Put it in a cement mixer with abucket of sharp crushed stone for an hour....
That's actually
precisely how to do it. No different than cleaning certain things or polishing rocks. Be careful when choosing the rocks, and know how your chain was hardened first, though. Fine gravel or "crush and run" make great abrasive media for this sort of rust removal.
Of course, if you have the time and patience, good old fashioned Naval Jelly (or anything of the sort) will knock rust out in a jiffy.
Regardless, after you get rid of the rust, you'll need to protect the chain with some form of oil or grease. There's almost never a reason to throw away a quality chain. Even if you push it beyond it's limits or damage it, you can still use the thing for less critical tasks in the future.
Cleaning up a good one is well worth the effort.
