Equilibrium, Kinetic, and Chromatographic Controls of the Solution

33 Equilibrium, Kinetic, and Chromatographic Controls of the Solution Composition Obtained during the i n situ

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Leaching of a U r a n i u m Orebody R. W. POTTER II, J. M. THOMPSON, M. A. CLYNNE, and V. L. THURMOND U.S. Geological Survey, Menlo Park, CA 94025

Extraction of uranium from uranium ore deposits in sand stones and arkosic sandstones is being accomplished economically by leaching the ores in place with the added advantage of mini mal physical disturbance of the rocks (1). In practice the leaching solution (lixiviant) consists of groundwater to which has been added an oxidant (air, O , or H O ) and a complexing agent, ((ΝH ) CO , Na CO , NaHCO, etc.), for the uranyl ion produced by the oxidation of the uranium ores (1, 2). The focus of current research is towards optimizing the lixiviant so as to get the maximum recovery of uranium while at the same time minimizing any unfavorable impact on the environ ment. In order to attain these goals, efforts are being put forth to develop empirical and/or theoretical models which are capable of predicting changes in composition of the lixiviant as a function of time and distance travelled through the orebody (2). The modelling that has been applied to date to the in situ leaching of uranium has primarily consisted of modelling the hydrologic behavior of the lixiviant as a function of time. A more sophisticated model has combined a chemical model with the hydrologie model to describe the dissolution of uranium and the pore volumes of lixiviant required to leach the orebody (2). This combined hydrologic-chemical model assumes equilibrium processes, although the basis for its construction was largely empirical data obtained from small scale pilot tests. Inherent in the construction of the simple hydrologie models or the combined hydrologic-chemical model is the assumption that the lixiviant moves as an essentially homogeneous mass (slug flow). Non-equilibrium effects on solution composition such as the kinetics of interaction of the minerals in the host rock with the lixiviant and chromatographic effects resulting from adsorp tion of ions from a moving solution onto mineral surfaces and/or ion exchange between the moving lixiviant and clays are generally disregarded or assumed to be minimal. These sim plistic assumptions have in large part resulted from a lack of 2

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0-8412-0479-9/79/47-093-761$05.00/0 This chapter not subject to U.S. copyright Published 1979 American Chemical Society Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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a v a i l a b l e f i e l d and l a b o r a t o r y d a t a on t h e v a r i a t i o n o f s o l u t i o n c o m p o s i t i o n as a f u n c t i o n o f t i m e and v o l u m e o f r o c k traversed. The p u r p o s e o f t h i s p a p e r i s t o p r e s e n t t h i s t y p e o f d a t a and t o c a l l a t t e n t i o n t o some o f t h e d i f f i c u l t i e s i n v o l v e d i n c o n s t r u c t i n g v i a b l e models d e s c r i b i n g the v a r i a t i o n o f s o l u t i o n c o m p o s i t i o n d u r i n g an i n s i t u l e a c h i n g o p e r a t i o n .


Initial

C o n d i t i o n s and

Predictions

A s u i t e o f 314 g r o u n d w a t e r - l i x i v i a n t s a m p l e s were c o l l e c t e d f r o m m o n i t o r i n g w e l l s l o c a t e d i n an u r a n i u m o r e b o d y b e f o r e and w h i l e t h e o r e b o d y was b e i n g in s i t u l e a c h e d . The c o l l e c t e d samples cover a 61-day p e r i o d . The m a j o r c h e m i c a l species i n i t i a l l y p r e s e n t i n t h e g r o u n d w a t e r were N a , K , M g , C a , S i 0 , C 1 " , S 0 4 , and H C 0 3 . In a d d i t i o n to t h e g r o u n d w a t e r a n a l y s e s , a s u i t e o f c o r e s f r o m t h e o r e b o d y was s t u d i e d to a s c e r t a i n the i n i t i a l m i n é r a l o g i e c o m p o s i t i o n o f the host formation. +

