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Distribution of an adsorbed anionic surfactant on the external and internal surfaces of a porous apatite mineral
Minerals Engineering, Vol. 6, Nos. 8--10, pp. 895--906, 1993
0892-6875/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
DISTRIBUTION OF AN ADSORBED ANIONIC SURFACTANT ON THE EXTERNAL AND INTERNAL SURFACES OF A POROUS APATITE MINERAL
R.B. BJORKLUND and H. ARWIN Laboratory of Applied Physics, Linkfping University, S-581 83 Linkfping, Sweden (Received 8 February 1993; accepted 15 March 1993)
ABSTRACT The adsorption of an anionic surfactant, disodium N-alkylsulfosuccinamate, onto polished apatite ore surfaces was studied by ellipsometry. Surfaces of varying porosity were created by partially dissolving planar apatite surfaces to different degrees in NaOH solution at pH 10. 6. Partially dissolved apatite was optically modelled as a semi-infinite substrate with a porous surface layer. The porous layer varied in thickness from 0 - 550 ,4 and in degree of porosity from 0 - 12 volume %. Adsorption of the surfactant in Ca(OH)2 solution on the non-porous surface resulted information of a thin film about 40 /1 thick which could be removed by flushing the measurement cell with fresh solution. Adsorption of the surfactant on the porous surfaces resulted in both reversible adsorption on the external surface and irreversible adsorption within the pores. The role of the pores in flotation is commented on and a proposal is made concerning how the selectivity for hydrophobization in apatite/calcite ore mixtures is possibly affected by solubility related differences in the microstructure of the minerals. Keywords Flotation, apatite, anionic surfactant, adsorption, ellipsometry, porous surface
INTRODUCTION Selective hydrophobization of the value minerals is an important factor in determining the effectiveness of separation in froth flotation. The ionic surfactants used as collectors are specifically adsorbed via electrostatic interaction with the mineral surface[l] or precipitate as a metal-surfaetant salt on the surface [2]. Salt precipitation during flotation lowers selectivity and this combined with the fact that the surface properties of the gangue minerals often are similar to the value minerals, makes selective hydrophobization a difficult task for flotation surfactants [3]. Several different aspects of the surfactant/mineral interaction, including the effect of preparation conditions and surface roughness [4], are important for understanding the fundamental processes in flotation. A number of experimental techniques have been employed to investigate the physico-chemieal surface properties of minerals. Examples are contact angle and electrokinetie measurements. These combined with adsorption studies based on surfactant concentrations before and after contact with the mineral surface, provide a good framework for understanding the relationship between mineral surface properties and flotation behavior. In addition, instrumental techniques such as FTIR have played an important role in identifying surfaetant complexes adsorbed on mineral surfaces at flotation conditions [5,6]. 895
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R.B. B1ORKLUNDand H. AaWIN
The interaction of anionic surfactants with sparingly soluble minerals such as apatite and calcite is a complicated system to study [7]. Since the concentration of Ca2+ can be on the order of 0.1 mM for calcite-containing ores, there is the possibility for Ca-surfactant salt precipitation on the surface. Dissolution of the minerals also leads to changes in the surface microstructure which affects the interaction with the surfactant. We recently reported results for adsorption of a monoalkylsulfosuccinamate surfactant on polished apatite and calcite ore surfaces [8]. We employed ellipsometry to observe in situ the adsorption (and desorption) of the surfactant and to monitor changes in microstructure (porosity) upon mineral dissolution. In the present study we used ellipsometry to follow adsorption of the surfactant as a function of the degree of porosity for apatite surfaces. Our goal was to investigate if ellipsometry can be used to differentiate between surfactant adsorption on the external and internal surfaces of apatite of varying porosity. Distribution of adsorbates on heterogeneous surfaces is an important parameter in several areas of technology. One example from industrial catalysis is the distribution of metal salts from the liquid phase on porous oxide carriers during preparation of catalysts determined by ion microprobe analysis and uv spectroscopy [9, 10]. A second example is the determination of the external and internal surface areas of zeolites by the t-plot method and benzene plugging technique [11]. We present here ellipsometry as a technique to monitor surfactant adsorption on porous apatite surfaces. It can be used to observe how the surfactant is distributed both within the pores and on the external surface of the mineral and the dependence on the degree of porosity. Ellipsometry provides a new method to study the interaction between surfactants and minerals, especially with respect to mineral microstructure, and thus has potential for increasing our understanding of the hydrophobization process.
EXPERIMENTAL Apatite was obtained as six-sided, greenish columns from an ore sample of very similar appearance to that pictured in Reference 12. Flouride was the predominant charge compensating anion in the apatite mineral. Small irregular pieces of the mineral, about 1 x 1 x 0.3 cm in dimension, were polished on an Ecomet4 variable speed grinder-polisher from Buehler Ltd using BUEHLER-METTM P1200 metallographic grinding paper. After polishing, the samples were glued to the ends of object glass slides so they could be positioned vertically in the measurement cell. The samples were attached in such a way to the slides that they could be repeatedly polished. The samples were cleaned prior to each run with a 50 % aqueous ethanol solution in an ultrasonic bath, dried with flowing N2 gas, and immediately placed in the measurement cell containing the initial solution. Di-sodium-N-alkyl (C16-C18) sulphosuccinamate, EMPIMIN MK/B, was kindly provided by Albright & Wilson Ltd from their Marchon, France factory. It was in liquid form having a solids content of 33.5 wt % and active matter content of 26.5 wt %. Pro analysi quality NaOH and Ca(OH)2 were obtained from Merck. Ellipsometric measurements were performed at room temperature in a quartz cell of about 8 rnl volume. The cell was open to the atmosphere and solutions were stirred by a teflon coated magnetic bar during measurements. The instrument used was a computerized single wavelength (546 nm, Rudolph Research, model 436) null ellipsometer with an angle of incidence of 68 o. Ellipsometry is based on reflection of polarized light at an oblique incidence. The difference in reflection for light polarized parallel (Rp) and perpendicular (Rs) to the plane of incidence results in a change in the state of polarization upon reflection [13]. The measured quantity is the complex reflectance ratio, p = Rp/Rs = tan,exp(iA), w h e r e , and A are the eUipsometric angles and i= ~/-1. By measuring and analyzing p, it is possible to determine optical and microstructural properties of a sample or of a surface film on a sample. The quartz cell equipped with tubes for pumping liquids in and out while maintaining a constant 5 ml solution volume is shown in Figure 1. The pumping speed was usually 20 ml/min. The basic solutions, NaOH and Ca(OH)2 (pH 10.6, [Ca2+] = 0.2 mM (2 x 10-4 M/l)), were always prepared just prior to use. A stock solution the surfactant was prepared periodically by dispersing 0.06 g EMPIMIN in 50 ml of NaOH solution using an ultrasonic bath. Small quantities of this solution were added to the measurement cell to obtain the desired surfactant concentration, usually 0.04 mM. Three samples of
Anionic surfactant distribution
897
apatite were investigated. The adsorption of the surfactant led to irreversible changes in the mineral surface and they were repolished between measurements. Since the ore was inhomogeneous, this led to small changes in the surfaces after each repolish.
