How to recycle tanks?

So I hear a rumor you can recycle tanks. I've tried looking in the manual and even asked in-game. Apparently you use your Engi Calc but I only get my old options that's always been there. No recycle when looking at a tank or anything. How do you recycle tanks?

 

Move up to the tank, use your calculator and discharge it. Like repairing it.

 

I've moved up to the tank and left click the mouse, and it'll do the calculator animation but nothing happens. I've held it for about 30 seconds.

 

it has to be a dead tank hull

 

Oh... that explains it. Thanks.