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Leaching of goethite in acid solutions Surana, Virendra Singh 1969

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LEACHING OF GOETHITE IN ACID SOLUTIONS by VIRENDRA SINGH SURANA .Sc* (Met; Engg.), Baharas H. Uni v e r s i t y , 1960 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, 1969, In presenting this thesis in p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t freely available for reference and Study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my wr i tten permi ss i on. Department of Metallurgy The University of B r i t i s h Columbia Vancouver 8,: Canada Date A p r i l 25, 1969 i ABSTRACT The leaching of goethite i n solutions of p e r c h l o r i c , sulphuric, and hydrochloric acids and i n p e r c h l o r i c acid i n presence of sulphur dioxide has been investigated. Suspensions of goethite powder (-65, +150 mesh) were leached over a wide range of temperatures, acid concentrations and sulphur dioxide pressures. Strong solutions of hydrochloric acid leached goethite powder more r a p i d l y than equivalent solutions of sulphuric a c i d , whilst at low concentrations sulphuric acid was the more active reagent. P e r c h l o r i c acid was found to leach the mineral only very slowly. The rate of d i s s o l u t i o n was found to increase with time i n the early stages of leaching with a l l concentrations of sulphuric acid, with strong solutions of hydrochloric a c i d , and with p e r c h l o r i c acid solutions containing sulphur dioxide. This phenomenon has been correlated with changes i n the surface morphology of the goethite during leaching. The rate data derived from the observed r e s u l t s have enabled the e f f e c t s of temperature, acid type, concentration and p a r t i a l pressure of sulphur dioxide on rate of leaching to be studied. A common mechanism of leaching i s proposed i n which protonation of the hydrated oxide surface i s followed by adsorption of an anion and desorption of the f e r r i c - a n i o n complex. i i ACKNOWLEDGEMENTS The author wishes to express h i s gratitude to Dr. I.H. Warren for h i s continued i n t e r e s t , d i r e c t i o n and encouragement necessary to bring t h i s work to i t s f i n a l form. Thanks are also extended to Dr. E. Peters for many h e l p f u l discussions as to the i n t e r p r e t a t i o n of the r e s u l t s , and to the t e c h n i c a l s t a f f . o f the Department of Metallurgy for t h e i r assistance i n some of the p r a c t i c a l aspects of the work. F i n a n c i a l support from National Research Council of Canada i n the form of a Research Assistantship i s g r a t e f u l l y acknowledged. i i i LIST OF CONTENTS Page INTRODUCTION I. General 1 I I . The F e r r i c Oxide Monohydrates 3 (a) The three a l l o t r o p i c modifications 3 (b) Preparation of the three f e r r i c oxide monohydrates 3 (c) Transformations from y-FeO.OH 4 (d) Structure of goethite 5 I I I . Review of Related Previous Work 6 IV. Scope of the Present Investigation 13 EXPERIMENTAL 14 I. The Mineral 14 (a) Chemical analysis 14 (b) X-ray d i f f r a c t i o n pattern 15 I I . The Reagents 16 I I I . The A n a l y t i c a l Method 16 IV. Autoclave Design 17 V. Experimental Procedure 22 RESULTS 24 I. Leaching of Goethite i n 26 (a) P e r c h l o r i c acid 26 (b) Sulphuric acid 26 (c) Hydrochloric acid 43 I I . Reductive D i s s o l u t i o n i n Acid Solutions 55 I I I . E f f e c t of Varying the Sample Weight 62 IV. E f f e c t of Adding F e + + + at the Start of the Run 62 DISCUSSION AND CONCLUSIONS 67 SUGGESTIONS FOR FURTHER WORK 81 REFERENCES 82 TABLES OF EXPERIMENTAL RESULTS 84 LIST OF TABLES Chemical analysis of the mineral goethite X-ray d i f f r a c t i o n pattern of the mineral goethite E f f e c t of [HCIOA] concentration on rate of leaching of goethite at 110 C E f f e c t of temperature on rate of leaching of goethite i n 1.0M HC10. 4 E f f e c t of [H2SO4] concentration on rate of leaching of goethite at 85°C E f f e c t of temperature on rate of leaching of goethite i n 0.15M H oS0. 2 4 E f f e c t of temperature on rate of leaching of goethite i n 1.8M H oS0. 2 4 E f f e c t of temperature on rate-law constants k' and tor leaching of goethite i n T.8M H 2S0^ E f f e c t of acid concentration and a c t i v i t y of hydrogen ion HC1 solutions on rate of leaching of 85°C E f f e c t of temperature on rate of leaching of goethite i n 0.15M HC1 E f f e c t of temperature on rate of leaching of goethite i n 2.0M HC1 E f f e c t of varying the p e r c h l o r i c a c i d concentration and the p a r t i a l pressure of SO2 on rate of leaching of goethite at 110°C E f f e c t of varying the sample weight and the p a r t i a l pressure of SO2 on rate of leaching i n 0.15M HCIO^ at 110°C A c t i v a t i o n energy of rate determining step i n leaching of goethite i n hydrochloric, sulphuric and p e r c h l o r i c acids Relative increase i n rate per unit of hydrogen ion a c t i v i t y at 85°C and 5% mineral d i s s o l u t i o n i n the three acids V LIST OF FIGURES No. Page 1. The titanium autoclave 18 2. Schematic diagram of glass d i s s o l u t i o n apparatus 21 3. Comparison of leaching of goethite i n the glass and the titanium reaction vessels (2M HC1 at 85°C) 25 4. E f f e c t of temperature on leaching of goethite i n 1.0M HC10 4 27 5. E f f e c t of HC10, concentration on leaching of goethite at 110°C 28 6. E f f e c t of HCIO^ concentration on rate of leaching at 110°C (Table 1) 29 7. Arrhenius p l o t for leaching of goethite i n 1.0M HC10, (Table 2) 30 8. E f f e c t of H2SO, concentration on leaching of goethite at 85°C * 32 9. E f f e c t of H„SO, concentration on rate of leaching at 85°C (Table 3) 33 10. E f f e c t of H„S0^ concentration on Amt. Fe dissolved/time Vs. time pl o t s at 85°C ( c . f . F i g . 9) 35 11. E f f e c t of temperature on leaching of goethite i n 0.15M H 2S0 4 36 12. E f f e c t of temperature on leaching of goethite i n 1.8M H 2S0 4 37 13. Amt. Fe dissolved/time vs. time plots f o r 0.15M IL^SO^ at d i f f e r e n t temperatures ( c . f . F i g . 11) 38 14. Amt. Fe dissolved/time vs. time pl o t s f o r 1.8M R^SO^ at d i f f e r e n t temperatures ( c . f . F i g . 12) 39 15. Arrhenius p l o t f o r leaching of goethite i n 0.15M H„S0, (Table 4) 40 16a. Arrhenius plot f o r leaching of goethite i n 1.8M ^SO^ (Table 5a) 41 16b. Arrhenius plot f o r rate constants k' and /~k" f o r leaching of goethite i n 1.8M H-SO. (Table 5b) 42 2 4 v i L i s t of Figures (cont) No. Page 17. E f f e c t of HC1 concentration oh leaching of goethite at 85°C 44 18. E f f e c t of HC1 concentration on Amt. Fe dissolved/time vs. time plots at 85°C-(c.f. F i g . 17) 45 19a. E f f e c t of HC1 concentration on rate of leaching of goethite at 85°C (Table 6) 46 b. Log-log p l o t between HC1 concentration and rate of leaching of goethite at 85°C 46 20. E f f e c t of hydrogen ion a c t i v i t y , ajj+, on rate of leaching of goethite i n HC1 solutions at 85°C (Table 6) 47 21. Plot showing the leaching rate of goethite i n d i l . HC1 solutions as a function of the product of a ^ and 47 22. E f f e c t of temperature on leaching of goethite i n 0.15M HC1 48 23. Arrhenius p l o t f o r the leaching of goethite i n 0.15M HC1 (Table 7) 49 24. E f f e c t of temperature on leaching of goethite i n 2.0M HC1 51 25. Amt. Fe dissolved/time vs. time plots f o r 2.0M HC1 at d i f f e r e n t temperatures (c.f . F i g . 24) 52 26. Arrhenius p l o t f o r leaching of goethite i n 2.0M HC1 (Table 8) 53 27. Relative s o l u b i l i t y of goethite i n HC1, H 2S0^, and HC104 54 28. E f f e c t of SO2 p a r t i a l pressure on leaching of goethite i n 0.15M HC104 at 110°C 56 29. E f f e c t of SO2 p a r t i a l pressure on Amt. Fe dissolved/time vs. time p l o t s for 0.15M HCIO4 (c.f. F i g . 28) 57 30. E f f e c t of p a r t i a l pressure on leaching of goethite i n 0.50M HC10 4 at 110°C 58 31. Amt. Fe dissolved/time vs. time plots f o r 0.50M HCIO^ at d i f f e r e n t SO2 p a r t i a l pressures ( c . f . F i g . 30) 59 32. E f f e c t of SO2 p a r t i a l pressure on rate of leaching of goethite i n 0.15M and 0.50M HC104 (Table 9) 61 v i i L i s t of Figures (cont) No. Page 33. E f f e c t of SO2 p a r t i a l pressure on leaching of goethite (2 gm. samples) i n 0.15M HCIO^ 63 34. Amt. Fe dissolved/time vs. time pl o t s f or leaching of goethite (2 gm. samples) i n 0.15M HCIO4 at d i f f e r e n t S0 2 p a r t i a l pressures ( c . f . F i g . 33) 64 35. Comparison of rates of leaching of 1 and 2 gm. goethite samples at various S0 2 p a r t i a l pressures (Table 10) 65 36. E f f e c t of addition of F e 1 1 1 ions at the s t a r t of the run 66 37. Electron-mircroprobe photograph of surface of unleached goethite 70 38. Electron-microprobe photograph of surface of goethite a f t e r leaching with p e r c h l o r i c acid 71 39. Electron-microprobe photograph of surface of goethite a f t e r leaching with sulphuric acid 72 40. Electron-microprobe photograph of surface of goethite a f t e r leaching with hydrochloric acid 73 41. Electron-microprobe photograph of surface of goethite a f t e r prolonged leaching with hydrochloric acid 74 1 INTRODUCTION I. GENERAL The removal of i r o n oxides from s i l i c a t e minerals has been reviewed previously''". The presence of i r o n oxide i n s i l i c a and alumino-s i l i c a t e s s e r i o u s l y detracts them from t h e i r commercial value for some important i n d u s t r i a l uses. I t i s w e l l known, for example, that s i l i c a 2 sand f o r high q u a l i t y glass making must not contain more than .008% Fe . Iron oxide also a f f e c t s the usefulness of K a o l i n i t e , A^O^. 23102-2^0 as a paper f i l l e r and coater. The d i s s o l u t i o n of i r o n oxide impurities i s thus important i n the preparation of s i l i c a t e raw materials such as glass sands and various clays. S i l i c a sands f o r glass making have been leached with a v a r i e t y of reagents e.g. hot o x a l i c acid, h y d r o f l u o r i c a c i d , and h y d r o f l u o s i l i c i c 3 acid f or d i s s o l v i n g both hydrated and dehydrated i r o n oxides . Leaching of i r o n oxide from a c i d i f i e d clay suspensions i s 4 achieved by the addition of sodium d i t h i o n i t e , Na2S20^. At room temperatue, the bulk of i r o n oxide present i n hydrate form as goethite or l e p i d o c r o c i t e , FeO.OH,is s o l u b i l i z e d by d i t h i o n i t e i n a few minutes. Dehydrated oxides such as hematite are r e l a t i v e l y unaffected under t h i s condition. In some hydrometallurgical processes, the i r o n oxides i n the ores constitute undesirable impurities rather than valuable ore mineral. In these cases i t may be necessary to design the process so as to minimize the d i s s o l u t i o n of i r o n oxides i n order to simplify or eliminate t h e i r subsequent removal i n the process. This i s possible only when the leaching c h a r a c t e r i s t i c s of i r o n oxides i n various media under d i f f e r e n t environmental conditions are thoroughly known. 