Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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SAG Mill Liners Breakage (26 replies)

Sandeep Bisht
1 year ago
Sandeep Bisht 1 year ago

What are the reasons and solution for liner breakage? Tell us about effect of changing liner geometry, adding some feature to liners, type of material and metallurgical operation of liners on deceasing break.

It may happen for both of them but since nowadays mostly rubber liners are used in ball mills so the breaking refers to SAG mills also this issue is critical in SAG mills.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

After a brief survey of SAG mill operation, CDI can tell whether we can guarantee to:

Improve ore throughput (>10%),
Liner shell life increase (>20%),
Reduce kW-hours/ton for the liner life cycle to designate P80 vs. existing system.
Eliminate liner breakage

This survey is at no cost to you. 

Standartenfurer
1 year ago
Standartenfurer 1 year ago

Steel liner breakage may have multiple causes, cast defect, design fault, wrong material selection, incorrect operation, etc. Discussing this can make a subject for half a semester at university. In brief, when liners are too hard, have porosities, sitting on uneven surface, or exposed to excessive ball impact, a favorable condition for cracks to occur is created. My colleagues in Brazil are really good in finding solutions for cast liners breakage. In Australia we developed alternative product (grates included) in Rubber, Polymet or Abrasion Resistant steel plate 500HB, even for SAG mills up to 34' diameter. This should be looked at on a case by case analysis; I don't think there is a generic answer to apply everywhere.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

To add to the litany:

Casting War page that cannot be accommodated by mill elastomer or grinding go-no-go ga.
Liner bolt torque is excessive for worn liner sectional modulus
Casting/liner design does not accommodate worn liner strength

Victor Bergman
1 year ago
Victor Bergman 1 year ago

The main factor to avoid liner breakage is to invest in quality and the correct material selection. If you accept low quality for a low price you will for sure pay later the double. Especially the shape and surface correctness of the backside are important to avoid this problem. I know one case where we had to install the liners in mortar because the backside quality was so poor. We are using 600HB rolled steel for slotted plates in cement and raw mills and we had no breakage issues so far. So above recommendation to use 500HB steel is a good option to avoid casting failures etc.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

This time I'm going to agree with him. Would have to know more of your operation to deliver a final opinion? In Bradken have enough experience to help you with your problem?

Sandeep Bisht
1 year ago
Sandeep Bisht 1 year ago

In these days because of larger face angels, overthrowing steel balls may not be the common reason for liner breakage but poor operation and casting issues can cause more damages and by poor operation I meant low charge level and low solid concentration in SAG mills.

Jean Rasczak
1 year ago
Jean Rasczak 1 year ago

I'd put liner failure to one of two causes. Liner Material incorrect for current ball hardness and size (possibly cheap steel and unsuited for any grinding material), OR a bolt failure, this is obvious with hard steel bolts when tops snap off but less obvious with ductile iron bolts that stretch, but as above results from incorrect material selection. As he says invest in quality even when deliveries problems seem unsurmountable, good workmen never blame their tools because they understand what to buy.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

Poor operational control would be the main cause of failure I think. You can be made of the best steel-correct hardness etc. but if you are continually getting bashed without any cushioning you're going to fail and much faster than if you were cushioned.

If you continuously run your mills half charged or emptier than full or under-loaded you will get ball on ball on liner instead of on slurry.

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

Loose liners due to bolt integrity, stretch and failure will result in broken Liners.
One experience I had some years ago was with a Manager who directed the Supervisors to run a variable speed Mill at 100% at all times resulting in broken liners and then he'd complain about the integrity of the steel. This overthrew the balls to impact the liners consistently until the lifters were worn 20%.

My experience has shown me to speed the Mill up as lifters wear. If you can run a mill at 100% speed with new lifters then you will lose efficiency as the lifters wear. Wrong design of lifters, there are many varied designs today, Yes, Ball charge needs to be maintained, densities need to be maintained. As with the whole plant, our job is to wear it out, not break it.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

I know of the opposite condition where the bolt torque was too high. As liner wore down, the liner sectional modulus became too weak and snapped the liners.

