a gas mixture including sulphur dioxide is recovered from sulphate waste material by burning a fuel to form a flame zone , introducing sulphate waste material into the zone and introducing pure oxygen or other oxygen - rich gas into the flame zone to support combustion of the fuel and to generate a flame of sufficient temperature to crack solid sulphate waste material and thereby liberate sulphur dioxide therefrom . solids exiting the flame zone are separated from the gaseous combustion products . the sulphate waste material may be taken from a plant for making titanium dioxide pigment by the sulphate route .