2 For years i t has been a common pr a c t i c e to free several i n d u s t r i a l products from impurity i r o n oxides by leaching with an acid or an acid i n combination with a reducing agent. Steel s t r i p s , f or example, are treated with sulphuric acid to remove oxide scale, although more recently the more r a p i d l y acting hydrochloric acid has begun to displace sulphuric 33 acid i n t h i s process. Recently, leaching of i r o n oxide as a process to recover i r o n , rather than as me.r§ly a means of removing iron-containing impurities has been a c t i v e l y investigated. Leaching with hydrochloric acid of a goethite-containing ore with the ultimate objective of producing i r o n powder i s being studied on the p i l o t plant scale^. The a p p l i c a t i o n of leaching processes with sulphur dioxide to the treatment of p y r i t e cinders containing non-ferrous metal values has been described by S h e r r i t t Gordon^. Several fundamental studies of the acid d i s s o l u t i o n of i r o n oxide have been made but many of the observed phenomena do not appear to have been f u l l y explained. The present study was undertaken to determine the leaching c h a r a c t e r i s t i c s of goethite, a-FeO.OH, at d i f f e r e n t temperatures i n various acid media and i n a c i d i f i e d s o l u t i o n of sulphur dioxide, and to e s t a b l i s h a reaction mechanism thereof. 3 I I . THE FERRIC OXIDE MONOHYDRATES ' (a) The Three A l l o t r o p i c Modifications: There are three a l l o t r o p i c modifications of f e r r i c - o x i d e monohydrate: ( i ) a-Fe20^.H20, i d e n t i c a l with the mineral goethite (a-FeO.OH) which gives QL-Fe^Oy or hematite on dehydration; ( i i ) y-Fe^^.^O, which gives yFe or a-Fe 20 3 on dehydration (depending on the condition of heating); and ( i i i ) 3-Fe 20 3.H 20, which gives a-Fe,^^ on dehydration, (b) Preparation of Various F e r r i c Oxide Monohydrates (i) a-Fe 20 3.H 20 The a-monohydrate i s obtained^ by oxidation of ferrous compounds, by slow hydrolysis of most f e r r i c s a l t s and by aging the r e s u l t i n g brown g e l under s u i t a b l e conditions. More highly hydrous preparations were obtained g by Albrecht by oxidation of ferrous bicarbonate s o l u t i o n with hydrogen 9 per oxide, oxygen or a i r at room temperature, and by Baudisch and Albrecht by slow oxidation of ferrous chloride at room temperature. The hydrous material loses ; most of i t s adsorbed water by heating i n a stream of dry a i r at 100°C for 48 hours, a f t e r which the composition i s e s s e n t i a l l y that f o r the monohydrate. By digesting cold p r e c i p i t a t e d hydrous f e r r i c oxide for 2 hours with 2N KOH at 150°C i n an autoclave Bohm^ obtained a l i g h t yellow powder which gave an X-ray d i f f r a c t i o n pattern for a-monohydrate with sharp l i n e s , ( i i ) Y-Fe 20 3.H 20 The y-¥e^p^.\{^) occurs i n nature as the mineral rubinglimmer or l e p i d o c r o c i t e which gives ferromagnetic y - F e ^ ^ on dehydration"'""'", y-monohydrate may be synthesized by the oxidation of fr e s h l y prepared hydrous 4 11 12 Fe^O^ , and hydrous 3Fe20.j.2FeO . The pure y-compound i s formed by oxidation of strong solutions of ferrous chloride i n the presence of pyridine 9 or of sodium azide i n weakly acid s o l u t i o n (pH 2.0 to 6.5) . ( i i i ) g-Fe 20 3.H 20 Aqueous solutions of f e r r i c chloride when allowed to stand at room temperature, or when heated slowly to 60-100°C, deposit a yellow hydrous p r e c i p i t a t e whose X-ray d i f f r a c t i o n pattern d i f f e r s from a - or y-'Fe^O^ or a - or Y-F&2(j2'H-20' I t : f r e c l u e n t l y regarded as f3-Fe20.j.H20. (c) Transformations from y-FeO.OH y- f e r r i c oxide monohydrate (y-FeO.OH) has been found to y i e l d d i f f e r e n t products on heating at 250°C depending on whether the monohydrate was i n a sealed tube (with some water) or was i n an open tube. Af t e r heating, the material i n the closed tube gives x-ray d i f f r a c t i o n l i n e s of paramagnetic a-Fe 20 3. The material i n the open tube i s y - F e 2 0 3 which i f f urther heated to 400°C undergoes an i r r e v e r s i b l e t r a n s i t i o n to a-Fe^O^. In the sealed tube with water, on the contrary, intermediate y-Fe20 3 never appears i n the end product nor at any stage during the transformation. 3-3 However, further work i n t h i s f i e l d has established that i n a sealed tube there i s an intermediate i r r e v e r s i b l e t r a n s i t i o n y-FeO.OH -> a -FeO.OH which f i n a l l y transforms to a -Fe20 3- The necessary condition for y-FeO.OH -»-a -FeO.OH t r a n s i t i o n i s that y-FeO.OH should be wet at the temperature. This condition i s r e a l i z e d i n a closed tube. In short the transformations could be represented as 5 Y-FeO.QU c o I c l -^ .1 I Y Irreversible TrcAnsih'on Wet wa». T)f.g 1 5 4 0 O ° C ^ -FeO.OH c o _c p O o CO CO ol- F e 3 0 3 (d) Structure of goethite The mineral goethite i s a dimorphic form of monohydrate iron 14 oxide and i s usually referred to as ot-FeO.OH. Structurally i t i s based on the arrangement of oxygen atoms i n hexagonal close-packing with Fe atoms i n center i of the resulting octahedral i n t e r s t i c e s . Each Fe atom i s coordinated by s i x oxygen atoms while each 0 atom i s linked to only three Fe atoms; thus to s a t i s f y Paulings rule^"^. In respect to the valence bond of oxygen the presence of a structural proton i n 2-fold coordination i s required. One can regard the c r y s t a l structure as consisting of double s t r i p s of FeCOH)^ i n C a x i a l direction with each double string linked to each other by a hydrogen bond between corner octahedral 0 atoms'^. 6 III REVIEW OF RELATED PREVIOUS WORK Although the present work i s aimed at studying the d i s s o l u t i o n c h a r a c t e r i s t i c s of goethite, previous i n v e s t i g a t i o n s of,the d i s s o l u t i o n of f e r r i c oxide by several workers are also discussed since some of the r e s u l t s have a d i r e c t bearing on the present work. Pryor and Evans have c a r r i e d out extensive studies on d i r e c t 17 and reductive d i s s o l u t i o n of a - f e r r i c oxide . They found that a-Fe^O^ dissolves comparitively r a p i d l y i n h y d r o f l u o r i c or concentrated hydrochloric acid which r e a d i l y form soluble complexes with f e r r i c ions. However, i n d i l u t e solutions of hydrochloric, sulphuric and p e r c h l o r i c a c i d , which, according to the authors, do not e a s i l y form these complexes, the rate of d i s s o l u t i o n f e l l o f f with time and the r e s u l t i n g s o l u t i o n was found to contain ferrous i r o n . This the authors a t t r i b u t e d to p r e f e r e n t i a l attack at surface defects i n v o l v i n g oxygen deficiency. To maintain e l e c t r i c a l n e u t r a l i t y these oxygen defects are associated with the presence of ferrous ions i n the oxide l a t t i c e . The l a t t i c e s t r a i n due to 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 into the s o l u t i o n . The f a l l i n rate with time was a t t r i b u t e d to the depletion of these defects at the surface and showed that the o r i g i n a l d i s s o l u t i o n rate could be restored by heating the p a r t i a l l y leached oxide to 1000°C, which would allow the d i f f u s i o n of the defects to the surface from the bulk of the oxide. 18 Azuma and Kametani have also extensively studied the d i s s o l u t i o n of f e r r i c oxide i n acid solutions and have noted a f a l l i n the irate of d i s s o l u t i o n with time. However, t h e i r reported f a l l i s only under ce r t a i n conditions, namely, low acid concentration . 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. Under these_conditions, according to t h e i r explanation, 7 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 ion i n s o l u t i o n increases. Therefore the weight percent of dissolved oxide would be expected to vary with the s o l i d to so l u t i o n r a t i o at s u b - c r i t i c a l acid concentration. They, i n f a c t , reported that with 0.1N HC1 the f i n a l concentration of i r o n i n the leach s o l u t i o n was the same (53 mg F e / l i t r e ) i r r e s p e c t i v e of whether the weight of the s t a r t i n g sample was 1 gm/litre or 2 gms/litre. However, on the contrary, Pryor and Evans, found that with 0.1N HC1 leach s o l u t i o n the r e s u l t s were almost independent of s o l i d to so l u t i o n r a t i o and i n f a c t within the range 2 gm/litre to 7.5 gm/litre the Fe content of the s o l u t i o n was d i r e c t l y proportional to the o r i g i n a l mass of the oxide taken. Another s t r i k i n g d i f f e r e n c e i n the studies of these two pai r s of workers i s the region i n which they observed the f a l l i n rate with time. Whereas Pryor and Evans observed the f a l l i n rate r i g h t from the beginning, the decrease observed by Azuma and Kametani did not commence u n t i l about 1% of the oxide was dissolved. As a matter of f a c t Azuma and Kametani found a continuous increase i n rate i n the beginning up to 1% i r o n dissolved a f t e r which the f a l l was observed. These are contradictory r e s u l t s and therefore a comparison of these two studies i s d i f f i c u l t but i t appears that the following two d i s t i n c t e f f e c t s can operate to reduce the rate of d i s s o l u t i o n . 1) depletion of e a s i l y soluble defects at the oxide surface, 2) the saturation of the solu t i o n with Fe so that the s o l u b i l i t y product of Fe(OH) i s approached. 8 Depending upon the ph y s i c a l c h a r a c t e r i s t i c s of the sample and • the experimental conditions one or both of these e f f e c t s may be observed. I t should, nevertheless, be noted that Pryor and Evans were in v e s t i g a t i n g the leaching c h a r a c t e r i s t i c s of f e r r i c oxide powder that had been subjected to a heat treatment which caused some of the f e r r i c i r o n to be converted to ferrous i r o n . And apparently they were d i s s o l v i n g mostly th i s ferrous i r o n i n t h e i r leaching experiments. Further, the f a l l i n rate with time was observed when less than 1% of the weight of the oxide sample was dissolved. 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 to be the same i n d i l u t e (0.01-.1N) solutions of hydrochloric, sulphuric and p e r c h l o r i c acids of equivalent concentrations. They also observed that the addition of .01 to . 