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Rate of solution of ferrous sulphide in hydrochloric acid solutions Berisford, Robert 1950

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L£z fa] a so ftr BATE OF SOLUTION OF FERROUS SULPHIDE IN HYDROCHLORIC ACID SOLUTIONS Submitted as p a r t i a l requirements for the degree of Master of Arts by Robert Berisford Department of Chemistry University of B.C. Apr i l , 1950 1. Acknowledgement I should like to take this oppor-tunity of expressing my sincere gratitude to Dr. J . Gilbert Hooley for his continued guidance and advice during the course of this investigation. Also to the Consolidated Mining and Smelting Company, I should like to express my gratitude for their summer research grant. 3-Abstract The rates of solution of laboratory prepared f e r r -ous sulphide i n solutions of varying concentrations of hydro-chloric acid were measured at. three different temperatures, 20, 30 and 40°C. It was found that the rate of solution is not a linear function of the acid concentration but at each through temperature passed thru a maximum at 7*5 N » The reaction was found to be f i r s t order in respect to the hydrogen ion concentration and the energy of activat-ion, (with one exception) i n the range 0 - 7*5 N was found to be 9700 + 150 calories per mole. Beyond 7.5 N the energy of activation rises to I67OO calories per mole. Each reaction conforms quite well to the expression - E/RT k - PZ e which is the expression proposed by the "Simple Collision Theory". The decrease in rate of solution has been attribu-ted to the absorption of ferrous chloride crystals on the surface of the unreacted sulphide. The mechanisms of solution have been discussed briefly. C ontents •I Tit l e 1 II Acknowledgement • • • • 2 III Abstract 3 IV Literature 4 V Theoretical Discussion 7 VI Experimental A. Preparation of FeS 1 1 B. Preparation of Samples ... 1 2 C. The Apparatus 1 3 D. Description of a Run 14 VII Results and Graphs 1 5 VIII Discussion of Results 2© IX Discussion of Mechanism 232 . . X Sugges t i ons 2 2 j XI Bibliography . 2 5 The Rate of Solution of Ferrous Sulphide in Hydrochloric Acid Solutions 4. L i t e r a t u r e According to Sneed and Haynard, (<t) the s o l u t i o n of ferrous sulphide i n acid solution i s a step wise-process and proceeds according to the equations FeS -L= S" + F e f 2 HC1 — H * f C I tt 2 H S " H HC1 —*- H f C I * 3 H2S A. L i p s c h i t z and R; von Hasslinger(2) state that p u r i f i e d ferrous sulphide reacts extremely slowly with cold d i l u t e acids. Active ferrous sulphide, which evolves hydro-gen sulphide with cold d i l u t e acids, contains m e t a l l i c iron. The hydrogen formed from the reaction between the metallic i r o n and d i l u t e acid, reduces the ferrous sulphide to hydrogen sulphide and free i r o n . A small amount of i r o n would act as an accelerator of the reaction. In equation form Fe° f 2HC1— FeCL2| H 2 FeS f H 2 — Fe° f H^S. This observation has been confirmed by G. Shaw(3) Tand a simi-l a r explanation can be applied to the observation of Casamajor 4, that the d i s s o l u t i o n of ferrous sulphide i n d i l u t e H 2 S 0 4 i s favored by the presence of zinc. L . M b s e r a n d M . B e h e P f o u n d t h a t o n e l i t r e o f 0.1 N H2SO4, s a t u r a t e d w i t h h y d r o g e n s u l p h i d e , d i s s o l v e d 0.2136 g m s . F e S o r 2 .4 x 10"3 m o l e a t 2 0 ° C . I t h a s b e e n n o t e d b y p r e v i o u s w o r k e r s , t h a t f e r r o u s s u l p h i d e i s m o r e r e a d i l y s o l u b l e i n d i l u t e t h a n i n c o n c e n t r a t e d m i n e r a l a