RECOVERY OF RARE EARTH ELEMENTS FROM SPENT FLUORESCENT LAMPS

Abstract

Exceptional properties such as optical, chemical, catalytic, magnetic, and luminescence of rare earth elements (REEs) have led to various applications. REEs are finite, site-specific, and often present with complex mineralogy in primary sources. Besides, investigating secondary rare earth sources is attractive because of economic benefits, rare earth dependency aspects, and resource conservation. The limited primary resources, strict environmental norms, and the growing demand for rare earth elements call for global phosphor recycling from waste fluorescent lamps. The spent fluorescent lamps were crushed in a custom-made prototype fluorescent crusher followed by sieving to retrieve fluorescent phosphor powder comprising about 30-40% rare earth (Y, La, Ce, Eu, Tb) values in the phosphor. Phosphor samples from spent compact fluorescent lamps (CFLs) and T5 fluorescent tubelights (TL) comprised approximately 32 and 42 % REEs, respectively. The two phosphor samples contained a mixture of Y2O3:Eu3+, BaMgAl10O17:Eu2+, CeMgAl11O19:Tb3+, phases with Al2O3 and SiO2 impurities. However, they differ due to the presence of additional calcium-based phosphate phases (Ca8Eu2(PO4)6O2, Ca3Eu(PO4)3, Ca3(PO4)2) in CFL and rare earth phosphate (LaPO4:Ce3+Tb3+) phase in TL phosphor. In this study, different processing routes comprising direct acid leaching, acid leaching followed by thermal treatment, acid/alkali baking, and microwave treatment of the phosphor powder are systematically investigated for recovery of Y, Eu, Ce, Tb, and La values. Acid leaching in 2M HCl, 80 °C, 1h yielded higher REE extraction in the CFL feed (82% Eu), with varying RE purity (77-97%) respective to oxalic acid dose compared to TL (69% Eu, 98-95% purity). Mechanical activation of 30 and 60 min prior to phosphor acid leaching caused partial dissociation of the LaPO4:Ce3+Tb3+ phase and yielded 34% Ce, 36% Tb, and 40% La. Transmission electron microscopy results revealed the formation of polycrystalline sites, which improved RE recovery. Mechanical activation-assisted acid leaching could not recover Eu, Ce, and Tb values from the complex spinel structures of the BaMgAl10O17:Eu2+ and CeMgAl11O19:Tb3+ phase. The conventional alkali roasting on waste CFL phosphor was investigated to understand the change in % RE extraction. Heat treatment with NaOH successfully dissociated Ce, Tb phase via substitution of rare-earth ion by Na+ ion to form rare earth oxide and water-soluble NaAlO2. Y, Eu, Ce, and Tb values were recovered from heat-treated mass in a two-step leaching process followed by oxalic acid precipitation recovery from the leach solution. 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 of mixed oxide (purity >95 %) of Y (79 %), Eu (7 %), Ce (5 %), and Tb (4 %) could be recovered from 100 units of discarded FLs. A novel microwave alkali treatment route was attempted with CFL phosphor to enhance phosphor decomposition reaction by volumetric heating. Microwave treatment of phosphor and NaOH (50 wt.-%) yielded approximately 42 % Y, 100 % Eu, 65 % Ce, and 70 % Tb recovery in just 5 min. Significant heat treatment time reduction (1 h to 5 min) in microwave furnaces showed that microwave has a high potential application in recovering Ce and Tb values from waste phosphor sample. However, Y recovery was reduced due to water-soluble NaYO2 phase formation during NaOH roasting. Conventional alkali roasting of acid-leached phosphor resisted Y loss and enhanced RE extraction by selectively leaching Y-Eu and Tb-Ce oxides. Then co-precipitation of impurities reduced the Tb-Ce oxide purity by 63%. Referring to the water insoluble nature of Ca, Ba sulphates, phosphor acid baking process was opted to enhance the recovery and purity of rare earth oxides. Sulphuric acid baking of CFL phosphor yielded an overall 91% RE extraction with 94.6% Y, 100% Eu, 50% Ce, and 43.1% Tb by 98% purity. The RE extraction studies were further explored on abundantly available TL phosphors. The REEs extraction was compared for TL phosphor via conventional alkali roasting/baking and acid baking processes. The YOX and BAM phases completely decomposed during acid and alkali baking with > 90% Eu and Y extraction. Prior, selective acid leaching of the YOX phase enhanced the acid baking process at a lower temperature (< 230 ℃). Phosphor acid baking showed a maximum 87% Tb, 36% La, and 17% Ce extraction at 500 °C, 0.5 h, 1 ml/g H2SO4, and alkali baking yielded 16.1% Tb, 86% La, and 73% Ce at 300 ℃, 0.5 h, 50 wt.