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Langmuir 1998, 14, 6512-6517
An Infrared Study of a Polyacrylate Flocculant Adsorbed on Hematite† F. Jones,‡ J. B. Farrow,§ and W. van Bronswijk*,‡ Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, and CSIRO Division of Minerals, P.O. Box 90, Bentley, Western Australia 6102 Received October 15, 1997. In Final Form: August 17, 1998 Hematite was flocculated at various pH values using a 100% polyacrylate of high molecular weight, and the adsorbed configuration of the polymer was determined from infrared (DRIFT) spectra. The positions of the symmetric and asymmetric carboxylate stretches for the polyacrylate sodium salt and the adsorbed flocculant, and their separation were used to determine the adsorbed configuration. The adsorption mechanism is dominated by chemisorption, except at neutral pH in the presence of calcium ions when electrostatic interactions also play a role. At pH 7 the polyacrylate adsorbs in an essentially monodentate configuration. At this pH there is a significant degree of hydrogen bonding between the CdO group of the flocculant and either the surface hydrogens of the hydroxylated hematite or the solvent (water). At higher pH (11-14) the monodentate configuration is much less evident and an asymmetric bidentate bridging structure is formed. In the presence of calcium ions at pH 7 a bidentate chelating structure occurs in addition to the monodentate structure. This chelating structure was not found at pH 13 and 14.
Introduction The Bayer process, used to refine alumina from bauxite, depends on the separation of the liquor (containing the dissolved aluminum) from waste solids prior to recovery of dissolved aluminum by precipitation of gibbsite, Al(OH)3. To do this flocculants, which are ultrahigh molecular weight water soluble polymers, are added to the slurry to aggregate the fine waste solids. These aggregates are normally orders of magnitude larger than the primary particles resulting in much faster settling rates and better solid-liquid separation. Aggregation of the fine solids is, in turn, dependent on the adsorption of the flocculant onto the solids. Allara and Nuzzo,1 in a detailed investigation of carboxylates adsorbed onto oxidized aluminum, used infrared spectroscopy to show that n-alkanoic acids (monocarboxylates) adsorb in a bridging bidentate (III) structure (Figure 1) which is not completely symmetrical but tilted somewhat from the surface. Other workers2-4 have studied similar systems, and Lee et al.4 concluded that a chelating (II) structure occurs for polyacrylate adsorbed onto alumina, from a comparison of their spectra with those of aluminum complexes in solution. Little is known, however, about the adsorption mechanisms of polyacrylate flocculants on residue minerals in Bayer liquors, which are strongly alkaline (up to 6 M NaOH). Flocculant adsorption may occur via an electrostatic adsorption mechanism involving adsorbed cations or may occur by direct chemisorption to the surface. To identify the mechanism of adsorption, we have obtained the infrared spectra of a 100% polyacrylate flocculant adsorbed on hematite at various pH values (7†
AJ Parker Cooperative Research Centre for Hydrometallurgy. Curtin University of Technology. § CSIRO Division of Minerals.
Figure 1. Metal-carboxylate structures.
14) and of its sodium salt. Diffuse reflectance infrared fourier transform (DRIFT) spectroscopy was used as it is a convenient means of obtaining surface specific infrared data,4-6 and hematite was chosen as the substrate since it is one of the major minerals found in Bayer residues.7,8 Attempts to use Raman spectroscopy were unsuccessful as the adsorbed species could not be detected using FT and dispersive instrumentation. Experimental Section Materials. The hematite used was a high-purity sample obtained from Kanto Chemicals. It was characterized by its surface area (BET, Quantachrome Autosorb I, using nitrogen), particle size distribution (Malvern Mastersizer), X-ray powder diffraction pattern (Philips X-pert diffractometer, Cu KR radiation), and elemental analysis after acid dissolution (ICP, Varian Liberty 220 emission spectrophotometer). The results are given in Table 1. The flocculant, AN995SH, was obtained from SNF Floerger (Australia). It is a sodium polyacrylate salt whose average molecular weight (multiangle laser light scattering9) and anionic content (13C nuclear magnetic resonance spectroscopy10) were found to be 1.4 × 107 g mol-1 and 100%, respectively.
