TAED activator for peroxide bleaching of recycled pulp

Transcription

1 TAED activator for peroxide bleaching of recycled pulp Yong Zou 1, Jeffery S. Hsieh 1*, Chhaya Agrawal 1 and Jane Matthews 2 1 Pulp and Paper Engineering, School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA Warwick International Group Limited Mostyn, Holywell, Flintshire, England CH8 9HE *Correspondence: jeff.hsieh@chbe.gatech.edu, Phone: ABSTRACT The aim of this research is to investigate the effects of ph, TAED charge, chelating agent-dtpa usage and reaction time of peroxide bleaching on the physical properties of recycled pulp. The results showed (a) Reaction time, ph and TAED charge had significant effects on the brightness; especially reaction time at low alkaline ph levels and low chemical charges; and the addition of DTPA increased tensile 10% and improved brightness stability; (b) for current mill process for bleaching recycled pulp-q(tby), compared to peroxide, TAED increased bulk (20%), reduction reaction time (50%), and reversion decreased; and lowered some of chemicals usage. Keyword: TAED, Recycled pulp, bleaching ITRDUCTI In the detergent and laundry industry, the peroxide activator TAED has been used for many years in combination with a persalt to provide effective bleaching at temperatures and residence times at which peroxide alone would be ineffective. TAED is colorless, odorless, stable when stored, safe, and easy to handle [1]. It has been established as a non-toxic, non-sensitizing, non-mutagenic product that biodegrades to form carbon dioxide, water, ammonia and nitrate; ethylene diamine is not detected during biodegradation [2]. With TAED, peroxide bleaching can be performed at both lower temperatures and lower alkalinity with significant benefits in terms of retained fiber quality. Efforts to remove chlorine from pulp and paper bleach plant operations have led to the increased use of hydrogen peroxide, which can brighten or bleach pulp, but is limited due to its decomposition rate and ability as a weak oxidizer. In recent years, TAED has been investigated as a bleaching activator; in the presence of peroxide at alkaline conditions, TAED generates the anion of peracetic acid [3]. TAED also has the potential to reduce the chemical cost for other additives, such as sodium hydroxide and peroxide stabilizers. While peroxide and peracids have very limited ph ranges in which they can be used, experience has shown that TAED works best at ; however, it can be used at much lower values (ph 7-8) without a significant loss in brightness, leading to a reduction in sodium hydroxide [4]. Stabilizers, such as sodium silicate and DTPA, are necessary for effective peroxide bleaching. These stabilizers are used to help reduce peroxide decomposition, but may not be necessary for TAED. TAED may also have the potential to improve pulp properties such as strength and bulk. Since TAED enhances the oxidation potential of peroxide, faster bleaching may result, allowing shortened reaction times to increase the overall throughput from bleaching operations. If the reaction times can be shortened, the use of TAED may be able to improve pulp strength by reducing the degradation of the fiber due to longer, less efficient bleaching. TAED reacts most favorably in pulp bleaching when the ph is close to However, it is still effective at ph values around 8, where peroxide alone has little to no effect [5]. In the presence of TAED, the perhydroxyl ion

2 reacts with the TAED to form the peracetate ion, as shown in Figure 1. Peroxide primarily works in alkaline conditions when the perhydroxyl ion, H 2 -, is formed: H 2 2 H H + Brightness, PC number, bulk, and tensile strength measurements were used to characterize the optical and physical properties of the pulp. By better understanding how these properties are affected by bleaching conditions, such as TAED charge, pulp type, and ph, it should be possible to improve and eventually optimize TCF bleaching for all pulp varieties. + 2H - base 2CH 3 C - H TAED DAED Figure 1: Formation of Peracetic Acid from TAED EXPERIMETAL Materials: The recycled fiber starts from approximately GE brightness to 57 GE brightness. The residual amount of peroxide remaining in the pulp filtrate after the bleach stage was determined by saving the pulp effluent prior to washing. In the presence of sulfuric acid and potassium iodide, a solution of peroxide was titrated with sodium hyposulfite. This study is only focused on using a QT sequence to determine the delignification effects of TAED in a single stage. Process: The peroxide experiments were performed in plastic bags at 55ºC and 20% consistency. For this experiment, 2.5% sodium silicate and 1.35% sodium hydroxide were added to the amount of water needed to achieve 12% consistency. The amount of pre-warmed water required to obtain 20% consistency was added to the pre-warmed pulp, and mixed well. The pulp was placed in a plastic bag, heat sealed, and fully immersed in a pre-heated constant temperature water bath. The pulp was mixed every fifteen minutes for the duration of the experiment. Bisulfite stages were performed in bags. Peroxide bleaching on the recycled fiber followed by hydrosulfite was required by our sponsor to simulate the mill conditions. At the end of the peroxide or TAED stage, 0.25% bisulfite was added to the pulp, placed in the water bath, and allowed to react for 15 minutes. At the end of this stage, the consistency was dropped to 4%, and 0.25% hydrosulfite was added. The pulp mixture ph was adjusted to 7.2 and placed in the water bath for 45 minutes.

