MAGNESIOCHROMITE (MgCr2O4) SYNTHESIS: EFFECT OF MECHANICAL AND MICROWAVE PRETREATMENT

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Magnesiochromite spinel synthesis based on oxides, hydroxides and nitrates of magnesium and chromium was investigated. The precursors reactivity was compared by means of effective rate constants calculated by Ginstling-Brounshtein equation. The possibility of use of this equation was confirmed by the dependences linearity with high linear approximation coefficients. The reactivity of MgO various forms (soft-burned, or caustic magnesite, and dead-burned, or periclase) in the spinel formation was compared. Oxide precursors (especially with the periclase participation) reacted with the substantially less rate in comparison with hydroxides and salts. The influence of a preliminary mechanical activation by impact-and-attrition (planetary mill) and attrition (ball-ring mill) action as well as a microwave treatment (2.45 GHz) was analyzed. The most positive effect of a mechanical treatment in a planetary mill that was associated with an activation of Mg and Cr compounds became apparent in the field of relatively low temperatures (700-1100 °С). For example, MgCr2O4 yield at 1000 °С and the joint impact treatment of oxides was twice as much than under the simple mixing. The subsequent temperature rising lead to some decrease of a pretreatment effect so long as diffusion coefficients in these conditions grew, so the reaction run rapidly even without a preliminary mechanical treatment. It was noticed that an appreciable reduction of MgO reactivity in the spinel formation after a certain attrition in a ball-ring mill linked to the plane sliding in cubic crystals and resulted in the removing of the most disordered and defective layer from grains and the plane surface uncovering. The combined method consisting of a mechanical treatment of magnesium and chromium nitrates mixture in a planetary mill and the subsequent burning in a thermal kiln (1000 °С) was considered as the most effective as it resulted in practically single product. The microwave treatment took up an intermediate position by the effectiveness.