c i d s . Shaw3 h a s n o t e d t h a t a m a x i m u m r a t e o f r e a c t i o n o c c u r e d a t 6608 N H C I f o r c y l i n d r i c a l s a m p l e s o f c o m m e r c i a l F e S a t 2 0 ° C . T h e r a t e o f r e a c t i o n b e t w e e n c y l i n d r i c a l s a m p l e s o f c o m m e r c i a l F e S a n d s u l p h u r i c a c i d r e a c h e s a m a x i m u m r a t e 860IO N . H o w e v e r , r e p e t i t i o n 4 " o f S h a w ' s w o r k , u s i n g f i n e l y g r o u n d s a m p l e s o f c o m m e r c i a l f e r r o u s s u l p h i d e (80/200 m e s h ) b y t h e p r e s e n t a u t h o r , f a i l e d t o s h o w a m a x i m u m r a t e w i t h H C I . T h e r a t e o f r e a c t i o n w a s f o u n d t o i n c r e a s e l i n e a r l y w i t h t h e / /2.S a c i d c o n c e n t r a t i o n f r o m N t o N . T h e r a t e o f r e a c t i o n b e t w e e n c o m m e r c i a l F e S ( g r o u n d t o 80/200 m e s h ) a n d h y d r o c h l o r i c a c i d w a s i n d e p e n d e n t o f r a t e o f s t i r r i n g . T h i s i n d i c a t e d t h e r e a c t i o n t o b e n o n d i f f u s i o n c o n t r o l l e d . T h e r a t e o f r e a c t i o n b e t w e e n s u l p h u r i c a c i d a n d g r o u n d 80/200 c o m m e r c i a l F e S w a s f o u n d t o b e d i f f u s i o n c o n -t r o l l e d . 6 L a b o r a t o r y p r e p a r e d F e S , p r e p a r e d b y J . A i s h , ? w a s f o u n d t o e x i s t i n t w o f r a c t i o n s , m a g n e t i c a n d n o n m a g n e t i c . T h e m a g n e t i c p o r t i o n w a s f o u n d t o d i s s o l v e m u c h f a s t e r i n d i l u t e H C I t h a n t h e n o n m a g n e t i c p o r t i o n . 6 T h e m e t h o d o f p r e p a r a t i o n o f t h e s u l p h i d e p l a y s a 6. p r e d o m i n a t i n g r o l e i n t h e r e a c t i v i t y o f t h e r e s u l t a n t p r o d u c t . F o r e x a m p l e , K r o w k o w s h i ^ r e p o r t s t h a t Z n S , i f p r e p a r e d f r o m a m e d i u m o f a c e t i c a c i d i s q u i t e i n s o l u b l e i n a c i d s o l u t i o n s . H o w e v e r , i f p r e c i p i t a t e d i n a b a s i c m e d i a t h e r e s u l t i n g ZnS i s q u i t e s o l u b l e . D o n g e s a n d N a t u r f o r s c h e ^ r e p o r t t h a t n i c k e l a n d c o b a l t s u l p h i d e s s h o w s i m i l i a r p r o p e r t i e s t o t h a t o f z i n c s u l p h i d e . A c c o r d i n g t o G . S . N i s h i h a r a , 1 0 t h e a t t a c k a b i l i t y o f p y r r o h o t e t e b y 0.125 N H2SG4 i s 100 w h e n t h a t o f p y r i t e i s u n i t y . A s a r u l e , t h e m i n e r a l a c i d s d i s s o l v e t h e p u r i f i e d s u l p h i d e w i t h o u t l e a v i n g a r e s i d u e a n d w i t h t h e e v o l u t i o n o f h y d r o g e n s u l p h i d e . I n t h e c a s e o f p y r r o f a d v t i t e , i t s d i s s o l u -t i o n i n h y d r o c h l o r i c a c i d m a y b e a c c o m p a n i e d b y t h e s e p a r a t i o n o f s u l p h u r a n d t h e s o l u t i o n m a y b e y e l l o w i n c o l o r , d u e t o t h e p r e s e n c e o f t r a c e s o f f e r r i c c h l o r i d e . P y r r o h o t i t e 1 1 i s a s o l i d s o l u t i o n o f s u l p h u r a n d f e r r o u s s u l p h i d e . I P S c h n e l l , ^ w h o n o t e d t h a t t h e r a t e o f s o l u t i o n o f f e r r o u s s u l p h i d e i n c o n c e n t r a t e d a c i d s o l u t i o n s i s l e s s t h a n i n d i l u t e s o l u t i o n s , c l a i m s t h i s i s d u e t o p r e f e r e n t i a l a d -s o r p t i o n o f c h l o r i d e i o n s o n t h e s u r f a c e o f t h e s u l p h i d e . 