Extraction of molybdenum from weak acid rhenium-containing solutions

Hydrometallurgy, 12 (1984) 111--116

111

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

E X T R A C T I O N O F MOLYBDENUM FROM WEAK ACID RHENIUMCONTAINING SOLUTIONS

L. KARAGIOZOV*

Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia 1184 (Bulgaria) and Ch. VASILEV

Higher Institute of Chemical Technology, Sofia 1156 (Bulgaria) (Received September 2, 1982; accepted in revised form October 25, 1983)

ABSTRACT

Karagiozov, L. and Vasilev, Ch., 1984. Extraction of m o l y b d e n u m from weak acid rheniumcontaining solutions. HydrometaUurgy, 12: 111--116. The possibility of extracting m o l y b d e n u m from weak acid (pH 2 to 3) rheniumcontaining solutions with the non-specific extractant Aliquat 336 is studied. A mechanism for the temperature influence on extraction and in particular for the extraction of m o l y b d e n u m and rhenium is proposed. It is shown that the selectivitycoefficient (t~= D M o / S R e ) depends on the concentrations of both elements and also on the NO~- concentration in solution. W h e n the concentration of m o l y b d e n u m and nitrate ions increases,/~ increases. Increasing temperature has the same effect. A modifier is essential. At low rhenium concentrations it is better to use high molecular weight alcohols (as modifier), and at high concentrations it is better to use acetophenone. The authors believe that the investigations can be generalized and that analogous results can be expected if trialkylamine (for example, trioctylamine) is used.

INTRODUCTION

Several methods of separating m o l y b d e n u m and rhenium by solvent extraction have been described in the literature. The use of di(2-ethylhexyl)phosphor ic acid as selective extractant for m o l y b d e n u m [1] and of quaternary ammonium salts (QAS) for rhenium at pH ~ 1 2 are known [2]. The possibility of selective stripping after extraction of both elements has been studied with Aliquat 336 [3] and with mixtures of trioctylamine (TOA) and Aliquat 336 [4]. This m e t h o d is suitable for concentrating weak feed solutions. In the work described in Ref. [ 5], the separation of m o l y b d e n u m and rhenium after extraction of b o t h elements with T O A was carried out. In the present paper we report the possibility of extraction of m o l y b d e n u m from weak acid (pH 2 to 3 -- HC1, HNO3 or H2SO4) rhenium-containing solu*To w h o m

correspondence should be addressed.

0304-386X/84/$03.00

© 1984 Elsevier Science Publishers B.V.

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tions with the non-specific extractant Aliquat 336. The object was to accomplish the separation on the basis of the difference between m o l y b d e n u m and rhenium anion complex extraction. EXPERIMENTAL Aqueous solutions of m o l y b d e n u m and rhenium were prepared from analytical grade Na2MoO4 and doubly recrystallised NH4ReO4, respectively. The concentrations of the elements were determined colorimetrically. The extraction of both elements was carried out with Aliquat 336, aviation kerosene being used as diluent. The pH of the aqueous phase ranged from 2 to 2.3, the phase ratio was 1 : 1 and the time for extraction was 10 min. The extraction was carried out in a dropping funnel with rotational motion. RESULTS AND DISCUSSION

Dependence o f molybdenum extraction on the ionic form o f the extractant The dependence of the distribution coefficient of m o l y b d e n u m , DMo, upon m o l y b d e n u m c o n t e n t of the aqueous solution was investigated. It was studied as a function of the ionic form of the extractant. Since the conditions of extraction for all experiments were the same, the experimental data for DMo are determined by the affinity of the opposite ion to the extractant. Aliquat 336 is a quaternary a m m o n i u m salt, R3NCH3X, where R = C7 to C10, and X - = OH-, CI-, NO~, ... (see below); i.e., X - is the ionic form of the extractant ("opposite ion"). We compared the experimental data for DMo for the condition (molar ratio) [Mo (aq)] : [Aliquat 336 (org)]: [X-] = 3 : 1 : 1. The molar ratio of [Mo (org)] : [Aliquat 336 (org)] = 3 : 1, as the anion of m o l y b d e n u m is Mo60,2~[6]. When we put the ions in order of affinity for the extractant (i.e., decreasing DMo) we find the following successions: for univalent ions: O H - < C1- < NO~ < SCN- < CIO~ < ReO~ for divalent ions: CO~- < C20~- < SO~- < Mo60,:~and: ReO~ < Mo60,:~ These orders are the same as those for the extraction of rhenium where DRe was considered [ 7 ]. The orders are also identical to those published for some of the anions [8,9], which were found using ion-exchange resins. The place of an ion in the order is determined by its hydration energy (the larger the energy, the smaller the affinity for the extractant).

