THE

EFFECT

OF U R A N I U M G . M.

OF C E R T A I N IN A C I D Nesmeyanova

COMPOUNDS

ON T H E

OXIDATION

MEDIA and

G.

M.

Alkhazashvili

Translated from Atomnaya Energiya, Vol. 10, No.6, pp. 587-591, June 1961 Original article submitted May 28, 1960

In the h y d m m e t a l l u r g i c a l t r e a t m e n t of ores the m a i n losses of uranium are due to the leaching process. It is therefore of interest to study substances which c a t a l y z e the transfer of uranium to solution. Studies into the effect of v a r i a b l e - v a l e n c y elements on the degree of solution of uranium showed that additions of vanadium, copper, and iron m a k e it possible to c o m p l e t e l y oxidize uranium in fairly low concentrations of sulfuric acid. In the case where copper and iron are contained in difficultly soluble minerals, these elements can be added in the form of soluble salts. The losses of uranium in the hydrometallurgical treatment of ores are m a i n l y determined by its degree of extration in the leaching process. Modern methods for treating pulps and solutions in many cases are characterized by a c o m p a r a t i v e l y low level of extraction of uranium from ores during leaching. This explains the interest of workers in intensifying the l e a c h i n g of uranium from ores by using new methods of autoclave and extraction leaching [1, 2], making additions of different substances to increase the extraction of uranium or shortening the t i m e of the process [3]. The study of one of the possible ways of intensifying the leaching process by additions of substances which have a c a t a l y t i c action on the transfer of uranium to solution is a very difficult problem. This difficulty is m a i n l y due to the c o m p l e x i t y of the c h e m i c a l composition of ores and the variety of c h e m i c a l processes which take place during leaching. It is characteristic that the catalysts are not active for all known oxidants in uranium chemistry but are specific for each oxidant separately [a]. Thus, when studying the c a t a l y t i c properties of mercury, silver, ferricyanide and copper ions it was found that mercury ions c a t a l y z e the oxidation of uranium by hypochIorite in a carbonate m e d i u m and the c o p p e r - a m m o n i a c o m p l e x is an efficient catalyst during oxidation by air. Only iron ions are active catalysts not only for any oxidant in an acid m e d i u m but for MnO 2, KC1C~, HNO~, etc. [4], which is due not to tile specific feature of the catalyst but to the mechanism of the process. In the present work we studied a number of additions; we did not study the mechanism of their action on the oxidation of uranium but determined the efficiency of the compounds used with regard to the degree of solution of uranium oxides. As additions c a p a b l e of a c c e l e r a t i n g the oxidation of uranium we chose compounds containing elements of v a r i a b l e v a l e n c y , which are found in uranium ores. The effect of these compounds was determined on pure uranium oxides and ore using V205, Co203, MnSO 4, CuSO 4, CoSO 4, FeSO4, and also minerals of iron (hematite, siderite, and e o v e l l i t e ) using the previously described method [4]. Additions of each of these compounds with respect to the weight of uranium m i x e d oxide or ore were 0.5%. The oxidizing agent was maganese dioxide, potassium chlorate, and nitric acid in a sulfuric acid medium. Preliminary data on the effect of divalent copper ions showed that at 90~ and t i m e o f experiment a hr the degree of oxidation of uranium by maganese dioxide with a sulfuric acid concentration of 10-25 g / l i t e r increases by 9-12%. According to the data of a number of authors [5, 6] the c a t a l y t i c activity of copper ions increases sharply in the presence of divalent iron. The results of the experiments showed that the intorduction into the reaction mixture of d i v a l e n t copper and iron compounds with a nitric acid concentration of 10 g / l i t e r increases the degree of oxidation of uranium from 28 to 100%, and at a concentration of 5 g 4 i t e r it increases from 12 to 82% (Fig. 1 ) . A similar a c c e l e r a t i o n of the reaction by these additions is observed when uranium is oxidized by manganese dioxide or potassium chlorate. In the absence of divalent copper and iron a change in the sulfuric acid concentration from 5 to 150 g / l i t e r increases the degree of oxidation of uranium by manganese dioxide from 52 to 86% and by potassium chlorate from 24 to 67%. With copper and iron ions in the solution (Fig. 2) the mixed oxide is c o m p l e t e l y oxidized by manganese dioxide at a sulfuric acid concentration of 10 g / l i t e r any by potassium chlorate at a concentration of 50 g / l i t e r .

583

,oo[ g.g

,

9

/

io/

so-

4 er

/

II

.=~..~

It

t

5

-2>

go.~=

o

5o

too

Isg

Concentration, g / l i t e r o

5o

leo

Concentration, g / l i t e r Fig. 1. Effect of iron and copper compounds on the oxidation of uranium by nitric acid (t = 90~ t i m e 1 hr): O-without additions; O - w i t h addition of FEZ+; A - w i t h addition of Fe z+ + CuZ+; ~ - h e m a t i t e .

