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Dissolution of iron oxide in aqueous solutions of sulphur dioxide Monhemius, Andrew John 1966

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DISSOLUTION OF IRON OXIDE IN AQUEOUS SOLUTIONS OF SULPHUR DIOXIDE by ANDREW JOHN MONHEMIUS B . S c , U n i v e r s i t y of Birmingham, 196^ A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of METALLURGY We accept t h i s t h e s i s as conforming t o the. re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA J u l y , I966 In p resent ing 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 of the requirements fo r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that 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 fo r reference and study . I f u r t h e r agree that permiss ion f o r ex -t e n s i v e copying of 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 granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n -c i a l gain s h a l l not be a l lowed wi thout my w r i t t e n p e r m i s s i o n . Department of M e t a l l u r g y  The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date J u l y 27, 1966 ABSTRACT A study, has been made of the d i s s o l u t i o n of n a t u r a l l y o c c u r r i n g a - i r o n oxide hydrate i n a c i d i f i e d aqueous s o l u t i o n s of sulphur d i o x i d e at 110°C. The d i s s o l u t i o n was found t o be:'independent of a c i d i t y a t low concentrations of sulphur d i o x i d e and i n v e r s e l y dependent on a c i d i t y at higher concentrations of sulphur d i o x i d e . Both homogeneous and heterogeneous c o n t r o l of the r e a c t i o n was observed. The a d d i t i o n of c u p r i c i o n t o the system c a t a l y s e d the r a t e . D i s s o l u t i o n i s thought t o occur v i a h y d r a t i o n of the oxide surface and subsequent r e a c t i o n of u n d i s s o c i a t e d sulphurous a c i d at the surface t o form a f e r r i c - s u l p h i t e complex. The r a t e determining step i s considered t o be the deso r p t i o n of the complex from the surface. A l i m i t e d study of the d i r e c t d i s s o l u t i o n of i r o n oxide hydrate i n 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 at temperatures between 120 and 150°C i s i n c l u d e d . Under these c o n d i t i o n s , the hydrated oxide surface i s thought t o undergo anion exchange during d i s s o l u t i o n . Work c a r r i e d out on the p r e p a r a t i o n and i d e n t i f i c a t i o n of the isomeric a- and y - i r o n oxide hydrates i s reported. i i i . ACKNOWLEDGEMENT The author wishes t o express h i s si n c e r e thanks t o Dr. I.H. Warren f o r h i s continued i n t e r e s t , guidance, and i n v a l u a b l e encouragement throughout the p e r i o d of study. .Thanks are a l s o extended t o Dr. E. Peters f o r h i s help w i t h the t h e o r e t i c a l i n t e r p r e t a t i o n s of the r e s u l t s , and t o members of 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 aspects of the work. F i n a n c i a l support from the Science Research C o u n c i l of Great B r i t a i n , i n the form of a N.A.T.O. Research Studentship, i s g r a t e f u l l y acknowledged. i v . TABLE OF CONTENTS Page INTRODUCTION 1 General 1 Review of l i t e r a t u r e 2 Scope of the present i n v e s t i g a t i o n 12 PART I : The S y n t h e t i c I r o n Oxide Hydrates Ik P r e p a r a t i o n of the i r o n oxide hydrates 15 I d e n t i f i c a t i o n of the i r o n oxide hydrates 19 - P r e p a r a t i o n of l e a c h i n g specimens 29 PART I I : The D i s s o l u t i o n of N a t u r a l 0<-Iron Oxide Hydrate i n Aqueous Solutions;, of Sulphur Dioxide 31 , EXPERIMENTAL 31 M a t e r i a l s 31 Autoclave design 33 Experimental procedure 35 A n a l y t i c a l methods 37 RESULTS ^0 D i r e c t d i s s o l u t i o n i n a c i d s o l u t i o n s ^0 P e r c h l o r i c a c i d ^0 Sul p h u r i c a c i d ko Reductive d i s s o l u t i o n i n a c i d s o l u t i o n s k-8 T y p i c a l r a t e curves ^9 The e f f e c t of v a r y i n g the p a r t i a l pressurei.of SO2 at constant a c i d i t y ^9 The e f f e c t of v a r y i n g . a c i d i t y a t constant'S0 2 c o n c e n t r a t i o n 55 Sulphate analyses 55 The e f f e c t of c u p r i c i o n on the system . . . . 55 DISCUSSION 63 D i r e c t d i s s o l u t i o n 63 Reductive d i s s o l u t i o n 70 TABLE OF CONTENTS (CONT'D) Page CONCLUSIONS 81 A p p l i c a t i o n of r e s u l t s 82 Suggestions f o r future work 83 REFERENCES 85 APPENDIX A: Tables of Experimental R e s u l t s 88 APPENDIX B: The Sulphur Dioxide-Water System . . . . . . . 9*4-v i . • LIST OF TABLES No. Page 1 • I n f r a - r e d a b s o r p t i o n s p e c t r a of the n a t u r a l and s y n t h e t i c i r o n oxide hydrates 21 2 D.T.A. curves f o r n a t u r a l and s y n t h e t i c i r o n oxide hydrates 26 3 Major peaks of X-ray d i f f r a c t i o n patterns of c<- and ^  -FeOOH 28 k X-ray d i f f r a c t i o n patterns of the s y n t h e t i c m a t e r i a l s 28 5 Q u a l i t a t i v e spectrographic examination of n a t u r a l g o e t h i t e 32 6 X-ray d i f f r a c t i o n p a t t e r n of the n a t u r a l g o e t h i t e 32 APPENDIX A I V a r i a t i o n of rat e w i t h c o n c e n t r a t i o n of s u l p h u r i c a c i d ' 88 I I E f f e c t of temperature on rat e of d i s s o l u t i o n i n s u l p h u r i c a c i d 88 I I I E f f e c t of v a r y i n g d i s s o l v e d sulphur d i o x i d e c o n c e n t r a t i o n a t v a r i o u s a c i d i t i e s 89 IV E f f e c t of a c i d i t y a t constant d i s s o l v e d sulphur d i o x i d e c o n c e n t r a t i o n 90 V Sulphate determinations 9° VI E f f e c t of v a r y i n g c u p r i c i o n co n c e n t r a t i o n at constant a c i d i t y and constant sulphur d i o x i d e c o n c e n t r a t i o n 91 V I I E f f e c t of v a r y i n g the conc e n t r a t i o n of d i s -solved sulphur d i o x i d e at constant a c i d i t y and constant c u p r i c i o n co n c e n t r a t i o n 91 V I I I E f f e c t of d i s s o l v e d sulphur d i o x i d e con-c e n t r a t i o n on copper i n s o l u t i o n 92 v i i . LIST OF TABLES (CONT'D) No. Page IX E f f e c t of temperature on c u p r i c c a t a l y s e d r e a c t i o n 92 X T y p i c a l heterogeneous r a t e s i n a c i d i f i e d aqueous sulphur d i o x i d e s o l u t i o n s 93 APPENDIX B BI V a r i a t i o n of K & w i t h temperature 96 B I I S o l u b i l i t y of sulphur d i o x i d e at various temperatures and various t o t a l pressures . . . . 96 v i i i . LIST OF FIGURES No. Page 1 Bending .v i b r a t i o n s of the 0H....0 group 20 2 Rate versus concentration of H 2S0 4 k-2 3 Rate versus a c t i v i t y of hydrogen i o n i n H 2S0 4 Itf k E f f e c t of temperature a t constant [H 2S0 4] . . . . kk 5 Arrhenius p l o t f o r d i s s o l u t i o n i n H 2S0 4 ^5 6 Comparison of rate of d i s s o l u t i o n of goet h i t e i n 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 . . . k6 7 E f f e c t of adding F e 2 ( S 0 4 ) 3 at the s t a r t of the run ^7 8 T y p i c a l r a t e p l o t s f o r d i s s o l u t i o n i n a c i d i f i e d sulphur d i o x i d e s o l u t i o n s ^0 9 Rate versus c o n c e n t r a t i o n of d i s s o l v e d sulphur d i o x i d e ( S e r i e s A l ) 52 10 Rate versus c o n c e n t r a t i o n of d i s s o l v e d sulphur d i o x i d e ( S e r i e s A2) 53 11 Rate versus c o n c e n t r a t i o n of d i s s o l v e d sulphur d i o x i d e ( S e r i e s A3) 5^ 12 Rate versus c u p r i c i o n conc e n t r a t i o n 57 13 E f f e c t of i n c r e a s i n g [S0 2 ] a a ^ at constant i n i t i a l c u p r i c i o n conc e n t r a t i o n 59 Ik E f f e c t of i n c r e a s i n g , [ S 0 2 ] o n on-[Cu] i n s o l u t i o n 59 15 Arrhenius p l o t f o r cu p r i c c a t a l y s e d r e a c t i o n . . 6 l 16 D i s s o l u t i o n of go e t h i t e i n d i l u t e p e r c h l o r i c a c i d i n the--presence of copper under hydrogen atmosphere 62 17 Rate versus con c e n t r a t i o n of d i s s o l v e d sulphur d i o x i d e a t va r i o u s a c i d i t i e s ( S e r i e s A l , A 2 , A 3 ) . 71 - 1 -INTRODUCTION General The problems a s s o c i a t e d w i t h the removal of i r o n oxides from 1 i n d u s t r i a l l y important s i l i c a t e m a t e r i a l s have r e c e n t l y been reviewed . D i t h i o n i t e s , or organic d e r i v i t i v e s of d i t h i o n i t e s , have t r a d i t i o n a l l y been used f o r t h i s purpose. Leaching w i t h d i t h i o n i t e s i s c a r r i e d out at room temperatures. These reducing agents are r e l a t i v e l y expensive when compared w i t h the p r i c e of sodium b i s u l p h i t e . P r e l i m i n a r y experiments had i n d i c a t e d t h a t l e a c h i n g of i r o n oxides w i t h b i s u l p h i t e could be very . r a p i d under t y p i c a l pressure l e a c h i n g c o n d i t i o n s . This study was undertaken t o t r y t o e s t a b l i s h the r a t e determining steps i n the r e a c t i o n o f b i s u l p h i t e i o n , w i t h i r o n oxides a t elevate d temperatures and pressures. I t was decided t o use s p e c i f i c a l l y hydrated i r o n oxides (FeOOH) since the b u l k of the leachable i r o n i n c l a y minerals i s present i n t h i s form. I t was a l s o hoped t h a t t h i s study would c o n t r i b u t e i n f o r m a t i o n which would a s s i s t i n as s e s s i n g the f e a s i b i l i t y of the s o l u t i o n mining 2 of hydrated i r o n oxide ores w i t h aqueous s o l u t i o n s of sulphur d i o x i d e . - 2 -Review of L i t e r a t u r e Most of the previous i n v e s t i g a t i o n s i n t o the r e a c t i o n s of f e r r i c i o n w i t h sulphurous a c i d have been c a r r i e d out on homogeneous systems at room temperatures, the only exception being very l i m i t e d s t u d i e s 5,9,10 using f r e s h l y p r e c i p i t a t e d f e r r i c hydroxide as the source of i r o n Although t h i s i n v e s t i g a t i o n i s concerned w i t h the r e a c t i o n s of s o l i d i r o n oxide hydrates w i t h aqueous s o l u t i o n s of sulphur d i o x i d e at e l e v a t e d temperatures, the previous work on homogeneous systems i s reviewed here si n c e i t i s p e r t i n e n t t o the present problem. A c e r t a i n amount of work has been c a r r i e d out on the r e a c t i o n s of s o l i d manganese oxides w i t h sulphurous a c i d . This work i s a l s o reviewed since i t introduces concepts which may be a p p l i c a b l e t o the present work. The l i t e r a t u r e i s discussed, as f a r as p o s s i b l e , i n c h r o n o l o g i c a l order. One of the e a r l i e s t r e p o r t s of r e a c t i o n s of the iron-sulphurous k a c i d system was t h a t of B e r t h i e r who observed the formation of a red s o l u t i o n when sulphur d i o x i d e was passed i n t o a suspension of f e r r i c hydroxide at room temperature. The red s o l u t i o n , which i s now thought t o be due t o the formation of a f e r r i c - s u l p h u r IV complex, s l o w l y becomes pale green on s t a n d i n g , and f e r r o u s ' s u l p h a t e (FeS0 4) and f e r r o u s 5 d i t h i o n a t e ( F e S 2 0 6 ) are formed. ( G e l i s ) 3 Heeren s t u d i e d the r e a c t i o n s between sulphurous a c i d and p y r o l u s i t e , Mn0 2, and a l s o manganite, Mn 203-H 20. He found t h a t the products of the o x i d a t i o n of sulphurous a c i d by these oxides were - 5 -sulphate and d i t h i o n a t e i n v a r i o u s p r o p o r t i o n s . He a t t r i b u t e d the v a r y i n g p r o p o r t i o n s of o x i d a t i o n products t o the f o l l o w i n g f a c t o r s : . i ) Temperature - h i g h temperatures caused high y i e l d s of sulphate by decomposition of d i t h i o n a t e . i i ) Composition of the m i n e r a l - presence i n the d i o x i d e of Mn203R20 gave more sulphate than d i t h i o n a t e . 9 Bassett and Henry i n v e s t i g a t e d , the e f f e c t s of a l a r g e number of o x i d i s i n g agents on sulphurous a c i d a t room temperatures. For the case of metal ions e a s i l y r e d u c i b l e t o a lower valency s t a t e , , or t o the metal, they found t h a t the p r o p o r t i o n s of d i t h i o n a t e and sulphate produced depended almost e n t i r e l y on the nature o f the metal i n v o l v e d and were independent of the c o n c e n t r a t i o n of the r e a c t a n t s , and, the a c i d i t y . They concluded t h a t t h e i r experimental r e s u l t s could be accounted f o r by p o s t u l a t i n g the formation of a complex a d d i t i o n product from the metal i o n and sulphurous a c i d . The complex ion,then decomposes very r a p i d l y by s e l f - o x i d a t i o n and r e d u c t i o n , i n v o l v i n g an e l e c t r o n t r a n s f e r from the non-metal group t o the metal i o n . The decomposition can take place i n two d i f f e r e n t and independent manners t o y i e l d e i t h e r sulphate or d i t h i o n a t e . They considered t h a t the complex i o n i s a.metal - s u l p h i t e complex. The•postulated mechanisms f o r the decompsition of t h i s complex m e t a l - s u l p h i t e i o n are summarised i n the f o l l o w i n g sequence of r e a c t i o n s : O x idation by f e r r i c i o n i s used as an example. The s t r u c t u r e of the complex i o n i s not known but i s represented here i n i t s simplest form: +++ = +++ i ) Fe + S0 3 Fe (S0 3 ) Complex Ion Formation +++ = e l e c t r o n ++ i i ) Fe (S0 3 ) -jp-- Fe ( S 0 3 ) S e l f Oxidation t r a n s f e r a n d R e d u c t i o n ++ ++ i i i ) 2Fe (S0 3 ") »»- 2Fe + S 2 0 6 D i t h i o n a t e Formation ++ ++ i v ) 2Fe (S0 3 ) 2Fe +• S0 3 + S0 3 r a p i d _ + v) S0 3 + H 20 S0 4 + 2H Sulphate Formation The authors consider t h a t t h i s mechanism i s i n agreement w i t h t h e i r experimental observations, since the same complex gives r i s e t o e i t h e r sulphate or dithonate by r e a c t i o n s of the same order. Because the r e a c t i o n s are of the same order, they s t a t e t h a t p r o p o r t i o n s of the two o x i d a t i o n products should be e s s e n t i a l l y independent of conc e n t r a t i o n and a c i d i t y , and depend almost e n t i r e l y . o n the nature of the o x i d i s i n g metal i o n . The nature of the metal i o n would presumably govern the r e l a t i v e p r o b a b i l i t i e s of the occurrence of r e a c t i o n s ( i i i ) and ( i v ) . T h e i r s u p p o s i t i o n f o r the s u l p h i t e i o n being the complex-forming anion, r a t h e r than the b i s u l p h i t e i o n , i s p a r t l y based on the f a c t t h a t a s i m i l a r complex i s thought t o be formed i n the r e a c t i o n of c u p r i c ions 8 w i t h s u l p h i t e ions under a l k a l i n e c o n d i t i o n s . The work of Bassett and Henry was extended by Bassett and 10 Parker . These authors s t u d i e d the o x i d a t i o n of sulphurous a c i d at room temperature by oxides of manganese, by f e r r i c and c u p r i c s a l t s , and by molecular oxygen in,the presence of va r i o u s d i s s o l v e d s a l t s . They showed t h a t o x i d a t i o n by s o l i d manganese oxides occured on,the oxide s u r f a c e . The proporti o n s of d i t h i o n a t e and sulphate formed were s p e c i f i c f o r each polymorph, but were independent of a l l other f a c t o r s examined. They concluded t h a t d i t h i o n a t e can only, be formed when the - 5 -oxide i s reduced i n one-electron steps. The mechanism they proposed i s e s s e n t i a l l y s i m i l a r t o t h a t proposed by Bassett and Henry, except t h a t i t i n v o l v e s the surface a d s o r p t i o n of s u l p h i t e i o n s , r a t h e r than complex i o n formation i n s o l u t i o n . The adsorbed s u l p h i t e ions are e i t h e r discharged i n two successive s i n g l e - e l e c t r o n steps t o y i e l d sulphate, or two of them a f t e r p a r t i a l discharge, u n i t e t o form a d i t h i o n a t e i o n . In the case of f r e s h l y prepared f e r r i c hydroxide, they concluded, from v i s u a l observation of a red c o l o u r a t i o n , t h a t the r e a c t i o n i n v o l v e d the formation of a complex . f e r r i c - s u l p h i t e i o n i n s o l u t i o n , i n s t e a d of a surface r e a c t i o n . Thus the f e r r i c hydroxide d i s s o l v e d before i t was reduced. The r e a c t i o n mechanisms proposed by Bassett and Henry and Bassett and Parker were pa r t of the evidence that l e d Higginson and Marshall"'"^ t o propound a general theory which a p p l i e s t o the o x i d a t i o n of certain nan-metallic compounds. The compounds i n question are those which, when o x i d i s e d or reduced i n homogeneous s o l u t i o n , give p r o p o r t i o n s of products which vary w i t h the o x i d i s i n g or reducing agent used. These authors took the r e a c t i o n mechanism which has been proposed f o r the o x i d a t i o n of hydrazine i n aqueous a c i d , and showed t h a t i t a l s o a p p l i e d t o the o x i d a t i o n of sulphurous a c i d . They, then used t h i s mechanism as a b a s i s f o r some ge n e r a l i z a t i o n s concerning c e r t a i n other o x i d a t i o n -r e d u c t i o n r e a c t i o n s . In t h i s general scheme, two mechanisms operate, and o x i d i s i n g agents were d i v i d e d i n t o three c l a s s e s , depending on whether t h e i r r e a c t i o n s i n d i c a t e d one, the other,, or both of these mechanisms. The - 6 -three c l a s s e s of o x i d i s i n g agents were c a l l e d " 1-equivalent, 2-equivalent, or 1 , 2-equivalent o x i d i s i n g agents". 