Langmuir 1992,8, 1709-1714
1709
Ellipsometric Study of Anionic Surfactant Adsorption on Apatite and Calcite Ore Surfaces Robert B. Bjorklund* and Hans Arwin Laboratory of Applied Physics, Linkoping University, S-58183 Linkoping, Sweden Received December 16, 1991. In Final Form: March 23, 1992
The adsorption of an anionic surfactant, disodium N-alkylsulfosuccinamate,onto polished apatite and calcite ore surfaces was studied by ellipsometry. Partial dissolution of the minerals in NaOH solutions at pH 10.5 caused a roughening of the surfaces which, in the case of apatite, was optically modeled as a semi-infinite substrate with a porous surface layer. Solutions containing Ca2+hindered the dissolution of the minerals, and contacting the surfactant with nondissolved surfaces in the presence of Ca2+resulted in reversible adsorption of a surface film. On partially dissolved apatite surfaces, in the absence of Ca2+ in the solution, an irreversible absorption of the surfactant was observed in the porous interface zone. In M Ca2+both surfactant absorption and calcium-surfactant salt precipitation the presence of 1.4 X was observed depending on the concentration of the surfactant. The calcium-surfactant salt could be washed away by flushing the cell with fresh Ca2+solution while the absorbed surfactant remained in the porous surface layer. Results on calcite surfaces were similar to those for apatite, but quantitative analysis of the data was not possible because of optical anisotropy and a much higher degree of dissolution.
Introduction
calcite, are sufficiently soluble that relatively high Ca2+ concentrations on the order of M are encountered in the flotation process. The relatively small size of the planar samples used and ability to flush the measurement cuvette with new solutions in the ellipsometry experiments made possible the comparison of the surfactant’s adsorption and desorption both in the presence of and in the absence of Ca2+. In addition, since interpretation of ellipsometry results is strongly dependent on optically modeling the system under study, the microstructure of the ore surface plays a prominent role in the analysis of our data.
Froth flotation is an important process for separating value minerals from the gangue minerals in the mining industry. The separation is based on the concentration of hydrophobic particles at air bubbles rising through aqueous dispersions of the ore. Since most mineral surfaces are hydrophilic, selective hydrophobization of the value minerals by various reagents is a critical factor in determining the effectiveness of the separation process. Anionic Surfactants are usually added to the ore dispersion to accomplish the hydrophobization of the value minerals. The polar group can adsorb on the surface of the mineral by electrostatic interaction’ and thus provide the necessary adsorption on the mineral to be floated. However, at the high pH conditions employed in the flotation process, precipitation of dissolved mineral cationsurfactant complexes on the ore surfaces results in nonselectivecoating of all particles present.2 This can decrease the efficiency of the flotation process and has led to the use of mixtures of ionic and nonionic surfactants where one function of the nonionic component is to disperse the calcium anionic surfactant salts and prevent nonselective precipitation on the ore ~ u r f a c e . ~ ~ ~ Several interrelated phenomena play important roles in the flotation process. These include reactions at the liquidlgas and liquid/solid interfaces along with properties of the bulk solution such as surface tension, complexation, and micellization. We have concentrated our study of the flotation process on the liquid/solid interface using ellipsometry5 to monitor events in the mineral surface/ solution interfacial region. The goal of the work was to compare the results for flotation reagent adsorption (and desorption) measured by ellipsometry with literature results describing studies on powder samples based on the difference in solution concentrations before and after adsorption. The surfactant used was a monoalkylsulfosuccinamate. The mineral surfaces studied, apatite and (1) Leja,J. Surface Chemistry ofFrothFlotation;PlenumPress: New York, 1982. (2) Ananthapadmanabhan, K. P.; Somasundaran, P. Colloids Surf. 1985, 13, 151. (3) von Rybinski, W.; Schwuger, M. J. Langmuir 1986, 2, 639. (4) von Rybinski, W.; Schwuger, M. J.; Dobias, B. Colloids Surf. 1987, 26, 291. (5) Azuun,R. M. A.;Bashara,N.M.EllipsometryandPolariredLight; North-Holland New York, 1977.
