Delignification and bleaching with peracids.

摘要

The purpose of the research is to investigate the preparation, stability, and application of the mixed peracid (Pxa), which is a mixture of peracetic acid and peroxymonosulfuric acid. The conversion ratio (amount of hydrogen peroxide converted to peracids) depends on the molar ratio of both sulfuric acid and acetic acid and the concentration of hydrogen peroxide. To reduce the cost of manufacturing the mixed peracid, its conversion ratio can be reduced to 70% without affecting bleaching efficiency. Control of the reaction temperature, mixing order and addition time are essential to improve the yield of the mixed peracid.; The conversion ratio of the peroxymonosulfuric acid will increase as a result of the increasing concentration of undissociated sulfuric acid (the reacting species). The equilibrium constant of peroxymonosulfuric acid formation was measured to be 1.88 at 4{dollar}spcirc{dollar}C.; The effects of altering pH, reaction temperature, time, and transition metals (both content and species) on the mixed peracid's stability were evaluated. The stability of the peracid decreases as the pH of the solution increases. Except in the case of Mn, application of a chelating agent (DTPA) reduces the metal-induced decomposition of both hydrogen peroxide and peracid. Among the transition metals commonly found in pulp, Mn is the most detrimental ion in the decomposition of peracetic acid.; Typical bleaching conditions for the mixed peracid bleaching is 0.2% DTPA, pH 2.5 to 6.5, 10% consistency, one hour at 70{dollar}spcirc{dollar}C for pulps without a pretreatment stage, and 30 minutes at 90{dollar}spcirc{dollar}C for those with a pretreatment stage (Q). The peracid charge is determined by delignification requirements. When preceded by a Q stage, the delignification selectivity in the mixed peracid stage is maintained even at high chemical charges.; As a delignifying agent to replace the chlorine and chlorine-containing compounds in the Total Chlorine-Free (TCF) bleaching sequence, a 45 percent delignification rate can be obtained by 1 percent of the mixed peracid and followed by an extraction stage. Surprisingly, a kappa number of 30 can be reduced to 6 by using 3 percent of the mixed peracid in a (PxaE) stage. In the delignification of chemical pulp, the mixed peracid can be used as an activated agent to improve the selectivity of subsequent oxygen delignification.; The General Energy and Material Balance System (GEMS) software package was used in a simulation of the sodium and sulfur balance in a mill recovery system. Use of the mixed peracid will affect the sulfur/sodium balance in the TCF closed mill recovery system. Limited amounts of the mixed peracid (0.5%) can be used in the TCF bleaching sequence without creating sulfur or sodium imbalances. Increasing the mixed peracid charge or molar ratio of sulfuric acid will upset the sulfur and sodium balances. Using oxidized white liquor in the bleach plant and dumping a part of the bleaching effluent will enable a mill to use higher amounts of the mixed peracid. (Abstract shortened by UMI.)