5N KC1 to hydrochloric acid leach s o l u t i o n did not increase the rate of d i s s o l u t i o n . This suggests that the important step i n the reaction i s the combination of adsorbed hydrogen ions with oxygen ions belonging to the oxide l a t t i c e : -0 ~~ + H +, = 0H~ s ads. or 0"~ + 2H + = H o0 s 2 The removal of one oxygen ion, as a hydroxyl ion, or as water does not involve any production or consumption of electrons. Consequently th i s reaction cannot create defects i n the form of ions of abnormal charge. Thus the rate of d i s s o l u t i o n i s controlled by the surface defects o r i g i n a l l y present i n the oxide surface. I f the oxygen ions, situated at the surface move into the s o l u t i o n , as hydroxyl ions (or as water) unaccompanied by i r o n , the surface of the oxide w i l l remain p o s i t i v e l y charged. This charge can be dissip a t e d only by i r o n ions passing into s o l u t i o n , since the temperature 9 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 the oxide surrounding the surface defects i s followed by a slower attack, on the more perfect parts of the c r y s t a l l a t t i c e . D i s s o l u t i o n of powdered samples by t h i s mechanism i s c a l l e d the " d i r e c t " d i s s o l u t i o n . At higher acid concentrations, according to Pryor and Evans, the rate of d i s s o l u t i o n of a-Fe20.j i n various acids decreased i n the order of HF > HCl y H 2 S 0 4 >^ H C 1 0 4 This order was i n accord with the order of decrease of the complexing a f f i n i t y of the anion for f e r r i c ion. Azuma and Kametani also found that the rate of d i s s o l u t i o n depended on the type of a c i d , even at low concentration. Besides d i r e c t d i s s o l u t i o n , Pryor and Evans described another 19 process, namely, "reductive d i s s o l u t i o n " meaning the process occurring when a f e r r i c oxide surface i s continuously reduced to the ferrous state permitting more rapid d i s s o l u t i o n . In order that the reductive d i s s o l u t i o n could occur, the oxide powder was i n contact with a conducting medium. Pryor and Evans, i n t h e i r experiments to e s t a b l i s h the mechanism of reductive d i s s o l u t i o n , 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 i r o n the anode. The leaching s o l u t i o n served as the e l e c t r o l y t e . They observed that both the rate and current e f f i c i e n c y of reductive d i s s o l u t i o n of a - f e r r i c oxide powder diminish with an increase i n the temperature of previous i g n i t i o n , with an increase i n the time of reduction, and with an increase i n the pH of the e l e c t r o l y t e . They concluded that at pH values below 2 the rate 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 conductivity of the oxide and hence by the number of surface defects. 10 Pryor proposed that the mechanism of reductive d i s s o l u t i o n of f e r r i c oxide involves the combination of hydrogen atoms discharged at the oxide surface, with oxygen ions belonging to the oxide l a t t i c e to form eit h e r hydroxyl ions or water molecules which pass into the bulk of the s o l u t i o n . This reaction l i b e r a t e s electrons which reduce an equivalent number of f e r r i c ions to the ferrous condition, thus creating a series of "metal-excess" type of defects i n the oxide surface which w i l l then become p a r t i c u l a r l y susceptible to d i r e c t acid attack. 21 Because of i t s extensive nature, the work of Azuma and Kametani warrants further discussion. They studied the d i s s o l u t i o n of f e r r i c oxide prepared by several d i f f e r e n t methods i n various inorganic acids. On the basis of the shapes of the curves on a double-log p l o t , they distinguished two types of d i s s o l u t i o n s , namely accelerated type and parabolic type. The "accelerated" type d i s s o l u t i o n consists of three stages:-(a) Stage I represents an i n i t i a l slow reaction during which the amount of the dissolved oxide i s approximately proportional to the cube root of time. This stage l a s t s up to the d i s s o l u t i o n of about 0.5% of the t o t a l oxide. (b) Stage II i s the accelerated region which goes up to about 30% d i s s o l u t i o n . The slope of the l i n e a r portion of Stage II varies from 1 to 2. (c) Stage III corresponds to the decay region where the d i s s o l u t i o n approaches completion. They did not observe the f i r s t stage when the rate of d i s s o l u t i o n was very f a s t at high acid concentration and elevated temperatures. In the "parabolic" type, d i s s o l u t i o n was found to proceed i n four stages: (a) Stages I and II are s i m i l a r to those of the "accelerated" process. 11 (b) Stage III l i e s between Stage II and the f i n a l s tage, where the amount of oxide d i sso lved i s approximately p ropor t iona l to the square root of the time — and hence the name "pa r abo l i c " type. (c) As the Stage III region terminates the curve becomes s im i l a r to that of the " a cce l e ra ted " process. whether a powder d i sso lves according to the " pa r abo l i c " type or the acce lerated type appeared to depend on themethod of preparat ion and hence on the cha r a c t e r i s t i c s of the p a r t i c l e s themselves. Most of the experiments ca r r i ed out by Azuma and Kametani were on powder specimens which d isso lved , i according to the " a cce l e ra ted " type and the rest of the d i scuss ion re fe rs to th i s type of d i s s o l u t i o n only . One very i n t e r e s t i ng feature of the i r study i s the fac t that the shapes of the curves are qu i te s im i l a r to each other i n sp i t e of a large d i f f e rence of rates between the runs due to d i f f e rence i n a c i d s , concentrat ions and temperatures. This i s true fo r powders prepared In a s p e c i f i c way. D i f ferences i n methods of preparat ion tend to change the shape of the curve. This i s only l o g i c a l because d i f f e r e n t methods of preparat ion give p a r t i c l e s of d i f f e r e n t phys i ca l cha r a c t e r i s t i c s and surface area which would na tu ra l l y have a very s i g n i f i c a n t e f f ec t on the o v e r a l l d i s s o l u t i o n c h a r a c t e r i s t i c s . Azuma and Kametani a lso found that the mechanism of d i s s o l u t i o n i s independent of the type of ac id and i t s concentrat ion, s ince ac t i v a t i on energies i n the range 2 0 - 2 Kcal/mole were obtained for a l l acids over a wide range of concentrat ions. They have a lso reported the existence of a c r i t i c a l concentrat ion, depending upon the type of a c i d , below which the d i s so lu t i on d id not go to completion no matter how long the durat ion of the runs was. Below the c r i t i c a l 12 concentrations, the shapes of the curves no longer matched each other and the amount of dissolved oxide increased with time to approach a f i n i t e value which was much less than that f o r complete d i s s o l u t i o n . The approximate value f o r the c r i t i c a l concentration i s 0.2N for HC1 and H„SO., and 0.8 N for 2 4 phosphoric acid. 22 Monhemius i n a study of the d i s s o l u t i o n of natural goethite, a-FeOOH, 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 rate c o n t r o l l i n g desorption of hydroxy-ferric complex. He also observed an i n i t i a l a ccelerating region i n the d i s s o l u t i o n plots for both sulphuric acid and a c i d i f i e d solutions of sulphur dioxide s i m i l a r to that obtained by Azuma and Kametani. I t was suggested that t h i s may be due to some c h a r a c t e r i s t i c s of the mineral i t s e l f although no evidence was put forward to that e f f e c t . He also proposed a dual mechanism to explain the reductive d i s s o l u t i o n of goethite suggesting that the rate determining step may be homogeneously co n t r o l l e d at low SO2 concentration while heterogeneously c o n t r o l l e d at high S0„ concentrations i n so l u t i o n . 13 IV. SCOPE OF THE PRESENT INVESTIGATION A considerable amount of good, work has been done on studying the d i s s o l u t i o n k i n e t i c s of hematite i n various media, although many of the r e s u l t s reported by d i f f e r e n t workers are quite contradictory to each other. However, very l i t t l e work has so f a r been done to investigate the leaching c h a r a c t e r i s t i c s of goethite. Previous i n v e s t i g a t o r s have reported the existence of a dual mechanism for the d i s s o l u t i o n of goethite namely a homogeneous control of the reaction rate at low acid concentration and a heterogeneous control at high acid concentrations. At the same time they have also indicated that t h e i r r e s u l t s of experiments performed under i d e n t i c a l conditions sometimes varied as much as 30% ; the reason of the s c a t t e r being a poorly designed apparatus. I t was therefore thought necessary to use an apparatus of improved design which can provide a better r e p r o d u c i b i l i t y of the r e s u l t s and v a r i f y whether such a dual mechanism does e x i s t . Further, the previous work has been mostly concerned with the leaching of goethite i n sulphuric acid and i n the a c i d i f i e d solutions of sulphur dioxide. I t was intended i n t h i s study to check these r e s u l t s and to extend the work using p e r c h l o r i c and hydrochloric acids. 