Bulk cleaning and especially rust removal from bolts, screws, and other small hardware

I have not read all the replies, so forgive me if this has already been suggested. I'm envisioning a drum on its side with a few fins on the interior that rotates on a slight incline, very much like a clothes dryer, and nuts & bolts & whatever can be tossed in along with an abrasive like a coarse sand. A motor then rotates the drum, and the tumbling action + time might do a really decent job of removing rust. Then when parts are sufficiently cleaned, they could be dumped onto a set of sifting screens with the parts staying up top on larger hole screens, like hardware cloth, and with a couple increasingly finer meshed screens below, the sand and likely iron oxide particles would come out the bottom, ready to be used again. Being a dry method it doesn't seem to messy to me, doesn't require any acids, alkali's or toxic chemicals and could be a "set it and forget it" automatic cleaner. It seems like the sort of thing that could be relatively easy to make.

I don't understand why Tennessee isn't part of the rust belt the way rust seems to grow here much better than other parts of the country

That electrolysis video looks like the way to go. I love a set-and-forget approach. Build a rig for it once and you can do batches regularly, with no nasty gick to worry about.

If you are looking for a good low voltage DC supply, consider doing a hatchet job on an old phone charger. We use them to run old PC fans in strategic places to move warm air around the house from our woodstove.

My 2 cents here: You have mentioned bulk so I suggest electrolysis. I use a parts cleaning basket to hold the bulk fastetners (in my case parts from a blacksmith post drill) and a mixture of washing soda. The basket surface being the - contact of the items being cleaned a juggle now and then to stir it up. The solution not extremely toxic, unlike some plating solutions. A battery charger will work for a power supply. I have an electroplating power supply source and an area where I do that, so that is what I use. A resist on parts that have names/dates etc can be covered with a resist to preserve that if you desire.
I have an ultrasonic that works on rust but again what quantity are you cleaning?
Tumbling nuts and screws is probably not something I would do in bulk, individual pieces depending on what they were maybe. I mass finish jewelry and use different media: stainless shot, ceramic, abrasive plastic pellets of different configurations and walnut shells. an assortment of bolts would be banging against themselves accelerating degradation of threads.
For large parts I recently purchased one of the power washer siphon sand blasters. The tool connects to my power washer and siphons a blasting media like a regular sandblaster eliminating the need for a large compressor while reducing dust immensely.
Vinegar an option as well, Coke can do a surprising job on surface rust on chrome but have never tried it on anything else.
Travis' comment on a  mixture of acetone and atf is spot on.  I agree that this is one homemade tip that is far above any ccmmercial penetraing oil. I own a parts store and can use anything, but this mixture has proven itself on even salt water soaked parts. It is also an excellent gun cleaning solvent.

Posting this photo because it looks like art to me. Also, notice that there isn't any dry red rust anywhere.  Most of the collections I work with are powder dry, covered in red rust and spiderwebs and dustbunny lints, and very unpleasant to handle (my fingers get dry and rusty and painful).  This collection came out of a small Folgers coffee can that was in my father-in-law's tool shed.  I never knew him; he died in the 1980s and I met his daughter in 2003.  So this collection is at least 35 years old, by which I mean, it's been sitting in an outdoor not-climate-controlled but enclosed space since at least 1985.  Most likely, it was a thirty year old collection in 1985.  So, why so shiny?

Every other coffee can in the collection from that closet has the typical dry red rust.  There's half a #10 can full of nuts like that.  But this man was a union machinist for Boeing.  He also grew up as poor as poor can be, as a dirt farmer from a big family in the Oklahoma dustbowl.  It's pretty obvious to me that his coffee can of threaded hardware got routinely oiled.  Waste oil, a squirt now and then of 3-in-1, I dunno how he did it.  But these screws were mailed into the future with a rust resistant coating.  I'm convinced of that.

Its a treat to find a properly oiled old bolt, or even a crusty greasy one, and find its as good as new. Counters out the disappointment of finding a rusty bent bolt in the "good bolts" bin.

Your quest  got me going,  so I found myself looking into tumblers.

This is the cleanest, nicest looking of the diy tire tumblers  that I have found:



This guys tumbler is rough looking,  but he is doing exactly what you want to do with it:



Among machinist,  a lot of them use a concrete cleaner with phosphoric acid as the active ingredient  , Crete-Nu by Savogran, but I can't find it anywhere, so it's probably discontinued.
I believe the phosphoric acid cleaners are still available.

I have successfully used a rust remover containing mostly phosphoric acid. Not too aggressive and I never used gloves, but it requires manual work with a (wire-)brush. And once diluted it turns into fertilizer.