1 i i i i i i i v The two mechanisms that were proposed.may be w r i t t e n as: Reactions w i t h 1-equivalent o x i d i s i n g agents:-JI 1-equiv. n+i 2X n+i 2X n+i x n + 1 n+i (X ) 2 I n i t i a l o x i d a t i o n process D i m e r i s a t i o n of r a d i c a l s n+2 n X + X D i s p r o p o r t i o n a t i o n of r a d i c a l s n+2 Further o x i d a t i o n of r a d i c a l Reactions w i t h 2-equivalent o x i d i s i n g agent:-n 2-equiv. n+2 X 1 , 2-equivalent o x i d i s i n g agents can react according t o both of these mechanisms. n X represents a simple compound of a non-metallic element i n o x i d a t i o n s t a t e "n" . The i n i t i a l product of a 1-equivalent o x i d a t i o n , X n 1 , i s u s u a l l y a f r e e r a d i c a l and can disappear by r e a c t i o n s ( i i ) , ( i i i ) or ( i v ) . Thus, from r e a c t i o n s ( i ) through ( v ) , i t may be seen that 2-equivalent reagents should o x i d i s e X n t o X n + 2 only, w h i l e 1-equivalent r n+i, n+2 o x i d i s i n g agents should produce [X J 2 and X i n p r o p o r t i o n s v a r y i n g w i t h the r e a c t i o n c o n d i t i o n s . n+2 I n the case of sulphurous a c i d , X represents sulphate, = r n+i , = SO4 , and LX J 2 represents d i t h i o n a t e , S 2 0 6 . - 7 -Higginson and M a r s h a l l found t h a t , f o r o x i d a t i o n - r e d u c t i o n r e a c t i o n s between t r a n s i t i o n metal ions or compounds and ions derived from n o n - t r a n s i t i o n elements, o x i d a t i o n may occur i n e i t h e r 1- or 2-equivalent steps, the 1-equivalent step occuring more o f t e n . In the case of f e r r i c i r o n , the o x i d a t i o n s t a t e which would be formed a f t e r a 2 -equivalent r e a c t i o n , i e F e + , i s unknown. Thus the r e d u c t i o n of f e r r i c i r o n must take place by 1-equivalent r e a c t i o n s and so the o x i d a t i o n products should i n c l u d e d i t h i o n a t e as w e l l as sulphate. In order t o s u b s t a n t i a t e t h e i r theory on the mechanism of 1-equivalent r e a c t i o n s , Higginson and M a r s h a l l s t u d i e d the r e a c t i o n s of f e r r i c i r o n w i t h sulphurous a c i d i n the presence of c u p r i c i o n . They showed th a t the mechanism of t h i s r e a c t i o n agreed w i t h the proposed 1-equivalent reagent mechanism. The suggested mechanism f o r t h i s system i n v o l v e d the production of the t h i o n a t e f r e e r a d i c a l ( H S O 3 ) , and c o n s i s t e d of the f o l l o w i n g sequence of r e a c t i o n s . ( i ) F e 3 + + , H S 0 3 " k i F e 2 + + HSO3 " ( i ) F e 2 + + HSO3 k - i Fe3+ + HSO3-( i i ) • 2 H S 0 3 k 2 — » > - H 2 S 2 0 6 ( i i i ) F e 3 + + HSO3 + H 2 0 -—w F e 2 + + S 0 4 = + 3H + ( i v ) C u 2 + + HSO3 + H 2 0 C u + + S 0 4 = + J H + (v) C u + + F e 3 + r a p i d C u 2 + + F e 2 + These authors s t a t e t h a t , under t h e i r c o n d i t i o n s , i e . [H ] = 0.08M at 25°C, there was no v i s u a l evidence f o r the formation of an i r o n ( I I I ) - s u l p h u r (IV) complex, n e i t h e r were they able t o i n t e r p r e t t h e i r k i n e t i c data on the b a s i s of the formation of such a complex. - 8 -A mechanism i n v o l v i n g , but not n e c e s s a r i l y dependent upon, the formation of an i r o n ( I I I ) - s u l p h u r (IV) complex was proposed by Karraker "-\ This mechanism p o s t u l a t e s the formation and r e a c t i o n s of a f e r r i c - b i s u l p h i t e complex, and a l s o the t h i o n a t e f r e e r a d i c a l : H + + H S 0 3 " Fe ( H S 0 3 ) 2 + F e 2 + + HSO3 F e 3 + + HSO3" S 0 4 2 " + F e 2 + + 3H + Karraker designed h i s r e a c t i o n c o n d i t i o n s to produce sulphate, r a t h e r than d i t h i o n a t e , by having a l a r g e excess of f e r r i c i r o n and low concentrations of both r e a c t a n t s . Under these c o n d i t i o n s , t h i o n a t e r a d i c a l s produced by r e a c t i o n ( i i i ) are l i k e l y t o react w i t h f e r r i c i o n t o produce sulphate, as i n r e a c t i o n ( v ) , r a t h e r than t o react w i t h another t h i o n a t e r a d i c a l and dimerise t o form' d i t h i o n a t e . The assumption of a f e r r i c - b i s u l p h i t e complex does not a f f e c t the r a t e expression f o r t h i s mechanism, but i t was i n c l u d e d because of the obvious colour change t h a t occurs on the a d d i t i o n of sulphurous a c i d t o f e r r i c i o n . This was considered strong evidence f o r the formation of a complex. The formation of a red c o l o u r a t i o n i n s o l u t i o n s c o n t a i n i n g f e r r i c ions and sulphurous a c i d under c e r t a i n c o n d i t i o n s i s w i d e l y reported, and the c o l o u r a t i o n i s g e n e r a l l y a t t r i b u t e d t o complex i o n formation. The composition of t h i s complex, or complexes, appears t o be u n c e r t a i n . Two types of complex are p o s s i b l e i n the f e r r i c iron-sulphurous K= 11 111 i v H 2 S 0 3 F e 3 + + HSO3" Fe ( H S 0 3 ) 2 + - ^ i HSO3. + F e 2 + - — HSO3 + F e 3 + + H20 - 9 -a c i d system: f e r r i c - s u l p h i t e complexes and f e r r i c - b i s u l p h i t e complexes. The s t r u c t u r e s of these two s e r i e s of complexes may be represented as [ F e 1 1 1 ( S 0 3 ) n ] 3 - 2 n and [ F e 1 1 1 ( H S 0 3 ) n ] 3 - n r e s p e c t i v e l y . Depending on the number of anions i n v o l v e d , the complexes could be a n i o n i c or c a t i o n i c . Bassett and Parker suggested t h a t f e r r i c - s u l p h i t e complexes w i t h n=3 and 12 n=2 are p o s s i b l e . Danilczuk and Swinarski c a r r i e d out a photometric I I I study of the f e r r i c - s u l p h i t e system. They showed th a t [Fe ( S 0 3 ) 3 J 3 ~ i s formed and i s s t a b l e under c o n d i t i o n s of excess S0 3~. At low con-c e n t r a t i o n s of s u l p h i t e i o n ( F e 3 + : S 0 3 ~ = 1:1 or 1:2), i t was suggested that a s e l f o x i d a t i o n - r e d u c t i o n of the complex takes place l e a d i n g t o the formation of complexes c o n t a i n i n g two and one s u l p h i t e i o n s , and a l s o t o the formation of d i t h i o n a t e and f e r r o u s i o n s . I t was concluded t h a t n e q u i l i b r i a e x i s t between [ F e I : C I ( S 0 3 ) n ] 3 - 2 n and F e + + and 2 SaOe". To t h i s author's knowledge, no d i r e c t evidence has been f o r t h -coming t o show t h a t complexes i n v o l v i n g f e r r i c and b i s u l p h i t e i o n occur. The c a t i o n i c complex [ F e I I I H S 0 3 J + + has been suggested"^. The formation of the t h i o n a t e f r e e r a d i c a l (HS0 3), or i t s anion ( S 0 3 " ) , was f i r s t p o s t u l a t e d by Franck and Haber 1 as o c c u r r i n g during the o x i d a t i o n of sulphurous a c i d by f r e e oxygen. The existence of t h i s f r e e r a d i c a l seems t o have been g e n e r a l l y accepted, and i t i s thought t o be formed during the r e a c t i o n s of most o x i d i s i n g agents w i t h sulphurous a c i d . Apart from r e a c t i o n s i n v o l v i n g a n i o n i c species derived from sulphurous a c i d , i t has a l s o been suggested that n e u t r a l sulphur d i o x i d e molecules can take p a r t i n the o x i d a t i o n r e a c t i o n s . For the case of - 10 -o x i d a t i o n by chromic and permanganic a c i d s , Bassett and Parker"*"^ p o s t u l a t e d t h a t the complex metal ion was formed by the a d d i t i o n of n e u t r a l sulphur d i o x i d e molecules r a t h e r than s u l p h i t e ions. The s t r u c t u r e of such a complex ion was suggested t o be: 0 ^ ^ 0 — » • S0 £ 14 Recent work c a r r i e d out on the l e a c h i n g of manganese dio x i d e w i t h sulphurous a c i d a l s o suggests t h a t u n d i s s o c i a t e d sulphur d i o x i d e molecules can enter i n t o the r e a c t i o n . In f a c t i t i s suggested th a t these n e u t r a l molecules react more q u i c k l y than charged b i s u l p h i t e ions at a c t i v e s i t e s on the manganese oxide surface. Summary of the r e a c t i o n s of the i r o n ( I I I ) - s u l p h u r IV System The r e a c t i o n s t h a t are thought t o take place i n the f e r r i c iron-sulphurous a c i d system are summarised i n the f o l l o w i n g paragraphs. I t must be emphasized t h a t these r e a c t i o n s have been proposed as a r e s u l t of s t u d i e s which were a l l c a r r i e d out at room temperatures, and the m a j o r i t y c a r r i e d out i n homogeneous systems: i ) The- f i r s t step i n v o l v e s the production of a t h i o n a t e f r e e r a d i c a l (HSO3). This may r e s u l t from the formation and subsequent decompositon of a f e r r i c - b i s u l p h i t e complex i o n which i s a s t a b l e s p e c i e s , +++ - ++ Fe + HSO3 Fe (HS0 3 ) Fe (HS0 3 ) +~ Fe + HS0 3 or may- be produced d i r e c t l y by the r e a c t i o n of f e r r i c i o n w i t h b i s u l p h i t e i o n +++ _ ++ Fe + HS0 3 ^ Fe + HS0 3 - 11 -i i ) The t h i o n a t e r a d i c a l may disappear by one or more of the f o l l o w i n g r e a c t i o n s : a) Back r e a c t i o n w i t h f e r r o u s i o n ++ +++ Fe + HS0 3 Fe + HS0 3 b) D i m e r i s a t i o n t o form d i t h i o n i c a c i d 2HSO3 - H 2 S 2 0 6 c) Reaction w i t h f e r r i c i o n and water t o form sulphate +++ ++ = + Fe + H 20 + HSO3 »- Fe + S 0 4 + 3H d) In the presence of copper, r e a c t i o n w i t h c u p r i c i o n and water t o form cuprous i o n ++ + = + Cu + H 20 + HSO3 Cu + S 0 4 + 3H The above r e a c t i o n s assume the formation of a f e r r i c b i s u l p h i t e complex, but a s i m i l a r sequence of r e a c t i o n s , r e s u l t i n g i n the same end products,, may be w r i t t e n i n v o l v i n g a f e r r i c s u l p h i t e complex and subsequent r e a c t i o n s of the SO3 r a d i c a l . - 12 -SCOPE OF THE PRESENT. INVESTIGATION The o r i g i n a l scope of t h i s i n v e s t i g a t i o n was planned t o be as f o l l o w s : -1) The synth e s i s and i n d e n t i f i c a t i o n of the two n a t u r a l l y o c c u r r i n g i r o n oxide hydrates, Goethite ( a-FeOOH) and-Lepidocrocite ( 7-FeOOH). 2) The p r e p a r a t i o n of b u l k compacts of the s y n t h e t i c oxide hydrate powders s u i t a b l e f o r l e a c h i n g . 3) A study and comparison of the d i s s o l u t i o n of the two hydrated oxides i n a c i d i f i e d s o l u t i o n s of sulphur d i o x i d e , at elevat e d temperatures and pressures.' The f i r s t p a r t of the i n v e s t i g a t i o n was completed s u c c e s s f u l l y . However i t was found impossible t o prepare compacts of the s y n t h e t i c • powders which had enough cohesion,to prevent d i s i n t e g r a t i o n d u r ing l e a c h i n g . Because of t h i s , i t was decided t o study the d i s s o l u t i o n of n a t u r a l l y o c c u r r i n g massive hydrated a - i r o n oxide. The study had t o be l i m i t e d t o the a-form of the oxide, since the 7-form very- r a r e l y occurs n a t u r a l l y i n pure massive form, a - i r o n oxide hydrate u s u a l l y being found w i t h i t . Although the sy n t h e s i s and i d e n t i f i c a t i o n of the oxides now has no bearing on the main pa r t of t h i s work, i . e . the d i s s o l u t i o n study, i t i s i n c l u d e d i n the f i r s t p a r t of t h i s t h e s i s f o r the sake of com-p l e t n e s s , and a l s o as a reference i n the event t h a t any f u r t h e r work i s t o be c a r r i e d out usi n g these s y n t h e t i c m a t e r i a l s . Thus 'this t h e s i s i s d i v i d e d i n t o two p a r t s , complete i n - 13 -themselves. The f i r s t p a r t deals w i t h the s y n t h e s i s , i n d e n t i f i c a t i o n , and p r e p a r a t i o n of compacts of the hydrated oxides. The second p a r t , which c o n s t i t u t e s the bulk of the work c a r r i e d out, deals w i t h the d i s s o l u t i o n of the n a t u r a l m i n e r a l , G o e t h i t e , i n a c i d . s o l u t i o n s and i n a c i d i f i e d s o l u t i o n s of sulphur d i o x i d e . - Ik -PART- I The Synthetic Iron Oxide Hydrates There are two naturally occuring iron oxide hydrates, Goethite and-Lepidocrocite. The synthetic materials corresponding to these two minerals are respectively a-Fe00H and 7-FeOOH. Qj-FeOOH may be regarded as a hydrated form of hematite, a-Fe203, and 7-FeOOH as a hydrated form of maghemite, y-Fe203. This part of the thesis deals with the work done on these synthetic iron oxide hydrates and is reported under the following sections: 1) Preparation of the iron oxide hydrates 2) Identification of the iron oxide hydrates 3) Preparation of leaching specimens. - 15 -1) P r e p a r a t i o n of I r o n oxide hydrates. Review of Methods  g -FeOOH There are three general methods of preparing Q; -FeOOH: i ) By the o x i d a t i o n of f e r r o u s compounds i n aqueous s o l u t i o n i i ) By the slow h y d r o l y s i s of some f e r r i c s a l t s i i i ) By,the aging of brown or yellow g e l s i ) Oxidation of f e r r o u s compounds Rapid o x i d a t i o n by the passage of a i r through s o l u t i o n s of 12 f e r r o u s c h l o r i d e or f e r r o u s carbonate, formed by the a d d i t i o n of sodium or ammonium carbonate t o f e r r o u s c h l o r i d e solutions"'"-', produces a-Fe00H. This can a l s o be achieved by the o x i d a t i o n of ferrous hydroxide s o l u t i o n l 6 IT w i t h oxygen , or a i r '. A more h i g h l y hydrous form may be prepared by the r a p i d o x i d a t i o n of ferrous bicarbonate s o l u t i o n w i t h peroxide, oxygen, or a i r at room temperature. This hydrous m a t e r i a l w i l l l o s e i t s excess moisture and become Ci-Fe00H i f heated i n a stream of dry a i r at 100°C 18 f o r 48 hours i i ) Slow h y d r o l y s i s of f e r r i c s a l t s The h y d r o l y s i s of s o l u t i o n s of f e r r i c s a l t s , such as n i t r a t e , sulphate, a c e t a t e , bromide and oxalate gives e i t h e r a-FeOOH or a-Fe 20 3 19 depending on the c o n d i t i o n s . At room temperature the r a t e of h y d r o l y s i s i s slow, s o l u t i o n s r e q u i r i n g s e v e r a l weeks t o become completely hydrolysed. H y d r o l y s i s i s more r a p i d at higher tempratures, but a-Fe 20 3 I s formed i n s t e a d of (X-FeOOH i f the s o l u t i o n s are b o i l e d ^ 0 _ The c o n c e n t r a t i o n of f e r r i c s a l t a l s o a f f e c t s the product. In g e n e r a l , i t i s not p o s s i b l e t o - 16 -20 ob t a i n pure a-FeOOH from h i g h l y concentrated s o l u t i o n s . a-FeOOH can a l s o be prepared from f e r r i c hydroxide, f r e s h l y p r e c i p i t a t e d from f e r r i c n i t r a t e s o l u t i o n , by adding i t t o 2N KOH s o l u t i o n and then passing 21,22 i n steam i i i ) Aging o f the brown or yel l o w gels The brown g e l , Fe 20 3nH 20, prepared by the d i r e c t n e u t r a l i s a t i o n of a f e r r i c s a l t w i t h an a l k a l i hydroxide, when aged i n a i r f o r a p e r i o d of about two years, gives a-FeOOH. Aging of the yellow g e l , Fe 20 3nH 20, prepared by the o x i d a t i o n of f r e s h l y p r e c i p i t a t e d f e r r o u s carbonate w i t h 23 hydrogen peroxide, f o r a p e r i o d of a week gives a-FeOOH 7-FeOOH The most w i d e l y used method of preparing pure 7-FeOOH i s the a d d i t i o n of a complexing agent, such as p y r i d i n e , soduim a z i d e , or u r o t r o p i n , t o a s o l u t i o n of fer r o u s c h l o r i d e . This s o l u t i o n i s then 15 19 21 22 24 o x i d i s e d w i t h a i r or a s o l u t i o n of sodium n i t r i t e ' ' . Other methods of p r e p a r a t i o n of 7-FeOOH are by the o x i d a t i o n of a fer r o u s s a l t 24 s o l u t i o n w i t h equivalent amounts of sodium iodate and sodium t h i o s u l p h a t e ' 19 and by the o x i d a t i o n of f r e s h l y prepared hydrous F e 3 0 4 , hydrous 3 F e 2 0 3 . 