Because of the size of mill component pieces, they are often not ideally perfect. This leads to flaw analyses: will the piece with the flaw survive the required life; is it ‘fit for purpose’? The mathematical flaw analyses for mill components are very conservative, and they make basic, theoretical, assumptions which often are only approximations. For example, the exact, detailed, geometry of a flaw may be approximated by some conservative upper bound description. These calculations are used to decide whether a piece may be accepted or should be scrapped, possibly leading to schedule delays, and monetary losses for all parties. Thus, it becomes very interesting to know just how conservative these calculations may be. Yet the people who make these calculations, rarely, if ever, follow this up.
To get an idea of the conservativeness of these flaw evaluation calculations, one needs to follow up on the life of the accepted flawed mill pieces. Often a flaw is calculated as being rejectable per fabrication specifications. However, the flawed piece is still accepted into use, based on ‘additional experience’, extra warranties, having back up pieces, etc. Tracing the operational life of such flawed pieces would provide significant information. However, mining project A&E companies, and their consultants, usually depart right after a mine starts operating. Follow up surveys are also complicated by changes of mine ownership, mine closures, equipment resale/reuse, etc. Thus, one has to make a concerted effort to keep track of such flawed pieces. Otherwise, each flaw calculation starts at ‘ground zero’ again.
I have chosen to track a few flawed pieces arising from my experience. Usually these are castings, however, I have included one interesting case involving shell plate.
Case 1 – 16.5’ diameter ball mill heads with integral trunnions
These are two ball mill heads, made of grey iron, originally for the Carr Forks project in Utah. Care was taken to eliminate shrinkage flaws in the head-to-trunnion knuckle region, but in doing so, the flaw area moved to the conical section of the heads. The castings were radiographed, and showed extensive flaws, such that they were immediately rejectable. However, even if the flaw areas were well beyond level 5 radiography, due to schedules, the heads were accepted for use, with the proviso that a new head casting be kept in stock. The Carr Forks project closed after just a few years of operation, and this mill moved to OK Tedi, in PNG. It has operated there ever since. As of the start of 2019, the mill heads have approximately a 35 year life, and continue with no problems. Using the early radiography rejection criteria, based solely on flaw area magnitude, these were the worst appearing castings in my experience, yet they have proved ‘fit for service’.
Case 2 – 32’ diameter SAG mill heads
The first 32’ mill heads for Mt. Isa had an extremely difficult time in manufacture, for Dominion Engineering. They were an Australian steel foundry’s first attempt at making ductile iron, and the project encountered an extremely high casting rejection rate. Many extra head segment castings were made, with little improvement. Finally, the best components were chosen for use, but these still contained flaws well over 1,000 sq.in. in area. These heads have now operated successfully, about 26 years. One head was replaced, but that was due to slurry erosion, in a completely different location.
Case 3 – 34’ diameter SAG mill heads
In the 1980s some European foundries were producing head segment castings, in ductile iron, to net thicknesses, without machining the head interior surfaces. When these surfaces were inspected, for the Kennecott project, these interior surfaces were found to contain large dross areas. This being one of the first times this phenomenon was encountered, by the Dominion engineers, the castings were put to use. Some of today’s specifications would have rejected all these head segments. Yet these 34’ SAG mill heads have now operated about 30 years, without any consequences. This is especially interesting since the A&E company, which accepted these heads, now has specifications which question/challenge any dross allowances.
Case 4 – 36’ diameter SAG mill heads
The Telfer project also had difficulty obtaining dross-free castings. Dross was found in the conical areas, and on bolting flanges. After much discussion, a ‘stress versus dross string length’ criterion was developed, and most castings were put into service. The bolting flange edges/extremities were cosmetically upgraded using titanium metal putty, which hardened to a suitable strength. Today, these castings have 15 years of operational life with no problems noted.
Case 5 – 40’ diameter SAG shell flange
During the manufacture, in Brazil, of the first 40’ SAG mill, for Cadia, it was discovered that some ingot forged flange plate contained compressed voids in a few spots. One of these spots appeared in a high stress area. This occurred because the ingots were not vacuum degassed, and on some, when the poor area was cropped/cut off, some voids remained, which then did not fuse completely under ingot volume reduction. Thus, flaws appeared as small areas of ‘pepper-like’ magnetic indications. After much discussion, and metallurgical speculation, Svedala made some destructive tests on samples, and demonstrated potential safety in use. This was accepted by the mine owner. Over the course of the 21 years of mill operation, to date, the high stress area was checked several times. At one point, 7 to 8 years into operation, it was noted that the area showed some slight change in the MT indications. Since it was possible that slurry access was occurring, with associated corrosion attack, a small weld buttering/overlay, of about 3 mm cover, was applied. Nothing further has been noted in the 21 years of operation.
Case 6 – 40’ diameter SAG mill heads
This is the most recent example, and actually demonstrates best the value (or loss) of monitoring history. Several Los Bronces head segments had some minor dross indications. However, the A&E company wanted no dross. This is ironic, as this was the same A&E that accepted the Kennecott heads (Case 3). However, the company did not keep performance history records. Also, as the company specifications were typographically cleaned and edited, over the intervening years, the requirement for investigating dross became for rejecting dross, with no consultation with the original group who wrote the specifications. During the ensuing Los Bronces discussions, academic consultants were all negative on the dross, due to theoretical calculations, but none had practical experience. As a compromise, to use the segments, they were cosmetically ‘refurbished’ with titanium metal putty, extended warranties were given, and inspection methods were established. To date, the mills have run for 10 years satisfactorily. As always, the A&E company left after start-up, and has no further history of these castings in operation. Thus, any future project flaw evaluations will continue to have no back-up from experience.
The above cases demonstrate the somewhat unchanging nature of ‘the state of the art’. They also demonstrate the conservativeness of current flaw evaluation methods, which, if not understood, lead to financial losses, and start-up delays. In evaluating flaws, one must differentiate between cosmetic appearance, and fitness for purpose. There are papers available in the mill design literature, that illustrate cosmetic inadequacies, especially of castings, but none, to my knowledge, that accurately tie such inadequacies to subsequent failures in operation.