Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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Converting a Polymet Mill Liner to Full Steel on SAG Mill (13 replies and 2 comments)

Victor Bergman
2 years ago
Victor Bergman 2 years ago

For a conversion of mill lining from polymet to steel; What effects will there be on power draw, grinding or other operating/performance variables?

I am looking at the possibility of converting a steel/rubber hybrid AKA Polymet lined SAG mill to steel liners. From a cost per item perspective it looks good, however I have limited understanding of how changing the liner will affect the grinding mechanics (if it does).
Does having the harder steel affect the breakage rate?
Does anyone have any information on this?
Does the liner change have a noticeable effect?
Did you see a change in recirculating load or mill weight?

Also Power draw:
The steel liners are heavier so I expect the power draw to increase. Will this be a constant increase when the mill is running?
If my current operating power is 7000kW will my new operating power be say 10% increase all the time or will it be a big increase on start-up and similar operating power to current polymet lined SAG?

Our mill was originally steel lined and we changed to rubber quite a few years ago. In the current economic climate I thought it would be worthwhile investigating steel liners to see if there were any benefits, while still getting the same performance. I am investigating the parameters of cost, wear life, robustness, installation and grinding performance. The next step is modelling

Any information you can share whether theory or experience would be appreciated.

Bob Mathias
2 years ago
Bob Mathias 2 years ago

Victor,

the power draw is a function of the internal diameter of the mill and the mill load conditions. Polymet liners can take up more internal space in the mill making it not available for grinding - so you will see an increase in power draw caused by the larger "open area" in the mill if the steel liners are actually smaller.

Example: a 26' ø × 11' EGL SAG mill with a 6" effective thickness polymet liner might draw 2,917 kW whereas as a 4.5" effective thickness steel liner might draw 2,991 kW. If you know the power draw and effective thickness of your current liners, use the ratio of (diam new / diam old)^2.5 to get a rough estimate the multiplier (for the same charge conditions and speed).

As mentioned by others, the internal geometry affects power draw, and what operators tend to do is draw the same power (as % of motor rating) by adjusting the filling or ball charge. Be aware that as the liner thickness changes, the % of critical speed changes unless you change the motor RPM.

Victor Bergman
2 years ago

The ratio idea for an estimate of power is a good idea. Unforturnately we don't have a VSD so I will have to make sure the models have high confidence.

Tony Verdeschi
2 years ago
Tony Verdeschi 2 years ago

I was reading that the mill in question was originally designed for steel lining - this is not uncommon, as this means structurally the mill is designed for max loading. What this could mean though, is that when fitted with Polymet liners, the bolts used are smaller, so if you now convert back to steel liners, don't forget to think about tooling and handling gear. I also noted a comment about liner weights, you will also have to think about liner handlers to make the reline duration cost effective and timely. Re the process, I also agree that there will be minimal power draw difference, and Polymets do typically run thicker liner plates, therefore a (slightly) smaller ID and hence less power draw. If your SAG is fitted with a Variable Speed Drive (VSD), this is easily compensated for with a small speed change. Motor torque should not really change, mill weight will of course increase, but as the mill was designed for steel liners, the support structure (bearings, gearing etc) should all be OK.

One aspect that I believe is the biggest influence is face angle of the lifter, as this is what effectively determines your charge lift, potential energy and toe position. The liner vendor should be able to model several different face angles to suit.

Also, one other point for your costing is the amount of steel you take out of the mill at a reline. You should be talking to the steel liner vendor about a kg or t off-take agreement, where you get a credit for every kg of unused liner taken out of the mill that is offset against the cost of new liners. The used liners are taken away and simply re-smelted and used in the next batch. That way, you pay only for the kg of wear lining you have actually used.

Also before fitting cast steel liners, please check the backing rubber on the inside of the mill - this may need replacing, as the rubber acts as a cushion for the steel liners, and prevents them from breaking when the bolts are tightened (remember tooling may also need to be considered, as you will need higher tightening loads on the fasteners to keep the liners in place- larger torque wrenches and sockets etc.)

Lastly, you will also need to consider your grinding media hardness / composition, as this could mean the difference between liners lasting their normal wear life or a mill full of shattered liners, especially if the trajectory is not correct (mill speed too high) and all the charge lands outside the "toe". This may also have an effect on the breakage characteristic of the ore, but this would depend on the ore hardness and F80-F100 to a degree...

Victor Bergman
2 years ago

Good point about the tooling and bolts, I checked out the liner handler but not bolt holes. The backing will prob need to be re-done if we do go ahead with steel which I will have to factor in to the cost analysis.

Victor Bergman
2 years ago
Victor Bergman 2 years ago

I do worry about getting the right steel hardness for the liners to prevent shattering.
I am getting current ore characterised to make sure data is up to date for models.

Bill Fraser
2 years ago
Bill Fraser 2 years ago

All the above are good points, but lets not forget safety. Steel liners and lifters weigh considerably more and take a greater deal of effort and time to do a reline. If the crews are not used to the weight for steel be sure to reinforce safe work procedures before the change. Reline time and risk should be considered.

