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Studies on the depressive action of chromate and dichromate salts on galena Okada, Susumu 1970

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STUDIES ON THE DEPRESSIVE ACTION OF CHROMATE AND DlCHROMATE SALTS ON GALENA by SUSUMU OKADA B.A.Sc. (Mining Engineering) Kyoto Univ e r s i t y , Japan, 19 67 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of Mineral Engineering We accept t h i s thesis as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA January, 19 70 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission f o r extensive copying of t h i s thesis f o r scholarl y purposes may be granted by the Head of my Department or by his representatives. I t i s understood that copying or pub l i c a t i o n of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. SUSUMU OKADA Department of Mineral Engineering The Unive r s i t y of B r i t i s h Columbia Vancouver 8, Canada ABSTRACT In the c o u r s e o f the e x p e r i m e n t a l work c o n d u c t e d i t was found t h a t when g a l e n a was t r e a t e d w i t h chromate s a l t s o r d i c h r o m a t e s a l t s i n an aqueous s o l u t i o n , l e a d chromate i d e n t i f i e d by i n f r a r e d s p e c t r o s c o p y was t h e main s p e c i e s a d s o r b e d on the g a l e n a s u r f a c e . The f o r m a t i o n o f chromium h y d r o x i d e o r chromium o x i d e was not o b s e r v e d . Thermodynamic c o n s i d e r a t i o n s o f the Pb-Cr-S - r^O b u l k system i n d i c a t e the v e r y s t a b l e c h a r a c t e r o f l e a d chromate as shown i n t h e p o t e n t i a l - p H diagram c o n s t r u c t e d i n t h i s s t u d y . I t i s a l s o seen t h a t no s t a b l e domain o f l e a d d i c h r o m a t e e x i s t s . The a d s o r p t i o n r a t e as w e l l as a d s o r p t i o n amount o f d i c h r o m a t e a t the p s e u d o - e q u i l i b r i u m c o n d i t i o n i n c r e a s e s w i t h the d e c r e a s e i n pH v a l u e . T h i s phenomenon s u p p o r t s the f a c t e x p e r i e n c e d i n g a l e n a f l o t a t i o n t h a t a d i c h r o m a t e s a l t i s a much more e f f i c i e n t d e p r e s s a n t than i s an e q u i v a l e n t chromate s a l t . The d e s o r p t i o n o f x a n t h a t e from g a l e n a s u r f a c e s c o v e r e d w i t h more than a monolayer t h i c k n e s s o f x a n t h a t e was found t o be p o s s i b l e by a d d i n g chromate s a l t t o the system, whereas chromate d e s o r p t i o n from g a l e n a s u r f a c e s due t o x a n t h a t e a d d i t i o n was not j o b s e r v e d . I t was f o u n d t h a t under comparable c o n d i t i o n s x a n t h a t e a d s o r b e d on l e a d chromate s u r f a c e s o x i d i z e d t o dixan'thogen much more r e a d i l y t h a n x a n t h a t e a d s o r b e d on g a l e n a s u r f a c e s . TABLE OF CONTENTS Page INTRODUCTION , - 1 LITERATURE SURVEY . . . . . . . 5 EXPERIMENTAL 11 Materials 11 A n a l y t i c a l Equipment 12 Experimental Procedures 13 (1) Adsorption and Desorption Tests 13 (2) Buffer Action Tests 14 (3) I d e n t i f i c a t i o n by Infrared Spectroscopy . . 14 EXPERIMENTAL RESULTS 16 A. E f f e c t of pH on the Concentrations of Chromate and Dichromate Ions 16 B. Changes i n pH of the Solutions Containing Galena 19 (1) Changes i n pH of Aqueous Solutions Containing Galena . . . . 19 (2) Changes i n pH of a Chromate Solution Containing Galena 2 3 C. Adsorption Rates of Chromate and Xanthate Ions by Galena 26 D. Adsorption of Chromate Ion and Xanthate Ion on Galena Surface 29 E. Enhanced Adsorption of Chromate Ion by Galena i n the Presence of Oxygen 34 F. E f f e c t of pH on Chromate Adsorption on Galena 37 V Page G. D e s o r p t i o n of Xanthate Ion or Chromate Ion from Galena Surface . . . . . . . 37 H. A d s o r p t i o n of Xanthate Ions by Lead Chromate 43 I . Thermodynamic C o n s i d e r a t i o n s of the Reactions I n v o l v e d i n the System Pb-Cr-S-H 20 44 J . I d e n t i f i c a t i o n of R e a c t i o n Products on the Surface of Galena T r e a t e d w i t h Potassium Chromate or,; Potassium Dichromate 46 K. I d e n t i f i c a t i o n of A d s o r p t i o n Products of Xanthate on Lead Chromate 5 3 DISCUSSIONS . . . . . . . . . . 57 A. Changes i n pH of the S o l u t i o n s C o n t a i n i n g G ci ~L 6 n 3, c o t t c b o c e < • • • • • • • • « « 57 B. Chemisorption of Chromate o r Dichromate Ions on Galena Surface 59 C. I n f l u e n c e o f Chromate on the A d s o r p t i o n of Xanthate on Galena Surface - 6 0 (1) Competitive A d s o r p t i o n 60 (2) A d s o r p t i o n Products o f Xanthate on Lead Chromate . . . . . . . . 62 CONCLUSIONS . . . . . . . . . 64 SUGGESTIONS FOR FUTURE WORK 66 REFERENCES . . . . . . . . 67 APPENDIX . . . . . . . . . . . . . 69 LIST OF TABLES Table Page I. Adsorption of Chromate Ion and/or Xanthate Ion on Galena Surface .33 I I . Adsorption of Chromate Ion on Galena Surface i n the Presence or Absence of Oxygen 36 I I I . Desorption of Chromate Ion from Galena Surface Due to Xanthate Adsorption 39 IV o Desorption of Xanthate Ion from Galena Surface Due to Chromate Adsorption (1) . . . . 40 V. Desorption of Xanthate from Galena Surface Due to Chromate Adsorption (2) . . . . 42 VI. Adsorption of Xanthate Ion on Pre- c i p i t a t e d Lead Chromate 45 VII. Reactions and Equilibrium f o r the System Pb-Cr-S-H 0 49-50 L I S T OF FIGURES F i g u r e Page . 1 A b s o r p t i o n S p e c t r a o f (a) P o t a s s i u m Chromate S o l u t i o n (pH 11.5) and (b) P o t a s s i u m D i c h r o m a t e S o l u t i o n (pH 4.3) 18 2 S t a b i l i t y o f Chromate I o n and D i c h r o m a t e I o n as a F u n c t i o n o f pH 2 0 3 A b s o r p t i o n S p e c t r a o f J ^ C r O ^ , K . E t . X . , and T h e i r M i x t u r e i n an Aqueous S o l u t i o n (pH 9.8) 2 2 4 Time Dependence o f pH o f r^O w i t h G a l e n a S u s p e n s i o n 24 5 Time Dependence o f pH o f t h e S o l u t i o n C o n t a i n i n g U n o x i d i z e d G a l e n a 2 5 6 T i t r a t i o n C u r v e s ( a ) O x i d i z e d G a l e n a , (b) U n o x i d i z e d G a l e n a , and ( c ) Water O n l y 2 5a 7 Time Dependence o f pH o f 2.0 x 10 _ i |M/L P o t a s s i u m Chromate S o l u t i o n C o n t a i n i n g G a l e n a . 2 7 -4 8 Time Dependence o f pH o f 2.0 x 10 M/L P o t a s s i u m Chromate S o l u t i o n C o n t a i n i n g U n o x i d i z e d G a l e n a . . 2 8 9 T i t r a t i o n C u r v e s o f Chromate S o l u t i o n s 30 10 A d s o r p t i o n R a t e s o f Chromate and X a n t h a t e on G a l e n a a t 25°C 31 11 E f f e c t o f pH on Chromate A d s o r p t i o n on G a l e n a 37a 12 P o t e n t i a l - p H D i a g r a m f o r t h e S y s t e m C r - H 2 0 . 47 13 P o t e n t i a l - p H D i a g r a m f o r t h e S y s t e m Pb-S-H 20 . . . . . 48 14 P o t e n t i a l - p H D i a g r a m f o r t h e S y s t e m P b - C r - S - H 2 0 51 v i i i Figure Page 15 Infrared Spectra of (a) Synthetic PbS Treated with K^CrC^, (b) Synthetic PbS Treated with K 2 C r 2 0 7 , (c) Galena Treated with K 2 Cr 2 07 , (d) PbCrOLi . 54 16 Infrared Spectra of (a) PbCrCv treated with K . E t . X . , (b) PbS treated with K 2 C r 2 0 7 , then K.Et.X., (c) Pb(EtX) 2 , (d) X„ , and (e) Cr 203 56 A - l Effect of the Concentration of Potassium Chromate on the Rest Potential of Galena Electrode 7 3 A - 2 Effect of pH on the Rest Potential of Galena Electrode in 1.0 x 10-3 M / L K CrO Solution 74 A-3 Effect of pH on the Rest Potential of Galena Electrode in 1.0 x IO" 3 M/L K 2 C r 2 0 7 Solution 75 A-4 Anodic Polarization Curves for Galena Electrode 7 6 A-5 Cathodic Polarization Curves for Galena Electrode 77 ACKNOWLEDGEMENT The author wishes to express his sincere appreciation to Dr. H. Majima for his guidance and encourage- ment given throughout the period of study. Thanks are also extended to the f a c u l t y , s t a f f and graduate students of the Mineral Engineering Department f o r many h e l p f u l discussions. F i n a n c i a l support from the Mines Branch, Department of Energy, Mines and Resources, Ottawa i s also appreciated. INTRODUCTION In the f l o t a t i o n of complex s u l f i d e ores contain- ing galena, chromate or dichromate s a l t s are commonly used as depressants. The hi s t o r y of studies involving the use of chromate i n f l o t a t i o n c i r c u i t s date to 1912, when Lowry and Greenway [1] introduced what would now be termed a "depressant" f o r the lead mineral by using one per cent sodium dichromate. The dezincing operation f o r lead concentrates i s important as a method of improving lead concentrate q u a l i t y and increasing zinc extraction into zinc concentrate i n zinc lead ore f l o t a t i o n . For example, at the Su l l i v a n Concentrator, dichromate s a l t had been used f o r the depression of galena i n the s e l e c t i v e f l o t a t i o n c i r c u i t . According to the example at the S u l l i v a n Concentrator, the lead concentrate was conditioned with dichromate, then with lime, xanthate, frother and copper sulphate. Galena was depressed under these conditions when the material was heated to 30-35°C, One of the examples of operation r e s u l t s [2] were: the grade of i n i t i a l lead concentrate, 5 8.1% Pb, 9.19% Zn, 9.7 2% Fe, the grade of zinc f r o t h product 29.3% Pb, 34% Zn, 9.7 2% Fe and those f o r f i n a l lead concentrate 62.45% Pb, 5.55% Zn, and 9.72% Fe. Even though t h i s type of reagent'? has almost l o s t p r a c t i c a l importance i n m i l l i n g operations, i t i s s t i l l 2 one of the most i n t e r e s t i n g t o p i c s on the d e p r e s s i o n of s u l f i d e m i n e r a l s . A l a r g e number of papers concerned w i t h the d e p r e s s i o n of galena and p y r i t e w i t h chromate or dicromate s a l t s have been pr e s e n t e d . The main r e s u l t s o b t a i n e d on galena by the p r e v i o u s r e s e a r c h e s are summarized as f o l l o w s : ( 1 ) When galena i s t r e a t e d w i t h a chromate or a dichromate s o l u t i o n , the h y d r o p h i l i c f i l m formed on galena i s l e a d chromate. However, some r e s e a r c h e r s suggest t h a t d e p r e s s i o n occurs as a r e s u l t of f o r m a t i o n of an oxide f i l m . ( 2) The chromate i s adsorbed i r r e g u l a r l y a t the m i n e r a l s u r f a c e , and the chemical attachment o f chromate io n s t o m i n e r a l i s independent o f xanthate i o n attachment, and a p p a r e n t l y takes p l a c e i n a d i f f e r - ent a r e a . (3) The chromate a d s o r p t i o n on the m i n e r a l s u r f a c e i s d e s c r i b e d by the F r e u n d l i c h e q u a t i o n . (H) The a d s o r p t i o n o f chromate on galena i n c r e a s e s w i t h an i n c r e a s e i n the c o n c e n t r a t i o n of the s o l u t i o n up t o pH 6 and becomes almost c o n s t a n t above t h i s pH v a l u e . ( 5 ) The i n c r e a s e o f the chromate a d s o r p t i o n c r e a t e s a p r o p o r t i o n a l decrease i n the y i e l d o f galena by f l o t a t i o n . ( 6 ) Dichromate i s a much more e f f i c i e n t depressant f o r galena than chromate. On the contrary i t i s also said that galena depression i s most e f f e c t i v e at pH 5-6 and above 8. ( 7 ) Galena depression occurs when a depressant adsorp- t i o n layer, approximately equal to a monolayer, i s formed; at t h i s point adsorption of xanthate remains the same as without the depressant. ( 8 ) The p o t e n t i a l of a galena electrode decreases when xanthate i s added and the addition of dichro- mate causes an increase i n p o t e n t i a l proportional to the logarithm of i t s concentration. A galena electrode treated with concentrated dichromate solut i o n shows no s i g n i f i c a n t p o t e n t i a l change by the addition of xanthate. As mentioned above, i n the d e t a i l e d mechanisms of galena depression with chromate or dichromate s a l t , some problems are yet unsolved. F i r s t l y , i t seems that there i s a discrepancy i n the prq,posed reaction product which causes galena depression. Secondly, Plaksin et a l . [ 3 ] stated that the increase of the chromate adsorption created the proportional decrease i n f l o t a t i o n y i e l d , and i t i s also well known that dichromate i s much more e f f e c t i v e f o r galena than chromate. On the other hand Bagdanov and Podnek [ 4 ] showed that galena depression by chromate s a l t was most e f f e c t i v e at pH 5-6 and above 8. Furthermore, i n the system 4 of galena, xanthate, and chromate, the behaviors of xanthate and chromate species on galena surface are not f u l l y understood. Considering these p o i n t s , i n the present work some experimental s t u d i e s and a thermodynamic i n v e s t i g a t i o n were made i n order t o get a b e t t e r understanding of the depression of galena. For these purposes an . spectrometer as w e l l as u l t r a v i o l e t spectrophotometer were used t o i d e n t i f y the surface products on galena. The p r i n c i p a l experiments were the ad s o r p t i o n t e s t s of chromate, dichromate, and/or xanthate ions on the galena s u r f a c e . Besides these experimental s t u d i e s , thermodynamic s t u d i e s were made of the p o s s i b i l i t y of the formations of lead chromate and chromium oxide. For t h i s purpose, a p o t e n t i a l -pH diagram was constructed f o r the system of Pb-Cr -S-r i^O, and the s t a b l e domains of lead chromate and chromium oxide were presented g r a p h i c a l l y . i LITERATURE SURVEY ,f As mentioned before, the hi s t o r y of studies involving the use of chromate or dichromate s a l t i n f l o t a t i o n c i r c u i t s date to 1912, when Lowry and Greenway [1] introduced what would now be termed "depressant" f o r the lead minerals by using one per cent sodium dichromate. Gaudin and his co-workers [5] studied the system, i n which galena was depressed with potassium dichromate i n the presence of iso-amyl xanthate. They also studied the separation of galena from sp h a l e r i t e . The f l o t a t i o n experiment was performed with the synthetic mixture of galena and sphalerite both sized to be -100+600 mesh. Galena was well depressed, while sphalerite was not. They assumed that galena was depressed by dichromate s a l t because the insoluble coating of lead chromate was formed on the galena surface. Coating may be responsible f o r the f a i l u r e to f l o a t , A few years l a t e r , however, Wark and Cox [6] showed that lead chromate, the mineral c r o c o i t e , was fl o a t e d by ethyl xanthate, i n d i c a t i n g that the interpre- t a t i o n given by Gaudin et a l . was not s a t i s f a c t o r y . Plaksin and Myasnikova [7], [8] employed radio- 51 active chromium, Cr , f o r the study of the e f f e c t of chromate on the f l o t a t i o n of galena and p y r i t e . Their r e s u l t s are summarized as follows: (1) The chromate . 6 a d s o r p t i o n on t h e m i n e r a l s u r f a c e i s g i v e n w i t h s u f f i c i e n t a c c u r a c y by t h e F r e u n d l i c h e q u a t i o n : a = ~kC^^n (a = adsorbed chromate amount, C = e q u i l i b r i u m c o n c e n t r a t i o n o f t h e chromate s o l u t i o n , k = c o n s t a n t w i t h t h e v a l u e o f 2.9 5 f o r PbS and 2.12 f o r F e S 2 , 1/n = c o n s t a n t w i t h v a l u e o f 0.42 f o r PbS and 0.83 f o r F e S 2 . (2) The i n c r e a s e o f t h e chromate a d s o r p t i o n c r e a t e s a p r o p o r t i o n a l d e c r e a s e o f t h e y i e l d o f m i n e r a l by f l o t a t i o n . (3) The a d s o r p t i o n i n c r e a s e d w i t h t h e i n c r e a s e o f t h e c o n c e n t r a t i o n o f t h e s o l u t i o n up t o pH 6 and becomes a l m o s t c o n s t a n t a t t h i s v a l u e and v a l u e s above. (4) The chromate i s a d s o r b e d i r r e g u a l r l y on t h e m i n e r a l s u r f a c e s , t a k i n g p r o b a b l y t h e f r e e p l a c e s i n t h e c r y s t a l l a t t i c e . From t h e s e e x p e r i m e n t a l r e s u l t s , t h e y c o n c l u d e d t h a t t h e d e p r e s s i v e e f f e c t o f chromate i o n s t o o k t h e form o f c h e m i c a l a t t a c h m e n t t o t h e m i n e r a l s u r f a c e s , as a r e s u l t o f w h i c h t h e m i n e r a l s u r f a c e became h y d r a t e d ; under t h e s e c o n d i t i o n s , s u r f a c e h y d r a t i o n was so g r e a t t h a t t h e m i n e r a l c o u l d no l o n g e r be f l o a t e d i n s p i t e o f t h e p r e s e n c e o f a x a n t h a t e a d s o r p t i o n l a y e r on i t s s u r f a c e o f a d e n s i t y up t o 30-33 p e r c e n t o f a complete monolayer. The d e p r e s s i v e e f f e c t o f p o t a s s i u m chromate and p o t a s s i u m d i c h r o m a t e i s r e g a r d e d as due t o a d s o r p t i o n o f chromate a n i o n s on t h e s u r f a c e o f g a l e n a and p y r i t e ; t h e s e a n i o n s a t t a c h t h e m s e l v e s c h e m i c a l l y t o t h e m i n e r a l s u r f a c e s . A c c o r d i n g t o t h e r e s u l t s o b t a i n e d by P l a k s i n and M y a s n i k o v a from s t u d y i n g t h e d e p r e s s i v e a c t i o n o f chromate i o n s on . 7 p y r i t e and galena by means of trace microradiography, the chemical attachment of chromate ions to mineral i s independent of xanthate ions attachment and apparently takes place i n d i f f e r e n t areas [3]. The study of the depressive action of chromates on galena and p y r i t e , which was c a r r i e d out at the Mekhanobr I n s t i t u t e of U.S.S.R. [ 9 ] , supports the hypothesis given by Plaksin et a l . Plaksin and Myasnikova [10] studied the depressive 51 action of chromate on galena and p y r i t e by using Cr isotope, with ethylxanthate as the f l o t a t i o n agent and pine o i l as the foaming agent. They found that the maximum chromate ion adsorption on the sulphide mineral surface r e s u l t e d i n a minimum of t h e i r extraction. In the contrast to references [7] and [8] the chromate adsorption was af f e c t e d by the pH of the so l u t i o n , was zero at pH 0, and reached 0.2 mg/g at pH above 6, a f t e r which i t remained constant. They hypothesized that the depressive action of chromate ion was at t r i b u t e d to the formation of very s l i g h t l y soluble normal or basic chromates on the mineral surface. Bogdanov and Podnek [4] also examined the e f f e c t of potassium chromate on galena and copper sulphides. Experiments on galena f l o t a t i o n with variable chromate s a l t consumption showed the t o t a l galena depression occurs when a depressant adsorption layer, approximately equal to a monolayer, i s formed; at t h i s point adsorption of butylxan- 8 thate remains the same as without a depressant. Depression of galena by potassium chromate was explained by the hydro- p h i l i z a t i o n e f f e c t of chromate ion, which covers the hydro- p h o b i l i z a t i o n e f f e c t of xanthate ions on the surface. Galena depression appeared most e f f e c t i v e at pH 5-6 and above 8. The sharp impairment of galena f l o t a t i o n at above pH 8 was explained as due to the f a c t that i n t h i s region potassium chromate i s i n solution with chromate ions, forming lead chromate, and to the depressive action of hydroxyl ions. Mitrofanov and Kushnikova [11] studied the adsorp- t i o n of chromium compounds on sulphide minerals by using 51 radioactive isotope Cr i n the form of dichromate. The e f f e c t of various f a c t o r s , such as pH, temperature, potassium dichromate concentration, contact time, etc. on the adsorp- t i o n of chromium compounds on sulphide minerals. They found that the adsorption of chromium compounds on galena increases with the increase i n temperature, concentration of potassium dichromate and contact time. They suggested that the adsorption of dichromate ion was evidently connected with HCrO^ and CrO", rather than with CrCOH)^ because sorption was reduced by approximately h a l f on min- era l s washed with HC1. Contrary to the conclusion of Mitrofanov et a l . , Stepanov and Nagirnyak [12] suggested that the depression of galena and p y r i t e occurs as a r e s u l t of formation of 9 oxide films on t h e i r surfaces. In the intermediate pH range, both dichromate and chromate ions are present i n the s o l u t i o n , regardless of which chromate s a l t i s added to the pulp; the e f f e c t of each of these ions i s determined by i t s concentration i n the pulp, and depends on the pH of the solut i o n . They made the thermodynamic calc u l a t i o n s f o r the ox i d i z i n g reactions of chromium s a l t s on xanthates. According to t h e i r c a l c u l a t i o n , under standard conditions the lower xanthates are oxidized, and the higher xanthates are not oxidized. Mitrofanov and Kushnikova [13], [14] have studied the e f f e c t of .potassium dichromate and sodium sulphide on adsorption of an amine on sulphides of heavy metals i n connection with t h e i r f l o a t a b i l i t y . According to the experiments using potassium