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Mechanism of fatty acid adsorption in salt-type mineral flotation
Minerals Engineering, Vol. 4, Nos 7-11, pp. 879-890, 1991 Printed in Great Britain
0892-6875/91 $3.00 + 000 © 1991 Pergamon Press plc
MECHANISM OF FATTY ACID ADSORPTION IN SALT-TYPE MINERAL FLOTATION
K. HANUMANTHA RAO and K.S.E. FORSSBERG Division of Mineral Processing, Lule~t University of Technology, S-95187 Luleh, Sweden
ABSTRACT Our recent studies on the interaction of oleate with salt-type mineral surfaces have been summarized based on adsorption, infrared and zeta-potential measurements. Monolayer coverage in the case of calcite and a bilayer formation in the cases of fluorite, apatite and scheelite, preceding the precipitation of calcium soap has been suggested for the adsorption mechanism of oleate on these minerals. The monolayer coverage is shown to correspond to a condensed state of oleate with a molecular coverage area of 33 ,~2 (liquid-crystal state). Depending on the surface potential and its magnitude at basic pH values, oleate is either chemisorbed on the surface calcium or monocoordinated through counter sodium and calcium ions for the monolayer filling. Keywords Salt-type minerals; oleate; adsorption; FTIR spectroscopy; zeta-potential INTRODUCTION The sparingly soluble calcium minerals such as calcite, fluorite, apatite and scheelite are largely concentrated by flotation processes. Invariably, fatty acids are used as collectors in these systems. For effective separations, selective adsorption of the collectors at mineral/water interface is essential. The mechanism of the adsorption of fatty acid based collectors on these minerals has been widely studied and a comprehensive review on this subject was published by Hanna and Somasundaran in 1976 [1]. Very recently, this subject has been critically reviewed by Finkelstein [2]. Finkelstein [2] observed that there is no consistency in the abstraction of the oleate by any mineral. The results are widely varied from one mineral to the other, and between two investigations on the same mineral. An attempt to quantify the published adsorption results has failed and only a qualitative feature of the oleate adsorption is discussed. Several shortcomings for the surface precipitation mechanism has been presented. It is felt that the first molecules of oleate adsorb on the surface of calcium minerals and not by a precipitation of calcium oleate at the surface. However, the available evidence is not sufficient for a mechanism based on chemisorption. In general, chemisorption, surface precipitation and bulk precipitation are thought to be the important reactions in these systems. However, a clear understanding on the chemistry of fatty acid interaction does not exist. The adsorption density levels attained several layers in these systems possibly due to a surface and bulk precipitation reactions, which make the interpretation of the shape of the traditional adsorption isotherms difficult. Our recent 879
880
K. HANUMANTHARAOand K. S. E. FORSSBERG
studies indicate that it is still possible to differentiate between adsorption and precipitation reactions [3-10]. In this paper, these recent findings have been summarized. MINERAL-REAGENT INTERACTION
Adsorption studies The adsorption isotherms of oleate on calcite at different pH values are presented in Figure 1 [4]. The isotherms show a linear increase of the amount adsorbed versus the logarithm of the equilibrium concentration, followed by a steep increase in the slope after a plateau corresponding to an oleate adsorption density of around 5/zmol m "2. A plateau was observed at 5 #mol m -2 by Sadowski [l 1], preceeding the infinite increase in the slope of the isotherm (Fig. 1). The steep increase in the slope of the isotherms characterize calcium oleate precipitation. The solubility product of calcium oleate for the precipitation step, considering the calcium ion concentration in the pulp is estimated to be around pK 14.0 [4].
20 °
pH Reference o 9.3 [4] x 10.0 ,x 11.0
fj tt
o
I
104
I
E
/
~x |
i
h
10.5
/
I
I
I
I
I Ill
I
I
10 .4
E Q U I L I B R I U M O L E A T E C O N C . (moles / 1)
Fig.1 Adsorption isotherms of oleate on calcite at various pH values. The adsorption isotherms of oleate on natural and synthetic fluorites at pH 10 are shown in Figure 2 [7,8]. The natural fluorite isotherm shows a plateau level at 10 #tool m -2 before a steep increase in the adsorption density. The steep increase in the adsorption density after the plateau level is determined to be due to the precipitation of calcium oleate from the analysis of pulp liquid. The composition of the pulp liquid after adsorption is shown in Figure 3. This figure shows that the negative logarithm of the ionic product of calcium and the square of the equilibrium oleate is constant at 14.39 for initial oleate concentration between 3x10 "5 and 6xl0"SM. This range of constant ionic product corresponds to the vertical step immediately after the plateau in the adsorption isotherm. Hence, this is characterized as the precipitation region of calcium oleate with a solubility product of 4.07x1015mo131 3 (pK=14.39). In the case of the synthetic sample, a break at 10 ~mol m "2 is once again observed in the adsorption isotherms at three different solid/liquid ratios as can be seen in Figure 2. An interesting observation is that at high solid concentrations (0.2% solids), the isotherm is seen to level off at 10 #mol m -2. At low solid concentrations (0.05 and 0.1% solids), the increase in the adsorption density after the plateau level is ascribed to the precipitation of calcium oleate. The solubility product of calcium oleate corresponding to these precipitation regions is estimated to be around pK 14.35, from the analysis of the pulp liquid after adsorption [8].
Fatty acid adsorption
in s a l t - t y p e m i n e r a l f l o t a t i o n
881
D X X
"8
D
DX
X
x
X
0 "~
A
~
i
A
A
A
Syntheticfluorite O 0"05%s°lids
a 1"0%S°lids
/~ 0.20 % Solids
10~
10.5
104
103
E Q U I L I B R I U M O L E A T E CONC. (moles / 1)
Fig .2 Adsorption isotherms of oleate on natural and synthetic fluorite samples at pH 10 [7,8]. 16
i0 -3
---
1° 15
"
"-5
Z ©
~
10-W
[-. Z L) Z O L) x Fluoride 0 Calcium
10-i0
I
I
I I i i iii 10-s
i
i
i
13 10.4
INITIAL OLEATE CONC. (moles / 1)
Fig.3 Results of the analysis of the liquid phase after adsorption as a function of initial oleate concentration [7]. Analyses are for the studies on natural fluorite sample as shown in Fig. 2. The adsorption isotherms of apatite at different pH values are shown in Figure 4 [5]. At pH 8, an infinite vertical line occurs at an equilibrium oleate concentration of 9x10"6M, characterizing precipitation of calcium oleate at an adsorption density slightly higher than 5 #mol m "2. The solubility product of this three dimensional phase is estimated to be pK 14.5. The isotherms at pH 9, 10 and 11 show two plateau levels, one at 5 #mol m "2 and the other at 10 #mol m "2. The isotherm shape is unchanged, irrespective of whether the solids are added to oleate solution or oleate is added to the apatite suspension. Figure 5 shows the oleate adsorption at different solid/liquid ratios. It can be seen that the adsorption density is independent of apatite content up to 10 #mol m "2, after which it is found to be dependent on the solids content. The oleate abstraction after 10 #mol m "2 is considered to be due to calcium oleate precipitation [5].
