of the stack gas is mixed with oxygen or oxygen enriched air and the mixture is fed to the furnace and to the converter preferably the mixture will contain 22 to 40 volume oxygen enriched air and 60 to 78 volume stack gas as shown in fig3 the recycled stack gas is supplied through pipe 90 and mixed with oxygen enriched air being supplied through pipe 82 to form the oxygen enriched recycled stack gas that is supplied through pipe 30 to the furnace and through pipe 35 to the scrubber and dryer 14 this improvement provides substantial reduction in the amount of nitrogen introduced into the system thus the reduction of pollution by vented so 2 and no x is particularly large because the system has been essentially closed to the limit permitted by the need to purge co 2 and n 2 there are other benefits as well the amount of moisture entering the system is reduced by eliminating that mixture which ordinarily enters with the air it is also possible to recover most of the sensible heat and sulfur values lost otherwise in the stack gas 44 in another improvement shown in fig4 oxygen enriched combustion air being supplied through pipe 30 is preheated indirectly in preheater 86 to a temperature of between 600 u00b0 and 1000 u00b0 c while spent acid being supplied through pipe 26 is indirectly preheated in preheater 88 to a temperature of between 100 u00b0 and 200 u00b0 c this reduces the amount of energy needed for direct preheat by combustion in the furnace the following examples illustrate the improvement in furnace effectiveness and reduction in inerts passed through the systems in examples 1 3 the numerical values obtained were obtained by simulating the process in a computer and making.