TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Unit: A unit as above in which the inclined wall (P3) is at an angle ( alpha ) of 35-45 degrees to the vertical, preferably a unit in the form of a sector in which the angle ( beta ) of the walls (P1 and P2) defining this sector is 30-360 (preferably 45-360) degrees . The diameter of the mill is such that the distance between the ends of the mill and the nearest wall of the lower zone (1) is 0.5-2 times the diameter of the largest particles in zone 1. The organs of fluidisation (3, 4, 5) of the lower, intermediate and upper zones (1, 2, 3) generate a loss of charge which is 0.2-1.5 (preferably 0.3-1.0) times the weight of the particle bed for the section of the zone considered. The mill has blunt protrusions on the stator (8) and corresponding blunt protrusions on the rotor (10), such that the gap between these protrusions is 1.0-1.5 times the size of the particles to be obtained at the outlet of the unit. EXAMPLE - Wood chips (2 x 3 x 5 cm) with a water content of 35 wt.% were processed at atmospheric pressure in a roasting milling unit corresponding to a conical sector with an angle of 90 degrees at the top, with a total height of 2.5 m, a maximum diameter (upper zone 3) of 4.6 m, a mill diameter (lower zone 1) of 0.5 m, intermediate (2) and upper zones (3) of diameter 0.5-3.6 m and 3.6-4.6 m respectively, an inclined wall at an angle of 43 degrees to the vertical, rates of fluidisation of 27.3 cm/s in zones 1 and 2 and 6.3 cm/s in zone 3, an average empty fraction of 0.47 in the fluidised bed, an average particle residence time of 20 minutes, and gas flow rates of 1305 Nm3/hour in zones 1 and 2 and 190 Nm3/hour in zone 3. The dimensions of the fluidisers in the three zones were based on a loss of charge for the organ concerned of 0.5 times the weight of the bed referred to the section concerned and a gas flow of 40-70 m/s at each outlet, giving orifice diameters of 5.6 mm with 590 orifices per m2 of grid section for zones 1 and 2, and 4.0 mm with 410 orifices per m2 for zone 3. This unit was designed to produce particles smaller than 500 microns with a water content of less than 0.5 wt.% at a rate of 20000 t/a or 2.31 t/hour.