My disposal was easy
I was the first to get a new tank from Triple M and they sent a truck from their factory to my home to pick up my old tank. They wanted to be sure that they were building the tank correctly since it was a rather odd shape. They asked if they could keep it and I agreed.Otherwise, I would have taken it to my local metal recycler and sold it for scrap. Unfortunately, since it is not "clean" metal, meaning that there are materials other than just aluminum, you will not get a top price. I think aluminum is going for about 25-30 cents per pound but you might get only half that value. You should check, but even if the value just pays for the car gasoline to get the tank there, at least you are rid of it.