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

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.