【作者】 陈滨；

【导师】 李小斌；

【作者基本信息】 中南大学 ， 有色金属冶金， 2008， 博士

【摘要】 氧化铝是世界经济中最重要的基础原材料之一。随着国民经济的发展，2007年我国氧化铝产量已近2000万吨，成为仅次于钢铁产量的特大型冶炼行业。我国铝土矿大多属高铝、高硅、低铁一水硬铝石型铝土矿，铝硅比为4-7之间，因此我国氧化铝企业大多采用烧结法或拜尔-烧结联合法生产氧化铝。目前，虽然一些氧化铝生产强化技术不断涌现，但是烧结法生产氧化铝中能耗高、成本高的问题并没有得到根本解决，这严重制约了烧结法生产氧化铝企业的经济效益。提高湿法系统中氧化铝浓度，是降低烧结法生产氧化铝能耗的一条重要技术路线，其关键是熟料的高浓度溶出与固液分离过程，而这两者的难点在于熟料高浓度溶出过程二次反应的控制和适宜絮凝剂的确定。因此，对熟料高浓度溶出与溶出后浆液的快速分离工艺以及相关过程的机理进行研究，对提高烧结法氧化铝生产技术经济指标具有重要意义。为此，本文就以下一些方面进行了系统研究：1．熟料溶出过程相关反应的热力学对熟料溶出过程相关的各种反应进行了详尽的归纳，并就其热力学性质进行了系统而详细的计算和分析。明确了溶出过程中原硅酸钙（β-2CaO·SiO₂）在三种不同碱性溶液中的稳定性次序由强到弱为NaOH>>NaAl（OH）₄>Na₂CO₃；明晰了各种二次反应发生的可能性；明确了不同类型的硅酸钙在铝酸钠溶液中的稳定性次序由强到弱为：CaO·SiO₂>3CaO·2SiO₂>2CaO·SiO₂；就不同饱和系数的水化石榴石在氢氧化钠和碳酸钠溶液中的稳定性进行了热力学计算和分析，得出了NaOH难于分解水化石榴石而Na₂CO₃则易于分解水化石榴石的结论；对原硅酸钙在溶出过程中可能发生的一系列水化反应、硅酸钙水化物与铝酸钠溶液各成分之间的反应进行了系统的热力学计算，结果表明，原硅酸钙最有可能水化成2CaO·SiO₂·1．17H₂O、2CaO·SiO₂·0．5H₂O和4CaO·3SiO₂·1．5H₂O；热力学上，硅酸钙水化物不跟NaOH反应，而与Na₂CO₃和NaAl（OH）₄的反应趋势较大。这些结果为熟料溶出工艺研究提供了理论指导。2．熟料溶出过程二次反应的动力学熟料溶出过程二次反应动力学的研究结果表明，根据动力学过程的特征，将该过程分为溶出时SiO₂进入铝酸钠溶液和Al₂O₃损失的两个动力学过程；对各实验条件下的动力学数据进行处理后可获得两个过程的动力学方程分别为：通过动力学方程，可以认为熟料溶出过程，因二次反应而导致SiO₂进入铝酸钠溶液和造成Al₂O₃损失的原因主要是NaAl（OH）₄与硅酸钙作用的结果。3．硅酸钙在不同碱性溶液中的行为为了更进一步探讨二次反应的机理，对硅酸钙在不同碱性溶液中的反应行为进行了系统的研究，其结果表明：相同条件下，硅酸钙在NaOH溶液中反应最弱，在Na₂CO₃溶液中反应最强，而在铝酸钠溶液中的稳定性介于二者之间，这与热力学分析结果相吻合；在铝酸钠溶液体系中，时间的延长、温度的提高都会使得硅酸钙的分解加剧，而碳酸钠对硅酸钙分解的影响远比在纯Na₂CO₃溶液中小，这是由于NaAl（OH）₄和Na₂CO₃的交互作用所致（Na₂CO₃分解硅酸钙的能力受到削弱而NaAl（OH）₄对硅酸钙的分解作用得以加强，导致NaAl（OH）₄成为在铝酸钠溶液中促使硅酸钙分解的主要因素）；由于β-2CaO·SiO₂是介稳的高温型物质，其结构具有热力学不稳定性，同时该物质中钙离子具有不规则配位，使其具有较高的活性。而当硅酸钙中CaO／SiO₂分子比下降以后，其在铝酸钠溶液中的反应活性降低，所以以3CaO·2SiO₂和CaO·SiO₂形式存在的硅酸钙要比2CaO·SiO₂稳定。4．熟料高浓度溶出工艺在理论研究的基础上，对工业熟料高浓度溶出过程进行了系统的实验研究。由实验结果得出的主要结论是，在保证溶出液Al₂O₃浓度>180 g．L^-1的条件下，增大液固比可使得溶出液SiO₂的浓度有所降低，但对Al₂O₃溶出率没有明显的影响；较低的溶出温度（75℃左右）和较短的溶出时间（5-10 min）有利于减小熟料高浓度溶出过程的二次反应程度，即有利于溶出液SiO₂浓度的降低和保持较高的Al₂O₃溶出率；游离Na₂O_k浓度和Na₂O_c浓度对熟料高浓度溶出过程的影响较小，而NaAl（OH）₄的影响则较大；Na₂Os浓度对熟料高浓度溶出过程SiO₂和Al₂O₃行为的影响很小；调整液中含适量SiO₂有利于熟料高浓度溶出过程提高Al₂O₃溶出率和降低SiO₂溶出率；控制适当溶出过程工艺条件，可以降低熟料高浓度溶出过程二次反应的程度。就添加剂对熟料高浓度溶出过程的影响进行的大量实验研究后，发现其中以主要官能团为-COOCa的A-1可在一定程度上抑制二次反应的进行。5．熟料高浓度溶出浆液的沉降分离工艺对工业熟料高浓度溶出后浆液沉降性能进行的详细实验结果表明，在保证溶出浆液沉降以后上清液中Al₂O₃浓度>180 g．L^-1的前提下，通过大量实验研究发现添加絮凝剂A-15可以实现赤泥的快速沉降，其适宜的添加量为125～250g/t干赤泥；而随着熟料粒度的减小，沉降速度变慢，赤泥压缩性变差；碳酸钠浓度对熟料高浓度溶出浆液的固液分离过程影响比较明显，随着碳酸钠浓度的提高，溶出浆液的沉降性能逐渐变差，但Na₂Oc浓度10～20g/L之间对沉降过程无显著影响；在调整液循环溶出过程中，循环次数越多，对沉降越不利，直至发生赤泥膨胀现象，熟料越细这一现象越明显；但通过絮凝剂A-3和A-15按1：7～1：9的体积比复配则可大大缓解调整液循环对浆液沉降性能的恶化作用。更多还原

【Abstract】 Alumina is one of the most important raw materials in the world economics. With the development of national economy, the alumina industry of China has developed with high speed since 2000’s in the 21^th century. The total output of alumina annually was nearly up to 20 megaton in year 2007. It is only inferior to the iron & steel yields in metallurgical industry. Due to the high content of silica in bauxite, sintering process and Bayer-Sintering combination process act special roles in our country’s alumina production. Nowadays, some new intensifying technologies of alumina production were come forth, but the problems of high energy consumption and cost is still under solved, this greatly decreases the benefits of the alumina company by sintering process. An efficient