Abstract: The present invention concerns a process and a plant for treating ore concentrate particles containing valuable metal and having at least arsenic and sulfur containing components. The process comprises a two-stage roasting process comprising a first roasting step (1) made in a first roasting reactor (16) and a second roasting step (3) made in a second roasting reactor (17). A gas mixture is formed from the first process gas component (2) obtained from the first roasting step(1)and from the second process gas component (4) obtained from the second roasting step (3). Post combustion of the gas mixture is made in a post combustion chamber (6). The post combustion operates with said reducing and sulphide rich first process gas component (2) and the second process gas component (4) as oxidizer gas in order to decompose SO3 in the gas mixture to reduce the SO3 content. The risk of accretion formation and corrosion in the post combustion chamber and in subsequent steps is reduced.