The Effect of Various Kinetic Factors for Fine Aluminium Trihydroxide Precipitation

ABSTRACT This study examined the effect of different variables on aluminium trihydroxide precipitation. These variables included low temperature (45–65°C), initial A/C ratio (0.4 and 0.5), seed size (0.1–1 μm) and seed concentration (3 g/L). A second order rate equation was used to model the experimental results obtained over a 24 hour precipitation period. That is: This equation was particularly useful when deviding the precipitation into the two stages, namely, nucleation and growth. Arrhenius rate constants of nucleation and growth are 2.72 and 34.6E+14 respectively. The activation energy for both stages of precipitation were 98,400 and 15,400 J/mol, respectively. The final product from precipitation at an initial A/C ratio of 0.5 exhibited extensive agglomeration, producing mosaic particles of 5 to 10 μm in size. Earlier in the precipitation period agglomeration was not extensive and products having high surface areas (greater than 6 m2/g) were obtained. The lower limit for which significant precipitation can be initiated was found to be at an initial A/C ratio of 0.4. As this A/C ratio is approached, there is a significant decrease in precipitation from those values predicted by the second order rate equation.