Application of air classification for upgrading of residues from dry off-gas de-dusting.

The residues from dry de-dusting off-gases are fine granular materials which either have to be used in some way or disposed of in landfill sites. When the off-gas originates from a high temperature process like a blast furnace, a sinter plant, a cement kiln or a combustion process, the off-gas contains various volatile components (e.g. heavy metals like Zn, Pb; alkali chlorides like KCl, NaCl) which condense on the dust particles when the off-gas cools down. As the specific surface area is higher for the smaller particles the condensed components are usually enriched in the fine particle fractions. In several cases of the use of the residues these volatile components are unwanted. Therefore, air classification of the residue is a feasible process to reduce such components in the bulk of the residue by separating off the finest fractions. The treatment of the sludge from wet off-gas cleaning of blast furnaces by hydro-cyclones is a well-established process. In this application the fine fraction which is enriched in Zn is separated from the sludge because Zn is an undesired element in the recycling of the material via the sinter plant. Air classification could be used in a similar way for the residues from dry off-gas de-dusting. Several possible applications for air classification of dry off-gas cleaning residues are discussed. This includes fly ash from biomass combustion, dust from the iron and steel production (blast furnace, sinter plant), as well as the bypass dust from cement production.