight variation of hydrogen ion and metal concentrations. It was reported [10] t h a t when the ratio of H ÷ to WO42- equals 2:1, 6:6, 7:6, 14:12 or 18:12, different species will form and the distribution of the different species changes accordingly. The general formula of the anion is HzWxOy q- [10]. In the case of isopolymolybdate ions, similar species were confirmed to exist [11]. When acid is added to a solution of sodium tungstate or molybdate, the following reaction will occur: p H ÷ + q MeO42- ~ (Hp_2rMeqO4q_r) (2q-p)- + r H 2 0 where Me denotes tungsten or m o l y b d e n u m . In the presence of an extractant, the anions will combine with protons to form isopolymetallic acid--extractant complexes and pass to the organic phase. In consequence, H ÷ is consumed so that the pH of the aqueous solution changes as the reaction proceeds and the equilibrium pH is always higher than the initial value. A part of the experimental data for the extraction of W and Mo by DBSO at 40°C is shown in Tables 3 and 4. In Table 3, the pH of the feeds was adjusted to 2.0 and changes to 2.5 at equilibrium, at the same time, the
142 TABLE 3 Equilibrium data for extraction of W by DBSO (40°C) Run no.
301 302 304 305 307 308
Before equilibration pH = 2.0
At equilibrium pH = 2.5
D(O/A) (WO3)o
vol. (ml)
cone. (g/l)
vol. (ml)
cone. (g/l)
org.
aq.
(WO~) O
(WO3) A
org.
aq.
(WO3)o
(WO3)A
7.95 8.20 3.90 4.10 4.10 3.60
10.0 15.0 15.0 15.0 10.0 10.0
0 0 0 0 0 0
1.07 2.72 10.4 19.7 49.8 58.7
9.10 8.93 3.80 4.15 4.45 3.85
8.55 13.9 14.8 14.6 9.35 9.55
1.15 4.14 29.6 37.3 44.1 47.3
0.025 0.280 2.84 9.68 32.3 42.4
(WO3)A
46.0 14.8 10.4 3.85 1.37 1.12
TABLE 4 Equilibrium data for extraction of Mo by DBSO (40°C) Run Before equilibration no. pH = 5.0
401 402 403 404 405
vol. (ml)
cone. (g/t)
org.
aq.
(MoO~)o
5.0 5.0 5.0 5.0 5.0
10.0 10.0 10.0 10.0 10.0
0 0 0 0 0
At equilibrium pH = 5.5
D(O/A) (MoO~) °
vol. (ml)
cone. (g/l)
(MoO3)A
(MoO 3)A
org.
aq.
(MoO~)o
(MoO3)A
2.40 4.83 9.35 19.7 39.0
5.45 5.50 5.45 5.60 5.45
9.40 9.44 9.40 9.39 9.39
2.16 4.23 8.35 13.4 13.8
1.31 2.65 5.09 13.0 33.6
1.65 1.60 1.64 1.03 0.411
volumes of the aqueous phase always decrease and those of the organic phase i n c r e a s e d u r i n g e q u i l i b r a t i o n . S i m i l a r c h a n g e s in p H a n d v o l u m e a r e a l s o o b s e r v e d in t h e c a s e o f M o { T a b l e 4). The changes of distribution coefficient of individual metals with pH v a l u e o f t h e a q u e o u s s o l u t i o n a t e q u i l i b r i u m a r e s h o w n in F i g . 2. F r o m t h e f i g u r e it is a p p a r e n t t h a t m o l y b d e n u m may be easily extracted at pH v a l u e s b e t w e e n 4 . 6 a n d 6, w h e r e a s t h e e x t r a c t i o n o f t u n g s t e n is i n s i g n i f i c a n t . T h e r e f o r e , it i n d i c a t e s a p o s s i b i l i t y t o d e s i g n a p r o c e s s t o s e p a r a t e t h e s e t w o m e t a l s . T h e p H o f t h e a q u e o u s s o l u t i o n is f i r s t a d j u s t e d t o a h i g h e r p H v a l u e t o r e m o v e t h e M o f i r s t , a n d t h e n m o r e a c i d is a d d e d t o l o w e r t h e p H , t o s a y a b o u t 2 . 5 , in o r d e r t o e x t r a c t a l l t h e t u n g s t e n f r o m t h e f e e d t o t h e o r g a n i c solvent. A s s h o w n in T a b l e 2, p e t r o l e u m s u l f o x i d e h a s a d e n s i t y v e r y n e a r t o t h a t of water and possesses a high viscosity. Therefore, a diluent has to be added
143
t o i m p r o v e its p h y s i c a l p r o p e r t i e s . In t h e case o f using P S O - - B u O H as a n e x t r a c t a n t - - d i l u e n t s y s t e m in t h e e x t r a c t i o n o f t u n g s t e n , it has b e e n f o u n d t h a t , c o n t r a r y t o t h e case o f using u n d i l u t e d D B S O , t h e o p t i m a l c o n d i t i o n f o r e f f i c i e n t e x t r a c t i o n shifts t o t h e high a c i d i t y range, as s h o w n in Fig. 3.
