March/April 2014

An electrifying idea

Electrical comminution promises lower power costs and more efficient liberation

By Eavan Moore

A Swiss company aims to send a jolt through the mineral processing sector. Selfrag AG has completed the pilot-scale engineering for an ore pre-treatment system that could lower the power draw of comminution by double-digit percentages. Using electric pulse fragmentation to weaken ore before it enters the milling circuit can speed up the grinding process and improve liberation results, according to research conducted with the Julius Kruttschnitt Minerals Research Centre (JKMRC).

In a study of Selfrag technology to be presented at the International Mineral Processing Congress in Santiago, Chile in October, a major gold producer, JKMRC and Selfrag used JKSimMet software to simulate modifications to an existing gold-copper circuit. Pre-weakening the ore fed to a SAG mill freed up spare capacity in the mill. Results suggested that by adding Selfrag’s technology, two pebble crushers and one ball mill could be removed from the circuit while maintaining the same overall throughput and yielding a five-kilowatt-hour-per-tonne energy reduction.

Underwater lightning

Although not yet commercialized for large-scale comminution in mining applications, the technology has existed for more than a decade. Selfrag’s product development is partly funded by sales of smaller, already commercialized versions used for ore testing, high-purity mineral crushing, and recycling. Both Selfrag and an Ontario-based company called CNT Mineral Consulting employ the same principle of electrical breakdown.

“What we’re doing basically is creating underwater lightning,” says Klaas Peter van der Wielen, mineral processing engineer at Selfrag, “the big difference being that we do it under controlled circumstances into rocks rather than from the clouds to the Earth.”

Selfrag uses an electric pulse generator to blast between 90 kilovolts (kV) and 200 kV into rock immersed in water. Usually the water must be treated to minimize its conductivity, so that both the rock and the water are dielectric, meaning they are not conductive. But when that much electricity is pushed through a dielectric, it causes semi-permanent changes to the atomic structure of the material. This process, called electrical breakdown, occurs in rocks before it does in water under the right electrical conditions, so that the electrical energy is deposited selectively inside the rocks with the water acting as an insulator.

“It literally rips electrons out of their shell, and this creates plasma, which is basically a cloud of free electrons,” explains van der Wielen. As with lightning, the plasma flow traces fractal patterns away from the initiation point to a ground electrode. The rapid heating that occurs in the plasma channels results in very high pressures, creating a shockwave that ultimately fragments the rock. The shockwave itself has effects similar to conventional explosive blasting, according to van der Wielen: a local crushing zone very close to the plasma channel, with radial and circumferential fractures further away and spalling at the edge.

Mineral processing benefits

Happily for miners, the plasma channels stream toward minerals with higher permittivity – the ability to transmit an electric field – which is a quality many high-value ores possess. Sulphide minerals like chalcopyrite and oxide minerals such as hematite and magnetite tend to affect electrical fields very strongly compared to gangue minerals, says van der Wielen.

“At the same time,” he adds, “the stream will also look for the easiest electrons to displace, and along grain boundaries there’s a higher likelihood of having electrons that aren’t quite bound into their atom properly. As well as being a weakness in terms of mechanical fracturing, [the grain boundary] also presents a nice electrical weakness that the stream prefers to travel along.”

Selfrag’s mineral processing benefits fall into two categories. First, the system promotes liberation at coarser fractions in tests run on quartz, graphite, diamond, and base metal ore. The second and better-understood benefit is that the cracks created by the shockwave tend to have significant weakening effects on rock. In Selfrag’s latest research, an energy input of two kilowatt hours per tonne produced a change in weakness of 55 to 125 per cent (as measured in A*b values). “Quite often,” van der Wielen says, “the cracking is so intense that where you had a rock with a compressive strength of something like 150 megapascals [typical for hard rock], after a treatment using one to three kilowatt hours per tonne, often you can actually crush it by hand. There’s no strength remaining whatsoever.”

The simulation results accumulating from Selfrag and its research partners suggest the electrical discharges are most effectively applied to SAG mill feed. This is both because its weakening effect is more pronounced at coarser particle sizes and because the improved grinding efficiency of the SAG mill could reduce or eliminate the need for downstream ball milling.

Noko Phala, principal metallurgist, R&D, AngloGold Ashanti, says that the limited data published so far suggest an energy reduction of 30 per cent could result from pre-weakening. If a 10-million-tonne-per-annum operation had grinding costs of $2 to $6 per tonne, and if power contributed half of that, the mine could save $3 to $9 million each year.

“Of course this excludes any potential scale reoptimizations to take into account the lower cost,” adds Phala. “This is the carrot that keeps us looking for new technologies.”

Piloting the craft

AngloGold Ashanti is watching the technology with interest but has held back from involvement pending further developments. The sign of a breakthrough, in Phala’s view, would be a continuously running pilot plant that realizes the value proposition as well as designs showing that scaling up to the 1,000-tonne-per-hour range is feasible. Such a plant would likely involve adding more discharge electrodes to the system, but that has not been engineered yet.

According to van der Wielen, the pilot plant is ready to go – with the help of a partner. Selfrag has done the engineering for a 10-tonne-per-hour demonstration plant and is now seeking interested suppliers, mining companies or consultancies to help build it.

Meanwhile, JKMRC and Selfrag have developed better ways to measure the relationships between energy input, size reduction, and pre-weakening effects. Their work in progress includes ways to minimize the cost of water treatment or to make the system less sensitive to water quality. “In principle our system can work with pretty dirty water; we have worked at conductivities up to 4,000 micro-Siemens/cm,” adds van der Wielen. “But the process is more efficient at lower process water conductivities so we will likely have a water treatment system alongside our Selfrag system.”

Another key challenge exists in maximizing the probability that every ore particle will receive energy; in the lab, the particles closest to the electrode benefit more than those further away. Van der Wielen also hopes to assess what kind of liberation effects occur at the same energy levels that produce weakening.

Though van der Wielen says large-scale commercialization is planned for 2016 or 2017, that progress will be dependent on finding funding and forming partnerships.

“No one is certain about timescales required to reach breakthroughs,” comments Phala. “All we are certain about is that if something is valuable, and its existence is not limited by the laws of physics, it will ultimately become a reality – and the electric pulse rock weakening technology certainly ticks all those boxes in my view.”

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