【作者】 阚洪敏；

【导师】 邱竹贤； 张廷安；

【作者基本信息】 东北大学 ， 有色金属冶金， 2008， 博士

【摘要】 低温铝电解是铝业界最活跃的研究课题之一。传统的霍尔-埃鲁法在电解生产铝时通常在950℃左右进行。该方法具有能耗高,操作复杂以及释放大量污染物等缺点。低温铝电解的采用能够在有效地提高电流效率的同时降低能耗。目前冰晶石体系低温铝电解的研究主要从降低电解质的初晶温度入手。但是随着电解温度的降低,出现一系列问题,例如电导率低,阴极结壳,氧化铝溶解度小和溶解速度慢等。在冰晶石体系低温铝电解研究进行的同时,一些国外的研究者开始了室温或者接近室温的铝电解方法(电沉积铝)的研究。采用的方法包括在有机溶液中进行铝的电沉积,以及把离子液体用于电解铝。然而,电解质的电化学电势窗口窄和电导率低使得在有机溶液中进行铝电解的方法受到了限制。离子液体制备的高放热反应造成操作困难,这限制了把离子液体用于电解铝的应用。课题在降低冰晶石体系电解质的初晶温度的基础上对低温铝电解中电解质的物理化学性质(密度和电导率等)进行了研究,同时对接近室温的铝电解的方法进行了研究。课题分别从氟化物体系(冰晶石体系)和氯化物体系进行了低温铝电解的研究。针对氟化物体系,并没有采用前人的经验公式计算电解质体系的初晶温度、密度和电导率,而是利用先进的实验方法测定电解质体系的初晶温度、密度和电导率,提高了数据的准确性和可靠性。课题利用34401A型数字万用表与计算机串口相结合的技术,根据步冷曲线确定电解质的初晶温度。采用东北大学炼铁研究所研制的RTW-09型熔体物性综合测定仪,根据阿基米德定律对密度进行了测定。选择热解氮化硼作为电导池,采用交流技术,利用PGSTAT30恒电位仪和BOOSTER20A电流扩展仪在高频范围内用小振幅的正弦波信号测量阻抗,用CVCC法对电导率进行了测定。不仅研究了AlF3、CaF2、LiF的加入对电解质物理化学性质的影响,而且研究了低浓度NaCl的加入对电解质物理化学性质的影响。针对氯化物体系,采用循环伏安法、计时电位法和计时电流法,研究了接近室温的铝的电化学沉积机理和成核过程,以便更好地理解该电解质体系中阴极上所发生的电化学反应。同时研究了电流密度对表面形貌以及沉积物与基体结合力的影响,以考察该体系作为铝电解的电解质的可能性。考虑到该体系的实用性,课题更注重在高电流密度下铝基体上电沉积过程的研究。针对氟化物体系,研究了Na3AlF6-AlF3-Al203-CaF2-LiF-NaCl体系的初晶温度、密度和电导率。实验结果表明：(1)在铝电解质中用氯化钠取代部分氟化锂能够有效地降低电解质的初晶温度。方差分析表明：NaCl和LiF对初晶温度都有显著影响,两者配合使用效果良好。(2)NaCl能有效地降低电解质密度,有利于工业生产,而LiF却使酸性电解质的密度稍有增加。在铝电解质中用氯化钠取代部分氟化锂能够有效地降低电解质的密度。(3)氟化锂和氯化钠能显著地提高电解质的电导率。当氟化锂和氯化钠的浓度分别增加1%时,平均电导率值分别增加为0.0276S/cm和0.024S/cm。当温度升高1℃时,电解质的电导率大约增加0.003S/cm。分子比增加有利于提高电导率。本文测得的电导率值低于Wang经验公式计算值,高于Choudhary经验公式计算值。同时根据电导率得到了部分体系的电导活化能。(4)从理论上解释了AlF3.A1203.LiF.NaCl对电解质的初晶温度、密度和电导率等物理化学性质的影响。(5)在实验的条件范围内,对Na3AlF6-A1F3-A1203-CaF2-LiF-NaCl体系所做的测定数据进行回归分析,分别得到初晶温度、密度和电导率的回归方程。针对氯化物体系,研究了AlCl3-NaCl体系的铝的电化学沉积机理和成核过程。实验结果如下所示：(1)循环伏安分析表明：Al(Ⅲ)离子通过4A12Cl7-+3e-→Al+7 AlCl4-和AlCl4-+3e-→Al+4 Cl-两个连续的步骤还原为金属铝；铝在钨电极上的电化学沉积需要一定的过电位。(2)计时电位分析表明：在一定的电流密度下,Al(Ⅲ)离子以两个连续的步骤还原,与循环伏安的研究结果吻合很好；铝在钨电极上电沉积发生成核和长大的电结晶过程。(3)计时电流分析表明：铝在钨电极上的电沉积成核的时间-电流曲线的特征依赖于所施加过电压的大小；铝在钨电极上发生电沉积,存在着明显的成核过程且为半球扩散控制成长的瞬时成核过程。(4)恒电流沉积分析表明：当铝在铝基体上恒电流沉积且电流密度在50～100mA/cm2时,镀层致密且与铝基体的结合较好；当电流密度大于200mA/cm.时,镀层有裂缝且与基体的结合较差。因此,AlCl3-NaCl熔盐体系作为电解质,在电流密度低于100mA/cm2时进行电沉积铝是完全可能的。(5)铝在铜基体上的电沉积表明：在金属铝形成之前,铝与铜形成金属间化合物并且金属间化合物的形成由铝原子向铜基体内的扩散控制。XRD结果表明：金属间化合物为AlCu.A14Cu9和A12Cu,电沉积的镀层厚度约为20μm。更多还原

【Abstract】 Low temperature aluminium electrolysis has been one of active research fields in recent years. Traditional Hall-Heroult electrolysis process for aluminium production usually operates at very high temperature (nowadays about 950℃) and unavoidably shows high energy consumption, complicated operation and pollutant emission. By the introduction of low-melting baths one might expect an increase in current efficiency and lower energy consumption. It is important to reduce liquidus temperature for study on low temperature aluminium electrolysis. But many problems will occur, such as low electrical conductivity, cathode shell, and low alumina solubility and alumina-solution rates when the temperature is too low. Furthermore, over the last decades, many researchers turned their interest in the methods at or near ambient temperatures to reduce pollutant emission and energy consumption. Aluminium electrodeposition in organic solutions was investigated for potential application in aluminium refining and recycling. Development in ionic liquids resulted in another potential approach for aluminium extraction and refining. But