Growth of MgO crystals from NaBO2 - KBO2 fluxes

Abstract MgO (periclase, magnesia) is a key component of refractory materials used for the lining of high-temperature aggregates in the metallic and non-metallic sectors alike. The primary source in production is magnesite (MgCO₃), which is thermally decomposed to magnesium oxide. However, the excellent refractory properties of magnesia are only achieved at specific crystallite sizes, which requires thermal post-treatment of MgO using sintering at temperatures of 1500 to 2200 °C in rotary or a shaft kiln or by fusion well above the melting point in an electric arc furnace, respectively. The present contribution is part of an exploratory study that investigates the possibility of obtaining periclase crystals with dimensions of at least 500 μm using alternative synthesis routes that demand lower temperatures. Therefore, the flux method using two alkali borate-based systems were successfully tested. The investigations included the determination of MgO solubilities in a pure NaBO 2 melt and a melt of eutectic composition in the system NaBO 2 -KBO 2 . It is worthy to note that synthesis runs employing pure potassium metaborate (KBO₂) as a solvent proved less successful and resulted in the concomitant formation of Mg-containing borates such as Mg 3 (BO 3 ) 2 . Temperatures and annealing times were systematically varied to identify the optimal growth conditions through slow evaporation of the solvents. The samples were characterized employing the following methods: optical microscopy, SEM, and PXRD. High-quality octahedra of sufficient size and free from flux inclusions were obtained after 96 hours and 120 hours at 1000 °C using the eutectic mixture. The investigation was conceived as a proof-of-concept study. Some issues related to a potential upscaling of the process are also discussed.