dichromate and t r i - 14 decylamme labeled with C , potassium dichromate increased the adsorption of tridecylamine on sphalerite and py r i t e at a l l pH values; however, on galena, t h i s increase was sh i f t e d toward a l k a l i n e pH values, with a maximum at about pH 10. Then, the following conditions f o r the separation of sphalerite from galena were determined: use of potassium dichromate 3-5 kg/ton, amine consumption 20 g/ton, pH 7.2, extraction of zinc into the zinc concentrate 68.0-72.0% and that into Pb concentrate 26.0-33.0%. Yamasaki et a l . [15] studied the action of potassium chromate on galena by means of electron d i f f r a c - 10 t i o n . They observed the formation of lead chromate f i l m on the galena surface when the galena specimen having 2mm x 2mm x 1mm was treated with 0.2 M/L of potassium chromate solut i o n f o r 24 hours. An aggregate of f i n e p o l y c r y s t a l l i n e lead chromate was.suggested as the surface product. An electrochemical survey of galena f l o t a t i o n was done by Pomianowski and Czubak-Pawlikowska [16]. They observed that the r e s t p o t e n t i a l of galena electrode decreased when potassium ethyl xanthate was added, and that an addition of dichromate caused the increase of the po t e n t i a l proportionate to the logarithm of i t s concentra- t i o n . Solov'eva and Khokhlova [17] also measured the electrode p o t e n t i a l of galena i n dichromate solutions. They found that the treatment of electrode with concentrated dichromate solu t i o n (1 g/L) resulted i n the covering of electrode with a lead chromate f i l m and no decrease of po t e n t i a l was observed a f t e r addition of xanthate. At 2 0 mg/L of dichromate s o l u t i o n , the addition of xanthate caused the decrease of p o t e n t i a l , and i t was e s p e c i a l l y large at pH around 7. Poole reported the e f f e c t of dichromate i n f l o t a t i o n at the Su l l i v a n m i l l at the year of 1920 [18]. EXPERIMENTAL Materials The galena, from Galena, Kansas was supplied by Ward's Natural Science Establishment Inc. The mineral specimen grade of galena was ground i n a mortar and pestle and screened into size f r a c t i o n s . Enough of the mineral f o r a l l the runs was ground i n i t i a l l y and stored i n a vacuum desiccator. Only - 200 + 270 mesh, - 270 +325 mesh and - 325 + 400 mesh" size f r a c t i o n s were used f o r adsorption t e s t s . Galena samples f o r i n f r a r e d spectroscopy, were pre- pared by a further grinding i n an agate mortar f o r 3 0 minutes. Pure potassium ethyl xanthate was synthesized by a standard method using potassium hydroxide, methanol and carbon disulphide and p u r i f i e d three times by r e c r y s t a l l i z - ation with ethyl alcohol and ethyl ether [32]. Xanthate so l u t i o n was always prepared j u s t before every experiment. Dixanthogen preparation was achieved by o x i d i z i n g the aqueous solut i o n of pure xanthate with iodine, extracting the product with ethyl ether and then drying i t under reduced pressure [33]. Potassium chromate, potassium dichromate, potassium hydroxide, lead sulphide of reagent grade supplied Mesh sizes r e f e r to Tyler standard screens. 12 from the Fisher S c i e n t i f i c Company were used. A l l solutions were made with d i s t i l l e d water. The pe r c h l o r i c acid was Baker and Adamson, 6 0%. Lead chromate used f o r the adsorption t e s t of xanthate was pr e c i p i t a t e d by the reaction between lead n i t r a t e and potassium chromate of reagent grade, followed by f i l t r a t i o n , r i n s i n g with d i s t i l l e d water and drying under reduced pressure. For the purging of dissolved oxygen from aqueous solutions, helium or nitrogen gas supplied by Canadian Liquid A i r Company Limited, was used without further p u r i f i c a t i o n . A n a l y t i c a l Equipment A Perkin-Elmer Model 450 UV-VIS-NIR Recording Spectrophotometer and matched 1.0 cm Corex c e l l s were used fo r the determination of xanthate, chromate and dichromate ion concentrations. The spectrophotometric determination of xanthate ion was based on i t s absorption peak at the wave length of 301 my. Chromate and dichromate ions were q u a n t i t a t i v e l y analyzed by measuring the o p t i c a l densities at 370 my and 345 my, res p e c t i v e l y . The i d e n t i f i c a t i o n of adsorbed species of chromate or dichromate ions on galena, synthetic lead sulphide and xanthate ions of lead chromate was achieved by means of "Helium: 99.995% , moisture <10 P.P.M., D.P. -76°F. s'c ii G Grade Nitrogen: 99.5 to 99.8%, argon content included, oxygen 2-5000 P.P.M., moisture 128 P.P.M., D.P. -40°F. i n f r a r e d spectroscopy. A Perkin-Elmer Double-beam Infrared Spectrometer, Model 5 21 was used f o r t h i s purpose. The measurements of r e s t p o t e n t i a l of galena elec trode was done with a Beckman Electroscan Model 30 and a matched electrode assembly. The measurement of pH were mad using the same equipment, but a combination electrode was used. Experimental Procedures (1) Adsorption and Desorption Tests Adsorption and desorption tests were conducted i n a c y l i n d r o c o n i c a l glass vessel whose diameter was 4.5 cm. and the length of c o n i c a l part was 7.0 cm. A bubbling t i p f o r nitrogen gas or a i r flow was inserted to allow gas bubbles to agitate the galena samples gently. Some adsorp- t i o n tests were carried, out i n a glove box under a c o n t r o l l atmosphere. A c e r t a i n amount of galena was put into the glass vessel, and the necessary amount of solu t i o n was added. Then the apparatus was put i n a water bath whose temperature was kept at 25 ± 0.5°C. Some experiments were done .at room temperature when i t was necessary. A f t e r a c e r t a i n period of a g i t a t i o n , the solution was f i l t e r e d using a M i l l i p o r e f i l t e r or separated from galena p a r t i c l e s by a centrifuge. Then the solu t i o n was subjected to a spectrophotometric determination of the concentrations of 14 xanthate, chromate or dichromate ions. Dixanthogen i n a solutio n or formed on s o l i d p a r t i c l e s was also determined spectrophotometrically when i t was necessary, a f t e r f i r s t e xtracting i t with hexane. The amount of dixanthogen was sometimes checked by pyridine c o l o r i m e t r i c method [19]. • (2) Buffer Action Tests In order to study the buffer action of galena, f i v e grams of galena powder (- 200 + 270 mesh) was put i n one hundred m i l l i l i t e r s of d i s t i l l e d water adjusted at various pH values. The same glass vessel was used as used i n adsorption t e s t s . Then the change i n pH of the solut i o n was recorded f o r t h i r t y minutes using a combination electrode. For pH adjustment, p e r c h l o r i c aqid and potassium hydroxide solutions were used. A i r and nitrogen gas were used to agitate the galena powders gently. (3) I d e n t i f i c a t i o n by Infrared Spectroscopy i I d e n t i f i c a t i o n of adsorption products of xanthate and of chromate or dichromate on natural galena, synthetic lead sulphide, and lead chromate was made by i n f r a r e d spectroscopy. Extremely f i n e powder of galena was made with an agate mortar to i d e n t i f y the adsorption products of dichromate on the galena surface. One gram of the sample The surface products on the surface of f i n e powder may be.different fromthose on the surface of large p a r t i c l e s , because the conditions of the surface a r e - d i f - ferent. 15 powder was immersed i n one hundred m i l l i l i t e r s of -2 1.0 x 10 M/L potassium dichromate solution f o r one hour at 25°C. A f t e r f i l t r a t i o n the sample was stored i n a vacuum desiccator. Synthetic lead sulphide was also used to i d e n t i f y the surface products of chromate and dichromate on galena. Lead chromate and chromate-treated lead sulphide were exposed to a 1.0 x 10 M/L xanthate solut i o n to i d e n t i f y the adsorption products of xanthate on lead chromate. One to two milligrams of these dried sample were then ground with f i v e hundred milligrams of potassium bromide and pressed into p e l l e t s at f i v e thousand p s i , and subjected to IR i d e n t i f i c a t i o n . EXPERIMENTAL RESULTS A. E f f e c t of pH on the Concentrations of Chromate and Dichromate Ions, and the E f f e c t of the Addition of Chromate on Xanthate S t a b i l i t y In the f l o t a t i o n of sulphide ores containing galena, both chromate and dichromate are used as depressants However, the r e l a t i o n s h i p between chromate and dichromate ions can be expressed as, 2CrO^ + 2H+ = 2HCrO~ = C r 2 0 ? + H 20 . . . . (4-1) The equilibrium condition of the reaction (4-2) can be calculated as Equation (4-3), using the necessary thermo- dynamic data given i n Latimer's book [20]. 2CrO~ + 2H+ = Cr 20^ + H 20 . . . . (4-2) pH = 7.3 + log (CrOp - 0.5 log (Cr 20^) . . . . (4-3) S i m i l a r l y the reaction (4-4) has the equilibrium condition which can be expressed by Equation (4-5). CrO^ + H + = HCrO" . . . . (4-4) pH = 6.46 - log (HCrO") + log (CrCO . . . . (4-5) In order to confirm such a r e l a t i o n s h i p , spectro- photometry determinations were made f o r chromate ion and dichromate ion concentrations. Figure 1 shows absorption spectra of chromate solution as well as dichromate s o l u t i o n . The peak of absorption band of chromate ions was found at the wave length of 37 0 my and that of dichromate ions at 345 my, thus the concentrations of each species can be determined by measuring the o p t i c a l densities at the wave length of 345 my and 370 my. A345 = e ' c C c + e d C d A370 = e c C c + e ' d C d Where A0,,c and A o n r are- absorbance at 345 my and OHO O / U 370 my r e s p e c t i v e l y , C c and are concentrations of chromate #> ion and dichromate ion, e amd e' are the molar e x t i n c t i o n ' • c c c o e f f i c i e n t of chromate at 37 0 my and 345 my, and and e'; are the molar e x t i n c t i o n c o e f f i c i e n t of dichromate at d 345 my and 37 0 my. The concentrations of chromate and dichromate ions were determined at the d i f f e r e n t l e v e l s of pH and were depicted i n Figure 2. The s t a b i l i t y of each ions ican be shown graphically as i n Figure 2, and i n the neutral s o l u t i o n , both ions coexist i n s o l u t i o n , regardless of which s a l t i s used. *e i s 0.482xl04/M and e' i s 0.240xl0 4/M. c c " " E d i s 0.315xl04/M and e'd i s 0.112xl0 4/M. The molar extinction coefficients of chromate at pH 9.5 and