Eco-Friendly Extraction of Rare-Earth Elements on Hydroxyapatite Using Ozone

Abstract

In order to extract reasonably rare-earth elements (REEs) as deep-sea resources, the effect of ozone (O₃) on REEs on hydroxyapatite (Abbreviated as Ca₃(PO₄)₂) selected as a simulated REEs mud was comprehensively examined in aqueous solutions including alkali metal salts (MCl or CH₃COOM, M = Li, Na, and K) and alkaline earth metal salts (MCl₂ or (CH₃COO)₂M, M = Mg, Ca, Sr, and Ba) ranging in temperature from 278 to 333 K. As a result, it was found that Ca₃(PO₄)₂ exists as a solid state and REEs except Ce maintain their dissolved morphologies during O₃ treatment. Moreover, it was confirmed that the leaching ratios of REEs from Ca₃(PO₄)₂ increase with an increase in the concentrations of alkali metal salts and alkaline earth metal salts in aqueous solutions. This phenomenon implies that their adsorption-desorption mechanisms are chemically controlled through cation exchange reactions between REEs on Ca₃(PO₄)₂ and other metal cations. In addition, the most effective cation to extract REEs from Ca₃(PO₄)₂ into an aqueous solution was found to be Ca ion among these additives. It was considered that the constant increase of the ΔH and ΔS values from La to Lu in the adsorption equilibrium reactions results from the increase in the hydration number of REEs due to the lanthanide contraction. On the basis of these results, the leaching experiments of REEs on Ca₃(PO₄)₂ were carried out to check whether the leaching promoting effect was accomplished by the combined use of O₃ and Ca ion or not. Only when used in conjunction with O₃ and Ca ion, it was found that the synergetic effect results in an almost fourfold increase in their leaching ratios. This promising result shows that it is possible to develop an innovative, modern, and eco-friendly extraction technology to separate REEs from Ca₃(PO₄)₂.