3 RESULTS AD DISCUSSI Four factors have been studied in a 2 4 experimental design includes the three primary variables analyzed in this work: reaction, ph, and TAED charge, as well as an additional variable-dtpa. The effect of a small addition of DTPA was analyzed in order to determine whether the metals management from the chelation stage, performed at the mill, was adequate or not. Table 1 showed the 2 4 experimental design in detail. Table 1 Experiment Design for T stage of QT % TAED Time (Hr) ph DTPA (%) %P %P %T/1.25%P %T/1.25%P %T/1.25%P %T/1.25%P %T/1.25%P %T/1.25%P %T/1.25%P %T/1.25%P ne can see by Figure 1 that while brightness loss still occurs, it is minimal (2-3%). However, the brightness was higher for the 1:1 TAED-peroxide ratio at than other TAED stages. It appears as though operating in excess of the stoichiometric ratio (1:2) minimizes the brightness loss, regardless of ph. This would hence indicate that excess TAED results in more complete peracetate generation and less peroxide decomposition. Similarly, this trend is witnessed for the PC number in Figure 1. Most significantly, TAED addition not only improves the PC number compared to peroxide-only bleaching, it also drops the PC number below At this point, the PC number appears to, with some slight fluctuations, reach an asymptotic limit. Recycled fiber also showed the lowest increase in bulk (Figure 2), which is most likely a result of fiber hornification, which would limit the diffusivity of the bleaching chemicals and prevent them from reaching the lignin compounds. Finally, Figure 3 shows the affect of TAED charge and ph on tensile strength. The trend is clear, as the fiber strength increases with increasing TAED charge and decreasing ph. The maximum tensile strength value of 54.6 occurs at the 1:1 ratio at. These data thus suggests that TAED can provide an increase in bulk coupled with an increase in tensile strength, which typically does not occur. The bulk increase indicates less fiber interaction; conversely, increased tensile strength indicates increased fiber to fiber bonding so as to strengthen the paper. The best explanation we can offer is that the TAED-activated peroxide further alters the fiber morphology after recycling

4 such that the fibers, individually weakened by the refining process, have significantly more interactions due to fraying and thus display greater strength as a whole. Further experimentation must be done. %GE PC umber Brightness PC umber 1.25%P - ph %T/1.25%P - ph %T/1.25%P %T/1.25%P - Figure 1: Brightness and Color Reversion for Recycled Sequence Bulk (cm 3 /g) % P - ph %T/1.25%P - ph %T/1.25%P %T/1.25%P - Figure 2: Bulk for Recycled Sequence

5 55 54 Tensile Strength () %P - ph %T/1.25%P - ph %T/1.25%P %T/1.25%P - Figure 3: Tensile Strength for Recycled pulp Sequence Table 2 showed that reaction time, ph and TAED charge had significant effects on the brightness; especially the reaction time plays a significant role at low ph level and the chemical charges. This may be due to a lower reaction rate for recycled pulp. Table 2 Effect of ph, TAED Charge and reaction time % TAED Time ph Brightness (Hr) (%GE) %T:1.25%P %T:1.25%P %T:1.25%P %T:1.25%P %T:1.25%P %T:1.25%P %T:1.25%P %T:1.25%P ne can see from Table 3 that the addition of DTPA did not have significant individual effect on brightness, but improved the brightness stability and increased the tensile strength. These changes in optical and physical properties by the DTAP addition might have been caused by an inefficient chelating stage.

6 Table 3 Effect of DTPA on ptical and Physical Properties for Recycled Pulp Bleaching Pre-Aging (%GE) Post-aging (%GE) Bulk (cm 3 /g) PC Tensile() 1.25%P - ph 11, 0.0% DTPA %T/1.25%P - ph 10.5, 0.05% DTPA %T/1.25%P -, 0.05 % DTPA %T/1.25%P -, 0.05 % DTPA CCLUSIS TAED is a peroxide activator with which bleaching can be performed at both lower temperatures and alkalinity with significant benefits in terms of retained fiber quality. This paper studies the effect of ph, TAED charge DTPA usage and reaction time of peroxide bleaching on the physical properties of recycled pulp. These preliminary results show that TAED-activated peroxide can provide better brightness, brightness stability, bulk, strength; and lower some of chemical requirements. furthermore, data from recycled have shown that use of excess TAED in bleaching is capable of providing brightness equal to or better than that of peroxide only bleaching. Reaction time, ph and TAED charge had significant effects on the brightness; especially reaction time at low ph levels and low chemical charges; and the addition of DTPA increased tensile and improved brightness stability; LITERATURE CITED 1. Woodman, J., Pollution Prevention Technologies for the Bleached Kraft Segment of the U.S. Pulp and Paper Industry, EPA600/R-93/110, August, pp Young, Raymond A., Environmentally Friendly Technologies for the Pulp and Paper Industry. J. Wiley, ew York: p (1998) 3. Special Report, Pulp and Paper Canada, 97(10): 15 (1996) 4. Jane Mathews, A new approach to textile bleaching, JSDC Volume 115, p.115. May/ June Amini, B, J.R. Anderson, and W. Wilkinson, Proceedings of the 81 st Annual Meeting, Technical Section: Canadian Pulp and Paper Association, CPPA Press, Canada: pp B (1996) 6. TAPPI Test Methods ( ) T Pan, G. X.; Spencer, L.; Leary, G. J. A comparative study on reactions of hydrogen peroxide and peracetic acid with lignin chromophores. Part 1. The reaction of coniferaldehyde model compounds. Holzforschung, Volume 54 (2000), pages