d i - chromate at pH 1.0 were determined s t a t i s t i c a l l y using 1 cm Corex c e l l s . . . . . (4-6) . . . . (4-7) Figure 1 Absorption Spectra of (a) Potassium Chromate S o l u t i o n (pH 11.5) and' (b) Potassium D i - chromate S o l u t i o n (pH 4.3). j . The ab s o r p t i o n s p e c t r a of potassium e t h y l xanthate potassium chromate, and t h e i r mixture i n an aqueous s o l u t i o n are shown i n Figure 3. The c o n c e n t r a t i o n _ 5 of potassium e t h y l xanthate was 6.0 x 10 M/L, and t h a t -4 of chromate was 2.0 x 10 M/L. The spectrum of the mixture was taken f o r t y minutes a f t e r m ixing, and the pH of the s o l u t i o n was 9.81. At any wave l e n g t h , the o p t i c a l d e n s i t y of the mixture s a t i s f i e d the r e l a t i o n s h i p expressed by Equation (4-7). A = C e + C e . . . . (4 x x c c where A i s the absorbance of the mixture, C c o n c e n t r a t i o n ' x of xanthate i o n s , C c o n c e n t r a t i o n of chromate i o n s , e the ' c ' c molar e x t i n c t i o n c o e f f i c i e n t of chromate s o l u t i o n , and e ' x the molar e x t i n c t i o n c o e f f i c i e n t of xanthate s o l u t i o n . Therefore, the r e s u l t i n d i c a t e s the s a t i s f a c t o r y s t a b i l i t y of the mixture of chromate and xanthate ions under these c o n d i t i o n s . B. Changes i n pH of the S o l u t i o n s Containing Galena (1) Changes i n pH of Aqueous S o l u t i o n s Containing Galena An attempt was made to determine the s t o i c h i o m e t r y of the ad s o r p t i o n r e a c t i o n of chromate ions on galena by a n a l y z i n g chromate i o n c o n c e n t r a t i o n and measuring pH. However, the change i n pH of the s o l u t i o n c o n t a i n i n g galena 0 2 4 6 8 10 12 14 pH Figure 2 S t a b i l i t y of Chromate Ion and Dichromate Ion as a Function of pH 21 alone was found to be very l a r g e i n the presence of oxygen. Both the changes i n pH and the amount of chromate ions adsorbed on galena surface were l a r g e r when oxygen was present i n s o l u t i o n than i n the absence of oxygen as mentioned l a t e r . From these observations i t would appear t h a t the r e a c t i o n system i s r a t h e r complicated. In f a c t , because the r a t i o s between chromate ions adsorbed and hydrogen ions consumed were not found to be a constant f o r the d i f f e r e n t l e v e l s of i n i t i a l pH and of i n i t i a l c o n c e n t r a t i o n of chromate i o n s , the s t o i c h i o m e t r i c determination of a d s o r p t i o n r e a c t i o n of chromate ions on galena surface was u n s u c c e s s f u l . Therefore, f i r s t of a l l , the change i n pH of water c o n t a i n i n g galena was s t u d i e d . F i v e gram q u a n t i t i e s of galena (- 270 + 325 mesh) were put i n one hundred m i l l i l i t e r s of double d i s t i l l e d water of v a r i o u s pH adjusted by per- c h l o r i c a c i d , and the changes i n pH w i t h time were measured con t i n u o u s l y during gentle a g i t a t i o n . Figure 4 shows the changes of pH of water c o n t a i n i n g galena i n the presence of oxygen. The value of pH s h i f t e d toward 7, r e g a r d l e s s of the i n i t i a l pH. S i m i l a r experiments were c a r r i e d out under the same experimental c o n d i t i o n s but w i t h galena prepared i n a glove box under a n i t r o g e n atmosphere. The water used was deaerated by n i t r o g e n gas bubbling. The r e s u l t obtained are shown i n Figure 5. In the a c i d i c range, the changes i n pH were s i m i l a r t o those i n the presence of oxygen, but the changes i n the a l k a l i n e r e g i o n were not so l a r g e .  In order to o b t a i n a b e t t e r understanding of the way i n which galena i s r e s p o n s i b l e f o r the pH of the s o l u t i o n , t i t r a t i o n experiments were designed. One hundred m i l l i l i t e r s of double d i s t i l l e d water c o n t a i n i n g f i v e grams of galena was adjusted t o pH 3 by p e r c h l o r i c a c i d , and was t i t r a t e d w i t h 0.01 N sodium hydroxide. The r e s u l t s are shown i n Figure 6. Curve (a) shows a t i t r a t i o n i n the pre- sence of oxygen, wh i l e curve (b) shows corresponding r e s u l t s without oxygen. The former has a simple shape compared t o the l a t t e r . A l l experiments mentioned above were c a r r i e d out without removal of oxygen contained i n the n i t r o g e n gas used. Since the o x i d a t i o n of a galena surface by oxygen contained i n n i t r o g e n gas might occur, one must leave room f o r some v a r i a t i o n . I t would be very i n t e r e s t i n g to examine the change i n pH of the s o l u t i o n a v o i d i n g oxygen contamination as completely as p o s s i b l e , however, f u r t h e r experiments were not done i n t h i s work. (2) Changes i n pH of a Chromate S o l u t i o n Containing Galena S i m i l a r experiments as mentioned above were c a r r i e d -4 out using 2 x 10 M/L potassium chromate s o l u t i o n . The r e s u l t s obtained i n the presence and absence of oxygen are shown i n Figure 7 and Figure 8, r e s p e c t i v e l y . In the pre- sence of oxygen, when s t a r t i n g pH values were 9.9 and 10.9, pH values d i d not change s i g n i f i c a n t l y a f t e r t h i r t y minutes Figure 4 Time Dependence of H„0 w i t h Galena Suspension Figure 5 Time Dependence of pH of the S o l u t i o n Containing Unoxidized Galena 0 1 - 2 3 4 5 " 6 • 7 0.01N NaOH (ml.) Figure 6 T i t r a t i o n Curves (a) Oxidized Galena '(b) Unoxidized Galena (c) Water Only 26 a g i t a t i o n , but when i n i t i a l pH was below 9, pH of the s o l u t i o n increased w i t h time. In the previous experiments, pH of the s o l u t i o n s which d i d not co n t a i n chromate s a l t changed from 3 to 3.5 and 4 to 5.6 a f t e r t h i r t y minutes s t a n d i n g , but i n those experiments, pH s h i f t e d from 3 to 4.5 and from 4 to 7, r e s p e c t i v e l y . This i n d i c a t e s t h a t pH s h i f t of the s o l u t i o n s c o n t a i n i n g chromate s a l t was l a r g e r than t h a t of the s o l u t i o n s without chromate s a l t . S i m i l a r phenomena were observed when the oxygen was excluded. The pH values of the s o l u t i o n s which d i d not co n t a i n chromate s a l t i n c r e a s e d g r a d u a l l y , whereas t h a t of the s o l u t i o n s c o n t a i n i n g chromate s a l t i ncreased r a p i d l y , when i n i t i a l pH was 3. Figure 9 shows a t i t r a t i o n curves f o r a chromate s o l u t i o n and f o r a chromate s o l u t i o n c o n t a i n i n g galena. C. A d s o r p t i o n Rates of Chromate and Xanthate Ions by Galena P l a n k s i n and Myasnikova [ 4 ] , and Bogdanov and K a k o v s k i i [9] reported t h a t the chemical attachment of chromate ions t o m i n e r a l s , such as p y r i t e , galena, e t c . , was independent of xanthate i o n attachment and apparently took place i n d i f f e r e n t areas. In order to o b t a i n an understanding of the ad- s o r p t i o n of chromate and xanthate on galena s u r f a c e , i n the system of galena-xanthate-chromate experiments were conducted to i n v e s t i g a t e the competitive a d s o r p t i o n between chromate 27 Figure 7 Time Dependence'of pH of 2.0 x 10 M/L Potassium Chromate S o l u t i o n Containing Galena ,• . 11* 6 1 I 1 1 \ I I I 0 5 10 15 2 0 25 • -3 0 • TIME, minute ' • -h Figure 8 Time Dependence of .pH of 2.0 x 10 M/L Potassium Chromate S o l u t i o n Containing Unoxidized Galena and xanthate i o n s , the d e s o r p t i o n of chromate ions due to the a d s o r p t i o n of xanthate from galena p r e t r e a t e d w i t h chromate, and the d e s o r p t i o n of xanthate from galena surface p r e t r e a t e d w i t h xanthate during the chromate ad s o r p t i o n t e s t . P r i o r to proceeding w i t h such ad s o r p t i o n and de- s o r p t i o n experiments, the a d s o r p t i o n r a t e s of chromate and xanthate ions on galena were s t u d i e d . The r e s u l t s obtained -4 f o r the s o l u t i o n s c o n t a i n i n g 1.6 x 10 M/L xanthate and -4 4.0 x 10 M/L chromate r e s p e c t i v e l y are shown i n Figure 10. Though the c o n c e n t r a t i o n of chromate ions was higher than t h a t of xanthate ions the a d s o r p t i o n of chromate ions i s f a i r l y slow compared t o t h a t of xanthate i o n s . I t i s p r e d i c t e d from these r e s u l t s t h a t xanthate a d s o r p t i o n i s completed before chromate a d s o r p t i o n when galena i s immersed i n a s o l u t i o n c o n t a i n i n g both xanthate and chromate i o n s . D. Ad s o r p t i o n of Chromate Ions and Xanthate Ions on Galena Surface In order t o understand the a d s o r p t i o n s t a t e of xanthate and chromate ions on galena, a competitive adsorp- t i o n study may provide u s e f u l i n f o r m a t i o n . Three kinds of s o l u t i o n s , one c o n t a i n i n g potassium chromate alone, one c o n t a i n i n g potassium xanthate alone, and the other c o n t a i n - i n g both chromate and xanthate i o n s , were used. The r e s u l t s obtained are shown i n Table I . 0 5 10 15 20 0.01N NaOH (ml.) Figure 9 T i t r a t i o n Curves of Chromate S o l u t i o n s (I) Chromate S o l u t i o n Only ( I I ) Containing Galena 31 10 bO 0 10 . 20 30 40 TIME, minute Figure 10 • Adsorption' Rates of Chromate and Xanthate on • >. Galena at 25°C. ' Galena (-325 + 400 mesh) 10 g. K 2CrO ) 4 (4.0 x ' 1 0 - 4 M/L) ' 50 mis., I n i t i a l pH 7.2 • K.Et.X.(1.6 x I O - 4 M/L) 50 mis., I n i t i a l pH 7,4 F i r s t of a l l , ten grams of galena (- 325 + 400 -4 mesh) was put i n t o f i f t y m i l l i l i t e r s of 4.0 x 10 M/L potassium chromate s o l u t i o n and aerated f o r t h i r t y minutes at 25°C. I n i t i a l pH of the s o l u t i o n was 7.35, and pH i n c r e a s e d s l i g h t l y t o 7.56 a f t e r t h i r t y minutes r e t e n t i o n . The amount of chromate ions adsorbed by galena was 1.24 x 5 10 moles per gram galena. The a d s o r p t i o n of xanthate by galena was s t u d i e d _5 u s i n g a n e u t r a l pH s o l u t i o n . When 8.0 x 10 M/L and 1.6 -4 x 10 M/L xanthate s o l u t i o n s were used, xanthate ions -7 -7 adsorbed were 3.5 x 10 moles/g galena and 7.7 x 10 moles /g galena, r e s p e c t i v e l y . I t i s seen t h a t the amount absorbed almost doubled when the i n i t i a l c o n c e n t r a t i o n of the s o l u t i o n was doubled. -4 In a s o l u t i o n c o n t a i n i n g 1.6 x 10 M/L potassium -4 xanthate and 4.0 x 10 M/L potassium chromate, the -7 a d s o r p t i o n of xanthate ions was 3.65 x 10 moles/g of galena _ g w h i l e the chromate a d s o r p t i o n was 0.79 x 10 moles/g of galena. The pH value changed from 7.2 5 t o 7.91 during the a g i t a t i o n . The a d s o r p t i o n of both chromate and xanthate were a l t e r e d c o n s i d e r a b l y under the competitive c o n d i t i o n s . The decrease measured i n the a d s o r p t i o n of chromate was 36 per cent, but i n xanthate 53 per cent. 33 TABLE I Adsorption of Chromate Ion and/or Xanthate Ion on Galena Surface Galena: -325+400 mesh 10.0 grams Xanthate and/or Chromate S o l u t i o n s : 5 0 mis. Retention Time: 30 minutes Temperature: 25°C. . I n i t i a l Concentration I n i t i a l pH Amount of Adsorption F i n a l pH CrO" (M/L) EtX (M/L) C r 0 4 (m/g) EtX (m/g) -4 -6 4.0 x. 10 0 7 .35 1.2 x 10 0 7 .56 0 8.0 x 10" 5 6 .85 0 3.5 x 10~ 7 6.98 0 1.6 x IO" 4 7 . 