the electrolyte properties (e.g. low electrochemical potential windows and low electrical conductivity) limited organic solutions applications. It is difficult to prepare ionic liquids due to the highly exothermic reaction. It limited ionic liquids applications for aluminium extraction and refining.We met the problem by studying fluoride system and chloride system. The purpose of the present work is to reduce liquidus temperature and density, increase electrical conductivity to provide a scientific basis for selecting suitable low temperature aluminium electrolyte composition for fluoride system. At the same time chloride system is investigated near ambient temperatures to obtain a new method for low temperature aluminium electrolysis.For fluoride system, in this study, advanced experiment methods were used. The liquidus temperature, density and electrical conductivity of Na3AlF6-AlF3-Al2O3-CaF2-LiF-NaCl system were measured, but not based on the empirical equations. To a great degree, the accuracy and reliability of the data were better. The liquidus temperature was measured with one Agilent 34401A meter connected to a computer. The method was based on the principle of thermal analysis. The whole process was controlled by computer software. The density measurement method was based on the Archimedes law. RTW-09 melt objects integrated detector made at Northeastern University was used to measure density of the electrolyte. Ac-techniques with a PGSTAT30 and a BOOSTER 20A are used to measure impedance with a sine wave signal with small amplitude in high frequency range. Electrical conductivity is gained by the Continuously Varying Cell Constant (CVCC) technique together with a pyrolytic boron nitride conductance cell. Not only the effects of AIF3, Al2O3, and LiF on liquidus temperature, density and electrical conductivity were discussed but also low concentration NaCl was discussed. For chloride system, the purpose of the present work is to determine the mechanism of the electrodeposition of aluminium from AlCl3-NaCl melts onto W substrate where the methods of cyclic voltammetry, chronopotentiometry and chronoamperometry have been employed. It is necessary to reach a better understanding of electrochemical reactions occurring at very cathodic potentials in the melts. The effects of deposition parameters such as deposition current density on surface morphology and adherence of the deposits were evaluated. We are especially interested in the electrodeposition of aluminium on Al substrates and at relatively high current densities due to their practical importance. Our work aims to explore the possibility of using AlCl3-NaCl melts as potential electrolytes for the electrolytic extraction and recycling of aluminium in aluminium industry.For fluoride system, the liquidus temperature, density and electrical conductivity of Na3AlF6-AlF3-Al2O3-CaF2-LiF-NaCl are discussed. By experiments and discussion, the main contents and results can be summarized as follows:(1) The addition of NaCl and LiF into the electrolyte reduced greatly the liquidus temperature. The Variance analysis showed it is necessary that LiF be partly replaced by NaCl. It not only improves the physicochemical properties of the electrolyte but also decreases cost and increases economic benefits.