Yan Hengwei,Liu Zhanwei,Ma Wenhui,Huang Liqiang,Wang Chengzhi,Liu Yingxin
The development of low-temperature aluminum electrolysis makes it possible to reduce energy consumption and greenhouse gas emissions. The KF-NaF-AlF 3 -based electrolyte is a promising low-temperature electrolyte for aluminum reduction. The liquidus temperatures of KF-NaF-AlF 3 electrolytes with a total cryolite ratio (CR t ) fixed at 1.3–2.0, and with different concentrations of NaF (with a step change of 5 mol.%), were investigated by the cooling curve method. The results showed that the liquidus temperature of KF-NaF-AlF 3 mixtures increases with the increase of the CR t , which has a bigger influence on the liquidus temperature than the traditional NaF-AlF 3 electrolyte currently used in aluminum smelters. When the content of NaF is 10–20 mol.%, the curves of the liquidus temperature vs. NaF content reach a peak. When the NaF content ranges from 30 to 40 mol.%, the liquidus temperature is relatively low (695–900°C). The precipitated order of the compounds in molten KF-NaF-AlF 3 electrolytes with a CR t of 1.3 is as follows: K 2 NaAlF 6 (or K 3 AlF 6 ) → Na 3 Al 5 F 14 → K 2 NaAl 3 F 12 → KAlF 4 .
Yan Hengwei,Yang Jianhong,Liu Zhanwei,Wang Chengzhi,Ma Wenhui
The cryolite ratio (CR) is an important parameter for the electrolyte in aluminum reduction cells. The measurement method for the CR of the KF-NaF-AlF 3 system acid (CR < 3) electrolyte by means of electrical conductivity was initially developed, and the formula for calculating the CR was deduced. This method has the advantages of simple operation and high precision. In addition, the relative standard deviations (RSD) of the measurement are < 1.2 pct, and the analysis error of the NaF or KF content has little effect on the determination of the CR.
KF-NaF-AlF 3 -based electrolyte is a promising low-temperature electrolyte for aluminum reduction. Alumina solubility in molten KF-NaF-AlF 3 -based electrolyte was determined as a function of the melt composition and temperature by measuring the weight loss of a rotating corundum disk and by using a LECO RO500 oxygen analyzer (LECO Corporation, St. Joseph, MI). The investigated temperature range is 1023 K to 1073 K (750 °C to 800 °C), and the total cryolite molar ratio (CRt = ([KF] + [NaF])/[AlF 3 ]) is 1.3 to 1.5; the content of NaF ranges from 0 mol pct to 50 mol pct. The effect of temperature, CaF 2 , and LiF on alumina solubility is discussed as well.
The surface tension and density of molten KF–NaF–AlF3-based electrolytes were determined as a function of the melting composition and temperature by the Archimedean method and maximum bubble-pressure method, respectively. The investigated temperatures range from (1013 to 1103) K, and the total cryolite molar ratio (CRt = ([KF] + [NaF])/[AlF3]) is 1.3 to 1.41; the mole fraction of NaF ranges from 0.15 to 0.4. The effects of temperature and the content of CaF2 and Al2O3 on surface tension and density were also investigated. The results indicate that the density and the surface tension tend to increase with the NaF content and CRt. The density and the surface tension fit the linear relationship with temperature.
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