01 0 7.7 x 10" 7 7 .03 -4 , ~ -4 -6 -7 4.0 x 10 1.6 x 10 7 . 26 0.8 x 10 3.7 x 10 7.73 Since the r e s u l t s obtained by U.V. spectrophotometer are accurate to w i t h i n ±1 %. Considering the experimental accuracy, the concentrations of CrO^", Cr 90^~, EtX", and d i -xanthogen, d e r i v e d from U.V. absorbance data and presented i n Tables I to VI, are c o r r e c t e d to two s i g n i f i c a n t f i g u r e s . the a d s o r p t i o n t e s t , helium bubbling was continued f o r a g i t a t i o n purpose. The c o n c e n t r a t i o n of the chromate - 4 s o l u t i o n was 4 . 0 x 10 M / L . A f t e r t h i r t y minutes r e t e n t i o n the s o l u t i o n was separated from the galena p a r t i c l e s and i t was subjected to spectrophotometric a n a l y s i s . The other f r a c t i o n , which weighed e x a c t l y the same as the f i r s t , was taken out from the glove box i n order to i n v e s t i g a t e the oxygen e f f e c t . This sample was t r e a t e d i n a chromate s o l u t i o n w i t h a i r bubbling f o r a g i - t a t i o n purpose. The experimental c o n d i t i o n s were a l l the same except f o r the a i r bubbling. Completion of the same experiment twice gave s i m i l a r r e s u l t s and the averages of these r e s u l t s are shown i n Table I I . I n i t i a l pH of a chromate s o l u t i o n was 7 . 6 0 ( i n a i r atmosphere). The amount of chromate ions adsorbed on — 6 galena surface was 1 . 2 7 x 10 moles per gram of galena i n — 6 helium atmosphere and i n c r e a s e d to 1 . 7 4 x 10 moles per gram of galena i n air.atmosphere. The a d s o r p t i o n of chromat f o r the l a t t e r case i s about 1 . 4 times of t h a t of the former. This f a c t supports the p r e d i c t i o n t h a t oxygen may be i n v o l v e d i n the formation of lead chromate on galena s u r f a c e . Such a phenomenon i s reasonable from the thermo- dynamic p o i n t of view which w i l l be discussed l a t e r . 36 TABLE I I A d s o r p t i o n of Chromate Ion on Galena Surface i n the Presence or Absence of Oxygen Galena: -270+325 mesh 5.0 gram Potassium Chromate S o l u t i o n : 4.0 x 10 M/L 50 mis. Retention Time: 3 0 minutes I n i t i a l Temperature: 2 5°C. Atmosphere I n i t i a l A d s o rption of Ra t i o F i n a l pH CrO" (moles/g) pH He 7 .60 1.2 x 10" 6 1.0 ' 7.72 A i r 7 .60 1.7: x i o - 6 1.4- 7 .80 37 F. E f f e c t of pH on Chromate Adsorption on Galena Figure 11 shows the e f f e c t of pH on chromate ads o r p t i o n on galena. Ten grams of galena (- 325 + 400 — 2 mesh) was put i n one hundred m i l l i l i t e r s of 1.0 x 10 M/L potassium chromate s o l u t i o n s which had v a r i o u s i n i t i a l pH val u e s . A f t e r gentle a g i t a t i o n f o r ten minutes at 25°C, the s o l u t i o n s separated from s o l i d p a r t i c l e s were used f o r spectrophotometric determinations and pH measurements. In Figure 11, the amount of chromate ions consumed are given i n terms of both i n i t i a l and f i n a l pH v a l u e s . At pH below 9 chromate ad s o r p t i o n decreased l i n e a r l y w i t h pH, and above pH 9 i t remained constant. G. Desorption of Xanthate Ions or Chromate Ions From Galena Surface As mentioned b e f o r e , the a d s o r p t i o n and desorption experiments were c a r r i e d out i n order to understand the adsorbed s t a t e s of xanthate and chromate on galena. Table I I I and Table IV show the r e s u l t s obtained f o r the d e s o r p t i o n t e s t s of chromate ions and xanthate i o n s , r e s p e c t i v e l y . Ten grams of galena (- 325 + 400 mesh), which was -4 p r e - t r e a t e d w i t h 4.0 x 10 M/L potassium chromate f o r f o r t y minutes, was taken i n t o t w e n t y - f i v e m i l l i l i t e r s of 1.6 x -4 10 M/L potassium e t h y l xanthate and a g i t a t e d f o r t h i r t y minutes at 25°C. The amount of xanthate ions adsorbed 37a 2' 3 ' 4 • 5 . 6 7 8 9 10 11 . • • pH Figure 11 E f f e c t of pH on Chromate Adsorption on Galena on galena was 3.56 x 10 moles per gram of galena. This value was l e s s than h a l f of the amount adsorbed on un- t r e a t e d galena. In t h i s experiment, no detectable amount of chromate ions from galena surface was desorbed counter- b a l a n c i n g the xanthate a d s o r p t i o n . This i s true f o r a l l the other xanthate a d s o r p t i o n t e s t s . _5 Then, ten grams of galena p r e t r e a t e d w i t h 8 x 10 M/L xanthate s o l u t i o n were put i n t w e n t y - f i v e m i l l i l i t e r s -4 of 4.0 x 10 M/L potassium chromate s o l u t i o n and a g i t a t e d f o r t h i r t y minutes at 25°C. The value of pH of the s o l u t i o n moved from 7.3 2 t o 7.9 3 during the a g i t a t i o n . The amount of chromate ions adsorbed on xa n t h a t e - t r e a t e d galena was — 6 0.96 x 10 moles per gram of galena and was about 77 per cent of t h a t adsorbed on the f r e s h galena. No xanthate ions were detected i n the r e s u l t i n g s o l u t i o n . However, -7 1.12 x 10 xanthate moles per gram of galena were detected, -4 when ten grams of galena p r e t r e a t e d w i t h 1.6 x 10 M/L - 2 xanthate s o l u t i o n were immersed i n a 1.0 x 10 M/L chromate s o l u t i o n f o r t h i r t y minutes. Other experiments w i t h more concentrated s o l u t i o n s were performed. The r e s u l t s obtained were s i m i l a r t o the above, and the r e s u l t s are a l s o shown i n Tables I I I and IV. I t i s expected t h a t dixanthogen i s i n v o l v e d i n t h i s galena, chromate and xanthate system. In f a c t , Table V shows t h a t about 40 per cent of t o t a l xanthate ions consumed stays as dixanthogen on the surface and i n a bulk s o l u t i o n . 39 TABLE I I I Desorption of Chromate Ion from Galena Surface Due t o Xanthate Adsorption Galena: -325+400 mesh 10.0 grams Potassium E t h y l Xanthate: 25 mis. Retention Time: 30 minutes Temperature: 25°C. I n i t i a l pH of Xanthate S o l u t i o n s : 6.3-7.8 Cone. of CrO = Ads. I n i t i a l Cone. CrO^ Des. EtX Ads. CrOJJCM/L) (mol es/g) of EtX"(M/L) (moles/g) (moles/g) 4.0 x IO -" 4 1.2 x I O - 6 1.0 x 10" 2 N.D. 3.6\- x 10 -7 4.0 x IO" - 4 1.1 x I O - 6 1.0 x 10~ 2 N.D. 1.4- x 10 -6 1.0 x 10" 2 - 1.6 x 10~ 4 N.D. 3.5 ^ x 10 -7 1.0 x 1 0 - 1 - 3.2 x 10" 4 N.D. 6 .6 x 10 -7 N.D. = Not Detectable Ads. = Adsorbed Des. = Desorbed Cone. = Concentration TABLE IV Desorption of Xanthate Ion from Galena Surface Due to Chromate Adsorption (1) Galena: -325+400 mesh 10.0 grams Potassium Chromate S o l u t i o n : 25 mis. Retention Time: 30 minutes Temperature: 2 5°C. I n i t i a l pH of Chromate S o l u t i o n s : 5.3-7.4 Cone. of EtX Ads. I n i t i a l Cone, EtX" Des. CrO~ Ads EtX"(M/L) (moles/g) of CrO =(M/L) (moles/g) (moles/g) 8.0 x 10" 5 3.5 x 10~ 7 4.0 x 10~ 4 N.D. 1.0 x 10 -6 1.6 x 1 0 - 4 7 . 6 x 10~ 7 1.0 x 10" 2 1.1 x 10~ 7 1.4-= • x 10 -6 1.0 x 10" 2 - 0 N.D. 0 1.0 x 10" 2 - 8.0 x 10~ 4 2.4- x 10~ 7 1 . 2 . x 10 -6 N.D. = Not Detectable Ads. = Adsorbed Des. = Desorbed Cone. = Concentration This experiment f o r xanthate d e s o r p t i o n was made using twenty grams of galena (- 270 + 325 mesh), one hundred -4 m i l l i l i t e r s of 1.0 x 10 M/L potassium xanthate. A f t e r s e p a r a t i n g s o l i d s from the s o l u t i o n , dixanthogen was ex- t r a c t e d w i t h f i f t y m i l l i l i t e r s of hexane, and subjected to a spectrophotometric determination. The amount of xanthate ions s t a y i n g as lead xanthate was c a l c u l a t e d from the d i f f e r e n c e between t o t a l xanthate ions consumed and d i - xanthogen on galena s u r f a c e . I n i t i a l pH of potassium e t h y l xanthate was 5.92 and t h a t of potassium chromate s o l u t i o n was 7.60. These r e s u l t s show that 66 per cent of lead xanthate and 40 per cent of dixanthogen i n i t i a l l y present were desorbed from galena surface due to adding potassium chromate. I t i s seen from these simple experiments t h a t the a d s o r p t i o n of xanthate on chromate-treated galena was much more a f f e c t e d than t h a t of chromate onto xanthate- t r e a t e d galena. I t i s a l s o seen t h a t xanthate d e s o r p t i o n from galena surface i s p o s s i b l e when the m i n e r a l i s t r e a t e d w i t h a concentrated s o l u t i o n of chromate, w h i l e no de- s o r p t i o n of chromate i o n i s observed due to xanthate a d s o r p t i o n . These r e s u l t s agree w i t h those obtained i n the compet i t i v e a d s o r p t i o n t e s t s of chromate and xanthate ions on galena and w i t h the r e s u l t s obtained from the a d s o r p t i o n experiment of xanthate ions on lead chromate. I t would have been very i n t e r e s t i n g t o i n v e s t i - gate the d e s o r p t i o n of xanthate, which i s l e s s than mono- TABLE V Desorption of Xanthate from Galena Surface Due to Chromate Adsorption (2) Twenty grams of galena (-270+325 mesh) were t r e a t e d w i t h 100 mis. of 1.0 x IO" 4 M/L potass ium e t h y l xanthate s o l u t i o n f o r ten minutes at 25°C. (as xanthate ion) Xanthate adsorbed on galena 7. 7 " x I O - 6 moles as lead xanthate 4. 1 ' x I O - 6 moles as dixanthogen 3.6 . x I O - 6 moles Dixanthogen i n an aqueous s o l u t i o n 1.2- x I O - 6 moles Then galena mentioned above was t r e a t e d w i t h 5 0 mis . of 1.0 x 10 M/L potassium chromate s o l u t i o n f o r ten minutes at 25°C. Chromate adsorbed on galena 1. 3<< X 10" 6 moles Xanthate remained on galena 4. 2A X 10" 6 moles as lead xanthate 2 . 7 X 10" 6 moles as dixanthogen . 1. 5 X 10" 6 moles Dixanthogen desorbed i n t o s o l u t i o n 0 . 