882
K. HANUMANTHARAOand K. S. E. FORKSBERG
pH
0 8.0 N
9,0
a
10.0 11.0
tx
E
m r~ <
' a ~ ?/ / ~ ~ & 10"6
10.5
~ ~A~ kll'i
,
, i i i tll
104
l0 ~
EQUILIBRIUM OLEATE CONC. (moles / 1)
Fig.4 Adsorption isotherms of oleate on apatite at various pH values [5].
(%) o 0.I x 0.2 tx 0.3 a 0.4 m 0.5 O 0.6
Solids
2O
I
0.7
+ 0.8
5 0 10.6
io "~
lO ~
lO ~
EQUILIBRIUM CONC. (moles / 1)
Fig.5 Adsorption of oleate on apatite at different solid/liquid ratios at pH 10 [5]. The adsorption isotherms of oleate on scheelite at pH I0 are shown in Figure 6 [6]. In the presence of a carbonate buffer, the surface calcium is expected to complex with the carbonate species, and for an exchange reaction with oleate ions to occur, a high concentration of oleate is probably required. This explains the shift in the adsorption isotherm to a higher equilibrium oleate concentration in the presence of a carbonate buffer. The infinite increase in the slope after 10 #tool m "e characterizes calcium oleate precipitation. The isotherms also show an initial increase in the slope around 5/~mol m "2. Figure 7 shows the oleate adsorption at pH 9 using a borate buffer solution, when the residual oleate concentration is measured after allowing the particles to settle, and the liquid centrifuged. It shows a plateau around 10 #mol m "2 before the precipitation step of calcium
883
Fatty acid adsorption in salt-type mineral flotation
oleate. This implies that the free calcium ions are not available for precipitation and are probably complexed with the borate species. This explains the plateau that precedes the precipitation step of calcium oleate when compared to the shape of the isotherms as presented in Figure 6.
302;
0
0
x
""
E
<
O
Z
5
0 10"
t
I
1 0 ~s
I II tl[ 1 0 "5
I
I I I IIHI 104
i
I I I )1 10"3
EQUILIBRIUM OLEATE CONC. (moles / l)
Fig.6 Adsorption isotherms of oleate on scheelite at pH 10: (o) pH adjusted with HCI/NaOH; (x) pH adjusted with carbonate buffer [6].
20
E
~3
O
]o-7
i
i i i ltlll 10 .6
I
I I I I I ) 11 10 s
I
I I I IIlll 104
t
I I I Ill 10 .3
EQUILIBRIUM OLEATE CONC. (moles / 1)
Fig.7 Adsorption isotherm of oleate on scheelite at pH 9 adjusted with borate buffer: (o) scheelite particles allowed to settle after equilibration; (x) scheelite suspension centrifuged after equilibration [6]. Cases and coworkers [I 2= 15] proposed an adsorption model where the surfactants containing more than 8 methylene groups condense two-dimensionally on the mineral surface due to strong lateral bonds. Because of surface heterogeneities, they also suggested that the two dimensional condensation initially takes place on the homogeneous domain that are formed
884
K. HANUMANTHARAp and K. S. E. FORSSl)ERG
with the most energetic sites at the smaller values of solution concentrations followed by successive two dimensional condensation on the less energetic homogeneous domain. Hence, in the monolayer range, the isotherm can take a shape of several small vertical steps or a smooth curve depending on the distribution of the surface energies. But, once the monolayer is covered, the bilayer formation always takes a single sub-vertical step due to the homogeneity of the surface caused by the monolayer formation. However, an infinite vertical step on the isotherm indicates a precipitation reaction in the solution. Because of the two dimensional condensation, if the oleate is assumed to be in a liquid crystal state (molecular coverage area, 33 N 2 [16]), the monolayer coverage corresponds to 5 #mol m "2. On the other hand, if the oleate molecule is in a hydrated crystal state (molecular coverage area, 20.5 A 2 [17]), monolayer coverage is expected around 8 #mol m "2 adsorption density. The adsorption isotherms show either a plateau or level o f f at 10 #mol m 2. Moreover, an initial break or an increase in the slope around 5 #mol m "2 is also observed. These breaks correspond to a mono and a bilayer formation for the two dimensional condensation of oleate on the mineral surface. However, the isotherms showing a steep increase in the adsorption density after bilayer formation can be described as calcium oleate precipitation. The solubility product of calcium oleate is seen to correspond to a p K region of 14.0-14.5, which agrees with the literature values [18-20]. In summary, the adsorption results show a monolayer coverage in the case of calcite and a bilayer formation in the cases of fluorite, apatite and scheelite, prior to the precipitation of calcium oleate. Infrared studies
The IR spectra in the case of calcite is found to be difficult to interpret due to the highly interfering carbonate absorption in the same frequency region as that of the carboxylate radical. No meaningful information about the bonding of the adsorbed oleate species is obtained even when a differential F T - I R is used and after subtracting the calcite spectrum from the spectrum of adsorbed oleate species [21]. The F T - I R spectra of the apatite at oleate concentrations equivalent to a monolayer capacity, show a characteristic peak at 1550 cm "1 regardless of the pH, thus indicating surface calcium oleate (1:1) [3, 21]. The double peaks charaterizing calcium oleate at 1571 and 1540 cm "1 have been observed at oleate concentrations greater than that required for a complete double layer. Thus, the spectra support the bilayered structure of oleate that precedes the precipitation of calcium oleate. Double layer formation was also confirmed from flotation recovery, as the maximum recovery was obtained at a monolayer coverage, and a sharp drop occurs in the recovery with increasing oleate concentrations [21]. The occurance of only one peak shows that it is a 1:1 calcium oleate complex that builds up the monolayer. The diffuse reflectance IR spectra in the case of the synthetic fluorite isotherm (0.1% solids, Fig. 2) at different adsorption densities are presented in Figure 8 [8]. The spectra at low adsorption densities show a band at 1557 cm "1. As the adsorption density increases, this band shifts to a lower value, and appears at 1555-54 cm "1 when the adsorption density is about 6 #mol m "2. No shift in the band is observed up to 10 # m o l m "2 with further increase in the adsorption density. The absorbance band that occurs between 1557-54 cm "1, when the adsorption density is below 10/zmol m "2, is assigned to the surface calcium oleate (1:1). Above 10 #tool m -2 adsorption density, the spectra show absorbance bands at 1576 and 1540 c m ' l corresponding to calcium oleate. These spectra further substantiate that calcium oleate is precipitated after the plateau in the isotherms. A similar IR spectra are observed for the adsorption results at 0.2% solids up to 10 #mol m -2 adsorption density [8]. Figure 9 shows the spectra of the synthetic fluorite where the adsorption studies have been made by changing the order of oleate addition [8]. At low oleate concentrations, the spectra are observed to be similar, irrespective of whether the solids are added to the oleate solution or oleate is added to the fluorite suspension. Moreover, the intensity of the surface calcium oleate band (1555 cm "1) is unchanged. The bands corresponding to precipitated calcium oleate species appear only after an initial oleate concentration of 2.2 x 10"4M. This implies
Fatty acid adsorption in salt-type mineral flotation
885
that the oleate presumably adsorbs on the surface prior to the nucleation of a new solid calcium oleate phase, since the solubility product is exceeded several times before the actual formation of calcium oleate.