7-T h e o r e t i c a l D i s c u s s i o n T h e b a s i c e q u a t i o n o f r a t e p r o c e s s e s a s d e v e l o p e d b y t h e " c o l l i s i o n t h e o r y s c h o o l o f t h o u g h t " i s g i v e n a s ^ 1 ^ - E / R T , k = P Z e (1) w h e r e k - s p e c i f i c r a t e c o n s t a n t P .= s t e r ' ^ ) f a c t o r Z B c o l l i s i o n f r e q u e n c y E = a c t i v a t i o n a l e n e r g y R = g a s c o n s t a n t T = a b s o l u t e t e m p e r a t u r e T h i s e q u a t i o n h a s b e e n a p p l i e d i n s e v e r a l c a s e s t o s o l i d l i q u i d h e t r o g e n e o u s s y s t e m s w i t h q u i t e g o o d r e s u l t s . ^ T h i s w o r k i s a n a t t e m p t t o a p p l y t h i s e q u a t i o n (1) t o t h e r a t e o f s o l u t i o n o f l a b o r a t o r y p r e p a r e d f e r r o u s s u l p h i d e i n h y d r o -c h l o r i c a c i d s o l u t i o n s . T h e s o l u t i o n o f f e r r o u s s u l p h i d e i n a c i d s o l u t i o n s m a y b e e x p r e s s e d b y t h e o v e r a l l e q u a t i o n F e S + 2Hi-* F e f + f H 2 S . (2) T h e e v a l u a t i o n o f t h e f a c t o r s i n t h e r a t e e x p r e s s i o n ( £ ) c a n h e d o n e a s f o l l o w s . 1) Z i s t h e n u m b e r o f s o l u t e m o l e c u l e s s t r i k i n g a u n i t a r e a o f t h e s o l i d a r e a i n a u n i t l e n g t h o f t i m e . F r o m k i n e t i c t h e o r y , t h e v a l u e o f Z 1 ^ i s 8 . z = N o /R T (3) w h e r e N s = n o . o f g r a m m o l e c u l e s o f s o l u t e p e r c u b i c c e n t i m e t e r o f s o l u t i o n . M = m o l e c u l a r w e i g h t o f t h e s o l u t e m o l e c u l e o r i o n . Z i s e x p r e s s e d i n c o l l i s i o n s p e r s e c o n d . / C M 2 2 ) P a n d E . F o r a f i r s t o r d e r p r o c e s s , t h e e x -p r e s s i o n f o r k , t h e s p e c i f i c r a t e c o n s t a n t i s 1 ^ k = 2 . ^ 0 l og 1 0 4 & . ( 4 ) t a - x w h e r e a = o r i g i n a l c o n c e n t r a t i o n o f t h e r e a c t a n t a - x - c o n c e n t r a t i o n o f t h e r e a c t a n t r e -m a i n i n g a f t e r a t i m e t . t = t i m e e x p r e s s e d i n m i n u t e s k h a s u n i t s o f r e c i p r o c a l t i m e . F o r t h e r e a c t i o n i n q u e s t i o n , t h e d e c r e a s e i n h y d r o g e n i o n c o n c e n t r a t i o n c a n b e f o u n d f r o m t h e v o l u m e o f h y d r o g e n s u l p h i d e e v o l v e d . I n g e n e r a l x - b V (5) w h e r e b = a c o n s t a n t V B v o l u m e o f h y d r o g e n s u l p h i d e i n c c . S u b s t i t u t i o n o f 5 i n 4 g i v e s k = 2 . 3 0 l og 1 0 a ( 6 ) t a - b V 9 -k c a n b e d e t e r m i n e d g r a p h i c a l l y f r o m a p l o t o f l o g ( a - b V ) v e r s u s t . T h e s l o p e o f t h e r e s u l t i n g s t r a i g h t l i n e i s - k / 2 . 3 0 . T a k i n g l o g s o f b o t h s i d e s o f (1) l o g k = l o g P Z * E _ _ (7) R T s o t h a t a p l o t o f l o g k v e r s u s 1/T s h o u l d g i v e a s t r a i g h t l i n e w h o s e s l o p e i s - E / R . E c a n b e f o u n d f r o m t h e e x p r e s s i o n E = - R t a n 0 (8) F r o m a p l o t o f l o g k a g a i n s t ^ t h e v a l u e o f l o g P Z c a n b e f o u n d b y e x t r a p o l a t i o n o f t h e l i n e t o t h e p o i n t w h e r e 1 - 0 . T h u s t h e s f c e r i c f a c t o r P ^ c a n b e f o u n d . T T h e e x p e r i m e n t a l r e s u l t s h a v e b e e n e x p r e s s e d a s r a t e s o f s o l u t i o n o f F e S i n m g . / c a r y m i n . T h e e x p e r i m e n t a l r a t e w a s d e t e r m i n e d f r o m a p l o t o f v o l u m e o f R " 2 S v e r s u s t i m e . T h e s l o p e o f t h e c u r v e ( d V ) t i m e s 3.66 g i v e s t h e r a t e o f s o l u t i o n . ( d t ) s i n m i l l i g r a m s p e r m i n . p e r c m . R = 3.66 . d V S d t ( 9 ) S = s u r f a c e a r e a i n s q u a r e c m s . 