【作者】 刘战伟；

【导师】 陈文汨； 李旺兴；

【作者基本信息】 中南大学 ， 冶金物理化学， 2011， 博士

【摘要】 砂状氧化铝由于具有较粗的粒度和较高的强度,在节能与环境保护要求日益严格的铝电解工业得到广泛的应用。要获得砂状氧化铝,必须促进氢氧化铝晶体的附聚和长大。附聚是获得粗粒氢氧化铝的重要前提,本论文着重研究了铝酸钠溶液种分附聚过程。应用红外光谱和拉曼光谱对铝酸钠溶液进行半定量分析,并首次探讨了结晶助剂(CGM)对铝酸钠溶液结构影响以及铝酸钠溶液微观性质与分解过程宏观指标的关联；考察了各种因素对种分附聚过程的影响并得出了有利于种分附聚的条件；研究了含有C=C键的长链十八碳羧酸表面活性剂和结晶助剂对种分附聚过程的影响,并通过对种分附聚工艺的改善得到了粒度较粗和强度较高的氢氧化铝产品。本论文的研究成果将对砂状氧化铝生产提供一定的技术支持和理论依据,其主要研究结果如下：1.建立了铝酸钠溶液拉曼光谱结构分析方法和确定了优化的操作条件。发明了以盖玻片为内标物的铝酸钠溶液拉曼光谱半定量分析方法。根据拉曼光谱图中铝酸钠溶液特征峰位置可以确定铝酸根离子的类型,根据铝酸钠溶液特征峰的强度或面积与盖玻片内标物荧光特征峰(920cm-1)强度或面积的比值可以半定量分析铝酸根离子含量。2.通过对铝酸钠溶液结构分析发现：溶液分子比1.4-3.2、苛性碱浓度120-260g/L的铝酸钠溶液,其主体铝酸根离子均为[Al(OH)4]-。在相同苛性碱浓度下,主体铝酸根离子[Al(OH)4]-和铝酸根二聚离子[Al2O(OH)6]2-浓度均随溶液分子比升高而逐渐下降。苛性碱浓度在120-220g/L时,同苛性碱浓度下低分子比铝酸钠溶液R值(620cm-1和535cm-1处特征峰强度之比)高,分解趋势大。超高苛性碱浓度(260g/L)铝酸钠溶液的R值随溶液分子比变化幅度较小,其值相对较高,相对于苛性碱浓度稍低的溶液(比如220g/L),该类溶液具有相对较高的分解趋势。在相同溶液分子比下,低苛性碱浓度铝酸钠溶液R值高,溶液分解趋势大。但中等浓度苛性碱(157-174g/L)和超高浓度苛性碱(260g/L)具有相对较高R值,其溶液分解趋势也相对较大。通过计算对比函数,发现添加剂可能通过改变溶液中主体铝酸根离子[Al(OH)4]-的浓度,从而对铝酸钠溶液结构产生微小影响。3.随着温度的增加,铝酸钠溶液分解率下降,氢氧化铝附聚效率增加,高温有利于-25μm粒级颗粒的附聚。随着铝酸钠溶液起始分子比的增加,溶液分解率下降,分解反应8h的氢氧化铝附聚效率先增加后降低。溶液起始分子比过低或过高时,附聚产物中均含有较多的-45μm粒级颗粒。随着铝酸钠溶液苛性碱浓度的增加,溶液分解率逐渐下降,附聚产物的粒度和氢氧化铝附聚效率也随之降低。随着晶种系数的增加,铝酸钠溶液分解率增加,氢氧化铝附聚效率先增加后降低。晶种系数过高或过低时,附聚产物中均含有较多的中小粒径颗粒。随着初始晶种粒度的增加,铝酸钠溶液分解率和氢氧化铝附聚效率减小。铝酸钠溶液分解率随搅拌速度的增加而增加。在适中的搅拌速度下,氢氧化铝附聚效率最高,附聚产物具有最好的粒度分布。高温78℃、较低溶液起始分子比1.45、低苛性碱浓度、晶种系数0.25和搅拌速度100r/min等条件有利于铝酸钠溶液的种分附聚过程。4.通过对一段法与二段法分解工艺所得产品形貌和粒度分布的比较,发现在铝酸钠溶液晶种分解过程中加强附聚就可以得到粒度较大和强度较高的氧化铝产品。并且对附聚机理进行了探讨,在附聚过程中,氢氧化铝晶种中细小颗粒优先附聚。5.当硬脂酸、油酸和亚油酸三种表面活性剂添加量较少,为10ppm时,不含有C=C键的饱和硬脂酸降低了铝酸钠溶液的分解率；当添加量大于20ppm时,表面活性剂中有无C=C键都能提高铝酸钠溶液分解率,且添加三种表面活性剂的铝酸钠溶液分解率基本接近。在温度较低(65℃)或溶液过饱和度较高的情况下,表面活性剂的加入可以对二次成核有一定的抑制作用；在温度较高(85℃)或溶液过饱和度较低的情况下,表面活性剂的加入可以促进附聚。晶种系数较低或较高时,添加表面活性剂增强附聚的效果都不是很明显。低搅拌速度和较细的初始晶种粒度有利于添加剂对附聚的增强效果。添加表面活性剂对附聚过程的增强作用主要体现在细粒子上。不管是抑制二次成核还是促进附聚,添加亚油酸的效果最好,油酸次之,硬脂酸最差,这与表面活性剂长链十八碳羧酸分子中C=C键有着紧密的关系。添加含有两个C=C键的亚油酸时,铝酸钠溶液的表面张力降低的幅度最大,油酸次之,不含有C=C键的饱和硬脂酸溶液表面张力降低的幅度最小。6.在铝酸钠溶液中添加CGM可以明显地增强氢氧化铝颗粒的附聚和粗化,从而有效地抑制二次成核,但是却降低了铝酸钠溶液的分解率。然而将其与可以提高铝酸钠溶液分解率但是对附聚的促进作用较小的表面活性剂混合添加则可以取长补短,既可增加溶液分解率,又使产品的粒度增加。通过对种分附聚工艺的改善有使粉状氧化铝变成砂状氧化铝的可能性。更多还原

【Abstract】 With the increasingly strict requirement of energy saving and environmental protection, sandy alumina is used widely in the industry of aluminum electrolysis, because of its coarse particle size and high strength. Agglomeration and crystal growth must be promoted for producing sandy alumina. Agglomeration is the important precondition for producing coarse aluminium hydroxide, this paper especially focuses on agglomeration process.In this paper, infrared and raman spectrum are applied in semi-quantitative analysis of sodium aluminate solution. It is investigated the effect of crystal growth modifier (CGM) on structure of sodium aluminate solution and the relationship between microscopic properties of sodium aluminate solution and macroscopic indices of precipitation process. Effect of operation factors on the agglomeration process is investigated and favorable conditions are