<p>I am operating a laser sintering machine, using polyamide 2200 powder (with a grain size of approximately 50 micrometers). During a print, a lot of powder goes unsintered and can theoretically be reused. However, using purely recycled powder degrades print quality to an unacceptable level.</p>

<p>Mixing a little used powder into a larger amount of fresh powder seems to work well though. What is the greatest ratio of used to fresh powder that still gives good results, and is there anything I can do (pre- postprocessing) to allow more powder to be reused?</p>

You'll find generally that mixing 40% new polyamide with 60% recycled polyamide will result in a reasonable finish and part. You will obviously want to use all new for parts requiring the best possible finish and mechanical properties, but this mixture will be very difficult to tell apart from a fully new mixture part:
http://www.paramountind.com/pdfs/eos_pa2200_mds.pdf
This is more detailed research showing how used powder changes and how that affects print quality here:
http://www.emeraldinsight.com/doi/abs/10.1108/13552540910960299
Searching for the research paper title may find a free source, but the linked resource does require a subscription or payment to that service.