Pulping & Bleaching
By Pulp & Paper Canada
Practically, pulp bleaching is a process that makes the final product whiter or brighter. Technically, bleaching a pulp decreases the visible light absorbance or increases the reflectance of a pulp, r...
By Pulp & Paper Canada
Practically, pulp bleaching is a process that makes the final product whiter or brighter. Technically, bleaching a pulp decreases the visible light absorbance or increases the reflectance of a pulp, resulting in an increase of the measured ISO brightness value.
The absorbance of visible light by wood pulp fibres is caused mainly by lignin; the other components of the fibre — cellulose and hemicelluloses — are essentially colourless. In the manufacture of (chemi)mechanical pulps, wood is fiberized with minimal lignin removal, and bleaching — or more properly brightening — of these pulps takes place solely by decolorization of lignin. During chemical pulping of wood, 80 to 90% of the lignin is removed but the residual lignin remaining after the pulping processes is highly coloured. Consequently, the subsequent bleaching stages are designed to completely remove the remaining lignin .
(Chemi)mechanical pulps are brightened with a lignin-retaining chemical, such as sodium hydrosulphite, while chemical pulps are bleached with lignin-degrading chemicals, such as chlorine dioxide and ozone. Hydrogen peroxide is unique; it is being used as either a lignin-preserving or a lignin-degrading chemical, depending of the process conditions.
This article provides highlights of current and emerging technologies for bleaching (chemi)mechanical and chemical pulps.
Bleaching of (chemi)mechanical pulps
Sodium hydrosulfite is the bleaching agent of choice for (chemi)mechanical pulps when a moderate (4-14 points) brightness increase is required . Hydrogen peroxide is preferred for higher brightness gains, e.g., between 10 and 20 points . Other chemicals such as sodium perborate and aminoboranes are still being evaluated but show some promise, as we will see later.
Bleaching of (chemi)mechanical pulps can be achieved in the refiner or in a post-refiner system or both. Refiner bleaching works because both the temperature and the mixing intensity in a refiner are very high. The high temperature forces the bleaching reactions to completion in a short time while the excellent mixing ensures a uniform distribution of the bleaching chemical within the pulp fibre suspension .
Refiner bleaching lends itself to mills having an occasional need for higher brightness pulps because it is easy to operate on an intermittent basis . However, one has to be careful as corrosion or scaling can be a problem.
Post refiner bleaching uses a tower to better control the bleaching conditions and to provide the retention time needed to reach the target brightness.
(a) Sodium hydrosulphite
Sodium hydrosulphite (Na2S2O4) brightens pulp through reductive reactions with pulp chromophores. The principal reducing species is believed to be a sulfur dioxide radical ion (SO2_-) formed by dissociation of S204-2. In addition, possible electron exchange between sulfur dioxide radical ions may give rise to SO2 and SO2-2, which are both reducing agents .
The normal ranges of operating conditions for hydrosulphite bleaching in a tower are: hydrosulphite charge: 0.1 – 0.8% (w/w); temperature: 40-75C ; retention time: 15-60 minutes; pH: 4.5 – 6.5; consistency: 3 – 10%.
These variables are all interdependent; for example, increasing the temperature or the pulp consistency allows decreasing the retention time, as does improving the mixing efficiency at the point of addition .
(b) Hydrogen peroxide
Hydrogen peroxide is the most widely used oxidative bleaching agent in (chemi)mechanical pulp bleaching .
The perhydroxyl anion, HOO-, is generally accepted as the active bleaching species in alkaline hydrogen peroxide systems. The anion is in equilibrium with undissociated hydrogen peroxide and the addition of alkali (usually sodium hydroxide) forces the equilibrium towards the anionic form .
The normal ranges of operating conditions are: peroxide application: 1 – 5% (w/w) ; pH range: 10.5 – 11.2 ; temperature: 45 -85C; retention time: 30-120 minutes; consistency: 4 – 35% .
A transition metal management stage is important for maximum peroxide bleaching efficiency. The stage decreases the amount of transition metals such as Mn, Fe and Cu that catalyze peroxide decomposition [5-6]. Of these metals, Mn is particularly problematic.
The main sources of metal ion contamination in pulp bleaching are: the pulp itself; the chemicals used to make the bleach liquor (iron is a contaminant in technical grade sodium hydroxide); process water; and finally the processing equipment (through wear and chemical attack) .
