Donhad was commissioned by an Australian client to conduct an audit of the SAG mill reline during a scheduled shutdown.
The scope of work was to replace discharge grates, feed-end liners and shell liners, and re-tension and/or replace pulp lifter bolts. Nothing unusual about that.
During the work, however, some unknown problems were revealed along with some less-than-perfect routine operational procedures.
Torque units and air tools
Donhad forged product specialist Brad McCracken explained, “We were able to nominate some concerns we had relating to torque units and air tools regarding calibration and operation of these tools onsite. Even minor inconsistencies between operators will result in variability in the applied bolt loads.”
“The pulp lifter and grate arrangements are the Achilles’ heel of most large SAG mills. Pulp Lifter bolt failures are common because the calibration and operation of the torque tools can have a significant impact on the longevity of the pulp lifter bolts.”
McCracken said it became apparent that when building liners back into the mill, night shift and day shift were building in a different order.
“Finishing on the discharge end can increase the risk of the bolts not being tensioned correctly due to actual or perceived pressure to inch the mill,” he said.
“Some tweaks to the order in which the liners are built would allow more time to tension the bolts on the discharge end which will improve the bolt tension outcome. Also, the bolts in the kits can be individually colour-coded prior to leaving Donhad’s factory.
“This makes bolt identification on site a lot easier. A chart can be supplied showing sizes and colours. The kits themselves can also be colour-coded for ease of identification.”
McCracken said the sealing washers being used on all of the M48 bolts were found to be an incorrect match for the cup washers.
“If the cup washer does not make metal to metal contact with the mill shell, any impact or movement can cause the bolt to lose tension, fatigue and fail. Having a correctly matching sealing washer will also save time when removing them” McCracken explained.
He said the practice of laying nuts and washers out at the face of the mill prior to assembly offered some further opportunities for improvement.
This removed trip hazards from the work area and improved the quality of the nuts and washers being used during assembly.
“There is a substantial risk of tripping when heavy torqueing equipment is being used and the nuts and washers get covered in mud, water, dust and grit when the holes are water blasted. Grit in the threads or on the face of the washer can contribute to the thread’s binding or loss of tension,” McCracken commented.
Pulp lifter bolts and recess washers
McCracken said most of the washers and bolts which had been brought up from spares were in poor condition and had to be cleaned up prior to being used.
“Even after being cleaned up they were still not ideal. Putting rusty washers and bolts in can create additional friction causing bolts to be under tensioned.”
During Donhad’s routine maintenance and service work, the company constantly watches for opportunities to recommend procedural and process improvement, according to McCracken.
He said shutdowns were a fantastic opportunity for Donhad to “run a microscope” over various reline processes and procedures.
“Because we spend hundreds of hours attending shutdowns at all types of mills, in many different countries, it’s rare that we don’t identify an improvement or two to recommend,” McCracken said.
“And when you consider about 1000 liner bolts per average mill, if we can find how to shave even 30 seconds off each bolt procedure, the overall downtime will be very significantly reduced. This would likely save the client hundreds of thousands of dollars.”
This article also appears in the December 2017 edition of Australian Mining.