Microwave acid baking process for recovery of rare-earth concentrate from phosphor of end-of-life fluorescent lamps

Abstract

A novel processing route of microwave-assisted acid baking followed by water leaching of waste phosphor retrieved from end-of-life fluorescent lamps is investigated. The waste phosphor consists of a mixture of monochromatic phosphor phases (Y2O3:Eu3+, BaMgAl10O17:Eu2+, CeMgAl11O19:Tb3+, LaPO4:Ce3+Tb3+), and the baking parameters like microwave power, baking duration, and acid dose were optimized using randomized block statistical design of experiments. The thermodynamic considerations of the sulfation process are evaluated by the thermogravimetric analysis of the phosphor-acid mixture. The estimated thermal decomposition activation energies for the predicted sulfation reactions are 652.9, 375.9, and 409.5 kJ/mol at 0.7, 0.85, and 1 mL/g acid dosage, respectively, in the temperature range of 231–308 °C. It was found that the microwave baking at 800 W for 3 min at 1 mL/g acid ratio yielded 82.5% overall rare-earth dissolution, including 93.6% terbium, 39.6% lanthanum, and ~100% europium and yttrium dissolution. Cerium dissolution was negligible in the investigated experimental conditions. The dissociation of the LaPO4:Ce3+Tb3+ phase governs the overall rare earth dissolution during the baking process. The XRD analysis of the products suggests that the lanthanum and cerium values interact with phosphoric acid at the phosphor surface to form polyphosphates (LaPO4, CePO4), deteriorates the dissolution efficiency. The partially reacted Ce (III) values were oxidized to a stable Ce (IV) state, which leads to its accumulation in the leach residue. The mass balance at optimal conditions indicates that 184 g phosphor of 100 tubular lights units yielded 73 g of 98% pure Y-Eu-Tb oxides. The specific energy expenditure for the present lab-scale study was calculated at 8 kWh/kg of phosphor. The cost estimations indicate that the process is economically viable with a total value generation of 14.47 US$/kg with 89% contribution of terbium and cost of processing is 0.69 US$/Kg phosphor.