+ +

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The h o s t f o r m a t i o n c o n t a i n i n g t h e o r e b o d y c o n s i s t s o f a p o o r l y c o n s o l i d a t e d , h i g h l y s o r t e d s a n d s t o n e composed c h i e f l y o f q u a r t z w i t h s u b o r d i n a t e amounts o f c l a y m i n e r a l s ( d o m i n a n t l y Ca-montmorillonite), potassium f e l d s p a r s , chert, plagioclase, pyrite-marcasite, z e o l i t e s ( d o m i n a n t l y c l i n o p t i l o l i t e ) and c a l cite. The c l a y m i n e r a l s and z e o l i t e s a r e d i s t r i b u t e d r a t h e r u n i f o r m l y as a f i n e m a t r i x t h r o u g h o u t t h e o r e b o d y . These phases c o n t a i n r e a d i l y a v a i l a b l e i o n exchange s i t e s . They t h e r e f o r e i n t r o d u c e a h i g h p o t e n t i a l f o r the c h r o m a t o g r a p h i c s e p a r a t i o n o f c h e m i c a l s p e c i e s by s e l e c t i v e a d s o r p t i o n o r i o n e x c h a n g e as t h e l i x i v i a n t flows through the f o r m a t i o n . Chromatographic separat i o n would i n v a l i d a t e the assumption t h a t the f l o w i s d o m i n a n t l y s l u g f l o w as u s e d i n t h e c u r r e n t m o d e l s (2). The l i x i v i a n t u s e d a t t h i s s i t e was m a n u f a c t u r e d by d i s s o l v i n g g a s e o u s ammonia and c a r b o n d i o x i d e i n t o g r o u n d w a t e r t h a t h a d p r e v i o u s l y b e e n pumped f r o m t h e o r e b o d y . T h i s s o l u t i o n was t h e n i n j e c t e d i n t o t h e f o r m a t i o n v i a i n j e c t i o n w e l l s and r e c o v e r e d from p r o d u c t i o n w e l l s a f t e r f l o w i n g t h r o u g h the o r e body. Samples f o r t h i s s t u d y were t a k e n f r o m m o n i t o r w e l l s l o c a t e d b e t w e e n t h e i n j e c t i o n and p r o d u c t i o n w e l l s a t distances r a n g i n g f r o m 2 t o 25 m e t e r s f r o m t h e i n j e c t i o n w e l l s . The samp l i n g i n t e r v a l was g e n e r a l l y once p e r day a t e a c h o f t h e f o u r w e l l s a l t h o u g h o c c a s i o n a l l y two s a m p l e s p e r day were c o l l e c t e d . U s i n g t h e i n i t i a l g r o u n d w a t e r p H , E h , and c o m p o s i t i o n s , c a l c u l a t i o n s o f m i n e r a l s t a b i l i t i e s were made u s i n g t h e c o m p u t e r p r o g r a m s SOLMNEQ (3) and WATEQ ( 4 ) . B o t h programs y i e l d e d r e s u l t s w h i c h showed t h a t t h e g r o u n d w a t e r c o m p o s i t i o n was c o m p a t i b l e w i t h t h e o b s e r v e d m i n e r a l a s s e m b l a g e s and w i t h t h e s t a t e of a l t e r a t i o n of the r e s p e c t i v e m i n e r a l s . The pH o f t h e g r o u n d water p r i o r to i n j e c t i o n o f the l i x i v i a n t ranged from 6.9 to 7.4 and was a p p a r e n t l y b u f f e r e d b y e q u i l i b r i a i n v o l v i n g c a l c i t e and aqueous C 0 s p e c i e s . P y r i t e was s e v e r a l o r d e r s o f m a g n i t u d e 2

Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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supersaturated based on the computer c a l c u l a t i o n as evidenced by the detectable Fe (0.12 ppm) and (0.3 ppm) present i n the formation waters. The i n i t i a l pH o f the l i x i v i a n t v a r i e d somewhat but i n general was greater than 9. Using the average pH, Eh, and com p o s i t i o n of the l i x i v i a n t , the host rock mineral s t a b i l i t i e s were again c a l c u l a t e d using SOLMNEQ and WATEQ. The c a l c u l a t i o n s i n d i c a t e d that c a l c i t e , which had been i n e q u i l i b r i u m with the groundwater, became g r e a t l y supersaturated. The potassium f e l d spar and p l a g i o c l a s e s o l u t i o n e q u i l i b r i a was s h i f t e d so that the f e l d s p a r s became unstable and tended to a l t e r w i t h the amount o f a l t e r n a t i o n being c o n t r o l l e d by the a p p l i c a b l e k i n e t i c rate law. On the b a s i s o f the above e q u i l i b r i u m c a l c u l a t i o n s and the p r o p e r t i e s of the l i x i v i a n t , one would p r e d i c t that as the l i x i v i a n t a r r i v e d at a monitoring w e l l the pH would i n c r e a s e , s i l i c a c o n c e n t r a t i o n most l i k e l y would be constant or would s l i g h t l y increase owing to the increased pH; the C a concentration would decrease owing to c a l c i t e p r e c i p i t a t i o n ; the Mg c o n c e n t r a t i o n would remain e s s e n t i a l l y unchanged; and NH^ and C 0 3 would simultaneously i n c r e a s e . The e f f e c t s o f p r e c i p i t a t i o n and d i s s o l u t i o n e q u i l i b r i a on the con c e n t r a t i o n s of d i s s o l v e d N a and K would probably not be as s i g n i f i c a n t as the ion-exchange r e a c t i o n s w i t h the c l a y s and z e o l i t e s . Based on the ion-exchange c h a r a c t e r i s t i c s of the m o n t m o r i l l o n i t e s , the concentraion o f K and N a would be expected to increase at approximately the same r a t e . The pre d i c t i o n s made above assume s l u g flow and that s t r i c t e q u i l i b r i u m c o n d i t i o n s c o n t r o l the s o l u t i o n composition. There are many problems i n v o l v e d w i t h making r e l i a b l e pre d i c t i o n s about the behavior o f uranium i n in s i t u leach environ ments. The f i r s t problem i s s p e c i f y i n g which uranium-bearing phases are present i n the orebody. I n the ore deposit we s t u d i e d , the major phases appear to be amorphous U 0 and u r a n i n i t e , although a s i g n i f i c a n t amount o f uranium i s present i n an u n i d e n t i f i e d form i n the rock m a t r i x r i c h i n c l a y s and z e o l i t e s . A second problem i s a lack o f i n t e r n a l l y c o n s i s t e n t thermodynamic data which makes i t d i f f i c u l t to c a l c u l a t e the s p e c i a t i o n o f the uranium i n the ammonium carbonate l i x i v i a n t . The most t r o u b l i n g aspect, however, centers on the dependence o f the s o l u b i l i t y and r a t e o f s o l u t i o n o f uranium upon the o x i d a t i o n s t a t e o f the l i x i v i a n t . At the s i t e we s t u d i e d there was an i n i t i a l Eh gradient o f over 200 mv across the t r a v e l path o f the l i x i v i a n t . I n modelling the i n s i t u leaching at t h i s s i t e i t i s necessary to c a l c u l a t e or p r e d i c t the capacity o f the oxidant to overcome t h i s pre-mining g r a d i e n t . However, the data necessary to accomplish t h i s are not a v a i l a b l e , hence only q u a l i t a t i v e p r e d i c t i o n s can be made. Oxidized uranium pre v i o u s l y m o b i l i z e d by the l i x i v i a n t might be re-reduced as i t i s transported through the reduced p o r t i o n o f the formation. The uranium concentration would then be expected t o decrease. The + +

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amount o f d e c r e a s e w o u l d be m o d e r a t e d b o t h by t h e c a p a c i t y and c o n c e n t r a t i o n o f the o x i d a n t . On t h e o t h e r h a n d , t h e c o n c e n t r a t i o n o f m o b i l i z e d uranium would remain unchanged, or i t w o u l d i n c r e a s e as t h e l i x i v i a n t moved i n t o t h e o x i d i z e d p o r t i o n o f the orebody.


