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The leaching of uranium from pitchblende ores by aqueous oxidation techniques 1951

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f\l„ / OF i" COFO. SE3;ES._/2L_ THE LEACHING OF -URANIUM FROM PITCHBLENDE ORES BY AQUEOUS OXIDATION TECHNIQUES by ERNEST PETERS A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Department of Mining and Metallurgy We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF APPLIED SCIENCE Members of the Department of Mining and Metallurgy THE UNIVERSITY OF BRITISH COLUMBIA September, 1951 Abstract Uranium exists i n carbonate s o l u t i o n i n i t s hexavalent state as the complex i o n U0 2(C03) 3 . In most ores uranium occurs as pitchblende (U 30g). I t has been found possible to di s s o l v e uranium i n carbonate-bicarbonate solutions from these ores by leaching them i n the presence of oxygen. The o v e r a l l reaction i s as follows: U 3 O 8 + \0Z + 3C03-= + 6 H C O 3 - -* 3 U 0 2 ( C 0 3 ) 3 + 3H 20 The objective of the present research was to examine the k i n e t i c s of t h i s reaction with a view to determining the mechanism of the reac t i o n and to e s t a b l i s h the variables upon which the rate depends. Two se r i e s of experiments were conducted on two types of materials: (a) Pitchblende specimens of measured surface area were suspended i n a s o l u t i o n containing sodium carbonate and sodium bicarbonate. Above the so l u t i o n a desired pressure of oxygen was maintained. (b) A pulp of low grade pitchblende ore i n carbonate, solutions was agitated i n an autoclave, i n the presence of oxygen maintained at the desired pressures. The rate of s o l u t i o n of uranium was measured by sampling and analyzing the solutions at regular time i n t e r v a l s . The e f f e c t of oxygen pressure, temperature, and reagent concen- t r a t i o n on the rate was studied i n each s e r i e s . The k i n e t i c r e s u l t s were examined by the methods of the absolute reaction rate theory. The f o l l o w i n g c o n c l u s i o n s w e r e d r a w n f r o m t h e l e a c h i n g t e s t s : (1) The a b s o l u t e r e a c t i o n r a t e s a r e a b o u t t h e same f o r a l l t h e m a t e r i a l s s t u d i e d . (2) The r e a c t i o n r a t e v a r i e s a s t h e s q u a r e r o o t o f t h e a b s o l u t e o x y g e n p r e s s u r e . (3) The a c t i v a t i o n e n e r g y i s a b o u t 12,000 - 3000 c a l o r i e s p e r gram m o l e . (4) Min imum amounts o f c a r b o n a t e and b i c a r b o n a t e a r e n e c e s s a r y t o d i s s o l v e t h e o x i d i z e d u r a n i u m . F u r t h e r i n c r e a s e s b e y o n d t h i s minimum amount r e s u l t i n o n l y m i n o r i n c r e a s e s i n t h e r a t e o f t h e r e a c t i o n . A mechan i sm f o r t h e l e a c h i n g r a t e i s p r o p o s e d i n w h i c h t h e o x i d a t i o n i s t h e r a t e c o n t r o l l i n g s t e p . P Acknowledgement The author wishes to express his appreciation to the National Research Council for financial aid in the form of a research assistantship. Appreciation i s also expressed to Eldorado Mining and Refining (1944) Limited, who supplied the pitchblende specimens and ore samples used in the research. The author i s indebted to Professor F.A. Forward and the staff of the Department of Mining and Metallurgy for their encouragement and advice. He i s especially indebted to Dr. J. Halpern who directed the research program, and who was of much assistance i n the writing of this thesis. The assistance of Ronald Dakers i n performing the analyses of the ores and residues i s also appreciated. Contents page Introduction 1 General 1 Historical 2 Application of the Sodium Carbonate Leach to Pitchblende Ores h Recent Research on Aqueous Oxidation at U.B.C. . . 5 The Chemistry of the Oxygen - Sodium Carbonate Leach 6 Side Reactions 7 Objectives of the Present Research 8 Materials 10 Ores . 10 Mineralogical Description 10 Pitchblende Specimens 12 Reagents 12 Equipment 13 Experimental Procedures 16 Pitchblende Specimens 16 Ores 1 6 Analytical Methods 17 Contents (continued) page Results 20 A. Leaching of Pitchblende Specimens 20 1. R e p r o d u c i b i l i t y . 20 2. P r o p o r t i o n a l i t y to Surface Area 20 3. E f f e c t of Sodium Bicarbonate Concentration . 21 4. E f f e c t of Temperature 25 5. E f f e c t of Oxygen Pressure 28 6. E f f e c t of Sodium Carbonate Concentration . . 30 7. E f f e c t of Sodium Sulphate Concentration . . 30 8. E f f e c t of Agitation 30 B. Leaching of Ores 32 1. R e p r o d u c i b i l i t y 32 2. E f f e c t of Temperature 32 3. E f f e c t of Oxygen Pressure 35 4. E f f e c t of M i l l i n g Time 35 5. E f f e c t of Sodium Carbonate Concentration . . 38 6. E f f e c t of Recycling the Leach Solution . . . 38 7. E f f e c t of Pulp Density 38 8. E f f e c t of P r i o r Roast . 40 9. E f f e c t of P r i o r F l o t a t i o n of Sulphides . . . 42 10. E f f e c t o f Sodium Sulphate Concentration . . 42 Discussion of Results * 44 Summary of Variables . 44 Contents (continued) page Numerical Values of Reaction Rates .45 Comparison of Absolute Reaction Rates of D i f f e r e n t Ores Z+6 Magnitude of Errors i n Reaction Rate Estimates . . 47 A c t i v a t i o n Energies 48 Reaction Mechanisms 49 Influence of Agitation 50 Determination of the C o n t r o l l i n g Step 51 Mechanism I 52 Mechanism I I 53 Discussion of Mechanisms 54 Approximations i n Theoretical Rate Calculations . 55 E f f e c t of Leach Reagents 56 Change i n C o n t r o l l i n g Mechanism 57 Conclusions . . . . . . . . . 58 Appendix A . . . . . 59 3 Appendix B 61 Appendix C 62 Appendix D . . . . . . . . 64 Contents (continued) page Appendix E 67 Appendix F 69 Bibliography 70 I l l u s t r a t i o n s F i g . 1. Botry o i d a l Pitchblende i n AS-2 Ore . . . 11 F i g . 2. No. 1 Autoclave 14 F i g . 3. No. 2 Autoclave 14 F i g . 4. T y p i c a l C a l i b r a t i o n Curves 18 F i g . 5 . E f f e c t of Surface Area 22 F i g . 6. E f f e c t of Sodium Bicarbonate 23 F i g . 7. Leaching Rate vs. Sodium Bicarbonate . . 23 F i g . 8. Pitchblende a f t e r a pure carbonate leach 24 F i g . 9. Pitchblende a f t e r a carbonate-bicarbo- nate leach 24 .Fig.10. E f f e c t of Temp, on a pure carbonate leach 26 F i g . 11. E f f e c t of Temp, on a % carbonate Ifo bicarbonate leach . , 26 Fig.12, E f f e c t of Temp, on a 5% carbonate % bicarbonate leach 27 Fig.13. Arrhenius Plots of Pitchblende Specimen Leaches 27 Contents (continued) page Fig.14. E f f e c t of Pressure 29 F i g . 15. Rate vs. Pressure 29 Fig.16. E f f e c t of Sodium Carbonate 31 Fig.17. E f f e c t of Sodium Sulphate 31 Fig.18. Comparison of Ore Leaching Curves . . . 33 Fig.19. E f f e c t of Temp, on Ore Leaching Curves . 34 Fig.20. Arrhenius Plot of Ore Leaching Rates . . 34 Fig.21. E f f e c t of Oxygen Pressure . . . . . . . 36 Fig.22. Leaching Rate vs. Pressure 36 Fig.23. E f f e c t of M i l l i n g Time 37 Fig.24. Leaching Rate vs. M i l l i n g Time 37 Fig.25. E f f e c t of Sodium Carbonate Concentration 39 Fig.26. E f f e c t of Recycling 39 Fig.27. E f f e c t of Pulp Density 41 Fig.28. E f f e c t of Prior Roast 41 Fig.29. E f f e c t of P r i o r F l o t a t i o n 43 Fig.30. E f f e c t of Sodium Sulphate 43 THE LEACHING OF URANIUM FROM PITCHBLMDE ORES BY AQUEOUS OXIDATION TECHNIQUES Introduction General T r a d i t i o n a l methods f o r e x t r a c t i n g uranium from i t s ores involve smelting or leaching with an acid such as n i t r i c , hydrochloric, or sulphuric. U n t i l recently the primary consideration has been the recovery of the radium which normally occurs and separates with the uranium. The cost of these methods of treatment has generally been such that only f a i r l y high grade ores or concentrates could be economically treated. The increased importance of and demand f o r uranium, coupled with the discovery of considerable deposits of low grade ores have c a l l e d f o r the development of more e f f i c i e n t and more economical methods f o r e x t r a c t i n g uranium from such ores. Preliminary work i n these laboratories and i n the labor a t o r i e s of the Bureau of Mines, Ottawa, suggested that the most favourable p o s s i b i l i t i e s i n t h i s d i r e c t i o n are offered by a basic leach 1 2 using a l k a l i n e carbonate leach s o l u t i o n s . Among the advantages of t h i s leach are the foll o w i n g : (a) The a l k a l i n e carbonate s o l u t i o n i s a f a i r l y s p e c i f i c solvent f o r uranium. Most other metals ( e s p e c i a l l y i r o n , which i s commonly present) are not dissolved and a f a i r l y clean leach s o l u t i o n i s obtained. (b) The reagent ( i . e . sodium carbonate) i s r e l a t i v e l y cheap and the leach solutions are non-corrosive, permitting the use of inexpensive equipment. (c) Calcium carbonate, which commonly occurs i n a s s o c i a t i o n with ( uranium, does not react with the a l k a l i n e leaching agent, whereas the presence of calcium carbonate causes unduly high reagent consumption i f an acid leach i s used. (d) The uranium can be r e a d i l y and completely p r e c i p i t a t e d from the leach solutions by one of several known methods, lea v i n g the solutions i n a condition suitable f o r r e c y c l i n g . H i s t o r i c a l The sodium carbonate leach has long been recognized as s u i t a b l e f o r the recovery of uranium (and vanadium) from carnotite (K2O.UO3.V2O5.3H2O) ores. The f i r s t mention of the carbonate leach occurs i n U.S. Patent No. 1,165,692, awarded to R.B. Moore. 1 This patent describes a process f o r the recovery of vanadium from carnotite by a sodium carbonate - caustic leach. The caustic prevents the uranium from d i s s o l v i n g i n the carbonate 1. Moore, R.B., 'Extracting Vanadium, Uranium, and Radium from Ore', U.S. Patents No. 1,165,692 and 1,165,693, Chemical Abstracts, Volume 10, 1916, page 561. 3 solution, thus effecting a uranium-vanadium separation. Uranium is subsequently recovered by the usual hydrochloric or nitric acid leach. H.L. Gibbs2 (U.S. Patent No. 1,999,807) proposes the use of an oxidizing agent such as H202 in the carbonate - caustic leach. A further method for the treatment of carnotite ores (K.B. Thews and F.J. Heinle)̂  involves boiling the ore with a solution of sodium carbonate in an autoclave. Sodium uranyl carbonate is subsequently precipitated by evaporation. Mellor^ describes several methods for extracting uranium which involve the fusion of the ores with sodium carbonate or sulphate, or a mixture of sodium carbonate and nitrate. The uranium is subsequently extracted with hot water. Liddell^ describes a method for the treatment of pitchblende ores which involves fusion with sodium sulphate followed by a water leach to remove soluble salts, and a dilute sulphuric acid leach to extract the uranium sulphate. 2. Gibbs, H.L. 'Recovery of Values from Carnotite Ores', U.S. Patent No. 1,999,807, April 30, 1945, Chemical Abstracts Vol. 29, 1935, p.3916. 