Effect of Tribocharging on Powder Packing

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Antonio Castellanos Mata, Javier Perez Vaquero, Miguel Angel Sanchez Quintanilla
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This report covers the results obtained in the second year of this project. At the end of the first year, we have developed an experimental set-up capable of dispersing a sample of about 10 g of powder in a gas stream, charging it by tribocharging, collecting the sample inside a Farday cage and measuring the charge in the collected sample. During this year, we have improved the set-up by building two tribochargers: a nylon cyclone tribocharger and a steel tube tribocharger. With this new set-up we are able to measure both the charge acquired by the particles in dispersion (but only when the steel tribocharger is used) and the ccharge remaining in the powder when collected.

We have run experiments with a set of seven materials stored at different humidities and dispersed using air and dry N2. From the results we have obtained we can conclude that in dispersion particles become electrically charged up to the maximum given by the electric field that would cause a corona discharge. However, when settled, the particle must lose most of their charge to keep the field created by the settled sample below the limit imposed by corona discharge. The remaining amount of charge on the settled powder depends on the sample mass and as a result, the specific charge of the collected sample tends to decrease when the collected mass is increased. A quantitative model for a simplified sample geometry has been developed.

We have found that the charge in the collected sample decays in a relatively short time, of the order of some minutes. We have decided to measure poured and tapped densities to evaluate the effect of charge on the packing as these experiments can be performed during the time span that powder remains charged. However, the results obtained are in conclusive. Possible explanations for the lack of a clear trend in the effect of specific charge and storage conditions on the solid fraction are given in the text.

During the Annual General Meeting, we were requested to move to smaller particle sizes (below 10 μm), storage humidites (below 10% RH) and dispersion gas humidities (dry nitrogen). The upgrades done in the tribocharging set-up to meet this condition are presented in the test, as well as some preliminary results.

Work has also continued in the other experiments planned in the project: individual particle size and charge distribution determination with particle tracking velocimetry (PTV) and scanning probe microscopy (SPM) experiments. However, no new significant results have been obtained and for these reason this report will focus on the results of the specific charge obtained with the tribocharging set-up. The state of the PTV and SPM experiments is discussed at the end of the report.