2005 年 12 巻 315 号 p. 106-114
Various burning conditions of paper sludge (PS) were examined for the purpose of waste-material recycling. Relationship between burning conditions and reaction products was investigated, and based on the results, optimal condition for recycling PS to use as a filler and a pigment of recycled paper was determined.
The PS ash with high whiteness could be obtained through the general burning process at a high temperature more than 700°C for a long time more than 5 hours. However, this PS ash was not suitable for a paper-filler and a paper-pigment since the pH and electric conductivity values increased too much at the slurry-mixing step due to the formation of gehlenite with high hardness.
Chemical changes of principal components of PS under a heating condition were examined by thermal analysis. Gehlenite was possibly formed from kaolin and calcium carbonate in PS via their thermally-caused change to mata-kaolin and calcium oxide. Since decarboxylation reaction depended on the partial pressure of carbon dioxide (CO2), transformation of calcium carbonate to calcium oxide could be suppressed by burning PS under CO2 atmosphere, although transforming reaction of kaolin to meta-kaolin hardly could because of independently from burning atmosphere. That is, decarboxylation temperature of calcium carbonate increased up to 800°C at CO2 concentration in air of 10% and 900°C at 100%.
Consequently, proper burning condition of PS for recycling as a paper-filler and a paper-pigment was established as follows. Burning temperature is 700 to 750°C. Burning atmosphere is 10% of CO2 and 90% of air (20% of oxygen (O2) and 80% of nitrogen (N2))