Air classification of blast furnace dust collected in a fabric filter for recycling to the sinter process

• Dust from a dry second cleaning stage for blast furnace top gas was investigated.
• Concentration of unwanted components, e.g. Zn, is particle size dependent.
• Air classification can be used for production of a low-contaminated dust fraction.
• Derived recovery–removal functions can be used in design of dust recycling.

Totally dry cleaning has become a common technology for top gas cleaning in blast furnaces in recent years. A significant advantage of totally dry gas cleaning is that the dust collected is obtained as dry powder, thus simplifying the recycling of the dust in the sinter plant and avoiding aqueous emissions. The concentration of some heavy metals, especially zinc, in the collected dust is usually higher than the maximum tolerable concentration for recycling to the sinter process. Therefore, a process for separation of dust with a low level of contamination from the rest is necessary to make partial recycling possible. This is possible because the limited components are more volatile and accumulate in the finer dust fraction. In wet blast furnace top gas cleaning, hydrocyclones are well established for this separation. For the separation of dry powder from the dry dedusting process air classification can be used. Dust from the top gas of a blast furnace with a fabric filter for dry top gas cleaning was split into several size fractions using a laboratory air classifier. The concentration of Ca, Cd, Cl, Cu, Fe, K, Na, Pb and Zn was analysed for each particle class and the loss on ignition was determined. A strong dependence of the concentration on the particle size was found for the more volatile metals, whereas the Fe and Ca concentration and the loss on ignition were quite evenly distributed. With the calculated recovery–removal-functions the possible recycling rate can be estimated for a given removal rate for the limited components.