You have a granular solid with $\mathrm{SG}=4$, which has particle sizes of $300 \mu \mathrm{m}$ and smaller. You want to separate out all of the particles with a diameter of $20 \mu \mathrm{m}$ and smaller by pumping water upward through a slurry of the particles in a column with a diameter of $10 \mathrm{~cm}$. What flow rate is required to ensure that all particles less than $20 \mu \mathrm{m}$ are swept out of the top of the column? If the slurry is fed to the bottom of the column through a vertical tube, what should the diameter of this tube be to ensure that none of the particles settle out in it?