UBC Theses and Dissertations

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UBC Theses and Dissertations

Study of the leaching of goethite and hematite. Roach, Gerald Ian Dunstan 1970

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A STUDY OF THE LEACHING OF GOETHITE AND HEMATITE by GERALD IAN DUNSTAN ROACH B.Met.(Hons). University of Sheffield, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF AEFB1EB SCIENCE i n the Department of Metallurgy We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA November, 1970. In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Co lumb i a , I a g ree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r agree tha p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f M e t a l l u r g y  The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada Date A p r i l 2 0 , 1971 1 ABSTRACT The d i r e c t d i s s o l u t i o n of v a r i o u s samples of he m a t i t e and g o e t h i t e i n h y d r o c h l o r i c , s u l p h u r i c and p e r c h l o r i c a c i d s u s i n g a p a r t i c u l a t e (-65, + 150 mesh) f e e d has been i n v e s t i g a t e d . The r e d u c t i v e d i s s o l u t i o n of h e m a t i t e samples u s i n g a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s was a l s o i n v e s t i g a t e d . G o e t h i t e and h e m a t i t e l e a c h e d by a common mechanism w i t h comparable r a t e s . The v a r i o u s shapes of l e a c h i n g curves o b t a i n e d f o r d i f f e r e n t h e m a t i t e samples can be c o r r e l a t e d w i t h t h e i r g r a i n s i z e t o p a r t i c l e s i z e r a t i o . An i n c r e a s i n g r a t e of d i s s o l u t i o n was found f o r a l a r g e number of g r a i n s per p a r t i c l e , and an a p p r o x i m a t e l y c o n s t a n t r a t e was observed f o r p a r t i c l e s c o n t a i n i n g r e l a t i v e l y few g r a i n s per p a r t i c l e . The e f f e c t of a c i d c o n c e n t r a t i o n on the r a t e of d i s s o l u t i o n f o r d i f f e r e n t a c i d s has l e d t o the modification of a p r e v i o u s l y proposed c h e m i c a l mechanism dependent on the complexing power of the anion of the a c i d w i t h i r o n . The i n c r e a s e i n d i s s o l u t i o n r a t e f o r a weak complexer was p r o -p o r t i o n a l t o the a c i d c o n c e n t r a t i o n e.g. HCIO^, and f o r a s t r o n g complexer, i f was p r o p o r t i o n a l t o the a c i d c o n c e n t r a t i o n squared e.g. HC1. The r a t e of d i s s o l u t i o n i n the presence of sulphur d i o x i d e was e x t r e m e l y r a p i d compared w i t h the a c i d d i s s o l u t i o n . i i ACKNOWLEDGEMENTS The aut h o r wishes t o express h i s thanks t o Dr. I.H. Warren f o r h i s c o n t i n u e d i n t e r e s t and p a t i e n c e throughout the p e r i o d of s t u d y . Thanks are a l s o extended t o members of f a c u l t y f o r h e l p f u l d i s c u s s i o n s , and t o the t e c h n i c a l s t a f f f o r t h e i r a s s i s t a n c e w i t h p r a c t i c a l a s p e c t s of the work. The author i s i n d e b t e d t o f e l l o w graduate s t u d e n t s f o r both t h e i r h e l p w i t h t h i s work and f o r t h e i r h o s p i t a l i t y d u r i n g the d u r a t i o n of t h i s work. F i n a n c i a l support from the Government of Canada i n the form of a N a t i o n a l Research C o u n c i l Research A s s i s t a n t -s h i p i s g r a t e f u l l y acknowledged and has been much a p p r e c i a t e d . i i i L I S T OF CONTENTS Page INTRODUCTION 1 I . GENERAL 1 I I . FERRIC OXIDES AND THE MONOHYDRATES 2 I I I . REVIEW" OF RELATED PREVIOUS WORK 3 a) D i r e c t A c i d D i s s o l u t i o n 3 b) R e d u c t i v e D i s s o l u t i o n 7 IV. SCOPE OF THE PRESENT INVESTIGATION 8 EXPERIMENTAL 10 I . SPECIMEN MATERIALS 10 ( i ) N a t u r a l M a t e r i a l s 10 ( i i ) S y n t h e t i c M a t e r i a l s 13 I I . X-RAY ANALYSIS lk ( i ) X-Ray D i f f r a c t i o n A n a l y s i s 1^ ( i i ) T r a n s m i s s i o n X-Ray A n a l y s i s 15 I I I . REAGENTS l 6 IV. SPECIMEN PREPARATION l 6 ( i ) P a r t i c u l a t e Feed l 6 ( i i ) P o l i s h e d S u r f a c e s 16 V. ANALYTICAL METHOD 17 V I . APPARATUS DESIGN 17 ( i ) The G l a s s Apparatus 17 ( i i ) The A u t o c l a v e 19 i v Page VI. EXPERIMENTAL PROCEDURE 21 ( i ) The Glas s Apparatus 21 ( i i ) The A u t o c l a v e 22 RESULTS 23 I . DIRECT DISSOLUTION OF IRON OXIDES 23 ( i ) P a r t i c u l a t e Feed 23 (a) H y d r o c h l o r i c A c i d 23 (b) S u l p h u r i c A c i d 37 (c) P e r c h l o r i c A c i d h i (d) Coarse P a r t i c u l a t e Feed U5 ( i i ) Powder Specimens ^5 ( i i i ) P o l i s h e d Specimens 1+5 ( i v ) M e t a l l o g r a p h y 50 (a) M assive Red Hematite 51 (b) B o t r y o i d a l Hematite 51 (c) B r a z i l i a n Hematite 5^ -(d) S y n t h e t i c Hematite 5h (e) G o e t h i t e 56 ( f ) Hematite S i n g l e C r y s t a l F l a k e s 60 I I . REDUCTIVE DISSOLUTION 63 ( i ) A c i d i f i e d S u l p h u r D i o x i d e S o l u t i o n s 65 ( i i ) Aqueous Su l p h u r D i o x i d e S o l u t i o n s 69 ( i i i ) P o l i s h e d S u r f a c e s 69 DISCUSSION 75 I . ANISOTROPIC DISSOLUTION 75 I I . DIRECT DISSOLUTION 75 Page ( i ) Hematite 75 ( i i ) P h y s i c a l A s p e c t s of the D i r e c t D i s s o l u t i o n 79 ( i i i ) D i r e c t D i s s o l u t i o n of G o e t h i t e 86 ( i v ) Chemical Mechanism 87 (v) R e d u c t i v e D i s s o l u t i o n . 9^ ( v i ) D i s s o l u t i o n Models 96 ( v i i ) A n i s o t r o p y of the C r y s t a l S t r u c t u r e 10k CONCLUSIONS 106 REFERENCES l r ) q APPENDIX A 1 1 0 APPENDIX B 1 1 2 APPENDIX C 1 1 5 APPENDIX D - 119 V I LIST OF FIGURES F i g u r e Page 1. Schematic diagram of g l a s s d i s s o l u t i o n a p p a r a t u s . 18 2. Comparism of l e a c h i n g i n the g l a s s apparatus and the a u t o c l a v e (2.kM HC1 at 8 0 ° C ) . 2h 3. T y p i c a l d i s s o l u t i o n curves f o r the v a r i o u s m a t e r i a l s i n 2.kM HC1 at 80°C. 25 h. Complete d i s s o l u t i o n curves f o r the B r a z i l i a n and Massive Red h e m a t i t e samples (3.6M HC1 at 80°C). 27 5. Complete d i s s o l u t i o n curves f o r b o t r y o i d a l h e m a t i t e and g o e t h i t e sam-p l e s (2.1+M HC1 at 80°C). 28 6. E f f e c t of HC1 c o n c e n t r a t i o n on the l e a c h i n g of massive r e d h e m a t i t e at 80°C. 29 7. E f f e c t of HC1 c o n c e n t r a t i o n on the l e a c h i n g of b o t r y o i d a l h e m a t i t e at 80°C. 30 8. E f f e c t of HC1 c o n c e n t r a t i o n on the r a t e of l e a c h i n g of the v a r i o u s samples at 80°C. 32 9. L o g - l o g p l o t of r a t e v e r s u s HC1 con-c e n t r a t i o n f o r the v a r i o u s specimens at 80°C. 33 10. E f f e c t of temperature on the l e a c h i n g of massive r e d h e m a t i t e i n 2. hM. HC1. 3h 11. E f f e c t of temperature on the l e a c h i n g of., b o t r y o i d a l h e m a t i t e i n 2 . kM HC1. 35 12. A r r h e n i u s p l o t s f o r l e a c h i n g t h e sam-p l e s i n 2.kM HC1. 36 13. T y p i c a l d i s s o l u t i o n curves of the sam-p l e s i n hU H 2 S 0 4 at 80°C. 38 lh. E f f e c t of H SO, c o n c e n t r a t i o n on the r a t e of l e a c h i n g at 80°C. 39 Cont i n u e d . F i g u r e 15 . A r r h e n i u s p l o t s f o r l e a c h i n g the sam-p l e s i n kM H„SO, . 2 k 16. T y p i c a l d i s s o l u t i o n curves f o r b o t -r y o i d a l h e m a t i t e and g o e t h i t e i n 5M HCIO^ at 90°C. 1 7 . E f f e c t of HC10, c o n c e n t r a t i o n on the l e a c h i n g at 90^C. 18. A r r h e n i u s p l o t s f o r l e a c h i n g the sam-p l e s i n 5M HCIO^. 1 9 . L e a c h i n g of coa r s e p a r t i c l e s i z e s y n t h e t i c h e m a t i t e (2.1+M HC1 at 80°C. I 20 . L e a c h i n g of h e m a t i t e and g o e t h i t e powders (1.2M HC1 at 80°C.) 21 . T y p i c a l d i s s o l u t i o n p l o t f o r the l e a c h -i n g of a polished b o t r y o i d a l h e m a t i t e sample (2.hM HC1 at Q0°C.) 22. a. P o l i s h e d massive r e d h e m a t i t e x5 b. As p o l i s h e d massive r e d h e m a t i t e x 255 . c. Leached massive r e d h e m a t i t e x 50. 2 3 . a. P o l i s h e d b o t r y o i d a l h e m a t i t e x 5. b. Cross s e c t i o n of l e a c h e d b o t r y o i d a l h e m a t i t e x 25 . 2k. a. H a l f l e a c h e d b o t r y o i d a l h e m a t i t e specimen x 5• b. B o t r y o i d a l h e m a t i t e l e a c h e d f o r v a r y i n g times x 5. 2 5 . S y n t h e t i c h e m a t i t e l i g h t l y e t c h e d x 250 . 26 . S y n t h e t i c h e m a t i t e l e a c h e d f o r v a r y -i n g times x 5• 27- a. Unleached g o e t h i t e x 5. b. L i g h t l y e t c h e d g o e t h i t e x 2 0 0 . v i i i C o n t i n u e d . . . F i g u r e Page 28. a. H a l f l e a c h e d g o e t h i t e specimen x 5. 61 b . G o e t h i t e l e a c h e d f o r v a r y i n g t imes x 5. 6 l 29- E l e c t r o n s c a n n i n g m i c r o s c o p e p i c t -u r es of g o e t h i t e l e a c h e d f o r v a r i o u s times x 30 0 0 . 62 30. a. E l e c t r o n probe p i c t u r e of l e a c h e d s i n g l e c r y s t a l h e m a t i t e f l a k e 6k x 255. b. E l e c t r o n m i c r o s c o p e p i c t u r e of a "bump" x 20,000 6k 31. T y p i c a l l e a c h i n g curves f o r the r e -d u c t i v e d i s s o l u t i o n of v a r i o u s h e m a t i t e samples i n a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s . 66 32. E f f e c t of v a r y i n g t h e SO^ p a r t i a l p r e s -sure i n a c i d i f i e d s o l u t i o n s . 67 33. E f f e c t of the SO p a r t i a l p r e s s u r e on the r a t e of l e a c n i n g . 68 3k. E f f e c t of i n c r e a s i n g the p e r c h l o r i c a c i d c o n c e n t r a t i o n . 70 35. E f f e c t of the S 0 p p a r t i a l p r e s s u r e on t h e r a t e of l e a c n i n g f o r v a r i o u s HCIO^ c o n c e n t r a t i o n . 71 36. E f f e c t of the SO^ p a r t i a l pressure on the d i s s o l u t i o n of massive r e d h e m a t i t e i n i n non a c i d i f i e d s o l u t i o n s . 72 37. E f f e c t of the S 0 2 p a r t i a l p r e s s u r e on the r a t e i n non a c i d i f i e d s o l u t i o n s . 73 38. Leached g o e t h i t e sample i n a c i d i f i e d S 0 2 s o l u t i o n x 5. 7^ 39. Types of d i s s o l u t i o n p l o t s o b t a i n e d f o r h e m a t i t e i n h y d r o c h l o r i c a c i d 76 1+0. a. B r a z i l i a n h e m a t i t e p a r t i c l e 81+ b. M assive r e d h e m a t i t e p a r t i c l e 81+ C o n t i n u e d . . F i g u r e kO. c. S y n t h e t i c h e m a t i t e p a r t i c l e d. B o t r y o i d a l h e m a t i t e p a r t i c l e kl. T h e o r e t i c a l d i s s o l u t i o n curves k2. C o r r e l a t i o n of d i s s o l u t i o n curves the t h e o r e t i c a l d i s s o l u t i o n c u r v e . 1*3. P l o t of CAmt Fe d i s s o l v e d ! v e r s u s time time f o r b o t r y o i d a l h e m a t i t e (2.4M HC1 at 80 °C.) LIST OF TABLES Table No. Page 1 Gravimetric Analysis of Natural Materials 11 2 Spectrographs Analysis of Natural Materials 12 3 Chemical Analysis of a Te^O^ powder 13 4 Chemical Analysis of a FeOOH powder 14 5 Activation Energies i n 2.4M HC1 31 6 Activation energies i n 4M B^SO^ 41 7 Activation Energies i n 5M HCIO^ 41 8 Rates of Dissolution of hematite materials i n hydrochloric acid 77 9 Correlation of rate to grain size for hematite dissolution i n 2.4M HC1 80 10 Activation Energies for Direct Dissolution i n various acids 88 1 INTRODUCTION I . GENERAL The d i s s o l u t i o n of i r o n , i n the forms of i t s o x i d e s or h y d r a t e d o x i d e s has "been a t o p i c of i n t e r e s t f o r many y e a r s . Oxides of i r o n are common t o a g r e a t e r or l e s s e r e x t e n t i n most ores and o f t e n as the major c o n s t i t u e n t i n c e r t a i n secondary p r o d u c t s . The d i s s o l u t i o n of t h e s e o x i d e s i s an unwanted e f f e c t i n many h y d r o m e t a l l u r g i c a l p r o c e s s e s which would n e c e s s i t a t e f u r t h e r p u r i f i c a t i o n p r o c e d u r e s . I t i s thus o f t e n " b e n e f i c i a l t o keep the d i s s o l u t i o n of t h e s e o x i d e s t o a minimum. Cl a y s and sands n o r m a l l y c o n t a i n i r o n o x i d e s as an u n d e s i r a b l e minor c o n s t i t u e n t . The o x i d e removal from the b u l k m a t e r i a l i s more e a s i l y a c c o m p l i s h e d i f they can be r e a d i l y l e a c h e d out from the c l a y or sand. S i l i c a sands f o r g l a s s making have been l e a c h e d by a v a r i e t y of a c i d s which a t t a c k b o t h the h y d r a t e d and d e h y d r a t e d i r o n oxides"! The r e -moval of i r o n o x i d e s from c l a y s u s p e n s i o n s i s a c h i e v e d by 2 sodium d i t h i o n i t e , Na^S^O^ , which r a p i d l y l e a c h e s both the h y d r a t e d and d e h y d r a t e d i r o n o x i d e s even at room t e m p e r a t u r e s . The i n c r e a s e i n the demand f o r i r o n powders has l e d t o the i n v e s t i g a t i o n of i r o n o x i d e d i s s o l u t i o n as a p o s s i b l e 3 source of i r o n by a l e a c h and a c e m e n t a t i o n r e a c t i o n . T h e r e f o r e t h e r e are p r o c e s s e s which r e q u i r e r a p i d o x i d e d i s s o l u t i o n and o t h e r s r e q u i r i n g slow d i s s o l u t i o n . This, has l e d to. jmany- i n v e s t i g a t i o n a of. th.e d i s s o l u t i o n c h a r a c t e r i s t i c s o f S o t E f e r r i c o x i d e and I t s : h y d r a t e s i n a c i d media. V a r i e d r a t e c u r v e s have Been o b t a i n e d by d i f f e r e n t w o r k e r s , A z urn a and Kametani. ' f i n d i n g S-shaped d i s s o l u t i o n curves f o r a Fe^O^ v h i c h were s i m i l a r t o tho s e o b t a i n e d b y G 8 7 Monhemius and Surana f o r a FeOOH. B a t h and o t h e r workers. have n o t i c e d a l i n e a r r a t e c u r v e . The p r e s e n t s t u d y was the f o u r t h i n a s e r i e s of s t u d i e s on the d i s s o l u t i o n of a Fe^O^ and a FeQOR i n a c i d s and a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s at th_e Department of 7 8 M e t a l l u r g y , U.B.C. Bath- and Surana s t u d i e d tJie d i r e c t G a c i d d i s s o l u t i o n of a Fe^O^ and a FeCLOR. Monhemius had s t u d i e d the r e d u c t i v e d i s s o l u t i o n of a FeO.OK i n a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s . The p r e s e n t work, was thus con-cerned w i t h the d i s s o l u t i o n of a ^ e 2 ^ 3 ^ n a c i < 1 I f l e < I s u l p h u r d i o x i d e s o l u t i o n s and c o r r e l a t i o n with. th_e work a c c o m p l i s h e d by these p r e v i o u s i n v e s t i g a t o r s . I I . FERRIC OXIDES AND THE MONO-HYDRATES F e r r i c o x i d e and the monhydrate possess v a r i o u s a l l o t r o p i c m o d i f i c a t i o n s . F e r r i c Oxides ( i ) a Fe^O^ - h e m a t i t e ; t r i g o n a l s t r u c t u r e ( i i ) y ^ e 2 ^ 3 ~ ma-&hemite ; s p i n e l t y pe s t r u c t u r e 3 B.rown h e m a t i t e i s . a synonym f o r l i m o n i t e , hydrous f e r r i c o x i d e . FeO COHInK^Q. F e r r i c Oxide Monohydrates C'i] a F e 2 0 ^ . H 2 0 - i d e n t i c a l t o g o e t h i t e , a FeOOH; orthorhombic s t r u c t u r e ( i i ) B F e 2 ° 3 - H 2 0 ( i i i ) y FegO^.H^O - l e p i d o c r o c i t e . D i m o r p h o u s form of g o e t h i t e . D e h y d r a t i o n of g o e t h i t e t o h e m a t i t e i s r e p o r t e d as o c c u r r i n g at 1 3 0 ° C ^. T u n e l l and P o s n j a k " ^ b e l i e v e t h a t the t r a n s i t i o n i s pH dependent, and they suceeded i n c o n v e r t i n g n a t u r a l h e m a t i t e t o g o e t h i t e i n . 