# Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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## Equation to calculate grinding mill ball size (3 replies and 1 comment)

sajid
6 months ago
sajid 6 months ago

My grinding mill has length and diameter of 4.57 m. the maximum size of the balls selected is 77 mm, as there are two or three kinds of ball sizes used..how to select the second ball size. are there any formula or equation that i can calculate the ball size.

Alex Doll
6 months ago
Alex Doll 6 months ago
2 likes by Helena Russell and David

Ball size is usually dictated by the size of the feed material you are grinding, and only minimally dependent on the dimensions of the grinding mills.

There are two equations commonly used to estimate the ball size, first is Azzaroni's equation and second is  Bond's equation.  Both predict a single size of ball to recharge an operating mill (there are different equations for the range of ball sizes for the first charge of a new mill).

The two equations are in Jaime Sepulveda's 'MolyCop tools' software as:

d = 6.06 (F80)^0.263 (ρ Wi)^0.4 (N D)^-0.25

d = 1.354 (F80)^0.5 [ρ Wi / (Nc D^0.5)]^⅓

where d is the make-up ball size in mm, F80 is feed size in µm, ρ is ore density (specific gravity) in t/m³, Wi is metric Bond work index for ball milling, N is mill rotation speed in rev/min; Nc is rotation speed as % of critical, and D is the mill diameter in US feet.

You'll get different answers from the two equations; just pick a size between the two and you should be close enough.

sajid
5 months ago

thanks for valuable information. i already calculated it for the first charge. but now i need calculation for the second charge. if you could give me some guideline it would be appreciated.

Maya Rothman
6 months ago
Maya Rothman 6 months ago

To maybe complement Alex's great answer see https://www.scribd.com/document/73445880/Grinding-Ball-Size-Selection

David
6 months ago
David 6 months ago

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