Results As p r e d i c t e d b y t h e e q u i l i b r i u m c a l c u l a t i o n s , t h e C a c o n c e n t r a t i o n d e c r e a s e d owing to c a l c i t e p r e c i p i t a t i n g from the lixiviant. H o w e v e r , t h e r a t e o f d e c r e a s e was m a r k e d l y s l o w e r than p r e d i c t e d . The S1O2 c o n c e n t r a t i o n d i d n o t b e h a v e as predicted. The i n i t i a l c o n c e n t r a t i o n o f s i l i c a was a p p r o x i m a t e l y 33 m g / L and d e c r e a s e d s t e a d i l y as a f u n c t i o n o f t i m e u n t i l i t r e a c h e d a v a l u e o f 7 m g / L where i t s t a b i l i z e d ( F i g u r e 1). T h i s b e h a v i o r c o u l d be t h e r e s u l t o f the p r e c i p i t a t i o n o f a s i l i c a - b e a r i n g p h a s e n o t c o n t a i n e d i n t h e c o m p u t e r c o d e s s u c h as b u d d i n g t o n i t e (NH4 f e l d s p a r ) , some ammonia c l a y s , o r l a y e r e d silicates. A t t h e t e m p e r a t u r e and pH o f t h e s o l u t i o n , a d i s s o l v e d s i l i c a v a l u e o f 7 mg/L i s below the s o l u b i l i t y o f q u a r t z , t h u s r u l i n g o u t t h e p r e c i p i t a t i o n o f q u a r t z as a c o n t r o l on t h e s i l i c a c o n c e n t r a t i o n . The u r a n i u m c o n c e n t r a t i o n s d i d n o t b e h a v e c o m p l e t e l y as p r e d i c t e d by t h e q u a l i t a t i v e ' a r g u m e n t s given above. U r a n i u m was m o b i l i z e d n e a r t h e c e n t e r o f t h e o r e b o d y ; and as i t m i g r a t e d o u t w a r d s t o w a r d s t h e p r o d u c t i o n w e l l s , t h e c o n c e n t r a t i o n i n s o l u t i o n d e c r e a s e d b y two o r d e r s o f m a g n i t u d e i n t h e r e d u c e d r e g i o n ( a s p r e d i c t e d ) and d e c r e a s e d i n t h e o x i d i z e d p o r t i o n by an o r d e r o f m a g n i t u d e . + +

The most s t r i k i n g d e p a r t u r e f r o m p r e d i c t i o n , a l t h o u g h e x p e c t e d , was t h e o b s e r v a t i o n t h a t t h e v a r i o u s s p e c i e s were chromatographically separated. The f i r s t s p e c i e s t o a r r i v e was H ( F i g u r e 2) w h i c h a p p a r e n t l y was g e n e r a t e d b y t h e r e a c t i o n s b e t w e e n ammonia, s i l i c a and s i l i c a t e s . I t s a r r i v a l time i s a b o u t 8 d a y s p r i o r t o t h e a r r i v a l o f NH3, t h e l a s t s p e c i e s t o arrive. C l o s e l y f o l l o w i n g H was ΗΟΟβ" w h i c h a r r i v e d a b o u t 6 d a y s p r i o r t o t h e a r r i v a l o f t h e NH3 ( F i g u r e 3 ) . At a p p r o x i m a t e l y t h e same t i m e as t h e HCO3" a r r i v e d a t t h e monitor w e l l , M g was a l s o a r r i v i n g ( F i g u r e 4) most l i k e l y as (MgHC03) or r e l a t e d s p e c i e s . A p p r o x i m a t e l y 5 days b e f o r e t h e NH3, t h e f i r s t C I " b e g a n t o a r r i v e ( F i g u r e 5 ) . The f i r s t a r r i v a l s o f N a ( F i g u r e 5) and K ( F i g u r e 6) a r e a p p r o x i m a t e l y t h e same, i . e . , a b o u t 3 d a y s p r i o r t o t h e NH3. The C a c o n c e n t r a t i o n began to decrease i n response to the HC03~ and C 0 3 i n c r e a s e a b o u t h a l f a day p r i o r t o i t s a r r i v a l (Figure 5). The s u p e r s a t u r a t e d c o n c e n t r a t i o n s w h i c h were o b s e r v e d , as w e l l as t h e s l i g h t i n c r e a s e o f C a 2 days p r i o r t o t h e NH3 a r r i v a l , s u g g e s t t h a t C a was b e i n g i o n e x c h a n g e d by t h e c l a y s a t a r a t e s l i g h t l y g r e a t e r , o r n e a r l y e q u a l t o , the k i n e t i c p r e c i p i t a t i o n r a t e o f c a l c i t e , thus e x p l a i n i n g i t s slow decrease i n c o n c e n t r a t i o n ( F i g u r e 5 ) . The +