3. Thews, K.B., and Heinle, F.J., 'Extraction and Recovery of Ra, V, and U from Carnotite', Ind.Eng.Chem. 15, 1159-61 (Nov. 1923). 4. Mellor, J.W., 'A Comprehensive Treatise on Inorganic and Theoretical Chemistry1, Longmans (1932) Vol. XII. 5. Liddell, D.M., 'Handbook of Non Ferrous Metallurgy; Recovery of the Metals', McGraw-Hill, 1945, 2nd edition. 4 In general, the methods of extracting uranium have y i e l d e d poor recoveries unless expensive n i t r i c or hydrochloric acids were used as leaching agents. In the past the alkaline-carbonate leach has been r e s t r i c t e d almost e n t i r e l y to carnotite or other ores i n which the uranium i s completely oxidized. Application of the Sodium Carbonate Leach to Pitchblende Ores Although uranium exhibits nearly a l l the valence states ranging from 0 to 8, i n i t s most stable compounds i t generally has a valence of 4 or 6 - corresponding to the oxides U0 2 and U O 3 . A c t u a l l y uranium forms about four d i f f e r e n t oxides as w e l l as a continuous single phase series ranging i n composition from U 20 5 to U0 3. In i t s most common ore, generally known as pitchblende, uranium occurs as an oxide corresponding c l o s e l y i n composition to the formula U 30g (sometimes considered (U0 2.2U0 3) ). This oxide f a l l s within the l i m i t s of composition of the si n g l e phase serie s noted above. In s o l u t i o n s i m i l a r valence properties are exhibited, the valences of 4 and 6 being stable and valences of 3, 5, and 8 being unstable. The tetrovalent compounds are inso l u b l e i n basic ( i . e . ca u s t i c , carbonate, or ammonia) so l u t i o n s . The hexavalent compounds of uranium are soluble i n carbonate solutions but are p r e c i p i t a t e d from ammonia or caustic solutions as insoluble uranates. In carnotite ores the uranium i s already present i n the hexavalent state, and i s therefore r e a d i l y dissolved by sodium carbonate s o l u t i o n s . This explains why the use of the sodium carbonate leach has generally been r e s t r i c t e d to these ores. In pitchblende ores, however, as noted above, the uranium i s 5 incompletely oxidized. It should s t i l l be possible to t r e a t these ores by the sodium carbonate leach, providing an o x i d i z i n g agent i s present, to convert the uranium completely to the hexavalent state. The Bureau of Mines, Ottawa, and Eldorado Mining and Refining (1944) Limited, Port Hope, have conducted a series of studies which have shown that successful leaching of uranium from pitchblende ores can be achieved with o x i d i z i n g agents such as potassium permanganate. The high cost of such reagents, however, discourages t h e i r use on low grade ores, p a r t i c u l a r l y when other oxidizable constituents such as sulphides are present. It therefore appeared that the use of gaseous oxygen or a i r , as the o x i d i z i n g agent, would be d i s t i n c t l y advantageous. Recent Research on Aqueous Oxidation at U.B.C. The oxidation of ce r t a i n sulphide minerals i n aqueous solutions 6 7 under oxygen pressure has been studied by R. Carter, W.K.A. Congreve,' R.B. Mcintosh,** J.F. Stenhouse,^ and J.E. Andersen.^ These i n v e s t i g a t o r s found that i n every case i t was possible to oxidize the sulphide minerals with gaseous oxygen under pressure. 6. Carter, R., 'Influence of Roasting Temperature on Gold Recovery from a Refractory Gold Ore 1, M.A.Sc. Thesis, University of B r i t i s h Columbia, 1949. 7. Congreve, W.K.A., 'Use of High Pressure Oxygen i n Ex t r a c t i o n Metallurgy; Report to the Research Committee', U n i v e r s i t y of B r i t i s h Columbia, 1949. 8. Mcintosh, R.B., 'Recovery of Cobalt from Taylor Gem Ore by Aqueous Oxidation,' M.A.Sc. Thesis, University of B r i t i s h Columbia, 1950. 9. Stenhouse, J.F., 'Humid Oxidation of P y r i t e ' , M.A.Sc. Thesis, University of B r i t i s h Columbia, 1950. 10. Anderson, J.E., 'Aqueous Oxidation of Galena 1, M.A.Sc. Thesis, University of B r i t i s h Columbia, 1951. 6 Two of the above investigators studied the fundamental o mechanisms involved i n the oxidation process. Stenhouse 7 studied the oxidation of p y r i t e by measuring the oxygen consumption. A continuously recording pressure gauge was attached to a small c y l i n d e r which supplied the oxygen f o r the r e a c t i o n . A rocking autoclave was used as a reaction v e s s e l . Andersen studied the oxidation of galena i n caustic solutions by measuring the lead concentration i n s o l u t i o n with a polarograph. The p)Atinum electrodes of the polarograph were placed w i t h i n the autoclave so that the lead concentration of the s o l u t i o n could be measured without removing any of the material. A s o f t i r o n a g i t a t o r was rotated by an external a l n i c o magnet. No studies had been made on the aqueous oxidation of incompletely oxidized oxides. However, the success of these techniques on sulphide minerals prompted an i n v e s t i g a t i o n of s i m i l a r methods i n the oxidation of pitchblende ores i n the presence of a sodium carbonate s o l u t i o n . The Chemistry of the Oxygen - Sodium Carbonate Leach U 30g i n the presence of oxygen dis s o l v e s i n sodium carbonate solutions as the complex ion U0 2(C03) 3 . The r e a c t i o n may be written as follows: U 30 8 + V2 °2 - 3 U0 3 I U0 3 + H 20 + 3 C 0 3 ~ -* U 0 2 ( C 0 3 ) 3 + 20H" II Hydroxyl ion i s produced as a r e s u l t of the s o l u t i o n of uranium. In the presence of excess sodium hydroxide, uranium may p r e c i p i t a t e from the s o l u t i o n as sodium uranate, i n accordance with the f o l l o w i n g reaction: 7 U 0 2 ( C 0 3 ) 3 + 40H- + 2 Na + ^=7 Na aUO» + 3 C 0 3 " + 2H 20 I I I insoluble Combining I, I I , and I I I we get 2U 30 8 + 0 2 + 9C0 3 + 6Na + 3 Na3U0*. + 3U0a(C0.Oa i n s o l u b l e soluble According to t h i s , only h a l f the uranium oxidized appears i n s o l u t i o n . It i s l i k e l y that t h i s state of a f f a i r s i s approached i f the s o l u t i o n of s u f f i c i e n t l y large q u a n t i t i e s of U 30g i n pure carbonate solutions i s attempted. However, i n view of the f a c t that the equilibrium i n equation I I I does not l i e completely to the r i g h t , i t i s l i k e l y that small amounts of U 30g can be dissolved. In the presence of excess bicarbonate t h i s problem i s averted, with the following r e a c t i o n taking place: U 3 O 8 + + 3 C 0 3 — + 6 HC0 3" — 3U0 2(C0 3) 3 + 3H 20 V In e f f e c t the hydroxyl ion that i s formed by the s o l u t i o n of uranium i s neutralized by the bicarbonate. From t h i s equation i t can be seen that the minimum reagent requirements f o r the s o l u t i o n of 1 g.p.l. U 30g are: 0.02 g.p.l. 0 2; 0.38 g.p.l. Na 2C0 3; 0.60 g.p.l. NaHC03. Side Reactions Most pitchblende ores contain materials which react with water to form acids. The most common examples are sulphides which oxidize t o form sulphuric acid. This r e s u l t s i n the conversion of a c e r t a i n amount of the carbonate to bicarbonate. The following r e a c t i o n occurs with p y r i t e : 2FeS 2 + 7 ^ + 8 C O 3 — + 7H20 — 2Fe(0H) 3 + 4 S 0 4 ~ + 8HC0 3" VI Only 0.114 gms. of sulphur per l i t r e i s necessary to provide s u f f i c i e n t bicarbonate i n a pure sodium carbonate s o l u t i o n f o r the complete 8 s o l u t i o n of one gram U 30g (equation V) to take, place. An a d d i t i o n a l 0.21 g.p.l. oxygen i s necessary to oxidize t h i s equivalent of p y r i t e , and 0.47 g.p.l. of sodium sulphate i s formed. In addition s i l i c a may dissolve as the s i l i c a t e i on S i 0 3 — , thereby converting some a d d i t i o n a l carbonate to bicarbonate. The reaction i s as follows: Si0 2 + H 20 + 2C0 3~ -* S i 0 3 ~ + 2HC03" VII On the other hand, basic oxides such as CaO and MgO ( l i k e l y t o be present i n roasted ores) react with carbonate solutions to form excess hydroxyl ions, as follows: CaO + H 20 + C0 3~ — CaC03 + 20H~ VIII Where t h i s occurs s u f f i c i e n t bicarbonate should be present to n e u t r a l i z e the hydroxyl ion formed. Objectives of the Present Research Preliminary aqueous oxidation - carbonate leaching t e s t s performed i n t h i s laboratory showed that uranium could be leached success- f u l l y by the same techniques as;were developed f o r the oxidation of 11 sulphides. The main objective of t h i s research was to study the chemical and p h y s i c a l variables involved i n t h i s leach. To achieve t h i s objective, two ser i e s of experiments were made: A. An i n v e s t i g a t i o n of the k i n e t i c s of the oxidation and s o l u t i o n of uranium using high grade specimens of pitchblende with measured surface areas. In t h i s way i t was hoped to c a l c u l a t e absolute r e a c t i o n 11. Halpern, J . , 'Uranium Ore Treatment Research Project; Progress Reports No. 1 and 2', Department of Mining and Metallurgy, U n i v e r s i t y of B r i t i s h Columbia, 1950-51. 9 rates, and to gain an i n s i g h t i n t o the mechanism of the leaching r e a c t i o n . B. An i n v e s t i g a t i o n of the leaching c h a r a c t e r i s t i c s of a t y p i c a l low grade pitchblende ore, and the e f f e c t of varying conditions on the leaching r a t e . I t was hoped that a fundamental understanding of the ore leaching process could be obtained by comparing these r e s u l t s with the information gained from the studies on pitchblende specimens. 10 Materials Ores Two ores of s i m i l a r o r i g i n , but of d i f f e r e n t grades, were used i n the leaching studies. The analyses are as follows: Ore U„Qs Fe S I n s o l ( S i 0 9 ) As P a 0 s C0 P AS-1 0.18 4.0 0.59 86.5 0.01 0.10 5.0 AS-2 0.41 3.9 0.50 85.9 0.01 0.10 6.0 In ad d i t i o n i t i s l i k e l y that traces of lead, radium, and other metals usually associated with uranium are present. Minera l o g i c a l Description The AS ores contain uranium as f i n e p a r t i c l e s of pitchblende which follow a f i n e network of f i s s u r e s i n the main body of the ore. Microscopic examination ( f i g . l ) revealed a t y p i c a l laminated b o t r y o i d a l form present i n f i s s u r e s ranging from 10 microns to 0.5 mm. i n thickness. An X-ray d i f f r a c t i o n pattern confirmed the uraninite structure, a face centered cubic l a t t i c e with a u n i t c e l l dimension of 5.49 angstroms. C a l c i t e appears to be the only mineral present i n the ore which i s d i r e c t l y associated with the pitchblende. P y r i t e , ilmenite, and magnetite are a l l present i n the l a r g e r f i s s u r e s as d i s t i n c t c r y s t a l s having no apparent as s o c i a t i o n with the pitchblende. The dominant gangue mineral i s a very hard red quartz (jasper) of d i s t i n c t l y primary o r i g i n . The ores were supplied by Eldorado and were crushed, sampled, and m i l l e d i n t h i s laboratory. 