Time to settle is your friend. Super fine particles remain suspended for hours. One solution is to have a drain a few inches up one side and a pump to transfer the water to another identical drum. Pour at the end of the day and let the fines settle overnight. Next morning pump the water off into the second drum so it is ready for the next batch and filter the solution from the night before's run to collect the atomized alloy.
In Europe I've seen the collection drum made as described above but made of titanium. This eliminates the filter step as the aqua regia is added to the titanium vessel for dissolving. The remaining water is not an issue because it dilutes the Hydrochloric Acid closer to it's azeotrope concentration resulting in less fuming.
