IMPURITY EFFECT OF CR-3+ ON CRYSTAL-GROWTH PROCESS OF AMMONIUM-SULFATE

Surface reaction kinetics in the crystal growth process of ammonium sulphate is investigated by estimating the interfacial concentration on the single crystal. It is clear that both NaN and BCF models can be applied to each surface reaction process of (100), (010) and (001) faces in pure solution with the same accuracy. In the presence of Cr3+, the growth rate remarkably decreases with Cr3+ concentration and finally stops at a concentration of 2ppm for (010) face, 2.5ppm for (001) face and 3ppm for (100) face. Under a constant concentration of Cr3+, crystal begins to grow when the supersaturation becomes larger than a critical value, after which the growth rate increases predominantly with supersaturation and attains the maximum, the value in the pure solution. As for the mechanism of the impurity effect, adsorption of Cr3+ on steps of the crystal surface is considered. Bow-shaped macrosteps were confirmed by microscopic observation. Applying the model of Cabrera and Vermilya, the distance between the adsorbed Cr3+ ions was roughly estimated as 14 nm for (100) and (010) faces, and 18 nm for (001) face. © 1990, The Society of Chemical Engineers, Japan. All rights reserved.