Pulp chemistry in industrial mineral flotation. Studies of surface complex on calcite and apatite surfaces using FTIR spectroscopy

Minerals Engineering, Vol. 2, No. 2, pp. 217-227, 1989 Printed in Great Britain

0892-6875/89 $3.00 + 0.00 © 1989 Pergamon Press pie

PULP C H E M I S T R Y IN INDUSTRIAL MINERAL FLOTATION. STUDIES OF SURFACE COMPLEX ON CALCITE AND APATITE SURFACES USING FTIR S P E C T R O S C O P Y B.-M. ANTTI and E. F O R S S B E R G Div. of Mineral Processing, Lulea U n i v e r s i t y of Technology, S-951 87 Lulea, Sweden

(Received

1 9 April

1988~ revision

26 Sept.

1988)

ABSTRACT Flotation experiments in a Hallimond tube have been performed on calcite and apatite at pH levels and oleate concentrations judged to be interesting from the point of view of adsorption isotherms. For the calcite system adsorption isotherms indicate precipitation of calcium cleate after monolayer formation at pH 9, 10 and 11, whereas in the apatite system a double layer of oleate is formed. This means that apatite, unlike calcite, is sensitive to collector reagent overdosage. With the aid of FTIR techniques it is possible to demonstrate the existence of a surface complex in the apatite system with a calciumoleate ratio of 1:1 at monolayer coverage of the mineral surface. In conditions corresponding to bulk precipitation of calcium oleate, this compound can be detected by FTIR analysis of unfloated material.

Ke2words Calcite flotation; apatite flotation; oleate; FTIR spectroscopy

INTRODUCTION The o b j e c t i v e of this i n v e s t i g a t i o n is to s t u d y the s u r f a c e c h e m i s t r y of calcite and apatite to gain a better u n d e r s t a n d i n g of the a d s o r p t i o n m e c h a n i s m i n v o l v e d and t h u s a c h i e v e more efficient separations. With a better u n d e r s t a n d i n g of how the c h e m i s t r y affects the s e p a r a t i o n process, it will be p o s s i b l e to improve the average a c h i e v a b l e grade and r e c o v e r y in a p h o s p h a t e plant. P r e v i o u s p u b l i c a t i o n s [I-4] have r e p o r t e d studies of calcite and apatite w i t h r e f e r e n c e to a d s o r p t i o n of o l e a t e and c o n s e q u e n t f l o t a t i o n p r o p e r t i e s at d i f f e r e n t pH levels. In the calcite system we studied the case of less than m o n o l a y e r c o v e r a g e of the m i n e r a l surface. The surface coverage 8 is <0.62 or 0.38, d e p e n d i n g on w h e t h e r the a d s o r b e d o l e a t e m o l e c u l e s are p r e s e n t as h y d r a t e d c r y s t a l s (surface area per m o l e c u l e = 20 A"2 ) or as liquid crystals (surface area per m o l e c u l e = 33 ~2) [5]. The a d s o r p t i o n e x p e r i m e n t s were p e r f o r m e d at pH 10.6, and on the basis of the a d s o r p t i o n isotherm together with a n a l y z e d oleate and c a l c i u m concentrations, a d s o r p t i o n at this surface coverage and pH was assumed to c o r r e s p o n d to a 1:1 c a l c i u m - o l e a t e complex. Model calculations with SOLGASWATER [6] and a d s o r p t i o n e x p e r i m e n t s w i t h a c o n s t a n t initial oleate c o n c e n t r a t i o n at v a r y i n g pH indicate that Ca(Of) 2 is formed at pH <10.3, while a 1:1 complex is formed on the mineral surface at h i g h e r pH (the model compound CaOHOI was used in the calculations). The a d s o r p t i o n e x p e r i m e n t s show that the a d s o r p t i o n process is l i k e w i s e pHd e p e n d e n t in the apatite system. M o n o l a y e r c o v e r a g e is a t t a i n e d at pH 8, after w h i c h Ca(Of) 2 is precipitated. At higher pH (9-11), a double layer of oleate m o l e c u l e s is formed instead before t h r e e - d i m e n s i o n a l p r e c i p i t a t i o n of Ca(Ol) 2 starts.

