Study On The Mechanism Of Ultrasonic Intensification On Decomposition Process Of Sodium Aluminate Solutions

Gibbsite crystallization from supersaturated caustic aluminate solution is an important step in Bayer process for alumina production from bauxite. Considerable research focusing on the gibbsite precipitation has been carried out under industrial relevant conditions for improved product yield without sacrificing quality. Ultrasonic processing was proved to have significant effect on decomposition of supersaturated sodium aluminate solution. By ultrasonic processing at low frequency, the decomposition rate of supersaturated aluminate can be enhanced, and the fine particles in the yield gibbsite are increased. However, the mechanism of the ultrasonic intensification was still not well understood, and the optimal conditions of ultrasonic processing still need to be clarified. The aim of this work is to probe the mechanism of ultrasonic processing on decomposition process of supersaturated sodium alumiante solutions to provide fundament for application of the ultrasonic processing.To clarify the characteristics of ultrasonic field of sodium aluminate solution, the characteristics of ultrasonic field of water was first studied. The parameters were defined to characterize ultrasonic field by energy distribution analysis and ultrasonic spectrum analysis. The thermal effects and chemical effects of ultrasonic processing and their relationship with ultrasonic spectrum were studied by the parameters defined. The methods to study ultrasonic field were concluded.With these methods, the characteristics of ultrasonic field of sodium aluminate solution were studied. It is found that quantity of heat produced by ultrasonic processing is decreased as the concentration increased orα_k increased. The total ultrasonic energy is decreased as concentration of the solution increased. The ultrasonic cavitation energy is also decreased with accruement of the concentration, while reach the highest at mediumα_k(about 1.5). As the concentration increased, the ultrasonic threshold is also increased, the increment is more significant in low concentration than that in high concentration. The cavitation is more easier to arise in medium α_k than in higher or lower α_k. The cavitation intensity can be increased by temperature rise or gas solution, the addition of fine particlescan also enhance the cavitation intensity, but the increase of the particle addition has no significant effect on cavitation intensity as the particle content higher than 0.1 g ? I"1.The sonoflorescence of sodium aluminate solution was studied to validate the conclusion drawn by ultrasonic spectrum analysis, it is found that the sonoflorescence area is smaller in higher concentration than in lower one. The effect of a k on sonoflorescence is more significant at lower a k than that at higher a k . The sonoflorescence intensity increases rapidly with the increment of a k as the a k below 1.48, and it has little change with a k as the ak above 1.48. Ultrasonic threshold increased with the increment of the concentration, and it is much higher at lower a k than that at medium a k, while it has no significant variation between medium a k and higher a k.Free radicals were detected by ESR DMPO trap in the sodium aluminate solution with ultrasonic processing. Raman and A127NMR detection reveal that the polymerization of ions in the solution are increased by ultrasonic processing. According to DFT molecular dynamic simulation and the characteristic of free radical reaction, the possible mechanism for ultrasonic intensification on decomposition of sodium aluminate liquor was deduced .The ultrasonic influence on the decomposition process of supersaturated sodium aluminate solutions was studied based on comprehension of mechanisms of the ultrasonic processing. It is found the decomposition rate can be accelerated only by low frequency-ultrasonic processing. The intensification of ultrasound can only realized under conditions of the following: in medium temperature, ultrasonic power high enough, sodium aluminate solution with medium a k and medium concentration. Ultrasonic cavitation is a necessary condition to realize ultrasonic intensification, but not a sufficient one. Ultrasonic processing can create more hexagonal lamellas to form crystal nucleus. There are no difference between the gibbsite obtained by decomposition process of sodium aluminate solution with and without ultrasonic processing by XRD detection.The methods to enhance the intensity of ultrasonic cavitation were discussed, two possible methodswere proposed: addition of agitation and processing with two ultrasonicprocessor. The possibility to solve the problem of increment of fineparticles during the ultrasonic intensification were also discussed.: tocontrol the circle of CSD and to agglomerate during the subsequentprocess.