Adsorption of barium ions by β-manganese dioxide and its activation in oleate flotation

International Journal o f Mineral Processing, 1 (1974) 267--275 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ADSORPTION OF BARIUM IONS BY H-MANGANESE DIOXIDE AND ITS ACTIVATION IN OLEATE FLOTATION

M.A. A R A F A ' , A.A. YOUSEF 1 and M.A. MALATI ~ ' The National Research Centre, Cairo, Egypt Medway and Maidstone College o f Technology, Chatham (Great Britain) (Accepted for publication February 14, 1974)

ABSTRACT Arafa, M.A., Yousef, A.A. and Malati, M.A., 1974. Adsorption of barium ions by ~-manganese dioxide and its activation in oleate flotation. Int. J. Miner. Process., 1: 267--275. The Ba 2+ ion adsorption isotherms on ~-MnO2 were of the Langmuir type. The endothermic heat of adsorption (40 kJ mo1-1 ) is ascribed to entropy contributions associated with the Na÷/Ba :+ ion-exchange mechanism. The Ba 2+ ion adsorption density was higher at pH 10 than that at pH 7, due to the more negative surface charge at the higher pH. Ba 2+ ions were found to reverse the sign of the ~ potential of the MnO: particles. More oleate was adsorbed bY ~-MnO~ in the presence of Ba 2+ ions than in their absence. The oleate adsorption isotherms on Ba~+-activated MnO 2 were of the Freundlich type and indicated an exothermic process. Hallimond flotation recovery of Ba~+-activated MnO~ was higher at pH 10 than at pH 7, although less oleate was adsorbed at the higher pH. At pH 7, Mn2+-activation led to higher recoveries than Ba~+-activation. It seems that the attraction between the surface and the activator plays an important rble in determining the flotation recovery.

INTRODUCTION The anionic flotation of manganese dioxide minerals and their activation h a v e r e c e i v e d m o r e a t t e n t i o n r e c e n t l y . B o n d a r e n k o e t al. ( 1 9 6 9 ) h a v e c o n c l u d e d , f r o m i n f r a r e d s p e c t r a , t h a t o l e a t e is a t t a c h e d t o p y r o l u s i t e o r p s i l o m e l a n e in t h e i o n i c f o r m a t h i g h p H v a l u e s a n d in t h e m o l e c u l a r f o r m a t l o w pH values. Akerkar and Altekar (1960) have reached similar conclusions from a study of the contact angle on pyrolusite. T h e a c t i v a t i n g e f f e c t o f s u l p h u r d i o x i d e h a s b e e n a s c r i b e d t o t h e M n 2+ i o n s f o r m e d b y i n t e r a c t i o n w i t h MnO2 p a r t i c l e s ( G a t e s , 1 9 5 7 ) . T h i s a s s u m p t i o n h a s b e e n r e c e n t l y c o n f i r m e d ( Y o u s e f e t al., 1 9 7 1 ) a n d t h e a c t i v a t i n g e f f e c t o f M n 2+ i o n s h a s b e e n e s t a b l i s h e d ( F u e r s t e n a u a n d R i c e , 1 9 6 8 ) . T h e a n i o n i c f l o a t a b i l i t y o f m a n g a n e s e d i o x i d e m i n e r a l s in t h e a p p a r e n t a b s e n c e o f a c t i v a t o r s m a y b e d u e t o t h e M n 2+ i o n s u s u a l l y f o u n d c o n t a m i n a t i n g t h e o x i d e s ( Y o u s e f e t al., 1 9 7 1 ) . I n a n e a r l i e r n o t e , w e h a v e c o m p a r e d t h e a c t i v a t i o n o f 6-MnO2 b y M n 2+ i o n s w i t h its a c t i v a t i o n b y B a 2+ i o n s ( M a l a t i e t al., 1 9 6 9 ) .

