Double layer conveyor belts

Columbus Group chief executive Donald Yates gives insight into how to improve the performance of conveyor belts in a way that may not have previously been considered.

Sometimes, making more of the small gains can really add up, particularly when questioning the long and established norms of how things can be done.

Moving ores around your mine site are often the tasks of thick, robust conveyor belts. But should this paradigm be put on the R&D test table?

Making things thicker will usually make things stronger. On the other hand, two layers separated by appropriate spacers is a well-accepted strong and reduced weight practice in a whole range of applications.

And the same double layer practical design start point can be applied to all those conveyor belt installations.

The gains can be overall lighter weights so less energy is used to drive them, but it can go further with other incorporated features like controlled dynamic sections that act like built in shock absorbers at the ore loading point, and even dust control along the whole length of the belt.

In looking at the whole life of a double layer conveyor belt, it can start onsite by being 3D printed with all the incorporated features in extra-long life auxetic (bullet resistant) materials and SMA (shape memory alloys) for active intervention control of the belt itself while running.

No longer should conveyor belts be just a passive element in the ore handling integrated processing works.

Then looking at the end-of-life of a double layer conveyor belt, the product package has just been recognised with a 2019 Sustainability Awards’ Highly Commended prize in waste elimination because these particular conveyor belts can be applied to growing meadows of seagrass that can generate substantial blue carbon credits.

Surely this is better that burying the worn belts on a mine site in the hope they will be forgotten when another purpose is readily at hand.

Pick and peel away liners and lifter bars

One aspect not forgotten is the AG and SAG mills are often fed by those conveyor belts. Typically, three or four times a year, the mill processing area near these mills goes silent because the same mills have been shutdown for a changeout of the internal liners and lifter bars.

Specialist crews arrive onsite and set about doing an internal rebuild of the mill, in an around-the-clock effort to restore the performance of the mill while minimalising the time of the mill being offline.

And when such SAG and AG mills might be generating a range of clear mining net activity profits between $5000 and $10,000 per hour, for every hour, then the shutdown of some days, is having a real detrimental effect on profit.

For example, three days or 72 hours of no mill activity, could be a loss of almost $400,000, which you cannot catch up. For the bigger mills, with say four complete changeouts per year, this is getting serious, with a loss in the order of $3 million per year just for the poor practices related to changeouts.

Now before saying that is an acceptable situation, it is possible to adopt an external, practical technology called ‘pick and peel away’ liners and lifter bars.

To restore the height difference between liners and the lifter bars, so achieving the same performance gains of a full changeout of the consumables within a SAG or AG mill. To do this in a theoretical perspective, the worn top and leading edge of a lifter bar are removed.

Similarly, the relative height of the mill liners, between the lifter bars, all bolted to the rotating shell, have to be reduced in height, so the relative lifter bar profiles and heights with respect to each other are restored.

Now to undertake this proposed ‘refreshing’ of the liners and lifter bars, it then becomes a simple process to just ‘remove’ a layer, which can be achieved externally just by undoing certain progressively shorter retaining bolts so that the no longer required, worn lifter bar layers and liners drop into the mill and breakup.

There is no need to enter the mill. There is no need to add new liners and lifter bars on such a regular basis as in the old changeout methods.

In standard practice, the internal liners and lifters are replaced in a procedure that causes a loss of some 12 days per year, and the new profile sets are only available for the purpose of grinding every three months before wearing down and performance is lost again.

With the proposed replacement of liner and lifter bar profiles, these sets of liners and lifter bars in sets of three are preloaded twice a year.

Then every two months, one set is dropped away exposing a refreshed liner and lifter bar combination inside the SAG or AG mill, without entering the mill, just by externally and progressively changing to shorter retaining bolts.

The bottom line savings are only two full changeouts per year, not four, with simple ‘refreshing of the profiles every couple of months. The bottom line gains are some 120 hours of extra profitable production per year from a SAG or AG mill fitted out with appropriately designed liners and lifter bars than do not have to be over-engineered to last longer in such a tough environment.

The ‘Pick and Peel Away or PaPa’ liner and lifter bar system can be designed for rubber only, rubber and steel and steel only consumables, in most preferred profiles.

Applying 3D printers to AG & SAG mills

It would seem that 3D printers are being called on to do so much more these days, and the production of liners and lifter bars for AG and SAG mills is no exception.

However, instead of the predictable individual production of replacement consumables that are manually installed on a regular basis changeout shutdown, to get the real benefits of 3D printers, now is the time to apply them in printing the liners and lifter bars, inside the AG or SAG mill in one print run.

The technologies are at the leading edge of 3D printing strategies. Multiple printers work together in printing very big robust structures in very little time.

It starts by removing all the internal liners and lifter bars plus the standard studs, bolts, nuts and seals.

They are no longer needed. Broken and loose bolts and leaking holes losing product are no longer the big issue. Enter a robust arm carrying the many 3D printers, that stretches from the discharge end, through the shell to the feed end of the main AG or SAG mill core body.

As the mill is inched slowly around, high pressure water jets clean away the milled product just before the structure re-enters the load at the bottom of the mill.

As each section is exposed, it is quickly and autonomously measured to develop a real time ‘map’ of the internal mill’s surface. This is compared to the required internal mill profile and the 3D printing build up reconstruction task is defined.

At this point the robust arm with effectively many hundreds of 3D printing composite overlay heads, with adjusting depth of print capabilities, puts down the appropriate liner and lifter bar profile. The composite overlay 3D printing bonds through the original holes in the AG or SAG mill shell (where the bolting structures were originally), and builds up a 3D printed robust overlay, usually of an interconnected structure and a tough long lasting fill nature, to complete the wanted liner or lifter bar profile.

Once done, the AG or SAG mill inches on and the process of clean, measure and then 3D print is repeated. It is as simple as that.

The repeat of the ‘Project 9’ procedure is recommended every three months to ensure that milling performance is maintained. And it is easy to optimise the internal liner and lifter bar profiles, just by changing the specification data base that drives the 3D printers.

Depending on the size of the AG or SAG mill, it is estimated that a gain of 320 or more hours of production per mill is possible.

That’s an amazing two weeks of extra production not lost to shutdowns. And no loose bolts with lost product to grab your attention. And no crews having to enter the mill at any time after the AG or SAG mill is prepared for introduction of the 3D printed system. 3D printing definitely has some serious cost-effective advantages.

Self rebuilding long life chutes

Not to rest on our laurels, the AG and SAG Mill 3D printing can be applied almost singularly to chutes. The chutes are already constructed of dynamic revolving hardened balls to share the wear.

But when wear is a problem, the modules can be easily replaced or a ball repair mechanism added to the underneath of the chute assembly.  Balls detected as showing irregular wear can be resized and rebuilt as needed, online, with no manual intervention.

Similar technology to that used in Project 9 SAG or AG mills rebuilds the surface with appropriate cleaning, measurement and 3D printing, without removal of the complete chute. Indeed, it can still act like a chute during the rebuild process.

This article also appears in the February edition of Australian Mining. 

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