How to Open Manholes on a Ball Mill dia 6m x 9 m length

Are the panels structural?

In a ball mill that size I would have doubts that it would be made of such light steel that it couldn't support itself with the access ports open.

Or if it's worn out and structurally damaged to the point it can't support itself without the ports bolted in place you have far greater issues to deal with than changing the balls.

Has this ball mill been inactive for a long period? I ask that because these mills can sag and this can pinch the bolts to the point they become oblong and break on attempted extraction. In this case they will need drilling with careful concentricity to avoid the thread and re-tapped to a larger round size after corrective work to correct the sag (at time, this damage can ruin a mill left for a decade or so)

If not sagged, you might try repeated thermal cycling with a torch followed by a oil quench (blast off the oil with a pressure washer before the next torch session. This can often degrade the corrosion mediated bond caused by the inexorable expansion of the rust in the threads. Often stainless bolts are used, but the steel shell will rust and the grip is strong enough to exceed the bolt strength.

In any event, call in an experienced ball millwright, they have the tools and experience to deal with this. Once you have observed the process, you will gain some experience and might be able to do it yourself.