Z < O <
01 I
I
I
I
I
1
I
I
19oo lSOO 17oo 16oo xs0o 14oo 13oo 12ooi~o 1800 1700 1600 1500 1400 1300 1260 WAVENUMBERS( c m ' b WAVENUMBERS (cm1) Fig.8 Diffuse reflectance FT-IR spectra at different adsorption densities ( # m o l m "2) for the synthetic fluorite isotherm at 0.1% solids as shown in Fig. 2 [8]. In the carboxylate region, the diffuse reflectance FT-IR spectra in the cases of natural fluorite and scheelite show the bands corresponding to sodium oleate (1561 cm "1) and calcium oleate (1539 and 1577 cm "1) from the beginning of the surface coverage [6,7,10]. From the adsorption studies, the precipitation of calcium oleate is observed after 10/~mol m "2, whereas the FT-IR spectra show that the surface is filled with sodium and calcium oleates even in the monolayer range. This discrepancy is explained by the negative zetapotential of the mineral surface and hence, monocoordination of oleate through counter sodium and calcium ions that exist at the interfacial region is considered rather than the growth of surface precipitate at low oleate concentrations, and in the monolayer range. In this case, the IR spectra cannot represent the true adsorbed oleate species as the counter calcium and oleate ions are expected to deposit calcium oleate on the surface during the sample preparation (air-drying). On the other hand, the synthetic fluorite sample showed a positive zeta-potential at pH 10 (Fig. 1 1) and the calcium oleate species are observed only after 10 #tool m "2 corresponding to the adsorption results. Hence, depending on the surface potential and its magnitude, the adsorption of oleate for the monolayer filling can be described as follows:
886
K. HANUMANTHARAO and K. S, E. FORSSBERG
-CaOH + "OOCR -CaOH + Na + "OOCR + OH" -CaOH + Ca ++ "OOCR + OH-CaOH2 + + "OOCR -CaO ~" Na + + "OOCR -CaO s" Ca ++ + "OOCR
-- -Ca + "OOCR + OH" -- -CaO Na OOCR" + H20 = -CaOCaOOCR +H20 -- -Ca + "OOCR + H20 -- -CaO Na OOCR, where 6 <1 = -CaO Ca OOCR, where 6 _<_1 INITIAL OLEATE " ^'2x10-5.M 0x 10-,M ~ 2 0 4 0
z < O m <
,900 ,800 ,700 ,600 ,500 ,400 ,300 ,200 ,gbo ,sbo ,400 WAVENUMBERS (cmq)
,~'00 ,5'00 ,,'00 ,3~0
1200
WAVENUMBERS (cm a)
Fig.9 Diffuse reflectance F T - I R spectra at different initial oleate concentrations with a change in the order of addition of oleate: (a) fluorite added to oleate solution; (b) oleate added to fluorite suspension [8].
Zeta=potential
studies
The zeta-potential of calcite, fluorite, scheelite and apatite as a function of pH are shown in Figure 10. The i.e.p, of both fluorite and apatite are found to be at pH 8.5 and 4.0 respectively. The zeta-potential of calcite and scheelite are found to be negatively charged through out the pH region studied. In the case of synthetic fluorite, a positive zeta-potential is shown in the pH region 3-1 1 (Figure I 1). The zeta=potential of the synthetic fluorite at different initial oleate concentrations as a function of pH is shown in Figure 1 1. It can be seen that the zeta-potential decreases as the oleate concentration increases at all pH values. Above pH 8, oleate exists mostly as monomers, and the decrease in zeta-potential with increasing oleate concentration shows the high a f f i n i t y of oleate towards the mineral surface. A similar zeta-potential curves in the presence of oleate have been obtained even when the mineral samples showed negative potential at basic pH region [4-8].
Fatty
acid adsorption
in s a l t - t y p e
mineral
ra
5(
0 A x
4( >
887
flotation
Calcite Fluorite Scheelit¢ Apadt¢
3(
g
2(
<
1( ( -1( )
< -2( t'.q -30 -4{
f
-5( -6(
I
)
I
I
[
I
I
I
I
3
4
5
6
7
8
9
10
11
12
pH
Fig. 10 Zeta-potential of calcite, fluorite, apatite and scheelite as a function of pH [4-7].
6(
O
5{
~o
o
4(
o
x
2c
~ -1( i
L~ N-!
OLEATE 0 NIL
-
e r,
2.18 x IO~M
5.45 x IO~_M -
O •
2.73x IO°_M 5.45x IO'~M I
)
I
I
I
)
I
I
[
3
4
5
6
7
8
9
10
11
12
pH
Fig.l I Zeta-potential of synthetic fluorite as a function of pH in the presence of different initial oleate concentrations [8]. The influence of oleate on calcite zeta-potential at pH 10 is shown in Figure 12 [4,20,22,23]. Initially, there is a decrease in the zeta-potential and between l x l 0 "s and lxl0"4M oleate concentration, the potential remains constant. The zeta-potential again decreases and reaches a minimum potential of -50 mV as the oleate concentration increases further. The minimum zeta-potential of calcite in the presence of oleate correspond to that of calcium oleate [4]. If the abstraction of oleate is in the form of precipitated calcium oleate and multilayers are formed on the surface, the zeta-potential should decrease as the oleate concentration increases, at least up to -50 mY. Hence, the adsorption of oleate on the surface could be deduced for the initial decrease in zeta-potential, and the precipitation of calcium oleate in the bulk solution between l x l 0 -s and lxl0"4M oleate concentration. The three dimensional growth of calcium oleate on the surface or the adsorption of calcium oleate precipitate on
888
K. HANUMANTHARAOand K. S. E. FORSSBERG
the top of the hydrophobic layer was thought to be the reason for the sudden decrease in the zeta=potential equivalent to that of calcium oleate beyond lxl0-4M oleate concentration [4]. 25 x [] •'~ 0
R a o e t a l [4] Marinakis et al [20] Zimmels et al [22] Mishra [23]
¢g ,.d < Z
-2.'
X
i
< -50
10-~
10 .4
INITIAL
OLEATE
CONC.
