Effect of Tribocharging on Powder Packing

Executive summary.

This reports presents the results of a series of experiments aimed at studying the factors affecting the amount of charge in dispersed and bulk powders, for how long the charge remains in the bulk powder before dissipating into the environment and the effect of the electric charge on the solid fraction of a bulk powder. The powders used in this study cover a range of particle sizes from 3 μm to roughly 1000 μm, although not all powders were used in all the experiments.

Experiment 1

In the first experiment presented in this report, particles are dispersed in a gas stream and charge due to collisions with a tribocharger. It is found that the main factor affecting the maximum amount of charge per particle is the particle size, since the charge is limited by the electric field on the surface of the particles and for the same charge on a particle, the electric field on its surface scales with the square of the diameter. In practical applications the particles may not experience enough number of collisions with solid surfaces to charge up to their maximum level, but the results presented in this report indicate than, on equal conditions, it is still the particle size the main parameter affecting the particle charge.

Experiment 2

In the second experiment we measure the charge distribution of the particles that come out of the tribocharger. We have found that the particle charge has a very wide distribution spanning both polarities. This finding may be explained if the charge transfer from the tribocharger to the dispersed particles causes a shift in a pre-existent charge distribution in the direction of the transferred charge. In consequence, the particles in a neutral bulk powder may carry electric charge, but on some of the particles the charge is positive and on the others is negative.

Experiment 3

In the third experiment we have measured the charge in a bulk powder formed by sedimentation of highly charged particles. We have found that while particle settles, the layer of bulk powder formed losses its charge. We propose a model that qualitatively describes the decay of the charge in the bulk powder based on the assumptions that the charge in the bulk powder has some mobility and that charge is dissipated on the surfaces of the bulk powder by neutralization with ions existing in the surrounding gas in order to keep the electric field on the surface of the powder at a value equal or below the breakdown field in the gas. The amount of charge in a bulk powder results from an equilibrium between charge dissipation into the surroundings and the accretion of new charge from the incoming particles and according to the model, depends on the charge on the particles that sediment, the mass flow rate at which they arrive and the effective electrical conductivity of the powder.

The effective electrical conductivity can be estimated from the typical time for charge dissipation, which is of the order of minutes, yielding a value of the effective electrical conductivity of the bulk powder of the order of nS/m.

Experiment 4

In the fourth set-up we measure directly the effective electrical conductivity of some powders as a function of consolidation and ambient humidity. The effective conductivity is found to be in the order of nS/m and it is highly dependent on humidity and to a lesser extend, on particle size. The strong dependence on humidity, specially for smaller particle sizes, may explain why the charge on bulk powders seem to be highly unpredictable in environments in which humidity is not controlled.

Experiment 5

In the fifth and last set-up, we measure the poured and tapped densities of charged and uncharged powders in order to determine if there is an effect of electric charge on the solid fraction, but within the accuracy of our experiment, we have found none.