Process for improving the sulphur yield of a complex for producing sulphur from a sour gas containing H.sub.2 S, the said complex comprising a sulphur plant and then an oxidation and hydrolysis unit followed by a purification unit

In a process for improving the sulphur yield of a complex producing sulfur from a sour gas containing H.sub.2 S, the complex comprising (1) a sulfur plant into which the sour gas is introduced together with a controlled quantity of a gas containing free oxygen and in which a controlled oxidation of the H.sub.2 S in the sour gas by means of the oxygen in the gas containing free oxygen is performed to produce sulphur and at the exit of which there is discharged a residual gas containing steam and, in an overall quantity of approximately between 0.2 and 6% by volume, sulphur compounds comprising H.sub.2 S, SO.sub.2 and at least one of the derivatives COS and CS.sub.2, (2) an oxidation and hydrolysis unit in which the residual gas originating from the sulphur plant is brought into contact with a catalyst for hydrolysis of the compounds:

• COS and CS.sub.2 to H.sub.2 S while operating at a sufficient temperature to produce a hydrolyzed residual gas containing H.sub.2 and SO.sub.2 and substantially free from COS and CS.sub.2 and finally (3) a purification unit through which the residual gas originating from the oxidation and hydrolysis unit is passed after the temperature of the said gas has been brought to the value required for its passage through the purification unit and while maintaining the molar ratio H.sub.2 S:SO.sub.2 in the residual gas entering the purification unit at a value substantially equal to 2:1, and the compounds H.sub.2 S and SO.sub.2 which it contains being made to react together to form sulphur and to obtain a purified residual gas at the exit of the purification unit, the improvement which comprises (a) maintaining the H.sub.2 S:SO.sub.2 molar ratio in the residual gas originating from the sulphur plant and entering the oxidation and hydrolysis unit at a value equal to or higher than 2:1 by varying the ratio of the flow rates of sour gas and of gas containing free oxygen which are introduced into the sulphur plant;
• (b) introducing a second gas stream containing free oxygen into the gas stream from the sulfur plant entering the oxidation and hydrolysis unit and oxidizing H.sub.2 S to SO.sub.2 and sulphur in the unit by contacting the gas stream with a catalyst for oxidation of the H.sub.2 S;
• and (c) maintaining the H.sub.2 S:SO.sub.2 molar ratio in the residual gas entering the purification unit at a value substantially equal to 2:1 by varying the flow rate of the gas stream containing free oxygen introduced into the oxidation and hydrolysis unit.