1 1 F i g u r e 1 B o t r y o i d a l P i t c h b l e n d e o c c u p y i n g f i s s u r e s i n AS-2 o r e . X75 12 Pitchblende Specimens The pitchblende samples used were hand-picked specimens of Great Bear Lake ore, also supplied by Eldorado. They were chosen f o r t h e i r high grade and corresponded f a i r l y uniformly to the following composition: U-sOa Insol (SiQ 2) S p e c i f i c Gravity 62.5^ 19.4$ 5.95 Pitchblende i s the name given to an intimate but heterogeneous mixture of the black uranium mineral uraninite (U 30g) and various impurities, mostly s i l i c a . The specimens used, which were the best a v a i l a b l e , appeared homogeneous i n the f r e s h l y fractured or ground state, but a f t e r leaching showed areas high i n s i l i c a , due to d i f f e r e n t i a l etching during the leach. The lack of homogeneity was r e f l e c t e d i n the f a i l u r e to achieve perfect r e p r o d u c i b i l i t y i n the measured leaching r a t e s . Reagents Sodium carbonate and other reagents used were commercial C P . products. D i s t i l l e d water was used i n a l l experiments. 13 Equipment Two autoclaves were used i n the leaching and oxidation tests. The f i r s t (designated as No. 1 autoclave) consists of a two gallon stainless steel, vertically stirred reaction vessel, heated by an internal steam c o i l . The outward appearance i s shown i n f i g . 2. The heating c o i l , a thermometer well, and a sampling tube, i n addition to the propellor type agitator, formed the substructure of the removable autoclave top. An oxygen inlet tube, equipped with a pressure gauge, was attached to one side, about 1^2 inches from the top. . A plug at the bottom provided a simple method for removing the charge. The superstructure of the autoclave top, clearly visible i n the photograph, consisted of a packed pressure gland, through which the s t i r r e r shaft passed, a pulley at the top end of the s t i r r e r shaft, an end bearing for the st i r r e r shaft, and a sampling valve. The end bearing was water cooled, and provided cooling water to the main gland by means of a small copper tube extending through the hollow st i r r e r shaft. The autoclave was designed for pressures up to 2 0 0 p.s.i., well in excess of the maximum pressure of 100 p.s.i. used during the course of the research. The second autoclave (designated as autoclave No. 2 and shown in f i g . 3) was of the same size and of similar design as the f i r s t . The substructures of the top (including the c o i l , sampling tube, thermometer well, and thermal regulator well) were constructed more compactly, so that they would f i t inside a two l i t r e glass liner. The two l i t r e liner was held firmly i n place by a tripod of two fiberboard rings. 14 F i g u r e 2 No. 1 A u t o c l a v e , r e a d y f o r a r u n . F i g u r e 3 No. 2 A u t o c l a v e . D e t a i l s o f t h e s u b s t r u c t u r e are v i s i b l e . The v a r i a c , t r a n s f o r m e r , and mercury s w i t c h are i n t h e background. 15 Since i t was desired to use d i f f e r e n t temperatures i n t h i s autoclave, steam (which was a v a i l a b l e only at 212°F) was not used f o r heating. Instead, heat v/as provided by a 2500 watt - 21 v o l t bare wire r e s i s t o r , placed between the two l i t r e glass l i n e r and the autoclave bottom. The heater was insulated from the autoclave, and surrounded with water when i n use. One end was connected to an i n s u l a t e d copper plug i n the bottom of the autoclave, while the other was grounded to the autoclave. Power was supplied to the heating c o i l by a 3KVA transformer having a 220 v o l t input and a 21 v o l t output r a t i n g . The input voltage was c o n t r o l l e d by a 220 volt v a r i a c . A mercury thermoregulator, used f o r temperature c o n t r o l i n some of the e a r l i e r runs, was abandoned because of i t s f r a g i l i t y and i n e f f i c i e n t operation. It was found that better temperature c o n t r o l could be achieved by simply s e l e c t i n g a suitable voltage s e t t i n g on the v a r i a c . 16 E x p e r i m e n t a l P r o c e d u r e s P i t c h b l e n d e S p e c i m e n s A l l t h e w o r k p e r f o r m e d o n t h e p i t c h b l e n d e s p e c i m e n s was done i n n o . 2 a u t o c l a v e . U s u a l l y two s p e c i m e n s w e r e w i r e d t o t h e i n t e r n a l c o i l o f t h e a u t o c l a v e and immersed i n 1250 m i s . o f l e a c h s o l u t i o n . The p i t c h b l e n d e s p e c i m e n s w e r e p r e p a r e d i n e a c h ca se b y g r i n d i n g c l e a n f l a t s u r f a c e s o n a f i n e emery w h e e l . S p e c i m e n s w e r e r e - u s e d a f t e r r e - g r i n d i n g t h e s u r f a c e s . The s u r f a c e a r e a was m e a s u r e d i n a f e w c a s e s , and l a t e r c a l c u l a t e d , a f t e r i t was e s t a b l i s h e d t h a t t h e a r e a was p r o p o r t i o n a l t o t h e 2/3 power o f t h e w e i g h t . The a u t o c l a v e was c l o s e d and h e a t e d t o t h e d e s i r e d t e m p e r a t u r e a f t e r w h i c h a d e s i r e d p r e s s u r e o f o x y g e n was i n t r o d u c e d and m a i n t a i n e d . Sample s o f t h e l e a c h s o l u t i o n w e r e w i t h d r a w n f r o m t h e a u t o c l a v e a t m e a s u r e d t i m e i n t e r v a l s f o r u r a n i u m a n a l y s e s and c a r b o n a t e t i t r a t i o n s . I n most c a s e s t h e s p e c i m e n s w e r e w e i g h e d b e f o r e a n d a f t e r t h e r u n t o o b t a i n a m a t e r i a l b a l a n c e c h e c k . The s u c c e s s i v e i n c r e m e n t s i n t h e u r a n i u m c o n c e n t r a t i o n o f t h e l e a c h s o l u t i o n s a m p l e s were c o r r e c t e d f o r t h e change i n s o l u t i o n vo lume due t o s a m p l i n g . The method o f c a l c u l a t i o n i s shown i n A p p e n d i x C . Ores B o t h a u t o c l a v e s were u s e d i n t h e o r e l e a c h i n g s t u d i e s . The t e m p e r a t u r e and p r e s s u r e v a r i a b l e s were s t u d i e d i n n o . 2 a u t o c l a v e , and a l l t h e o t h e r v a r i a b l e s i n n o . 1 a u t o c l a v e . I n n o . 2 a u t o c l a v e a c h a r g e o f 500 grams o r e and 1000 m i s . l e a c h s o l u t i o n was u s e d . The o r e was p r e v i o u s l y m i l l e d f o r one h o u r i n 17 a rod m i l l . Samples were withdrawn in the same way as i n the study of the pitchblende specimens. Since an amount of pulp proportional to the leach solution was withdrawn in this case, no correction of .solution analyses due to the volume change was necessary. Where d i s t i l l a t i o n of water from the inside to the outside of the liner occurred, a correction for the volume change was necessary, and could be calculated from carbonate titrations and uranium analyses of the samples. The method of calculation i s illustrated by the second sample calculation i n Appendix C. The procedure used i n No. 1 autoclave was similar except that a charge of 1500 grams ore and 3 or 4/2 l i t r e s of leach solution was used. Analytical Methods Leach solutions were analyzed by a carbonate - peroxide method 12 similar to those described by Rodden. A five ml. sample of leach solution was diluted to 100 mis. after the addition of five mis. 30$ hydrogen peroxide. The optical density of the resulting solution was determined i n a Beckman model DV spectrophotometer at 370 millimicrons. D i s t i l l e d water was used as a standard. The exact procedure was altered from time to time to suit the particular conditions of individual leaches. The optical densities of standard solutions were determined i n most cases to obtain a calibration for each procedure. Typical calibration curves are shown in f i g . 4. 12. Rodden, C.J., •Analytical Chemistry of the Manhatten Project', Nuclear Energy Series, Manhatten Project Technical Section, Division VIII Vol. 1, 1st ed., McGraw-Hill, 1951. 18 O -10 -20 -SO U30, CONTENT GRAMS PER LITER F i g u r e 4 T y p i c a l c a l i b r a t i o n c u r v e s showing the r e l a t i o n between Beckman s p e c t r o p h o t o m e t e r r e a d i n g s and t h e uranium c o n t e n t o f the samples. 19 Ore head samples and residues were analyzed f o r uranium by 13 the c e l l u l o s e column method. In t h i s method elements that i n t e r f e r e i n the caustic - peroxide spectrophotometry determination are adsorbed from van ether n i t r a t e s o l u t i o n by a c e l l u l o s e column. The change i n carbonate and bicarbonate concentration during the leach was determined by t i t r a t i n g samples of the leach so l u t i o n with 0.1 N. HC1. The end point (pH = 80) f o r the conversion of C 0 3 " to HC0 3" and t h a t (pH = 4) f o r the conversion of HC0 3~ to C0 2 were determined using phenolphthalein and methylorange, re s p e c t i v e l y , as i n d i c a t o r s . In l a t e r experiments more accurate determinations of the end points were made with a Beckman t i t r i m e t e r . 13. Rabbitts, F.T., et a l , 'The Determination of U 30g i n Ores and Solutions; Cellulose Column Method', Mines Branch, Department of Mines and Technical Surveys, Canada, Memo. No. 105. 20 Results A. Leaching of Pitchblende Specimens 1. Reproducibility In the work on the pitchblende specimens, a number of runs were repeated at various times to ascertain the reproducibility of the leaching rates. The following effects were noted: (a) Reproducibility for successive leaches on the same specimens was good. Maximum errors i n rate measurements were probably no greater than ± 5%. (b) Reproducibility for widely separated leaches on the same specimens was relatively poor due to a tendency for the rate to decrease with re-use of the specimen. In general, in a series of f i v e runs, the last would be about 15$ slower than the f i r s t under identical condit ions. In one case a decrease in rate of 35$ was noted i n two reproducibility runs separated by a series of eight runs. In general, successive leaches on one pair of specimens were used i n the study of each variable. The results obtained in this way were sufficiently reproducible for meaningful interpretation. The reproducibility within a given series was checked by repeating the f i r s t run after completion of the series. The change in rate was generally much smaller than the changes caused by the variable which i t was desired to study in the series. 2. Proportionality to Surface Area Since the pitchblende specimens decreased i n size with successive leaches, to obtain comparable rates i t was necessary to ascertain that the 21 leaching rate was d i r e c t l y proportional to surface area. This would be the case i n a true heterogeneous reaction. Two successive leaches were performed, the f i r s t using two specimens, and the second using only one of these two specimens. The t o t a l leaching rate i s pl o t t e d against the measured surface area i n f i g . 5. The p l o t shows that t h e leaching rate i s proportional to the surface area. 3. E f f e c t of Sodium Bicarbonate Concentration Since the chemistry of the sodium carbonate leach i n d i c a t e s that the presence of bicarbonate i n the leach s o l u t i o n i s necessary f o r the oxidized uranium to be dissolved, a study was made of the e f f e c t of t h i s reagent. F i g . 