Is it okay to use silicone to seal gas furnace heater supply ducts? If so, do you know how long the silicone will need to cure before the system is useable?

Some background: I've been trawling around my basement recently and found that a lot of my duct connections have exceeded my wildest dreams of what poor craftsmanship looks like. Lots of connections were "sealed" using literal duct tape - which has since crusted off and is not holding on to anything. The joints where circular ducts attach to box ducts are the worst offenders.

Thanks for your time.