# Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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## Estimate ball size change efficiency gains (2 replies)

Charlie
11 months ago
Charlie 11 months ago

Hi,

If I calculated the grinding ball I need on this ball mill grinding circuit to be about 1/2 the diameter of the ball this mill is already using, is there a way to estimate the grinding efficiency gains I could expect of switching to that smaller grinding ball? I calculated the smaller grinding media will provide about 5X greater a grinding surface area. I hope there is a way to approximate what production improvements are ahead.

Thanks

Marshal Dienes
11 months ago
Marshal Dienes 11 months ago
There is no formula for this but the gain is related to two things.
1. An increase in the ratio of grinding surface area per mass of media as diameter decreases.
2. Avoiding excessive scats, meaning that the feed and ball size are a good match.
In terms of motor power draw smaller balls will pull less power for the same charge level as bigger balls. Imagine you have the larger/big balls in a bucket and the small balls in another and you try to hand-stir them.  You can understand which task will require the most power.  The same goes inside the mill i.e. the smaller grinding media consumes low energy (kW).

So you will see a drop in power but throughput will not decrease if you keep the same ball charge level with the smaller balls.  The can be of 2-3% max, which is enormous in the grinding world.

LBERNAL
11 months ago
LBERNAL 11 months ago

- Although most of primary mills are using bigger ball size that they need, I see difficult a drastic change to the half size
- Depending of the feed PSD and hardness probably you need a fraction of bigger balls to crush the coarse mineral coming from tertiary crushing or Sag mill, (typically 25%-35) and the remaining small balls, such scheme generates a balance balls charge and also has greater grinding surface compared when only one big size is fed to the mill. You must to compare the grinding surface existing inside the ball mill not the surface of the feeding balls.
- Speaking of forged steel balls, it is important to consider the hardness: bigger ball has external high hardness but decrease to the centre, then these big balls easily are losing weight and generating scrap instead small balls that are necessary for fine grinding. This is other reason to add two ball size to the mill
- When casted steel balls are used, specially Cr or Ni alloy, the hardness is similar between surface and centre. The wear is kept constant and less scrap is  produced. Still in this case two size ball to feed the mill is better
- One experience: mill feed iron ore – ¼”, ball size 2.5”, the throughput is 185 tph versus feed of two ball size 2” (33%) and 1.5” (67%) and the throughput increased to 200 tph (+8%)

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