Fig. 2. Effect of copper and iron compounds on the oxidation of uranium by potassium chlorate ( ) and manganese dioxide ( - - - ) in a sulfuric acid m e d i u m (t = 90~ t i m e l h r ) : O-without additions; O-with addition of Fe z+ + Cu2+; e - s i d e r i t e and covellite.

IOO

,00

G G i-i

I

(" ..~r r..._~-- -~

o 5~

t o

I

2

3

4

T i m e , hr. Fig. 3. Effect of copper and iron compounds on the extraction of uranium into solution and the duration of the process when treating U30 s with sulfuric acid using potassium chlorate ( ) and manganese dioxide ( - - - ) as the oxidants (t = 90~ Hz804 concentration 10 g/liter): O-without additiom; | addition of Cu 2 + + Fe 2 + .

SO

100

150

Concentration, g / l i t e r Fig. 4. Effect of compounds of Mn 2+ (9 Mn 4§ (O), Co 2+ ((9), Co 3+ (A) and V5+ ( 0 ) on the oxidation of uranium by potassium chlorate in a sulfuric acid m e d i u m (O-without additions; t = 90~ time 1 hr).

Although it has been found that with a sulfuric acid concentration of I0 g / l i t e r the degree of oxidation of uranium by chlorate increases b y 18%, the fact of a c c e l e r a t i o n of the r e a c t i o n under these conditions makes it possible to reduce the effect of acidity on the oxidation of uranium. A study was there m a d e of the region of c o m p a r a t i v e l y low concentrations of acid during the solution of uranium oxide. It was found (Fig. 3) that with a sulfuric acid c o n c e n t r a tion of 10 g / l i t e r manganese dioxide oxidizes uranium in the presence of copper and iron in 1 hr, and potassium c h l o rate in 2 hr, Since the additions of copper and iron ions were very small, their effect on the process can be assumed to be c a t a l y t i c . Since the oxidizing properties of divalent copper with regard to uranium are weak, then even the increase in the content of copper and iron ions in the solution should not change the character of the chain reaction. This is due to the fact that the degree of oxidation of uranium due to oxidation by these additions cannot be increased by 50% and more. It can be assumed that the mechanism of the c a t a l y t i c effect of copper and iron ions on the o x i d a tion of uranium is connected with a number of reactions, for e x a m p l e : 2 F e 2+ + MnO2(4H*) --> 2 F e 3+ 4- Mn 2§ + 2 H 2 0 ; F e ~§ -b C n 2+ <---~C u + + Fea~; 2Cu + -t- MnO~( + 4 H § --+ 2Cu 2§ + Mn 2§ + 2H=O; U 4+ -1- 2 F e a+ ---> U 6* + 2 F e z§ .

584

10o

r - - ~ f "

Fig. 5. Effect of compounds of Co ~" (O) and V 5+ ( e ) on the oxidation of uranium by manganese dioxide in a sulfuric acid m e d i u m ( O - additions absent; t = 90~ time 1 hr).

so 9 ~0 O

0

fO0

SO

/so

Concentration, g/liter" I00

Fig. 6. Dependence of uranium extraction from ore contained in a mixture with minerals of iron on the concentration of acid and additions of vanadium pentoxide ( t=90~ . . . . t=20~ O, O - as the o x i d a n t - m a n g a n e s e dioxide (without the introduction and with the introduction of vanadium pentoxide respect ively):Z~, e - as the o x i d a n t - nitric acid (without the introduction and with the introduction of vanadium pentoxide respectively).

O

o

E C)