217

218

B.-M. ANT'rl and E. FO~.~sBm~e

In the w o r k d e s c r i b e d in the present report, FTIR techniques were used to study the form in which the oleate molecules are adsorbed to the surfaces of calcite and apatite in flotation. The flotation experiments were performed in a Hallimond tube at certain relevant pH levels, and the c o l l e c t o r r e a g e n t concentration was c h o s e n to a t t a i n the desired degree of surface coverage according to the adsorption isotherms. As the adsorption isotherm for calcite was too l i m i t e d to g i v e e n o u g h i n f o r m a t i o n , more comprehensive adsorption experiments were performed on that material after the FTIR measurements. In e a r l i e r IR investigations, oleate adsorption on calcite has been studied [7,8] by the KBr tablet or NuJol mull technique. Peck, like F u e r s t e n a u and Miller, found evidence of precipitated calcium oleate and calcium laurate on calcite surfaces at pH 9, and proposed the following reaction mechanism: CaCO 3 + 2RCO0- = Ca(RCOO)2(surf)

+ CO32-.

Fuerstenau and Miller also examined the adsorption process at pH 12.5, where the IR s p e c t r a g a v e i n d i c a t i o n s of 1:1 C a - l a u r a t e complex for which they a s s u m e d the f o r m u l a C a O H L a . T h i s s a m p l e p r e p a r a t i o n t e c h n i q u e has b e e n criticized on the grounds that it may influence the surface complex formed in the f l o t a t i o n e n v i r o n m e n t . The s a m p l e p r e p a r a t i o n t e c h n i q u e u s e d in the p r e s e n t work (separation of froth phase, drying at room temperature, mixing with KBr w i t h o u t m o r t a r i n g or p r e s s i n g ) g i v e s c o n d i t i o n s w h i c h o u g h t to resemble those found in flotation [9]. EXPERIMENTS Material The following materials were tested 1.

CaCO 3 (purum),

2.

CaCO 3 (p.a.)

3.

Calcite

precipitated

(Norvijaur)

<38 ~m

in the calcite experiments:

BET surface 11.5 m2/g BET surface

0.6 m2/g

BET surface

8.4 m2/g

The FTIR spectra showed that these were chosen on account of its large specific the a d s o r p t i o n e x p e r i m e n t s , as it had experiments [I].

equivalent, so m a t e r i a l No. 1 w a s surface. Material No. 3 was used in p r e v i o u s l y b e e n u s e d in f l o t a t i o n

For the apatite experiments w e u s e d the <38 ~m f r a c t i o n of a m i l l e d fluoroapatite crystal from Canada (95% pure) with BET surface of 6.4 m2/g for the F T I R m e a s u r e m e n t s , and the <75 ~m f r a c t i o n of the same material (BET surface 1.7 m2/g) for the adsorption experiments [3]. Reagents Sodium oleate (analar grade) from Riedel-De Haen was used without purification. Ca(Ol) 2 was prepared by p r e c i p i t a t i o n from 10 -3 M solutions of sodium oleate and calcium chloride. The precipitate was filtered, washed with isopropanol and recrystallized from benzene, after which the product was dried at 6 0 ° C o v e r n i g h t . HCI (p.a.) and NaHO (p.a.) were used for pH adjustment where necessary. Method The f l o t a t i o n e x p e r i m e n t s w e r e performed in a Hallimond tube. One gram of mineral was conditioned with 100 ml of oleate solution for 10 minutes, after which the tube was filled to 250 ml with water of the correct pH and oleate concentration, and f l o t a t i o n was s t a r t e d w i t h n i t r o g e n gas at 9 lh -I . F l o t a t i o n was d i s c o n t i n u e d a f t e r 30 s e c o n d s . The f l o t a t i o n p r o d u c t and u n f l o a t e d m a t e r i a l w e r e c o l l e c t e d separately, filtered, and dried at room temperature before the FTIR measurements. A d s o r p t i o n of oleate on both minerals is a quick process. Kinetic experiments show that equilibrium is a t t a i n e d w i t h i n 30 m i n u t e s . In p r a c t i c e the experiments were performed in such a way that 0.5 g apatite or 0.2 g calcite were shaken up in 100 ml oleate solution of specified oleate concentration and pH. After 60 minutes the pH was measured, the suspension was filtered through

