A process and apparatus for the incineration of organic material, and particularly agricultural waste products, to produce useable energy, consists of exposing the material to elevated temperature in an excess of air in a cylindrical furnace which terminates at the top in a frustum of a cone, the furnace having tangential inlet means at the bottom and discharge means comprising a concentric hollow cylinder projecting into the bottom portion of the furnace; the process and apparatus are particularly useful in the preparation of siliceous compositions from materials such as rice hulls and the process can be conducted under conditions whereby a highly reactive amorphous form of silica is produced, which silica is useful as a component of hydraulic cements.
