The high-temperature reaction products of the CaO-CoO system in an atmospheric environment are combined with commercial-grade CaCO₃ and Cr₂O₃ reagents. These mixtures are then pressed using a hand press to form samples for high-temperature reaction studies. The samples are sintered in an electric furnace at 600°C and held at 1300°C. X-ray diffraction analysis is performed at room temperature, comparing both sintered and unsintered samples. A hexavalent compound, CaCrO₄, forms between 600°C and 1200°C, but it disappears above 1200°C, transforming into CaCr₂O₃. The addition of TiO₂ delays this reaction by forming a solid solution, though this approach is not economically viable or practical for brick production. As an alternative, gaseous reactions were investigated using high-temperature X-ray diffraction.
To study the in-situ reactions at elevated temperatures, high-temperature X-ray diffraction was employed. A mixture of commercial-grade CaCO₃, Cr₂O₃, and 1% (by mass) Na₂CO₃ was prepared in acetone using an agate mortar. The resulting powder was placed on a platinum fixture (MaC Co., Ltd.) attached to a high-temperature X-ray diffraction camera. The sample was heated to 1250°C using an R resistor, with a heating rate of 10°C/min and a cooling rate of 20°C/min. X-ray diffraction was conducted every 50°C from 500°C to 1250°C. If a reaction occurred, the temperature was maintained until the diffraction process was complete; otherwise, the heating continued.
As the temperature increased, Cr₂O₃ began to form around 1000°C, coinciding with the disappearance of CaCrO₄. At 1050°C, all hexavalent chromium was converted to trivalent chromium, indicating a very rapid transformation. The sample was cooled to 1250°C and held for 30 minutes. At 100°C, the trivalent compound CaCr₂O₄ transformed back into the hexavalent compound CaCrO₄, which remained stable until 50°C. Post-test X-ray diffraction at room temperature revealed that most compounds were hexavalent, with only a small fraction remaining trivalent.
During the heating phase from 110°C to 1250°C, CaCrO₄ was converted to CaCr₂O₃, and the trivalent state persisted until 500°C. After the test, only trace amounts of hexavalent chromium were detected under both atmospheric conditions. The sample was heated in an electric furnace at 10°C/min in air to 1350°C and held for 30 minutes. It was then cooled to 500°C under various gas atmospheres, including CO, by adjusting the bypass switch at a flow rate of 150–20 mL/min. The results showed that cooling in a CO atmosphere was most effective in reducing hexavalent chromium in the refractory bricks.
By reducing the samples under different atmospheres, hexavalent chromium can be significantly reduced when heated to over 1000°C and cooled under vacuum or nitrogen. This demonstrates that controlled cooling environments help minimize hexavalent chromium content. In cement kiln applications, refractory bricks containing hexavalent chromium were tested under various cooling conditions. Carbon reduction tests 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 these processes.
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