Grinding & Classification Circuits

Grinding & Classification Circuits 2017-03-23T09:46:37+00:00
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Scats production in overflow ball mill (13 replies)

4 months ago
mohamedkrafi 4 months ago

I have lot of scats in overflow ball please help if I transformed overflow ball in grate discharge ball mill I will less scats production  or not? (D80=10mm, P80=140 microns)

4 months ago
David 4 months ago

Hi, this is a great topic.

I went thru the same problem of ball mill scats over production last week. I need to ask you, is the 10mm something you saw yourself or something you were told it was? This is important as if you have not seen if yourself, you can not trust it is really 10mm. There should be no scats at 10 mm unless you are using much too small a grinding ball or the mill charge is much too low.

For example, I was told by a client his ball mill feed was P80 = 9mm but when I went to look at the feed belt, I saw this below.  The conveyor belt is 15 mm thick, therefore the rocks on it are more like 20-30mm.

I suppose you have a reverse spiral installed on the trunnion? A trommel? 

Asking a grinding mill is to grind a feed that is coarser than the optimum size of feed swells the Rowland EF4—Oversized Feed and kills the mill's efficiency.

Reasons your ball mill could be scatting heavily:

  • Too large a feed
  • Not enough grinding media
  • Too small a grinding ball (what size is your?)

Sometimes, you solve scatting by charging the ball mill correctly (I saw cases where the mill was up to 15% under power).

Other times, you need to enlarge the ball size diameter ie: using a 75 mm ball instead is 35 mm.

Other times, you must control and reduce the feed size (crushing circuit product).

Therefore, no, changing to a grate discharge ball mill from overflow would not improve the situation. It would only cause the grates to pull with unground oversized rock chips/scats.

I hope others here can complement this answer.

4 months ago
mohamedkrafi 4 months ago

Thank you dear DAVID

The last stages of vibrating screening is 12 mm D80=de to 10 mm measured by exploitation team The ball mill size added: 2/3 is of 100mm and 75 mm 1/3 Actually the initial work index changed and we tried to calculate the operational work index according to a follow-up of the parameters below but I don’t have any conclusions according to this follow-up for an absorbed power and for the same tonnage I find a scats production is different (the feed ore coming the same area)

 D80 = 10 mm   
Minerai Bounhass
DateP80 Overflow  cycloneabsorbed powerScats production T/day% ScatsFeed ball mill T/H




Value for 42 tph

Specific Gravity



Ball Work index(initial)


11.78 is changed after

Feed F100



Feed F80



Product P80 (Overflow cyclone)



Circulation Load



Capacity (Dry solid rate)



type of grinding



Circuit type



Discharge type



Liner type



Ball size



Ball charge weight


Up to 55t

Ball charge by volume

% v/v

Up to 35%

Total charge by volume

% v/v


Percentage solids inside the mill

% w/w


Operating Mill speed



Operating Mill speed

% Critical


Power at Pinion



Percentage Losses at drive (gearbox+Motor)


approx 7

Installed Motor power



4 months ago
David 4 months ago

Thanks for the data. What size rock do you consider 'scat'?

Do you see your problem:
Feed F100µm = 20000
and Power at Pinion = 472 kW (72% of installed).

I do not see mill dimensions so, I can't tell if the mill is large enough.

Difficult getting to 90um with those 100 mm balls.

I bet your cyclone overflow %Solids is pretty low?

4 months ago
mohamedkrafi 4 months ago

Dear David

The specifications this above ( F100= 2000 mm and 472 KW power in Pinion for a word index of 11.78 it is just the contractual data of ball mill supplier.

