Mineral/reagent interactions" an X-ray photoelectron spectroscopic study of adsorption of reagents onto mixtures of minerals

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1 Clay Minerals (1999) 34, Mineral/reagent interactions" an X-ray photoelectron spectroscopic study of adsorption of reagents onto mixtures of minerals G. C. ALLEN*, J. R. EASTMAN *t, K. R. HALLAM*, G. J. GRAVELING *~, K. V. RAGNARSDOTTIR $ AND D. R. SKUSE ~ *lntelface Analysis Centre, University of Bristol, *Department of Chemistry, University of Bristol, ~Department of Geology, University of Bristol, and ~Central Research Laboratory, ECC International plc, Cornwall, UK (Received 24 April 1997," revised 23 March 1998) ABSTRACT: Reagent-based selective separations of minerals are of great industrial importance. The basis of these separations is the competitive differential adsorption of the reagent onto a number of minerals. An X-ray photoelectron spectroscopic (XPS) study of the adsorption of polyacrylamide onto kaolin, feldspar and quartz is described. Initial data confirmed the utility of XPS for the study of polyacrylamide adsorption onto silica glass slides. Further results indicated the utility of the technique for the direct measurement of adsorption of polyacrylamide onto individual aliquots of kaolin, feldspar and quartz. Adsorption was relatively greatest onto kaolin. Feldspar displayed less adsorption and quartz very little. An imaging method was used to characterize, at least relatively, the competitive adsorption of polyacrylamide onto kaolin in the presence of mixtures of kaolin and quartz. This is a potential new technique for study of competitive adsorption. Reagent-based processes are widely used in the processing of industrial minerals (Somasundaran & Moudgil, 1987). In one class of water-based selective processes, impure mixtures of minerals are purified by virtue of differences in their surface chemistry via selective adsorption of either a flocculant (in the selective flocculation process) (Attia, 1987) or a flotation reagent (in the froth flotation process) (Leja, 1983). In the laboratory, study of these selective processes has largely comprised either empirical performance determination using bench-scale mimics of industrial processes (Read, 1972) or adsorption studies using pure minerals chosen to resemble the probable nature of the minerals present in the real impure mineral mixture (Wakeman et al., 1989). Both of these approaches are of great value in a process development environment. However, both approaches also have disadvantages when used to increase the level of understanding of a particular mineral separation process. The principle disadvantages are that empirical small-scale mimics of industrial separation processes give little information about the adsorption behaviour of process chemicals and that adsorption studies using pure minerals suffer the criticism that reagent adsorption behaviour in a single reagent/single mineral situation is likely to be different from that encountered in practice where a competitive single reagent/several minerals adsorption regime exists. Furthermore, adsorption studies are usually performed indirectly by determination of the amount of non-adsorbed reagent left in solution after incubation of known quantities of reagent and mineral and subsequent separation of the mineral. Thus, this approach cannot give mineral specific information about competitive adsorption. As a consequence of the above, we have recently embarked on an investigation into the utility of The Mineralogical Society

2 52 G.C. Allen et al. X-ray photoelectron spectroscopy (XPS) to study directly the competitive adsorption of process reagents onto in situ mixtures of minerals. The process chosen for study was the polyacrylamide mediated selective flocculation of kaolin from aqueous mixed slurries of kaolin, quartz and feldspar. Such slurries are encountered in the processing of the primary kaolin deposits of southwest England. Here we report preliminary data which establish the utility of XPS for characterizing, directly, the adsorption of polyacrylamide onto a model glass surface and onto conventional pure samples of kaolin, quartz and feldspar. Subsequently we report a single initial result where imaging (small area) XPS was used to characterize