Mid Sweden University

The influence of different types of iron salts (i.e., ferrous or ferric cations with sulphate, nitrate or chloride anions) on the reaction between coarse thermomechanical pulp and acid hydrogen peroxide (Fenton chemistry) was studied when the initial pH was 3.2 and 5.3. Also, ferric ions chelated with EDTA or EDG at different molar ratios were compared with ferrous sulphate when the initial pH was extended from about 3 to 8. Different anions of ferric ion salt gave a similar catalytic effect. At an initial pH of 7–8, the ferric-EDTA catalyzed reaction resulted in similar or higher hydrogen peroxide consumption and more detectable hydroxyl radicals than the ferrous sulphate catalyzed reaction, but less reaction with the pulp was indicated. Between pH 5–8, using Fe-EDG as a catalyst gave higher hydrogen peroxide consumption and more detectable hydroxyl radicals than if using ferrous sulphate; however, the measured effect on the pulp was similar or less.

The acid hydrogen peroxide system has been evaluated in more detail in the presence of a coarse Norway spruce thermomechanical pulp (TMP). The effects of parameters on the reactions involved were determined, such as pH, retention time, pulp consistency, type of iron catalyst, and the amount of dissolved organic material. Residual hydrogen peroxide, chemical oxygen demand, dissolved iron, and total fiber charge were quantified and the hydroxyl radical activity was monitored. The results showed that a catalyst of free ferrous ions (Fe²⁺) gave more reactions with the lignocellulosic material than ferric ions (Fe³⁺) chelated with EDTA. Furthermore, there were somewhat more reactions with the TMP when there was less dissolved organic material in the liquid phase from the start. Hydroxyl radicals were detected in all cases, but the level differed depending on the conditions used. It is likely that the iron must bind with the TMP or be in close proximity for the hydroxyl radicals to react with the pulp.

The potential of using acid hydrogen peroxide under Fenton conditions to lowerthe electrical energy consumed during the production of Black spruce (Piceamariana) thermomechanical pulp (TMP) was investigated in pilot scale. Thechemical system, which consisted of ferrous sulphate, hydrogen peroxide andoptionally an enhancer (such as a chelating agent), was evaluated as an inter-stagetreatment. The produced TMPs were thoroughly characterised in order to explainthe effect of the chemical system on fibre development and to be able to propose amechanism for the impact on refining energy reduction. The possibility to improvethe optical properties by washing, chelating and sodium dithionite or hydrogenperoxide bleaching the treated pulps was evaluated. The system of lignocellulosicmaterial, a Norway spruce (Picea abies) TMP, and Fenton chemistry was alsoevaluated in a model study to understand more about how conditions such as e.g.initial pH, dissolved organic material and reaction time affect the reactions.Ferrous and ferric ions (free and chelated) and different anions were evaluated.Moreover, it was examined whether hydroxyl radicals could be detected andmonitored.The results obtained in pilot scale showed that it is possible to significantly reducethe specific energy consumption by approximately 20% and 35% at a freenessvalue of 100 ml CSF or a tensile index of 45 Nm/g by using 1% and 2% hydrogenperoxide respectively. The energy reduction was obtained without any substantialchange to the fractional composition of the pulp, although tear strength wasslightly reduced, as were brightness and pulp yield. No major differences betweenthe reference pulp and the chemically treated pulps were found with respect tofibre length, width or cross-sectional dimensions. However, the acid hydrogenperoxide-treated pulps tended to have more collapsed fibres, higher flexibility, alarger specific surface area and a lower coarseness value. The yield lossiiiaccompanying the treatment was mainly a consequence of degradedhemicelluloses. It was also found that the total charge of the chemically treatedpulps was higher compared to the reference pulps; something that may haveinfluenced the softening behaviour of the fibre wall.A washing or chelating procedure could significantly reduce the metal ion contentof the chemically treated TMPs. The amount of iron could be further reduced to alevel similar to that of untreated pulps by performing a reducing agent-assistedchelating stage with dithionite. The discoloration could not, however, becompletely eliminated. The brightness decrease of the treated pulps was thereforenot only caused by the greater iron content in the pulp, but was also dependent onthe type of iron compound and/or other coloured compounds connected with theacid hydrogen peroxide treatment. Oxidative bleaching using hydrogen peroxidewas more effective than reductive bleaching using sodium dithionite in regainingthe brightness that was lost during the energy reductive treatment.From the model study and by using a chemiluminescence method, it could beconcluded that hydroxyl radicals were present in the system of Fenton chemicalsand lignocellulosic material (TMP). Initial pH, retention time, pulp consistency,type of catalyst (free or chelated) and dissolved organic material had an impact onthe reactions between TMP and acid hydrogen peroxide. Different anions(sulphate, nitrate and chloride) of ferric ion salt gave a similar catalytic effect.There appeared to be more reactions with the TMP when there was less dissolvedorganic material in the liquid phase from the start. A catalyst of ferrous sulphatehad a greater impact on the pulp (increased total fibre charge and carbonyl groups,more dissolved organic material in filtrate) than ferric ions chelated withethylenediaminetetraacetic acid at an initial pH of about 3-7. If using ferric-EDG(ethanol diglycinic acid) as catalyst, the measured effect on the pulp was similar orless compared to using ferrous sulphate. Ferric-EDG, however, gave higherhydrogen peroxide consumption and more detectable hydroxyl radicals than usingferrous sulphate (initial pH 5-8). It is likely that the iron catalyst must bind to theTMP, or be in close proximity to it, for the hydroxyl radicals to be able to react withthe material.A mechanism was proposed: the hydroxyl radicals generated in the Fentonreaction will probably attack and oxidise the available outer fibre surfaces,weakening these layers, and simultaneously dissolve some of the organic material.This can facilitate fibre development, give a better bonding pulp and reduce theelectrical energy required during refining.