268 Ba 2÷ ion activation is of particular interest since Ba 2÷ ions occupy lattice sites in the minerals cryptomelane and psilomelane (Malati, 1971). Schuhmann and Prakash (1950) have studied the activation of quartz by Ba 2÷ ions in oleate flotation over a wide pH range. They have ascribed the activation to the formation of a surface barium oleate with a Ba2÷: oleate ratio of 1:1. In the present investigation, the adsorption isotherms of Ba 2÷ ions on/3-MnO2 are reported at pH 7 and at three temperatures and are compared with an isotherm at pH 10 and 35°C. The effect of Ba 2÷ ions on the ~ potential of the MnO2 particles at the two pH values is given. The enhancement of the adsorption of oleate in presence of Ba 2+ ions is reported and their effect on the flotation recovery of ~-MnO2 is depicted and compared with the activating effect of Mn 2÷ ions. MATERIALS The ~-MnO2 was prepared by heating manganese(II) nitrate as described earlier and was identified as ~-MnO2 by X-ray diffraction (Yousef et al., 1971). The solid assayed 61.5% Mn and could be represented by MnOl.~2. The size fraction --0.037 + 0.010 mm, prepared by sieving and desliming, was used for the adsorption measurements and flotation tests. The surface area of this fraction, determined by the B.E.T. k r y p t o n or nitrogen adsorption, was found to be 11.8 m 2 g--1. Sodium oleate solutions were prepared from pure oleic acid by saponification taking the usual precautions (Malati and Estefan, 1967). Stock solutions of barium chloride were made from Johnson Matthey Specpure solid in redistilled water. C e t y l t r i m e t h y l a m m o n i u m bromide (CTAB) was purified as described earlier (Yousef et al., 1970). EXPERIMENTAL In the adsorption tests, 5.00 g of the solid were mechanically shaken with 100.0 cm 3 of a standardised solution of sodium oleate and/or barium chloride for three hours in an air thermostat. Solutions were generally 10--2M in sodium chloride. An aliquot of the centrifugate or filtrate was analysed for its oleate concentration using the CTAB titration (Yousef et al., 1970). Barium concentration in the filtrate was determined by complexometric titration at pH 11 using cresolphthalexon as indicator (Andregg et al., 1954). In a set of experiments, the adsorption of Ba 2÷ ions was determined using 14°Ba-labelled solutions. Details of the technique and counting equipment have been published elsewhere (Malati et al., 1970). The adjusted pH of the pulp was measured and rechecked after adsorption, using a Metrohm E187 potentiometer and a combined glass electrode. A modified Hallimond tube (Suliin and Smith, 1966) was thermostated and used for the flotation tests. Three grams of the solid were conditioned with 60 cm 3 of the activator-collector solution and the pH was adjusted. No frother was

269

added and purified air was passed at a rate of 1.25 cm 3 sec -1 . Recovery was calculated from the weights of the floated and unfloated fractions. Electrophoretic mobility of the solid particles in barium chloride solutions was measured by the microelectrophoretic m e t h o d and the ~"potential was calculated using Smoluchowski's equation (Yousef et al., 1971). When the ratio of the particle radius to the thickness of the double layer was less than 300, the f potential was computed from a graphical representation of Henry's equation (Overbeek, 1952). RESULTS

AND

DISCUSSION

The zero point of charge of the ~-MnO2 particles was found at pH 4.6 (Yousef et al., 1971), indicating that the particles were negatively charged at the pH values covered in this ihvestigation. These particles are expected to adsorb cations from aqueous solution, the adsorption density increasing with an increase in pH. The adsorption isotherms of Ba 2÷ ions on ~-MnO2 in 10-2M NaC1 solution are given in Fig.1 at pH 7 and 10. The isotherms obtained by using 14°Ba-labelled solutions are in fair agreement with the corresponding isotherms calculated from the results of complexometric titrations.

I

Z4

~

I

/

t

I

I

I

~

f

16

o

E =k

o I

'~

+~

x

~ ~ I°'+'

o

I

I 8

E < 4

Equilibrium

( one entrotion

Fig.1. Adsorption isotherms • at pH 10 and 35°C - - • - - at pH 7 and 35°C - - o - - at pH_7 and 20°C

of o x ~

Ba at at at

i 10 retool/I

2÷ i o n s o n ¢~-MnO2. pH 10 and 35°C using l~Ba p H 7 a n d 3 5 ° C u s i n g ~4°Ba pH 7 and 10°C

