The mixture of hydrate of lime and carbonate of lime is first placed in a closed chamher where it is wholly or partially converted into nitrate of calcium by the action of preheated air, superheated steam and an electric current of suitable strength and pressure, the mixture being afterwards further subjected while in a state of agitation to a stronger electric current in the presence of hot and/or cold air currents. For the reasons stated the reactions should be effected in the presence of a catalyst