【作者】 王丹琴；

【导师】 李小斌；

【作者基本信息】 中南大学 ， 冶金工程， 2012， 博士

【摘要】 铝酸钠溶液分解是从铝土矿提取氧化铝过程中至关重要的工序之一,直接影响产品的物理和化学性能以及分解过程的效率,是当前氧化铝工业上的技术瓶颈之一。要实现铝酸钠溶液的强化分解,必须首先在铝酸钠溶液的结构及其分解理论上寻求突破。本论文通过计算铝酸根阴离子的迁移数,结合红外光谱分析,研究了铝酸根离子的电迁移性质与其结构间的关系,分析了铝酸钠溶液分解过程中各种铝酸根离子的反应行为,并最终确定了有利于铝酸钠溶液分解过程的离子类型。通过调控铝酸钠溶液的离子结构,使其向含有更多有利于分解过程的铝酸根离子转化,并最终促进了铝酸钠溶液的分解过程。论文的主要研究结果如下：(1)铝酸钠溶液的电导率随着Na2O质量浓度的升高先变大后减小,在100～120g/L出现一极大值,符合强电解质电导率变化规律。根据电解质溶液离子迁移数的计算方法,计算了铝酸钠溶液中各种铝酸根离子的离子迁移数,结合红外光谱,研究了铝酸根阴离子的导电能力和结构的关系。研究发现：较低Na2O质量浓度(<175g/L)下,铝酸根阴离子的迁移数为0.6左右,红外光谱显示在这个浓度范围内离子结构主要为Al(OH)4-；中等Na2O质量浓度(175-330g/L)下,铝酸根阴离子的迁移数为0.2左右,此时溶液结构为A1(OH)4-和[Al2(OH)8(H2O)2]2-；高Na2O质量浓度(>330g/L)下,铝酸根阴离子的离子迁移数接近于0,对应的溶液结构主要是Al2O(OH)62-和[Al2(OH)8(H2O)2]2-,只有少量Al(OH)4-。铝酸钠溶液中铝酸根离子聚合度随溶液碱浓度的增大而增大,铝酸根离子的电迁移能力则随离子聚合度的增大而降低。(2)高浓度或高苛性比的铝酸钠溶液中,铝酸根离子主要以聚合形式存在,随着溶液的稀释,铝酸钠溶液中聚合离子逐渐减少,而单体的铝酸根离子Al(OH)4-逐渐增多,稀释到一定程度时,铝酸钠溶液中只存在Al(OH)4-。随着温度的升高,铝酸钠溶液的电导率逐渐升高,这是因为温度升高时,导电能力较强的Al(OH)4-离子有所增多,同时880cm-1峰也逐渐增强,说明温度的升高有利于550cm-1处对应的Al-O-Al结构的Al2O(OH)62-向Al(OH)4-和880cm-1处Al-O伸缩振动带对应的[Al2(OH)8(H2O)2]2-离子转化。(3)研究发现,碳酸钠和硫酸钠均会改变铝酸钠溶液的结构。当C(Na2CO3)>10g/L,C(Na2SO4)>5g/L时,铝酸钠溶液中四面体离子Al(OH)4-明显减少,低波数段Al-O-Al结构的Al20(OH)62-离子明显增多。同时,加入碳酸钠和硫酸钠后,铝酸钠溶液粘度升高,电导率下降,并对晶种分解产生抑制作用。由此说明,铝酸钠溶液中聚合离子增多,不利于铝酸钠溶液的晶种分解。硅可以取代铝酸钠溶液中Al-O-Al结构中的Al,形成Al-O-Si结构和Al-O-Si-O-Al-O结构等多种形式硅氧铝键以及由氢键相连的大分子结构,导致铝酸钠溶液粘度增大,分解率降低。(4)铝酸钠溶液晶种分解过程中,720、880cm1处Al-OH振动带对应的四面体铝酸根离子百分比及浓度不断下降；550cm-1处对应的Al20(OH)62-离子百分比及浓度略有上升。衰减全反射(ATR)方法测得铝酸钠溶液分解过程中氢氧化铝表面附液中主要为Al(OH)4-离子,且随着分解时间的延长,该离子的浓度基本保持不变。结合本体溶液和氢氧化铝附液中铝酸根离子的结构变化,认为铝酸钠溶液分解过程中有利于氢氧化铝析出的含铝离子是Al(OH)4-等Al-OH四面体离子,若调控溶液中其它复杂含铝离子结构向该类结构转变,将有利于氢氧化铝的析出。(5)通过磁场处理铝酸钠溶液后,铝酸钠溶液的电导率和离子结构都会发生明显的变化。在磁场强度为50～70kA／m及150～200kA／m范围内时,铝酸钠溶液的电导率达到最大值,铝酸钠溶液中导电能力最强的离子A1(OH)4-的浓度达到最大。在磁场强度为52kA／m下处理铝酸钠溶液并进行晶种分解,当晶种包含有部分细颗粒时,磁场提高铝酸钠溶液的分解率为6%左右,但其分解产品的粒度也变细,究其原因是磁场使铝酸钠溶液中二聚离子减少、Al(OH)4-增多,并最终加速了铝酸钠溶液的二次成核过程；当晶种为均一的粗颗粒时,磁场提高铝酸钠溶液的分解率为2-3%,且产品形貌规则,颗粒大小均匀。除此之外,晶种和杂质的加入,也可以改变溶液中铝酸根离子间的转化并最终影响分解过程。更多还原

