Investigation on the sodium hydroxide replacement with calcium hydroxide on the optical and physical properties of recycled pulp bleached with hydrogen peroxide

Today, use of recycled fiber is currently an important lignocellulosic source for paper production. In conventional (chemical) deinking and bleaching processes, sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) are commonly used as the active alkali and bleaching agent, respectively. In this research, the recycled waste papers (70% ONP/30% OMG) were repulped by conventional method and deinked by combined washing and flotation processes. The prepared pulps were bleached with a conventional bleaching using calcium hydroxide (Ca(OH)2), as a substitute for NaOH, at different levels of 2 (C1), 4 (C2), 6 (C3), 8 (C4), and 10% (C5). In addition to determining its optimal amount of consumption, its performance in H2O2 bleaching compared to NaOH was evaluated. Conventional bleaching with H2O2 was also performed separately in the presence of NaOH, as a basis agent (control sample), and the optical and physical properties of standard handsheets (60 gm-2) were evaluated. The results of using Ca(OH)2 in place of NaOH in recycled pulp bleaching showed that its usage was desirable in terms of brightness value and similar physical properties, compared to those of conventional H2O2 bleaching system, while improving the performance of the bleaching process in the presence of NaOH. In general, experimental runs of C1 and C2 showed the highest degree of brightness, thickness (caliper), and bulk, but with increasing the usage of Ca(OH)2 to more than 4%, the papers with weaker optical and physical properties achieved.