Kinetics and Mechanisin of Hydrochloric Acid Leaching of Rare Eearth Oxide Used for a Fluorescence Substance

Leaching behavior of rare earth component used for fluorescence substance has been examined kinetically in aqueous hydrochloric acid solutions. The kinetics of leaching reaction was also investigated by changing the temperature, the stirring velocity, Cl-ion concentration and the hydrochloric acid concentration. The leaching degree of rare earth elements used for a fluorescence substance was larger in the order of Y>Eu>La>Tb.>Ce. Yttrium and europium oxides in a fluorescence substance were dissolved parabolically with leaching time in aqueous hydrochloric acid solutions, and leaching reaction was also accelerated with increasing temperature and stirring velocity. The apparant activation energies calculated frm Arrhenius plots were 20.3kJ/mol for yttrium oxide and 18.5kJ/mol for europium oxide.According to these lower apparant activation energies, the rate-determing step of leaching reaction could be considered to be the diffusion process through Nernst's boundary layer on the surface of rare earth oxide. From the experimental results obtained, the leaching rate equations for yttrium oxide and europium oxide in aqueous hydrochloric acid solutions could be expressed as follows: leaching rate for yttrium oxide=κ'·S'·C·V (χ)·exp (-20.3/RT) and leaching rate for europiumoxide=κ''·S''·C·V (χ)·exp (-18.5/RT), whereκ' andκ'' are a rate constant, S' andS'' the surface area, Cthe hydrochloric acid concentration, V (x) the function of stirring velocity, Rthe gas constant andTthe absolute temperature.