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In Situ Mechanistic Study of SDS Adsorption on Hematite for Optimized Froth Flotation Bianfang Bai,† Nick P. Hankins,*,‡ Michael J. Hey,† and Sam W. Kingman‡ School of Chemistry and School of Chemical, Environmental and Mining Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
An in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) technique has been developed to study the adsorption of sodium dodecyl sulfate (SDS) onto pure, colloidal hematite as a function of both concentration and pH. The absorption bands of methyl and methylene groups are most reliable for quantifying adsorption unambiguously. The infrared spectral results have been correlated with ζ-potential studies to establish an absence of adsorption above the iso-electric point of 7.3, and to indicate that the mechanism of adsorption is physical, involving reversible electrostatic and hydrophobic forces. In the same way, there is no evidence for the existence of chemisorption. This is supported in the spectral results by the absence of shifts in relative band intensities and the absence of new adsorption bands. At low pH and high SDS concentrations, the adsorption is sufficient to cause a reversal in sign of the ζ-potential. Spectral absorption measurements should be made under in situ conditions to obtain unambiguous results. Studies of adsorption on natural hematite indicate a high depletion of SDS from solution at low pH; the measurements are consistent with precipitation losses due to the presence of calcium and ferric ions. This precipitation leads to a drop in flotation recovery. At high pH, adsorption occurs onto mineral particles which have become coated with carbonate mineral, leading to a flat variation of recovery with pH. The results have enabled optimum flotation conditions to be defined. Recovery can be directly related to adsorption density at moderate pH below the iso-electric point. The optimum pH for flotation is 5; extremes of pH are detrimental to flotation recovery of pure mineral and should be avoided. Low pH causes surfactant precipitation, while high pH will lead to surface transformation of the mineral. A collector concentration of 0.05 times the critical micelle concentration, corresponding to partial monolayer coverage on the hematite surface, is sufficient to ensure good recovery. Introduction Flotation processes are principally used to separate or concentrate minerals during their production; they are both energy intensive and highly inefficient in their use of energy and materials. It is estimated that flotation accounts for 10% of total energy consumption in mineral processing; in this respect, it is second only to grinding. These factors are compounded by the huge scale of operations. For example, in the year 2000 the Palabora mine in South Africa processed up to 130 000 tonnes/day of 0.9% copper sulfide ore by flotation.1 The associated collector (surfactant) consumption was 400 g/tonne or 52 tonnes/day, with a cost of $0.5-1.0 per pound of collector or $58 000 per day. It is clear that even small improvements in the efficiency of flotation will lead to significant savings in the cost of energy and collector use, and this offers large incentives to seek such improvements. The overall aim in flotation should be to maximize recovery and selectivity, with a minimum cost of reagents and time (energy). Only through a fundamental understanding of collector adsorption mechanisms, both qualitative and quantitative, can the process of flotation be correctly interpreted, designed, and optimized. The * To whom correspondence should be addressed. Tel.: 0115 9514197.Fax: 01159514115.E-mail:
[email protected]. † School of Chemistry. ‡ School of Chemical, Environmental and Mining Engineering.
work presented here is part of a concerted approach at the University of Nottingham to address this. The novelty of the approach taken lies in the integrated nature of the study, so that the understanding which results from the fundamental study can be applied immediately in a consistent and systematic way. Mineral froth flotation is undoubtedly the most important and versatile mineral processing technique available, and its use and application both continue to widen. It has permitted the mining of otherwise uneconomic low-grade and complex ore-bodies. It is a selective process, and can be used to achieve specific separations from complex ores. It is widely used for mineral sulfides, such as chalcopyrite (CuFeS2), sphalerite (ZnS), and galena (PbS), as well as for mineral oxides, such as hematite (Fe2O3), ilmenite (FeTiO3), quartz (SiO2), and cassiterite (SnO2). The process of flotation depends on the fact that the surface of a desired mineral particle is rendered hydrophobic by the selective adsorption of surface-active agents (surfactants), termed collectors. An air-bubble may then attach itself to the particle and lift it toward the froth. Regulators help to control the selectivity of the process, by modifying the action of the collector. Over the past several decades, a great deal of research has been carried out to understand the process of flotation, and to elucidate the underlying mechanisms of collector adsorption. These mechanisms include physical forces, such as electrostatics, hydrogen bonding, and
