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Cadmium sulfide has often been used as a pigment in plastics such as high-density polyethylene (HDPE). Removal of CdS after the useful life of plastics is desired since it poses an environmental hazard in the waste phase of these plastics. In this study, a process is investigated to convert the insoluble CdS in the polymer matrix into CdCl 2 by treatment with HCl, followed by extraction of CdCl 2 with 1,4-butanediol (BDO). Since the diffusivity of CdCl 2 in HDPE is low, reducing diffusion length and enhancing interfacial area by dispersing BDO by laminar mixing of molten HDPE with BDO facilitate the extraction process. The overall mass transfer process during extraction is modeled as a combination of consecutive steps: diffusion of CdCl 2 through HDPE to the BDO dispersed in molten HDPE, followed by transport of the dispersed, CdCl 2 containing BDO to the bulk BDO phase surrounding the HDPE. This mechanism achieves reduction of CdCl 2 in HDPE from 1128 ppm to less than 100 ppm in 12 min. The somewhat higher melt flow index (MFI) of the HDPE after extraction (8.3 g/10 min) as compared with the MFI of the starting HDPE (6.3 g/10 min) shows some extent of degradation. However, the MFI is acceptable for HDPE recycling.

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