【Abstract】 The precipitation of gibbsite from caustic aluminate solution is one of the most important operations during the alumina production process, which influences the precipitation efficiency and the physical and chemical properties of products directly. It has been the rate-limiting step in the Bayer process used for alumina refining since the innovation of Bayer process. The fundamental understanding of the structure of aluminate solution and the precipitation mechanism is essential for substantial advances in the precipitation process, although some improvements have been achieved over the years by incremental process modifications. In this thesis, the intrinsic relationship between the electric conductivity and the structure of sodium aluminate solutions was investigated by calculating the transference number of aluminate anions (t(Al)) in the solution of different alkali concentration and molar ratio of Na2O to Al2O3. Furthermore, combining with the characterization of FTIR analyses of the solution, the changes of aluminate ions in the Bayer seeded precipitation process were analyzed and thus the aluminate species favoring gibbsite precipitation were approximately confirmed. The gibbsite precipitation process was eventually enhanced by transferring other aluminate ions to those which were beneficial for gibbsite crystallization. The main results and progresses are listed as follows:(1) The electric conductivity of sodium aluminate solution firstly increases with increasing concentration of alkaline, and then reaches the maximum when the concentration of C(Na2O) is between100-120g/L, illustrating that the aluminate solution belongs to strong electrolyte solution. The relationship between electric conductivity and ion structure of sodium aluminate solution was investigated and the results show that in the solution of low concentration of C(Na2O)<175g/L, the aluminate anions dominantly exist in the form of Al(OH)4-and t(Al) is approximately0.6; that in the solution of medium concentration of C(Na2O) from175g/L to330g/L, the aluminate anions mainly exist in the form of Al(OH)4-and [A12(OH)8(H2O)2]2-,in which t(Al) is approximately0.2; and that in the solution of high concentration of C(Na2O)>330g/L, aluminate anions exist dominantly in the form of Al2O(OH)62-and [Al2(OH)8(H2O)2]2-with a little Al(OH)4-, where t(Al) is almost zero. This illustrates that the electric conduction ability of the aluminate anion in the sodium aluminate solution is closely related to its structure. It has been studied that the degree of polymerization of aluminate anions rises with the increasing concentration of the alkaline and the transference number of the aluminate anions declines with the increasing degree of polymerization of aluminate anions.(2) The aluminate species maily exist in the form of dimeric or polymeric species in the solution of high concentration of alkaline or high causitic ratio. The concentration of dimeric or polymeric species decreases with the diluting process and the concentration of Al(OH)4-increases gradually and eventually becomes the predominant species in alkaline aluminate solutions. The concentration of tetrahedral aluminate ions corresponding to the bands at720cm-1and880cm-1increases remarkably as the temperature rises, while the concentration of Al2O(OH)62-corresponding to the band at about550cm-1declines slightly, suggesting that the tetrahedral aluminate ions are transferred by Al2O(OH)62-.(3) Na2CO3and Na2SO4can change the ion structure of the aluminate solution, the concentration of Al(OH)4-declines remarkably, while the concentration of Al2O(OH)62-increases obviously according to the FTIR analyses when the concentration of Na2CO3is above10g/L and the concentration of Na2SO4is above5g/L. Besides, the viscosity of sodium aluminate solution is increased, the electric conductivity is reduced and the gibbsite precipitation is inhibited after adding Na2CO3and Na2SO4. Consequently, higher concentration of polymeric aluminate anions is harmful to the precipitation process of gibbsite from sodium aluminate solution. It is obtained that Al in Al-O-Al is replaced by Si and formed Al-O-Si, Al-O-Si-O-Al band, and larger molecular structures connected by hydrogen bonding are also formed, resulting in the increase of the viscosity and the decrease of the precipitation ratio.(4) The concentration of tetrahedral aluminate ions corresponding to the bands at720cm-1and880cm-1declines remarkably as the precipitation process proceeds, while the concentration of Al2O(OH)62-corresponding to the band at about550cm-1increases slightly, the content of Al(OH)4-in the attached-liquid on the aluminum trihydroxide surface has not diminished apparently, suggesting that Al-OH tetrahedral aluminate ions centered at about720and880cm-1are the most beneficial aluminate ions resulting in gibbsite precipitation from sodium aluminate solution, while the dimeric aluminate species centered at about550cm-1are disadvantageous to gibbsite precipitation. The gibbsite precipitation process can be enhanced by transferring other aluminate ions to ions which are beneficial for gibbsite crystallization.(5) The electrical conductivity of the sodium aluminate solution with low caustic molar ratio of Na2O to Al2O3varies with the intensity of the applied magnetic field and reaches a maximum in the presence of a magnetic field ranging from50to70kA/m and from150to200kA/m. IR spectra of sodium aluminate solutions suggest that magnetic field treatment modifies the structure of the aluminate solution. Moreover, the gibbsite precipitation from sodium aluminate solution would be promoted when the aluminate solution is magnetically treated with a magnetic field of52kA/m. The precipitation ratio increases about6%compared with the nonmagnetically treated aluminate solutions when the seeds include a fraction of finer particles, and the gibbsite products include more fine particles, indicating the secondary nucleation process is enhanced in the early stage of the precipitation process. The precipitation ratio increases about2～3%compared with the nonmagnetically treated aluminate solutions when the seeds with uniform size are added. And the products are made up of globular particles in uniform size. It can be concluded that the precipitation of gibbsite from sodium aluminate solution could be promoted by adjusting the structure and the physicochemical properties of aluminate solutions by introducing suitable magnetic field. Moreover, adding seed or impurities can also adjust the aluminate ions and eventually affect the gibbsite precipitation process.更多还原