Phase transformation of transient B4C to CaB6 during production of CaB6 from colemanite

The aim of this project is the production of calcium hexaboride (CaB6) from colemanite and petroleum coke and the determination of the temperature-dependent phase transformation of boron carbide (B4C), which appears as an intermediate compound to CaB6. This process makes use of the advantages of the “B4C method” as a special case of the “carbothermic method” for the direct production of CaB6 from natural raw materials.Colemanite (2CaO·3B2O3·5H2O), taken from Eti holding Co. in Turkey, and petroleum coke are used as raw materials. As-received crushed colemanite is processed by calcination between temperatures of 400 °C and 600 °C. X-ray diffraction analyses show that the obtained material contains amorphous boron oxide (B2O3) and calcium oxide (CaO) as well as calcium carbonate (CaCO3) and SrSO4 as additional crystalline components.The production of CaB6 was carried out by sintering the powder mixture under argon between temperatures of 1400 °C and 2000 °C. The products were analysed with an X-ray diffractometer.The experimental results and thermodynamic calculations show that B4C is produced as an intermediate product and remains present up to temperatures of 1700–1850 °C as the boron source for the formation of CaB6. The decomposition reactions of B4C to CaB6 in this temperature range were recorded by X-ray diffraction analyses and agree with thermodynamic calculations.It is concluded that the “carbothermic method” can be directly used for the production of CaB6 from the as-received unmilled raw material obtained from the calcination of colemanite at 400 °C and petroleum coke.