10 .3
10 .2
( m o l e s / 1)
Fig.12 Zeta-potential of calcite as a function of initial oleate concentration at pH 10. The results on the effect of sodium oleate on the zeta-potential of fluorite samples at pH 10 are presented in Figure 13 [7,8]. The decrease in the zeta-potential is observed to occur in two stages with increasing oleate concentration, even though the minimum potentials reached in both cases are the same. The first stage is correlated to a successive oleate adsorption up to a bilayer formation (10 #mol m'2). The second decrease up to -50 mV is thought to be the growth of the three dimensional condensation of calcium oleate on the surface. The change in the zeta-potential accounts for the increase in the surface charge density due to the change in the molecular area of the adsorbed molecule from the liquidcrystal state to the precipitated phase. This hypothesis is supported by the fact that the constant zeta-potential of -50 mV corresponds to the zeta-potential of calcium oleate, based on the adsorption results and the IR spectra [8]. A similar zeta-potential results for apatite and scheelite in the presence of oleate have been reported [5,6]. CONCLUSIONS The shape of the adsorption isotherms of oleate on fluorite, apatite and scheelite at basic pH values indicate a plateau level around l0 #mol m "2, which corresponds to a bilayer formation for the two dimensional condensation of oleate on heterogeneous surfaces. The infinite increase in the slope of the isotherms after the bilayer formation characterizes calcium oleate precipitation. In the case of calcite, the precipitation of calcium oleate occurs after a monolayer coverage. The zeta-potential studies support the conclusions drawn from the adsorption isotherms. The evidence for the chemisorption of oleate on surface calcium is obtained by diffuse reflectance FT-IR studies. Here, the samples show a positive potential at the adsorption pH value of 10. When the samples show a negative zeta-potential, the IR spectra indicate sodium and calcium oleate species from the beginning of surface coverage. However, a monocoordination of oleate through counter sodium and calcium ions is suggested for the monolayer filling as opposed to the growth of surface precipitate. In these cases, the IR spectra cannot give conclusive evidence for the nature of oleate bonding, as the counter
Fatty acid adsorption in salt-type mineral flotation
889
calcium and oleate could transform into compound formation during the sample preparation (air-drying). 2O
-1C
<
x Natural fluorite O Synthetic fluorite
~x,X O
-30
'
......
i;,
.
.
.
.
.
.
.
. . . .
103
INITIAL OLEATE CONC. (moles / 1)
Fig.13 Zeta-potential of natural and synthetic fluorites as a function of initial oleate concentration at pH 10 [7,8]. In summary, monolayer coverage in the case of calcite, and a bilayer formation in the cases of fluorite, apatite and scheelite, prior to the precipitation of calcium oleate is suggested as the adsorption mechanism of oleate on these minerals. The adsorption density at monolayer coverage corresponds to a condensed state of oleate with a molecular coverage area of 33 A 2. ACKNOWLEDGEMENT The authors are grateful to the Swedish Mineral Processing Research Foundation (MinFo) for financial support.
REFERENCES
.
.
3.
.
.
HE
Hanna H.S. & Somasundaran P., Flotation of salt-type minerals, Flotation, A.M. Gaudin memorial volume (ed. M.C. Fuerstenau), AIME, New York, vol. 1, 197-272 (1976). Finkelstein N.P., Review of interactions in flotation of sparingly soluble calcium minerals with anionic collectors, Trans. IMM, Sec. C, 98, 157 (1989). Hanumantha Rao K., Antti B-M., Cases J.M. & Forssberg K.S.E., Studies on the adsorption of oleate from aqueous solution onto apatite, Developments in Mineral Processing, Proc. XVIth Int. Miner. Process. Congr. (ed. E. Forssberg), Elsevier, Amsterdam, Vol 10A, 625-636 (1988). Hanumantha Rao K., Antti B-M., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part I. Adsorption and electrokinetic studies of calcite in the presence of sodium oleate and sodium metasilicate, Coll. Surf., 34,227 (1988/89). Hanumantha Rao K., Antti B-M., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part II. Adsorption and electrokinetic studies of apatite in the presence of sodium oleate and sodium metasilicate, b~t. J. Miner. Process., 28, 59 (1990). 4:7/11-Q
890
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15. 16. 17. 18. 19. 20. 21. 22. 23.
K. HANUMANTHARAO and K. S. E. FORSSBERG
Hanumantha Rao K., & Forssberg K.S.E., Mechanism of oleate interaction on salttype minerals: Part III. Adsorption, electrokinetic and diffuse reflectance F T - I R studies of scheelite in the presence of sodium oleate, Coll. Surf., - in press. Hanumantha Rao K., Cases J.M., de Donato P., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part IV. Adsorption, electrokinetic and diffuse reflectance F T - I R studies of natural fluorite in the presence of sodium oleate, J. Colloid Interface Sci., - in press. Hanumantha Rao K., Cases J.M., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part V. Adsorption and precipitation reactions in relation to the solid/liquid ratio in the synthetic fluorite-sodium oleate system, J. Colloid Interface Sci., - in press. Hanumantha Rao K. & Forssberg K.S.E., Adsorption of oleate species on sparingly soluble calcium minerals from aqueous solutions: A comparision of theoretical and experimental thermodynamic chemical equilibria, Proc. 3rd Int. Syrup. Beneficiation Agglomeration, Bhubaneswar, India, January 16-18 (1991 ). Hanumantha Rao K. & Forssberg K.S.E., Adsorption of oleate from aqueous solution onto fluorite, Proc. X V I I t h Int. Miner. Process. Congr., Dresden, GDR, Sept. 23-28 (1991 ) - in press. Sadowski Z., The effect of dispersant reagents on the sodium oleate adsorption at the salt minerals-water interface, 8th Int. Syrup. Surfactants in Solution, Gainesville, Florida, June 10-15, (1990). Predali J.J. & Cases J.M., Thermodynamics of the adsorption of collectors, Proc. Xth Int. Miner. Process. Congr., Inst. Min. Metall., London, 473-492 (1973). Cases, J.M., Adsorption des tensio-actifs a l'interface solide-liquide: Thermodynamique et influence de l'heterogeneite des adsorbants, Bull. Mineral., 102 (5-6), 684 (1979). Cases J.M., Poirier J.E. & Canet D., Adsorption a l'interface solide-solution aqueuse des tensio-actifs ioniques: les systemes a forte liaison namale adsorbant-adsorbant et surface heterogene, Solid-Liquid Interactions in Porous Media (ed. J.M. Cases), Technip, Paris, 335-370 (1985). Cases J.M., Levitz P., Poirier J.E. & Van Damme H., Adsorption of ionic and nonionic surfactants on mineral solids from aqueous solutions, Advances in Mineral Processing (ed. P. Somasundaran), SME Publication, Littleton, Col., 171-186 (1986). Belle J. & Bothorel P., Thermodynamical study of the phospholipid chain structure in mono and bilayers, Nouv. J. Chim., 1 (4), 265 (1977). Parekh B.K. & Aplan F.F., The critical surface tension of wetting of minerals coated with collectors, Proc. XVth Int. Miner. Process. Congr., Cannes, 1985, Societe de l'Industrie Minerale, Vol. 2, 3-15 (1985). Yoke J.T., The solubilities of calcium soaps, J. Phys. Chem., 62, 753 (1958). Du Rietz C., Chemisorption of collectors in flotation, Proc. Xlth Int. Miner. Process. Congr., Cagliari, Instituto di Arte Mineraria e Preperazione dei Minerali, Rome, 395-403 (1975). Marinakis K.I. & Shergold H.L., The mechanism of fatty acid adsorption in the presence of fluorite, calcite and barite, Int. J. Miner. Process., 14, 161 (1985). Antti B-M. & Forssberg K.S.E., Pulp chemsitry in industrial mineral flotation. Studies of surface complex on calcite and apatite surfaces using FTIR spectroscopy, Miner. Engng., 2 (2), 217 (1989). Zimmels Y., Lin I.J. & Friend J.P., The relation between stepwise bulk association and interracial phenomena for some aqueous surfactant solutions, Coll. Poly. Sci.. 253,404 (1975). Mishra S.K., Electrokinetic properties and flotation behaviour of apatite and calcite in the presence of sodium oleate and sodium metasilicate, Int. J. Miner. Process., 9, 59 (1982).
