GALLATIN, Tenn. (AP) — Three workers have been critically burned in an explosion at a Middle Tennessee chemical plant, and federal investigators are on their way to the scene. The Friday morning accident that injured five is the third this year at the Hoeganaes (HAY'-gan-eez) Corp. plant where a January flash fire killed two workers, one of whom died last week. The Gallatin facility makes metal powders, and the two previous accidents occurred after flammable dust in the air combusted.