Ball Mill Loading (dry milling)
When charging a ball mill, ceramic lined mill, pebble mill, jar mill or laboratory jar use on a jar rolling mill it is important to have the correct amount of media and correct amount of product.
Charging a Dry Mill – The general operation of a grinding mill is to have the product impacted between the balls as they tumble. If there are insufficient solids in the mill then the media will strike itself and generate excessive media wear. If too much product then the solid may buffer the impact too much. As with most of the independent variable in ball milling it is best to do some empirical testing and then adjust your parameters. The starting point for ball mill media and solids charging generally starts as follows:
- 50% media charge
- Assuming 26% void space between spherical balls (non-spherical, irregularly shaped and mixed-size media will increase or decrease the free space)
- 50% x 26% = 13% free space
- Add to this another 10%-15% above the ball charge for total of 23% to 25% product loading.
So as a rule of thumb we use 25% solid loading.
Empirical Check: Once the mill has been loaded and run for a few minutes, open the cover and look down into the mill. You should see some discernible layer of product on top of the media. If you can actually see the media then is too little solids charge. Most solids will decrease in volume as the particle size is reduced, so you may have to add more solids and check the solids volume again after a few hours. Once the quantity is finally determined, you can simply load the mill with the total amount of solids during the initial mill charging.
With proper solids loading of about 25% solids (red), the grinding media (gray) will always have product to impact between the pieces of media.
With too low a solids loading the media will stike themselves resuling in ineffective millign and increase media and mill wear.