I’ve heard the shots go off while working elsewhere in the plants. Blanks (shock blast only) or do they load a solid fragible clay or salt load above the powder ?
IIRC, they used blanks.
It was a neat place to visit and we frequently walk out there when hunting; it was maintained beautifully until corporate decided to shut 'er down. The employees would collect the soot that was knocked loose from the stacks, dispose of it and then tidy up to a degree that I always found astonishing. Once the TLC stopped - there were some persnickety guys who worked there for many years and treated it like it was their own - it got shabby fast.
When I last saw it, I thought it would still be there long after I was gone, but there isn't anything left of it now but a few patches of concrete in a field full of weeds.