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Some aspects of the acid dissolution of hematite Bath, Murray Damon 1968

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SOME. ASPECTS OF THE ACID DISSOLUTION OF HEMATITE by MURRAY DAMON BATH B.Sc.(Eng.), University of the Witwatersrand, I963. 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 thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1968 In presenting th is thesis in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the- L ibrary shal l make i t f ree ly avai lab le for reference and study. I fur ther agree that permission for extensive copying of th is thesis for scholar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or publ icat ion of th is thesis for f inanc ia l gain shal l not be allowed without my wri t ten permission. Department of Metallurgy The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada Date May 1, 1968 i i ABSTRACT The dissolution of hematite ( o< -Fe 20 3) i n hydrochloric acid solutions has been investigated. Sintered compacts, single crystals and particulate specimens were subjected to leaching over a range of temperatures and acid concentrations. The effects of varying the hydrogen and chloride ion concentrations independently were also investigated. The dissolution was found to be highly anisotropic, the basal (0001) plane dissolving at a rate an order of magnitude greater than that of any of the other surfaces examined. Qualitative experiments indicated that t h i s effect i s also characteristic of dissolution i n n i t r i c , sulphuric and perchloric acids. The effect i s attributed to the presence of a greater number of active dissolution sites on the basal plane, resulting from the anisotropic nature of the hematite c r y s t a l structure, and possibly also from the presence of a greater number of dislocations terminating on the basal plane. The reaction rate was found to depend strongly on the acid concentration, increasing as the 2.5th power of the acid normality i n the range 0.2-7.ON, a n ( i to vary independently with hydrogen and chloride ion concentrations. At a c i d i t i e s below 2N, the rate appears to vary with the product of hydrogen and chloride ion a c t i v i t i e s , while at concentrations i n the range 2-7N, a l i n e a r dependence on hydrogen ion a c t i v i t y was observed. A tentative mechanism i s proposed, i n which protonation of the hydrated oxide surface i s followed by adsorption of a chloride ion and desorption of a fe r r i c - c h l o r i d e complex. i i i ACKNOWLEDGEMENTS The author wishes to express his sincere thanks to Dr. l.H. Warren for his continued interest, guidance and encouragement throughout the period of study. Thanks are also extended to members of faculty and fellow graduate students for helpful discussions, and to the technical staff for their assistance with practical aspects of the work. Financial support from the Transvaal and Orange Free State Chamber of Mines, in the form of a Research Scholarship, and from the National Research Council of Canada, under Grant No. A -2U55, is gratefully acknowledged. i v TABLE OF CONTENTS Page INTRODUCTION 1 I. General 1 II. Review of literature . . . . . . . . . . . . 3 (a) Dissolution of ferric oxide . . . . . . 3 (b) Dissolution of other corundum-type oxides . . . . . . . . . . . . . . . . . 13 (c) Dissolution of other oxides Ik III. Scope of the present investigation . . . . . 16 EXPERIMENTAL 18 I. Specimen materials and reagents 18 Ca) Synthetic polycrystalline and powder specimens .18 (b) Natural single crystal specimens . . . . 18 Cc) . Reagents 20 II. Specimen preparation 20 (a) Sintered polycrystalline compacts . . . 20 (t>) Synthetic powder specimens 22 Cc) Natural crystal specimens 25 III. Apparatus design 25 IV. Experimental procedure 27 (a) Orientation of single crystals 27 Cb) Dissolution tests 29 V. Analytical method ..... . . . . 31 RESULTS 32 I. Tests on sintered polycrystalline compacts . 33 • II. Tests on synthetic powder specimens . . . . . 35 (a) Effect of temperature 35 (b) Effect of acid concentration 39 Cc) Effect of varying hydrogen ion con-centration at constant chloride ion con-centration hk (d) Effect of varying chloride ion con-centration at constant hydrogen ion con-centration h6 V TABLE OF CONTENTS (CONT'D) Page I I I . Tests on single c r y s t a l specimens 48 (a) Effect of c r y s t a l orientation 48 (b) Effect of temperature 55 (c) Effect of acid concentration . . . . . . 56 I'd) Dissolution of single c r y s t a l sphere . . 56 DISCUSSION 60 (a) Solution anisotropy . 60 (b) Reaction mechanism 64 CONCLUSIONS . . . . . 70 Suggestions for further work 71 REFERENCES 72 APPENDIX A: Tables of experimental results 76 APPENDIX B: X-ray d i f f r a c t i o n data 85 APPENDIX C: Estimation of hydrogen ion a c t i v i t e s i n HCl solutions 84 •vi LIST OF TABLES No. Page 1 Chemical analysis of reagent grade cX-Fe 2 0 3 • 18 2 Chemical analysis of natural hematite crystals 19 3 Lattice constants of ©<-Fe 20 3 and o<-A1203 . • 29 APPENDIX A A l Effect of temperature on rate of dissolution of o< -Fe 2 0 3 powder i n HCl solutions 76 A2 Effect of HCl concentration on rate of d i s -solution of c X -Fe P 0 3 powder at 85°C 77 A3 Effect of varying hydrogen ion concentration at constant chloride ion concentration on rate of dissol u t i o n of oC-Fe 2 0 3 powder at 85°C . 78 Ah Effect of varying chloride ioncconcentration at constant hydrogen ion concentration on rate of dissolution of c< -Fe 2 0 3 powder at 85°C 79 A5 Effect of c r y s t a l orientation on rate of d i s -solution of natural hematite i n 5.ON HCl at 85°C 8 0 A6 Effect of temperature on rate of dissolution of ( 0 0 0 1 ) , (.1010).-..and ( 2 2 5 3 ) Surfaces'of natural hematite i n 5.ON HCl 8 l A7 Effect of HCl concentration on rate of dissolution of the basal ( 0 0 0 1 ) surface of natural hematite at 85°C 82 APPENDIX B B l X-ray d i f f r a c t i o n results for various cX-Fe 2 0 3 specimens 83 APPENDIX C CI Hydrogen ion a c t i v i t e s i n HCl solutions at 85°C 86 • ..vii LIST OF FIGURES No. Page 1 Variation of open porosity with bulk density of sintered c<-Fe 203 compacts 23 2 Optical micrograph of sintered c < - F e 2 0 3 compact . . 2k 3 Schematic diagram of dissolution apparatus . . . . 26 k Sintered p o l y c r y s t a l l i n e compacts of o< -Fe 203 a f t e r -leaching under various conditions . . . . . . . . . 5 Typical rate curves. Effect of temperature on rate of dissolution of c X-Fe 2 0 3 powder i n 5.ON HCI . . . 36 6 Arrhenius plot for dissolution of o< -Fe 203 powder . i n 5.0N HCI 37 7 Arrhenius plot for dissolution of ©<-Fe 203 powder i n 0.5N HCI .38 8 Effect of HCI concentration on rate of dissolution of c<-Fe 2 03 powder at 85°C - t y p i c a l rate curves . kO 9 Effect of HCI concentration on rate of dissolution of cX-Fe 2 0 3 powder at 85°C kl 10(a) .Effect of hydrogen ion a c t i v i t y on rate of dissolution of c< -Fe 20 3 powder at 85°C k2 10(b) Effect of hydrogen ion a c t i v i t y on rate of d i s -solution of ©<-Fe s0 3 powder i n d i l u t e HCI solutions. ( I n i t i a l part of Figure 10(a) plotted on larger scale) 43 10(c) Dissolution rate of ©<-Fe 2 0 3 powder i n di l u t e HCI solutions as a function of the square of hydrogen ion a c t i v i t y kj> 11. Effect of hydrogen ion concentration on rate of dissolution of ex. -Fe a0 3 powder at 85°C, and various constant chloride ion concentrations. . . . kj 12 Effect of chloride ion concentration on rate of dissolution o f o ^ - F e 2 0 3 powder at 85°C and various constant hydrogen ion concentrations . . . . . . . . k'J 13 Sim p l i f i e d (0001) stereographicprojection of o<-Fe s03 with orientations of ea s i l y etched surfaces superimposed . . . . . . . . . . . . . . . 50 y i i i LIST OF FIGURES (CONT'D) No. Page lk Specimen M8B a f t e r etching i n 5.ON HCl . . . . . . 51 15 Specimen M6 after etching in 5.ON HCl . . . . . . . 51 16 Typical rate curves for dissolution of natural single c r y s t a l hematite - effect of orientation on dissolution i n 5.ON HCl at 85°C 53 17 Basal surfaces of natural hematite c r y s t a l after leaching i n 5.ON HCl at 85°C <jk 18 Arrhenius plots for dissolution of (0001), (1810), (22^3) surfaces of natural hematite in 5.ON HCl . . 57 19 Effect of HCl concentration on rate of dissolution of (0001) surface of natural hematite at 85°C . . . 58 20 Single c r y s t a l sphere of natural hematite at various stages during dissolution i n 6.ON HCl at 95°C . . . 59 CI Mean a c t i v i t y c o e f f i c i e n t of hydrochloric acid i n aqueous solutions at 85°C, determined by ex-trapolation of l i t e r a t u r e data in-the range 0-60°C . 85 1 INTRODUCTION I. GENERAL The dissolution of oxides i s of interest i n several f i e l d s , including hydrometallurgy, structural ceramics and nuclear fuels technology. . In.hydrometallurgy, leaching of low-grade oxide ores has long been standard practice. Many studies of the leaching of oxide minerals have been of a somewhat empirical nature, although extensive k i n e t i c studies 1 2 3 have been carried out on the acid dissolution of U0 2 , Cu 20 and Mn02 , among others. The dissolution of oxides i s also involved i n the preparation of certain s i l i c a t e raw materials, for example glass sands and various clays, 4 where the treatment i s usually aimed at the removal of iron oxides. In some hydrometallurgical processes, the oxides i n the ore constitute undesirable impurities rather than valuable ore minerals. In these cases i t may be desirable to design process conditions so as to minimize t h e i r d i s s o l u t i o n , i n order to simplify or eliminate subsequent p u r i f i c a t i o n steps i n the process. The u t i l i z a t i o n of ceramic structural members i n corrosive environments depends for i t s success on a knowledge of the dissolution • 5 . characteristics of the materials. A recent paper by Dawihl and K l i n g l e r describes a study of the corrosion resistance of A1 20 3 bodies to concentrated acids. In nuclear fuels technology, the necessity for reprocessing spent fuel elements for the recovery of valuable constituents has led to the 2 development of leaching processes for oxide fuels. Several studies have been carried out i n t h i s context,.for example those on the dissolution of 6 y U0 2 and Th0 2. BeO may be used as a moderator i n certain homogeneous ceramic fuel elements, so that any study of the reprocessing of such a 8 f u e l must- take into account the dissolution characteristics of BeO. Koch has recently carried out a k i n e t i c study of i t s dissolution i n various acids. Iron, oxides are present i n most ores i n greater or lesser amounts, and i n some secondary products, such as pyrite cinder, as the major con-stituent. Hydrometallurgical treatment of these materials may be complicated by the unwanted dissolution of iron oxides, making subsequent p u r i f i c a t i o n steps necessary. On the other hand, i n the beneficiation of non-metallic minerals such as glass sands and clays, the maximum dissolution of iron oxides i s the desired end. The mechanism by which these oxides dissolve i s of interest from both these aspects. The acid dissolution of f e r r i c oxide has received some atten-9 —14 t i o n , but certain of the observed phenomena do not appear to have been f u l l y explained. For instance, the very rapid dissolution i n acids whose 9 anions form strong complexes with the f e r r i c ion has been noted , .but not explained. The unusual S-shaped rate curves obtained by Azuma and ^ • 12., Kametani' " warrant.further investigation, as does the fact that the activation energies for the dissolution reaction obtained by these workers 13 are about twice the magnitude of those obtained by Furuichi et a l . I t was with the hope of providing at least a p a r t i a l answer to some of these problems that t h i s study was undertaken. 3 I I . -REVIEW OF THE LITERATURE Besides studies on the dissolution of f e r r i c oxide, investigations of several other- oxides are also discussed, since some of the results appear to have a bearing on the present work. (a) Dissolution o f F e r r i s Oxide The most extensive previous investigations of the dissolution of 9;10>11 X2 f e r r i c oxide have been by. Pryor and Evans ( 1 9 ^ 9 ) ; Azuma and Kametani ( 1 9 6 4 ) , and by Furuichi, Sato and Okamoto 1 3.(1965). The work of Simnad ' 14 and Smoluchowski on the effect of proton .irradiation on the dissolution of f e r r i c oxide, and that of Monhemius,15. on the dissolution of goethite (o£-FeOOH), -is also Of interest, Pryor and Evans carried out careful studies of both the reductive and direct dissolution.of f e r r i c oxide, as part of an extensive investiga-t i o n into certain aspects of the pas s i v i t y of metals. They examined the 9 direct-acid dissolution of chemically precipitated c<-Fes03 a f t e r i g n i t i o n at temperatures up.to 1000°C. -.They found that -Fe 2 0 3 dissolves compara-t i v e l y rapidly i n hydrofluoric or concentrated hydrochloric acids, .both of which form soluble complexes of high s t a b i l i t y with the f e r r i c ion. However, the bulk of t h e i r study was concerned with the dissolution.of the surface layers (about 1$ by.weight of the oxide) i n di l u t e solutions of hydrochloric, sulphuric or perchloric acids, which do not easily form these complexes. In these solutions the rate of dissolution was found.to decrease with time, and the. resulting solutions contained ferrous iron. The authors attributed the presence of t h i s ferrous iron to preferential attack at surface defects involving oxygen deficiency.. To maintain e l e c t r i c a l n e u t r a l i t y , part of the iron would have to be i n the ferrous state, and the l a t t i c e s t r a i n k associated with these defects would lead to the p r e f e r e n t i a l removal of ferrous ions from the l a t t i c e . They attributed the f a l l i n rate with time to the depletion of these defects at the surface, and showed that the o r i g i n a l dissolution rate could be restored by heating the p a r t i a l l y leached oxide to 1000°C, allowing defects to diffuse to the surface from the bulk of the oxide. 