2 F e 0 2 6 , F e 2 S 3 and F e S 2 7 . Experimental P r e p a r a t i o n 1 - a-FeOOH (NH 4) 2C0 3 (96g) was d i s s o l v e d i n water (1 l i t r e ) . FeS0 4-7H 20 (278g) was d i s s o l v e d in.water (2 l i t r e ) . A few drops of concentrated s u l p h u r i c a c i d were added t o the sulphate s o l u t i o n t o produce a c l e a r - 17 -blue-green s o l u t i o n . The s l i g h t brown residue was f i l t e r e d o f f . The ammonium carbonate s o l u t i o n was added dropwise t o the sulphate s o l u t i o n at room temperature over a p e r i o d of about 8 hours (approximately 30 drops per minute). During t h i s time-.the s o l u t i o n was a g i t a t e d and a r a p i d flow of a i r was maintained through i t . The orange-brown p r e c i p i t a t e was f i l t e r e d o f f , washed w i t h water and d r i e d at 60°C. The y i e l d was 6 l . 5 $ of the t h e o r e t i c a l amount. Pr e p a r a t i o n 2 - a-FeOOH A s o l u t i o n of potassium hydroxide (363.6gm) i n water (500ml) was added t o a s o l u t i o n of FeS04-7H20 (200gm) i n water (3 l i t r e ) . The s o l u t i o n s were pre-heated t o 70°C before mixing. The mixture was kept at 70°C and o x i d i s e d by bubbling a i r through.the a g i t a t e d s o l u t i o n f o r 2 l / 2 hours. During o x i d a t i o n the p r e c i p i t a t e turned from a blue-grey t o a yellow-ochre c o l o u r . A f t e r o x i d a t i o n the p r e c i p i t a t e was f i l t e r e d , washed w i t h d i s t i l l e d water and d r i e d a t 60°C. The y i e l d was 95$ of the t h e o r e t i c a l amount. P r e p a r a t i o n 3 - • 7-FeOOH P y r i d i n e (500ml) was s l o w l y heated t o i t s b o i l i n g p o i n t and then allowed t o c o o l t o room temperature i n a carbon d i o x i d e atmosphere. Pure f e r r o u s c h l o r i d e s o l u t i o n ( 125ml) , which had stood over powdered, i r o n to remove excess a c i d , was added dropwise while a current of carbon d i o x i d e was maintained through the apparatus. A yellow c r y s t a l l i n e p r e c i p i t a t e of t e t r a p y r i d i n e f e r r o u s c h l o r i d e (T.F.C.) was formed. • F e C l 2 + 4 C 5H 5N ^- Fe(C 5H 5W ) 4 C l 2 - 18 -The s o l u t i o n was allowed t o stand f o r one hour under a carbon dio x i d e atmosphere. The T.F.C. was f i l t e r e d o f f and d i s s o l v e d i n water (3 l i t r e ) , forming a dark brown-green s o l u t i o n . A i r was then bubbled through t h i s s o l u t i o n , s l o w l y f o r 15 minutes and then r a p i d l y f o r 30 minutes. .A yellow-brown p r e c i p i t a t e was formed, which was f i l t e r e d o f f , washed, and d r i e d at 60°C. Y i e l d = 3.03 gm. Pre p a r a t i o n k - 7-FeOOH P y r i d i n e (20ml) and water (100ml) were added t o saturate d f e r r o u s c h l o r i d e s o l u t i o n (100ml). The s o l u t i o n was a g i t a t e d and a i r was passed through r a p i d l y f o r 30 minutes. A red-brown p r e c i p i t a t e was formed, which was f i l t e r e d o f f , , washed, and d r i e d at 60°C. Y i e l d • = 3.50 gm. Pre p a r a t i o n 5 - 7-FeOOH Ferrous c h l o r i d e (80gm) was d i s s o l v e d i n water (2 l i t r e ) and the s o l u t i o n was f i l t e r e d . Urotropine (Hexamethylenetetramine) (112gm) was d i s s o l v e d i n water (400ml) and f i l t e r e d . The Urotropine s o l u t i o n was added t o the fer r o u s c h l o r i d e s o l u t i o n . .A s o l u t i o n of sodium n i t r i t e (28gm) i n water (400ml) was added dropwise w i t h constant s t i r r i n g . The s o l u t i o n was then heated t o 60°C and h e l d a t t h i s temperature f o r 30 minutes, w i t h o c c a s i o n a l s t i r r i n g . A red-brown p r e c i p i t a t e formed•during h e a t i n g . Gas was evolved.as the temperature reached 60°C. A f t e r $0 minutes, the s o l u t i o n was allowed t o c o o l , 3 hours a f t e r the beginning of the h e a t i n g , the red-brown p r e c i p i t a t e was f i l t e r e d o f f , washed u n t i l no c h l o r i d e was present i n the f i l t r a t e , and then d r i e d at 60°C. The y i e l d was 62$ of the t h e o r e t i c a l amount. - 19 -2) ' I d e n t i f i c a t i o n of the Iron oxide hydrates. Attempts were made t o p o s i t i v e l y i d e n t i f y the products o f the preceding syntheses by the use of 3 d i f f e r e n t a n a l y t i c a l techniques, namely: I n f r a - r e d a b s o r p t i o n a n a l y s i s D i f f e r e n t i a l thermal a n a l y s i s - X-ray d i f f r a c t i o n The r e s u l t s obtained u s i n g each of these three techniques are reported and discussed i n the next three sub-sections. I n f r a - r e d a b s o r p t i o n a n a l y s i s . A review of the l i t e r a t u r e i n d i c a t e d t h a t i n f r a - r e d absorption a n a l y s i s could be used.to d i s t i n g u i s h between the isomeric forms of FeOOH. The sp e c t r a of a l l the s y n t h e t i c preparations and of samples of the n a t u r a l minerals were determined. The bonds i n the FeOCH molecule are the Fe -0 bond and the 0-H bond. However, there i s a l s o hydrogen bonding between hydroxyl groups and oxygen atoms of d i f f e r e n t molecules,-0-H... .0. The iron-oxygen 28 bonding was studied.by Bass and Benedict , and i t s a b s o r p t i o n spectrum was found t o l i e i n the range 7OOO-I5OOO cm - 1, which i s outside the range used i n t h i s i n v e s t i g a t i o n . The wavelength range s t u d i e d i n t h i s work l a y between 600 and 4000 cm"1. The absor p t i o n bands of the hyd r o x y l and hydroxyl-oxygen bonds occur w i t h i n t h i s range. A hyd r o x y l bond w i t h no as s o c i a t e d hydrogen bonding shows a sharp a b s o p r t i o n peak at about 2.75^ M, due t o s t r e t c h i n g v i b r a t i o n s along the bond a x i s . The s t r e t c h i n g v i b r a t i o n of the hydroxyl group i s commonly^.symbolised by \)-0H. As the hydrogen bonding between the - 20 -hydroxyl group and the neighbouring oxygen atom becomes stronger, the \)-0H peak becomes broader and weaker, and the frequency s h i f t s t o lower va l u e s . I f the hydroxyl-oxygen hydrogen bond has a s t r e n g t h comparable t o the hyd r o x y l bond i t s e l f , then i t should.be p o s s i b l e t o observe an abs o r p t i o n due t o the bending v i b r a t i o n s of the 0-H->-«0 group. (See F i g . 1) sr ^ y y o D o o r< cs o In plane bending Out of plane bending or s c i s s o r i n g or wagging FIGURE I Bending v i b r a t i o n s of the 0H----0 group Absorptions due t o bending v i b r a t i o n s are observed i n compounds w i t h s t r o n g l y hydrogen bound hydroxyl-oxygen groups i n the range 800-1100cm~^. The symbol f o r bending v i b r a t i o n s of t h i s group i s S-OH. Se v e r a l workers have studied, the i n f r a - r e d a b s o r p t i o n spectra of Goethite and L e p i d o c r o c i t e 2 2 ' 2 ^ 5 0 , 3 1 , 3 2 , 3 3 _ T h e r e p o r t e d band frequencies are as f o l l o w s : -\)-0H. (cm - 1) S-OH (cm - 1) Goethite 3049-95 885 and 797 Lepidocrocite 3120 1025 I t may be seen t h a t 6-OH i n Goethite i s reported as g i v i n g r i s e t o a double a b s o r p t i o n band, whereas i n L e p i d o c r o c i t e i t gives only a s i n g l e band. Sample - l Wave Number cm Natu r a l G o e t h i t e * 3800 - 2800 1 1100 1 895 795 1 b.w. ! v.w. s. s. | Natu r a l L e p i d o c r o c i t e * 3200 - 2800 \ 1640 1160 1025 1 89O 79O 1 b.w. I b.w. | w. w. 1 w. m. 1 Preparation 1 (a-FeOOH) 3160 1 1630 , 1135 1020 1 885 795 1 b.s. | b .w. , m. m. I s . s. j Preparation 2 (a-FeOOH) 3i4o | 1640 j | 895 795 j b.s. | b . w . , 1 s * s * 1 ' Preparation 3 ( 7-FeOOH) 3050 | 1620 1 II55 1020 j | 750 b.s. | b.w. ' b.w. s. I m. 1 | Preparation 4 ( 7-FeOOH) 3440 1 | 1155 1020 j b.s. 1 1 v.w. m. 1 j 1 Preparation 5 ( 7-FeOOH) 3000 - 2800 | j 1160 1020 | b.s. 1 | w. w. | 1 * Purchased from Wards KEY: b.w. = Broad and weak w. = Weak b.s. = Broad and strong m. = Medium v.w. = Very weak s. - Strong 1 Table 1 I n f r a - r e d absorption spectra of n a t u r a l and s y n t h e t i c i r o n oxide hydrates £3 - 22 -R e s u l t s The samples were prepared f o r a n a l y s i s both as KBr p e l l e t s and as N u j o l mulls w i t h equal success. The frequencies of the observed a b s o r p t i o n bands are reported i n Table I . I t may be seen t h a t the i n f r a - r e d absorption spectra of the n a t u r a l and s y n t h e t i c samples of FeOOH can be grouped i n t o d i s t i n c t sets of bands. These bands are i n d i c a t e d by the dotted l i n e s i n Table I . 3000cm """range: These bands, which were evident i n every sample, were very broad, and the peak frequencies somewhat s c a t t e r e d . 1600-I650cm range: These bands are broad and weak and are evident i n only three of the s y n t h e t i c m a t e r i a l s and one of the n a t u r a l 30 m i n e r a l s . Absorption at t h i s frequency i s a t t r i b u t a b l e t o adsorbed water I ll60-1000cm~1 range: In t h i s range a double band i s evident i n a l l but the n a t u r a l g o e t h i t e and one of the s y n t h e t i c a -FeOOH preparations (Prep. 2). The double band has a weak peak a t approximately 1150cm"1 and a f a i r l y strong peak at 1020cm"1. 79Q-9QQcm~1 range: A second double band, w i t h peaks at ^890 and 795cm"1, i s present i n the two n a t u r a l minerals and the two sy n t h e t i c a -FeOOH.preparations. These bands are strong i n the n a t u r a l g o e t h i t e and the s y n t h e t i c m a t e r i a l s , but weak i n tue n a t u r a l l e p i d o c r o c i t e . D i s c u s s i o n The broad bands found i n the 3000cm 1 range are a t t r i b u t a b l e t o -OH, and the broadness i s caused by strong hydrogen bonding. The - 23 -broadness of these bands was greater in.the natural minerals than in the synthetic materials, indicating that the hydrogen bonding is stronger in the former than the latter. The double bands in the range 1150-790cm"1 may be attributed to the bending vibration, 8-OH, of the 0-H 0 g r o u p 2 9 , 5 0 ' 5 5 T h e double band at 89O and 795cm"1 is very characteristic of c<-Fe00H. It is 3^ not known why this band should show two peaks. Glemser and Hartert consider that i t is a consequence of the rhombic crystal structure of goethite, since cc-AlOOH, a -GaOOH, and £-Zn(OH)2, which are a l l rhombic, a l l give double bands in this range, whereas the non-rhombic In(0H )3 gives only- one band. The 6-OH vibration in Lepidocrocite is reported in the literature as giving rise to only- one band at 1020cm"1. It may,.be seen from the results, that the present investigation has shown that two peaks occur in this region, a strong peak at 1020cm"1 and a weak peak at approximately 1150cm - 1. It is suggested that this second peak is also due to S-OH of Lepidocrocite, for the following reasons: i) Lepidocrocite has a rhombic crystal structure i i ) BShmite ( 7-AIOOH), which is isomorphous with Lepidocrocite, shows a double band with peaks at 1145 a*id 1073cm"1 25 # Conclusions i) The results obtained in this investigation for the infra-red absorption spectrum of Goethite ( a-FeOOH) agree with those reported in the literature. The spectrum for this material consists of a broad, - 2k -r a t h e r undefined band i n 3500-3000cm"1 range due t o V^OH w i t h a s s o c i a t e d hydrogen bonding, and two strong,. well-defined,peaks at 89O and 795cm_1> .due t o &-0H. i i ) The ab s o p r t l o n spectrum of L e p i d o c r o c i t e ( 7-FeOOH) i s s i m i l a r t o a-FeOOH in . the 3000cm"1 range, but it- shows a strong peak a t 1020cm"1 and a weak peak a t approximately 1150cm"1. Both these peaks are thought t o be due t o 8-OH. i i i ) The broadness of the \)-OH peaks i s g r e a t e r i n the n a t u r a l minerals than in- the s y n t h e t i c m a t e r i a l s , i n d i c a t i n g stronger hydrogen bonding i n , t h e former m a t e r i a l s . i v ) The n a t u r a l g o e t h i t e sample contains v i r t u a l l y pure Cc-Ee00H, as does p r e p a r a t i o n 2. v) The n a t u r a l l e p i d o c r o c i t e sample and p r e p a r a t i o n 1 c o n t a i n mixtures of a- and 7-FeOOH. v i ) Preparations 3, k, and 5 c o n t a i n pure 7-FeOOH. v i i ) I n f r a - r e d a n a l y s i s i s a good method f o r d i s t i n g u i s h i n g between the isomeric forms of i r o n oxide hydrate. i i ) D i f f e r e n t i a l Thermal A n a l y s i s Since g o e t h i t e and l e p i d o c r o c i t e , a-FeOOH and 7-FeOOH, have 1 d i f f e r e n t c r y s t a l s t r u c t u r e s , i t should t h e o r e t i c a l l y be p o s s i b l e t o d i s t i n g u i s h between them by d i f f e r e n t i a l thermal a n a l y s i s . On hea t i n g , g o e t h i t e dehydrates d i r e c t l y t o hematite, a-Fe 2 0 3 , whereas l e p i d o c r o c i t e f i r s t dehydrates t o maghemite, 7-Fe 2 0 3 . At a higher temperature 7-Fe 20 3 - 25 -transforms t o a-Fe 20 3. In most cases, these d i f f e r e n c e s are apparent by D.T.A. The D.T.A. curve f o r g o e t h i t e c o n s i s t s of a s i n g l e endothermic 18 37 38 peak o c c u r r i n g between 3^5-400°C >•'*>•' , The curve f o r l e p i d o c r o c i t e i s reported t o c o n s i s t of an endothermic peak o c c u r r i n g about 350°C, f o l l o w e d 3T 38 by a v a r i a b l e exothermic peak, u s u a l l y at about 450°C ' . •The p a r t i c l e s i z e i s known t o a f f e c t the temperatures of these t r a n s f o r m a t i o n s , w i t h a decrease i n p a r t i c l e s i z e lowering the t r a n s f o r m a t i o n temperatures. The above mentioned f i g u r e s are f o r c o a r s e l y c r y s t a l l i n e m a t e r i a l s . 36 P a t i c l e s i z e can cause the peaks, t o v a r y between 300-400°C f o r go e t h i t e , and 300-360°C f o r the endothermic, and 370-450°C f o r the exothermic transformations of l e p i d o c r o c i t e ^ . • A d i f f i c u l t y a r i s e s when t r y i n g t o d i s t i n g u i s h between.poorly c r y s t a l l i s e d g o e t h i t e and l e p i d o c r o c i t e . P o o r l y c r y s t a l l i s e d g oethite i s s a i d t o dehydrate f i r s t t o amorphous Fe 203, which then c r y s t a l l i s e s t o C2-Fe 203, producing an exothermic peak on the curve. This type of curve 23,39 can be e a s i l y confused w i t h the l e p i d o c r o c i t e - t y p e curve - 26 -R e s u l t s The temperatures at which peaks were observed i n the D.T.A. curves of the v a r i o u s samples may be found i n Table 2. Sample N a t u r a l Goethite N a t u r a l = L e p i d o c r o c i t e * P r e p a r a t i o n 1 ( a-FeOOH) Pre p a r a t i o n 3 ( 7-FeOOH) Pr e p a r a t i o n 4' ( 7-FeOOH) Pr e p a r a t i o n 5 ( 7-FeOOH) Temperature (°C) 440 (EXQ) 400 (Exo) 320 (Endo) 67O-(Exo) 210 (Exo) 275 (Endo) 440 (Exo) 592 (Exo; 285 (Endo) 415 (Exo) 285 (Endo) 430 (Exo) Table 2: D.T.A. curves f o r n a t u r a l and s y n t h e t i c i r o n oxide hydrates Purchased from Wards Conclusions -i ) The sample of s o - c a l l e d n a t u r a l l e p i d o c r o c i t e appears t o c o n s i s t mainly of go e t h i t e i i ) P r e p a r a t i o n 1 could be l e p i d o c r o c i t e , 7-FeOOH i i i ) P reparations 4 and 5 give curves i n reasonable agreement w i t h the reported values f o r l e p i d o c r o c i t e i v ) For the purposes of t h i s i n v e s t i g a t i o n , D.T.A. i s not a conc l u s i v e method of i d e n t i f y i n g the s y n t h e t i c a l l y prepared i r o n oxide hydrates. i i i ) X - r a y - D i f f r a c t i o n . Since i t had been shown t h a t the two is omers a- and 7-FeOOH - 2 7 -could be r e a d i l y , d i s t i n g u i s h e d by t h e i r i n f r a - r e d - a b s o r p t i o n specta, only a very l i m i t e d " X - r a y . i n v e s t i g a t i o n was c a r r i e d out. The major peaks i n the reported X-ray d i f f r a c t i o n p a t t e r n s are l i s t e d i n Table 3 . The r e l a t i v e i n t e n s i t i e s reported by d i f f e r e n t sources show considerable v a r i a t i o n , but there i s general agreement on the© values of the peaks. S y n t h e t i c a-FeOOH (Prep. 2 ) and 7-FeOOH (Prep, k) were examined. The r e s u l t s may be found i n Table k. The peaks i n the d i f f r a c t i o n p a t t e r n s of these m a t e r i a l s were r a t h e r broad, but i t i s thought t h a t reasonable agreement i s shown between the ©values of .the s y n t h e t i c m a t e r i a l s and the reported v a l u e s . General Conclusions on the p r e p a r a t i o n and i d e n t i f i c a t i o n of the i r o n  oxide hydrates: 1) I n f r a - r e d a b s o r p t i o n a n a l y s i s i s a good.technique f o r d i s t i n g u i s h i n g c o n c l u s i v e l y between, the isomeric forms of i r o n oxide hydrate. 