1 M E C 1 at 1 0 0 ° C a f t e r a few weeks . I l l . REVIEW OF RELATED PREVIOUS WORK a) D i r e c t A c i d D i s s o l u t i o n L i t e r a t u r e r e v i e w s on the d i r e c t a c i d d i s s o l u t i o n of f e r r i c o x i d e and i t s monohydrate have been a d e q u a t e l y 7 6 covered by Bath and Monhemius . There are c e r t a i n p i e c e s of l i t e r a t u r e , t o g e t h e r w i t h some new a r t i c l e s t h a t have a d i r e c t b e a r i n g on the p r e s e n t work and are worth r e i t e r a t i n g . P r y o r and Evans , i n s t u d i e s on the f i r s t 1% of d i s s o l u t i o n of s y n t h e t i c a l l y p r e p a r e d a F e 2 0 , noted t h a t the d i s s o l u t i o n r a t e d e c r e a s e d w i t h i g n i t i o n t e m p e r a t u r e of the o x i d e between 3 8 5 ° C and 1 0 0 0 ° C . T h i s decrease i n r a t e was a t t r i b u t e d t o the l o s s of p o r o s i t y caused by s i n t e r i n g at the h i g h e r t e m p e r a t u r e s which would reduce the s p e c i f i c s u r -f a c e a r e a . A d i s s o l u t i o n mechanism based on l a t t i c e d e f e c t s 4 at the s u r f a c e was proposed t o account f o r the i n i t i a l h i g h r a t e of d i s s o l u t i o n . The d i s s o l u t i o n r a t e f e l l as the a t t a c k p r o g r e s s e d ; t h i s d e c l i n e was a t t r i b u t e d t o the s m a l l e r number of d e f e c t s i n the i n t e r i o r of the specimens. Support of t h i s mechanism was f u r n i s h e d by d i s -s o l u t i o n s t u d i e s of i r r a d i a t e d a Fe^O^ . The i r r a d i a t e d samples had a h i g h e r r a t e of d i s s o l u t i o n than the u n - i r r a d i a t e d a ^ e2^3 "but b o t h samples were found t o have an i n c r e a s i n g d i s s o l u t i o n r a t e w i t h i n c r e a s i n g t i m e . No e x p l a n a t i o n was g i v e n f o r t h i s l a t t e r phenomenon which was not observed i n the work of P r y o r and Evans. The a u t h o r s a l s o e n v i s a g e d s u r f a c e d e f e c t s d i f f u s i n g out t o account f o r the i n i t i a l l y s i m i l a r r a t e s of the i r r a d i a t e d and n o n - i r r a d i a t e d sample; t h i s b e i n g i n d i r e c t c o n t r a s t t o P r y o r and Evans who assumed t h a t the d e f e c t s were c o n c e n t r a t e d i n the s u r f a c e l a y e r s . The d e f e c t s produced by t h e r m a l and i r r a d i a t i o n t r e a t m e n t s may d i f f e r markedly which c o u l d p o s s i b l y e x p l a i n t h i s d i s -crepancy. 5 More r e c e n t work by Azuma and Kametani on s y n t h e t i c a Fe^O^ has d i s t i n g u i s h e d two t y p e s of d i s s o l u t i o n c u r v e s , " a c c e l e r a t e d " and " p a r a b o l i c " t y p e s . In the case of " a c c e l e r a t e d " d i s s o l u t i o n , t h r e e s t a g e s o c c u r r e d : -( i ) an i n i t i a l r e a c t i o n i n which the d i s s o l v e d amount was n e a r l y p r o p o r t i o n a l t o time ( i i ) an " a c c e l e r a t e d " r e g i o n ( i i i ) a d e c e l e r a t i o n as d i s s o l u t i o n nears c o m p l e t i o n . 5 Four stages tcere. e v i d e n t i n th.e " p a r a b o l i c " d i s s o l u t i o n process:: -( i l and ( i i ) the same as the " a c c e l e r a t e d " type ( i i i ) a p a r a b o l i c r a t e where d i s s o l u t i o n was p r o p o r t i o n a l t o the square r o o t of time ( i v ) a f i n a l d e c r e a s i n g r a t e stage I n v e s t i g a t i o n s were c a r r i e d out by Azuma and Kametani t o t r y and r e l a t e the shape of the " a c c e l e r a t e d " d i s s o l u t i o n curve t o the a n i s o t r o p i c d i s s o l u t i o n n a t u r e of a Fe^O^', t h i s proved t o be u n s u c c e s s f u l . S i m i l a r " a c c e l e r a t e d " c u r v e s have been o b t a i n e d i n l e a c h i n g s t u d i e s of n a t u r a l g o e t h i t e ^ ' ^ . 7 D i s s o l u t i o n curves o b t a i n e d by Bath on s y n t h e t i c h e m a t i t e showed a c o m p l e t e l y d i f f e r e n t fomn. The amount o f m a t e r i a l l e a c h e d i n c r e a s e d l i n e a r l y w i t h time t o about 30% d i s s o l u t i o n b e f o r e s l o w l y d e c r e a s i n g with, i n c r e a s i n g t i m e . The curves o b t a i n e d are v e r y s i m i l a r t o t h e o r e t i c a l d i s -s o l u t i o n curves f o r spheres of i s o t r o p i c m a t e r i a l , the ob-s e r v e d r a t e d e c r e a s i n g as the s u r f a c e a r e a d e c r e a s e s . 12 A l a t e r paper by Azuma and Kametani showed t h a t the d i s s o l u t i o n r a t e of a Fe^O^ decre a s e d w i t h i n c r e a s i n g c a l c i n i n g t e m p e r a t u r e . A suggested reason f o r the decrease i n r a t e was g i v e n t h a t t h e r e was a l o s s of p o r o s i t y at t h e h i g h e r c a l c i n i n g t e m p e r a t u r e s . A l t h o u g h t h e r e i s obvious, v a r i a t i o n s i n the r a t e curves f o r the d i s s o l u t i o n of a Fe^O^ i n a c i d s , most workers found the same' v a r i a t i o n i n r a t e w i t h d i f f e r e n t a c i d s . The 6 h 5 12 r a t e of dias.olutJ.on i s generally^Jh\e.lieye.d ' ' t o i n c r e a s e t r i t h . t h e o r d e r : KCIO^ < K 2 S 0 4 <, EC1 < HF t h i s o r d e r B e i n g the same as th.at f o r the i n c r e a s i n g complex-i n g a f f i n i t y of the anion f o r the f e r r i c i o n . 13 A r e c e n t paper on zero p o i n t charges of a Fe^O^ proposes two p o s s i b l e mechanisms f o r th.e a t t a c k of the a F e ^ ^ s u r f a c e . When the anions i n s o l u t i o n are weak such t h a t t h e i r i n t e r a c t i o n w i t h the i r o n atom i s much l e s s th.an the OE l i -gand i n the aquocomplex, (e . g . NO^ , ClO^ 1 th.e f o l l o w i n g mechanism was proposed: H +, NO ~ [^FetEgO). ( 0 H ) 2 ] ^ [ ^ F e C E 2 0 ) OH] The anions are not s p e c i f i c a l l y adsorbed on the o x i d e s u r f a c e but s t a y as c o u n t e r i o n s i n the o u t e r H e l m h o l t z l a y e r The p o s i t i v e s u r f a c e charge on the oxide i s o b t a i n e d by the c h e m i s o r p t i o n of H + i o n s and the o x i d e i s then r e v e r s i b l e t o H + a l o n e . I f . t h e anions p r e s e n t i n s o l u t i o n are more e l e c t r o n e g a t i v e than NO^ ( e . g . CI ) they can r e p l a c e the h y d r o x y l l i g a n d s of the s u r f a c e aquocomplex t h r o u g h c o v a l e n t i n t e r a c t i o n w i t h the m e t a l i n a d d i t i o n t o E + a d s o r p t i o n on the s u r f a c e . E + , C I " [ > F e ( H 2 0 ] ( 0 E ) 2 < % . [ ^ F e ( . H 2 0 ) 2 C l ] The m a j o r i t y of w o r k e r s ^ ^,14 j j a v e found a c t i v a t i o n e n e r g i e s of about 20 ± 2 k c a l s / m o l e f o r the d i s s o l u t i o n of a F e ^ ^ and a FeOOH i n v a r y i n g a c i d s and v a r y i n g c o n c e n t r a t i o n s . T h i s 7 l e d t o thje Idea of a s i m i l a r mechanism "being a s c r i b e d t o the d i s s o l u t i o n which, was independent o f thle i a c l d -used., lyiore r e -cent work has shown t h a t t h e r a t e may depend on b o t h thle a n i o n 7 and c a t i o n e.g. a ^ 2 ^ 3 d i s s o l u t i o n i n d i l u t e HC1 , whereas i n 8 o t h e r cases C d i s s o l u t i o n of a FeOOE i n H^SO^I o n l y a c a t i o n dependencyof r a t e i s found. Using the mechanism suggested by the work on the zero p o i n t c h a r g e , I t may be p o s s i b l e t o p r e d i c t the a f f e c t of the ani o n on the r e a c t i o n . b) R e d u c t i v e D i s s o l u t i o n g Monhemius and Surana s t u d i e d the r e d u c t i v e d i s -s o l u t i o n of a FeOOH i n aqueous s u l p h u r d i o x i d e s o l u t i o n s a c i d i f i e d w i t h p e r c h l o r i c a c i d . S i m i l a r shaped curves t o those f o r a c i d , a t t a c k of a FeOOH were o b t a i n e d , t h a t Is an i n c r e a s i n g r a t e of d i s s o l u t i o n w i t h i n c r e a s i n g t i m e . Work on the r e d u c t i v e d i s s o l u t i o n of manganese d i o x i d e ^ by aqueous s u l p h u r d i o x i d e s o l u t i o n s gave l i n e a r d i s s o l u t i o n c u r v e s . High l e a c h i n g r a t e s were a c h i e v e d and a mechanism i n v o l v i n g t h e r e a c t i o n of u n d i s s o c i a t e d SO^ mo l e c u l e s was proposed. In s t u d i e s ^ ' ^ on o x i d a t i o n of s u l p h u r o u s a c i d by m e t a l s , manganese o x i d e s and f e r r i c and c u p r i c s a l t s , s u g g e s t e d t h a t b i s u l p h i t e i o n s adsorb on the s u r f a c e . These i o n s are then d i s c h a r g e d t o g i v e s u l p h a t e (by a two s u c c e s s i v e s i n g l e e l e c t r o n s t e p s ) or d i t h i o n a t e (by p a r t i a l d i s c h a r g e of two b i s u l p h i t e i o n s ) . The p r o p o r t i o n s of d i t h i o n a t e and s u l p h a t e v a r i e d f o r each polymorph. I n the case of the r e d u c t i v e d i s -8 s o l u t i o n of manganese d i o x i d e , o n l y s m a l l amounts of s u l p h a t e were oBse r v e d ; i n th\e. case o f d J'eO.'Q.H, l a r g e amounts of s u l p h a t e were fo u n d . F e r r i c h y d r o x i d e has- Been shown t o d i s s o l v e i n s u l p h u r o u s a c i d t o form a r e d f e r r I c - ^ s u l p h i t e Ion complex i n solution"*"^. T h i s I s th\en reduced t o the f e r r o u s s t a t e i n the s o l u t i o n and not at the s u r f a c e . Monh.emi.us proposed th.at d i s s o l u t i o n of a FeQOE p r o -ceeded By two p o s s i b l e mech.anis.ms-, 1.1 The replacement of a h y d r o x y l l i g a n d t o form a. f e r r i c s u l p h i t e complex i i l The replacement of two h y r o x y l l i g a n d s t o form a f e r r i c s u l p h i t e complex These complexes then desorb and are reduced i n s o l u t i o n . The o v e r a l l r a t e i n v o l v e s b o t h of these mechanisms, mechanism ( i . 1 B e i n g predominant at l o w s u l p h u r d i o x i d e con-c e n t r a t i o n s , (< 10$ S0 2I, and mechanism Cii) at h i g h e r s u l p h u r d i o x i d e c o n c e n t r a t i o n s . High, r a t e s of l e a c h i n g of a FeOOH and a Fe^O^ are o B t a i n e d By sodium d i t h i o n i t e even at room t e m p e r a t u r e . IV. SCOPE OF THE PRESENT INVESTIGATION O r i g i n a l l y the i n t e n t i o n of the p r e s e n t i n v e s t i g a t i o n was t o st u d y the r e d u c t i v e d i s s o l u t i o n of n a t u r a l a F e 2 0 ^ i n a c i d i f i e d aqueous s u l p h u r d i o x i d e s o l u t i o n s and t o compare the r e s u l t s o B t a i n e d t o those f o r a FeOOH. C o r r e l a t i o n of the d a t a f o r the d i r e c t d i s s o l u t i o n 9 of a ^ e2^3' a n c* a FeilQIL \ca'a hoped to help i n p r e d i c t i n g t h e d i f f e r e n c e s i n t h e r e d u c t i v e d i s s o l u t i o n of a Fe^Q-^ and a FeOQH. U n f o r t u n a t e l y - tILer'e i s ; no g e n e r a l agreement "Between the previous; i n v e s t i g a t o r s ; on the! shapes: of the l e a c h i n g curves f o r a FegO^ i t s e l f making c o r r e l a t i o n v i r t u a l l y i m p o s s i b l e with, the a FeOOK d a t a . As- o n l y s y n t h e t i c a Fe^Q^ had been used by p r e v i o u s i n v e s t i g a t o r s , and as n a t u r a l a Fe^O^ , r a s " t o be used f o r the r e d u c t i v e d i s s o l u t i o n , d i r e c t a c i d l e a c h i n g of n a t u r a l h e m a t i t e was- u n d e r t a k e n . Runs on s y n t h e t i c ; a Fe^O^ and n a t u r a l a FeOQH were a l s o u n d e r t a k e n t o f a c i l i -t a t e the c o r r e l a t i o n . As the a n i s o t r o p y of d i s s o l u t i o n of a Fe^O^ was b e l i e v e d t o a f f e c t the d i s s o l u t i o n r a t e , a b r i e f l o o k at the a n i s o t r o p i c aspect of d i s s o l u t i o n was a f f o r d e d . The p r e s e n t work thus c o v e r s a s p e c t s of the d i r e c t a c i d d i s s o l u t i o n of h e m a t i t e and g o e t h i t e i n v a r i o u s a c i d s , and the r e d u c t i v e d i s s o l u t i o n of h e m a t i t e and g o e t h i t e i n aqueous s u l p h u r d i o x i d e s o l u t i o n s . 10 EXPERIMENTAL I . SPECIMEN MATERIALS Samples of n a t u r a l l y o c c u r r i n g and s y n t h e t i c a l l y produced h e m a t i t e and n a t u r a l l y o c c u r r i n g g o e t h i t e were u t i l i z e d i n t h i s s t u d y . ( i ) N a t u r a l M a t e r i a l s a) Massive Red Hematite A sample of massive r e d h e m a t i t e from the M i n n e s o t a , M i c h i g a n ore d e p o s i t s i n the U.S.A. was s u p p l i e d by Ward's N a t u r a l S c i e n c e E s t a b l . I n c . , New York. The m a t e r i a l con-s i s t e d of a f i v e pound b l o c k of h e m a t i t e , g r e y i s h i n c o l o u r , and w i t h some n o t i c e a b l e t h i n q u a r t z s t r e a k s . b) B o t r y o i d a l Hematite A s m a l l sample of b o t r y o i d a l h e m a t i t e was o b t a i n e d from the Thompson-Sutton Geology C o l l e c t i o n , Geology Dept., U.B.C. The m a t e r i a l was dark grey and showed the v e r y c h a r a c t e r i s t i c b o t r y o i d a l (bunch of grapes) s t r u c t u r e . c) B r a z i l i a n Hematite The l a r g e n a t u r a l c r y s t a l s of h e m a t i t e used o r i g i n * a t e d from I t a b i r n , Minas G e r a i s , B r a z i l . The o r i g i n a l m a t e r i a l showed two l a r g e c r y s t a l s , showing s e v e r a l w e l l developed f a c e s , i n t e r g r p w n w i t h each o t h e r and w i t h a p o l y c r y s t a l l i n e m a t r i x , The two l a r g e c r y s t a l s were s e p a r a t e d from the m a t r i x and each o t h e r by c a r e f u l diamond sawing. 11 d) "Massive G o e t h i t e " The sample of b,rovn f i b r o u s g o e t h i t e . o r i g i n a t e d , l i k e the massive r e d h e m a t i t e , from the M i n n e s o t a , M i c h i g a n , ore d e p o s i t s and was s u p p l i e d hy Ward's N a t u r a l S c i e n c e E s t a b . I n c . , New York. The specimen showed a t y p i c a l b o t r y o i d a l morphology and c o n t a i n e d a l a r g e amount of q u a r t z . The g o e t h i t e was s e p a r a t e d from the q u a r t z as f a r as was f e a s i b l e by diamond sawing. Chemical A n a l y s i s of N a t u r a l Specimens Powdered samples of "the n a t u r a l ores were a n a l y s e d g r a v i m e t r i c a l l y f o r t h e i r i r o n and s i l i c o n c o n t e n t s and s p e c t r o g r a p h i c a l l y f o r o t h e r i m p u r i t i t e s by Can Test L t d . , Vancouver. The g r a v i m e t r i c r e s u l t s are shown i n Table 1, w i t h i r o n and s i l i c o n v a l u e s b e i n g c o n v e r t e d t o t h e i r o x i d e s . TABLE 1 G r a v i m e t r i c A n a l y s i s o f N a t u r a l M a t e r i a l s Hematite vt% Fe vt% S i F e 2 0 3 S i 0 2 T o t a l a) Ma s s i v e Red h e m a t i t e 6 2 . 2 5 . 1 6 8 8 . 8 7 1 1 . 0 8 9 9 - 9 5 b) B o t r y o i d a l h e m a t i t e 6 6 . 6 2 2 . 0 95.23 h .29 9 9 . 52 c) B r a z i l i a n h e m a t i t e 6 9 .lU 0 . 6 2 9 8 . 6 0 1.31+ 9 9 . 9h vt% Fe vt% S i FeOOH S i 0 2 T o t a l d) G o e t h i t e kh.k5 1 2 . 8 0 7 0 . 6 1 27.^1 9 8 . 02 The a n a l y s i s i n d i c a t e s l a r g e amount s of s i l i c a p res -e n t , e s p e c i a l l y i n the g o e t h i t e . Though the s i l i c a would not be a n t i c i p a t e d t o a f f e c t the d i s s o l u t i o n n o t i c e a b l y , i t c o u l d 12 c r e a t e problems i n sampling and thus cause e r r o r s ¥h.en d i f f e r e n t d i s s o l u t i o n runs are compared. A l a r g e volume of the p a r t i c u l a t e m a t e r i a l was p r e p a r e d and mixed t h o r o u g h l y each time b e f o r e a sample f o r l e a c h experiments was removed. The s p e c t r o g r a p h i c r e s u l t s shown i n Table 2 s i g n i f y t h a t the m a t e r i a l s were r e l a t i v e l y p u r e , c a l c i u m , magnesium, aluminium and manganese ( a l l p r o b a b l y p r e s e n t as t h e i r r e s p e c t i v e o x i d e s ) b e i n g the o n l y o t h e r i m p u r i t i e s i n any p r o p o r t i o n . TABLE 2 S p e c t r o g r a p h i c A n a l y s i s M a ssive Red B o t r y o i d a l B r a z i l i a n G o e t h i t e A l . 5 . 3 . 3 . 3 Ca . 5 . 5 . 1 .k Cr . 0 0 0 6 N . D . N.D. . 0 0 1 Cu . 0 0 0 5 . 0 0 0 5 . 0 0 0 5 . 0 2 Mg . l . 0 2 . 0 1 . 2 Mn . 2 . 2 . 0 5 . 3 Mo . 0 0 8 t r a c e t r a c e . 0 0 5 N i . 0 0 2 N.D. t r a c e N.D. Ag . 003 t r a c e t r a c e t r a c e Sn . 0 0 3 N.D. N.D. N.D. V . 0 6 . 0 2 . 0 1 . 0 1 A l l o t h e r elements were e i t h e r p r e s e n t as a t r a c e or not de-t e c t e d . 13 The e f f e c t of t h e s e i m p u r i t i e s , on the e x p e r i m e n t a l r e s u l t s has been assumed t o be n e g l i g i b l e . ( i i ) S y n t h e t i c M a t e r i a l s a) S y n t h e t i c Powders The s y n t h e t i c h e m a t i t e powder specimens were p r e -pared from reagent grade a Fe^O^ (Baker and Adams) w i t h o u t any f u r t h e r p u r i f i c a t i o n . Table 3 i n d i c a t e s the maximum l i m i t s of i m p u r i t i e s . TABLE 3 Chemical A n a l y s i s of Reagent Grade a Fe^O^ vf % I n s o l u b l e i n HC1 0.20 s u l p h a t e (S0^~) 0.20 copper (Cu) 0.005 Z i n c ' (Zn.) 0.005 Substances not p r e c i p i t a t e d by NH^OH (as s u l p h a t e s ) 0.10 The s y n t h e t i c a FeOOH used was o b t a i n e d from N o r t h -ern Pigments of Canada. The powder was i n a c i r c u l a r needle form and was f i n e r than 325 mesh. The c h e m i c a l a n a l y s i s i s g i v e n i n T a b l e U . 