means for reducing energy consumption is to increase Al₂O₃ concentration of wet method system, its pivotal technique involve the processes of clinker-leaching with high Al₂O₃ concentration and the separation of solid and liquid, however, the difficulties of these two processes are respectively the control of secondary reaction during clinker-leaching and the determination of suited flocculent. Therefore, it has great meanings to work over the secondary reaction mechanism, the technologies of clinker-leaching with high Al₂O₃ concentration and the sedimentation of leached slurry for increasing the technology economic index of alumina production by sintering process. In this case, the following aspects have been studied systematically in the dissertation:1. Thermodynamic calculation and analysis of clinker-leaching processThermodynamic computation and discussion of reactions involved in the clinker-leaching process are described in detail. The stability ofβ-2CaO·SiO₂ in three alkaline liquors and different types of calcium silicate in sodium aluminate liquor have been confirmed in thermodynamic terms, results are respectively as follows: NaOH>>NaAl（OH）₄>Na₂CO₃ and CaO·SiO₂>3CaO·2SiO₂>2CaO·SiO₂. Meanwhile, the results of investigation on the reactions between hydrogarnet with different saturation coefficient and NaOH or Na₂CO₃ show that Na₂CO₃, instead of NaOH, can easily decompose hydrogarnet and hydrogarnet in NaOH solution is more stable than in Na₂CO₃ solution. Finally, the hydration ofβ-2CaO·SiO₂ and reactions between calcium silicate hydrate and three alkaline liquors have all been analyzed. Its results indicate that hydration ofβ-2CaO·SiO₂ to afford 2CaO·SiO₂·1.17H₂O, 2CaO·SiO₂·0.5H₂O and 4CaO·3SiO₂·1.5H₂O is thermodynamically most favorable. NaOH contributes less dependence on the decomposition of calcium silicate hydrate, in contrast, Na₂CO₃ and NaAl（OH）₄ remarkably influence on the decomposition of calcium silicate hydrate. All these results lay a theoretic basis for the investigation of new clinker-leaching technology.2. The kinetics of secondary reaction during clinker-leachingThe kinetics of secondary reaction during clinker-leaching was researched in this part. Results demonstrate that secondary reaction process is divided into two sections according to the characteristics of dynamic process, namely, one is the process that SiO₂ dissolves into sodium aluminate solution and the other is the process about loss of Al₂O₃. After comprehensively treated, the two kinetic equations can be expressed as follows:It can be inferred from kinetic equation that reaction between calcium silicate and NaAl（OH）₄ is the main reason to result in SiO₂ entering into sodium aluminate solution and loss of Al₂O₃.3. Behaviors of calcium silicate in different alkaline solutionsIn this part:, the behaviors of calcium silicate in different alkaline solutions were systematically studied in order to further discover the mechanism of secondary reaction. The results illustrate that the stability sequence of calcium silicate in three alkaline solutions is NaOH>NaAl（OH）₄>Na₂CO₃, which is coincident with the thermodynamic