THE COAL MILL ON KILN NO. 2 WAS DAMAGED WHEN AN EXPLOSION TOOK PLACE IN THE DUCTWORK LEADING TO THE PRIMARY AIR FAN. STONE AND COAL HAD BUILT UP IN THE MILL'S REJECT TROUGH. THE STONE AND COAL HEATED TO EMBERS. A SPARK OR HOT PIECE OF STONE WAS CARRIED INTO THE DUCTWORK FOR THE PRIMARY AIR FAN AND TOUCHED OFF AN EXPLOSION. NO EMPLOYEES WERE IN THE AREA. THERE WAS NO FIRE.