12 Azuma and Kametani , .in a similar study, also noted a f a l l i n rate with time, but only under certain conditions, namely low acid concentra-tion and a high r a t i o of f e r r i c oxide to solution volume. They attributed the decreasing rate to the fact that under the above-mentioned conditions, the s o l u b i l i t y product of f e r r i c hydroxide i s approached as the concentration of f e r r i c iron i n solution increases. For this reason, the weight percent of dissolved oxide would be expected to vary with the p a r t i c u l a r experimental conditions i . e . s o l i d to solution r a t i o and a c i d i t y , as was i n .fact observed by these authors. Because of t h i s , comparison of these two studies i s d i f f i c u l t , but i t appears that two d i s t i n c t effects can-operate to reduce the rate of dissolution: (a) depletion of e a s i l y soluble defects at the oxide surface, • +++ and (b) the saturation of the solution with Fe- so that the s o l u b i l i t y product of Fe(0H)3 i s reached. Depending on the p a r t i c u l a r experimental conditions, either or both of these effects may be operative. The f a l l i n rate observed by Pryor and Evans commenced almost immediately, being evident after only 0.05$> by weight of the oxide had been dissolved, whereas the decrease observed by.Azuma and Kametani did not 5 begin u n t i l a t l e a s t 1$> d i s s o l u t i o n had occurred. I t i s p o s s i b l e that the d i f f e r e n c e could be explained by assuming a higher concentration, of defects i n the m a t e r i a l used by. Azuma and Kametani, but as n e i t h e r group of i n v e s t i -gators gave analyses f o r F e + + i n t h e i r oxides, t h i s cannot be confirmed. The s i t u a t i o n i s f u r t h e r complicated by the complex r a t e curves obtained by the Japanese workers. I t seems l o g i c a l t o conclude t h a t two d i f f e r e n t e f f e c t s were observed by the two groups of i n v e s t i g a t o r s . •Assuming that the decreasing r a t e observed by'Pryor and Evans was i n f a c t due t o d e p l e t i o n of surface d e f e c t s , then continued d i s s o l u t i o n would presumably have y i e l d e d rate curves of s i m i l a r shape t o those observed by.Azuma.and Kametani. On the other hand, . i t i s probable that these l a t t e r i n v e s t i g a t o r s d i d not note t h i s i n i t i a l decrease i n r a t e because t h e i r experiments were designed t o examine the d i s s o l u t i o n t o completion, and were not s e n s i t i v e enough t o detect changes o c c u r r i n g i n the d i s s o l u t i o n of the f i r s t 0 . - 1 $ of the oxide. The f a l l i n rate observed by them a f t e r the d i s s o l u t i o n of 1% or more, was due, as they suggested,.to the approach of the system t o the s o l u b i l i t y product of f e r r i c hydroxide. The i n i t i a l rate of d i s s o l u t i o n was found by. Pryor and Evans t o be the same i n d i l u t e ( 0 . 0 1 - 2 . O N ) s o l u t i o n s of h y d r o c h l o r i c , s u l p h u r i c and p e r c h l o r i c a c i d s of equivalent concentration. They a l s o observed that the a d d i t i o n of 0 . 0 1 t o 0 . 5 N K C 1 t o h y d r o c h l o r i c a c i d ( 0 . 0 1 t o 0 . 1 N ) d i d not i n f l u e n c e the r a t e . At higher a c i d concentrations, the rate was found t o depend on the p a r t i c u l a r a c i d , decreasing i n the order: BF > HCI > H 2 S 0 4 > H C 1 0 4 i n accord w i t h the order of decrease of the complexing a f f i n i t y of the anion f o r f e r r i c i o n . . Azuma and Kametani, however, found that the r a t e of 6 dissolution depended on the type of acid, even at low concentrations. This difference, once again, can probably be attributed to the fact that Pryor and Evans were examining the removal of a defective surface layer, while the Japanese investigators observed the bulk dissolution. Pryor and Evans concluded that the rate-determining step i n the reaction was the combination of adsorbed hydrogen ions with oxygen ions i n the l a t t i c e : Os" + Hads = ° H" •or: 0"- •+ 2H+ d s = H20 Since the removal of one oxygen ion, as water or hydroxyl ion, does not involve the production or consumption of electrons, the reaction cannot create defects. Thus the rate of dissolution should be controlled by the number of defects o r i g i n a l l y present i n the oxide surface. .The transfer of an oxygen ion into solution leaves the surface p o s i t i v e l y charged. This charge can be dissipated only by the passage of iron ions into solu-t i o n , since the temperature i s too low to permit rapid s e l f - d i f f u s i o n . The' i n i t i a l l y rapid removal of oxide surrounding the surface defects i s followed by a slower attack on the more perfect parts of the l a t t i c e , due to the stronger i n t e r i o n i c forces associated with the defect-free l a t t i c e . I t i s t h i s slower "bulk dissolution" which was probably observed by Azuma and Kametani. Pryor and Evans 1 0'' 1 1 coined the term "reductive dissolution" to describe the process occurring when a f e r r i c oxide surface i s continuously reduced to the ferrous state, so permitting more rapid dissolution. They used a c e l l i n which f e r r i c oxide powder f l o a t i n g on a pool of mercury constituted the cathode and a sheet of pure iron the anode. They found that both the rate and the current eff i c i e n c y decreased with an increase 7 i n the temperature of previous i g n i t i o n , and w i t h an increase in. the pH of the e l e c t r o l y t e . They concluded t h a t at pH values below.2.0, the r a t e of reductive d i s s o l u t i o n i s c o n t r o l l e d by the surface c o n d u c t i v i t y of the oxide, and hence by the number of surface d e f e c t s . The r a t e of reductive d i s s o l u t i o n was found t o be very much higher than that of d i r e c t d i s s o l u t i o n . P r y o r 1 1 proposed t h a t the mechanism of reductive d i s s o l u t i o n i n v o l v e s combination of hydrogen atoms, discharged at the f e r r i c oxide s u r f a c e , w i t h oxygen ions belonging t o the oxide l a t t i c e , t o form e i t h e r hydroxyl ions or water molecules which pass i n t o the bulk of the s o l u t i o n . .This r e a c t i o n l i b e r a t e s e l e c t r o n s which reduce an equivalent number of f e r r i c ions t o the ferrous c o n d i t i o n , thus c r e a t i n g a s e r i e s of metal-excess type d e f e c t s i n the oxide surface, which then becomes p a r t i c u l a r l y suscep-t i b l e t o d i r e c t a c i d a t t a c k . 14 Simnad and Smoluchowski i n v e s t i g a t e d the e f f e c t of proton i r r a d i a t i o n on the d i s s o l u t i o n rate of cX. -Fe 203. i n 1.0N HCI s o l u t i o n s . T h e i r specimens were prepared by o x i d i z i n g pure i r o n sheet, and were presumably f i n e l y _ p o l y c r y s t a l l i n e . The i r r a d i a t i o n .treatment was found t o increase s i g n i f i c a n t l y the rate of d i s s o l u t i o n , presumably due t o an increased c o n c e n t r a t i o n of l a t t i c e d e f e c t s . The i n i t i a l rates of d i s s o l u t i o n of i r r a d i a t e d and u n i r r a d i a t e d specimens were much the same, but a f t e r a •layer approximately 6000 angstroms t h i c k had been removed from the surface, the d i s s o l u t i o n of the i r r a d i a t e d specimens was considerably a c c e l e r a t e d . -It was suggested t h a t the i n i t i a l slow a t t a c k r e s u l t e d from a lower concen-t r a t i o n of defects i n the surface l a y e r due t o t h e i r r a p i d outward d i f f u s i o n . The a p p a r e n t " d i s s o l u t i o n rate of both i r r a d i a t e d and u n i r r a d i a t e d specimens increased r a p i d l y - w i t h time. No explanation was o f f e r e d f o r t h i s observation. 8 Some comment seems necessary on the fact that while Pryor and Evans invoke the "slow" diffusion, of oxygen vacancies at ordinary tempera-tures to explain t h e i r observed f a l l i n rate, - Simnad and Smoluchowski attribute the fact that the surface layers of irradiated and unirradiated o4-Fe2C-3 dissolve at the same rate, to the "rapid" d i f f u s i o n of defects from the surface of the -irradiated specimens. These two views appear contradictory, and i t must be assumed that the nature and number-of the defects i n the l a t t e r investigators'.specimens were very different from those i n the f e r r i c oxide studied by Pryor and Evans. I t i s to be expected that material subjected to proton i r r a d i a t i o n would contain a high concen-t r a t i o n of defects of various types, including vacancies and i n t e r s t i t i a l s , which could contribute to an enhanced dissolution rate. I t i s also reasonable to expect that some of the defects formed i n t h i s way would be unstable and would disappear by recombination. Presumably.this would occur more e a s i l y at the surface, so accounting for the i n i t i a l l y i d e n t i c a l dissolution rates of irradiated and unirradiated specimens. 12 The work of Azuma and Kametani warrants further discussion, because of the complex shape of the rate curves they obtained. They studied the dissolution of f e r r i c oxide powders i n several inorganic acids over a wide range of concentrations and temperatures. On the basis of the shapes of the rate curves, they distinguished two types of dissolution, an "accelerated type", and a "parabolic type". In the "accelerated type", dissolution was found to occur i n three stages: (i ) an i n i t i a l reaction i n which the dissolved amount was nearly proportional to the cube root of time. ( i i ) an accelerated region and ( i i i ) ; a f i n a l stage during which d i s s o l u t i o n approached completion. On a p l o t of l o g weight percent of oxide d i s s o l v e d against l o g time, these ra t e curves appeared S-shaped. In. the " p a r a b o l i c type", . d i s s o l u t i o n was found t o proceed i n f o u r stages: (1) and (2) s i m i l a r t o those of the "accelerated type"; (3) p a r a b o l i c r a t e d i s s o l u t i o n d u r ing which the d i s s o l v e d amount of oxide was n e a r l y p r o p o r t i o n a l t o the square root of time, and (k) a f i n a l stage. •Whether a powder d i s s o l v e d according t o the " p a r a b o l i c type" or t o the " a c c e l e r a t e d type" appeared t o depend on the method of p r e p a r a t i o n , and hence on the c h a r a c t e r i s t i c s of the p a r t i c l e s themselves. Extensive experiments were c a r r i e d out only on powder specimens which d i s s o l v e d according t o the "accelerated type", and the r e s t of t h i s d i s c u s s i o n r e f e r s to t h i s type of d i s s o l u t i o n . Azuma and Kametani confirmed t h a t the d i s s o l u t i o n r a t e s . i n v a r i ous ac i d s decrease i n the same order as.the order of decrease of the complexing a f f i n i t y of the anion f o r the f e r r i c i o n . They a l s o suggested that the a c t u a l mechanism i s independent of the type of a c i d and i t s c o n c e n t r a t i o n , since a c t i v a t i o n energies i n the range 2 0 - 2 kcals/mole were obtained f o r a l l the a c i d s i n v e s t i g a t e d , over a wide range of concentrations. These i n v e s t i g a t o r s a l s o observed that there was a c r i t i c a l c o n c e n t r a t i o n , depending on the type of a c i d , below which the d i s s o l u t i o n d i d not appear t o go t o completion. For concentrations below the c r i t i c a l , the shapes of the d i s s o l u t i o n curves were no longer c o i n c i d e n t , and the 10 amount of dissolved oxide increased with time to approach a f i n i t e value which was less than that for complete dissolution. The f i n i t e f i n a l concentration of F e + + + was found to "be a function of acid concentration, suggesting an equilibrium between f e r r i c oxide and the acid. I t was con-cluded that the s o l u b i l i t y product of f e r r i c hydroxide was the co n t r o l l i n g factor below the c r i t i c a l concentration. This suggested that the weight percent of oxide dissolved would vary with the r a t i o of the amount of oxide to volume of solution. This was confirmed by experiment. Thus the c r i t i c a l value should not be a constant for the acid, but dependent on the conditions of the experiment. Taking into account the l i m i t of concentra-t i o n of Fe + + +,.the dissolution was found to proceed along the accelerated course even when the a c i d i t y was as low as 0.05W. .The shape of the curves obtained for dissolution i n phosphoric acid appeared to be s i g n i f i c a n t l y d i f f e r e n t . The i n i t i a l period of slow dissolution was not observed, and after about 100 hours the concentration +++ of Fe i n solution decreased abruptly due to the p r e c i p i t a t i o n of an insoluble f e r r i c phosphate. A s i g n i f i c a n t feature of t h i s study i s the s i m i l a r i t y i n shape of the curves obtained for dissolution i n a l l acids (except phosphoric), despite wide differences i n absolute rate due to type of acid, concentration and temperature. This holds true f o r powder prepared i n a spe c i f i c way -differences i n preparation appear, to change the shape of the curves somewhat. I t i s also s i g n i f i c a n t that the shape of the curves cannot be explained by •the assumption of uniformly dissolving, .isotropic, spherical p a r t i c l e s . The above observations suggest that the shape of the curves.is not due to any "chemical" effect i n the system but dependent on some physical property of the powder p a r t i c l e s themselves. 