Drain
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Fresh solution~
Stirrer
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Fig. 1 Measurement cell for the ellipsometry experiments containing 5 mL solution. Since ellipsometry is a fairly new technique for studying the flotation process, we briefly describe here the major differences between our experimental set-up and the actual flotation conditions. Our experiments were done on planar apatite surfaces which were intended to serve as models for the powdered mineral. The solutions in contact with the surfaces were quite dilute as compared with the actual flotation process and the liquid to solid ratio was about 16. Adsorption studies using null ellipsometry are based on the relative changes in the ellipsometric angles upon film formation. Thus it was necessary to attain stable values for the angles prior to addition of the surfactant. Since changes in the surface microstructure caused by dissolution led to changes in the angles, it was necessary to have a solution pH of 10.6, which is somewhat higher than that employed in the flotation process, in order to surpress changes in the surface microstructure prior to adding the surfactant. It should be emphasized that contacting the surface with a saturated apatite solution at lower pH was not sufficient to maintain a stable surface. Even at equilibrium there was a mass transport back and forth across the solid/solution interface which altered the surface microstructure. In this respect the extreme surface sensitivity of ellipsometry to changes in the surface microstructure is a disadvantage. However, it is also this sensitivity to events occurring on the surface which makes the technique a useful complement to mineral studies based on changes in the solution composition. A second important feature of the experiments was the ability to pump fresh solution through the measurement cell. This allowed us to follow the desorption of the surfactant which gave information about the strength of adsorption. The adsorption studies were done by adding the surfactant to a fixed volume of solution in the cell. It was also possible to flow a surfactant-containing solution through the cell, making possible adsorption studies in solutions which were free from ions originating from the mineral sample. Greater freedom to establish the composition of the solution in contact with the mineral sample is thus another feature of the ellipsometry experiment.
RESULTS AND DISCUSSION We begin presentation of the results with a brief description of the partial dissolution of the apatite surface in NaOH solution, which we have described more completely elsewhere [8]. We then apply this information to create surfaces of varying porosity by controlled dissolution of the mineral. This is followed by the main topic of the paper which is the adsorption of the surfactant in Ca(OH)2 solution on the different surfaces.
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BJORKLUND and H. ARWIN
Partial Dissolution Figure 2 shows the time variations in A and ~ for an apatite surface when the solution was changed from Ca(OH)2 to NaOH in the cell. We have previously assigned the observed increases in the angles to the formation of a porous interface zone which results in a roughening of the apatite surface [8]. It is most probable that the dissolution occurred preferentially at certain sites as has been described for synthetic fluorapatite [14] leading to a three-dimensional porous interface zone.
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Fig.2 Time dependence of A and ~ for an apatite surface first in Ca(OH)2 solution, [Ca2+] = 0.2 mM, and then in flowing NaOH solution at pH 10.6. Letters A - E indicate the points where the dissolution was stopped by reintroducing the Ca(OH)2 solution into the cell in later separate experiments. A rough surface can be optically modelled as a semi-infinite substrate with a porous surface layer• In order to quantify the results shown in Figure 2 we performed an optical analysis to determine the thickness and composition of a surface layer under the assumption that the optical properties of the constituents of the layer were known. The constitutients were the bulk mineral and the ambient media (the NaOH solution). The latter has a refractive index ofna = 1.334 as determined by an Abbe refractometer. The (assumed isotropic) refractive index, ns, of apatite was obtained from A and ~ at t = 0 in a simple two-phase (ambient-substrate) model in which [13] n s = no since l+tanZo
(1)
where p = t a n , exp iA and O = 680 is the angle of incidence. For apatite. ~ was found to be 1•626 i0.016. This value is somewhat higher than in our previous report [8] which is due to better polishing characteristics for this apatite sample. The fact that the refractive index has a small imaginary part is probably due to model mismatches. Apatite is slightly anisotropic and the mineral surface may have an initial roughness even after polishing.
Anionic surfactant distribution
899
For a mixture of two materials, the optical properties are not a simple volume fraction weighted average of the constitutients. Instead they are obtained by more sophisticated theories. We have used the Bruggeman effective medium theory in which the effective refractive index, neff, is obtained from [15]
[ L
n s
+
+
I
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+
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where fs is the volume fraction of the apatite and fa = 1-fs is the porosity. In Figure 3 we show the data from Figure 2 plotted as ~ versus A. The lines show values for a range of porosities and film thicknesses calculated in an optical model with a porous film on an apatite substrate. The refractive index of the porous film was obtained from Eq. (2). At the end of the dissolution (point E) the values were about 12 volume % porosity in an interface zone which was 550 /k thick. We conclude from Figure 3 that the process occurring when a polished apatite surface was in contact with a NaOH solution can adequately be described by formation of a porous surface film. A more detailed description of the surface microstructure cannot be determined from the data. As we have described previously [8], a possibility is the selective dissolution of the mineral at certain sites leading to a threedimensional porous interface zone. I
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Fig.3 Calculated ¢ - A curves for formation of a porous surface layer on apatite in the thickness range 0 - 500 A, and with porosities in the range 0.05 - 0.20. Further details are given in the text. The dots are the experimental results from Figure 2.
Controlled Dissolution Since the apatite dissolution could be stopped at any time by re-introducing the Ca(OH)2 solution, it was possible to create surfaces of varying porosity relative to the initial polished surface. The different levels of porosity obtained are indicated at positions A-E in Figures 2 and 3. Surfactant adsorption was then studied on the different porous surfaces. Since the surfactant caused irreversible changes in the surface, it was not possible to create the different surfaces sequentially. Instead, each surface was produced by partially dissolving a freshly polished surface to the appropriate positions on the curve shown in Figure 2. The agreement between the initial values of A and ~ after polishing for the different samples was good, A = 1.1" + 0.20 and ~ = 26.00 + 0.20.
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BJORKLUND and H. ARWIN
Surfactant Adsorption For the initial polished surface (point A in Figures 2 and 3) the addition of surfactant to the Ca(OH)2 solution caused an increase in A, as shown in Figure 4, and no change in ~. For clarity we omit ¢ in reporting results for the adsorption experiments since it was nearly constant throughout. A reached a steady-state value during the adsorption and flushing the cuvette with fresh Ca(OH)2 solution caused it to return to slightly above the original baseline as shown in the figure.
