Magnesium chloride is converted to high-purity magnesium oxide and HCl by feeding an aqueous magnesium chloride solution to a spray dryer operated as a thermohydrolysis reactor at a temperature of at least 300° C. Heat for the reactor is generated by burning a combustible gas that contains at least 30 mole % hydrogen, based on total moles of combustible compounds in the gas. A solid magnesium oxide product and an HCl-containing gas stream are withdrawn from the reactor. The solid product contains at least 97.5 wt. % MgO and less than 2.5 wt. % total magnesium hydroxychloride and magnesium chloride, based on total MgO, magnesium hydroxychloride, and magnesium chloride. The process enables production of high-purity magnesium oxide at relatively low spray-dryer operating temperatures. Also disclosed is a carboxylic acid manufacturing process in which magnesium chloride is processed to regenerate magnesium oxide and HCl.