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Figure 2. pH as a function of time. Solid arrow indicates the arrival of H while the open arrow indicates the arrival of thefirstdetectable NH . +

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TIME/DAYS Figure 3. HC0 ~ concentration as a function of time. Solid arrow indicates the arrival of HCOf while open arrow indicates the arrival of the first detectable NH . 3

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TIME/DAYS Figure 4. Mg concentration as a function of time. The solid arrow indicates the arrival of Mg while the open arrow indicates the first arrival of detectable NH . ++

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Figure 5. Na , Cl~, and Ca concentration as a function of time. Solid arrows indicate the arrivals of Na , CI', and Ca while the open arrow indicates the arrival of the first detectable NH . +

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Figure 6. K concentration as a function of time. Solid arrow indicates the arrival of K while the open arrow indicates the first arrival of detectable NH . +

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m o b i l i z e d u r a n i u m b e g a n t o a r r i v e a b o u t one day p r i o r t o t h e NH3. This high r e t e n t i o n time i s suggestive of e x t e n s i v e i n t e r a c t i o n w i t h t h e c l a y s and z e o l i t e s . The e x p l a n a t i o n f o r t h e d e c r e a s e i n c o n c e n t r a t i o n o f u r a n i u m as i t moved i n t o t h e o x i d i z e d r e g i o n may r e s u l t f r o m e x t e n s i v e i o n - e x c h a n g e w i t h t h e c l a y s and z e o l i t e s , t h u s f i x i n g t h e u r a n i u m i n t h e s e p h a s e s .


