Process of epitaxial crystal growth for CaSO₄ · 0.5H₂O on a surface of dissolving fluorapatite crystals studied by scanning electron microscopy

Abstract

Summary:A process for the epitaxial crystal growth for CaSO 4 • 0.5H 2 O 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 2 O crystals. Confirmation of an aggregate mechanism for CaSO 4 • 0.5H 2 O 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 2 O were formed, (2) they coalesced into needlelike 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 2 O coatings and the peculiarities of the etching process for the FAP crystal surface under conditions of CaSO 4 • 0.5H 2 O coatings formation were also studied.

Textured fluorapatite bonded to calcium sulphate strengthen stomatopod raptorial appendages

“…One possible two-step mechanism for this phase transition would be for CAS to first stabilize ACP, which could then crystallize using the [100] CAS plane as a template that lowers the driving force for crystal nucleation and leads to a lower total free energy of the system. Epitaxial relationships between FAP and hydrated CAS have previously been observed for synthetically grown FAP crystals 41 . Moreover, the stronger binding energy of the FAP/CAS interface suggests that fluorine ions play a role to further optimize the stiffness and strength of the biomineralized complex near the outer impact surface.…”

Influence of Aluminum on Morphologies and Crystallization Kinetics of Hemihydrate Calcium Sulfate in the Hemihydrate Process of Phosphoric Acid Production

“…Relationship between the population density and particle size in an MSMPR crystallizer. where β is a shape factor and has a value of 16π/3 for a spherical particle, γ is the interfacial tension (mJ/m 2 ), V m is the molar volume (cm 3 •mol −1 ), N A is Avogadro's number (mol −1 ), f(θ) is a correction factor which takes into account nucleation modes, and R is the gas constant (8.314 J/mol•K). When purely homogeneous nucleation takes place, f(θ) = 1.…”

“…In addition, the utilization value of hemihydrate calcium sulfate (HH) is also higher than that of dihydrate calcium sulfate (DH). Crystallization of HH plays an important role in the hemihydrate process of wet-process phosphoric acid production. , Due to the excessive viscosity of concentrated phosphoric acid solution, the surface of the phosphate ore is often covered by HH products, which will lead to emergency shutdown in serious cases.…”

Toughening, micromechnics, and multi-scale structural analyses of high-performance biotools

“…One possible two-steps mechanism for this phase transition would be for CAS to first stabilize ACP, which could then crystallize using the[100] CAS plane as a template that lowers the driving force for crystal nucleation and leads to a lower total free energy of the system. Epitaxial relationships between FAP and hydrated CAS have previously been observed for synthetically grown FAP crystals182 . Moreover, the stronger binding energy of the FAP/CAS interface suggests that fluorine ions play a role to further optimize the stiffness and strength of the biomineralized complex near the outer impact surface.In addition to this chemical strategy, the mechanical response of dactyl segments impact regions is further tailored through preferred crystalline orientation, and this strategy is detected in both species.…”