An anode producer has several ring furnaces to stabilise the anodes by carbonization for later processing in a melting bath. The hot exhaust gases pass through double walls in between the anodes where the gases are contaminated. The purification is done in a regenerative thermal oxidiser (RTO). The waste gases also contain particulate matter, aerosols and halogenous hydro-carbons. This requires a specially constructed RTO, combustible pre-filters for the particulate matter, as well as a downstream absorber to remove inorganic fluorides. The absorber uses a packed bed of limestone which is regenerated with a peeling drum. The loaded grain surface is removed through the peeling process and the residual limestone is redirected to the fixed bed.