Abstract: A process for the recovery of sulfur from an acid gas stream is disclosed. The process includes a Claus sulfur recovery step in combination with a direct reduction step and a biological sulfur recovery step to provide a sweet gas stream having a very low concentration of hydrogen sulfide and sulfur dioxide. The process includes reacting an acid gas steam with oxygen under such oxidation conditions to yield a combustion gas comprising hydrogen sulfide and sulfur dioxide. The combustion gas is reacted under Claus reaction conditions to yield a reaction gas comprising sulfur. Sulfur is recovered from the reaction gas to yield a Claus tail gas comprising hydrogen sulfide and sulfur dioxide. The Claus tail gas is reacted under direct reduction reaction conditions to yield a direct reduction gas comprising sulfur. Sulfur is recovered from the direct reduction reaction gas to yield a tail gas comprising a concentration of hydrogen sulfide.

Abstract: A combined process for the recovery of sulfur from an acid gas stream. The combined process includes both a Claus unit and a unit for treating the Claus tail gas with caustic to remove hydrogen sulfide therefrom and to biologically oxidize the hydrogen sulfide using certain types of sulfur bacteria to make elemental sulfur. The combined process provides for an exceptionally low concentration of hydrogen sulfide in the finally treated sweet gas.