2 ) Pure a-FeOOH may be synthesised-by P r e p a r a t i o n 2 . 3) ' Pure 7 -FeOOH maybe sy t h e s i s e d by-Preparations 3, k, and 5- P r e p a r a t i o n 5 w a s found t o give the highest y i e l d of m a t e r i a l . - 28 -Table 3: Major peaks of X-ray d i f f r a c t i o n p a t t e r n s of .a-and ,7-FeOOH  (ASTM card index) O-FeOOH -7-EeOOH 0 d A 9 X 0 d A e 5 . 0 0 11.2 60 6.26 8 . 9 100 4 .21 13.3 100 3.29 17.1 90 2.69 21.2 70 2.47 23 .1 60 2.58 2 2 . 0 20 1.94 2 9 . 9 70 2.44 23.4 80 1.72 3^-3 50 Table 4 : X-ray d i f f r a c t i o n p a t t e r n s of the s y n t h e t i c m a t e r i a l s • a-FeOOH (Prep. 2) 7-Fe00H (Prep. 4) O d' A e ' m O d- A e V i 4 . 9 8 11.2 20 6.34 8 . 8 90 4.20 13.3 100 3-31 17.0 100 2.70 21.0 30 2.48 23.0 80 2.58 22.0 20 1.94 29.9 70 2.46 23.2 60 1.72 3 4 . 3 30 - 29.-3. Preparation- of Leaching Specimens In order t o prepare samples of the s y n t h e t i c m a t e r i a l s s u i t a b l e f o r l e a c h i n g , t e s t s were c a r r i e d out t o determine the s u i t a b i l i t y of the powders f o r compacting, machining, and c u t t i n g . This was followed by t e s t s on the prepared samples under autoclave c o n d i t i o n s . Compacting A bulk p r e p a r a t i o n of a-FeOOH ( P r e p a r a t i o n _ l ) was made, and the t e s t s c a r r i e d out w i t h t h i s powder. Three types of compactions were t r i e d -a) 18 gm of powder were h y d r o s t a t i c a l l y compressed i n o i l , enclosed i n a rubber tube, t o a pressure of 35000 p s i . The cohesion of the compact was good and i t s d e n s i t y was 2.2Cgm/cc, 53$ of t h e o r e t i c a l . b) l6gm of powder were s t a t i c a l l y compressed t o a pressure of 117,000 p s i . The d e n s i t y of t h i s compact was s l i g h t l y greater, 2.56 gm/cc, 6l.5$ of t h e o r e t i c a l , but i t was more b r i t t l e than ( a ) . c) A s t a t i c a l l y compressed b i l l e t was crushed t o pass a 16 mesh screen and recompressed s t a t i c a l l y t o 117000 p s i . The d e n s i t y of the m a t e r i a l was 2.64 gm/cc, 63-5$ °f t h e o r e t i c a l , but the cohesion of the b i l l e t was poor. Machining, c u t t i n g and mounting I t was found p o s s i b l e t o machine the h y d r o s t a t i c a l l y pressed b i l l e t s t o round s e c t i o n , u s i n g a standard metal-working l a t h e at slow speed and slow h o r i z o n t a l movement of the t o o l . - 30 -The b i l l e t s were s l i c e d , i n t o s e c t i o n s , u s i n g a j e w e l l e r ' s saw w i t h a very f i n e blade. -A sm a l l j i g was used t o ho l d the b i l l e t and to guide the blade v e r t i c a l l y during c u t t i n g . .Sections of the compacts were mounted i n epoxy r e s i n w i t h one surface exposed. The exposed,face was then ground f l a t . .Specimen T e s t i n g Mounted s e c t i o n s , prepared i n the above manner, were t e s t e d t o determine whether they would withstand t y p i c a l pressure l e a c h i n g c o n d i t i o n s . I n i t i a l l y , t e s t s w i t h these specimens were s u c c e s s f u l , no c r a c k i n g or d i s i n t e g r a t i o n of the compacts being observed. However a f t e r a p e r i o d of a few weeks, specimens prepared i n an i d e n t i c a l manner t o the e a r l i e r ones began t o d i s i n t e g r a t e during l e a c h i n g . I t was found impossible t o prevent t h i s . The reasons f o r t h i s d i s i n t e g r a t i o n are not known, but i t i s conjectured t h a t the i r o n oxide hydrate powder underwent some aging process which s u f f i c i e n t l y changed i t s surface p r o p e r t i e s t o reduce the cohesion of the compacts. I t was undesirable t o work w i t h f r e s h l y prepared oxide powders because there was no guarantee t h a t the l e a c h i n g c h a r a c t e r i s t i c s of one batch would be the same as another, and so i t was decided t o abandon the idea of working w i t h s y n t h e t i c m a t e r i a l and use n a t u r a l l y o c c u r r i n g massive g o e t h i t e . P a r t I I of t h i s t h e s i s deals w i t h the work c a r r i e d out using t h i s m a t e r i a l . - 31 -PART I I The D i s s o l u t i o n of N a t u r a l g-Iron Oxide  Hydrate i n Aqueous. S o l u t i o n s of Sulphur Dioxide EXPERIMENTAL M a t e r i a l s (a) Reagents: Reagent-grade chemicals were used e x c l u s i v e l y . Nitrogen and sulphur d i o x i d e were su p p l i e d by Matheson Co. and used without f u r t h e r p u r i f i c a t i o n . (b) N a t u r a l m i n e r a l : A q u a n t i t y of massive n a t u r a l g o e t h i t e was purchased from Ward's N a t u r a l Science Establishment. Wet chemical a n a l y s i s showed th a t the mineral contained 56$ i r o n , i n d i c a t i n g an FeOOH content of 89 %. I n f r a - r e d a b s o r p t i o n a n a l y s i s showed th a t the i r o n oxide hydrate content of the min e r a l was pure ct-FeOOH. A q u a l i t i a t i v e spectrographic a n a l y s i s i n d i c a t e d t h a t s i l i c o n was the major i m p u r i t y , some aluminium.was present, a l s o t r a c e s of sodium, manganese, ti t a n i u m , copper, n i c k e l and c o b a l t (See Table 5). The X-ray d i f f r a c t i o n p a t t e r n of the min e r a l showed good agreement w i t h the reported p a t t e r n f o r g o e t h i t e •(See Table 6 ). The major peak f o r s i l i c a was a l s o observed. - 32 -Table 5 : Q u a l i t a t i v e s p e c t r o g r a p h s examination of n a t u r a l goethite I r o n Major Co n s t i t u e n t S i l i c o n Intermediate C o n s t i t u e n t +1$ Aluminium Minor C o n s t i t u e n t Approx. 1$ Sodium Manganese T r a c e g Approx. 0.01$ Titanium Copper N i c k e l Approx. 0.001$ Cobalt I n s o l u b l e s Approximately 10$ Table 6: . X-ray d i f f r a c t i o n p a t t e r n of the n a t u r a l g o e t h i t e Reported This study o d A e 0 d A e 5 . 0 0 11.2 60 5-03 11.1 20 4 . 2 1 13.3 1Q0 4 .21 13.3 100 2.69 21.2 70 2.70 21 .0 30 2.58 2 2 . 0 20 2.44 23.4 80 2.46 23.2 25 1.72 34.3 50 1.72 3 4 . 2 15 - 33 -I n i t i a l l y the m i n e r a l was cut on a diamond saw i n t o r e c t a n g u l a r b l o c k s , and mounted i n epoxy r e s i n w i t h one face exposed. The exposed face was ground f l a t and used as the l e a c h i n g specimen. This method of specimen p r e p a r a t i o n was found t o be u n s a t i s f a c t o r y due t o poor r e p r o d u c t i b i l i t y of r e s u l t s amongst d i f f e r e n t specimens. The l a c k of r e p r o d u c i b i l i t y was thought t o be due t o e i t h e r one, or both of the f o l l o w i n g reasons: a) Inhomogeneous d i s p e r s i o n of i m p u r i t i e s i n the mi n e r a l b) P r e f e r e n t i a l l e a c h i n g along c e r t a i n c r y s t a l planes. In-order t o overcome these inhomogenieties of the m i n e r a l , i t was decided t o work w i t h a p a r t i c u l a t e feed. The mi n e r a l was crushed i n t o s m a l l pieces w i t h a sledge-hammer, and then ground down t o the r e q u i r e d s i z e u s i n g a hand-muller. The p a r t i c l e s were s i z e d by wet screening t o remove adhering f i n e s . The -65 + mesh f r a c t i o n was chosen and used f o r a l l l e a c h i n g t e s t s . A f t e r screening and d r y i n g , p a r t i c l e s of i r o n from the g r i n d i n g t o o l s were removed w i t h a hand magnet. Leaching was c a r r i e d out on.weighed samples of t h i s m a t e r i a l . E i t h e r one or two gram samples were used. • Sampling was done using a 7 -point technique, and the bulk m a t e r i a l was r o l l e d on a polythene sheet during sampling t o ensure complete mixing. The surface area of the p a r t i c l e feed used was estimated t o be 90cm2/gm + 30$-Autoclave Design A 1 l i t r e s t a i n l e s s s t e e l (316SS) autoclave w i t h a glass - 34 -l i n e r was used. B a f f l e s , c o n s i s t i n g of four glass rods set at 90° "to one another, were h e l d i n place against the sides of the l i n e r by two annular T e f l o n r i n g s . S t i r r i n g was provided by a Magnedrive u n i t , s u p p l i e d by Autoclave Engineers. This u n i t had a completely enclosed s t i r r i n g s h a f t , which was actuated by means of-an e x t e r n a l r o t a t i n g magnet. The gas i n l e t tube, of l / 8 " copper t u b i n g , was attached t o the top of the s t i r r e r shaft housing. This housing was made of 3 l 6 s s . Sulphur d i o x i d e , n i t r o g e n , or compressed a i r could be d e l i v e r e d through the- i n l e t tube. The s t a i n l e s s s t e e l s t i r r i n g shaft was sheathed by Te f l o n rod, d r i l l e d t o give a snug f i t over the s h a f t . A g i t a t i o n was provided by a T e f l o n b l o c k which was threaded on t o the bottom of s t i r r e r s h a f t sheath. The s t i r r i n g speed used f o r a l l runs was 900 R.P.M. V i s u a l t e s t s showed t h a t t h i s s t i r r i n g speed, plus the e f f e c t of the b a f f l e s , provided extremely t u r b u l e n t a g i t a t i o n of the s o l u t i o n . The i n s i d e face of the s t a i n l e s s s t e e l head was covered by T e f l o n sheet, which was bonded t o the s t e e l s u r f ace. The th e r m i s t o r w e l l was of copper, covered w i t h Tygon p a i n t and sheathed w i t h Tygon t u b i n g . The sample tube i n s i d e the autoclave was of polyethylene w i t h a s i n t e r e d g l a s s f i l t e r f i t t e d on the i n l e t end of the tube. The e x t e r n a l p a r t of the sample tube was of l / 8 " copper tube, and the sample valve constructed from Bery]lium-copper. The gas e x i t l i n e was l / V ' 3l6ss t u b i n g . The pressure gauge, 0-100 p s i Duragauge 3l6ss Type, was connected t o the gas e x i t tube, on the pressure side of the exhaust v a l v e . Heating was provided by a c i r c u l a r gas burner c o n t r o l l e d by a s o l e n o i d v a l v e . This v a l v e was activated by a Thermistemp temperature c o n t r o l l e r (Yellowsprings Instrument Co.) The temperature of the contents of the - 35 -autoclave was measured by a high temperature t h e r m i s t o r probe, i n s e r t e d i n t o the t h e r m i s t o r w e l l . Using t h i s system, the temperature was c o n t r o l l e d t o w i t h i n 1 1.5°C. Experimental Procedure The experimental procedure c o n s i s t e d of the f o l l o w i n g steps: i ) A one or two gram sample of m i n e r a l , reagents, and s u f f i c i e n t water t o give a s o l u t i o n volume of 655™! were put i n t o the autoclave. i i ) The v e s s e l was sealed, and the s t i r r e r s t a r t e d , i i i ) The exhaust valve was opened and n i t r o g e n was allowed t o f l u s h through the v e s s e l f o r the whole of the warm-up p e r i o d . i v ) The s o l u t i o n was allowed t o b o i l under the n i t r o g e n atmosphere f o r 10-15 minutes, i n order t o d i s p l a c e any d i s s o l v e d oxygen. v) The exhaust v a l v e was then c l o s e d down u n t i l there was j u s t a s m a l l bleed of gas escaping from the v e s s e l . This bleed was continued throughout each run. v i ) Heating was resumed. When the s o l u t i o n was a few degrees below r e a c t i o n temperature., the n i t r o g e n supply was turned o f f and sulphur d i o x i d e was introduced a t the re q u i r e d pressure. For runs c a r r i e d out i n the absence of sulphur d i o x i d e , i . e . the d i r e c t d i s s o l u t i o n experiments, a n i t r o g e n atmosphere was main-t a i n e d throughout the run. v i i ) The s o l u t i o n was allowed t o s t a b i l i s e at the r e a c t i o n temperature ( u s u a l l y about 10 minutes a f t e r the i n t r o d u c t i o n of the - 36 -sulphur d i o x i d e ) , and the f i r s t sample was taken. .The co n c e n t r a t i o n of i r o n i n this sample was used as the blank f o r the c a l c u l a t i o n of the rate curve f o r the run. v i i i ) Samples f o r i r o n determination were taken at r e g u l a r i n t e r v a l s throughout a run, u s u a l l y s i x samples per run. • Sampling procedure i n v o l v e d f i r s t t a k i n g and d i s c a r d i n g 2 ml of s o l u t i o n . This was t o clear, the sample tube of unreacted s o l u t i o n . Then a f u r t h e r 5ml sample was taken and used f o r the i r o n determination. i x ) Sulphate estimations were c a r r i e d out on a lOQml sample taken at the end of the run. The continuous bleed of gas through the system, mentioned i n (v) above, was necessary t o create a s l i g h t excess pressure i n the s t a i n l e s s s t e e l head- of the autoclave. This excess pressure prevented water vapour from e n t e r i n g the head, and so only dry sulphur d i o x i d e was i n contact w i t h the s t e e l . C o r r o s i o n of the s t a i n l e s s s t e e l under these c o n d i t i o n s was so low t h a t the amount of i r o n e n t e r i n g the s o l u t i o n from the autoclave during a blank run of 3 hours d u r a t i o n was v i r t u a l l y unmeasurable, and c e r t a i n l y n e g l i g i b l e when compared w i t h the rate of l e a c h i n g of i r o n from the ore. . Water vapour l o s s through the exhaust valve during a run was n e g l i g a b l e . This technique had the disadvantage that the maximum pressure t h a t could be used i n the system was the d e l i v e r y pressure of the sulphur d i o x i d e . The sulphur d i o x i d e was i n l i q u i d form i n the d e l i v e r y tank and so the maximum pressure a v a i l a b l e was the vapour pressure of l i q u i d sulphur di o x i d e a t ambient temperature. This v a r i e d s l i g h t l y , depending on ambient temperature, but never exceeded 36 p s i g . Because of t h i s - 37 -l i m i t a t i o n i n the maximum pressure of sulphur d i o x i d e a v a i l a b l e , the range of a c i d i t y and b i s u l p h i t e i o n concentration t h a t could be used i n t h i s study was somewhat l i m i t e d . -A system i n which higher sulphur d i o x i d e pressures could have been used would have been p r e f e r a b l e . A n a l y t i c a l Methods a) Iron The concentration of i r o n in. the sample s o l u t i o n s was measured c o l o r i m e t r i c a l l y by means of the orange-red f e r r o u s complex of 1-10-orthophenanthroline. To ensure t h a t a l l the i r o n i n the s o l u t i o n was i n the f e r r o u s s t a t e hydroxylamine hyd r o c h l o r i d e was used as a reducing agent. The s o l u t i o n s were buffered-at pH4.5 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 o l u t i o n . A composite reagent was used c o n s i s t i n g of 20$ by volume of a 0.15$ orthophenanthroline s o l u t i o n , 20$ by volume of a 1$ hydroxylamine hyd r o c h l o r i d e s o l u t i o n , and 60$ by volume of acetate b u f f e r . Pml a l i q u o t s of sample s o l u t i o n (or s m a l l e r , i f necessary) were p i p e t t e d i n t o 25ml p o r t i o n s of the composite reagent and made up t o 100ml w i t h water. The o p t i c a l d e n s i t y of each s o l u t i o n was measured on a Beckman Model B Spectrophotometer, u s i n g l i g h t of wavelength 510myV.. The c o n c e n t r a t i o n of iron.was read, d i r e c t l y from a c a l i b r a t i o n curve pre-pared by u s i n g standard i r o n s o l u t i o n s . b) Sulphate kO Sulphate i o n was estimated by the method, due t o Belcher The sulphate was f i r s t p r e c i p i t a t e d by a d d i t i o n of standard barium - 38 -c h l o r i d e s o l u t i o n . The excess barium c h l o r i d e was estimated by adding an excess of standard E.D.T.A. s o l u t i o n . The excess E.D.T.A. was t i t r a t e d i n a l k a l i n e s o l u t i o n w i t h standard magnesium c h l o r i d e s o l u t i o n , u s i n g Superchrome Black as an i n d i c a t o r . I n t h i s way, the poor end-point which occurs when barium c h l o r i d e i s t i t r a t e d