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<p>Molten salt electrolysis in calcium chloride (CaCl<sub>2</sub>) is a promising alternative route for white phosphorus production, but its efficiency is often limited by sluggish dissolution of phosphate precursors. In this work, the dissolution behavior of hydroxyapatite (HAp) in molten CaCl<sub>2</sub> was systematically investigated through concentration evolution, phase analysis, kinetic modeling, and off–gas absorption experiments. The dissolved phosphorus concentration increased rapidly at the initial stage and then evolved more gradually with time, while the terminal–stage dissolved phosphorus concentration increased from 0.511&#xa0;±&#xa0;0.013 wt pct at 1073&#xa0;K (800&#xa0;°C) to 1.185&#xa0;±&#xa0;0.058 wt pct at 1223&#xa0;K (950&#xa0;°C). Phase analysis showed that HAp dissolution does not proceed by simple direct dissolution, but involves the formation and evolution of chlorophosphate intermediates, mainly Ca<sub>2</sub>PO<sub>4</sub>Cl and apatite-type chlorophosphate phases assignable to Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl. Kinetic comparison indicated that the dissolution behavior of HAp is more consistent with diffusion through an intermediate product layer than with a shrinking-core process without a product layer. For the dissolution experiment at 1123&#xa0;K (850&#xa0;°C), a two-bottle NaOH absorption system revealed stage–dependent off–gas evolution, with most OH<sup>−</sup> consumption occurring during heating and the first 5 hours of isothermal holding. Compared with previously reported fluorapatite (FAp) dissolution, HAp exhibits a faster initial increase in dissolved phosphorus concentration despite relatively close terminal–stage dissolved phosphorus concentrations. These results are consistent with a self–agitation effect associated with <i>in-situ</i> gas generation during HAp dissolution, which may weaken liquid–phase mass–transfer resistance in the early stage and thereby alter the dominant kinetic limitation.</p>

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Dissolution Behavior of Hydroxyapatite in Molten CaCl2

  • 期刊论文

  • 发表日期：

    2026年5月29日

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  • Yue Wang,
  • Liwen Zhang,
  • Qi Fang,
  • Haijun Wang,
  • Xiaoli Xi,
  • Zuoren Nie

摘要

Molten salt electrolysis in calcium chloride (CaCl2) is a promising alternative route for white phosphorus production, but its efficiency is often limited by sluggish dissolution of phosphate precursors. In this work, the dissolution behavior of hydroxyapatite (HAp) in molten CaCl2 was systematically investigated through concentration evolution, phase analysis, kinetic modeling, and off–gas absorption experiments. The dissolved phosphorus concentration increased rapidly at the initial stage and then evolved more gradually with time, while the terminal–stage dissolved phosphorus concentration increased from 0.511 ± 0.013 wt pct at 1073 K (800 °C) to 1.185 ± 0.058 wt pct at 1223 K (950 °C). Phase analysis showed that HAp dissolution does not proceed by simple direct dissolution, but involves the formation and evolution of chlorophosphate intermediates, mainly Ca2PO4Cl and apatite-type chlorophosphate phases assignable to Ca5(PO4)3Cl. Kinetic comparison indicated that the dissolution behavior of HAp is more consistent with diffusion through an intermediate product layer than with a shrinking-core process without a product layer. For the dissolution experiment at 1123 K (850 °C), a two-bottle NaOH absorption system revealed stage–dependent off–gas evolution, with most OH− consumption occurring during heating and the first 5 hours of isothermal holding. Compared with previously reported fluorapatite (FAp) dissolution, HAp exhibits a faster initial increase in dissolved phosphorus concentration despite relatively close terminal–stage dissolved phosphorus concentrations. These results are consistent with a self–agitation effect associated with in-situ gas generation during HAp dissolution, which may weaken liquid–phase mass–transfer resistance in the early stage and thereby alter the dominant kinetic limitation.