0743-7463/92/2408-1709$03.00/0
Experimental Section Mineral samples were ores, comprised mainly of apatite or calcite,readily identified by color. Flouride was the predominant charge compensating anion in the apatitemineral. Smallirregular pieces of the minerals about 0.5 X 0.5 X 0.2 cm in dimension were polished on a Ecomet4 variable-speed grinder-polisher from Buehler LM. using BUEHLER-MET P1200 metallographic grindingpaper. The polishing yielded quite good opticalsurfaces for the apatite samples, but the softer calcite samples contained many imperfections which resulted in a higher noise level for measurements on calcite. Polishing on a TEXMET grinding wheel using diamond-in-oil slurries produced better optical surfaces but coated the sampleswith an oil film which was difficult to wash off and led to erroneous results during the early stages of the work. 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 ultrasound bath, dried with flowing Nz gas, and immediately placed in the measurement cell containing the initial solution. Disodium N-alkyl(Cl&l&ulfosuccinamate, EMPIMIN MK/B, was kindly provided by Albright & Wilson Ltd. from their Marchon, France,factory. It was in liquid form having a solid content of 33.5 wt % and active matter content of 26.5 w t %. Pro analysi quality NaOH and Ca(0H)swere obtained from Merck. Ellipsometric measurements were performed at room temperature in a quartz cell of about 8-mL volume. The cell was open to the atmosphere, and solutionswere stirred by a Tefloncoated magnetic bar during measurements. The instrument used was a single-wavelength (546nm, Rudolph Research, model 436) null ellipsometer with an angle of incidence of 68’. Ellipsometry is based on the reflection of polarized light at an oblique incidence. The differencein reflectionfor light polarized parallel (R,)and perpendicular to the plane of incidence results in a
0 1992 American Chemical Society
1710 Langmuir, Vol. 8, No. 7, 1992 change in the state of polarization upon reflection. The measured quantity is the complex reflectance ratio, p = R,/R, = tan J, exp(iA), where J, and A are the ellipsometric angles related to the polarizer and analyzer positions and i = -11/2. By determining and analyzing p, it is possible to determine optical and microstructural properties of a sample or of a surface film on a sample. The angular 'swing" of the polarizer and analyzer for determining the null positions was usually f 3 O for apatite samples and f4O for ~ a l c i t e . ~ The quartz cell was equipped with tubes for pumping liquids in and out while maintaining a constant 5-mL solution volume. The pumping speed wm usually 20 mL/min. The basic solutions NaOH and Ca(OH)2 were always prepared just prior to use. A stock solution was prepared periodically by dispersing 0.10 g of EMPIMIN in 100 mL of NaOH solution using an ultrasound bath. We chose Ca(0H)Z as the source for Ca2+to be introduced into the cell as a substitute for the Ca2+dissolvedfrom the miner& in the actual process. This maintained a common anion, OH-, with the NaOH used to adjust the pH. The pH was 10.5 for all measurements. For Ca2+-freemeasurements this was achieved with only NaOH, and for measurements in 1.4 X 10"' M Ca2+the pH was from Ca(OH)2 (pH 10.37) plus NaOH. Three samples each of apatite and calcite were measured on. The adsorption of the flotation reagent led to irreversible changes in the mineral surface, and they were repolished between measurements. Since the ores were inhomogeneous, this led to small changes in the surfaces after each repolish. While the general results presented here are representative for the samples, particular details varied between samples. This was especially true for the solubility of the apatite surfaces as determined by the changes in the ellipsometric parameters when the samples were first placed in NaOH solution. However, these differences in solubility, which are differences in porosity according to our optical model, seemed to only affect the relative magnitude of surfactant adsorption among the samples and not the general features.
Results and Discussion Since we were interested in measuring the relative changes in the parameters A and #, it was necessary to introduce the flotation reagent a t a time point when these parameters had attained stable values. This requirement was difficult to achieve in Ca2+-freesolutions, especially for calcite, because of mineral dissolution. We therefore describe our results according to the following three topics: dissolution of the minerals in NaOH, reagent adsorption before dissolution, and reagent adsorption after dissolution. Mineral Dissolution in NaOH. Increases in A and # were observed for both apatite and calcite samples when they were placed in NaOH solution. This is seen in Figure 1 for apatite and in Figure 2 for calcite. The increases could be halted and the parameters could be stabilized by flushing the cell with solutions containing Ca2+as shown in Figure 2. When the ambient was again changed back to the original NaOH solution, the increases in the parameter values resumed. Since introduction of Ca2+ resulted in a stabilization of the parameters, we conclude that the curves shown in Figures 1and 2 are related to the dissolution of the mineral surfaces.'+1° A layer-by-layer dissolution would not affect the ellipsometric readings, so the results in Figures 1 and 2 most probably show that the dissolution caused roughening of the surface. A rough surface can be optically modeled as a semiinfinite substrate with a porous surface layer. In our preliminary analysis we found that only the results for (6) Chin, K. 0. A.; NancoUas, G. H. Langmuir 1991, 7, 2175. (7) Compton, R.G.;Daly, P. J.; House, W. A. J. Colloid Interface Sci. 1986, 113, 12. (8) Amankonah, J. 0.; Somasundaran, P.; Ananthepadmanabhan, K. P.colloids Surf. 1985, 15, 295. (9) Oberndorfer, J.; Dobias, B. Colloids Surf. 1989, 41, 69. (10)Attia, Y. A.; Fuerstenau, D. W. Colloids Su-f. 1988/89, 34, 271.