11 Monhemius15, i n a study of the dissolution of natural goethite, o<.-Fe00H, suggested that i n acid solutions hydration of the oxide surface i s followed by protonation and then by adsorption of an anion. This i s followed by the rate-controlling Resorption of the hydroxy-ferric complex. He also observed an i n i t i a l accelerating region i n the rate curves for dissolution i n both sulphuric acid and acid sulphur dioxide solutions, similar to that obtained by Azuma and Kametani. I t was suggested that t h i s must be due to a property of the mineral i t s e l f , although no evidence was presented to support t h i s view. Furuichi, Sato and Okamoto13 studied the effects of ageing and roasting of f e r r i c oxide precipitates, on t h e i r subsequent diss o l u t i o n • i n sulphuric acid solutions. They found that precipitates roasted between 350 a n (i 400°C dissolved with average rates (over a period of 20 hours), independent of t h e i r previous ageing history, but dependent on roasting temperature, while those roasted below.350° had average dissolution rates depending on both ageing and roasting treatments, the rate decreasing with .increasing ageing time and roasting temperature. This behaviour was attributed to the change from an amorphous to a c r y s t a l l i n e structure, and the development of a higher degree of c r y s t a l l i n i t y with higher roasting temperatures. •These authors obtained two-stage dissolution curves having an i n i t i a l non-parabolic region followed by a parabolic region. For oxides roasted above J>^>0°C, the non-parabolic region was not observed. The i n i t i a l stage was considered to correspond to the dissolution of amorphous material, while the parabolic region represented the dissolution of w e l l - c r y s t a l l i z e d oxide. The f i r s t stage was found to be f a i r l y well described by a cubic rate equation based on a mechanism involving the adsorption of hydrogen ions 12 on the surface as the r a t e - c o n t r o l l i n g s t e p . : A c t i v a t i o n energies ranged from k.Q t o 12.3 kcals/mole, depending.on ageing and r o a s t i n g h i s t o r y . In the p a r a b o l i c r e g i o n , i . e . the d i s s o l u t i o n of w e l l - c r y s t a l l i z e d m a t e r i a l , the r a t e was found t o be approximately p r o p o r t i o n a l t o a c i d c o ncentration and unaffected by the a d d i t i o n . o f excess f e r r i c ions t o the s o l u t i o n . .The a c t i v a t i o n energies c a l c u l a t e d from the temperature depen-dence of the s o l u t i o n rate were of the order of 10-12 kcals/mole, depending somewhat on ageing and r o a s t i n g treatments. D i s s o l u t i o n i n t h i s r e g ion was explained on the b a s i s of a d i f f u s i o n mechanism, .in which d i f f u s i o n of a proton i n t o the surface of the oxide p a r t i c l e i s the rate c o n t r o l l i n g step. The values obtained f o r the a c t i v a t i o n energy were compared w i t h those f o r the d i f f u s i o n of protons i n o6Al 2 0 3 ;H20 (12.9 k c a l s / m o l e ) 1 6 , and i n cKr^OOH (16.5 k c a l s / m o l e ) 1 7 , .and considered t o be i n good agreement. •Pryor and Evans 9 noted t h a t the amorphous nature of i r o n oxide p r e c i p i t a t e s p e r s i s t e d even a f t e r r o a s t i n g at temperatures up t o 385° C . The maximum r o a s t i n g temperature employed by F u r u i c h i et a l was kOO°C, so t h a t i t i s q u i t e p o s s i b l e , a l l o w i n g f o r v a r i a t i o n s due t o the c o n d i t i o n s of p r e c i p i t a t i o n , t h a t the m a t e r i a l they i n v e s t i g a t e d was i n no case a t r u e , c r y s t a l l i n e «<-Fe 2 0 3 . . This might account, i n p a r t , f o r the low a c t i v a t i o n energies (10-12 kcals/mole), obtained by them, as compared w i t h the values observed by.Azuma and Kametani f o r o<-Fe 203 (20 - 2 kcals/mole), and by Monhemius 1 5 f o r rX-FeOOH (18.2 kcals/mole). The very low a c t i v a t i o n energy (k.8 kcals/mole), observed f o r the d i s s o l u t i o n of m a t e r i a l roasted at low temperatures (110°C), suggests a mechanism c o n t r o l l e d by the d i f f u s i o n of an i o n i n the aqueous phase. This i s reasonable, since the p r e c i p i t a t e under these c o n d i t i o n s would probably c o n s i s t of a l o o s e , porous aggregate of very f i n e p a r t i c l e s . The steady increase i n a c t i v a t i o n energy from k.8 13 t o 12 ..3 kcals/mole w i t h i n c r e a s i n g r o a s t i n g temperature p o s s i b l y represents a gradual t r a n s f e r of c o n t r o l from d i f f u s i o n i n the aqueous phase t o d i f f u s i o n . i n the " s o l i d " as the s o l i d becomes l e s s and l e s s porous. (b) D i s s o l u t i o n of Other Corundum-Type Oxides oC-Fe 203 i s a member of the group of oxides having the general formula R 203 and the corundum s t r u c t u r e . This s t r u c t u r e was f i r s t analyzed by W. H. Bragg and W. L. B r a g g 1 8 , and l a t e r by P a u l i n g and H e n d r i c k s . 1 9 More recent work has confirmed the r e s u l t s of these i n v e s t i g a t o r s . Hematite has a rhombohedral s t r u c t u r e , w i t h the oxygen atoms i n approximately c l o s e -packing, and the i r o n atoms occupying two-thirds of the octahedral holes i n the oxygen l a t t i c e . S i x oxygen atoms form an octahedral group around the metal atom, and each.oxygen atom i s surrounded by four metal atoms. Besides oC-Fe203 and <X-Al 203, the group i n c l u d e s C r 2 0 3 , V 203, T i 2 0 3 , oc/Ga 20 3 and Rh 203 . 2 C? Because of the s i m i l a r i t y of t h e i r s t r u c t u r e s , the d i s s o l u t i o n of these oxides might be expected t o have some features i n common w i t h the d i s s o l u t i o n o f < X - F e 2 0 3 . However, a search of the l i t e r a -t ure f a i l e d t o l o c a t e any work on the d i s s o l u t i o n of these oxides. Some work has been done, however,.on the a c i d a t t a c k ofcX>Al 203. In an e t c h - p i t study of flux-grown corundum, Champion and Clemence 2 1 observed a marked d i f f e r e n c e i n the ease w i t h which the various c r y s t a l planes were attacked c h e m i c a l l y . The b a s a l planes were found t o be most r e a d i l y a t t a c k e d , and the rhombohedral planes the l e a s t . Dawihl and K l i n g l e r 5 , .in a study of the c o r r o s i o n r e s i s t a n c e of p o l y c r y s t a l l i n e O C - A 1 2 0 3 and s i n g l e c r y s t a l sapphire i n h y d r o c h l o r i c , n i t r i c and s u l p h u r i c a c i d s , a l s o noted a marked p r e f e r e n t i a l a t t a c k on the b a s a l plane. This plane appeared t o d i s s o l v e a t a rate approximately kO times as great as that of the (1120) prism plane. Ik (c) .Dissolution of Other Oxides ( i ) U0 2 -Mackay and Wadsworth1 leached sintered U0 2 specimens i n sulphuric acid solutions under oxygen atmospheres. The rate was observed to be a function of the concentration of hydrogen ions, and to be d i r e c t l y proportional to the p a r t i a l pressure of.oxygen. The rate was also apparently independent of the pa r t i c u l a r acid used. The authors concluded that a U0 2 surface s i t e reacts with a molecule of water to form a hydroxyl complex, which i n turn can dissociate with the characteristics of a weak acid. A rate-determining step was proposed, involving the reaction between an oxygen molecule and the hydroxyl complex on the U0 2 surface. ( i i ) ,Si0 2 - Most investigations into the s o l u b i l i t y of quartz have been made i n alkalin e or neutral.media. However, a b r i e f discussion of some of t h i s work i s included here, since i t appears that the adsorption of water on the surface i s the rate-controlling step, just as i t i s i n acids. 22 Siebert et a l , i n a study, of the kinetics of the dissolution of the basal plane of quartz i n water at high temperatures and pressures, came to the conclusion that the rate-controlling step i s the reaction of water with the quartz surface, although the possibility.-of d i f f u s i o n through a polymerized .layer of s i l i c i c acid on the surface could not be excluded. 23 These findings are i n agreement with those of Hooley for the dissolution of quartz in.strongly alkaline solutions. 24 -In a study of the silica-water system, Kennedy quotes evidence of previous workers to the effect that the basal plane of quartz may dissolve i n water or HF solutions at a rate 100 times as great as the hexagonal prism plane. Bergman et a l 2 5 , 2 s have studied the dissolution of 15 fine quartz powders in'HF solutions. They found that p a r t i c l e s with diameters greater than exhibited S-'shaped dissolution curves, an upward trend i n the curve appearing a f t e r the dissolution of.5-10$ of the quartz. Since the p o s s i b i l i t y of an autocatalytic effect had been ruled out, as had the p o s s i b i l i t y of a change i n average p a r t i c l e shape or size, i t was concluded that the unusual dissolution curves reflected a change i n the dissolving material other than one of available surface. Electron micro-, scopy of replicas of p a r t i a l l y dissolved p a r t i c l e s revealed the development of d i s t i n c t c r y s t a l faces, and t h i s was taken as evidence that quartz possesses a number of preferred directions of dissolution normal to the c r y s t a l faces produced. The p a r t i c l e s also showed surfaces on which no dissolution appeared to have occurred, evidence that the rate of etching of the preferred faces was many times that of the other surfaces. The preferred faces would accordingly tend to grow.in area at the expense of the other surfaces, the proportion of fast-dissolving surface would increase rapidly, ,and an increase i n the rate of dissolution during etching would result. This ef f e c t , coupled with the apparent decrease i n rate as the dissolution neared completion, would give rise to the S-shaped rate curves. ( i i i ) . Z i n c - f e r r i t e - In a study of the dissolution of z i n c - f e r r i t e i n sulphuric acid, Nil- and Hisamatsu observed that f e r r i t e of oxygen-deficient composition dissolved more rapidly than that of stoichiometric composition. They concluded that anion vacancies or cation i n t e r s t i t i a l s would be effective dissolution centres, and that the rate would be controlled by t h e i r concentrations. -Additional support for t h i s view was provided i n an extension of t h i s work 2 8 to a study of the effect of reducing agents i n ++ the system. The addition of Fe metal powder, or of Fe ions, or the contact of the f e r r i t e with a cathodically polarized electrode, a l l tended' to increase the dissolution rate. This was ascribed to an increase i n the anion vacancy concentration at the surface. 1 6 I I I . SCOPE OF THE PRESENT INVESTIGATION The present investigation was o r i g i n a l l y intended to comprise the preparation of dense sintered specimens of O£-Fe203 and a study of t h e i r reductive dissolution using sulphur dioxide as a reducing agent. .As a preliminary to the main investigation, some tests were carried out on the direct acid dissolution of hematite. The results of these t e s t s , together with a review of the l i t e r a t u r e , suggested that there were aspects of the direct d i s s o l u t i o n process that were not well understood, and that further work i n t h i s d i r e c t i o n might be rewarding. For instance, Azuma and Kametani 1 2 offered no explanation for the S-shaped rate curves they obtained for the complete dissolution of f e r r i c oxide powders i n various acids, except to point out that they could not be explained by the assumption of uniformly-dissolving spherical p a r t i c l e s . I t was hoped that the present study might provide an explanation for t h i s observation. Previous i n v e s t i g a t o r s 9 ' 1 2 have noted the very rapid dissolu-t i o n of f e r r i c oxide i n hydrofluoric acid and concentrated hydrochloric acid, and ascribed i t to the fact that the anions i n these solutions form stable complexes with the f e r r i c ion. However, the mechanism by which the dissolution rate i s actually increased has not been explained, and i n the present study t h i s effect i s examined by varying the anion concentration of the solutions independently.of the hydrogen ion concentrations. Hydrochloric acid was chosen for most of the test work for two reasons: ( i ) conveniently measurable rates could be obtained i n th i s system, and ( i i ) . i t provided the opportunity to examine the role of complex formation i n the overall dissolution process. 1 7 In order to maintain a known, constant surface area during leach-ing, and to f a c i l i t a t e microscopic examination of leached surfaces, sintered discs of c<_Fe203.were used i n the e a r l i e r t e s t s , i n preference to the r e l a t i v e l y fine powders used by previous investigators. However, for reasons which w i l l be discussed -later, these gave neither l i n e a r nor reproducible dissolution curves. Qualitative results only are therefore presented f o r experiments on these specimens. The bulk of the experimental work was done on either s i n g l e - c r y s t a l hematite specimens or on closely-sized synthetic cK-Fe 203 powders. 