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hydrophobic associative forces between surfactant tails; and chemical forces, such as chemisorption, surface precipitation, chelation, and solution speciation of the collector.2,45 In addition to collector adsorption, interactions with dissolved mineral species, and other reagents such as frothers, activators, and depressants, have been studied, as well as the foam properties. Despite this, a great deal of confusion and contradiction still exists concerning adsorption mechanisms, particularly for oxide and sulfide minerals. For oxide mineral flotation, physical forces of interaction undoubtedly play a major role, while the contribution of chemical interactions remains uncertain. Researchers agree that the primary forces are the Coulombic (electrostatic) interactions between surfactant headgroups and the mineral surface, and the hydrophobic surfactant tail-tail interactions, but uncertainties remain about the nature of adsorbed aggregates. Harwell, Hankins, and co-workers have developed a comprehensive model for the process of surfactant adsorption on mineral oxides.4-6 For mineral oxides, the exact adsorbate structure and the nature of the adsorbent-adsorbate bond is expected to depend on the nature and hydrophobicity of the solid surface, the net surface charge, the solution pH, the solution ionic strength and composition, and the surfactant molecular structure and orientation. The original model considered the formation of surfactant aggregates on local surface patches of known charge density.6 The work of Hankins et al.5 developed this approach further by incorporating the site-binding model for both the charging of oxide surfaces and the association of charged counterions. This allowed the development of a comprehensive model. The model was able to predict experimentally measured variations in the adsorption level as a function of surfactant concentration, solution pH and counterion (electrolyte) concentration. This model has since been applied within the oil industry to minimize surfactant loss, due to adsorption, during micellar-enhanced oil recovery. Recently, a significant amount of work on collector adsorption has been executed by Fuerstenau and co-workers in the U.S.,7 and Cases and co-workers in France.8 However, these latter contributions were not aimed at optimizing flotation in a systematic way. The use of in situ observations, made directly on the adsorbed molecular layers under actual conditions, is vital if meaningful conclusions are to be made from macroscopic observations. Ex situ observations, i.e., those made under conditions dissimilar to the ones which pertain during the adsorption process, create large uncertainties in the reliability of data obtained. This is true, for example, when the adsorption sample must be dried prior to analysis. Early in situ qualitative infrared studies of surfactant adsorption on minerals were performed at Nottingham University’s School of Chemistry.9 Recently, a lot of attention has been given to the use of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). This technique allows in situ measurements in aqueous solutions; where feasible, it is highly useful for the study of flotation surface chemistry. It can be applied in three ways: (1) direct adsorption onto an internal reflection element (IRE) made from a mineral of interest; (2) vacuum deposition of the mineral onto the IRE; or (3) pressing a mineral surfactant suspension against an IRE. ATR-FTIR spectroscopy is particularly attractive
for the elucidation of surfactant adsorbate structures in the presence of ionic surfactant solutions. ATR-FTIR allows: (a) in situ analysis of adsorption; and (b) simultaneous quantitative and qualitative analyses of both the surfactant and any coadsorbed molecules. Despite this, the potential use of ATR-FTIR for optimizing flotation remains largely untapped. Indeed, hardly any studies have attempted to correlate studies of adsorption mechanism with flotation optimization in a systematic way. Studies of Surfactant Adsorption during Oxide and Hematite Flotation: Current State of the Art. A number of spectroscopic techniques have already been used for adsorption studies, such as proton NMR,10 fluorescence probe spectroscopy,11 UV-visible,12 FTIR,9,13 surface-enhanced Raman scattering,14 and X-ray photoelectron spectroscopy.15 However, these techniques have largely been applied qualitatively, such as identifying the chemical nature of surface compounds and the structure and orientation of the adsorbed entity. Many studies have actually been performed ex situ, since the samples are dried, frozen, or evacuated, rendering the measurements unrealistic. The in situ studies have used pure or cleaved crystal surfaces16,17 or vacuum-deposited amorphous layers;18-22 less frequently, layers of mineral particles have been used.23 The range of systems studied quantitatively is limited, and includes anionics and cationics on Al2O3 and TiO2.19,23 Research into surfactant adsorption on minerals over the past fifty years has yielded valuable information on the adsorption characteristics of minerals, and the mechanisms governing them,4-6,24-30 including the insoluble metal oxide hematite.31 Early studies of the adsorption onto hematite of anionic surfactants (such as carboxylic acids, soaps, and hydroxamates) and cationic surfactants (such as quaternary ammonium salts) were interpreted in terms of physical adsorption, via an electrostatic mechanism.32 Kallay et al.33 measured the adsorption density of SDS on a synthetic hematite as a function of pH, using a calorimetric technique. For pH
