”The Ilmenite particles were synthesized to nano size by high energy ball milling. The Ilmenite particles are brought to nano size by top-down approach. 25 grams of Ilmenite(9.8µm) were placed in 80ml tungsten carbide mixing jar together with 5 tungsten carbide milling balls (10mm diameter) and one tungsten carbide milling ball (15mm diameter); gives a ball to powder weight ratio of 10:1 milling of particles takes place at 400 rpm for 48 hours. Methanol was added into the jar to prevent powder sticking to the balls and walls of the jar. After completion of milling hours the powder is taken out of the jar and it is collected in a vacuum container after methanol is allowed to evaporate.”

http://www.sersc.org/journals/IJBSBT/vol5_no4/21.pdf

Look up 'comminution limit'. You'll probably not be able to do this.  Wet milling is the only possibility.

Which ball mill, how many sample, correct initial size, which nanozie? 100 nm? 900 nm? 10 nm???