At present the secondary cruscher is F100=12mm and work index will be the order of 15 kWh/T according to this above data

Diameter ball mill= 3.2m  the length is 4.5 m

thanks lot for your help

Best regards

4 months ago
David 4 months ago


  1. I need the ACTUAL power draw of your mill now (not vender specs).
  2. I need to ask you, is the 10mm something you saw yourself or something you were told it was?
  3. If you say at present the secondary crusher is F100=12mm --> Can you provide a photo of that?
  4. I have asked you what size rock do you consider 'scat'?
  5. What is cyclone overflow %Solids?
  6. I suppose you have a reverse spiral installed on the trunnion? A trommel? 
4 months ago
mohamedkrafi 4 months ago
  • I asked you for the ACTUAL power draw of your mill now (not vender specs).
    in the above table you have the parameters measured of the last three months.
  • I need to ask you, is the 10mm something you saw yourself or something you were told it was?
    Exploitation team gave me this information about D80=10mm I don't see  the photo I will you send you the photos in the next days
  • I have asked you what size rock do you consider 'scat'?
    Between 2mm to 8mm see attached file
  • What is cyclone overflow %Solids?
    33% flotation feed
  • I suppose you have a reverse spiral installed on the trunnion? A trommel?
    yes we have trommelés.jpg
4 months ago
David 4 months ago


I had not captured the absorbed power data were your kW; thank you. 
At an average of 522 kW you are still low of full power it seems to me.

Do you have a photo of the trommel and holes condition? If your trommel has holes smaller than the F80 = you will drown in scats. You should redrill your trommel to 12 mm holes & keep it clean.

After that, take your ball mill density up closer to 80%

+ Your target grind P80 is 140 um?

Furthermore, if the F80 you have is true; the 100mm ball should be eliminated.

I am also inclined to say you should increase your 75mm ball charge closer to 600 kW

4 months ago
mohamedkrafi 4 months ago

After plant start up et after flotation optimisation our target grind is 140 microns instead 90 microns

See attached video about feed and outlet ball mill

Best regards

Bill Fraser
4 months ago
Bill Fraser 4 months ago

Hi think your ball mill trommel has its holes too small. What you are discharging is small and smaller than fresh feed. It should/would go to the cyclone feed pump if your holes were larger. Also, reviewing the conversation above; you should eliminate that 100 mm ball and increase your mill's power-draw. 

4 months ago
David 4 months ago

Mohamed, if yours scats are 8 mm, you need to enlarge your trommel holes to I would say around 15 mm.

Furthermore, your 100 mm ball needs to be eliminated. By using a 75 mm ball instead of 100 mm you will effectively 2X the number of balls you load. Well if you are at 50/50 100 and 75, you will double on 50% therefore a net of 33% in "good hammers". This change should not go unnoticed.

Be sure to increase the grinding media charge to as much as the mill's motor will allow (trunnion diameter dependent).  This should be near 600 kW.

After all that, you will increase the mill operation density to near 80% and monitor.

Do you have a ball mill feed video? 

4 months ago
mohamedkrafi 4 months ago


I think that to increase the aperture to 15 mm knowing that the ore feeding  d100 = 10 mm all the scats  is going not to outside of trammel but  it’s  going to pass towards the pump and the hydro cyclone, the pump of and  hydrocyclone lining  will be destroyed, hydro cyclone  Under flow will be  blocked and we are going to find afterward these scat in the flotation cells 


4 months ago
David 4 months ago

My good Mohamed,

Have you inspected your trommel for holes status? You said above scats of 2mm to 8mm in size are coming out. That makes me think the holes are plugged.

If your cyclone feed pump can not take some 15mm rocks, you have a problem there also. ie: rubber lined casing/impeller? Switch to metal.

You forgot/ignored the full recipe/prescription. That 100 mm ball needs to be eliminated and your low ball charge (mill power draw) must be increased. You must change those first.

Just letting the <15 mm scats back into the mill (larger trommel holes) will not magically make them get ground without more properly sized grinding media.

IF your F100 = 10 mm as you claim and your mill cannot deal with that:
ADD STEEL TO YOUR MILL if you have kW available.

Maybe others have better tips/tricks but, this I what I would do in your situation. It has always worked.

PS: I do not even use trommels on my ball mills.

4 months ago
Khosana 4 months ago

Hi Krafi,

I would suggest that you firstly study your feed PSD vs ball charge size distribution before opting for the alternative. Plus I'm not sure if a grate discharge will give you the same grind as the overflow could give you.

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