Effect of the Heat Treatment on the Chromium Partition in CaO-MgO-SiO₂-Cr₂O₃ Synthetic Slags

Mg-spinel phase is known to be important for control of Cr leaching from Cr-containing slags. The objective of the present study is to get an understanding of the phase relationships in the CaO-MgO-SiO₂-Cr₂O₃ system with a view to control the precipitation of Cr-spinel in the slag phase. The equilibrium phases in CaO-MgO-SiO₂-Cr₂O₃ slag system in the range of 1673 K to 1873 K (1400 °C to 1600 °C) have been investigated experimentally and compared with the results from thermodynamic calculations. The slag compositions close to the industrial slag systems were chosen. The Cr₂O₃ and MgO contents in the slag were fixed to be 6 and 8 wt pct, respectively. The basicity (CaO/SiO₂) of the slag was varied in the range of 1.0 to 2.0. The slags were synthesized at a pre-determined oxygen partial pressure (10⁻⁴) or air (2.13 × 10⁴ Pa) at a temperature above the liquidus temperature. The samples were then soaked at targeted temperatures for 24 hours in controlled atmosphere in order to achieve the equilibrium state before quenching in water. Four different heat-treatment regimes (defined as Ia, Ib, II.a and II.b) in Section II–D) were used in the present experiments. The lower oxygen partial pressure was maintained by a suitable mixture of CO and CO₂ gases. Phases present and their compositions in the quenched slags were studied using scanning electron microscopy coupled with energy-dispersive spectroscopy and X-ray diffraction techniques. The chromium content in the phases present was analyzed using wavelength-dispersive spectrometer. The experimental results obtained are compared with the calculation results from Factsage software. The size of spinel crystals increased drastically after slow-cooling from 1873 K (1600 °C) followed by annealing at 1673 K (1400 °C) for 24 hours (heating regimes II) compared to samples being quenched directly after soaking at 1873 K (1600 °C) (heating regime I.a). It was found that the amount of foreign elements in the spinel phase, and other phases decreased after soaking at oxygen partial pressure of 10⁻⁴ Pa resulting in phases with less defects and foreign oxide contents compared to those treated in air. The size of spinel crystals was found to be larger in samples with lower basicity.