Acid Leaching of SHS Produced Magnesium Oxide/Titanium Diboride

The stoichiometric self-propagating high-temperature synthesis (SHS) thermite reaction involving magnesium (Mg), titanium dioxide (TiO₂), and boron oxide (B₂O₃) forms MgO and titanium diboride (TiB₂) as final products. Selective acid leaching is used to remove the MgO leaving TiB₂ powder. This study investigates the acid leaching of SHS-produced MgO/TiB₂ powders and a stoichiometric mixture of commercially obtained MgO and TiB₂ powders. Leaching was conducted at pH levels of 4.0, 2.5, and 1.0 by the introduction of concentrated aliquots of HNO₃. This method maintains a minimum pH target throughout the leaching process, thereby sustaining a dynamic concentration to remove the oxide. The optimal leaching conditions were determined to be at 90°C at a minimum pH target of 2.5 for the SHS-produced product. At these conditions, conversion percentages of 83%–84% of MgO were measured with only trace amounts of TiB₂ measured in the solution (<100 μg/L). Conversion percentages for each leaching condition and dissolution mass of solid MgO and TiB₂ at each pH are also reported. Results from powder X-ray diffraction confirm the removal of MgO and minimal dissolution of TiB₂, and indicate the formation of unidentified compounds. Inductively coupled plasma mass spectrometry (ICP) was used to analyze the ionic composition and extent of leaching. Scanning electron microscopy was used to observe the particle morphology of the leached powders.