sulphurbentonite
You do not state flow rates, sp.g., pressures, viscosities, etc.
Looking up sulphur bentonite (which I suspect is the solution) on Google leads me to submit the following.
You know how a set of gears mesh (contact each other). It is the residue of the abrasive material interferring (coheasivly) with that meshing of the gears that can/will produce problems. Wearing away at the gears at what ever speed your operating then finally there are no gears and no pump.
For abrasive material pumps look elsewhere, such as diapharam or parastaltic pumps. Do a google search.
Good Luck
pennpoint