Looking for suggests for an application i am working on. Our process involves using a high viscosity, highly abrasive material in which we control the flow using a pneumatic actuated metal seated ball valve.  Initially we had issues with the valves failing to open (prior to switching to the metal seated valve). We operate at a pressure around 1500 psi and the valve cycles about once per minute.  The issue that we are having now are leaks occuring at the stem.
Metal-seated ball valves such as VTI or Mogas are frequently used successfully in applications such as this.  Ceramic valves >EXIST< for abrasive services. 
You didn't say how hot the application is.  If it's not too hot, and o-ring-sealed bushing below the packing helps to keep the grit out of the packing. 
Thw pneumatic actuator is good because it probably does not put a side-load on the valve stem. Rack and pinion wont side-load the stem, nor will a scotch yoke.  Make sure your mounting bracket and coupling are concentric anyway. 
Live-loaded packing will help since it has a spring to keep compression on the packing as it wears.  THe packing stackup should have a junk ring as the first element next to the process:usually this is a braided ring that can absorb grit instead of trying to seal around it. 

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