Efficient Removal of CaCO<sub>3</sub> by Hydrochloric Acid Treatment under pH Monitoring in Paper Sludge Suspension

Abstract

In this study, we aimed to realize a removal of CaCO3 by hydrochloric acid treatment at the optimum amount while pH monitoring was performed on the PS suspension. Initially, as a standard measurement, suspensions prepared with commercially CaCO3 powder were reacted with hydrochloric acid. The CaCO3 powder in suspension was taken to react in about 10 minutes. Furthermore, it was also found that CaCO3 powder could be almost completely removed by adding hydrochloric acid to less than the suspension pH of 2. Using these results, we attempted to efficiently remove CaCO3 in paper sludge （PS） with hydrochloric acid under monitoring pH of suspension. X-ray diffraction and X-ray fluorescence elemental analysis were performed on samples of PS calcined after hydrochloric acid treatment, and CaCO3 in the source PS was almost removed. The removed CaCO3 from the PS was recovered from the treated solution as CaCl2･2H2O. Scanning electron microscopy （SEM） of samples of the PS before and after hydrochloric acid treatment revealed that paper-derived fibers in the PS were hardly dissolved by hydrochloric acid treatment at pH 2. In addition, we confirmed partially self-ignition of the acid treated PS because of the organic-derived fibers remaining in the acid treated PS.