6 shows the leaching curves at f i v e percent sodium carbonate and d i f f e r e n t concentrations of sodium bicarbonate. F i g . 7 shows the change of rate with increasing bicarbonate concentrations. The following e f f e c t s of bicarbonate concentrations are noted: (a) . When one percent sodium bicarbonate was added to a f i v e percent sodium carbonate leach s o l u t i o n , the leaching rate more than doubled. (b) The surface of the specimens was l e f t a grey colour a f t e r being subjected to a leach with some bicarbonate present. When pure carbonate leach solutions were used, the surface was l e f t a brown colour, presumably due to the formation of some in s o l u b l e sodium uranate (see equation I I I page 7). F i g s . 8 and 9 show the types of surface etches produced on the pitchblende specimens with a pure carbonate, and a carbonate bicarbonate leach. (c) A more nearly l i n e a r leaching rate i s produced when bicarbonate i s present i n the leach s o l u t i o n . In a serie s of 22 F i g u r e 5 The e f f e c t of s u r f a c e a r e a on t h e t o t a l l e a c h i n g r a t e . C o n d i t i o n s : 100°C; 60 p s i g . oxygen p r e s s u r e ; 5<?o N a 2 C 0 3 ; Ifo NaEC0 3. 23 I O I I— I I 1 L _ O 30 60 90 I20 ISO TIME - MINUTES F i g u r e 6 The e f f e c t of sodium b i c a r b o n a t e on the l e a c h i n g o f uranium from p i t c h b l e n d e specimens. C o n d i t i o n s : 100 ° C ; 60 p s i g . oxygen p r e s s u r e ; 5 $ NacCOg. 0 1 •10 ( ) •08 I s ( * Ul <0 •06 - 9: & •OJ 5 1 0 02 C c > / - • 1 2 PERCENT r- 1 3 Nai -/CC h t... -i r F i g u r e 7 L e a c h i n g R a t e v s . sodium b i c a r b o n a t e c o n c e n t r a t i o n . F i g u r e 8 The appearance o f the s u r f a o e o f a p i t c h b l e n d e specimen a f t e r an o x i d i z i n g l e a c h w i t h 5% sodium c a r b o n a t e s o l u t i o n . T75 F i g u r e 9 The appearance o f t h e s u r f a o e of a p i t c h b l e n d e specimen a f t e r an o x i d i z i n g l e a c h i n a s o l u t i o n c o n t a i n i n g 5$ sodium c a r b o n a t e and 5$ sodium b i c a r b o n a t e . 25 f i v e runs made with pure carbonate s o l u t i o n s , two of the runs showed a rapid slowing down i n the leaching r a t e as the leach proceeded, while the other three were much more nearly l i n e a r . Carbonate t i t r a t i o n s revealed that the solutions i n the former two runs contained a s l i g h t excess of caustic over the carbonate • content i n the leach solutions. In the l a t t e r three runs which showed more l i n e a r rates, a s l i g h t excess of bicarbonate was present. F i g . 10 c l e a r l y reveals t h i s e f f e c t , confirming the i behavior anticipated e a r l i e r (see page 7) i n the discussion of the chemistry of the leach process. r (d) The rate of leaching increased as the sodium bicarbonate concentration was increased from one to f i v e percent, but showed a tendency to l e v e l o f f with f u r t h e r increase i n bicarbonate concentration. 'J; 4.v E f f e c t of Temperature The effect' of temperature on the leaching r a t e was studied at three d i f f e r e n t concentrations of sodium bicarbonate, as shown i n f i g s . 10, 11, and 12. F i g . 13 shows the Arrhenius plot f o r each of these s e r i e s . The r a t e of leaching increased with temperature. A c t i v a t i o n energies of 9 to 12"^; k i l o c a l o r i e s per gram mole are calculated from the Arrhenius p l o t s . For subsequent c a l c u l a t i o n s , the l a s t s e r i e s , with f i v e percent bicarbonate, w i l l be considered the most accurate f o r the f o l l o w i n g reasons: (a) The f i r s t s e r i e s , without bicarbonate, does not have l i n e a r leaching r a t e s , and shows evidence of incomplete s o l u t i o n of the oxidized products. In t h i s case i t appears that an equilibrium reaction i s involved.- A u n i d i r e c t i o n a l reaction i s 26 F i g u r e 10 The e f f e c t o f t e m p e r a t u r e on t h e l e a c h i n g o f ur a n i u m from p i t c h b l e n d e specimens. C o n d i t i o n s : 60 p s i g . oxygen p r e s s u r e ; 5fo N a g C 0 3 . F i g u r e 11 The e f f e c t o f temperature on t h e l e a c h i n g o f uranium from p i t c h b l e n d e specimens. C o n d i t i o n s : 60 p s i g . oxygen p r e s s u r e ; 5$ N a 2 C 0 3 ; l | NaHC0 3. 27 /o / / A > .c I > <* ier \, / M_ £ u o Q w ?/ X i 5 g r A _ • ... < i_r • 1 3 .90 120 H ~IME -MINU7 o • M 1 40 — —t F i g u r e 12 The e f f e c t of temperature on the l e a c h i n g of uranium from p i t c h b l e n d e specimens. C o n d i t i o n s : 60 p s i g . oxygen p r e s s u r e ; 5% Na 2C0 3; 5% NaHC0 3. F i g u r e 13 A r r h e n i u s p l o t s o f the t e m p e r a t u r e e f f e c t a t t h r e e b i c a r b o n a t e c o n c e n t r a t i o n s . 28 desired i n thfe c a l c u l a t i o n of absolute r e a c t i o n rates. (b) The second series, with one percent sodium bicarbonate s o l u t i o n , consists of only three runs. The general reproduci- b i l i t y of these runs was too poor to permit use of a s e r i e s of only three runs f o r conclusive i n t e r p r e t a t i o n . (c) The t h i r d s e r i e s , with f i v e percent bicarbonate, consists of f i v e runs, in c l u d i n g one to t e s t the r e p r o d u c i b i l i t y of the s e r i e s . A f a i r l y good Arrhenius p l o t gives an a c t i v a t i o n energy of 12,300 c a l o r i e s per gram mole, with probable l i m i t s of e r r o r of t 1,000 c a l o r i e s . Subsequent calc u l a t i o n s are based on t h i s s e r i e s . 5. E f f e c t of Oxygen Pressure The e f f e c t of oxygen pressure was studied using a s o l u t i o n containing f i v e percent sodium carbonate and f i v e percent sodium b i c a r - bonate. The rate curves obtained f o r runs at d i f f e r e n t pressures are shown i n f i g . 14. In f i g . 15 the r a t e i s p l o t t e d as a f u n c t i o n of oxygen pressure, and also as a f u n c t i o n of the square root of the oxygen pressure. The l a t t e r plot c l e a r l y i ndicates that the r a t e i s proportional to the square root of the oxygen pressure, i n d i c a t i n g that the oxygen p a r t i c i p a t i n g i n the reaction i s dissociated. 14, 15 The same e f f e c t was noted by previous i n v e s t i g a t o r s i n the aqueous oxidation of sulphide minerals. 14. Andersen, J.A., o p . c i t . 15. Stenhouse, J.F., o p . c i t . 29 zo / ? £ / / V PRESSURE -y ' P-26 P-27 o - 4 ( P-2B IO • -0 • P-23 oo -+ £ 1 u t X j P-30 60 — X TR A  TR A  ( ,* Et \to \ ° 0 H f j *: l n | J% i" 9 /<= C o Jl - o & to 7"/ Arf MINUTEi F i g u r e 14 The e f f e c t o f p r e s s u r e on t h e l e a c h i n g o f uranium from p i t c h b l e n d e specimens. C o n d i t i o n s : 1 0 0 ° C ; 5% N a 2 C 0 3 ; 5% NaHC0 3. OXYGEN PRESSURE - PS.I. F i g u r e 15 Rate v e r s u s oxygen p r e s s u r e and r a t e v e r s u s square r o o t o f oxygen p r e s s u r e . 30 6. E f f e c t of Sodium Carbonate Concentration Two ser i e s of experiments were made t o determine the e f f e c t of varying the sodium carbonate concentration. The sodium bicarbonate was kept constant at f i v e percent, and the carbonate v a r i e d between one and f i v e percent. The corresponding duplicate runs were averaged, and the leaching curves obtained i n t h i s way are shown i n f i g . 16. The f o l l o w i n g e f f e c t s were noted: (a) Only small increases i n leaching rates occurred with increased carbonate concentrations. (b) At low carbonate concentrations, under the temperature conditions e x i s t i n g i n the autoclave, some of the bicarbonate was converted to carbonate by e x p e l l i n g C0 2. The r e a c t i o n i s as follows: 2HC03- C0 2t + H 20 + C 0 3 — This i s probably a thermal e f f e c t . 7. E f f e c t of Sodium Sulphate Concentration The e f f e c t of a n e u t r a l s a l t , sodium sulphate, was studied i n a s e r i e s of two runs, to i n d i c a t e whether or not a s a l t e f f e c t was present. F i g . 17 shows the extraction curves of both runs. No change i n the leaching rate was noted. 8. E f f e c t of Ag i t a t i o n One run was attempted without a g i t a t i o n , except f o r three minutes of a g i t a t i o n p r i o r to the taking of each sample at 30 minute i n t e r v a l s . The leaching rate was about h a l f the normal rate with f u l l a g i t a t i o n . Apparently a g i t a t i o n has a strong e f f e c t , which may be c r i t i c a l under c e r t a i n conditions. The high a c t i v a t i o n energy revealed i n the temperature study indicates that the standard a g i t a t i o n conditions 31 20 15 - PUN % P-/7->P~34- P-/8*P33 P-/S->P-30 IV. 5% » - o — — o a . S u ^ * P- ' K I a.: . ^ ' •JO t : 0 _ i _ 30 1 60 90 120 - MINUTES _ l F i g u r e 16 The e f f e c t of sodium c a r b o n a t e c o n c e n t r a t i o n on the l e a c h i n g o f uranium from p i t c h b l e n d e specimens. C o n d i t i o n s : 100°C; 60 p s i g . oxygen p r e s s u r e ; 5% NaHCO,,. 10 P35 No Na.SC i o P36 /OY. Na.SO. -x- « o c | 30 _!_ 60 TIME | 90 /20 MINUTES L 150 F i g u r e 17 The e f f e c t o f sodium s u l p h a t e on the l e a c h i n g o f uranium from p i t c h b l e n d e specimens. C o n d i t i o n s : 100°C; 60 p s i g . oxygen p r e s s u r e ; 5% Na 2C03; Vfo NaHC03. 32 of t h i s research were not c r i t i c a l . B. Leaching of Ores 1. R e p r o d u c i b i l i t y In the work on the ores, the r e p r o d u c i b i l i t y of extraction rates was p e r f e c t l y s a t i s f a c t o r y on each ore. However, since two grades of ore were used, comparable leaching rates were necessary i f a l l the e f f e c t s studied could be applied to both ores. F i g . 18 shows that the leaching rates are i n fac t very close, p a r t i c u l a r l y i n the range of 30 to 70 percent extraction, the i n t e r v a l chosen as a measure of the rate. 2. E f f e c t of Temperature A serie s of s i x runs was made at f i v e d i f f e r e n t temperatures on AS-2 ore. The e x t r a c t i o n curves are shown i n f i g . 19, and an Arrhenius p l o t , which corresponds to an a c t i v a t i o n energy of 9,700 c a l o r i e s per gram mole, i n f i g . 20. For comparative purposes, Arrhenius plots are also shown f o r two other pitchblende ores studied i n t h i s laboratory. These are Great Bear Lake (Hutch) concentrate (4.72$ U 30g) and B-C ore (0.12$ U 30g). The two ores were found t o have s i g n i f i c a n t l y higher a c t i v a t i o n energies than the AS-2 ore. The reason for t h i s apparent discrepancy i s not clear, but may be that the accuracy of the a c t i v a t i o n energies deter- mined f o r G.B.L. and B-C ores i s poor. In the case of B-C ore the uranium concentration was too low to permit accurate determination by the methods used. The G.B.L. (Hutch) concentrate was very high i n sulphides which were oxidized during the leach. Some copper was also found to dissolve i n the leach solutions. I t i s possible that these f a c t o r s a f f e c t e d the rates of uranium leaching, or i n t e r f e r e d with the accurate determination of these r a t e s . 