Bjorklund and Arwin Ca I
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Figure 1. Time dependence of A and J, for an apatite surface first in Ca(OH)2 solution, [CaZ+l= 1.4 X lo4 M, and then in NaOH solution at pH 10.5. NalCal Ne lCal Na I Ca
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Figure 2. Time dependence of A and $ for a calcite surface in NaOH solution at pH 10.5. During periods marked Ca the cell was flushed with Ca(OH)2 solution, [Ca2+]= 1.4 x 104 M.
apatite could be explained by such a simple model. The data on calcite, although qualitatively very similar, did not fit the model. This is partly because calcite was less stable in NaOH, and partly because calcite is very anisotropic (no= 1.661 and n, =: 1.4858 at h = 546 nmll), and our isotropic models may fail to describe the system in such cases. Therefore, we present in Figure 3 an analysis of only the apatite data from Figure 1. The basic idea in the analysis is to seek thickness and composition of a surface layer under the assumption that the optical properties of the constituents of the layer are known. The constituents are the bulk mineral and the ambient media (the NaOH solution). The latter has a (11) Driscoll, W.G.,Ed.; Handbook of Optics; McGraw-Hill: New York, 1978; pp 10-26.
Adsorption on Apatite and Calcite Ore Surfaces 1-
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Langmuir, Vol. 8, No. 7, 1992 1711 C a I S I Ce
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refractive index of n, = 1.334 as determined by an Abbe refractometer. The (assumed isotropic) refractive index, n,, of apatite is obtained from A and $ a t t = 0 in a simple two-phase (ambient-substrate) model in which5
where p = tan $ exp(iA) and 4o = 6 8 O is the angle of incidence. For apatite, n, was found to be 1.586 - i0.023. The fact that the refractive index has a small imaginary part is probably due to model mismatches. Apatite is also anisotropic but to a much lesser extent, and the mineral surface may have an initial roughness even after polishing. For a mixture of two materials, the optical properties are not a simple volume fraction weighted average of the constituents. Instead they are obtained by more sophisticated theories. We have used the Bruggeman effective medium theory in which the effective refractive index, n,ff, is obtained from12
where f, and fa = 1- f a are the volume fractions of the two constituents in the film. In Figure 3, we show the same data as in Figure 1 but plotted as $ versus A. The lines show calculated values for a range of porosities and film thicknesses. We conclude from Figure 3 that the process occurring when a polished apatite surface is inserted in a NaOH solution can adequately be described by formation of a porous surface film. Under the conditions in our experiment, the final film thickness was 375 A with an average porosity of about 22%. However, the growth of the surface film was not uniform with respect to the porosity of the layer. The initial 150 A of the film had a porosity of 40%, and this value slowly decreased during the remainder of the porous layer growth. Since our apatite ore samples were both physically and chemically nonhomogenous, it is not surprising that the thickness and porosity of the surface layer formed during dissolution varied. Similar effects have been observed for fluorapatite as variation in the dissolution rate caused by impurities and variation in crystallinit? and for calcite where surface roughness was altered by the coalescence of microscopic terraces (pro(12)Aspnes, D.E. Thin Solid Films 1982,89, 249.
TIME (min)
Figure 4. Time dependence of A and $ for an apatite surface after 1 h in a Ca(0H)Z solution, [CaZ+l= 1.4 X lo4 M, during the following treatments: (a) 1X M surfactant added, S, (b) cell flushed with Ca(0H)Z solution, (c) 2 X 10" M surfactant added, 25, (d) cell flushed with Ca(0H)Z.