The purpose of pretreating (chemi)mechanical pulps with a chelating agent is to remove as much of the transition metals present before the addition of bleach liquor . Typically the pentasodium salt diethylenetriaminepentaacetic acid (Na5DTPA) is used in this role. The pretreatment is usually carried out at a low consistency (e.g., 3.5%) at a pH of 4.0 – 6.0, after refining or grinding, at a dosage between 0.1 and 0.6%. Before bleaching, the pulp is thickened (e.g., to 20-35%) to wash the chelated metals out of the pulp. When a thickening step before bleaching is not possible, the addition of the chelant still decreases the amount of peroxide decomposition which occurs during bleaching, though not as significantly .
Reductant-assisted DTPA chelation (Qy) is an effective way to improve the removal of manganese from mechanical pulps . A sodium hydrosulphite charge as low as 0.1% improves manganese removal and brightness gain during subsequent peroxide bleaching. This process has been demonstrated to be particularly useful for dealing with seasonal variations in brightness.
However, the removal of transition metals is never complete (especially for iron), and the pulp and the bulk of the system still contain enough metals to accelerate the peroxide decomposition. To reduce the catalytical activity of the remaining metals, sodium silicate is added, typically at a dosage of 3%.
Because EDTA and DTPA are poorly biodegradable, zeolites (non-toxic water insoluble inorganic compounds) have been proposed as an alternative or complement to chelating agents for the control of transition metal catalyzed decomposition of hydrogen peroxide in the bleaching of mechanical pulps . There is still debate as to the value of zeolites. Dyhr and Sterte  concluded that zeolites alone were not viable substitutes for the chelating agents currently used to minimize the transition metal catalyzed decomposition of hydrogen peroxide in bleaching processes. However, another recent study showed that the use of zeolites resulted in similar brightness gains compared to those obtained with DTPA. Of the five zeolites tested, the one with equivalent molar ratio of aluminum-silicon was found to achieve the best performance in chelating manganese .
(c ) Other bleaching agents
The work of Pedneault at al.  showed the potential of common but rarely used reducing chemicals. For example, amine borane (CH3)3CNH2_BH3 and borane ammonia complex BH3_NH3 performed better than sodium hydrosulphite in the bleaching of TMP from balsam fir and black spruce in a 1:3 ratio.
Sodium percarbonate generates hydrogen peroxide in aqueous solution. According to Leduc et al. , sodium percarbonate achieved a brightness gain of 16 points, comparable to the gains obtained by hydrogen peroxide.
Formamidine sulphinic acid has also shown potential for bleaching TMP. However, its colour stripping ability has made it more appealing for bleaching recycled pulp with a high content of coloured paper.
During refiner bleaching, transition metals contaminants cannot be removed before adding the bleaching liquor, although it may be possible to add a chelant in a preceding impregnation. Furthermore, sodium silicate cannot be added as a component of the bleach liquor because of the risk of scale formation on the refiner plates . These factors, coupled with the high operating temperature in the refiner, limit the potential brightness gains achievable with hydrogen perox
ide with this technology . In refiner bleaching using sodium hydrosulphite, typical ISO brightness gains are on the order of 4 to 6 points, which represents approximately 50 to 95% of the total brightness gain required with this chemical .
According to data by Leduc et al.  sodium perborate was more effective than peroxide in refiner bleaching; at an equivalent oxidizing charge, up to 14 points ISO were gained using peroxide while up to 20 points ISO were gained using perborate. Most strength properties were also improved.
Bleaching of chemical pulps
The removal of lignin in chemical pulp bleaching is accomplished by a mutli-stage application of bleaching chemicals. In the earliest days of bleaching, lignin removal was achieved with forms of elemental chlorine such as chlorine and hypochlorite. Elemental chlorine free (ECF) bleaching, in which chlorine dioxide replaces all of the elemental chlorine, has now become the dominant way to bleach chemical pulp; totally chlorine-free (TCF) bleaching continues to be of marginal interest in North America and will not be discussed here. Worldwide production of ECF pulp has increased for the eleventh consecutive year, reaching 63 million tons in 2001. This change has virtually eliminated the formation of poly-chlorinated aromatic compounds (dioxins and furans), and substantial reduced the adsorbable organic halides (AOX) and colour in the bleaching effluent. Chlorine dioxide is, however, more expensive and less efficient than elemental chlorine and therefore the switch to ECF bleaching has increased bleaching costs.
Advances in kraft pulp bleaching have been extensively reviewed recently  and several developing trends in Canadian mills since 1996 have been comprehensively analyzed [15,16]. One of the most important developments is the implementation of oxygen delignification, using pressurized oxygen and alkaline to remove about 50% of the lignin before the application of chlorine dioxide. The merits of oxygen delignification and its recent development have very well been documented .
