Utilization of paper sludge in preparation of high-purity calcium formate

Paper sludge generated from recycling of printed paper is a negative value waste since cellulosic fibers in the sludge are too damaged to be used in paper making and, more important, the sludge has very high content of ash (up to ~ 35%) and ink residues. To be able to utilize cellulosic fibers in bio-conversion industries which depend on saccharification of the fibers into sugars, de-ashing of the sludge is necessary. The major component of the ash is fillers used in paper making, especially calcium carbonate. In the current work, high-purity calcium formate was prepared utilizing calcium carbonate in the paper sludge — as a zero-value precursor — via treatment with formic acid under controlled conditions. The synthesized calcium formate was characterized using Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM–EDX). Calcium formate synthesized using paper sludge was compared to that prepared by reacting pure precipitated calcium carbonate and formic acid using the aforementioned tools of analyses. The results confirmed successful preparation of calcium formate with high-purity (99%) with orthorhombic crystal structure from paper sludge as pure as that prepared using pure commercially available precipitated calcium carbonate. The results and approach used in the current work could convert paper sludge to value-added products with high profit and at the same time reduce the huge amounts of land-filled sludge. Graphical abstract