the differential adsorption of polyacrylamide onto a mixed quartz and kaolin system in a competitive adsorption situation. MATERIALS AND METHODS All water used was deionized. Polyacrylamide The polyacrylamide flocculant used was a 90:10 (mole:mole) acrylamide:sodium acrylate copolymer having molecular mass x 106. This material was supplied as a dry powder by Allied Colloids Ltd. (Low Moor, Bradford, West Yorkshire, BD12 0JZ, UK) under the brand name Magnafloc E24. Primary stock solutions (0.5 wt%) were prepared by first wetting the E24 powder (5 g) with a minimum of methanol (15 g). This wetted powder was subsequently added slowly to water (980 g) with vigorous stirring over 20 min. Primary stock solutions were allowed to condition for 2 h prior to use, stored in the dark and discarded after 48 h. Further dilutions were performed as necessary and discarded at the end of each working day. Kaolin Kaolin matrix (75 kg) was collected from Lee Moor china clay pit, Devon, UK (samples of kaolin, feldspar and quartz available from DRS at the above address). Kaolin was liberated as slurry from matrix by mixing aliquots of matrix (25 kg) with water (50 kg) in a cement mixer for 2 h and subsequent screening at 60 mesh (250 gm). Aliquots of the resultant slurry were combined and screened at 300 mesh (53 gm) and ph 9.0 (NaOH). A 5 gm cut was performed on this coarse material using a cluster of hydrocyclones (10 mm cylinder, 2.6 mm vortex finder, 2.0 mm spigot, Richard Mozely Ltd, Cardrew, Redruth, Cornwall, TR15 1SS, UK) to yield a fine particle size kaolin (7.5 kg) having 75 wt% of particles <2 ~tm as determined by sedigraph (Model S100, Micrometrics, 1 Micrometrics Drive, Georgia, USA ). Feldspar Feldspar granules (~3 mm diameter, 134 g total weight) were hand-picked from sand at Lee Moor china clay pit, combined with water (50 g) and the ph adjusted to 9.0 (NaOH). This mix was ball milled (80 rpm for 48 h) using a cylindrical mill of internal dimensions 113 cm 2 x 12 cm equipped with 225 cylindrical, teflon-coated grinding pellets of dimensions 0.8 cmzx 1.2 cm. The resultant slurry was diluted to 2000 cm 3, mixed and transferred to a cylindrical settling tube of internal dimensions 44 cm2x 50 cm and allowed to sediment at 21~ for 320 min. The supernatent was collected and dried. Quartz Massive quartz (20 kg) was hand picked from Lee Moor china clay pit. Aliquots of this sample were subjected to initial comminution using the following sequence: sledge hammer, jaw crusher (gap 4 mm, two passes), roller crusher (gap 3 ram, two passes). The product of this initial comminution (1 kg, varying from dust to 2 3 mm particles) was ball-milled (42 rpm for 48 h) using a cylindrical mill of internal dimensions 227 cm 2 x 17 cm equipped with 4.4 kg of teflon-coated spherical grinding beads of varying diameters between 1-3 cm. The resultant slurry was diluted to 8000 cm 3, mixed and transferred into four cylindrical settling tubes of internal dimensions 44 cmzx50 cm and allowed to sediment at 21~ for 320 min. The supernatant liquids were collected, combined and dried. Mineral slurries for flocculation experiments Mineral slurries (10 wt% solids) were compounded with sodium silicate dispersant (0.4 g of sodium silicate per 100 g of mineral), calcium chloride (0.11 g per 100g of mineral) and

3 Imaging XPS study of adsorption 53 deionized water. The sodium silicate was branded P84 and supplied by Brunner Mond UK Ltd., PO Box 4, Mond House, Northwich, Cheshire, CW8 4DT, UK, as a 40 wt% solution. This was diluted to 20 wt% prior to use. Polyacrylamide adsorption onto glass slides Glass slides (0.5 cm 2) were placed in aliquots of polyacrylamide solution (20 cm3). A further slide was placed in water (20 cm 3) as a control. These slides were incubated for two days at 21~ Subsequently, the slides were removed from solution, washed with water and allowed to dry. Each slide was mounted on a stub using adhesive copper tape prior to XPS analysis. Adsorption of varying concentrations of polyaerylamide onto minerals (ph 9.0) Aliquots of mineral slurry (1000 g) were stirred whilst the ph was adjusted to 9.0 (0.1 M NaOH). Aliquots of these ph 9.0 slurries (100 g) were added to aliquots of polyacrylamide solution with stirring. The resultant flocculated slurries were centrifuged at 300 