33 c too j 7 5 %. ! '•50 5 ORE o A -s Dec _X 1 25 1 i o SC /SO T/Mir I MJA///TJTC F i g u r e 18 Comparison o f l e a c h i n g c u r v e s o f AS-1 and AS-2 o r e s . C o n d i t i o n s : 100°C; 30 p s i g . oxygen p r e s s u r e ; 5% N a g C 0 3 34 F i g u r e 19 The e f f e c t o f temperature on t h e l e a c h i n g of AS-2 o r e . C o n d i t i o n s : 30 p s i g . oxygen p r e s s u r e ; 5$ NagCOg. O \ c B-L • HUTCH , I ' A -S2 • C. ORE n & r " n M • ] _ C ) \ R A T E  in  V : O > R A T E  in  • , i l l ; V -2L ) - • • i 1 1 1 V 2-5 2 6 2 7 2 s 2 9 SO x/03 F i g u r e 20 A r r h e n i u s p l o t s o f the temperature e f f e c t on t h e l e a c h i n g r a t e o f t h r e e o r e s . 35 It should be noted that small errors i n the rate determination may lead to large errors i n the estimation of the a c t i v a t i o n energy. In general, a greater accuracy i s indicated f o r the A-S ores than f o r the other ores studied. 3. E f f e c t of Oxygen Pressure The e f f e c t of oxygen pressure on AS-2 ore was studied i n a s e r i e s of four runs. The e x t r a c t i o n curves are shown i n f i g . 21. The extraction rate i s plotted against the oxygen pressure, and against the square root of the oxygen pressure, i n f i g . 22. The rate i s seen to be d i r e c t l y proportional t o the square root of the oxygen pressure. The same r e l a t i o n was observed i n the leaching of the pitchblende. 4. E f f e c t of M i l l i n g Time The e f f e c t of m i l l i n g time was studied on AS-1 ore i n a s e r i e s of f i v e runs. The extraction curves are shown i n f i g . 23. The leaching rate as a function of m i l l i n g time i s shown i n f i g . 24. Screen analyses f o r each m i l l i n g time are given i n Appendix F. It i s a w e l l known mineral dressing p r i n c i p l e that i n the m i l l i n g of coarse homogeneous p a r t i c l e s , the t o t a l surface area of the p a r t i c l e s increases i n d i r e c t proportion to m i l l i n g time. I f the pitchblende i s d i s t r i b u t e d throughout the ore i n f a i r l y coarse, homogeneous p a r t i c l e s , the t o t a l surface area of the pitchblende would then increase i n proportion to the m i l l i n g time. F i g . 24, which consists of a l i n e a r plot of leaching rate against m i l l i n g time, thus implies that leaching rate i s proportional t o the surface area of the pitchblende p a r t i c l e s i n the ore. (& s i m i l a r r e l a t i o n was observed i n the case of the pitchblende specimens (page 22, f i g . 5). 36 O 30 60 90 120 /50 /SO 2/0 240 TIME - MINUTES F i g u r e E l The e f f e c t of oxygen p r e s s u r e on the l e a c h i n g o f AS-2 o r e . C o n d i t i o n s : 100 bC; 5% N a 2 C 0 3 . OK YCE/V PRESSURL P S .1. i I y | SO • /5 k 3 IO s •\- f <a • 1K •75 m * § ? | . in * -5" K kj A IP > (a G . -r y ^ ay O 1 o 2-5 SQUARE / ?007 OF 5 OXYGEN PRESS yffE 75 F i g u r e £2 L e a c h i n g r a t e v e r s u s oxygen p r e s s u r e and r a t e v e r s u s square r o o t o f oxygen p r e s s u r e . 37 /oo RUN MILLING TIME UL-46 90 MIN- — 75 MIN — 60 M/N — 45 MIA/ — 30 MIN — UL-50 UL5/ UL-52 UL-53 90 /20 TIME - M/NUTES F i g u r e 23 The e f f e o t of m i l l i n g time on t h e l e a c h i n g o f AS-1 o r e . C o n d i t i o n s : 100°C, 30 p s i g . oxygen p r e s s u r e . O 1 1 1 1 I I I 15 30 45 60 75 90 i MILLING TIME - MINUTES F i g u r e 24 L e a c h i n g r a t e v e r s u s m i l l i n g t i m e . 38 5. E f f e c t of Sodium Carbonate Concentration The e f f e c t of sodium carbonate concentration was studied i n a ser i e s of f i v e runs on AS-1 ore. No other reagent was i n i t i a l l y present i n the leach s o l u t i o n . The extraction curves f o r t h i s s e r i e s i n which the sodium carbonate concentration was varied between one and f i v e percent are shown i n f i g . 25. The following e f f e c t s are noted: (a) The leaching rate increases s i g n i f i c a n t l y with increase / i n sodium carbonate concentration i n the range of low concen- t r a t i o n s ( l to 3%). T h i s , i s not s u r p r i s i n g since the uranium i s insoluble i n the absence of carbonate. (b) Only small increase i n leaching rate, s i m i l a r to that noted i n the work on pitchblende specimens (page 31, f i g . 16), i s obtained by increasing the sodium carbonate concentration further, say up to f i v e percent. The i n d i c a t i o n i s that an optimum concentration i s reached and that f u r t h e r increase i n the carbonate concentration has no e f f e c t on the rate. 6. E f f e c t of Recycling the Leach Solution The e f f e c t of r e c y c l i n g the leach solutions was studied i n a s e r i e s of experiments i n which AS-1 leach solution was recycled four times, a fresh l o t of ore being leached each time. The e x t r a c t i o n curves f o r t h i s s e r i e s are shown i n f i g . 26. There does not appear to be any s i g n i f i c a n t change i n rate as a r e s u l t of r e c y c l i n g the leach s o l u t i o n . Minor variations i n rate may be due to increasing bicarbonate concentration as the s o l u t i o n i s recycled. 7. E f f e c t of Pulp Density The e f f e c t of pulp density was studied i n a series of three runs. 39 O 30 60 90 /20 /SO TIME - MINUTES F i g u r e £5 The e f f e c t o f sodium oarbonate c o n c e n t r a t i o n on the l e a c h i n g o f AS-1 o r e . C o n d i t i o n s : 100°C; 30 p s i g . oxygen p r e s s u r e . F i g u r e 26 The e f f e c t o f r e c y c l i n g l e a c h s o l u t i o n on the l e a c h i n g of AS-1 o r e . C o n d i t i o n s : 100°C, 30 p s i g . oxygen p r e s s u r e ; 5fo NagCOg. \ 40 The extraction curves are shown i n f i g . 27. The rate of leaching appears to be s l i g h t l y f a s t e r at the higher pulp density. The f o l l o w i n g reasons are suggested: (a) At the higher pulp density a higher concentration of sulphides i s present. The oxidation of these sulphides r e s u l t s i n the production of a greater amount of bicarbonate with a subsequent increase i n the leaching rate. (b) At higher pulp d e n s i t i e s a g i t a t i o n conditions may be more favourable. Ho work was done on the a g i t a t i o n v a r i a b l e . However, a g i t a t i o n i s known to have an e f f e c t on the leaching rate. 8. E f f e c t of P r i o r Roast One batch of AS-2 ore was roasted two hours at 700°C p r i o r to leaching. The r e s u l t i n g leaching curve i s shown, together with a comparable leaching curve f o r an unroasted ore i n f i g . 28. The leaching rate of the roasted ore was s u b s t a n t i a l l y slower than that of the unroasted ore. The following reasons are suggested: (a) The decomposition of c a l c i t e (CaCOs) known to be present i n the ore resulted i n the production of excess hydroxide (equation VIII, page 8), thus l i m i t i n g the s o l u b i l i t y of uranium. The leaching rate increased somewhat when s u f f i c i e n t sulphuric acid and sodium bicarbonate were added to n e u t r a l i z e the hydroxide formed, but the rate was s t i l l w e l l below that f o r an unroasted ore. (b) The pitchblende reacted with the c l o s e l y associated c a l c i t e during the roast to form insoluble calcium uranate or 41 TIME - MINUTES F i g u r e £7 The e f f e c t of p u l p d e n s i t y on t h e l e e c h i n g of AS-1 o r e . C o n d i t i o n s : 100°C; 30 p s i g . oxygen p r e s s u r e ; 5% NagCOg. /OO 0 U/V/?OAST£ D 75 k k SO <* c t i J 25 20/n/s '/OOgms NaHCO, a.<id.ec I 0 C 1 j /OO 200 1 300 J • 400 1IML MINUTES F i g u r e £8 The e f f e c t o f a p r i o r r o a s t on t h e l e a c h i n g o f AS-£ o r e . R o a s t i n g temperature = 700°C. C o n d i t i o n s o f l e a c h : 100°C; 30 p s i g . oxygen p r e s s u r e ; 5$ N a 2 C 0 3 . 42 diuranate according to the following equation: U 30 8 + 3CaC03 + \oz — 3CaU04 + 3C0 2 t Reaction with i r o n to form i n s o l u b l e f e r r a t e s or with s i l i c a to form i n s o l u b l e s i l i c a t e s i s also p o s s i b l e . 9. E f f e c t of P r i o r Flotation of Sulphides Most of the sulphides were removed from one sample of AS-1 ore by f l o t a t i o n . A f l o t a t i o n concentrate weighing 23.7 grams and having a uranium analysis of 0.50$ U 30 8 accounted f o r 4.4$ of the uranium i n the o r i g i n a l 1500 gm. sample of ore. The extraction curve i s compared with that of an untreated sample i n f i g . 29. The small d i f f e r e n c e between the two extraction curves i s probably within the l i m i t s of r e p r o d u c i b i l i t y . 10. E f f e c t of Sodium Sulphate Concentration One sample of AS-1 ore was leached with t e n percent sodium sulphate added.to the normal leach s o l u t i o n . The r e s u l t i n g leaching curve i s shown i n f i g . 30. As i n the case of the pitchblende specimens (page 30) no difference i s noted. U3 /oo TIME - MINUTES F i g u r e 29 The e f f e c t o f p r i o r f l o t a t i o n o f s u l p h i d e on t h e l e a c h i n g o f AS-1 o r e . C o n d i t i o n s : 100°C; 30 p s i g . oxygen p r e s s u r e ; 5% Na 2C03. f I /OO j \ 75 - r I i 50 L >L -66 No Na iSOf — o h ^ 1 UL 76 _ /Of. Na SO, — X t ) 25 i 0 1 J O 30 6( TlMr • ? i 120 vii MI/-rrc /50 F i g u r e 30 The e f f e c t o f sodium s u l p h a t e on the l e a c h i n g o f AS-1 o r e . C o n d i t i o n s : 100°C; 30 p s i g . oxygen p r e s s u r e ; 5% NagCOg. 44 Discussion of Results Summary of Variables In the studies on the leaching of pitchblende specimens, and ores, the following f a c t o r s are shown to increase the rate of leaching markedly: (1) Increasing temperature. (2) Increasing oxygen pressure. (3) The presence of some carbonate i n i t i a l l y , i n the leach s o l u t i o n . (4) The presence of some bicarbonate i n i t i a l l y , i n the leach s o l u t i o n . (5) Increased m i l l i n g time (of ores). The following f a c t o r s strongly reduced the leaching r a t e : (1) Excess hydroxyl i o n i n the leach s o l u t i o n s . (2) Roasting of the ores. The following f a c t o r s had only a small e f f e c t on the extraction rates: (1) Increasing bicarbonate concentrations beyond one to two percent. (2) Increasing carbonate concentrations beyond two percent. (3) Oxidation of sulphides, presence of NagSO^, r e c y c l i n g and other chemical factors i n f l u e n c i n g the concentration of carbonate and bicarbonate by chemical reactions. The following two fa c t o r s were not studied conclusively: (1) A g i t a t i o n . (2) E l e c t r i c a l P o t e n t i a l . 