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Influence o f concentrations o f molybdenum and rhenium on their extraction from c o m m o n solution A number of solutions which contained different concentrations of both elements were prepared. The solutions contained 0.5, 1.0, 1.5, 2.0 and 3.0 g/1 Mo, and 0.5, 1.0 and 1.5 g/1 Re, together with 10 g/1 C1- (NH4C1). Figure 1 shows the dependence of the extraction of b o t h elements u p o n the concentration of m o l y b d e n u m and rhenium in the solution. We can see that the extraction is different for each element. The rhenium concentration depends strongly on the concentration of m o l y b d e n u m in the initial aqueous solution. When the concentrations of the elements were low they were extracted fully b y the extractant, b u t when their concentration was high then the deficiency of the extractant resulted in a competitive extraction, and Mo60~9 t o o k the place of ReO~.

I

~is0 60

o

~o

~0 20 ,

I

~0

l

I

,

2,0 N~g,,,, I

f

30

Fig. 1. Dependenceof molybdenumand rhenium extraction, E (%), upon their concentration. Organicphase -- 0.5% (by volume) Aliquat 336 + 10% (by volume) acetophenone (ACPh). If we consider the selectivity coefficient ({~ = DMo/DRe ) as a function of concentrations we can see that ~ >> 2 (Fig. 2, full line). When the concentration of m o l y b d e n u m increases,/~ increases also, as the extractant is loaded more fully with m o l y b d e n u m , which takes the place of rhenium. When the extractant is saturated the distribution coefficient and ~ decrease. It is known that high molecular weight alcohols decrease the extraction of rhenium. Experiments c o n d u c t e d with addition of iso-CsH~7OH show a lower value of ~max (= 65, see Fig. 2, d o t t e d line), b u t ~ values are better at lower concentrations of the elements in solution. Here we have to note the fact that at a rhenium concentration ~ 0.1 g/1 the selectivity coefficient, ~, is less than 1. This may be a consequence of the possibility that the m o l y b d e n u m complex includes ReO~ as the ionic form of the extractant (cf. the influence of temperature).

114

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6C

5C

t,O

3(

2( lC

1,0

2,0

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Fig. 2. Dependence of selectivity coefficient, fl, u p o n the c o n c e n t r a t i o n of m o l y b d e n u m and r h e n i u m in solution. Organic phase -- 0.5% Aliquat 336 + 10% ACPh (full line) or 5% iso-CsH ~ O H (dotted line).

Dependence of extraction of the elements on temperature The dependence log DMe/T-1 is shown in Fig. 3 for m o l y b d e n u m and rhenium. When the temperature increases the extraction of m o l y b d e n u m increases also (lines 2 and 3); however, the dependence of rhenium extraction on temperature is the opposite (line 1). The value for AHMo was calculated and it depends on the molar ratio [Mo (aq)] : [Aliquat 336 (org)]. For line 2, AHMo = 42.3 + 1.6 kJ/mol and for line 3, AHMo = 6.7 -+ 0.8 kJ/mol. AHRe = - 2 0 . 9 -+ 1.2 kJ/mol. In general, extraction decreases when the temperature increases (as is the case with rhenium). This effect probably depends on the fact that at higher temperature the ion sheath of hydrated molecules is destroyed. This leads to a decrease in the difference between the competitive p o w e r of ions (using QAS), and the selectivity coefficient of each ion will tend towards 1, i.e., the extraction will tend towards 50%. The increase of extraction with increase of t e m p e r ature observed in the extraction of m o l y b d e n u m is a relatively rare dependence. The extracted m o l y b d e n u m polyanion is large and we might imagine that, when the temperature is increased, the dehydration of the m o l y b d e n u m ion is more pronounced than that of the smaller chloride ion. This can result in a weak effect, which leads to increasing extraction when the temperature is