2 X 10" 6 moles Xanthate i o n desorbed i n t o s o l u t i o n 3 . 3 X 10" 6 moles 43 l a y e r coverage, from galena s u r f a c e . However, the s e n s i t i v - i t y of a n a l y t i c a l technique used i n t h i s study was i n - s u f f i c i e n t f o r such a purpose. H. Adsorption of Xanthate Ions by Lead Chromate In order to examine whether chromate ions are re l e a s e d or not to a s o l u t i o n from lead chromate when xanthate ions are adsorbed on i t , a d s o r p t i o n t e s t s of xanthate on lead chromate were conducted. One gram of p r e c i p i t a t e d f i n e l e a d chromate was placed i n an Ehrenmyer f l a s k which contained one hundred m i l l i l i t e r s of xanthate s o l u t i o n and a g i t a t e d mechanically f o r t h i r t y minutes. The s o l u t i o n was then subjected to spectrophoto- m e t r i c determination of xanthate ions as w e l l as chromate i o n s . This was done f o r s o l u t i o n s of d i f f e r e n t xanthate c o n c e n t r a t i o n s . The r e s u l t s obtained are shown i n Table VI. When the i n i t i a l xanthate c o n c e n t r a t i o n was 1.0 _2 x 10 M/L, the amount of xanthate ions adsorbed per one -4 gram of lead chromate was 1.8 x 10 moles while the amount _5 of chromate m the s o l u t i o n was found to be 3.9 x 10 _ 3 moles a f t e r t h i r t y minutes a g i t a t i o n . When 1.0 x 10 M/L _5 xanthate s o l u t i o n was used 1.4 x 10 moles of xanthate ions were adsorbed per one gram of lead chromate. This i s approximately one tenth of the amount adsorbed when 1.0 x 10 M/L xanthate s o l u t i o n was used. The amount of chrom- _5 ate ions r e l e a s e d i n the l a t t e r case was 4.7 x 10 moles. :0n the other hand, when no xanthate was added, 5.8 x 10 ^ moles of chromate were found i n the s o l u t i o n a f t e r t h i r t y minute a g i t a t i o n . By using hexane as a so l v e n t f o r dixanthogen, i t was a l s o found t h a t more than 80 per cent of t o t a l xanthate consumption stayed as dixanthogen i n t h i s system. These r e s u l t s show tha t the r e l e a s e of chromate ions from lead chromate was decreased s l i g h t l y by xanthate a d s o r p t i o n on l e a d chromate. I . Thermodynamic Considerations of the Reactions Involved In The System Pb-Cr-S-H 20 In order to get i n f o r m a t i o n on the adsorbed specie of chromate or dichromate s a l t on galena, i t might be use- f u l to i n v e s t i g a t e the system Pb-Cr-S-I-^O thermodynamically For t h i s purpose, the p o t e n t i a l - p H diagram f o r the system Pb-Cr-S-H 20 at 25°C was constructed. Thermodynamic data used were c i t e d from Latimer [20] and Pourbaix [21]. The p o t e n t i a l - p H diagrams f o r the system Cr-I^O and Pb-S-H 20 are shown i n Figure 12 and 13 r e s p e c t i v e l y . The expected r e a c t i o n s and t h e i r e q u i l i b r i u m c o n d i t i o n s are given i n Table V I I . The p o t e n t i a l - p H diagram f o r the system Pb-Cr-S-H 20 i s shown i n Figure 14. In these f i g u r e s the s o l i d l i n e s represent the u n i t a c t i v i t y of the d i s s o l v e d 45 TABLE VI Adsorption of Xanthate Ion on P r e c i p i t a t e d Lead Chromate Lead Chromate ( p r e c i p i t a t e d ) : 1.0 0 gram Potassium E t h y l Xanthate S o l u t i o n : 100 mis. Retention Time: 30 minutes Temperature: 2 5°C. I n i t i a l Cone, of Xanthate (M/L) Adsorption of Xanthate (moles/g) CrO~ i n Soln.(moles) Pb i n Soln. 1.0.x 10" 2 1.0 x I O - 3 0 1.8 x 10" 4 1.4 x 10" 5 0 3.9 x IO" 5 4.7 -x 10" 5 5.8. x 10" 5 1 P.P.M. 2 P.P.M. 1 P.P.M. ions and the broken l i n e s an a c t i v i t y of 10 . The thermo- dynamic data f o r lead t h i o s u l p h a t e and b a s i c lead t h i o s u l p h a t were not a v a i l a b l e , t h e r e f o r e lead sulphate and b a s i c lead sulphate were used i n s t e a d . I t i s seen from these f i g u r e s t h a t galena i s o x i d i z e d t o lead sulphate or b a s i c lead s u l p h a t e , and t h a t beyond t h a t p o t e n t i a l lead chromate i s formed, i g n o r i n g the k i n e t i c f a c t o r s . Furthermore, lead chromate has a l a r g e s t a b l e domain. This i n d i c a t e s t h a t even i n the pH range where dichromate i o n i s s t a b l e , f o r example pH 1, the formation of l e a d chromate i s favored and there i s no lead dichromate s t a b l e domain. This i s e a s i l y checked by mixing the dichromate s o l u t i o n and lead n i t r a t e d i s s o l v e d i n a d i l u t e d n i t r i c a c i d whose pH i s 1. As mentioned above the formation of lead chromate i n the bulk system i s very much favored and lead chromate once formed has a very s t a b l e domain. J . I d e n t i f i c a t i o n of Reaction Products on the Surface of Galena Treated w i t h Potassium Chromate or Potassium Dichromate As mentioned p r e v i o u s l y , many i n v e s t i g a t o r s exam- ined the surface products on galena t r e a t e d w i t h a chromate or dichromate s o l u t i o n by e l e c t r o n d i f f r a c t i o n and r e p o r t e d t h a t lead chromate was formed on galena s u r f a c e . They used a s i n g l e c r y s t a l of galena and t r e a t e d i t w i t h a con- Figure 12 P o t e n t i a l - p H diagram f o r the system Cr-H-O H 2 S pH Figure 13 P o t e n t i a l - p H diagram f o r the system Pb-S-H„0 49 TABLE VII Reactions and E q u i l i b r i u m Formulae For The System Pb-Cr~S-H 20 1. Pb0 2 + C r O ^ + 4H + + 2e = PbCrO^ + 2H 20 E = 1.919 - 0.1182pH + 0.0295 l o g [ C r O ^ ] 2. 2Pb0 2 + C r 2 0 y = + 6H + + 4e = 2PbCr0 4 + 3H 20 E = 1.704 - 0.0886'pH + 0.0148 l o g [ C r 2 0 y = ] 3. Pb0 2 + H 2 C r 0 4 + 2H + + 2e = PbCrO^ + 2H 20 E = 1.706 - 0.0591pH + 0.0295 l o g [I^CrO^] 4. Pb0 2 + HCrO^" + 3H + + 2e = PbCr0 4 + 2H 20 E = 1.729 - 0.0886 pH + 0.0295 l o g [HCrO^ -] 5. PbCrO^ + S 0 4 = + 8H + + 3e = PbS0 4 + C r 3 + + 4H 20 E = 1.318 - o.l576 pH + 0.0197 l o g [ S O ^ ] - 0.0197 l o g [ C r 3 + ] 6. 2PbCr0 4 + 2 S 0 4 = ':+ 10H + + 6e = C r 2 0 3 + 2PbSC>4 + 5H 20 E = 1.153 - 0.0985 pH + 0.0197 l o g [ S 0 4 = ] 7. PbSO^'PbO + 2 C r 0 4 = + 2H + = 2PbCr0 4 + S 0 4 = + H 20 pH + | l o g CS0 4 =] - | l o g [ C r 0 4 = ] = 11.053 8. 2PbCr0 4 + S 0 4 = + 8H + + 6e = C r ^ + PbSO^'PbO + 4H 20 E = 1.094 - 0.0788 pH + 0.0098 l o g CS0 4 =] 9. Pb0 2 + HS0 4" + 3H + + 2e = PbS0 4 + 2H 20 E = 1. 626 - 0.0886 pH + 0.0295 l o g [HS04*~] 10. C r 2 0 7 = + H 2 ° + 2 H + = 2H 2Cr0 4 pH + l o g [H 2Cr0 4_l - | l o g [ C r 2 0 y ] = -0.091 11. 2 C r 0 4 = + 2H + = C r 2 0 7 = + H 20 pH + | l o g [ C r 2 0 7 = ] - l o g [ C r 0 4 = ] = 7.296 5 0 12,.: P h 0 3 = + 2H + = Pb0 2 + H 20 pH - | l o g [ P b 0 3 = ] = 15.660 12.- 2Pb0 2 + S0^~ + 6H + + 4e = PbSO^'PbO + 3H 20 E = 1.593 - 0.0886 pH + 0.0148 l o g [ S 0 4 = ] 14. 2Pb0 3 = + S 0 4 = + 10H + + 4e = PbSO^'PbO + 5H 20 E = 2.518 - 0.1480 pH + 0.0296 l o g [ P b 0 3 = ] + 0.0148 lof [ s o 4 = ] 15. 2 C r 0 4 = + 10H + + 6e = C r 2 0 3 + 5H 20 E = 1.310 - 0.0985 pH + 0.0197 l o g [ C r 0 4 = ] 16. H 2 C r 0 4 + 6H + + 3e = C r 3 + + 4H 20 E = 1.330 - 0.1182 pH - 0.0197 l o g [ C r 3 + ] + 0.0197 l o g [ H 2 C r 0 4 ] 17. PbSO^'PbO + S 0 4 = + 2H + = 2PbS0'4 + H 20 pH - j l o g [ S 0 4 = ] = 3.02 18. C r 2 0 3 + 6H + = 2 C r 3 + + 3H 20 pH + j l o g [ C r 3 + ] = 2.80 19. PbSO^ + 8H + + 6e = P b + + + S + 4H 20 E = 0.280 - 0.0788 pH - 0.0098 l o g [ P b + + ] 20. P b + + + S + 2e = PbS E = 0.354 + 0.0295 l o g [ P b + + J 21. PbSO^ + 8H + + 8e = PbS + 4H 20 E = 0.299 - 0.0591 pH 22. PbS0 4'PbO + S 0 4 = + 18H + + 16e = 2PbS + 9H 20 E = 0.321 - 0.0667 pH + 0.0037 l o g [ S 0 4 = ] 23. PbS + 2H + + 2e = Pb + H 2S E = -0.301 - 0.0591 pH - 0.0295 l o g [H 2S] 50a 9+ + + 24,. Cr + e = Cr E = -0.408 - l o g [ C r + + ] + l o g [ C r 3 + ] 25. C r 2 0 3 + 6H + + 2e = 2 C r + + + 3H 20 E = 0.088 - 0.1773 pH - 0.0591 l o g [ C r + + ] 26. PbCr0 4 + S 0 4 = + 2H + = PbSO^ + H 2 C r 0 4 pH - | l o g E S 0 4 = ] + | l o g [ H 2 C r 0 4 ] = -0.415 51 Figure 14 P o t e n t i a l - p H diagram f o r the system Pb-Cr-S-H-0 ..centrated chromate or dichromate s o l u t i o n . In a f l o t a t i o n •-.•condition, the same product i s expected to be formed on ::galena surface when a chromate or dichromate s a l t i s used as a depressant; but to prove t h i s , i t i s necessary to i n - v e s t i g a t e whether the same r e a c t i o n product i s obtained ;when galena i s t r e a t e d w i t h a d i l u t e s o l u t i o n which contains the same l e v e l of chromate or dichromate per u n i t surface area. .The author used a n a t u r a l galena powder and a l s o s y n t h e t i c galena t r e a t e d w i t h a chromate or dichromate s o l u - t i o n and t r i e d t o i d e n t i f y the products formed on galena surface by making p e l l e t s f o r use i n a i n f r a r e d spectrophotometer. To make a p e l l e t , an extremely f i n e galena powder was prepared according t o the procedure mentioned before. F i v e hundred m i l l i g r a m s of the powder was immersed i n f i f t y _ 2 m i l l i l i t e r s of 1.0 x 10 M/L dichromate s o l u t i o n whose pH was 4.2 and was mechanically s t i r r e d f o r one hour at room temperature. A f t e r f i l t e r i n g and d r y i n g the sample i n a vacuum d e s i c c a t o r , the p e l l e t was made w i t h KBr. The same procedure was a p p l i e d f o r lead sulphide which was t r e a t e d i n a dichromate and a chromate s o l u t i o n , and the pH values were 4.2 and 7.6 r e s p e c t i v e l y . The s p e c t r a of these three p e l l e t s which were scanned on a Perkin-Elmer Model 521 a g a i n s t KBr blanks are shown i n Figure 15 - ( a ) , (b) and ( c ) . As a comparison, a spectrum of lead chromate i s shown i n Figure 15 - ( d ) . The spectrum of lead sulphide t r e a t e d w i t h a -:chromate s o l u t i o n at pH 9,8 was the same as ( c ) . Spectra ( a ) , (b) and (c) show the absorption band v at 848 cm ^ which i n d i c a t e s the formation of lead chromate on galena s u r f a c e . The a b s o r p t i o n bands f o r chromium oxide chromium hydroxide, lead d i o x i d e , or lead hydroxide were not found, K. I d e n t i f i c a t i o n of Adsorption Products of Xanthate on Lead Chromate The same•IR s p e c t r o s c o p i c technique, which was employed f o r the i d e n t i f i c a t i o n of lead chromate on galena s u r f a c e , was used i n order to i d e n t i f y the a d s o r p t i o n products of xanthate on lead chromate. As the author mentioned before the s p e c t r a of powdered n a t u r a l galena t r e a t e d w i t h a chromate or dichromate s o l u t i o n d i d not give a very c l e a r a b s o r p t i o n band. Therefore i n t h i s case, lead chromate p r e c i p i t a t e d and s y n t h e t i c galena t r e a t e d w i t h potassium dichromate s o l u t i o n were used i n s t e a d of n a t u r a l galena t r e a t e d w i t h a chromate or dichromate s o l u t i o n . In order to t r e a t lead chromate w i t h xanthate, one gram of lead chromate and s y n t h e t i c galena p r e t r e a t e d w i t h a potassium dichromate s o l u t i o n whose pH was 5.6 were _2 placed m f i f t y m i l l i l i t e r s of 1.0 x 10 M/L potassium e t h y l xanthate whose pH was 6.8 r e s p e c t i v e l y and s t i r r e d f o r one hour at room temperature; the m a t e r i a l s were I n f r a r e d • S p e c t r a of (a) S y n t h e t i c PbS t r e a t e d w i t h K 2 C r 0 4 (pH 7.6), (b) S y n t h e t i c PbS t r e a t e d w i t h K Cr„0 ? (pH 4-. 