Industrial mineral flotation

219

a 0.22 um milllpore filter, and the solution w a s a n a l y z e d for t o t a l c o n c e n t r a t i o n of c a l c i u m and oleate. The oleate c o n c e n t r a t i o n w a s m e a s u r e d spectrophometrlcally [10], w h e r e a s the c a l c i u m c o n c e n t r a t i o n was a n a l y z e d using atomic a b s o r p t i o n s p e c t r o m e t r y . T h e q u a n t i t y of o l e a t e a d s o r b e d is c a l c u l a t e d f r o m the d i f f e r e n c e in o l e a t e c o n c e n t r a t i o n b e f o r e a n d after adsorption. All t h e F T I R s p e c t r a w e r e o b t a i n e d by d i f f u s e r e f l e c t a n c e of r a d i a t i o n c o l l e c t e d by a H a r r i c k a c c e s s o r y and a n a l y z e d in a B r u k e r IFS 88 F o u r i e r T r a n s f o r m Infrared S p e c t r o m e t e r . The m e a s u r e m e n t s w e r e made w i t h the kind coo p e r a t i o n of P r o f e s s o r J . M . C a s e s at the Centre de Recherche s u r la V a l o r i s a t i o n des M i n e r a i s in Nancy, France. Two d i f f e r e n t methods were used to prepare the samples for measurement: 150 mg KBr was very t h o r o u g h l y mixed w i t h 2% by mass of the sample. The m i x t u r e was t r a n s f e r r e d to a die 13 mm in d i a m e t e r a n d p r e s s e d w i t h a force of 98 kn.

150

mg of p o w d e r m i x t u r e (0.5 or 15% s a m p l e by t r a n s f e r r e d to the m e a s u r e m e n t cell in powder form. K B r a c t s as a (RKBr/Rsample)-

reference

phase,

the

absorbency

mass

being

in

KBr)

defined

was

as

log

R E F E R E N C E SPECTRA A r e f e r e n c e series was run in order to be able to e v a l u a t e the FTIR spectra f r o m the v a r i o u s f l o t a t i o n products. The c h a r a c t e r i s t i c a b s o r p t i o n bands of the r e f e r e n c e spectra [11] are listed below. S o d i u m oleate and c a l c i u m oleate T h e s p e c t r a w e r e r u n on K B r tablets c o n t a i n i n g c o n c e r n e d and are shown in Figures I and 2.

2% by mass

of

the p r o d u c t s

05

0.~-

~

~ 0.2.

~

0.1.

0.0.

-0,1

3400

' 3600

'

2~0

'

2200

'

1600

1&O0

I0~

WAVENUMBERS CM-1

Fig.1 Diffuse (~r tablet with In both spectra,

reflectance FTIR s p e c t r u m o f sodium o l e a t e 2% NaO1 by mass; r e s o l u t i o n 4 cm-1; 200 scan)

g e n e r a l l y speaking:

most peaks at wave h y d r o c a r b o n chain,

numbers

from 3040

to 2800

cm -I are related to the

m o s t peaks at wave numbers c a r b o x y l a t e radical.

from 1800

to 1400 cm -I are related to the

C h a r a c t e r i s t i c bands in the 3040-2800 cm -I wave number range are 2921

cm -I

2852 cm -I

asymmetric symmetric

s t r e t c h i n g v i b r a t i o n in the CH 2 radical s t r e t c h i n g v i b r a t i o n in the CH 2 radical

220

B.-M. ANrn and E. FOgSSBERO

2956 cm -I

a s y m m e t r i c stretching v i b r a t i o n in the CH 3 radical

2870 cm -I

symmetric stretching v i b r a t i o n in the CH 3 radical

3010 cm -I

stretching v i b r a t i o n bond.