0892-6875/91 $3.00 + 000 © 1991 Pergamon Press plc
MECHANISM OF FATTY ACID ADSORPTION IN SALT-TYPE MINERAL FLOTATION
K. HANUMANTHA RAO and K.S.E. FORSSBERG Division of Mineral Processing, Lule~t University of Technology, S-95187 Luleh, Sweden
ABSTRACT Our recent studies on the interaction of oleate with salt-type mineral surfaces have been summarized based on adsorption, infrared and zeta-potential measurements. Monolayer coverage in the case of calcite and a bilayer formation in the cases of fluorite, apatite and scheelite, preceding the precipitation of calcium soap has been suggested for the adsorption mechanism of oleate on these minerals. The monolayer coverage is shown to correspond to a condensed state of oleate with a molecular coverage area of 33 ,~2 (liquid-crystal state). Depending on the surface potential and its magnitude at basic pH values, oleate is either chemisorbed on the surface calcium or monocoordinated through counter sodium and calcium ions for the monolayer filling. Keywords Salt-type minerals; oleate; adsorption; FTIR spectroscopy; zeta-potential INTRODUCTION The sparingly soluble calcium minerals such as calcite, fluorite, apatite and scheelite are largely concentrated by flotation processes. Invariably, fatty acids are used as collectors in these systems. For effective separations, selective adsorption of the collectors at mineral/water interface is essential. The mechanism of the adsorption of fatty acid based collectors on these minerals has been widely studied and a comprehensive review on this subject was published by Hanna and Somasundaran in 1976 [1]. Very recently, this subject has been critically reviewed by Finkelstein [2]. Finkelstein [2] observed that there is no consistency in the abstraction of the oleate by any mineral. The results are widely varied from one mineral to the other, and between two investigations on the same mineral. An attempt to quantify the published adsorption results has failed and only a qualitative feature of the oleate adsorption is discussed. Several shortcomings for the surface precipitation mechanism has been presented. It is felt that the first molecules of oleate adsorb on the surface of calcium minerals and not by a precipitation of calcium oleate at the surface. However, the available evidence is not sufficient for a mechanism based on chemisorption. In general, chemisorption, surface precipitation and bulk precipitation are thought to be the important reactions in these systems. However, a clear understanding on the chemistry of fatty acid interaction does not exist. The adsorption density levels attained several layers in these systems possibly due to a surface and bulk precipitation reactions, which make the interpretation of the shape of the traditional adsorption isotherms difficult. Our recent 879
880
K. HANUMANTHARAOand K. S. E. FORSSBERG
studies indicate that it is still possible to differentiate between adsorption and precipitation reactions [3-10]. In this paper, these recent findings have been summarized. MINERAL-REAGENT INTERACTION
Adsorption studies The adsorption isotherms of oleate on calcite at different pH values are presented in Figure 1 [4]. The isotherms show a linear increase of the amount adsorbed versus the logarithm of the equilibrium concentration, followed by a steep increase in the slope after a plateau corresponding to an oleate adsorption density of around 5/zmol m "2. A plateau was observed at 5 #mol m -2 by Sadowski [l 1], preceeding the infinite increase in the slope of the isotherm (Fig. 1). The steep increase in the slope of the isotherms characterize calcium oleate precipitation. The solubility product of calcium oleate for the precipitation step, considering the calcium ion concentration in the pulp is estimated to be around pK 14.0 [4].
20 °
pH Reference o 9.3 [4] x 10.0 ,x 11.0
fj tt
o
I
104
I
E
/
~x |
i
h
10.5
/
I
I
I
I
I Ill
I
I
10 .4
E Q U I L I B R I U M O L E A T E C O N C . (moles / 1)
Fig.1 Adsorption isotherms of oleate on calcite at various pH values. The adsorption isotherms of oleate on natural and synthetic fluorites at pH 10 are shown in Figure 2 [7,8]. The natural fluorite isotherm shows a plateau level at 10 #tool m -2 before a steep increase in the adsorption density. The steep increase in the adsorption density after the plateau level is determined to be due to the precipitation of calcium oleate from the analysis of pulp liquid. The composition of the pulp liquid after adsorption is shown in Figure 3. This figure shows that the negative logarithm of the ionic product of calcium and the square of the equilibrium oleate is constant at 14.39 for initial oleate concentration between 3x10 "5 and 6xl0"SM. This range of constant ionic product corresponds to the vertical step immediately after the plateau in the adsorption isotherm. Hence, this is characterized as the precipitation region of calcium oleate with a solubility product of 4.07x1015mo131 3 (pK=14.39). In the case of the synthetic sample, a break at 10 ~mol m "2 is once again observed in the adsorption isotherms at three different solid/liquid ratios as can be seen in Figure 2. An interesting observation is that at high solid concentrations (0.2% solids), the isotherm is seen to level off at 10 #mol m -2. At low solid concentrations (0.05 and 0.1% solids), the increase in the adsorption density after the plateau level is ascribed to the precipitation of calcium oleate. The solubility product of calcium oleate corresponding to these precipitation regions is estimated to be around pK 14.35, from the analysis of the pulp liquid after adsorption [8].
Fatty acid adsorption
in s a l t - t y p e m i n e r a l f l o t a t i o n
881
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D
DX
X
x
X
0 "~
A
~
i
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A
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Syntheticfluorite O 0"05%s°lids
a 1"0%S°lids
/~ 0.20 % Solids
10~
10.5
104
103
E Q U I L I B R I U M O L E A T E CONC. (moles / 1)
Fig .2 Adsorption isotherms of oleate on natural and synthetic fluorite samples at pH 10 [7,8]. 16
i0 -3
---
1° 15
"
"-5
Z ©
~
10-W
[-. Z L) Z O L) x Fluoride 0 Calcium
10-i0
I
I
I I i i iii 10-s
i
i
i
13 10.4
INITIAL OLEATE CONC. (moles / 1)
Fig.3 Results of the analysis of the liquid phase after adsorption as a function of initial oleate concentration [7]. Analyses are for the studies on natural fluorite sample as shown in Fig. 2. The adsorption isotherms of apatite at different pH values are shown in Figure 4 [5]. At pH 8, an infinite vertical line occurs at an equilibrium oleate concentration of 9x10"6M, characterizing precipitation of calcium oleate at an adsorption density slightly higher than 5 #mol m "2. The solubility product of this three dimensional phase is estimated to be pK 14.5. The isotherms at pH 9, 10 and 11 show two plateau levels, one at 5 #mol m "2 and the other at 10 #mol m "2. The isotherm shape is unchanged, irrespective of whether the solids are added to oleate solution or oleate is added to the apatite suspension. Figure 5 shows the oleate adsorption at different solid/liquid ratios. It can be seen that the adsorption density is independent of apatite content up to 10 #mol m "2, after which it is found to be dependent on the solids content. The oleate abstraction after 10 #mol m "2 is considered to be due to calcium oleate precipitation [5].