3.66 = a c o n v e r s i o n f a c t o r r e l a t i n g t h e v o l u m e o f H 2 S w i t h t h e a m o u n t o f F e S g o i n g i n t o s o l u t i o n . 10. From equation 2 i t i s to be seen that at ® 9 C . and one atmosphere pressure one mole of FeS gives r i s e to 22.4 1. of or at 20°C. 8 24.G 1? Hence, the evolution of 1 c c . of H2S at 20° and one atmosphere corresponds to the s o l u t i o n of 87.9 x 103/24 x 10 3 * 3 .66 rag. of FeS. Further, from equation 2, the evolution of one mole of H2S requires the consumption of 2 moles of H* at standard conditions. The production of one c c . of M2S at 20° C, and one atmosphere pressure requires the consumption of 2/24 x 10^ « $.35 x 10""5 miles of H*-* The constant b, i n equation 6 , i s 8,35 x 10-5 mpj.es of H-+/eev<<«f H2S. Calculation of Surface Areas Let a maximum screen opening through which a par-t i c l e has passed and b •«* minimum screen opening of the sieve through which the p a r t i c l e td.ll not pass. The assumptions made are 1. On the average,/ each p a r t i c l e i s the shape of a cube. 2. The length of one edge of -the cube i s ; 3 p cm. • long.; .' ' Let o = L. The surface area of each p a r t i c l e i s S3- ** 6l? and the mass L 3 . I The surface area of U grams of the powdered sulphide i s then given as •„ S SL.S 1 6L 2 *^|-The screen dimensions were those reported i n the A.G.S. Year Book 1 8 1921-22. Experimental Ao Preparation of FeS, The ferrous sulphide was prepared i n the following ffiannero A solution containing s i x t y grams of sodium s u l -phide per 0*5 l i t r e of b o i l e d d i s t i l l e d water was mixed w.ith a solu t i o n of ferrous ammonium sulphate (100 grams per l i t r e ) and the ferrous sulphide p r e c i p i t a t e d according to the equa-t i o n FeS0 4.(NH4)2S0 4.6H 20 f Na 2S.9H 20 - > FeS 4- Na 230 4 f (HH 4) 230 4 f 15H 20. The hydrated ferrous sulphide was allowed to remain In contact with the warm so l u t i o n (75° ** 80°) f o r a period of 5 - 6 minutes and then f i l t e r e d and washed on a suction f i l -t e r . The washing was continued u n t i l the washings gave a nega fcive test f o r sulphate i on on the addition of a few drops of B a C l 2 solution. The washed ferrous sulphide w a s then transferred to a Kjeldahl f l a s k and dried i n a furnace at 375 - 400 degrees Centigrade for 3^ - 4 hours under reduced pressure. When the sulphide was dry, the furnace w a s turned o f f and allowed to co o l to room temperature (the system, s t i l l under reduced pres sure). When the sulphide w a s c o o l ? i t w a s removed and pre* pared for rate studies. The y i e l d was about 95% of the predicted t h e o r e t i -c a l value. The r e s u l t i n g product was a grey black material • roughly the s i z e of a #5 sieve. I t was found necessary to prepare 20 - 21 gram l o t s at a time since l a r g e r amounts made the f i l t r a t i o n exceedingly slow. The f i l t r a t i o n and washing must be c a r r i e d out as quickly as possible to reduce the r i s k of a i r oxidation of the moist s u l p h i d e . ^ The sulphide must also be cool before i t i s removed from the furnace. In one or two instances when t h i s precau-t i o n was not taken, the sulphide was oxidized-'to an orange powder* Preparation of Samples The prepared samples were ground i n a mortar and passed through a s e r i