<PICT:0448433/III/1> <PICT:0448433/III/2> <PICT:0448433/III/3> In a process for bringing about chemical or physical changes by the reaction of a gaseous medium and finely divided solid material, the gas and the solid are introduced into a reaction zone, and mechanical propelling power is applied within said zone so as to make and maintain a uniform mixture in the form of a suspension of the solid in the gas, and to circulate the suspension so that all parts of the mixture are subjected to substantially the same treatment. In one form of the apparatus used for the production of red lead from litharge, barium peroxide from barium oxide or lead zinc or magnesium chromates from chromic oxide and the corresponding oxides a reaction chamber 1, Fig. 1, having a semi-circular top and bottom is provided with a fan 2 having solid blades, the solid material being fed in through the inlet pipe 3 and the gas, e.g. oxygen through a pipe 4 provided with a valve 12. The chamber 1 may be jacketed and may be provided with electric heating elements 19. The products of the reaction are removed by the pipe 6 provided with a suitable cover 13. When used for the manufacture of red lead, the chamber 1 is heated to 425--450 DEG C. and oxygen is the gas used. For a continuous reaction, the reaction chamber 1a, Fig. 2, is provided with a horizontal fan 2a having solid blades the gaseous medium being introduced through the pipe 4a by the fan 20 and the solid material through the pipe 3a by the feeder 21. If other solids are used in the reaction they are fed into the chamber by the feeders 22, 23 and other gaseous reagents through pipes 30. The mixture of the solid in suspension in the gas is circulated in the chamber by the fan 2a and the treated solids finally pass through an opening 7a into a settling chamber 8a. The products of the reaction are removed from the chamber 8a by a conveyer 27 and the gases leave by the pipe 26. When the apparatus is used for the production of iron or iron sponge, air is passed into the chamber 1a by the pipe 4a and coal or coke dust is fed by the feeder 22 into this pipe in such proportion so that the fuel is completely combusted to carbon dioxide. When the chamber has been heated to the desired temperature, iron ore is fed into the chamber by the feeder 21 and more coke or coal dust by the feeder 23. The ore and coke dust may be mixed in the proper proportions and the mixture, preferably preheated, is fed into the chamber. The gases leaving by the pipe 26 are burned to preheat the air and the charge before they enter the chamber. In another form of apparatus, three reaction chambers connected to each other by openings such as 7a are provided, the fourth chamber being a settling chamber similar to 8a. In another form of apparatus, the ore is first heated in a reaction chamber similar to the chamber 1a, passed into a settling chamber similar to 8a, through a second reaction chamber where the ore is further heated, a second settling chamber and then through a third reaction chamber where the ore is finally reduced, the reduced ore passing into a third settling chamber from which it is removed by a conveyer. The hot gases from the reaction chamber may be used to preheat the air which is passed into the reaction chambers. In another apparatus for bringing about reactions between gases by the aid of catalyst, e.g. converting sulphur dioxide to sulphur trioxide with the aid of iron oxide, the finely divided oxide, freed from dust, is fed into the heated reaction chamber 1d the fan being already in motion. The sulphur dioxide, mixed with the required portion of dry air, and preheated to the desired temperature is introduced into the chamber by the inlet 4d. The mixture of catalyst and gas in the form of a suspension produced by the fan 2d passes by the outlet 7d into the settling chamber 8d, the catalyst settling in the chamber being passed by a worm 27s into the inlet 3d. If desired the sulphur trioxide gases may be passed by the pipe 36d through a similar reaction chamber in contact with zinc oxide or roasted zinc ore to produce zinc sulphate. Instead of carbon, in the form of coke dust, coal dust, charcoal dust or dried sawdust, other reducing agents such as hydrogen methane and volatilized oils may be used. The apparatus described above may be used for the preparation of anhydrous aluminium chloride by treating a mixture of aluminium oxide powdered coke or charcoal with chlorine, sufficient air or oxygen being also fed into the chamber to burn the coke &c. to form carbon dioxide. The cooking of gypsum, the reduction of sulphates, such as the sulphates of barium calcium and sodium, to sulphides, the production of hydrogen by the treatment of ferric oxide with superheated steam, or water gas with finely divided lime or calcium hydroxide, the roasting of sulphide ores of zinc and copper, the carbonization of coal dust and the distillation of oil shale is also described.