d i r e c t l y w i t h E.D.T.A. .was avoided. Because the i r o n • i n the s o l u t i o n would complex w i t h the E . D . T i A . and thus give high r e s u l t s , a m o d i f i c a t i o n of the method due to H u n t ^ was used t o remove a l l c a t i o n s from the s o l u t i o n . P r i o r t o the a d d i t i o n of the barium c h l o r i d e s o l u t i o n , the sample was passed through a 20cm column of Amberlite 120 C a t i o n exchange r e s i n . The r e s i n was used i n the hydrogen i o n s u b s t i t u t e d s t a t e , and when saturated w i t h i r o n , i t was regenerated by passing through 5N h y d r o c h l o r i c a c i d s o l u t i o n . This method was found t o be completely s u c c e s s f u l i n removing a l l measurable t r a c e s of i r o n from the samples. Procedure: Sulphate determinations were c a r r i e d out on a 100ml sample th a t was taken a t the end of the run. Immediately a f t e r withdrawal from the autoclave, n i t r o g e n was bubbled through the sample f o r 15-20 minutes t o d i s p l a c e d i s s o l v e d sulphur d i o x i d e . The sample was then passed s l o w l y through the ion-exchange column. A sample (25ml) of the ion-exchanged s o l u t i o n was heated t o b o i l i n g . Standard 0.01N B a C l 2 s o l u t i o n (20ml) was added and the s o l u t i o n was allowed t o stand f o r one hour. Standard 0.01M E.D.T.A. s o l u t i o n (25ml) was then added p l u s an ammonium chloride-ammonia b u f f e r s o l u t i o n (20ml). Twelve drops of Superchrome Black i n d i c a t o r were added and the s o l u t i o n was t i t r a t e d t o a c l e a r c e r i s e - r e d colour w i t h standard 0.01M MgCl 2 s o l u t i o n . The ammonium chloride-ammonia b u f f e r s o l u t i o n c o n s i s t e d - 39 -of l6.5gm NH 4C1 and 226ml concentrated ammonia s o l u t i o n made up t o 1 l i t r e , and the Superchrome Black i n d i c a t o r was a O.yjo s o l u t i o n i n ethanol. Each sulphate determination was c a r r i e d out i n d u p l i c a t e or t r i p l i c a t e . Using the above procedure, i t was found p o s s i b l e t o measure lmg of sulphur i n standard f e r r o u s ammonium sulphate samples t o an accuracy of ± 2$. - ko -RESULTS For the sake of c l a r i t y the r e s u l t s are presented only as diagrams accompanying the t e x t i n t h i s s e c t i o n . They are presented i n ta b u l a t e d form i n Appendix A. The t a b l e s i n the appendix f o l l o w the same sequence as the f i g u r e s i n . t h i s s e c t i o n . D i r e c t D i s s o l u t i o n i n A c i d ; S o l u t i o n s A l i m i t e d study was c a r r i e d out t o determine the behavior of g o e t h i t e i n a c i d i c s o l u t i o n s under non-reducing c o n d i t i o n s . The d i s s o l u t i o n i n 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 o l u t i o n s was i n v e s t i g a t e d , lgm samples of the m i n e r a l were used i n a l l cases. P e r c h l o r i c a c i d D i s s o l u t i o n i n p e r c h l o r i c a c i d was found to be very slow. At 130°C and an a c i d s t r e n g t h of 0.7M, the r a t e of d i s s o l u t i o n was found t o be only 6 x 10" 5 moles Fe per hour (See F i g . 6 ) . At lower a c i d i t i e s and temperatures, the r a t e was too slow t o be a c c u r a t e l y measurable. S u l p h u r i c a c i d The e f f e c t s of a c i d i t y and temperature on the d i s s o l u t i o n of g o e t h i t e i n s u l p h u r i c a c i d s o l u t i o n s were s t u d i e d . Under c e r t a i n c o n d i t i o n s , namely low a c i d i t i e s and higher temperatures, d e v i a t i o n s from l i n e a r i t y were observed i n the p l o t s of conc e n t r a t i o n versus time (See F i g . k). However a l l r a t e s t h a t are reported have been measured on the i n i t i a l l i n e a r p o r t i o n s of the conc e n t r a t i o n p l o t s . - kl -Varying a c i d i t y at constant temperature The e f f e c t of v a r y i n g the conc e n t r a t i o n of s u l p h u r i c a c i d i n s o l u t i o n at a constant temperature was i n v e s t i g a t e d . The a c t i v i t y of hydrogen i o n at each a c i d c o n c e n t r a t i o n has been assumed t o he equal, t o the molar con c e n t r a t i o n of a c i d . The rate of d i s s o l u t i o n i s p l o t t e d versus a c i d c o n c e n t r a t i o n i n F i g . 2 and versus hydrogen i o n a c t i v i t y i n F i g . 3 (Table .1 Appendix A). I t may be seen that the r a t e increases approximately l i n e a r l y w i t h hydrogen i o n a c t i v i t y . The r a t e of d i s s o l u t i o n i n s u l p h u r i c a c i d i s f a s t e r than i n p e r c h l o r i c a c i d at s i m i l a r molar concentrations. At 130°C and an a c i d c o n c e n t r a t i o n of approximately 0.7M, the rate of d i s s o l u t i o n i n s u l p h u r i c a c i d i s of the order of 35 times f a s t e r than i n p e r c h l o r i c a c i d . (See F i g . 6). Varying temperature at constant a c i d i t y The r e s u l t s f o r v a r i a t i o n s i n the temperature at a constant s u l p h u r i c a c i d c o n c e n t r a t i o n may be found i n F i g . k. An Arrhenius p l o t f o r these r e s u l t s i s given i n F i g . 5 (Table I I Appendix'A). The slope of t h i s l i n e corresponds t o an a c t i v a t i o n energy of 18.2 Kcals/mole. The d e v i a t i o n s from l i n e a r i t y of the r a t e curves at higher temperatures, evident i n F i g . k, were thought t o be due t o the f a c t t h a t the e q u i l i b r i u m c o n c e n t r a t i o n of f e r r i c sulphate was being approached. In order t o s u b s t a n t i a t e t h i s i d e a , a run was done i n which f e r r i c sulphate was added t o the s o l u t i o n i n i t i a l l y . The r e s u l t s of t h i s experiment are p l o t t e d i n F i g . 1. 1.9 x 10~ 4 moles of i r o n , as f e r r i c sulphate, were added at the s t a r t of t h i s run. I t may be seen t h a t the concentration of i r o n i n s o l u t i o n does approach a maximum value, i n d i c a t i n g that the d[Fe] dt (Mxl0 5/min) h J 2 A 1 J 0 Temp. = 130°C lgm.Goethite samples [H 2S0 4]M Figure 2 Rate versus concentration of H 2S0 4 ro Figure 3• Rate versus a c t i v i t y of hydrogen ion i n H2S04 ( - kk -[Fe] (MxlO 4) Time (min) Figure k. - E f f e c t of temperature at constant [H g S 0 4 ] Figure 5. Arrhenius p l o t f o r d i s s o l u t i o n i n H2S04 - k6 -[Fe] (MxlO 4) Rate =21.8 moles Fexl0 4/hr. Temp 130°C • [H 2S0 4] = 0.7M o [ H C I O 4 ] = 0.7M Igm Goethite samples Rate = 0.6 moles o-180 Time (min) Figure 6 Comparison of r a t e of d i s s o l u t i o n of Goethite i n 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 - 7^ -[Fe] (MxlO 4) 28 _ 2k 20 16 12 Temp = l40°C [H 2S0 4] = 0.146M lgm Goethite samples ^ \ 1.9x10" M Fe added as F e 2 ( S 0 4 ) 3 at s t a r t of run o -O" o F e 2 ( S 0 4 ) 3 added o o o 60 180 300 Time (min) Figure 7 E f f e c t of adding F e 2 ( S 0 4 ) 3 at the s t a r t of the run - 48 -s o l u b i l i t y of f e r r i c sulphate may be becoming rat e determining. A run c a r r i e d out under the same c o n d i t i o n s , but without the added f e r r i c sulphate, i s p l o t t e d on the same diagram. Reductive D i s s o l u t i o n i n A c i d S o l u t i o n s The main body of the l e a c h i n g experiments was. c a r r i e d out i n aqueous s o l u t i o n s of sulphur d i o x i d e , 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 . A knowledge of the t o t a l pressure of the system, the temperature, and the hydrogen i o n co n c e n t r a t i o n , enabled the c a l c u l a t i o n of the concentration of d i s s o l v e d , s u l p h u r d i o x i d e and the conc e n t r a t i o n of b i s u l p h i t e ion t o 42 be made from the data of Beazley et a l . (For a d i s c u s s i o n of the sulphur dioxide-water system, and d e t a i l s of these c a l c u a t i o n s , see Appendix B.) Most of these experiments were c a r r i e d out at a temperature of 110°C. This temperature was chosen f o r the f o l l o w i n g reasons: i ) Conveniently measurable r a t e s were obtained, i i ) Sulphur d i o x i d e could only be introduced t o the system at pressures above the vapour pressure of water. As the maximum sulphur d i o x i d e pressure a v a i l a b l e was J6 p s i . g . , i t was d e s i r a b l e t o keep the vapour pressure of water, and hence the temperature, as low as p o s s i b l e i n order t o maximise the range of sulphur d i o x i d e pressures t h a t could be used. i i i ) The decomposition temperature of goe t h i t e ( a-FeOOH) to hematite ( a-Fe 20a) i n aqueous s o l u t i o n s i s v a r i o u s l y . r e p o r t e d 43,44,45 as o c c u r r i n g between 120-150°C. Thus i t was thought p r e f e r a b l e t o work at a temperature at which g o e t h i t e would be the r m a l l y s t a b l e . - 4g -One gram samples of the p a r t i c u l a t e g o e t h i t e were used f o r the m a j o r i t y of the runs. However i n order t o determine areas of heterogeneous and homogeneous c o n t r o l , c e r t a i n runs were repeated using two gram samples. I t may be seen from the f o l l o w i n g r e s u l t s t h a t these experiments i n d i c a t e d that homogeneous c o n t r o l of the r e a c t i o n occurred under c e r t a i n c o n d i t i o n s . For t h i s reason, the r a t e s are reported i n homogeneous u n i t s r a t h e r than heterogeneous u n i t s . A set of t y p i c a l r a t e s , observed under c o n d i t i o n s of heterogeneous c o n t r o l , and reported i n heterogeneous u n i t s , may be found i n Table X (Appendix.A). T y p c i a l Rate- Curves A set of ra t e curves, t y p i c a l of those observed i n aqueous sulphur d i o x i d e s o l u t i o n s , , may be found i n F i g . 8. The shape of these p l o t s i e . a curved p o r t i o n f o l l o w e d by a l i n e a r p o r t i o n , was c h a r a c t e r i s t i c f o r v i r t u a l l y every r u n . c a r r i e d out w i t h sulphur d i o x i d e . I t was a l s o observed f o r some of the runs c a r r i e d out with s u l p h u r i c a c i d s o l u t i o n s . The l i n e a r p o r t i o n began approximately one hour a f t e r the s t a r t of the run. The curved p o r t i o n . e x i s t e d f o r the same time p e r i o d , i e . the f i r s t hour, f o r a l l c o n d i t i o n s s t u d i e d , i r r e s p e c t i v e of the ra t e of d i s s o l u t i o n of i r o n . Because of t h i s , i t was thought to-be due t o some c h a r a c t e r i s t i c of the m i n e r a l i t s e l f . The e f f e c t of v a r y i n g the p a r t i a l pressure of sulphur d i o x i d e at constant  a c i d i t y Three s e r i e s of experiments were done under these c o n d i t i o n s at three d i f f e r e n t 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 s , namely 7.0 x 10~2M, - 50 -[Fe].MxlO* Time (min) Figure 8 T y p i c a l , r a t e p l o t s f o r d i s s o l u t i o n i n a c i d i f i e d sulphur d i o x i d e s o l u t i o n s - 51 -1.4 x 10 1M and 2.8 x 10 1M. These s e r i e s are named S e r i e s A l , A2 and A3 r e s p e c t i v e l y . The r e s u l t s , as p l o t s of r a t e of d i s s o l u t i o n of i r o n versus the concentration of d i s s o l v e d sulphur d i o x i d e f o r each a c i d i t y may be found i n F i g s . 9, 10, 11. (Table: I I I Appendix A.) I t may be seen t h a t the curves are a l l of the same general shape, i n v o l v i n g an i n i t i a l l y slow increase, i n rate w i t h sulphur dioxide c o n c e n t r a t i o n . This i s f o l l o w e d by a steeper s e c t i o n i n which the r a t e increases approximately l i n e a r l y w i t h sulphur d i o x i d e c o n c e n t r a t i o n . An attempt was made t o d e l i n i a t e areas of homogeneous and heterogeneous c o n t r o l by r e p e a t i n g runs under the same c o n d i t i o n s , u s i n g one and two gram samples of the m i n e r a l . These r e s u l t s i n d i c a t e d t h a t , i n general f o r S e r i e s A l and A2, i e . low a c i d c o n d i t i o n s , the r a t e appeared t o be mainly homogeneously c o n t r o l l e d at low sulphur d i o x i d e c o n c e n t r a t i o n s . Series-A3 appeared t o be mainly heterogeneously c o n t r o l l e d throughout. The r e p r o d u c i b i l i t y of the r e s u l t s i n these s e r i e s was v a r i e d . Under some c o n d i t i o n s the r e p r o d u c i b i l i t y was good-within 5$, but under other c o n d i t i o n s v a r i a t i o n s of up t o 30$ were observed. In general the r e p r o d u c i b i l i t y was best i n S e r i e s A l the lowest a c i d s e r i e s . The poor r e p r o d u c i b i l y i n the other s e r i e s was mainly evident at low concentrations of d i s s o l v e d sulphur d i o x i d e . The l a c k of r e p r o d u c i b i l i t y i n these areas was thought t o i n d i c a t e t h a t a complex k i n e t i c s i t u a t i o n was o c c u r r i n g . d[Fe] dt (Mxl05/min) Series A l Temp 110°C [HCIO4] = 7.0xlO~ M O - lgm Goethite sample 6 - -2gm Goethite sample © - 1 and 2gm Goethite samples 3 6 1— 3-5 0.5 1.0 1.5 2.0 2.5 3.0 4.0 [ S 0 2 ] a q (MxlO) Figure 9 Rate versus concentration of d i s s o l v e d sulphur d i o x i d e d [ F e l — (Mxl0 5/min) Series A2 Temp 110°C [HC10 4] = l.kxlO~xM O- lgm Goethite sample 2gm .Goethite sample. 1 and 2gm Goethite samples 2.5 3.5 k.O [S02]aq_ (MxlO) V J I V j J Figure 10 Rate versus concentration of d i s s o l v e d sulphur d i o x i d e d[Fe] ~cTE (Mxl05/min) 1.0-. 0 - 5 _ 3-'5 ^ . 0 [ S 0 2 ] a Q (MxlO) Figure 11 Rate versus concentration of d i s s o l v e d sulphur d i o x i d e - 55 -The e f f e c t of v a r y i n g a c i d i t y at constant sulphur d i o x i d e concentration A number of runs was c a r r i e d out at d i f f e r e n t a c i d i t i e s at a constant low c o n c e n t r a t i o n of d i s s o l v e d sulphur d i o x i d e . These r e s u l t s are reported i n Table IV (Appendix A)• I t may be seen t h a t they confirm the observation t h a t , at low concentrations of d i s s o l v e d sulphur d i o x i d e , the r a t e of d i s s o l u t i o n i s v i r t u a l l y independent of the a c i d i t y of the s o l u t i o n . Sulphate analyses In order t o t r y t o e s t a b l i s h the stoichiometry of the r e a c t i o n , the l e a c h s o l u t i o n s of a number of runs were analysed f o r t h e i r sulphate content. In each case, the sample f o r sulphate a n a l y s i s was taken 5 hours a f t e r the beginning of the run. The r e s u l t s are reported i n Table V (Appendix A ) . The molar r a t i o of sulphate t o i r o n was always greater than 1.5:1, the average value being 1.77:1 The e f f e c t of c u p r i c i o n on the system Since copper i s known t o c a t a l y s e the homogeneous o x i d a t i o n of sulphurous a c i d by f e r r i c i o n , s e v e r a l s e r i e s of experiments were performed t o determine the e f f e c t of c u p r i c i o n on the l e a c h i n g of g o e t h i t e w i t h aqueous s o l u t i o n s of sulphur d i o x i d e . Cupric i o n was added as c u p r i c p e r c h l o r a t e . The r e s u l t s are reported under the f o l l o w i n g headings: i ) V arying c u p r i c i o n c o n c e n t r a t i o n under c o n d i t i o n s of constant a c i d i t y and constant sulphur d i o x i d e c o n c e n t r a t i o n - 56 -i i ) Varying sulphur d i o x i d e concentration at constant a c i d i t y and constant i n i t a l c u p r i c i o n concentration i i i ) The e f f e c t of temperature on,the c u p r i c c a t a l y s e d r e a c t i o n i v ) The e f f e c t of cuprous i o n i n the absence of sulphur d i o x i d e . i ) V arying the c u p r i c i o n c o n c e n t r a t i o n at constant a c i d i t y and constant  sulphur d i o x i d e concentration S e r i e s BI was c a r r i e d out under c o n d i t i o n s of low a c i d i t y and low concentration-of d i s s o l v e d sulphur d i o x i d e . The r e s u l t s of t h i s s e r i e s may be found i n Fig. 12, p l o t t e d as r a t e versus c u p r i c i o n con-c e n t r a t i o n . Under these c o n d i t i o n s i n the absence of copper, the r e a c t i o n i s homogeneously c o n t r o l l e d . The r a t e i s c a t a l y s e d by the a d d i t i o n of c u p r i c i o n t o the system. At low concentrations of c u p r i c i o n , the rate remains homogeneously c o n t r o l l e d , but i t becomes heterogeneously c o n t r o l l e d at higher c u p r i c concentrations. This has been i l l u s t r a t e d i n F i g . 