2 ) , (c) Galena t r e a t e d w i t h K 2 C r 2 0 7 (pH'4.3), (d) PbCrO^. washed w i t h d i s t i l l e d water and d r i e d i n vacuum. A f t e r making p e l l e t s w i t h KBr, sp e c t r a were obtained. They are •shown i n Figure 16 - (a) and (b). Spectra of lead xanthate (c) and chromium oxide (e) are a l s o shown i n the same f i g u r e . Spectra (a) and (b) have four s i g n i f i c a n t absorp- t i o n bands r e s p e c t i v e l y at 1200 cm ^, 1106 cm ^, 1016 cm and 84 8 cm ^. The abso r p t i o n bands due to chromium oxide, dixanthogen, lead d i o x i d e , or lead hydroxide could not be obtained. The band at 848 cm ^ i s considered to represent lead chromate and others represent lead xanthate. However, these three a b s o r p t i o n bands i n s p e c t r a (a) and (b) do not agree e x a c t l y w i t h those i n spectrum ( c ) . P o l i n g and Le j a s t u d i e d the surface products on deposited lead sulphide t r e a t e d w i t h potassium e t h y l xanthate by i n f r a r e d s p e c t r o - photometer and suggested a 1:1 c o - o r d i n a t i o n between the surface metal atom and xanthate r a d i c a l i n the f i r s t mono- l a y e r . The d e v i a t i o n of ab s o r p t i o n peak f o r xanthate a d s o r p t i o n products on lead chromate may i n d i c a t e the p o s s i b i l i t y of 1:1 c o - o r d i n a t i o n between xanthate and lea d i n t h i s system. However, t h i s matter w i l l be discussed l a t e r . WAVENUMBER, CM 5 6 1400 1200 1000 800 600 Figure 16 I n f r a r e d Spectra of (a) PbCrCv t r e a t e d w i t h K.Et.X. (pH 7.2), (b) PbS t r e a t e d w i t h K9Cr2_0 7 (pH 6.3), then K.Et.X. (pH 7.2), (c) PMEtX) , (d) X , and (e) Cr 0 3 DISCUSSIONS A. Changes i n pH of the S o l u t i o n s Containing Galena As shown i n Figures 4 and 5, the explanations f o r b u f f e r a c t i o n can be given i n s e v e r a l ways. F i r s t l y when oxygen was excluded, i n a l k a l i n e s o l u t i o n s , no s i g n i f - i c a n t l a r g e s h i f t i n pH was observed, whereas the changes i n pH were f a i r l y l a r g e i n a c i d i c s o l u t i o n s . N e g l e c t i n g the o x i d a t i o n of galena by oxygen contained i n n i t r o g e n gas used i n these experiments, the s h i f t of pH to the a c i d i c s i d e might be due to a d i s s o l u t i o n of galena i n the per- c h l o r i c a c i d used to a d j u s t pH and the p r e f e r e n t i a l a d s o r p t i o n of hydrogen on galena. The d i s s o l u t i o n of galena i s expressed by the next equation: PbS + 2H + = P b + + + H 2S . . . . (5-1) Secondly, when oxygen i s present, the i n i t i a l o x i d i z e d product of galena i s thought to be lead t h i o s u l p h a t e or b a s i c lead t h i o s u l p h a t e . A number of i n v e s t i g a t o r s s t u d i e d the o x i d a t i o n r e a c t i o n of s u l f i d e minerals i n aqueous s o l u t i o n s i n the connection w i t h the f l o t a t i o n or hydro- meta l l u r g y of s u l f i d e ores. Hagihara et a l . [22] i n d i c a t e d the formation of lead sulphate. L e j a , L i t t l e and P o l i n g [23] suggested the formation of lead t h i o s u l p h a t e and, Eadington 5 8 and Prosser [24] reported the formation of b a s i c lead t h i o - •„:sulphate. In the l e a c h i n g of p e n t l a n d i t e or galena i n ammonical s o l u t i o n s at elevated temperature, Forward et a l . [25] found t h a t the i n i t i a l o x i d i z e d product was m e t a l l i c s a l t s of t h i o s u l p h a t e , and tha t the f i n a l product was m e t a l l i c s a l t s of sulphate. Therefore, i f the c o n d i t i o n s o f f l o t a t i o n are considered, the r e s u l t s obtained by Lej a et a l . , and Eadington et a l . seem to be more reasonable than t h a t by Hagihara et a l . [22] and by Abramov [26]. As pH of a s o l u t i o n i s in c r e a s e d , lead t h i o s u l p h a t e may convert to b a s i c t h i o s u l p h a t e . When the d i s s o l u t i o n of adsorbed species i s di s c u s s e d , s o l u b i l i t y data of adsorbed species must be used. S o l u b i l i t y data, however, are not a v a i l a b l e f o r the adsorbed species of lead t h i o s u l p h a t e or b a s i c lead t h i o s u l p h a t e on galena. The s o l u b i l i t y of le a d t h i o s u l p h a t e _ 3 i s r eported to be 0.94 x 10 M/L, while no data i s given on the s o l u b i l i t y of b a s i c lead t h i o s u l p h a t e . When the d i s s o l u t i o n of those o x i d i z e d species proceeds i n an a c i d i c s o l u t i o n , hydrogen i o n i s consumed. Therefore, i n the a c i d i c s i d e , hydrogen i o n consumption by the d i s s o l u t i o n of galena i t s e l f and the o x i d i z e d products of galena surface may cause the increase i n pH, and at the same time, the p r e f e r e n t i a l hydrogen i o n adso r p t i o n [27] may cause the pH to in c r e a s e . In a l k a l i n e s o l u t i o n s , then, the s h i f t of pH i s expla i n e d by the formation of lead hydroxide adsorbed or -deposited on the galena surface as w e l l as the p r e c i p i t a t i o n A .of lead hydroxide i n a bulk s o l u t i o n . As bulk lead . s u l f i d e i s l e s s s o l u b l e i n comparison to bulk lead hydroxide, and assuming t h i s tendency i s true f o r adsorbed s p e c i e s , .the formation of lead hydroxide on f r e s h galena surface i s not favored. This assumption seems reasonable because the s h i f t of pH i n the a l k a l i n e s i de was not l a r g e when oxygen was avoided. When an o x i d i z e d f i l m i s formed on galena s u r f a c e , the f i l m i s d i s s o l v e d by i n c r e a s i n g pH, f o l l o w e d by the r e a c t i o n of lead ions d i s s o l v e d w i t h hydroxyl i o n s . Thus PMOlDg i s p r e c i p i t a t e d on the galena surface and i n a bulk s o l u t i o n as w e l l . This e x p l a n a t i o n can be supported by the f a c t t h a t s l i g h t l y o x i d i z e d galena, or at l e a s t when oxygen i s present i n the system, galena can be f l o a t e d by s u l p h y d r y l c o l l e c t o r s , whereas no f l o t a t i o n i s observed i n a strong a l k a l i n e s o l u t i o n . B, Chemisorption of Chromate or Dichromate Ions on Galena Surface As mentioned i n the previous s e c t i o n , from the thermodynamic p o i n t of view, the formation of lead chromate on galena surface i s very much favored when galena i s t r e a t e d w i t h chromate s a l t or dichromate s a l t . In a c i d i c The s o l u b i l i t y of lead hydroxide i s 6.4 x 10 M/L and t h a t of galena i s 1.2 x 10 _ 6M/L. 60 •region, where dichromate i o n i s very s t a b l e , l e a d chromate 1*8 a l s o very s t a b l e . Furthermore, as seen from the i n f r a r e d s p e c t r a 'given i n Figure 15, a l l s p e c t r a have an a b s o r p t i o n band at 848 cm which i s assigned to Cr-0 symmetrical s t r e t c h [ 2 8 ]. However, the weak peaks at 7 95 and 935 cm ^ are a l s o observed. Both peaks may be assigned to Cr-O-Cr asymmetric s t r e t c h , and may i n d i c a t e the formation of lead dichromate on galena s u r f a c e . Anyway i t i s obvious t h a t the main product on galena surface i s lead chromate when galena i s t r e a t e d w i t h chromate or dichromate s a l t . On the other hand, as shown i n Figure 11, the amount of chromate uptake on galena i n c r e a s e d almost l i n e a r l y w i t h dichromate c o n c e n t r a t i o n . In a c i d i c r e g i o n , the amount of chromate ad s o r p t i o n was very l a r g e , and t h i s i n d i c a t e s the h y d r o p h i l i c property of galena s u r f a c e . Therefore, i t i s very understandable t h a t dichromate s a l t i s a more e f f i c i e n t depressant than chromate s a l t i n f l o t a t i o n . C. I n fluence of Chromate on the A d s o r p t i o n of Xanthate on Galena Surface CD Competitive Adsorption I t has been shown t h a t lead chromate i s formed on the galena surface when galena i s t r e a t e d w i t h a chromate s o l u t i o n . Due to the h y d r o p h i l i c property of t h i s f i l m , the f l o a t a b i l i t y of galena i s suppressed. However, Wark and 61 sCox [6] reported t h a t c r o c o i t e , PbCrO^, was f l o a t e d by xanthate, and P l a k s i n and h i s co-worker [4] suggested t h a t the chemical attachment of xanthate i o n and t h a t of chromate i o n took place i n d i f f e r e n t areas. In the present work, the a d s o r p t i o n r a t e s of xanthate ions and chromate ions on galena, the competitive a d s o r p t i o n of these two s p e c i e s , the d e s o r p t i o n of xanthate ions or chromate ions from galena surface due to chromate or xanthate treatment were completed. Furthermore the r e s t p o t e n t i a l of galena e l e c t r o d e i n a chromate s o l u t i o n , and the products on galena surface were examined to i n v e s t i - gate the depression of galena w i t h chromate or dichromate i o n and the p o s s i b i l i t y of f l o t a t i o n of chromate-treated galena by xanthate. Under the c o n d i t i o n s described i n Figure 10, xanthate i s adsorbed more r a p i d l y than chromate. Neverthe- l e s s , i n competitive a d s o r p t i o n t e s t s , as shown i n Table I , xanthate adsorption i s more suppressed than chromate a d s o r p t i o n . I t might be p o s s i b l e to e x p l a i n such a phenomenon i n s e v e r a l ways. However, i t i s reasonable to consider t h a t the a d s o r p t i o n r a t e of xanthate i s r e t a r d e d , even i f i t s i n i t i a l r a t e i s f a s t , by the i n h i b i t i o n due to the hydro- p h i l i c f i l m