Breakdown strength of
various materials as a function of the pulse rise
time

How selFrag works

In the course of only a few microseconds, a HV discharge (of 0 to 400 kV, with an energy of 10 – 300 J per cm) is deposited in the discharge plasma channel, which initially measures only a few µm. Pressures of up to 10¹⁰ Pa are generated in the process.

They have an effect similar to chemical explosives (such as TNT) ignited in a borehole. As a result of the discharge, high-pressure shock waves propagate through the solids causing them to be destroyed due to mechanical stress. Reflections from acoustic inhomogeneities induce tensile stress in the areas of grain boundaries, inclusions or composite interfaces, causing
the material to break predominantly at these inhomogenities. Consequently, composite materials are fragmented into their components with a high degree of selectivity. In all applications, the solids must be submerged in a dielectric liquid such as water, oil or other organic liquids.

For practical reasons, selFrag uses water as the liquid of choice. Materials exposed to electric voltage exhibit variable breakdown strengths as a function of the pulse rise time, i.e. the time it takes for the high voltage to reach its peak. An HV discharge occurs first in the material with the lowest breakdown strengths. For example, when the pulse rise time is less than 500 ns, the breakdown strength of water exceeds that of most classical nonconducting solids such as ceramics, glass, and many minerals, so the discharge occurs in the solids first.

Selective Fragmentation offers key benefits as compared to mechanical methods:

• Predominant fracturing along grain interfaces
• Liberation of morphologically intact minerals or micro-fossils
• Recovery of mono-mineral fragments
• Very clean surfaces of liberated minerals
• Minimal damage to liberated target specimens
• High yield of available target specimens
• Preservation of natural particle size distribution
• Narrow range of particle size distribution and choice of mean value
• Single step processing - from rock to micro-grain
• Only a few minutes to process one kilogramme of sample
• Semiautomatic programmable processing
• Very low production of undesired fines below 50 µm
• No dust production
• No (cross-) contamination of samples
• No moving parts, so virtually no wear
• Easy cleaning - simply scrub and rinse
• Higher speed and quality of downstream analytical process steps

The selFrag process opens up new possibilities for analyzing mineral samples in
scientific and materials testing applications.

Fragmentation using high-voltage (HV) discharge

Historical background

The use of high voltage (HV) pulsed power technology for fragmentation dates back some 30 – 40 years. Russian scientists were the first to conduct systematic research in this field. In 1995, the German Karlsruhe Research Centre (Forschungszentrum Karlsruhe, FZK) embarked on a far-reaching R&D program to explore possible industrial applications for selective fragmentation.

Since then, the FZK has built several specially designed pilot plants and has investigated the fragmentation of many different materials in the fields of mineralogy, raw materials and composites. FZK has demonstrated the advantages of this method as compared to mechanical fragmentation, and the great potential that exists for the commercialisation of this technology. selFrag is now making this patented technology commercially available under a worldwide licence from FZK.