Grinding & Classification Circuits

Grinding & Classification Circuits 2017-03-23T09:46:37+00:00
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Analyzing the Effect of Ball Size on SAG Grinding (8 replies)

Bob Mathias
1 year ago
Bob Mathias 1 year ago

A complex analysis at a pilot plant scale is very hard to duplicate in actual production. I'm not sure you can avoid the trial and error method. The bottom line is the kilowatt hours per ton required to achieve maximum recovery. Several SAG mills have cracked after ball size AND percentage ball charge were increased so best to proceed slowly and change only one factor at a time. Always useful when one is trying to monitor effect of individual parameters.

Most SAG grinding operations currently use ball size between 5.5 and 6 inch maximum size, However, in most cases the use of these ball sizes has not been justified by a phenomenological analysis of the real effect on the grinding process, but rather by trial and error. The analysis of the influence of ball sizes in SAG grinding has not been developed in detail by considering the effect on process parameters inside the mill, grinding efficiency or internal classification. Historically a conventional grinding reference has been made on the empirical procedure developed by Azzaroni (1980). Further to Azzaroni’s methodology, Sepulveda (Sepulveda, 2007) developed the SPEC methodology, incorporating the ball size effect on the grinding efficiency parameters using a population balance model included on Moly-Cop tool spreadsheets.

Bob Mathias
1 year ago
Bob Mathias 1 year ago
Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I have not seen documentation that 6 inch steel does a better job than 5 inch balls (as in lowering the kWh/t to achieve the same grind). I do not believe that it is better for ores needing less than 15 kWh/t in the SAG mill because impact grinding is such a small percentage of the total SAG energy. If there is data available on this please share it. Otherwise we will continue to smash liners and run up costs for no good purpose.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

It's make me happy to see that this is an interesting theme.
I agree there's not a lot of documentation that 6 inch does a better job than 5 inch balls, but i haven't seen any documentation that 5 inch ball does better job than 4 inch balls.
first of all, let's create the environment of the study, we're talking on Copper industry (head grade: 0.8%, met recovery: 85%, throughput: 3000 - 4000 t/h, our main business case is on the throughput more than the recovery).
In this paper (published on IMPC 2014) you can see 3 test (let's clarify that this is just the beginning of the studies) with 3 completely different ores (soft medium and hard) showing that there's an effect mainly on the coarse particles so finally the T80 of the SAG is coarser with 6 inch balls than in 5 inch balls.
Analyzing the breakage function, you could see that the breakage rate increase more in the coarse and decrease in the fine ore (proportionally, the increase is bigger than the decrease), and the main objective in SAG grinding is to obtain the maximum throughput more than the fine product generation, the fine product generation is ball mill labor, finally if the ball mills has no problems then the T80 makes no problem.

As you say, "lowering kWh/t to achieve the same grind", well you can standardize the result to same P80 (T80) using using Bond's equation, and to talk about the liners, as i said on the conference if you change the balls you can't be quiet and you need to change the liners design (mechanical resistance, abrasion resistance, face angle, etc...).

In the last hand, the 6" balls consumption is lower than 5" balls consumption, as the industrial data can show you on the paper.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I prefer fundamentals over simulations and programs. Your point about 5 Vs 4" steel is great. The function of steel is twofold. First to raise the SG of the total charge so it will draw the required power and second to break the big pieces as fast as they enter the mill. If there is no build up of coarse pieces, then 4 inch steel is good. Build up, forces lower tonnage, so only then do you need to increase the size of steel balls used. Most of our benchmark data was obtained using 5 inch steel so that is why it is known to be good. Once it is realized that about 75 to 80 of SAG energy is consumed in abrasion grinding and that less than 25% is consumed in impact breakage it is concluded that large steel may be overused in many plants. In addition, this situation exists in many plants because the SAG mill is too small and the ball mills too big for high efficiency. Making a really coarse SAG product is not an energy efficient path to be on.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

I have studied the Starkey Test Method in University. Anyway, you have to consider in Chile (specifically at Los Bronces) our SAG mill is 22 MW.. so is not small enough.
Then, as i said at IMPC big balls for very fine ore it's bad idea for coarse rocks then it's almost logical. You could see that in the breakage curves on the paper.