14 TABLE k Chemical A n a l y s i s of S y n t h e t i c a FeOQH Powder wt % . Average a FeOOH c o n t e n t 99 .5 Water s o l u b l e s a l t s 0 .5 A c i d i n s o l u b l e s a l t s t r a c e Copper (Cu) .015 b) S y n t h e t i c S i n g l e C r y s t a l F l a k e s S i n g l e c r y s t a l s p r e p a r e d as f l a k e s were a c q u i r e d from E . I . du Pont de Nemours & Co., Delaware. These f l a k e s were made by o x i d a t i o n of f e r r i c c h l o r i d e d i s s o l v e d i n a sodium c h l o r i d e f l u x . No seed c r y s t a l s were u s e d , c r y s t a l l i -z a t i o n p r o c e e d i n g v i a s e l f - n u c l e a t i o n . The h a b i t o f the c r y -s t a l s was s t r i c t l y d i c t a t e d by the s t r u c t u r e , the t h i n dimen-s i o n c o r r e s p o n d i n g t o the l a r g e c - a x i s . I I . X-RAY ANALYSIS ( i ) X-ray D i f f r a c t i o n A n a l y s i s A l l the mat.erials used i n t h i s study were examined u s i n g an X-ray d i f f r a c t o m e t e r to a s c e r t a i n the n a t u r e o f the i r o n p r e s e n t . I n a l l cases i r o n r a d i a t i o n was used i n con-j u n c t i o n w i t h a manganese f i l t e r . The r e s u l t s o b t a i n e d appear i n Appendix B and agree w i t h the r e s u l t s i n the p u b l i s h e d d a t a f o r h e m a t i t e and g o e t h i t e . I t was noted t h a t peaks c o r r e s p o n d i n g t o a SiO 15 (quartz \ were well defined in the natural massive red hematite and the goethite, hut were less perceptible for the other natural materials. This would be anticipated from the chemical analysis. ( i i ) Transmission X-ray Analysis Specimens of ^  1 mm. thickness of the varying mater-i a l s were exposed to Fe K q radiation and the re s u l t i n g trans-mission X-ray pattern analysed. The single c r y s t a l a Fe^O^ flakes and the B r a z i l i a n hematite gave very clear spot patterns as would be anticipated. The massive red hematite gave a series of spots, but without any noticeable formation of Debye rings. This is i n d i c a t i v e of a coarse grain s i z e ; only a few grains being in the path of the X-ray beam. Original compacts of the synthetic material, when exposed to the beam, resulted in very spotty Debye rings being observed. Thus a finer grain size than the massive red a Fe 20^, possibly of the order of (10-100^) would be a n t i c i -pated. The botryoidal hematite and the goethite gave smooth continuous rings, the hematite rings being noticeably broader than those of the goethite. A fin e r grain size of the hematite is l i k e l y , both grain sizes possibly being less than I31 . The transmission X-ray analysis, thus indicated a decreasing grain size in the order, B r a z i l i a n a Fe^O^ > massive red a Fe^O^ > synthetic aFe^O^ > , a FeOOH > botryoidal a Fe 0 . 16 I I I . REAGENTS Apart from tn.e n a t u r a l i r o n o x i d e s used i n t h i s s t u d y , a l l o t h e r m a t e r i a l s were of reagent grade. De-i o n i z e d water was used f o r a l l s o l u t i o n s . N i t r o g e n was used as s u p p l i e d by Canadian L i q u i d A i r L t d . , the s u l p h u r d i o x i d e (anhydrous q u a l i t y ) was o b t a i n e d from Matheson of Canada L t d . and was used w i t h o u t f u r t h e r p u r i f i c a t i o n . IV. SPECIMEN PREPARATION ( i ) P a r t i c u l a t e Feed N a t u r a l Specimens The m a t e r i a l was f i r s t c r u s h e d u s i n g a hammer or g y r a t o r y c r u s h e r and s u b s e q u e n t l y hand m u l l e d t o a p p r o x i m a t e l y the r e q u i r e d s i z e . The m a t e r i a l was wet s c r e e n e d t o the s t i p u l a t e d p a r t i c l e s i z e s . S y n t h e t i c Specimens Compacts which had been p r e p a r e d by M. Bath by s i n t e r i n g a^s^O^ powder i n a i r at 1200°C f o r 60 h o u r s , were ground i n a hand m u l l e r . The powder was then wet s c r e e n e d t o the a p p r o p r i a t e s i z e . ( i i ) . P o l i s h e d S u r f a c e s R e c t a n g u l a r b l o c k s of th.e v a r i o u s m i n e r a l s were cut w i t h the diamond saw. These b l o c k s were then mounted i n B a k e l i t e or "koldmount" and p o l i s h e d by c o n v e n t i o n a l m e t a l l o -g r a p h i c t e c h n i q u e s . 17 V. ANALYTICAL METKOD The progress - of d i s s o l u t i o n -was. f o l l o w e d by s p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n s of the i r o n c o n t e n t of samples t a k e n at i n t e r v a l s t h roughout the e x p e r i m e n t . The orange-red f e r r o u s 1-10 o r t h o p h e n a n t h r o l l n e complex, which has an a b s o r p t i o n peak at a wavelength of 510 m u, was u t i -l i z e d i n t h i s s t u d y . The a n a l y t i c a l p r o c e d u r e has been a d e q u a t e l y o u t l i n e d i n the p r e v i o u s t h e s i s of B a t h , Monhenius and Surana(See Appendix A ) • V I . APPARATUS DESIGN ( i ) The G l a s s Apparatus D i r e c t a c i d d i s s o l u t i o n experiments were performed i n a g l a s s r e a c t i o n f l a s k m a i n t a i n e d at a c o n s t a n t temperature i n a water t h e r m o s t a t . A schematic drawing i s shown i n F i g . I of the assembly used. The f l a s k was made from a Ik mm. g l a s s t u b e , the s m a l l e r d iameter b e i n g found t o i n c r e a s e the e f f e c t i v e n e s s of the magnetic s t i r r e r , e n a b l i n g the p a r t i c u l a t e f e e d t o be a d e q u a t e l y suspended. G l a s s rods were r e q u i r e d t o reduce the v o r t e x caused by the magnetic s t i r r e r . The r e a c t i o n f l a s k was f i t t e d w i t h a n i t r o g e n i n l e t tube and a r e t u r n t u b e , b o t h h a v i n g s t o p c o c k s . The sample tube was f i t t e d w i t h a f r i t t e d g l a s s f i l t e r t o p r e v e n t any removal of the powder on s a m p l i n g . A t e f l o n covered magnetic s p i n bar was used t o s t i r the s o l u t i o n s b e i n g r o t a t e d by a magnetic s t i r r i n g u n i t s i t u a t e d below the t h e r m o s t a t v e s s e l . 18 C O O L I N G WATER I NLET N I T R O G E N INLET M M E R S I O N HEAT ER S A M P L I N G TUBE FRITTED G L A S S FILTER REFLUX CONDENSER STIRRER MOTOR C O N T A C T THERMOMETER REACT ION FLASK SPIN BAR M A G N E TIC STIRRER Fig. 1. Schematic diagram of glass dissolution apparatus. 19 The water thermostat teas;, heated b y a 10Q watt immersion h e a t e r connected t o th.e mains: s u p p l y v i a a v a r i a c , and the tem p e r a t u r e c o n t r o l l e d by a m e r c u r y - i n - g l a s s con-t a c t thermometer connected t o the h e a t e r by a mercury r e l a y . The water was s t i r r e d c o n s t a n t l y . T h i s arrangement kept the temperature w i t h i n .2°C of the r e q u i r e d v a l u e . The r e a c t i o n v e s s e l was normally open t o the atmosphere v i a a r e f l u x condenser. For s a m p l i n g d u r i n g a r u n , the system was c l o s e d from the atmosphere and a s m a l l o v e r - p r e s s u r e of n i t r o g e n i n t r o d u c e d t o f o r c e the s o l u t i o n out t h r o u g h the sample tube. Loss of s o l u t i o n due t o e v a p o r a t i o n d u r i n g a run was found t o be n e g l i g i b l e . ( i i ) The A u t o c l a v e The main c y l i n d e r of the 200Q ml c a p a c i t y a u t o -c l a v e was f a b r i c a t e d from m i l d s t e e l , i t h a v i n g a round bottom. The i n s i d e of the c y l i n d e r was bonded t o g l a s s by the R i t t e r P f a u l d e r Co. New Y o r k , t o g i v e a c o r r o s i o n r e s i s t a n t i n t e r i o r . The a u t o c l a v e head was machined from H a s t e a l l o y C, the i n t e r i o r s u r f a c e b e i n g c o v e r e d w i t h f i b r e - g l a s s r e i n f o r c e d t e f l o n a d hesive sheet donated by Dodge F i b r e s C o r p o r a t i o n . Two 1/8" s t a i n l e s s s t e e l gas l i n e s were f i t t e d t o the a u t o -c l a v e head u s i n g low p r e s s u r e s t a i n l e s s s t e e l f i t t i n g s . The sample tube c o n s i s t e d of a f r i t t e d g l a s s f i l t e r c onnected t o a 1/8" heavy w a l l e d t a n t a l u m t u b e , the j o i n t b e i n g made by a t i g h t l y f i t t i n g t e f l o n tube. The t a n t a l u m tube passed through a p r e s s u r e f i t t i n g on the a u t o c l a v e head. A 20 water c o o l e r was a t t a c h e d t o the o u t e r end of the sample tube and the sample was removed by opening a t a n t a l u m needle v a l v e connected t o the end of the sample tube. The i n t e r n a l p r e s s u r e r a i s e d the s o l u t i o n out th r o u g h the sample tube when the v a l v e was opened. The p r e s s u r e gauge used was g r a d u a t e d from 0-100 p . s . i . i n 1 p . s . i . s u b - d i v i s i o n s , t h e gauge b e i n g connected t o the gas e x i t l i n e of the a p p a r a t u s . The t h e r m i s t o r w e l l was made of t e f l o n ; the t h e r m i s t o r p r o b e , which c o n t a i n e d a 1/8" s t a i n l e s s s t e e l p i p e t h r e a d f i t t i n g , s l i d i n t o the w e l l and screwed i n t o the a u t o -c l a v e head t o m a i n t a i n the p r e s s u r e s e a l . V i g o r o u s m i x i n g and gas d i s p e r s i o n were o b t a i n e d by a t e f l o n covered t a n t a l u m s h a f t f i t t e d w i t h two p r o p e l l e r s and one i m p e l l e r . T h i s arrangement was found t o be s a t i s - " f a c t o r y i n k e e p i n g the p a r t i c u l a t e f e e d i n s u s p e n s i o n . The t a n t a l u m s h a f t passed t h r o u g h a p a c k i n g g l a n d on the o u t e r s i d e of the a u t o c l a v e head. The p a c k i n g m a t e r i a l was g r a p h i t e l u b r i c a t e d t e f l o n c o r d s u p p l i e d by John Crane- P a c k i n g , Vancouver. The s h a f t was kept i n l i n e by b e a r i n g s spaced i n a h o u s i n g j u s t above the p a c k i n g g l a n d and was r o t a t e d by a l / 5 t h H.P. v a r i a b l e speed motor. T h i s arrangement was found t o be s a t i s f a c t o r y f o r a l l p r e s s u r e s used w i t h i n the a u t o c l a v e . The normal s t i r r i n g speed of the s h a f t was a p p r o x i m a t e l y 900 r.p.m. The head was s e a l e d t o the c y l i n d e r by a t e f l o n g a s k e t , the whole b e i n g clamped t o g e t h e r by a hardened s t e e l s p l i t r i n g . E i g h t compression screws i n the s p l i t t i n g were 21 t i g h t e n e d t o ensure a p r e s s u r e seal.. The a u t o c l a v e teas heated By two semi.-*circular h e a t -i n g u n i t s w h i c h were clamped t o g e t h e r round a s t e e l c y l i n d e r . The h e a t i n g c o i l s and c y l i n d e r were c o n t a i n e d i n a l a r g e p a i n t can, v e r m i c u l a t e B e i n g used t o h e l p i n s u l a t e the c o i l s from the can. The a u t o c l a v e s l i d i n t o the' s t e e l c y l i n d e r and r e s t e d on the m e t a l top p l a t e welded t o t h e i n n e r c y l i n d e r . The h e a t i n g elements were j o i n e d i n s e r i e s and con -n e c t e d t o a V a r i a c t r a n s f o r m e r v i a a "Thermistemp" temperature, c o n t r o l l e r . The t h e r m i s t e r proBe i n the. a u t o c l a v e , which was connected t o the c o n t r o l l e r , a c t i v a t e d the h e a t e r . U s i n g t h i s system the. temperature i n the a u t o c l a v e was c o n t r o l l e d w i t h i n ± 0.5°C of the d e s i r e d v a l u e . VII.EXPERIMENTAL PROCEDURE ( i ) The Glas s Apparatus The r e a c t i o n f l a s k , c o n t a i n i n g 250 mis of s o l u t i o n and the magnetic s p i n B a r , was. immersed i n the water B a t h which had Been a d j u s t e d t o the r e q u i r e d t e m p e r a t u r e . A f t e r the system had r e a d j u s t e d t o the pre s e t t e m p e r a t u r e , 0.2 gms of the sample was added, the head f i t t e d , and a l l the r e q u i r e d c o n n e c t i o n s completed. The s p i n Bar was s e t i n t o m otion and su B s e q u e n t l y samples were removed at r e g u l a r i n t e r v a l s , n o r m a l l y t h i r t y or s i x t y m i n u t e s , By a p p l y i n g a s l i g h t over p r e s s u r e of n i t r o g e n . Two 10 mis samples were t a k e n from the f l a s k each t i m e , the second B e i n g c o l l e c t e d i n a sample B o t t l e which was im m e d i a t e l y s t o p p e r e d . The f i r s t sample was r e t u r n e d t o the 22 f l a s k _ as soon as, possible a f t e r . the oyer p r e s s u r e of n i t r o g e n had Been r e l e a s e d . A 1-5-5/an 1 sample was pipe.tted from the c o l d second sample f o r c o l o r i m e t r l c I r o n a n a l y s i s , t h e excess s o l u t i o n B e i n g r e t u r n e d t o t h e f l a s k . Runs were n o r m a l l y con-* t i n u e d u n t i l at l e a s t ' 1Q% of the s t a r t i n g m a t e r i a l Bad d i s -s o l v e d . ( i i 1 The A u t o c l a v e A 1 gm sample and 1200 mis of l e a c h s o l u t i o n were added t o the a u t o c l a v e , the a u t o c l a v e head -was s e c u r e d t o tBe main Body and the whole assemBly lowered i n t o t h e f u r n a c e . The f u r n a c e h e a t e r and the a u t o c l a v e s t i r r e r were s t a r t e d . A i r i n the a u t o c l a v e was f l u s h e d out w i t h n i t r o g e n , t h i s pro--cedure B e i n g r e p e a t e d when the p r e s e t temperature had Been reached so as t o remove, any d i s s o l v e d oxygen t h a t Bad Been d i s p l a c e d on h e a t i n g . The system was then c l o s e d and a l l o w e d t o come t o t h e r m a l e q u i l i B r i u m . . When the water vapour p r e s s u r e had r e - e s t a B l i s h e d i t s e l f , s u l p h u r d i o x i d e was i n t r o d u c e d at the r e q u i r e d p r e s s u r e . A f t e r t e n m i n u t e s , a B l a n k sample was t a k e n By opening the sample v a l v e and removing two 10 mis samples, the f i r s t sample B e i n g d i s c a r d e d and the second Being c o l l e c t e d i n a sample B o t t l e which was i m m e d i a t e l y s t o p p e r e d . Samples were c o l l e c t e d u s u a l l y at t h i r t y minute i n t e r v a l s t h r o u g h o u t the run u n t i l at l e a s t 5$ d i s s o l u t i o n had o c c u r r e d . The s u l p h u r d i o x i d e p r e s s u r e and the s t i r r i n g speed were main-, t a i n e d c o n s t a n t throughout the r u n . 23 RESULTS The d i s s o l u t i o n d a t a o b t a i n e d has been p r e s e n t e d m a i n l y i n the form of g r a p h s , n u m e r i c a l r e s u l t s b e i n g grouped i n the Appendix C. C o r r e c t i o n s were made t o a l l o w f o r the removal of samples during a r u n . P r e v i o u s workers have shown t h a t v a r i a t i o n i n sample weight does not a f f e c t the r a t e of d i s s o l u t i o n u n l e s s the s o l u b i l i t y p r o d u c t was approached. Care was taken t h a t t h i s s i t u a t i o n never o c c u r r e d . R e s u l t s have t h e r e f o r e been e x p r e s s e d as % i r o n d i s s o l v e d i n a l l cases f o r ease of com-p a r i s m . I d e n t i c a l d i s s o l u t i o n e x p eriments were c a r r i e d out i n the g l a s s apparatus and the a u t o c l a v e t o ensure c o n s i s t e n c y of r e s u l t s . As F i g . 2 i n d i c a t e s , t h e r e i s e x c e l l e n t agree-ment i n "the da t a from both s o u r c e s . I . DIRECT DISSOLUTION OF IRON OXIDES ( i ) P a r t i c u l a t e Feed Un l e s s o t h e r w i s e s t a t e d , the p a r t i c u l a t e f e e d used was i n the s i z e range (+ 1 5 0 - 6 5 ) mesh. A l l of the runs were c a r r i e d out i n the g l a s s a p p a r a t u s , the m a j o r i t y at 80°C. Rates were measured at a g i v e n p e r c e n t a g e d i s s o l u t i o n from the t a n g -ent of the d i s s o l u t i o n c u r v e , 5% d i s s o l u t i o n b e i n g n o r m a l l y chosen. (a) H y d r o c h l o r i c A c i d T y p i c a l d i s s o l u t i o n curves f o r the v a r y i n g i r o n o x i d e s samples i n 2.ku HC1 at 80°C are shown i n F i g . 3. I t i s n o t i c e a b l e t h a t a v a r i e t y of shapes are o b t a i n e d as w e l l as marked d i f f e r e n c e s i n d i s s o l u t i o n r a t e s . 24 Time (mins.) F i g . 2. Comparison of l e a c h i n g i n the g l a s s apparatus and the a u t o c l a v e (2.1+M HC1 at 80°C) 25 26 B o t r y o i d a l h e m a t i t e d i s s o l v e d markedly f a s t e r than any of the o t h e r samples, i n c l u d i n g th.e g o e t h i t e , the r a t e of d i s s o l u t i o n i n c r e a s i n g w i t h t i m e . G o e t h i t e d i s s o l v e d i n a s i m i l a r manner, hut at a p p r o x i m a t e l y h a l f the r a t e of b o t r y o i d a l h e m a t i t e . The d i s s o l u t i o n r a t e of g o e t h i t e was i n i t i a l l y s l o w e r than t h a t o f the B r a z i l i a n h e m a t i t e p a r t i c l e s , but as d i s s o l u t i o n proceeded the r a t e s u r p a s s e d t h a t of the B r a z i l i a n h e m a t i t e . The o t h e r h e m a t i t e samples e x h i b i t e d an almost l i n e a r r a t e c u r v e , the p l o t s b e i n g c o m p l e t e l y l i n e a r f o r the slower d i s s o l v i n g samples. The B r a z i l i a n h e m a t i t e d i s -s o l v e d more than t w i c e as q u i c k l y as any of t h e o t h e r s p e c i -mens, b e i n g of a comparable r a t e t o g o e t h i t e . To e s t a b l i s h the d i s s o l u t i o n curves more f u l l y , a s e r i e s of runs t o t o t a l d i s s o l u t i o n were c a r r i e d o ut. As can be seen from F i g s , h and 5s the d i s s o l u t i o n r a t e s de-c r e a s e d f o r a l l samples as d i s s o l u t i o n neared c o m p l e t i o n . T h i s decrease i n r a t e can be a t t r i b u t e d t o the r a p i d decrease i n s u r f a c e a r e a as the d i s s o l u t i o n p r o c e s s nears i t s con-c l u s i o n . E f f e c t of A c i d C o n c e n t r a t i o n V a r i a t i o n of a c i d c o n c e n t r a t i o n s i n the range 1M to 5M was s t u d i e d . No changes i n curve shapes from those ob-t a i n e d i n 2.kM HC1 were found f o r each m a t e r i a l . The r a t e s of d i s s o l u t i o n i n c r e a s e d s i g n i f i c a n t l y w i t h i n c r e a s i n g a c i d c o n c e n t r a t i o n as i n d i c a t e d by a t y p i c a l s et of r e s u l t s i n F i g . 