IN THE CRACKING OF WASTE SULFURIC ACID TO PRODUCE SO2CONTAINING GASES DEVOID OF IMPURITIES DELETERIOUS TO SO3 CONVERSION, THE WASTE ACID CONTAINING ABOUT 30-60% BY WEIGHT H2SO4 IS CONCENTRATED TO ABOUT 60-75% BY WEIGHT H2SO4 PRIOR TO CRACKING, THE LATTER STEP BEING IMPROVED FROM THE STANDPOINT OF ENERGY CONSUMPTION BY PASSING THE CRACKING GASES HAVING A TEMPERATURE OF ABOUT 1000* C. IN DIRECT CONTACT WITH THE 30-60% BY WEIGHT ACID TO EVAPORATE WATER THERFROM. THE CRACKING GASES ARE THEN COOLED TO CONDENSE OUT WATER AND IMPURITIES. ORGANIC IMPURITIES ARE SCRUBBED OUT OF THE RESULTANT GAS WITH H2SO4, THE LATTER, WHEN LOADED WITH IMPURITIES BEING PASSED TO THE CRACKING STAGE. RESIDUAL SO2 CONTAINED IN THE CONDENSED OUT WATER IS ENTRAINED BY AN AIR STREAM, AND THE RESULTANT SO2 PLUS AIR IS USED AS COMBUSTION AIR IN THE CRACKING STAGE.