Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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How Coarse can Ball Mill Feed be (12 replies)

2 years ago
Gruppen 2 years ago

We are looking at a 1 MTPA comminution circuit that goes from a tertiary crush to a single ball mill (closed on a cyclone cluster) to produce a 100 µm P80 product. We're doing a trade-off study. Closing the tertiary on the screen or leaving it open! (The crushing plant runs 11 h per day, the mill 24 h/d.) The mill power (and media consumption) is higher and the mill grinding length is a bit longer in the open case. Is there any other side-effects associated with feeding a ball mill with 16 kW-h/tonne feed which is coarser (closed P80 of 13.4 mm vs. open P80 of 15.4 mm i.e.2 mm coarser)? Aside from the additional capital and operating costs are there any operational issues associated with feed which is coarse i.e. coarser than the selection function maximum which is at -2 mm.

Bill Rico
2 years ago
Bill Rico 2 years ago

Have you looked at crushing finer? 13.4mm a quite coarse given the low throughput. With a triple deck screen you should be able to get to sub 8-10mm surely? Unless this stuff is crazy hard/abrasive.

High scatting rate would be the primary concern I think, and as the crusher liners wear, the situation will get worse and your throughput rate/grind size will vary. Wouldn't recommend it if this is going to be an issue.

2 years ago
Gruppen 2 years ago

What we're looking at is a jaw, open circuit secondary cone and an open circuit tertiary. Screens before the cones if the capacity is a problem but no recycle on the secondary or tertiary. The idea is the very cheapest get-into-operation solution. The cost of the recycle conveyors and larger screens is being avoided (and being transferred to mill op costs). What I really am interested in is whether the scatting problem is a definite or a maybe if the Bond Ball Mill Work Index were 16 kW-h/tonne? Is it a show stopper or a just more work for the FEL? What % of the feed might end up in the scats bunker? Yes the p80 being presented to the ball mill could be less, 8-10 mm as you mentioned but I'm looking at this coarse transfer size case.

Maya Rothman
2 years ago
Maya Rothman 2 years ago

If feed coarser than the selection function maximum which is at - 2 mm and ball mill is biggest, it is possible to increase circulating load to get an acceptable product P80 for ball mill. Need to check the pumps and hydro-cyclones to increase the circulating load. It is also possible to increase the number of balls of larger diameter in the mill, if the inverter is slightly increased speed.

You can also slightly increase the dilution of the pulp into ball mill.

Kumar Choudhry
2 years ago
Kumar Choudhry 2 years ago

I also agree with increasing the circulation load can assist to achieve the required p80 but do not forget that high circulating load will also lead to lower than targeted throughputs in terms of milling. The mill will be backing up due to high circulating load. Normally you need to determine the bond work index for the material you will be treating. With proper Bond work index data then your desired p80 and Milling throughputs will be easily achieved. Physical characteristics of the ore to be treated is very important and we often look at cyclone efficiency forgetting the Kilowatt hour required to reduce one short tonne of ore of an infinite size to 80% passing 100 microns.

Marshal Meru
2 years ago
Marshal Meru 2 years ago

We had designed a similar Plant to yours using closed-circuit secondary crushing only, to save on costs, and closed-circuit milled with 90mm, accepting a scats "loss"/lower grade reintroduction, as a consequence of preferential grinding. This concentrator was uprated later to series milling when increasing throughput to beyond 2.5x. I am sure that the ball size should improve the selection function peak; however, tertiary crushing/HPGR should give you a much better product than 13mm, still dry screening? Whereas, secondary closed circuit crushing to 16-18mm should be possible?

Alan Carter
2 years ago
Alan Carter 2 years ago

Not clear whether you are using a wet or a dry grinding system if dry, perhaps some grinding aids could do the trick (depends on the material you are grinding) Any more information you can share ?

2 years ago
Gruppen 2 years ago

But it's a wet grinding process. We're making a 3D model of the system and applying value engineering as we go. For instance the secondary cone crusher feed chute, the screen and the discharge chute add 20 m in length (and two trestles) to the conveyor; so we would now cost the screen and additional chutes, conveyor and structural steel against a cone crusher that can handle the volume of the unscreened feed. We're trying to find the lowest capital comminution system for 1 MTPA. Almost regardless of operating costs!

