Study On The Fundamentals And Technology Of Sandy Alumina Production By Sinter Process

Because the dry de-fluorination technology using sandy alumina as absorbent, developed by Alcoa in 1970s, has been spread and improved in aluminum industry in the whole world, the sandy alumina production has become one of the main trends in alumina refineries. But, the deposits of bauxite in China are predominantly diasporic type with high silica, which needs rigorous digestion conditions and only could higher caustic ratio green aluminate liquor be gained, hence it is very difficult to produce sandy alumina. Especially, the researches on theory and technology for sandy alumina production by such process are scarce, whereas the alumina production ability in China by Sinter process is more than 40%. Therefore the dissertation focuses on the theory and technology of sandy alumina production by Sinter process. According to its own process characteristics of Sinter process we began with study on the thermodynamics and kinetics in carbonization and seed precipitation process, combining with process characteristic variation brought by the intensified Sinter process for alumina production and the improvements of facilities. Based on the conscientious laboratory research and industrial scale test, a technical route for sandy alumina production has been proposed and a qualified sandy alumina produced successfully in the biggest Sinter process production line (850kt/a) of China for the first time. The main achievements made are as following:The carbonization precipitation has a similar mechanism with the seed precipitation process. The thermodynamics calculation in carbonization indicated that this process is mainly the result of reactions of both OH^- neutralized by CO₂ and NaAl(OH)₄ hydrolyzed and the contribution by the direct reaction of CO₂ with NaAl(OH)₄ may not be significant. This mechanism is furthermore approved by the laboratory experimental result. Controlling the super-saturation of liquor could guarantee a better condition for the new nuclei to grow up and avoid generating much more fine particles effectively, which is required in sandy alumina production. The essentiality of this conclusion also means that a great deal of theoretic researches on seed precipitation for manufacture of sandy alumina could be used to the carbonization process as references.The kinetics of seed precipitation process shows that the activation energy of seed precipitation is 94.60kJ/mol, indicating the surface reaction as controlling step.So, the temperature has a key effect on precipitation rate and morphology of the product in the process. Decreasing precipitation temperature, thus increasing solution super-saturation and decreasing addition of seed are favorable to growth of particles. The finer the particles are, the faster the particles grow via agglomeration in the seed precipitation. The coarser hydrate could be used as seed in the later period of seed precipitation in order to modify the product morphology.Due to the complicities of the Sinter process and the not being synchronistical operating cycles of equipments and variations in industrial production, it isn't realistic to hold on extreme stable operating conditions and solutions with totally balanced concentration and caustic ratio. Considering the fact that the hydrate got in carbonization is exceeded 80% of the total amount products and combining with the research in laboratory and industry, a technical route merging the characteristics of both carbonization and seed precipitation for producing sandy alumina with Sinter process has been put forward, i.e., giving priority to the carbonization conditions to control the amount of fine particles produced, guaranteeing the fine seeds grow up firstly and the coarse particles be decorated slowly during seed precipitation.According to this technical route, a modification was made for the Sinter process in Zhongzhou Branch Company of CHALCO and since then the qualified sandy alumina has been produced, providing a higher carbonization ratio and a same seed precipitation ratio(>51%) compared with those before. It has been reputed as "zhongzhou module" for producing sandy alumina with Sinter process in CHALCO.