7.5" ..5< O n 5.0-
WO3:20 G/L 40 °
MOO3:20 G/L
2.5
--'1i ~~ 2.0
4.0
6.0
pN
Fig. 2. Effect of equilibrium pH on distribution coefficients of W and Mo using undiluted DBSO and initial aqueous concentrations as indicated; O/A = 1.
500 40 °
t O c5
INITIAL AQ. CONCN.: / (wO3)A 26.7 G / L ~ ' / 250/_/ 41.0
]
o
0. 0.3 0.5 0.7 0.9 I.I ACIDITY AT EQUILIBRIUM, (H+)A, N Fig. 3. Effect of (H~)~ on D(O/A) at different initial concentrations of (WO3)A; solvent: PSO :BuOH = 1:1; O/A = 1.
Extraction isotherms T h e e q u i l i b r i u m d i s t r i b u t i o n s o f t h e t w o m e t a l s b e t w e e n s u l f o x i d e and a q u e o u s p h a s e s w e r e d e t e r m i n e d . T h e results o b t a i n e d f o r t u n g s t e n a n d m o l y b d e n u m , w h e n p r e s e n t alone, are s h o w n in Figs. 4 a n d 5. T h e s e results w e r e o b t a i n e d u n d e r t h e e x p e r i m e n t a l c o n d i t i o n s s h o w n in Tables 3 a n d 4.
144 T h e curves in Figs. 4 a n d 5 (also t h e n u m e r i c a l values in Tables 3 a n d 4) s h o w t h a t at a p p r o p r i a t e p H c o n d i t i o n s p u r e d i b u t y l s u l f o x i d e c a n efficiently e x t r a c t t u n g s t e n a n d m o l y b d e n u m f r o m dilute h y d r o c h l o r i c acid s o l u t i o n ; t h e d i s t r i b u t i o n c o e f f i c i e n t s h a v e b e e n f o u n d to increase w i t h decreasing c o n c e n t r a t i o n s o f b o t h metals. F r o m this f a c t o n e m a y a n t i c i p a t e t h e p o s s i b i l i t y o f a high r e c o v e r y o f m e t a l values even at low c o n c e n t r a t i o n s . T h e d a t a also reveal t h a t t h e loading c a p a c i t y o f t h e organic p h a s e is high, a n d c o n s e q u e n t l y t h e c o n c e n t r a t i o n o f t u n g s t e n or m o l y b d e n u m in t h e
40.
X -
Y
- 40
\
d
[
2
pH =2"5
,:,
>..: 20 [o
20
o
2b 4'0 X, (WO a)A, G/L
Fig. 4. Distribution coefficient of tungsten, D(O/A), and equilibrium curve at 40°C (experimental conditions and data are shown in Table 3).
1541~ N~
40 °
pH=S.S
-I.5
8 O -I.0 v C5
2 5
0
.0.5
-Y
0
2'0
40
X, ( MoO3)A, G/L Fig. 5. Distribution coefficient of molybdenum, D(OZA), and equilibrium curve at 40°C (experimental conditions and data are shown in Table 4).