% NaOH. Insoluble Si3Tb5 phase formation and partial decomposition of CMAT phase in the alkali baking route leads to lower Tb extraction. The acid baking of the acid-leached phosphor yielded 72% Tb, 95% La, 63% Ce, ~ 95% Y, and Eu extraction at 300 °C, 0.5 h,1.05 mL/g H2SO4. Lower Tb extraction during the acid baking of acid-leached phosphor can be attributed to the formation of Ba3Tb(PO4)3 and BaTbO3 phases. The acid baking process is suitable for extracting Tb, Eu, and Y, i.e., heavy REEs, whereas the alkali baking process is suitable for La and Ce, i.e., light REEs having comparatively higher basicity. The thermal analysis of phosphor and flux interaction depicts that alkali baking is less energy-intensive than acid baking. In comparison, the one-step acid baking process and the two-step alkali baking process yielded desirable extraction results (87% Tb, 78.5% total RE and 50.4% Tb, 59.3% total RE respectively). The product yield in the acid and alkali baking route of phosphor (1-step process), and acid-leach phosphor (2-step process) is 36%,29% (acid baking), and 39%, 28.2% (alkali baking), respectively, with ~ 98% mixed rare-earth oxide purity. Alkali-assisted microwave heat treatment of TL feed was optimized at different microwave exposure times to avoid Si3Tb5 phase formation, which hinders Tb extraction. The best lixiviants for phosphor and alkali roasted acid leach residue leaching was HCl and HNO3, respectively. Leaching of Eu–Y values followed by microwave treatment of leach residue with sodium hydroxide was found promising for REEs extraction. The microwave route yielded 94% La, 9% Ce, 45% Eu, and 62% Tb extraction with 50% NaOH and 65% Eu, 95% Tb extraction with double (100%) NaOH dosage. In another part, an investigation of alkali-assisted mechanical activation, followed by thermal (microwave) treatment for RE recovery from TL phosphor was carried out. The activation energy for the thermal decomposition of the phosphor was determined as 498.9 kJ mol-1, 381.6 kJ mol-1, and 202.2 kJ mol-1 for Na2CO3, NaOH, and CaCl2, respectively. Mechanical activation with Na2CO3 reduced the decomposition temperature for microwave processing and yielded 99% Y, 97% La, 0.6% Ce, 99% Eu, and 80% Tb values. Another novel processing route of microwave-assisted acid baking followed by water leaching for TL phosphor was also investigated. The mixture of monochromatic phosphor phases in TL phosphor 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 ºC. It was found that the microwave baking at 800 W for 3 min at 1 mL/g acid ratio yielded 82.5% overall RE dissolution, including 93.6% Tb, 39.6% La, and ~100% Eu-Y dissolution. The dissociation of the LaPO4:Ce3+Tb3+ phase governs the overall RE dissolution during the baking process. The XRD analysis of the products suggests that the La and Ce values interact with phosphoric acid at the phosphor surface to form polyphosphates (LaPO4, CePO4), deteriorating the dissolution efficiency. The partially reacted Ce (III) values were oxidized to a stable Ce (IV) state, which led to its accumulation in the leach residue. The microwave acid baking process is best for Tb extraction; however, it lacks individual separation of Y, Eu, and Tb. 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 cost estimations indicate that the process is economically viable with a total value generation of 14.47 US$/kg with 89% terbium contribution, and the processing cost is 0.69 US$/Kg phosphor. A two-step process consisting of acid leaching followed by NaOH or Na2CO3 microwave treatment of leach residue was found best concerning overall extraction and separation of Y-Eu, La-Tb and Ce oxides. The material balance shows that 50 g mixed oxide of Y, Eu, and ∼8-11 g of La, Ce, Eu, Tb, mixed oxide with purity over 97 %, can be recovered from 100 units of discarded tubular lights. Fluorescent lamps are the potential secondary source of Y, Eu, La, Ce and Tb values. RE recovery processing of piled up spent fluorescent lamps is a boon for developing country like India. It may bring socio-economic and political change with a revolution in medical, defence, automobile, and other sectors. It is high time to consider fluorescent lamp recycling and rare earth recovery for nation transformation.