The potential of using acid hydrogen peroxide for lowering the

electrical energy consumption during production of Black spruce (Picea

mariana) thermomechanical pulp (TMP) was investigated. The chemical

system, which consisted of ferrous sulphate, hydrogen peroxide and

optionally an enhancer [ethylenediaminetetraacetic acid (sodium salt),

3,4-dimethoxybenzyl (veratryl) alcohol or oxalic acid/sodium oxalate],

was evaluated as an inter-stage treatment where the primary refiner was

used as a mixer. The approach has the advantage of minimising the

capital investment needed for implementation, thus being directly

applicable in a thermomechanical pulping process consisting of two or

more refiners in series.

The results obtained in a pilot plant trial revealed that is was

possible to significantly reduce the specific energy consumption by

approximately 20-and 35% to a freeness value of 100 ml CSF by using 1

and 2% hydrogen peroxide, respectively. The energy reduction could be

obtained without any substantial change in fibre length, fractional

composition of the pulp or tensile strength of the paper. The tear

strength was slightly reduced however, as was the pulp yield. The major

drawback with the acid hydrogen peroxide system was a reduction in

brightness by at least 6 brightness units. The addition level of

ferrous sulphate was too high and the possibility to reducing the

discoloration should be considerable when the chemical system is

optimized.

The acid hydrogen peroxide system has the potential to significantly

reduce the specific energy consumption in the production of softwood

thermomechanical pulps (TMPs). A drawback of the chemical system is

discoloration of the pulp during refining. The work presented in this

study evaluates the possibility to regain the lost brightness by

washing, chelating and sodium dithionite or hydrogen peroxide bleaching

of the treated pulps.

A washing or chelating procedure can reduce the metal ion content of

the chemically treated TMPs considerably, though brightness can be

increased by a maximum of two ISO units. The amount of iron can be

further reduced to a level similar to that of untreated pulps by

performing a reducing agent-assisted chelating stage (Q(Y)) with

dithionite. The discoloration cannot,, however, be completely

eliminated. The brightness decrease-of the treated pulps is thus not

only caused by higher iron content in the pulp, but is also dependent

on the type of iron compound and/or other coloured compounds connected

with the acid hydrogen peroxide treatment.

