Phase transformation of transient B 4C to CaB 6 during production of CaB 6 from colemanite

Abstract

The aim of this project is the production of calcium hexaboride (CaB 6) from colemanite and petroleum coke and the determination of the temperature-dependent phase transformation of boron carbide (B 4C), which appears as an intermediate compound to CaB 6. This process makes use of the advantages of the “ B 4 C method” as a special case of the “ carbothermic method” for the direct production of CaB 6 from natural raw materials. Colemanite (2CaO·3B 2O 3·5H 2O), 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 (B 2O 3) and calcium oxide (CaO) as well as calcium carbonate (CaCO 3) and SrSO 4 as additional crystalline components. The production of CaB 6 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 B 4C is produced as an intermediate product and remains present up to temperatures of 1700–1850 °C as the boron source for the formation of CaB 6. The decomposition reactions of B 4C to CaB 6 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 CaB 6 from the as-received unmilled raw material obtained from the calcination of colemanite at 400 °C and petroleum coke.