Grinding circuit uptime after a full liner change
The liner change went to plan. The two weeks afterwards did not, and the reason was in the bolt torque log.
A full mill reline is one of the most rehearsed jobs in a concentrator. This one came in under the window, which should have been the end of the story. Nine days later the mill was down again with loose liner bolts in two rows.
What the log showed
Every bolt had been torqued to specification and signed off. What the log did not record was the re-torque after the first run-in period, because the procedure treated re-torque as optional when the initial torque had been verified. On a new liner set, it is not optional. Rubber and composite liners bed in, the stack compresses, and bolts that were correct at zero hours are loose at twenty-four.
The fix was procedural
- Mandatory re-torque at 24 and 72 running hours, with a recorded value rather than a tick
- Torque values specified per row, since the loading is not uniform
- Two spare bolt sets held on site — a single missing bolt should never extend a shutdown
Nothing here is novel. But a procedure that permits the crew to skip a step under time pressure will eventually have that step skipped, and the cost of finding out is a second unplanned shutdown inside a fortnight.
