A method for processing slags containing iron and non-ferrous metals, to produce clean slag free of detrimental substances and non-ferrous metals and suitable for use as a raw material or construction material. Slag is reduced in a reduction furnace with the help of reducing agents so that at least 5% of the iron of the slag is reduced into metal. Some of the non-ferrous metals, such as zinc, lead, arsenic and cadmium, vaporize. The contents of the reduction furnace are continuously mixed to prevent separation of a metallic phase from the slag. The generated slag-metal mixture is tapped off from the reduction furnace, cooled, crushed and ground to a finer size. Finally, a metal fraction is separated from a clean slag.