We also noted excessive bolt torques promoted by mill suppliers. Our analysis showed liners could be fastened with 50% less torque and still keeps bolt stress within safe Goodman (mean and alternating stress) fatigue range.

Similar conditions occur when stress risers are built into casting geometry, which have propagated cracks and then migrate through bolt hole patterns.

Sandeep Bisht
1 year ago
Sandeep Bisht 1 year ago

The shell liners also can be strengthened by adding some feature like ribs or extra thickness in critical points. About bolts I agree with you. A new Design of a shell liners have been fastened by two bolts while the old one had 3 bolts with the same weight.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

The only reason for the bolt is to keep the plate in place and not to add stress to the plate's material. Some of the suppliers are offering plates with a large free space on the backside in order to offer cheaper due to less weight. But the breakage risk is much higher if the plates are wearing and fastened with super high torque. We are using 4.6 grade bolts only and we do not experience any problems. If you have to buy new liners pay attention to the space on the back it should not be more than 5mm.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Further to my above comments, we do Finite Element Analysis (FEA) of SAG mill lifter designs with designated bolt torque, worn liner patterns, and Discrete Element Modeling (DEM) of granular flow forces imparted on liners, to assure against poor liner or bolt stress capacity.

Informed SAG mill designers understand geometries importance in lifter/liner designs and mill comminution efficiency, power draw, and wear life performance, noting a trend to larger lifters at greater pitch, which can impose higher forces on lifter systems and result in higher wear rates.

Our goal of advanced liner design is to maximize total through-put, and wear life as measured by Total kW-Hours/Ton vs. liner metal mass used in Tons/kg over the liners life.

Most mills are not optimized for the above and have antiquated bolt patterns to prove it. Recall, not many years ago, lifters were pitched at 2 times their diameter in feet (i.e. a 40 ft. diameter SAG mill required 80 lifters). Not so today as milling efficiency has become an important operating measure.

A good DEM codecan demonstrate the ore collision spectra that signifies rock breakage patterns/rates by each rock size, and can simultaneously track the liner wear pattern to offer the statistics of optimal liner geometries with-regards-to power draw, through-put, and service life.

The above comment should also include the liner metallurgy specification and grinding ball impact breakage indices. Improved SAG mill liner design also includes impact and abrasion evaluation with-regards-to hardness, and ductility of liners and balls. Why are some mills designed with 350 Brinell hardness while other mills successfully use 700 Brinell?

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

Slightly away from liner breakage topic! You are obviously an expert in your field. I understand most of your comments.

Excuse my ignorance, all the plants where I have worked 100% of the feed enters the SAG mill ground and later classified via trommels and cyclones,

Would the systems be more efficient if the ore was classified,(scalp or screen) prior to milling, with only the coarse entering the SAG while fines report directly to the ball mills. Surely this would optimize production and reduce wear rate.

Secondly, I believe SAG Mills were introduced as an alternative to tertiary crushing, but now we add tertiary crushers at the other end, we now call them pebble or scat crushers, seems we have done a 360.

Have we shot ourselves in the foot?
On the few occasions we had liner breakage the problem was operator and wear orientated.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Maybe we are getting off liner breakage, but, we are on the milling topic.

Mine operations I know have made such an evaluation and find it is less expensive to pass it through the SAG than build the infra-structure to classify, transfer, and maintain the crusher undersize to ball mill feed. If quite fine, it transports in the SAG at a higher rate through the mill than rock that does not fit the grate size. If the fines content is high and significantly inhibits coarse ore comminution in the SAG, then your argument may be valid.

Recycle crushers are installed to put edges back on the rock that has been ground down to a rounder and more competent product. SAG mills tend to break most rock via attrition. Sharp edges are easier to break than rounded geometries.

Crusher fractures rock, often on a major axis while SAG mill "mill" it bit-by-bit from its edges. Round rock takes many passes to render it to a smaller size by small attrition and abrasion. It is morphologically more competent, but has the same basic strength rating. A primary crusher produces edges and uses much less power to break to smaller size fraction, than a mill, as does secondary crusher.