in the CH radical nearest the double

C h a r a c t e r i s t i c bands in the 1800-1400 cm -I wave number range are 1465 cm -I

b e n d i n g v i b r a t i o n of h y d r o g e n in the CH 2 radical

1450 cm -I

a s y m m e t r i c d e f o r m a t i o n of the CH 3 radical

1375 cm -I

symmetric d e f o r m a t i o n of the CH 3 radical

1710 cm -I

symmetric C-O v i b r a t i o n s

in the COOH radical.

In ionized fatty acids the band at 1710 cm -I is replaced by two new bands due to v i b r a t i o n s in the CO0- radical: 1610 - 1550 cm -I

a s y m m e t r i c stretching v i b r a t i o n

1420 - 1300 cm -I

symmetric stretching v i b r a t i o n

2.2.

1.6,

o

t

1.0.

.u 0.4,

-02

340o

3o'00

26'00 2250 18'00 WAVENUHBERS C1~-1

1400

1000

Fig.2 Diffuse r e f l e c t a n c e FTIR s p e c t r u m of calcium oleate (KBr tablet with 2% Ca(Ol) 2 by mass; r e s o l u t i o n 4 cm-1; 200 scan) C a l c i u m carbonate The FTIR spectrum is shown in F i g u r e 3. This spectrum was run d i r e c t l y on a powder mixture c o n t a i n i n g 15% f l o t a t i o n product by mass. It is c h a r a c t e r i z e d by a very strong band at 1430 cm -I and a band of m e d i u m strength at 873 cm -1. These a b s o r p t i o n bands are related to v i b r a t i o n s in the carbonate radical; the b a n d at 873 cm -I is s p e c i f i c to calcium c a r b o n a t e and is commonly used to d e t e r m i n e the p r e s e n c e of calcite in p h o s p h a t e minerals. Apatite T h e F T I R s p e c t r u m ( F i g u r e 4) w a s run d i r e c t l y on a 150 mg powder mixture c o n t a i n i n g 0 . 5 % a p a t i t e by m a s s . T h e p h o s p h a t e r a d i c a l in ion c o m p o u n d s absorbs at 1080 and 980 cm -l. The double peak at 2360 and 2341 cm -I found in the calcite and apatite spectra is d u e to the p r e s e n c e of a t m o s p h e r i c C O 2 ( g ) and is n o t r e l a t e d to t h e a b s o r b e n c y of the samples. T h e a b s o r p t i o n bands of interest from the point of view of e l u c i d a t i n g the a d s o r p t i o n m e c h a n i s m a r e e v i d e n t l y t h o s e t h a t c o r r e s p o n d to a s y m m e t r i c v i b r a t i o n in t h e C O 0 - r a d i c a l . If a 1:1 c o m p l e x is formed on the mineral surface, we expect an a b s o r p t i o n band at 1560 cm -I (cf. sodium oleate). If on the other hand Ca(Of) 2 is p r e c i p i t a t e d and adsorbed on the surface, one would expect to find a double peak at 1577 and 1540 cm -I .

Industrial mineral flotation

221

1.4

,2

I

10-

~

08.

0.4.

0.2,

0"30400

'

3000

'

26'00

'

2200

'

1800

'

I~0

'

1000

WAVENUMaERS CM-~

Fig.3 Diffuse reflectance FTIR spectrum of calcium carbonate (KBr r e f e r e n c e ; 15% CaCO 3 by m a s s ; r e s o l u t i o n 4 c m - 1 ; 200 s c a n ) 04-

0.3-

~

02-

-- 0,1-

0.0-

-0,1

3400 ' 3~00

2s'~

'

zi~

WAVENUMBERS

Fig. 4

,e'~

'

,4~

'

,~

CM-1

Diffuse reflectance FTIR spectrum of apatite, <38~m (KBr reference; 0.5% apatite by mass; resolution 4 cm-1; 200 scan)