attained. Effect of surfactant and CGM on the agglomeration process is investigated and coarse aluminum hydroxide products with high strength are attained by improvement of agglomeration process. The results in the paper may be a technical support and theoretical guidance for alumina production. Main conclusions are drawn as follows:1. The determination method for structure of sodium aluminate solution is developed and suitable operation conditions are obtained. Using cover-glass as internal standard, a semi-quantitative method of raman spectrum for the analysis of sodium aluminate solution is invented, the type of aluminate ion can be determined according to the position of characteristic peak of sodium aluminate solution in raman spectrum and the content of aluminate ion can be analyzed by semi-quantitative method according to the ratio of intensity and area of characteristic peak of sodium aluminate solution to that (920cm-1) of the cover-glass.2. Through the analysis of structure of sodium aluminate solution, it is found that main aluminium-containing ion is [Al(OH)4]- when molecular k) of liquor is 1.4-3.2 and caustic alkali concentration (Nk) of liquor is 120-260g/L. At the same Nk, the concentration of [Al(OH)4]- and [Al2O(OH)6]2- decreases gradually with increasing ak of the liquor. When Nkis 120-220g/L, the R value (Ratio of intensity of characteristic peak of 620cm-1 to that of 535cm-1) is high and the tendency of precipitation is strong at the same Nk when ak of the liquor is low. The change of R is small with the change of ak when the Nk of sodium aluminate solution is very high (260g/L), but the R is high relative to that of the liquor of lower Nk (220g/L), the liquor of high R has stronger precipitation tendency. At the same ak of the liquor, the R is high and the precipitation tendency is strong when the Nk of sodium aluminate solution is low, but the R of liquor of moderate Nk(157-174g/L) and very high Nk (260g/L) is higher and the precipitation tendency is relatively strong. Through calculating contrast function, it is found that additive has limited effect on the structure of sodium aluminate solution by changing the concentration of [Al(OH)4]-.3. The precipitation yield of sodium aluminate solution decreases and the agglomeration efficiency of gibbsite particles increases with the increasing liquor temperature, high temperature is advantageous for agglomeration of particles less than 25μm. The precipitation yield decreases and the agglomeration efficiency at 8h increases firstly and then decreases with the increasing ak of liquor, the agglomerates contain much more particles less than 45μm when ak is too high or low. The precipitation yield, the particle size of agglomerates and the agglomeration efficiency all decrease with the increasing Nk of the liquor. The precipitation yield increases and the agglomeration efficiency increases first and then decreases with the increasing mass ratio Ks of crystal seeds, the agglomerates contain