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).