Ken,
You are correct the kettles to make salt just disappear or were broken but all the salt works have in common are near by a water of a river.

I think they got shipped there by barge,pulled by steamboat on near the Grand River-due to to its weight,size.--Pre-War

To get rid of them;to keep the kettles from being used to make salt by the Union.

My bet is on they end up rafted them,to deep river water,and would sink them into mud at the bottom--and they have never been found.