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* Dr. H. U. Suess, Degussa GmbH, ** Dr. M. Janik, AQura GmbH, both in 63457 Hanau,Germany

Key words: bleaching, hydrogen peroxide, decomposition, carbonate ions, magnesium sulfate

1. Abstract
In presence of an excess of hydrogen peroxide sodium carbonate as well as sodium bicarbonate rapidly react into an equilibrium of carbonate ions and peroxo carbonate ions.

With 13C enriched carbonates this reaction can be monitored by 13C NMR. Even at only 30C this mixture decomposes slowly, confirming the instability of hydrogen peroxide in the presence of carbonate ions. At elevated temperature the presence of carbonate ions leads to significant losses of peroxide. The addition of magnesium salts precipitates carbonate as magnesium carbonate. In high temperature bleaching, like disperser bleaching, this significantly improves the bleaching result and decreases peroxide demand.

2. Introduction
Losses of hydrogen peroxide in bleaching processes are typically ttributed to transition metal decomposition. In recycling plants the enzyme catalase - originating from biological activity (bacteria growth) - triggers peroxide losses. Stabilization of peroxide bleaching therefore focuses on chelation agents (like DTPA) for the chelation of transition metals and biocides (like glutaric dialdehyde) for enzyme destruction. However, there is an additional still widely unrecognized source for peroxide losses: carbonate ions. The negative impact of carbonate ions in bleaching processes was already published [1]. The presence of carbonate ions negatively affects brightness increase in E(o)p stages, it consumes peroxide in disperser bleaching of secondary fibers and it limits the brightness development in mechanical pulp bleaching. The effects only become pronounced at a temperature higher than about 70C. Therefore a negative bleaching result is mostly not related to the presence of carbonate ions.

3. Peroxocarbonate
Sodium percarbonate is used in huge amounts in laundering agents. This compound is not a peroxo compound but a hydrogen peroxide addition product. In the crystals hydrogen peroxide replaces water yielding a compound with the average formula Na2CO3 + 1.5 H2O2. Dissolving this product in water generates an alkaline bleaching liquor.

Fig. 1: Carbonate (163 ppm) and peroxocarbonate (159 ppm) peaks in a solution of Na2 13 CO3 and an excess of H2O2 (20 times) in D2O at 30C (pH 11.7) after 50 minutes

The existence of the peroxocarbonate anion in aqueous solution is sometimes doubted.
Indeed mixing of stoichiometric amounts of Na2CO3 (13C enriched) and H2O2 (in D2O) only shows one carbon peak in the 13C NMR, the one for the carbonate anion.

However, this changes once an excess of hydrogen peroxide is added. At 30C in a mixture of Na2CO3 with the 20 fold excess of H2O2 within minutes a second carbon peak develops. The intensity of this second signal increases while the peak for the carbonate anion decreases. Figure 1 shows the signals for both carbonate species. They belong to the CO3 2- anion and to the CO4 2- anion. Within about one hour an equilibrium is established.

Fig. 2: Increase of the peroxocarbonate peak and decrease of the carbonate peak over time in a solution of Na213CO3 and a 20 fold excess of H2O2 (D2O, 30C)

Figure 2 shows the decrease of the peak for the carbonate ion and the parallel increase of the peroxocarbonate's peak. The existence of the peroxocarbonate ion was already described by Flanagan [2]. Later Richardson [3] monitored the speed of the generation of peroxo bicarbonate from NaH13CO3 and H2O2 in ethanol/water. However, in both papers the instability of these solutions was not recognized.