270 The isotherms have a Langmuir shape. It may be assumed that the low coverage form of the isotherm applies: x/m=

(1)

a . b . c

where x / m is the a m o u n t of Ba 2+ ions adsorbed g - l , C is the equilibrium concentration and a is a constant. The constant b is equal to b ~ e x p ( Q / R T ) , where bl is another constant, Q is the heat of adsorption, R is the gas constant and T is the absolute temperature. The logarithm of the initial slopes of the isotherms in Fig.1 may be taken as ln(ab~) + Q / R T . By plotting the logarithms of the slopes against l I T in Fig.2, Q has been estimated as 40 kJ mo1-1 . The endothermic nature of adsorption was also observed for the adsorption of Mn 2÷ ions on ~-MnO2 (Yousef et al., 1971) and for the adsorption of the alkaline earth ions by quartz (Estefan and Malati, 1973a). The endothermic nature indicates positive entropy contributions to the free energy of adsorption. In the present work, the a m o u n t of H + ions released on equilibrating/3-MnO2 with Ba 2+ solution was appreciably smaller than the a m o u n t of Ba 2÷ ions adsorbed. Since the solutions were 10-2M in NaC1, it may be assumed that the adsorption process involves a Na÷/Ba2÷ ion-exchange: 2Na÷(O.H.L.) + Ba2+(soln.) # Ba2÷(O.H.L.) + 2Na+(soln.)

(2)

where the h y d r a t e d ion is either in the outer Helmholtz layer (O.H.L.) or in the bulk of the solution. Assuming t h a t the ions in O.H.L. are relatively immobile, the adsorption of Ba 2+ ions, represented by eq.2, will be accompanied by a marked increase in entropy (Malati et al., 1974). Preliminary results have indicated that the adsorption affinity of Ba 2+ ions to/3-MnO2 was greater than that of Ca 2÷ ions. It seems that the affinity sequence:

-6-

-6"5

-7

-7"5

Reciprocal

!

|

i

:3"3

3"4

;3"5

of a b s o J u t o

temperature

103//T " K

F i g . 2 . E s t i m a t i o n o f t h e h e a t o f a d s o r p t i o n o f Ba ~÷ i o n s o n ~-MnO2 : t h e v a r i a t i o n o f In(initial s l o p e o f a d s o r p t i o n i s o t h e r m ) w i t h t h e r e c i p r o c a l o f t h e a b s o l u t e t e m p e r a t u r e .

271 Ba 2÷> Sr 2+> Ca 2÷, observed in the case of v-MnOz (Gabano et al., 1965) and of hydrous manganese dioxide (Posselt et al., 1968) is also found in the case of ~3-MnO2. This order is the order of the increase in the radii of the hydrated cations (Malati and Estefan, 1966) and this affinity sequence seems to indicate that h y d r a t e d cations participate in the ion-exchange mechanism. However, a recent kinetic study using Specpure MnO2, in the Na÷ form, has shown that a fast Ca 2÷ ion adsorption process is followed by a slower process (Rophael and Malati, 1972). The fast process probably involves a Na+/Ca2÷ ionexchange. This mechanism is similar to the mechanism suggested above for Ba 2÷ ion adsorption in eq.2. The ability of Ba 2÷ ions to reverse the charge of the MnO2 particles is evident from Fig.3, which also indicates that the minimum Ba 2÷ ion concentration required to reverse the charge is appreciably higher at pH 10 than the corresponding concentration at pH 7. This follows from the more negative surface charge at pH 10 compared to that at pH 7 (Arafa, 1969). The marked increase in the adsorption density of Ba 2÷ ions at pH 10 compared to that at pH 7 is clear from Fig.1. This increase is expected in view of the more negative ~ potential and surface charge at the higher pH. In solutions 10--4M in BaC12 and 10-2M in NaC1 and at pH 7, the adsorption isotherms of oleate follow the Freundlich equation (Fig.4). The enhancement of the oleate adsorption as a result of activation is clear when the results are i

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1

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> E O

D. O

-e---

O

N --40

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entration

1 0- 4 of

BaCI 2

10-3 rnol/I

Fig.3. The ~" p o t e n t i a l o f ~-MnO2 particles in 10-2M NaCl s o l u t i o n at 27°C as a f u n c t i o n o f Ba 2÷-ion c o n c e n t r a t i o n . o e q u i l i b r i u m pH 7 . equilibrium pH 10