As can be seen from a comparison between the spectra for NaOl, Ca(O1)2 and CaCO 3, the absorption band of the carbonate radical lies within the same wave number range as that of the carboxylate group. This means that we must utilize differential spectra to obtain information about the bonding oleate molecules to the mineral surfaces. The spectra of pure mineral (Sm) and floated mineral (Sfm) a r e r e c o r d e d s e p a r a t e l y and s t o r e d in the c o m p u t e r ' s m e m o r y . In subtracting Sfm - Sm we select a peak that is characteristic of the mineral and subtract until that peak disappears. In the p r e s e n t case we c h o s e the calcium carbonate peak at 873 cm -I. Absorption conditions in the apatite system are somewhat more favourable, but subtraction of spectra was nevertheless felt to give the best information. In t h i s c a s e t h e p e a k at 1061 cm -I was c h o s e n as the s t a r t i n g p o i n t for subtraction.

RESULTS AND DISCUSSION Calcite system All s p e c t r a w e r e run d i r e c t l y flotation product by mass.

on

150

mg

powder

mixtures

containing

15%

As was mentioned above, the calcite mineral absorbs in the same wave number range as the C-O stretching of the carboxylate radical. This, in combination with incomplete adsorption data, made it impossible to obtain direct results concerning a surface complex on the calcite surface. ME ~ - - E

222

B.-M. ANTTI and E. FORSSBERG

It is however p o s s i b l e to use the intensity of the a b s o r p t i o n b a n d at 2 9 2 5 cm -I as a measure of how much oleate has been adsorbed on the mineral surface in e a c h e x p e r i m e n t [12]. F i g u r e 5 s h o w s the s p e c t r u m of c a l c i t e a f t e r f l o t a t i o n in the 3200-2800 cm-- wave number range. A base line has been drawn in at a tangent to the base of the spectrum, and the d i f f e r e n c e in absorbency between the peak and the base line calculated. The results are shown in Table 1 .

~(- 42)

>

Ir

,..i

~2bo

~oo

28bo

WAVE NUMBER Icm'll

Fig.5

TABLE I

Graphic r e p r e s e n t a t i o n of method used to obtain a b s o r b e n c y (AcH 2) at 2925 cm -I

A b s o r b e n c y of f l o t a t i o n p r o d u c t at 2925 cm -I as a f u n c t i o n of C i and pH

10.2

pH

ii Remarks

C i (M) 2" 10 -4 M

0.049

0.042

0.038

< monolayer

8" 10 -4 M

0.123

0.080

0.141

~ monolayer

1 . 4 " i 0 -s M

0. 176

0.180

> monolayer

As the purpose of this project was to study the surface complex on the mineral s u r f a c e in c o n j u n c t i o n w i t h f l o t a t i o n as a f u n c t i o n of pH, n o e x a c t d e t e r m i n a t i o n of f l o t a t i o n r e c o v e r y was made for the experiments. R e c o v e r i e s w e r e low (<50%) in the f l o t a t i o n experiments with an initial oleate concentration of C i = 2 x i 0 -4 M ( c o r r e s p o n d i n g to l e s s t h a n m o n o l a y e r coverage), while 100% recovery was obtained at the two higher c o n c e n t r a t i o n s ( c o r r e s p o n d i n g r e s p e c t i v e l y to m o n o l a y e r and more than m o n o l a y e r coverage). The results of the H a l l i m o n d tube flotation experiments thus indicate that an o v e r d o s e of oleate, i.e. an oleate c o n c e n t r a t i o n higher than that needed for a m o n o l a y e r on the mineral surface, does not seem to have any adverse effect on r e c o v e r y . T h i s is confirmed by the IR analysis of the quantity of adsorbed oleate at varying oleate c o n c e n t r a t i o n and pH according to Table I. As Figure 6 s h o w s , t h e r e is an almost linear c o r r e l a t i o n b e t w e e n the increase in the quantity of oleate a d s o r b e d and the initial oleate concentration. More detailed a d s o r p t i o n e x p e r i m e n t s with oleate on calcite [4] were p e r f o r m e d in an attempt to explain the results d e s c r i b e d above. The a d s o r p t i o n isotherms at pH 9, 10 a n d 11 a r e s h o w n in F i g u r e 7. T h e y s h o w that the a d s o r p t i o n density is higher at pH 9 and 10 than at pH 11. In addition, t h r e e - d i m e n s i o n a l p r e c i p i t a t i o n of calcium oleate takes place immediately after the formation of

