Tuesday, 25 November 2008

Characterising Electron Transfer Mechanism in Tribo-Electrification of Pyrite Through Contact Angle Measurements

R. K. Dwari* and K. Hanumantha Rao*

Division of Mineral Processing, Luleå University of Technology, SE-971 87 LULEÅ, Sweden






Abstract


Coal beneficiation by tribo-electrostatic method depends on tribo-charging attributes of coal and ash forming minerals. The tribo-electrification behaviour of pyrite mineral contacted with different materials has been investigated through charge measurements and the charge acquisition is probed through surface energy calculations from liquid contact angle data. Liquid contact angle on pyrite powder after tribo-electrification is determined by Krüss tensiometer using Washburn’s equation. The sample holder in tensiometer is specially fabricated with different materials serving the purpose of tribo-electrification and contact angle measurement. The acid and base parameters of pyrite surface determined with van Oss acid-base approach using liquid contact angle data after tribo-electrification with different materials revealedthe charging phenomena and electron transfer mechanism. The results showed an explicit correlation between the charge generated by pyrite powder and surface acceptor (acid)-donor (base) electronic state underlying the work functions of contacting surfaces. Thus a method for characterising the changes in surface energetic structure of solids during contact electrificatio in terms of surface acid-base parameters has been illustrated for the first time.










INTRODUCTION


Coal is the single largest fossil energy source used worldwide and accounts for more than 60% of the total commercial energy consumed. The major portion of the coal used for such power generation is not clean enough to maintain the rigorous environmental standards required these days world over. The problem is the high sulphur content in coal used in most of the western countries or ash as is the case in countries like India. The sulphur content in coals globally variesfrom 0.38 to 9% and rarely exceeds this range. The sulphur content in non-coking coals of India is generally below 0.5% and mostly in the form of pyrite. However, the North-Eastern coals contain 2-7% sulphur and about 75-90% of this sulphur is organic in nature [1]. Coals in the Western countries also contain pyritic sulphur and removal of this sulphur alone could reduce the total sulphur content to less than 1%, well below the pollution control standard. Electrostatic separator with tribo-charging technique has great potential for the separation of pyrite from coal in fine size materials, but organic sulphur is not accessible to this dry physical separation technique as it is covalently bound with carbon in macerals. There have been some investigations carried out using tribo-electrostatic method but has not achieved commercial status in coal beneficiation industry. The tribo- or contact electrification of materials is a wellknown effect, which is a function of the ability of material to accept or donate electrons when it is in dynamic contact with other material. The net charge acquisition by the material in tribo-electrification depends on the work function difference between the contacting materials. In recent years, there has been a broad understanding that electron transfer during contact charging is due to surface properties rather than bulk properties of materials and the amount and polarity of charge transfer between two dissimilar materials is controlled partly by their surface chemistry. The ability of particles to donate or accept electrons is an inherent property of particles based on their work function and physical form and it is logical to believe that there must be an underlying tendency for a material to charge, which is a consequence of its surface energetic electron donating/electron accepting properties. Accordingly, contact electrification is also explained as acid-base interactions between surfaces involving protons in the case of Brönsted acids-bases or electrons in the case of Lewis acidsbases. The acid-base interactions associate charge rearrangement at the interface and, when the interacting surfaces are abruptly separated, some fraction of charge may remain on the surface. The surface acidity and basicity are thus related to surface acceptor and donor electronic states and thereby to work functions. The work functions of metals can be easily determined reliably but they are not established for insulating materials. The published work function values for the same material varied because of different surface states caused by impurities, crystal imperfections, defects, etc. Despite the fact that the surface physico-chemical properties will influence the extent of charging, characterization of surface energetic electron donating/accepting properties before and after contact charging has not been illustrated till today, primarily due to the lack of standard instrumentation and procedures. Many authors quantified the acid-base character of mineral surfaces by solvatochemistry, zeta-potential, inverse gas chromatography (IGC), liquid contact angles, etc. In the context of triboelectrification, electron donating-accepting tendencies of pharmaceutical powders have been investigated by IGC and obtained a correlation between the charges generated in tribo-electric studies of powders and acid/base parameters determined by the IGC. However, no studies have been reported up today on the characterisation of electron transfer emphasising the changes in surface acid-base properties of contacting surfaces before and after tribo-electrification. The pyritic sulphur could be in the form of pyrite and marcasite. Both pyrite and marcasite have similar chemical composition but possess different crystalline forms of cubic and orthorhombic respectively. Pyrite is the most commonly reported mineral while marcasite has often been mentioned as occurring in lesser amount except in Victorian brown coal of Australia, where it is the only sulphide mineral. According to Esposito et al. [8], there is a difference in the properties of coal pyrite and ore pyrite in terms of morphology, specific gravity and surface area. A detailed understanding of the surface properties of pyrite after tribocharging with different materials is necessary for an efficient rejection of pyrite from coal. The aim of the present work is, therefore, to understand and quantify the surface energetic structure of pyrite, which is one of the most important ashforming minerals in coal in terms of acidity and/or basicity, before and after tribo-electrification with different materials, and thereby to identify the optimum tribo-charger material in dry coal beneficiation. In the present investigations, triboelectrification of pyrite and electron transfer between the two contacting materials in terms of surface acid-base properties were studied using glass, copper, aluminium and brass as tribo-charging media. Investigations on marcasite and coal pyrite will be considered in a future work.



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