duced by polishing a t an angle to the 100 face) to yield For apatite macroscopic terraces during diss~lution.~ samples placed in NaOH solutions the porous layer thickness a t steady state was 200-400 A and the porosity 1 5 2 5 % . Calcite behaved qualitatively very similar (Figure 2), but, as mentioned above, could not be modeled as nicely. However, we conclude that also calcite undergoes surface dissolution in NaOH, leading to formation of a porous surface layer. The detailed microstructure of the surface layer cannot be determined from our data. One possibility is roughening of the outermost surface. Another possibility would be selective dissolution of the mineral at certain sites, as has been described for fluorapatite,6 leading to a threedimensionalporous interface zone. The latter model seems to fit better for the discussion in the final section. Reagent Adsorption before Dissolution. The crushed ore introduced into the flotation cell always undergoes a partial dissolution process. The dissolution is not as extensive as described in the previous section, but it does involve a disappearance of some of the original ore surface. The opposite situation is where minerals are introduced to solutions containing Ca2+which hinders the dissolution process. We have chosen the interaction of the surfactant with such nondissolved surfaces as the starting point for the investigation of the hydrophobization process. The effect of the surfactant on nondissolved apatite was as shown in Figure 4. Additions of the surfactant resulted in increases in A which were reversible upon flushing with Ca2+solution. From an ellipsometric point of view, an increase in A with constant $ corresponds to adsorption of a surface film with relatively low refractive index. In Figure 4 we show a calculated thickness scale under the assumption of nf = 1.45 for the surface film. The thicknesses of the surface films are then of the order 30-40 A. The adsorptionldesorption processes shown in Figure 4 exhibit some of the characteristics observed for the partially dissolved apatite surfaces discussed below. The nondissolved surfaces are the simpler of the two systems consisting of essentially a single interface with the solution. After dissolution a porous surface layer is formed, as shown in Figure 3, which creates additional surface adsorption sites. Both systems have a common outer surface interface with the solution, and it is this interface which dominates the interaction with the surfactant for the nondissolved surfaces. When the sample was partially dissolved in stages, it was pos,sibleto observe the transformation from the adsorption type shown in Figure 4 to the type which we later characterize as a pore fillinglabsorption by the surfactant.
Bjorklund and Arwin
1712 Langnuir, Vol. 8,No. 7,1992
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Results were similar on calcite surfaces. Addition of the surfactant caused increases in A as shown in Figure 5 which, however, were not fully reversible upon flushing the cell with fresh Ca2+solution. Adsorptionldesorption of surfactant seemed to be accompanied by partial mineral dissolution similar to that shown in Figure 2. The small decreases in $ upon surfactant addition, most clearly seen at the second addition, can possibly be explained by surfactant interaction with the slightly porous surface. Reagent Adsorption after Dissolution. Apatite surfaces placed in the NaOH solution attained stable values for A and $after 1-3 h. Flushing the cell with fresh NaOH did not change the parameters. The overall behavior when partly dissolved mineral surfaces were exposed to the surfactant was found to be rather complicated. Several processes probably occurred simultaneously. We therefore present our results and discuss them in parallel. As a starting point in each case, we assume a microstructure as shown in Figure 6a. The bulk mineral has an outer surface and a porous interface zone (surface layer) which are in contact with the solution. To evaluate the ellipsometric data, we use an optical model as shown in Figure 6b. In the analysis we also involve a second surface layer with
after 3 h in NaOH solution during the following treatments: (a) M surfactant cell flushed with fresh NaOH solution, (b) 2 X added, S, (c) additional 2 x 10" M surfactant added, 25, (d) additional 4 X 10" M surfactant added, 45,(e) cell flushed with NaOH solution.
refractive index n, on top of the porous layer as shown in Figure 6c,d. In the discussion, we assume n, and np to be constant and the variables are the thicknesses tf and t, of the two films and nf. An increase in nf is equivalent to a decrease in porosity, which optically is equivalent to replacing less polarizable molecules with more polarizable molecules. The analysis takes into account four processes: (I) Reversible Ca-induced surfactant precipitationladsorption at the outer interface. This is assumed to be the same effect as seen in Figure 4 and causes A to increase with constant $. Simple calculations verified this in the model shown in Figure 6d using n, = 1.45. (11) Irreversible surfactant adsorption inside the porous interface zone. Due to this adsorption (or absorption), nf increases because the surfactant molecules have a higher refractive index than the water molecules which they replace. An increase in nfcauses A to decrease in the models in Figure 6b,d, but $ remains essentially constant. (111)Mineral dissolution at the inner interface. This will increase tf and is ellipsometrically detected as increases in both A and $. (IV) Mineral dissolution at the outer interface. This decreases tf and causes both A and $ to decrease. Note that process I occurs alone in the experiment shown in Figure 4 and that processes I11 and IV together make up the kinetics in Figure 1 leading to the microstructure at steady state as in Figure 6a. The four processes and the optical models in Figure 6b,d can now be used to explain the kinetics of the ellipsometric data on apatite presented in Figures 7-9.Figure 7 shows the changes in A and $ when a partly dissolved apatite surface after 3 h in NaOH solution was exposed to the surfactant. A decreased after each addition of the surfactant. We interpret the results as "bulk absorption" of surfactant in the porous surface zone. This densification increases nf which causes A to decrease. This is consistent with the calculations shown in Figure 3,where a decrease in porosity (increase in nf) at constant film thickness gives rise to a decrease in A. The decrease in $ observed in Figure 7 after the surfactant was added is small but significant and could indicate dissolution at the outer interface, causing tf to decrease as the rate of
Langmuir, Vol. 8, No. 7, 1992 1713
Adsorption on Apatite and Calcite Ore Surfaces N a b
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