results of this dissertation under the same leaching condition. Leaching in sodium aluminate solution, the prolongation of reacting time and elevation of reacting temperature will increase the decomposition of calcium silicate. The influence of Na₂CO₃ on the decomposition of calcium silicate in sodium aluminate solution is less than in Na₂CO₃ solution, which due to mutual action between NaAl（OH）₄ and Na₂CO₃. Attributing to decline of calcium silicate’s reactive activity after decrease of CaO/SiO₂ of calcium silicate, 3CaO·2SiO₂ and CaO·SiO₂ in sodium aluminate solution are more stable than 2CaO·SiO₂.4. Technology of clinker-leaching with high Al₂O₃ concentrationBased on the mentions above, particular research on the technology of clinker-leaching with high Al₂O₃ concentration has been carried out. Experimental results display that SiO₂ concentration in leached slurry climbs down with the increase of liquor-solid ratio to a certain extent under assuring the Al₂O₃ concentration>180 g.L^-1, however, liquor-solid ratio exerts little influence on the leaching rate of Al₂O₃. Lower reacting temperature（about 75℃） and less reacting time（5-10 min） avail to reduce the extent of secondary reaction. Na₂Os, as well as Na₂O_c and free Na₂O_k concentration, have insignificant impact on the process of clinker-leaching with high Al₂O₃ concentration, but NaAl（OH）₄ has profound effect on the behaviors of SiO₂ and Al₂O₃ during clinker-leaching. Original liquor contains proper amount of SiO₂ is propitious to the elevation of leaching rate of Al₂O₃ and the reduction of leaching rate of SiO₂. Clinker-leaching with high Al₂O₃ concentration under suitable leaching conditions can make the extent of secondary reaction on the decline. It was also found that additive type A-l（"-COOCa" as main functional group） can restrain secondary reaction from happening to some extent in this part.5. Technology of sedimentation of leached slurry with high concentrationIn the last part, the detailed investigation on sedimentation of slurry obtained after clinker leached with high Al₂O₃ concentration was completed. As seen from results, it was found that adding flocculent type A-15 can make rapid settlement of red mud come true under assuring the Al₂O₃ concentration>180 g.L^-1, its proper additive amount is 125-250g/t dry red mud. The sedimentation rate reduces and the compressibility of red mud deteriorates with the fall of clinker size. Na₂CO₃ concentration exerts remarkable influence on the separation process of solid and liquor, sedimentation performance of leached slurry becomes bad gradually with the elevation of Na₂CO₃ concentration, but Na₂O_c concentration in the range of 10-20g/L exposes inapparent impact on this process. During circulation leaching with the use of liquor which is used to extract clinker, the increase of circulation-leaching amount make against the settlement process, especially when clinker size changes into more fine. However, the deterioration of slurry’s sedimentation performance can be greatly alleviated through combining flocculent type A-3 and A-15 according to the ratio by volume of 1:7- 1:9.更多还原