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Fig.4 Time dependence of A for surface A (defined in Figures 2 and 3) in 0.2mM Ca(OH)2 solution during the following treatments: (a) 0.04 mM surfactant added, S and (b) cell flushed with fresh Ca(OH)2 solution. Adsorption of the surfactant on surface A, which was freshly polished, was restricted to the external surface area of the mineral. Although the surface contained several relatively large cracks, these did not contribute to the light reflected to the detector, and can be considered to be a part of the external surface according to Gregg and Sing [16]. Thus it was the planar, non-porous apatite which was monitored during adsorption A. The optical model which is applicable to adsorption of a surface film on the planar mineral surface is shown in Figure 5b. If we assume a surfactant refractive index of r~ = 1.45, it is possible to calculate a thickness scale for the adsorbed layer as shown in Figure 4. Since 40 A is approximately the length of the surfactant hydrocarbon chain, it is possible that the surfactant, standing vertically, was adsorbed as a monolayer on the surface. However, the calculated thickness is an average value for the surface and does not give any additional information about the adsorption geometry. Flushing the cell with fresh solution caused the film to desorb and A returned to nearly its original value. The discrepancy between the A values before and after adsorption may be due to a small quantity of surfactant remaining on the surface or some mineral dissolution during the adsorption period. Although we describe our observations in Figure 4 as surfactant adsorption, we cannot be certain that it is not a Ca-surfactant salt precipitation occurring. At higher surfactant concentrations, above 0.08 mM, the light beam reaching the detector exhibited an instability which was probably caused by scattering from a precipitation in the solution. For simplicity we will use the term adsorption throughout the text to describe film formation on the surface and surfactant filling of the porous interface zone. Adsorption of the surfactant on porous surfaces was performed. The results for the surfaces representing the rapidly changing A values in Figures 2 and 3, positions B and C, are shown in Figure 6. For position B (7 % porosity, 300 A thick porous interface zone), little change in A was observed during adsorption of the surfactant but a significant decrease occurred when the cuvette was flushed with fresh solution.
Anionic surfactant distribution
901
For position C (10 % porosity, 425 ,~, thick porous interface zone), A decreased upon surfactant adsorption and decreased further during the flush.
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Fig.5 Physical (a) and optical (b) models for surface A (defined in Figures 2 and 3) after adsorption of the surfactant. Physical (c) and optical (d) models for apatite surfaces B - E (defined in Figures 2 and 3) after partial dissolution in NaOH solution and adsorption of surfactant on the external and internal surfaces in Ca(OH)2 solution. The optical model for adsorption on a porous surface is presented in Figure 5d. The surfactant can adsorb on both the external mineral surface and within the pores created by the partial dissolution (Figure 5c). The former is equivalent to adsorption on the non-porous surface and results in an increase in A with no change in ~. Adsorption in the pores causes an increase in the effective refractive index of the porous interface zone from neff, as obtained from Eq. (2), to n'cff since polarizable surfactant molecules replace less polarizable solvent species. The result is that A decreases with no change in ~. We have previously verified this model experimentally and also shown that adsorption within the pores was mostly irreversible [8]. Thus the changes in A observed upon surfactant adsorption and desorption can be used to determine if the adsorbed surfactant occupies adsorption sites on the external surface or within the pores. An increase in A was observed upon surfactant adsorption on surface A. Since the surface was nonporous, adsorption could be described by the model shown in Figure 5b. The adsorption was reversible and A returned to nearly its original baseline. Adsorption on surface B mused a small increase in A. However, flushing the cuvette led to a decrease in A below its original baseline. We interpret these results as indicating that adsorption of the surfactant on both external and internal surfaces occurred on surface B. External surface adsorption seemed to dominate although it is difficult to quantify the two types of adsorption using our simple optical models. When the surface film on the external surface desorbed during the flush, the decrease in A revealed that a significant irreversible adsorption of surfactant within the porous interface zone had occurred. A similar result was observed on surface C. However the sharp decrease in A observed when the surfactant was introduced to the solution clearly indicated that more adsorption in the porous layer was occurring as compared to surface B.
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Time dependence of A for surfaces B and C (defined in Figures 2 and 3) in 0.2mM Ca(OH)2 solution during the following treatments: (a) 0.04 mM surfactant added, S and (b) cell flushed with fresh Ca(OH)2 solution.
Continuing the dissolutions yielded two additional surfaces, D and E, shown in Figures 2 and 3. These are characterized by an increase in both the thickness of the porous interface zone and the degree of porosity relative to surface C. Adsorption of the surfactant on D yielded an initial decrease in A followed by an increase and decrease as shown in Figure 7. This was similar to the adsorption curve observed for surface C except that the increase in A indicated a slightly larger contribution from adsorption on the external surface. That adsorption within the pores was substantial was confirmed by the large decrease in A when the cuvette was flushed with fresh Ca(OH)2 solution. A similar result was seen on surface E where the increase in A observed after the inital decrease during surfactant adsorption indicated a somewhat larger contribution for adsorption on the external surface than was observed for D. Our conclusion from these results is that ellipsometry can be used to determine at least qualitatively how an adsorbed surfactant is distributed between the external and internal surfaces of an apatite mineral. However, the simple optical model that we have used forces us to compare two different quantities when analyzing the distribution of the surfactant. We can, for example, determine the approximate pore filling by the surfactant for surface E by using the A values before (3.20) and after surfactant adsorption and desorption (2.70) from Figure 7. It is then possible to use these angles together with Figure 3 to determine that the volume porosity decreased from about 12 % to 9 % upon surfactant adsorption (if we ignore the small difference in refractive index between the surfactant and the solution). The film thickness for the external surfactant adsorption can be calculated from the change in A when the adsorbed surfactant desorbed from the surface (3.00 - 2.70 = 0.30 in Figure 7). Such a change in A is equal to a film thickness of about 25 ,~ according to the optical model in Figure 5b. Thus we are faced with the problem of comparing a change in volume % caused by the surfactant adsorbed on the internal surface
Anionic surfactant distribution
903
and a film thickness for the external adsorption if we desire a more quantitative description of the surfaetant distribution. Apart from the problem of quantifing the surfactant distribution, the question remains if the results can be correlated with a physical model describing the evolution of the porous interface zone as the apatite dissolves. Even more important is the relevance of the results on the planar, polished mineral surface to the actual flotation system.
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Fig.7 Time dependence of A for surfaces D and E (defined in Figures 2 and 3) in 0.2mM Ca(OH)2 solution during the following treatments: (a) 0.04 mM surfactant added, S and (b) cell flushed with fresh Ca(OH)2 solution. One possible physical model is based on previous studies of dissolution behavior of fluorapatite [14] and calcite [17]. From these investigations it can be concluded that the dissolution rate is strongly influenced by impurities and variations in crystallinity, and that the surface morphology of the minerals plays an important role in the dissolution process. For fluorapatite it was found that dissolution occurred in three dimensions and followed a spiral dislocation mechanism [14]. The dissolution rate was thus dependent on the number of dislocations emerging at the mineral surface. Cleaved surface planes of Iceland spar crystals were found to be unreactive, and a surface roughening by polishing or acid treatment was necessary in order to obtain dissolution in the pH range 6 - 7 [17]. Cutting the surface at a known angle, 0, to the (100) cleavage plane created surfaces which exhibited increasing dissolution rates for increasing values of 0. This was due to the creation of a greater number of terraces and it was observed by SEM that macroscopic terraces were formed during dissolution by coalescence of microscopic terraces.
