Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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Low Ball Charge, Coarse Grind (8 replies)

12 months ago
antz001 12 months ago

If one has a low ball charge in a ball mill, what advice would you suggest? We do not add steel balls to the mill, thus the grind is too coarse. I am responsible for the metallurgical plant. The people above me just wants "tons"! I know what to do! I would reduce tonnage to assist the mill with grinding in order to improve the grind. It is a gold plant. You require 80% -75um. The grind is currently at 40-45% -75um. The CIL Tanks are sanded up. (i.e. 6 Tanks - 3 online/3 offline) Please provide any suggestions? The tonnage is 105-110 tph.

The Mill Dimensions are as follows:

Primary Mill: 5,79m IS x 7,35m EGL
Motor size: 2,35 MW 
Currently using a 100mm ball

The Bond Ball Mill work index is: 18,7kWh/t (hard rock blend)

The Plant age is: 13 years old

The plant has not been maintained, thus equipment failure is a major problem.

1) Mill 0
Voltage: 6,600 volt
Ampere: 292 ampere (full load capacity)
Power Factor: 0,70
Note: It was advised not to operate the motor more than 75% of its design capacity - 220 ampere

2) Mill 1
Voltage: 11,000 volt
Ampere: 154 ampere (full load capacity)
Power factor: 0,70

Note: It was advised not to operate the motor at more than 75% of its design capacity - 115 ampere

Mill 0 is the primary mill (i.e. feed to the mill)

The discharge of Mill 0 reports to a common mill feed sump.

The cyclone feed pump, pumps to a cluster cyclone, the underflow reports to Mill 1. The overflow (i.e. product) reports to a Linear Trash Screen, underflow reports to a Thickener (1 and 2). Underflow from the Thickener feeds CIL Tanks 0/1/2/3/4/5.

Alex Doll
12 months ago
Alex Doll 12 months ago
5 likes by Jim, Maya Rothman and Bill Fraser and 2 more...

Assuming you have the ball mill in closed-circuit with a classifier, then you need to both adjust the classifier settings (eg. hydrocyclone outlet diameters) and boost the grinding power draw from the ball mill.  

The main ways to increase the power draw from a ball mill are to increase the level in the mill (add balls) or to increase the mill speed (can be done either with a variable speed drive or by changing the mill pinion to one with a different number of teeth).

Checking Bond's ball size equation suggests the 100 mm ball is too coarse.  Assuming your feed ore is 4 mm nominal F80, then you should be adding between 45 mm and 50 mm balls.  Too coarse a ball results in lower efficiency and a coarser product.

Hope this helps.

Alex Doll
12 months ago
Alex Doll 12 months ago

Do these mills have one motor each or two motors (twin-pinions)?  A single 2.35 MW motor is much too small for a mill of those dimensions.

12 months ago
antz001 12 months ago
1 like by David

Each Mill has one motor and pinion

The current power draw from each Mill is as follows:

  1. Mill # 0 - 196 ampere
  2. Mill # 1 - 87 ampere
Alex Doll
12 months ago
Alex Doll 12 months ago

Ok, so the power draw in the mill #0 is 1568 kW (=6.6 kV × 196 A × √3 × p.f.), and I've assumed it is a SAG mill where the ball charge is less than the total mill filling.

and the power in mill #1 is 1160 kW (=11 kV × 87 A × √3 × p.f.) which I've assumed is a ball mill of the same 5.8×7.35m dimensions and where the ball charge is equal to the total mill filling.

You can check the equation is correct by plugging in the rated Amps and you should get the rated power of each motor (#0 rated at 292 Amps = 2337 kW, some minor round-off).

These motors are too small for these mills.  I've modelled your mill#0 in and made some assumptions (5% v/v ball charge, 13.45 RPM = 75% of critical speed) and I get that your 75% operating limit (the purple line) restricts you to operating at or below 15% v/v filling in the mill.  The way to improve this power draw is to fix whatever is causing this 75% operating limit so that you can raise the filling (and the power draw).

The power split between your SAG mill (#0) and ball mill (#1) is terrible - I see why you can't achieve the 74 µm P80 target.  To make things operate better requires more power draw on the mill #1, at least 1618 kW (which is 121 Amps).  If you don't achieve 121 Amps in mill #1, then there is no way to improve mill #0 or the overall circuit efficiency.

Sorry to deliver bad news, but you probably already know all this 🙁 .
Craig Lockhart
12 months ago
Craig Lockhart 12 months ago
5 likes by Jim, Maya Rothman and Sugar Watkins and 2 more...

Is your primary mill Fully Autogenous?  Overflow or grate discharge?

Your mills are desperately under-powered and in need of grinding balls.  The secondary mill is only operating at 56% of FLA (not 75%).  I agree with Alex's comments regarding increasing the mill speed, but without media, the mills will not grind at any speed.  Smaller media will help also, as long as you have tight control on the top-size of feed to the secondary mill (slotted grates, screen, trommel).  Smaller media will also increase liner life if your mill is rubber lined.

If your secondary mill is not capable of grinding the feed it is receiving, the circulating load will continue to build until you exceed the capacity of your cyclone apexes and cause them to rope and purge oversize out the top until the excess circulating load is relieved - coarse oversize material will then pass to your trash screen, which only catches the largest material, but the material passing the screen is still extremely coarse for the leach tanks.  High circulation of coarse material will substantially accelerate wear on your cyclones, feed pump, and slurry piping.

