The article that it is possible to transform FeO to Fe3 O4 by vapour. The aim of this conversion is to build-up the lodestone ( magnetite ) concentrate such as the mining and metallurgical waste-products containing FeO-component. The thermodynamical calculation of the equilibrium composition in the temperature range of 100°C to 1000°C shows that at 200°C the highest of thermodynamic effeciency of the conversion, i.e. the values: 89,7 % of Fe3O4 and 7,1 % Fe2O3 was achieved. The last stage of preparation of the lodestone concentrate is the process of magnetic separation of Fe3O4 by the water operation in the low-intensity magnetic field.