6 and 7 f o r massive r e d h e m a t i t e and b o t r y o i d a l h e m a t i t e . 27 0 1200 2^00 3600 Time (mins.) Fig.4. Complete dissolution curves for the B r a z i l i a n and Massive red hematite samples (.3 . 6M HC1 at 80°C). 2 8 100 75 io Fe dissolved 50 25 O Botryoidal hematite, • Goethite, 360 Time (mins, 720 1080 F i g . 5 . Complete d i s s o l u t i o n curves f o r B o t r y o i d a l h e m a t i t e and G o e t h i t e samples (2. i+M HC1 at 80°C) . 29 F i g . 6 . E f f e c t of HC1 c o n c e n t r a t i o n on the l e a c h i n g of Massive red h e m a t i t e at 80°C. 3Q F i g . 7. E f f e c t of HC1 c o n c e n t r a t i o n on the l e a c h i n g of B o t r y o i d a l h e m a t i t e at 80°C. 31 Rates f o r 5$ d i s s o l u t i o n were c a l c u l a t e d from the r e s p e c t i v e d i s s o l u t i o n curves and p l o t t e d a g a i n s t t h e i r a c i d c o n c e n t r a t i o n s i n F i g . 8 . The r a t e s a l l showed an i n c r e a s e w i t h a c i d c o n c e n t r a t i o n i n a s i m i l a r manner, the r a t e r a p i d l y -i n c r e a s i n g w i t h i n c r e a s i n g a c i d c o n c e n t r a t i o n . The b o t r y o i d a l h e m a t i t e had the l a r g e s t r a t e , f o l l o w e d by the B r a z i l i a n hema-t i t e and then the g o e t h i t e . I f r a t e d ata at 20% d i s s o l u t i o n had been used, the g o e t h i t e would have had a h i g h e r r a t e than t h a t f o r B r a z i l i a n h e m a t i t e due t o the i n c r e a s i n g d i s s o l u t i o n r a t e w i t h time f o r g o e t h i t e . The r a t e of d i s s o l u t i o n v e r s u s a c i d c o n c e n t r a t i o n f o r a FeOOH at 20% d i s s o l u t i o n s t i l l showed the same form of curve as at 5% d i s s o l u t i o n . A l o g - l o g p l o t of t h i s d a t a g i v e s s t r a i g h t l i n e s f o r the v a r i o u s m a t e r i a l s w i t h s l o p e s i n the range of 1 . 9 t o 2.3, Figure. 9 • E f f e c t of Temperature D i s s o l u t i o n experiment at 65°C, 72°C, 80°C and 85°C were c a r r i e d out t o e s t a b l i s h a c t i v a t i o n e n e r g i e s f o r the v a r i o u s samples. A l l samples showed i n c r e a s i n g l e a c h i n g r a t e s w i t h i n c r e a s i n g t e m p e r a t u r e ; F i g u r e 10 and 11 show t y p i c a l i n c r e a s e s f o r the two t ypes of curve shapes. A r r h e n i u s p l o t s f o r 5% d i s s o l u t i o n gave a s e r i e s of almost p a r a l l e l s t r a i g h t l i n e s ( F i g u r e 12) i n d i c a t i n g s i m i l a r a c t i v a t i o n e n e r g i e s . The a c t i -v a t i o n e n e r g i e s from the s l o p e s are l i s t e d i n Table 5-TABLE 5 A c t i v a t i o n E n e r g i e s at 5% D i s s o l u t i o n Sample Ea ( k c a l s / m o l . ) Massive r e d h e m a t i t e B o t r y o i d a l h e m a t i t e 23.4 ±2.2 23.2 ±2.6 Cont i n u e d . . 32 0 1.2 2.4 3.6 4.8 F i g . 8 . E f f e c t of HC1 c o n c e n t r a t i o n on the r a t e of l e a c h i n g of the v a r i o u s samples at 80°C. 33 -1.0 log Rate [log(/oFe/min) ] -2.0 -3.0 O Botryoidal hematite, D B r a z i l i a n hematite, Goethite, A Massive Red hematite, V Synthetic hematite. 0.2 , - 0.4 log LHC1J 0.6 F i g . 9 . L o g - l o g p l o t of r a t e v e r s u s HC1 c o n c e n t r a t i o n f o r the v a r i o u s specimens at 80°C. F i g . E f f e c t of temperature on of Massive r e d h e m a t i t e the l e a c h i n g i n 2.kM HC1. 3 5 Time (mins) F i g . 11. E f f e c t of temperature on the l e a c h i n g of B o t r y o i d a l h e m a t i t e i n 2. 1+M HC1 . Fig.12. Arrhenius plots for leaching the sample in 2.kM HC1. C o n t i n u e d . . . Sample Ea C k-cala/moi. 1 B r a z i l i a n h e m a t i t e 22 .7 ±1.2 S y n t h e t i c h e m a t i t e 22.6" ±2.4 G o e t h i t e h e m a t i t e 21.1 ±1.0 '(h) S u l p h u r i c A c i d D i s s o l u t i o n s t u d i e s i n 4M H^SO, demonstrated the 2 4 same ty p e s of "behaviour as i n h y d r o c h l o r i c a c i d though the r a t e s were markedly l o w e r . T y p i c a l d i s s o l u t i o n p l o t s are shown i n F i g u r e 13. Due t o lack, of c e r t a i n specimen m a t e r i a l s , runs were o n l y c a r r i e d out on the " b o t r y o i d a l and massive r e d h e m a t i t e s and g o e t h i t e . E f f e c t of A c i d C o n c e n t r a t i o n A l l the r a t e s i n c r e a s e d w i t h a c i d c o n c e n t r a t i o n , though not as markedly as w i t h h y d r o c h l o r i c a c i d . F i g u r e 14 shows a p l o t of d i s s o l u t i o n r a t e at 5% v e r s u s a c i d concen-t r a t i o n . An a p p r o x i m a t e l y l i n e a r dependence seems to be i n -d i c a t e d . E f f e c t of Temperature I n c r e a s i n g r a t e s w i t h i n c r e a s i n g t emperature were observed and A r r h e n i u s p l o t s f o r the d i f f e r e n t m a t e r i a l s are shown i n F i g u r e 15. A c t i v a t i o n e n e r g i e s f o r the v a r i o u s m a t e r i a l s are l i s t e d i n Table 6. Time (mins.) F i g . 1 3 . T y p i c a l d i s s o l u t i o n curves of the samples i n kM HgSO^ at 80°C. 2.8 2.9 3.0 l/T x 1(T Fig.15. Arrhenius plots for leaching the samples in km H^ SO, . 2 k 41 TABLE 6 A c t i v a t i o n E n e r g i e s at 5% D i s s o l u t i o n M a s sive Red a 18.7 ± 2.5 k c a l s / m o l e B o t r y o i d a l a F e 2 ° 3 20.2 ± 1.8 k c a l s / m o l e G o e t h i t e 18.4 ± 2.0 k c a l s / m o l e (JC I P e r c h l o r i c A c i d The shapes of the d i s s o l u t i o n curves f o r HCIO^ tended t o he a l l s t r a i g h t l i n e s over the normal l e n g t h of run owing t o o n l y a v e r y s m a l l p r o p o r t i o n of the i r o n d i s -t o s t i m u l a t e the r e a c t i o n . C u r v i n g c o u l d he seen i n some of t l i e p l o t s a f t e r s e v e r a l h o u r s , though n o t i c e a b l y l e s s so than the p r e v i o u s a c i d s ( F i g u r e l6)_ . V a r i a t i o n s i n a c i d c o n c e n t r a t i o n r e -s u l t e d i n a l i n e a r dependence between HCIO^ c o n c e n t r a t i o n and r a t e ( F i g u r e 17). A r r h e n i u s p l o t s f o r the b o t r y o i d a l a Fe^O^ and a FeOOH were c a l c u l a t e d from d a t a o b t a i n e d at 85, 90 and 95°C; 2% d i s s o l u t i o n r a t e v a l u e s b e i n g used. The r e s u l t s , F i g u r e 18, show a l a r g e amount of e r r o r , however, the r e -s u l t s are s i m i l a r t o those o b t a i n e d f o r the p r e v i o u s a c i d s . TABLE 7 A c t i v a t i o n E n e r g i e s at 2% D i s s o l u t i o n s o l v i ng. T e s t s were thus c a r r i e d out i n 5N HC10V at 90°'C B o t r y o i d a l a Fe^O G o e t h i t e 21.1 ± 5.2 k c a l s / m o l e 22.1 ± 4.5 k c a l s / m o l e Time (hours) F i g . 1 6 . T y p i c a l d i s s o l u t i o n curves f o r B o t r y i o d a l h e m a t i t e and G o e t h i t e i n 5M HC10, at 90°C. 43 F i g . 1 7 . E f f e c t of HCIO^ c o n c e n t r a t i o n on the r a t e of l e a c h i n g at 90°C. F i g . l 8 . Arrhenius plots for leaching the samples in 5M HC10, . 45 (_al Co a r s e P a r t i c u l a t e Feed i D i s s o l u t i o n runs, on iCUtl2 mesh g o e t h i t e , s y n t h e t i c and massive r e d h e m a t i t e i n 2.UM EC1 showed t h a t a d i f f e r e n c e i n l e a c h i n g BeB.avi.our c o u l d o c c u r . B o t h 'geoth.lte and massive r e d h e m a t i t e r e t a i n e d t h e i r previous- shape of d i s s o l u t i o n c u r v e s whereas s y n t h e t i c h e m a t i t e a l t e r e d . P r e v i o u s l y the d i s s o l u t i o n had remained c o n s t a n t with, t i m e , t e n d i n g t o de-c r e a s e i n the f i n a l s t a g e s , But F i g u r e 19. i n d i c a t e s an i n -c r e a s i n g d i s s o l u t i o n r a t e w i t h time f o r the c o a r s e r f e e d m a t e r i a l . There was. s l i g h t f r a g m e n t a t i o n d u r i n g the course of l e a c h i n g i n a l l c a s e s , more so i n the case of s y n t h e t i c h e m a t i t e . Runs f o r the same l e n g t h of time i n water r e v e a l e d t h a t o n l y about 3% of the m a t e r i a l fragmented from the o r i g i n a l s i z e s o l e l y By m e c h a n i c a l a g i t a t i o n . Cii) Powder Specimens Leach runs on powder specimens ' (.-325 mesh) of h e m a t i t e and g o e t h i t e i n h y d r o c h l o r i c a c i d are shown i n F i g u r e 20. I t can Be seen t h a t Both d i s s o l v e v e r y r a p i d l y , most l i k e l y due t o the v e r y l a r g e s u r f a c e a r e a , lower t e m p e r a t u r e s and more d i l u t e a c i d s o l u t i o n s would Be r e q u i r e d to determine the shapes more a c c u r a t e l y . ( i i i ) P o l i s h e d Specimens P r e v i o u s workers had d i f f i c u l t y i n p r o d u c i n g r e -p r o d u c a B l e r a t e d a t a from p o l i s h e d s u r f a c e s of n a t u r a l m a t e r i a l s , a p a r t from s i n g l e c r y s t a l s , due to t h e i r inhomo-4 8 g e n e i t y . T h i s waa found t o he the. caae In the. p r e s e n t work except f o r h o t r y o i d a l h e m a t i t e where, r e p r o d u c a h l e r e s u l t s were o b t a i n e d . A l l runs were c a r r i e d out i n 2 . 4M HC1, the s i z e , of the specimens b e i n g a p p r o x i m a t e l y 1 sq.cm. Mas s i v e Red Hematite Though the d a t a was u n r e p r o d u c a b l e , a p p r o x i m a t e l y l i n e a r r a t e curves were o b t a i n e d i n n e a r l y every case. B o t r y o i d a l Hematite A t y p i c a l . r a t e curve f o r complete d i s s o l u t i o n i n 2 .1+M HC1 at 80°C i s shown i n F i g u r e 21 and i n d i c a t e s an almost i d e n t i c a l shape t o t h a t o b t a i n e d on the p a r t i c u l a t e f e e d . D u r i n g the l e a c h i n g the s u r f a c e l o s e s i t s m e t a l l i c l u s t r e and becomes more r e d i s h as the l e a c h i n g p r o c e e d s . A red powder c o a t i n g appears on the s u r f a c e and a f t e r about t h r e e hours i t c o u l d be seen t h a t t h i s c o a t i n g was s l o w l y b e i n g s t r i p p e d : o f f the s u r f a c e , though s t i l l l e a v i n g the r e d c o l o u r a t i o n on the s u r f a c e . I f the l e a c h i n g was stopped when a l l the specimen was l e a c h e d out of the mounting compound, a v e r y f i n e r e d " s i l t " was found on f i l t e r i n g the l e a c h s o l -u t i o n . No such r e d " s i l t " was found i f the d i s s o l u t i o n was a l l o w e d t o go t o c o m p l e t i o n Ci.e. when the i r o n c o n c e n t r a t i o n became c o n s t a n t w i t h t i m e ) . Rate curves at 65°C, 72 OC,80°C, 85°C were ob-t a i n e d , the s u r f a c e areas of the o r i g i n a l f a c e s b e i n g kept as c o n s t a n t as p o s s i b l e , t h i s b e i n g a c h i e v e d by c a r e f u l diamond sawing and g r i n d i n g . The a c t i v a t i o n energy o b t a i n e d 20.3 +3.2 k c a l s / m o l e i s i n agreement w i t h t h a t found p r e -49 80 60 -mgms Fe i n solution. kO -20 500 1000 Time (mins.) 1500 Fig.21, T y p i c a l d i s s o l u t i o n p l o t f o r the l e a c h i n g of a p o l i s h e d B o t r y o i d a l h e m a t i t e sample (2.kM HC1 at 80°C). 50 v i o u s l y , though the e r r o r i s s i g n i f i c a n t l y g r e a t e r . B r a z i l i a n Hematite S t r a i g h t d i s s o l u t i o n curves were o b t a i n e d f o r these specimens, the r a t e v a r y i n g g r e a t l y from one specimen to an o t h e r . No attempt was made t o o r i e n t a t e t h e s e specimens i n a p a r t i c u l a r d i r e c t i o n and thus the d i f f e r e n c e s i n r a t e s are p r o b a b l y a s s o c i a t e d w i t h the a n i s o t r o p y of h e m a t i t e . The p i t t i n g and f a c e t i n g observed d i d not a f f e c t the i i n e a r t y of the d i s s o l u t i o n l i n e s . S y n t h e t i c Hematite Though no r e p r o d u c a b l e d a t a was o b t a i n e d , d i s s o l u t i o n curves showing an i n c r e a s e i n d i s s o l u t i o n r a t e w i t h time was c l e a r l y i n d i c a t e d . The s u r f a c e became n o t i c e a b l y r e d as l e a c h i n g proceeded. G o e t h i t e The l e a c h i n g curves f o r g o e t h i t e showed t h a t the d i s s o l u t i o n r a t e was i n c r e a s i n g w i t h time even though r e -p r o d u c i b i l i t y was poor. Areas of the g o e t h i t e s u r f a c e would turn- y e l l o w as leaching p roceeded, the c o l o u r a t i o n not b e i n g homogeneous over the whole s u r f a c e . A f t e r c o n t i n u e d l e a c h i n g a y e l l o w c o l o u r a t i o n was o b t a i n e d over the whole s u r f a c e . ( i v ) M e t a l l o g r a p h y The inhomogeneity of the n a t u r a l samples caused some t e a r i n g and p i t t i n g of the p o l i s h e d s u r f a c e due t o t h e 51 differences in polishing c h a r a c t e r i s t i c s of quartz and the various iron minerals. In certain cases cracks were evident on the polished surface. A l l experiments vere carried out in 5M HC1 at 80°C unless, otherwise stated. Ca 1 Massive Red Hematite The as polished structure (.Fig. 22a) shows a varied, but very large grain size throughout th.e specimen. At higher magnifications (Fig• 22b), s i l i c a p a r t i c l e s can be seen i n t e r -mixed with a FegO^j electron microprobe analysis confirmed the presence of s i l i c o n in these p a r t i c l e s . Although the surface was s t i l l quite rough, p i t t i n g was not p a r t i c u l a r l y prevalent in any areas. After'leaching for one hour, (Fig.22c) p i t t i n g was clearly evident. These 'pits' on closer observation were due to certain of the smaller grains being almost t o t a l l y leached away rather than being pits in an actual grain. Many grains showed almost no sign of attack whereas others had been leached uniformly across their width. (b) Botryoidal Hematite The as polished specimens: showed the t y p i c a l morpho-l o g i c a l growth structure of botryoidal materials.. The un-leached specimens showed no obvious indication of grain s i z e , Fig.23 a showing the material as i t appeared In the un-t leached state. On prolonged leaching the material was covered by a red surface layer,Fig. 2Ua showing a comparism. between the leached and unleached material. Fig.22 a.Poll shed massive red hematite, x 5 Fig.22 b.Polished massive red hematite, x 255 Fig.22 c.Leached massive red hematite, x 50 53 Fig.23 b. Cross section of leached botryoidal hematite, x 25 54 F i g . 24h shows a sample etched f o r 0, 1 5 , 30, 45 and 60 minutes i n 2 . 4M HC1 at 80°C showing the f o r m a t i o n of the r e d c o l o u r a t i o n on the s u r f a c e . A t r a n s v e r s e s e c t i o n through the specimen showing t h i s s u r f a c e l a y e r i s seen i n F i g . 23 b-T h i s r e d l a y e r appeared t o i n c r e a s e i n t h i c k n e s s w i t h time t o a g i v e n d e p t h , and then remain c o n s t a n t t h rough the r e s t of the run as the specimen was l e a c h e d . No p r a c t i c a l measurements on the d i f f e r e n c e i n t h i c k n e s s of t h i s l a y e r w i t h time or a c i d c o n c e n t r a t i o n was a c c o m p l i s h e d , though the l a y e r o f t e n appeared as t h i c k as 0.1 mm. A f t e r a l e a c h e d specimen was washed and a l l o w e d t o d r y , i t was found t h a t much of t h i s r e d l a y e r c o u l d he e a s i l y brushed o f f . The r e d m a t e r i a l removed appeared t o be a v e r y f i n e powder. U n f o r t u n a t e l y not enough m a t e r i a l was o b t a i n e d f o r a n a l y s i s , but i t i s w e l l known t h a t h e m a t i t e , when ground f i n e , i s i n f a c t a r e d powder. Cc ) B r a z i l i a n Hematite P r e v i o u s workers have shown t h a t p i t s and f a c e t i n g occur when s i n g l e c r y s t a l specimens are l e a c h e d , the form of p i t t i n g or f a c e t i n g depending on the r e l a t i v e o r i e n t a t i o n of the l e a c h i n g p l a n e . Thus no work was u n d e r t a k e n i n t h i s a r e a , however, i t was noted t h a t when p o l i s h e d sample was l e a c h e d , i t d i d m a i n t a i n i t s b r i g h t grey m e t a l l i c c o l o u r . (d) S y n t h e t i c Hematite The as p o l i s h e d s t r u c t u r e r e v e a l e d the presence of g r a i n b o u n d a r i e s , a l i g h t e t c h i n p e r c h l o r i c a c i d making these F i g . 