One item for the lifter angle modelling. Be sure the vendor models the angle through the life of the lifter so you understand the impacts on throughput and grind size on start-up and as they wear. If this is modeled well you will understand when it is economical to change them. Too often plants lose tonnes by running worn out lifters than if they shut down and changed them. This is where polymet do well as you change every second row each down. The result is a far more consistent grind and throughput opposed to the high and then low lift you get from combined steel liner/lifters. Low lift conditions results in low throughput and more fines produced. Considering you don't have a VSD, you can't adjust speed to compensate for lifter wear which makes consistent performance harder to achieve. Adding more charge helps throughput, but you will still have more fines.

Helena Russell
2 years ago
Helena Russell 2 years ago

Because of different wear characteristic of rubber vs. steel, power draw will change during the life of liners. If steel liners have longer life then lifter release angle will increase slower than hybrid lifters and the period with max capacity will last longer.
We also are aware that removing scats improves liner life and improves grind and power consumption. In SAG mills pebble ports help with this but also increases recirculating loads. A properly sorted trommel / screen deck should assist here.
Before changing maybe is better to answer: Rubber vs Steel liners, which one is the best option to have a high throughput in SAG Mill?
The advantage of polymet is exactly the opposite that you indicate. Due the complete liner is ligher you can replace this weight for more ore to draw more power if the volume let you that. In other word polymet give you the opportunity to increase production this strategy has been followed by various mine in Chile like Pelambres.

David Kano
2 years ago
David Kano 2 years ago

Another thing to consider is the extension of the current liners life. This can be substantial with the addition of a trunnion or trommel magnet system. On one installation I believe the life of the lifters was extended by nearly two months. (also removal of the ball chips increased the liner life of the classifiers and allowed for a better grinding media size distribution in the ball mill). (no, I don't sell them) If you need more information please feel free to let me know and I will dig through my archives for some of the reports.

Paul Morrow
2 years ago
Paul Morrow 2 years ago

If your mill runs at fixed speed, then the extra weignt of the steel liners will increase its accelerating time. If it is driven by a synchronous motor through an air clutch, then there will be a small increase in clutch wear during acceleration, ( insignificant) There will be no increase in power draw once the clutch is closed. If the mill is driven by a WRIM through a gearbox, It would be wise to review the new accelerating time for the drive train with the capability of the motor, gearbox, rheostat combination. Again once the mill is up to speed, there will be no increase in power draw unless you take other action to increase it.

Bill Fraser
2 years ago
Bill Fraser 2 years ago

Great comments above. If I may add, hybrid liners tend to feature very robust lifters in order to protect the rubber shell plates. This equates to reduced SAG mill power draw capability via the reduction in effective mill diameter. Steel liners (I assume yours will be Cr-Mo steel) do not have these design constraints hence there is more freedom to select the best liner geometry for your application. Liner handling remains an issue of course but if you have a proper liner handling machine plus a good relining crew you are in business.

I hope you will find my comments useful.

Maya Rothman
2 years ago
Maya Rothman 2 years ago

An advantage of polymet (hybrid) lifters is that a harder steel can be used without the danger of cracking due to brittleness. This means that is has a longer life than a pure steel lifter. You need to do an NPV over the entire lifter life to determine which material and profile is best for you. This is typically done with DEM simulations that include the impact of wear (to get the change in throughput as the lifter wears).

John Koenig
2 years ago
John Koenig 2 years ago

There is also the option to have separate Hi lifters and Lo lifters and shell plates for cast steel liners. I have not seen this option on Polymet liners, but I'm sure this can be done. This gives you the flexibility of selective relining - only replacing the liners that are actually worn, saving reline time and money. The old school top-hat liners means that once the lifter (the top part of the hat) is worn, the complete liner is replaced, where the shell liner part (the "brim" of the hat) is still generally OK - this is wasteful. Separate Hi's and Lo's also allow the operators to run different patterns: Hi-Lo-Hi or Hi-Hi-Lo etc. all Hi's or all Lo's. We have put this system in large SAG's, but I m sure this is applicable in most other sizes of mills as well.

Preening of cast steel liners is well known by most liner suppliers and in MU experience not a real issue with CrMo liners. Typically now rubber infills are used to fill the gaps between liners, and it is rare with current casting technology that castings are oversize. Also correct liner layout is essential, so consulting with the mill OEM and / or designers and the liner vendors is recommended.

David Kano
2 years ago
David Kano 2 years ago

Great comments from all.
Please also note that depending on the chemistry and metallurgy of the liners and mill ball sze, there could be peening of liners. If the space between liners if not adequately sized, the inter-liner gaps may be closed, resulting in; (a) the liners being very difficult to remove. This translates to very harsh and long reline periods.

Mill reline downtime could have very significant impact on your NPV. Removing of peened liners is more tedious if the liners have to be removed prematurely. (b) If the mill has flanges along the shell, the peened liners may exert excessive force on flange bolts. This may not be a problem since the mill was designed for steel liners, but may, depending on the age of the mill and the maintenence philosophy for the mill flanges, need to be looked at. Mention has been made of appropiate tooling. Appropiate torquing of liners is also very critical. There is need to have an appropiate torrquing and retorqing regime to avoid looseness and premature breakage of the liners.

Victor, there are many of good comments here, just one thing to add with regards to your requirement to reline the mill with new rubber backing, this is a big job and their are some options on how to go about doing so. Aside from the obvious option if relining the complete mill backing at once, you can also consider having your steel liner supplier securing the rubber backing right to the liner fit surface itself. This will allow for time savings during the reline which I assume would be a larger than average reline for your site.

David
2 years ago
David 2 years ago
David
2 years ago
David 2 years ago

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