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 NaO, followed by water leaching. The thermodynamic analysis revealed that in the presence of NaO at appropriate temperatures, the CrO in the ferronickel slag can be converted to NaCrO, instead of NaCrO, 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 NaO 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 CrO in the ferronickel slag to NaCrO 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.