Conclusions B e f o r e computer models can r e a c h the r e q u i r e d s o p h i s t i c a t i o n t o be t r u l y c a p a b l e o f p r e d i c t i n g t h e b e h a v i o r o f an o r e b o d y as i t i s in s i t u l e a c h e d , t h e r e a r e s e v e r a l s i g n i f i c a n t p i e c e s o f i n f o r m a t i o n w h i c h must be i n c o r p o r a t e d i n t o t h e m o d e l . First and f o r e m o s t i s t h e e f f e c t o f t h e m i n é r a l o g i e a s s e m b l a g e on t h e process. T h i s n e c e s s i t a t e s the a c c u r a t e c h a r a c t e r i z a t i o n o f the host formation p r i o r to i n s i t u l e a c h i n g . Secondly, a better and i n t e r n a l l y c o n s i s t e n t d a t a b a s e i s n e e d e d f o r t h e k i n e t i c and t h e r m o d y n a m i c p r o p e r t i e s o f u r a n i u m compounds and t h e i r s p e c i a t i o n and o x i d a t i o n . In a d d i t i o n , r e l i a b l e k i n e t i c data f o r i o n exchange r e a c t i o n s are a l s o r e q u i r e d . T h i r d l y , the l i x i v i a n t c a n n o t b e t r e a t e d as a homogeneous mass as i t moves t h r o u g h t h e f o r m a t i o n b u t r a t h e r some c o r r e c t i o n s a r e r e q u i r e d f o r chromatographic e f f e c t s . I f a l l of these elements are i n c o r p o r a t e d i n t o a b a s i c h y d r o l o g i e flow m o d e l , then d e t a i l e d m o d e l l i n g o f the b e h a v i o r o f t h e l e a c h i n g s y s t e m w i l l be p o s s i b l e . The d e v e l o p m e n t o f such comprehensive models w i l l not o n l y a i d i n the o p t i m i z a t i o n o f s o l u t i o n c o m p o s i t i o n s f o r t h e most e f f e c t i v e u r a n i u m r e c o v e r y , b u t w i l l a l s o a l l o w a more r e a l i s t i c e n v i r o n m e n t a l impact assessment and c o r r e c t i v e m e a s u r e s i f r e q u i r e d . Abstract The process of in situ l e a c h i n g o f uranium ore bodies o f f e r s a method of e x t r a c t i n g uranium that i s economically v i a b l e and more environmentally acceptable than current s u r f a c e mining technology. Models capable of p r e d i c t i n g s o l u t i o n composition as a f u n c t i o n of time and d i s t a n c e t r a v e l l e d are r e q u i r e d to optimize uranium recovery as w e l l as to evaluate environmental impact. The current approach i s to t r e a t the problems as being strictly c o n t r o l l e d by e q u i l i b r i u m c o n s i d e r a t i o n s and to e i t h e r d i s r e g a r d or to consider n e g l i g i b l e the k i n e t i c and chromatographic e f f e c t s . We have c o l l e c t e d 314 ground-water samples from w e l l s l o c a t e d across an uranium ore body being leached. These samples r e f l e c t c o n d i t i o n s before and during the l e a c h i n g of an uranium ore body. The samples were analyzed by a v a r i e t y of methods f o r d i f f e r e n t elements and s p e c i e s , Na, K, Mg, Ca, SiO , Cl, U, NH , and HCO , i n order to b e t t e r understand the 2

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processes involved and how well the behavior is predicted by existing equilibrium models. The observed data agrees well only with the prediction for Ca. The magnitude of the SiO effect was significantly larger than expected, possibly due to the precipitation of an ammonium feldspar not considered in the equilibrium calculations. Even when corrected for the regional Eh gradient, the behavior of uranium differed substantially from the qualitative predictions. The concentration of Na, K, and Mg were controlled by ion exchange reactions; however, these concentrations were also strongly affected by chromatographic effects which were observed for all of the species studied. The most striking was the 8-day separation between the arrival of NH and H. These effects are significant and invalidate normal assumptions that the flow is slug flow. In order to be truly predictive, models for the complex leaching process must contain equilibrium, kinetic, and chromatographic parameters. Development of such comprehensive models w i l l not only aid in the optimization of solution compositions for the most effective uranium recovery, but w i l l also allow a more r e a l i s t i c environmental impact assessment.


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Literature Cited 1. 2. 3.

4.

White, L. In-situ leaching opens new uranium reserves in Texas. Eng. Mining J. July, 1975, 73-81 (1975). Shock, D.A. The Vail Conference on in-situ leaching of uranium. In Situ 1, 103-113 (1977). Kharaka, Y.K. and Barnes, Ivan. "SOLMNEQ: Solution-mineral equilibrium computations." U.S. Geol. Survey Comp. Contrib., PB-215899, NTIS, Springfield, Virginia, 82 p. (1973). Truesdell, A.H. and Jones, B.F. WATEQ, A computer program for calculating chemical equilibria of natural waters. U.S. Geol. Survey J. Res. 2, 233-248 (1974).

Disclaimer: The reviews expressed and/ or the products mentioned in this article represent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. RECEIVED

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1978.


Equilibrium, Kinetic, and Chromatographic Controls of the Solution

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