Abstract

The phase equilibria in the NaF–AlF₃–CaF₂ system with a fixed sodium-fluoride-to-aluminum fluoride molar ratio of 1.17 are studied. This composition is eutectic; its melting temperature is 680°C. Samples were prepared using individual salts. Aluminum fluoride is purified from oxygen-containing impurities using ammonium fluoride and glassy carbon crucible. The study is performed by thermal analysis, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). To perform XRD and DSC analyses, the melt is frozen on an alumina stick in order to prepare quenched samples. The liquidus temperatures are determined by measuring cooling curves, which are plotted on the thermopower–time coordinates using an APPA 502 multimeter. The solidus temperatures were studied with a STA 449C Jupiter (NETZSCH, Germany) synchronous thermal analyzer, which is calibrated using pure salt supplied by NETZSCH. A single-crystal sapphire is used to calibrate the sensitivity. A MiniFlex 600 X-ray (Rigaku, Japan) diffractometer equipped with a copper anode is used for XRD studies to determine the phase composition of samples. A quasi-binary (NaF–AlF₃)_eut–CaF₂ phase diagram is constructed. Seven samples are studied; in this case, the potassium fluoride content is varied from 0 to 6.5 mol %. The diagram is characterized by a eutectic point at a calcium fluoride content of 0.5 mol % and a temperature of 675°C. The phase diagram also has two peritectic points, which correspond to the decomposition of the complex Ca₂AlF₇ and NaCaAl₂F₉ compounds. When reaching the eutectic point, the liquidus temperature abruptly increases as the calcium fluoride content increases. This results in the low solubility of CaF₂ in the low-melting NaF–AlF₃ eutectic at temperatures below 750°C. The data obtained are of interest for developing a technology of low-temperature electrolysis of aluminum.

中文翻译：

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(0.54NaF–0.46AlF3)eut–CaF2 系统中的相平衡

摘要

NaF–AlF ₃ –CaF _{2 中的}相平衡研究了固定氟化钠与氟化铝摩尔比为 1.17 的系统。这种成分是共晶的；其熔点为680°C。使用单独的盐制备样品。使用氟化铵和玻璃碳坩埚将氟化铝从含氧杂质中提纯。该研究通过热分析、差示扫描量热法 (DSC) 和 X 射线衍射 (XRD) 进行。为了进行 XRD 和 DSC 分析，将熔体冷冻在氧化铝棒上以制备淬火样品。液相线温度是通过测量冷却曲线来确定的，这些曲线使用 APPA 502 万用表绘制在热电 - 时间坐标上。固相线温度使用 STA 449C Jupiter（NETZSCH，德国）同步热分析仪进行研究，使用耐驰提供的纯盐校准。使用单晶蓝宝石校准灵敏度。配备铜阳极的 MiniFlex 600 X 射线（日本理学）衍射仪用于 XRD 研究，以确定样品的相组成。准二元 (NaF–AlF₃ ) _eut –CaF ₂相图被构建。研究了七个样本；在这种情况下，氟化钾含量在 0 到 6.5 mol% 之间变化。该图的特征在于氟化钙含量为 0.5 mol% 和温度为 675°C 时的共晶点。相图中也有两个包晶点，对应于复合物 Ca ₂ AlF ₇和 NaCaAl ₂ F ₉化合物的分解。当达到共晶点时，液相线温度随着氟化钙含量的增加而突然增加。这导致 CaF ₂在低熔点 NaF-AlF ₃中的低溶解度在低于 750°C 的温度下共晶。获得的数据对开发铝的低温电解技术很有意义。