The invention concerns a method for treating iron-and-steel plant sludge comprising adjusting the available CaO content of the sludge so as to obtain an available CaO content of 3% to 8% based on the sludge weight, adding a carbon-containing fuel, introducing the sludge onto one of the upper stages of a multistage furnace, heating the multistage furnace, gradually transferring the sludge onto the lower stages, and adjusting the temperature of the exhaust gases of the multistage furnace to at least 500.degree. C. and that of the treated sludge at the output of the multistage furnace to a temperature ranging between 700.degree. C. and 800.degree. C.