14 EXPERIMENTAL 1 • THE MINERAL (a) Chemical Analysis The powdered sample used i n this study was prepared from massive nat u r a l goethite obtained from Ward's Natural Science Establishment Inc., New York. The mineral was crushed into small pieces with a sledge hammer and then ground down to the required s i z e using a hand muller. The p a r t i c l e s were siz e d by wet screening to remove adhering:-fines. Gravimetric, analysis (as reported by Coast Eldridge's P r o f e s s i o n a l Services D i v i s i o n i n Vancouver) showed that the mineral contained 58.62% Fe i n d i c a t i n g an FeO. OH content of 93.16%. The semiquantitative spectographic analysis indicated the presence of s i l i c o n as the major impurity followed by magnesium, manganese, and titanium, and also traces of molybdenum, lead, copper and strontium (Table A). None of these impurities would be expected to a f f e c t the r e s u l t s of the experiment. Table A Chemical Analysis of the Mineral Goethite Weight % Fe 58.62 S i 5.0 Mg 0.5 Mn 0.1 T i 0.1 Mo 0.01 Pb 0.01 Cu 0.005 Sr 0.005 15 (h) X-ray D i f f r a c t i o n Pattern An x-ray d i f f r a c t i o n pattern of the ore sample was obtained and the 29 and the r e l a t i v e intensity values compared with the reported 29 and re l a t i v e intensity values of a-FeO.OH as given i n the ASTM card (Table B). Although the r e l a t i v e i n t e n s i t i e s showed some va r i a t i o n , there was an excellent agreement on the 29 values of the peaks. Table B X-ray D i f f r a c t i o n Pattern of the Mineral Goethite (Using k Cu radiations) Reported This Study o dA 29 (degrees) I / I , • & K ; 29 (degrees) I/I 4.97 17.8 60 4.97 17.8 69 4.18 21.2 100 4.17 21.3 100 3.36 26.5 60 3.37 26.4 73 2.69 33.3 70 2.69 33.3 83 2.58 34.7 55 2.58 34.7 73 2.48 36.2 40 2.48 36.2 68 2.44 36.8 80 2.44 36.8 74 2.18 41.4 60 2.18 41.3 75 1.72 53.4 70 1.71 53.4 72 1 16 I I . THE REAGENTS A l l chemicals used i n t h i s study were reagent grade. Deionised..i water was used f o r a l l solutions. The nitogen used was ordinary cylinder grade supplied by Canadian L i q u i d A i r Ltd., and the SO^ (anhydrous quality) was obtained from Matheson of Canada Ltd., and used without further p u r i f i c a t i o n . I I I . THE ANALYTICAL METHOD The progress of d i s s o l u t i o n was followed by spectrophotometry 23 determination of the i r o n content of samples taken at regular i n t e r v a l s throughout the experiment. Ferrous i r o n and 1-10 orthophenanthroline form an orange-red 35 complex having a strong absorption peak at a wavelength of 51C>mM Absorption i n solutions of th i s complex obeys Beer's Law and can be used for the quantitative determination of ferrous i r o n . Since most of the ir o n was expected to be i n the t r i v a l e n t s t a t e , hydroxylamine hydrochloride was added to each of the samples to reduce a l l i r o n to the ferrous state. Sample solutions were buffered at pH 4.5 with a sodium acetate a c e t i c acid buffer since the colored complex has been shown to be most stable i n the pH range 2-9. 3 5 Procedure A composite reagent was made up containing 20% by volume of orthophenanthroline s o l u t i o n , 20% by volume of a hydroxylamine hydrochloride sol u t i o n and 60% by volume of acetate buffer. Suitable aliquots of sample s o l u t i o n were pipetted into 100 ml volumetric f l a s k s and the volume made up with the composite reagent. The o p t i c a l density of each sol u t i o n was measured on a Beckman Model B Spectrophotometer using l i g h t of 510my wavelength. 17 The concentration of i r o n was read from a plo t c a l i b r a t e d by using standard i r o n solutions prepared from 99.9% pure i r o n wire. IV. AUTOCLAVE DESIGN A 2000 ml capacity, s e r i e s 4500 pressure reactor (Fig. 1) manufactured by Parr Instrument Company, I l l i n o i s , USA, was used for most of the leaching experiments. O r i g i n a l l y the reactor was purchased with the head, cy l i n d e r and the Inner wetted parts a l l made of titanium whereas the external valves and f i t t i n g s made of s t a i n l e s s s t e e l . This reactor could be sa f e l y and conveniently used f o r tests i n v o l v i n g l i q u i d s and gases under pressure up to 1000 psig and temperature up to 350°C. A f l a t Teflon gasket seals the head to the cy l i n d e r . The head i s clamped to the cy l i n d e r with a s t e e l r i n g that i s s p l i t i n t o two halves which s l i d e into place without disturbing any of the valves or attachments on top of the head. The s t e e l band which holds the s p l i t r i n g i n place i s rais e d i n t o p o s i t i o n from the bottom of the reactor. The closure i s sealed by tightening s i x compression screws with a hand-wrench. Vigorous mixing and rapid gas dispersion are attained i n the reactor by an i n t e r n a l s t i r r i n g shaft f i t t e d with two p r o p e l l e r s . The flow pattern of t h i s s t i r r e r e f f e c t i v e l y prevents the accummulatiori of so l i d s i n the bottom of the ve s s e l . A Teflon guide-bearing s t a b i l i z e s the shaft and a un i v e r s a l coupling at the top compensates f o r any misalignment. The titanium shaft which drives the s t i r r e r i s sealed by precut Rulon cones held i n a removable packing cup. This i s a s e l f s ealing gland i n which the force applied to a set of three 45-degree cones comes from within the autoclave i t s e l f . The s t i r r i n g shaft i s driven by a 1/15 h.p. var i a b l e speed b a l l bearing motor which can run at any speed between 0 and 5000 rpm. 18 STIRRER SHAFT PULLEY THERMISTOR WELL STIRRER SHAFT GUIDE BEARING PYREX GLASS LINER F i g . 1. The t i t a n i u m autoclave. 19 A 1500 watt e l e c t r i c heater f o r t h i s apparatus i s b u i l t into ' an insulated s t a i n l e s s s t e e l s h e l l . The reactor s l i d e s into the heater and rests on the aluminium top plate. Automatic temperature control was provided by a Thermistemp Temperature Co n t r o l l e r Model 71. (Yellow Springs Instrument Co. Inc., Yellow Springs, Ohio) which i s actuated by an i r o n -constantan thermistor probe which s l i d e s into the thermistor well i n the reactor. Using t h i s system the temperature was con t r o l l e d within ± 1°C. The pressure gauge used had a 4 1/2" d i a l graduated from 0 to 100 p s i i n 1 l b . subdivisions. This was connected to the gas e x i t tube on the pressure side of the release valve. The gas connection to the reactor i s made with s t a i n l e s s s t e e l tubing.*.".. The sample tube ins i d e the reactor was made of titanium. The i n t e r n a l pressure r a i s e s the reactant through t h i s tube and discharges i t from the sampling valve . During the leaching experiments i t was found that strong solutions of sulphuric acid v i o l e n t l y attack the titanium surface of the autoclave y i e l d i n g titanium ions which strongly catalyse the leaching reaction. This •, of course, was undesirable. In order to prevent a d i r e c t contact between the leach l i q u o r and the i n t e r i o r of the titanium reactor, the l a t t e r was spray-coated with a 5 m i l . thick Teflon coating. This worked quite s a t i s f a c t o r i l y i n p l a i n acids but the presence of S0^ i n the reactor presented a new problem. At high pressures, the SO^ gas found i t s way through the micro-pores of the Teflon coating and forced i t o f f the reactor wall. Therefore, as an a l t e r n a t i v e , a t i g h t f i t t i n g Pyrex glass l i n e r to cover the i n t e r i o r of the autoclave and a s t e e l - r e i n f o r c e d Teflon s t i r r e r were used to r e s i s t SO^ attack on thin Teflon coatings. 20 The leaching experiments i n the aforementioned reactor seemed to go s a t i s f a c t o r i l y for cpmnaratiyely/ low concentration ^SO^ solutions. However, with strong sulphuric acid solutions i t was feared that the bare-i n t e r i o r of the titanium sampling tube may get attacked and thus influence the d i s s o l u t i o n rates. In order to avoid such an occurrence a l l tests involving strong ^ ^ ^ 4 a n c* a^-so strong HC1 were performed i n an a l l - g l a s s reaction vessel which was e a r l i e r designed and used i n this department. The salient features of the apparatus are shown i n figure 2. The glass reaction f l a s k i n which the dissolution experiments were carried out was maintained at a constant temperature i n a water thermostat. The one l i t r e capacity f l a t bottom conical f l a s k was f i t t e d with a f r i t t e d glass f i l t e r . The leaching solution and the mineral sample i n the fla s k were agitated by a Teflon covered magnetic bar turned by a magnetic s t i r r e r positioned below the thermostat vessel. The thermostat water was heated by a 100 watt immersion heater connected to the main?1 supply through a Variac. The temperature was controlled by a mercury-in-glass contact thermometer connected to the heater c i r c u i t through a mercury relay. To maintain a uniform temperature, the water bath was continuously stirred, by a variable speed s t i r r e r . This system provided a temperature control within - 0.2°C. Throughout the run, the reaction vessel was open to the atmosphere through a reflux condenser. However, while taking a sample during the run, a nitrogen pressure of about 10 p s i was applied i n the fla s k to force the solution out through the sample tube. 21 COOLING WATER I NLET NITROGEN INLET IMM ERSION HEAT ER SAMPLING TUBE FRITTED GLASS FILTER REFLUX CONDENSER STIRRER MOTOR CONTAC T THERMOMETER REACTION FLASK SPIN BAR MAGNE TIC STIRRER F i g . 2. Schematic diagram of gl a s s d i s s o l u t i o n apparatus. 22 V. EXPERIMENTAL PROCEDURE The use of the glass apparatus and the titanium autoclave for d i s s o l u t i o n experiments involved s l i g h t l y d i f f e r e n t procedures. They w i l l be discussed separately. The Glass Apparatus The experimental procedure consisted of the following steps: 1) The thermostat was f i l l e d with water, the immersion heater turned on and the temperature c o n t r o l l e r set at the required temperature. 2) A 0.2 gm sample of the mineral and 600 ml of leaching s o l u t i o n of the required concentration together with the magnetic spin bar were put into the r e a c t i o n f l a s k . 3) When the thermostat temperature reaches the required value, the reaction f l a s k was lowered i n the thermostat, various connectiony made, and the spin bar set into motion by the magnetic s t i r r e r underneath. 4) The system was allowed to come to a thermal equilibrium. 5) The samples were taken at i n t e r v a l s of 15, 30, 60 or 120 minutes -depending on the expected rate of d i s s o l u t i o n - by applying a pressure of 5 to 10 p s i of nitrogen to the f l a s k to force the leaching s o l u t i o n out through the f r i t t e d glass f i l t e r at the lower end of the sample tube. The f i r s t 10 ml. of the sample s o l u t i o n were immediately returned to the reaction f l a s k through the funnel and a second sample of 10 ml. taken. This technique ensured that the sample was not contaminated by traces of the previous s o l u t i o n remaining i n the sample tube. Samples were c o l l e c t e d i n a sample b o t t l e which was immediately stoppered to prevent vapour loss during cooling. 6) A f t e r cooling, 2 to 5 ml. of the sample s o l u t i o n , depending on the i r o n content expected, was pipetted into a 100 ml. volumetric f l a s k , and the small excess returned to the reaction f l a s k . 23 Usually the runs were continued long enough to dissolve about 3 to 5% of the starting material. The samples were analyzed for iron as described i n the e a r l i e r section. The Titanium Autoclave 1) A one gram sample of the mineral and 1200 ml. of the leaching solution of desired strength were put into the Pyrex glass l i n e r i n the autoclave body. 2) The autoclave was sealed by clamping the head on the autoclave body by the s i x compression screws. 3) The assembly was then gently lowered i n the furnace and various connections made. 4) The s t i r r i n g started and the heating turned on. The solution was heated under a nitrogen atmosphere. 