Further strategies and technologies to minimize bleaching chemical consumption are evolving in an effort to reduce the cost of ECF bleaching without impairing selectivity; they are highlighted here.
Improved chlorine dioxide bleaching efficiency
Ideally, all the oxidizing power of chlorine dioxide would be used for bleaching reactions. In practice, the industrial process wastes much of the chlorine dioxide by forming chlorite and chlorate in a complex series of reaction pathways. Chlorine dioxide bleaching efficiency can be improved by accelerating the bleaching reaction or by preventing the formation of unwanted reaction by-products. Recently, it has been found that the rate and efficiency of chlorine dioxide delignification are improved by using aldehydes. These additives convert the chlorite formed in situ during chlorine dioxide bleaching back into active chlorine dioxide . The amount of wasteful chlorite formation and therefore the benefit from aldehyde-enhanced bleaching is however pulp specific.
Setting up technology teams
Chemistry of hexenuronic acid groups
One of the most significant findings of recent years, for which the 2003 Marcus Wallenberg prize has been awarded to Johanna Buchert, Anita Teleman, Maija Tenkanen and Tapani Vuorinen, is the role of hexenuronic acid in pulp bleaching. Hexenuronic acid groups (HexA) form during alkaline pulping in xylan chains and are relatively stable when exposed to alkali conditions. Consequently, they may prevent the degradation of xylan chains at high temperatures . However, they also increase bleaching chemical consumption, decrease brightness, increase brightness reversion and hinder metal ion removal. An acid treatment selectively removes most of the HexA before bleaching but this reaction is strongly influenced by temperature and pH. Models are available to predict the effect of pH, temperature, reaction time and consistency on HexA removal, and on effluent and pulp properties . These models are also useful for selecting the acid hydrolysis conditions needed to achieve an optimum kappa number reduction. This acid treatment stage allows bleach plants to conserve bleaching chemicals and improve the brightness stability of bleached kraft pulps especially those made from hardwoods which contain larger quantities of HexA after pulping than softwoods.
The use of more than one oxidant within a stage can also improve bleaching efficiency. Both peroxide and oxygen have become widely used in extraction stages to increase delignification. Combining ozone and chlorine dioxide in a single stage is a more recent example of taking advantage of complementary chemistries. Ozone is able to react with virtually any lignin functional group, while chlorine dioxide reacts with predominantly with free phenolic groups. Combining ozone and chlorine dioxide in either a (DZ) or a (ZD) stage improves the delignification efficiency and lowers the overall charge of oxidants. Additional benefits of the (DZ) or (ZD) combination are a reduction of AOX and colour in effluent. However, its effect on bleached yield has not been evaluated, although a detrimental effect on bleached yield is assumed .
Improved efficiency of oxygen-alkali extraction
Most chemical pulp bleaching sequences contain an oxygen-alkali extraction stage to lower the cost of chemical pulp bleaching. The stage has been improved since its introduction in 1980 by increasing the temperature, the pressure and the retention time .
Hydrogen peroxide in ECF bleaching
Hydrogen peroxide has become essential for improving the overall efficiency of ECF bleaching. It is especially important to mills which have limited chlorine dioxide capacity. Hydrogen peroxide is versatile. In addition to common approaches such as using peroxide in the first and second alkaline extraction stages and a pressurized peroxide stage, peroxide is used in the last high-density storage tower under conditions which increase brightness and decrease reversion without affecting pulp physical properties . Peroxide processes can be accelerated by additives. One example is a silicomolybdate-activated peroxide process which has been used industrially since 1994 to alleviate insufficient chlorine dioxide capacity or to decrease AOX. These processes often do not result in a net bleaching cost saving but address a mill specific operating hurdle .
The authors wish to thank Richard Berry, Jean Bouchard, Barbara van Lierop and John Schmidt for their comments and review of this manuscript.
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One of a kind! Paprican’s bleaching pilot plant can process any type of pulp with any of the chemistries presently used by the industry. View 1 (left) shows some of the tanks and towers as well as the belt washer and chlorine dioxide generator. View 2 (above) shows the top of the pressurized tower which is used for oxygen delignification work.
In refiner bleaching using sodium hydrosulphite, typical ISO brightness gains are on the order of 4 to 6 points, which represents approximately 50 to 95% of the total brightness gain required with this chemical
Models are available to predict the effect of pH, temperature, reaction time and consistency on HexA removal, and on effluent and pulp properties