e s of standard sieves* The portion of the sulphide, which remained between two selected screens w<$S5 used i n a given set :o£y:r$ns« This assured a f a i r l y con-stant d i s t r i b u t i o n of p a r t i c l e s i z e s and afforded a manner by which the surface area of the powdered samples couM be c a l -culated (See t h e o r e t i c a l discussion). For example that portion of the sulphide which remained between the 20 and 50 sieves was c a l l e d 20/50 FeS. Ferrous sulphide, prepared i n the above manner, was found to consist of two portionsj d i f f e r i n g i n t h e i r magnetic properties* The ground sulphide was separated i n t o two (magnetic and non magnetic) portions by means of a permanent « n r H K l > 0 F i g . I - Diagram of Apparatus .A B C D E F G H I K S J Reaction f l a s k Y-shaped adapter Mercury seal 50 ml. acid burette CaCl2 drying tower Stopcock Connection to drying tower 300 m l o gas eudiometer S t i r r i n g rod 3-way stopcock 1 4 . F r o m t h e d r y i n g t u b e , t h e g a s w a s l e d t o a 300 m l . e u d i o m e t e r , i n w h i c h t h e v o l u m e s o f g a s g e n e r a t e d w e r e m e a s u r e d . E x p e r i m e n t a l D e s c r i p t i o n o f a R u n T h e r e a c t i o n f l a s k ( A ) w a s p l a c e d i n t h e w a t e r b a t h a n d f i r m l y h e l d i n p l a c e b y m e a n s o f a c l a m p . O n e g r a m o f t h e p r e p a r e d s u l p h i d e w a s i n t r o d u c e d f r o m a w a t c h g l a s s a n d t h e r e a c t i o n f l a s k f i t t e d w i t h t h e Y s h a p e d a d a p t e r ( B ) , m e r c u r y s t i r r i n g s e a l ( G ) , a n d t h e s m a l l a c i d e u d i o m e t e r ( D ) . T h e r e a c t i o n f l a s k w a s c o n n e c t e d t o t h e d r y i n g t o w e r ( E ) b y a r u b b e r c o n n e c t i o n a t ( I ) . T h e t h r e e - w a y s t o p c o c k (J) w a s t u r n e d s o a s t h e g a s m e a s u r i n g e d u i o m e t e r ( K ) w a s c o n n e c t e d t o t h e r e s t o f t h e a p p a r a t u s , a n d t h e l e v e l o f t h e m e r c u r y i n t h e e u d i o m e t e r a d j u s t e d t o z e r o v o l u m e . F i f t y m l . o f a c i d s o l u t i o n o f d e s i r e d c o n c e n t r a -t i o n w e r e p l a c e d i n e u d i o m e t e r D . T h e s m a l l e u d i o m e t e r w a s t h e n s e a l e d w i t h t h e r u b b e r s t o p p e r L a n d s t o p c o c k ( G ) o p e n e d . T h e s t i r r e r w a s s e t i n m o t i o n , s t o p c o c k ( H ) c l o s e d a n d t h e s y s t e m t e s t e d t o s e e i f i t w a s a i r t i g h t . T h i s w a s d o n e b y l o w e r i n g t h e l e v e l o f t h e m e r c u r y i n ( K ) a n d t h u s c r e a t i n g a p a r t i a l v a c u u m i n t h e c l o s e d s y s t e m . W e r e t h e s y s t e m a i r t i g h t t h i s p a r t i a l v a c u u m r e m a i n e d i n t h e s y s t e m . U p o n b e i n g c e r t a i n t h a t t h e s y s t e m w a s a i r t i g h t , H w a s o p e n e d a l l o w i n g t h e p r e s s u r e i n t h e s y s t e m t o r e t u r n to atmospheric conditions, the l e v e l of the mercury i n K, re-adjusted to zero volume. Stopcock I was then opened, allowing the acid s o l u t i o n to run i n t o the reaction f l a s k and simultaneously, stop clock % was started. The volume of hydrogen sulphide so produced was measured, at atmospheric pressure, at one minute i n t e r -v a l s . Results In table I are l i s t e d the r e s u l t s f o r the i n v e s t i -gation of laboratory prepared magnetic ferrous sulphide. Graphs I - VTI are p l o t s of l o g k against 1/T f o r each acid concentration. From