‡
(1) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (2) Buckland, A. D.; Rochester, C. H.; Topham, S. A. J. Chem. Soc., Faraday Trans. 1 1980, 76, 302. (3) Parfitt, R. L.; Farmer, V. C.; Russell, J. D. J. Soil Sci. 1977, 28, 29, 40. (4) Lee, D. H.; Condrate Snr, R. A.; Reed, J. S. J. Mater. Sci. 1996, 31, 471.
(5) Weissenborn, P. K. Ph.D. Thesis, Curtin University of Technology, Perth, Western Australia, 1993. (6) Gong, W. Q.; Parentich, A.; Little, L. H.; Warren, L. J. Colloids Surf. 1991, 60, 325. (7) Yamada, K.; Harato, T.; Shiozaki, Y. Light Metals 1980, 39. (8) Li, L. Y.; Rutherford, G. K. Int. J. Miner. Proc. 1996, 48, 169. (9) Scott, J. P.; Fawell, P. D.; Ralph, D. E.; Farrow, J. B. J. Appl. Polym. Sci. 1996, 62, 2097.
10.1021/la971126l CCC: $15.00 © 1998 American Chemical Society Published on Web 10/02/1998
IR Study of Polyacrylate Flocculant on Hematite Table 1. Properties of the Kanto Chemicals Hematite Sample surface area (m2 g-1) particle size (µm) d10 d50 d90 chemical composition (wt %) Fe2O3 TiO2 SiO2 total oxides XRD pattern a
Langmuir, Vol. 14, No. 22, 1998 6513 Table 2. Infrared Band Assignments for Polyacrylates and Polyacrylate Salts assignment OH stretch CH stretch CdO stretch COO- asymmetric stretch CH bend COO- symmetric stretch
8.25 0.24 0.70 9.50 98.80 1.45 0.08 100.23 hematite onlya
XRD pattern matched that of JCPDS-PDF #33,664.
Analytical grade (or equivalent) sodium hydroxide and hydrochloric acid were used to control pH. MilliQ water was used to prepare solutions, and nitrogen gas (ultrahigh purity) was used with an in-line filter for sample preparation. Flocculant Adsorption. The technique used for adsorbing flocculant on hematite was modified from that developed by Weissenborn,5 with particular care taken to exclude CO2. Sodium hydroxide solutions of known concentration (0.001, 0.1, and 1 M, respectively), or water, were placed in contact with hematite for 24 h to adsorb any carbonate in solution. The supernatant was then decanted (under a nitrogen atmosphere) onto a fresh, carbonate-free hematite sample, placed in an airtight container, and sonicated. The flocculant was prewetted with ethanol (1 mL gm-1), and then water was added slowly with gentle agitation to obtain a 0.5% w/v stock solution. This was diluted to 0.1% w/v with the appropriate sodium hydroxide solution before it was added to the slurry, under a nitrogen atmosphere, and the slurry was capped, sonicated once more, and allowed to reach equilibrium (24 h). No agitation other than sonication was applied. Slurry pH values were not adjusted, and final pH values were determined by measurement with a pH meter (Orion SA520, Orion pH electrode), except for the pH 14 slurry where titration was used. Solids were separated by centrifugation (overnight at 2000 rpm), the supernatant being decanted under nitrogen and the solids dried under reduced pressure (∼15 kPa) at 50 °C to remove bulk water. Washing of the solids was found not to affect the position of IR bands found in the 2000-700 cm-1 region, and spectra are reported for unwashed samples as loss of flocculant from the surface is minimized, more closely reflecting the situation in aqueous solution, i.e., the presence of water molecules and sodium and hydroxyl ions. Hematite samples without adsorbed flocculant were prepared in the same manner. Infrared Spectroscopy. FTIR spectra were obtained using a Bruker IFS 66 instrument, Harrick “Praying Mantis” DRIFT accessory, and MCT (mercury cadmium telluride) detector. A resolution of 4 cm-1, with two times zero filling to give a 2 cm-1 plotting interval, was used, and 256 scans were accumulated. Samples (0.024 ( 0.06 g) were intimately mixed with previously dried KBr (1.176 ( 0.05 g) by gentle grinding so as to minimize particle break up. To maximize reflectance and minimize spectral distortion, the sample-KBr mixtures were passed through a 150 µm stainless steel sieve11 and the