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R . B . BJORKLUND and H. ARwlN
Our results for surfactant adsorption on apatite surfaces of varying porosity indicate that the surfactant adsorbed only on the external surface of the freshly polished mineral (Figure 4) and exhibited a predominately internal adsorption on surfaces C - E (Figures 5 and 6). A slightly increased external surface adsorption was observed on E relative to C and D. These results seem to indicate that the surface zone changed its morphology as the apatite was dissolved. This was in agreement with the dissolution experiments reported for fluorapatite and calcite [14,17] and the quantitative description of the porous zone shown in Figure 3. From Figure 3 it can be concluded that as the apatite surface dissolved, certain pores became deeper and the total porosity of the surface zone increased only slightly from C to E. A simple physical model to describe our controlled dissolution experiments would involve dissolution initiated at particular defects, such as dislocations, giving several growing pores on the mineral separated by external surface. Eventually, the volume in the surface zone occupied by pores begins to level off as pores grow into each other and create new external surface. That a considerable external (highly reflecting) surface was always present on the apatite was evidenced by the relatively small decrease (about 10%) in reflected light intensity reaching the detector during the dissolution shown in Figure 2. With regard to the relevance of the adsorption experiments described here to the actual flotation process, we want to stress that our approach has been to simplify the solid/liquid interface. By dealing with a planar surface in contact with a liquid phase of well-defined composition, it should be possible to determine the elementary steps involved in the surfactant-mineral interaction. A good example is the information obtained from a comparision of the rates of adsorption on the internal and external surfaces shown in Figures 4, 6, and 7. One would expect that adsorption on a porous surface would proceed by a several step mechanism involving adsorption on the surface terraces, diffusion to the pore and finally migration into the pore. However, it is quite clear from Figure 6 that, when both processes occur simultaneously, adsorption within the pores is the faster of the two. This despite the fact that adsorption on the non-porous surface is quite rapid as shown in Figure 4. One possible explanation for the faster adsorption on the internal surface is possibly the difference in sticking coefficients for the surfactant on the two different surfaces. At the high pH employed for the study, the mineral surface is most likely negatively charged and very few collisions of the surfactant with the external surface results in adsorption. The probability that surfactant molecules entering pores directly from the liquid phase remain in the porous zone is greater since these encounter a three-dimensional array of Ca2+ ions in the pore. A second interesting point is if adsorption of the surfactant in the internal surface contributes anything to the hydrophobization of the mineral. Since air bubbles can only contact the external surface of mineral particles [18], it would appear likely that surfactant adsorbed in the pores has no effect on the flotation process. However, internal surfactant adsorption may be very important in determining the effectiveness of separating two minerals having different solubilities. For an ore containing apatite and calcite, which possess the same degree of microporosity in the dry state, the increase in porosity for the calcite component upon contact with the conditioning solution is considerably greater than for the less soluble apatite. This is evidenced by a substantial decrease in reflected light intensity from dissolving calcite surfaces in ellipsometry experiments [8], which is related to a decrease in external surface area. Thus, when the surfactant is introduced to the flotation cell it initially adsorbs to a greater extent on the internal calcite surface as compared to the internal apatite surface. This leads to a relatively lower adsorption on the external calcite surface since the surfactant concentration in the immediate vicinity of the calcite particle is depleted. The result is a lower degree of hydrophobization of the calcite as compared to apatite.
CONCLUSIONS Ellipsometry can be used to study two aspects of the flotation process - mineral dissolution and surfactant adsorption. As a surface sensitve technique, it is a valuable complement to studies based on composition changes in the solution in contact with the mineral. By monitoring the pore development of apatite during partial dissolution it was possible to produce surfaces of varying porosity. These served as model surfaces to study the effect of porosity on the adsorption of an anionic surfactant. The optical model developed to describe the porous surfaces made it possible to qualitatively determine the distribution of the adsorbed
Anionic surfactantdistribution
905
surfactant between the external and internal apatite surfaces. There exists some possibility to obtain a more quantitative description of the surfactant distribution, but this would require further refining of the optical model. For porous surfaces where surfactant adsorption on both the external and internal surfaces was possible, it was observed that adsorption within the pores was the faster process. This was interpreted as being due to more effective trapping of the surfactant by the pores as compared to the planar external surface. Based on the ellipsometry results it can be concluded that an important factor in obtaining selective hydrophobization of the apatite component in apatite/ealcite ore mixtures is the greater surfactant adsorption on the internal surface of the more soluble calcite. However, it should be emphasized that the ellipsometry studies were done on model apatite surfaces in dilute solutions at a pH somewhat higher than the actual flotation process. In addition, it would be desirable to check the results with an independent technique, such as cross sectional transmission electron microscopy which is often used to confirm film thickness determinations by ellipsometry in multi-layer semiconductor devices. However, despite advances in the use of TEM to study minerals [19], it seems unlikely that quantitative information about the thickness and porosity of the apatite porous interface zone and the concentration of the surfactant adsorbed on the surface and in the pores could be obtained using TEM.
ACKNOWLEDMENT The work reported here was partially supported by the Swedish National Board for Industrial and Technical Development (NUTEK).
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2. 3. 4. 5.
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7. 8. 9. 10. 11. 12. 13. 14. 15.
Leja J., Surface Chemistry of Froth Flotation. Plenum Press. New York (1982). Ananthapadmanabhan K.P. & Somasundaran P., Surface precipitation of inorganics and surfactants and its role in adsorption and flotation. Colloids Surf., 13, 151 (1985). von Rybinski W. & Schwuger M.J., Adsorption of surfactant mixtures in froth flotation. Langmuir 2, 639 (1986). Ducker W.A., Pashley R.M. & Ninham B.W., The flotation of quartz using a double-chained cationic surfactant. J. Colloid Interface Sci., 128, 66 (1989). Rao K.H., Cases J.M., de Donate P. & Forssberg K.S.E., Mechanism of oleate interaction on salt-type minerals 4. Adsorption, electrokinetic, and diffuse reflectance FTIR studies of natural fluorite in the prescence of sodium oleate. J. Colloid Interface Sci., 145, 314 (1991). Gong W.Q., Parentich L.H., Little L.H. & Warren L.J., Adsorption of oleate on apatite studied by diffuse reflectance I R transform spectroscopy. Langmuir 8, 118 (1992). Somasundaran P., Amankonah J.O. & Ananthapadmanabhan K.P., Mineral-solution equilibria in sparingly soluble mineral systems. Colloids Surf., 1S, 309 (1985). Bjorklund R.B. & Arwin H., Ellipsometric study of anionic surfactant adsorption on apatite and calcite ore surfaces. Langmuir 8, 1709 (1992). Schwarz J.A. & Heise M.S., Preparation of metal distributions within catalyst supports IV. Multicomponent effects. J. Colloid Interface Sci., 135, 461 (1990). Goula M.A., Kordulis Ch. & Lycourghiotis A., Influences of impregnation parameters on the axial Me/y-alumina profiles studied using a novel simple technique. J. Catal., 133, 486 (1992). Sayari A., Crusson E., Kaliaguine S. & Brown J., External surface areas of H-ZSM-5 zeolites. Langmuir 7, 314 (1991). Woolley A.R., Bishop A.C. & Hamilton W.R., The Hamlyn Guide to Minerals, Rocks and Fossils. p. 86. Hamlyn Publishing Group Ltd. London (1974). Azzam R.M.A. and Bashara N.M., Ellipsometry and Polarized Light., North-Holland. New York (1977). Chin K.O.A. & Nancollas G.H., Dissolution of fluorapatite. A constant - composition kinetics study. Langmuir 7, 2175 (1991). Aspnes D.E. Optical properties of thin films. Thin Solid Films, 89, 249 (1982).
