Use is made of a high-speed agitator comprising vessel 2 rotated at a low speed and bladed agitating element 3 rotated at a high speed in direction reverse to that of the vessel 2, the bladed agitating element 3 having a rotary axis arranged parallel to, and located apart from, the rotary axis of the vessel 2. Petroleum residuum such as solvent deasphalting residuum is agitated together with a grinding auxiliary and water in the high-speed agitator so that the petroleum residuum is ground. Thereafter, a dispersant is added thereto to form a slurry and the viscosity thereof is adjusted to a given value. A stabilizer is further added thereto to obtain a stable slurry. The dispersant and the stabilizer may be placed in the high-speed agitator prior to the grinding of the petroleum residuum. Thus, there is provided a process in which a high-concentration petroleum residuum-water slurry with a desirable particle size distribution, being cheap and highly stable, can easily be obtained by a one-stage grinding.

A stream containing kerogen-rich and mineral-rich oil shale particles flow in a downward helical course such that the stream is confined in a manner to give the stream an outer and upper side having a first depth and comprising a substantial amount of said kerogen-rich particles, and an inner and lower side having a second depth and comprising a substantial amount of said mineral-rich particles, the first depth being greater than the second depth. The kerogen-rich and mineral-rich particles are subsequently separated in order to recover the kerogen-rich particles. The mineral-rich particles are further processed using froth flotation.