SmartEar technology from Metso Process Technology & Innovation is designed to measure a grinding mill’s performance by analyzing its acoustic emissions. SmartEar helps to prevent damage to mill liners caused by the impact of the balls, and provides an estimate of the ball fill level when used in conjunction with SmartSAG, a dynamic model that performs online estimations for the key parameters of a SAG mill. The SmartEar system includes microphones, a signal transducer, transmission system, and software to process the information. The transduced signal can be transmitted in several ways such as direct transmission through cables, optical fiber, or wirelessly.

The system’s software processes the signal in terms of audible intensity (dB) and frequencies (Hz), and uses three pre-configured filters to monitor the charge of the mill, liner impacts, and abnormal events. SmartEar can operate as a stand-alone system or can interface with any control system.

A paper presented at the 12th Mill Operators’ Conference, sponsored by AusIMM and held in early September at Townsville, Queensland, Australia, described how the Pueblo Viejo gold mine in the Dominican Republic has included SmartEar in its effort to actively improve its mill control strategies since start-up. One of these initiatives was to install an acoustic monitoring system on the SAG mill. After SmartEar was installed in January 2013, data from the system was used to optimize the mill’s performance by minimizing any interval in which the mill is either running too quietly or too loudly.

A mill running ‘too quietly’, according to authors D. La Rosa, D. Shuen, M. Wortley and R. Sales, is an indication that the charge in the mill is too high for efficient breakage or that the mill speed is too low and grinding media are not striking the toe of the charge. Conversely, when the acoustic emissions from the mill are high, this can indicate that the balls are striking above the toe of the charge, resulting in inefficient grinding and excessive liner and lifter wear. Adjusting mill operating conditions to keep acoustic emissions in the ‘sweet spot’ between these limits results in more efficient grinding.

The SmartEar outputs used to measure mill conditions include an impact counter, which measures steel-on-steel impacts on the mill shell. The impact counter uses both frequency and time domain analysis to determine the number of ball impacts per minute. This method of analysis excludes parts of the sound signal not associated with steel-on-steel impacts and gives an accurate measure of this critical control parameter.

After the acoustic system was installed, Pueblo Viejo collected data to determine the upper and lower limits for the impact count variable. The live data are available to the mill operators and used as a key-operating variable to control the mill.

According to the authors, through continuous improvement and the use of advanced technologies like SmartEar, Pueblo Viejo—a joint venture between Barrick (60%) and Goldcorp (40%)—is improving the control and operation of its SAG mill, resulting in reduced liner wear, more efficient mill performance and optimized energy consumption.