45 Each of the f a c t o r s l i s t e d above may influence the leaching reaction i n one of two ways: (a) By l i m i t i n g the extent t o which oxidation or s o l u t i o n of the uranium can occur ( i . e . by a f f e c t i n g the equilibrium). (b) By infl u e n c i n g the rate of the leaching reactions. An examination of the chemistry involved i n the leaching process (pages 6, .7 and 8) reveals that c e r t a i n minimum concentrations of oxygen, carbonate, and bicarbonate, are necessary to oxi d i z e and di s s o l v e a given amount of uranium. In addition, p r e c i p i t a t i n g agents such as hydroxide must be absent. I f these conditions are met, then the leaching reaction proceeds u n i d i r e c t i o n a l l y ( i . e . without r e - p r e c i p i t a t i o n of the uranium or the se t t i n g up of an equilibrium) and any changes i n temperature, pressure, or concentration of reagents, influence only the k i n e t i c s of the re a c t i o n . Studies on the k i n e t i c s of the reaction can thus be used to derive a mechanism' ftor the rea c t i o n by comparing the measured rates with those c a l c u l a t e d from fundamental p r i n c i p l e s . Numerical Values of Reaction Rates The average absolute reaction rates measured from the pitchblende specimens at 100°C and 60 p . s . i . g . oxygen pressure are given i n the 16 following table f o r three bicarbonate concentrations. 16. The f i r s t column consists of averages from Appendix A. 46 Na 2C0 3 NaHC03 Ratg Rate percent percent mgms/cm. /min. molecules/cm. 2/sec. 5 0 .031 1.11 x 1 0 1 5 5 1 .080 2.86 x 1 0 1 5 5 5 .132 4.72 x 1 0 1 5 Comparison of the Absolute Leaching Rates of D i f f e r e n t Ores The absolute reaction rates f o r the extraction of uranium from the ores, leached with f i v e percent sodium carbonate, have been estimated on the basis of three assumptions: (a) The uranium occurs as pitchblende p a r t i c l e s of about the same grade as the pitchblende specimens. (b) The average s i z e of the pitchblende p a r t i c l e s i s about 400 mesh, or 37 microns, a f t e r 50$ of the uranium has been extracted from the ores. (c) The p a r t i c l e s are assumed t o be cubic i n shape. 17 The measured rates, and the estimated absolute r e a c t i o n rates f o r three ores, on the basis of these assumptions, are: 17 19 Ore Measured Rate, Estimated Rate Estimated Rate percent per minute mgms/cm.2/sec. molecules/cm. 2/sec. (at 30 psig) (at 30 psig) (at 60 psig) AS-2 .646 .030 1.5 x 1 0 1 5 G.B.L.(Hutch) .717 1 8 .033 1.7 x 1 0 1 5 B-C .96 1 8 .044 2.2 x 1 0 1 5 17. Measured rate for AS-2 ore i s an average takenVrom Appendix B. 18. Peters, E., »The Leaching of Uranium from Pitchblende Ores; Progress Report No.3*, Department of Mining and Metallurgy, University of B r i t i s h Columbia, 1951. 19. F i n a l estimated rates are based on a pressure of 60 p . s . i . g . f o r bett e r comparison with the pitchblende specimens. The square root r e l a t i o n - ship between the leaching rate and the pressure was applied. 47 The absolute rates f o r the three ores are seen to be very s i m i l a r . In a d d i t i o n they compare very c l o s e l y with the absolute rates measured f o r the pitchblende specimens. The i n d i c a t i o n i s thus very strong that these pitchblende ores have s i m i l a r leaching c h a r a c t e r i s t i c s . Magnitude of Errors i n Reaction Rate Estimates The assumptions made i n the above absolute r e a c t i o n rates are j u s t i f i a b l e only i n the absence of better methods f o r estimating surface areas. The following errors are;.involved: 1. The measured surface area of the pitchblende specimens i s based on t h e i r macroscopic measurements. Chemical methods of measuring absolute surface areas of polished surfaces ind i c a t e that the true surface area may be several times as large as the measured area i n such cases. 2. The assumption that the pitchblende p a r t i c l e s i n the ore are of the same grade and density as the pitchblende specimens was made i n the absence of any evidence on t h i s point. The error involved would not a f f e c t the estimated rates by more than 50$. 3. The assumption that the average size of the pitchblende p a r t i c l e s i s 400 mesh a f t e r 50$ uranium e x t r a c t i o n i s an estimate based on the screen analyses which show most of the ore to be -325 mesh. At 50$ extraction most of the very f i n e p a r t i c l e s w i l l have disappeared. An error not greater than a f a c t o r of two may be involved i n t h i s estimate. This error would be d i f f e r e n t f o r each ore, since the screen analyses vary. 4. The assumption that the surface area of small p a r t i c l e s i s equal to the surface area of cubes of the same mesh s i z e , i s i n error. 48 20 Gaudin reports that i r r e g u l a r f i n e p a r t i c l e s may have a surface area (measured by absolute chemical methods) from 1.3 to 2.0 times that of cubes of the same screen s i z e . In general, the i n d i c a t e d absolute r e a c t i o n rates are probably correct to w i t h i n a f a c t o r of f i v e , and the r e l a t i v e rates probably w i t h i n a f a c t o r of two. The accuracy i s considered adequate f o r purposes of comparison of the rates with each other and with c a l c u l a t e d rates. A c t i v a t i o n Energies Temperature studies on the pitchblende specimens and three ores produced a wide range of experimental a c t i v a t i o n energies. The a c t i v a t i o n energy term of the rate equation i s therefore subject to a wide magnitude of e r r o r . The following i s a summary of the numerical values of the a c t i v a t i o n energies and the a c t i v a t i o n energy terms: ( l ) Pitchblende Specimens Na?C0.^ NaHC03 A c t i v a t i o n Energy A c t i v a t i o n Energy percent percent ( c a l o r i e s per gm.mole) Rate Term at 100°C £- E/RT 5 0 11,400 2.1 x l O " 7 5 1 9,150 4.3 x l O " 6 5 5 12,300 6.3 x 10- 8 (2) Ores Ores B-C 14,600 2.8 x 10-9 G.B.L. (Hutch) 16,000 4.4 x l o A Q AS-2 9,700 . 2.1 x l O " 6 20. Gaudin, A.M., ''Principles of Mineral Dressing', 1st ed. McGraw-Hill, 1939. 49 I t can be seen from the above table that the leaching rates are very s e n s i t i v e to changes i n the a c t i v a t i o n energy. Since the leaching rates f o r a l l the ores and the pitchblende specimens studied agreed so cl o s e l y , i t i s considered very u n l i k e l y that the a c t i v a t i o n energies do, i n f a c t , vary over such a wide range of values. I t i s more l i k e l y that some of the a c t i v a t i o n energy values determined are i n considerable e r r o r . (An a c t i v a t i o n energy, based on two rate measurements 30°C apart, and each subject to 10% error, w i l l r e s u l t i n a possible e r r o r of 4000 c a l o r i e s i n the r e s u l t i n g a c t i v a t i o n energy.) The value of 12,300 c a l o r i e s per mole i s considered the most accurately determined of a l l the values, f o r reasons already given. I t also agrees with the weighted average of a l l the values given i n the t a b l e . It has therefore been chosen as the basis of subsequent c a l c u l a t i o n s . Reaction Mechanisms 21 According to Eyring, a reaction at a surface may be separated i n t o f i v e steps, the slowest of which w i l l determine the o v e r a l l r a t e . The steps are: (a) Transport of reactants to the surface. (b) Adsorption of the reactants on the surface. (c) Reaction on the surface.. (d) Desorption of the reaction products from the surface. (e) Transport of the products away from the surface. In the a l k a l i n e - carbonate leach of pitchblende, the o v e r a l l 21. Glasstone, S., L a i d l e r , K., and Eyring, H., 'The Theory of Rate Processes*, McGraw-Hill, New York and London, 1941. 50 leach r e a c t i o n has been given as u30g + \qz + 3 C 0 3 ~ + 6HCO3- -* 3U0 2(C0 3)3 + 3H20 In the l i g h t of the above steps, t h i s r e a c t i o n can be written i n the fol l o w i n g steps: 1. \pz (gas) -*• ̂ fcOz (solution) (solution of oxygen) 2. V2g2 (solution) -»• ̂ 0 2 (near surface) (diffusion through solution) 3. U3O* + %>02 3U03 (adsorption - reaction) 4. U0 3 + H 20 -* U0 2(0H) 2 (hydration - desorption) 5. U0 2(0H) 2 + CO3— + 2HCO3- — U0 2(C0 3) 3-- + 2H 20 (complexing of uranium as a homogeneous re a c t i o n i n solution 6. U0 2(C0 3 )3 (near surface) U 0 2 ( C 0 3 ) 3 ( d i f f u s i o n ) The slowest step i n t h i s s e r i e s w i l l determine the rate of the o v e r a l l r e a c t i o n . Influence of Agitation In the above s e r i e s , steps 1, 2, and 6 are influenced by a g i t a t i o n i n the following way: 1. Increased a g i t a t i o n increases the t o t a l area of the g a s - l i q u i d i n t e r f a c e . Step 1, which i s a heterogeneous reaction at t h i s i n t e r f a c e , would increase i n rate p r o p o r t i o n a l l y with the i n t e r f a c e area. 2. The t o t a l distance necessary f o r reactants and products to d i f f u s e would decrease with increased a g i t a t i o n . The d r i v i n g force f o r d i f f u s i o n , represented by the concentration gradient, would be correspondingly greater, with a r e s u l t i n g increase i n the rate. Although one t e s t was performed on the e f f e c t of a g i t a t i o n (page 30) the following features indicate that none of the steps affected by 51 a g i t a t i o n represents the c o n t r o l l i n g mechanism under the conditions described i n t h i s research: 1. A single t e s t with a g i t a t i o n present only ten percent of the time showed a reaction rate nearly h a l f as f a s t as t e s t s i n which f u l l time a g i t a t i o n was employed. 2. D i f f u s i o n and s o l u t i o n mechanisms have much lower a c t i v a t i o n 22 energies than any of those observed. Eyring i n d i c a t e s that aqueous d i f f u s i o n mechanisms have a c t i v a t i o n energies no greater, than 5,000 c a l o r i e s per mole. In conclusion, the evidence indicates that none of steps 1, 2, and 6 are the c o n t r o l l i n g mechanism under the conditions described i n t h i s research. Determination of the C o n t r o l l i n g Step Of the three remaining steps, the f i f t h can also be eliminated as .the rate c o n t r o l l i n g step, due to i t s homogeneous nature. Conclusive evidence of a heterogeneous c o n t r o l l i n g step i s given by the proportio- n a l i t y of the reaction rate to surface area. The remaining two steps are: 1. The adsorption, or oxidation step, expressed by the equation: u30g + "^0 2-*3U0 3 (Mechanism I) 2. The hydration, or desorption step, expressed by the equation: . U 30 8 + H 20-> U0 2(0H) 2 (Mechanism II;) 22. Glasstone, S., et a l , i b i d . 52 Mechanism I Since i n the oxidation step only tetravalent uranium atoms a c t u a l l y take part, the equation of the step can be written U0 2 + ^ 0 2 — U0 3 The unit process involves the r e a c t i o n of d i s s o c i a t e d ( i . e . atomic) oxygen as i n d i c a t e d by the p r o p o r t i o n a l i t y of the experimental rate to the square root of the oxygen pressure. The rate equation f o r t h i s step i s given as rate(molecules/cm. 2/sec.) = 3 C Q C U O . KT . . e - H * / R T 2 2 h f 0 2 . % > 2 where C Q ^ i s the concentration of oxygen Cyo^is the concentration of uranium atoms on the surface K i s Boltzman's constant T i s the absolute temperature h i s Planck's constant f * i s the p a r t i t i o n f u n c t i o n of the a c t i v a t e d complex f n i s the p a r t i t i o n f unction of oxygen fyo^is the p a r t i t i o n f unction of surface U0 2 molecules e i s the base of n a t u r a l logarithms H* i s the enthalphy of a c t i v a t i o n R i s the gas constant The concentration of tetravalent uranium atoms i s equal to the concentration of a l l uranium atoms on the surface, since any uranium atom i s capable of r e a c t i n g . The f a c t o r of three takes into account the f a c t that three atoms are a c t u a l l y d i s s o l v e d f o r each atom oxidized. 