Treatment of flue gases

Claim 1

IN A PLROCESS FOR TREATING HOT COMBUSTION GAS WHICH CONTAINS A FINITE AMOUNT BUT LESS THAN ABOUT 1% OF SULPHUR DIOXIDE IN WHICH SULPHUR DIOXIDE IS ABOSRBED FROM THE GAS IN AN ABSORPTION STEP WITH A SOLUTION OF AMMONIUM SULPHITES WHICH CONTAINS ADDED AMMONIA AND IN WHICH ABSORBENT SOLUTION IS WITHDRAWN FROM THE ABSORPTION STEP AND ACIDIFIED IN AN ACIDIFICATION STEP WITH SULPHURIC ACID THEREBY RELEASING SULPHUR DIOXIDE GAS AND DORMING AN AMMONIUM SULPHATE SOLUTION WHICH CONTAINS DISSOLVED SULPHUR DIOXIDE, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF PASSING, IN A COOLING STEP, SAID HOT COMBUSTION GAS AT AN INITIAL TEMPERATURE ABOVE ABOUT 200*F.F IN DIRECT CONTACT WITH AN AQUEOUS, ESSENTIALLY AMMONIUM SULPHATE SOLUTION AT A TEMPERATURE BELOW 200*F. WHICH CONTAINS DISSOLVED SULPHUR DIOXIDE WHEREBY SAID HOT COMBUSTION GAS IS COOLED TO A TEMPERATURE BELOW 200*F. AND THE SULPHUR DIOXIDE AND WATER VAPOR CONTENTS THEREOF ARE AUGMENTED AND THE CONCENTRATION OF AMMONIUM SULPHATE IN SAID AQUEOUS ESSENTIALLY AMMONNIUM SULPHATE SOLUTION IS INCREASED; CONTINUOUSLY WITHDRAWING CONCENTRATED AMMONIUM SULPHATE SOLUTION FROM SAID COOLING STEP; TREATING PART OF SAID WITHDRAWN AMMONIUM SULPHATE SOLUTION TO RECOVER CRYSTALLINE AMMONIUM SULPHATE THEREFROM; CIRCULATING THE REMAINDER OF SAID WITHDRAWN AMMONIUM SULPHATE SOLUTION TO SAID COOLING STEP FOR DIRECT CONTACT THEREIN WITH HOT COMBUSTION GAS; PASSING COOLED COMBUSTION GAS FROM THE COOLING STEP TO THE ABSORPTION STEP WHEREIN SULPHUR DIOXIDE IS ABSORBED IN A DILUTE SOLUTION OF AMMONIUM SULPHITES; DISCHARGING COOLED COMBUSTION GAS SUBSTANTIALLY FREE FROM SULPHUR DIOXIDE FROM THE ABSORPTION STEP TO THE ATMOSPHERE; WITHDRAWING SILUTE SOLUTION OF AMMONIUM SULPHITES FROM THE ABSORPTION STEP; ACIDIFYING SAID WITHDRAWN SOLUTION OF AMMONIUM SULPHITES WITH SULPHURIC ACID THEREBY RELEASING SULPHUR DIOXIDE GAS AND FORMING DILUTE AMMONIUM SULPHATE SOLUTION WHICH CONTAINS DISSOLVED SULPHUR DIOXIDE; RECOVERING SAID RELEASED SULPHUR DIOXIDE GAS; AND PASSING SAID DILUTE AMMONIUM SULPHATE SOLUTION WHICH CONTAINS DISSOLVED SULPHUR DIOXIDE TO SAID COOLING STEP FOR DIRECT CONTACT THEREIN WITH HOT COMBUSTION GAS.