145
s t r i p p i n g l i q u o r c a n a l s o b e h i g h . T h i s is d e s i r a b l e in v i e w o f e n e r g y c o n s u m p t i o n if it is n e c e s s a r y t o e v a p o r a t e t h e s o l u t i o n f o r t h e r e c o v e r y o f t h e m e t a l v a l u e in t h e f o r m o f s a l t s . F r o m t h e f o r m o f t h e e x t r a c t i o n i s o t h e r m s it can be seen that only a few theoretical stages are required to attain a high percentage of recovery. Figure 6 indicates that with petroleum sulfoxide and butanol as extracting
50-
X
d
X" +
\ t_9
6
>: 25
:¢ /
40 °
I
f o
Xo (W03) A, G/L Fig. 6. Equilibrium curves for the extraction of tungsten b y PSO--BuOH at different acidities; equilibrium HC1 concentrations as indicated; solvent: PSO :BuOH = 1:1. 1.0
<0 . 8 S ~ 40 °
pH=3.2
0
0.6-
×
~ 0.4-
o.I
S'" 0
, 1.0 X, ( MoO3)A, G/L
2.0
Fig. 7. Equilibrium curves for the extraction of molybdenum by petroleum sulfoxide with different diluents; o, PSO :BuOH = 1:1; x, PSO :CsH17OH = 1:1.
146 s o l v e n t f o r t u n g s t e n , t h e p o w e r o f e x t r a c t i o n increases w i t h increasing acidity. In c o m p a r i s o n w i t h Fig. 4, t h e e x t r a c t i o n o f t u n g s t e n is f a v o u r e d b y a s t r o n g e r acid w i t h P S O - - b u t a n o l r a t h e r t h a n w i t h DBSO. Figure 7 s h o w s t h e e q u i l i b r i u m curves o f m o l y b d e n u m o b t a i n e d w h e n using PSO as e x t r a c t a n t a n d a l c o h o l as d i l u e n t . It s e e m s t h a t m o l y b d e n u m can still be e x t r a c t e d at m u c h l o w e r a c i d i t y t h a n t u n g s t e n ; t h e s e p a r a t i o n o f t h e t w o m e t a l s is also possible. Figure 7 f u r t h e r indicates t h a t b o t h b u t a n o l a n d o c t a n o l m a y be used as diluent; h o w e v e r , t h e f o r m e r s h o w s higher e x t r a c t i n g p o w e r t h a n t h e latter.
Separation of tungsten from molybdenum Synthetic solution When t h e feed s o l u t i o n was m a d e o f m i x e d p u r e t u n g s t a t e and m o l y b d a t e o n l y , t h e p l o t o f s e p a r a t i o n f a c t o r vs. e q u i l i b r i u m a q u e o u s p H value is s h o w n in Fig. 8 ( u p p e r curve). B e t w e e n p H values 3 a n d 5, t h e s e p a r a t i o n f a c t o r s are fairly high and t h e curve s h o w s a m a x i m u m at p H 4. In c o n t r a s t to t h e result o b t a i n e d b y E s n a u l t [4] e m p l o y i n g D 2 E H P A as e x t r a c t a n t , t h e use o f D B S O enables t h e s e p a r a t i o n o f t h e t w o metals even if t h e feed s o l u t i o n bears a high WO3/MOO3 ratio. T h e e f f e c t o f an even larger ratio (WO3/MOO3) in the feed s o l u t i o n o n t h e d i s t r i b u t i o n c o e f f i c i e n t s a n d h e n c e t h e s e p a r a t i o n f a c t o r s was also e x a m i n e d . T h e e x p e r i m e n t a l results are illustrated b y t h e e x a m p l e s in T a b l e 5. E x p e r i m e n t a l d a t a w e r e also o b t a i n e d w i t h p e t r o l e u m s u l f o x i d e - - b u t a n o l as e x t r a c t a n t (see T a b l e 6). TABLE 5 Separation factors when DBSO is used as extractant Aqueous feed (g/l)
pH
WO 3
MoO 3
WOflMoO~
61.6 93.2
2.42 0.48
25.5 194
4.8 4.4
D(O/A)
~(Mo/W)
WO3
MoO 3
0.537 0.178
5.63 6.35
10 36
TABLE 6 Separation factors when petroleum sulfoxide (PSO) is used as extractant; solvent used: PSO :BuOH = 1:1 Aqueous feed (g/l)
pH
WO3
MoO 3
WOflMoO3
7.74 2.26
2.16 2.54
3.!58 0.890
2.2 2.2
D(O/A)
2(Mo/W)
WO3
MoO~
0.0581 0.126
4.20 3.47
72.3 27.5
147 By close examination of the data in Tables 5 and 6, and the trend of the distribution of W and Mo in Figs. 4 and 5 (irrespective of different pHs) the following generalizations can be made: (1) At a proper pH value, the distribution of either metal in the organic phase will be favoured by a low concentration, whether the aqueous solution contains a single metal or both. (2) With undiluted DBSO as extractant, the optimal pH for the separation of the two metals is around 4, whereas this optimal point will shift to higher acidity, if PSO--BuOH is used as solvent. (3) A comparison of DBSO-- and PSO--diluent indicates that PSO is not only a cheaper, but may also be a better extractant than the former. Solution from scheelite
The extraction behavior of a solution obtained by decomposition of scheelite by hydrochloric acid is also studied. The solution usually contains 50--100 g/1 WO3 and 1--5 g/1 MOO3. The distribution coefficients of both metals between DBSO and aqueous phases at different pH values were determined, and the resulting plot of separation factor vs. equilibrium aqueous pH is also shown (lower curve of Fig. 8). Although the curve is slightly lower than that of the synthetic solution, the maximum separation factor is still about 12, which is found at the same pH as the latter. The slight lowering of separation factor curve may be explained by the influence of other impurities originally present in scheelite.
