Selective recovery of chromium from ferronickel slag via alkaline roasting followed by water leaching

Highlights: • A novel technological route for selective recovery of chromium from ferronickel slag was developed. • The Cr₂O₃ in the ferronickel slag was converted to NaCrO₂ by alkaline roasting with addition of Na₂O₂. • The loose structure of NaCrO₂ was beneficial to subsequent water dissolution. • The formation of highly toxic Cr (VI) was restrained in the process. -- Abstract: Chromium was selectively recovered from ferronickel slag by roasting the slag with addition of Na₂O₂, followed by water leaching. The thermodynamic analysis revealed that in the presence of Na₂O₂ at appropriate temperatures, the Cr₂O₃ in the ferronickel slag can be converted to NaCrO₂, instead of Na₂CrO₄, which prevents the formation of highly toxic Cr (VI). The experimental results confirmed that under the optimal alkaline roasting and water leaching conditions of the mass ratio of ferronickel slag to Na₂O₂ of 1, roasting temperature of 600 °C, roasting time of 1 h, leaching temperature of 50 °C, leaching time of 1 h, and liquid-to-solid ratio of 10 mL/g, 92.33% of Cr was leached with 64.28% of Na and 11.16% of Si and only 0.06 wt % Cr was left in the leaching residue. The high leaching percentage of Cr was a result of the transformation of Cr₂O₃ in the ferronickel slag to NaCrO₂ with a loose structure during alkaline roasting that was beneficial to water dissolution. Compared to the traditional alkaline roasting process, the proposed more environmentally friendly method did not produce toxic Cr (VI) during recovery of chromium and the resulting residue has potential to be used as a good construction material.