Nonprocess-element control in the liquor cycle using an ash leaching system

To lengthen run time between washes and reduce thermal cycling of the recovery boiler, the Evadale, Texas, mill has been attempting to reduce the levels of nonprocess elements (NPEs) in the liquor cycle. The mill has periodically purged electrostatic precipitator ash in an effort to reduce and control potassium (K) and chloride (Cl) concentrations. Additional efforts to lower chloride concentrations involved changing the makeup chemical from diaphragm- to membrane-grade caustic and improving the operation of the chlorine dioxide generator spent acid salt filter. The change in makeup chemical NPE content successfully reduced the amount of boiler plugging due to sticky ash formation, but did not provide the desired amount of time between boiler washes. In an effort to further reduce the amounts of potassium and chloride in the liquor cycle, an ash leaching system was installed. The leaching system partially dissolves precipitator ash in hot water. The potassium and chloride are selectively leached into the liquid phase and remain in solution. Sodium sulfate crystals are separated from the leaching slurry using a decanting centrifuge and returned to the liquor cycle via a mix tank. The potassium- and chloride-rich liquid is sewered. Since the implementation of these nonprocess-element reduction methods, the chloride content of the incoming electrostatic precipitator ash has been reduced by roughly 55% and the potassium content by approximately 50%. Approximately 80% of the soda entering the ash leaching process is recovered into the liquor cycle.