12 by p l o t t i n g r e s u l t s obtained u s i n g one and two gram samples of g o e t h i t e . I t may be seen t h a t as the r a t e becomes f u l l y heterogeneously c o n t r o l l e d , the increase i n r a t e slows down, but then at higher c u p r i c c o n c e n t r a t i o n s , the r a t e begins t o increase f a i r l y r a p i d l y again, s t i l l remaining heterogeneously c o n t r o l l e d . i i ) V a rying the sulphur d i o x i d e c o n c e n t r a t i o n at constant a c i d i t y and  constant i n i t i a l c u p r i c i o n c o n c e n t r a t i o n S e r i e s B2 was c a r r i e d out t o determine the e f f e c t of - 57 -d[Fe] 5 ~cTE (MxlO /min) 10.0 Figure 12 Rate versus c u p r i c i o n c o n c e n t r a t i o n - 5 8 -i n c r e a s i n g the concentration of d i s s o l v e d sulphur d i o x i d e under c o n d i t i o n s of constant a c i d i t y - a n d constant i n i t i a l c u p r i c i o n con-c e n t r a t i o n . The r e s u l t s may be found i n F i g . 13 (Table V I I Appendix A). I t i s evident t h a t there i s an optimum concentration of d i s s o l v e d sulphur d i o x i d e . At concentrations g r e a t e r than the optimum, the r a t e begins t o decrease. An experiment was c a r r i e d out t o determine the amount of copper i n s o l u t i o n at v a r i o u s sulphur d i o x i d e concentrations. The i n i t i a l reagent concentrations were the same as f o r S e r i e s B2. The s o l u t i o n was brought up t o r e a c t i o n temperature under a n i t r o g e n atmosphere and a sample was taken. Sulphur d i o x i d e was then introduced at a c e r t a i n pressure. At the end of one hour, a f u r t h e r sample was taken and the sulphur d i o x i d e pressure increased. This was repeated three times and the samples analysed f o r t h e i r copper content. The r e s u l t s are p l o t t e d i n ' F i g . Ik (Table V I I I Appendix A ) . I t may be seen t h a t the c o n c e n t r a t i o n of copper i n s o l u t i o n decreases as soon as sulphur d i o x i d e i s introduced t o the system. The copper c o n c e n t r a t i o n then remains constant u n t i l a t higher sulphur d i o x i d e c o n c e n t r a t i o n s , i t begins t o decrease again. -This second decrease occurs at approximately the same sulphur d i o x i d e con-c e n t r a t i o n as the decrease i n r a t e observed i n S e r i e s B2. This i s evident from a comparison of F i g s . 13 and Ik. i i i ) The e f f e c t of temperature on the c u p r i c c a t a l y s e d r e a c t i o n In order t o determine the a c t i v a t i o n energy of the c u p r i c c a t a l y s e d r e a c t i o n , a s e r i e s of experiments was performed i n which the r e a c t i o n temperature was v a r i e d , the reactant concentrations being kept - 59 -d[Fe] (MxlQ 5/min) 4.0 J 2.0 | , , , I ~o ITo 2 T 0 5 T 0 4.0 [S02] (MxlO) £lC]_ Figure 13 E f f e c t of i n c r e a s i n g [S0P] at constant i n i t i a l c u p r i c i o n  concentration a C^ [Cu] (MxlO 3) [ S 0 2 ] A Q (MxlO) Figure 14 E f f e c t of i n c r e a s i n g [S02] on [Cu] i n s o l u t i o n - 60 -constant. The concentration of d i s s o l v e d sulphur d i o x i d e was kept constant by v a r y i n g the p a r t i a l pressure of sulphur d i o x i d e i n accordance w i t h the p u b l i s h e d s o l u b i l i t y data of sulphur d i o x i d e (See Appendix B). The r e s u l t s of t h i s s e r i e s are p l o t t e d on an Arrhenius diagram, F i g . 15 (Table IX Appendix A ) . The slope of t h i s l i n e corresponds t o an a c t i v a t i o n energy of 23.1 KCal/mole. i v ) The e f f e c t of cuprous i o n i n the absence of sulphur d i o x i d e To determine whether cuprous i o n could enter i n t o the r e a c t i o n , a run was c a r r i e d out under c o n d i t i o n s of h i g h c u p r i c i o n con*-, c e n t r a t i o n and low a c i d i t y . .The reducing atmosphere was provided by i n t r o d u c i n g hydrogen i n t o the system.instead of sulphur d i o x i d e . In t h i s way, the e f f e c t of d i s s o l v e d sulphur d i o x i d e was e l i m i n a t e d , w h i l e s t i l l m a i n t a i n i n g the reducing c o n d i t i o n s necessary f o r the formation of some cuprous i o n . The r e s u l t s of t h i s run ( F i g . 16), i n d i c a t e d t h a t i t i s p o s s i b l e t o d i s s o l v e i r o n from g o e t h i t e i n d i l u t e p e r c h l o r i c a c i d s o l u t i o n i n the presence of cuprous i o n . Figure 15 Arrhenius p l o t f o r c u p r i c c a t a l y s e d d i s s o l u t i o n - 62 -[PeJ MxlO 4 Time (min) Pigure 16 D i s s o l u t i o n of go e t h i t e i n d i l u t e p e r c h l o r i c a c i d i n the presence of copper under H 2 atmosphere - 63 -DISCUSSION Direct Dissolution The results of the direct dissolution experiments indicate that:-i) The rate of dissolution of goethite in sulphuric acid increases approximately l inear ly with hydrogen ion act iv i ty over the range of acid concentrations studied. i i ) The activation energy for dissolution in.sulphuric acid is 18.2 KCal/mole. i i i ) The rate of dissolution in sulphuric acid is faster than in perchloric acid of the same molar concentration. Similar results to these were reported by Azuma and Kametani for the dissolution of hematite, ct-Fe 203, in acid media They showed that the activation energy of 20^2 KCals/mole was v i r tua l ly independent of the type of acid used, that the rate of dissolution increased with increasing hydrogen ion, and that the absolute rate of dissolution depended upon the particular acid used. They demonstrated that there was a direct relationship" between the absolute rates of dissolution and the s tab i l i ty constants for complexes formed by fer r i c ions with the anions of the various acids. In the following discussion an attempt is made to relate the results of this study and of Azuma and Kametani by postulating a general mechanism which is operative during the direct dissolution of both hydrated a -Fe203 (Goethite) and anhydrous a -Fe 2 0 3 (Hematite) in acid - Gk -media. I t i s considered t h a t the f i r s t step i n the d i s s o l u t i o n of both hematite and g o e t h i t e i n v o l v e s h y d r a t i o n of the oxide s u r f a c e , so tha t they both take up s i m i l a r surface c o n f i g u r a t i o n s i n aqueous s o l u t i o n designated i n the f o l l o w i n g manner: . F e ^ + H 20 ^ Fe^-OH [ l ] ^ O H ( S ) (aq) ^OH(S) In t h i s c o n d i t i o n the oxide surface w i l l probably have an o v e r a l l negative charge due t o the hydroxyl groups on the surface. In a c i d s o l u t i o n t h i s i s l i k e l y t o r a p i d l y undergo p r o t o n a t i o n : ^OE . .OH r . Fe^-OH + H ^ - Fe^-OH + H 20 [2] ^ O H ( S ) (aq) © (S) ( aq) This e q u i l i b r i u m , i n v o l v i n g the f i r s t p r o t o n a t i o n of f e r r i c hydroxide, has been reported t o be the p r i n c i p a l e q u i l i b r i u m kl o c c u r r i n g i n s o l u t i o n s of pH J t o 1 at 25°C . At pH approaching 1 or l e s s , a second p r o t o n a t i o n becomes i n c r e a s i n g l y l i k e l y -/ O H + ® F e ^ O H , ^ + H ^ = £ ^ FeOH + H 20 [ 5 ] ® (s) (a1) ® (S) (aq) In order t o account f o r the enIon dependency shown on the rate of d i s s o l u t i o n i n va r i o u s a c i d s , i t i s suggested t h a t anion a d s o r p t i o n takes place at the sur f a c e , f o l l o w i n g r e a t i o n [3J ® _ FeOH + X ^ F e — O H [k] ® (S) (aq) ® (S) © Assuming that,, a t any time, the conc e n t r a t i o n of FeOH on - 65 -the surface i s s m a l l , i e . r e a c t i o n [k] takes place r a p i d l y f o l l o w i n g r e a c t i o n [3J, then an o v e r a l l e q u i l i b r i u m maybe w r i t t e n -s OH + ^ /X F e ^ O H + H + X ^ 5 -* Fe^-OH + H 2 0 [5] © (S) (aq) (aq) © (S) (aq) Reaction [5] i s , i n e f f e c t , an anion exchange, r e s u l t i n g i n the exchange of a hydroxyl i o n f o r an anion,, X , at the oxide surface. F e r r i c hydroxide has been shown t o have an anion exchange c a p a c i t y i n 48 a c i d s o l u t i o n Two d i f f e r e n t mechanisms l e a d i n g t o the d i s s o l u t i o n of f e r r i c i o n are p o s s i b l e f o l l o w i n g r e a c t i o n [5]. Mechanism (a) Reaction [5] i s f o l l o w e d d i r e c t l y by a r a t e - c o n t r o l l i n g desorption of the h y d r o x y - f e r r i c complex: k« + Fe^-OH — - — » - Fe X OH, 4 [6] © (S) (aq) The r a t e equation f o r t h i s mechanism may be w r i t t e n as: d (FeXOH +) = k 6K 5[Fe<^8l ] [X~] [H +] [7] dt 9 I f i t i s assumed t h a t the whole oxide surface undergoes the f i r s t p r o t o n a t i o n ( r e a c t i o n 12J), then [Fe— OHJ should be a constant, © and may be regarded as the t o t a l number of s i t e s . According t o t h i s mechanism, the r a t e of d i s s o l u t i o n w i l l depend on both the co n c e n t r a t i o n of hydrogen i o n , and the co n c e n t r a t i o n of anion i n s o l u t i o n . - D i f f e r e n c e s i n absolute r a t e s of d i s s o l u t i o n i n d i f f e r e n t a c i d s may be accounted f o r by v a r i a t i o n s i n K 5. The value of K 5 should be r e l a t e d t o the - 66 -complexing a f f i n i t y of the anion, X , f o r f e r r i c i o n , since Azuma and Kametani have shown th a t there i s a d i r e c t r e l a t i o n s h i p between the rate of d i s s o l u t i o n and the s t a b i l i t y constant of the f e r r i c - a n i o n complex. Mechanism (b) The assumption t h a t ' K 5 v a r i e s f o r d i f f e r e n t anions may be i n c o r r e c t and th a t i n f a c t , K 5 may be la r g e f o r a l l anions, so th a t r e a c t i o n [5J l i e s w e l l t o the r i g h t . I f t h i s i s the case, i t i s suggested that d i s s o l u t i o n may proceed v i a another p r o t o n a t i o n of the s i t e s c o n t a i n i n g an adsorbed.anion: F e * d F + H + r f r — FeX + H20 [8] © (S) ( a q j ®(S) . (aq.) ® k 9 + + FeX FeX [9] ®(S) (aq) In t h i s case, the r a t e equation i s : ++ ^OH + d(FeX ) = k 9 K 8 [Fe^—OHj.[H J [10] dt © D i s s o l u t i o n o c c u r r i n g v i a t h i s mechanism should depend on the c o n c e n t r a t i o n of hydrogen i o n i n s o l u t i o n , but should be independent of the conc e n t r a t i o n of anion. • D i f f e r e n c e s i n . the absolute rate of d i s s o l u t i o n f o r d i f f e r e n t anions would be due t o d i f f e r e n t values of K 8. The experimental evidence i s not considered s u f f i c i e n t at present t o favour e i t h e r one of these mechanisms (a) or (b). F i g . 3 shows t h a t over the range of concentrations s t u d i e d , the d i s s o l u t i o n of i r o n from g o e t h i t e i n s u l p h u r i c a c i d s o l u t i o n f o l l o w s an approximately l i n e a r dependence on the a c t i v i t y of hydrogen i o n . This might suggest t h a t - 67 -mechanism (b) was ope r a t i v e . However mechanism (a) could a l s o show a l i n e a r dependence on hydrogen i o n i f [X ] remained constant 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 • (See equation-[7]) This would be the case i f sulphate, r a t h e r than b i s u l p h a t e , was the adsorbing anion i n s u l p h u r i c a c i d s o l u t i o n s , since the second d i s s o c i a t i o n constant of s u l p h u r i c a c i d i s s m a l l compared w i t h the f i r s t . F u r ther s t u d i e s are necessary t o enable a d i s t i n c t i o n t o be -made between these two mechanisms. The a d d i t i o n of anion a t a f a i r l y h i g h a c i d s t r e n g t h should give a good i n d i c a t i o n as t o which mechanism i s op e r a t i v e , since d i s s o l u t i o n o c c u r r i n g v i a mechanism (a) should be de-! pendent on the anion c o n c e n t r a t i o n whereas v i a mechanism (b) i t should be independent of anion c o n c e n t r a t i o n . The p o s t u l a t i o n t h a t the rate c o n t r o l l i n g step i s the desorption of a complex species from the oxide surface i s made since the a c t i v a t i o n energy f o r the process has been shown t o be s i m i l a r f o r d i f f e r e n t a c i d s , i n d i c a t i n g a s i m i l a r r a t e c o n t r o l l i n g step i n each case. The reported value of 20- 2 KCal/mole f o r hematite and the value determined i n t h i s study of 18.2 KCal/mole f o r g o e t h i t e are considered t o be i n good agreement. The values of these a c t i v a t i o n energies are high enough t o i n d i c a t e t h a t a chemical desorption i s t a k i n g p l a c e , r a t h e r than a d i f f u s i o n process. The spread i n values between d i f f e r e n t a c i d s , i e . - 2 KCal/mole i s thought t o be s u f f i c i e n t t o account f o r the observed d i f f e r e n c e s i n rates i n d i f f e r e n t a c i d s . The change i n absolute r a t e has been p o s t u l a t e d t o be e i t h e r a f u n c t i o n of the p r e - e q u i l i b r i u m anion exchange r e a c t i o n ( r e a c t i o n [5]), or the p r e - e q u i l i b r i u m proton a t t a c k ( r e a c t i o n [8.]). The enthalpy i n v o l v e d i n these p r e - e q u i l i b r i u m r e a c t i o n s w i l l - 6 8 -be a p a r t of the measured a c t i v a t i o n energies. Changes i n t h i s enthalpy-due t o involvement of d i f f e r e n t anions i n these e q u i l i b r i a could account f o r the observed spread i n the a c t i v a t i o n energies. This argument may .be demonstrated s e m i - q u a n t i t a t i v e l y as f o l l o w s -The rat e of d i s s o l u t i o n v i a e i t h e r mechanism (a) or (b) may be expressed i n terms of the a c t i v a t i o n energy by the Arrhenius equation: Ed + AH Rate = Ae RT where A - the Arrhenius constant E^ = the a c t i v a t i o n energy of the desorption step AH = the enthalpy of the p r e - e q u i l i b r i u m r e a c t i o n The measured a c t i v a t i o n energy w i l l ! c o n s i s t of the sum of E^ and AH. I t has been shown th a t at 130°C, the r a t e of d i s s o l u t i o n i n su l p h u r i c a c i d i s of the order of 35 times f a s t e r than i n p e r c h l o r i c a c i d a t s i m i l i a r molar concentrations. „ , , » T T Ed + AHi 7 RT Thus: Rate„ c_ Ae H g S ° 4 = Ed +AH 2 = 35 Rate A e ~ R T HC10 4 where AHx a n d A H 2 are the e n t h a l p i e s a s s o c i a t e d w i t h the p r e - e q u i l i b r i a i n v o l v i n g H 2S0 4 and HC10 4 r e s p e c t i v e l y . I f i t i s assumed th a t E^ i s v i r t u a l l y independent of the p a r t i c u l a r desorbing s p e c i e s , then: - A H r + A H g _ = l o g 35 2.303RT 2.303RT - 6 9 -. . AH 2 -AHi - 2 . 3 0 3 R T l o g 35 = ^.575(^03)(1.544) = 2.85 KCal Thus the d i f f e r e n c e between, the e n t h a l p i e s a s s o c i a t e d w i t h the p r e - e q u i l i b r i a i n v o l v i n g these two a c i d s i s l e s s than the reported spread of 4 KCals i n the measured a c t i v a t i o n energies f o r d i s s o l u t i o n i n various a c i d s . - 70 -Reductive D i s s o l u t i o n S o l u t i o n s of sulphur d i o x i d e i n water have t r a d i t i o n a l l y been r e f e r r e d t o as sulphurous a c i d s o l u t i o n s . Although the anions of t h i s a c i d , b i s u l p h i t e (HS0 3~) and s u l p h i t e ( S O 3 - ) , have been i d e n t i f i e d i n s o l u t i o n , u n d i s s o c i a t e d sulphurous a c i d has not (See Appendix B). I t i s b e l i e v e d t h a t the bulk of the sulphur d i o x i d e i n s o l u t i o n e x i s t s as n e u t r a l molecules w i t h an a s s o c i a t e d h y d r a t i o n sheath of water molecules. However, f o r the purposes of t h i s d i s c u s s i o n , the term sulphurous a c i d , and i t s t r a d i t i o n a l l y accepted formula H 2 S O 3 , w i l l be used, i n s t e a d of the more c o r r e c t term, hydrated sulphur d i o x i d e (S0 2xH 20). I t i s considered t h a t the re d u c t i v e d i s s o l u t i o n of goe t h i t e i n sulphurous a c i d , as i n the case of d i r e c t d i s s o l u t i o n , i n v o l v e s h y d r a t i o n of the oxide surface as a p r e r e q u i s i t e t o f u r t h e r r e a c t i o n -s>9 - • /OH Fe^—OH + H 20 ^ F e — O H [ l ] (S) (aq) ^OH(S) The e f f e c t of a c i d i t y and d i s s o l v e d sulphur d i o x i d e c o n c e n t r a t i o n The r e s u l t s f o r S e r i e s A l , A2, and A3, i n which the p a r t i a l pressure of sulphur d i o x i d e was v a r i e d a t three d i f f e r e n t a c i d i t i e s , are p l o t t e d on the same diagram ag a i n s t the conc e n t r a t i o n of d i s s o l v e d sulphur dioxide-See F i g . 