1. The wall of the milling jars and balls should be harder (or more tough) than the milled powders to restrict the contamination from the wall and balls. Considering the contamination of stainless steel balls, we suggest zirconia balls.
2. About the working time, that depends on the start grain size, hardness, and the final fineness you want to achieve.
3. Solvent: Normally use water or ethanol (only with materials susceptible to easy hydration). Here we can use water, because al2o3 is so stable material against chemicals.
For this type of experiments, generally trial and error approach is used.
Below is our test experience for reference,
Materials: Alumina particles 2~3mm, use ethanol as solvent
Alumina jars+zirconia balls
Rotation speed: 400rpm
Grinding time: 6hrs
Discharging granularity: averange 4.3 micron