Lessons learned from ball-milling at home

[pcm81]

Recently I've developed interest in ball-milling and below i am posing some of the lessons learned, hoping to provide some guidance to others interested in the topic.

 

This is not meant to be any sort of "ultimate guide" or an "end all how-to" rather a jump-start document.

 

Ball-milling can be used for 2 separate reasons (mixing and milling) and 2 separate approaches are required:

 

In both cases size of the drum determines batch quantity as well as the largest size of the media that can be used. I found that diameter of the media being no bigger than 1/8th (might get away with 1/6th) of the inner diameter of the jar works well.

 

1. Mixing

Ball-milling can me used to mix wet or dry compounds into a homogeneous mixture. The choice of the media is guided by the type of materials being mixed (usually non-sparking media is needed if the mixture is for pyro applications) as well as the diameter of the jar. Media should be large enough to make it easy to separate from the mixture. Media in size range of 1/4 to 1.25 inches works well. Lead or ceramic media works well. Media needs to be more dense than the mixture being mixed. Media is usually significantly bigger (more than 1000x) than particle size of compounds being mixed.

 

2. Milling

For milling applications several factors should be considered. Hardness of the material being milled. Milling media needs to be harder or at-least less fragile than material being milled. Size of the material being milled also plays an important role in choice of the media. Media should be no less than 10x larger than material being milled, however when media is 1,000 or more times bigger than the material being milled the material being milled will tend to stick to itself at the same rate as it is being broken down, hence rendering milling process useless. If the media is less that 10x the size of the material being milled you are risking to wear out the milling media faster than milling the material; also, media will not have enough impact force to crush the material. It is possible to start with say 1 inch media and then add 1/8th inch media when the particle size has been reduced to microscopic scale. Hardened Antimony&Lead alloy media is a popular choice for non-sparking media. Lead without Antimony will not harder after hear treatment. Unlike other metals, lead does not work harden; in fact the opposite result is achieved. Hence lead media needs to be re-hardened after extensive use via heat treatment (quenched). Pure lead will not harden as the result of heat treatment.

 

For the 6" harbor freight ball-mill i found that 3/8th, 1/2, 5/8th and 3/4 inch steel media works well for milling non-combustible compounds (cement). However 1.25 inch media is already too big to be efficient. 1.25 inch media can be used with a very small load of material, to reduce it to grit size that 0.75" media can handle.

 

Depending on the RPM of the drum either only sheering or sheering and impact action of the media on material can be achieved. Drum should rotate fast enough that the ball media is picked up and lifted by the wall to about 10 or 11 o'clock position, then dropping back to 6 o'clock crushing the material. At the same time, material trapped between most exterior layer of the media and layers closer to the center of the jar will also experience shearing force. Turning the jar too slow or too fast will result only in sheering and no crushing force.

 

For specialized milling applications (with high budget >$500) particles down to nano-scale size can be produced using planetary or vibrating high energy ball mills. Regular ballmills like $60 HF or even $200

 

Hope this helps other newbies like me.

[drdecibel]

Please take a look at my post under brass media and tell me what you think.

 

Thanks