Metals Management

Although transition metals cause the decomposition of H2O2, a controlled decomposition

with the well-defined generation of radicals would be desirable from the

point of improving delignification. However, to date, no such selective generation

has been described. A manganese containing complex [46] has been described as

catalyst for peroxide bleaching. Unfortunately, synthesis of this manganese complex

is rather difficult, therefore its industrial use would be far too costly. Typically,

the radicals produced by metal-catalyzed decomposition are unselective, and fiber

damage dominates as a result of cellulose depolymerization. In consequence, metal

impurities must be removed from the pulp before any subsequent peroxide

treatment [36,46,47]. The amounts of transition metals present in pulp differ

widely, as levels depend on the wood species and the soil on which the wood was

grown. Normally, manganese and iron are the dominant metals, and others such

as copper and cobalt are present only in trace amounts (around 1ppm). In sulfite

pulping, the removal of metal is straightforward since, under the acidic and reducing

conditions of the pulping process, the metals become water-soluble and are

easily removed during brownstock washing.

In kraft pulping, the transition metal ions become insoluble as they are reduced

to a low state of oxidation and precipitate as sulfides. The sulfides are very insolu-

860 7Pulp Bleaching

ble under alkaline and neutral conditions and cannot be removed by washing.

During oxygen delignification, the metals may be raised to a higher state of oxidation,

although the resulting hydroxides are still insoluble under the conditions of

oxygen stage washing. However, they become water-soluble under mild to strong

acidic conditions. In conventional bleaching processes, the transition metals are

removed during the acidic bleaching stages. Since H2O2 typically is applied in ECF

bleaching only after the first D stage, the metal profile normally is already sufficiently

low, and no specific measures for metal removal are required. The effect of

pH value on the elimination of iron and manganese from a softwood kraft pulp is

shown graphically in Fig. 7.118. Compared with iron, the removal of manganese

is clearly much easier. Strong acidic conditions are required to reduce the quantity

of iron, which is very likely bound to lignin or lignin–carbohydrate structures.

The iron is therefore not directly available for to decompose H2O2, and consequently

traces remaining in the pulp after chelation do not have a negative effect

on the bleaching process.

7 6 5 4 3 2

Initial

Fe Mn

Metals [ppm]

pH value

Fig. 7.118 Removal of iron and manganese from softwood

kraft pulp with increasing acidity. All trials conducted at 3%

consistency, 60 °C, 0.5 h with H2SO4 for acidification.

The removal of metals is far more important in TCF bleaching, because H2O2 is

applied early in the sequence, and at much higher charges. Since strongly acidic

conditions have the disadvantage of removing not only metals such as manganese

but also magnesium (which protects against loss of viscosity), metals removal at

the mill scale is typically carried out at moderate pH with chelants such as diethylene

triamino penta-acetate (DTPA). The impact of increasing amounts of chelant

is shown in Fig. 7.119, where DTPA addition maintains a high level of magnesium.

Typically, a chelation stage (Q) is operated at medium consistency, a temperature

between 50 °C and 70 °C, a pH of about 6, and a retention time of about 1h.

As mentioned, it can be assumed that any remaining traces of metals are tightly

7.6 Hydrogen Peroxide Bleaching 861

0 0.25 0.5 1

DTPA (%)

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