these graphs i t i s possible to compute the. energy of a c t i v a t i o n E and the value of the PZ f a c t o r . These are tabulated i n table I. Graph VIII i s a p l o t of rate of s o l u t i o n of ferrous sulphide i n hydrochloric acid of various concentrations. The reported rates are ± r e l a t i v e ^ . Table II shows the rate of s o l u t i o n of ferrous sulphide i n sulphuric acid of various concentrations. Tables I I I , IV and V show the rate of s o l u t i o n of commercial ferrous sulphide i n hydrochloric a c i d . Graphs IX, X and H are representative p l o t s of tfzS -a volume against time. 16. Table I T z kr E PZ P R L.5W 20 0.198 2.91 x 10~3 14900 3.98X10 8 2.01xl0 9 0.490 30 0.201 4.98 x 10"3 0.821 40 0.205 1.58 x 10 - 2 2.20 3N 1.42x10^ 20 0.396 3.06 x 10"3 9650 5.62 x # 0.907 3°, 0.403 8.30 x 10-3 2.07 40 0.410 1.15 x 10-3 3.50 20 0.594 3.79 x 10" 3 9600 6.31 x l O 4 1.28x305 2.00 30 O.606 6.33 x 10-3 3.04 40 0.618 l i l 3 x 10" 2 6.76 6N 20 O.792 3.07 x 10-3 9750 8.92 x l ^ 1.13 xio5 2.20 30 0.804 6.17 x 10"3 3.90 40 0.816 1.00 x 10- 2 7.00 20 O.99O 5.3 x 10"3 9850 1.26x30? 1.27K1O5 3.20 30 1.008 6.35x 10-3 3.54 40 1.020 1 1.60x10-3 -12.60 T a b l e I. (Cont'd) T Z kr £ • P Z . ..P. R 9& -20 1.188 5.16 x 10" 4 I49OO 3.97x10 7 3.36x10^ 0.84 30 1.201 7.20 x 10~4 0.68 4 0 1.230 5.1 x 10"^ 4.50 20 I.38O 3.38 x 1G~4 16700 4.47 x K ) 8 3 . 2 4 x l O 8 0.25C 30 1 . 4 1 0 5»35 x l O " 4 0.65C 4 0 1 . 4 4 1 3.10 x 10-3 1 03*60 T a b l e I. R a t e o f s o l u t i o n o f magnetic l a b o r a t o r y p r e p a r e d f e r r o u s s u l p h i d e i n h y d r o c h l o r i c a c i d . M e s h 20/50. C a l c u l a t e d s u r f a c e a r e a 2 1 . 8 c m 2 , kr » - s p e c i f i c r a t e c o n -s t a n t s . S e n e r g y o f a c t i v a t i o n , P t h e steric f a c t o r . H i s t h e r a t e i n tqg.cm? per m i n . g - « . e a l ^ t e s - /sola*, .tap •reel**-18. Table II Concentration Rate • 1 M. . 1 .21 2 2 . 9 2 ' 4 5 . 3 7 6 3 . 3 6 8 0 T = 40° C' . 2 0 / 5 0 FeS Rate of solution of prepared FeS, 2 0 / 5 0 i n R^ SO/^  at 40° C, i n mg . / c m 2/ min e y • Table III Mesh ' Area Rate 10/20 8 . 8 cm2 1 . 4 0 5 0 / 3 0 52 " 0 . 6 4 30/120 82 " 0 . 6 2 120/200 124 1 1 0 . 3 8 Rate of solution, mg./cm2/min. of commercial FeS of various mesh sizes i n 3N HCI at 20° C. Table IV Mesh Area Rate 10/20 8 . 8 cm2 4 . 0 7 50/80 52 " 1 . 0 4 80/120 82 " 1.81 120/200 124 " 1 .26 Rate of solution, mg./cm2/min. of commercial FeS of various mesh sizes i n 6N HCI at 20° C. 1 9 . Table V Mesh Area Rate 10/20 8 .8 cm2 1 .33 . 50/SO 52 " 0 . 2 6 7 30/120 82 " 0.183 120/200 124 " 0 . 1 2 5 Rate of solution, mg./cm2/min. of commercial FeS of various mesh sizes i n 9N HCI at 20° C. Examination of the p a r t i c l e s of ferrous sulphide, which were insoluble i n acid whose concentration was greater than 7 .5 W, under a moderately powerful microscope, revealed that there was an absorbed layer of small c r y s t a l s on the surface of the unreacted ferrous sulphide. No such absorbed c r y s t a l s could be observed on the surface of ferrous sulphide i n acid concentrations below 7.5N hydrochloric a c i d . Discussion of Results It is to be seen from graph #8 that the rate of solution of laboratory prepared ferrous sulphide i n hydro-chloric acid reaches a maximum at 7»5N concentration and drops off sharply as the acid concentration i s increased. This decrease i n rate i s accompanied by a sharp increase in the activational energy of the process. It i s true, that the PZ factor also increases sharply, but the value of the exponential factor decreases much more rapidly. The obser-vation that above 7«5N