an apparatus for the sulfatizing roasting of a selenium - containing raw material , especially the anode slime from copper electrolysis , with sulfuric acid at a raised temperature , the apparatus comprising a furnace chamber having a feed pipe for feeding a slurry of a finely - divided , selenium - containing raw material and sulfuric acid into the furnace chamber , an outlet for withdrawing the roasted solid from the furnace chamber , an outlet for discharging the selenium bearing gases produced during the sulfating roasting , members for heating the slurry in the furnace chamber , and devices for transferring , as a layer , the slurry fed into the furnace chamber from the slurry feed point towards the outlet for solid , the members for transferring the slurry being one or more combinations of a tray which supports slurry and a scraper which scrapes the tray , at least some of the slurry - heating members being mounted in the tray in order to heat it .