PURPOSE: To improve the water-based separation performance of a heavy hydrocarbon oil by grinding petroleum coke particles and iron compd. particles to prepare a paste or slurry containing a ground material of ≤75 μm particle size in the presence of an oil.

CONSTITUTION: Petroleum coke particles and iron compd. particles are ground in the presence of an oil to prepare a paste or slurry containing ground material having ≤75 μm particle size. The obtd. paste or slurry is used as a hydroconversion catalyst. In this method, it is preferable to mix 10 to 90 wt.% iron salt or oxide with 90 to 10 wt.% cokes particles. The paste or slurry produced by grinding the iron compd. in the presence of an oil is supplied to a heavy hydrocarbon oil flow containing 0.1 to 5 wt.% petroleum cokes particles to form an additive of petroleum cokes and iron compd. to obtain a hydroconversion catalyst.