882
K. HANUMANTHARAOand K. S. E. FORKSBERG
pH
0 8.0 N
9,0
a
10.0 11.0
tx
E
m r~ <
' a ~ ?/ / ~ ~ & 10"6
10.5
~ ~A~ kll'i
,
, i i i tll
104
l0 ~
EQUILIBRIUM OLEATE CONC. (moles / 1)
Fig.4 Adsorption isotherms of oleate on apatite at various pH values [5].
(%) o 0.I x 0.2 tx 0.3 a 0.4 m 0.5 O 0.6
Solids
2O
I
0.7
+ 0.8
5 0 10.6
io "~
lO ~
lO ~
EQUILIBRIUM CONC. (moles / 1)
Fig.5 Adsorption of oleate on apatite at different solid/liquid ratios at pH 10 [5]. The adsorption isotherms of oleate on scheelite at pH I0 are shown in Figure 6 [6]. In the presence of a carbonate buffer, the surface calcium is expected to complex with the carbonate species, and for an exchange reaction with oleate ions to occur, a high concentration of oleate is probably required. This explains the shift in the adsorption isotherm to a higher equilibrium oleate concentration in the presence of a carbonate buffer. The infinite increase in the slope after 10 #tool m "e characterizes calcium oleate precipitation. The isotherms also show an initial increase in the slope around 5/~mol m "2. Figure 7 shows the oleate adsorption at pH 9 using a borate buffer solution, when the residual oleate concentration is measured after allowing the particles to settle, and the liquid centrifuged. It shows a plateau around 10 #mol m "2 before the precipitation step of calcium
883
Fatty acid adsorption in salt-type mineral flotation
oleate. This implies that the free calcium ions are not available for precipitation and are probably complexed with the borate species. This explains the plateau that precedes the precipitation step of calcium oleate when compared to the shape of the isotherms as presented in Figure 6.
302;
0
0
x
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E
<
O
Z
5
0 10"
t
I
1 0 ~s
I II tl[ 1 0 "5
I
I I I IIHI 104
i
I I I )1 10"3
EQUILIBRIUM OLEATE CONC. (moles / l)
Fig.6 Adsorption isotherms of oleate on scheelite at pH 10: (o) pH adjusted with HCI/NaOH; (x) pH adjusted with carbonate buffer [6].
20
E
~3
O
]o-7
i
i i i ltlll 10 .6
I
I I I I I ) 11 10 s
I
I I I IIlll 104
t
I I I Ill 10 .3
EQUILIBRIUM OLEATE CONC. (moles / 1)
Fig.7 Adsorption isotherm of oleate on scheelite at pH 9 adjusted with borate buffer: (o) scheelite particles allowed to settle after equilibration; (x) scheelite suspension centrifuged after equilibration [6]. Cases and coworkers [I 2= 15] proposed an adsorption model where the surfactants containing more than 8 methylene groups condense two-dimensionally on the mineral surface due to strong lateral bonds. Because of surface heterogeneities, they also suggested that the two dimensional condensation initially takes place on the homogeneous domain that are formed
884
K. HANUMANTHARAp and K. S. E. FORSSl)ERG
with the most energetic sites at the smaller values of solution concentrations followed by successive two dimensional condensation on the less energetic homogeneous domain. Hence, in the monolayer range, the isotherm can take a shape of several small vertical steps or a smooth curve depending on the distribution of the surface energies. But, once the monolayer is covered, the bilayer formation always takes a single sub-vertical step due to the homogeneity of the surface caused by the monolayer formation. However, an infinite vertical step on the isotherm indicates a precipitation reaction in the solution. Because of the two dimensional condensation, if the oleate is assumed to be in a liquid crystal state (molecular coverage area, 33 N 2 [16]), the monolayer coverage corresponds to 5 #mol m "2. On the other hand, if the oleate molecule is in a hydrated crystal state (molecular coverage area, 20.5 A 2 [17]), monolayer coverage is expected around 8 #mol m "2 adsorption density. The adsorption isotherms show either a plateau or level o f f at 10 #mol m 2. Moreover, an initial break or an increase in the slope around 5 #mol m "2 is also observed. These breaks correspond to a mono and a bilayer formation for the two dimensional condensation of oleate on the mineral surface. However, the isotherms showing a steep increase in the adsorption density after bilayer formation can be described as calcium oleate precipitation. The solubility product of calcium oleate is seen to correspond to a p K region of 14.0-14.5, which agrees with the literature values [18-20]. In summary, the adsorption results show a monolayer coverage in the case of calcite and a bilayer formation in the cases of fluorite, apatite and scheelite, prior to the precipitation of calcium oleate. Infrared studies
The IR spectra in the case of calcite is found to be difficult to interpret due to the highly interfering carbonate absorption in the same frequency region as that of the carboxylate radical. No meaningful information about the bonding of the adsorbed oleate species is obtained even when a differential F T - I R is used and after subtracting the calcite spectrum from the spectrum of adsorbed oleate species [21]. The F T - I R spectra of the apatite at oleate concentrations equivalent to a monolayer capacity, show a characteristic peak at 1550 cm "1 regardless of the pH, thus indicating surface calcium oleate (1:1) [3, 21]. The double peaks charaterizing calcium oleate at 1571 and 1540 cm "1 have been observed at oleate concentrations greater than that required for a complete double layer. Thus, the spectra support the bilayered structure of oleate that precedes the precipitation of calcium oleate. Double layer formation was also confirmed from flotation recovery, as the maximum recovery was obtained at a monolayer coverage, and a sharp drop occurs in the recovery with increasing oleate concentrations [21]. The occurance of only one peak shows that it is a 1:1 calcium oleate complex that builds up the monolayer. The diffuse reflectance IR spectra in the case of the synthetic fluorite isotherm (0.1% solids, Fig. 2) at different adsorption densities are presented in Figure 8 [8]. The spectra at low adsorption densities show a band at 1557 cm "1. As the adsorption density increases, this band shifts to a lower value, and appears at 1555-54 cm "1 when the adsorption density is about 6 #mol m "2. No shift in the band is observed up to 10 # m o l m "2 with further increase in the adsorption density. The absorbance band that occurs between 1557-54 cm "1, when the adsorption density is below 10/zmol m "2, is assigned to the surface calcium oleate (1:1). Above 10 #tool m -2 adsorption density, the spectra show absorbance bands at 1576 and 1540 c m ' l corresponding to calcium oleate. These spectra further substantiate that calcium oleate is precipitated after the plateau in the isotherms. A similar IR spectra are observed for the adsorption results at 0.2% solids up to 10 #mol m -2 adsorption density [8]. Figure 9 shows the spectra of the synthetic fluorite where the adsorption studies have been made by changing the order of oleate addition [8]. At low oleate concentrations, the spectra are observed to be similar, irrespective of whether the solids are added to the oleate solution or oleate is added to the fluorite suspension. Moreover, the intensity of the surface calcium oleate band (1555 cm "1) is unchanged. The bands corresponding to precipitated calcium oleate species appear only after an initial oleate concentration of 2.2 x 10"4M. This implies
Fatty acid adsorption in salt-type mineral flotation
885
that the oleate presumably adsorbs on the surface prior to the nucleation of a new solid calcium oleate phase, since the solubility product is exceeded several times before the actual formation of calcium oleate.
