Further Understanding Of The Effect Of Transition Metal Ions On Hydrogen Peroxide Bleaching

Hydrogen peroxide bleaching has been extensively used in high-yield pulps' bleaching. Unfortunately, hydrogen peroxide can be decomposed under alkaline condition, especially in the situation of the presence of transition metal ions, because of their catalyzing on the decomposition of the hydrogen peroxide. Thus bleaching efficiency is decreased. Experiments show that transition metal ions' valence is also responsible for the decomposition of hydrogen peroxide.Among transition metal ions, manganese ion is the most harmful species. Manganese has seven valence electrons (3d54s2) and exists in positive oxidation states of 2, 3, 4, 6, and 7. The catalytic effect of manganese in various oxidation states is different. Under acid condition Mn( II) is more stable, while under causdic condition it is easily oxidized to Mn(III). Among Mn(II), Mn(III) and Mn(IV), Mn(II) is a stable valence. It has not any catalyzing effect on the decomposition of hydrogen peroxide. About Mn(III) and Mn(IV), they both have sharply catalyzing effect on the decomposition of hydrogen peroxide. Comparatively, Mn(III) is more serious. Sodium silicate and magnesium sulfite are conventional stabilizers. If they are used together, they can prevent hydrogen peroxide from being decomposed. The best dosage of sodium silicate is about 0.5% (on solution) . While sodium silicate consistency is above 0.5%, residual hydrogen peroxide is almost unchanging. Adding sequesters DTPA or EDTA together with bleaching agents at the beginning of bleaching can decrease the free Mn(III) produced during bleaching. Then residual hydrogen peroxide is increased. For manganese, the sequestering effect of EDTA is better than that of DTPA. It can be reflected by the increase of residual hydrogen peroxide and the increase of the brightness of pulp. Adding reducing agent such as sodium thiosulphate can reduce Mn(III) to Mn( II) effectively, so the ineffective decomposition of hydrogen peroxide is decreased.Transition metal ions iron presents in two styles Fe( II) and Fe(III). They all have the catalyzing effects on the decomposition of hydrogen peroxide, but lighter than manganese. For iron, it has the responsibility for the decrease of pulp's brightness, while manganese has the responsibility for the decomposition of hydrogen peroxide. Because Fe3+ is brown, it can effect directly the brightness of pulp. Iron can also effect the viscosity of pulp. Fe( II )is unstable, it can be oxidized to Fe(III) whether under acetic condition or caustic condition. But under acetic condition it respectively stable. Fe( II) and Fe(III) almost have the sameeffects on the decomposition of hydrogen peroxide. Comparatively the effect of Fe(III) is more stronger. Fe( II) and Fe(III) can be stabilized by sodium silicate and magnesium sulfite. The effect of stabilizing Fe( II) is further better than that of stabilizing Fe(III). Adding sequesters DTPA or EDTA together with bleaching agents at the beginning of bleaching can also effectively decrease the effects of iron on pulp. For iron, the sequesting effect of DTPA is better than that of EDTA. With the increase of sequesters' dosage, the brightness increases by and by. pH value has a great effect on the sequesting effect of sequesters. While pH value is 11-12, the sequesting effects of DTPA on manganese and iron arrive the best effects. Under caustic condition, sodium hydrosulfite can reduce effectively Fe(III) to Fe(II), so the bad effect of Fe(III) on pulp's brightness is decreased. Under acetic condition, cellulose itself has reducing effects. With the more acetic, the stronger reducing effects of pulp, the higher brightness can be got. During bleaching adding reducing agent sodium thiosulphate can reduce Fe(III) to Fe( II) effectively, so the ineffective decomposition of hydrogen peroxide is decreased.