Thermodynamics of chromite ore oxidative roasting process

Abstract

To explicate the thermodynamics of the chromite ore lime-free roasting process, the thermodynamics of reactions involved in this process was calculated and the phrases of sinter with different roasting times were studied. The thermodynamics calculation shows that all the standard Gibbs free energy changes of the reactions to form Na₂CrO₄, Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ via chromite ore and Na₂CO₃ are negative, and the standard Gibbs free energy changes of the reactions between MgO, Fe₂O₃ and SiO₂ released from chromite spinel to form MgO·Fe₂O₃ and MgO·SiO₂ are also negative at the oxidative roasting temperatures (1 173–1 473 K). The phrase analysis of the sinter in lime-free roasting process shows that Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ can be formed in the first 20 min, but they decrease in contents and finally disappear with the increase of roasting time. The final phase compositions of the sinter are Na₂CrO₄, MgO·Fe₂O₃, MgO·SiO₂ and MgO. The results indicate that Na₂CrO₄ can be formed easily via the reaction of Na₂CO₃ with chromite ore. Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ can be formed as intermediate compounds in the roasting process and they can further react with chromite ore to form Na₂CrO₄. MgO released from chromite ore may react with iron oxides and silicon oxide to form stable compounds of MgO·Fe₂O₃ and MgO·SiO₂, respectively.

Keywords

chromite ore / oxidative roasting / thermodynamics / sodium chromate