Some common misconceptions about mechanical seals 11-3 Two hard faces are a sensible choice if there are dirt or solids in the product you are pumping. Dirt and solids cannot penetrate these faces unless they open. The trick to sealing solids and slurry is to keep the lapped seal faces together.
Dual seals are a good choice for a slurry application.
Putting a clean liquid between two seals is not going to stop solids from clogging the inner seal. Since the barrier fluid is at a higher pressure than the stuffing box pressure you will probably end up diluting your product. Putting the seal outside the stuffing box can keep the springs and other parts from clogging in an abrasive slurry. As the seal faces wear the seal is going to have to move into the slurry that will restrict its movement. You should not use ceramic seal faces in a mechanical seal.