usually the first approximation is using the Stokes flow formula, but it is also important to know the size distribution. The size used may be that for which 96% of the particles are smaller, based on experieince with pulverized coal dust transport. A lower velocity will cause the flow to behave as 2-phase flow, with corresponding layout and flow peculiarities at bends.
I seem to recall that there were issues back in the 1960's when there was "missing" radioactive dust at the nuclear reprocessing plants in the USA, later found out to be layout in the transport ducts and also in the ventilaton system ducts. You would do well to contact the agencies and companies that have experience in this particular field. An old reference book on this sort of topic was " Fluidization and fluid-particle systems " by Frederick Zenz

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick