Understanding the solidification and leaching behavior of synthesized Cr-containing stainless steel slags with varying Al2O3/SiO2 mass ratios

Abstract

This study investigated the effect of Al2O3/SiO2 mass ratios on the equilibrium crystallization behavior of synthesized CaO–SiO2–MgO–Al2O3–Cr2O3 stainless steel slags to understand the selective concentration behavior of Cr into a primary Mg(Cr,Al)2O4 spinel phase during slag solidification and to determine the leaching stability of Cr-containing slags. The spinel solid solution was precipitated within the temperature range of 1600-1400 °C, where the Cr/(Cr+Al) mole ratio in the Mg(Cr,Al)2O4 spinel phase gradually decreased for slags with higher Al2O3/SiO2 mass ratios. When the Al2O3/SiO2 mass ratio increased from 0.125 to 0.5, the Cr content in the amorphous glass phase gradually decreased, with a subsequent increase in the Cr content in the crystalline phase. For slags with a unit Al2O3/SiO2 mass ratio and MgO mole percent comprising less than the combined sum of the Cr2O3 and Al2O3 mole percents, the Cr content in the amorphous glass phase increased, which was correlated with the enhanced substitution of Cr3+ with Al3+ in the spinel. The trend of the amount of Cr-related ions in the leachate was consistent with the trend of Cr in the amorphous glass phase: the amount decreased for slags with Al2O3/SiO2 mass ratios from 0.125 to 5 and then increased for slags with an Al2O3/SiO2 mass ratio of 1. The results suggest that the addition of appropriate amounts of Al2O3 to stainless steel slags could be conducive to stabilizing Cr into the primary spinel phase to minimize Cr leaching into the environment.