i have quite a collection of empty cartridges.....it seems a shame to throw them away....

I am assuming you mean for edible inks. You can get refils here http://www.kopykake.com/store/edible-pictures-on-cakes-products/edible-inks-refill-kits/edible-ink-refills/

Many schools will recycle them and they gain benefits from recycling them. I know our school does and I am sure there are probably some close to you. We also sell refills, but I got the feeling you were not looking for that answer!

You sure can! I just bought a package of refills from icingimages.com
Super easy to refill!

Here is a picture of a image I did from the refill pack.

http://cakecentral.com/gallery/2104267

HTH

Michael