18 EXPERIMENTAL I . SPECIMEN MATERIALS AND REAGENTS (a) , S y n t h e t i c - P o l y c r y s t a l l i n e and Powder Specimens The s i n t e r e d p o l y c r y s t a l l i n e and powder specimens used i n t h i s study were prepared from reagent grade c< -Fe 20 3 (Baker and Adams), without f u r t h e r p u r i f i c a t i o n . The maximum l i m i t s of i m p u r i t i e s i n t h i s m a t e r i a l are shown i n .Table I . TABLE I . Chemical a n a l y s i s of reagent gradeo^. -Fe 20 3. weight $ .. I n s o l u b l e i n HCl 0.20 s u l f a t e (S0 4) 0.20 copper (Ou) 0.005 z i n c (Zn) 0.005 Substances not p r e c i p i t a t e d by NH4OH (as s u l f a t e s ) ; 0.10 An x-ray d i f f r a c t i o n p a t t e r n was made t o confirm the i d e n t i t y of 29 the m a t e r i a l . The r e s u l t s agreed w e l l w i t h published data f o r c X — F e 2 0 3 (see Table B l , Appendix B). (b) N a t u r a l S i n g l e C r y s t a l Specimens A group of n a t u r a l hematite c r y s t a l s o r i g i n a t i n g i n T t a b i r a , Minas Gerais, B r a z i l , was obtained. -These c r y s t a l s showed s e v e r a l well-developed face s , some of which bore evidence of n a t u r a l e t c h i n g . .The group c o n s i s t e d 19 of two large crystals p a r t i a l l y intergrown with each other and with a po l y c r y s t a l l i n e matrix. .The two large crystals were separated from each other and from the matrix by careful diamond sawing. X-ray d i f f r a c t i o n : An x-ray d i f f r a c t i o n pattern of some of the f i n e l y ground material showed a l l the major peaks for hematite, with no extraneous peaks, i n 29 agreement with published data (see Table B l , Appendix B). Chemical analysis: Portions of the single crystals were submitted to Coast Eldridge, Engineers and Chemists Ltd., for:, chemical, analysis . Reported results are shown i n Table 2(a), while Table 2(b) shows the values converted to the equivalent oxides. .TABLE 2. Chemical analysis of natural hematite crys t a l s . (a) Values as reported (b) Values converted to equivalent oxides weight jo weight jo Fe (total) 69.14 Fe 20 3 92.10 F e + + 4.62 FeO 5.94 A1 20 3 0.6i A1 20 3 0.61 S i 0 2 .1..54 S i 0 2 1.54 Total 99-99 The presence of a r e l a t i v e l y large amount of ferrous iron (4.62$) i s somewhat puzzling. c?C-Fe 20 3 i s known to deviate from stoichiometry, 20 usually exhibiting a s l i g h t metal excess, thought to be manifested as 3 0 vacancies i n the oxygen l a t t i c e . • However, the variation from stoichio-metry i s small, usually barely detectable by chemical a n a l y t i c a l methods. Microscopic examination of polished sections, of the crystals f a i l e d to reveal the presence of intergrowths or precipitates of•other.minerals, and no explanation can be offered for the presence of such a;large amount of divalent iron i n the crystals. The only other impurities present i n s i g n i f i c a n t quantities were S i 0 2 and AI2O3, neither of which would be expected to affect the results of the experiments. (c) Reagents Except for the natural single c r y s t a l s , a l l materials used i n t h i s study were reagent grade. D i s t i l l e d water was used for a l l solutions. Nitrogen was. ordinary cylinder grade, supplied by Canadian Liquid A i r Ltd. I I . SPECIMEN PREPARATION . (a) Sintered P o l y c r y s t a l l i n e Compacts Although these specimens did not y i e l d reproducible dissolution rates, t h e i r method of preparation w i l l be described, since qualitative results are quoted. Suitable test specimens had to s a t i s f y the following requirements: (a) Negligible open porosity. The presence of open or interconnected pores would lead to an.uncertain s o l i d - l i q u i d interface area, and possibly to large changes i n surface area during leaching. (b) Freedom from cracks or flaws. Again, these imperfections would lead to uncertainties i n surface area. 21 (c) Homogeneous and reproducible microstructure. I n i t i a l l y a density of 95-96$ of the theo r e t i c a l value was aimed at, since i t was f e l t that at t h i s value, open porosity would be negligible. The t h e o r e t i c a l density ofo^.-Fe 203, as determined from l a t t i c e parameter measurements,19 i s 5-25 gm/cm3, so that 95$ corresponds to a density of 5.00 gm/cm . i n fact when satisfactory specimens were eventually produced, i t was found that those with a density greater than 93$ of the theo r e t i c a l value had negligible open porosity. Several methods of producing compacts were t r i e d , including reactive hot pressing using synthetic goe'thite . (©<-FeOOH) as starting material, conventional hot pressing, sintering of die-pressed compacts, and sintering of hydrostatically pressed compacts. Of these, only the l a s t was f u l l y successful. The specimens produced by the other methods were either badly cracked or showed marked inhomogeneities i n polished sections. The method f i n a l l y adopted for the production of test specimens consisted i n tamping 200 gm l o t s of reagent-grade f e r r i c oxide powder into a t h i n rubber tube, which was then sealed and placed i n a cylinder of o i l . A hydrostatic pressure of 6000 p s i was applied to the o i l by means of a piston, and the pressure maintained for two minutes. The c y l i n d r i c a l green compacts (about 3 inches long by l i inches i n diameter) were then sintered i n a i r at temperatures ranging from 1000 to 1100°C, and for times ranging from 12 to 38 hours. 0 The sintered cylinders were sl i c e d into l / 8 inch thick discs, using a diamond saw, and the discs ground f l a t on both sides with 2^ 0 mesh carborundum paste on an iron lapping wheel. A Mullard ultrasonic d r i l l i n g u n i t , f i t t e d with a one inch diameter thin-walled tubular cutting t o o l was 22 used to trim the discs, into perfect c i r c l e s . .The same unit was used to d r i l l a l / 8 inch mounting hole i n the centre of each disc. The surfaces of the specimens were finished'by conventional metallographic techniques, O.O^jx alumina being used for the f i n a l p olish. Porosity determinations were made by rapidly weighing the shaped specimens immediately after a two-hour immersion i n b o i l i n g water. Bulk densities were determined by drying overnight at 110°C/) weighing, and measuring thickness and diameter with a micrometer. Figure 1 shows the relationship between sintered density and open porosity. Points on the curve represent specimens prepared by sintering at temperatures from 1000 to 1100°C and for times from 12 to 38 hours. I t i s evident that specimens with density greater than 4.9 gm/cm3 (93$ t h e o r e t i c a l ) , have open porosity of less than 0.2$ by volume. Microscopic examination, of a polished section showed that the discrepancy between actual and theoretical density i s accounted .for by the presence of small, spherical pores within individual grains, and occasional larger.voids at grain boundaries (Figure 2). (b) .Synthetic Powder Specimens Powder samples were prepared by crushing and grinding, i n an agate mortar, compacts which had been sintered i n a i r at 1200°C for 60 hours. The powder was screened wet, and the plus 325 mesh, .minus 270 mesh, ( 4 4 - 53 /0 fra c t i o n was separated for dissolution t e s t s . Wet screening was employed to reduce the amount of very fine material adhering to the larger p a r t i c l e s . The narrow size range (44yu - 53 p-) was chosen so as to reduce as far as possible effects due to the change i n d i s t r i b u t i o n of p a r t i c l e size during dissolution. Microscopic examination of the powder showed roughly equiaxed grains, free from adhering fine dust. 23 Figure 1. V a r i a t i o n of open p o r o s i t y w i t h bulk d e n s i t y f o r s i n t e r e d o<-Fe 203 compacts. Figure 2. S i n t e r e d o C - F e 2 0 3 compact, l i g h t l y etched w i t h hot phosphoric a c i d t o show g r a i n boundaries (700x). Chemical analysis, carried out "by. Coast Eldridge, Engineers and Chemists, Ltd., showed 70-0$ t o t a l iron and 0.05$ ferrous iron. .(c) Natural Crystal' Specimens In order to confirm that the two natural hematite crystals used i n t h i s study were i n fact single c r y s t a l s , >Laue back-reflection x-rays were taken .at several points on each. Identical patterns were obtained at the various points, and the sharp spots were taken to indicate a f a i r l y high degree, of c r y s t a l l i n e perfection. .Test specimens of various crystallographic orientations were cut from these two c r y s t a l s , conventional Laue back-reflection methods being used for the- orientation. .These specimens were then mounted i n "Koldmount" self-curing resin with the oriented surfaces exposed, and the surfaces polished by conventional metallographic techniques. -A more detailed description of the c r y s t a l orientation w i l l be found under '"Experimental Procedure". I I I . APPARATUS 'DESIGN Dissolution experiments were carried out i n a glass reaction flask maintained at a constant temperature i n a water thermostat. The main features of the apparatus are shown i n Figure 5. The one-litre capacity flask was f i t t e d with a nitrogen i n l e t tube and a sample tube, both having glass stopcocks. For tests on powder specimens, the lower end of the sample tube was f i t t e d with a f r i t t e d glass f i l t e r . The solution i n the vessel was s t i r r e d by means of a Teflon-covered magnet rotated by a magnetic s t i r r e r unit below the thermostat vessel. Heat was supplied by a 100 watt immersion heater, connected to the mains supply through a variable 26 R E F L U X C O N D E N S E R NITROGEN INLET STIRRER M O T O R R E A C T I O N F L A S K IMMERSION HEATER S A M P L E T U B E CONTACT T H E R M O M E T E R F R I T T E D G L A S S FILTER M A G N E T I C STIRRER Figure 5- Schematic diagram of d i s s o l u t i o n apparatus. 2 7 transformer. The temperature was c o n t r o l l e d by a mercury-in-glass contact thermometer, connected t o the heater c i r c u i t through a mercury r e l a y . The water bath, contained i n an i n s u l a t e d enclosure, was s t i r r e d continuously by a v a r i a b l e speed s t i r r e r . Temperature was maintained w i t h i n 0 . 2°C of the d e s i r e d value by t h i s arrangement. Normally the v e s s e l was open t o the atmosphere, through a r e f l u x condenser. However, i n order t o take a sample during a t e s t , the appropriate stopcocks were c l o s e d , and a pressure of 5-10 p s i of n i t r o g e n was a p p l i e d to the f l a s k t o force the s o l u t i o n out through the sample tube. Evaporation from the f l a s k during a run was n e g l i g i b l e . Powder specimens were introduced i n t o the f l a s k by washing through the funnel (see Figure 3) w i t h a small q u a n t i t y of the t e s t s o l u t i o n . S i n g l e c r y s t a l and s i n t e r e d compact specimens were held i n place i n the s o l u t i o n by means of a glass rod extending t o the bottom of the f l a s k . . IV. • EXPERIMENTAL PROCEDURE (a) .Orientation of S i n g l e C r y s t a l s ( i ) Randomly oriented single c r y s t a l fragments. P r i o r t o the systematic i n v e s t i g a t i o n of the e f f e c t of o r i e n t a -t i o n on the rate of d i s s o l u t i o n , a number of s i n g l e c r y s t a l fragments of hematite were mounted with random o r i e n t a t i o n i n B a k e l i t e mounts. The exposed surfaces were ground f l a t , p o l i s h e d , and etched f o r 1 0 minutes i n b o i l i n g 5-ON HCI. Microscopic examination revealed wide v a r i a t i o n s i n the degree of a t t a c k on d i f f e r e n t specimens. B a c k - r e f l e c t i o n Laue x-rays were taken of those specimens showing strong a t t a c k . 2 8 f i i ) S p e c i f i c a l l y oriented c r y s t a l s . In.order to investigate the effect of c r y s t a l orientation on the rate of diss o l u t i o n , i t was necessary to prepare a series of specimens having surfaces p a r a l l e l to various c r y s t a l planes. Conventional back-r e f l e c t i o n Laue x-ray methods were used to determine the orientation of the specimens. FeK,,^  radiation was used for a l l x-rays, with an exposure time of 20-40 minutes at 30KV and 10 mA. The external morphology of the crystals allowed an estimate to be made of the position of the t r i a d axis. When t h i s had been confirmed and accurately located, a reference basal plane was cut on each c r y s t a l . Once t h i s had been done, the positions of the other planes were ea s i l y determined. .A simple two-circle goniometer was constructed to simplify this part of the work. Specimens having surfaces l y i n g i n the following crystallographic planes were cut: (0001) (2243) •(1010) (1120) (1011) The specimens were mounted i n "Koldmount" resin, and the oriented surfaces ground f l a t using 600 mesh alundum paste on an iron lapping wheel. .The surfaces were finished by polishing with I^JCL alumina, 1 diamond paste and f i n a l l y O.O^ja. alumina. Where the same specimen was used for more than one t e s t , the surface was ground and polished, as described above, before each test. As a f i n a l check, -Laue x-ray patterns were taken of a l l mounted 29 specimens, and i t was confirmed that the o r i e n t a t i o n i n each case was w i t h i n 5° of the nominal plane. The methods a v a i l a b l e f o r c u t t i n g and g r i n d i n g d i d not a l l o w the p r e p a r a t i o n of more a c c u r a t e l y o r i e n t e d specimens, except p o s s i b l y by tedious t r i a l and e r r o r methods. However, i n view of the very l i m i t e d quantity, of m a t e r i a l a v a i l a b l e , i t was decided not t o attempt t o improve on these o r i e n t a t i o n s . The specimens were considered t o be s u f f i -c i e n t l y a c c u r a t e l y o r i e n t e d f o r the purpose of the present i n v e s t i g a t i o n . Since no t a b l e s of c r y s t a l l o g r a p h i c angles or standard stereo-graphic p r o j e c t i o n s f or o<. -Fe 203 could be found i n the l i t e r a t u r e , the 31 values given by W i n c h e l l for©£-Al 20 3 were used f o r the o r i e n t a t i o n work.. 32 The d i f f e r e n c e s i n the l a t t i c e constants of these two isomorphous oxides are s u f f i c i e n t l y s m a ll that the Laue patterns foro<.