Industrial mineral flotation

223

a monolayer at all pH levels. The measured total concentrations of calcium in equilibrium with precipitated calcium oleate are of the order of Ixi0 -4 M at pH 9 and 10, and 3x10 - D at pH 11, which gives a solubility product for calcium oleate of 1.6x10 -14 M 3. This figure is r o ~ h l X an order of magnitude higher than that given by Du Rietz [13] (1.4x10 -lD M ~) or that obtained at pH 8 in the a p a t i t e s y s t e m (1.2x10 -15 M 3) The d i f f e r e n c e can m o s t p r o b a b l y be ascribed to the fact that the measured total calcium concentration is greater than the free concentration owing to complex formation with various species in solution. Ai-CH 2 ) 0.2'

S /'

0.1

f

0.(

10 -t.

, i i ,,,,~1 i i Jlllll t0"3 104 INITIAL CONCENTRATION (moles/I)

Fig.6 Absorbency of flotation product at 2925 cm -I as a function of initial oleate concentration at varying pH (o pH 9; A pH 10.2; x pH 11) 20 , t j

pH °9.4-.0.1 x 10.1..0.2

/

~ , ,0.9-.0.,

/o

i

"

0

X

f,

lO,.6

16-s

lO-~

EQUILIBRIUM CONCENTRATION [ m o l e l / I )

Fig.7

Adsorption isotherms for oleate on calcite, <38 um, as a function of pH

Apatite system The adsorption isotherms for oleate on apatite at pH 8-11 are shown in Figure 8. In this case the pH has a greater effect on adsorption than in the calcite system. At pH 8 a monolayer of oleate ions forms first on the mineral surface, after which three-dimensional precipitation of calcium oleate takes place. At pH 9, 10 and 11 there is no precipitation, a double layer of oleate ions being formed instead. Up to monolayer coverage the polar part of the oleate ion is b o n d e d to the m i n e r a l surface; this takes place step by step because the mineral surface is heterogeneous (the adsorption isotherm slopes s o m e w h a t ) . When the double l a y e r is f o r m e d t h r o u g h a t t a c h m e n t of o l e a t e ions by hydrophobic b o n d i n g to the h y d r o c a r b o n c h a i n s in the o l e a t e ions of the monolayer, the isotherm makes a vertical Jump because after the formation of the monolayer the mineral surface must be regarded as homogeneous. The next p l a t e a u c o r r e s p o n d s to a f u l l y f o r m e d d o u b l e layer, and here the mineral surface is once more hydrophilic. The a d s o r p t i o n d e n s i t y c o r r e s p o n d i n g to monolayer coverage is 5 umoles/m 2 at pH 9,_10 and 11 and slightly higher at pH 8. This c o r r e s p o n d s to an area of 33 R Z per oleate molecule, which is the

224

B.-M. ANTTI and E. FORSSBERG

value found for such molecules when they are in liquid crystal phase [5]. 25

pH = 8.0 x 9.0

,., 20

¢' 10.0 0 11.0

E

"6 E

::L 15 ¢v

~ lO

10 0

5 i

0i

,,

i

i

iiiill

I0-~ 10-3 Equilibrium c0ncentration.(molesA)