272

i

I

i

I

l

i

+0"8

c~

-~ 0"4 0 E "o h o "o o

0

om 0 - 4 I

I 0"4

I

I 0'8

Log equilibrium concentration

it.2

I i~mol/

I

Fig.4. Freundlich isotherms of the adsorption of oleate by ~oMnO2 from a solution 10-4M in BaCl~ and 10-2M in NaCl at pH 7. x at 10°C (283°K) • at 20°C (293°K) o at 35°C (308°K)

compared with those obtained in absence of activator (Fig.5). Fig.4 also indicates that the adsorption of oleate is exothermic, i.e., similar to the adsorption of oleate in absence of activators (Yousef et al., 1971). At pH 10, the adsorption of oleate from solutions 1 0 - 2 M in NaC1 and 10--4M in BaC12 does not follow the Freundlich equation. However, the oleate adsorption density is appreciably less than that at pH 7. The activation of ~-MnO2 in oleate flotation is demonstrated by Fig.6 which depicts the flotation recovery as a function of oleate concentration in 1 0 - 4 M activator solution. The recovery increases with the increase in oleate concentration at constant Ba 2÷ ion concentration, indicating the r61e of lateral interactions between the hydrocarbon chains in fixing the oleate to the surface. However: the rise in the recovery curve becomes less steep at higher oleate concentrations, i.e., as the concentration ratio: (activator)/(collector) drops. Although the amount of oleate adsorbed at pH 10 was lower than the amount adsorbed at pH 7, Fig.6 shows that the recoveries are higher at the higher pH.* This may be ascribed to the higher Ba 2÷ ion adsorption density at the higher pH as mentioned above. * Flotation tests on Mn2+-activated MnO~ at pH 10 were not attempted because the hydrolysis products formed at this pH would complicate the picture.

273

:L

4

/ /

c~ "w

= c~

E

/.,Y •

10

Equilibrium

210

concentration

pmoi/I

Fig.5. A d s o r p t i o n i s o t h e r m s o f s o d i u m oleate on ~-MnO2 at p H 7 and 35°C. in a b s e n c e o f o t h e r salts x in 10-2M NaCI s o l u t i o n • in a s o l u t i o n 10-4M in BaC12 and 10-~M in NaCl

I

f

'

I

I

|

I

I

t

I

I

80

40

I

0"4 Initial

concentration

0"8 of

I

I

I "2 sodium

oleate

mmol/I

Fig.6. H a l l i m o n d f l o t a t i o n r e c o v e r y o f ~-MnO~ at r o o m t e m p e r a t u r e as a f u n c t i o n o f oleate concentration. o in 10-4M BaCl~ s o l u t i o n at p H 7 • in 10-4M BaC12 s o l u t i o n at p H 10 x in 10-4M MnCl~ s o l u t i o n at p H 7

274

Fig.6 also demonstrates the higher recoveries of Mn2+-activated particles compared with Ba2÷-activated ones. When the results of oleate adsorption shown in Fig.4 are compared with the corresponding results in presence of MnC12 solutions (Yousef et al., 1971), it is found that generally somewhat more oleate is adsorbed by Mn2÷-activated particles compared to the Ba 2÷activated particles. One is t e m p t e d to ascribe this to the lower solubility of manganese(II) oleate compared to barium oleate (Du Rietz, 1965), However, no correlation was f o u n d between the solubility of the alkaline-earth oleates and the enhancement of oleate adsorption by quartz in presence of the alkalineearth ions (Estefan and Malati, 1973b). The higher activating efficiency of Mn 2÷ ions may be ascribed to the stronger 'affinity between the adsorbed Mn 2÷ ions and the surface of/~-MnO2 compared with the corresponding affinity of Ba 2÷ ions. The higher adsorption density of Mn 2÷ ions compared to Ba 2÷ ions is evident when the results shown in Fig.7 and 1 are compared. It seems that the surface-activator attraction plays a prominent rSle in fixing the oleate to the surface of the particles, as has been suggested in the case of quartz activation (Malati and Estefan, 1967).

"~20 G

2

Equilibrium

4

6

concentration

retool/

8 l

Fig.7. A d s o r p t i o n i s o t h e r m s o f M n 2. ions o n &MnO~ at p H 7. o at 35°C (308°K) x at 20°C (293°K) • at 10°C (283°K) CONCLUSIONS

In the activation of negatively-charged/3-MnO2 particles at pH values > 5, the activating effect of cations such as Ba 2÷ or Mn 2÷ ions is to anchor the collector oleate anions to the surface which is thus rendered hydrophobic. Two major attractions are involved: ( 1 ) Attraction between a negative surface site and the activating cation. (2) Attraction between the activating cation and the oleate anion. The former attraction seems to play a prominent rSle. Lateral interactions between the hydrocarbon chains of the adsorbed oleate are also important in rendering the mineral surface hydrophobic.

