Study On The Preparation Of Coarse Boehmite By Seeded Precipitation

Abstract:The seeded precipitation in sodium aluminate solution is one of the most important sub-processes in Bayer Process. It also has been the research priority in alumina industry. Promoting the precipitation ratio of gibbsite can improve equipment productivity and reduce energy cost but brings much fine gibbsite, and then it has to keep a very low precipitation ratio compared with the theoretical one to obtain sandy alumina in practical. Therefore, a large amount of alumina trihydrate is always cycling with the sodium aluminate solution, limiting the equipment productivity and increasing the energy cost. This paper adopted a new two-step technology, promoting firstly the precipitation ratio and then preparing the coarse boehmite from fine gibbsite. This paper mainly focuses on the preparation of coarse boehmite from fine gibbsite in the sodium aluminate solution. The morphology, the precipitation ratio particle size distribution and the formation of boehmite were analysed by SEM, XPD, FT-IR and BET. Results can be seen as follows:(1) The temperature, time and the seeds of boehmite benefit the formation of boehmite. Temperature is the main fact that affect the formation of boehmite. More than90%boehmite is formed at140℃for more than11h, at180℃for2h, at200℃for40min and at240℃for only5min.(2) The temperature, the time, the increase of alkali concentration at high temperature, increase of seed amount and weak stirring are favorable to the agglomeration of particles. Meanwhile, particles can agglomerate obviously at high temperature. Elevating the temperature can reduce the reaction time.(3) Layered structure of alumina trihydrade is destroyed from the surface of particles to inside, and boehmite is generated in the forms of nanoflakes, and then coarse, nanoflakes and porous boehmite is formed.(4)The precipitation ratio varies in the form of "W". Total population of particles decreases, among wich fine particles Between0μm and30μm disappear completely and the population of coarse particles increase. The agglomeration mechanism of "dissolution-precipitation-agglomeration" is then inferred.(5) Under T≥180℃and pNa2O≤50g/L, more than90%boehmite were prepared without fine particles below30μm and with the highest specific surface area of141.05m/g, pore volume of0.115cm3/g and average pore size of3.3nm.