Effect Of Transition Metal Ions On Pulp Oxygen Delignification And Its Control

Oxygen delignification is polluting pulp bleaching methods widely used in today’s Pulp and Paper Industry. But its poor reaction selectivity, when the delignification rate more than 50%, it will cause serious degradation of cellulose. In order to ensure product quality, In industrial processes delignification rate was usually controlled at 30-40%. A major factor affecting the oxygen delignification selectivity is transition metal ion. In oxygen delignification process, the transition metal ions promoting the degradation of carbohydrates in the pulp and reduce oxygen delignification selectivity. To protect the carbohydrates and improve the selectivity, the most effective way is add magnesium to the pulp. However, the mechanism of magnesium sulfate as carbohydrate protector still has a lot of controversy, so the study of cellulose protective agent in oxygen delignification hard to get a breakthrough. In order to improve the oxygen delignification selectivity and product competitiveness, this paper focus on the effect of transition metal ions in oxygen delignification, exploring the negative effect of transition metal ions on oxygen delignification and cellulose protection of magnesium sulfate. According to the mechanisms of transition metal ions and magnesium, new and efficient oxygen delignification protective agent will be developed.In this paper, transition metal ions is added to pulp or removed by chelating and acid washing, and then oxygen delignification. Through analyzing viscosity and kappa number, residual alkali, dissolved lignin and other indicators after oxygen delignification, the effect of transition metal ions on oxygen delignification has been explored. The results show that in oxygen delignification process, whether there is magnesium additives, transition metal ions do not affect the pulp lignin removal. Transition metal ion will accelerate the degradation of cellulose, reducing the viscosity of the pulp. The order of ransition metal ions decrease the viscosity is Mn2+> Fe2+> Cu2+> Fe3+. Adding MgSO4 can effectively increase the viscosity. When Mn2+, Fe2+, Cu2+ content was about 100mg/kg,200mg/kg and 50mg/kg, Mg2+ added has the best efficiency on viscoosity protection. Complete removal of transition metal ions is good for the selectivity improvement, and, Mg2+ addition can greatly improve the selectivity of oxygen delignification. There are other mechanisms of magnesium sulfate as carbohydrate protective agent, in addition to magnesium hydroxide adsorption transition metal ion.This paper focus on the different adding way of magnesium hydroxide (Mg2++NaOH, NaOH+Mg2+ and NaOH+Mg(OH)2) to produce different particle sizes of magnesium hydroxide. And the effect of particle diameter of magnesium hydroxide on protective efficiency of MgSO4 was studied. and explore the impact of the synthesis of magnesium hydroxide in situ of pulp fibers on oxygen delignification. The results show that the minimum magnesium hydroxide particle size is forming by adding magnesium before sodium hydroxide, the average particle diameter is 850nm. With the magnesium hydroxide particle size reducing, the efficiency of magnesium on the cellulose protection is increasing. With the increasing of transition metal ion content, the size of magnesium hydroxide particle is more important for improving the efficiency of cellulose protection. The smaller magnesium hydroxide particle size, the higher efficiency of cellulose protection. When the magnesium hydroxide particle size decreases from 2953nm to 850nm, in the amount of 800mg/kg Fe3+ added, the viscosity from 673mL/g increased to 886mL/g. In Fe2+ addition amount is 200mg/kg, viscosity from 615mL/g increased to 784mL/g. In Mn2+ addition amount of 100mg/kg, viscosity from 627mL/g increased to 775mL/g. In situ synthesis magnesium hydroxide in pulp fiber is better than synthesis in solution. For Radiata pine kraft pulp kappa number of 34.3, in situ synthesis of magnesium hydroxide in the fiber surface can make the viscosity increase from 726mL/g to 941mL/g, while in solution only up to 896mL/g。This paper using bamboo pulp as cellulose model compound, By analyzing the Dp and molecular weight of cellulose, glucose content in the waste after oxygen delignification conditions, exploring the key factors affecting cellulose degradation. This study found that even without lignin, cellulose still degrade in oxygen delignification process. Transition metal ions promoting the degradation of cellulose, and magnesium sulfate can reduce the degradation of cellulose. Transition metal ions promoting the high molecular weight cellulose degradation to low molecular weight, so that the cellulose content of small molecules is increased, the cellulose molecular weight distribution broadens. The magnesium sulfate addtion can prevent the degradation of large molecular weight cellulose,so that cellulose molecular weight distribution becomes narrower.Papers focus on several transition metal ions control agents, investigating its impact on oxygen delignification, in order to find an metal ion control agents which suitable for oxygen delignification. And then mixed with magnesium sulfate, developing a efficient cellulose-protecting agent of oxygen delignification. The results showed that, adding A12(SO4)3, DTPA, HEDP, CDTA and EDTA and APAM into the oxygen delignification process is ineffective on transition metal ions control。 CM is effective on control transition metal ions,can improve the selectivity of oxygen delignification. Optimum dosage of CM was 0.5%, and with the smaller CM particle size, the selectivity of oxygen delignification improve significantly. CM has a synergistic effect with magnesium sulfate, and when CM and magnesium complex 2:1, viscosity increased from 620mL/g to 964mL/g, the selectivity increased from 1.6 to 5.1.