-Fe 20 3 could be con-v e n i e n t l y analyzed w i t h the a i d of the corundum data (see Table 3). TABLE 5 32 L a t t i c e constants f o r o<^Al 203 and <?C-Fe203  ( r e f e r r e d t o hexagonal axes).  c a c/a c * - A l 2 0 3 12.99 U.76 2.73 OC-Fe 2 0 3 .13.75 5.03 2.73 (b) D i s s o l u t i o n Tests 12 I t has been shown that under c o n d i t i o n s of low a c i d i t y and high r a t i o of f e r r i c oxide to s o l u t i o n volume, the d i s s o l u t i o n rate may f a l l w i t h time, due t o the s a t u r a t i o n of the s o l u t i o n w i t h f e r r i c i r o n . In t h i s study, experimental c o n d i t i o n s were chosen t o avoid t h i s . Large s o l u t i o n volumes (^>00 ml) and small amounts of f e r r i c oxide (0.1-0.2 gm) were used. The experimental procedure consisted of the following steps: ( i ) the temperature controller was set at the required temperature. ( i i ) the reaction f l a s k , containing ^00 ml of solution of the required concentration, was immersed i n the thermostat, and the various connections made. ( i i i ) the'system was allowed to come to thermal equilibrium. (iv) the specimen was attached to the specimen holder and immersed i n the solution. In the case of experiments on powder samples, a weighed quantity, usually 0.100 gm, occasionally 0.200 gm, of the material, was introduced into the flas k by washing i t through .the funnel (see Figure 3) with a small quantity of the solution. (v) samples were taken at intervals. of 15, J>0 or 60 minutes, depending on the expected rate, by applying a pressure of.5-10 p s i of nitrogen to the f l a s k to force a sample out through .the sample tube. .The f i r s t 10 ml of solution removed was immediately returned to the flas k v i a the funnel, and a second sample of 6-7 nil taken. This was to ensure that the sample was not contaminated by traces of the previous sample remaining i n the tube. Samples were collected i n a sample bottle which was immediately stoppered to prevent vapor loss during cooling. After cooling, 1-5 ml, depending on the iron content expected, was pipetted from the sample, and the small excess returned to the flask. Runs were usually continued long enough to dissolve a minimum of 3$ of the starting material. Samples were analyzed f o r iron as described i n the following section. -At the end of each run, i n the case of single c r y s t a l or compact specimens, the specimen was removed from the solution, washed with water and then alcohol, dried, and examined microscopically. Photomicrographs of the specimen surfaces were taken a f t e r many of the runs. 31 V. ,ANALYTICAL METHOD The progress of dissolution was followed by spectrophotometry determination of the iron content of samples taken at regular intervals throughout the experiment. Ferrous iron and 1-10 orthophenanthroline form an orange-red 3 3 complex having a strong absorption peak at a wavelength of 510 mu Absorption i n d i l u t e solutions of t h i s complex obeys Beer's Law and can be used for the quantitative determination of ferrous iron. Since most of the iron was expected to be i n the t r i v a l e n t state, hydroxylamine hydro-chloride was added to each of the samples to reduce a l l iron to the ferrous state. Sample solutions were buffered at pH 4.5 with a sodium acetate-acetic acid buffer, since the coloured complex has been shown to be most 3 3 stable i n the pH range 2-9. Procedure: A composite reagent was made up containing 0.3 gm/l. ortho-phenanthroline, 2.0 gm/li hydroxylamine hydrochloride and buffer. Suitable aliquots of sample solution were pipetted into 100 ml volumetric f l a s k s , 25 ml of composite reagent added, and the volume made up with d i s t i l l e d water. The solutions were allowed to stand for at least 30 minutes (to ensure f u l l development of the colour), before the op t i c a l densities were measured on a Beckman Model B spectrophotometer at 510 mji. The concentration of iron was read from a cal i b r a t i o n curve prepared using standard ferrous ammonium sulphate solutions, checked against a basic standard prepared from 99-9$ pure iron wire. RESULTS For the sake of c l a r i t y , most of the results i n t h i s section are presented .in the form of graphs, numerical results being grouped together i n a separate appendix. This section i s divided into three parts, each describing a separate phase of the work, as follows: I. Tests on p o l y c r y s t a l l i n e sintered compacts of .o<C-Fe203. I I . Tests on synthetic powder specimens. I I I . Tests on natural hematite single c r y s t a l s . 33 I. TESTS ON SINTERED POLYCRYSTALLINE COMPACTS No quantitative results are presented for t h i s phase of the investigation, since they are not considered informative enough to warrant inclusion. Reproducibility of rates i n the leaching tests was poor, and non-linear rates were obtained, making interpretation d i f f i c u l t . The reason for both the above mentioned effects was found to be the strongly anisotropic nature of the dissolution. This i s c l e a r l y demonstrated i n Figure k, which shows specimens af t e r p a r t i a l leaching i n 1.0N HCI,.5.ON HCIO4 and 5.ON H 2S0 4. These micrographs show that while certain grains have been .strongly attacked, others s t i l l exhibit traces of polishing scratches, indicating that v i r t u a l l y no dissolution has occurred. This selective attack was observed,in specimens leached i n HCI, \HNO3, H2SO4 and H C I O 4 , at concentrations ranging from 1.0N to 6.ON. The rate of attack was q u a l i t a t i v e l y observed.to be most rapid i n HCI and .slowest i n H C I O 4 , the rates i n HNO3 and H 2S0 4 being intermediate. The poor rep r o d u c i b i l i t y of leaching rates of these specimens i s believed to result from the loosening of grains from the surface by dissolution of neighbouring grains, resulting i n an increased area available for dissolution. This would result i n a steadily increasing apparent rate of dissolution, as was observed i n some tests, and i n poor reproducibility between runs under i d e n t i c a l conditions. Although these tests did not give quantitatively reproducible re s u l t s , they served to indicate the strongly anisotropic nature of the dissolution, and to point out the d e s i r a b i l i t y of taking t h i s into account in subsequent work. 34 Figure 4. Sintered p o l y c r y s t a l l i n e compacts of oC-Fe 2 0 3 after leaching under various conditions (700x). (a) 1.0N HCI, 1 6 minutes at 100°C. (b) 5.ON HC10 4, 90 minutes at 100°C. fc) 5.ON HC10 4, 90 minutes at 100°C. •(d) 5.ON H 2S0 4, 5 minutes at 100°C . 35 I I . SYNTHETIC POWDER SPECIMENS The effects of the following variables on the rate of dissolution of c<-Fe203 powder (-270 mesh, +325 mesh) in hydrochloric acid solutions were investigated: (a) temperature (b) acid concentration (c) varying hydrogen ion concentration at constant chloride ion concentration (d) varying chloride ion concentration at constant hydrogen ion concentration. Except for those designed to examine the effect of temperature, a l l runs were carried out at 85°C. Linear rate curves were obtained for diss o l u t i o n amounting to approximately• 4-0$ of the o r i g i n a l material. After t h i s , the apparent rate decreased as the available surface area of the powder began to decrease s i g n i f i c a n t l y . In the following rate curves, the number of milligrams of iron dissolved from a 0.100 gm-sample of<?<-Fe203 powder is plotted against time, due allowance being made for the iron content of samples removed during a run. A l l rates were measured on the i n i t i a l linear part of the curves. (a) Effect of Temperature Typical rate plots for dissolution i n 5.ON HCl at various tem-peratures are shown i n Figure 5. The rate i s seen to increase markedly with temperature. The effect of temperature on the rate of dissolution i n 0.5N HCl was also investigated. The results of these tests are presented i n Table A l , .Appendix A, while Figures 6 and 7 are Arrhenius plots for dissolution i n 5.ON and 0.5N HCl respectively. Activation energies determined from the slopes of these l i n e s are 21.6- 1.0 and 19.4- 1.8 kcals/mole.respectively. Fe i n s o l u t i o n (mgms.) d i s s o l u t i o n of ©< -Fe 20 3 powder in.5.ON'HCl. 37 38 3 9 (b) Effect of Acid Concentration The effect on the dissolution rate of varying the hydrochloric acid concentration i n the range 0.2N to 7.ON was investigated. Figures 8 and 9 show that the rate increases rapidly with acid concentration. A log-log plot of t h i s data gives a straight l i n e with a slope of 2.5 over th the whole range of experimental conditions, indicating an apparent 2.5 power dependency of rate on HCl concentration. However, when the rate i s plotted against a c t i v i t y , as i n Figure 10(a), the rate appears to depend on the square of the hydrogen ion a c t i v i t y (or the product of the hydrogen and chloride ion a c t i v i t i e s ) i n the range 0..2-2.0N, and d i r e c t l y on a c t i v i t y i n the range 2.0N-7.0N. Figure 10(b) shows the i n i t i a l part of t h i s plot ( 0 - 2.ON) on a larger scale,, while i n Figure 10(c) the rate i n the range 0 - 2.ON i s plotted against the square of the hydrogen ion a c t i v i t y . Appendix C describes the estimation of the a c t i v i t i e s i n HCl solutions, while numerical rate data are given i n Table A2,.Appendix A. Figure 8 E f f e c t of H C I concentration on rate of d i s s o l u t i o n of CX.-Fe203 powder at 85°C - t y p i c a l rate curves. o hi HCl concentration (normality) Figure 9 . Effect of HCl concentration -on rate of dissolution of -Fe 2 0 3 .powder at 85°C . 1.0 2.0 '.. • . aH* Figure 10 (b) - Effect of hydrogen ion a c t i v i t y on rate of dissolution ofo<-Fe 20 3 powder i n dilue HCl solutions. ( I n i t i a l part of Figure 10(a) plotted on larger scale.) 0.0k : ; : ,(aH+); • Figure 10 (c) Dissolution rate ofo<-Fe 20 3 powder i n dilute HCl solutions as a function of the square of hydrogen ion a c t i v i t y . kk (c) Effect of Varying Hydrogen Ion Concentration at  Constant Chloride Ion Concentration To investigate the independent effects of hydrogen and chloride ions, tests were carried out i n which part of the HCI was replaced by an equivalent amount of KC1, so allowing the hydrogen ion concentration to be varied while keeping the t o t a l chloride ion concentration constant. Three sets of experiments were performed, with t o t a l chloride ion concen-trations of 5.ON, 3-ON and 2.ON respectively. In each case the rate was found to increase, apparently l i n e a r l y , with increase i n hydrogen ion concentration [see Figures 11(a), (b), and(c)]. I t was also noted that the rate of dissolution i n solutions i n which part of the HCI had been replaced by.an equivalent amount of KC1 was less than that i n solutions of HCI of the same t o t a l ionic strength. The range over which the hydrogen ion concentration could be varied by t h i s method was limited by the rather low s o l u b i l i t y of KC1 i n concentrated HCI solutions. Numerical results are given i n Table A3, Appendix A. 0 OhO 'min.) 0 03 0 CD §> 0 020 (a) [Cl ] .= 2.ON OJ 0 010 Rat 1— 1 1 —1 1 L_ 1 1.0 1.2 1.4 1.6 1.8 2.0 2.2 •rt B CO OJ •p 0.100 -0.090 0.080 0.070 • [Cl ] = 3.ON 0.060 0.050 —1 1 L _ l ; 1 1 ,1 2.0 2.2 2.4 .2.6 2.8 3.0 3.2 O.3IO -0.300 7 O.29O -•H a w O..280'«-§> a "~ 0.270 CD •P & .0.260 0.250 Figure 11. Effect of hydrogen ion concentration on rate of dissolution of OC-Fe203 powder at 85°C and various constant chloride ion concentrations. (d) Effect of Varying Chloride Ion Concentration at  Constant Hydrogen Ion Concentration Since the results of the foregoing experiments appeared to indicate that the dissolution rate was dependent on the concentrations of both hydrogen and chloride ions, tests were carried out to determine the effect of varying the chloride ion concentration independently of the hydrogen ion concentration. Dissolution rates were determined i n 1.0N, 2.ON and k.ON HCI solutions to which varying amounts of KC1 had been added. Results are shown graphically i n Figures 12 (a), (b), and (c), and numerical results are given i n Table Ah,-Appendix A. The rate increases approximately l i n e a r l y with chloride ion concentration at a given hydrogen ion concen-t r a t i o n . Unfortunately, the dissolution rates i n these mixed HC1-KC1 solutions cannot be represented as functions of hydrogen and chloride ion a c t i v i t i e s , since only very scanty thermodynamic data are available at 25°C, and none at a l l at the experimental temperature (85°C). The degree of dissociation i n these solutions i s also unknown. I t i s clear from the available a c t i v i t y . c o e f f i c i e n t data for HCI solutions, and for HC1-KC1 s o l -utions at 25°C, that large deviations from unity are to be expected i n the more concentrated solutions. This makes i t impossible even to estimate the a c t i v i t i e s i n these solutions, and therefore i n Figures 11 and 12, d i s -solution rates are plotted as functions of added chloride (or hydrogen) ion at various concentrations of added hydrogen (or chloride) ion. 1.0 1-5 2.0 2.5 0.070 o. 060 — 0.050 o.oko (b) [H +] = 2. ON 0.050 1 1 1 1 2.0 2.5 5-0 5.5 O..27O 0.260 0.250-0.24O 0.250 0.220 •H a 0.210 / (c) [H +] = 4.0N &_ 0.200 0) -p nj 0.190 O / K 0.180 1 1 1 4.5 5.0 5-5 T o t a l [CI ] - normality. -Figure 12. E f f e c t of chloride ion concentration on rate of d i s s o l u t i o n of O 4 - F e 2 0 3 powder at 85°C and various constant hydrogen ion concentrations. 