If centrifuging of the slurry, even at a lower rpm, results a packed gel, you'd have to use a Dialysis bag. The final wash solution can be Methanol or Ethanol. 

To achieve the product in powder format, I recommend freeze-drying of the washed sample.

In addition to the above worthy comments, you  have to wash slurry of chitosan coated powder on a #60 sieve with deionized water and/or other solvent(s), then the sample is dried using freeze dryer.