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16. 17. 18. 19.
R . B . BJORKLUND and H. ARWlN
Gregg S.J. & Sing K.S.W., Adsorption, Surrface Area and Porosity. Academic Press. London (1982). Compton R.G., Dale), P.J. & House W.A., The dissolution of iceland spar crystals: the effect of surface morphology. J. Colloid Interface Sci., 113, 12 (1986). Schulze H.J., Physico-chemical Elementary Processes in Flotation., Elsevier, Amsterdam (1984). MeLaren A.C., Transmission Electron Microscopy of Minerals and Rocks., Cambridge University Press, Cambridge (1991).
0892-6875/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
DISTRIBUTION OF AN ADSORBED ANIONIC SURFACTANT ON THE EXTERNAL AND INTERNAL SURFACES OF A POROUS APATITE MINERAL
R.B. BJORKLUND and H. ARWIN Laboratory of Applied Physics, Linkfping University, S-581 83 Linkfping, Sweden (Received 8 February 1993; accepted 15 March 1993)
ABSTRACT The adsorption of an anionic surfactant, disodium N-alkylsulfosuccinamate, onto polished apatite ore surfaces was studied by ellipsometry. Surfaces of varying porosity were created by partially dissolving planar apatite surfaces to different degrees in NaOH solution at pH 10. 6. Partially dissolved apatite was optically modelled as a semi-infinite substrate with a porous surface layer. The porous layer varied in thickness from 0 - 550 ,4 and in degree of porosity from 0 - 12 volume %. Adsorption of the surfactant in Ca(OH)2 solution on the non-porous surface resulted information of a thin film about 40 /1 thick which could be removed by flushing the measurement cell with fresh solution. Adsorption of the surfactant on the porous surfaces resulted in both reversible adsorption on the external surface and irreversible adsorption within the pores. The role of the pores in flotation is commented on and a proposal is made concerning how the selectivity for hydrophobization in apatite/calcite ore mixtures is possibly affected by solubility related differences in the microstructure of the minerals. Keywords Flotation, apatite, anionic surfactant, adsorption, ellipsometry, porous surface
INTRODUCTION Selective hydrophobization of the value minerals is an important factor in determining the effectiveness of separation in froth flotation. The ionic surfactants used as collectors are specifically adsorbed via electrostatic interaction with the mineral surface[l] or precipitate as a metal-surfaetant salt on the surface [2]. Salt precipitation during flotation lowers selectivity and this combined with the fact that the surface properties of the gangue minerals often are similar to the value minerals, makes selective hydrophobization a difficult task for flotation surfactants [3]. Several different aspects of the surfactant/mineral interaction, including the effect of preparation conditions and surface roughness [4], are important for understanding the fundamental processes in flotation. A number of experimental techniques have been employed to investigate the physico-chemieal surface properties of minerals. Examples are contact angle and electrokinetie measurements. These combined with adsorption studies based on surfactant concentrations before and after contact with the mineral surface, provide a good framework for understanding the relationship between mineral surface properties and flotation behavior. In addition, instrumental techniques such as FTIR have played an important role in identifying surfaetant complexes adsorbed on mineral surfaces at flotation conditions [5,6]. 895
896
R.B. B1ORKLUNDand H. AaWIN
The interaction of anionic surfactants with sparingly soluble minerals such as apatite and calcite is a complicated system to study [7]. Since the concentration of Ca2+ can be on the order of 0.1 mM for calcite-containing ores, there is the possibility for Ca-surfactant salt precipitation on the surface. Dissolution of the minerals also leads to changes in the surface microstructure which affects the interaction with the surfactant. We recently reported results for adsorption of a monoalkylsulfosuccinamate surfactant on polished apatite and calcite ore surfaces [8]. We employed ellipsometry to observe in situ the adsorption (and desorption) of the surfactant and to monitor changes in microstructure (porosity) upon mineral dissolution. In the present study we used ellipsometry to follow adsorption of the surfactant as a function of the degree of porosity for apatite surfaces. Our goal was to investigate if ellipsometry can be used to differentiate between surfactant adsorption on the external and internal surfaces of apatite of varying porosity. Distribution of adsorbates on heterogeneous surfaces is an important parameter in several areas of technology. One example from industrial catalysis is the distribution of metal salts from the liquid phase on porous oxide carriers during preparation of catalysts determined by ion microprobe analysis and uv spectroscopy [9, 10]. A second example is the determination of the external and internal surface areas of zeolites by the t-plot method and benzene plugging technique [11]. We present here ellipsometry as a technique to monitor surfactant adsorption on porous apatite surfaces. It can be used to observe how the surfactant is distributed both within the pores and on the external surface of the mineral and the dependence on the degree of porosity. Ellipsometry provides a new method to study the interaction between surfactants and minerals, especially with respect to mineral microstructure, and thus has potential for increasing our understanding of the hydrophobization process.
EXPERIMENTAL Apatite was obtained as six-sided, greenish columns from an ore sample of very similar appearance to that pictured in Reference 12. Flouride was the predominant charge compensating anion in the apatite mineral. Small irregular pieces of the mineral, about 1 x 1 x 0.3 cm in dimension, were polished on an Ecomet4 variable speed grinder-polisher from Buehler Ltd using BUEHLER-METTM P1200 metallographic grinding paper. After polishing, the samples were glued to the ends of object glass slides so they could be positioned vertically in the measurement cell. The samples were attached in such a way to the slides that they could be repeatedly polished. The samples were cleaned prior to each run with a 50 % aqueous ethanol solution in an ultrasonic bath, dried with flowing N2 gas, and immediately placed in the measurement cell containing the initial solution. Di-sodium-N-alkyl (C16-C18) sulphosuccinamate, EMPIMIN MK/B, was kindly provided by Albright & Wilson Ltd from their Marchon, France factory. It was in liquid form having a solids content of 33.5 wt % and active matter content of 26.5 wt %. Pro analysi quality NaOH and Ca(OH)2 were obtained from Merck. Ellipsometric measurements were performed at room temperature in a quartz cell of about 8 rnl volume. The cell was open to the atmosphere and solutions were stirred by a teflon coated magnetic bar during measurements. The instrument used was a computerized single wavelength (546 nm, Rudolph Research, model 436) null ellipsometer with an angle of incidence of 68 o. Ellipsometry is based on reflection of polarized light at an oblique incidence. The difference in reflection for light polarized parallel (Rp) and perpendicular (Rs) to the plane of incidence results in a change in the state of polarization upon reflection [13]. The measured quantity is the complex reflectance ratio, p = Rp/Rs = tan,exp(iA), w h e r e , and A are the eUipsometric angles and i= ~/-1. By measuring and analyzing p, it is possible to determine optical and microstructural properties of a sample or of a surface film on a sample. The quartz cell equipped with tubes for pumping liquids in and out while maintaining a constant 5 ml solution volume is shown in Figure 1. The pumping speed was usually 20 ml/min. The basic solutions, NaOH and Ca(OH)2 (pH 10.6, [Ca2+] = 0.2 mM (2 x 10-4 M/l)), were always prepared just prior to use. A stock solution the surfactant was prepared periodically by dispersing 0.06 g EMPIMIN in 50 ml of NaOH solution using an ultrasonic bath. Small quantities of this solution were added to the measurement cell to obtain the desired surfactant concentration, usually 0.04 mM. Three samples of
Anionic surfactant distribution
897
apatite were investigated. The adsorption of the surfactant led to irreversible changes in the mineral surface and they were repolished between measurements. Since the ore was inhomogeneous, this led to small changes in the surfaces after each repolish.