Z < O <
01 I
I
I
I
I
1
I
I
19oo lSOO 17oo 16oo xs0o 14oo 13oo 12ooi~o 1800 1700 1600 1500 1400 1300 1260 WAVENUMBERS( c m ' b WAVENUMBERS (cm1) Fig.8 Diffuse reflectance FT-IR spectra at different adsorption densities ( # m o l m "2) for the synthetic fluorite isotherm at 0.1% solids as shown in Fig. 2 [8]. In the carboxylate region, the diffuse reflectance FT-IR spectra in the cases of natural fluorite and scheelite show the bands corresponding to sodium oleate (1561 cm "1) and calcium oleate (1539 and 1577 cm "1) from the beginning of the surface coverage [6,7,10]. From the adsorption studies, the precipitation of calcium oleate is observed after 10/~mol m "2, whereas the FT-IR spectra show that the surface is filled with sodium and calcium oleates even in the monolayer range. This discrepancy is explained by the negative zetapotential of the mineral surface and hence, monocoordination of oleate through counter sodium and calcium ions that exist at the interfacial region is considered rather than the growth of surface precipitate at low oleate concentrations, and in the monolayer range. In this case, the IR spectra cannot represent the true adsorbed oleate species as the counter calcium and oleate ions are expected to deposit calcium oleate on the surface during the sample preparation (air-drying). On the other hand, the synthetic fluorite sample showed a positive zeta-potential at pH 10 (Fig. 1 1) and the calcium oleate species are observed only after 10 #tool m "2 corresponding to the adsorption results. Hence, depending on the surface potential and its magnitude, the adsorption of oleate for the monolayer filling can be described as follows:
886
K. HANUMANTHARAO and K. S, E. FORSSBERG
-CaOH + "OOCR -CaOH + Na + "OOCR + OH" -CaOH + Ca ++ "OOCR + OH-CaOH2 + + "OOCR -CaO ~" Na + + "OOCR -CaO s" Ca ++ + "OOCR
-- -Ca + "OOCR + OH" -- -CaO Na OOCR" + H20 = -CaOCaOOCR +H20 -- -Ca + "OOCR + H20 -- -CaO Na OOCR, where 6 <1 = -CaO Ca OOCR, where 6 _<_1 INITIAL OLEATE " ^'2x10-5.M 0x 10-,M ~ 2 0 4 0
z < O m <
,900 ,800 ,700 ,600 ,500 ,400 ,300 ,200 ,gbo ,sbo ,400 WAVENUMBERS (cmq)
,~'00 ,5'00 ,,'00 ,3~0
1200
WAVENUMBERS (cm a)
Fig.9 Diffuse reflectance F T - I R spectra at different initial oleate concentrations with a change in the order of addition of oleate: (a) fluorite added to oleate solution; (b) oleate added to fluorite suspension [8].
Zeta=potential
studies
The zeta-potential of calcite, fluorite, scheelite and apatite as a function of pH are shown in Figure 10. The i.e.p, of both fluorite and apatite are found to be at pH 8.5 and 4.0 respectively. The zeta-potential of calcite and scheelite are found to be negatively charged through out the pH region studied. In the case of synthetic fluorite, a positive zeta-potential is shown in the pH region 3-1 1 (Figure I 1). The zeta=potential of the synthetic fluorite at different initial oleate concentrations as a function of pH is shown in Figure 1 1. It can be seen that the zeta-potential decreases as the oleate concentration increases at all pH values. Above pH 8, oleate exists mostly as monomers, and the decrease in zeta-potential with increasing oleate concentration shows the high a f f i n i t y of oleate towards the mineral surface. A similar zeta-potential curves in the presence of oleate have been obtained even when the mineral samples showed negative potential at basic pH region [4-8].
Fatty
acid adsorption
in s a l t - t y p e
mineral
ra
5(
0 A x
4( >
887
flotation
Calcite Fluorite Scheelit¢ Apadt¢
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g
2(
<
1( ( -1( )
< -2( t'.q -30 -4{
f
-5( -6(
I
)
I
I
[
I
I
I
I
3
4
5
6
7
8
9
10
11
12
pH
Fig. 10 Zeta-potential of calcite, fluorite, apatite and scheelite as a function of pH [4-7].
6(
O
5{
~o
o
4(
o
x
2c
~ -1( i
L~ N-!
OLEATE 0 NIL
-
e r,
2.18 x IO~M
5.45 x IO~_M -
O •
2.73x IO°_M 5.45x IO'~M I
)
I
I
I
)
I
I
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3
4
5
6
7
8
9
10
11
12
pH
Fig.l I Zeta-potential of synthetic fluorite as a function of pH in the presence of different initial oleate concentrations [8]. The influence of oleate on calcite zeta-potential at pH 10 is shown in Figure 12 [4,20,22,23]. Initially, there is a decrease in the zeta-potential and between l x l 0 "s and lxl0"4M oleate concentration, the potential remains constant. The zeta-potential again decreases and reaches a minimum potential of -50 mV as the oleate concentration increases further. The minimum zeta-potential of calcite in the presence of oleate correspond to that of calcium oleate [4]. If the abstraction of oleate is in the form of precipitated calcium oleate and multilayers are formed on the surface, the zeta-potential should decrease as the oleate concentration increases, at least up to -50 mY. Hence, the adsorption of oleate on the surface could be deduced for the initial decrease in zeta-potential, and the precipitation of calcium oleate in the bulk solution between l x l 0 -s and lxl0"4M oleate concentration. The three dimensional growth of calcium oleate on the surface or the adsorption of calcium oleate precipitate on
888
K. HANUMANTHARAOand K. S. E. FORSSBERG
the top of the hydrophobic layer was thought to be the reason for the sudden decrease in the zeta=potential equivalent to that of calcium oleate beyond lxl0-4M oleate concentration [4]. 25 x [] •'~ 0
R a o e t a l [4] Marinakis et al [20] Zimmels et al [22] Mishra [23]
¢g ,.d < Z
-2.'
X
i
< -50
10-~
10 .4
INITIAL
OLEATE
CONC.
10 .3
10 .2
( m o l e s / 1)
Fig.12 Zeta-potential of calcite as a function of initial oleate concentration at pH 10. The results on the effect of sodium oleate on the zeta-potential of fluorite samples at pH 10 are presented in Figure 13 [7,8]. The decrease in the zeta-potential is observed to occur in two stages with increasing oleate concentration, even though the minimum potentials reached in both cases are the same. The first stage is correlated to a successive oleate adsorption up to a bilayer formation (10 #mol m'2). The second decrease up to -50 mV is thought to be the growth of the three dimensional condensation of calcium oleate on the surface. The change in the zeta-potential accounts for the increase in the surface charge density due to the change in the molecular area of the adsorbed molecule from the liquidcrystal state to the precipitated phase. This hypothesis is supported by the fact that the constant zeta-potential of -50 mV corresponds to the zeta-potential of calcium oleate, based on the adsorption results and the IR spectra [8]. A similar zeta-potential results for apatite and scheelite in the presence of oleate have been reported [5,6]. CONCLUSIONS The shape of the adsorption isotherms of oleate on fluorite, apatite and scheelite at basic pH values indicate a plateau level around l0 #mol m "2, which corresponds to a bilayer formation for the two dimensional condensation of oleate on heterogeneous surfaces. The infinite increase in the slope of the isotherms after the bilayer formation characterizes calcium oleate precipitation. In the case of calcite, the precipitation of calcium oleate occurs after a monolayer coverage. The zeta-potential studies support the conclusions drawn from the adsorption isotherms. The evidence for the chemisorption of oleate on surface calcium is obtained by diffuse reflectance FT-IR studies. Here, the samples show a positive potential at the adsorption pH value of 10. When the samples show a negative zeta-potential, the IR spectra indicate sodium and calcium oleate species from the beginning of surface coverage. However, a monocoordination of oleate through counter sodium and calcium ions is suggested for the monolayer filling as opposed to the growth of surface precipitate. In these cases, the IR spectra cannot give conclusive evidence for the nature of oleate bonding, as the counter
Fatty acid adsorption in salt-type mineral flotation
889
calcium and oleate could transform into compound formation during the sample preparation (air-drying). 2O
-1C
<
x Natural fluorite O Synthetic fluorite
~x,X O
-30
'
......
