Process for preparing sulfur dioxide

A process for the preparation of essentially pure sulfur dioxide from waste sulfuric acid containing metal salt impurities and at least 50% by volume sulfuric acid, which consists essentially of burning finely divided liquid sulfur with an oxygen containing gas, containing at least 70% by volume oxygen, to produce not flame gases and directly heating the waste sulfuric acid with the hot flame gases to a temperature of 1,000.degree. to 1,500.degree. C to obtain a combustion gas consisting essentially of SO.sub.2, water, O.sub.2, metal salt impurities and a small amount of SO.sub.3 , cooling the hot gases to a temperature of about 300.degree. to 500.degree. C by direct contact with a salt melt quench coolant to condense metal salt impurities and to separate the metal salt impurities from the combustion gas.