40 °
~15C~
SYNTH.
~lOv
c~ 5
SOLN. F R O M SCHEELI TE
3:o
4'.o
s.o
6:o
pH
Fig. 8. Effect of equilibrium pH on ~ = D(Mo)/D(W) during the extraction of tungsten and molybdenum by undiluted DBSO; initial aqueous concentration: o, synthetic solution: (WO3)A, 20 g/l, (MoO~)A, 2.5 g/l; A, solution from scheelite: (WO3)A, 53.3 g/l, (MoO3)A, 2.3 g/1.
148
Effect of diluen ts The addition of any small amount of water-immiscible organic diluent w i l l s e r i o u s l y i m p a i r t h e e x t r a c t i o n o f b o t h m e t a l s b y D B S O , as s h o w n in F i g . 9 a n d T a b l e 7.
40 °
40 °
~4 7 c~
v
C5 3
I.O2
0
I00
5'0
I O0 5'O DBSO °,o (V)
0
Fig. 9. Effect o f addition of diluent to DBSO on the distribution of W and Mo: for W, the diluent is dibutyl ether; for Mo, the diluent is toluene; initial aqueous concentration: (WO3) A 25.7 g/l, (MoO~) A 8.51 g/l; O/A = 0.5. TABLE 7 Effect of diluents on the extraction of W and Mo by DBSO and DOSO Extractant
DBSO 1 M DBSO 1 M DBSO DOSO 1 M DOSO 1 M DOSO 1MDOSO 1 M DOSO
Diluent
None BuOH Bu~O None Xylene Bu:O BuOH BuOH+EtOH
Temp. (°C)
40 13 25 80 25 25 25 25
Miscibility (%) Solvent in water
Water in solvent
6.46 8 0.5 1 0.013 insol. 0.06 1
29.5 16 3 1 0.04 0.06 10 20
Extractability
strong medium weak medium nil nil nil weak
H o w e v e r , in c a s e o f t h e a d d e d d i l u e n t , s u c h as B u O H w h i c h is p a r t i a l miscible with water, the reduction of the percentage extraction of the metals w i l l b e less. T h e u s e o f u n d i l u t e d d i o c t y l s u l f o x i d e , w h i c h h a s a h i g h e r melting point and much smaller mutual solubilities with water than DBSO ( s e e T a b l e 7), g r e a t l y r e d u c e d t h e p e r c e n t a g e e x t r a c t i o n o f t h e m e t a l s . H o w -
149
ever, the addition of BuOH and EtOH to increase its miscibility with water did improve its extracting power somewhat. This fact leads us to suspect that the species extracted into the solvent may carry water molecules in solvated form. A preliminary test was done and the results obtained are illustrated in Table 7 also. This may give a clue to induce further study in order to gain a clear insight into the mechanism of the extraction of both metals. The effect of diluent on petroleum sulfoxide is quite similar to that on DBSO, see Fig. 10 and Table 8. In this table, the petroleum sulfoxide is pre150 / X
~100-
70
o cl
/0
503 0 O/o P S O
0
0.2
0.4.
0.6
0.8
(H+)A '
1.0 N
Fig. 10. E f f e c t s o f p e r c e n t P S O in s o l v e n t a n d e q u i l i b r i u m a c i d i t y , (H+),~, o n D ( O / A ) ; initial a q u e o u s c o n c e n t r a t i o n : (WO3) A 26.7 g/l; d i l u e n t is b u t a n o l ; O / A = 1. TABLE 8 Effect of diluents on the extraction of tungsten; solvent: PSO :diluent = 1:1 (by vol.), shaken with 3.5 N HC1 before use Diluent
Before equilibration acidity [ H+] 0
MiBK Bu20 BuOH
At equilibrium
vol (ml) aq.pH
0.01 M 1.4 0 . 0 3 M 1.4 0 . 7 1 M 2.0
WO 3 (g/l)
org.
aq.
org.
aq.