rpm for 10 rain. The supernatant liquids were replaced with clean water and the sediments re-suspended. This washing process was repeated once. The samples were prepared for XPS by drying the flocculated slurry directly onto stainless steel stubs Competitive adsorption of polyacrylamide onto kaolin and quartz An aliquot of mineral slurry (100 g, 10 wt% solids) comprising kaolin (5 g) and quartz (5 g) was prepared at ph 4.0. This slurry was added to an aliquot of polyacrylamide at 1000 ppm. The resulant flocculated slurry was treated as described in the above paragraph. XPS Conventional, (large area, footprint -1 cm2), XPS was performed using a Kratos XSAM800 photoelectron spectrometer (Kratos Analytical Ltd., Wharfside, Trafford Wharf Road, Manchester, M17 IGP, UK). Imaging, (small area, resolution 10 gm), XPS was performed using a VG Scientific Escascope photoelectron spectrometer (VG Scientific, The Birches Industrial Estate, t~ 6 ~a 7e 2 g ppm 500 ppm 100 ppm _ ~ ~ ~ ' Control o o o ~ o o o O O~ ~O to Binding energy (ev) FIG. 1. Smvey XPS spectrum of a polyacrylamidetreated glass slide Binding energy (ev) FIG. 2. Smoothed Nls XPS spectra for glass slides treated with polyacrylamide solutions of varying concentration.

4 54 G. C. Allen et al. kao o q Binding energy (ev) FIG. 3. Survey XPS spectra of kaolin, quartz and feldspar. lmberhorne Lane, East Grinstead, West Sussex, RH19 1UB, UK). Both were equipped with dual anode X-ray sources (Mg/A1) and concentric hemispherical electron energy analysers. During the acquisition of spectra the main chambers were kept at 8 9tort. RESULTS AND DISCUSSION Polyacrylamide adsorption onto glass slides The survey XPS spectra for an uncoated, control, glass slide showed bands in the Ols, Si2p and Cls regions. The Ols and Si2p bands were ascribed to the silica glass. The Cls bands were attributed to adventitious carbon from the apparatus vacuum pump oil. Further peaks were present in the Cls and Nls regions of the survey spectra of those slides treated with polyacrylamide (Fig. 1). Because of the complex nature of the Cls spectra recorded, the Si2p signal from the glass slides was used to determine the extent of sample charging in each case. This was calculated, using the adventitious carbon Cls peak, for the control slide and all glass slide samples were then charged referenced to Si2p at 103 ev. Deconvolution of these Cls peaks indicated the presence of a peak at 285.4_+0.2 ev (charge corrected) assigned to the C atoms in the skeletal chain of the polyacrylamide. A further broad peak at ev (charge corrected) was established as being due to the C atoms in the pendant groups of the polymer. The width of this peak was thought to be due to the varying amide/acrylate nature of the polymer functional groups. The N ls region of the XPS spectra of each of the three polyacrylamide treated slides showed a band with a binding energy of 399.7_+0.2 ev (charge corrected). This band was assigned to the

5 imaging XPS study of adsorption 8" z Z 0 5 [ [] Kaolinite] LA Feldspar I 4 3 [] tion (Fig. 2). Since the surface area of each of the slides was the same and the analysis area covered the whole slide, the increased intensity was probably caused by increased adsorption of polyacrylamide. The intensity of the Cls and Ols (513 ev) regions of the spectra from the treated slides also increased in intensity with increased polyacrylamide solution concentration. This is further evidence for increased polyacrylamide adsorption with increased dose. The above data indicate the utility of XPS as a method for the direct measurement of the adsorption of polyacrylamide onto inorganic surfaces Poly(acrylamide) concentration (ppm) FIG. 4. Adsorption of polyacrylamide onto kaolin and feldspar. acrylamide N atom. The intensity of the Nls band increased with increased polyacrylamide concentra- Adsorption of varying concentrations of polyacrylamide onto minerals (ph 9.0) Survey XPS spectra of kaolin, quartz and feldspar were acquired (Fig. 3) and compared with spectra from samples of the same minerals treated with varying concentrations of polyacrylamide. As expected, all the spectra from the untreated Fro. 5. XPS images from kaolin/quartz/polyacrylamide mixtures; (a) Si2p; (b) A12p; (c) Nls; (d) composite S2p, Nls map showing absence of N on quartz.