much more median and small particles when Ks is too high or low. The precipitation yield and the agglomeration efficiency all decrease with the increasing particle size of initial seeds. The precipitation yield increases with the increasing agitation rate, the agglomeration efficiency of gibbsite products is highest and particle size distribution of agglomerates is best at a moderate agitation rate. High temperature (78℃), lower initialαk(1.45), low Nk, Ks 0.25 and agitation rate 100r/min are favorable for the agglomeration process.4. By comparing with morphology and particle size distribution of products from one-stage process and two-stage process, it is found that alumina products with bigger particle size and higher strength can be attained by enhancing agglomeration in seeded precipitation process of sodium aluminate solution. The agglomeration mechanism is also discussed, the agglomeration begins from the smallest particles.5. The precipitation yield of sodium alumiante solution decreases by adding saturated stearic acid without C=C bond when the dosage of stearic acid, oleic acid and linoleic acid surfactants is less(10ppm). The precipitation yield increase and they are close by adding three different surfactants when the dosage is above 20ppm. The secondary nucleation can be controlled by adding surfactants when temperature is lower(65℃) and degree of supersaturation is higher. The agglomeration can be promoted by adding surfactants when temperature is higher(85℃) and supersaturation degree is lower. The strengthening effect of agglomeration is not remarkable by adding surfactants when Ks is too high or low. The low agitation rate and fine initial seed are advantageous for strengthening effect of surfactants on agglomeration. The strengthening effect of surfactants on agglomeration is mainly reflected on fine particles. Whether the secondary nucleation is controlled or agglomeration is promoted, the best effect can be attained by adding linoleic acid, oleic acid and stearic acid. Among linoleic acid, oleic acid and stearic acid, stearic acid is the worst one. It has a close relation with C=C bond of surfactants. The surface tension decrease extent of sodium alumiante solution is the greatest by adding linoleic acid with C=C bond and it is the smallest by adding stearic acid without C=C bond.6. When CGM is added into sodium aluminate solution, both agglomeration and coarsening of particles are markedly enhanced and also allow for secondary nucleation control, but the precipitation yield of the liquor decreases. The precipitation yield of liquor can be increased, but the promotion of agglomeration is weak when surfactant is added into sodium aluminate solution. However, both precipitation yield of liquor and particle size of aluminum hydroxide product increase at the same time when adding CGM with anionic surfactant. It is possible for powdery aluminum hydroxide to become sandy aluminum hydroxide by improvement of agglomeration process.更多还原