TY - BOOK

T1 - Triboelectric charge behavior of minerals at microscale for understanding the triboelectrostatic separation process

AU - Mirkowska, Monika Joanna

N1 - no embargo

PY - 2016

Y1 - 2016

N2 - Triboelectrostatic separation (TS) is based on differences in the electrostatic charging behavior of insulating materials. Although the TS process is well established, there is no comprehensive model available so far which allows reducing the number of trial-and-error experiments for the selection of proper process parameters. The multiplicity of factors influencing particle charging, the complexity of electrostatic interactions and its local character, along with the lack of appropriate research tools are serious challenges that have to be overcome. The goal of research presented here is to identify, describe, and classify factors which play a role during electrification of insulators, wherein, the quartz - calcite system is chosen as a model system. Microscale triboelectrification studies of atomically flat single crystals are performed by means of atomic force microscopy (AFM) based measurements. A combination of local charging using contact-mode AFM and Kelvin probe force microscopy analysis is employed to detect the local surface potentials before and after charging of the single crystals. Three different methods of surface contact charging are applied: rubbing, static charging, and a matrix of static charging. Rubbing simulated frictional contact, whereas static charging and the matrix of static charging correspond to non-frictional contacts at a single point or a multi-apex system, respectively. In some cases, a bias was applied to the tip during charging. This emulated contact with an already charged object or that in an external electric field. Also, the time evolution of the surface potential was measured in order to monitor the loss / redistribution of the introduced charge. Single crystals were charged by contact with titanium nitride tips of commercial AFM cantilevers and with quartz or calcite µm-sized mineral particles, which were attached to the free end of a commercial AFM cantilever. The main focus is set to investigate the influence of the type of contact, material of contact partners, contact force, charging speed, the number of rubbing lines during rubbing, or contact points during the matrix of static charging under different environmental conditions (in temperature range of 30-50°C and in humidity range of 5-60%r.H.). The most important observation of this work is a significant difference in the tribocharging behavior of quartz and calcite single crystals. Quartz tends to charge negatively, whereas calcite tends to charge positively. Particular emphasis should be placed on the results at low humidity: During tribocharging under dry nitrogen atmosphere, a strong localized charge was created on both the quartz, and the calcite surfaces. From the results of the studies it can be concluded that in order to obtain decent mineral concentration via TS of the quartz - calcite system, the process should be carried out in the following way: at low humidity, immediately after comminution processes, which also need to be carried out under dry conditions and the charging unit should allow to create fresh surfaces of the minerals, however, with low contact forces. Moreover, some conclusions from the investigation may have a wider impact, beyond the quartz - calcite system. For instance, the used methodology can be applied to investigate other systems of primary and even secondary raw materials to improve their separation. Further, the knowledge acquired can be used in other areas as well, where electrostatic charge is created due to contact. For example, it can be applied in order to prevent electrostatic discharge, which is known to cause damage to electronic circuits, and can also lead to explosions in factories which store insulating powders. On the other hand the observed charging effects like the localized, bipolar, and non-localized charge patterns might represent common features concerning triboelectrostatic charging in general. Therefore,

AB - Triboelectrostatic separation (TS) is based on differences in the electrostatic charging behavior of insulating materials. Although the TS process is well established, there is no comprehensive model available so far which allows reducing the number of trial-and-error experiments for the selection of proper process parameters. The multiplicity of factors influencing particle charging, the complexity of electrostatic interactions and its local character, along with the lack of appropriate research tools are serious challenges that have to be overcome. The goal of research presented here is to identify, describe, and classify factors which play a role during electrification of insulators, wherein, the quartz - calcite system is chosen as a model system. Microscale triboelectrification studies of atomically flat single crystals are performed by means of atomic force microscopy (AFM) based measurements. A combination of local charging using contact-mode AFM and Kelvin probe force microscopy analysis is employed to detect the local surface potentials before and after charging of the single crystals. Three different methods of surface contact charging are applied: rubbing, static charging, and a matrix of static charging. Rubbing simulated frictional contact, whereas static charging and the matrix of static charging correspond to non-frictional contacts at a single point or a multi-apex system, respectively. In some cases, a bias was applied to the tip during charging. This emulated contact with an already charged object or that in an external electric field. Also, the time evolution of the surface potential was measured in order to monitor the loss / redistribution of the introduced charge. Single crystals were charged by contact with titanium nitride tips of commercial AFM cantilevers and with quartz or calcite µm-sized mineral particles, which were attached to the free end of a commercial AFM cantilever. The main focus is set to investigate the influence of the type of contact, material of contact partners, contact force, charging speed, the number of rubbing lines during rubbing, or contact points during the matrix of static charging under different environmental conditions (in temperature range of 30-50°C and in humidity range of 5-60%r.H.). The most important observation of this work is a significant difference in the tribocharging behavior of quartz and calcite single crystals. Quartz tends to charge negatively, whereas calcite tends to charge positively. Particular emphasis should be placed on the results at low humidity: During tribocharging under dry nitrogen atmosphere, a strong localized charge was created on both the quartz, and the calcite surfaces. From the results of the studies it can be concluded that in order to obtain decent mineral concentration via TS of the quartz - calcite system, the process should be carried out in the following way: at low humidity, immediately after comminution processes, which also need to be carried out under dry conditions and the charging unit should allow to create fresh surfaces of the minerals, however, with low contact forces. Moreover, some conclusions from the investigation may have a wider impact, beyond the quartz - calcite system. For instance, the used methodology can be applied to investigate other systems of primary and even secondary raw materials to improve their separation. Further, the knowledge acquired can be used in other areas as well, where electrostatic charge is created due to contact. For example, it can be applied in order to prevent electrostatic discharge, which is known to cause damage to electronic circuits, and can also lead to explosions in factories which store insulating powders. On the other hand the observed charging effects like the localized, bipolar, and non-localized charge patterns might represent common features concerning triboelectrostatic charging in general. Therefore,

KW - Triboaufladung (TA)

KW - Kontaktaufladung

KW - Rasterkraftmikroskop (AFM)

KW - Raster-Kelvin-Sonden-Mikroskopie (KPFM)

KW - Nichtleitern

KW - Kalzit

KW - Quarz

KW - Tribo-elektrostatische Sortierung

KW - Electrification

KW - Triboelectrification

KW - Contact charging

KW - Atomic Force Microscopy (AFM)

KW - Kelvin Probe Force Microscopy (KPFM)

KW - Dielectrics

KW - Calcite

KW - Quartz

KW - Triboelectrostatic separation

M3 - Doctoral Thesis

ER -