Conclusions and Contributions
The general conclusion can be formulated as follows:
Because virtual instrumentation makes the measurement of process variables both accurate and versatile, it enables a deeper insight into electrostatic phenomena, paving the way for the improvement of existing technologies and the development of new ones.
The main scientific and technical contributions of the thesis are:
1. Accurate measurement of the electric charge carried by single mm-size particles.
1.1. Increase the precision of the measurement by using a standard Faraday pail and an electrometer, in association with a custom-designed virtual instrument;
1.2. Evaluate the efficiency of various charging mechanisms: corona discharge, electrostatic induction, triboelectrification;
1.3. Refine the model of particle charging/discharging in roll-type corona-electrostatic separators.
2. Continuous measurement of the triboelectric charge of insulating powders processed in compressed-air devices
2.1. Monitor the charge of insulating particles by using a modified Faraday cage and an electrometer, in association with a custom-designed virtual instrument;
2.2. Characterize the tribocharging of different powders;
2.3. Evaluate the capability of the tribocharging device;
2.4. Elaborate the methodology for the statistical control of a tribocharging process.
3. Non-contact measurement of the electric potential at the surface of granular insulating layers
3.1. Characterize the charge-decay from particles in contact with a grounded electrode, by using an electrostatic voltmeter, in association with a custom-designed virtual instrument;
3.2. Predict the electrostatic separability of d ifferent mixtures of solid insulators.
4. Contact measurement of the electric potential of the electrodes of an electrostatic separator
4.1. Visualize the variation in time of the electric potential of the electrodes of an electrostatic separator after a spark discharge, by using a high-voltage probe and a digital oscilloscope;
4.2. Evaluate various high-voltage supplies in view of their use with electrostatic separators.
5. Detection of the metal content of the granular materials processed in a corona-electrostatic separators;
5.1. Monitor the high-voltage at the electrodes of a metal-insulator corona-electrostatic separator, by using a high-voltage probe and an electrometer, in association with a custom-designed virtual instrument;
5.2. Identify a linear relationship between the metal content of the processed material and the standard deviation of the high-voltage at the electrodes of a corona-electrostatic separator.
5.3. Increase the resolution of the metal sensor from 5% to 0.5%, by using a low-pass filter.
6. Experimental modelling of corona-electrostatic separation processes:
6.1. Establish a linear-interaction model of the process by using Taguchi's methodology;
6.2. Analyse the robustness of the electrostatic-separation process;
6.3. Evaluate the influence of humidity on the outcome of the corona-electrostatic separation.
Further development of accurate measurement techniques for electrostatic process variables using virtual instruments will pave the way for both the improvement of existing technologies and the expansion of new ones.
1. Development of a dynamic Faraday cage and an ad-hoc virtual instrument will enable accurate measurement of the electric charge carried by single free-falling mm-size particles in electrostatic separation applications. Such measurements could contribute to the refinement of the existing models of particle charging and trajectories in roll-type and free-fall electrostatic separators.
2. The powder charge measurement systems described in this thesis will be used in conjunction with appropriate experimental design techniques in order to evaluate the effects of both control and noise variables on the efficiency of any tribo-charging process. The same measurement systems could be recommended as standards for industrial statistic process control.
3. By using high-voltage probes with a wider measurement domain (up to 30 kV), the surface potential measurement schemes will make possible a better charge-decay characterization of a larger number of materials and under various operating conditions. An important point to be solved is the recording of charge-decay characteristics starting at Vt=0, as this would enable further advancements in modelling of particle behaviour in corona-electrostatic roll-type separators. Such results could also be of use in predicting the feasibility of electrostatic separation of different mixtures of solid insulators.
4. Studies are needed to establish if high-voltage monitoring schemes developed for metal content measurement in roll-type corona-electrostatic separators could be expanded to other applications.