EPD-processing

Electric-pulse disaggregation (EPD) is a mineral separation technique that liberates all mineral grains from any rock irrespective of its lithology or grain-size distribution. Normal mechanical crushing of whole-rock samples is replaced by the rending effect of an explosion, which is produced by applying an electric current from a high-voltage power source. A voltage greater than the 100 kV necessary for the electrical breakdown of rock samples is achieved by using capacitors that are charged in parallel but discharged in series. The sample sits in a water bath and the rapid distribution of electric pulses through the sample leads to explosions, which occurs preferentially along grain boundaries (zones of weakness). As a result, individual, undamaged mineral grains can be recovered in their original shape and form regardless of grain size.

A number of industrially and scientifically important minerals occur in a wide range of rock types as very subordinate (<1%), mostly unevenly distributed components; these are the so-called accessory minerals. Often they are precious metal-bearing phases, such as Au-Ag-tellurides and platinum-group minerals (PGM), or industrially important minerals, such as diamonds and gold, or they may carry rare-earth and/or radioactive elements. Accessory minerals provide much information on their host rock - for example, on its isotopic composition, absolute age and genesis. Apart from the fact that they are sometimes representative or characteristic of a particular type of ore or ore body, accessory minerals may be used as exploration guides and to assist in the study of ore-forming processes.

However, to gain such information it is necessary to recover a concentrate of the accessory minerals. This usually requires a time-consuming process of crushing, in the course of which the recovered accessory mineral fraction may become contaminated by other components or the minerals are broken and their original forms are destroyed.

This technique of mineral liberation quickly releases all mineral grains in their natural size distributions and preserves their original shapes. Further concentration of different grain-size fractions after such treatment can be achieved by gravitational, electromagnetic, flotation or other methods. EPD can be used to release individual mineral phases or aggregates and provides an opportunity for detailed study of their morphology and shape, crystal structure, physical and textural features, and chemical composition.


Technical description of EPD apparatus

The compressive force of normal mechanical crushing is replaced in EPD by the tension that is caused by the rending effect of an explosion. The tension is created by the direct application of an electric current to the rock sample.


Fig. 1.

Generalized circuit diagram of electric-pulse disintegration apparatus showing basic components. Capacitors, C, are charged in parallel and discharged in series. Other electrical components: L, inductance coil; S, spark gap. Sieve size (in water bath, left) is varied according to grain size of sample.

A generalized circuit diagram (Fig. 1) shows the basic components: a high-voltage power source is used as input to a Marx circuit, which consists of a number of pulse capacitors. The capacitors, which are charged in parallel, allow the build-up of a voltage that is much higher than the input voltage, the increase depending on the number of capacitors in the circuit.

The electric discharge is triggered by means of "spark gaps", which, in practice, consist of two steel balls separated by air. When the spark gaps are triggered, the array of capacitors discharges in series. Inductance coils regulate the timing of the discharges. The discharge time for the circuit is approximately 1 ms at 20 kV and 0.1 ms at 40 kV.

The water bath (EPD-chamber) in which the sample is immersed acts as the last capacitor in the system. With the instrument mentioned above, at low voltages (<50 kV) the electrical discharge initially passes through the water bath and the electrons travel around the rock. At >50 kV, however, the rock breaks down electrically. The breakdown is achieved when the electric current increases rapidly in a widening discharge channel that is filled by a high-density plasma with the density of solid material. The plasma exerts a physical pressure on the rock, which results in an explosion. The explosion occurs preferentially along zones of weakness in the solid material (rock) and along the grain boundaries of mineral phases, particularly when the minerals have different electrical conductivities. The discharge moves through the rock along grain boundaries because they present a better conducting path. The product consists of unbroken, individual mineral grains in their original shape and form, regardless of grain size.

Use of the method may cause secondary contamination. Melted electrode material (i.e. from the steel, copper or aluminium used as the electrodes) can be found in the finest grain-size fraction. Less commonly, melted accessory minerals from the sample may appear in the finest fraction, although they are easily identified in scanning-electron microscope (SEM) or electron-microprobe images. Particularly careful attention should be given to this phenomenon when natural mineral parageneses are reconstructed.

    The EPD-crushers that are used by CNT-MC Inc. are assembled from high-voltage equipment of different capacity. In addition, EPD-chambers of different design, materials, and volume are available for crushing.


Examples of application

CNT MC Inc.