Production of the Al–B Master Alloy by Aluminothermal Reduction of KBF4 and B2O3 in a Medium of the Molten Salt Flux

Abstract

The process of obtaining the Al–B master alloy by the aluminothermal reduction of KBF₄ and B₂O₃ using fluoride fluxes KF–AlF₃ and KF–NaF–AlF₃ at 983 and 1123 К, respectively, and chloride-fluoride fluxes KCl–NaCl–KF at Т = 1173–1223 К have been studied. All experiments were carried out under the same conditions: the stirring rate of molten mixture was 400 rpm, the duration of synthesis was 30 min. The maximum amount of boron (1.5%) in the Al–B alloy was obtained using KBF₄ (3% per B) as a boron-containing raw material in the KF–AlF₃ medium with a molar (cryolite) ratio (CR) of KF/AlF₃, equal to 1.3, at Т = 983 К, while the boron extraction degree does not exceed 75%. The comparable results were obtained in the experiments with the KF–NaF–AlF₃ flux (CR = 1.5) at T = 1123 K. However, with an increase in the concentration of the loaded boron, the degree of its extraction significantly decreased, which is associated with the decomposition at a higher temperature not only KBF₄, but also NaBF₄, which is less thermally stable; the latter is formed as a result of an exchange reaction in the melt. Therefore, the use of sodium salts as a component of the flux is not recommended. The Al–B master alloys obtained by reducing the KBF₄ in the fluoride fluxes were solid solutions of B in Al containing the intermetallic compound AlB₂. The lowest amount of boron in aluminum with the minimum extraction degree was obtained in experiments with B₂O₃ in the molten KF–AlF₃ with CR = 1.5. Nevertheless, the results of scanning electron microscopy indicate a uniform distribution of B along the Al matrix and the absence of intermetallic compounds; however, a large amount of Al₂O₃ was found, which is the product of B₂O₃ reactions with both the liquid Al and the flux (KF–AlF₃).