# Grinding & Classification Circuits

Grinding & Classification Circuits2018-06-17T09:51:34-04:00
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## How to design a Ball Mill (2 replies)

Partha@Mech
3 months ago
Partha@Mech 3 months ago

E= 10 Wi (1/√P80- 1/√F80)

E= Specific Energy, Wi= Work Index,P80= Output,F80= Feed

After this we need to multiply certain factors such wet/dry Factor, Separation Factor, Dia. factor,  Open/Closed Circuit Factor etc..

E= 10 Wi (1/√P80- 1/√F80 * C1*C2*C3*C4*C5*C6

After This we multiply throughput Volume (Q)

Specific Power (P)= QE which gives Total Power consumption (KW).

We assume Critical Speed as 70% of Nominal RPM with 30% of Grinding Charge.

Question1:- How do we determine Nominal RPM optimum for Grinding?

Question2:- How do we determine Dia. Of Mill?

Question3:- How do we determine Length. Of Mill?

Question4:- How do we determine L/D ratio for wet closed Mill with specific throughput?

Question5:- How do we determine the F80 for Grinding Mill so that the P80/P68 is in 106/75 microns?

Question6:- Does Length & Diameter determine Critical Speed/ Nominal RPM?

Alex Doll
3 months ago
Alex Doll 3 months ago

Before replying, a warning is in order:  get an experienced professional opinion before you go buying a mill.  There are a lot of ways that a design like this can go wrong if you don't know what you are doing.  For example, work index is not fixed and will vary depending on the closing mesh that you ran the experiment at.  You also need a rod mill work index to design a ball mill operating on a coarse feed, above about 4 mm.

Q1:  You design for a typical percentage of critical speed, usually 75% of critical. Then you iterate the mill diameter using a Morrell C-model or Nordberg equation to get the RPM that corresponds to 75% for that mill diameter.

Q2:  You first work out the specific energy consumption using the Bond equation you showed, then multiply that by the desired throughput to choose the mill motor.  Then you use a Morrell C-model or Nordberg equation to iterate mill geometries to fit the motor.

Q3:  As per Q2.

Q4:  There are only rules of thumb for this: if you are doing coarse grinding, then you want a shorter mill; if you are doing fine grinding, then you want a longer mill.  Eg. 200 µm product would be D:L of 1:1.5; 75 µm product would be a range between 1:1.8 and 1:2.

Q5:  Ball mill performance suffers with coarse feed, so keep the F80 below 5 mm.  Design your upstream crushers/rod mills to achieve this, if possible.  The coarsest you can run a ball mill on typical ore is about 10 mm before the performance becomes unacceptable.

Q6:  Only diameter.  See https://www.sagmilling.com/tools_millspeed

SmartDog
3 months ago
SmartDog 3 months ago

Note: "We assume Critical Speed as 70% of Nominal RPM" is backwards.  Critical speed is the rpm that cascading stops and the charge is pinned.  So it should read "We assume Nominal RPM as 70% of Critical Speed".  Except you would probably want to run at 75% Critical Speed as that would give a slightly smaller mill.

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