Keeping an ear out for success

The novel application of the acoustic emissions technique is providing industry with a mechanism to monitor machine noise in real time and to maximise efficiency. Maryrose Cuskelly reports.

Photograph of acoustic analyser, which looks like a round metal disc, attached to a metal drum

Process productivity article from resourceful: Issue 6, March 2014

The acoustic emissions analyser, developed by CSIRO, monitors passive sound waves generated in mineral processing.

Installed by Xstrata Technology on the fine grinding mill at the Ernest Henry mine in north Queensland, the technology allows for real-time monitoring of the process, meaning reduced downtime and maximised efficiency.

Lindsay Clark, General Manager for Mineral Processing at Xstrata Technology, remembers when plant operators literally put their ears to the side of an IsaMill to aurally monitor its performance.

The sounds coming from the mill’s chambers, he says, gave an indication of the distribution of grinding media and how efficiently the mill was working.

Given this practice, it’s not surprising that Xstrata Technology’s interest was piqued when it learnt that CSIRO was investigating the link between acoustic waves generated in mineral processing and machine performance.

‘Once we learned that CSIRO had technology that could pick up the sound waves,’ he says, ‘we thought we could put it together to determine the media levels inside the mill.’

Xstrata Technology has now been working in partnership with CSIRO to develop the acoustic emissions analyser for almost three years and commercialisation of the technology is close.

Dr Steven Spencer is a research scientist and project leader who has been working on the analyser since 2006, investigating the links between acoustic waves, grinding media and slurry characteristics, and particle size distribution in grinding mills.

The effectiveness of a mill, he says, is dependent on the even distribution of grinding material.

‘The analyser sits on the mill’s surface and monitors the passive acoustic waves generated by the grinding media – usually sand or ceramic pebbles – when it is thrown towards the outside edge of the mill by the action of the turning rotor,’ Dr Spencer says.

It is able to cut through the noise generated by the mill to pick up sound the grinding media is making within the machine, rather than the sound of the machine itself, to diagnose the mill’s performance.

‘There are eight different grinding chambers along the length of the mill and we have a sensor over each of those and we are able to pick up the acoustics coming off each one. It tells us basically how many grinding media are in each of those chambers,’ he says.

‘The second thing is that we’re able to pick up if any of the stirring discs are wearing down.’

Any change in the state of the mill’s moving parts will change the characteristics of the acoustics that the analyser picks up and allows for problems to be rapidly diagnosed and rectified. The technology can also shed light on the slurry within the mill, providing data on the density and size of the particles. The analysis happens online and in real time with output on a second-to-second basis.

‘Essentially something is tapping on the inside of the mill and we’re listening on the outside,’ Dr Spencer says.

Catherine Jackson, project leader in acoustics at CSIRO, says the advantage of acoustic monitoring is that it’s non-intrusive and, because the analyser is mounted on the exterior of the mill, it’s also easy to install.

‘It doesn’t disturb anything that’s happening in the process or in the mill, but you can still gauge local process conditions,’ she says.

‘It’s also a passive monitoring system, so it’s relatively low cost in comparison with other acoustic methods, because you don’t have to have any source input. It’s based purely on the acoustics of the system.’

Such innovations are vital for the ongoing sustainability of the minerals and mining industry, not least because of falling productivity. Ore grade generally is decreasing, says Dr Spencer, and mineral deposit grain size is also tending to be smaller.

‘Consequently, you’ve got to put more and more energy into grinding it [the ore] into smaller sizes and it costs vast amounts of money,’ he says.

Online monitoring of grinding media load and discharge particle size distributions has the potential to lower grinding energy costs and increase throughput. The decreased maintenance downtime achievable with online machine wear monitoring can substantially improve the efficiency of grinding processes as well.

Both these goals can be achieved through acoustic emissions analysis, potentially having a significant impact on the profitability of mining operations.

‘Even if you only achieved a 5 per cent increase in the grind,’ says Ms Jackson, ‘that would be a substantial saving to any mine site.’

At the Ernest Henry mine in north Queensland, Xstrata Technology is employing the acoustic emissions analyser in milling copper and magnetite ores, however, it has wide application in many industrial processes across most mineral ores.

It is also being used at a platinum mine in South Africa.

Lindsay Clark is confident that the technology will be widely taken up in the minerals and mining industry.

‘We had an IsaMill users meeting in September last year in Cape Town, South Africa,’ he says.

‘We gave a presentation on the acoustic emissions project to all of our users and we will, potentially, gain significant business out of that.’

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