48 I I I . SINGLE CRYSTAL SPECIMENS To investigate further the anisotropic nature of the dissolution process, a series of experiments was performed using natural single c r y s t a l hematite. The effects of the following variables on the rate of dissolution were investigated: (a) c r y s t a l orientation (b) temperature (c) acid concentration (d) The above experiments were a l l carried out on oriented sur-faces of single c r y s t a l specimens. In addition, the dissolution of a single c r y s t a l sphere was investigated. (a) .Crystal Orientation ( i ) Randomly oriented c r y s t a l surfaces Microscopic examination of randomly oriented single c r y s t a l surfaces, a f t e r etching i n b o i l i n g 5-ON HCl for 10 minutes, showed wide variations i n the degree and nature of attack from specimen to specimen. These differences were f e l t to be due to orientation differences, and to confirm that one (or possibly more) crystallographic plane(s) was dissolving at a far greater rate than any of the others, the orientations of those specimens showing severe attack were determined. Of the f i f t e e n specimens examined, seven showed surfaces etched to a greater or lesser degree, while the others were v i r t u a l l y unattacked. .Back-reflection Laue patterns were obtained from those specimens showing 1+9 evidence of etc h i n g . Figure 13 i s a s i m p l i f i e d (0001) stereographic p r o j e c t i o n f o r C< -Fe 203 on which have been p l o t t e d the o r i e n t a t i o n s of the seven etched specimens. The surfaces a l l appear t o be oriented c l o s e t o the b a s a l plane, suggesting t h a t t h i s plane, or p o s s i b l y a high index plane c l o s e t o i t , i s being most r a p i d l y attacked. In general, i t was noted that those surfaces l y i n g close t o the (0001) plane appeared, a f t e r e t c h i n g , t o c o n s i s t of pyramidal f a c e t s , w i t h each of the three pyramidal faces approximately e q u a l l y developed, while those s i t u a t e d f u r t h e r from the (0001) plane tended t o have one facet developed at the expense of the others. This i s i l l u s t r a t e d i n Figures ik and 15 which show two of these surfaces, one (M8B) l y i n g close t o the (0001) plane, and the other (M6) s i t u a t e d some distance from i t (see Figure 13). These f a c e t s apparently correspond t o p a r t i c u l a r c r y s t a l l o g r a p h i c planes., since t h e i r o r i e n t a t i o n on any one specimen was constant over the whole su r f a c e , and was reproduced when the surface was p o l i s h e d and r e -etched. Evidence was obtained i n a l a t e r experiment t o show t h a t the fa c e t s represent the (1011) or the (1012) planes i n the c r y s t a l . ( i i ) S p e c i f i c a l l y o r i e n t e d c r y s t a l surfaces Figure 16 shows t y p i c a l d i s s o l u t i o n curves f o r s i n g l e c r y s t a l s p e c i -mens of f i v e d i f f e r e n t o r i e n t a t i o n s i n 5.ON HCI at 85°C. Within the l i m i t s of experimental e r r o r , the p l o t s are l i n e a r . Note t h a t although specimen areas vary, the p l o t s i n t h i s f i g u r e have been normalized so that each 2 represents the d i s s o l u t i o n of one cm of surface. Table A 5 , Appendix A ~2 -1 gives a c t u a l rates i n terms of mgm.cm ,.minute .Figure 1J>. Simplified (0001) stereographic projection of cX.-Fe203 with orientations of e a s i l y etched surfaces superimposed. M6 etc. represent sample numbers - see also Figures lk and 15. Figure 1^• Specimen M6 a f t e r e t ching i n 5.ON HCI f o r 10 minutes (700x) (see Figure 13). It is clear from these results that the rate of dissolution of the (0001) plane is very much greater than that of any of the other planes investigated,.at least in strong HCl solutions. It was not possible to extend this series of experiments to other acids or to solutions of lower concentration because of the difficulty in measuring the rather low rates (due to the small available surface area). The rates on the other surfaces are a l l of the same order, apparently decreasing in the order: (1010) > ( 1120) — (2243) >(1011) and approximately an order of magnitude smaller than the rate on the basal plane. However, caution should probably be exercised in drawing conclusions from the relative magnitudes of these rates, as the reproducibility was not good (see Table A5/ Appendix A). It is possible that the true rates on surfaces other than (0001) are in fact lower than indicated by the measured values. Due to inherent imperfections in the crystals, it was impossible to produce a flaw-free test surface by polishing. Fine cracks and pores were always present, and i t is considered that dissolution of basally oriented regions exposed by these defects would contribute disproportionately to the overall dissolution rate. Microscopic examination did in fact reveal regions of severe attack, obviously initiated at surface defects, in otherwise essentially unetched surfaces. On the other hand, these defects would be of no significance in the basal specimens, since the areas exposed at cracks and pores would be of slow dissolving orientations, and would not affect the overall observed rate. Microscopic examination of the surfaces after leaching revealed extensive etch-pit formation in the basal specimens. Figures 17(a) and (b) are optical micrographs of a basal surface after leaching in 5.ON HCl at 85°C for 3 hours. The sharp-edged triangular (or hexagonal) pits had the Figure 16. T y p i c a l rate curves f o r d i s s o l u t i o n of n a t u r a l s i n g l e c r y s t a l hematite - e f f e c t of o r i e n t a t i o n o n • d i s s o l u t i o n i n 5.ON HCI at 85°C. Figure 1 7 . Basal surfaces of n a t u r a l hematite c r y s t a l a f t e r l e a c h i n g i n 5.ON HCl at 85°C f o r 3 hours (260x). 55 same orientation over the whole surface, and since they were reproduced af t e r repolishing and etching, they are assumed to correspond to the i n t e r -section of dislocations with the surface. In some areas of the surface, these p i t s appeared to be concentrated within shallow, conical depressions (Figure 17(a) . These l a t t e r features were also reproduced after repolishing and etching, and therefore probably also represent dislocations. The micro-graphs show' large areas, r e l a t i v e l y free of p i t s , i n which dissolution appears to have occurred by the stripping of layers p a r a l l e l to (0001). No etch p i t s were observed i n specimens other than (0001), possibly because dissolution was not continued for a su f f i c i e n t length of time. (b) Effect of Temperature Two possible explanations were considered for the fact that dissolution of the basal plane i s an order of magnitude greater than that of any of the other planes investigated: ( i ) .The number (or density) of "active dissolution s i t e s " i s very much greater on the basal plane. ( i i ) The rate-determining step i n the dissolution process i s different for the basal plane. •If ( i ) i s true, then the activation energy for dissolution should be the same for a l l planes, while i f ( i i ) i s the case, then a measurable difference might exist between the activation energy for dissolution of the basal plane on the one hand, and a l l other planes, on the other. To test t h i s hypothesis, the effect of temperature on the rates of dissolution of the (0001), (1010) and (22^3) surfaces were measured i n 5.ON HCl solutions. For each of these surfaces the rate of dissolution was -determined at 75* $5 a n c * 95°C. The results are i l l u s t r a t e d (as. Arrhenius plots) in : Figure - 1 8 , while numerical data are given i n Table A6. The activ a t i o n energies for dissolution determined from these data are: ( 0 0 0 1 ) : 2 2 . 6 t ; 2 . 2 kcals/mole (1010) : 2 0 - ' 7 kcals/mole (22^3) : 2k.1 - 2 . 3 kdals/mole (c) Effect of Acid Concentration A series of tests was carried out to determine the effect of acid concentration:on the rate of dissolution of the basal plane. The rate was found to increase with concentration i n the range 3.ON to 6.ON, as shown i n Figure 19 and i n Table A7, Appendix A. The plot of log rate against log of acid concentration has a slope of approximately 3 . (d) , Dissolution of Single Crystal Sphere A small sphere, 6 . 5 mm i n diameter, was ground from a natural single c r y s t a l of hematite and i t s dissolution i n 6.ON HCI examined. Figure 20 shows the appearance of the sphere before dissolution and at various stages during the test. These macrographs show one of the two p a r a l l e l "faces" developed on opposite sides of the sphere. Laue x-rays confirmed that-these areas were centred around the poles of the c-axis. V i r t u a l l y no attack was evident on the rest of the spherical surface, even af t e r 90 hours i n 6.ON HCI at 95°C. Also evident i n these photographs i s the pronounced faceting of the surface during the l a t e r stages of dissolution. These facets appeared to correspond to either the (1011) or to the ( 1012) planes i n the c r y s t a l . A Laue back-reflection x-ray normal to one set of facets gave the ( 1012) pattern, but microscopic examination suggested the presence of two sets of facets, possibly (1011) and ( 1 0 1 2 ) . Figure 19- E f f e c t of HCl c o n c e n t r a t i o n on rate of d i s s o l u t i o n of (0001) surface of n a t u r a l hematite a t 85°C. CO 59 (a) before d i s s o l u t i o n (b) a f t e r 2 hours (c) a f t e r 19 hours (d) a f t e r kl hours Figure 20. S i n g l e c r y s t a l sphere of n a t u r a l hematite at v a r i o u s stages during d i s s o l u t i o n i n 6.ON HCI at 95°C. (approx. l O x ) . 6o DISCUSSION ' (a) Solution anisotropy Many c r y s t a l l i n e materials exhibit differences i n chemical r e a c t i v i t y 34 on different c r y s t a l planes. This has been ascribed to the effect of differences i n atomic spacing or packing (and i n some cases, composition) between dif f e r e n t c r y s t a l planes, on the ease of adsorption .of reacting ions •< or molecules. Since the heterogeneous reaction of a s o l i d with i t s gaseous or aqueous environment necessarily includes the adsorption, of an ion .or molecule as one of the steps i n the reaction sequence, t h i s dependence on c r y s t a l orientation i s not unexpected. 35 The anisotropy of surface r e a c t i v i t y has been observed i n metals, 34^36 5 covalent crystals and ionic c r y s t a l s . The effect in.metals i s not very marked, the r e a c t i v i t y of the various surfaces (as measured by solu-b i l i t y , i n acids, for example) d i f f e r i n g at the most by a factor of about "35 three. The effect appears to be most pronounced i n crystals of low symmetry, but i s evident even i n cubic metals. However, the r e l a t i v e magnitudes of the reaction rates on various planes also appear to depend 37 on the experimental conditions, p a r t i c u l a r l y the composition of the reactant. Solution anisotropy appears to be more marked i n non-metallic materials. For instance, evidence has been presented for the dissolution of the basal.plane of quartz at a rate approximately two orders of magnitude 24 greater than that of any other plane , while the basal surface of o<-Al 20 3 (corundum) has been shown to dissolve i n acids kO times as rapidly as the (1120) prism plane.' In the present study, the appearance of the surfaces of sintered 6 1 a^-Fe203 specimens a f t e r leaching i n a c i d solutions suggested that d i s s o l u -t i o n of t h i s material i s highly anisotropic.