engleski

Anomalous nucleation events during crystallization of zeolite A under marginal alkalinities : A population balance analysis

Influence of the simultaneous change of batch alkalinity (A = [Na2O/H2O]b) and total batch silica concentration (tsc = [SiO2/H2O]b) of the reaction mixture, at two characteristic batch molar ratios, y = [SiO2/Al2O3]b (1.3 and 2.0), on the kinetics of the nucleation, crystal growth and kinetics of crystallization and on particulate properties of crystallized zeolite A, was investigated. It was found that the mentioned processes during crystallization from high and medium alkaline systems (0.0165 ≤ A ≤ 0.046) takes place in accordance with previously described experimental evidences, but that the anomalous kinetics of nucleation during crystallization of zeolite A from low alkaline systems (A ≤ 0.01) was not either predicted theoretically or, till recently, evidenced experimentally. Results of the analysis of distribution of nuclei in gel matrix have shown that in low alkaline systems, most of the nuclei are concentrated in the "centers" ("cores") of gel particles and rest of nuclei are distributed between the "surfaces" and "centers" of gel particles. Population balance analyzes of the crystallization processes, showed that the nucleation in the low alkaline systems takes place in two steps as a consequence of the mentioned distribution of nuclei in the gel matrix. Another consequence of the anomalous distribution of nuclei in the gel matrix is that more than 95 wt. % of zeolite A is formed by the growth of the nuclei released from the gel matrix during the first step of nucleation, and thus that the maximum rate of nucleation in the second step is reached at the crystallization time when about 80 – 90 wt. % of gel has been transformed into zeolite A. This is, at the same time, the reason that the crystal size distributions of the products obtained by crystallization of zeolite A from low-alkaline systems are characterized by two separate crystal populations.

Zeolite A ; alkalinity ; nucleation ; crystal growth ; population balance

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