5) When the solution attained the required "temperature, the nitrogen supply was turned off and sulphur dioxide introduced at the required pressure. For runs carried out i n absence of SO2, a nitrogen atmosphere was maintained throughout the run. 6) The solution was allowed to s t a b i l i z e at the required temperature and sulphur dioxide pressure for about ten minutes and then the f i r s t sample was taken. The concentration of iron i n this sample was used as the blank for p l o t t i n g the iron dissolved vs. time plot. 7) Samples for iron determination were taken at regular intervals throughout the run u n t i l about 8-10% of Fe was i n solution. 24 RESULTS Most of the r e s u l t s i n this section are presented only as graphs and p l o t s , the numerical r e s u l t s being grouped together i n separate tables at the end of the thesis. Before going further, i t i s pertinent at t h i s stage to c l a r i f y three points: 1) A l l d i s s o l u t i o n experiments i n the glass apparatus were ca r r i e d out using a 0.2 gm. mineral sample with 600 ml. of the leaching s o l u t i o n . On the other hand a 1 gm. sample and 1200 ml. of the leaching s o l u t i o n were used i n a l l experiments (except for those aimed at checking the e f f e c t of the sample-weight i t s e l f ) c a r r i e d out i n the titanium autoclave. However, for the sake of uniformity and i n order that the r e l a t i v e rates of d i s s o l u t i o n could be compared a l l the r e s u l t s have been normalized on the basis of one gm. sample. 2) While doing a run, the f i r s t sample of the leaching s o l u t i o n was taken out only a f t e r the system has reached an equilibrium with respect to the desired temperature, and SO^ pressure (where a p p l i c a b l e ) . This sample was taken as the "blank" to determine the amount of i r o n dissolved i n subsequent 30, 60 minutes or longer i n t e r v a l s . 3) In order to check that the runs c a r r i e d out i n glass apparatus and the titanium autoclave could be compared independently, or i n other words the reaction v e s s e l i n i t s e l f does not play any part i n the leaching process, two runs were c a r r i e d out - one i n the glass apparatus and the other i n the titanium autoclave - under exactly i d e n t i c a l conditions of sample weight, acid strength and temperature. The d i s s o l u t i o n plots are shown i n Figure 3. It i s c l e a r that the d i s s o l u t i o n proceeded i n exactly s i m i l a r fashion i n both the vessels. Figure 3: Comparison of leaching of goethite i n glass and titanium reaction vessels (2MHC1 at 85°C). 26 I. LEACHING OF GOETHITE IN (a) P e r c h l o r i c Acid E f f e c t s of acid concentration and temperature on leaching of a-Fe0.0H powder (-65, + 150 mesh) i n p e r c h l o r i c acid were investigated. The p l o t s of amount of ir o n dissolved versus time at d i f f e r e n t temperatures (Fig. 4) and at d i f f e r e n t acid strengths (Fig. 5) were e s s e n t i a l l y s t r a i g h t l i n e s . However, one run i n 2M HCIO^ at 90°C c a r r i e d out for 48 hours gave a s l i g h t l y curved p l o t as shown i n F i g . 27. The rates were i n general a l l very slow. As shown i n Figure 5, (Table 1), the rate increases with increasing concentration of p e r c h l o r i c acid. The rates of d i s s o l u t i o n are pl o t t e d against the p e r c h l o r i c acid concentration (which i s equivalent to the a c t i v i t y of hydrogen ion) i n F i g . 6. I t may be seen that the rate increases approximately l i n e a r l y with the concentration of p e r c h l o r i c acid or a c t i v i t y of hydrogen ion for the range 0.15 M to 1.5 M. Di s s o l u t i o n p l o t s i n 1.0 M p e r c h l o r i c acid at various temperatures are shown i n Figure 4. The rate i s seen to increase with temperature. The r e s u l t s of these tests are presented i n Table 2. Figure 7 i s the Arrhenius p l o t f o r leaching i n 1.0M HCIO^. The slope of t h i s l i n e corresponds to an a c t i v a t i o n energy of 17.8 * 2.9 Kcals/mole. (b) Sulphuric Acid As i n the case of p e r c h l o r i c acid, e f f e c t s of acid strength and temperature on d i s s o l u t i o n of goethite i n sulphuric acid were studied. At any temperature, the rate of d i s s o l u t i o n i n sulphuric acid was found to be about 15 to 20 times as f a s t as i n p e r c h l o r i c acid of equivalent concentration. Further, the plo t s between Fe dissolved vs. time f or sulphuric acid were not l i n e a r (Fig. 8, 11, and 12) even f or short d i s s o l u t i o n periods as they were F i g . 4:, E f f e c t of temperature on leaching of goethite i n 1.0 M HC10,. 4 F i g . 6: E f f e c t of HC10, concentration on rate of leaching of goethite at 110 (Table 1 ) . 30 F i g . 7: A r r h e n i u s p l o t f o r l e a c h i n g o f g o e t h i t e i n 1.0M HC10. ( T a b l e 2). 31 for p e r c h l o r i c acid d i s s o l u t i o n . Instead they have a tendency to curve up as the d i s s o l u t i o n proceeds i n d i c a t i n g an apparent increase i n rate of leaching of goethite with time. A l l these and other curved plots obtained i n t h i s study could be described by the equation of the type Amt. Fe dissolved = k't + k " t 2 (1) where t i s the time of leaching, and k' and k" are constants for each p l o t . The values of k' and k" may depend on the type of a c i d , i t s concentration, and the temperature of leaching. Equation (1) could be written as A m t ^ F e = k' + k" t (2) So that i f Amt. Fe/t i s plotted against t the r e s u l t would be a s t r a i g h t l i n e having a slope = k", and with an intercept on y-axis = k'. I t i s therefore easy to c a l c u l a t e g r a p h i c a l l y the values of k' and k" for each p l o t . By d i f f e r e n t i a t i n g equation (1) i t i s possible to get the rate equation of the d i s s o l u t i o n of i r o n as d[Amt. F e j a q = fe, + dt By i n s e r t i n g the values of k' and k" obtained graphically and the value of t corresponding to a f i x e d amount of i r o n dissolved i n eq. (3) i t was possible to c a l c u l a t e the rate of leaching for each p l o t . Rate data obtained i n t h i s manner were used to construct Figure 9 (Table 3) which gives the e f f e c t of varying the acid concentration on rate of d i s s o l u t i o n of goethite i n ^SO^ at 85°C. The a c t i v i t y of hydrogen ion at each concentration has been assumed to be equal to the molar concentration of the acid. I t may be seen that the rate increases approximately l i n e a r l y with the hydrogen ion a c t i v i t y except for a small deviation at low concentrations. 33 0.50 0.40 0.30 Rate (mgms Fe/min.) 0.20 0.10 0.00 • • • / / • • 0 1 2 3 4 5 H SO, cone. [ M ] F i g . 9: E f f e c t of H SO, concentration on rate of leaching at 85°C. (Table 3). 34 The Amt. Fe/time versus time plots f o r various ^SO^ concentrations at 85°C are shown i n F i g . 10. The r e s u l t s for d i s s o l u t i o n i n 0.15 M I^SO^ at various temperatures are shown i n F i g . 11 (Table 4). The e f f e c t of temperature on d i s s o l u t i o n i n 1.8 M ^SO^ was also investigated. The r e s u l t s of these tests are shown i n Table 5 and i n F i g . 12. The plots can be seen to be curving up (Fig. 11 and 12) and the Amt. Fe/t vs. t p l o t s for these curves resulted i n good s t r a i g h t l i n e s as shown i n F i g . 13 and 14 r e s p e c t i v e l y , confirming that the leaching p l o t s are following the r e l a t i o n s h i p 2 Amt. Fe (in solution) = k't + k"t Figures 15 and 16(a) (Tables 4 and 5a) are Arrhenius pl o t s for d i s s o l u t i o n i n 0.15M and 1.8M ^SO^ r e s p e c t i v e l y . A c t i v a t i o n energies determined from the slopes of these l i n e s are 18.1 -1.3 and 19.8 - 1.6 KCalories/mole r e s p e c t i v e l y . I t should be noted here that each a c t i v a t i o n energy value reported here i s obtained by p l o t t i n g log Rate vs 1/T°K (for a fixed amount of i r o n dissolved, at d i f f e r e n t temperatures) and i s an average of the a c t i v a t i o n energies of the two possibly d i f f e r e n t mechanisms represented by rate constants k' and k". In f a c t separate a c t i v a t i o n energy values were calculated by p l o t t i n g log k' vs. 1/T°K and log v^ k" vs. 1/T°K for each acid concentration to obtain a c t i v a t i o n energies corresponding to k' and k". However, as could be seen i n F i g . 16b (Table 5b) the two a c t i v a t i o n energy values are e s s e n t i a l l y i d e n t i c a l i n d i c a t i n g that there i s perhaps only one mechanism of d i s s o l u t i o n but the morphology changes necessitated the use of two terms i n the-rate-law equation. Therefore, for the reason of s i m p l i c i t y , hereonwards log rate vs. 1/T°K have been plotted to obtain the average a c t i v a t i o n energy values. Amt. Fe/t mgms Fe/min.) 0 60 120 180 240 300 360 420 Leaching Time t (min.) Fig. 10: E f f e c t of H2SO4 concentration on Amt. Fe dissolved/time vs. time plots at 85°C. (c.f. F i g . 9). Fe dissolved (mgms) 0 0 ON Leaching Time (minutes) F i g . 11: E f f e c t of temperature on leaching of goethite i n 0.15M H SO F i g . 13: Amt. Fe dissolved/time vs. time plots for 0.15M H.SO, at d i f f e r e n t temperatures (c.f. F i g . 11). F i g . 14: Amt. Fe dissolved/time vs. time pl o t s for 1.8M H^SO^ at d i f f e r e n t temperatures ( c . f . F i g . 12.) 40 -0.8. -2-0 I | 1 L _ 2.55 2.65 2.75 1/T x 10 3 F i g . 15: Arrhenius p l o t f or leaching of goethite i n 0.15M H„S0, (Table 41 42 2.70 2.80 2.90 1/T°K x 10 3 F i g . 16(b). Arrhenius p l o t f or rate constants k' and /k" for leaching of goethite i n 1.8M H SO. (Table 5b). 43 (c) Hydrochloric Acid Effect of acid concentration on the rate of dissolution at 85°C i n the range IM to 4M HCl i s shown i n Figures 17 and 19(a) (Table 6). As i n the case of sulphuric acid di s s o l u t i o n , the curved plots between Amt. Fe dissolved vs. Time have been found to follow the relationship: Amt. Fe = k't + k " t 2 [See the lin e a r Amt./t vs. t plots i n Figure 18]. As i s clear i n Figures 17 and 19(a) the rate increases rapidly with the acid concentration. A log-^log plot [Fig. 19b] of this data gave a straight l i n e with a slope of 2.2 over the entire range of experimental condition indicating an apparent 2. 