(Figure k). This anisotropy i s thought to be the cause of the poor r e p r o d u c i b i l i t y and increasing rates observed for. these specimens, as previously discussed. The single c r y s t a l experiments confirm that the basal plane d i s s o l v e s , at l e a s t i n hydrochloric a c i d solutions, at a rate approximately an order of magnitude greater than that of any of the other surfaces examined. It has been postulated that some materials dissolve by d i f f e r e n t mechanisms on d i f f e r e n t c r y s t a l surfaces, .e.g. germanium i n the HF-H 20 2-H 20 system i s thought to dissolve as G e + + on the (100) and (111) surfaces, and '36 as GeOF2 on the (110) surface. In an attempt to detect a possible difference i n the reactions occurring on the various surfaces of c K - F e 2 0 3 , a c t i v a t i o n energies were measured for.the d i s s o l u t i o n of (1000), (1010) and (22^3) surfaces (Figure 18). Although the experimental errors are r e l a t i v e l y large, the a c t i v a t i o n energies f o r the d i s s o l u t i o n of the various surfaces are comparable. On the basis of t h i s evidence, i t i s concluded that the. r e l a t i v e l y high rate of d i s s o l u t i o n of the (0001) surface of <5*v-Fe203 r e s u l t s from the presence of a greater density of "active d i s s o l u t i o n .sit e s " as compared with other surfaces, rather than from a difference i n the reactions occurring. In the present study, etch-pits, presumably representing the points of emergence of d i s l o c a t i o n s , were observed on the basal surfaces of (_^-Fe 203 c r y s t a l s a f t e r p a r t i a l d i s s o l u t i o n i n HCl solutions (Figure 1 7 ) . No etch-pits were observed on any of the other surfaces, possibly because d i s s o l u t i o n was not continued long enough. I t i s assumed that d i s l o c a t i o n s emerging on the basal plane are revealed as etch-pits because of the more rapid d i s s o l u t i o n normal to t h i s plane. However, the p o s s i b i l i t y also 62 exists that t h i s plane dissolves most rapidly precisely,because of the •presence of a greater density of dislocations. Evidence to the contrary i s presented i n Figure 17(a), .in which large areas, apparently free of etch-pits, are evident. Dissolution has ce r t a i n l y occurred on these areas, as evidenced by the step-like layers revealed on the previously smooth-polished surface. These steps presumably correspond to the c r y s t a l planes p a r a l l e l to the.basal plane, rendered v i s i b l e because of a s l i g h t deviation (up to 5°) of the surface from the true basal orientation. However, the fact that rapid dissolution appears to occur even i n the absence of d i s -locations does not preclude the p o s s i b i l i t y that a higher density of dislocations emerging, on the basal plane may contribute to a higher d i s s o l u -tion rate. I t i s suggested that the two effects may to some extent be com-plementary, the dislocations providing points at which the removal of a layer p a r a l l e l . t o (0001) can begin. This hypothesis i s supported by the 3§*5.^.91. results of Sunagawa .who has made extensive studies of the surfaces of naturally grown and etched hematite cr y s t a l s , using phase contrast microscopy. He has concluded that growth occurs mainly by the two-dimensional spreading of layers p a r a l l e l to (0001), and that etching occurs by the reverse of t h i s process, .starting at steps i n the surface, .or at screw dislocations which terminate at the surface, and removing layers p a r a l l e l to the basal plane. The anisotropy of'dissolution of o<-Fe203 observed i n t h i s study can be invoked to suggest a possible explanation.for.the experimental 12 results of Azuma and Kametani , .who obtained -S--shaped rate curves for the dissolution of OC -Fe 20 3 powders i n acids. I f the powder p a r t i c l e s can be considered approximately spherical, then as dissolution proceeds they w i l l tend.to develop "c r y s t a l faces" normal to the direction of most rapid 63 d i s s o l u t i o n . As d i s s o l u t i o n continues on these " f a c e s " , they w i l l grow at the expense of the s l o w e r - d i s s o l v i n g regions of the s u r f a c e , and i n so doing w i l l provide a continuously i n c r e a s i n g area f o r r a p i d d i s s o l u t i o n . This r e s u l t s i n an a c c e l e r a t i n g apparent r a t e of d i s s o l u t i o n , which continues u n t i l the decrease i n the t o t a l a v a i l a b l e surface begins t o outweigh the increase i n r a p i d l y d i s s o l v i n g s u r f a c e , and the apparent r a t e decreases. The net r e s u l t i s an S-shaped d i s s o l u t i o n curve. This mechanism has been 26 suggested by Bergman t o account f o r the S-shaped d i s s o l u t i o n curves obtained i n a study of the d i s s o l u t i o n of f i n e quartz p a r t i c l e s i n HF s o l u t i o n s . Since o£ -Fe^O^ has been shown t o d i s s o l v e a n i s o t r o p i c a l l y , the above i s considered t o be a reasonable explanation f o r Azuma and Kametani's 15 r e s u l t s . I t i s probable that the i n c r e a s i n g r a t e observed by Monhemius i n the i n i t i a l stages of d i s s o l u t i o n of g o e t h i t e ( <=< -FeOOH) r e s u l t s from the same e f f e c t . Goethite has a s t r u c t u r e s i m i l a r t o that of hematite, and might be expected t o d i s s o l v e a n i s o t r o p i c a l l y . In a recent study of the d i s s o l u t i o n of c o v e l l i t e (CuS) i n su l p h u r i c a c i d - f e r r i c sulphate s o l u t i o n s , Thomas and Ingraham observed an i n i t i a l slow d i s s o l u t i o n p e r i o d f o l l o w e d by a l i n e a r r a t e p e r i o d . They a l s o noted that the m a t e r i a l d i s s o l v e d a n i s o t r o p i c a l l y . I t i s suggested that these two observations can be connected on the b a s i s of the above mechanism. In g e n e r a l , f i n e p a r t i c l e s of any m a t e r i a l which has a s t r o n g l y p r e f e r r e d d i r e c t i o n of d i s s o l u t i o n would be expected t o give r a t e curves of t h i s type. However, the p r e c i s e shape of the curves would o b v i o u s l y depend on the degree of ani s o t r o p y , p a r t i c l e size and size d i s t r i b u t i o n , and p a r t i c l e shape. Bergman noted that the .effect in the d i s s o l u t i o n of quartz was most marked w i t h i n a c e r t a i n p a r t i c l e size range, becoming l e s s pronounced as the p a r t i c l e size was increased or decreased. This i s suggested as an explanation f o r the absence of the e f f e c t i n the present s e r i e s of t e s t s on powderedo<-Fe 00 0. 6k The facets developed on single c r y s t a l surfaces oriented close to the basal plane (Figures lk, 15 and 20) appear to correspond to either the (1011) or (1012) planes i n the c r y s t a l . These would appear to be the most stable surfaces i n acid solutions. Further evidence for t h i s i s provided by the fact that of a l l the surfaces examined, the (1011) dissolved at the slowest rate (Table A5). (b) Reaction mechanism The results of the experiments on particulate o<. -Fe 203 indicate that: ( i ) The rate of dissolution i n HCl solutions increases with + + temperature, the observed activation energies being 19.4-1.8 and 21.6-1.0 kcals/mole i n 0.5 and 5-ON solutions respectively. ( i i ) The rate of dissolution increases with acid concentration, the apparent order of the increase depending on whether normality or hydrogen ion a c t i v i t y i s used as the: measure of concentration (see discussion below). ( i i i ) The dissolution rate increases approximately l i n e a r l y with hydrogen ion concentration at constant chloride ion concentration. (iv) The rate increases approximately l i n e a r l y with chloride ion concentration at constant hydrogen ion concentration. The activation energy observed for dissolution i n 0.5 and 5.ON HCl solutions agrees reasonably well with the value 20-2 kcals/mole obtained by 12 Azuma and Kametani for the dissolution of f e r r i c oxide p a r t i c l e s i n various inorganic acids. I t i s also high enough to indicate that the rate i s controlled by a surface reaction, rather than by diffusion i n the aqueous phase. I t compares with values of 13.7 kcals/mole for the dissolution of 8 15 BeO i n HCl , 18 kcals/mole for goethite ( ©<-Fe00H) i n H 2S0 4 , and 6 5 + . .ki 22-1 kcals/mole for c o v e l l i t e i n f e r r i c sulphate-sulphuric acid. A l l of these reactions are considered to be chemically controlled. I t is concluded that d i f f u s i o n i n the aqueous phase plays no rate-controlling role i n the reaction examined i n t h i s study. This i s i n contrast to the work of Furuichi et a l , on the dissolution of aged and calcined f e r r i c oxide p r e c i p i t a t e s , i n which d i f f u s i o n i n the aqueous phase appeared to be an important factor. This difference i s probably due to the r e l a t i v e l y much larger p a r t i c l e size employed i n the present study. The marked dependence of rate on HCl concentration was noted 9 by Pryor and Evans , who suggested that i t was the result of.the high com-plexing a f f i n i t y of the chloride ion for f e r r i c iron. In the present investigation, the rate appears to depend on the 2.5 power of the HCl normality (or molarity) i n the range 0.2-7N. However, i f hydrogen ion a c t i v i t y i s taken as the measure of concentration, i t appears that at lower a c i d i t i e s (0.2-2N), the rate varies with the square of the hydrogen ion a c t i v i t y (or with the product of hydrogen and chloride ion a c t i v i t i e s ) . While at higher concentrations ( 2 - 7 N ) , the rate depends d i r e c t l y on the hydrogen ion a c t i v i t y . The rate data of Azuma and Kametani were replotted as a function of hydrogen ion a c t i v i t y , and a similar dependence was noted. I t should however be noted that there i s some uncertainty i n the a c t i v i t y values used (see Appendix C). The effects on the rate of independently varying the hydrogen and chloride ion concentrations i n mixed HC1-KC1 solutions unfortunately cannot be considered i n terms of the a c t i v i t i e s of these ions. Only very scanty a c t i v i t y data are available for HC1-KC1 solutions at 25°C , and apparently none at a l l at higher temperatures. However some qualitative conclusions may be drawn from the results of the tests i n mixed solutions. 66 Figures 11 and 12 show the effects of varying the hydrogen (chloride) ion concentration at constant chloride (hydrogen) ion concentration. Changes i n both hydrogen and chloride ion concentrations appear to affect the rate. This suggests that chloride as well as hydrogen ions take part i n the reaction. However, i t seems more l i k e l y that, at least at concentrations above 2N, the observed effect of chloride ion (Figure 12) i s due to changes i n the hydrogen ion a c t i v i t y i n the solutions, rather than to a direct dependence of the rate on chloride ion a c t i v i t y . The available data (at 25 QC) on a c t i v i t i e s i n HC1-KC1 solutions indicate that the a c t i v i t y coefficient of HCI (and therefore also of H+) 42 i s increased by the addition of KC1 to concentrated HCI solutions. In addition, the evidence of Figure 10(a), showing rates i n pure HCI solutions, indicates that, at least at concentrations above 2N, the reaction i s f i r s t order i n hydrogen ion. The second-order dependency at lower concentrations (0.2-2N) implies that either two hydrogen ions, or one hydrogen and one chloride ion are involved i n the reaction. Unfortunately, the tests i n mixed HC1-KC1 solutions were not extended into t h i s concentration range, so d e f i n i t e conclusions regarding the roles of H + and C l ions i n this region cannot be drawn. I t i s now w e l l established that oxide surfaces are hydrated to a greater or lesser extent i n aqueous solutions. Several investigations have been made of the adsorption of ions at the f e r r i c oxide-solution interface, and models proposed for the nature of the oxide surface at 4j this interface. Parks and de Bruyn view the process as occurring i n two steps; a surface hydration, followed by dissociation of the surface hydroxide. The hydration step may be visualized as an attempt 67 by the exposed surface atoms to complete their coordination shell of nearest neighbours. Exposed cations accomplish this by pulling an OH ion or water molecule and the oxygen ions by pulling a proton from the aqueous solution, the net result being that the surface is covered by a hydroxyl layer with the cations buried below the surface. The process by which the surface charge is established may be viewed as either an adsorption of H+ or 0H~ ions, or as a dissociation of surface sites which may assume a positive or negative charge. To explain the results of a study of the kinetics of proton kk adsorption at the ferric oxide-solution interface, Onoda and de Bruyn postulated the existence of a goethite-like interphase separating the bulk anhydrous oxide from the solution phase. The composition of this transition region would be expected to vary with the pH of the solution, and i t is not certain that the surface would have the form suggested by Onbda and de Bruyn under the highly acid conditions of the present study, since their experiments were performed in solutions of relatively high pH. However, for present purposes, i t is sufficient to assume simple hydration of the surface as suggested by Parks and de Bruyn, and shown schematically below: Fe j V 0 Fe 0 OH OH OH OH 68 The'' as vidence a v a i l a b l e does not allow a r e a c t i o n mechanism t o be deduced w i t h c e r t a i n t y . However, a t e n t a t i v e mechanism which i s co n s i s t e n t w i t h the observed r e s u l t s i s suggested f o r the d i s s o l u t i o n of f e r r i c oxide i n HCI s o l u t i o n s . For convenience, a s i t e on the oxide surface can be represented by: 0-Fe-OH 1 s In s t r o n g l y a c i d s o l u t i o n s , t h i s i s l i k e l y t o be p a r t l y protonated, and an e q u i l i b r i u m can be w r i t t e n as f o l l o w s : , K x I + 0-Fe-OH+H ^ O-Fe + H 20 [ l ] s ^ 1 s The surface s i t e c o n s t i t u t e s an ion-exchange p o s i t i o n , which may adsorb a c h l o r i d e i o n : O-Fe-Cl [2] + - • K £ O-Fe + C l = 5 = s s The complexed f e r r i c i o n may now desorb from the surface. To account f o r the f a c t that the a c t i v a t i o n energy f o r the d i s s o l u t i o n i s s i m i l a r i n d i f f e r e n t a c i d s , t h i s i s p o s t u l a t e d t o be the r a t e -determining step i n the sequence: — K l ~ s 0-Fe-Cl — F e 0 C 1 a q [ 5 ] The o v e r a l l r e a c t i o n can be represented by. | 0-Fe-OH + H + + C l ~ 6 * - FeOCl n + H 20 [k], s aq and the r a t e equation i s : d ( F e ) KiKskx [ dt sO-Fe-OH] a H + . a ^ - - - [5] where [ |gO-Fe-OH] represents the number of a c t i v e s i t e s on the surface , and a„+ and a the a c t i v i t i e s 'of H+ .and C l ~ ions i n the s o l u t i o n . C l I f the number of s i t e s on the surface i s assumed .constant then thi + rate i s seen t o depend on the H and C l i o n i c a c t i v i t i e s . The above 69 would therefore appear to be a reasonable q u a l i t a t i v e explanation of the mechanism of d i s s o l u t i o n i n the range 0-2N. D i f f e r e n t absolute rates i n various acids can be a t t r i b u t e d to differences i n the value of K 2, i . e . e f f e c t i v e l y , differences i n the complexing a f f i n i t y of the anion for f e r r i c i r o n . Azuma and Kametani have shown a p o s i t i v e c o r r e l a t i o n between the rates i n various acids and the association constants of +++ t h e i r anions f o r Fe . I t i s not suggested that the species FeOCl aq n e c e s s a r i l y e x i s t s as such, but i t i s l i k e l y that an intermediate of t h i s type w i l l precede the formation of the f i n a l product. The l i n e a r dependence of rate on hydrogen ion a c t i v i t y observed at higher HCl concentrations may r e s u l t from the s h i f t i n g of the e q u i l i b r i u m of equation [2] well to the r i g h t . D i s s o l u t i o n may then proceed v i a another protonation of the surface s i t e : Fe-O-Cl + H * a F e C l + + + OH [6] 's (or p o s s i b l y l sFe-0-Cl + H + ^ -E*. | sFe-OH-Cl +) T h i s would be followed by the rate-determining desorption of the Fe complex: ++ k2 • ++ FeCl FeCl [7] s aq The rate equation i s then: d ( F e ) = k 2 K 3 [ | s F e - 0 - C l ] a R+ dt 70 CONCLUSIONS (1) The dissolution of c<-Fe203 in HCI solutions is highly anisotrop the basal surface dissolving at a rate an order of magnitude greater than that of any of the other surfaces examined. The effect is also evident in dissolution in H2SO4, HN03, and HC104. This anisotropy is attributed to a difference in the relative numbers of active dissolution sites available on the various crystal surfaces. This is due ultimately to the highly anisotropic nature of the hematite crystal structure, and possibly also to a greater density of dislocations on the basal plane. (2) The use of pressed polycrystalline specimens for dissolution rate studies on anisotropic materials can give rise to confusing results due to preferential leaching of faviourably oriented grains leading to an increase in available surface area. (3) The dissolution rate is controlled by a surface reaction. (k) The strong dependence of the rate on HCI concentration is due to the large increase in hydrogen ion activity with acid concentration. The rate of dissolution depends on the anion as well as the hydrogen ion concentration in solution. Whether this implies direct participation of the anions in the reaction, or whether it is a secondary effect due to changes in hydrogen ion activity brought about by the addition of chloride ions, has not been conclusively determined. (5) ,A tentative mechanism for the dissolution reaction involves protonation of the hydrated oxide surface, adsorption of a chloride ion, andvthe rate-determining desorption of the ferric-chloride complex., Tl Suggestions for future work (1) The role of the anion i n the dissolution reaction at low acid concentrations is.not yet clear. To some extent t h i s i s due to the choice of KC1 as a source of excess CI ions i n the solutions. A better choice would have been L i C l , since the L i + ion i s closer i n size to the H + ion + than i s the K ion, and the a c t i v i t y c o e f f i c i e n t of HCl i n mixed HCl-LiCl solutions i s l i t t l e affected by the addition of L i C l at a constant L i C l + HCl molality. An investigation of dissolution i n solutions of t h i s type might throw more l i g h t on the reaction mechanism. Extension of t h i s work to other acids would determine whether or not the anion effect i s r e s t r i c t e d to HCl solutions. (2) The dissolution of goethite ( <=< -FeOOH) i n SO^ solutions has 15 recently been investigated. Since i t appears that the i n i t i a l step i n the dissolution of hematite i s the hydration of the surface to give a structure analogous.to that of goethite, i t would be informative to compare the dissolution of hematite i n SO^ solutions. (3) High purity synthetically grown single crystals ofo^-AlgO^ and oC-Cr^O^ (isomorphous with o< -FegO^) are commercially available. Dissolution tests on material of th i s kind would eliminate the uncertainties of the present study on natural c r y s t a l s , and by testing specimens i n which various amounts of deformation had been introduced, i t might be possible to decide whether dislocations contribute s i g n i f i c a n t l y to the enhanced dissolution of the basal plane, or whether t h i s i s due en t i r e l y to the anisotropy of the rhombohedral c r y s t a l structure. 7 2 REFERENCES 1. T. L. Mackay and M. E. Wadsowroth, Trans. 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A., 183 ( 1 9 5 8 ) . 3 6 . B. Schwartz, J . Electrochem. S o c , .114, 285 (I967). 3 7 - R. Glauner and R. Glocker, Z. K r i s t . , 8 0 , 377 ( 1 9 3 1 ) -3 8 . I . Sunagawa, M i n e r a l . J . (Tokyo), 3 ( 2 ) , 59 (I960). 3 9 . • I . Sunagawa,.Am. M i n e r a l o g i s t , .4_J_, 1139 (I962). 40. I . Sunagawa,. Am. M i n e r a l o g i s t , 4_7, 1332 (I962). 4 l . G. Thomas and T. R. Ingraham, Can. Met. Quart., 6 , 153 (I967). 4 2 . J . E. Hawkins, J.-Am. Chem. Soc., 5_4, 4 4 8 0 ( 1 9 3 2 ) . 4 3 . G. A. Parks and P. L. de Bruyn, J . Phys. Chem., 6 6 , 967 ( 1 9 6 2 ) . 4 4 . G. Y. Onoda and P. L. de Bruyn, Surface Science, 4 , .48 (I966). 4 5 . H. S. Harned and R. W. E h l e r s , .J. Am. Chem. S o c , 5 5 , 2179 (1933) 46. G. ?tkerlof and J . W. Teare, J . Am. Chem. S o c , 5 9 , 1855 ( 1 9 3 7 ) . 74 G. Faita and T. -Mussini, J. Chem. Eng. Data,.9, 332-35 (1964). "International Critical Tables", Vol. III,.McGraw H i l l Book Co., Inc., New York, 1928, p. 54. H. S. Gutowsky and A. Saika, J. Chem. Phys.,.21, 1688 (1953). G. C. Hood, 0. Redlich and C. A. Reilly, J. Chem. Phys., 22, 2067 (1954). 75 APPENDICES A, B AND C Note: Inmost of the tests on powder specimens, 0.1 gm of ©< -Fe203 powder was used. However, in some tests, where the expected rate was low, 0.2 gm samples were used. A l l rates given in the following tables have been normalized to apply:to the dissolution of 0.1 gm samples. 76 APPENDIX A TABLE A l Effect of temperature on the rate of dissolution of oC-Fe 2 0 3 powder i n HCl solutions 1 (Figures 5, 6 and 7V. 5.ON HCl (0.1 gm samples) 0.5N HCl (0.2 gm samples) Temp. °C Rate mgms/min. Temp. °C Rate mgms/min. 65 0.050 75 0.056xl0" 2 75 0.130 80 O . O 8 5 X I O " 2 85 0.312 85 0.108xl0~2 95 0.688 90 0.257x10"2 77 APPENDIX A ..TABLE A2 Effect'of HCI concentration on the rate of  dissolution of ck.-FegQ3 powder at 85°C. (Figures 8: , 9 and 10). HCI concentration Normality N hydrogen ion a c t i v i t y a H + Rate mgms/min. 0.2 0.15 0.011x10"2 P.5 0.35 O . l l x l O " 2 *"* 1.0 , 0.72 0.006 0.005 2.0 1.78 0.033 3.0 3.39 0.096 k.o 5.89 0.191 0.179 5.0 10.0 O..312 6.0 16.9 0.556 7.0 27.0 0.783 N< See Appendix C for estimation of a c t i v i t i e s . 0.2 gm. samples. 0 78 . APPENDIX A TABLE A3 . Effect of varying hydrogen ion concentration at constant chloride ion.concentration.on rate of dissolution of oc -Fe 20 3 powder at 85°C • .(Figure 11(a), (b) and (c)). r c i " ] = 5. ON . t c i " ] . = 3. ON [Cl'] .= 2. ON ' [H +] Rate [H +] • Rate [H +] Rate mgms/min. mgms/min. mgms/min. 3 . 9 0.253 2 i 0 0.053 1.0 0.012 4.0 0.257 O.O58 1.5 0.022 4 .25 0.264 2.25 0.056 1.75 •0.025 •O..272 • 0.062 2.0 0.033 4-5 O.285 2,5 0.075 4.75 O.298 2.75 0.081 •5.0 O.312 3.0 O.O96 * 0.2 g ;m sample 79 APPENDIX A TABLE Ak. Effect of chloride ion concentration at  constant hydrogen ion concentration on  rate of dissolution of o<-FepQ3 powder  at 85°C. (Figures j g ( a ) , (b) and ( c ) ) . [H +] = 1.0N [H +] = 2. ON •[H +] = 4. ON [ci-] Rate mgms/min. [ci-] Rate mgms/min. [ci-] Rate mgms/min. 1.0 o. 006 2.0 .0.033 4.0 0.191 1.5 0.008* 2-5 0.042 0.179 2.0 0.012 3-0 0.053 0.058 4.25 0.200 2-5 0.016 3.5 0.065 4.5 . 4.75 0.219 0.236 * 0.2 gm samples 5.0 0.257 5.25 0.270 80 APPENDIX A T A B L E A3 E f f e c t of c r y s t a l o r i e n t a t i o n on the  rate of d i s s o l u t i o n of n a t u r a l hematite  i n 5.ON HCI at 85°C. (Figure 16). C r y s t a l Rate o r i e n t a t i o n .mgm.cm. 2 m i n . ^ . x l O 2 (0001) 2.48 2.61 (1120) 0.33 0..30 (1011) 0.18 0.21 (1010) 0.58 0.55 (2243) 0.34 0.33 0.32 TABLE A6 Effect of temperature on the rate of dissolution  of (0001), (1010) and (22^5) surfaces of natural  hematite i n 5.ON HCI (Figure 18)• Temp.°C Rate (mgm.cm. 2 min." 1 x IP 2)  (0001) (1010) • (22X3) 75 1.0U b.2'8 0.12 , . : 0.1k 85 2.48 C.55 0.32 2.61 O.58 0.33 • 0.34 95 6.15 1.39 0.87 82 APPENDIX A TABLE AT Effect of HCI concentration on the  rate of dissolution of the basal  (0001) surface of natural hematite  at 85°C. (Figure 19). HCI concentration-*- Rate ^ 1 2 normality mgm.cm." min." xlO 3.0 0.56 4.0 1.22 1.70 1.39 5.0 2.48 2.6l 6.0 4.76 APPENDIX, B TABLE B l X-ray d i f f r a c t i o n r e s u l t s f o r various  c?C-Fe 203 specimens used in t h i s study.  Radiation FeKpc , Mn F i l t e r . ASTM Powder Di f f r a c t i o n 'File Baker and Adams Reagent Grade c?<-Fe203 Synthetic Powder Specimen (Sintered 60 hrs. B r a z i l i a n Hematite Crystals P o l y c r y s t a l l i n e Sintered Specimen (Sintered ;12 hrs. at 1200°C) . a t 1100°C) o dA 0 dA dA O dA O dA V l x 3.66 25 3.67 35 3.67 35 3.67 50 3.67 35 2.69 : 100 2.69 100 2.69 95 : 2.70 . 95 2.69 100 50 2.51 70 2.51 .100 2.51 100 2.51 40. 2.201 .30 2.20 30 2.20 50 2.20 30 2.20 19 I.838 40 •1.84 40 1.84 60 1.84 20 1.84 30 I.69O 60 I .69 45 I .69 75 I .69 30 1.69 40 1.596 16 1.60 15 1.60 20 1.60 15 1.60 10 1.484 35 1.48 30 1.48 40 1.48 20 1.48 20 1.452 35 1.45 25 1.45 30 1.45 30 I .45 25 CO Note: Since the d i f f r a c t i o n peaks were r e l a t i v e l y narrow and sharp, t h e i r heights were taken to be an approximate measure of t h e i r i n t e n s i t y , relative to the height of the strongest peak. Sk APPENDIX C Estimation of Hydrogen 'Ion A c t i v i t y i n HCi. Solutions The a c t i v i t y of an electrolyte which ioBi7.ee as •+ + - _ X = n X '+ n X i s defined by a •- a + „a_ where a + and a_ are the ionic a c t i v i t i e s . The mean ionic a c t i v i t y i s defined by I + a n where n = n + n . For HCI, which Ionizes as HCI •>> H + :+ C l " , HCI H - C l " 1/2 and a~ = fa .a ) + H + C l " The mean molality, m , i s defined by 2/n •+ . +n n" m" = mf n . n" ) = in i n the case of a .1 - 1 e l e c t r o l y t e + ± The mean a c t i v i t y c o e f f i c i e n t V = a_ , so that 0 + ID~ + 1/2 m y - = fa + .. a _) H C l If I f we assume a + = a H C l " + then a , = m \> H Several groups of investigators have calculated values of + ~ from measurements of the EMF of the c e l l M2/HC.ifm)/A@Cl - Ag . Harned and Ehlers' determined.activity coefficients i n solutions ranging from Q.QOk in to 4 m between 0 and 60"',C. Akerlof and k6 Teare expended these measurements to the range 3 m to .16 m, between kl \ ± 0 and 50°C. Faita and MuRPini have recently redetermined V i n the range O.OO555 m to 9.25.I m and 0 to 50°C. In order to obtain values of 6 5 the a c t i v i t y c o e f f i c i e n t f o r use i n t h i s study, i t was necessary t o ex t r a p o l a t e the above data t o the experimental temperature (85°C). + For p a r t i c u l a r values of i n the re l e v a n t concentration range was p l o t t e d as a f u n c t i o n of temperature (on l o g - l o g c o o r d i n a t e s ) , and the l i n e s e x t r a p o l a t e d t o 85°C. Values of ^  " determined i n t h i s way are p l o t t e d as a f u n c t i o n of m o l a l i t y i n Figure Cl.. 6 0 5 0 4 0 r 3 0 2 0 10 • Horned ond E h l e r s 4 9 oFoito and M u s s i n i 4 7 • flkerlof ond Tea re 4 6 10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 100 m (HCI MOLALITY) Figure C 1: Mean a c t i v i t y c o e f f i c i e n t of h y d r o c h l o r i c a c i d i n aqueous s o l u t i o n at 85°C', determined by e x t r a p o l a t i o n .of l i t e r a t u r e data i n the range 0 - 60°C . Density data i n the I n t e r n a t i o n a l C r i t i c a l Tables were used t o convert m o l a r i t y values t o m o l a l i t i e s . E a r l y c o n d u c t i v i t y measurements and more recent vapour pressure determinations suggest t h a t d i s s o c i a t i o n i s incomplete i n HCI s o l u t i o n s as d i l u t e as 0.1 molar. However, agreement between values f o r the degree of d i s s o c i a t i o n determined by these two methods i s very poor. A p p l i c a -t i o n of proton magnetic resonance methods' t o the study of concentrated 86 acid solutions has yielded evidence that HCl i s f u l l y dissociated even 49,50 i n 12 molar solutions. In view of the c o n f l i c t i n g evidence, i t was decided to assume for the purposes, of t h i s study that dissociation i s complete i n a l l solutions. Hydrogen ion a c t i v i t i e s were calculated from a H + = m^t where a-^+ = hydrogen ion a c t i v i t y m = molality y ± = ionic a c t i v i t y c o e f f i c i e n t . Table C 1 shows values of m, ^ ± and the calculated values of for the concentration range of t h i s study. TABLE C 1 Hydrogen Ion A c t i v i t i e s i n HCl Solutions at 85°C HCl HCL v ,± aH+ = mv± normality: molality.m 1 I 0.2 0.20 0.75 0.15 0.5 0.51 O.69 0.35 1.0 1.02 O.71 0.72 2.0 2.09 O.85 1.78 3-0 3.20 1.06 3.39 4.0 4.56 1.35 5.89 5.0 5=57 1.80 10.0 6.0 6.84 2.48 16.9 7-0 8:18 3.30 27.0 

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