17. I t may be seen that at low concentrations of d i s s o l v e d sulphur d i o x i d e the rate increases s l o w l y , and i s v i r t u a l l y independent of a c i d i t y . At higher sulphur d i o x i d e c o n c e n t r a t i o n s , the rate begins t o increase more r a p i d l y i n an approximately l i n e a r f a s h i o n , and a marked dependency, on a c i d i t y becomes evident. Figure 17 Rate versus concentration of d i s s o l v e d sulphur d i o x i d e at various a c i d i t i e s ( S e r i e s A l , Ag, and A3) - 72 -The range of hydrogen i o n conc e n t r a t i o n i n which these s e r i e s of runs were c a r r i e d out was 7 x 10~ 2M t o 2.8 x 10 - 1M. Under these c o n d i t i o n s , p r o t o n a t i o n of the sur f a c e , s i m i l a r t o th a t p o s t u l a t e d f o r d i r e c t d i s s o l u t i o n , i s thought t o occur-^OH + /-OH F e ^ O H + H v Fe^-OH + H 20 [2j ^ O H ( S ) (aq.) ®(S) (aq.) At 110°C, the apparent d i s s o c i a t i o n constant of sulphurous -3 a c i d has been c a l c u l a t e d t o be Ka = 1.37 x 10 (See Appendix B). The low value of t h i s constant i n d i c a t e s t h a t the m a j o r i t y of the d i s s o l v e d sulphur d i o x i d e w i l l remain u n d i s s o c i a t e d i n s o l u t i o n , and i t i s thought l i k e l y t h a t n e u t r a l sulphurous a c i d molecules are the r e a c t i n g species at the oxide s u r f a c e . Furthermore, i n order t o account f o r the observed hydrogen ion dependency at higher concentrations of d i s s o l v e d sulphur d i o x i d e , i t i s p o s t u l a t e d t h a t the sulphurous a c i d can react a t the oxide surface v i a two d i s t i n c t mechanisms, (a) and (b) -Mechanism (a) Reaction i n v o l v i n g the removal of two' hydroxyl groups from the s u r f a c e , and r e s u l t i n g i n the formation of a f e r r i c s u l p h i t e complex on the s u r f a c e -^ 0 H K 1 X + F e — O H + H 2S0 3 ~- FeS0 3 + 2H 20 [ l l ] ® (S) (aq) (S) (aq) -The next, and r a t e - c o n t r o l l i n g step, i s considered t o be the desorption of t h i s complex from.the surface i n t o s o l u t i o n -+ k i p + r n FeS0 3 , Q S = = — • FeS0 3 [12] l b i (aq) - 73 -The r a t e equation f o r these two steps, assuming R e a c t i o n [ l l ] i s i n e q u i l i b r i u m , may be w r i t t e n as: + ^OH d Fe(S0 3) = k 1 2K:n [Fe^_0H] [H 2S0 3 J [13] dt © Mechanism (b) Reaction i n v o l v i n g the removal of one hy d r o x y l group from the oxide s u r f a c e , r e s u l t i n g i n the formation of a h y d r o x y l - f e r r i c s u l p h i t e complex ^ 0 H K + Fe^—OH', N + H 2S0 3 ^ 1 4 - FeS0 3(OH) + H 20 +H [lk] ® (S) 2 3(aq)"= 3 V '(S) Again, the r a t e c o n t r o l l i n g step i s considered t o be des o r p t i o n of the complex from the surface i n t o s o l u t i o n -FeS0 3(0H) N k l 5 » FeS0 3(OH) [15] (S) (aq) Assuming Reaction [lk] i s i n e q u i l i b r i u m , the rat e equation f o r the d i s s o l u t i o n of i r o n v i a t h i s mechanism may be w r i t t e n as-^ O H d (FeS0 3(OH)) = k 1 5 K 1 4 [ F e — O H ] • [H 2S0 3 ] dt ® , [16] EH*1 I f both of these mechanisms (a) and ( b ) , c o n t r i b u t e t o the d i s s o l u t i o n of i r o n from goe t h i t e i n sulphurous a c i d , t h e i r r a t e equations may be combined t o give an o v e r a l l r a t e equation-^ O H OH d (Fe) = k 1 P K 1 j F e — O H ] [H 2S0 3 ] + k 1 5 K 1 4 [Fe—OH] [H 2S0 3 ] [17] dt ® ®  ^ O H I f i t i s assumed t h a t the number of s i t e s , i e . [ F e — O H ] , - 7^ -remains constant f o r a given surface area of m i n e r a l , then i t may be seen t h a t the r a t e of d i s s o l u t i o n of i r o n i s a f u n c t i o n of two terms, both d i r e c t l y p r o p o r t i o n a l t o the c o n c e n t r a t i o n of sulphurous a c i d . However one term i s independent of, and the other i n v e r s e l y p r o p o r t i o n a l t o , the c o n c e n t r a t i o n of hydrogen i o n . The r e s u l t s show th a t the r a t e i s v i r t u a l l y independent of hydrogen i o n concentration at low concentrations of d i s s o l v e d sulphur d i o x i d e , suggesting t h a t mechanism (a) i s predominant. As the sulphur d i o x i d e c o n c e n t r a t i o n i s increased, an inverse dependency of r a t e on hydrogen i o n c o n c e n t r a t i o n becomes evident, i n d i c a t i n g t h a t mechanism (b) becomes i n c r e a a i n g l y predominant. The observed hydrogen i o n 1 dependency i s l e s s than [ H + J . This observation agrees w i t h the above rate equation, since i t i s comprised of two terms,, one of which i s independent of hydrogen i o n c o n c e n t r a t i o n . Thus the o v e r a l l r a t e should show a dependency on hydrogen i o n c o n c e n t r a t i o n which l i e s between the l i m i t s set by these two terms. The mechanisms o u t l i n e d above can be used t o show why homogeneous c o n t r o l of the r e a c t i o n i s observed under c e r t a i n c o n d i t i o n s . Homogeneous c o n t r o l i s g e n e r a l l y evident at low concentrations of d i s s o l v e d sulphur d i o x i d e . Under these c o n d i t i o n s , i t has been suggested t h a t mechanism (a) i s predominant, and t h a t the r a t e c o n t r o l l i n g step i s the desorption of the c a t i o n i c f e r r i c s u l p h i t e complex. For the r e a c t i o n t o become homogeneously c o n t r o l l e d , the r a t e c o n t r o l l i n g heterogeneous step must reach an e q u i l i b r i u m . This means t h a t r e a d s o r p t i o n of the c a t i o n i c f e r r i c s u l p h i t e complex would have t o be r a p i d compared w i t h i t s r a t e of - 75 -desorption. Since hydrated i r o n oxide i s known t o have anion exchange p r o p e r t i e s under a c i d c o n d i t i o n s , i t i s u n l i k e l y t h a t a c a t i o n i c complex could be r a p i d l y readsorbed back on t o the oxide surface. I t i s thought t o be more l i k e l y t h a t a f t e r d e s o r p t i o n , the f e r r i c s u l p h i t e complex rea c t s w i t h b i s u l p h i t e i o n t o form an a n i o n i c f e r r i c s u l p h i t e complex, p o s s i b l y : -F e S 0 3 + + HS0 3" : F e ( S 0 3 ) 2 " + H + [ l 8 ] I t i s suggested t h a t F e ( S 0 3 ) 2 ~ i s the species t h a t can adsorb back r a p i d l y on t o the oxide surface and create an e q u i l i b r i u m s i t u a t i o n , s o : t h a t a homogeneous r e a c t i o n then becomes r a t e c o n t r o l l i n g . The p o s s i b l e sequences of r e a c t i o n s i n s o l u t i o n are numerous. However each sequence should r e s u l t i n the same o x i d a t i o n 11 products, namely sulphate and d i t h i o n a t e , but p r o p o r t i o n s of these species formed w i l l probably depend on the mechanism i n v o l v e d . • The mechanism which w i l l predominate depends mainly on which f e r r i c - s u l p h u r IV species are s t a b l e under the p a r t i c u l a r c o n d i t i o n s . As was pointed out i n the l i t e r a t u r e review s e c t i o n , two s e r i e s of f e r r i c - s u l p h u r IV complexes are p o s s i b l e - those i n v o l v i n g s u l p h i t e ions and having the general 3 - 2 n formula F e ( S 0 3 ) n , and those i n v o l v i n g b i s u l p h i t e i o n s , the general 3 n formula being F e ( H S 0 3 ) n ~ . U n t i l i n f o r m a t i o n i s a v a i l a b l e on the s t a b i l i t y of these complexes under v a r i o u s c o n d i t i o n s , statements made concerning homogeneous r e a c t i o n s i n t h i s system must be p u r e l y s p e c u l a t i v e . - 76 -However, since i t has been suggested.that the a n i o n i c f e r r i c d i s u l p h i t e complex, Fe(,S0 3) 2~, i s predominant under c o n d i t i o n s of low sulphur d i o x i d e concentrations,.- a mechanism w i l l be p o s t u l a t e d i n v o l v i n g t h i s complex. . Even i f t h i s assumption i s i n c o r r e c t , the general p r i n c i p l e s i n v o l v e d should apply t o mechanisms i n v o l v i n g other complexes. The f i r s t step i n the r e d u c t i o n of f e r r i c i r o n i s thought t o i n v o l v e the c o l l i s i o n of two f e r r i c d i s u l p h i t e complexes. This i s i n 10 agreement w i t h the work of Bassett and Parker . Depending on the movement of e l e c t r o n s between the two complexes, e i t h e r sulphate or d i t h i o n a t e could be formed as o x i d a t i o n products. The r e a c t i o n i n v o l v i n g sulphate production may be w r i t t e n as-2 F e ( S 0 3 ) 2 * 2 F e + + + S 0 3 + J S 0 3 ~ [19] Sulphur t r i o x i d e would react immediately w i t h water t o form sulphate S 0 3 * H 20 S 0 4 = + 2H + [20] Under a c i d c o n d i t i o n s , f r e e s u l p h i t e ions would be very unstable and probably r e a c t w i t h protons t o form b i s u l p h i t e 3S0 3~ + 3H + m> 3HS03~ [21J The production of d i t h i o n a t e i n v o l v e s the discharge of two s i n g l y charged s u l p h i t e f r e e r a d i c a l s -2Fe(S0 3) 2'*'~ 2 F e 2 + + 2S0 3~ + 2S03" [22] - 77 -D i t h i o n a t e can then r e s u l t from d i m e r i s a t l o n of these f r e e r a d i c a l s -2S0 3" S 2 0 6 ~ [23] As b e f o r e , the s u l p h i t e ions would probably react t o form b i s u l p h i t e 2S0 3~ + 2H + ** 2HS0 3" [2k] In order t o t r y t o e s t a b l i s h the stoichiometry of the r e a c t i o n s , sulphate determinations were c a r r i e d out on the f i n a l l e a c h s o l u t i o n s obtained at the end of a number of runs. The sulphate t o i r o n molar r a t i o s t h a t were determined were; a l l g r e a t e r than 1.5 t o 1 (See Table V Appendix A ) . I f sulphate p r o d u c t i o n occurs v i a r e a c t i o n s [19] and [20] then the maximum sulphate t o i r o n molar r a t i o would be 0.5 t o 1. This must be the maximum r a t i o t h a t can occur i n t h i s system since the formation of sulphate from the a n i o n i c species of sulphurous a c i d i n v o l v e s the discharge of two e l e c t r o n s . These can only be discharged on t o two f e r r i c ions t o form f e r r o u s i o n s , r e s u l t i n g i n a sulphate t o i r o n r a t i o of 0.5: 1. The reasons f o r the r e l a t i v e l y h i g h concentrations of . sulphate determined i n t h i s study are not known, but i t i s thought t h a t i t may be due t o o x i d a t i o n of sulphurous a c i d by p e r c h l o r i c a c i d . I t i s a l s o p o s s i b l e t h a t t r a c e s of c h l o r i c a c i d , which i s a strong o x i d i s i n g agent, were present i n the p e r c h l o r i c a c i d . Oxidation by r e s i d u a l d i s s o l v e d - 78 -oxygen i n the water i s thought t o he u n l i k e l y , since the s o l u t i o n was always b o i l e d under a n i t r o g e n atmosphere, before i n t r o d u c t i o n of the sulphur d i o x i d e . The e f f e c t of c u p r i c i o n The m a j o r i t y of the runs i n which copper was added t o the system were c a r r i e d out under c o n d i t i o n s of low a c i d i t y and low d i s s o l v e d sulphur d i o x i d e concentration-conditons under which mechanism (a) i s p o s t u l a t e d t o be the dominant d i s s o l u t i o n mechanism. Under these c o n d i t i o n s , the ra t e was found t o be homogeneously c o n t r o l l e d i n the absence of copper. I t was found t h a t a d d i t i o n s of c u p r i c i o n t o the system could c a t a l y s e the rate of d i s s o l u t i o n . I t may be seen from F i g . 12 t h a t , at low c u p r i c i o n con c e n t r a t i o n s , the r a t e remains homogeneously c o n t r o l l e d , but that at higher concentrations, the c o n t r o l becomes heterogeneous. •The r e a c t i o n s of c u p r i c i o n i n the f e r r i c iron-sulphurous a c i d system have been p o s t u l a t e d t o be^"-++ =, + = + Cu + S 0 3 + H 20 Cu + S 0 4 + 2H [25] r, + ^ 3 + r a p i d 1 „ + + „ + + r Cu + F e f + - — C u + Fe [26] Under the c o n d i t i o n s of t h i s study, the c o n c e n t r a t i o n of f r e e f e r r i c i o n i n s o l u t i o n i s l i k e l y t o be very low. I t i s thought more l i k e l y t h a t cuprous i o n formed by Reaction [25] w i l l r e a ct r a p i d l y w i t h the c a t i o n i c f e r r i c s u l p h i t e complex as i t desorbs from the - 79 -oxide s u r f a c e : -F e S 0 3 + + C u + r a p i d - FeS0 3 + C u + + [27] Ferrous s u l p h i t e , which i s a n e u t r a l molecule, should not have the same tedency t o readsorb back on t o the oxide as does a n i o n i c f e r r i c d i s u l p h i t e formed by Reaction [ l 8 ] . I t i s more l i k e l y t h a t ferrous s u l p h i t e would react w i t h a proton and form f e r r o u s i o n and b i s u l p h i t e i o n . I t i s considered t h a t the r e s u l t s i l l u s t r a t e d i n F i g . 32-' can be explained by p o s t u l a t i n g competition between Reactions [l8] and [27]. As the c o n c e n t r a t i o n of c u p r i c i o n i n s o l u t i o n i s increased, Reaction [27] becomes i n c r e a s i n g l y predominant. The rate can speed-up due t o d e p l e t i o n of the " i n s o l u b l e " f e r r i c d i s u l p h i t e complex, u n t i l the r a t e of d e s o r p t i o n of the f e r r i c s u l p h i t e complex becomes rat e c o n t r o l l i n g (Reaction [12]). An a c t i v a t i o n energy measurement was c a r r i e d out under c o n d i t i o n s where f u l l y heterogeneous c o n t r o l was observed i n the presence of c u p r i c i o n ( F i g . 15)- The determined value of 23.1 KCals/mole i s thought t o be i n good agreement w i t h the reported value of 20- 2 KCal/mole f o r the d i r e c t d i s s o l u t i o n of hematite i n a c i d media. Under these c o n d i t i o n s , i t has a l r e a d y been p o s t u l a t e d t h a t the r a t e c o n t r o l l i n g step i s the d e s o r p t i o n of c a t i o n i c species from the oxide surface. The upward t r e n d i n the r a t e w i t h i n c r e a s i n g c u p r i c i o n , a f t e r heterogeneous c o n t r o l has been achieved i s not understood (See F i g . 12). I t i s suggested t h a t cuprous i o n may be able t o react at the surface of the oxide and reduce f e r r i c i r o n t o the ferrous s t a t e . At s u f f i c i e n t l y h i g h copper c o n c e n t r a t i o n s , there may, be enough cuprous io n formed,to cause ap p r e c i a b l e d i s s o l u t i o n v i a t h i s mechanism. Some - 80 -support f o r t h i s argument i s provided by the experiment performed i n which the reducing c o n d i t i o n s were created by a hydrogen atmosphere in s t e a d of sulphur d i o x i d e (See F i g . 16). Under these c o n d i t i o n s , the observed d i s s o l u t i o n must in v o l v e r e a c t i o n s w i t h copper, presumably as cuprous i o n at the oxide surface. Further study on t h i s system i s r e q u i r e d . The e f f e c t of i n c r e a s i n g the c o n c e n t r a t i o n of d i s s o l v e d sulphur d i o x i d e under c o n d i t i o n s of constant a c i d i t y and constant c u p r i c i o n concentration ( S e r i e s B2) i s shown i n F i g . 13. The decrease i n the r a t e at high concentrations of d i s s o l v e d sulphur d i o x i d e i s considered t o be due t o the c o n d i t i o n s becoming s u f f i c i e n t l y reducing t o p r e c i p i t a t e copper metal from s o l u t i o n . In t h i s way, the c a t a l y t i c e f f e c t of c u p r i c i o n would be reduced due t o a decrease i n i t s c o n c e n t r a t i o n . Q u a l i t a t i v e support f o r t h i s argument i s shown i n F i g . Ik. The c o n c e n t r a t i o n of copper i n s o l u t i o n was measured at v a r i o u s con-c e n t r a t i o n s of d i s s o l v e d sulphur d i o x i d e . A comparison of F i g s . 13 and Ik shows th a t the c o n c e n t r a t i o n of copper i n s o l u t i o n does i n f a c t begin t o decrease at approximately the same value of d i s s o l v e d sulphur d i o x i d e t h a t the observed decrease i n the rate of d i s s o l u t i o n of i r o n .occurs. - 81 -CONCLUSIONS 1) I t i s suggested t h a t the same mechanism i s operative during the d i r e c t d i s s o l u t i o n of both hydrated and anhydrous f e r r i c oxide i n a c i d media of pH l e s s than one. This mechanism i n v o l v e s an i o n exchange of surface hydroxyl groups f o r a c i d anions. The r a t e determining step i s considered t o be the desorption of a f e r f i c - a n i o n complex species from the oxide surface. 