Thermal Stability of H2O2 and Bleaching Yield

The temperature in bleaching can be varied within a wide range. Logically, a lower

temperature results in a slow bleaching reaction, but this can be compensated for

by extending the retention time. Peroxide bleaches at ambient temperature, and

this allows an application in steep bleaching with a time range of days. Mechanical

pulp and sulfite pulp is bleached on an industrial scale under such conditions,

but these are rare exemptions. Typically, bleaching with H2O2 employs a temperature

range between 70 °C and 90 °C. The huge amounts of pulp handled in continuous

processes does not allow long residence times, or the bleaching towers

would need to be very large. Temperature and time are interrelated. The trend to

use narrow water loops with a high level of internal recycling, leads to high tem-

7.6 Hydrogen Peroxide Bleaching 863

peratures within the loops. Pulping and refining processes are operated above 100 °C,

and today even screening and cleaning of the pulp is conducted at a temperature

close to the pulp’s boiling point. In mechanical pulp bleaching, the temperature

typically is above 70 °C, but in chemical pulp bleaching it can be as high as 90 °C.

Consequently the time required for bleaching becomes short. A very high temperature

(>95 °C) is critical because H2O2 decomposes thermally. An example of this

reaction at a concentration typical of a bleaching process is shown in Fig. 7.121.

0,0 0,5 1,0 1,5 2,0

0,01

0,1

Temperature [°C]

70 85 100

peroxide residual [g/L]

Time [h]

Fig. 7.121 Decomposition of diluted alkaline H2O2 in deionized

water at pH 10.5 with temperature and time. Starting

concentration 2.5 g L–1; pH adjustment with NaOH.

The normal residence time for a peroxide stage is about 1.5 h. Depending on

the temperature and the amount of H2O2 to be consumed, this time may be

shorter and/or extended to 2–3 h. Pressure and very high temperature were

recommended for the consumption of large amounts of H2O2 in ECF and TCF

bleaching [50,51]. However, pressure is required only in so far as it allows a

bleaching temperature above 100 °C. At a temperature below the boiling point of

water, an increased pressure has no impact on peroxide performance. On the

other hand, a very high temperature in peroxide bleaching has a negative impact

on pulp quality. The energy of activation for cleavage of the oxygen–oxygen bond

of H2O2 is rather low; therefore, the side reaction “thermal decomposition” or

homolytic cleavage increases strongly with temperature (see Fig. 7.120). The aftermath

of this bond cleavage is the formation of other radicals, which trigger cellulose

chain cleavage. Viscosity losses are also observed which, together with the

improved solubility of lower molecular-weight compounds present in the pulp at

high temperature and alkalinity, leads to yield losses [52,53]. Thus, extreme temperatures

should be avoided in peroxide bleaching.

864 7Pulp Bleaching

An example of the impact of high temperature in peroxide-supported extraction

stages is provided in Tab. 7.48. The aggressive conditions allow less chlorine dioxide

to be used, but the impact on viscosity and yield is pronounced. Consequently,

in ECF bleaching the mill practice is to keep the temperature level below 90 °C

during the peroxide stages. The exemption is TCF bleaching, where a very high

temperature and even pressure must be applied to compensate for the absence of

an effective delignification agent such as chlorine dioxide. In this situation, the

consequences of a lower yield and decreased pulp strength must be accepted.

Tab. 7.48 Impact of very high temperature in peroxide stages on

pulp yield, effluent load, and viscosity. eucalyptus kraft pulp,

bleached under standard (Eop 0.4% H2O2, P 0.2% H2O2) and

hot conditions (Eophot 0.5% H2O2, Phot 0.8% H2O2) to a

brightness of >89% ISO.

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