Thanks for your comments.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

The 'Starkey' test as known in Chile is really the SPI test done in a 12 inch diam. SAG mill. The second generation of this test is the SAGDesign test, done in a 19.2 inch diam. mill running at plant load and speed. Here the SAG test is followed by a Bond BM Wi on SAG ground ore. This gives great information for design because the SAGDesign SAG test accuracy is within 5% of the true value, based on both repeat tests and plant bench mark data. SAGDesign methodology is patented.

Helena Russell
1 year ago
Helena Russell 1 year ago

The fact is not a lot of research work (not much published as far as I'm aware) has been completed in recent years regarding the effect of ball size. In fact, in my view Chile in general and people such as yourself are leaders in this field. The data you have collected is arguable the most advanced. Arrium (previously Moly-cop) also hold a lot of useful data. Levi and Jamie have the keys to much of this development which I believe is ongoing. There is of course room for further development of the Morrell models to incorporate the effect of ball size relative to measured ore properties and hence advanced breakage rate models. In regards to matching ore properties, the SMC Test database is arguably the most comprehensive database out there that would assist in this development. I'm unaware of anyone who has developed such models.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Very interesting topic here. I missed your presentation at IMPC so I will take a closer look to the paper. It is strange to see opposite direction (increase or decrease ball size) to increase SAG throughput. I am coming from the same underground school of thought of Mr. Starkey - there is more attrition and much less impact in the SAG mill than people thinks. Some people may argue with this but that may explains some strange results that we can observe in some plant trials. I think we need to go back to basic as the Bond formula is suggesting and be careful to jump on fast conclusion during plant trials in this fast pace industry:

1. Feed Size: Ball size depends on the amount of fines/coarse materials going into the mill. There is a compromise to be done and the definition of fines depends of each application.

1. Lifter Profile: Maintenance has win the battle against performance - high and aggressive profile last longer. However, the lifter must be design according to the ball size used. Right now, all lifter profiles have been optimized for 5 in balls. If you use 4 or 6 in balls, you need a different lifter. That is usually not done.

1. Critical Speed: There is a tendency to use speed as a control variable. More speed, more tonnage. However, if you use 4 in balls and do not change the profile at high speed, any DEM will show you that the 4 in balls will be projected on the liner then people stops the trial concluding that 4 in balls is no ood. Does it mean that the 4 in balls can not be used? Therefore, speed might required to be decrease below 75%. 80% shall not be the operating target.

1. Mill Diameter: Operating a 32 ft or a 42 ft mill diameter is not the same. That factor is also into the Bond formula. Trajectory is not the same when falling into the toe of the charge.

1. Filling void: Attrition becomes important the voids are filled with ore with smaller balls which has a greater surface area than larger balls. That will change the apparent density of the charge. That goes along with what Mr Starkey is saying and also, the Molycop Tools. Larger balls and creating impact at the toe of the charge with aggressive lifters prevents that. That was the tendency for the past few years. In addition, we are now pre-crushing the feed to 1-2" but still using 3" grates! Three or four years ago, Mr Starkey crashed stop a mill with that practice: there was no ore in the second half of the mill near the discharge - only balls. Is that efficient?

1. Total Charge: Are you operating at 26% total load? Or maximum absorbed power? We did a crash stop two years ago when the mill was operating at maximum power. The total charge was... 40%! Is that too much? Mr Starkey will say yes (and I agree with him) from a paper he published back in 2003.

1. Ore Hardness: Is your ore soft? Hard? Both? How variable is it? I think that unfortunately, we don't know what is required for hard ore. It was astonishing to see a paper last week where the plant was decreasing the size of the balls with a hard ore (about 15 kWh/t from SAGDesign SAG hardness) to increase the production and decrease the SAG product on a low aspect ratio SAG mill (Ball mill converted to a SAG).

DEM is progressing into that direction now to better understand the grinding mechanism and hope that it will solve a part of those SAG mysteries without waiting another 20 years to understand what is going inside the SAG. We are changing the operating conditions for which the SAG mills were designed for so all I am saying is that with new ways of operating, we may have to change some practices to make it work better.

I will stay in touch with you in the future.

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