2.power dependency of rate on HCl concentration. However, when the rate i s plotted against a c t i v i t y , a ^ , as i n Fig. 20, the rate appears to depend on the product of hydrogen and chloride ion a c t i v i t i e s i n the lower concentration range (Fig. 21) and d i r e c t l y on the hydrogen ion a c t i v i t y i n the higher concentration range (Fig. 20). The effect of temperature on leaching of goethite i n 0.15M and 2.0M HCl was also investigated. The results are shown i n figures 22 and 24 respectively. The rates of leaching i n 0.15M HCl are i n general very slow and the Amt. of Fe dissolves vs. Time plots were nearly straight lines (Fig. 22). However }it i s possible that i f the runs were continued for very long periods to enable a substantial amount of Fe to go i n solution, the plots would be found to be s l i g h t l y curved. Figure 23 (Table 7) gives the Arrhenius plot for dissolution i n 0.15M HCl. The activation energy as determined from the slope of this l i n e i s 19.0 - 9.4 KCals/mole. This rather large scatter i n the rate values and hence the activation energy may be due to the d i f f i c u l t y of attaining even a f a i r degree of accuracy i n Fe dissolved (mgms) 0 30 60 90 120 150 180 Leaching Time (min.) F i g . 17: E f f e c t of HC1 concentration on leaching of goethite at 85°C. Amt. Fe/t (mgms Fe/min.)-^ 0 30 60 90 120 150 180 ^ Time t (min.) Fig. 18: E f f e c t of HCl concentration on Amt. Fe dissolved/time vs. time plots at 85°C ( c . f . F i g . 17). 46 Fig. 19(b): Log-log plot between HC1 cone, and rate of leaching of goethite at 85°c. 47 4.0 3.0 2.0 Rate (mgm Fe/min.) 1.0 U 0.0 aH-:+ 1.0 Fig. 20: Effect of hydrogen ion a c t i v i t y , a on rate of leaching of goethite i n HC1 solutions at 85°C. (Table 6). 0.5 Rate (mgm Fe/min.) 0.0 I l i I 0. 0 1.0 2.0 3.0 4.0 aH+ a C l ' Fig. 21: Plot showing the leaching rate of goethite i n d i l HC1 solutions as a function of the product of a. and a c l-. H+ O - 120°C 0 60 180 300 420 Leaching Time (min.) F i g . 22: E f f e c t of temperature on leaching of goethite i n 0.15M HCl. F i g . 23: Arrhenius p l o t for the leaching of goethite i n i n 0.15M HCl (Table 7). 50 estimating Fe at such low concentrations. The d i s s o l u t i o n i n 2.0M HC1, on the other hand, i s quite rapid and the Amount Fe dissolves vs. Time plots are appreciably curved (Fig. 24). Here again, as i n case of other curved p l o t s , the d i s s o l u t i o n was found to proceed according to the r e l a t i o n s h i p ? Amt. Fe i n s o l u t i o n = k't + k"t [See Figure 25] An Arrhenius p l o t f o r these r e s u l t s corresponding to 25 mg. Fe dissolved i s given i n F i g . 26 (Table 8). The slope of t h i s l i n e corresponds to an a c t i v a t i o n energy of 22.5 - 2.2 KCals/mole. The r e s u l t s of the experiments, c a r r i e d out on leaching of goethite i n p e r c h l o r i c , sulphuric and hydrochloric acids thus could be summarised as follows (a) Strong solutions of hydrochloric acid leach goethite more r a p i d l y than equivalent solutions of sulphuric a c i d , whilst at low concentrations sulphuric acid i s the more active reagent. P e r c h l o r i c acid leaches the mineral only very slowly. F i g . 27 shows the comparative amounts of i r o n leached under approximately equivalent conditions. (b) The general shape of the plots of i r o n leached against time may be d i f f e r e n t f o r d i f f e r e n t acids and may even change with acid concentration. For example even during prolonged leaching i n 2M HCIO^ the plot does not show the phenomenon of increasing rate of leaching with time exhibited by hydrochloric and sulphuric acids of the same concentration (Fig. 2 7 ) . (c) Leaching with d i l u t e hydrochloric acid (0.15M) appears to resemble that with p e r c h l o r i c a c i d , but i n d i l u t e and strong sulphuric acid and i n strong hydrochloric a c i d , the rate of leaching apparently increases with time. F i g . 24: E f f e c t of temperature on leaching of geothite i n 2.0 M HCl. 53 F i g . 26:. Arrhenius p l o t f o r l e a c h i n g of g o e t h i t e i n 2M HCl (Table 8). 54 50 40 3 0 Fe dissolved (mgms.) 20 10 • - 2 M HC1, 90°C O - 2 M H 2 S 0 4 , 85°C ^ - 2M HC10., 90°C 4 15 30 45 60 L_ 0 60 120 180 240 - J 300 O 720 1440 2160 2880 ^ Leaching Time (min.) F i g . 27: Relative s o l u b i l i t y of goethite i n HC1, f^SO^, and HCIO^ 55 II REDUCTIVE DISSOLUTION IN ACID SOLUTIONS The e f f e c t s of the following v a r i a b l e s on rate of d i s s o l u t i o n of goethite i n aqueous s o l u t i o n of sulphur dioxide a c i d i f i e d with p e r c h l o r i c acid were investigated:. 1. Strength of acid 2. SO^ p a r t i a l pressure A l l the runs were c a r r i e d out at 110°C for the following reasons: I) The sulphur dioxide could only be introduced to the system at pressures above the vapour pressure of water at that temperature. As the maximum sulphur dioxide pressure a v a i l a b l e from the c y l i n d e r at room temperature i s 36 p sig i t was desirable to keep the vapour pressure of water, and hence the temperature, as low as p o s s i b l e , i n order that a large range of SO^ pressures i s a v a i l a b l e for the experiments. i i ) At temperatures much below 110°C, there was not enough Fe dissolved to enable an accurate measurement of i r o n i n s o l u t i o n . i i i ) The decomposition of goethite (a-FeO.OH) to hematite (a-Fe20 3) i n 36 37 aqueous solutions i s v a r i o u s l y reported as occurring between 120-150°C. ' ' Thus i t was thought preferable to work at temperatures atwhdch goethite would be thermally s t a b l e . Two s e r i e s of experiments were c a r r i e d out at two d i f f e r e n t p e r c h l o r i c acid concentrations, namely 0.15M and 0.50M, at d i f f e r e n t p a r t i a l pressures of SO^. The r e s u l t s as plots of Amount of i r o n i n s o l u t i o n vs. Time of leaching for various sulphur dioxide p a r t i a l pressures are given i n Figures 28 and 30. As observed e a r l i e r i n the case of cone. HCl and H^SO^ d i s s o l u t i o n s , the p l o t s are a l l curved upwards with varying degrees of curvatures, but they a l l f a l l on good s t r a i g h t l i n e s (Fig. 29, 31) 56 F i g . 28: E f f e c t of SO p a r t i a l pressure on leaching of goethite i n 0.15 M HCIO. at 110°C. 0.30 F i g . 29: E f f e c t of S0 2 p a r t i a l pressure on Amt. Fe dissolved/time vs. time plots for 0.15M HC10, (c.f. F i g . 28) . 58 F i g . 30: E f f e c t of SO p a r t i a l pressure on leaching of goethite i n 0.50 M HC10. at 110°C. F i g . 31: Amt. Fe dissolved/time vs. time plots for 0.50MHC10£ at d i f f e r e n t SO p a r t i a l pressures (c.f. F i g . 30). 60 when Amount Fe dissolved/t i s plotted against t for each curve. This indicates that here also the dissolu t i o n follows the same general pattern as described by equation (1). The rate values (at 5 mg Fe i n solution) derived from Figures 28, 29, 30, and 31 are plotted against the p a r t i a l pressure of SO^ for each of the two a c i d i t i e s as shown i n Figure 32. (Table 9). The plots are f a i r l y good straight l i n e s indicating a linea r relationship between the rate of dis s o l u t i o n and S0„ p a r t i a l pressure at both a c i d i t i e s . 61 F i g . 32: E f f e c t of SO2 p a r t i a l pressure on rate of leaching of goethite i n 0.15M and 0.50 M HC10, (Table 9). 62 I I I . EFFECT OF VARYING THE SAMPLE WEIGHT For a l l the runs described i n the preceding section, one gram samples were used. However, to check the e f f e c t of sample weight, or, i n other words, to a s c e r t a i n the homogeneous or heterogeneous nature of the rate determining step of the d i s s o l u t i o n reaction, a few runs were repeated using 2 gm sample i n 0.15 M HCIO^ - everything else remaining the same (Fig. 33). The Amt. Fe dissolved/time vs Time plo t s f or the 2 gm sample are shown i n Figure 34. The general shapes of the plots (Fig. 33) are s i m i l a r to those obtained f o r 1 gm. sample i n 0.15 M HCIO^ at various p a r t i a l pressures of SO^ (Fig. 28). The rate values (for 10 mg Fe i n solution) derived from Figures 33 and 34 are plotted against SO^ p a r t i a l pressure i n F i g . 35. (Table 10). The corresponding rate values for a 1 gm sample are also p l o t t e d i n t h i s f i g u r e for the sake of comparison. As could be e a s i l y seen, f o r each SO2 pressure, the rate of d i s s o l u t i o n for 2 gm. sample i s twice as much as for 1 gm. sample under i d e n t i c a l experimental conditions. This indicates that the rate c o n t r o l l i n g step of the d i s s o l u t i o n reaction i s heterogeneous. IV. EFFECT OF ADDING Fe"1"1"1" AT THE START OF THE RUN To confirm that the increase i n rate with time as observed for most of the experiments i s not due to some a u t o c a t a l y t i c e f f e c t of f e r r i c i r o n already dissolved, two runs were c a r r i e d out under exactly i d e n t i c a l conditions of sample weight, temperature, and strength of acid. However, i n one run f e r r i c c hloride was added to the s o l u t i o n i n i t i a l l y , whereas there was no pre-addition i n the other-run. The r e s u l t s of the experiments are p l o t t e d i n f i g u r e 36. 75 mg of i r o n as FeCl^^H^O was added at the s t a r t of one of the runs. I t may be seen that the two p l o t s are exactly 63 F i g . 33: E f f e c t of SO p a r t i a l pressure on leaching of goethite (2 gm samples) i n 0.15 M HC10 . F i g . 34: Amt. Fe dissolved/time vs. time p l o t s f o r leaching of goethite (2 gm samples) i n 0.15 M HC10 at d i f f e r e n t SO p a r t i a l pressures ( c . f . F i g . 33.) 65 Fig. 35: Comparison of rates of leaching of 1' and 2 gm. g o e t h i t e samples a t various SO. partial pressures (Table 10). 