Federico Cangialosi

The role of weathering on fly ash charge distribution during triboelectrostatic beneficiation

Triboelectrostatic beneficiation of coal combustion fly ashes with high-unburned carbon contents can produce low-carbon ash products having value as mineral admixtures and meeting technical requirements for replacing cement in concrete. This capability is a result of establishing bipolar charge on mineral ash versus carbon particles where, typically, unburned carbon attains positive surface charge and ash attains negative surface charge under the tribocharging conditions employed in triboelectrostatic technologies. However, long-term exposure of fly ash to weathering conditions, such as moisture or high humidity, before beneficiation is known to dramatically diminish carbon-ash separation efficiencies. Although experimentation has shown that water soluble surface species can be redistributed on fly ash particles after exposure to moisture, which could affect the extent of charging and polarities, measurement of the actual amount of charge and polarity on particles after weathering exposure versus after removal of surface moisture has not been accomplished. Hence, a new experimental methodology was developed and applied to measure charge distributions on tribocharged ash and carbon particles in a fly ash that had been exposed to weathering conditions for 6 months before and after removal of the surface moisture. Weathered ash particles were found to have an average zero charge, whereas carbon particles attained an average negative charge, opposite of the normal polarity for carbon. Although the extent of uncharged particles decreased and ash particles attained an average negative charge after drying, carbon particles attained only an average zero charge. These changes were reflected in very small increases in carbon-ash separation efficiency, in contrast to previous beneficiation tests in which fly ash drying led to significant increases in carbon-ash separation efficiency. It is suggested that removal of surface moisture in the absence of other processes like surface ion redistribution would beneficially impact carbon-ash triboelectrostatic beneficiation.

Modelling Of Tribo-electrostatic SeparationFor Industrial By-products Recycling

Tribo-electrostatic separation is a dry technology which allows the elimination of impurities from industrial wastes on the basis of their surface charging characteristics. In this paper results of investigations aiming to numerically simulate the tribo-electrostatic separation of fine particles for industrial by-products recycling are described. After charging in a pneumatic transport line, the mixture of the particles to be separated is injected in a separation chamber where a DC electric field is created using two parallel plate electrodes. The particle/gas flow inside the chamber was simulated using an extended commercial computational fluid dynamics (CFD) code. The three-dimensional turbulent flow was calculated. Based on the Lagrangian approach, the trajectories of the powder particles (<100 microns) were modelled considering electric and aerodynamic forces. Comparison of the simulation results with experiments carried out with a bench-scale separation unit are presented for the case of silica beads, whose electric characteristics resemble those of coal fly ashes. The effects of particle charge, electric field and injection velocity were investigated, revealing that a proper choice of separator geometry and flow parameters allows unwanted fine particles in industrial wastes like fly ash to be efficiently removed.