What was found
AT FL280 WITH ENGINE AND WING ANTI-ICE ON, THE CABIN ALTITUDE STARTED TO CLIMB AT ABOUT 700 FPM. ACCOMPLISHED COM PER RED BORDER CHECKLIST AND WENT TO FULL CLOSED MANUALLY. AIRCRAFT PERFORMED AN EMERGENCY DESCENT AND CABIN PRESSURE CONTROL WAS REGAINED AT 13,000 FEET. LANDING AT GTF WAS UNEVENTFUL. MAINTENANCE FOUND THE RIGHT DUCT TORN AND BLOWN FROM CLAMP. REPLACED THE DUCT AND CLAMP. AIRCRAFT RETURNED TO SERVICE.