This is a few years late as I've only recently started making biochar, so I apologise!
I second John Elliott's solution. I use a powerful stainless steel Electrolux stick blender with a longish shaft. I dump my 12 litres of char out of the retort straight into a 20L bucket half filled with urine, and then pound it for a couple of minutes with a piece of 2x4 to initially break it up into smaller chunks. Then, using the stick blender at an angle, and set to turbo, it soon creates a vortex and the char becomes a very fine slurry which is then poured onto the garden or around plants. The particle size would be around 1 mm and it takes just a minute or two to complete. I use the Back to Eden garden method so the char gets itself to the root zone naturally, with rain, planting activity and worms. Where the bed is empty, I rake it in to a depth of about 100 to 200mm.
The beauty of this method is that there is no dust and the newly made biochar is at its most adsorbant (is this the right word?!). It fairly hisses while it sucks up the liquid. It also ensures that the biochar is fully charged with nitrogen and moisture to kick things off with a bang in the soil. The reason I like to micronise it is because I'm only making biochar on a small scale (12 litre batches), so micronising it gives me the biggest surface area and therefore the most bang for the small volumes I'm producing.
