Despite, and sometimes because of their enormous size, autogenous mills can become bottlenecks in mineral processing. Monitoring mill lining wear requires shutting down and, until now, for an engineer to step inside. These workers have had to physically measure the changes inside these three-storey-tall cylinders – a process that is time-consuming, dangerous and inaccurate.
Now, an Australian company called Scanalyse is offering a better way inside. Scanalyse provides cost- and performance-related information about mills using a 3D laser scanner capable of gathering as many as ten million point measurements within a three-minute period. According to Tom Gabardi, the company’s North American regional manager, their MillMapper service was developed six years ago at Australia’s Curtin University. Up to now, operators have relied on a range of methods for checking liner thickness – everything from using throughput-based projections to wooden liner models to ultrasonic gauges – but measuring the wear on mill liners both accurately and comprehensively has been a challenge with them all. This has led to difficulty in knowing exactly when costly liner replacements must occur.
Quicker, safer and more thorough
Time is a major consideration when a mill shutdown can cost a mine well over $50,000 an hour. Gabardi adds that you would normally send a human inside to take measurements, requiring a number of safety steps and extending the downtime.
“You have to get a confined space permit issued, lock out the mill, pull the feed cart, do a visual inspection to make sure no suspended material is going to fall on anyone, and test the atmosphere inside to prepare for human entry into the mill,” he says. These preparatory stages can add up to an hour or more of expensive downtime during a mill inspection, in addition to the mill inspection itself which can be 15 minutes to 30 minutes, according to Gabardi. By contrast, the total time to detach the feed cart from the mill, use a special beam to insert the scanner and complete the scan remotely can be as short as 15 minutes.
Valérye Desbiens, a process engineer at Cliffs Natural Resources’ Bloom Lake iron mine, highlights another disadvantage to conventional techniques: the size of the mill means you simply cannot reach most of the liner panels.
“Maybe we’d check two to six shell liner plates, and I was able to reach only two to four grate liner plates. So we checked less than 10 per cent of the total mill surface,” she says. These are limitations MillMapper does not have. Rather than requiring direct contact with the liner surface, the scanner uses a laser beam to measure its distance from points of the mill’s interior and uses these measurements to calculate liner thickness. Depending on the charge inside the mill, this is about 60 per cent to 80 per cent of the surface.
Desbiens offers an on-site example of the benefits: in July 2011, while still using Bloom Lake’s old inspection method – wood models of the liner plates – she found one crack but did not find the second, which a subsequent MillMapper scan detected. “So the scanner was very helpful for us,” she says.