10-6

Fig.8

Adsorption

10-5

isotherms

for

oleate

on

fluoroapatite,

<75 ~m, as a function of pH At pH 8 the solubility product of calcium oleate is 1.2x10 -15 M 3 ([Ca]to t = Ix10 -5 M), w h i c h is in good a g r e e m e n t with the value given by Du Rietz [12]. At higher pH, p r e c i p i t a t i o n occurs after formation of the d o u b l e layer. W i t h a l l o w a n c e made for normal energy terms it is not p o s s i b l e for more than two layers of oleate ions to be a d s o r b e d on the mineral surface [5]. The flotation e x p e r i m e n t s were p e r f o r m e d at pH 8 and 10 w i t h v a r y i n g o l e a t e c o n c e n t r a t i o n s up to 8 = 2 a c c o r d i n g to the a d s o r p t i o n isotherms. At pH 8 a steady increase in f l o t a t i o n recovery was o b t a i n e d up to 100% at 8 = I, after w h i c h the r e c o v e r y f e l l by a b o u t 25% at d o u b l e d o s a g e . At pH 10 m a x i m u m f l o t a t i o n was o b t a i n e d as e x p e c t e d at @ = I, with a sharp d r o p in r e c o v e r y w h e n the oleate c o n c e n t r a t i o n was increased. The FTIR spectra of floated products at 8 ~ 1 and at pH 8 and 10 are p r e s e n t e d in Figures 9 and 10. The spectra show that regardless of pH there is only one c h a r a c t e r i s t i c peak on the d i f f e r e n t i a l s p e c t r u m in the wave number range in question, and that it occurs at 1550 cm -i. At the high r e s o l u t i o n used, the peak is almost obscured by noise from water vapour a d s o r p t i o n . H o w e v e r , by v a r y i n g the s u b t r a c t i o n of w a t e r vapour from the s p e c t r u m one can clearly identify which peaks are c h a r a c t e r i s t i c of the sample. The o c c u r r e n c e of only o n e p e a k l e a d s to the a s s u m p t i o n that it is a 1:1 C a - o l e a t e complex w h i c h builds up the monolayer. The frequency is d i s t i n c t from 1560 cm -I, found in NaOl, which is reasonable since there is a d i f f e r e n t cation in the s u r f a c e complex and there is no sign of the double peak c h a r a c t e r i s t i c of Ca(Ol) 2. In earlier IR studies of oleate a d s o r p t i o n on salt-like minerals, the p r e s e n c e of bands at 1550 or 1555 cm -I was interpreted as a sign of c h e m i s o r b e d oleate ion. A band at 1560 cm -I indicates p h y s i s o r b e d oleate ion, w h i l e p h y s i s o r b e d Ca(Ol) 2 ought to give a double band at 1570 and 1540 cm -I [16]. It is thus reasonable to assume an ion exchange m e c h a n i s m

that the a d s o r p t i o n of oleate

=CaOH + RCO0- #

takes place

by

=Ca + -OOCR + OH-

w h e r e = d e s i g n a t e s surface species. This m e c h a n i s m d e p e n d e n c e found for the a d s o r p t i o n process.

also

accounts

for

the

pH

Figure 11 shows the FTIR s p e c t r u m obtained from u n f l o a t e d m a t e r i a l at 8 2 and pH I0. Here we find the double peak c h a r a c t e r i s t i c of Ca(Of) 2 (1571 and 1 5 4 0 c m - ' ) . T h e i n i t i a l o l e a t e c o n c e n t r a t i o n is somewhat higher than that r e q u i r e d for a complete double layer, so Ca(Ol) 2 is p r e c i p i t a t e d in the bulk phase a c c o r d i n g to the a d s o r p t i o n isotherm. If

we

compare

the

quantities

of

adsorbed

oleate

as

A ( - C H 2)

at 2925

cm -I

Industrial mineral flotation

225

a c c o r d i n g to F i g u r e 5, we find:

r - I 0

pH 8

pH I 0

0.016

0.011 0.020

~ 2

pH 8 ftoat

;jt

0.015

HzOlO)~ r"

