The new process was evaluated for sub-micron grinding in the laboratory. In order to satisfy the requirements for an ink-jet application, the target particle size d90 was < 140 nm which requires the use of small media of max. 0.3 mm and high-efficiency grinding equipment in order to reach the target quality with a useful productivity rate.
The lab strived to use 0.1 mm media in a high-performance bead mill operated in re-circulation for this application, which required installation of a 0.05 mm (50 μm) gap size screen as a bead separator.
As a consequence, the pre-dispersion process had to deliver a particle size distribution with a d100 < 50 μm. This goal was achieved by introducing 100 kWh/t at a net productivity rate of 100 kg/h, as shown by laser diffraction and also a wet-sieve test, where no residue was found when passing the slurry through a 50 μm filter bag.
This was confirmed by the subsequent fine grinding which was carried out with 0.1 mm media in re-circulation without any productivity or consistency issues. Fine-grinding required an additional 800 kWh/t to reach the final target of d90 < 140 nm, which for a 15 liter mill equates to a productivity of approx. 35 kg/h.
Another sample of the pre-dispersed slurry was milled with 0.2 mm media under similar conditions. Due to the larger media, this process required 1,200 kWh/t at a rate of 20 kg/h. This showed that the fine-grinding process using 0.1 mm media was 33 percent more energy efficient and 75 percent more productive.