Chlorine-Free Bleaching of Bagasse Kraft Pulp by Mliox Method
Conventional kraft pulps need chlorine chemicals to be bleached to full brightness. The reactivity of the residual lignin of kraft pulps towards alkaline peroxide bleaching was significant increased by pretreatment with peroxyformic acid (PFA) alone or in combination with other non-chlorine bleaching sequences. Three different Milox bleaching sequences have been developed for unbleached bagasse kraft pulp. The sequences are based on prebleaching with peroxyformic acid (PFA), which renders the residual lignin of pulp more susceptible to attack by other non-chlorine bleaching chemicals. Sequential bleaching with peroxyformic acid followed by alkaline peroxide alone in one or two stage or separately by acid treatment produced bleached pulp with 81.9-83.9% ISO brightness and good properties. Introducing permanganate delignification stage in the sequence increased the brightness of bagasse pulp to 87.5% ISO brightness. It also further reduced the consumption of hydrogen peroxide to 23.9% of fully bleached bagasse pulp. Moreover, the effect of removal of transition metals ions (Fe, Cu, and Mn) from the pulp by different methods and effect of this removal on the bleaching process was investigated. The strength and optical properties of resulted bagasse bleached pulp are tested and compared with that obtained from CEH sequence conventional bleaching method. The results showed that the strength and optical properties of resulted bagasse bleached pulp by Milox method are similar to those obtained from conventional bleaching method (CEH sequence).