Effect of grinding and heating on Ni2+ uptake properties of waste paper sludge

Uptake properties of Ni2+ were examined for unmilled and milled paper sludge calcined at various temperatures to develop a new usage of waste paper sludge. Since paper sludge mainly consists of cellulose ([C6H10O5]n) fibers, calcite (CaCO3), kaolinite (Al2Si2O5(OH)4) and talc (Mg3Si4O10(OH)2), amorphous and crystalline CaO(MgO)-Al2O3-SiO2 compounds are formed by calcining paper sludge. Wet and dry milling treatments were performed to accelerate solid-state reaction to form the above mentioned target compounds. The crystalline phases originally present decompose at increasing calcining temperature (up to 800 °C) in the order cellulose <kaolinite <calcite <talc. An amorphous phase was formed in the samples below 800 °C and the temperatures changed depending on the milling conditions in the order dry milled <wet milled <unmilled. Gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8) crystallized by calcining at 900 °C in all the samples, formed by solid-state reaction of kaolinite with calcite. The 29Si and 27Al MAS NMR spectra of the amorphous samples were similar to those of crystalline phases, suggesting that the local structure of the amorphous phase resembles the crystalline structures. Maximum Ni2+ uptake occurred in the dry-milled samples calcined at 600 °C, and wet-milled and unmilled samples calcined at 700 °C, respectively. The dry-milled and calcined at 600 °C sample showed the highest Ni2+ uptake (4.54 mmol/g) and an uptake rate of 3.5 μmol/(g min).