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

In the study, the solid solubility limits of Cr 3+ in the CaAl 12-x Cr x O 19 phase and leachability of any Cr 6+ safety were investigated. Analytical tools and leaching experiments were used to investigate the Al 3+ substitution in hibonite (CaAl 12 O 19 ) structure by Cr 3+ . The results showed that at Al:Cr=9:3, all the generated phases were chrome-hibonite (Ca 1.06 Cr 2.87 Al 9.07 O 18.99 ) and were also supported by the XPS and Cr 6+ leaching experiments results. However, when proportion of Cr in system increases to Al:Cr=7:5, the primary phases are Ca 1.09 Cr 3.92 Al 8.01 O 18.98 and alumina-chrome solid solution (Al 0.77 Cr 1.22 O 3 ) along with a small amount of wuboraite solid-solution (CaAl 2±x Cr 2±x O 7 , a Cr 3+ phase) and chrome-hauyne (Ca 4 Al 6 CrO 16 , a Cr 6+ phase). At Al:Cr=7:5, distilled water and 0.1 mol/L HCl leach around 1.9 mg/L Cr 6+ , but HCl showed relatively faster leaching of Cr 6+ than water and below the Chinese national standard of waste disposal of 3 mg/L. The maximum solid-solubility limit of Cr 3+ in CaAl 12-x Cr x O 19 could be considered as x =3.9.