Reductive Heating Experiments on BOF-Slag: Simultaneous Phosphorus Re-Distribution and Volume Stabilization for Recycling

Abstract

Basic-oxygen furnace (BOF) slag is the second abundant by-product from the steel-making process. It is not readily recycled for high phosphorus content and volume instability. To eliminate its phosphorus-hosting dicalcium silicate (C 2 S) and volumetrically unstable free-lime, reductive heating experiments on BOF-slag are carried out at 1300-1600 8C using quartz-sand or serpentine as a basicity modifier. In the products, C 2 S and free-lime are consumed through reacting with CaO and MgO, which are produced by Ca-ferrite and Mg-wustite reduction, respectively, to form merwinite and akermanite. The products are also characterized by silicatemetal segregation and the extent of that increases with increasing temperature and decreasing particle size of starting mixtures. Thermodynamic calculations show that phosphorus in C 2 S is mostly converted to Fe 2 P in the metal domain. Compared to the slag-serpentine experiments, the slag-quartz experiments resulted in higher extents of silicate-metal segregation with lower phosphorus contents of %0.1% in the silicate domain, which therefore is recyclable. To optimize silicate-metal segregation for recycling BOF-slag, it is suggested (i) bringing the BOF-slag composition within the low-temperature side of the C 2 S-merwinite-akermanite Alkemade triangle by adding quartz-sand; (ii) reductively heating to 1500-1600 8C; and (iii) cooling slowly then quenching to suppress C 2 S crystallization from melts.

Effect of Al2O3 Addition on Mineralogical Modification and Crystallization Kinetics of a High Basicity BOF Steel Slag

“…Accelerated carbonation treatment of slag has been demonstrated as an efficient way to eliminate the free lime by reacting with CO2 [11][12][13]. Basicity modifier such as SiO2 was examined as an effective agent by transforming the free lime and diacalcium silicate to merwinite and akermanite [14,15]. Recently, the present authors reported a novel lime stabilization method by cooling the slag in air, where wustite in the slag is oxidized to hematite and then stabilizes the free lime by forming calcium aluminoferrite [16].…”