It is practically difficult to find titanium sponges with low and stable aluminum impurities on the market even though it is the precondition to prepare high-purity titanium. Analysis indicates that almost all the aluminum impurities in the titanium sponge are inherited from the magnesium used to reduce titanium tetrachloride. However, it remains elusive for decades why magnesium produced through the silicothermic reduction method contains a high content of aluminum impurities with large fluctuations. By recourse to thermodynamic calculations and comparative experiments, we demonstrate that fluorite, a material used as a catalyst in the silicothermic reduction method to produce magnesium, is the chief culprit for the pest aluminum and propose a mechanism to rationalize the observed phenomena. Our findings indicate that one practical way to produce qualified magnesium for the production of high-purity titanium is to abandon fluorite during the production of magnesium with the silicothermic reduction method.

尽管这是制备高纯钛的前提，但市场上很难找到铝杂质含量低且稳定的海绵钛。分析表明，海绵钛中的铝杂质几乎全部来自还原四氯化钛所用的镁。然而，硅热还原法生产的镁为何铝杂质含量高且波动大，几十年来一直是个谜。通过热力学计算和比较实验，我们证明了萤石（一种在硅热还原法生产镁中用作催化剂的材料）是有害铝的罪魁祸首，并提出了一种机制来合理化所观察到的现象。我们的研究结果表明，生产用于高纯钛生产的合格镁的一种实用方法是在硅热还原法生产镁的过程中放弃萤石。