Weak acid leaching of MgB$_2$ to purify magnesiothermic boron powder

We have established a new procedure for recovering boron contained in oxidized MgB$_2$ phase. Incorrect conditions of synthesis or conservation, or excess impurities in precursors, can lead to MgO formation with consequent degradation of superconducting properties, thus representing a possible problem in the industrial production of MgB$_2$ conductors following the ex-situ PIT process. A procedure has been developed in order to dissolve MgO (due to the oxidization of Mg in MgB$_2$ phase) and to decompose the MgB$_2$ phase into elemental B and a soluble Mg salt. The process can also be successfully applied to low-grade B in order to enhance the purity, providing that the B is first reacted into MgB2. In fact, MgB$_2$ synthesis converts residual B$_2$O$_3$ (present in B) and other impurities to leachable compounds. For the present work, an ill-preserved MgB$_2$ powder produced in 2008 was treated with glacial acetic acid. The reduction of impurities and secondary phases was monitored along each step of the proposed process by X-ray diffraction (XRD) technique and by energy dispersion spectroscopy (EDS) analysis. The purification process was successfully applied on two different MgB2 powders: the first was synthesized using commercial B (H. C. Starck), the second using a different magnesiothermic B with a low purity grade prepared by freeze-drying process at CNR-SPIN. An interesting result is that, after acidic treatment of MgB2, the resulting B particles are finer and more amorphous than pristine B particles. The increased surface area of B undoubtedly offers a more reactivity of powders versus the weak acid, because the impurities within the B particles are exposed to the direct action of the acid and not longer protected by B crust.