Process of Epitaxial Crystal Growth for CaSO_4 · 0.5H_2O on a Surface of Dissolving Fluorapatite Crystals Studied by Scanning Electron Microscopy

摘要

A process for the epitaxial crystal growth for CaSO_4 · 0.5H_2O on a surface of dissolving natural fluorapatite (FAP) crystals under conditions of wet-process phosphoric acid production has been studied by scanning electron microscopy (SEM). With the SEM working in a secondary electron mode, we were able to establish the main stages of the coating formation, including direct measurements of its thickness and growing rate values for individual needle-like CaSO_4 · 0.5H_2O crystals. Confirmation of an aggregate mechanism for CaSO_4 · 0.5H_2O epitaxial crystals growth, established earlier under conditions of chemical crystallization, was achieved for the first time. The mechanism includes the following steps: (1) ultramicrocrystals of CaSO_4 · 0.5H_2O were formed, (2) they coalesced into needle-like crystals by means of mutual aggregation, (3) the needle-like crystals united into star-like and besom-like aggregates, and (4) the latter formed macro-aggregates. We have found that these steps occur simultaneously, resulting in multilayer coating formation. During the investigation, the phenomenon of FAP crystals aggregation by means of mutual intergrowing of epitaxial CaSO_4 · 0.5H_2O coatings and the peculiarities of the etching process for the FAP crystal surface under conditions of CaSO_4 · 0.5H_2O coatings formation were also studied.