acid, small crystals become absorbed to the surface of the unreacted ferrous sulphide would ex-plain why a decreasing fraction of the hydrogen ions have sufficient energy to penetrate the absorbed layer. The thickness of the crystal barrier seems to become effective at 7.5N and increases sharply with increasing acid concen-tration. This is in keeping with a common ion effect. The calculation of surface areas as followed i n this investigations assumes a symetric distribution of par-t i c l e sizes about the average of the two screen size openings The value of the P factor i s quite large. It was expected to be of the order of unity. The po s s i b i l i t y of a chain reaction must be considered. It is to be noted from Table II, that the rate of solution of ferrous sulphide i s not a linear function of the sulphuric acid concentration, but reaches a maximum i n acid concentration 4 to 6 molar. Tables III, IV and V indicate that the rate of solu tion of commercial ferrous sulphide per unit area decreases slightly with increased surface area. The calculated area of 10/20 mesh may well be i n error considering the assumptions involved. General Discussion The solution of ferrous sulphide may be considered from two points of view. It may be a process requiring the c o l l i s i o n with the surface of an activated hydrogen ion. This is a heterogeneous process and should be dependant upon the surface area and the concentration of the acid solution. It would be f i r s t order in respect to the hydrogen ion since as the reaction proceeds, the number of collisions with the surface decreases. A layer of absorbed crystals at the solid solution interface would prevent the hydrogen ions from reaching the surface of the sulphide. This would decrease the rate of solution. On the other hand, the jferrous sulphide must dis-solve and ionize. The reaction with the hydrogen ions would then proceed in solution. Such a process may be represented by the following equations: FeS(s) S = / Fe + + (1) S= i H f^=^ HS' (2) HS" f H + =^= H2S (3) Steps 2 and 3 are apparently bimolecular processes and, i f they occur i n sol u t i o n may be homogeneous. The s o l u b i l i t y product of ferrous sulphide i s 4 X 1 C T 4 0 at 2 0 ° C . The concentration of the sulphide i o n would be of the order of 1 0 ~ 2 0 moles per l i t r e while, i n a one molar solution, the hydrogen ion concentration i s 10 2 ^ times as great. Steps 2 and 3 would be pseudo f i r s t order i n respect to sulphide ion concentration. Process 1 would be the rate c o n t r o l l i n g step, and any way i n which t h i s step were affected would affec 4t the over a l l process. An absorbed layer of ferrous chloride c r y s t a l s may be s u f f i c i e n t to prevent the i n i t i a l s olution of the ferrous sulphide. Suggestions The r e s u l t s presented here marks only a small amount of the necessary work to c l a r i f y the.rate of s o l u t i o n of FeS i n a c i d solution. The remaining work to be done should take the form 1. Dependence of rate of s o l u t i o n upon surface area. This presents one d i f f i c u l t y (at l e a s t ) . Ferrous sulphide p a r t i c l e s , may be h a l f magnetic and h a l f non magnetic. How to separate the two kinds of the sulphide and s t i l l r e t a i n large sized p a r t i c l e s (having a small surface area) presents a b i t of a problem. A p l o t of rate of sol u t i o n i n mg. per 2 min per cm f o r each acid concentration against the calculated surface area would indicate the dependanee of rate upon sur-face area. A marked difference i s shown i n the rates of solution of c y l i n d r i c a l and