| The knowledge and technology about nano materials and their preparation were reviewed briefly and the effects of aggregation, dynamics and additives on preparing the nano alumina trihydroxide (ATH) by carbonation were summarized.The viscosity of the suspension increases during carbonation precipitation of the ATH while the solid content rises and particle size diminishes, and the suspension evolves into a viscous Non-Newtonian fluid. The rheology of the suspension from carbonation was investigated and the results showed that the suspension acted as a dilatant and a pseudo-plastic non-Newtonian fluid at low and high shear rate respectively; it also was a time-dependent dilatant non-Newtonian fluid with a low thixotropy at high shear rate. The study indicated that the aluminium trihydroxide particles in suspension formed a stable network structure which could be retained or restored after temperature rise or shear destruction; the viscosity of the suspension fluctuated when the shear stress approached the network structure destruction level. Related analyses showed that the network structure was the origin of the complex fluid behavior and also the reason for nano particles formation during carbonation precipitation and their stability in suspension.Based on the achievements about micro-mixture and mass transfer in Non-Newtonian fluid, a conclusion was made that the high viscosity affects the mass transfer and carbonation process evidently, which caused the low yield of the reaction, particle aggregation and emergence of by-product. At high viscosity, conventional methods did not improve the mass transfer effectively in contrast with depressing viscosity. The rheology of ATH aqueous suspension at various solid contents, temperatures, ion concentrations, pH values and with surfactants was studied. The viscosity of the suspension increased along with the solid content (Max.33wt%), especially when the content above9.4wt%. The viscosity reduced along with the temperature’s rise and the pH value changed the strength of ATH net structure and viscosity both. The viscosity altered a little with ion concentration. None of the viscosity, temperature, pH value and ion concentration improved fluidity.Surfactants were utilized to stabilize and decrease the viscosity of suspension, and were adopted by some reactions. Combined with the knowing about the surfactants, a way was proposed to intensify the mass transfer with them. Surfactants were selected to improve fluidity of nano ATH filter cake, among which polymer dispersers show merits. These surfactants improved fluidity so much that the suspension of12.9wt%became a Newtonian fluid. An aqueous suspension of solid content up to64wt%was obtained with the surfactant. Those surfactants that decreased the viscosity of the aqueous suspension were inactive more or less under the caustic conditions, in which a disperser plus CTAB kept their properties and led to obtaining a concentrated ATH caustic suspension of40wt%content with good fluidity at pH above12and [Na+]=2.7mol-·L-1. The surfactants were utilized in the carbonation reaction, The surfactants reduced the suspension’s viscosity greatly, so the mass transfer was intensified, reaction time was shorten, yield increased. The characteristics of the nano ATH showed the water content of the ATH filter cake fell by TGA, and the nano ATH was well dispersible hexagon shaped slice with even size distribution, size about100nm×10nm by TEM and BET surface area over80m2/g. The TGA and FTIR showed that the nano ATH adsorbs the surfactants of1.5wt%in dry mass. The related studies not only realized disperse and adjustable fluidity of the ATH suspension but also reduced the equipment investment and energy cost greatly during production and application process.A simplified model was put forward after the analysis of the current aggregation dynamics and viscosity models, along with the pH and temperature curves which could bring a clear prospect about the carbonation process. |
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