66 i d e n t i c a l i n nature and would n i c e l y f i t one over the other i f 75 mg Fe value i s added to one or subtracted from the other. This means that addition of f e r r i c ions to the s o l u t i o n has no e f f e c t on the rate of leaching i n d i c a t i n g thereby the absence of any a u t o c a t a l y t i c e f f e c t . DISCUSSION AND CONCLUSIONS As seen c l e a r l y from the curved nature of the plots between Amt. Fe dissolved vs. Time of d i s s o l u t i o n , the rate of leaching of goethite i s increasing under most of the leaching conditions, (except for leaching i n p e r c h l o r i c acid and i n d i l u t e hydrochloric acid for which leaching i t s e l f i s very slow). A s i m i l a r increase i n rate of leaching of goethite was also 22 reported by Warren and Monhemius , at l e a s t u n t i l about 4% of the t o t a l Fe was dissolved. Azuma and Kametani also observed such an increase i n 18 rate i n the leaching of s t r u c t u r a l l y s i m i l a r hematite. This increasing rate of leaching may be ascribed to the difference i n chemical r e a c t i v i t y on d i f f e r e n t c r y s t a l planes which i n turn may be due to the differences i n atomic spacings or packing between d i f f e r e n t c r y s t a l planes, and on the ease of adsorption of reacting ions or molecules. Difference i n chemical r e a c t i v i t y of d i f f e r e n t planes may r e s u l t i n rapid leaching of some planes i n preference to others. This preferred leaching of c e r t a i n surfaces causes an increase i n area with time and therefore an increasing rate of leaching. The anisotropy of surface r e a c t i v i t y has been observed for , 25 ., , 26,27 . . . , 28 _,, c c metals , covalent c r y s t a l s , and i o n i c c r y s t a l s . The e f f e c t i n metals i s not very marked, the r e a c t i v i t y of the various surfaces d i f f e r i n g 25 at the most by a factor of three . The e f f e c t appears to be more pronounced i n c r y s t a l s of low symmetry. Solution anisotropy appears to be more 68 marked i n non-metallic materials. For example d i s s o l u t i o n of basal plane of quartz proceeds at a rate two order of magnitude greater than that of 29 any other plane while the basal planes of a-A^O^ (corrundum) have been 28 shown to d i s s o l v e 40 times as r a p i d l y as the prism plane The increase i n rate of d i s s o l u t i o n with time observed i n t h i s study i s opposite to the decrease i n rate reported by Pryor and Evans while leaching a-Fe^O^. This s t r i k i n g d i f f e r e n c e i n the leaching c h a r a c t e r i s t i c s may be due to the d i f f e r e n t pre-treatments given to the minerals used i n these two studies. Whereas Pryor and Evans subjected t h e i r mineral to a heat-treatment converting some of the f e r r i c i r o n to ferrous i r o n , no such treatment was given to the goethite mineral used i n this study. Further, Pryor and Evans were i n v e s t i g a t i n g the d i s s o l u t i o n of t h i s ferrous i r o n from the a - F e r r i c oxide powder within the range up to 1% Fe dissolved, while t h i s study was mainly concerned with the leaching of f e r r i c i r o n i n the goethite up to about 10% Fe dissolved. Strongly anisotropic leaching has been observed 24 recently with hematite and because of the s t r u c t u r a l s i m i l a r i t y i t seems l i k e l y that goethite would show the same e f f e c t . To account for the r e s u l t s obtained on leaching of goethite i t appears to be necessary to postulate a leaching mechanism involving both uniform d i s s o l u t i o n of the mineral surface and under c e r t a i n conditions p i t t i n g , f a c e t t i n g or a n i s o t r o p i c d i s s o l u t i o n . The changing mode of leaching i n d i f f e r e n t acids i s c l e a r l y i l l u s t r a t e d by a series of electron-microprobe photographs of a mounted goethite sample taken before and a f t e r leaching. F i g . 37 shows the polished surface of the sample before leaching at (600) magnification. Leaching i n 2M p e r c h l o r i c a c i d , at 85°C and for nearly 50 hours, E; r o c ee d£ di b y a 69 slow and approximately uniform attack a l l over the mineral surface as shown i n F i g . 38. F i g . 39 shows the e f f e c t of leaching i n 2M H^O^ at 85°C for nearly 6 hours. The surface has c l e a r l y roughened r e s u l t i n g i n an increased surface area as the leaching continues. An increasing rate of leaching with time i s therefore not s u r p r i s i n g . F i g . 40 i s the electron-microprobe photograph of the goethite surface which was leached i n 2M HC1 at 99i°C for 1 hour. The surface roughening i s even more pronounced as compared with leaching i n equivalent strength t^SO^. Figures 38, 39, 40 and 41 represent the mineral surface from which approximately equivalent amounts of Fe were dissolved i n various acids. F i g . 41 depicts the mineral surface a f t e r prolonged leaching i n HC1. Extensive p i t t i n g can be seen to have developed. The increasing rate of leaching with time i s therefore more pronounced i n the case of HC1 as compared to ^SO^ or HCIO^. The r e s u l t s of the d i r e c t d i s s o l u t i o n with p e r c h l o r i c acid ind i c a t e that: 1) the rate of leaching increases with temperature, the observed a c t i v a t i o n energy being 17.8 - 2.9 KCals/mole, 2) the rate of leaching increases with acid concentration and i s , i n f a c t , d i r e c t l y proportional to the a c t i v i t y of hydrogen ion, and 3) there i s an approximately uniform attack of the mineral surface. In sulphuric acid, the rate of leaching 1) increases with temperature with an average a c t i v a t i o n energy of 19.9- 1.6 KCals/mole, 2) i s , apart from a small deviation at low acid concentration, d i r e c t l y proportional to the a c t i v i t y of hydrogen ion, and 70 F i g . 37: Electron-microprobe photograph of surface of unleached g o e t h i t e . (X600). 71 F i g . 38. Electron-microprobe photograph of surface of g o e t h i t e a f t e r leaching w i t h p e r c h l o r i c a c i d . (X600) F i g . 39: Electron-microprobe photograph of surface of g o e t h i t e a f t e r l e a c h i n g w i t h s u l p h u r i c a c i d . (X600) 73 Fig. 40: Electron-microprobe photograph of surface of goethite after leaching with hydrochloric acid. (X600) 74 F i g . 41: Electron-microprobe photograph of surface of g o e t h i t e a f t e r prolonged l e a c h i n g w i t h h y d r o c h l o r i c a c i d . (X300) 75 3) involves possibly a multiple - uniform, and p i t t i n g or anisotropic -attack at a l l concentrations (low as wel l as high). In hydrochloric acid the rate of leaching: 1) increases with temperature with an average activation energy of 22.5 - 2.2 KCals/mole, 2) i s , apart from a small deviation at low acid concentration, d i r e c t l y proportional to the a c t i v i t y of hydrogen ion, and 3) the leaching involves a uniform attack at low concentrations and a multiple-uniform, and p i t t i n g or anisotropic - attack at high concentrations. The activation energies a l l f a l l i n the range 17.8 to 22.5 KCals/mole and the mean activation energy of 20 * 2.5 KCals/mole i s , approximately i n -dependent of the concentration of acid and the kind of acid. This i s i n excellent agreement with the average activation energy of 20 - 2 KCals/mole 18 obtained by Azuma and Kametani for the dissolution of powdered f e r r i c -oxide i n various inorganic acids. These values are high enough to indicate that the rate i s controlled by a surface reaction rather than by d i f f u s i o n i n the aqueous phase. This also compares with the values of 20 - 2 KCals/mole for d i s s o l u t i o n of hematite i n H C l ^ and of 18 KCals/mole previously observed 22 for goethite i n H^O^. The fact that the reaction i s surface controlled, or, i n other words, heterogeneous i n nature i s further confirmed by comparing the results obtained for 1 gm. and 2 gm. samples (Fig. 35) i n otherwise exactly i d e n t i c a l conditions. I t i s thus concluded that neither d i f f u s i o n nor a chemical reaction i n the aqueous phase plays a rate-controlling role i n the reaction examined i n this study. 76 The approximate f i r s t order dependence of rate of leaching on hydrogen ion a c t i v i t y observed i n this study was also noted by Azuma 18 22 and Kametani during leaching of haematite with various acids .. Bath confirmed the f i r s t order dependence at high acid concentrations but found a deviation at low concentrations s i m i l a r to that described here for goethite. A simple approximation of the rate of leaching i n any of the three acids therefore i s given by [ a ^ « a c t i v i t y of hydrogen ion] However,in this equation K must d i f f e r for each acid (Table 11). The order of the increase of leaching rate i n different acids i s the same as that of the increase of the complexing power of the anions of the acid for f e r r i c ion as observed by Pryor and Evans^ and l a t e r established by 18 Azuma and Kametani for hematite leaching. The anion, i n addition to influencing the value of K, also changes the morphology or mode of leaching; and, since over a broad range of acid concentration the rate of leaching i s apparently dependent only on a ^ , i t i s possible that only a certain maximum quantity of each anion i s required to s p e c i f i c a l l y " a c t i v a t e " the surface of the oxide. This has been experimentally observed i n case of hydrochloric acid leaching i n that the rate i s dependent on (the product of) hydrogen and chloride ion a c t i v i t y at low concentration of the acid (Fig. 21). However, when there are enough anions (chloride ions, i n this case) present so that a l l the available s i t e s have been activated, the rate then depends only on the a c t i v i t y of hydrogen ion (Fig. 20), and therefore a linear relationship i s observed between rate and hydrogen ion a c t i v i t y at higher concentration of HCl. 77 The evidence available does not allow a reaction mechanism to be deduced with certainty. However, a tentative mechanism which i s consistent with these observations i s suggested for the leaching of goethite i n hydrochloric, perchloric, and sulphuric acids and i n a c i d i f i e d solutions of sulphur dioxide. 31 As postulated by Onoda and de Bruyn goethite i s formed by the hydration of f e r r i c oxide i n aqueous solutions and the exact composition of the surface would be expected to vary with the pH of the solution. However, for the present i t i s s u f f i c i e n t to assume the 32 surface structure of the type as suggested by Parks and de Bruyn and as shown below: Fe Fe Thus, a s i t e on the oxide surface could be represented by Jg 0-Fe-OH. The f i r s t step i n the dissolution of goethite i n acid solutions has been considered to be protonation and the equilibrium could be represented by 0-Fe + + 2H20 (5) 0-Fe-OH + H o0 + K l s 3 78 The surface site constitutes an ion exchange position which may adsorb an anion such as a chloride ion, C l " 0-Fe+ + C l " ^ r - ^ - £ . I O-Fe-Cl (6) In solutions of low acid concentration, the complex fe r r i c ion on the surface may now desorb O-Fe-Cl k3, » FeOCl (7) s aq v ' To account for the fact that the activation energy for the dissolution i s similar in different acids, step (7) i s postulated to be the rate determining one. The over a l l reaction can be represented by 0-Fe-OH + H + + C l " + H.O • ^ FeOCl + 2H„0 (8) is l aq 2 and the rate equation i s —IT" ' K i K 2 k i 1 L°"Fe"0Hl V" a C l " ( 9 ) If the number of sites on the mineral surface i s constant then the rate of leaching depends on the activity of H + and Cl in solution as observed 24 by Bath i n the leaching of hematite with dilute solutions of hydrochloric acid. The same relationship has also been varified by the linear plot obtained between rate of dissolution vs. a ^ S Q J - a s s n o w n i n F i8« 21.for the dissolution of goethite in dilute HC1 in the present study. This would therefore appear to be a reasonable qualitative explanation of the mechanism of dissolution in HC1 in the low concentration range. It i s possible to explain qualitatively the small deviation from linearity observed in the low concentration region of the rate vs. concentration