CRJ200 CABIN PRESSURIZATION BECAME UNCONTROLLABLE. FLT CREW PERFORMED EMER DESCENT AND RETURNED FOR LNDG.

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WHEN CLBING THROUGH FL240 WE NOTICED OUR CABIN ALT WAS CLBING RAPIDLY. WE QUICKLY RECEIVED A 'CABIN ALT' CAUTION MESSAGE (CABIN ALT IS AT 8500 FT). THE FO RAN THE QRH CABIN ALT MESSAGE CHKLIST (FOR THE CAUTION MESSAGE). MEANWHILE CABIN ALT CONTINUED TO RISE TOWARD 10000 FT WITHIN ABOUT 30 SECONDS AFTER THE INITIAL 'CABIN ALT' CAUTION MESSAGE. I DONNED MY OXYGEN MASK WHILE THE FO RAN THE CHKLIST. HE WAS NOT ABLE TO FINISH THE CHKLIST BEFORE WE RECEIVED A TRIPLE CHIME AND A MASTER WARNING 'CABIN ALT' MESSAGE (CABIN ALT AT 10000 FT). I THEN CALLED FOR THE CABIN ALT IMMEDIATE ACTION ITEM CHKLIST. THE FO DONNED HIS OXYGEN MASK; ESTABLISHED CREW COM AND PROCEEDED WITH THE NEW CABIN ALT MASTER WARNING MESSAGE CHKLIST. I DECLARED AN EMER WITH ATC AND REQUESTED A LOWER ALT. ATC IMMEDIATELY TURNED ME TOWARD ZZZ AND GAVE ME AN ALT OF 12000 FT. I PERFORMED AN EMER DSCNT OF APPROX 6000 FPM. BEFORE REACHING 12000 FT; ATC ASSIGNED US 10000 FT. UPON REACHING 10000 FT THE FO AND I REMOVED OUR OXYGEN MASKS. THE FO CALLED THE FLT ATTENDANT TO CHK ON HER; AND THE STATUS OF OUR PAX. ONCE WE KNEW THEY WERE OK; I XFERRED THE CTLS TO THE FO; I THEN MADE A PA TO INFORM THE PAX OF THE SIT; AND TO LET THEM KNOW THAT WE WERE RETURNING TO ZZZ; AND THAT IT WAS SAFE FOR THEM TO REMOVE THEIR OXYGEN MASKS. ATC CONTINUED TO GIVE US LOWER ALTS AND ASSIGNED US AN ILS APCH TO RWY 27L IN ZZZ. WE FINISHED THE QRH CHKLISTS; DSCNT AND APCH CHKLISTS; AND BRIEFED THE APCH TO ZZZ. AT THIS POINT WE WERE STILL 10 MINS OUT OF ZZZ. I HAD TIME TO CALL MY DISPATCH; MAINT DEPT; AND OPS TO INFORM THEM OF OUR SIT. THE CABIN ALT WAS ERRATIC IN MANUAL MODE; AND WOULD NOT STABILIZE. IT FLUCTUATED UP AND DOWN BY THOUSANDS OF FEET. I SWITCHED IT TO AUTO MODE AROUND 5000 FT AGL AND THE CABIN PRESSURE STABILIZED AT 1100 FT. THE FO THEN XFERRED THE CTLS BACK TO ME. WE MADE A NORMAL LNDG WITHOUT COMPLICATIONS. AS A SAFETY PRECAUTION I HAD THE FIRE TRUCKS FOLLOW US TO THE GATE SINCE THE OXYGEN CANISTERS HAD BEEN USED AND THE BURNING SMELL WAS APPARENT IN THE FLT DECK.