Depending on storage temperature the peak for the peroxo compound vanishes within several hours or days. In the experiment above after a long weekend not even peroxide traces could be detected in the mixture. This points at the instability of the peroxo compound. Obviously the percarbonic acid anion decomposes relatively easily into carbonate and oxygen. It is speculated that the peroxo carbonate anion generated as an equilibrium with the carbonate anion (CO3 2- + H2O2 CO4 2- + H2O) decomposes via a similar reaction path as the perhydroxyl anion with excess peroxide:

CO4 2- + H2O2 CO3 2- + H2O + O2

Fig. 3: Generation of peroxo bicarbonate and decrease of the bicarbonate peak over time in a solution of NaH13CO3 and a 20 fold excess of H2 O2 (D2O, 30C, pH 8.3)

Fig. 4: Increase and decrease of the peroxo bicarbonate concentration over extended time, ratio of the 13C peaks of bicarbonate and peroxo bicarbonate

The generation and decomposition of the peroxo compound is concentration and temperature dependent. Bicarbonate reacts similar. Mixing of sodium bicarbonate with an excess of hydrogen peroxide (in D2O) also generates a second peak in 13C NMR. The signal for H13CO3 - is at 162 ppm, it decreases rapidly. The peroxo bicarbonate peak is at 159 ppm.

Figure 3 shows the rapid generation of the equilibrium within about 25 minutes at 30C. The highest amount of peroxo bicarbonate is reached at about a ratio of 80 to 20. The decline of the peroxo bicarbonate concentration takes more time.

Figure 4 shows this development in a 4 days analysis. In contrast to the effects seen with soda ash, the peroxo bicarbonate is more stable. Its concentration decreases but even after 4 days (at 30C) this peak is still visible. Because there is an excess of peroxide in the mixture, decomposition losses are initially compensated. It is therefore not possible to directly correlate the decrease in the amount of peroxo compound to its thermal stability. This would only be possible in a stoichiometric 1:1 mixture.

Even in absence of catalyse and with very good chelation alkaline hydrogen peroxide solutions used in bleaching processes do decompose. Two reasons for the decomposition can be named: the temperature (thermal decomposition) and the pH (alkaline decomposition). The higher both factors are, the more intense the losses. The presence of carbonate ions is a third reason for peroxide losses [1].

Because the carbonate ions affect the pH level, no simple method exists to attribute peroxide losses to just the presence of carbonate ions. In order to demonstrate the effects of pH, temperature and carbonate ions several decomposition experiments were made. In bleaching processes amounts of chemical applied may vary within a rather wide range.

Peroxide concentration can be lower than about 5 g/L and as high as 60 g/L; caustic soda amounts between 10 g/L and 40 g/L, depending on consistency and brightness targets. The tests were conducted with an input of 20 g/L peroxide and 20 g/L NaOH (0.5 mol/L). To eliminate the impact of metal traces analytical grade chemicals were used. De -ionized and boiled water was used to exclude an impact of (air) carbon dioxide. For the same reason only freshly prepared solutions of caustic soda were applied and the experiments were conducted under nitrogen in a closed flask.

Figure 5 shows the decrease of the peroxide concentration over time in solutions with 0.5 mol/L of caustic soda, soda ash and sodium bicarbonate at 50C and at 90C. pH was high with NaOH (pH 12.5) and Na2CO3 (pH 11) and rather low with bicarbonate (pH 9). Because of the low pH stability of the bicarbonate solution at 50C is good. Already at 50C peroxide in the sodium carbonate solution is less stable than in caustic soda, despite of the lower pH.

At 90C the lower pH of the bicarbonate solution cannot compensate the accelerating impact of the carbonate ions. Bicarbonate and carbonate solutions lose peroxide significantly faster than the caustic soda solution. In case, some of the caustic soda is replaced by an equimolecular amount of soda ash, the decomposition of peroxide is only marginally accelerated at 50C (Fig. 6). On the other hand, at 90C the negative impact of carbonate becomes significant again. Decomposition accelerates and slows down only after most of the peroxide is decomposed.