2) The d i s s o l u t i o n of hydrated f e r r i c oxide i n a c i d i f i e d aqueous s o l u t i o n s of sulphur d i o x i d e i s thought t o i n v o l v e r e a c t i o n of n e u t r a l sulphurous a c i d molecules at the oxide surface. I t i s suggested t h a t t h i s r e a c t i o n may proceed i n two d i f f e r e n t ways, depending on the c o n c e n t r a t i o n of d i s s o l v e d sulphur d i o x i d e , t o produce e i t h e r a c a t i o n i c or a n e u t r a l f e r r i c - s u l p h i t e complex on the oxide surface. 3) Depending on the c o n d i t i o n s , i t was observed t h a t the rate of d i s s o l u t i o n i n aqueous sulphur d i o x i d e s o l u t i o n s could be e i t h e r heterogeneously or homogeneously c o n t r o l l e d . I t was found t h a t , under c o n d i t i o n s of homogeneous c o n t r o l , a d d i t i o n s of c u p r i c i o n t o the system could c a t a l y s e the r a t e u n t i l f u l l y heterogeneous c o n t r o l was achieved. A mechanism has been p o s t u l a t e d t o e x p l a i n these r e s u l t s . k) I t i s thought that-under c o n d i t i o n s of heterogeneous c o n t r o l , the rate-determining step i s the desprption of the f e r r i c - s u l p h i t e complex from the oxide s u r f a c e , and t h a t r e d u c t i o n t o f e r r o u s i r o n takes place i n the s o l u t i o n . - 82 -A p p l i c a t i o n of R e s u l t s I t has. been shown th a t q u i t e h i g h r a t e s of d i s s o l u t i o n of hydrated, f e r r i c oxide i n aqueous s o l u t i o n s of sulphur d i o x i d e can be achieved at modest temperatures and pressures. For example, at 120°C the r a t e of d i s s o l u t i o n i n aqueous sulphur d i o x i d e s o l u t i o n ( [ S 0 2 ] a a ^ = 0.139M) i n the presence of p e r c h l o r i c a c i d and cu p r i c i o n ([HC10 4] = 0.07M, r + + - , [Cu ] = 0.003M) i s approximately 25 times f a s t e r , t h a n i n s u l p h u r i c a c i d s o l u t i o n of s i m i l a r molar c o n c e n t r a t i o n . I t i s thought t h a t i r o n - o x i d e i m p u r i t i e s i n c l a y minerals, may be l e a c h e d . r a p i d l y - w i t h such s o l u t i o n s under m i l d pressure l e a c h i n g c o n d i t i o n s , e s p e c i a l l y . a s the surface area of a suspension of c l a y p a r t i c l e s would be extremely l a r g e . Tests should be made on commercial c l a y s t o determine whether t h i s process can o f f e r an a l t e r n i t i v e t o the t r a d i t i o n a l d i t h i o n i t e l e a c h i n g processes. The a p p l i c a t i o n of the r e s u l t s of t h i s study t o the problem.of the s o l u t i o n mining of deposits of i r o n oxide minerals- i s l e s s c l e a r l y d e f i n e d . Aqueous sulphur d i o x i d e s o l u t i o n s should.be cheap t o produce, and t h i s i s a prime r e q u i s i t e f o r i n s i t u • l e a c h i n g processes, since reagent l o s s i s l i k e l y t o be con s i d e r a b l e . Provided t h a t the ore-body could be s u f f i c i e n t l y s h a t t e r e d , so t h a t the surface area presented t o the l e a c h solution-was l a r g e , and th a t the temperature w i t h i n the ore-body could be maintained - at above 100°C., then aqueous sulphur d i o x i d e should be a s u i t a b l e l e a c h i n g agent. I r o n oxide minerals c o n t a i n i n g copper i m p u r i t i e s should be eminently s u i t a b l e s i n c e these i m p u r i t i e s would c a t a l y s e the ra t e of d i s s o l u t i o n . - 83 -Suggestions For Future Work This study has opened up s e v e r a l areas of work th a t could be u s e f u l l y persued: 1) A comparison of the behavior of g o e t h i t e and hematite under d i r e c t and r e d u c t i v e d i s s o l u t i o n c o n d i t i o n s . This should show whether i n f a c t the oxide surfaces do take up s i m i l a r c o n f i g u r a t i o n s i n aqueous s o l u t i o n and react s i m i l a r l y . 2) The present study should be extended t o i n c l u d e higher concentrations of sulphur d i o x i d e i n s o l u t i o n . A l s o work at higher tem-peratures should prove i n t e r e s t i n g since g o e t h i t e i s reported,to become unstable i n aqueous s o l u t i o n i n the temperature range 120-150°C. Quite marked increases i n the r a t e of d i s s o l u t i o n may occur under c o n d i t i o n s where the g o e t h i t e c r y s t a l s t r u c t u r e i s unstable. 3) A k i n e t i c study of the homogeneous r e a c t i o n s of f e r r i c i o n w i t h aqueous sulphur d i o x i d e at e l e v a t e d temperatures should be c a r r i e d out. I t should i n c l u d e a,study of f e r r i c - s u l p h u r IV complexes, to determine which complexes, i f any, are formed and under what c o n d i t i o n s they occur. The r e a c t i o n s o c c u r r i n g i n the heterogeneous systems cannot be f u l l y understood u n t i l t h i s type of i n f o r m a t i o n i s a v a i l a b l e . k) The f e a s i b i l i t y of the anion exchange mechanism p o s t u l a t e d to occur during the d i r e c t d i s s o l u t i o n of i r o n oxide i n a c i d s could be i n v e s t i g a t e d by measuring the r a t e of d i s s o l u t i o n i n a c i d s o l u t i o n s -of s u f f i c i e n t l y low pH f o r anion exchange t o occur, and then observing 1the e f f e c t of i n c r e a s i n g the anion c o n c e n t r a t i o n . - 84 -5) A study of the r e a c t i o n s of c u p r i c i o n w i t h i r o n oxide under reducing c o n d i t i o n s created by a hydrogen atmosphere may lea d t o a commercially f e a s i b l e method of l e a c h i n g i r o n oxides. - 8 5 -REFERENCES 1. Warren I.H., "Unit Processes in Hydrometallurgy", Vol. I, p. 300, Gordon and Breach (1964). 2. Warren I.H., Peters E. , Byer ley J . , "Solution Mining", Reports submitted to Imperial O i l (Jan-April 1965). 3 . Heeren, Pogg. Annalen, £, 55 (1826). 4 . Berthier, Ann. Chim., Phys. X, 78 (184-3). 5 . Gelis, B u l l . - S o c , Chim., 5_, 333 ( I 8 6 3 ) . 6. Carpenter H.C.H., J . Chem. S o c , 8 l , 1 (1902) . 7. Franck J . , and Haber F., Ber. Berl. Akad., 250 (1931) . 8 . Albu H.W., and Schweinitz H.D. von, Ber. 6j5_, 729 (1932). 9 . Bassett H. and Henry A . J . , J . Chem. S o c , 9 l 4 (1935). 10. Bassett H. and Parker'W.G., J . Chem. S o c , 1540 (1951). 11. Higgicjmson <W.C .E. and Marshall J.W., J . Chem. S o c , 447 ( I 9 5 7 ) . 12. Danilczuk E. and Swinarski A., Roczniki Chem., 35_, 1563 ( I 9 6 I ) . 13. Karraker D.G., J . Phys. Chem., 67, 87I ( I 9 6 3 ) . 14. Herring A.P. and Ravitz S.F., Trans. A.I.M.E., 232, 191 ( I 9 6 5 ) . 15. G r i f f i t h R.H. and Morcom A .R., J . Chem. S o c , 786 (1945). 16. Mellor J.W.,'Comp. Treat, on Inorg. and Theoret. Chem.", Vol XIII, p. 8 5 9 f f . , Longmans (1934). 17. Mayne J.E.O., J . Chem. Soc, 129 (1953) . 18. Kulp J ;L . and Trites A.F., Am. Mineralogist, 3 6 , 23 (1951). 19. Weiser H.B., "Inorg. Col lo id Chem.", Vol. II, p. 3 9 , Wiley (1935). 2 0 . Kataoka I., Mem. Fac. Agriculture, No. 5 , Kochi Univ. ( I 9 5 9 ) . 21 . Glemser 0 . , Ber.-Dtsch. Chem., JO, 2117 (1937). It 22'. Glemser 0 . and Hartert E., Z... fur Anorg. Chemie, 283, 111 (1956). 23 . Gheith M.A.,. Am J . Sc i . , 250, 677 (1952). - 86 -24. Baudisch 0 . and A lbrecht W.H . , J . Am. Chem. S o c , 5_4, 943 (1932). 2 5 . Van Schuylenborgh J . and Arens P . L . , R e c u e i l Trav. Chem., 6 9 , 1557 (1950) . 26. Welo L .A . and Baudisch 0 . , P h i l Mag. [ 7 ] , 17, 753 (1934). 27 . V e i l S . , Acad. des Sc . Compt. Rendus, 186, 753 (1928). 28. Bass A .M. and Benedict W . S . , Astrophys . J., 116, 652 (1947). 2 9 . Cabannes-Ott . C , Acad. des . S c . Compt. Rendus, 224, 2491 (1957)-3 0 . Cabannes-Ott C , Annales de Chemie, [13] 5_, 95^ ( i 9 6 0 ) . 3 1 . Duval C. and Lecomte'; J . , B u l l Soc . Chim. 8 , 713 (194l) . 3 2 . Duval C.. .and Lecomte J . , J . Chim Phys, 5_0, C64 (1953). It 3 3 . Har ter t E . and Glemser 0 . , Z . fu r E lekt rochemie , 6 0 , 746 ( I 9 5 6 ) . 34. Glemser 0 . and Har ter t E . , Naturwissenschaften, 4 0 , 552 (1953). 3 5 . F reder ickson L . , A n a l . Chem., 26 , I883 (1954). 3 6 . Pesnjak E. and Merwin H . E . , Am. J . S c i . , 4j_, 311 (1919). 37 . Mackenzie R . C . , "The D.T. Inves t igat ion of C l a y s " , p. 299, Mineral . Soc . London (1957)• 3 8 . Kulp J . L . and T r i t e s A . F . , Am. M i n e r a l o g i s t , 3 6 , 23 ( I95I) . 3 9 . K e l l y W . C . , Am. M i n e r a l o g i s t , 4 l , 353 (1956). 4 0 . Belcher R. , Gibbons D . , and.West T . S . , Chem, and Indus t . , 73 , 127 and 850 (1954). 4 l . . H u n t W . G . , Am. Waterworks Assn . J . , 4_5_, 535 (1953). 42. Beazley W.B. et a l , Dominion For. . S e r . B u l l . 93 ,Canada Dept. of Mines and Resources (1938). 4 3 . Schmalz R . F . , J . Geophys. R e s . , 64 No. 5 , 575 (1959)-44. Smith F .G . and Kidd D . J . , Am. M i n e r a l o g i s t , 3_4, 403 (1949)-4 5 . Gruner J . W . , Econ. G e o l . , 2 6 , 442 (1931)-46. Azuma K. and Kametani H . , Trans A . I . M . E . , 230, 853 (1964) . 47 . Gayer K .H . and Wootner L . , J . Phys. Chem., 6 0 , I569 (1956). - 87 -L8. Schofield R.K., J. S o i l S c i . , 1, 1 (19^9). L9. Ostwald W.,. J . prakt Chem., (2), 5_2, 3 l k (I885). 50. BarthK.,. Z. Phys. Chem., 9, 176 (I892). 51. Falk M. and Giguere D-.A., Can J. Chem., 36, 1121 (1958). 52. Jones L.H. and McLaren E., J . Chem, Phys., 28, 995 (I958). 53. Cotton F.A. and Wilkinson G., "Advanced Inorganic Chemistry", p. L23, • Interscience (I962). 5L. Campbell W.B. and Maass 0., Can. J . Res. 2, k2 (1930). 55. Campbell W.B. and Maass 0.,. Pulp and Paper Mag., 599 (193°). - 88 -APPENDIX A  Tables of Experimental Resul ts Table I: V a r i a t i o n of rate wi th concentrat ion of su lphur ic a c i d  ( F igs . 2 and 3) Temp = 130°C lgm,Goethite samples [ H 2 S 0 4 ] (MxlO) a R + (MxlO) d[~Fe]/dt (Mxl05/min; 1.46 1.46 0.25, 0.21 2.87 2.87 1.49, 1.34 4 .08 4 .08 2.32 5.47 5.47 3.18,.3.O6 8.62 8.62 4.28, 4 .10 Table I I : E f f e c t of temperature on rate of d i s s o l u t i o n i n su lphur ic  a c i d (F igs . 4 and 5")" [H 2 S0 4 ] = 0.146M lgm Goethite samples Temp 103 d[Fe]/dt l o g d[Fe] °C . T°K (MxlOS/min) ( dt > 119 2.55 128 2.49 140 2.42 147 2.38 1.97 -4.706 2.46 -4.610 4.50 -4.347 6.84 -4.165 [HC10 4] Weight of sample S 0 2 p.p. (psig) [S0 2] (MxlO' 0.07 M 2 gm O.Ik M 1 gm 2 gm d ( F e ) (MxlO 5/min) at K 0.28 M 1 gm 10 15 18 20 25 30 0.83 1.39 2.04 2.30 2.63 3.22 3.84 0.380 0.385 0.390 0.710 0.380 1.800 0.655 0.610 1.340 2.080 2.570 0.327 0.329 0.474 0.357 0.497 0.510 0.681 1.050 1.720 0.324 0.474 0.560 0.743 0.624 1.082 0.750 0.280 0.237 0.332 0..407 0.364 O.597 0.465 0.485 0.692 1.178 0.624 I .030 Table I I I E f f e c t of varying dissolved sulphur dioxide concentration at d i f f e r e n t a c i d i t i e s Temperature 110°C (Figs. 9,10,11,17) - 90 -Table IV: Effect of a c i d i t y at constant dissolved sulphur dioxide  c oncent rat i on Temp = 1 1 0°C [ S 0 2 1 „ = O . I 3 9 M lgm Goethite . aq. samples [HCIO4] (MxlO) d[Fe]/dt (MxlO6/min) 0 . 2 8 4 . 0 0 0 . 7 0 3.84 O . 9 8 4 . 5 0 1 . 4 0 4 . 0 0 2 . 1 1 3 . 9 5 2 . 8 0 3 . 3 4 -Av. 3 . 9 4 Table V: Sulphate determinations Temp. = 1 1 0°C lgm Goethite samples Determinations made af t e r 3 hour run [HCIO4] [ S 0 2 ] a q ' [ S 0 4 ] (MxlO) (MxlO) [FeT 0.70 1.39 1-53 0.70 1.34 1.86 i.4o 1.39 1.66 2.11 1.39 1.91 2.86 O.83 1.76 1.40 2.63 I . 9 2 Av. 1.77 - 91 -Table VI : • E f f e c t of vary ing cupr ic ion concentrat ion at constant a c i d i t y and constant sulphur d iox ide concentrat ion (F ig 12) Temp = 110°C [HC10 4] = 0.Q7M [ S 0 2 L „ = 0.139M [ C u(C10 4) 2] d[Fe]/dt (MxlO 4) (MxlO5/min) lgm sample 2gm sample 0 0.385 0.380 3.8 1.155 1.140 7-7 1-368 15.3 1.820 3.620 30.6 2.250 4.550 61.0 .4.600 9.270 5.320 Table VI I : E f f e c t - o f vary ing the concentrat ion of d i s s o l v e d sulphur  d iox ide at constant a c i d i t y and constant cupr i c ion  concentrat ion ( F i g . 15) Temp = 110°C [HC10 4].= O.O7M - [ C u(C10 4) 2] = 3.06xlO _ 3 M lgm Goethite samples S 0 2 p .p . [S0 2 ] a q d[Fe]/dt (psig) (MxlO) (Mxl0 5 /min) 10 1-39 2.250 15 2.04 2.400 20 2.63 3.820 25 3.22 3.550 30 3.84 2.320 - 92 -Table VIII: Effect of dissolved sulphur dioxide concentration on  copper in s o l u t i o n ( F i g . 14) Temp = 110°C [HC104] = 0.07M lgm Goethite sample S0 2 p.p. '[S0 2] aq [Cu] in soln. (psig) (MxlO) (MxlO - 3) 0 0 3.06 10 1.39 2.51 25 3.22 2.51 30 3.84 2.31 Table IX: Effect of temperature on cupric catalysed reaction  (Fig. 15) [HC104] = 0.07M [S0 2 ] a q = 0.139M [Cu(C10 4) 2]. = 3.06xlO _ 3M lgm Goethite samples Temp 103_ d[Fe]/dt log ,d[Fe] °C T°K (MxlO5/min) 1 dt 95 2.72 0.617 -5.210 100 2.68 1.275 -4.894 110 2.61 2.250 -4.648 120 2.545 4.420 -4.354 - 93 -Table X: T y p i c a l heterogeneous rates i n a c i d i f i e d aqueous sulphur dioxide so lu t ions The fo l low ing rates were measured under condi t ions of f u l l y heterogeneous c o n t r o l . They have been computed.on the bas i s of a s p e c i f i c surface area f o r the minera l of °.0 cm2/gm. This value was c a l c u l a t e d knowing the p a r t i c l e s i ze range (208-104/4) and the densi ty of the minera l (approx. 4.2 g_m/cc). The p o s s i b l e e r ro r i n t h i s c a l c u l a t i o n i s thought to be - 30$. Temp = 110°C [S0 2 ] [HC10 4] [Cu(C10 4 ) 2 ] d[Fe]/dt (M)' q (M) (MxlQ3) (Mxl07/cm2/min) 0.204 0.07 0.79 0.322 0.07 1.49 0.204 0.14 0.4l 0.139 0.28 0.37 0.139 0.07 1-53 2.02 0.139 0.07 3.06 2.50 0.139 0.07 6.10 5.45 94 -APPENDIX .B  The Sulphur Moxide-Water System Solutions of sulphur dioxide in water have tradit ional ly been referred to as solutions of sulphurous acid, H 2 S O 3 . The equi l ibr ia that were thought to be involved in such solutions were: H20 + S0 2 ^ H 2S0 3 [1] H 2S0 3 ^ H + + HSO3" [2] HSO3 ^ H + S0 3 [3] The second ionisation, reaction [3], was usually neglected •^9 50 since Ostwald and Bartlr showed that solutions of sulphurous acid act as i f monobasic even to high di lutions. "51 "52 Modern investigators^ have attempted to identify H 2S0 3 in solution by infra-red absorption techniques but without success. It has been concluded that, in aqueous solutions of sulphur dioxide, undissociated sulphurous acid either does not exist or is present in only inf initesimal quantities. Undissociated dissolved sulphur dioxide is thought to exist in solution as a clathrate of the gas hydrate type^ 5. The equi l ibr ia existing in aqueous solutions of sulphur dioxide .are thought to be best represented as: S0 2 + xH20 ^ - S02-'XH20 [4] S0 2-xH 20 _ HS03" + H 3 0 + + (x-2)H20 [5] The apparent dissociation constant for "sulphurous acid" is given.by: - 95 -K a = [H+j[HS03-] [ s o 2 J Q - [HSO3_] [6] Campbell and Maass-^ calculated values of Kg. from conductivity data obtained at various concentrations of dissolved sulphur dioxide and temperatures up to 90°C. They showed that K a decreased with temperture, but was v i r t u a l l y independent of the concentration of dissolved sulphur dioxide up, to Qf>S02. These data are presented i n Table BI. -The s o l u b i l i t y of sulphur dioxide i n water at temperatures up to 120°C has been determined by Campbell and Maass . -The s o l u b i l i t y data f o r the range of temperatures used i n t h i s study are presented i n Table B I I . A review of the l i t e r a t u r e including a compilation of reported data on the sulphur dioxide water system may be found i n a p u b l i c a t i o n by Beazley et a l Knowing the t o t a l pressure i n the system, the concentration of dissolved sulphur dioxide i n solutions used i n t h i s study was determined d i r e c t l y , from graphs p l o t t e d from the data i n Table B I I . The effects of v a r i a t i o n s i n i o n i c strength on the s o l u b i l i t y of sulphur dioxide were neglected. The values of K a at temperatures greater than 90°C were found by p l o t t i n g log Ka versus ^. This gave a s t r a i g h t - l i n e r e l a t i o n s h i p which could be extrapolated to higher temperatures. Knowing Ka, the concentration of b i s u l p h i t e (HS03~) could be cal c u l a t e d from equation [ 6 ] , assuming that [H+] was equal t o the concentration of p e r c h l o r i c a c i d i n s o l u t i o n . - 96 -Table BI: Variation of Kg v i th temperature Temp °C K a x l 0 3 5 27-9 15 22.1 25 17.5 50 15.1 50 8.6 70 4.6 90 2.5 Table BII: So lubi l i ty of sulphur dioxide in water at various  temperatures and various tota l pressures^. 90°c 100°C 110°C 120°C cm.Hg $S02 aq .cm.Hg $S02 aq cm.Hg $S02 aq cm.Hg $S02 aq 97-1 1.05 127.5 1.03 166.3 1.01 213.0 1.00 144.5 2.02 182.5 1.97 230.0 1.95 284.5 I.89 186.6 3.00 230.8 2.95 283.0 2.87 344.0 2.82 251.8 3.81 280.1 5.72 358.0 5-65 529.0 6.03 

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