The high-temperature reaction products of the CaO-CoO system in the atmosphere are combined with commercial-grade CaCO₃ and Cr₂O₃ reagents. These mixtures are then pressed using a hand press to form samples for high-temperature reactions. The samples are placed in an electric furnace, sintered at 600°C, and then heated to 1300°C. After cooling, their mineralogy is analyzed using X-ray diffraction at room temperature, along with the unsintered samples. A hexavalent compound, CaCrO₄, forms within the temperature range of 600°C–1200°C, but disappears above 1200°C, forming CaCr₂O₃. Adding TiO₂ delays this reaction by forming a solid solution, but this approach is not economically viable due to cost and quality issues in the brick. Instead, gaseous reactions were studied using high-temperature X-ray diffraction.
High-temperature X-ray diffraction was used to investigate the CaO-Cr₂O₃ system in the atmosphere, aiming to observe in situ reactions at elevated temperatures. A mixture of commercial-grade CaCO₃, Cr₂O₃, and 1% (by mass) Na₂CO₃ was prepared in acetone using an agate mortar. The resulting powder was loaded onto a platinum fixture equipped with a high-temperature X-ray diffraction camera. The sample was heated to 1250°C using a resistance heater, with a heating rate of 10°C/min and a cooling rate of 20°C/min. X-ray diffraction was performed every 50°C between 500°C and 1250°C. If a reaction occurred, the temperature was held constant for 180 seconds to allow continued X-ray analysis; otherwise, it was increased further.
As the temperature rose, CaCrO₄ began to form at 1000°C, coinciding with the disappearance of CaCr₂O₄. At 1050°C, all hexavalent chromium was converted to trivalent, indicating a rapid transformation. The sample was cooled to 1250°C and held there for 30 minutes. At 100°C, the trivalent compound CaCr₂O₄ transformed back into the hexavalent form CaCrO₄, which remained stable until 50°C. Post-test X-ray diffraction at room temperature showed that most compounds were hexavalent, with only a small amount of trivalent chromium remaining.
During the heating phase from 110°C to 1250°C, CaCrO₄ was converted to CaCr₂O₄, and the trivalent state persisted until 500°C. After testing, only trace amounts of hexavalent chromium were detected under both atmospheric and wide-angle X-ray conditions. The sample was heated at 10°C/min in air to 1350°C and held for 30 minutes. Then, it was cooled to 500°C at a rate of 150–20 mL/min by switching the bypass valve, under different gas atmospheres. Post-testing analysis showed that cooling in a CO-rich atmosphere was most effective in reducing hexavalent chromium in the refractory bricks.
Hexavalent chromium can be reduced by heating the sample to 1000°C or higher under high-temperature X-ray diffraction, followed by cooling in a vacuum or nitrogen atmosphere. This demonstrates that controlled cooling environments significantly reduce the presence of hexavalent chromium. In cement kiln applications, cooling tests were conducted on refractory bricks containing hexavalent chromium under various atmospheres. Carbon reduction experiments indicated that CO gas is the most effective medium for reducing hexavalent chromium. Therefore, solid carbon is used as a source of CO gas in such processes.
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