Abstract: A system for transferring solid material to and from a high pressure reactor as a water slurry is disclosed. In a wet oxidation reaction system comminuted solid hydrocarbonaceous material, such as shale, coal, tar sand, wood or waste, flows in a continuous circulation loop to mix and supply a slurry of water and a high concentration of comminuted solid hydrocarbon-containing particles to the reactor. Such a slurry is reacted with oxygen at high temperature and pressure to extract hydrocarbon fluids as a gaseous phase and to remove metal, sulfur and nitrogen components from the material in the residual liquid. Flow into and out of the reactor vessel is through hydraulically actuated cylinders each isolated from atmosphere and the reaction vessel by a pair of full flow gate valves having no valve seats, throats or stems subject to abrasion.