It takes many more machines and their maintenance to render the size fraction to fine milling. A crusher can take rock down in size by about 6:1 reduction for each stage. A mill does better than 10:1, but with much less power efficiency.

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

Re-circulating load in the crushing and grinding circuits don't seem to get the attention they deserve, to me, re-circulating is no different than double or triple handling and as we all know double handling in mining can be the difference between success and failure. We seem to have a lot of re-circulating load in grinding sections these days.

Also, over the years I've noticed that we don't get rid of the small steel from Mills, square and half balls, small worn ones at the required rate, which adds weight to the mills and wastes power, so many Mets now add balls to the Mill by what the power draw tells them without reading the ore being treated at that particular time. Checking viscosity compared to density for any one ore.

Automation is agreat innovation but still requires experienced Operators to maintain efficiency.

I have been in a couple Mills lately. Overflow type where the ball charge was a meter below the discharge level. All the small scats and rubbish didn't get the opportunity to discharge.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

I have two comments, regarding SAG mills to your conclusions:

Re-circulating load applies "edges" on rocks to allow SAG to break edges off the parent rock. A little academic exercise - calculate the force needed to break a 2 mm slice off a 25 mm sphere. The tangential loading force at the cross-section has a diameter of 13.5 mm or more than half the rock diameter. The fractured piece requires more than 6 times the force than to break a 2 mm x 13.5 mm cross-section of rock.

I argue more balls the better. My argument is that the mill charge breaks most rock by its specific gravity. Higher density increases comminution where higher steel content is key, not ball size. Balls do some cataract damage, but it pales to attrition breakage when the charge direction changes and applies high shear action at the toe region when hydraulic pressure is maximum. However, you must believe in the results of DEM code as I published at SAG 2011. Our simulations can mimic mill throughput, power draw, mill liner wear, and size reduction by particle size. Ball mills are different.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

You appear to referring to re-circulating through a crusher, what about re-circulating load that does not go through a crusher but merely cyclone classification?

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

I assume you are referring to a ball mill, since cyclone discharge recycle normally goes back to ball mill feed. I am less versed in ball mill grinding. Ball mills break rock by compression fracture, while SAG mills predominantly break by shear. Some compression fracture does occur, but, it is not the major comminution mechanism. Cataracting and cascading of balls, onto ore also aid, but, we believe they have a minor role as well.

I am aware of some SAG mills that have a conveyor connecting the trommel overflow back to mill feed. This will not be very effective since the rocks are very rounded and will become a hindrance to SAG production and fill the mill with grate size rocks. They are often called critical size, should be called critical shape. If they had edges at same size the mill would digest them. What amount of force does it take to break a 10x10 mm rock cross-sectional area that has 70 mPA strength?

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

To your question on pre classifying ore prior to SAG or AG milling, just that thing has been proposed in the past. If Essar Steel Minnesota ever gets around to finishing their taconite plant we will have an excellent case to prove whether or not that practice is justified.

Their flowsheet is to screen primary crushed ore at 8 inches (203 mm) The minus ore will be crushed to -1 1/2 inches (38 mm) (below critical size). The plus ore will be handled separately and the two will be stored in separate stockpiles. The feed to their SAG mills will be a mix of the two, the plus as media rock and the minus as rock to be ground.

The ultimate product from their SAG mill will be about 6 Mesh (3.3 mm). Further grinding will be by ball mill after magnetic separation at that size.

The same ore has a history of being processed to similar size in conventional dry and wet SAG mills at Butler Taconite before it closed so there will be a chance for direct comparison of the two flowsheets. The new flowsheet looks very promising on a pilot plant scale.

My understanding of pebble crushing is that there is a population of rock in a SAG/AG mill that is critical sized, too large to be crushed by the largest rocks or balls and too small to do any significant grinding on its own. The crushing takes the pebbles down through this critical size to a point where they can be ground more efficiently instead of being ground by attrition.