9.90 9.55 10.85
10.0 10.0 10.0
0 0 0
52.9 52.9 54.4
acidity aq. pH
D (O/A)
vol (ml)
WO 3 (g/l)
org.
aq.
org.
aq.
10.15 10.10 11.45
14.9 10.8 50.7
37.6 42.2 4.99
0,57 9.90 0.62 9.55 0.60 N 9.60
TABLE 9 Mutual solubilities of diluents and water (25°C) Diluent
Aqueous phase wt. % diluent
Xylene Bu20 MiBK BuOH EtOH
1.7 7.31 ~
Organic phase w t . % H 2O
Source
1.9 20.4
Ref. [ 1 2 ] Ref. [ 1 2 ]
0.40 0.26 10.2
150
equilibrated with hydr ochl or i c acid of m o d e r a t e c o n c e n t r a t i o n so that a controlled equilibrium acidity can be maintained w i t h o u t the precipitation o f isopolymetallic acid prior to the extraction. The mutual solubilities of diluent--water systems are shown in Table 9. It indicates that there exists a certain degree of parallellism between mutual solubility and extractability. Synergistic behavior When a mixture of extractants, such as PSO and D2EHPA, is used, the separation factor for one metal with respect to t he ot her is greatly enhanced. The e n h a n c e m e n t of ~ by using a mixed e xt ract ant is due to an increase of DMo on the one hand and a decrease of D w on the ot her hand under proper extraction conditions. An example is given in Table 10. TABLE 10 Separation of tungsten from m o l y b d e n u m Extractant
pH
PSO only D2EHPA only PSO--D2EHPA a
1.5 1.4 1.4
Aqueous feed (g/l)
WO~
MoO~
70 70 70
0.774 0.316 0.142
DMo
Dw
~(Mo/W)
0.655 2.25 3.15
0.0192 0.156 0.0117
34.1 14.4 269
al:l
Extraction mechanism In weak acid solutions the species of bot h tungsten and m o l y b d e n u m are very c o m p l e x and their ionic states have been studied by m any researchers. Based on the data presented by T y t k o [10] and under the present extracting conditions, the tungsten m a y be considered to exist mainly as paratungstate and meta-tungstate, and the m o l y b d e n u m as h e p t a m o l y b d a t e . The main ionic equilibrium and extraction equations m ay be as follows: n H ÷ + m MeO42- -~ M e m O yH q - + w H 2 0
(1)
(Mere Oy Hq - )
~n,m
(H+)n (MeO42_)m
M e m O y H q - + q H ÷ + e R2SO ~ MemOyHp+ q" (R2SO)e
(la)
(2)
Combining eqns. (1) and (2) gives h m H ÷ + m MeO42- + e (R2SO) ~- M e m O y H z ( R 2 S O ) e + w H 2 0 Kex, rn,e =
(MemOyHz(R2SO)e) (H+)hm (MeO42_)m (R 2 s o ) e
(3)
(3a)
151 where Me is tungsten or m o l y b d e n u m , and R2 SO is dialkyl sulfoxide. Based on the above equations the expression for the distribution coefficient for the metals is as follows:
(H+)hmKex,m,e(Me042-) m-'
(R2SO) e
D = m 1 + E(MeO42-) rn-' ~ n , m (H+) h m
(4)
n
According to the expression derived from the assumed mechanism, D depends on the pH, and the concentrations of R2SO and the metallic ions. The change of experimental values of D with these parameters agrees well with the prediction. In order to determine the value of e in eqn. (4), slope analysis was used, and the resulting data are plotted in Fig. 11. In the case of dilute aqueous solutions, when the acidity of the solution and the initial concentration are kept constant, D may be considered to be affected only by extractant concentration. At low extractant concentrations, the activity coefficients can be conveniently taken as near enough to unity to be ignored. From the experimental straight line fitted by least squares, the molecular ratios of the extracted complexes were estimated. The slope of the line for tungsten is 2.09 and that for m o l y b d e n u m is 1.96. The data of linear dependent analysis are shown in Table 11. 0
--0.5
SLOPE ° / / ° 2.09 -0.5
--I.0 £3
£3
© -I.0 0_J
W~o /
SLOPE
-I .5
-2.0
-os
o --1.5 0-J -2.0
b
5s
LOG [DBSO'I
i
1.0
-2.4
Fig. 11. Slope analysis; initial aqueous concentrations: ©, (WO3)A, 25.6 g/l; z~, ( M o O a ) A 8.51 g/l; solvent: DBSO in M in diluent.
