A novel green additive CaAl12-xCrxO19 to eliminate Cr(VI) in environmentally friendly refractory castables: Chromium solubility and its leachability

Abstract

In the study, the solid solubility limits of Cr3+ in the CaAl12-xCrxO19 phase and leachability of any Cr6+ safety were investigated. Analytical tools and leaching experiments were used to investigate the Al3+ substitution in hibonite (CaAl12O19) structure by Cr3+. The results showed that at Al:Cr=9:3, all the generated phases were chrome-hibonite (Ca1.06Cr2.87Al9.07O18.99) and were also supported by the XPS and Cr6+ leaching experiments results. However, when proportion of Cr in system increases to Al:Cr=7:5, the primary phases are Ca1.09Cr3.92Al8.01O18.98 and alumina-chrome solid solution (Al0.77Cr1.22O3) along with a small amount of wuboraite solid-solution (CaAl2±xCr2±xO7, a Cr3+ phase) and chrome-hauyne (Ca4Al6CrO16, a Cr6+ phase). At Al:Cr=7:5, distilled water and 0.1 mol/L HCl leach around 1.9 mg/L Cr6+, but HCl showed relatively faster leaching of Cr6+ than water and below the Chinese national standard of waste disposal of 3 mg/L. The maximum solid-solubility limit of Cr3+ in CaAl12-xCrxO19 could be considered as x=3.9.