24 To. Botryoidal hematite leached for increasing times, x 5 56 more prominent. The g r a i n s i z e was found t o he i n the o r d e r of 5 0 - lOOu and a s m a l l degree of p o r o s i t y was e v i d e n t . On l e a c h i n g , i t was n o t i c e a b l e t h a t d i f f e r e n t g r a i n s were a t t a c k e d at d i f f e r e n t r a t e s , F i g . 25 .showing t h a t w e l l d e v e l o p e d f a c e t -i n g has o c c u r r e d i n some g r a i n s whereas others show no e v i d e n c e of a t t a c k . On l e a c h i n g f o r a p r o l o n g e d t i m e , the s u r f a c e was found t o go r e d i n a s i m i l a r manner t o t h a t of the b o t r y o i d a l h e m a t i t e . F i g . 2 6 shows a sample e t c h e d f o r i n -c r e a s i n g l e n g t h s of time i n k.8M H C 1 at 8 0 ° C showing a g a i n the p r o d u c t i o n of the r e d c o l o u r a t i o n . I t was found on c l o s e r e x a m i n a t i o n however, t h a t i n t h i s case the l a y e r was not q u i t e as u n i f o r m as b o t r y o i d a l h e m a t i t e . (e) G o e t h i t e The as p o l i s h e d s t r u c t u r e of g o e t h i t e showed a more marked m o r p h o l o g i c a l growth s t r u c t u r e than b o t r y o i d a l h e m a t i t e . ( F i g . 2 7 a ) . On l e a c h i n g , t h i s s t r u c t u r e becomes more e v i d e n t ( F i g . 2 7 b ) . S i l i c a i s a g a i n e v i d e n t b e i n g w e l l i n t e r m i n g l e d w i t h a FeOOH i n many p l a c e s . The s u r f a c e of the g o e t h i t e specimens showed a v e r y c l e a n smooth s u r f a c e l i k e the b o t r y o i d a l hemat i t e. P r o l o n g e d l e a c h i n g produced a y e l l o w c o l o u r a t i o n of the s u r f a c e , F i g . 2 8 a showing a l e a c h e d and u n l e a c h e d p o r t i o n of the specimen. I t i s n o t i c e a b l e t h a t the c o l o u r a t i o n i s not u n i f o r m a c r o s s the l e a c h e d m a t e r i a l as was the case f o r bo-t r y o i d a l h e m a t i t e . The c o l o u r i n g appears t o f o l l o w t o some ex t e n t the m o r p h o l o g i c a l s t r u c t u r e and the f a c t t h a t some areas 57 Fig.25. L i g h t l y etched synthetic hematite, x 250 58 Fig. 26. Synthetic hematite leached for increasing times, x 5 59 Fig. 27 b. Li g h t l y etched goethite. x 200 60 appear almost .the o r i g i n a l c o l o u r seems t o suggest a c e r t a i n degree of p r e f e r r e d o r i e n t a t i o n a c r o s s the specimen. A specimen l e a c h e d f o r v a r y i n g l e n g t h s of time i s shown i n F i g . 28b, i n d i c a t i n g how the y e l l o w c o l o u r a t i o n o c c u r s . As i n the case of b o t r y o i d a l h e m a t i t e , the c o l o u r -a t i o n was found t o be due t o the presence of a y e l l o w powder on the s u r f a c e of the specimen. The c o l o u r of g o e t h i t e i n powder form i s known t o be y e l l o w . M e t a l l o g r a p h i c e x a m i n a t i o n of the s u r f a c e a f t e r l e a c h i n g i n d i c a t e s t h a t " t u n n e l s " i n t o the b u l k of the s p e c i -men are formed. F i g . 29a shows an a r e a l e a c h e d f o r t e n minutes i n 5N HC1 at 80°C. I t can be seen t h a t an a n i s o t r o p i c a t t a c k appears t o be t a k i n g p l a c e . F i g . 24 b,c, d, show a FeOOH l e a c h e d f o r p r o g r e s s i v e l y l o n g e r t i m e s . ( f ) Hematite S i n g l e C r y s t a l F l a k e s On l e a c h i n g , t h e s e f l a k e s d e c r e a s e d i n t h i c k n e s s q u i t e u n i f o r m l y w i t h o u t any g r e a t change i n t h e i r o t h e r dimen-s i o n s . As l e a c h i n g of the f l a k e s neared completion', the f l a k e s became t r a n s l u c e n t and b l o o d r e d i n c o l o u r ; the f l a k e s were o r i g i n a l l y opaque and g r e y . The o r i g i n a l s u r f a c e topography was e x t r e m e l y smooth and remained so as l e a c h i n g proceeded. E l e c t r o n probe p i c t u r e s of the s u r f a c e a f t e r about 50$ d i s s o l u t i o n r e v e a l e d "bumps" on the s u r f a c e ( F i g . 30a). These "bumps" appeared t o be c o n i c a l i n n a t u r e , h a v i n g u s u a l l y s i x d i s t i n c t i v e f l a t f a c e s . No e t c h p i t s were apparent. These "bumps" were randomly d i s t r i b u t e d a c r o s s 61 Fig. 28 b. Goethite specimen leached for increasing times. Fig. 29 a.Leached goethite. 5 min,5MHCl. x 800 Fig. 29 bleached goethite 5 min,5MHCl,x 3000 Fig.29 c.Leached goethite . 10 min,5MHCl. x 3000 Fig.29 d.Leached goethite. 15 min.5MHCl. x 3000 63 the s u r f a c e . C e r t a i n specimen r e v e a l e d v e r y few "bumps" whereas o t h e r s showed a l a r g e amount, e s p e c i a l l y c l o s e t o the specimen edge. The "bumps" v a r i e d i n s i z e , t h o s e at the edge n o r m a l l y b e i n g s m a l l e r than at t h e c e n t r e . Steps on the s u r f a c e were often apparent b e f o r e and a f t e r l e a c h i n g , o f t e n b e i n g a s s o c i a t e d w i t h "bumps". No v a r i a t i o n i n e l e c t r o n d i f f r a c t i o n p a t t e r n s between the "bumps" and the normal specimen s u r f a c e c o u l d be d e t e c t e d . T r a n s m i s s i o n e l e c t r o n m i c r o s c o p e p i c t u r e s ( F i g . 30b) d i d not r e v e a l any c l u e as t o t h e i r o r i g i n . The s i z e and f r a i l n e s s of the specimens made i t v i r t u a l l y i m p o s s i b l e t o i n t r o d u c e any d i s l o c a t i o n by way of s t r a i n i n g or hardness i n d e n t a t i o n s . I I . REDUCTIVE DISSOLUTION The s o l u b i l i t y of s u l p h u r d i o x i d e i n aqueous s o l u t i o n s d e creases w i t h i n c r e a s i n g t e m p e r a t u r e , c o n v e r s e l y the r a t e of d i s s o l u t i o n of i r o n o x i d e i n c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . The maximum p r a c t i c a l p r e s s u r e of s u l p h u r d i o x i d e a v a i l a b l e from the c y l i n d e r was k2 p . s . i . The m a j o r i t y of the runs were c a r r i e d out at 110°C because o f : i ) An a c c u r a t e l y measureable r a t e was o b t a i n e d a f t e r o n l y a few ho u r s . i i ) The s u l p h u r d i o x i d e over p r e s s u r e was s t i l l h i g h enough to a l l o w m e a n i n g f u l v a r i a t i o n s i n the p a r t i a l p r e s s u r e of s u l p h u r d i o x i d e . F i g . 30 a. Electron-probe picture of a leached single c r y s t a l hematite flake, x 255 Fig. 30 b. Electron microscope picture of a "bump", x 20,000 i i i ) A l l p r e v i o u s work on a FeOOH had t e e n c a r r i e d out at t h i s t e mperature and thus f o r ease of c o m p a r i s o n , t h i s temperature was again s e l e c t e d f o r a Fe^O^ d i s s o l u t i o n . T y p i c a l l e a c h i n g curves f o r b o t r y o i d a l a Fe^O , massive r e d a Fe^O and g o e t h i t e o b t a i n e d i n a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s at 110°C u s i n g a 1 gm. p a r t i c u l a t e f e e d sample are shown i n F i g . 3 1 . These curves are v e r y s i m i l a r t o those o b t a i n e d i n d i r e c t a c i d d i s s o l u t i o n e x p e r i m e n t s . B o t r y o i d a l a Fe^O^ and a FeOOH showed i n c r e a s i n g r a t e s w i t h time whereas massive r e d a Fe^O^ showed almost a l i n e a r r e l a t i o n s h i p between i r o n d i s s o l v e d and t i m e . The r a t e s of d i s s o l u t i o n i n c r e a s e d i n the same or d e r as the d i r e c t d i s s o l u t i o n . M a ssive r e d a Fe 20 < a FeOOH < B o t r y o i d a l a Fe^O^ I t was n o t i c e d t h a t on the runs f o r g o e t h i t e and b o t r y o i d a l h e m a t i t e , the tef l o n p a r t s of the a u t o c l a v e had a f i n e l a y e r of y e l l o w or r e d m a t e r i a l a d h e r i n g to.them at the end of the run. The i n t e r i o r of the a u t o c l a v e remained c l e a n a f t e r a run on massive r e d h e m a t i t e . Owing t o the l a c k of d i f f e r e n t m a t e r i a l s , the m a j o r i t y of the runs were c a r r i e d out u s i n g massive r e d hematite ( i ) A c i d i f i e d S u l p h u r D i o x i d e S o l u t i o n s T y p i c a l d i s s o l u t i o n p l o t s of the amount of i r o n d i s s o l v e d v e r s u s time f o r v a r y i n g p a r t i a l p r e s s u r e of s u l p h u r d i o x i d e are shown i n F i g . 32. A l l s o l u t i o n s had been a c i d i f i e d by .5M HCIO^. The r a t e of d i s s o l u t i o n can be seen t o i n c r e a s e w i t h the p a r t i a l p r e s s u r e of s u l p h u r d i o x i d e and a p l o t of PSO 66 15 io Fe dissolved. 10 5 0 F i g . 3 1 . T y p i c a l l e a c h i n g curves f o r the r e d u c t i v e dissolution.-'-of v a r i o u s h e m a t i t e samples i n a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s . Time (mins.) F i g . 3 2 . E f f e c t of v a r y i n g the SO^ p a r t i a l p r e s s u r e i n a c i d i f i e d s o l u t i o n s . 69 v e r s u s 5% r a t e shows a l i n e a r r e l a t i o n s h i p ,Fig.33. I n c r e a s i n g the a c i d i t y of the s o l u t i o n i n c r e a s e d the observed r a t e , Fig.34 showing curves f o r .5M, 1M and 2M HCIO^ at a c o n s t a n t p a r t i a l p r e s s u r e of s u l p h u r d i o x i d e . The r a t e s of d i s s o l u t i o n f o r the d i f f e r e n t a c i d c o n c e n t r a t i o n s a gain i n c r e a s e d i n a l i n e a r f a s h i o n w i t h t h e p a r t i a l p r e s s u r e of s u l p h u r d i o x i d e , ( F i g . 35). A p l o t of l o g r a t e v e r s u s l o g [HCIO^J gave a s t r a i g h t l i n e w i t h a s l o p e of a p p r o x i m a t e l y .33 i n d i c a t i n g a 1/3 , rate.dependence p r o p o r t i o n a l t o [HCIO^J ( i i ) Aqueous Su l p h u r D i o x i d e S o l u t i o n s The shape of the r a t e curve f o r massive r e d a Fe^O^ i s s i m i l a r to t h a t o b t a i n e d i n a c i d i f i e d s o l u t i o n s . V a r i a t i o n s i n the p a r t i a l p r e s s u r e of s u l p h u r d i o x i d e a f f e c t e d the r a t e i n the same manner as f o r a c i d i f i e d s o l u t i o n s Fig.36, a p l o t of PSOg v e r s u s r a t e again g i v i n g a s t r a i g h t l i n e , Fig.37. ( i i i ) P o l i s h e d Specimens D i s s o l u t i o n of p o l i s h e d specimens obeyed the same k i n d of b e h a v i o u r as had been seen i n the a c i d d i s s o l u t i o n . B o t r y o i d a l h e m a t i t e t u r n i n g r e d , g o e t h i t e t u r n i n g y e l l o w (see F i g . 38) and massive r e d h e m a t i t e r e t a i n i n g i t s grey c o l o u r a t i o n and showing e x t e n s i v e p i t t i n g . 70 71 F i g . 3 6 . E f f e c t of the S 0 2 p a r t i a l p r e s s u r e on the d i s s o l u t i o n of Massive r e d h e m a t i t e non a c i d f i e d s o l u t i o n s . 73 Fig. 38. Goethite sample leached i n a c i d i f i e d SO solution, x 5 75 DISCUSSION I... ANISOTROPIC DISSOLUTION The phenomenon of a n i s o t r o p i c d i s s o l u t i o n i n c e r -t a i n c r y s t a l l i n e m a t e r i a l s . , m e t a l l i c or non m e t a l l i c i n n a t u r e , T X 8 19 has been w i d e l y r e p o r t e d i n the l i t e r a t u r e ' ' . T h i s phenomenon has been a t t r i b u t e d t o the e f f e c t of d i f f e r e n c e s i n atomic s p a c i n g s or p a c k i n g i n the c r y s t a l p l a n e s on the ease of a d s o r p t i o n of r e a c t i n g i o n s or m o l e c u l e s . Non m e t a l l i c c r y s t a l s and c r y s t a l s of low symmetry n o r m a l l y e x h i b i t t h i s phenomenon t o a much g r e a t e r e x t e n t . P r e v i o u s workers i n v e s t i g a t i n g the d i r e c t d i s s o l u -t i o n of h e m a t i t e i n a c i d s have shown t h a t h e m a t i t e e x h i b i t s 7 2 0 a n i s o t r o p i c d i s s o l u t i o n . The b a s a l p l a n e (0001) of h e m a t i t e appears t o be the most r a p i d l y l e a c h e d p l a n e , the d i s s o l u t i o n r a t e of t h i s p l a n e b e i n g a p p r o x i m a t e l y an o r d e r of magnitude f a s t e r than any of the o t h e r p l a n e s examined. I I . DIRECT DISSOLUTION ( i ) Hematite The d i s s o l u t i o n curves f o r h e m a t i t e o b t a i n e d i n t h i s s t u d y , which are t y p i f i e d by the curves o b t a i n e d i n 2.hu HC1, t o g e t h e r w i t h a t y p i c a l d i s s o l u t i o n curve of Azuma and Kametani are shown c o l l e c t i v e l y i n F i g . 3 9 . The v a r y i n g c o n d i t i o n s f o r these p l o t s are l i s t e d i n Table 8 , the temperature i n a l l cases b e i n g 80°C. 76 30 20 io Fe dissolved. 10 0 F i g . 3 9 - Types of d i s s o l u t i o n p l o t s o b t a i n e d f o r h e m a t i t e i n h y d r o c h l o r i c a c i d . 77 TABLE 8 M a t e r i a l S i z e (mesh.) A c i d App. r a t e % Fe/min. ^ M a s s i v e r e d - 70 + 140 2 . 4M HC1 0 .04 O B o t r y o i d a l - 70 + 140 2 . 4M HC1 0 .35 O B r a z i l i a n - 70 + 140 2 . 4M HC1 0 .11 V S y n t h e t i c - 70 + 140 2 .km. HC1 0 .035 • S y n t h e t i c powder < 325 2 . 4M HC1 10 . 0 • S y t h e t i c * - 100 1M HC1 0 .8 * Azuma and Kametani From F i g u r e 39 , i t i s c l e a r t h a t a wide range of observed d i s s o l u t i o n r a t e s have been o b t a i n e d , v a r y i n g from ^ 10% Fe/min. f o r powdered h e m a t i t e to ^0.04% Fe/min. f o r the massive r e d h e m a t i t e . The observed d i s s o l u t i o n r a t e i s r e l a t e d t o the a v a i l a b l e a r e a of the d i s s o l v i n g m a t e r i a l , the r a t e i n c r e a s i n g w i t h i n c r e a s i n g s u r f a c e a r e a . The powdered m a t e r i a l would thus be e x p e c t e d t o d i s s o l v e much more r a p i d l y than the o t h e r s i z e s due t o i t s l a r g e r s u r f a c e a r e a . The d i s -s o l u t i o n r a t e s o b t a i n e d on the powdered m a t e r i a l were much g r e a t e r than f o r any of the o t h e r samples. The s y n t h e t i c h e m a t i t e used by Azuma and Kametani was unwashed m a t e r i a l , c o a r s e r than 100 mesh. The r a p i d d i s -s o l u t i o n a c h i e v e d by t h e i r specimens cannot be a t t r i b u t e d d i r e c t l y t o the l a r g e a v a i l a b l e s u r f a c e a r e a (though any a t t a c h e d f i n e s would i n c r e a s e trie surface, area and the. oh-s e r v e d r a t e 1. 78 A l l trie o t h e r d i s s o l u t i on curves are f o r d i f f e r e n t types' of Hematite In the s i z e range —' &5 + -^ O mesh, and thus the i n i t i a l s u r f a c e areas would he comparable. E x a m i n a t i o n of the washed and d r i e d p a r t i c l e s showed t h a t the b o t r y o i d a l h e m a t i t e was more l e n t i c u l a r i n sh.ape whereas the o t h e r samples were more r e g u l a r i n t h e i r d i m e n s i o n s . The s u r f a c e a r e a of the b o t r y o i d a l h e m a t i t e may be s l i g h t l y l e s s than f o r the o t h e r forms of h e m a t i t e . The b o t y o i d a l h e m a t i t e d i s s o l v e d at almost an o r d e r of magnitude f a s t e r than the massive r e d h e m a t i t e a f t e r the i n i t i a l s t a g e . The shape of the d i s s o l u t i o n curve f o r b o t r y -o i d a l h e m a t i t e i s s i m i l a r t o the " a c c e l a r a t e d " shape o b t a i n e d by Azuma and Kametani. A l l the o t h e r curves are o r i g i n a l l y l i n e a r , but showing a decrease i n d i s s o l u t i o n r a t e as d i s -7 s o l u t i o n nears c o m p l e t i o n . Bath, o b t a i n e d s i m i l a r curve shapes i n h i s s t u d y . A d i s s o l u t i o n r a t e t h a t i n c r e a s e s i n time ( " a c c e l e -r a t e d " t y p e ) i s o f t e n a s s o c i a t e d w i t h an i n c r e a s i n g s u r f a c e a r e a ; d e c r e a s i n g r a t e s w i t h i n c r e a s i n g time ( " d e c l e r a t i n g " type) are more i n d i c a t i v e of a d e c r e a s i n g s u r f a c e a r e a . The a c t i v a t i o n e n e r g i e s of a l l the m a t e r i a l s i n d i r e c t d i s s o l u t i o n i n h y d r o c h l o r i c a c i d were almost i d e n t i c a l . A s i m i l a r c h e m i c a l mechanism may thus be o p e r a t i n g even f o r the 7 two d i f f e r e n t types of curve shapes o b t a i n e d . Bath , by measur-i n g the a c t i v a t i o n e n e r g i e s of the l e a c h i n g of d i f f e r e n t p l a n e s i n h e m a t i t e , had p o s t u l a t e d t h a t though the l e a c h i n g r a t e s v a r i e d f o r d i f f e r e n t c r y s t a l o r i e n t a t i o n s , the same c h e m i c a l 79 mechanism was. prohalily- o p e r a t i n g on th.e d i f f e r e n t c r y s t a l planes:. Chemical compositions, of the v a r y i n g m a t e r i a l s were s i m i l a r except f o r the h i g h s i l i c a c o n t e n t of the massive re d h e m a t i t e . No c a t a l y s i s or i n h i b i t i o n of d i s s o l u t i o n would he a n t i c i p a t e d from e i t h e r the major ( s i l i c a ) or minor im-p u r i t i e s . These f a c t o r s suggest t h a t a p h y s i c a l r a t h e r t h a n a c h e m i c a l phenomenon may he r e s p o n s i b l e f o r the v a r i a t i o n i n the shapes of the d i s s o l u t i o n curves and thus a p h y s i c a l s o l u -t i o n was f i r s t i n v e s t i g a t e d . ( i i ) P h y s i c a l A s p e c t s of the D i r e c t D i s s o l u t i o n A l l the h e m a t i t e m a t e r i a l s used had the same c r y s t a l s t r u c t u r e , the t r i g o n a l a Fe^O^. T h i s s t r u c t u r e c o n s i s t s of the oxygen i o n s i n the a n i o n l a t t i c e b e i n g arranged i n a s l i g h t l y d i s t o r t e d hexagonal p a c k i n g , w h i l e s u c c e s s i v e c a t i o n l a y e r s c o n t a i n e q u a l numbers of i r o n atoms a l l i n s i x f o l d c o o r d i n a t i on. There was a n o t i c e a b l e d i f f e r e n c e i n the m o r p h o l o g i c a l s t r u c t u r e of the v a r i o u s samples a s s o c i a t e d w i t h t h e i r mode of f o r m a t i o n . One of the major a f f e c t s of the n a t u r e of f o r m a t i o n of t h e s e m a t e r i a l s i s the v a r i a t i o n i n g r a i n s i z e . T r a n s m i s s i o n X-ray s t u d i e s i n d i c a t e d a v e r y c o a r s e g r a i n s i z e f o r the massive r e d h e m a t i t e , but a v e r y f i n e g r a i n s i z e f o r the b o t r y o i d a l h e m a t i t e U s i n g o p t i c a l m i c r o s c o p y , an i n d i c a t i o n of the v a r i o u s g r a i n s i z e s was o b t a i n e d f o r the " d e c e l a r a t i n g " r a t e specimens. 