115

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1,2

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~"------.,,.--~.._..~..-3

+

l

+.+

I

i

Fig. 3. D e p e n d e n c e o f distribution coefficient, DMo , upon the temperature. 1: 0.2% A l i q u a t 336, aqueous phase - - 0.1 g/l Re. 2: 0.4% A l i q u a t 3 3 6 + 5% iso-CsH1TOH, aqueous phase - - 2.0g/1 Mo. 3: 0.5% A l i q u a t 3 3 6 + 5% iso-CsHt~OH, aqueous phase - - 4.8 g/1 Mo.

increased; however, in this case AHMo will not depend on the molar ratio [Mo (aq)] : [Aliquat 336 (org)]. In some cases the increase in extraction when the temperature increases is a consequence of a change in the extracted ionic form. The observed dependence in the case of molybdenum probably is a consequence of a change in the molybdenum complex in the organic phase. The mechanism of molybdenum extraction from acid aqueous solutions with Aliquat 336 dissolved in kerosene can be described by the following equations

[6]: 3 R4NC1 (org) + Mo60~- ~ (R4N)~Mo6019" R4NC1 (org) + 2 C1- (aq)

(1)

which are valid when the inequality [Mo (aq)] < 2 [R+NC1 (org)] is valid. When [Mo (aq)] > 2 [R+NC1 (org)] we have: 2 R4NCI (org) + Mo60~- (aq) -~ (R+N)2M06019 (org) + 2 C1- (aq)

(2)

When the temperature is increased, the extractant loaded with the molybdenum complex is set free. The molybdenum complex obtained at a molar ratio [Mo (aq)] :[Aliquat 336 (org)] = 5:1 (Fig. 3, line 3) must be described by eqn. (2), and the complex will be almost free from the extractant. When the proportion of extractant which is solvated is low, the temperature dependence must be very weak. This was indeed observed (Fig. 3), and the observed temperature dependence agrees with the proposed explanation [6].

Influence of concentration of attendant impurity In Fig. 4 the dependence of molybdenum and rhenium extraction on the concentration of CI-, NO~" and SO~-, respectively, is shown. When the solution contains NO~ ~ is increased, since NO~ has a different influence on the metals.

116

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Fig. 4. Influence of the concentration of CI-, NO~-, and SO4:- on extraction of molybdenum (a) and rhenium (b). Organic phase -- 0.3% Aliquat 336 + 10% ACPh; aqueous phase -- 2.0 g/1 H o (a), 0.1 g/1 Re (b). (Fig. 4b was previously published in Ref. [10])•

CONCLUSION

We can conclude that the extraction of m o l y b d e n u m from rheniumcontaining solutions is possible with an extractant which is non-specific (Aliquat 336). The selectivity coefficient,/3, will be increased if the solutions contain a high concentration of NO~ and also when the temperature of the solutions is higher than room temperature. At a low concentration of rhenium it is better to use high molecular weight alcohols as modifier, and at higher concentrations it is better to use acetophenone. The investigations with Aliquat 336 can be generalized and analogous results may be expected when trioctylamine is used. REFERENCES 1 2 3 4 5

6 7 8 9 10

Zelikman, A. and Nerezov, V., Zvetnie Metali, 1 (1968) 65. Churward, P. and Rosenbaum, J., J. Metals, 15 (1963) 648. Karagiozov, L., Vasilev, Ca., Chimbulev, M. and Kunev, D., Metalurgia, 8 (1977) 22. Karagiozov, L. and Vasilev, Ch., Hydrometallurgy, 4 (1979) 51. Karagiozov, L. and Vasilev, Ch., Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metall., (3) (1981) 44. Karagiozov, L. and Vasilev, Ch., J. Inorg. Nucl. Chem., 43 (1981) 199. Karagiozov, L., Ph.D. Dissertation, VKhTI, Sofia, 1977, p. 148. Shmidt, V.S., Extraczia Amminami, Moscow, Izd. Atomizdat, 1970, p. 312. Ivanov, I. and Gindin, L., Izv. Sib. Otd. Akad. Nauk SSSR, Set. Khim., 3 (7) (1967) 100. Karagiozov, L., Vasilev, Ch., Kunev, D. and Chimbulev, M., God. Vissh. Khimikotekhnol. Inst., Sofia, 24 (2) (1981) 265--273.