Abstract
Rare Earth (RE) phosphors waste contains valuable rare Earth elements (REEs), such as cerium, terbium, yttrium, and europium. In industry, the process of NaOH roasting followed by acid leaching is usually used to extract the REEs from the waste in China. Using this process, the leaching efficiencies of cerium and terbium are clearly lower than those of other REEs, which results in uneven extraction of REEs in the waste and low total REE leaching efficiency. The key reason is that the trivalent cerium and terbium in the waste are oxidized into RE oxides during NaOH roasting, which are difficult to dissolve in acid solution. To solve this problem, an optimized process of controlling the oxygen concentration during NaOH roasting is proposed in this paper. The influences of the oxygen concentration, roasting temperature, roasting time, mass ratio of waste phosphor to NaOH, HCl solution concentration, acid leaching temperature, acid leaching time, and liquid–solid ratio on the REE leaching efficiency were investigated. Under the optimum conditions, the leaching efficiencies of cerium and terbium increased dramatically and the total REE leaching efficiency is 99.11%.
Highlights
The useful life of a three-primary color fluorescent lamp is 10,000–20,000 h
The results show that oxidation of trivalent cerium and terbium in the phosphor can be effectively avoided in the process
After washing and acid leaching, the leaching efficiencies of cerium and terbium are >98% and the total rare Earth elements (REEs) leaching efficiency is greater than 99%
Summary
The useful life of a three-primary color fluorescent lamp is 10,000–20,000 h. Traditional chemical methods mainly involve the direct acid leaching process [7,8,9,10,11,12], in which yttrium and europium in the red powder can be leached compared to other REEs (mainly cerium and terbium included) in the blue and green powders. Even if the enhanced leaching methods are used, the leaching efficiencies of REEs in blue and green powders cannot be improved significantly This is because the yttrium and europium in the red powder exist in the form of oxides, which can dissolve in acid solution. Some iron powder was added in the mixture of alkali and waste phosphor in the process to avoid the oxidation of RE during roasting. In order to avoid oxidation of cerium and terbium in phosphors REE waste to highvalence oxides, and to ensure that the impurities are not introduced during the whole extracting system, an optimized process is proposed to extract REEs (Ce, Tb, Y, Eu) from phosphors waste by a combined alkali roasting–acid leaching process
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