Selectivity in Electric Pulse Fragmentation

Electric pulse fragmentation is a novel comminution methodolgy utilised in SelFrag systems where highly energetic electric pulses are used fragment composite solid objects into their component parts. EPF has a high selectivity and can offer enhanced liberation of components from a material allowing easier or increased recovery. These advantages are due to the selective nature of the fragmentation, i.e. the electric discharges are preferentially attracted to specific regions within a particle, concentrating the fragmentation energy where it can have the most beneficial effect, for example in removing components from electronic waste, selectively breaking metaliferous minerals from an ore, or disaggregating a scientifically valuable specimen without compressive crushing.

 

This article is designed to give a short introduction into the nature of the processes’ selectivity, and how this can be exploited to our advantage.

 

There are several stages to a fragmentation event which will be described in more detail below. These are:

  1. Polarization (& Electrostriction)
  2. Field distortion & dielectric permitivity
  3. Discharge
  4. Electrical Breakdown

  1. Polarization (& Electrostriction)

A dielectric material is an electrical insulator that can be polarized by an applied electric field. When a dielectric material is placed into an electric field, electric charges do not flow through the material as they do in a conductor, but instead slightly shift from their average equilibrium positions causing dielectric polarization of discrete components within the material. Particles with high electrical permitivity (Eo) have a stronger surface charge than low Eo particles. This creates a charge imbalance at the boundaries of these components, which causes a distortion in the electrical field.

Schematic of a rock within an electrical field. Individual minerals becomes polarised and a charge imbalance develops at their boundary with the matrix material.

 

  1. Field distortion & dielectric permitivity

Minerals in rocks are dielectric particles and they can be considered as randomly-aligned electrical domains. When subjected to a strong electrical field, these domains align slightly. When materials of different dielectric permittivity are next to one another, the strength of the field distortion at the interface varies due the difference in dielectric constants of each material.

 

  1. Discharge

As discussed previously, plasma streamers develop when the voltage on the working electrode is sufficiently high. Theselectivity of the process arises as these streamers seek the ‘fastest’ route to the grounding electrode which is where there are perturbations in the electrical field. The streamer is thus forced toward more conductive particles along field distortion caused by charged surfaces.

Strong field distortion caused by the presence of high permitivity particles means the discharge pathway intersects these minerals preferentially on it’s way to the ground electrode, focussing breakage energy into the mineral boundaries.

 

  1. Electrical Breakdown

Once the streamer has grounded via the pathway of high permitivity minerals, the resulting plasma channel and shock wave act to weaken and disintegrate the material. The main fracturing will be along the primary discharge pathway, but other fractures will be generated along the paths of ‘failed’ streamers.

 

 

The explosive expansion of the plasma channel creates a shockwave which disaggregates the material from the inside.

 

Produced by Lightning Machines, Images by Alexander Weh.