Fig. 5: Decrease of the peroxide concentration in presence of NaOH, Na2CO3 and NaHCO3 at 50C and 90C. Start with 20 g/L H2O2, all alkalis at 0.5 mol/L

Fig. 6: Decrease of the peroxide concentration at 50C and 90C in presence of 0.5 mol/L caustic soda or a 0.4 mol/L NaOH/0.1 mol/L Na2CO3 mixture

4. Decomposition reaction in bleaching processes
The effects seen in the 13C NMR spectra – the decrease of the peak for the peroxo carbonate over time by decomposition - are mirrored in bleaching processes. Industrial peroxide bleaching is typically conducted at temperatures between 45C and 90C. The low end applies for repulping in deinking plants. Mechanical pulp is bleached between 60C and 75C and chemical pulp between 70C and 85C. The highest temperature (90C to 95C) is used in disperser bleaching in deinking units and in TCF bleaching of chemical pulp.

As already mentioned and visualized in Figure 5, high temperature triggers peroxide decomposition. Decomposition of peroxide by caustic soda is a known fact; the graph in addition shows the impact of carbonate.

Therefore the result of high temperature bleaching should improve with an elimination of carbonate ions. An option is precipitation. Magnesium sulfate reacts fast with carbonate ions. The product, magnesium carbonate, formula 4 MgCO3 . Mg(OH)2 . 4 H2O, is very insoluble in water, it precipitates immediately.

In deinking plants air is used in froth flotation. Air contains carbon dioxide which is readily absorbed in the alkaline liquor and sodium carbonate is generated. Consequently rather high amounts of carbonate can be found in the process water of deinking plants [4]. The addition of magnesium sulfate visibly improves disperser bleaching at 90C. Figure 7 has an example. The addition of magnesium sulfate increases the pulp's lightness by about one point. The impact on peroxide stability is significant. The amount remaining doubles to >40% with magnesium addition. This indicates a high potential for savings.

Fig. 7: Peroxide residual and lightness of deinked pulp resulting with increasing magnesium sulfate addition in disperser bleaching with 1% H2O2, 0.5 % NaOH, 90C, 15 min, 25% consistency

5. Conclusion
Peroxocarbonate is generated from carbonate ions and hydrogen peroxide. Its limited stability at elevated temperature causes peroxide decomposition. The precipitation of carbonate ions with magnesium sulfate stabilizes high temperature peroxide bleaching. This decreases peroxide demand and improves brightness.

6. Materials and Methods
13C NMR spectra were recorded with a Bruker Avance 400 or DRX 500 MHz spectrometer using an invers gated pulse sequence with a 30 flip angle, a delay of 5 s, 8 or 16 scans and a temperature of 30C.

Na2 13CO3 and NaH13CO3 enriched to 99% 13C (commercial products (Isotec, USA), ultra high concentrated hydrogen peroxide (93%), commercial D2O (Merck, Germany).

Stability tests were conducted with analytical grade chemicals (NaOH, NaHCO3, Na2CO3) from Merck, Germany, under nitrogen atmosphere to exclude air (CO2).

Bleaching trials were conducted with pulp samples from a deinking mill, taken after dewatering by a press ahead of the disperser. Disperser bleaching was simulated in plastic bags by preheating pulp samples for 5 minutes in a water bath. Magnesium sulfate solution was premixed with the pulp; all other chemicals were added at once directly after.

7. References
[1] H. U. Suess, J. D. Kronis; Impact of carbonate ions on H2O2 performance in pulp bleaching, Intn. Pulp Bleaching Conf., Poster Session, Portland, OR. (2002)

[2] J. Flanagan, D. P. Jones, P. Griffith, A. C. Skapski, A. P. West; On the existence of peroxocarbonates in aqueous solution, J. Chem. Soc. Chem. Commun. 20-21. (1986)

[3] D. E. Richardson, H. Yao, K.M. Frank, D.A. Bennett, Equilibria, kinetics and mechanism in the bicarbonate activation of hydrogen peroxide: Oxidation of sulfides by peroxomonocarbonate, J. Am. Chem. Soc., 122, 1729-1737. (2002)

[4] H. U. Sss, B. Hopf, K. Schmidt; Optimising peroxide bleaching of deinked pulps in the disperser, 2002. Presented at: PTS-CTP Deinking Symposium, Mnchen (German version in: Wochenbl. f. Papierfab. 130 (11/12) 738 – 745. (2002)

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