Oxidative bleaching with hydrogen peroxide (P) is more effective than

reductive bleaching with sodium dithionite in regaining the brightness

lost during the energy reductive treatment. By using a Q(Y) P sequence,

a hydrogen peroxide charge of 3.8% was needed to reach an ISO

brightness of 75% for the chemically treated pulps. The corresponding

hydrogen peroxide charge for the untreated TMP reference was 2.5%.

By using Fenton chemistry (hydrogen peroxide, ferrous iron, pH<4) as an inter-stage treatment during production of Black spruce thermomechanical pulp, the specific energy consumption can be reduced by at least 20% to a constant tensile strength. The energy reduction can be achieved with preserved fibre length and cross-sectional dimensions. However, the fibre cross-sectional shape is affected, and fibres of the chemically treated pulps had the highest collapse index. A drawback with the acid hydrogen peroxide system is a decrease in pulp yield and a reduction in brightness. Different approaches for lowering the iron content of the chemically treated pulps prior to bleaching are presented. The discoloration is advantageously reduced (but not completely eliminated) by hydrogen peroxide bleaching.

The potential of using acid hydrogen peroxide under Fenton conditions to lower the electrical energy consumed during the production of Black spruce (Picea mariana) thermomechanical pulp (TMP) was investigated. The chemical system, which consisted of ferrous sulphate, hydrogen peroxide and optionally an enhancer (3,4-dimethoxybenzyl alcohol, ethylenediaminetetraacetic acid or oxalic acid/sodium oxalate), was evaluated as an inter-stage treatment where the primary refiner was used as a mixer. The produced TMPs were thoroughly characterised in order to explain the effect of the chemical system on fibre development and to be able to propose a mechanism for the impact on refining energy reduction. The possibility to improve the optical properties by washing, chelating and sodium dithionite or hydrogen peroxide bleaching the treated pulps was evaluated.

The results obtained in a pilot plant trial show that it is possible to significantly reduce the comparative specific energy consumption by approximately 20% and 35% at a freeness value of 100 ml CSF or a tensile index of 45 Nm/g by using 1% and 2% hydrogen peroxide respectively. The energy reduction is obtained without any substantial change in the fractional composition of the pulp, though tear strength is slightly reduced, as are brightness and pulp yield. No major differences between the reference pulp and the chemically treated pulps were found with respect to fibre length, width or cross-sectional dimensions. However, the acid hydrogen peroxide-treated pulps tend to have more collapsed fibres, higher flexibility, a larger specific surface area and a lower coarseness value. The yield loss accompanying the treatment is mainly a consequence of degraded hemicelluloses. It was also found that the total charge of the chemically treated pulps is higher compared to the reference pulps, something that may have influenced the softening behaviour of the fibre wall.

A washing or chelating procedure can reduce the metal ion content of the chemically treated TMPs considerably. The amount of iron can be further reduced to a level similar to that of untreated pulps by performing a reducing agent-assisted chelating stage (QY) with dithionite. The discoloration cannot, however, be completely eliminated. The brightness decrease of the treated pulps is thus not only caused by higher iron content in the pulp, but is also dependent on the type of iron compound and/or other coloured compounds connected with the acid hydrogen peroxide treatment. Oxidative bleaching with hydrogen peroxide (P) is more effective than reductive bleaching with sodium dithionite in regaining the brightness lost during the energy reductive treatment. Using a QY P sequence, a hydrogen peroxide charge of 3.8% was needed to reach an ISO brightness of 75% for the chemically treated pulps. The corresponding hydrogen peroxide charge for the untreated TMP reference was 2.5%.

The radicals generated in the Fenton reaction will probably attack and weaken/soften the available outer fibre wall layers. This could facilitate fibre development and consequently lower the electrical energy demand for a certain degree of refinement.