275

REFERENCES Akerkar, D.D. and Altekar, V.A., 1960. Contact angle studies at pyrolusite surface. J. Mines Metals Fuels, 8: 69--72. Anderegg, G., Flaschka, H., Sallmann, R. and Schwarzenbach, G., ]954. Metal indicators. 7. A phthalein responding to alkaline-earth ions and its analytical application. Helv. Chim. Acta, 37: 113--120. Arafa, M.A., 1969. Factors affecting the adsorption of some flotation reagents on some Egyptian minerals with special reference to the activation of manganese dioxide. Thesis, Cairo University, Cairo, 260 pp. Bondarenko, P.O., Vainschenker, I.A. and Kriveleva, E.D., 1969. Infrared spectroscopic study of manganese mineral reaction with sodium oleate solution. Obogashch-Rud., 1 4 : 1 4 - - 1 6 (in Russian). Du Rietz, C., 1965. Chemisorption of collectors. In: Surface Chemistry.Munksgaard, Copenhagen, pp. 21--37. Estefan, S.F. and Malati, M.A., 1973a. Adsorptive behaviour of alkali and alkaline-earth cations onto quartz surface in anionic flotation. 2nd Cairo Solid State Conference "Recent Advances in Science and Technology of Materials". The American University, Cairo, p.160 (abstr.). Estefan, S.F. and Malati, M.A., 1973b. Activation of the oleate flotation of quartz by alkaline-earth ions. Trans. Inst. Min. Metal. Sec. C., 82: 237--240. Fuerstenau, M.C. ant ~ L~ice, D.A., 1968. Flotation characteristics of pyrolusite. Trans. Am. Inst. Min. Metall. Pet. Eng., 241: 453--457. Gabano, J.P., Etienne, P. and Laurent, J.F., 1965. Surface properties of ~-manganese dioxide. Electrochim. Acta, 10: 947--963. Gates, E.G., 1957. Agglomeration flotation of manganese ore. Min. Eng., 9: 1368--1372. Malati, M.A., 1971. The solid state properties of manganese dioxides. Chem. Ind., 446--452. Malati, M.A. and Estefan, S.F., 1966. The r~le of hydration in the adsorption of alkalineearth ions onto quartz. J. Colloid Sci., 22: 306--307. Malati, M.A. and Estefan, S.F., 1967. Activation of quartz by alkaline-earth cations in oleate flotation. J. Appl. Chem., Lond., 17: 209--212. Malati, M.A., Mazza, R.J., Sherren, A.J. and Tomkins, D.R., 1974. The mechanism of adsorption of alkali metal ions on silica. Powder Technol., 9:107--110. Malati, M.A., Yousef, A.A. and Arafa, M.A., 1969. Adsorption of Ba ~+and Mn 2÷ cations and of sodium oleate by ~-manganese dioxide. Chem. Ind., 459--460. Malati, M.A., Yousef, A.A. and Estefan, S.F., 1970. l~tude par traceur radioactif du m6canisme de flottation des minerais de manganese et du quartz ~ l'aide des acides gras. Chim. Ind., 103: 1347--1353. Overbeek, J.Th.G., 1952. Electrokinetic Phenomena, In: H.R. Kruyt (Editor), Colloid Science, Elsevier, Amsterdam, p.209. Posselt, H.S., Anderson, J.F. and Weber, W.J., 1968. Cation sorption on colloidal hydrous manganese dioxide. Environ. Sci. Technol., 2: 1087--1093. Rophael, M.W. and Malati, M.A., 1972. Characterisation of manganese dioxides. I. The rate and activation energy of adsorption of calcium ions by ~-manganese dioxide. Chem. Ind., 768--769. Schuhmann, R. Jr. and Prakash, B., 1950. Effect of BaC12 and other activators on soap flotation of quartz. Trans. Am. Inst. Min. Metall. Pet. Eng., 187: 591--600. Suliin, D.B. and Smith, R.W., 1966. Hallimond tube investigation of fluoride activation of beryl and feldspars in cationic collector systems. Trans. Inst. Min. Metal., Sect. C. 75: 333--336. Yousef, A.A., Arafa, M.A. and Malati, M.A., 1970. Determination of sodium oleate for adsorption measurements. Chem. Ind., 649--650. Yousef, A.A., Arafa, M.A. and Malati, M.A., 1971. Adsorption of sulphite, oleate and manganese(II) ions by ~-manganese dioxide and its activation in flotation. J. Appl. Chem. Biotechnol., 21: 200--207.