(2) The result showed increase in NaCl concentration reduced bath density. It is beneficial to industry production. But LiF increased density of the acidic melt. NaCl replace LiF partly in electrolyte system, the density was reduced greatly. (3) The experiments showed that electrical conductivity was increased greatly with NaCl and LiF added. Increasing lwt%LiF resulted in corresponding increase of 0.0276 S/cm for superheat condition 15℃. For NaCl, it was 0.024 S/cm. Electrical conductivity was increased 0.003 S/cm with 1℃increased. In this study, electrical conductivity was lower than that which is predicted by the Wang Model and higher than that which is predicted by the Choudhary Model. Activation energy of conductance was obtained based on the experiment results.(4) The effects of AIF3, Al2O3, LiF, and NaCl on liquidus temperature, density and electrical conductivity were discussed in theory.(5) In experimental range, the regression equations were developed on the basis of experimental data.For Chloride system, the mechanism of the electrodeposition of aluminium from AlCl3-NaCl melts is investigated. By experiments and discussion, the main contents and results can be summarized as follows:(1) The voltammetric studies showed Al (Ⅲ) was reduced in two consecutive steps: 4Al2Cl7-+3e-→Al+7 AICl4-, AlCl4-+3e-→Al+4 Cl-.Certain nucleation overpotential was required during the deposition of aluminium on W electrode.(2) Chronopotentiometry analysis showed that Al (Ⅲ) was reduced in two consecutive steps under certain current density. This is in reasonable agreement with cyclic voltammograms. The electrochemical deposition process of aluminium on tungsten electrode has been investigated and found to proceed by a nucleation and growth mechanism.(3) The current-time characteristics of nucleation on tungsten showed a strong dependency on overpotential. Initially, the current decreases rapidly with the time, then the current began to increase and became flat gradually. These transients showed the typical nucleation characteristics during the deposition of aluminium on tungsten electrode. Chronoamperometric analysis showed that the deposition of aluminium exhibited instantaneous three-dimensional nucleation with hemispherical diffusion-controlled growth of nuclei.(4) By using constant current deposition, the electrodeposits obtained on aluminium substrates between 50 and 100mA/cm2 were quite dense and well adherent to the aluminium substrates. Those obtained at the current density higher than 200mA/cm2 had intergranular crevices growth with relatively poor adherence. Our studies showed that AlCl3-NaCl melt system can be possibly used as potential electrolytes for the electrolytic extraction and recycling of aluminium at less than 100mA/cm2.(5) The electrochemical deposition of aluminium on copper substrates in AlCl3-NaCl melts indicated that the formation of intermetallic compounds was occurred before the metal aluminium was formed. The formation of intermetallic compounds was controlled by the process of which aluminium atoms were diffusing into copper electrode. XRD showed intermetallic compounds were AlCu, Al4Cu9 and Al2Cu. Coating thickness was about 20μm.更多还原