Recovery of nanosize zinc from phosphor wastes with an ionic liquid
Hsin-Liang Huang A B , H. Paul Wang A C E , Edward M. Eyring D and Juu-En Chang A CA Department of Environmental Engineering, National Cheng Kung University, Tainan City, Taiwan, Republic of China.
B Department of Safety, Health and Environmental Engineering, National United University, Miao-Li City, Taiwan, Republic of China.
C Sustainable Environment Research Center, National Cheng Kung University, Tainan City, Taiwan, Republic of China.
D Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA.
E Corresponding author. Email: wanghp@mail.ncku.edu.tw
Environmental Chemistry 6(3) 268-272 https://doi.org/10.1071/EN08098
Submitted: 28 November 2008 Accepted: 22 April 2009 Published: 18 June 2009
Environmental context. Very fine phosphor ashes are discharged from particulate collection systems (such as bag houses) in the cathode ray tube or television disassembling processes. Effective recovery of ZnO and ZnS nanoparticles from the phosphor ash can be achieved by extraction with a room temperature ionic liquid. By synchrotron radiation X-ray absorption spectroscopy, the obtained molecular scale data turn out to be very useful in revealing speciation of zinc in the extraction process, which also facilitates the development of a simple nanoparticle recovery method.
Abstract. An effective, simple method has been developed for the recovery of ZnO and ZnS nanoparticles from hazardous phosphor ash waste. Experimentally, zinc (77%) in the phosphor ash (that contains mainly zinc (91%)) can be recovered by extraction with a room temperature ionic liquid (RTIL) ([C4mim][PF6], 1-butyl-3-methylimidazolium hexafluorophosphate). Component fitted X-ray absorption near edge structure (XANES) spectra of zinc indicate that metallic zinc (Zn) (9%) in the phosphor ash can be dissolved to form a Zn2+–1-methylimidazole ([mim]) complex during extraction with the RTIL. ZnS and ZnO nanoparticles (60–61%) can also be extracted from the phosphor. Over the 298–523 K temperature range, desired ZnO/ZnS ratios (0.3–0.6) can be obtained since interconversion of ZnS to ZnO in the RTIL is temperature dependent. The Fourier transformed extended X-ray absorption fine structure (EXAFS) data also show that the nanosize ZnS extracted in the RTIL possesses a Zn–S bond distance of 2.33 Å with coordination numbers (CNs) of 3.6–3.7. At 523 K, in the RTIL, ~30% of the ZnS is oxidised to form octahedral ZnO (with a bond distance of 2.10 Å and a CN of 6.1) that may coat the surfaces of the ZnS nanoparticles. This work exemplifies the utilisation of X-ray absorption spectroscopy (EXAFS and XANES) to reveal speciation and possible reaction pathways in a nanoparticle extraction process (with a RTIL) in detail.
Additional keywords: EXAFS, ionic liquid, nano ZnO, nano ZnS, phosphor, XANES.
Acknowledgements
The financial support of the Taiwan National Science Council, Bureau of Energy, National Cheng Kung University (Excellence Project), and National Synchrotron Radiation Research Center (NSRRC) is gratefully acknowledged. We also thank J. F. Lee of the NSRRC for his experimental assistance with EXAFS.
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