i;,
.
.
.
.
.
.
.
. . . .
103
INITIAL OLEATE CONC. (moles / 1)
Fig.13 Zeta-potential of natural and synthetic fluorites as a function of initial oleate concentration at pH 10 [7,8]. In summary, monolayer coverage in the case of calcite, and a bilayer formation in the cases of fluorite, apatite and scheelite, prior to the precipitation of calcium oleate is suggested as the adsorption mechanism of oleate on these minerals. The adsorption density at monolayer coverage corresponds to a condensed state of oleate with a molecular coverage area of 33 A 2. ACKNOWLEDGEMENT The authors are grateful to the Swedish Mineral Processing Research Foundation (MinFo) for financial support.
REFERENCES
.
.
3.
.
.
HE
Hanna H.S. & Somasundaran P., Flotation of salt-type minerals, Flotation, A.M. Gaudin memorial volume (ed. M.C. Fuerstenau), AIME, New York, vol. 1, 197-272 (1976). Finkelstein N.P., Review of interactions in flotation of sparingly soluble calcium minerals with anionic collectors, Trans. IMM, Sec. C, 98, 157 (1989). Hanumantha Rao K., Antti B-M., Cases J.M. & Forssberg K.S.E., Studies on the adsorption of oleate from aqueous solution onto apatite, Developments in Mineral Processing, Proc. XVIth Int. Miner. Process. Congr. (ed. E. Forssberg), Elsevier, Amsterdam, Vol 10A, 625-636 (1988). Hanumantha Rao K., Antti B-M., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part I. Adsorption and electrokinetic studies of calcite in the presence of sodium oleate and sodium metasilicate, Coll. Surf., 34,227 (1988/89). Hanumantha Rao K., Antti B-M., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part II. Adsorption and electrokinetic studies of apatite in the presence of sodium oleate and sodium metasilicate, b~t. J. Miner. Process., 28, 59 (1990). 4:7/11-Q
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15. 16. 17. 18. 19. 20. 21. 22. 23.
K. HANUMANTHARAO and K. S. E. FORSSBERG
Hanumantha Rao K., & Forssberg K.S.E., Mechanism of oleate interaction on salttype minerals: Part III. Adsorption, electrokinetic and diffuse reflectance F T - I R studies of scheelite in the presence of sodium oleate, Coll. Surf., - in press. Hanumantha Rao K., Cases J.M., de Donato P., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part IV. Adsorption, electrokinetic and diffuse reflectance F T - I R studies of natural fluorite in the presence of sodium oleate, J. Colloid Interface Sci., - in press. Hanumantha Rao K., Cases J.M., & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals: Part V. Adsorption and precipitation reactions in relation to the solid/liquid ratio in the synthetic fluorite-sodium oleate system, J. Colloid Interface Sci., - in press. Hanumantha Rao K. & Forssberg K.S.E., Adsorption of oleate species on sparingly soluble calcium minerals from aqueous solutions: A comparision of theoretical and experimental thermodynamic chemical equilibria, Proc. 3rd Int. Syrup. Beneficiation Agglomeration, Bhubaneswar, India, January 16-18 (1991 ). Hanumantha Rao K. & Forssberg K.S.E., Adsorption of oleate from aqueous solution onto fluorite, Proc. X V I I t h Int. Miner. Process. Congr., Dresden, GDR, Sept. 23-28 (1991 ) - in press. Sadowski Z., The effect of dispersant reagents on the sodium oleate adsorption at the salt minerals-water interface, 8th Int. Syrup. Surfactants in Solution, Gainesville, Florida, June 10-15, (1990). Predali J.J. & Cases J.M., Thermodynamics of the adsorption of collectors, Proc. Xth Int. Miner. Process. Congr., Inst. Min. Metall., London, 473-492 (1973). Cases, J.M., Adsorption des tensio-actifs a l'interface solide-liquide: Thermodynamique et influence de l'heterogeneite des adsorbants, Bull. Mineral., 102 (5-6), 684 (1979). Cases J.M., Poirier J.E. & Canet D., Adsorption a l'interface solide-solution aqueuse des tensio-actifs ioniques: les systemes a forte liaison namale adsorbant-adsorbant et surface heterogene, Solid-Liquid Interactions in Porous Media (ed. J.M. Cases), Technip, Paris, 335-370 (1985). Cases J.M., Levitz P., Poirier J.E. & Van Damme H., Adsorption of ionic and nonionic surfactants on mineral solids from aqueous solutions, Advances in Mineral Processing (ed. P. Somasundaran), SME Publication, Littleton, Col., 171-186 (1986). Belle J. & Bothorel P., Thermodynamical study of the phospholipid chain structure in mono and bilayers, Nouv. J. Chim., 1 (4), 265 (1977). Parekh B.K. & Aplan F.F., The critical surface tension of wetting of minerals coated with collectors, Proc. XVth Int. Miner. Process. Congr., Cannes, 1985, Societe de l'Industrie Minerale, Vol. 2, 3-15 (1985). Yoke J.T., The solubilities of calcium soaps, J. Phys. Chem., 62, 753 (1958). Du Rietz C., Chemisorption of collectors in flotation, Proc. Xlth Int. Miner. Process. Congr., Cagliari, Instituto di Arte Mineraria e Preperazione dei Minerali, Rome, 395-403 (1975). Marinakis K.I. & Shergold H.L., The mechanism of fatty acid adsorption in the presence of fluorite, calcite and barite, Int. J. Miner. Process., 14, 161 (1985). Antti B-M. & Forssberg K.S.E., Pulp chemsitry in industrial mineral flotation. Studies of surface complex on calcite and apatite surfaces using FTIR spectroscopy, Miner. Engng., 2 (2), 217 (1989). Zimmels Y., Lin I.J. & Friend J.P., The relation between stepwise bulk association and interracial phenomena for some aqueous surfactant solutions, Coll. Poly. Sci.. 253,404 (1975). Mishra S.K., Electrokinetic properties and flotation behaviour of apatite and calcite in the presence of sodium oleate and sodium metasilicate, Int. J. Miner. Process., 9, 59 (1982).