an iron furnace!
More specifically it is the most complete remaining iron furnace, although it is in fact not all there. The "boiler" looking coils on top make it more complete than the remaining early iron furnaces around.

A better pic for more info from the sign.

I didn't take piece meal pics of the sign itself that I parked near for the shots.

Here you can see from the pics, but probably not read, the shape of the furnaces.
Nice work all. Interesting guesses and good job to Mike in SC for identifying a landmark in Vintondale, PA which is a part of PA that most people miss, including locals!