Mineral selective magnetizing flash roasting in oxidizing atmosphere was tested successfully for the improved low intensity magnetic separation of siderite from ankerite and other gangue minerals within ore. The magnetization process was simulated with particle bed tests permeated with artificial combustion gas. The phase transition and the gas composition were measured simultaneously during temperature and time-dependent neutron diffraction experiments. The physical background of the rapid phase transition and the nature of the SO2 absorption are discussed. Apart from the scientific background, the effects on the process technique are outlined.