The heat effect of extraction The extracting equilibrium process involves dissolution, the complexing reaction and phase equilibration, and is very complicated. From eqn. (4),
152 T A B L E 11 L i n e a r d e p e n d e n t analysis System
Numbers of d a t a
Degree of freedom
Incidence coefficients
D e g r e e of reliance
Critical correlation coefficient
Significance
Slope I n t e r c e p t C o r r e l a t i o n coefficient Mo--DBSO
6 6
4 4
1.91 1.91
-1.92 -1.92
0.995 0.995
0.05 0.01
0.811 0.917
*** ***
W--DBSO
9 9
7 7
2.09 2.09
-0.895 -0.895
0.987 0.987
0.05 0.01
0.666 0.798
*** ***
it is clear that D is related to Kex,m,e , (MeO42-)A, pH and (R2SO). When the temperature changes, (MeO42-)A, pH and (R2SO) will change only very slightly. It m a y be considered that D is linear with respect to Kex, m,e. By making such assumptions and considering all above processes as being pseudomacroscopic, the relation between distribution coefficient and temperature may be written as follows: lnD = lnD °
AHp ( 1 ) R "T
(5)
where T is temperature, AHp the total heat of extraction, and R the gas constant. From a plot of ln D vs. 1 / T (Fig. 12), AHp values for extraction of tungsten and m o l y b d e n u m from dilute hydrochloric acid solution were estimated from experimental data to be AHp, w = - 1 9 kJ/mol (-4.5 kcal/ mol), and AHp,Mo = - 1 5 kJ/mol (-3.5 kcal/mol).
2.0
v
W03
SLOPE
2260
pH= 2.5
SLOPE
1770
pH=5.5
Z d
1.0
0 3.0xlO-3
3.1xlO-3 l / T , °K-1
3.2x10 -3
Fig. 12. Plot of In D vs. 1/T; initial aqueous concentrations: o, ( W O 3 ) A 20 g/l; A, ( M o O 3 ) A 8.5 g/l; initial phase ratio: O / A = 1 .
153 CONCLUSIONS 1. D i b u t y l s u l f o x i d e a n d p e t r o l e u m s u l f o x i d e have b e e n f o u n d to be useful in t h e e x t r a c t i o n a n d s e p a r a t i o n o f t u n g s t e n a n d m o l y b d e n u m . T h e loading c a p a c i t i e s o f b o t h e x t r a c t a n t s are s u f f i c i e n t l y high to be used in practice. 2. T h e d i s t r i b u t i o n c o e f f i c i e n t s o f b o t h m e t a l s d e p e n d s t r o n g l y on t h e a c i d i t y o f t h e a q u e o u s phase. By v a r y i n g t h e a c i d i t y (or p H ) it is possible t o e x t r a c t o n e m e t a l a n d leave t h e o t h e r in t h e r a f f i n a t e in c o u n t e r c u r r e n t o p e r a t i o n . D u e t o t h e s a m e d i s t r i b u t i o n b e h a v i o r , either m e t a l can be easily s t r i p p e d . 3. U n d e r t h e p r e s e n t e x p e r i m e n t a l c o n d i t i o n s , t h e e x t r a c t e d species m a y be H z M e m O y " (R2SO)e, w h e r e e = 2 f o r D B S O and Me = W or Mo. F r o m t h e p h e n o m e n a e x p e r i e n c e d in l a b o r a t o r y , it s e e m s t h a t t h e e x t r a c t i n g p o w e r o f t h e solvent c o n t a i n i n g d i a l k y l s u l f o x i d e as e x t r a c t a n t is s o m e w h a t r e l a t e d t o t h e m u t u a l s o l u b i l i t y o f t h e solvent a n d water. 4. With t h e a d d i t i o n o f D 2 E H P A to p e t r o l e u m s u l f o x i d e , a synergistic e f f e c t a p p e a r s t o exist. A t t h e p r o p e r p H c o n d i t i o n s t h e s e p a r a t i o n f a c t o r ~ ( m o / W ) m a y be increased m a n y f o l d .
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