i'm kind of late to the conversation, but i know where some of the salt is going, because i took it away.
over on the east side are the containment ponds for morton, among other salt distributors.
i would show up in my 48 foot flat bed, take care of the paperwork, pull into the loading area and then they would load me with 14 or 16 pallets of bagged salt--- 47,000 pounds per truckload. i would then drive to home improvement stores in places like sioux falls, where they would sell it to people to put on their driveways. the setup was good for a lot of trucks per day. i don't know how many, but they had harness gantries for tarping the load, and only big operations do that.
lots of salt in the east comes from salt mines underground, but why drill a mine when you have the salt flats?
47,000 pounds doesn't sound like much, until you do 100 trucks a day for 60 years.