2,5

5

tO

Concentration, g / l i t e r As can be seen from these equations, the reactions of oxidation between the solid and liquid substances proceed more readily than between the two solids (UzO 8 and MnO2). The established effect of these ions indicated that compounds of copper and iron contained in ores in the form of various minerals, depending on the degree of solution of the minerals, also help the oxidation of uranium during acid leaching. The data given in Figs. land 2 show that the introduction of h e m a t i t e in amounts of 0.5% of the weight of the mixed oxide a c c e l e r a t e the process, but several times less than the introduction of ions of iron oxide, added in the form of a readily soluble salt. The slight effect of h e m a t i t e is due to its poor solubility in solutions of m i n e r a l acids [7]. At the same t i m e the readily soluble minerals of iron and copper (siderite and c o v e l l i t e ) have an effect which is almost equal to the effect of salts of these elements on the oxidation ofuranium by chlorate or by manganese dioxide. Since ores contain a number of elements with variable valency, it would be desirable to study their effect on the oxidation of uranium as a function of the leaching conditions. On the other hand, it is interesting to establish the effect on this process of oxidation products contained in the solution at the m o m e n t of leaching. It was found that in the oxidation of uranium by potassium chlorate (Fig. 4) compounds of MnO z and C o c o 3 are slightly soluble in solutions of mineral acids but, being oxidants for uranium, they a c c e l e r a t e the reaction to an almost equal extent. The degree of solution of the mixed oxide at a concentration of sulfuric acid of 150 g / l i t e r is increased to 34-38%. This points to the c a t a l y t i c character of the reactions, since due to the oxidation of uranium by additions of the MnO z or CozO3 only 3% of the uranium dissolves. Whereas during the oxidation of uranium by potassium chlorate additions of Co20 s at a concentration of sulfuric acid of 5-25 g / l i t e r do not affect the process (see Fig. 4), during the oxidation of uranium mixed oxide by manganese dioxide the degree of oxidation of uranium at a sulfuric acid concentration of 5 g / l i t e r increases by 20% (Fig. 5). Readily soluble compounds such as MnSO4 and CoSO 4 have a different effect on the process. Whereas additions of Mn z+ at sulfuric acid concentrations of 50-150 g / l i t e r increase the extraction of uranium by 30% and above, ions of Co 2+ help the oxidation of uranium by potassium chlorate only with an acid concentration of 150 g / l i t e r .

585

The greatest catalytic effect is shown by compounds of vanadium which, with an acid concentration of 10 g / l i t e r can completely dissolve uranium mixed oxide when the oxidants are not only chlorates but also manganese dioxide (see Fig. 5). The character of the curves for the oxidation of uranium by both oxidants in the presence of vanadium is almost the same, which points to the possibility of uniform extraction of uranium into solution when using (under identical conditions) oxidants which are different in their nature. In connection with this the obtained results were checked on uranium ore containing 67% uranium (Fig. 6). The ore was mixed with twice the amount of iron minerals (hematite, siderite, pyrites, limonite,and magnetite). The obtained mixture was treated with a solution of sulfuric acid with an oxidant. It was found that when using manganese dioxide as the oxidant at temperatures of 20 and 90~ with sulfuric acid concentrations of 2.5-10 g / l i t e r the extraction of uranium into solution depends little on the concentration of the latter. The introduction of vanadium pentoxide into the reaction mixture does not change this relationship although it increases the extraction of uranium at a t e m perature of 2O~ A completely unexpected effect is shown by additions of VzOs during the oxidation of uranium by nitric acid in a sulfuric acid medium at a temperature of 20~ In this case the degree of solution of uranium with a concentration of mixed sulfuric and nitric acids of 10 g / l i t e r approaches 95%. At a temperature of 90~ there is also an increase in the extraction of uranium but not as much as at 2O~ SUMMARY 1, A study has been made of the effect of compounds of cobalt, vanadium, manganese, a mixture of divalent copper and iron on the degree of oxidation and solution of uranium-mixed oxides by various oxidants in an acid medium. 2. The greatest effect is Shown by compounds of vanadium, copper, and iron, which can completely oxidize uranium at fairly low (5-10 g / l i t e r ) concentrations of sulfuric acid. 3. It has been shown that minerals of copper and iron in the uranium ores accelerate the oxidation of uranium but their effect depends on the solubility of these minerals in acids. If the ore contains low solubility minerals, containing elements with variable valency, then these elements can be added to the process in the form of readily soluble salts. 4. It has been shown that divalent manganese has the same catalytic effect on the oxidation of uranium mixed oxides by potassium chlorate as MnCh and Co203. LITERATURE 1. 2. 3. 4. 5. 6. 7.

CITED

Robinson et al. Transaction of the Second International Conference on the Peaceful Use of Atomic Energy (Geneva, 1958). Selected reports of non-Soviet scientists, Vol. 7 (Moscow, Atomic Energy Press, 1959) p. 95. R. Bailes, I. Magnet. Mines. Mag. 4~7, No. 6, 51 (1957). De-Seza et al, Transactions of the Second International Conference on the Peaceful Use of Atomic Energy (Geneva, 1958). Selected reports of non-Soviet scientists, Vol. ? (Moscow, Atomic Energy Press, 1959) p. 38. G . M . Nesmeyanova and G. M. Alkazashvili, Atomnaya 4nergiya 8, No. 4, 330 (1960). I . N . K u z ' m i n a and E. L. Yakhontova. Zh. prikl, khim. 26, No. 4, 348 (1953). A . P . Snurnikov and V. D. Ponomarev, Tsvetnye rectally , No. 8, 22 (1956). G . M . Nesmeyanova and N. K. Chernushevich. Atomnaya gnergiya 9, No. 2, 137 (1960).

All abbreviations of periodicals in the above bibliography are letter-by-letter transliterations of the abbreviations as given in the original Russian journal. Some or all of this periodical literature m a y well be available in E n g l i s h translation. A complete list of the cover-tocover English translations appears at the back of this issue.

586