The plant comprises at least two storage, metering and distribution devices (1, 5), one for the limestone or dolomite component and the other for quicklime, respectively, which are connected to the inlet (21) of a mixer mill (16) whose outlet (22) feeds an expansion and drying device. The invention lies in that the expansion device (27), in which the quicklime being hydrated dries the limestone or dolomite component, is housed in an enclosure (30) confining the atmosphere in order that the latter may be heated during the exothermic hydration of the quicklime and may restore the heat thus stored to the said component in order to evaporate the residual water which it contains, this enclosure communicating (between 30 and 35) with the open air to remove the air saturated with water vapour and to replace it with unsaturated air, and in that a third storage, metering and distribution device (8) for the washing sludge and wet dust produced by the filtration is connected to the inlet (21) of the mixer mill (16). <IMAGE>
