Continuous process for the extraction of polyamide-6

A continuous process for preparing polyamide-6 from caprolactam, which comprises the steps of:

• (a) polymerizing caprolactam at elevated temperature to obtain a polymer melt mixture of polyamide-6, caprolactam, and oligomers of caprolactam;
• (b) granulating the polymer melt mixture of polyamide-6, caprolactam, and oligomers of caprolactam to obtain chips of polyamide-6 that also contain caprolactam and oligomers of caprolactam;
• (c) following step (b), extracting from the chips of polyamide-6, the extractable caprolactam and oligomers of caprolactam, using a stream of extraction water having at most 15% by weight of caprolactam monomers or oligomers to obtain a wet polyamide-6 product containing about 10% water by weight, a stream of extract water rich in extractable caprolactam and oligomers of caprolactam, and a first stream of recycled circulating water containing a low amount of caprolactam and caprolactam oligomers relative to the stream of extract water, and passing the first stream of recycled circulating water containing a low amount of caprolactam and caprolactam oligomers from the extraction to the granulation of step (b), whereby passing the first stream of recycled circulating water containing the low amount of caprolactam and caprolactam oligomers, as a sole source of water for the granulation, avoids any need to add fresh water to the granulation, avoids any need to evaporate said water to recover the polyamide-6, the extractable caprolactam and the oligomers of caprolactam and serves to pre-extract caprolactam and oligomers of caprolactam from the polymer melt following step (a);
• and
• (d) following the granulation of step (b) removing a second stream of recycled circulating water containing a higher amount of caprolactam and caprolactam oligomers in the range of 4 to 6%, relative to the low amount in the first stream of recycled circulating water, and passing the second stream of recycled circulating water from the granulation to the extraction.