Marshal Dienes
2 years ago
Marshal Dienes 2 years ago

At the top size you are dealing with crushers are much cheaper to run than ball mills. There are other things to consider. That is hard ore. Your ball size needs to be matched to the ore size. If it turns out you need 4 inch (100 mm) balls to handle the scats your mill liners will last a much shorter time than if you need 2 inch (50 mm) balls. In fact with rubber liners, the 4 inch balls will beat the living daylights out of the liners.

We're working with a plant where exactly what you describe for a crusher circuit going into a ball mill with hard ore is going on. If the crusher gaps or liner wear get out of hand scats can be 10 to 20% of new feed. Unless there is some preferential grinding phenomenon going on and you can throw away the scats, that high rate is not to your advantage.

Don't even think about crushing scats from a ball mill. There is so much metal (ball chips) in there the metal detection system is bypassing the crusher all the time. Of course if your ore isn't magnetic, that isn't a problem. Tramp magnets will pick the worst of the metal.

Finally don't forget to watch what kind of crushers you buy. Some are optimized for aggregate and tend to make a coarse product with a minimum of fines and some are optimized for mining and make a finer product with lots of included fines. You want the latter. The manufacturer may not tell you which is which.

2 years ago
Gruppen 2 years ago

I spread-sheeted the Bond new ball size formula and varied F80 and WI. To me hard is WI>18. So it seems the predicted size should be less than 4". For a 16' diameter ball mill do you have an opinion on what new ball size would be half-way liveable in terms of liner life?

Tony Verdeschi
2 years ago
Tony Verdeschi 2 years ago

A quick check of the Rowland optimum feed size for this ball mill is between 3 and 4 mm; operating either circuit you describe will be at least 15% inefficient compared a more conventional ball mill feed size. The screen doesn't matter in terms of energy efficiency; only more crushing (or a small rod mill) that will improve the energy efficiency. I doubt the opex benefit of energy efficiency will matter given the capex needed. Ball size sounds right; I get 3.5 inch top size.

Sugar Watkins
2 years ago
Sugar Watkins 2 years ago

I note the question is -6 months old, but there have been some really interesting recent comments.
I agree that anything with a BWI>18kWh/t is a hard ore, you mention a WI of -16kWh/t - so I am assuming for my comments that the ore is of medium competency. You also mention a crusher P80 (closed) of 13.4mm or a P80 (open) of 15.4mm. My rough calculation with the above (using the open case) agrees with your ~16' (5.3m) mill size, installed power around 2.5MW.

Feeding >15mm to a ball mill this size is not typical - much larger ball mills (>21' or 7.0m diameter) typically get 12-16mm feed (from what I have seen in my experience)

With regards to 3.5"- 4" media, typically for Ball mills of this size (16'), max size of ball used should be around the 50-60mm. I feel that a 4” ball in a 16’ Ball Mill is too large and more suited to SAG Milling. You risk damaging liners (rubber, composite or full steel) as well as trommels and feed chutes in the process. To ensure the final grind, extra EGL in the mill may be required as you rightly point out.

You asked if there are any untoward effects in feeding larger crusher P80 to the Ball mill. The issues I see are:


If the EGL is not long enough, you will not get the residence time and hence the grind you are after,
The hydro-cyclones will have a hard time if set for the desired P80
Increased re-circulating load
Larger than normal feed spouts needed to handle the coarser feed
High wear on trammel screens


High wear on your sump pumps will wear faster
If using 3.5-4" media - high wear on liners and a high percentage of ball scats / broken balls.

Another option may to consider scrapping the tertiary, and perhaps even secondary crushers (depending on Primary crusher CSS) plus screening / scalping capacities) and using HPGR as secondary crushing / primary grinding before feeding to the Ball mill. There are several iron ore flowsheets in operation that run Primary-Secondary-HPGR (dry), then wet for Ball and Regrind milling before going to product and tails.

2 years ago
Gruppen 2 years ago

By the way this project has been stalled due to tenure issues but the challenge (minimum cost of getting into production, even symbolically) is still worthy of consideration. The design can allow for retrofit of closed-circuit crushing, or quaternaries, HPGR, rods or even SABC but these must be paid for by sold Dore. He has suggested that squeezing down the CSS (P80 9 mm) and having a 4 mm trommel where the oversize could be re-processed (I would say parked) might be do-able at WI 16. One assumption in all this is that a ball mill is available which has lots of grinding length, and even if this were not the case a certain pre- or post-ball mill oversize could be parked providing the capital for parking was less than closing the tertiary or some other solution. The other project setting criterion is that the maintenance and operation should be suited to a remote, let's say island, location.

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