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).