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| CSIRO | SOLVE | Issue 8 | Aug 06 | |
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ARTICLE
 ACOUSTIC ANALYSIS: Good Vibrations By Sylvia Pivali and Rebecca Thyer
Novel technology that deploys active and passive acoustic waves will allow operators of large industrial plants to ‘hear’ how their machinery is performingJust as a mechanic can listen to a car engine to gauge how smoothly it is running, new technology is allowing the operators of large-scale mineral processing plants to similarly ‘tune in’ to their facility’s performance. Of course, it is a lot more complex than adjusting a timing belt or changing a spark plug, and the sounds that count – those that can indicate the performance or level of wear-and-tear and other sources of potential downtime – need ‘ears’ that can detect frequencies well above the human hearing range. The ‘ears’ developed by researchers at CSIRO Minerals are a range of on-line analysers that use active and passive acoustic waves able to cut through the background noise generated by masses of heavy machinery. These analyses are constantly being fine-tuned by CSIRO Minerals’ On-Line Analysis and Control team, which has been working with the minerals and energy industries for more than 20 years. Project leader Michael Millen says that effective on-line analysers are becoming increasingly important tools in the minerals processing industry. “They provide industry with a tool that can rapidly respond to changes in processes or machine condition, and this allows operators to optimise processes, minimise downtime, maximise efficiency…and save money,” he says. Illustrations of how this technology works are the UltraPS and Acoustic Emission (AE) analysers. The UltraPS analyser uses active acoustic waves at ultrasonic frequencies to measure the size distribution of solid particles in mineral slurries. The AE analyser monitors passive acoustic waves, at frequencies in and above the human hearing range, to sense the condition and state of processes within noisy processing machines.
Active waves are generated by the analyser to be injected into slurries or materials, whereas passive acoustic waves are naturally generated within machinery or objects containing moving parts or material by particle impacts, friction, fluid flow and bubble collapse. These elastic waves move through a device and can be sensed outside, much as earthquakes generated inside the earth can be felt on the surface at large distances from the point of origin. Mr Millen says the UltraPS analyser has been developed specifically for use in grinding circuits within minerals processing plants, where ore is transported and processed in slurry form. Grinding is the key to separating valuable minerals from unwanted material. “However, under-grinding results in poor separation and over-grinding results in energy wastage,” Mr Millen explains. “Getting it right is important as a grinding mill is the biggest single energy user in a minerals plant.” The UltraPS analyser measures particle size by sending pulsed acoustic waves through the slurry and monitoring the magnitude and velocity of transmitted sound vibrations. Software algorithms are used to derive particle size distributions from the transmitted acoustic waves. Key features of the analyser include rapid analysis – less than a minute – and an ability to operate in undiluted opaque slurries at very high solids concentration. Although the standard model is working in a number of mineral-processing plants in Australia and South Africa, research is well advanced to develop a multiplexed version of the analyser to monitor multiple slurry streams, Mr Millen says. And a version is also under development for alumina refineries: “The main challenge here is to develop an analyser that can withstand the hostile environment of a refinery, where high temperature and corrosive slurry streams are used.” CSIRO’s AE analyser also monitors acoustic waves to assess processing units’ operation and state of wear. It is being developed for processing applications, including dense medium cyclones – the primary machines used to separate coal from other material in coal preparation plants. The AE analyser monitors acoustic waves generated by particle strikes on the cyclone’s internal walls, picking them up by sensors mounted on the outside shell. CSIRO Minerals researcher Dr Steven Spencer says modern statistical analysis techniques are then applied to extract information such as separation efficiency and the condition (wear and tear) of the cyclone liners. Obtaining this information is fast – less than 10 seconds – and the analyser’s simple robust sensors do not need to be inside the processor being analysed: “It is expected to have wide applications in many industrial processes where noise in and above the audible frequency range is influenced by process and machine condition,” he says. CSIRO is also developing an AE analyser for monitoring process efficiency and wear-and-tear patterns inside rock breakage units, such as autogenous and semi-autogenous grinding mills, the primary grinding units in many ‘hard rock’ mineral-processing plants. The technology has been tested at a number of processing plants, including coal preparation plants, as part of an Australian Coal Association Research Program project. In the next year the team aims to develop a commercial prototype AE analyser for coal preparation plants. Dr Spencer says the ability to cheaply, non-intrusively and continuously monitor key processing variables and wear states in enclosed processing vessels is a significant step forward for the industry. “The potential increase in process throughput and product quality, coupled with a decrease in shutdown maintenance costs, could translate into substantial cost savings.” APPLICATION On-line analysers use acoustic waves to cut through the noise generated by heavy machinery and ‘hear’ how well it is running BENEFIT Inexpensive, non-intrusive and continuous monitoring of the wear state of equipment means increased throughput and decreased downtime For further information contact: |
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