Recovery of rare earth elements from fluorescent lamp phosphors

Bumhira, Levie (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.


ENGLISH ABSTRACT: In recent decades, the production and usage of fluorescent lamps has rapidly increased. This is attributed to several benefits they offer in comparison to incandescent bulbs. The lamps are known to have lower energy consumption (about 75 % less than incandescent bulbs) and longer life expectancy. The rapid growth in production and usage has resulted in large volumes of waste fluorescent lamps being discarded every year. Recycling of spent fluorescent lamps offers a number of economic benefits apart from the well-known environmental benefits. The primary objective of this study is to identify a viable process for rare earths recovery from end of life fluorescent powders in the South African context. The study focused on the use of hydrometallurgical unit operations in the recovery of four rare earth metals namely yttrium (Y), europium (Eu), cerium (Ce) and terbium (Tb). A two-staged leaching process was employed followed by solvent extraction to recover the metals from solution. Leaching tests indicated that the red phosphor component (Y2O3:Eu3+) could be easily dissolved into solution during the first step of acid leaching. Over 98% Y and 89 % Eu recoveries were achieved using sulphuric acid as the lixiviant. Ce and Tb were not leached at this stage. The effect of ultrasound and alkali fusion on Tb and Ce leaching was then investigated. Alkali fusion followed by acid leaching produced the best recoveries with Ce and Tb recoveries exceeding 96 % and 99 % respectively. Poor recoveries of the rare earths (<10 %) were obtained using ultrasound assisted digestion. Solvent extraction tests showed that DEHPA could be used to recover rare earths from aqueous solutions obtained after the first and second leaching stages. The results showed that about 11 countercurrent equilibrium stages are required to achieve more than 95 % yttrium extraction at pH -0.25, O/A ratio of 1 and a temperature of 25 ºC. Eu and residual Y could only be extracted after pH adjustment to 0.5 using 5 M sodium hydroxide. More than 92 % Eu and 99 % Y extraction was achieved using an O/A ratio of 1.5 and 1M extractant concentration. A multiple stage stripping process can then be used to recover more than 95% of the rare earths from the organic phase prior to precipitation and calcination. Ce and Tb solvent extraction results showed that all the targeted rare earths could only be recovered as a mixed product at this stage. A mixed rare earth product was produced using 1 M DEHPA, a pH of 0.5, O/A ratio of 1 and a temperature of 25 °C. Complete extraction of Y and Eu was assumed at these conditions since both rare earths could not be detected in the remaining aqueous solution. More than 95 % Ce and 98 % Tb were extracted in a single stage solvent extraction process. The rare earths can undergo stripping using 5 M sulphuric acid prior to precipitation and calcination to recover the metals as rare earth oxides.

AFRIKAANSE OPSOMMING: In onlangse dekades het die produksie en gebruik van fluoresseerlampe spoedig verhoog. Dit word toegeskryf aan die verskeie voordele wat dit inhou in vergelyking met gloeilampe. Die lampe is bekend dat dit laer energie verbruik (omtrent 75 % minder as gloeilampe) en ’n langer lewensduur het. Die spoedige groei in produksie en gebruik het tot groot volumes fluoresseerlampafval gelei, wat elke jaar weggegooi word. Herwinning van gebruikte fluoresseeerlampe bied ’n aantal ekonomiese voordele, bo en behalwe die welbekende omgewingsvoordele. Die hoofdoel van hierdie studie was om ’n lewensvatbare proses vir seldsame aardelementherwinning uit einde van lewe fluoresseerpoeiers in die Suid-Afrikaanse konteks te identifiseer. Die studie het gefokus op die gebruik van hidrometallurgiese eenheid bedrywighede in die herwinning van vier seldsame aardelemente naamlik yttrium (Y), europium (Eu), seriuim (Ce) en terbium (Tb). ’n Twee-fase logingsproses is gebruik gevolg deur oplossingekstraksie om die metale uit die oplossing te herwin. Logingstoetse het aangedui dat die rooi fosfor komponent (Y2O3-Eu3+) maklik in die oplossing kon oplos gedurende die eerste fase van suurloging. Meer as 98 % Y en 89 % Eu herwinning is bereik deur swaelsuur as logingsmiddel te gebruik. Ce en Tb is nie geloog in hierdie fase nie. Die effek van ultraklank en loogsoutsmelting op Tb- en Ce-loging is toe ondersoek. loogsoutsmelting gevolg deur suurloging het die beste herwinning opgelewer met Ce- en Tb-herwinning wat 96 % en 99 %, onderskeidelik, oorskry. Swak herwinning van die seldsame aardelemente (<10 %) is verkry deur ultraklank ondersteunde vertering te gebruik. Oplossingekstraksietoetse het gewys dat DEHPA gebruik kan word om seldsame aardelemente uit waterige oplossings te herwin na die eerste en tweede loogfases. Die resultate het gewys dat omtrent 11 teenstroomewewigfases nodig is om meer as 95 % yttriumekstraksie by pH -0.25, O/A verhouding van 1 en ’n temperatuur van 25 °C, te behaal. Eu en residuele Y kon slegs geëkstraheer word na ’n pH aanpassing na 0.5 deur 5 M natriumhidroksied te gebruik. Meer as 92 % Eu en 99 % Y ekstraksie is bereik deur O/A verhouding van 1.5 en 1 M ekstraheermiddelkonsentrasie. ’n Meervoudige-fase stropingproses kan dan gebruik word om meer as 95 % van die seldsame aardelemente uit die organiese fase voor presipitasie en kalsinering te herwin. Ce en Tb oplossingekstraksieresultate het gewys dat al die geteikende seldsame aardelemente slegs herwin kon word as ’n gemengde produk in hierdie fase. ’n Gemengde seldsame aardelement produk is vervaardig deur 1 M DEHPA, ’n pH van 0.5, O/A verhouding van 1 en ’n temperatuur van 25 °C te gebruik. Algehele ekstraksie van Y en Eu is aangeneem by hierdie toestande aangesien beide seldsame aardelemente nie in die oorblywende waterige oplossing waargeneem kon word nie. Meer as 95 % Ce en 98 % Tb is geëkstraheer in ’n enkel-fase oplossingekstraksieproses. Die seldsame aardelemente kan stroping ondergaan deur 5 M swaelsuur te gebruik voor presipitasie en kalsinering om die metale as seldsame aardoksiedes te herwin.

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