<p>The content of aluminium impurity in primary magnesium produced via silicothermic reduction often fluctuates uncontrollably. Such variability harms the market credibility and economic performance of producers, as well as constraining the material’s suitability for certain high-end applications. Existing purification methods are either inefficient or prohibitively expensive. Inspired by the frequent detection of Ca–Al–F–O compounds in troublesome deposits formed during the reduction process, we propose using CaO to trap aluminium impurities in magnesium vapour, where it reacts with AlF to form Ca₁₂Al₁₄F₂O₃₂, as supported by thermodynamic calculations. By constructing a miniaturized silicothermic reduction apparatus in our laboratory, we show that CaO can reduce aluminium impurities in magnesium down to 6.3 ppm, achieving over 90% removal efficiency. In industrial-scale applications, by using calcined dolomite—a cost-effective, readily available raw material in magnesium plants—the percentage of primary magnesium with aluminium impurities satisfying the Mg9998 grade could be raised from nearly zero to 83%. Compared with the current most popular high-purity magnesium production technology, our approach lowers extra purification costs by ~96%, enabling large-scale, ultra-low-cost production of high-purity magnesium with low aluminium.</p>