Recycling steel wastewater sludges as raw materials for preparing lightweight aggregates

The cost for treatment and disposal of hazardous stainless steel sludges has been soaring up recently in Taiwan, becoming a great concern for steel industry. Thus most steel plants have piled up the wastes on-site to await cost-effective recycling/disposal means. Two steel sludges with similar ternary component (silicon oxide, alumina, and flux) compositions, but considerably different particle size distribution, were individually added in clay and sintered, leading to a conclusion that particle size distribution of the sludge powders play a critical role in both glassy surface formation and pellet bloating during sintering. Glassy surface formation closely relates to the almost complete disappearance of quartz crystalline phase after sintering. All fired pellets readily meet official toxicity characteristic leaching procedure leaching limits of toxic metals As, Hg, Cd, Pb, Cu, Ag, Cr, Se, Ba, and Ni. Iron oxide components in steel sludges act as a flux for pellet sintering. Although the green pellets do not abide by Riley's rule of thumb in aspect of chemical compositions, some of them can be successfully sintered and bloated into lightweight aggregates. Physical properties of fired pellets, including particle density, water sorption capacity, and compressive strength, are also presented in the present study.