Triboelectric charge characteristics and electrical separation of industrial minerals

University dissertation from Luleå : Luleå tekniska universitet

Abstract: Despite the fact that triboelectric separation process has not been completely perceived, it pronounced to be environmentally amiable and economically viable for minerals and wastes beneficiation, especially when fine particles with relatively similar electrical properties are being to be separated. Since the method mainly depends on the charges developed on particles for a successful separation, it is necessary to develop different charge polarities on minerals undergoing separation. This can be achieved by finding a way to manipulate their electro-physical properties and increasing the differences among the energetic levels, e.g., work function, and electrical properties of participating materials, a perfect tribocharging medium and suitable operational conditions. Accordingly different pretreatment methods, such as chemical, thermal, humidity, irradiation and doping, with reference to tribocharging medium have been suggested and applied for gaining the foregoing goals. The optimization of operational and mechanical variables of separating device is also necessary for achieving remarkable separation of minerals from raw material. The studies on identification of triboelectric charge on mineral particles and the factors affecting this charge, monitoring the imparted charges on different particle size fractions of various industrial minerals by utilizing different tribocharging systems and tribocharging materials under different conditions, measuring the main electrical properties, i.e., dielectric constant and electrical conductivity for all particle size fractions of mineral samples before and after chemical treatment, exploring the trajectories of mineral particles through the electric field of the free fall electrostatic separator before treating, investigating the role of different organic and inorganic chemical species on charge acquisition and separation behaviour of minerals, envisaging the contact electrification from the chemical Lewis acid-base concept by measuring the electrokinetics of mineral particles in both aqueous and non-aqueous media and attempting to correlate the physical concept of tribo/contact electrification with the Lewis acid-base, and finally madding efforts to induce the fundamental findings related to the charge characteristics and separation behaviours of the examined minerals to a complex raw material containing different silicates, are dealt within this thesis in order to investigate the possibilities for applying triboelectrostatic separation method for separating complex mineral systems. The results reveal that, in spite of the simplicity in theory, the contact/tribo electrification and consequently the triboelectric separation are complex processes and special care must be taken into account to apply this technique for beneficiation of minerals. From the findings it can be concluded that a relation exists between the triboelectric charges accumulated on mineral grains and the work function of both the minerals and tribocharger medium. The electrokinetic results indicate that the minerals can be arranged in triboelectric series since their donicity and point of zero charges were found to be in good agreement with triboelectric charges acquired by them. Accordingly the values of their energetic levels, i.e., Fermi energy and work function are estimated by the electrokinetic and charge measurement data. These results have led to arrange the mineral samples within a series in which quartz, feldspar, wollastonite, apatite and calcite are cited due to their tendencies to be charged negatively, i.e, the one having higher donor value must possess higher tendency to be charged negatively. For separation point of view the separation of quartz from all other mineral samples was found to be possible by using Teflon cyclone as the tribocharger. However, separation of calcite from apatite can be achieved when copper, steel and Perspex tribocharging devices are utilized. Chemical conditioning was found to be useful for enlarging the differences among the charges acquired by mineral grains and H3BO3, Na2WO4, and BaCl2 were found to be effective for changing the electrical beneficiation potential of quartz and calcite. However, salicylic acid was detected to be good for separating feldspar from wollastonite. Finally, inducing the findings related to single mineral systems to a complex ore, i.e., Banmossen silicate deposit, Sweden, containing mainly feldspars, wollastonite and quartz minerals, revealed that triboelectric separation can be counted as an alternative for processing such a complex ore with the aid of chemical or other pretreatment methods. The separation results showed that a wollastonite product with an assay of more than 70% and recovery of 40% is accomplished from the feed material containing 28% wollastonite by subjecting the materials one time through the electric field of separator when the raw material is pretreated by salicylic acid in a controlled relative humidity and temperature of 15% and 80°C respectively. The result is more significant considering that the dry conditioning process has been utilized. This study reveals that, notwithstanding the truth that triboelectric charge has not completely understood even at the beginning of third millennium, triboelectric separation deserves to be considered in more details and must be examined to different mineral systems, especially when the technology offers possibilities to use new separation machines which are able to separate fine particles. More research and development projects are recommended to develop the triboelectric separation process for beneficiation of complex raw materials during new century.