In any case, pebble crushing works, though metal in the SAG mill pebbles is a significant problem for crushing. Especially if your ore is also magnetic as ours is.

In 1992 National Steel Pellet Company published a paper at SME that compared a normally fed SAG mill circuit with a fully autogenous circuit equipped with pebble crushing. Their conclusion was that the AG circuit with crushing allowed a feed rate increase of 10% compared to the SAG circuit once the control wrinkles were worked out. This conclusion was on a full size converted mill in a plant with 9 other SAG mill lines. If you have access to Onemine you can download this paper.

Pebble Crushing Test in a SAG Mill Circuit, Healey, D. E., SME, 1992, Preprint No. 92-12

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

I'd like to see the effect of the different lifter profiles have on the charge trajectory.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Any specific details, assuming you are inquiring about SAG. The combinatorics is:

Charge Filling (balls are most important to control peening and ball breaking)
Lifter Pitch
Lifter Height
Mill RPM
Lifter Face Angle
Lifter Root or Fillet Radius
Lifter Top Geometry
Lifter Plate angle
Wear Bars
Influenced by Straight or Spiral End Cones & their geometries
Ball & Ore largest size distribution

What is your objective function beyond trajectories?

Wear Life
Comminution rate
Power Efficiency/tons
Breakage properties for ore size & ball size distributions - break larger or small sizes.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

There are a lot of people out there selling lifters etc that are better than their competitors, lifters such as high, high/low, wide spacing etc. How does a person know which is most suitable (apart from ore type specifics) and at 85% critical which lifter would do what and to what effect.

I am curious as to which lifters is the most effective and why are the others not so? E.G., why would I choose a wide spacing lifter over a high/low lifter?

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

We practiced mill optimization, with in-house DEM code, for about 6 years before selling the technology to a major mill supplier. Men, software, hardware, and written agreement, not to compete in comminution for 5 years, was our penalty. We could model a complete mill with fluid, and product reduction down to a size that quantified production rate as a function of wear morphology, power draw over wear cycle, size reductions achievable, with optimization procedures. The methods were published in SAG 2001 with 4 papers.

Then we were awarded contract to modify 40 ft. SAG in Australia, published in SAG 2006.

Now 12 years later, we are at it again. A new, faster, more comprehensive granular dynamics code, called ROCKY, with wear and double coupled fluid phase. Runs about 300 times faster, have 5 x times more machines. (Sidebar: name ROCKY came from CSIRO division head - planned a techno/commercialcollaboration). New rock breakage model based on JKMRC A*b, t10 protocol.

I guess this is a long way of saying it’s somewhat proprietary. See Sag 2015 - 3 papers.

You pose a simple question that has a hard and complex answer. It is not only a trajectory solution, but, also simultaneous selection of best practice for ore, ball, wear life, power and comminution within the limits of the machine. We can be precise about the trajectory that should land on the toe region and not peen the liner which is a function of fill level, liner shape.

Different mill liner designs can place the ore on the same toe point. Isn't our goal maximizing throughput at minimum cost?

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

Ok, a lot of the mills I have been inside of apart from the natural wear and attrition of the liners and lifters most seem to use one particular lifter which may only differ in lifter face angle. They all seem to have the high lifter. All of them are chasing the same thing- as you say maximum throughput at minimum cost AND good grind size. What would make them decide to change lifters to a high/low configuration or any other, assuming the ore has not changed? Of course the mill is run so as to enable impact to the toe.

Don't Polysius sell/construct HPGR's. Do you think HPGR will play a bigger role in grinding and comminution, even replace mills in the circuit?

Victor Bergman
1 year ago
Victor Bergman 1 year ago

Yes, correct. We have 2 different HPGR's, one for cement applications and the other one for minerals. The main differences are the wear protection (minerals: tungsten carbide studs) and hydraulic pressure. Many of these machines are sold in minerals projects and they have many advantages. The specific power consumption is much lower and the product has a lot of micro fractures that makes further processing in a ball mill quite easy. But talking about HPGR's was not the topic of this discussion.

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