Toller, you don't value your life much do you?
Discarded tanks may not pass a hydro test. Refrigerant recovery tanks
must get hydrostatically tested every 5 years to be legal. If those
discarded tanks won't pass the test they are not safe to use. You
might get away with it, but why take the chance? I am not talking
about the law either. I know an A/C contractor who had a tech leave a
refrigerant tank on a roof under a unit so he wouldn't have to lug it
up & down every time he serviced that building. The tank blew, landed
on a car two blocks away. Hefty fine, also had to pay damages, his
company name was on the tank. Lucky no one got hurt or killed!
Propane, like refrigerant, is stored in tank as part gas, part liquid.
If the tank ruptures for any reason, you have a missle and a bomb. Not
worth the chance to me. "Some days you gets the bear, some days the
bear gets you." If you understood the physics involved, or even saw
the results of a failure just once, you would not do it or tell any one
else how to.
Not trying to be an ass, just concerned.