80 E l e c t r o n mis.cros.copy and ;X-*ray l i n e , h r o a d e n i n g t e c h n i q u e s were used i n the case o f the b o t r y o i d a l h e m a t i t e because of the e x t r e m e l y f i n e g r a i n s i z e . T able 9 compares the r e l a t i v e r a t e s and shapes the d i s s o l u t i o n p l o t s w i t h the approximate g r a i n s i z e s . TABLE 9 M a t e r i a l Rate % Fe/min. Curve Shape G r a i n S i z e M a ssive Red .oh D e c e l e r a t i n g > 1000 y B o t r y o i d a l • 35 A c c e l e r a t i n g ^ 0 . l y B r a z i l i a n .11 De c e l e r at i n g S i n g l e c r y s t a l S y n t h e t i c .035 D e c e l e r a t i n g 30 - 70y The s i z e of p a r t i c l e s used i n these runs was between 105u and 210y . From the above d a t a , i t would appear t h a t the f a s t d i s s o l u t i o n and " a c c e l e r a t e d " d i s s o l u t i o n curve may be a t t r i -b u t a b l e t o the f i n e g r a i n s i z e . The specimens used by Azuma and Kametani were p r e -pared by r o a s t i n g f e r r i c s u l p h a t e i n a i r at 700°C f o r twenty-f o u r hours. At t h i s t e m p e r a t u r e , s i n t e r i n g i n a Fe^O^ i s v e r y slow and thus a v e r y f i n e g r a i n s i z e and a l a r g e amount of p o r o s i t y would be a n t i c i p a t e d . The s y n t h e t i c h e m a t i t e used i n t h i s study had been s i n t e r e d f o r s i x t y hours at 1200°C t o g i v e a 30-70y g r a i n s i z e . There was s t i l l some p o r o s i t y i n t h i s m a t e r i a l . An " a c c e l e r a t e d " d i s s o l u t i o n curve f o r s y n t h e t i c 81 h e m a t i t e was. o h t a i n e d . by u s i n g an 8-10. me.s,h C238Q-2QQQu 1 p a r t i c l e s i z e wh ere as. .mass;i"ve. r e d h e m a t i t e s t i l l gave a de-c e l e r a t i n g r a t e c u r v e . The shape of the d i s s o l u t i o n curve f o r s y n t h e t i c h e m a t i t e can thus he a l t e r e d by v a r y i n g the g r a i n s i z e t o p a r t i c l e s i z e r a t i o . The " d e c e l e r a t e d " curve was ob-t a i n e d by h a v i n g o n l y a few g r a i n s per p a r t i c l e whereas the " a c c e l e r a t i n g " curve was o b t a i n e d by h a v i n g a l a r g e number of g r a i n s per p a r t i c l e . T h i s agrees w i t h the d a t a f o r the o t h e r m a t e r i a l s c o n s i d e r e d . I n c r e a s i n g the c a l c i n i n g temperature of h e m a t i t e causes a decrease i n l e a c h i n g r a t e . T h i s has been a t t r i b u t e d t o a decrease i n p o r o s i t y of the m a t e r i a l , but the i n c r e a s e i n the g r a i n s i z e may a l s o be a f a c t o r . Azuma and Kametani n o t i c e d t h a t above 900°C, the c a l c i n i n g can cause a change i n the type of d i s s o l u t i o n c u r v e . At t h i s t e m p e r a t u r e , s i n t e r -i n g i n a Fe^O^ i s q u i t e r a p i d and hence the p a r t i c l e s i z e t o g r a i n s i z e r a t i o may change enough t o account f o r the a l t e r -a t i o n of the d i s s o l u t i o n c u r v e , a p a r t i c l e s i z e of > 100 mesh b e i n g used i n t h e i r s t u d i e s . The d i s s o l u t i o n of the p o l i s h e d s e c t i o n s of h e m a t i t e i n d i c a t e s a p o s s i b l e r e a s o n f o r the d i f f e r e n c e i n shape. L e a c h i n g of the massive r e d h e m a t i t e , though c a u s i n g some p i t t i n g , d i d not l e a d t o a r e d c o l o u r a t i o n of the s u r f a c e as i n the case of the b o t r y o i d a l and s y n t h e t i c h e m a t i t e s ( F i g -ures 2k and 2 5 ) . T h i s c o l o u r a t i o n , which was due t o f i n e h e m a t i t e p a r t i c l e s on the s u r f a c e , o c c u r r e d r a p i d l y f o r b o t r y o i d a l h e m a t i t e , but s l o w l y f o r s y n t h e t i c h e m a t i t e . 82 As. h e m a t i t e l a a n i s o t r o p i c , p r e f e r e n t i a l l e a c h i n g of favourab.ly- o r i e n t a t e d grains- occurs-. The p r e f e r a b l y o r i e n t a t e d g r a i n s , -with a b a s a l (0001] o r i e n t a t i o n , w i l l be l e a c h e d an ord e r of magnitude f a s t e r compared w i t h o t h e r o r i e n t a t e d g r a i n s , thus e x p o s i n g new g r a i n s f o r a t t a c k . T h i s p r o c e s s can thus r e -peat i t s e l f undermining s l o w l y d i s s o l v i n g g r a i n s and l e a v i n g a v e r y porous l a y e r of f i n e h e m a t i t e p a r t i c l e s on the s u r f a c e which would impart the c h a r a c t e r i s t i c r e d c o l o u r of h e m a t i t e . These p a r t i c l e s w i l l c o n t i n u e t o d i s s o l v e at t h e i r much sl o w e r r a t e or may be removed from the s u r f a c e by a b r a s i o n . T h i s would e x p l a i n the s l i g h t r e d c o l o u r a t i o n of the s o l u t i o n and the r e d c o a t i n g found on the t e f l o n s p i n bar at the end of a d i s s o l u t i o n experiment on b o t r y o i d a l h e m a t i t e but not on the ot h e r specimens. V i s u a l o b s e r v a t i o n of the f o r m a t i o n and removal of t h i s porous l a y e r was seen on the d i s s o l u t i o n experiments on the b o t r y o i d a l h e m a t i t e p o l i s h e d s e c t i o n s . In p o l y - g r a i n e d p a r t i c l e s t h e r e appears t o be a r a p i d l e a c h i n g of f a v o u r a b l y o r i e n t a t e d g r a i n s which exposes an i n c r e a s e d number of f a v o u r a b l y o r i e n t a t e d g r a i n s . As a r e s u l t , the a c t u a l s u r f a c e a r e a of f a s t l e a c h i n g g r a i n s w i l l increase causing the " a c c e l e r a t e d " type of d i s s o l u t i o n c u r v e . The u n f a v o u r a b l y o r i e n t a t e d g r a i n s w i l l c o n t i n u e t o d i s s o l v e s l o w l y , and a f t e r a l l the f a v o u r a b l y o r i e n t e d g r a i n s have been l e a c h e d , a slower r a t e t o complete d i s s o l u t i o n w i l l occur as the s e are d i s s o l v e d . The p a r t i c u l a t e f e e d f o r s y n t h e t i c ,massive r e d and B r a z i l i a n h e m a t i t e s c o n t a i n e d r e l a t i v e l y few, and i n many 83 i n s t a n c e s , o n l y one g r a i n of h e m a t i t e per p a r t i c l e . The d i s -s o l u t i o n b e h a v i o u r would thus approximate more t o the d i s s o l u -t i o n of a s i n g l e c r y s t a l than the p r e v i o u s l y o u t l i n e d b e h a v i o u r . Hematite has no c l e a v a g e p l a n e s , though i t does undergo p a r t i n g on i t s p r i m a r y t w i n n i n g p l a n e (0001) and to a l e s s e r e x t e n t on the secondary t w i n n i n g p l a n e ( l O l l ) . I f t h i s was c o m p l e t e l y t r u e , the m a j o r i t y of the p a r t i c l e s i n the f e e d would have a f a s t d i s s o l v i n g (OOOl) p l a n e , slow d i s s o l v -i n g ( l O l l ) p l a n e , or b o t h , exposed. F i g u r e 40a shows t h a t a l l the exposed f a c e s of the B r a z i l i a n h e m a t i t e are v e r y smooth. The massive r e d h e m a t i t e showed at l e a s t one c l e a r l y f r a c t u r e d s u r f a c e , F i g u r e 40b, as the g r a i n s i z e of the ore was l a r g e r than the p a r t i c l e s i z e . The s y n t h e t i c h e m a t i t e , whose g r a i n s i z e was s l i g h t l y l e s s than the p a r t i c l e s i z e showed a c l e a n but p i t t e d s u r f a c e , F i g u r e 40c. A l l t h r e e samples d i f f e r e d markedly from th e B o t r y o i d a l h e m a t i t e p a r t i c l e s u r f a c e , F i g u r e 404. B ath had shown t h a t though p i t t i n g and f a c e t i n g d i d occur d u r i n g d i s s o l u t i o n of o r i e n t a t e d c r y s t a l f a c e s , a l i n e a r d i s s o l u t i o n p l o t was s t i l l o b t a i n e d . T h i s appeared to i n d i c a t e t h a t the f a c e t i n g d i d not a p p r e c i a b l y a f f e c t the e f f e c t i v e s u r -f a c e a r e a of d i s s o l u t i o n . When the B r a z i l i a n h e m a t i t e d i s s o l v e s , d i s s o l u t i o n w i l l t a k e p l a c e p r e d o m i n a n t l y on the exposed (.0001) f a c e s . These f a c e s may undergo p i t t i n g and f a c e t i n g , but t h e r e w i l l be no l a r g e i n c r e a s e i n the s u r f a c e a r e a as i n the case of b o t r y o i d a l h e m a t i t e . The p a r t i c l e s , b e i n g s i n g l e c r y s t a l s , would show no r e d c o l o u r a t i o n as the p a r t i c l e s would r e t a i n t h e i r dense c r y s t a l s t r u c t u r e t h r o u g h o u t . A s i m i l a r argument Fig.40 a. B r a z i l i a n hematite p a r t i c l e . x255 Fig.40 b. Massive Red hematite p a r t i c l e . x255 Fig. 40 c. Synthetic hematite p a r t i c l e . x255 Fig. 40 d. Botryoidal hematite p a r t i c l e . x255 85 would als.o a p p l y to., the s y n t h e t i c and mas,s.i.y e. r e d hematites.. The l e a c h e d p o l i s h e d surface, of massive r e d h e m a t i t e showed no red c o l o u r a t i o n . T h i s c o u l d he t h a t as s u r f a c e g r a i n s are undermined or l o o s e n e d , t h e y would s t i l l he v e r y l a r g e and p o s s i b l y f a l l away b e f o r e b e i n g reduced t o a f i n e enough s i z e t o cause a r e d c o l o u r a t i o n , or t h a t as t h e r e would be so few f i n e g r a i n s i n any g i v e n a r e a , no n o t i c e a b l e c o l o u r a t i o n would be obs e r v e d . The s y n t h e t i c specimens, be-cause of the s m a l l g r a i n s i z e d i d show the r e d c o l o u r a t i o n , F i g u r e 25 . The B r a z i l i a n h e m a t i t e p a r t i c l e s , as they o r i g i n a t e d from a s i n g l e c r y s t a l , would be expected t o have a l a r g e num-ber of the p a r t i n g (.0001) f a c e s exposed, more so than the massive r e d h e m a t i t e which may a l s o f r a c t u r e around g r a i n b o u n d a r i e s . The s y n t h e t i c h e m a t i t e would be expected t o have p a r t e d m a i n l y around g r a i n b o u n d a r i e s and have v e r y (0001) f a c e s r e v e a l e d . As the observed l e a c h i n g r a t e i s p r i m a r l y on the s u r f a c e a rea of (.0001) f a c e s , an i n c r e a s e i n r a t e a c c o r d i n g t o S y n t h e t i c < M a s s i v e r e d < B r a z i l i a n would be a n t i c i p a t e d . T h i s o r d e r agreed w i t h the r e s u l t s , ob-t a i n e d e x p e r i m e n t a l l y ( F i g u r e 3 ) . In the i n i t i a l d i s s o l u t i o n s t a g e , B r a z i l i a n h e m a t i t e would have a l a r g e p r o p o r t i o n of i t s s u r f a c e a r e a as (0001) f a c e s . B o t r y o i d a l h e m a t i t e , which would p a r t a l o n g g r a i n b o u r n d a r i e s , would have a s m a l l e r a r e a of (0001) f a c e s o r i g i n -a l l y exposed. B r a z i l i a n h e m a t i t e might thus be ex p e c t e d t o few c l e a n dependent l e a c h i n g 86 leach, as f a s t a s , or f a s t e r t h a n , b o t r y o i d a l h e m a t i t e i n the o r i g i n a l d i s s o l u t i o n s tage u n t i l the s u r f a c e area of (OOOl) f a c e s i n the h o t r y o i d a l h e m a t i t e has i n c r e a s e d s u f f i e n t l y due t o p r e f e r e n t i a l a t t a c k o f the f a v o u r a b l y o r i e n t a t e d g r a i n s . The e x p e r i m e n t a l r e s u l t s ( F i g u r e 3 ) showed t h a t the o r i g i n a l l e a c h i n g r a t e s were comparable. No a l l o w a n c e was made t h a t t h e p a r t i c l e s may undergo some f r a g m e n t a t i o n ' d u r i n g a run. S i m i l a r runs c a r r i e d out i n water showed o n l y a v e r y s m a l l amount of f r a g m e n t a t i o n o c c u r r e d , and thus i f any major f r a g m e n t a t i o n does o c c u r , i t i s p a r t i a l l y a s s o c i a t e d w i t h the d i s s o l u t i o n p r o c e s s . ( i i i ) D i r e c t D i s s o l u t i o n of G o e t h i t e G o e t h i t e i s a "secondary" m i n e r a l o f t e n formed by w e a t h e r i n g of i r o n o x i d e s and i t i n v a r i a b l y has a v e r y f i n e g r a i n s i z e . T h e - t r a n s m i s s i o n X-ray a n a l y s i s showed t h i s t o be t r u e f o r the specimen s t u d i e d and a g r a i n s i z e of about l y was observed u s i n g the e l e c t r o n m i c r o s c o p e . G o e t h i t e has an a c t i v a t i o n energy f o r d i r e c t d i s s o -l u t i o n i n v a r i o u s a c i d s s i m i l a r t o t h a t of h e m a t i t e and i t has been p o s t u l a t e d t h a t b o t h a Fe^O^ and a FeOOH r e a c t by s i m i l a r g c h e m i c a l mechanism . G o e t h i t e , which has an ort h o r h o m b i c s t r u c t u r e , i s a l s o a n i s t r o p i c i n n a t u r e . The oxygen atoms are arranged i n a h e x a g o n a l l y c l o s e - p a c k e d l a y e r w i t h the i r o n atoms i n the o c t a h e d r a l i n t e r s t i c e s . Assuming t h a t a FeOOH and b o t r y o i d a l h e m a t i t e r e a c t by s i m i l a r c h e m i c a l mechanisms, an " a c c e l e r a t e d " d i s s o l u t i o n curve and a sl o w e r r a t e , because of the l a r g e r g r a i n s i z e , would 87 be expect e d. T h i s was. found toJhe. t h e cage e x p e r i m e n t a l l y ; , F i g u r e 3. The p o l i s h e d s u r f a c e s showed the same manner of a t t a c k as f o r b o t r y o i d a l h e m a t i t e except t h a t t h e c h a r a c t e r -i s t i c y e l l o w c o l o u r of g o e t h i t e was o b s e r v e d , F i g u r e 28. One n o t i c e a b l e d i f f e r e n c e was t h e inhomogenelty of t h i s y e l l o w c o a t i n g , F i g u r e 28a. T h i s may w e l l be a s s o c i a t e d w i t h some degree of p r e f e r r e d o r i e n t a t i o n of the g r a i n s i n t h e s p e c i -men and a l s o to the l a r g e s i l i c a c o n t e n t . The g o e t h i t e was found t o have i d e n t i c a l l e a c h i n g c h a r a c t e r i s t i c s t o the b o t r y o i d a l h e m a t i t e throughout t h i s study.. ( i v ) P h e m i n a l Mechanism A c t i v i a t l o n e n e r g i e s o b t a i n e d f o r t h e d i f f e r e n t samples i n the d i f f e r e n t a c i d s are l i s t e d below (Table 10 I , and are compared t o those o b t a i n e d by previous, w o r k e r s . There i s no s i g n i f i c a n t change i n the v a l u e s w i t h a c i d c o n c e n t r a t i o n , the r e s u l t s i n t h i s s t u d y comparing f a v o u r a b l y t o those i n p r e v i o u s s t u d i e s . Rate of d i s s o l u t i o n f o r s u l p h u r i c and p e r c h l o r i c a c i d i n c r e a s e d p r o p o r t i o n a l l y w i t h the a c i d c o n c e n t r a t i o n i n the range 1-5M. A l i n e a r dependence on [H + ] c o n c e n t r a t i o n appeared t o be i n d i c a t e d i n t h e s e c a s e s . H y d r o c h l o r i c a c i d showed an i n c r e a s i n g r a t e w i t h a c i d c o n c e n t r a t i o n up t o 6 M. Th i s dependence on HC1 c o n c e n t r a t i o n had been s t u d i e d 7 p r e v i o u s l y and i t was shown t h a t the r a t e was dependent on the anion and the c a t i o n . 