Drain
.~_ _.~~"
Fresh solution~
Stirrer
Lightbeam
Fig. 1 Measurement cell for the ellipsometry experiments containing 5 mL solution. Since ellipsometry is a fairly new technique for studying the flotation process, we briefly describe here the major differences between our experimental set-up and the actual flotation conditions. Our experiments were done on planar apatite surfaces which were intended to serve as models for the powdered mineral. The solutions in contact with the surfaces were quite dilute as compared with the actual flotation process and the liquid to solid ratio was about 16. Adsorption studies using null ellipsometry are based on the relative changes in the ellipsometric angles upon film formation. Thus it was necessary to attain stable values for the angles prior to addition of the surfactant. Since changes in the surface microstructure caused by dissolution led to changes in the angles, it was necessary to have a solution pH of 10.6, which is somewhat higher than that employed in the flotation process, in order to surpress changes in the surface microstructure prior to adding the surfactant. It should be emphasized that contacting the surface with a saturated apatite solution at lower pH was not sufficient to maintain a stable surface. Even at equilibrium there was a mass transport back and forth across the solid/solution interface which altered the surface microstructure. In this respect the extreme surface sensitivity of ellipsometry to changes in the surface microstructure is a disadvantage. However, it is also this sensitivity to events occurring on the surface which makes the technique a useful complement to mineral studies based on changes in the solution composition. A second important feature of the experiments was the ability to pump fresh solution through the measurement cell. This allowed us to follow the desorption of the surfactant which gave information about the strength of adsorption. The adsorption studies were done by adding the surfactant to a fixed volume of solution in the cell. It was also possible to flow a surfactant-containing solution through the cell, making possible adsorption studies in solutions which were free from ions originating from the mineral sample. Greater freedom to establish the composition of the solution in contact with the mineral sample is thus another feature of the ellipsometry experiment.
RESULTS AND DISCUSSION We begin presentation of the results with a brief description of the partial dissolution of the apatite surface in NaOH solution, which we have described more completely elsewhere [8]. We then apply this information to create surfaces of varying porosity by controlled dissolution of the mineral. This is followed by the main topic of the paper which is the adsorption of the surfactant in Ca(OH)2 solution on the different surfaces.
898
R.B.
BJORKLUND and H. ARWIN
Partial Dissolution Figure 2 shows the time variations in A and ~ for an apatite surface when the solution was changed from Ca(OH)2 to NaOH in the cell. We have previously assigned the observed increases in the angles to the formation of a porous interface zone which results in a roughening of the apatite surface [8]. It is most probable that the dissolution occurred preferentially at certain sites as has been described for synthetic fluorapatite [14] leading to a three-dimensional porous interface zone.
ca I
.a • °°" "" • ' • ' E " D.
• • "°'°°
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.'-S
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O}
.•..•..o..
26.5
"'-
..o.o'.'. •''" ......°'°°• ..o o.." "o ~•• °"
25.9
I
80 Time
!
160
(min)
Fig.2 Time dependence of A and ~ for an apatite surface first in Ca(OH)2 solution, [Ca2+] = 0.2 mM, and then in flowing NaOH solution at pH 10.6. Letters A - E indicate the points where the dissolution was stopped by reintroducing the Ca(OH)2 solution into the cell in later separate experiments. A rough surface can be optically modelled as a semi-infinite substrate with a porous surface layer• In order to quantify the results shown in Figure 2 we performed an optical analysis to determine the thickness and composition of a surface layer under the assumption that the optical properties of the constituents of the layer were known. The constitutients were the bulk mineral and the ambient media (the NaOH solution). The latter has a refractive index ofna = 1.334 as determined by an Abbe refractometer. The (assumed isotropic) refractive index, ns, of apatite was obtained from A and ~ at t = 0 in a simple two-phase (ambient-substrate) model in which [13] n s = no since l+tanZo
(1)
where p = t a n , exp iA and O = 680 is the angle of incidence. For apatite. ~ was found to be 1•626 i0.016. This value is somewhat higher than in our previous report [8] which is due to better polishing characteristics for this apatite sample. The fact that the refractive index has a small imaginary part is probably due to model mismatches. Apatite is slightly anisotropic and the mineral surface may have an initial roughness even after polishing.
Anionic surfactant distribution
899
For a mixture of two materials, the optical properties are not a simple volume fraction weighted average of the constitutients. Instead they are obtained by more sophisticated theories. We have used the Bruggeman effective medium theory in which the effective refractive index, neff, is obtained from [15]
[ L
n s
+
+
I
:o
:"' /1 a
+
(2)
where fs is the volume fraction of the apatite and fa = 1-fs is the porosity. In Figure 3 we show the data from Figure 2 plotted as ~ versus A. The lines show values for a range of porosities and film thicknesses calculated in an optical model with a porous film on an apatite substrate. The refractive index of the porous film was obtained from Eq. (2). At the end of the dissolution (point E) the values were about 12 volume % porosity in an interface zone which was 550 /k thick. We conclude from Figure 3 that the process occurring when a polished apatite surface was in contact with a NaOH solution can adequately be described by formation of a porous surface film. A more detailed description of the surface microstructure cannot be determined from the data. As we have described previously [8], a possibility is the selective dissolution of the mineral at certain sites leading to a threedimensional porous interface zone. I
I
I
I
I
I
I
°.,1 .5ooA o.1 .E / ~J / Porosity / ~ /
26.6
I
1
2
I
A (degrees)
I
3
I
/
0.2
.
I
4
Fig.3 Calculated ¢ - A curves for formation of a porous surface layer on apatite in the thickness range 0 - 500 A, and with porosities in the range 0.05 - 0.20. Further details are given in the text. The dots are the experimental results from Figure 2.
