Peracetic Acid in Pulp Bleaching

Bleaching of pulp with peracids is limited on an industrial scale to the application

of peracetic acid, CH3COOOH. In the past, other per-compounds were also promoted

for bleaching, among these being Caro’s acid, H2SO5, or mixtures of Caro’s

acid and peracetic acid. The interest in their application stems from the search for

alternatives to chlorine bleaching. The application of peracids was tested in ECF

and TCF bleaching sequences [1]. With the increasing knowledge about ECF

bleaching and its good environmental performance, peracids became less interesting.

Today, peracetic acid occupies a niche in TCF bleaching. The application of

peracetic acid in TCF bleaching was introduced because of the need to modify residual

lignin to allow its destruction in hydrogen peroxide bleaching, and to

improve the economics of TCF sequences. Typically, a peracid treatment is applied

under weak acidic conditions. This results in an improved delignification and a higher

brightness in the following alkaline peroxide stage.

Peracetic acid has a sharp pungent odor. It has a boiling point of 103 °C and a

vapor pressure of 3325 Pa at 25 °C. It is a weaker acid than acetic acid, and is produced

by mixing (glacial) acetic acid with hydrogen peroxide. The addition of a

strong acid (e.g., sulfuric acid) accelerates the formation of the equilibrium between

acetic acid, hydrogen peroxide, water and peracetic acid:

2 CH 3 COOH _ H 2 O 2 _ H 2 O _ CH 3 COOOH _ CH 3 COOH _ 2 H 2 O

880 7Pulp Bleaching

The equation shows that the equilibrium can be shifted to the right by applying

high concentrations of hydrogen peroxide. However, there will always be an excess

of hydrogen peroxide and unreacted acetic acid in the mixture. This increases the

cost for the application of equilibrium peracetic acid in bleaching, because the

reaction conditions for peracid bleaching will not allow the reaction of hydrogen

peroxide. Another important factor is product safety. Storage and handling of

higher concentrations of peracetic acid with a high H2O2 content are restricted

due to its potential hazards. This prevents application in mill practice.

The peracetic acid equilibrium is shifted to the right by distillation under vacuum.

The resulting peroxide conversion is greater than 90%. The distillation products

are the most volatile compounds, water and peracetic acid (boiling point

103 °C). This distillate must be cooled to prevent re-formation of the equilibrium.

The ideal storage temperature for the mixture is below 0 °C; therefore, storage

tanks require both insulation and refrigeration. Cooled distilled peracetic acid is

commercially available with a content of 35–40% peracetic acid in water. Because

of the absence of a strong acid, re-formation of the equilibrium is very slow. An

accidentally higher storage temperature (e.g., ambient temperature) would not

constitute a safety hazard, but the resulting “new” equilibrium would produce a

lower concentration of peracetic acid.

Another economical alternative for peracetic acid application is on-site mixing

of peracetic acid with hydrogen peroxide. At a temperature slightly above ambient,

and with acid activation, the equilibrium is established within a few hours. Mixtures

with a content >8% peracetic acid and <40% H2O2 are commercially produced

on-site. In order not to waste the content of hydrogen peroxide in this equilibrium,

the Paa treatment must be followed by the peroxide stage, without intermediate

washing. Following an addition of caustic soda, the unused hydrogen peroxide

content in the pulp reacts in the subsequent P step.

The reactions of peracid with lignin follow mainly an electrophilic pathway.

With regard to reactivity, peracetic acid (CH3COOOH) has an advantage over

Caro’s acid (H2SO5). Peracetic acid has a pKa value of 8.2, and is only partly dissociated

at neutral or moderately acidic pH. Peracetic acid reacts via hydroxylation

(OH+), splitting into a cation and an anion, acetate (CH3COO–). In contrast, Caro’s

acid has two pKa values of 1and 9.3. Thus, it is completely dissociated

(SO5

2– + 2H+). An electrophilic reaction is only possible via the mono anion

(HSO5

– OH+ + SO42–), which is present only at very low concentration. This

explains the slow reaction of Caro’s acid with lignin. A comparison of both compounds

at identical active oxygen content favors Paa. The final kappa number is

lower, and the final brightness higher after the Paa-P treatment.

The demand for peracetic acid is moderate. Figure 7.137 illustrates an example

of a TCF bleaching application. An input of 0.1–0.5% peracetic acid is sufficient

for the activation. Paa is applied at moderately acidic pH and at a temperature of

about 80 °C. Because the peracid reaction is slow, a retention time of 1h is not

sufficient to consume a charge of more than 0.5% at 80 °C. On the other hand,

because of the high temperature, peracetic acid is hydrolyzed into acetic acid and

7.7 Peracetic Acid in Pulp Bleaching 881

0 0.1 0.3 0.5

peracetic acid (%)

brightness (%ISO)

Дата добавления: 2015-10-21