EE was welding side rails on belt. Suspect piece of hot slag went under skirting at feeder hopper end. After welding crew was on site 40 add'l min. cleaning up & no sign of fire. Shift ended 6:45 pm. Slag evidently smoldered & started fire on skirting. EE usually runs belt but since was end of shift, he didn't. Damage consisted of burning up a belt & some rollers on conveyor.