On what basis were you advised to operate at 75% of full load?  If it is a motor limitation (e.g. reduced cooling efficiency at high altitude or cut/shorted windings) you may want to investigate increasing the air flow on the motor ventilation system to enable a higher limit.  If there is a structural weakness in the mills such as:  a) Shell erosion - if it cannot be repaired by welding, metal liners can add strength - make sure this is considered in the operating limitation. b) Trunnion damage - consider using rubber or composite mill liners (especially near mill openings) and replace the weight savings with additional media charge.  In any case, you should be looking at all means possible to increase utilization of available grinding horsepower.  Are you limited by power generation or transmission capacity?

If maintenance continues to be a problem, make sure you arm yourself with financial information such that you understand the cost of lost production for each minute the equipment is idle or the feed is shut off.  It will enable you to demonstrate the benefit of spending money where an improvement in reliability and production will be gained.  In this way, maintenance can be viewed as a cash-positive investment rather than simply a "cost".

Feel free to add more detail to your query for more accurate, detailed, and concise responses...

I hope this helps.  Good luck.


12 months ago
antz001 12 months ago

Hello Craig/Alex

The Mills that are installed at the Plant are Balls Mills. They are overflow type mills.

Your advice is greatly appreciated.

You have identified the problems associated with the coarse grind. I agree 100% with your comments.

The Mills need the required ball addition. (i.e. add the correct ball charge).
We are using the incorrect ball size. (i.e. 100 mm) I have been advised by Alex to make use of 45 or 50 mm size balls for finer grind.

My dilemma is that I would like to add steel balls to the Mills to improve the grind but the Management does not seem to understand! All they want is tons!

They purchase a few steel balls every other month (i.e. 30 tons) I am expected to charge the (x2) Mills with this amount of steel balls to obtain the correct grind.

Since I am unable to get sufficient steel balls, practically I know, if I cut the tonnage to the Mills, I would be helping the Mills to grind much more effectively.

When both Mills 0 and 1 are in operation, the desired tonnage is 120 tph.
When only one Mill (either 0 or 1) in operation, the desired tonnage is 80 tph.

Would you be able to advise me at what tonnage I can operate these Mills, either single or both?

The Mills are rubber lined.

I do not want to lower the tonnage that I damage the mill liners.

Waiting patiently for your response and feedback.

Thanking you.

Kind regards,


Craig Lockhart
12 months ago
Craig Lockhart 12 months ago


To run your mills most effectively, the media must be replaced in the mills at the same rate it is consumed, preferably a set amount each day, with occasional adjustments to maintain the power target.  I generally assume ~0.5 lbs/ton for a ball mill with ore of moderate hardness and abrasiveness - your ore is a little on the hard side and probably more abrasive also.  At 1.0 lbs/ton (2 mills), 110 tph and assuming 85% run time, your mills should consume about 40 tons of steel per month.  Forged grinding balls should cost ~$1000/ton delivered, depending on your location and delivery costs, so an additional $10K/month could get you back on track.  If your mills were drawing full (75%) power (an additional ~800 kW) and assuming $0.10/kWh, each operating month would see an additional $54,000 in electricity costs.  $64,000 is ~49 oz Au at $1,300/oz.

Given the state your production is in currently, I would assume you can easily gain 10% throughput (to achieve the 120 tph target), 10% additional recovery (better liberation and getting all 6 leach tanks operating at once), 5% additional run-time, and save tens of thousands $$ a month in unnecessary maintenance and labor costs.  Could these production gains generate an additional 49 oz Au per month?

Given that both mills combined are drawing only a little more than one at FLA, you should probably be limited to 80-90 tph, but still will not be achieving target grind.  Your ball consumption probably won't decrease simply by virtue of reduced throughput either.  Liner damage should not be a problem for overflow mills at reduced throughput (no significant increase in energy of shell impacts).

As Alex suggested, it would probably be best to favor charging the secondary ball mill - it is in closed circuit and will likely give you the most efficient use of grinding media and horsepower.  I recommend 2 tons to Mill (1) for every 1 ton to mill (0).

If you want to try something bold, perhaps you could try loading un-crushed (ROM) feed onto the primary mill (0) feed belt to run it fully autogenous, and charge all of your available balls in the secondary mill.  It will be tricky to run due to overflow discharge, and may have trouble with "media" retention, so be sure to put a screen on the discharge before the pump box to scalp oversize.  This would work better if the mill were grate discharge with metal shell liners.

If you are allowed to share production and ore characteristics data (e.g. grind/recovery curve, head grade, etc.) perhaps you can contact David directly through the website ("Contact" option under "About 911Metallurgy") and he can give you my email address.


12 months ago
David 12 months ago


What size material are in feeding into your first ball mill?

"It was advised not to operate the motor more than 75% of its design capacity"

  • who is "it" that has set that 75% power limit?
  • what technical evidence/proof (written report) have they provided you that this limit is real VS arbitrary?
  • are your mills Overflow or grate discharge?

You need to go after that voodoo 75% power limit. Hire and outside electrical specialist to have that verified and removed. That is a killer holdback for your production.

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