of chromate adsorbed species on galena. O x i d a t i o n of xanthate by chromate i o n i n an aqueous s o l u t i o n i s expected to be very slow from the f a c t t h a t the U.V. spectrum of the mixture of chromate and xanthate s o l u t i o n s i s very s t a b l e as shown i n Figure 1. Such a slow o x i d a t i o n of xanthate i n chromate s o l u t i o n i s q u i t e understandable from the e l e c t r o n t r a n s f e r theory of chromium r e d u c t i o n [30]. However, i f a semiconductor l i k e galena i s i n v o l v e d i n the system, the enhanced o x i d a t i o n r a t e of xanthate by a c a t a l y t i c e f f e c t due t o galena i s p r e d i c t a b l e . Pomianowski and Pawlikowska [16] showed th a t the r e s t p o t e n t i a l of galena was r a i s e d s t r o n g l y by the chromate a d d i t i o n . This phenomenon i n d i c a t e s the p o s s i b i l i t y of o x i d a t i o n of xanthate to dixanthogen by chromate ions i n the presence of galena. In f a c t , the formation of dixanthogen amounted to about 80 per cent of the t o t a l xanthate adsorbed. This was detected by a n a l y z i n the r e s u l t a n t s o l u t i o n of xanthate a d s o r p t i o n t e s t w i t h the a i d s of U.V. spectrophotometry. The amount of dixanthogen formed i n the presence of chromate i n the system i s much high e r than t h a t without chromate. (2) A d s o r p t i o n Products of Xanthate on Lead Chromate I t has been shown th a t lead xanthate and dixantho gen are formed when lead chromate i s t r e a t e d w i t h a xanthat s o l u t i o n , as shown i n Figure 13 and Table V. However, as shown i n Figure 13, the one a b s o r p t i o n band of lead xanthate i s l o c a t e d at 1200 cm ^ and t h i s i n d i c a t e s a c e r t a i n s h i f t from the a b s o r p t i o n band at 1210 cm f o r l e a d xanthate which can be formulated by PbCEtX^. L e j a et a l . [29] obtained an i n f r a r e d spectrum of lead s u l f i d e t r e a t e d w i t h a xanthate s o l u t i o n and suggested a 1:1 co- •ordination between the surface metal atom and the xanthate ^ r a d i c a l i n the f i r s t monolayer from the s h i f t i n C-O-C band frequency from about 1210 t o 1195. As the same type of s h i f t i n frequency was observed i n t h i s experiment, a 1:1 c o - o r d i n a t i o n between the surface lead atom and the xanthate r a d i c a l may be suggested on lead chromate s u r f a c e . As mentioned b e f o r e , the formation of dixanthogen can be e x p l a i n e d i n terms of the high r e s t p o t e n t i a l of lead chromate. (Cf. Figure A - l . ) CONCLUSIONS In order to o b t a i n a b e t t e r understanding of the 'depressive a c t i o n of chromate or dichromate s a l t on galena, some experimental work was performed and the thermodynamics were considered. The main r e s u l t s obtained under the c o n d i t i o n s employed i n t h i s study are as f o l l o w s : (1) Galena shows a b u f f e r a c t i o n i n an aqueous s o l u t i o n . However i n the absence of oxygen at a l k a l i n e range, no s i g n i f i c a n t pH change was observed. (2) The a d s o r p t i o n amount of chromate on galena stays almost constant above pH 9 and i t i n c r e a s e s w i t h the i n c r e a s e i n dichromate c o n c e n t r a t i o n i n s o l u t i o n . (3) When galena i s t r e a t e d w i t h chromate s a l t or -dichro- mate s a l t , l e a d chromate i s formed on the galena surface as the main adsorbed s p e c i e s . (4) The p o t e n t i a l - p H diagram of Pb-Cr-S-^O system at 25°C. was c o n s t r u c t e d . This i n d i c a t e s t h a t l e a d chromate i s a very s t a b l e substance whereas lead dichromate i s very unstable. (5) Xanthate can adsorb on chromate-treated galena surface and a l s o on lead chromate s u r f a c e . 65 (6) Xanthate adsorbed on chromate-treated galena or lead chromate appeared mostly as dixanthogen. (7) The d e s o r p t i o n of chromate from galena surface due to xanthate a d d i t i o n i s very s l i g h t . (8) The des o r p t i o n of xanthate from galena surface which i s covered w i t h more than a monolayer t h i c k - ness of xanthate, due t o chromate a d d i t i o n i s p o s s i b l e . However the desorbed amount of xanthate i n such a case i s only 10-20% of the amount of chromate adsorbed. (9) When xan t h a t e - t r e a t e d galena i s subjected t o desorp- t i o n t e s t by chromate, i t i s found t h a t both xanthate ions and dixanthogen were desorbed. SUGGESTIONS FOR FUTURE WORK In t h i s study, when much xanthate was adsorbed on galena s u r f a c e , xanthate ions were desorbed by adding chromate s a l t . However, i t may be very h e l p - f u l t o i n v e s t i g a t e the p o s s i b i l i t y of the de s o r p t i o n of xanthate when the amount of xanthate a d s o r p t i o n i s much l e s s . For t h i s purpose, t o use xanthate i n c l u d i n g r a d i o a c t i v e s u l f u r may be very u s e f u l . The mechanism of depression of galena by chromate or dichromate s a l t i s not c l e a r enough. I t may be very u s e f u l to study more about galena-chromate- xanthate system and to i n v e s t i g a t e the r o l e of dixanthogen i n t h i s system. REFERENCES 1. Lowry, A. and Greenway, H.H., A u s t r a l i a n Patent 5065 (1912). 2. 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Chem., 1956 , 34_; 1419. 31. B r o d i e , J.B., The E l e c t r o c h e m i c a l D i s s o l u t i o n of Galena, M.A.Sc. Thesis, U n i v e r s i t y of B r i t i s h Columbia, 1968. F o s t e r , L.S., M i n e r a l and M e t a l l u r g i c a l Research T e c h n i c a l Paper, No. 2, U n i v e r s i t y of Utah, 1927. Dewitt, C C , and Roper, E.E., J . AM Chem. Soc. 1932, 54, 445. A P P E N D I X ELECTROCHEMICAL STUDY E l e c t r o c h e m i c a l Measurements on N a t u r a l Galena Specimen E l e c t r o c h e m i c a l measurements were made on n a t u r a l .galena specimens, mounted i n p l a s t i c and mechanically p o l - ished t o expose a measured surface area to s o l u t i o n s , chromate, dichromate, and 1M HCIO^. This a c i d , HCIO^, was chosen .because i t i s completely i o n i z e d at t h i s d i l u t i o n and the p e r c h l o r a t e i o n i s very s t a b l e and does not s i g n i f i c a n t l y complex metal c a t i o n s . P o l a r i z a t i o n s t u d i e s were performed w i t h a Beckman model TM30 E l e c t r o s c a n instrument equipped w i t h a p o t e n t i o - s t a t . A t h r e e - e l e c t r o d e system was used i n v o l v i n g a calomel e l e c t r o d e i n a Luggin c a p i l l a r y w i t h the p o i n t l o c a t e d very c l o s e t o the specimen to e l i m i n a t e as much as p o s s i b l e of the IR p o t e n t i a l drop i n the s o l u t i o n . The p o t e n t i a l between the calomel e l e c t r o d e and the specimen was c o n t r o l l e d and could be scanned w i t h the p o t e n t i o s t a t , while c u r r e n t passed only between the specimen and an a u x i l i a r y e l e c t r o d e . The s o l u t i o n was u s u a l l y kept almost oxygen-free by bubbling w i t h helium i n a covered c e l l . The helium from the c e l l could be passed through a gas t r a i n to absorb gases such as I^S, which are given o f f under c e r t a i n con- d i t i o n s by the specimen. The normal k i n d of experiment 71 "that was performed c o n s i s t e d of determining the p o l a r i z a - t i o n curve. The p o t e n t i a l between the specimen and the reference e l e c t r o d e was v a r i e d at a s e l e c t e d scan r a t e and the curr e n t was recorded. This curve could be d i v i d e d i n t o anodic and cathodic regions corresponding to the d i r e c t i o n ;of c u r r e n t i n the specimen, and the p o t e n t i a l d i v i d i n g the anodic and cathodic regions i s known as the " r e s t " p o t e n t i a l of the specimen. The r e s t p o t e n t i a l of galena e l e c t r o d e was measured i n a chromate s o l u t i o n at pH 10.8, where chromate s a l t stays as chromate i o n i n a s o l u t i o n . The r e s u l t i s shown i n Figure A - l . I t incre a s e d by adding chromate s a l t and stayed almost _3 constant at co n c e n t r a t i o n 1.0 x 10 M/L. I t may be s a i d t h a t the r e s t p o t e n t i a l increases w h i l e lead chromate i s formed on galena specimen. Figure A-2 shows pH dependence of the r e s t p o t e n t i a l _ 3 of galena e l e c t r o d e i n 1.0 x 10 M/L chromate s o l u t i o n . As seen from the f i g u r e , pH dependence i n a c i d i c r e g i o n i s d i f f e r e n t from t h a t i n a l k a l i n e r e g i o n . An i n c l i n a t i o n i n a c i d i c r e g i o n was 46mV/pH and i n a l k a l i n e r e g i o n 66my/pH. _3 When 1.0 x 10 M/L l^C^O,., s o l u t i o n was used, an i n c l i n - a t i o n was 46mV/pH i n a l k a l i n e and 7 0mV/pH i n a l k a l i n e s o l u t i o n . These r e s u l t s i n d i c a t e t h a t the r a t i o H+/e i s 4/5 i n a c i d i c r e g i o n and 9/8 i n a l k a l i n e r e g i o n . However i t must be emphasized t h a t the r e s t p o t e n t i a l of galena e l e c t r o d e i n aqueous s o l u t i o n s of various pH values a l s o changes. 72 Figure A-4 shows a example of anodic p o l a r i z a t i o n ..vcurves of galena e l e c t r o d e i n various s o l u t i o n s . The ^ p o l a r i z a t i o n curve of galena e l e c t r o d e i n 1M HCIO^ was st u d i e d by Brodie [31]. When chromate or dichromate s a l t was added, the curren t d e n s i t y was suppressed s i g n i f i c a n t l y . This may i n d i c a t e the formation of passive f i l m on galena s u r f a c e . Figure A-5 shows cathodic p o l a r i z a t i o n curve of galena e l e c t r o d e . 120 L i i i i ,1 i i i • 0 • 4 .8 12 16 20 24 CONCENTRATION of K 2 C r 0 4 , ( l O - 4 M/L) Figure A - l E f f e c t of the Concentration of Potassium Chromate on-the Rest P o t e n t i a l of Galena E l e c t r o d e . ( I n i t i a l pH of the S o l u t i o n : 10.8)   76 0. 0.2 0.4 0.6 0.8 1.0 1.2 Eh ( V o l t ) Figure A-4 Anodic P o l a r i z a t i o n Curves f o r Galena E l e c t r o d e i n (a) IM HC10„ (b) 4.0 x 10 4 M/L K 2CrC\ (pH 10.0) (c) 4.0 5 IO" 4 M/L K 2Cr^ 7 (pH 2.9) scan r a t e 2.8 mV/sec. He'bubbled -0.6 -0.4 -0.2 ' • 0 0.2 0.4 Eh (v) Cathodic P o l a r i z a t i o n Curves f o r Galena E l e c t r o d e i n (a) 1M HClOh (b) 4.0 x IO" 4 M/L K 2CrOLi (pH 10.0) (c) 4-.0 x IO- 4 M/L K 2 C r 2 0 7 (pH 2 . 9 ) scan rate' '2.8 mV/sec. He bubbled

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