Recovery of rare earths from waste cathode ray tube (CRT) phosphor powder by selective sulfation roasting and water leaching

Abstract

Until recently, most displays, such as television and computer screens, were based on cathode ray tubes (CRTs). With the introduction of new types of displays including liquid crystal displays, CRTs have been widely replaced, leading to a gradual build-up of hazardous CRT powder waste. In this paper, a new approach is introduced where the valuable rare-earth elements (REEs) (i.e. yttrium and europium) in the powder are selectively recovered, leaving behind a zinc-rich residue and glass for further recycling. The main rare-earth compound in the waste powder is Y2O2S:Eu3+ (YOS). The fine-grained CRT powder was mixed with zinc sulfate monohydrate (ZnSO4·H2O) and roasted at 600–900 °C for short periods of time. In this way, Y2O2S:Eu3+ was transformed into water-soluble rare-earth sulfates. Meanwhile, ZnS underwent a two-stage reaction with ZnSO4·H2O, where first a partially water-soluble intermediate (ZnO·2ZnSO4) and then the water-insoluble ZnO was formed. Addition of a sufficient amount of ZnSO4·H2O ensured the recovery of ≥95% of the rare earths in a subsequent water leaching step, but there was also co-dissolution of about 5% of the total amount of zinc present. Several purification methods were tested and compared to separate the REEs from the zinc impurity in the solutions. In addition to the conventional precipitation methods with sulfides and oxalic acid, a novel liquid-liquid exchange reaction between REE-containing leachate and zinc-loaded Versatic Acid 10 was tested. Finally, a complete flow sheet is proposed for the (almost complete) valorization of REEs as well as the total zinc present.