the drawing shows a heating chamber useful in the method of this invention . more specifically , the drawing shows an upstanding elongate heating chamber 1 into a lower portion of which is introduced a carrier gas by way of pipe 2 and solid particles to be heated by way of pipe 3 . the solid particles and / or carrier gas can be at any desired temperature but are preferably preheated , and if not preheated at all or to the desired extent can undergo additional heating as desired by use of heaters 4 and 5 . the outlet from heating chamber 1 for both particles and gas is provided by pipe 6 . if desired , at least a portion of the already combusted and therefore hot particles , can be externally recycled by way of pipe 7 for reintroduction to a lower portion of chamber 1 as shown by pipe 8 . provision should be made as shown by pipe 9 for removing part or all of the recycle stream from pipe 8 so that the extent of external recycle can be controlled or eliminated if desired . a solid - gas separation device 10 may be provided so that gas can be removed by pipe 11 from the particles to be recycled thereby externally recycling primarily only heated particles . the control of the carrier gas being injected by way of pipe 2 is important in order to obtain the internal backmixing desired for this invention . it may also be desirable in order to achieve the internal backmixing results of this invention to be able to remove a particle - gas stream from an intermediate portion of the chamber as shown by pipe 12 . the use of external recycle by way of pipe 8 , intermediate solids and gas removal by way of pipe 12 , gas removal by way of device 10 , and apportionment of solids between pipes 8 and 9 can all be carried out at the same time or alone or in any combination of two or more thereof as is desirable to achieve combustion of the carbon on the particles in chamber 1 while utilizing internal backmixing . the particular conditions which will allow internal backmixing as desired by this invention vary widely depending upon the type of heating chamber employed , the type of carrier gas employed , the type and size range of the solid particles to be treated , and the like , so that a precise set of conditions for all oil shale particles is extremely difficult if not impossible to set out quantitatively . however , the carrier gas and solid particle introduction into chamber 1 must be controlled in a manner which will be disclosed in greater detail hereinafter so that as the various particles travel upwardly in chamber 1 towards outlet 6 a substantial portion undergo at various times and in various locations between introduction at pipe 5 and outlet 6 one or more backmixing conditions in which the particle falls downwardly a finite distance and then resumes its upward travel , ultimately reaching outlet 6 . there can be a buildup of larger solid particles , e . g ., larger than about 4 mesh , in the bottom of chamber 1 . this buildup of particles will be fluidized and can be controlled by being withdrawn , in whole or in part , by way of pipe 12 . however , at any given time and location over the interior length of chamber 1 there can be one or more particles traveling downward at the same time a number of particles are traveling upward thereby causing substantial backmixing of relatively hotter downwardly moving particles with relatively cooler upward moving particles . this provides much better and uniform heating throughout the chamber and accomplishes more complete combustion of the carbon on the particles before they reach outlet 6 . generally , the particles should be predominantly less than 4 mesh ( u . s . sieve size ). it is not required that all particles be less than 4 mesh just that the substantial majority meet this requirement , for example , at least about 80 % of the particles . it is also preferable that the carrier gas be controlled in relation to the solid particles so that the amount of particles falling downwardly for backmixing in said chamber at any given time is equivalent to at least about one - third by weight of the particles being fed into the chamber by way of pipe 3 . the carrier gas can be air , oxygen , mixtures of two or mores gases , and the like , and can carry one or more oxidizing agents for enrichment . generally , the carrier gas is introduced into the heating chamber by way of pipe 2 at a rate of at least 5 feet per second and at a ratio of pounds of particles being fed into the chamber by way of pipe 3 to cubic feet of gas being fed into the chamber by way of pipe 2 of at least about 1 / 1 but less than about 100 / 1 . when there is external recycling of solid particles by way of pipe 8 the carrier gas introduction by way of pipe 2 should be at least 10 feet per second and the pounds of particles to cubic feet of carrier gas introduced ratio should be from about 2 / 1 to about 20 / 1 . with external recycle of particles the predominant particle size can be reduced to less than 60 mesh . colorado oil shale having previously been retorted at a temperature in the range of 800 ° to 1 , 000 ° f . and being in the size range of from about 4 mesh to less than 320 mesh is passed into a heating chamber substantially as shown in the drawing . air is introduced into the bottom of the chamber by way of pipe 2 at a rate of 15 feet per second and is carefully controlled so that the pounds of oil shale particles introduced by way of pipes 3 and 8 to cubic feet of air introduced by way of pipe 2 is about 10 to 1 , the solid particles being preheated to at least 1 , 100 ° f . before entry into the heating chamber . substantial heat is recovered from chamber 1 for use in heating water for heat exchange purposes . reasonable variations and modifications are possible within the scope of this disclosure without departing from the spirit and scope of this invention .