fine l y ground commercial ferrous sulphide i n concentrated hydrochloric acid. For each concen-tration of acid and each surface area, specific rates can be calculated and E's and PZ determined. The change i n activational energy with surface area may be of some use i n predicting the mechanism. For each surface area and acid concentration the rate may be determined at different rates of s t i r r i n g and added viscosity changers. This information may prove useful i n predicting the mechanism. The reaction may be s i m i l i a r l y studied using the non magnetic portion of the sulphide. Similiar investigation may be extended to sulphideS of cobalt, nickel or zinc. The presence of absorbed ferrous chloride crystals on the surface of the ferrous sulphide can be checked by com-parison of X Radiogram pictures of the untreated ferrous sul-phide and the sulphide which has been exposed to a concentra-ted (J^7»5N) hydrochloric acid solution. The reaction should also be studied over a greater range of temperatures. The temperature range should include five or six pointsfor i t i s f e l t that drawing the best straight line between three points i s not too accurate. Such new temperatures would have to be below room temperature s i n c e a t 40°, t h e v a p o r p r e s s u r e o f HCI i s q u i t e h i g h . T h e r a t e o f r e a c t i o n i s f a s t a n d b e c o m e s q u i t e d i f f i c u l t t o f o l -l o w a c c u r a t e l y w i t h t h e e x i s t i n g a p p a r a t u s . A n u m b e r o f q u e s t i o n s a r i s e f r o m t h e r e s u l t s . (1) D o e s t h e c o n c e n t r a t i o n o f s u l p h i d e i n c r e a s e w i t h i n c r e a s e d s u r f a c e a r e a ? (2) I s t h e p o i n t o f m a x i m u m r a t e i n d e p e n d a n t o f p a r t i c l e s i z e ? B I B L I O G R A P H Y 1 . S n e e d a n d M a y n a r d , G e n e r a l C o l l e g e C h e m i s t r y . V a n N o s t r a n d 1 9 4 4 , p . 3 6 * 6 . 2 . L i p s c h i t z a n d v o n H a s a l i n g e r . M o n a t c h 2 0 , 2 1 2 , 1 9 0 5 . 3 . S h a w , G . B . A . T h e s i s . U . B . C . 1 9 4 7 . 4 . C a s a m a j o r , P . C h e m . N e w s , 4 4 , 4 4 , 1 8 8 1 . 5 . M o s e r a n d B e h e r . Z e i t . A n o r g . C h e m . 1 3 4 , 4 9 , 1 9 2 4 . 6 . P r e s e n t A u t h o r . B . A . T h e s i s . U . B . C . 1 9 4 8 . 7 . A i s h , J . B . A . T h e s i s . U . B . C . 1 9 4 8 . 8 . K r o w k o w s k i , C . A . 2 8 , 5 3 1 4 . 9 . D o n g e s a n d N a t u r f o r s c h e , C . A . 4 1 , 2 6 5 7 . 1 0 . N i s h i h a r a . E c o n . G e o l . 9 , 4 8 3 , 1 9 1 4 . 1 1 . P o r t e r , J . D . J . C h e m . E d . 2 6 . ' A p r i l 4 8 , 2 0 2 . 1 2 . S c h n e l l . J . C h e m . E d . 8 . M a r c h , 1 9 3 0 . 1 3 . C . N . - H i n s h e l w o o d . K i n e t i c s o f C h e m i c a l C h a n g e . O x f o r d U n i v e r s i t y P r e s s , 1 4 . M o e l w y n - H u g h e s , K i n e t i c s o f R e a c t i o n s i n S o l u t i o n . O x f o r d U n i v e r s i t y P r e s s , 1 9 4 7 . 1 5 . H e r t z . . . W e i d . A n n . 1 7 , 1 7 7 , 1 8 8 2 . 1 6 . G l a s s t o n e . T e x t b o o k o f P h y s i c a l C h e m i s t r y . D . V a n N o s t r a n d , 1 9 4 6 . 1 7 . W i n k l e r a n d H i n s h e l w o o d . J o u r n . A m . C h e m . S o c . A u g u s t , 1 9 3 5 , 1 1 4 7 . 1 8 . H a n d b o o k o f P h y s i c s a n d C h e m i s t r y . 3 0 E d i t i o n 1 9 4 7 . 2 6 1 7 . C h e m i c a l R u b b e r P u b l i s h i n g C o m p a n y , 1 9 . S . M e y h a r a . P r o c . P h y s . M a t h . S o c . J a p a n 3 5 8 , 6 4 , 1 9 4 0 . 

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