0 3 to about 0 7 times preferably from about 0 4 to about 0 6 times the diameter of the circle which they describe during rotation the vertical distance between bottom 7 of vat 1 and lower impeller 4 in fig1 is equal to about one half of the diameter of the circle produced by the impellers during rotation bottom cover 7 may be planar or curved with the hollow facing upward which improves the circulation of the pulp in the vat as for the vertical distance between upper impeller 3 and cover 9 of vat 1 it is substantially equal to the distance between the two impellers 3 and 4 when in use nickel ore in the form of pulp is introduced through conduit 6 which is connected to the lower side of the center opening of bottom 7 of vat 1 the dryness of this pulp that is the weight percentage of dry matter in the pulp must be at least equal to about 50 and even be higher than 70 as far as possible as for the ore itself it must have as fine a particle size as possible thus a particle size distribution such that 80 of the particles pass through a sieve whose openings are between 250 microns and 3 millimeters is considered satisfactory in this case the pulp is propelled on the one hand toward the top by lower impeller 4 and on the other hand toward the bottom by upper impeller 3 in this way the mineral particles collide with each other in a zone situated about mid distance between the two impellers 3 and 4 this collision causes a further break down of.