plot (Fig. 9) for the dissolution of goethite in sulphuric acid. At very low concentration, solutions of sulphuric acid w i l l contain appreciable quantities of SO^- ions. SO^- anions are known to complex very strongly with f e r r i c ions [Stability constant K = F e S 04+ ~ 165 at 25°C] [Fe+++][S04=] 39 79 and are readily adsorbed by the protonated surface of goethite resulting in a high rate of leaching. However, at higher concentrations, the sulphuric acid behaves as a monobasic acid dissociating into H + and HSO^ ions. HSO, anions have a relatively poor complexing af f i n i t y with f e r r i c ++ ions [Stability constant K • C F e H S 04 1- ^ 6 at 25°C] 3 9 and thus the [Fe+^+HHSOTJ rate of leaching is low. Therefore, as the sulphuric acid concentration goes up, the concentration of relatively poorly complexing species HSO^ also goes up and the rate of leaching gradually f a l l s and f i n a l l y attains a steady relationship with H + concentration of the solution. Different overall rates in various acids can be attributed to the differences in the values of equilibrium constant i.e. effectively, differences in the complexing power of the anion for f e r r i c ion. The linear dependence of the rate on hydrogen ion activity observed at higher HCl concentrations may be due to the shifting of equilibrium of equation (6) well to the right with the surface sites becoming saturated with anions. Dissolution may then proceed by a further protonation of the activated surface This would be followed by the rate-determining desorption of the f e r r i c complex OH-Fe-Cl+ £2_»» FeOHCl+ (11) Is aq where k 2 is the reaction rate constant. The rate equation in strong acid solution is then: d[Fel dt ^ — = k 2K 3 [ |sO-Fe-Cl] a ^ (12) 80 The rate of leaching now depends on K^, which may depend on the "activation" of the surface by each anion, and this i n turn could be related to the complexing power of anions for the f e r r i c ion. From Fig. 20 i t can be seen that a l l the active surface sit e s may be saturated by about 1 mole/litre of chloride ions provided by 1 mole/litre of HCl after which the rate increases l i n e a r l y with a^ + . Confirmation of the saturating adsorption effect with an anion was obtained by leaching goethite i n a solution containing one mole/litre of hydrochloric acid and one mole/litre of perchloric acid. The observed rate of leaching was approximately equivalent to that observed i n a 2M solution of hydrochloric acid. The reductive dissolution with sulphur dioxide may proceed by a s i m i l a r adsorption mechanism involving bisulphite ion. This i s quite reasonable to assume because the rate of dissolution has been found (Fig. 32) to be dependent on both a c t i v i t y of hydrogen ion and a c t i v i t y of bisulphite ions i n the solution as would be predicted by eq. (9). 81 SUGGESTIONS FOR FURTHER WORK 1] D i f f e r e n t anions and t h e i r r e l a t i v e complexing power have a very s i g n i f i c a n t e f f e c t on the rate of leaching of goethite i n acid s o l u t i o n s . I t i s considered l i k e l y , that the surface s i t e s of the mineral become saturated with anions as the anion concentration i n the s o l u t i o n increases. Once a l l the active s i t e s have been saturated the leaching rate no more depends on the anion concentration. Further work should be c a r r i e d out to determine these saturation concentrations f o r various anions. 2] Hydrofluoric a c i d i s l i k e l y to d i s s o l v e goethite at a very f a s t rate as the f l u o r i d e ions are known to complex very strongly with f e r r i c ions. Detailed work should be persued to study the c h a r a c t e r i s t i c s of leaching of goethite i n h y d r o f l u o r i c acid. 3] As observed throughout the present i n v e s t i g a t i o n , the o v e r a l l rate of leaching i s strongly affected by the changes i n the surface morphology of the mineral. A c l o s e r look at the mineral by the help of electron-microprobe might reveal some i n t e r e s t i n g f a c t s as to how the surface s i t e becomes to be activated before i r o n i s released into s o l u t i o n , and throw some l i g h t on the actual meachanism of the removal of the surface layers as the leaching proceeds. 82 REFERENCES 1. Warren, I.H. "Unit Processes i n Hydrometallurgy", p. 300, Gordon and Breach Science Publishers, Inc., New York, N.Y., 1964. 2. Johnstone, S.J., "Minerals f o r Chemical and A l l i e d Industries", p. 436, Chapman and H a l l , London (1954). 3. Gutierrez, L.G., and Gazulla, O.R.F., Anales r e a l Soc. espan. f i s . y. guim 687-98 (1951). 4. Gruber, H., U.S. 1,036,831, Aug. 27, 1912. 5. Gravenor, CP., et a l , C.I.M. B u l l . 57, 421 (1964). 6. Kunda, W., et a l , Conference of M e t a l l u r g i s t s , Kingston (1967). 7. Weiser, H.B. "Inorganic C o l l o i d Chemistry", Vol. I I , p. 39 Wiley (1939). 8. Ber., 62B 1476 (1929). 9. Baudisch, 0. and Albrecht, W.H. , J . Am. Chem. Society 54, 943, (1932). 10. Bohm, Z., Anorg. Chem.149, 212 (1925); c . f . , also, Krause, 174, 154; 176, 398 (1928). 11. Sosman and Posnjak, J . Wash. Acad S c i . ; 15, 332 (1925); Herroun and wilson, Proc. Phys. Soc. (London), 41, 106, (1928). 12. Welo, L.A., and Baudisch, 0., P h i l . Mag. (6) 50, 400 (1925); J . Bio. Chem., 61, 269 (1924). 13. Welo, L.A. and Baudisch, 0., P h i l . Mag. (7) 17, 753 (1934). 14. Jurinak, J . J . , Journal of C o l l i o d Science, 19, 477-487 (1964). 15. Pauling, L., The Nature of the Chemical Bond" 3rd ed., p. 548, C o r n e l l University Press, New York, 1960. 16. Francombe, M.H., and Rooksby, H.P., Clay Minerals B u l l . 4, 1, (1959). 17. Pryor, M.J. and Evans, U.R., J . Chem. Soc. 3330 (1949). 18. Azuma, K., and Kametani, H., Trans. AIME, 230, 853-862 (1964). 19. Pryor, M.J. and Evans, U.R. , J . Chem. S o c , 1266 (1950). 20. Pryor M.J., J . Chem. S o c , 1274 (1950). 21. Azuma, K. and Kametani, H., Trans. AIME, 242 1025, (1968). 83 22. Warren, I.H., and Monhemius, A.J., Conference of M e t a l l u r g i s t s , Toronto (1966). 23. Vogel, A.I., Text Book of Quantitative A n a l y s i s , Longmans, Green and Co., London. 24. Bath, M.D., M.A.Sc. Thesis, Dept. of Metallurgy, Un i v e r s i t y of B r i t i s h Columbia (1968). 25. Yamamoto, M., S c i . Rept., Research Inst., Tohoku Uni v e r s i t y , Ser. A., 10, 183 (1958). 26. Gatos, H.C., Science, 137, 311 (1962). 27. Schwartz, B. , and J . Electrochem. S o c , 114, 285 (1967). 28. Dawihl, W., and Klinger, E., Ber. deut. Keram. Ges., 44, 1 (1967). 29. Kennedy, G.C., Econ. Geol., 45, 629 (1950). 30. Thomas, G., and Ingraham, T.R., Can. Met. Quart., 6, 153 (1967). 31. Onoda, G.Y., and de Bruyn, P.L., Surface Science 4, 48 (1966). 32. Parks, G.A., and de Bruyn, P.L., J . Phys. Chem., 66, 967 (1962). 33. Perkins, E., and Mobley, L.S., Iron and Steel Engineer, A p r i l , 156 (1965). 34. Bragg, W.L., and Bragg, W.H., "X-rays and C r y s t a l Structure", G. B e l l and Sons , London (1924). 35. Fortune, W.B., Mellon, M.G., Ind. Eng. Chem., Aanal. Ed., 10, 60 (1938). 36. Schmalz, R.F., J . Geophys. Res., 64, No. 5, 575 (1959). 37. Smith, F.G., and Kidd, D.J. , Am. Mineralogist, 34, 403, (1949). 38. Gruner, J.W., (1949) Econ. Geol. 26 442 (1931). 39. L i s t e r , M.W., and Rivington, D.E., Can. J . Chem. 33, 1591 (1955). 0 84 Table 1 E f f e c t of [HCIO^] concen t r a t i on on r a t e of l e ach ing of g o e t h i t e at 110°C (F igures 5, 6) [HCIO^] concen t r a t i on Rate ( M o l a r i t y , M) ( m g m F e / m l n ) 0.15 0.0045 0.50 0.0107 0.75 0.0192 1.00 0.0222 1.50 0.0330 Table 2 E f f e c t of temperature on r a t e of l e ach ing of g o e t h i t e i n 1.0M HC10 (F igures 4, 7) Temp. 10 Rate l og Rate °C T°K (mgms Fe/min) 120 2.5432 0.0420 - 1.3768 115 2.5760 0.0282 - 1.5498 110 2.6096 0.0222 - 1.6536 100 2.6795 0.0120 - 1.9208 85 Table 3 E f f e c t of [H~S0,] concentration on rate of leaching of goethite at 85°C (Figures 8, 9) [I^SO^] concentration Rate (Molarity) (mgm Fe/min) 1.0 0.1135 2.0 0.1975 3.0 0.252 4.0 0.360 5.0 0.435 86 Table 4 E f f e c t of temperature on r a t e of l e a ch ing of g o e t h i t e i n 0.15M H„SO (F igures 11 , 15) Temp. 10 Rate log Rate °C T°K (mgm Fe/min) 120 2.5432 .0922 - 1.0353 110 2.6096 .0540 - 1.2676 100 2.6795 .0270 - 1.5686 90 2.7533 .0139 - 1.8570 87 Table 5 (a) E f f e c t of temperature on rate of leaching of goethite i n 1.8M H SO, (Figures 12, 16a) 3 Temp. 10 Rate log Rate °C T°K (mgm Fe/min) 90 85 80 75 2.7533 2.7917 2.8313 2.8719 0.2628 0.17235 0.12175 0.0790 - 0.5804 - 0.7637 - 0.9146 - 1.1024 88 Table 5(b) E f f e c t of temperature on rate-law constants k' and/~k" f o r leaching of goethite i n 1.8M H.SO. (Figures 12, 14, 16b) Temp. °C 10 T°K Uniform attack k' log k 1 (mgm Fe/min) P i t t i n g attack /k" . log /k" J (mgm Fe/min) 90 85 80 75 2.7533 2.7917 2.8313 2.8719 ,06885 ,04445 .02955 ,02125 -1.1621 -1.3521 -1.5295 -1.6727 ,02594 ,01655 .01183 .00771 -1.5860 -1.7812 -1.9270 -2.1129 89 Table 6 E f f e c t of acid concentration and a c t i v i t y of hydrogen ion i n HC1 solutions on rate of leaching at 85°C (Figures 17, 19a, 20) HC1 A c t i v i t y of hydrogen Rate Normality ions, A Y & ^ ^ ( m g m Fe/min) 1.0 0.72 .157 1.5 1.21 .3415 2.0 1.78 .6645 3.0 3.39 1.63 4.0 5.89 3.56 90 Table 7 E f f e c t of temperature on rate of leaching of goethite i n 0.15M HC1 (Figures 22, 23) Temp. 10 Rate log Rate °C T°K (mgm Fe/min) 120 2.5432 0.0189 -1.7235 115 2.5760 0.0118 -1.9281 110 2.6096 0.00735 -2.1337 100 2.6795 0.0050 -2.3010 91 Table 8 E f f e c t of temperature on rate of leaching of goethite i n 2.0M HCl (Figures 24, 26) Temp. 10 Rate log Rate °C T°K (mgm Fe/min) 90 2.7533 1.0175 + 0.0074 85 2.7917 0.6685 - 0.1749 80 2.8313 0.3920 - 0.4067 70 2.9138 0.1661 - 0.7796 92 Table 9 E f f e c t of varying the p e r c h l o r i c acid concentration and the p a r t i a l pressure of SO on rate of leaching of goethite at 110°C (Figures 28, 30, 32) S0 2 p.p. Rate (mgm Fe/min) 0.15M HC10. 0.50M HC10 2.3 .045 .067 5.3 .064 .081 8.3 .081 .121 15.3 .118 .175 23.3 .162 .211 93 Table 10 Effect of varying the sample weight and the partial pressure of S0» on rate of leaching in 0.15M HC10 at 110°C (Figures 28, 32, 33, 35) Rate (mgm Fe/min) 1 gm. sample 2 gm sample 2.3 .045 .082 5.3 .064 .127 8.3 .081 .166 15.3 .118 .248 23.3 . 162 .337 S02 p.p. psi 94 Table 11 A c t i v a t i o n energy of rate determining step i n leaching of goethite i n h ydrochloric, sulphuric and p e r c h l o r i c acids, and r e l a t i v e increases i n rate per unit of hydrogen ion a c t i v i t y at 85°C and 5% mineral d i s s o l u t i o n i n the three acids Acid A c t i v a t i o n Energy . . Increase i n rate ( a ) (Kcals/mole) ^ } (mg Fe/min/a^) P e r c h l o r i c 17.8 ± 2.9 .0047 Sulphuric 19.9-1.6 .085 Hydrochloric 22.5-2.2 .16 

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