Non Process Element Control in the Liquor Cycle Through the Use of an Ash Leaching System

摘要

To improve run-time between washes and reduce thermal cycling of recovery boilers, the Evadale, Texas mill has been attempting to reduce the levels of non-process elements (NPE) in the liquor cycle. The mill has periodically purged electrostatic precipitator ash in an effort to reduce and control concentrations of potassium and chlorides. An additional effort to lower chloride concentrations was made by changing the make-up chemical from diaphragm to membrane grade caustic. Improvements were also made in the operation of the chlorine dioxide generator spent acid salt filter. The change in makeup chemical NPE content successfully reduced the amount of boiler pluggage due to sticky ash formation, but did not allow the desired amount of time between boiler washes. In an effort to further reduce the amount 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. The 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 non-process element reduction methods, the chloride content in the incoming electrostatic precipitator ash has been reduced by roughly 55% and the potassium content has been reduced by about 50%. Approximately 80% of the soda entering the ash leaching process is recovered into the liquor cycle.