88 TABLE lQ A c t i v a t i o n E n e r g i e s f o r D i r e c t D i s s o l u t i o n ( K c a l / m o l l ACID M a t e r i a l HC1 H„S0 ; HC10, M a s s i v e r e d a F e 2 ° 3 23.4 18, . 7 ± 2 , • 5 B o t r y o i d a l a F e 2 ° 3 2 3.2 20 , . 2 ± 1 , .8 21. . 1 ± 5 . 2 B r a z i l i an a F e 2 ° 3 22 .7 S y n t h e t i c a F e 2 ° 3 22 .6 18, . 4±2 , .0 22, .1±4.5 G o e t h i t e 21 .1 S y n t h e t i c ' 7 a F e 2 ° 3 1 9 . 5 - 2 1 . 6 S i n g l e c r y s t a l 7 . a F e 2 0 3 20-24 8 G o e t h i t e 22 . 5 19. .8 Synthet i c ^  a F e 2 ° 3 21 .9 18, . 2 19. . 2-21.4 The l o g r a t e - l o g a c i d c o n c e n t r a t i o n p l o t f o r the t h r e e a c i d s f o r the h o t r y o i d a l a Fe^O^ samples gave s t r a i g h t l i n e s w i t h s l o p e s of one f o r the p e r c h l o r i c and s u l p h u r i c a c i d s , and a p p r o x i m a t e l y two f o r h y d r o c h l o r i c a c i d . 7 The mechanism proposed by Bath e x p l a i n e d a d e q u a t e l y the a t t a c k by h y d r o c h l o r i c a c i d . The s u r f a c e i s f i r s t assumed to undergo h y d r a t i o n . T h i s h y d r a t i o n i s b e l i e v e d t o be r a p i d . The f a c t t h a t t h e r e i s no obvious d i f f e r e n c e s i n r a t e f o r the a Fe^O^ and a FeOOH powders; the r a t e f o r b o t r y o i d a l a Fe^O^ was g r e a t e r than f o r a FeOOH; and a Fe 0_ and a FeOOH behave B 2 3 i d e n t i c a l l y i n the v a r i o u s a c i d s , appears t o c o n f i r m t h i s r a p i d h y d r a t i o n . 89 A h y d r a t e d s u r f a c e w i l l he r e p r e s e n t e d by : -I -0-Fe,-OH 1 s Thi s s u r f a c e may he p r o t o n a t e d -0-Fe-OH.- + H o 0 + •—* I -O-Fe + + 2H„0 s • ' 3 * s 2 T h i s s u r f a c e s i t e may then adsorb an a n i o n such as CI , + K ? -O-Fe + CI ==± I -O-Fe-Cl s * 1 s and t h i s complex may then d e s o r b , [ ^ - O - F e - C l — ^ FeO C l ( a q ) T h i s l a s t step was c o n s i d e r e d r a t e d e t e r m i n i n g The o v e r a l l r e a c t i o n i s : -0-Fe-OH + H o 0 + + C I - - * FeO C l ( a q ) + 2H o0 s 3 2 and the r a t e e q u a t i o n i s , d (F e ) aq — = k 2 K l K 2 H s-0-Fe-0H] a H + . a c l -dt T h i s r a t e e q u a t i o n g i v e s the c o r r e c t r a t e dependence of a c i d c o n c e n t r a t i o n f o r h y d r o c h l o r i c a c i d . U n f o r t u n a t e l y , the r a t e e x p r e s s i o n does not s a t i s f y the cases f o r p e r c h l o r i c or s u l p h u r i c a c i d s where a d i r e c t de-pendence on a c i d c o n c e n t r a t i o n was found. 13 Ahmed and M a k s i n o i r , i n t h e i r s tudy of zero p o i n t charge of h e m a t i t e i n v a r i o u s a c i d s , p o s t u l a t e d t h a t the s u r f a c e 40 can undergo p r o t o n a t i o n to form an aquocomplex or be a t t a c k e d d i r e c t l y by the a n i o n . A d u a l mechanism may thus be o p e r a t i v e The s u r f a c e may f i r s t undergo a p r o t o n a t i o n as p r e v i o u s l y d e s c r i b e d , . K | g-0-Fe-0H + H 30 | g-0-Fe (B^O) + H 20 Th i s aquocomplex may e i t h e r d e s o r b , | g - 0 - F e ( H 2 0 ) + -+1 0 - F e C H 2 0 ) + ( a q ) or absorb an a n i o n X , + K ? I -0-Fe(H„0) + X ^ I -0-Fe-(H„O) X ' s I s 2 and the r e s u l t i n g complex then d e s o r b s . l s-Q-Fe(H 20) X 0-Fe(H 20) X(aq) Assuming once a g a i n t h a t d e s o r b t i o n i s the r a t e l i m i t i n g s t e p , the o v e r a l l r e a c t i o n may be w r i t t e n , 2| -0-Fe-OH + 2H 0 + + C l ~ - * O-Fe (H_0) + ( a q ) + O-Fe (H_0) X (aq + 2H 20 and the r a t e w i l l be g i v e n by, d l F e J dIO-Fe H 0 + J djO-Fe HO X j . = i + £ dt dt dt = k l \ M s - 0 - F e - 0 H j a H + + k g K g K± [|g-0-Fe-0HjaR+ a^-= K x [| g-0-Fe-0H]a H+ ( k 1 + k 2 K 2 & x ~ ) . I f the a n i o n i s a good complexer with, i r o n , K 2 w i l l be l a r g e and as the ani o n w i l l r a p i d l y absorb on the s u r f a c e , 91 I | ^ -O-Fe-H^O*] w i l l be low. THe r a t e e q u a t i o n can then be s i m p l i f i e d t o , d l F e ] = k- K . K . I | -0-Fe-OH] a„+a Y-d t = K a - f a -ct H X i f |he number of s u r f a c e s i t e s remain c o n s t a n t . •I C h l o r i d e i o n s complex r e a d i l y w i t h i r o n and w i l l l e a d to the r a t e e x p r e s s i o n g i v e n above. T h i s e x p r e s s i o n i s i d e n -t i c a l to t h a t o b t a i n e d p r e v i o u s l y which i s known to f i t the ex-p e r i m e n t a l r e s u l t s for h y d r o c h l o r i c a c i d . In the case of a weak anion w i t h low complexing a f f i n i t y f o r i r o n , w i l l be ve r y s m a l l and the r a t e de-pendence w i l l be g i v e n by, d[Fe] dt = k 1 K x l| s-0-Fe-0H l a H + - k g a R+ which i n d i c a t e s the r a t e i s p r o p o r t i o n a l to the a c t i v i t y of a^ + or a p p r o x i m a t e l y p r o p o r t i o n a l t o the a c i d c o n c e n t r a t i o n . Such a weak complexer i s the CIO^ i o n - e x p e r i m e n t a l r e s u l t s f o r p e r c h l o r i c a c i d obeyed t h i s mechanism. S u l p h u r i c a c i d , at c o n c e n t r a t i o n s g r e a t e r than 1M i s known to d i s s o c i a t e p r e d o m i n a t e l y t o : -H SO. H + + HSO " 2 4 4 The a n i o n HSO^ i s a weak complexer w i t h i r o n and so 22 a l i n e a r dependence of r a t e on a c i d c o n c e n t r a t i o n would be expected as was found e x p e r i m e n t a l l y . T h i s w i l l not be the case f o r weak s u l p h u r i c a c i d s o l u t i o n s where s u l p h a t e i o n s , which are s t r o n g c o m p l e x e r s , are a l s o p r e s e n t . The r a t e dependence on the a c i d c o n c e n t r a t i o n ( f o r s i n g l y charged anions) can be summarized as. follows: i ) I f the a n i o n i s a s t r o n g complexer, e.g. CI , Br , 2 r a t e = k a„+ a n , _ k [ a c i d ] a H ^-L a i i ) I f the ani o n i s a weak complexer, e.g. CIO^ , NO^ r a t e = kg a j j + kg [ a c i d ] A p l o t of l o g r a t e v e r s u s l o g [ a c i d ] s h o u l d g i v e a s l o p e of ^2 f o r s t r o n g complexers and ^1 f o r weak com p l e x e r s . T h i s was found t o be t r u e e x p e r i m e n t a l l y . Azuma and Kametani^used v a r i o u s a c i d s i n t h e i r work. Val u e s of the s l o p e o b t a i n e d from t h e i r p l o t of l o g r a t e v e r s u s l o g J a c i d ] are l i s t e d below, t o g e t h e r w i t h r e s u l t s from the p r e s e n t work: A c i d Ions i n . s o l u t i o n Complexin, A b i l i t y g S l o p e HF H + HF 2" St r o n g 1.06 HC1 + H C l " S t r o n g 1.92 HC1 H + C l ~ S t r o n g 1.9^ HC1* H + C l " S t r o n g 1 .92-2.2 HBr H + B r ~ S t r o n g 1.96 HN0 3 H + N0 3" Weak 0.93 HCIO^ H + Weak 0.93 C o n t i n u e d . . . 9 3 C o n t i n u e d . A c i d Ions i n S o l u t i o n Complexing A b i l i t y HClOj^* E 280 U» H 3 P ° 4 H + CLO, H + HSO, H + ? H + ? Weak Weak Slope 1.0 1.0 0 . 56 0.59 * P r e s e n t work, The s t r o n g complexers a l l showed v a l u e s of ^2 as expected except f o r h y d r o f l u o r i c a c i d . H y d r o f l u o r i c a c i d d i s s o c i a t e s a c c o r d i n g t o , 2HF H + + HF 2~ Assuming the [ R + J and {HF^ J c o n c e n t r a t i o n are e q u i v a l e n t , a„+ = { a c i d ] 2 HF. [ a c i d ] As the r a t e i s g i v e n by, Rate = k aTT+ a v a H A J- V Rate = k [ a c i d ] 2 [ a c i d ] 2 = k [ a c i d ] a Thus, owing t o t h e way h y d r o f l u o r i c a c i d d i s s o c i a t e s a l i n e a r dependence o f a c i d c o n c e n t r a t i o n and a s l o p e of ^ 1 would be exp e c t e d . T h i s agrees w i t h the e x p e r i m e n t a l r e s u l t s . The cases of the weak complexing a c i d s agree w i t h the proposed mechanism. 94 The work, by- Azuma and Kametani on H~SO. and H„PO, 2 k 3 1+ p r e s e n t s some d i f f i c u l t y , In the c o n c e n t r a t i o n range t h e y used f o r s u l p h u r i c a c i d .05-1. 5M, v a r i o u s a n i o n s - w i l l be p r e s e n t i n s o l u t i o n dependent on the pH. At the h i g h e r c o n c e n t r a t i o n s , t h e r e was some d e v i a t i o n from a l i n e a r p l o t t o g i v e a h i g h e r . v a l u e of s l o p e . T h i s s u g gests a change of r a t e dependence on 8 6 a c i d c o n c e n t r a t i o n . Both Surana and Monhemius found a l i n e a r dependence of r a t e on a c i d c o n c e n t r a t i o n i n the range s t u d i e d i n t h i s work. I f doubly charged anions are i n s o l u t i o n , e.g. SO. HPOj^ , a m o d i f i c a t i o n t o a l l o w f o r t h i s excess n e g a t i v e charge would be r e q u i r e d and cause a d i f f e r e n t r a t e dependence. (v) R e d u c t i v e D i s s o l u t i o n In the r e d u c t i v e d i s s o l u t i o n of a Fe^O^ i n aqueous and a c i d i f i e d s u l p h u r dioxide s o l u t i o n s , i r o n i s reduced from i t s o r i g i n a l f e r r i c s t a t e to the f e r r o u s s t a t e . T h i s r e d u c t i o n i s accompanied by the f o r m a t i o n of d i t h i o n a t e s and s u l p h a t e s . B a s s e t t and P a r k e r " ^ had shown t h a t f e r r i c h y d rox-i d e was l e a c h e d by s u l p h u r o u s a c i d g i v i n g complex f e r r i c s p e c i e s i n s o l u t i o n from which i r o n was s u b s e q u e n t l y reduced to the f e r r o u s s t a t e . No attempt was made i n t h i s study t o a s c e r t a i n whether t h i s r e d u c t i o n o c c u r r e d p r i o r t o or a f t e r the d e s o r b t i o n of the i r o n complex. The r e s u l t s of t h i s study i n d i c a t e a l i n e a r de-pendence of d i s s o l u t i o n r a t e on the p a r t i a l p r e s s u r e of sulphur d i o x i d e i n the aqueous and a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s . 95 The p a r t i a l p r e s s u r e of s u l p h u r d i o x i d e i n aqueous s o l u t i o n s i s r e l a t e d t o the hydrogen i o n and " b i s u l p h i t e i o n con-c e n t r a t i o n by?"*" [H +] [HSO " ] Ka = — « 10~5 at 110°C .: ^ CS0 2 " [HS0 3"]) •where C i s the t o t a l c o n c e n t r a t i o n of s u l p h u r d i o x i d e i n S 0 2 s o l u t i o n and i s d i r e c t l y p r o p o r t i o n a l t o t h e s u l p h u r d i o x i d e p a r t i a l p r e s s u r e . The r a t e i s thus a l s o d i r e c t l y p r o p o r t i o n a l t o the p r o d u c t of the hydrogen i o n and b i s u l p h i t e i o n con-c e n t r a t i o n . 6,8 I t has been proposed p r e v i o u s l y t h a t a s i m i l a r mech-anism t o t h a t f o r d i r e c t d i s s o l u t i o n may o c c u r . As b i s u l p h i t e i s a s t r o n g complexing a n i o n f o r f e r r i c i o n s , l i n e a r dependence of r a t e on the pr o d u c t of the hydrogen i o n and b i s u l p h i t e i o n c o n c e n t r a t i o n s would be e x p e c t e d . As t h i s p r o d u c t i s d i r e c t l y p r o p o r t i o n a l t o the s u l p h u r d i o x i d e p a r t i a l p r e s s u r e , such a l i n e a r dependence on the i o n i c p r o d u c t e x i s t s . The c o n c e n t r a t i o n s of the ani o n and hydrogen i o n i n n o n - a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s are about 0.04M as most of the s u l p h u r dioxide i s as uridissociated. . m o l e c u l e s at 100°C. -With t h e s e low c o n c e n t r a t i o n s , i t i s d i f f i c u l t t o e n v i s a g e a r e a c t i o n mechanism s i m i l a r t o t h a t f o r d i r e c t d i s s o l u t i o n t o g i v e the r a t e s o bserved. S t u d i e s of the d i s s o l u t i o n of p y r o l u s i t e i n s u l p h u r o u s 15 a c i d l e d t o the i d e a of u n d i s s o c i a t e d s u l p h u r d i o x i d e m o l e c u l e s 96 t a k i n g p a r t i n the d i s s o l u t i o n mechanism. I t was shown t h a t as much as 9Q% of the observed r a t e c o u l d he a t t r i b u t e d to the a d s o r p t i o n of s u l p h u r d i o x i d e m o l e c u l e s and the sub-sequent d e s o r p t i o n of an i r o n - s u l p h u r d i o x i d e complex from the s u r f a c e . Such a mechanism would have a l i n e a r dependence on the p a r t i a l pressure' of s u l p h u r d i o x i d e . From the p r e s e n t s t u d y , i t i s i m p o s s i b l e t o say which i s the a d s o r b i n g s p e c i e s . A c i d i f y i n g the s o l u t i o n s would r e s u l t i n a decrease i n b i s u l p h u t e i o n c o n c e n t r a t i o n from i t s a l r e a d y low v a l u e , however, an i n c r e a s e i n r a t e + 1/3 w i t h [H ] was i n d i c a t e d . T h i s suggests t h a t t h e r e may be two independent mechanism o p e r a t i n g . The r e d u c t i v e d i s s o l u t i o n has been shown t o be r e a d i l y 6 c a t a l y s e d by copper and i t i s possible t h a t hydrogen i o n s may have a c a t a l y t i c a f f e c t on the d i s s o l u t i o n r e a c t i o n . More work would be r e q u i r e d t o u n d e r s t a n d the r o l e of the hydrogen i o n i n the r e d u c t i v e d i s s o l u t i o n . ( v i ) D i s s o l u t i o n Models The e x p e r i m e n t a l r e s u l t s i n d i c a t e d t h a t t h e r e are two d i f f e r e n t d i s s o l u t i o n mechanisms o p e r a t i n g , one c a u s i n g an i n -c r e a s e i n s u r f a c e area and the o t h e r c a u s i n g a decrease i n s u r f a c e a r e a . a) " A c e l e r a t e d " Type As the g r a i n s i z e was always of the order or l a r g e r than the p a r t i c l e s i z e , models based on the l e a c h i n g of s i n g l e 9_7 c r y s t a l s were s t u d i e d f o r a p o s s i b l e e x p l a n a t i o n . The f o l l o w i n g t h r e e s i m p l e models were c o n s i d e r e d : a) I s o t r o p i c d i s s o l u t i o n of a s i n g l e c r y s t a l sphere b) A n i s o t r o p i c d i s s o l u t i o n of a s i n g l e c r y s t a l sphere c) A n i s o t r o p i c d i s s o l u t i o n of a s i n g l e c r y s t a l pyramid In the a n i s o t r o p i c c a s e s , the (0001) p l a n e was con-s i d e r e d as the f a s t d i s s o l v i n g p l a n e . The shape of the d i s s o -l u t i o n curves o b t a i n e d a r e shown i n F i g . 4 1 , the m a t h e m a t i c a l models used are i n Appendix D. Due t o the a n i s o t r o p i c n a t u r e of d i s s o l u t i o n , case Ca) s h o u l d not a p p l y , but F i g . 41 i n d i c a t e s t h a t cases (a) and (c) gave i d e n t i c a l c u r v e s . The p a r t i c l e s o f t e n have w e l l d e f i n e d f l a t f a c e s and as a r e s u l t case (b) would not be a p p l i c a b l e . I f the p a r t i c l e s are randomly shaped or p y r a m i d a l i n n a t u r e (such as the B r a z i l i a n h e m a t i t e p a r t i c l e s ) , and p o s s e s s a f l a t (OOOl) f a s t l e a c h i n g p l a n e , t h e s e p a r t i c l e s c o u l d be grouped i n t o a number of s m a l l spheres w i t h t h e i r (OOOl) p l a n e s exposed. These spheres of p a r t i c l e s would then behave as a s p h e r i c a l p a r t i c l e u ndergoing i s o t r o p i c a t t a c k . P l o t t i n g d i s s o l u t i o n curves t o g e t h e r w i t h the i s o t r o p i c d i s s o l u t i o n curve ( F i g . 42) shows t h e r e i s good agreement. From t h i s c o n s i d e r a t i o n , i t would appear t h a t i f a p a r t i c u l a t e f e e d m a t e r i a l d i s s o l v e s a c c o r d i n g t o the i s o t r o p i c d i s s o l u t i o n curve f o r a s p h e r i c a l p a r t i c l e , a n i s o t r o p i c d i s s o l u t i o n s h o u l d not n e c e s s a r i l y be r u l e d out. An " a c c e l e r a t e d " type d i s s o l u t i o n curve i s not a n e c e s s a r y c r i t e r i a f o r a n i s o -t r o p i c d i s s o l u t i o n . 98 99 /otime to complete dissolution. F i g . 4 2 . C o r r e l a t i o n of d i s s o l u t i o n curves t o the t h e o r e t i c a l d i s s o l u t i o n c u r v e . b) " A c c e l e r a t e d " Type The " a c c e l e r a t e d " type d i s s o l u t i o n specimens have a l a r g e number of g r a i n s per p a r t i c l e . The a c c e l e r a t e d r e g i o n extends to about 30% d i s s o l u t i o n . A s i m p l i f i e d model has been c o n s i d e r e d i n an attempt t o d e s c r i b e the i n c r e a s e i n the observed r a t e d u r i n g t h i s i n i t i a l p e r i o d . In t h i s model, o n l y g r a i n s h a v i n g a b a s a l p l a n e ex-posed are c o n s i d e r e d t o be l e a c h e d . I f the o r i g i n a l s u r f a c e a r e a of a p a r t i c l e i s A, and the average c r o s s s e c t i o n a l area of a g r a i n i s a, then the number of g r a i n s , n, on the s u r f a c e i s g i v e n by: A n = — a Assuming t h a t a f r a c t i o n , <j> , of t h e s e g r a i n s have t h e i r b a s a l p l a n e exposed, the number of f a s t l e a c h i n g g r a i n s w i l l be , n f = As a g r a i n i s l e a c h e d , new s u r f a c e w i l l be exposed, some of which w i l l have the c o r r e c t o r i e n t a t i o n f o r f a s t d i s -s o l u t i o n . C o n s i d e r i n g a t r a n s v e r s e s e c t i o n t h rough a d i s -s o l v i n g p a r t i c l e . o r i g i n a l surface 101 The increase i n surface area from y to y + dy w i l l he liven by, SS = n f.