Recycling : Reduce, Reuse and Recycle

Basic oxygen furnace (BOF) slag industrial waste from steel-making is generated by 7-9 million tons each year in Korea. BOF slag is reused as construction materials (e.g., land banking and road base). When BOF slags have contact with water however, BOF slag generates highly alkaline leachate (pH>12) by dissolved CaO located on the slags surface. This highly alkaline leachate adversely affects the surrounding environment thus, BOF slag should be appropriately treated prior to reuse. This study aims to mitigate alkaline leachate generation from BOS slag focusing on prevention of CaO dissolution through carbonation. A carbonation test was conducted where BOF slag and NaHCO3 solutions (0.05, 0.1, 0.5, 1.0 M) were agitated with the ratio of 1:5 (g-slag: mL-solution) for 48 hr. The pH and Ca2+ concentration was measured for the leachate. Residual free CaO was measured and x-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) were conducted for the treated BOF slags. No significant difference of free CaO concentration was detected after carbonation indicating that NaHCO3 treatment did not significantly affect reduce free CaO concentrations in BOF slags. However, the leachate pH of BOF slags slightly reduced from 12.0 to 11.3 at 1 M NaHCO3 concentration. TGA and XPS analysis also confirmed that CaCO3 was precipitated on the BOF slag surface after carbonation, preventing additional free CaO dissolution. The results suggested that produced CaCO3 was precipitated on BOF slag forming a coating layer on the surface of BOF slag. The further decrease in leachate pH of BOF slag treated with CO2 gas was obtained (from 12.0 to 10.3). CO2 gas treating solution (pH 7.2) which led to leaching out more Ca2+ than NaHCO3 did subsequently producing more CaCO3 coating on the BOF slags. This finding is expected to contribute to developing appropriate treatments for safe and proper BOF slag reuse.