o=

-0.005 1900

Fig.9

I~

Weighted differential

1700 1 6 0 0 1500 1/.00 Wovenumber$ CM-1

t3~

1200

s p e c t r u m (floated apatite - pure apatite}, pH 8, 8 - I

pH 10 floor 0.0066

x 0.0~ -

-o.oors~ 19oo

Fig.10

,~

Weighted differential

1550 HZ0 (Ok,

,~00 I ~

i~

,~

Wovenumber$ CM-I

,3~

,200

spectrum (floated apatite - pure apatite), pH 10, @ - I

In all the e x p e r i m e n t s the initial oleate c o n c e n t r a t i o n was slightly h i g h e r than r e q u i r e d for 8 = I or e = 2. A c c o r d i n g to the a d s o r p t i o n isotherms the a d s o r p t i o n d e n s i t y is e v i d e n t l y greater at pH 8 than at other pH levels up to m o n o l a y e r c o v e r a g e , a n d t h i s is c o n f i r m e d by the IR a n a l y s i s above. If we

226

B.-M. ANTTI and E. F O ~ S B G

allow for the fact that the oleate system is partly present as free oleic acid at pH 8, this enhanced a d s o r p t i o n density can be explained by the formation of a 1:1 c o m p l e x b e t w e e n oleate ions and calcium ions at the mineral surface, after which oleic acid is p h y s i c a l l y a d s o r b e d to t h i s m o n o l a y e r . T h e F T I R s p e c t r u m ( F i g u r e 9) c o n t a i n s indications of a peak at 1710 cm -I , w h i c h is c h a r a c t e r i s t i c of symmetric C-O v i b r a t i o n in the C O O H r a d i c a l . At pH 10 a double layer starts to form under the influence of a surplus of oleate. The particles that float display a p r e d o m i n a n t l y h y d r o p h o b i c s u r f a c e and a 1:1 complex.

pH 10 non float 001-

I

~ o

{G) F H20

-00025-

q

-0015

19oo

18~o

17'oo

lr~O

lr~

1~o

13bo ,2o0

Wavenumbers CM-I

Fig.t1

If we 8), we lower oleate

Weighted differential

spectrum (unfloated apatite - pure apatite), pH 10, 8 - 2

compare the a d s o r p t i o n isotherms of calcite and apatite (Figures 7 and find that the a d s o r p t i o n density c o r r e s p o n d i n g to m o n o l a y e r coverage is for apatite than for c a l c i t e . In p r a c t i c e a p a t i t e r e q u i r e s a l o w e r c o n c e n t r a t i o n than does calcite for a given flotation recovery [I 7].

CONCLUSIONS The a d s o r p t i o n isotherms for oleate on calcite and apatite indicate that the c o m p o s i t i o n a n d s u r f a c e s t r u c t u r e of the mineral influence the process of adsorption. Calcite shows a higher a d s o r p t i o n density at m o n o l a y e r c o v e r a g e t h a n does apatite (6.5 and 5 u m o l e s / m 2 respectively). In the calcite system p r e c i p i t a t i o n of calcium oleate after m o n o l a y e r takes place at pH 9, 10 and 11, w h e r e a s in the apatite system a double layer of oleate ions is formed. This means that apatite, unlike calcite, is sensitive to c o l l e c t o r r e a g e n t overdosage. FTIR analysis of floated apatite at m o n o l a y e r level indicates the p r e s e n c e of a 1:1 c a l c i u m - o l e a t e complex, and the p r o b a b l e m e c h a n i s m for c o n s t r u c t i o n of the m o n o l a y e r is =CaOH + RCO0-

~

=Ca + -OOCR + OH-

In the case of calcite it is very difficult, to o b t a i n information about the structure of a d s o r p t i o n b a n d s of the c a r b o n a t e and CO0r e a s o n a b l e to suppose that the m o n o l a y e r is both minerals.

even with an FTIR spectrometer, the surface complex because the radicals overlap. It is however built up in a similar manner on

ACKNOWLEDGEMENTS

The authors wish to thank P r o f e s s o r J.M.

Cases

for allowing them to visit his

Industrial mineral flotation

227

l a b o r a t o r y and for making his e q u i p m e n t and staff available to this project. F i n a n c i a l support from the Swedish Mineral R e s e a r c h F o u n d a t i o n (MinFo) and the Swedish National Board for Technical Development (STU) is g r a t e f u l l y acknowledged.

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