53 The numerical value of the reaction r a t e i s calculated from t h i s equation, using an a c t i v a t i o n energy of 12,300 c a l o r i e s per gram mole, i n Appendix D. The calculated t h e o r e t i c a l r e a c t i o n rate f o r t h i s mechanism corresponds to 4 x 1 0 ^ molecules/cm. 2/sec., a f a c t o r of eight f a s t e r than the measured rate on pitchblende specimens i n a f i v e percent carbonate, f i v e percent bicarbonate leach s o l u t i o n . Mechanism I I The desorption step has been expressed by the equation U0 3 + H 20 — U0 2(0H) 2 The rate equation f o r t h i s reaction can be written as rate = C T t n ,C„ rt. KT . ft- . e " H * / R T U0 3'°H 20- £± h f U 0 3 * % 2 0 where C J J Q ^ i s the concentration of hexavalent uranium atoms on the surface ^H20 ^ s ^ e concentration °f water fU"03 the p a r t i t i o n function of hexavalent surface uranium molecules % 2 0 ^ s t n e P a r t , i t i o n function of l i q u i d water. The other symbols have the same s i g n i f i c a n c e as i n the rate equation of mechanism I. The concentrations and p a r t i t i o n functions are based on a standard state of 1 molecule per cm. or per cm. . The t h e o r e t i c a l value for the rate of t h i s mechanism at 1 0 0 ° C and an a c t i v a t i o n energy of 12,300 c a l o r i e s per gram mole i s c a l c u l a t e d 54 i n Appendix E. The calculated rate i s equal to 1.2 x 10 molecules per square centimeter per second. This i s a f a c t o r of 3000 slower than the measured rate on pitchblende specimens i n a f i v e percent carbonate, f i v e percent bicarbonate leach s o l u t i o n . Discussion of Mechanisms With an a c t i v a t i o n energy of 12,300 c a l o r i e s per "'gram mole, there i s a strong i n d i c a t i o n , from the c a l c u l a t i o n of t h e o r e t i c a l rates, that the oxidation step (Mechanism I) i s the c o n t r o l l i n g step i n t h i s reaction. No allowance has been made f o r the e f f e c t of widely scattered experimental a c t i v a t i o n energies, such as those l i s t e d previously. The f o l l o w i n g table shows the calculated rate from both mechanisms through a . range of a c t i v a t i o n energies covered by those obtained by experiment. A c t i v a t i o n Energy. Calculated Rate Calculated Rate Measured Rate,., ( c a l o r i e s per gram Mechanism I Mechanism I I (molecules/cm./sec. mole) (molecules/cm ./ (molecules/cm?/ sec.) sec.) 9,000 3.3 x 10 1 8 9.3 x 10 1 3 1.11 x 10 1 5 10,000 8.7 x 10 1 7 2.6 x 10 1 3 to 11,000 2.2 x 10 1 7 6.7 x 10 1 2 4.72 x 10 1 5 12,000 5.9 x 10 1 6 1.8 x 10 1 2 13,000 1.5 x 10 1 6 4.7 x 10 1 1 14,000 4.0 x 10 1 6 1.2 x 10 1 1 15,000 1.0 x 10 1 5 3.1 x 10 1 0 16,000 2.8 x l O 1 ^ 8.3 x 10 9 The above t a b l e shows that there i s better agreement with mechanism I f o r a l l a c t i v a t i o n energies above 10,000 c a l o r i e s per gram 55 mole. Good agreement between mechanism I I and the measured r e a c t i o n rate would be obtained i f the a c t i v a t i o n energy were about 7,000 c a l o r i e s per gram mole. 23 Andersen found that a desorption mechanism such as mechanism I was the c o n t r o l l i n g step i n the aqueous oxidation of lead i n galena. He found an experimental a c t i v a t i o n energy of 6,820 c a l o r i e s per gram mole i n t h i s case. In the leaching of pitchblende, an a c t i v a t i o n energy of the same order of magnitude would be expected f o r a s i m i l a r c o n t r o l l i n g mechanism. 2.L. In contrast t o both these mechanisms, Stenhouse found an a c t i v a t i o n energy of 1,840 c a l o r i e s per gram mole i n the oxidation of i r o n i n p y r i t e . He proposed a c o n t r o l l i n g mechanism invo l v i n g the d i f f u s i o n of oxygen through a layer of i r o n oxide - an in s o l u b l e product of the rea c t i o n . In the oxidation of pitchblende, i n s o l u b l e impurities such as lead and s i l i c a could b u i l d up on the reacting surface, t o the point where a s i m i l a r mechanism could play a r o l e . No evidence of t h i s type of e f f e c t was noticed. Approximations i n Theoretical Rate Calculations In a d d i t i o n t o the possible e r r o r involved i n the a c t i v a t i o n energy, c e r t a i n other approximations were made i n c a l c u l a t i n g the t h e o r e t i c a l r e a c t i o n rates: 23. Andersen, J.E., op. c i t . 24. Stenhouse, J.F., op. c i t . 56 (a) The p a r t i t i o n functions of s o l i d reactants were considered equal to unity. They could be calculated from the E i n s t e i n model f o r s p e c i f i c heat of s o l i d s , i f the s p e c i f i c heat equation f o r the s o l i d were known. In no case i s the p a r t i t i o n function of a s o l i d reactant at the normal b o i l i n g point l i k e l y to be greater than ten. (b) The p a r t i t i o n f u n c t i o n of the activated complex was considered equal to unity. I t i s l i k e l y t o be of the same order as the s o l i d reactant, and would therefore cancel most of the error involved i n (a). (c) The rate equation does not take into account the p a r t i c i p a t i o n of the leach reagents, which are known to have an e f f e c t on the leaching rate. E f f e c t of Leach Reagents In the steps outlined f o r the oxidation - leaching of uranium from pitchblende, only step f i v e involves the reagents. I f step four i s completely u n i d i r e c t i o n a l , as i t i s believed to be, changes i n the concentrations of the reagents would have no e f f e c t on the r a t e . If the indicated steps are correct, the reagents must have xome influence on a step p r i o r to the u n i d i r e c t i o n a l step four. This influence may not nec e s s a r i l y be caused by d i r e c t p a r t i c i p a t i o n of the reagents. Thermodynamically, the most stable surface any uranium oxide can present to atmospheric oxygen i s a U3O8 surface. Possibly the reagents contribute to the s t a b i l i t y of the U0 3 surface i n contact with the leach s o l u t i o n . t? 57 Change i n C o n t r o l l i n g Mechanism The c o n t r o l l i n g mechanism i n a reaction such as described herein may change from one step to another by the a l t e r i n g of the 25 p h y s i c a l or chemical conditions of the experiment. Stenhouse and 26 Andersen both found that the rate of oxidation dropped r a p i d l y when the caustic concentration was increased beyond a c e r t a i n maximum. 26 Andersen showed that at the higher caustic concentration the reaction rate was highly dependent on a g i t a t i o n , while at the lower caustic concentration, a g i t a t i o n played no role at a l l . He pointed out that the reduced oxygen s o l u b i l i t y i n higher concentrations of caustic, resulted i n a c o n t r o l l i n g step i n v o l v i n g the s o l u t i o n or d i f f u s i o n of 25 oxygen. Stenhouse ' suggested the same p o s s i b i l i t y , though he did not study the a g i t a t i o n v a r i a b l e . These aqueous oxidation reactions could be studied conclusively by adjusting the conditions so that the d i f f e r e n t possible c o n t r o l l i n g mechanisms are dealt with separately. The order of magnitude" of the a c t i v a t i o n energy i s sometimes s u f f i c i e n t to i n d i c a t e the type of mechanism involved. However, not enough work has been done to point t h i s out conclusively. 25. Stenhouse, J.F., i b i d . 26. Andersen, J.E., op. c i t . 58 Conclusions 1. Theoretical considerations i n d i c a t e that the c o n t r o l l i n g mechanism i n the a l k a l i n e carbonate leaching of uranium from p i t c h - blende ores under the conditions described herein i s the oxidation step. Most of the e f f e c t s on the rate of leaching noted i n t h i s study are explained by t h i s mechanism. 2. At an a c t i v a t i o n energy of 12,000 c a l o r i e s per gram mole ( approximately the value observed i n t h i s study), the leaching rate can be doubled by r a i s i n g the temperature 15 ° C The m i l l i n g time must be nearly doubled to achieve a s i m i l a r increase i n leaching rate. 3. The optimum reagent concentrations are about f i v e percent sodium carbonate and f i v e percent sodium bicarbonate. 4. Under conditions of very poor a g i t a t i o n , d i f f u s i o n may become the c o n t r o l l i n g mechanism, with a correspondingly lower a c t i v a t i o n energy. 5. The leaching rate varies i n proportion to the square root of the oxygen p a r t i a l pressure. 6. There i s no apparent d i f f e r e n c e i n leaching behavior of the low grade pitchblende ores, the high grade concentrate, or the p i t c h - blende specimens that were examined i n t h i s research. 59 Appendix A Experimental Leaching Rates from Pitchblende Specimens Run Variable Reagents Conditions Remarks Rate under NaHC03 Na 2C0 3 Temp, Press. mgms/cm.2 study Percent Percent °C. psig. per minute P-5 Temp. N i l 5 100 60 Standard .0294 P-6 11 11 11 110 68 - .0392 P-7 11 11 i» 121 75 - .0666 P-8 11 11 11 90 52 - .0194 P-9 11 i t 11 100 60 Reprodu- .0385 Temp.) NaHC03) c i b i l i t y P-10 1 5 100 60 — .0685 P-L2 11 1 5 85 50 - .048 P-13 11 1 5 115 70 - .125 P-14 NaHC03 3 5 100 60 - .102 P-15 i • 5 5 100 60 - .107 P-16 Na 2C0 3 5 N i l 100 60 Drove o f f C0 2 .083 P-17 11 5 1 100 60 Drove o f f C0 2 l e s s severely .064 P-18 11 5 3 100 60 - .122 P-19 None 1 5 100 60 Reprodu- c i b i l i t y .067 P-20 Area 1 5 100 60 - .0622 P-21 Temp. 5 5 100 60 Standard .165 P-22 11 5 5 84 52 - .056 P-23 11 5 5 70 50 - .031 P-24 11 5 5 115 70 - .235 P-25 t ? 5 5 100 60 Reprodu- .120 c i b i l i t y .116 P-26 Pressure 5 5 100 60 Standard P-27 11 5 5 100 30 - .0862 P-28 t» 5 5 100 10 - .0282 P-29 11 5 5 100 100 — .144 I 60 Appendix A (cont) Run Variable Rea eents Conditions Remarks Rate £ under NaHC03 Na 2C0 3 Temp Press. mgms/cm. study- Percent Percent °C. psig. per min. P-30 Pressure 5 5 100 60 Reprodu- .103 c i b i l i t y P-31 NaHC03 3 5 100 60 - .101 P-32 « t 1 5 100 60 - .074 P-33 Na 2C0 3 5 3 100 60 - .094 P-34 11 5 1 100 60 - .093 P-35 Na 2S0 4 5 1 100 60 - .090 P-36 5 1 100 60 10$ Na2S0,f .090 P-37 A g i t a t i o n 5 1 100 60 A g i t a t i o n .037 on 10$ of time 61 Appendix B Experimental Leaching Rates from A-S Ores Run Ore Na 2C0 3 Percent Cycle Temp. Press Remarks Rate Percent Solids °C. p s i g . Percent per min. UL-35 AS-2 5 25 1 100 30 Roasted 2 hrs.800°C. 1$ NaHC03 added. 0.067 UL-46 AS-1 5 25 1 100 30 M i l l e d 90 min. 1.18 UL-48 5 50 1 100 30 1:1 Pulp Density 1.01 UL-50 5 25 1 100 30 M i l l e d 75 min. 1.27 UL-51 ' 1 5 25 1 100 30 M i l l e d 60 min. 0.74 UL-52 5 25 1 100 30 M i l l e d 45 min. 0.55 UL-53 5 25 1 100 30 M i l l e d 30 . min. 0.33 UL-56 AS-2 5 25 1 100 30 M i l l e d 60 min. 0.67 UL-59 AS-1 5 33 1/3 2 100 30 Recycle 0.59 UL-62 11 5 33 1/3 3 100 30 Recycle 0.845 UL-63 11 5 33 1/3 4 100 30 Recycle 0.67 UL-66 11 5 33 1/3 1 100 30 Std. Run 0.50 UL-70 11 5 33 1/3 1 100 30 P r i o r f l o - t a t i o n of Py r i t e ' 0.76 UL-72 1? 