If you ever get a chance, legitimately, to document the plant itself as well, do it, and then pair it with the portraits.
Consider seeing if you can get permission to take photos at the gate a few weeks before the place closes. Look in the little bars that usually are around such a place, as well.
Then go back a year later and shoot them again; over half of the bars will likey be closed up.
I lucked into such a project many years ago.
My dad managed manufactiring production a place that made gear for mining and pulp and paper and sewer treatment facilities. At one time over 600 employees. He would go in on Saturdays when the factory was shut down to work on reports, and I would be allowed to photgraph the facility.
Sadly I was only into 35mm gear at the time, and a larger format would have expressed some images much better. Once I as allowed in to the foundry while they were pouring, and came away with great images of that outing.
That was now 25years ago. The plant is a smaller opertion. It is managed by a childhood friend. I should probably try to get back there again.