THE MAN WAS INSPECTING DUCT INSIDE OF #3 RAW MILL SEPARATOR, WAS WEARING FULL FALL PROTECTION DEVICES AND HAD FOLLOWED CSE RULES. HE INADVERTENTLY SLID FROM A HORIZONTAL RUN INTO A VERTICALLY SLOPING RUN OF CHUTE. HIS FALL PROTECTION PREVENTED INJURY, BUT HE COULDN'T PULL HIMSELF UP THE SAFETY ROPE. NO INJURY WAS INCURRED. TRAPPED FOR APPROX 1 HR.