Controlled Dissolution Since the apatite dissolution could be stopped at any time by re-introducing the Ca(OH)2 solution, it was possible to create surfaces of varying porosity relative to the initial polished surface. The different levels of porosity obtained are indicated at positions A-E in Figures 2 and 3. Surfactant adsorption was then studied on the different porous surfaces. Since the surfactant caused irreversible changes in the surface, it was not possible to create the different surfaces sequentially. Instead, each surface was produced by partially dissolving a freshly polished surface to the appropriate positions on the curve shown in Figure 2. The agreement between the initial values of A and ~ after polishing for the different samples was good, A = 1.1" + 0.20 and ~ = 26.00 + 0.20.
900
R.B.
BJORKLUND and H. ARWIN
Surfactant Adsorption For the initial polished surface (point A in Figures 2 and 3) the addition of surfactant to the Ca(OH)2 solution caused an increase in A, as shown in Figure 4, and no change in ~. For clarity we omit ¢ in reporting results for the adsorption experiments since it was nearly constant throughout. A reached a steady-state value during the adsorption and flushing the cuvette with fresh Ca(OH)2 solution caused it to return to slightly above the original baseline as shown in the figure.
c.
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s
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I
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.
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• -. • •o
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0
I
!
40
80 Time (min)
Fig.4 Time dependence of A for surface A (defined in Figures 2 and 3) in 0.2mM Ca(OH)2 solution during the following treatments: (a) 0.04 mM surfactant added, S and (b) cell flushed with fresh Ca(OH)2 solution. Adsorption of the surfactant on surface A, which was freshly polished, was restricted to the external surface area of the mineral. Although the surface contained several relatively large cracks, these did not contribute to the light reflected to the detector, and can be considered to be a part of the external surface according to Gregg and Sing [16]. Thus it was the planar, non-porous apatite which was monitored during adsorption A. The optical model which is applicable to adsorption of a surface film on the planar mineral surface is shown in Figure 5b. If we assume a surfactant refractive index of r~ = 1.45, it is possible to calculate a thickness scale for the adsorbed layer as shown in Figure 4. Since 40 A is approximately the length of the surfactant hydrocarbon chain, it is possible that the surfactant, standing vertically, was adsorbed as a monolayer on the surface. However, the calculated thickness is an average value for the surface and does not give any additional information about the adsorption geometry. Flushing the cell with fresh solution caused the film to desorb and A returned to nearly its original value. The discrepancy between the A values before and after adsorption may be due to a small quantity of surfactant remaining on the surface or some mineral dissolution during the adsorption period. Although we describe our observations in Figure 4 as surfactant adsorption, we cannot be certain that it is not a Ca-surfactant salt precipitation occurring. At higher surfactant concentrations, above 0.08 mM, the light beam reaching the detector exhibited an instability which was probably caused by scattering from a precipitation in the solution. For simplicity we will use the term adsorption throughout the text to describe film formation on the surface and surfactant filling of the porous interface zone. Adsorption of the surfactant on porous surfaces was performed. The results for the surfaces representing the rapidly changing A values in Figures 2 and 3, positions B and C, are shown in Figure 6. For position B (7 % porosity, 300 A thick porous interface zone), little change in A was observed during adsorption of the surfactant but a significant decrease occurred when the cuvette was flushed with fresh solution.
Anionic surfactant distribution
901
For position C (10 % porosity, 425 ,~, thick porous interface zone), A decreased upon surfactant adsorption and decreased further during the flush.
ne=1.334
Solution
I~ ~ n- 145 f (a|
Apatmte } (b)
External
Internal
n/~/~/,
na:1.334 nf =1.45
(c)
(d)
neff > neff
tef~
Fig.5 Physical (a) and optical (b) models for surface A (defined in Figures 2 and 3) after adsorption of the surfactant. Physical (c) and optical (d) models for apatite surfaces B - E (defined in Figures 2 and 3) after partial dissolution in NaOH solution and adsorption of surfactant on the external and internal surfaces in Ca(OH)2 solution. The optical model for adsorption on a porous surface is presented in Figure 5d. The surfactant can adsorb on both the external mineral surface and within the pores created by the partial dissolution (Figure 5c). The former is equivalent to adsorption on the non-porous surface and results in an increase in A with no change in ~. Adsorption in the pores causes an increase in the effective refractive index of the porous interface zone from neff, as obtained from Eq. (2), to n'cff since polarizable surfactant molecules replace less polarizable solvent species. The result is that A decreases with no change in ~. We have previously verified this model experimentally and also shown that adsorption within the pores was mostly irreversible [8]. Thus the changes in A observed upon surfactant adsorption and desorption can be used to determine if the adsorbed surfactant occupies adsorption sites on the external surface or within the pores. An increase in A was observed upon surfactant adsorption on surface A. Since the surface was nonporous, adsorption could be described by the model shown in Figure 5b. The adsorption was reversible and A returned to nearly its original baseline. Adsorption on surface B mused a small increase in A. However, flushing the cuvette led to a decrease in A below its original baseline. We interpret these results as indicating that adsorption of the surfactant on both external and internal surfaces occurred on surface B. External surface adsorption seemed to dominate although it is difficult to quantify the two types of adsorption using our simple optical models. When the surface film on the external surface desorbed during the flush, the decrease in A revealed that a significant irreversible adsorption of surfactant within the porous interface zone had occurred. A similar result was observed on surface C. However the sharp decrease in A observed when the surfactant was introduced to the solution clearly indicated that more adsorption in the porous layer was occurring as compared to surface B.
902
R. ]3. BJORKLUND and H. ARWIN
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Fig.6
Time dependence of A for surfaces B and C (defined in Figures 2 and 3) in 0.2mM Ca(OH)2 solution during the following treatments: (a) 0.04 mM surfactant added, S and (b) cell flushed with fresh Ca(OH)2 solution.
Continuing the dissolutions yielded two additional surfaces, D and E, shown in Figures 2 and 3. These are characterized by an increase in both the thickness of the porous interface zone and the degree of porosity relative to surface C. Adsorption of the surfactant on D yielded an initial decrease in A followed by an increase and decrease as shown in Figure 7. This was similar to the adsorption curve observed for surface C except that the increase in A indicated a slightly larger contribution from adsorption on the external surface. That adsorption within the pores was substantial was confirmed by the large decrease in A when the cuvette was flushed with fresh Ca(OH)2 solution. A similar result was seen on surface E where the increase in A observed after the inital decrease during surfactant adsorption indicated a somewhat larger contribution for adsorption on the external surface than was observed for D. Our conclusion from these results is that ellipsometry can be used to determine at least qualitatively how an adsorbed surfactant is distributed between the external and internal surfaces of an apatite mineral. However, the simple optical model that we have used forces us to compare two different quantities when analyzing the distribution of the surfactant. We can, for example, determine the approximate pore filling by the surfactant for surface E by using the A values before (3.20) and after surfactant adsorption and desorption (2.70) from Figure 7. It is then possible to use these angles together with Figure 3 to determine that the volume porosity decreased from about 12 % to 9 % upon surfactant adsorption (if we ignore the small difference in refractive index between the surfactant and the solution). The film thickness for the external surfactant adsorption can be calculated from the change in A when the adsorbed surfactant desorbed from the surface (3.00 - 2.70 = 0.30 in Figure 7). Such a change in A is equal to a film thickness of about 25 ,~ according to the optical model in Figure 5b. Thus we are faced with the problem of comparing a change in volume % caused by the surfactant adsorbed on the internal surface
Anionic surfactant distribution
903
and a film thickness for the external adsorption if we desire a more quantitative description of the surfaetant distribution. Apart from the problem of quantifing the surfactant distribution, the question remains if the results can be correlated with a physical model describing the evolution of the porous interface zone as the apatite dissolves. Even more important is the relevance of the results on the planar, polished mineral surface to the actual flotation system.