4 33 1/3 1 100 30 (E f f e c t of) 0.46 UL-73 T t 3 33 1/3 1 100 30 (Na 2C0 3 ) 0.41 UL-74 t t 2 33 1/3 1 100 •' 30 11 0.37 UL-75 11 1 33 1/3 1 100 30 11 0.26 UL-76 » f 5 33 1/3 1 100 30 10$ Na 2S0 i f 0.55 Ul-109 AS-2 5 33 1/3 1 101 30 No.2 Auto- clave 0.645 UL-110 11 5 33 1/3 1 81 22 r t 0.288 UL-111 • t 5 33 1/3 1 61 18 11 0.149 UL-112 11 5 33 1/3 1 115 40 11 1.07 UL-113 11 5 33 1/3 1 100 30 1 t 0.698 UL-114 11 5 33 1/3 1 71 20 1 t 0.217 UL-115 ? t 5 33 1/3 1 100 1 t 0.448 . UL-116 1 • 5 33 1/3 1 100 l\ 1 t 0.326 UL-117 11 5 33 1/3 1 100 60 1 t 0.760 62 Appendix C Sample Calculations (a) Conversion of o r i g i n a l data to absolute units i n the extraction of uranium from pitchblende specimens: P-25 • Specimen weight = 14.6- grams. Wt. loss = 0.2838 gmsf^ -v 2 Surface Area = 14.5 fimV '. Sample Time D B Grams U30g Grams U30g Milligrams - per l i t r e T o t a l per cm.2 1 0 ..009 .009 > 0 .026 0 0 2 20 min. .019 .019 .033 2.3 3 40 .029 .028 . .053 .066 4.4 4; 60 .0:39 .037 f. .079 .099 ' 6.8 5 80 .053 .049 .114 .143 9.9 6' 100 ' .064 .059 .143 .179 12.3 7 120 .077 • ..069 .172'; r .201 .215 14.8 8 140 .090 1079 . 2 8 0 w 17.3 9 160 .101 .087 .224 • 19.3 Standard;, .058 = • .140 Standard .130 • .350 A .- Op t i c a l Density from Beckman Spectrophotometer. A B - D corrected f o r volume change due to sampling, (a) - Note material.balance check i n t h i s run. (b) Conversion of o r i g i n a l data to percent U30g extracted i n the leaching of A-S ores i n Autoclave number 2. UL-117.: Residue Analyses = .031$ U30g. F i n a l - e x t r a c t i o n = 92.2$. 63 Appendix C. (Contd.) Sample Time B Phenol. 1 c" M.0.° D i f f . D E Percent U 30g Extracted 1 0 .113 13.5 27.1 13.4 .106 30.2 2 5 .147 12.65 26.35 13.45 .137 39.0 3 10 .175 12.9 26.95 13.8 .159 45.3 4 20 .211 12.8 27.25 14.1 .188 53.5 5 30 .244 12.75 27.5 14.35 .214 61.0 6 45 .287 12.8 27.95 14.7 .245 69.8 7 60 .324 12.85 28.4 15.05 .271 77.2 8 90 .373 12.7 29.15 15.85 .296 84.3 9 120 .430 13.9 31.9 17.3 .312 88.9 10 150 .483 14.75 34.25 18.7 .324 92.2 A - O p t i c a l Density from Beckman Spectrophotometer. B - Phenolphthalein end point i n mis. f o r 3 ml. sample t i t r a t e d with 0.1 N. HC1. C - Methyl Orange end point f o r a 3 ml. sample t i t r a t e d with 0.1 N. HC1. D - Difference between phenolphthalein and methyl orange.end points, l e s s half the t o t a l calculated t i t r a t i o n f o r the carbonate t i e d up as the uranium complex ion U 0 2 ( C 0 3 ) 3 . This column would be a constant i n the absence of d i s t i l l a t i o n w i t h i n the auto- clave . E - O p t i c a l Density readings corrected f o r d i s t i l l a t i o n c a l c u l a t e d from D. Percent extraction i s based on the r e l a t i o n between the residue analysis and the o p t i c a l density of the f i n a l sample. 64 Appendix D Calculation of Rate from Mechanism I From the reaction U0 2 + 3j£0 2 U0 3 the rate equation can be written as 2' Ik -H*/RT rate(molecules/cmf/sec.) = 3 . C Q . C ^ Q . KT . f * . e 2 2 h H 1 f f where the symbols have the same sig n i f i c a n c e as on pages 52 and 53. At equilibrium, the concentration of oxygen i n s o l u t i o n i s , relat e d to the concentration i n the gas phase by the following equation: C 0 2 ( U q ) = f Q 2 ( l i q ) . e - H l / R T C Q (gas) f Q (gas) II where f represents the p a r t i t i o n f u n c t i o n of the material designated by the subscript, and represents the enthalphy of sol u t i o n of oxygen. We can now substitute f o r C Q 2 i n I, obtaining the following equation: rate = 3 C * C • ̂  - • I I I If t h i s i s the c o n t r o l l i n g mechanism, the experimental a c t i v a t i o n energy i s equal to (H]_ + H*)-RT. Subs t i t u t i o n , the rate 2~ becomes: At 100°C (373°K) the following substitutions may be made: rate - 3e-CQ C •J)r-pHr -e " i » Appendix D (continued) 65 1. e = 2.72 (base of natural logarithms). 2. ^U02" ^ a s P e c i m e n having a density of 6.0 at 62.5$ U 30 8 ; ( there are 6.02 x 1 0 2 3 x 6.0 x .625 = 2.68 x 1 0 2 1 molecules U 30 8 842 21 per cc, or 8.05 x 10 atoms uranium per cc. Since there i s probably no way to d i f f e r e n t i a t e between tet r a v a l e n t and hexavalent uranium atoms, a l l of them must be considered capable of r e a c t i n g . The separation of 100 planes i n the U 30g l a t t i c e i s 2.7 A°. The separation of 111 planes i s 3.12 A°. Assuming the c r y s t a l separates equally e a s i l y on these two planes, the average separation of planes represented by the surface i s 2.91 A". The number of planes per cm. 8 7 at t h i s separation i s 10 = 3.44 x 10' planes per cm. The number of 2.91 atoms uranium on 1 cm.2 of surface i s then 8.05 x 102"*" = 2.34 x l O ^ atoms/cm. . 3.44 x 10^ . * . C U Q 2 = 2.34 x 10 1 4. 3. ^Og^ 2 , •1'ne c o n c e n t r a t i o n of oxygen i n molecules per cc at 60 psig. and 100°C i s C A = 6.02 x 10 2 3 x 273 y 60 = 8.03 x 10 1 9 22400 373 14.7 ^ Q . % C Q 2 = 8.96 x 10y. 4. KT = 1.38 x 10" 1 6 x 373 = 7.76 x 10 1 2 (~1 ) h 6.62 x lO-X'f sec 5. f * = p a r t i t i o n function of the activated complex — 1 6. f = p a r t i t i o n function of the s o l i d reactant ~ 1 U0 2 7. f A . The p a r t i t i o n function of gaseous oxygen can be 2 3/2 s p l i t up i n the following way: fQ (translation) = (2frmKT) = 2.46 x 10 26 Appendix D (continued) 66 f n (vibration) — 1 u2 f n (rotation) = 8 TT^IKT = 177 (J 2 2" .*. f Q = (2.46 x 1 0 2 6 x 177) = 2.085 x l O 1 ^ . 8. e . This i s the a c t i v a t i o n energy term. At an a c t i v a t i o n energy of 12,300 c a l o r i e s per gram mole, t h i s term i s equal to 6.3 x 10~ 8. Su b s t i t u t i n g these values i n t o equation IV, we get rate = 3 x 2.72 x 8.96 x 10 9 x a.34 x 1 0 1 4 x 7.76 x 1 0 1 2 x 6.: x 10-* 2.085 x 101Lv = 6.37 x 10 3 x 6.3 x 10" 8 16 2 = 4 x 10 molecules U0 2 per cm. per second. Observed rate = 4.72 x 1 0 ^ molecules/cm. 2/sec. Factor of difference =8.5 The t h e o r e t i c a l r a t e i s somewhat f a s t e r than the observed rea c t i o n rate, but the agreement i s reasonable, i n view of the estimates involved i n the c a l c u l a t i o n s . 67 Appendix E Cal c u l a t i o n of the Rate from Mechanism I I From the hydration equation: U0 3 + H 20 -* U0 2(0H) 2 the rate equation can be written as: rate = C„ n . C T T n . KT . f * . e'11*^7 I H 2o U0 3 — — n 1 H 2 0 , 1 U 0 3 where the symbols have the same s i g n i f i c a n c e as on pages 52 and 53. The following reactions w i l l have reached e q u i l i b r i u m i f t h i s i s the c o n t r o l l i n g mechanism: (1) 0 2 gas -» 0 2 s o l u t i o n (2) U0 2 + \ o z - U0 3 From these e q u i l i b r i a , the fo l l o w i n g equations can be drawn: =  f 0 2 ( s o l n ) ^ e-H]_/RT n (soln) (gfe) f Q 2 ( g a s ) C - C ft f u ° 3 e-H 2/RT m • CU0 3 " CU0 2' ° 0 2 * =- * 6 1 1 1 (soln) & U0 2 0 2 soln where H-̂  i s the enthalpy of sol u t i o n of oxygen H 2 i s the enthalpy of oxidation of U0 2 i n s o l u t i o n . S u b s t i t u t i n g these values, the rate equation becomes: -(Hi+ H 2 + H*)/RT rate = C 0 J 8 « . C U 0 2 . C ^ Q . KT . f * . e T IV h _ _ 0 2- 1U0 2- 1H 20 In terms of the experimental a c t i v a t i o n energy, t h i s equation becomes: 1/2 A -E/RT rate - e . C ^ . C ^ . C ^ . • 6 V f 0 2 ' f U 0 2 , f H 2 0 Appendix E (continued) 68 Comparing t h i s equation with mechanism I, we f i n d : r a t e ( m e c h . I I ) " r a t e ( m e c h I ) • i • 5fe2 i 1 H 2 0 V i The f o l l o w i n g substitutions may be made: • ( l ) r a^ e(mech I) = ^ x m ° l e c u l e s / c n i . 2 / sec. W C H 2 0 = 6.02 x 1 0 2 3 = 3.34 x 1 0 2 2 . S ^ '"(3) f H Q = M H p O where S R Q i s the entropy of water i n a 2 t R standard state of 1 molecule per cm. . S H 2 Q = 121.55 2 7' 2 3 at 100°C (calculated from data) '. f . « „ 121.55 = q 7 v i n 2 D * ' H 2 0 £ ft j , ' x i U * The calculated rate i s therefore: 16 22 12 2 rate = 4 x 10 x 3.34 x 10 = 1.2 x 10 molecules/cm. /sec. 3 x 3.7 x 1 0 2 6 Observed Rate = 4.72 x 1 0 1 5 . 3 Factor of difference 4 x 10 . The t h e o r e t i c a l r a t e based on t h i s mechanism and an a c t i v a t i o n energy of 12,300 c a l o r i e s per gram mole i s about 4,000 times too slow, compared with the observed reaction rate. 27. Hougen, O.A., and Watson, K.M., Chemical Process P r i n c i p l e s , Part I I , Wiley and Sons, New York, 1947. 28. Kelley, K.K., Contributions to the Data on T h e o r e t i c a l Metallurgy, B u l l e t i n 476, U.S. Govt. P r i n t i n g O f f i c e , Washington, 1949. 69 Appendix F Screen Analyses of AS-1 Ore RUN NO. — UL-46 UL-50 UL-51 UL-52 UL-53 MILLING TIME •*+ SCREEN SIZE I 90 75 60 45 30 + 48 mesh - - - - 0.3$ . -48 + 65 " - - - - 2.0$ -65 +100 *• - - - 1.3$ 32.5$ -100 +150 " - trace 0.4$ 11.4$ 17.-4$ -150 +200 " trace 0.5$ 6.0$ 18.9$ 10.1$ -200 +270 " 1.0$ 6.1$ 15.4$ 13.8$ 6.5$ -270 +325 " 0.1$ 1.3$ 5.3$ 4.5$ 2.7$ -325 98.9$ 92.1$ 72.9$ 50.1$ 28.5$ TOTALS 100 $ 100 $ 100 $ 100 $ 100 $ MILLING CONDITIONS : Rod Charge = 60 l b s . Ore Charge = 1500 grams. L i q u i d Charge = 1 l i t r e water. M i l l Speed = 40 r.p.m. 70 Bibliography Gaudin, A.M. 'Princip l e s of Mineral Dressing', 1st ed., McGraw-Hill, 1939. Gibbs, H.L., 'Recovery of Values from Carnotite Ores', U.S. Patent 1,999,807, A p r i l 30, 1935, Chemical Abstracts 29, 3916, 1935. Glasstone, S., La i d l e r , K., and Eyring, H., 'The Theory of Rate Processes', McGraw-Hill, 1941. Halpern, J., 'The Chemistry of the Alkaline Carbonate Leach', GR-1 Memo. , Department of Mining and Metallurgy, U n i v e r s i t y of B r i t i s h Columbia, 1951. Halpern, J . , 'Uranium Ore Treatment Research Project; Progress Reports No. 1 and 2, Department of Mining and Metallurgy, University of B r i t i s h Columbia, 1950-51. L i d d e l l , D.M., 'Handbook of Non Ferrous Metallurgy; Recovery of the Metals', 2nd ed., McGraw-Hill, 1945. Mellor, J.W., 'A Comprehensive Treatise on Inorganic and Th e o r e t i c a l Chemistry, Vol. XII', Longmans, London, 1932. Moore, R.B., 'Extracting Vanadium, Uranium, and Radium from Ores', U.S. Patents 1,165,692 and 1, 165,693, Chemical Abstracts, 10; 561, 1916. Pauling, L., 'The Nature o f the Chemical Bond', C o r n e l l U n i v e r s i t y Press, Ithaca, New York, 1948. Rabbitts, F.T., Guest, R.J., Jordan, J.E., Kornelsen, E.D., Proula, E., La Chance, G.R., and Rice, W.B., 'The Determination of U 30 8 i n Ores and Solutions; C e l l u l o s e Column Method', Mines Branch, Department of Mines and Technical Surveys, Canada, Memo. 105. Rodden, C.J., ' A n a l y t i c a l Chemistry of the Manhattan Project', Nuclear Energy Series, Manhattan Project Technical Section, D i v i s i o n VIII Vol. 1, 1st ed., McGraw-Hill, 1951. Thews, K.B., and Heinle, F.J., 'Extraction and Recovery of Ra, V, and U from Carnotite', Ind.Eng.Chem. 15, 1159-61, November, 1923. Wells, A.F., ' S t r u c t u r a l Inorganic Chemistry', 2nd ed., Oxford U n i v e r s i t y Press, London, 1950.

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