The nominal 50 kgh(-1) dry coal biodepyritization pilot plant was built at Porto Torres (Sassari, Italy) in the area of EniChem S.p.A. Chemical Works, with financial support of the Commission of European Communities and with the participation of the DMT of Essen (Germany), the Mining and Mineral Processing Department of the University of Cagliari (Italy), the Technical University of Delft (The Netherlands) and the Warren Spring Laboratory of Stevenage (UK). The plant went on stream in early September 1992. Test runs were carried in the range 6.5-41.5% solids concentration. For every solids concentration the steady state was attained in about 10 d. For all the test runs the assays showed that more than 90% pyrite removal is achieved in the first five bioreactors; for a pulp flow rate of 6.94 x 10(-2) dm(3)s(-1) (250 lh(-1)) and useful bioreactor volume of 7.5 m(3), this corresponds to a residence time of 540 345 s, i.e., 6.254 d, and to a pyritic iron biosolubilization rate of 36 mg dm(-3) h(-1). The pyrite biosolubilization rate constant is 1.53 x 10(-2) h(-1). The power input per bioreactor operating on a 40% solids pulp is 4 kW with cos phi = 0.76. Hence, the power required for processing 100 kgh(-1) coal at 40% solids in the bioreactor section is 4 x 5 = 20 kW h, i.e. 200 kW h per tonne dry coal.