Evaluation of alkali processing for the recycling of rare earth values from spent fluorescent lamps

Phosphor samples collected after crushing and sieving of discarded fluorescent lamps comprise approximately 31 % rare earth elements in the form of $Y_{1.90}Eu_{0.10}O_3$, $Al11Ce_{0.67}MgO_{19}Tb_{0.33}$, and $Al_{10.09}Ba_{0.96}Mg_{0.91}O_{17}$: $Eu^{2+}$ phase. Direct leaching and mechanical activation assisted leaching are incapable of recovering $Ce$, $Tb$ values from the $Al_{11}Ce_{0.67}MgO_{19}Tb_{0.33}$ phase. Heat treatment with $NaOH$ was found successful for dissociation of $Ce$, $Tb$ phase via substitution of rare-earth ion by $Na^+$ ion to form rare earth oxide and water-soluble $NaAlO_2$. $Y$, $Eu$, $Ce$, and $Tb$ values were recovered from heattreated mass in a two-step leaching process followed by recovery from the leach solution by oxalic acid precipitation. Over 95 % extraction rate was attained after heat treatment at 400 °C with 150 wt-% $NaOH$ for 1 h. It was found that $Y$, $Eu$ containing phase does not take part in the heat treatment process whereas the $Ce$, $Tb$ phase undergoes a solid-state chemical reaction with $NaOH$ via product layer diffusion model with 41.5 kJ/mol activation energy. Approximately 15 g mixed oxide (purity >95 %) of $Y$ (79 %), $Eu$ (7 %), $Ce$ (5 %), and $Tb$ (4 %) could be recovered from 100 units of discarded FLs. Microwave treatment of phosphor and $NaOH$ (50 wt-%) yielded approximately 42 % $Y$, 100 % $Eu$, 65 % $Ce$, and 70 % Tb recovery in just 5 min. Approximately 9 g of REO and 5 g of cerium enriched leach residue were recovered from the microwave route within 5 min and depicted high microwave potential application in the recovery of $Ce$ and $Tb$ values from waste phosphor sample.