e.i 2.9
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Fig.7 Time dependence of A for surfaces D and E (defined in Figures 2 and 3) in 0.2mM Ca(OH)2 solution during the following treatments: (a) 0.04 mM surfactant added, S and (b) cell flushed with fresh Ca(OH)2 solution. One possible physical model is based on previous studies of dissolution behavior of fluorapatite [14] and calcite [17]. From these investigations it can be concluded that the dissolution rate is strongly influenced by impurities and variations in crystallinity, and that the surface morphology of the minerals plays an important role in the dissolution process. For fluorapatite it was found that dissolution occurred in three dimensions and followed a spiral dislocation mechanism [14]. The dissolution rate was thus dependent on the number of dislocations emerging at the mineral surface. Cleaved surface planes of Iceland spar crystals were found to be unreactive, and a surface roughening by polishing or acid treatment was necessary in order to obtain dissolution in the pH range 6 - 7 [17]. Cutting the surface at a known angle, 0, to the (100) cleavage plane created surfaces which exhibited increasing dissolution rates for increasing values of 0. This was due to the creation of a greater number of terraces and it was observed by SEM that macroscopic terraces were formed during dissolution by coalescence of microscopic terraces.
904
R . B . BJORKLUND and H. ARwlN
Our results for surfactant adsorption on apatite surfaces of varying porosity indicate that the surfactant adsorbed only on the external surface of the freshly polished mineral (Figure 4) and exhibited a predominately internal adsorption on surfaces C - E (Figures 5 and 6). A slightly increased external surface adsorption was observed on E relative to C and D. These results seem to indicate that the surface zone changed its morphology as the apatite was dissolved. This was in agreement with the dissolution experiments reported for fluorapatite and calcite [14,17] and the quantitative description of the porous zone shown in Figure 3. From Figure 3 it can be concluded that as the apatite surface dissolved, certain pores became deeper and the total porosity of the surface zone increased only slightly from C to E. A simple physical model to describe our controlled dissolution experiments would involve dissolution initiated at particular defects, such as dislocations, giving several growing pores on the mineral separated by external surface. Eventually, the volume in the surface zone occupied by pores begins to level off as pores grow into each other and create new external surface. That a considerable external (highly reflecting) surface was always present on the apatite was evidenced by the relatively small decrease (about 10%) in reflected light intensity reaching the detector during the dissolution shown in Figure 2. With regard to the relevance of the adsorption experiments described here to the actual flotation process, we want to stress that our approach has been to simplify the solid/liquid interface. By dealing with a planar surface in contact with a liquid phase of well-defined composition, it should be possible to determine the elementary steps involved in the surfactant-mineral interaction. A good example is the information obtained from a comparision of the rates of adsorption on the internal and external surfaces shown in Figures 4, 6, and 7. One would expect that adsorption on a porous surface would proceed by a several step mechanism involving adsorption on the surface terraces, diffusion to the pore and finally migration into the pore. However, it is quite clear from Figure 6 that, when both processes occur simultaneously, adsorption within the pores is the faster of the two. This despite the fact that adsorption on the non-porous surface is quite rapid as shown in Figure 4. One possible explanation for the faster adsorption on the internal surface is possibly the difference in sticking coefficients for the surfactant on the two different surfaces. At the high pH employed for the study, the mineral surface is most likely negatively charged and very few collisions of the surfactant with the external surface results in adsorption. The probability that surfactant molecules entering pores directly from the liquid phase remain in the porous zone is greater since these encounter a three-dimensional array of Ca2+ ions in the pore. A second interesting point is if adsorption of the surfactant in the internal surface contributes anything to the hydrophobization of the mineral. Since air bubbles can only contact the external surface of mineral particles [18], it would appear likely that surfactant adsorbed in the pores has no effect on the flotation process. However, internal surfactant adsorption may be very important in determining the effectiveness of separating two minerals having different solubilities. For an ore containing apatite and calcite, which possess the same degree of microporosity in the dry state, the increase in porosity for the calcite component upon contact with the conditioning solution is considerably greater than for the less soluble apatite. This is evidenced by a substantial decrease in reflected light intensity from dissolving calcite surfaces in ellipsometry experiments [8], which is related to a decrease in external surface area. Thus, when the surfactant is introduced to the flotation cell it initially adsorbs to a greater extent on the internal calcite surface as compared to the internal apatite surface. This leads to a relatively lower adsorption on the external calcite surface since the surfactant concentration in the immediate vicinity of the calcite particle is depleted. The result is a lower degree of hydrophobization of the calcite as compared to apatite.
CONCLUSIONS Ellipsometry can be used to study two aspects of the flotation process - mineral dissolution and surfactant adsorption. As a surface sensitve technique, it is a valuable complement to studies based on composition changes in the solution in contact with the mineral. By monitoring the pore development of apatite during partial dissolution it was possible to produce surfaces of varying porosity. These served as model surfaces to study the effect of porosity on the adsorption of an anionic surfactant. The optical model developed to describe the porous surfaces made it possible to qualitatively determine the distribution of the adsorbed
Anionic surfactantdistribution
905
surfactant between the external and internal apatite surfaces. There exists some possibility to obtain a more quantitative description of the surfactant distribution, but this would require further refining of the optical model. For porous surfaces where surfactant adsorption on both the external and internal surfaces was possible, it was observed that adsorption within the pores was the faster process. This was interpreted as being due to more effective trapping of the surfactant by the pores as compared to the planar external surface. Based on the ellipsometry results it can be concluded that an important factor in obtaining selective hydrophobization of the apatite component in apatite/ealcite ore mixtures is the greater surfactant adsorption on the internal surface of the more soluble calcite. However, it should be emphasized that the ellipsometry studies were done on model apatite surfaces in dilute solutions at a pH somewhat higher than the actual flotation process. In addition, it would be desirable to check the results with an independent technique, such as cross sectional transmission electron microscopy which is often used to confirm film thickness determinations by ellipsometry in multi-layer semiconductor devices. However, despite advances in the use of TEM to study minerals [19], it seems unlikely that quantitative information about the thickness and porosity of the apatite porous interface zone and the concentration of the surfactant adsorbed on the surface and in the pores could be obtained using TEM.
ACKNOWLEDMENT The work reported here was partially supported by the Swedish National Board for Industrial and Technical Development (NUTEK).
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2. 3. 4. 5.
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