| There are some impurities contained in domestic alumina, especially a certain amount of K2O and Li2O in alumina which would lead to the enrichment of the high-concentrations LiF and KF in electrolytic cell during aluminum produced. Meanwhile the enrichment of LiF and KF in electrolytic cell can affect the liquidus temperature of the samples and the solubility of alumina. In order to provide theoretical basis for controlling alumina concentration and superheating accurately, and to achieve a high efficient and energy-saving aluminum production process, it is necessary to explore the liquidus temperature of the electrolyte and solubility of alumina in the system of Na3A1F6-A12O3-A1F3-KF-LiF-CaF2-MgF2 molten salt.Based on the principle of single variable, this thesis studies the respective effect of cryolite ratio, and of the additive amount of LiF, KF, CaF2, MgF2 on the liquidus temperature of the electrolyte, and deduces the regression equation by means of least squares, as well as the effects of the additive amount of LiF and KF, keeping time and the temperature on the solubility of alumina. Through a systemic study by the analytical method of least squares, it shows that:(1) The effect of cryolite ratio on the liquidus temperature is obvious, i.e the liquidus temperature of electrolyte rises as the cryolite ratio increases. When the cryolite ratio is constant, as the contents of LiF, KF, CaF2 and MgF2 increase within the test scale, the liquidus temperature of cryolite-alumina system can be decreased correspondingly, and the effects of LiF and MgF2 on decreasing the liquidus temperature is stronger than those of KF and CaF2.(2) Considering the real situation of electrolytic aluminum industry, the regression equation is applied to simulate the empirical equation of the liquidus temperature in the system of Na3A1F6-A12O3-A1F3-CaF2-MgF2-LiF-KF, in which the cryolite ratio is 2.2-3.0, the content of alumina is 3%, the content of CaF2 is 0 wt%-5 wt%, the content of MgF2 is 0 wt%-5 wt%, the content of LiF is 0 wt%-7 wt% and the content of KF is 0 wt%-7 wt%.t/℃= 1000.67-1.488[ExA1F3]-10.114[LiF]-3.119[KF]-0.717[CaF2]-7.92[MgF2] ＋0.1451[ExA1F3][LiF]＋0.1251[ExA1F3][KF]-0.0796[ExA1F3][CaF2] ＋0.0968[ExA1F3][MgF2]-0.1729[ExA1F3][ExA1F3]＋0.0293[LiF][LiF] ＋0.0416[KF][KF]-0.1325[CaF2][CaF2]＋0.005[MgF2][MgF2]In the equation, t is the liquidus temperature, ℃; the mass percentage content of each additives is given in square bracket, and [ExAIF3] reprents the content of A1F3. The correlation coefficient in this empirical equation is 0.9876.(3) The effect of temperature on the solubility of alumina is obvious, i.e the solubility of alumina in the melt increases as the temperature rises. When the temperature is constant, LiF can increase the solubility of alumina within the test scale, whereas LiF can decrease the solubility of alumina.(4) The 73 batches of test results are summed and processed by the means of least squares for a purpose of a better study of the combined effect of LiF and KF. Accordingly the empirical equation for solubility of alumina is calculated:S= 151.46-3.224[LiF]＋5.333[KF]-0.351* T-0.0114[LiF][KF] ＋2.995* 10-3*[LiF]* T-5.416*10-3* [KF]* T＋9.56*10-3*[LiF] [LiF] ＋0.0363* [KF] [KF]＋2.079* 10-4* T2The equation is applicable to the range CR=2.4,0≤[LiF]≤6 wt%,0<[KF]≤6 wt%, [CaF2]= 3 wt% and [MgF2]= 2 wt%. In the equation, t is the temperature of the molten salt, ℃; the mass percentage content of each additives is given in square bracket. The correlation coefficient in this empirical equation is 0.9563. |
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