| Carbonization is one of the most important working procedures in alumina production by Sintering process. Whether the control of production conditions is suitable or not will expose direct impact on the purity and granularity of alumina. Attrite index of the alumina produced by Sintering process in our country at present is still far from the national criterion of sandy alumina. This paper profoundly investigated the kinetic mechanism of carbonization process.The carbonization process is divided into two parts: one is the reaction between sodium hydroxide and carbon dioxide, the other is the process of aluminum trihydroxide precipitation. Two processes were investigated respectively. The investigation on the previous process showed that the apparent activation energy of this process was only 8.057 KJ.mol-1, which suggested that it was a diffusion controlling process; then the relationship between gas flow rate and reaction rate (rd=K(Q/VL)2/3 )was deduced theoretically under the consumption; The research on the latter showed that the relationship between precipitation ratio and reaction time can be described by formula Aulami. With the increase of carbon dioxide concentration, precipitation ratio of carbonization process remarkably goes up; In the range of 75~90C, the effect of temperature on precipitation process is not notable; The additions of seed improve the precipitation rate of early stage. After comprehensively analyzing and deducing, the kinetic equation of carbonization process can be expressed as follows:Where: Ï„-precipitation time, Min; T-temperature, K;A-Al2O3 concentration in liquor when time is Ï„ , g.L-1; Ae-Al2O3 equilibrium concentration in liquor, g.L-1; fr-instantaneous seed coefficient; Nk-caustic alkali concentration, g.L-1; The research on nucleation process of carbonization process showed that the change of supersaturation of liquor takes on a trend of increasing at first, then coming down. With improvement of carbon dioxide concentration, the induction period of reaction process will be curtailed; with the increase of temperature, thereis not a determinate orderliness; with increase of seed coefficient, induction period will be diminished; carbonization process without adding seed is not similar with seed precipitation process, but carbonization process with adding seed is close to the process of seeded precipitation; the apparent activation energy of nucleation of carbonization process obtained by regression calculation is 123.679KJ/mol, it can be seen that this process is a crystallization process controlled by chemical reaction. Meanwhile, this paper put forwards the new idea of carbonization precipitation by adding additives with low silica modulus which aim at solving the problem of complicated process, long time and high energy consumption appeared in desilication process adopting the method of two-stage deep desilication. It can be seen that the addition of A can inhibit the SiO2 precipitation. Under the condition of optimal addition, the SiO2 content in alumina production when precipitation ratio is about 90% can be close to the national secondary criterion (0.04%); the mechanism probably is that this additive can improve the SiO2 meta-stable concentration. |