©Sy where 0 i s the circumference of the exposed grain at y. A fr a c t i o n <j> of this new surface area w i l l be readily leach-able and hence the increase in fast dissolving area is given by: 6S f = nf<f>0<Sy A 2 = £ 0 . T 6y a The actual rate of dissolution of the basal plane i s a constant and can be represented by: y = kt where t = time to penetrate to a depth, y, and k i s the rate expressed as the depth of penetration/sec. Thus , <Sy = k6t and 6S = - 0<J)2 k6t X St, A 2 integrating, S- = — 0<J> kt + const. x a O r i g i n a l l y , at t = 0, the area of fast leaching grains is <j>A, „ Q A0<|>2 kt + 4>A Hence , S_ - — f a The observed rate, K, is dependent on the surface area and the ov e r a l l rate as indicated by the "decelerated" dis-solution . 102 K. = c o n s t . C s u r f a c e a r e a ) ( r a t e ) = C — 0c(>2 kk* t + C<)>Ak' 3-•where k* i s the r a t e c o n s t a n t and C i s a c o n s t a n t Now, K = s o 1 dt Hence, [Fe] = C - 0<f>2 kk' t 2 + C<J>Ak't + const S O I Q, As t h e r e i s no i r o n i n s o l u t i o n at t = 0, t h i s i n t e -g r a t i o n c o n s t a n t i s z e r o . The amount of i r o n a p p e a r i n g i n s o l u t i o n i s g i v e n by an e x p e s s i o n of the form: [Fe] . = a t 2 + 6t s o l T Fe 1 A p l o t of _ _ s o l v e r s u s time s h o u l d g i v e a l i n e a r p l o t . t • Surana had shown t h a t t h i s r e l a t i o n h e l d i n h i s study and i n the p r e s e n t study a s i m i l a r dependence up t o 30% d i s s o l u t i o n was found ( F i g . 4 3 ) . At g r e a t e r than 30$ d i s s o l u t i o n , t h i s p l o t d e v i a t e d from l i n e a r i t y p r o b a b l y due t o the decrease i n the number of available g r a i n s f o r l e a c h i n g towards the c e n t r e of the p a r t i c l e , g Surana had a l s o shown t h a t A r r h e n i u s p l o t s f o r the r a t e c o n s t a n t s a and 3 gave s i m i l a r a c t i v a t i o n e n e r g i e s . From t h i s a n a l y s i s , one would expect the same temperature dependence f o r a and 6 . The c o n s t a n t 3 i s r e l a t e d d i r e c t l y t o the s u r f a c e a r e a A, AO however a i s r e l a t e d t o the r a t i o — . As A i s a f u n c t i o n of a 0.3 Amt.Fe/t io Fe/min.) 0.2 o.i o / (i<#) 60 120 Time (min.) 180 Fig.. 43. P l o t of (Amt Fe d i s s o l v e d ) v e r s u s time time f o r B o t r y o i d a l h e m a t i t e (2.4M HC1 at 80°C). 104 p a r t i c l e s s i z e and — i s a f u n c t i o n of g r a i n s i z e , a de-pendence of a on the p a r t i c l e s i z e t o g r a i n s i z e r a t i o i s i n d i c a t e d . A s m a l l e r g r a i n s i z e f o r a g i v e n p a r t i c l e s i z e s h o u l d r e s u l t i n a h i g h e r o b s e r v e d r a t e . T h i s r e s u l t had been i n d i c a t e d by the e x p e r i m e n t a l r e s u l t s . Though c e r t a i n f a c t o r s have not been t a k e n i n t o account such as the number of g r a i n s b e i n g l e a c h e d w i l l be i n c r e a s i n g ; t h e r e w i l l be some degree of c o m p e t i t i o n f o r g r a i n s as l e a c h -i n g can s t a r t at many p l a c e s on the specimen s u r f a c e ; t h i s s i m p l e model i s u s e f u l i n d e s c r i b i n g the e x p e c t e d shape of the d i s s o l u t i o n c u r v e . ( v i i ) A n i s o t r o p y of the C r y s t a l S t r u c t u r e A model of . the hem at i t a c r y s t a l l a t t i c e was examined i n the hope t h a t i t might e x p l a i n the reason f o r the f a s t d i s -s o l u t i o n of the p r i m a r y t w i n n i n g p l a n e s {0001}, the r e l a t i v e l y f a s t d i s s o l u t i o n of the {1010} p l a n e s , and the slow d i s s o l u t i o n of the o t h e r c r y s t a l l o g r a p h i c p l a n e s i n c l u d i n g the secondary t w i n n i n g planes {1011}, as found by Bath^, The most obvious s i m i l a r i t y between the {0001} and the {1010} p l a n e s i s t h a t they c o n t a i n o n l y one k i n d of atom i n the p l a n e , i r o n or oxygen. These were the o n l y p l a n e s i n the s t r u c t u r e t h a t c o n t a i n e d s o l e l y i r o n or oxygen-atoms. The s p a c i n g between s u c c e s s i v e p l a n e s of oxygen and i r o n atoms was much c l o s e r f o r the {1010} p l a n e s uian the {0001} p l a n e s which c o u l d e x p l a i n the f a s t e r d i s s o l u t i o n r a t e of the l a t t e r . The {10l0} p l a n e s c o n t a i n e d b o t h i r o n and oxygen atoms. 105 A c e r t a i n amount of d i r e c t bonding between i r o n and oxygen atoms o c c u r r e d i n trie p l a n e . From t h i s c o n s i d e r a t i o n , i t would appear t h a t the a t t a c k occurs p r e f e r e n t i a l l y on c r y s t a l l o g r a p h i c p l a n e s c o n t a i n i n g o n l y one type of atom. 106 CONCLUSIONS 1 ) The shapes of v a r i o u s d i s s o l u t i o n p l o t s f o r h e m a t i t e i n a c i d media can he a t t r i b u t e d t o the a n i s o t r o p y of h e m a t i t e and t o d i f f e r e n c e s i n g r a i n s i z e of m i n e r a l specimens. i i ) A l a r g e number of g r a i n s per p a r t i c l e • w i l l cause a n " a c c e l e r a t e d " d i s s o l u t i o n curve whereas a " d e c e l e r a t e d " or almost l i n e a r d i s s o l u t i o n curve i s o b t a i n e d i f the p a r t i c l e s i z e i s of the o r d e r ' o f the g r a i n s i z e . i i i ) D i f f e r e n t a c i d s have v a r y i n g r a t e dependences w i t h a c i d c o n c e n t r t i o n i n the d i r e c t d i s s o l u t i o n of h e m a t i t e . i v ) A mechanism, which g i v e s the v a r i o u s r a t e dependence on a c i d c o n c e n t r a t i o n f o r d i f f e r e n t a c i d s , has been proposed. The d i s s o l u t i o n r a t e i s c o n t r o l l e d by the d e s o r p t i o n of a f e r r i c i o n complex. v) The r a t e dependence f o r v a r i o u s a c i d s i s a s s o c i a t e d w i t h the complexing power of the a n i o n . I f the ani o n i s s t r o n g , the r a t e w i l l be dependent on the a c t i v i t y p r o d u c t of hydrogen i o n and a n i o n . For anions which complex weakly w i t h f e r r i c i r o n , the r a t e i s dependent o n l y on the hydrogen i o n con-c e n t r a t i o n . v i ) Hematite and g o e t h i t e r e a c t by s i m i l a r c h e m i c a l mechanism w i t h no a p p r e c i a b l e d i f f e r e n c e i n r a t e f o r the same p a r t i c l e s i z e . v i i ) The r e d u c t i v e d i s s o l u t i o n of h e m a t i t e in'aqueous 107 and a c i d i f i e d s u l p h u r d i o x i d e s o l u t i o n s i s an a n i s o t r o p i c d i s s o l u t i o n p r o c e s s . v i i i ) The r a t e of the r e d u c t i v e d i s s o l u t i o n i s p r o p o r t i o n a l to the s u l p h u r d i o x i d e p a r t i a l p r e s s u r e . i x ) I n c r e a s i n g the a c i d i t y w i t h p e r c h l o r i c a c i d i n -c r e a s e s the rate at a f i x e d p a r t i a l p r e s s u r e of s u l p h u r d i o x i de. x) The mechanism of the r e d u c t i v e d i s s o l u t i o n appears t o he complex, u n d i s s o c i a t e d SOg, HSO^ i o n s or b o t h , may he the r e a c t i n g s p e c i e s . S uggestions f o r F u r t h e r Work i ) By t h e r m a l l y t r e a t i n g f i n e g r a i n e d h e m a t i t e i t s h o u l d he p o s s i b l e t o o b t a i n a u s e f u l v a r i a t i o n i n g r a i n s i z e t o enable the a c t u a l dependence of r a t e on the g r a i n s i z e t o p a r t i c l e s i z e r a t i o . i i ) Doubly charged anions such as S 0^ and HPO^ appear t o behave i n a d i f f e r e n t manner to t h a t of the s i n g l y charged a n i o n s . S t u d i e s on the a t t a c k of t h e s e anions may h e l p i n the u n d e r s t a n d i n g of the d i r e c t d i s s o l u t i o n p r o c e s s . i i i ) A study of t h e r e a c t i o n of f e r r i c i o n s i n s o l u t i o n w i t h aqueous s u l p h u r d i o x i d e at e l e v a t e d t e m p e r a t u r e s may h e l p i n d e t e r m i n i n g the mechanism of the r e d u c t i v e d i s s o l u t i o n . i v ) Sodium d i t h i o n i t e r e a c t s r a p i d l y w i t h a Fe^O^ a^ low'' t e m p e r a t u r e s a study of t h i s r e a c t i o n may l e a d t o a b e t t e r under-s t a n d i n g of the aqueous s u l p h u r d i o x i d e system. v) The r e d u c t i v e d i s s o l u t i o n of a Fe^O^ by aqueous s u l p h u r d i o x i d e s o l u t i o n s i s v e r y prone t o c a t a l y s i s ; an i n v e s t i g a t i o n of v a r i o u s c a t a l y s t s such as a n t h r a q u i n o n e s u l p h a t e may be v e r y i n t e r e s t i n g . I t would be e s p e c i a l l y i n t e r e s t i n g t o see whether the c a t a l y s i s works by a c t i v a t i n g the s l o w l y d i s s o l v i n g p l a n e s or by homogeneously enhancing the d i s s o l u t i o n . An i d e a of w hich p r o c e s s i s t a k i n g p l a c e c o u l d be o b t a i n e d from d i s s o l u t i o n c u r v e s of f i n e g r a i n e d m a t e r i a l . 109 REFERENCES 1. G u t i e r r e z , L.G. and G a z u l l a , O.R.F., A n a l e s . r e a l Soc. espan. f i s . y. guim 687-98 1951. 2. Gruber, H. U.S. . 1 ,036 ,831 (Aug. 27, 1912). 3. Gravenor, C P . et a l . C.I.M. B u l l , 5_7, 421, (1964). 4. P r y o r , M.J. and Evans, U.R., J . Chem. Soc. 3330 (1949). 5. Azuma, K. and Kametani, H., Trans. AIME, 230, 853, (1964). 6. Monhemius, A . J . M.A.Sc. T h e s i s , Dept. M e t a l l u r g y , U.B.C. (1966). 7. B a t h , M.D., M.A.Sc. T h e s i s , Dept. of M e t a l l u r g y , U.B.C. (1968). 8. Surana, V., M.A.Sc. T h e s i s , Dept. of M e t a l l u r g y U.B.C. (1968). 9. P o s n j a k , E. ,and Merwin, H.E., J . Am. Chem. S o c , 4_4, 1965, (1922). 10. T u n n e l l , G. and P o s n j a k , E. Econ. Geol. 26, 337,(1931). 11. Simnad, M. and Smoluchowski, R., J . Chem. Phys. 2_3, 1 9 6 l , (1955). 12. Azuma, K. and Kametani, H., Trans. AIME, 242, 1025, (1968). 13. Ahmed, S.M. and M a k s i m o i r , Dept. Energy and Resources Re-se a r c h R e p o r t , Ottawa, RI96 (Feb. 1968). 14. Thomas, G. and Ingraham, T. R., Can. Met. Quart. 6_, 153, (1967). 15. H e r r i n g , A.P. and R a v i t z , S.F., Trans. AIME, 232, 191, (1965). 16. B a s s e t t , H. and P a r k e r , W.G., J . Chem. Soc. 1540, (1951) 17. B a s s e t t , H . a n d Henry, A . J . , J . Chem. Soc. 914, (.1935). 18. G a t e s , H.C. S c i e n c e , 137, 311, (1962). 19- S c h w a r t z , B., J. E l e c t r o c h e m . S o c , 114 , 2 8 5 ., (19 67 ) . 20. P r o s s e r , A.P., Warren, I.H. et a l . I.M.M. 7_8, C21, (1969). 21. B e a z l e y , W.B. et a l . Dominion F o r . S e r . B u l l . £3, (1938 ), 22. F o r t u n e , W.B. and M e l l o n , M.G., Ind. Eng. Chem. A n a l . Ed. 10_, 60, (19 38). 110 APPENDIX A ANALYTICAL METHOD7 "The p r o g r e s s of d i s s o l u t i o n was f o l l o w e d by s p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n of the i r o n c o n t e n t of samples taken at r e g u l a r i n t e r v a l s t hroughout the e x p e r i m e n t . F e r r o u s i r o n and 1-10 o r t h o p h e n a n t h r o l i n e form an orange-red complex h a v i n g a s t r o n g a b s o r p t i o n peak at a wavelength of 510 my. A b s o r p t i o n i n d i l u t e s o l u t i o n s of t h i s complex obeys Beer's Law and can be used f o r the q u a n t i t a t i v e d e t e r m i n a t i o n of f e r r o u s i r o n . S i n c e most of the i r o n was e x p e c t e d t o be i n t h e t r i v a l e n t s t a t e , h y dro-xylamine h y d r o c h l o r i d e was added t o each of the samples t o reduce a l l i r o n t o the f e r r o u s s t a t e . Sample s o l u t i o n s were b u f f e r e d at pH 4.5 w i t h a sodium a c e t a t e - a c e t i c a c i d b u f f e r , s i n c e the c o l o u r e d complex has been shown t o be most s t a b l e 2 2 i n the pH range 2-9-P r o c e d u r e : A composite reagent was madeup c o n t a i n i n g 0.3 g m / l i o r t h o p h e n a n t h r o l i n e , 2.0 g m / l i h y d r o x y l a m i n e h y d r o c h l o r i d e and b u f f e r . S u i t a b l e a l i q u o t s of sample s o l u t i o n were p i p e t t e d i n t o 100 ml v o l u m e t r i c f l a s k s , 25 ml of composite reagent added, and the volume made up w i t h d i s t i l l e d water. The s o l u t i o n s were a l l o w e d to s t a n d f o r at l e a s t 30 minutes ( t o ensure f u l l development of the c o l o u r ) , b e f o r e the o p t i c a l d e n s i t i e s were measured on a Beckman Model B s p e c t r o p h o t o -meter at 510 my. The c o n c e n t r a t i o n of i r o n was r e a d from a c a l i b r a t i o n curve p r e p a r e d u s i n g s t a n d a r d f e r r o u s ammonium s u l p h a t e s o l u t i o n s , checked a g a i n s t a b a s i c s t a n d a r d pre p a red from 99-9$ pure i r o n w i r e . " 112 APPENDIX B X-ray d i f f r a c t i o n results for the various specimens used i n this study, FeK radiation with a Mn f i l t e r was used a throughout. The heights of the d i f f r a c t i o n peaks were taken as a measure of t h e i r intensity and related to the height of the strongest peak. This approximation was more accurate for the large grain size materials. Noticeable broadening occurred for botryoidal a Fe 0 . 113 TABLE BT, a F e c Q 3 ASTM Powder Massive Red Botryoidal B r a z i l i a n Synthetic D i f f r a c t i o n F i l e a F e 2 0 3 a F e 2 0 3 a F e 2 ° 3 a F e 2 0 3 dA* I/I. I/I. I/I. I/I. I/I. 3.66 2.69 2. 51 2 . 201 1.838 1. 690 1. 596 1.U8U l . 452 25 100 50 30 ho 60 16 35 35 30 100 55 35 H5 65 20 35 35 25 90 100 20 45 ho 20 15 30 35 95 100 30 25 1+0 15 ho 30 35 100 80 h5 35 TO 20 1+0 35 114 TABLE B2. a FeOOH ASTM Powder N a t u r a l D i f f r a c t i o n a FeOOH F i l e dA° I / I 5.0 20 15 1+. 21 100 100 3. 37 20 Combined 2. 69 80 70 2.57 20 20 2.1+8 20 30 2.1+1+ 70 75 2.25 20 10 2 .18 1+0 35 APPENDIX C 115 TABLE CI E f f e c t of HC1 C o n c e n t r a t i o n on Rate (% Fe/min.) at 80°C M[ HC1] B o t r y o i d a l B r a z i l i a n M a s sive Red S y n t h e t i c G o e t h i t e 1.2 .052 .027 .01 . 007 .015 2.1* .18 .11 .04 .035 . 101 3.6 • 39 . 252 .095 . 080 .24 4.8 .66 .445 .177 .156 .U2 TABLE C2 E f f e c t of. Temperature on Rate {.% Fe/min.) i n 2 . 4M HC1 Temp . °C. B o t r y o i d a l B r a z i l i a n M a s s i v e Red S y n t h e t i c G o e t h i t e 85 .263 .17 . .066 .055 .166 80 .18 .11 .01+ .035 . 101 72 .071 .054 .017 .015 .05^ 65 .04 .026 .0095 .007^ . 029 116 TABLE C3 Effect of H^SO^ Concentration on Rate  (% Fe/min.) at 80°C. M[H SO^] Botryoidal Massive Red Goethite 1 .023 .0045 .014 2 .044 .0085 .026 4 .076 .016 .046 5 .096 .020 .058 TABLE C4 Effect of Temperature on Rate j% Fe/min.) in 4M H^ SO, 2—4 Temperature Botryoidal Massive Red Goethite °C. 85 .126 .025 .068 80 .076 .016 .046 72 .035 .011 .026 65 .021 .006 .013 TABLE C5 E f f e c t of HCIO^ C o n c e n t r a t i o n on Rate MtHClO^] B o t r y o i d a l G o e t h i t e 3 .0051 .0029 5 .0076 .0045 7 .0114 .0066 TABLE C6 E f f e c t of Temperature on Rate C$ Fe/min.)  i n 5M HCIO^ Temperature B o t r y o i d a l G o e t h i t e °C. 95 90 85 .0126 .0076 .0057 .0057 .0045 .0026 118 TABLE CT E f f e c t of the P a r t i a l P r e s s u r e of SO 2 on the L e a c h i n g of Ma s s i v e Red Hematite at 110°C S 0 2 U n a c i d i f i e d . 5M HCIO^ 1 M HCIO^ 2 M HXIO^ p.p. 3 .0085 4 .012 .027 5.5 .025 6 . 5 .0125 7 .028 .0U2 10 .0192 .037 .049 .057 14 .025 .048 . 0 6 .076 20 .0357 .07 .09 .105 119 APPENDIX D Isotropic Dissolution of a Single Crystal Sphere Let F be the fraction reached at time t of a unit sphere, then 1-F is the fraction remaining. Thus, F = | n [1 - (1 - kt) 3] 3 3 2 2 k t = 4 n [kt - k V + ~ H where k is the fraction dissolved/unit time. Values of F for various times can then be calculated Anisotropic Dissolution of a Single Crystal Sphere 120 Dissolution is assumed to take place soley on the basal plane. After a time t, two parallel basal planes w i l l have developed, each a dis-tance (r -r x) from the centre of the sphere. The mass of the element considered, 6M = npd 2 6x Thus, the mass of material removed at time t, M = 2nd •{rZ - (r-x) 2} dx - 2Hp ( | - rx) x = depth of penetration /.unit time x = kt. Hence values of M for various t can be calculated. Anisotropic Dissolution of a Single Crystal Pyramid Height = b (0001) Height - b 121 Mass of element 6M = p-j a.a sin 60°.6x /§ X 2 M = T p (b - x) dx 4 o M = ^ [b 2 x - bx 2 + | 3 ] As before, x = kt Hence, /3 r,2 , _ t, 2 2 , k t 3 3 M = 7 [b kt - bk t + 4 3 This expression is of the same form as that for the irotropic dis-solution of a sphere. 

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