6 56 G.C. Allen et al. minerals showed bands in the Ols, Si2p and Cls regions, whereas spectra from kaolin and feldspar also showed bands in the Al2s region. Additional peaks in the Cls and Nls regions were observed in the spectra of kaolin and feldspar samples treated with polyacrylamide. These peaks were assigned to adsorbed polyacrylamide. The intensity of these peaks increased with increased added polyacrylamide concentration. The Nls region showed a band at 399.7_+0.2 ev (charge corrected). Given that the surface areas of each mineral were approximately the same and that the analysis area was the same, this Nls band was used to quantify polyacrylamide adsorption. The data are summarized in Fig. 4. These data indicate that polyacrylamide adsorption onto kaolin and feldspar increased with increased polyacrylamide dose to a plateau level and that the plateau-adsorbed amount onto kaolin was greater than onto feldspar. No polyacrylamide adsorption onto quartz was detected. In more general terms, these data, along with the data for glass slides, confirm that XPS can be used to quantify directly the adsorption of polyacrylamide onto both macroscopic planar and colloidal inorganic surfaces. Competitive adsorption of polyacrylamide onto kaolin and quartz A back-scattered electron image of a polyacrylamide (1000 ppm) treated mixed mineral system comprising kaolin and quartz in equal weights was recorded using the Escascope. Survey and narrow scan spectra were then recorded from the image area. The Si2p ( ev), A12p ( ev) and Nls (406.9 ev) peaks were then used to image the spatial distribution of these atoms across the image area. In these images (Fig. 5a-d) the concentration of any particular atom at any location in the image is directly related to the image intensity at that position. Hence, bright areas of the image are associated with high concentrations of a particular atom. The region of the image with enhanced Si and reduced A1 corresponds to quartz (north-north east of centre). Whereas the region with high Si and A1 corresponds to kaolin. The kaolin region shows considerable enhanced Nls activity compared to the quartz region. Thus, these variations in N concentration indicate, at least in a qualitative fashion, the differential competitive adsorption of polyacrylamide onto kaolin in mixtures of kaolin and quartz. In more general terms these data demonstrate a potential new technique which could be useful for the study of competitive adsorption of reagents onto minerals. CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER WORK The data presented above for polyacrylamide adsorption onto glass slides and single mineral samples indicate the utility of XPS for quantitative study of polymer adsorption. The imaging data for polyacrylamide adsorption onto kaolin and feldspar indicate a new and potentially useful technique for the study of selective adsorption processes. However, further technique development is necessary prior to useful exploitation of the method. In particular, development needs to be focused on evaluating the potential of the imaging technique for quantitative rather than qualitative measurements of competitive adsorption. ACKNOWLEDGMENTS The authors acknowledge the University of Bristol and ECC International plc for financial support. REFERENCES Attia Y.A. (1987) Flocculation in Biotechnology and Separation Systems. Elsevier, The Netherlands. Leja J. (1983) Surface Chemistry of Froth Flotation. Plenum, New York. Read A.D. (1972) Use of high molecular weight polyacrylamides in the selective flocculation separation of a mineral mixture. Brit. Polym. ~L 4, Somasundaran P. & Moudgil B.M. (eds.) (1987) Reagents' in Mineral Technology. Marcel Dekker, New York. Wakeman R.J., Thuraisingham S.T. & Tarelton E.S. (1989) Colloid science of solid-liquid separation technology, Filtrat. Separ. 26,