General
Raw materials
Chemicals
Lignin reactions
Carbohydrates reactions
Extractives reactions
Mixing of chemicals
Washing between stages
Degree of water cycle closure in bleaching
General |
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Raw materials |
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Chemicals |
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Lignin reactions |
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Carbohydrates reactions |
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Extractives reactions |
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Mixing of chemicals |
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Washing between stages |
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Degree of water cycle closure in bleaching |
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The wood raw material affects the bleaching requirements of the pulp. Due to the chemical composition, a gentler bleaching is sufficient for hardwood pulps in comparison with softwood pulps. The unit processes that precede bleaching: cooking, washing, screening and oxygen delignification, affect the bleachability, cleanliness and strength properties of the pulp and therefore the optimal bleaching technique. The bleachability of the pulp is also affected by impurities in the bleach plant filtrate, the structure of the water cycle and the degree of closure.
Mixing the chemicals and washing the pulp between stages play an important part in the bleach plant. If the chemicals are badly mixed into the pulp it leads to an uneven bleaching effect. In the worst case the pulp may resemble marble cake. The lignin dissolved in the previous stage is washed out by the washers; otherwise it would needlessly consume chemicals in the subsequent stage.
One important factor affecting bleaching is the consistency of quality of the
incoming pulp 
. Pulp quality is affected by the process conditions
of cooking, washing, screening and oxygen delignification and their uniformity.
For example, regardless of having the same kappa number, pulp which has been
cooked with sufficiently high residual alkali throughout the cook and at a high
white liquor sulfidity is easier to brighten. Sufficient residual alkali (mill-specific,
pH about 10-11) guarantees that the lignin does not precipitate back onto the
fiber surface. High sulfidity is also beneficial for the initial viscosity,
but a large dose of alkali is not. Uneven pulp quality raises chemical consumption.
The impurities carried in the pulp consume bleaching chemicals; therefore effective washing and screening before the bleach plant are essential. Screening should be so effective that fiber bundles entering bleaching brighten without an excess charge of chemicals.
Oxygen delignification between cooking and the bleach plant decreases the amount of shives to a fair extent.
Wood species have different chemical compositions, so different pulps require different kinds of bleaching to brighten them. Thanks to their lower lignin content, milder bleaching is sufficient for hardwood pulps compared with softwood pulps.
The bark content of chipped wood and the fiber bundles originating from knot wood have an impact on bleaching. The bark of birch and spruce is more difficult to handle than that of pine. The lignin content of knot wood is greater than that of other stem wood. These fiber bundles are the most difficult parts of wood to bleach. Furthermore, shives may travel from an uneven cook to bleaching. In order to dissolve the lignin from these pulp impurities during bleaching, very much bleaching chemicals is needed. As the final brightness points consume an inordinate amount of chemicals and are critical for the yield and viscosity of pulp, the pulp cleanliness needs to be handled in some other way than by adding bleaching chemicals. The degree of decay of the wood also increases bleaching chemical consumption.
The kappa number or lignin and hexenuronic acid content of the incoming pulp affects the bleaching chemical requirement. The higher the kappa number of the pulp entering bleaching, the more bleaching agents are needed to attain a given brightness level. On the other hand, the bleach plant is the origin of most of the wastewater effluent from the pulp mill, and bleaching chemicals are more expensive than cooking and oxygen delignification chemicals.
For successful bleaching and in order to get the desired results, the effectiveness and chemical nature of the bleaching agents must be known, as should their reactivity with lignin and carbohydrates.
The reactivity of residual lignin towards a certain chemical is dependent on the chemical structure of the residual lignin and the modifications the lignin has been subjected to during the cooking process. The residual lignin in pulps cooked by different methods can differ chemically from each other to a considerable extent, so bleaching also requires a different chemical dose or sequence in order to be successful. Therefore identifying the structure of the residual lignin is important. The kinds of reactive groups that are in the lignin should be known in order to aid the selection of bleaching agents that affect the groups in question.
Both acidic and alkaline stages are always used in bleaching 
. The acid and alkali stage together form a unified
whole. In the acidic stage the lignin dissolves. Although the majority of the
reaction products are removed in the subsequent washing stage, alkaline extraction
is still necessary. The carboxyl groups generated in the acidic stage are also
neutralized. Therefore most of remaining wood extractives usually dissolve in
the first alkaline stage.
Lignin reactions can be most simply classified into reactions
In principle, the main delignifying bleaching chemicals
Due to the acidic conditions in the Z and D bleaching stages, the initial step of delignifying bleaching reactions is an attack on sites of high electron density followed by nucleophilic reactions.
Reactions of molecular chlorine with pulp carbohydrates are much slower than those with lignin. Carbohydrate chains can be broken by oxidation reaction involving HOCl and Cl2 and also by attack of chlorine radicals.
Because of the generation of reactive radicals, it is difficult to avoid degradation of carbohydrates; especially during oxygen-alkali delignification, but also in the ozone stage. In the latter case, the most harmful radical intermediates are hydroxyl (HO•) and perhydroxyl (HOO•) radicals formed by direct decomposition of O3 in water or indirectly by reaction of O3 with an organic substrate.
The main oxidizing attack of bleaching chemicals occurs within the polysaccharide chains but can also be directed to the reducing end groups of the chains. However, in kraft pulps, although new aldehydic end groups are formed via cleavage of the glycosidic linkages, the end groups are mainly of the carboxylic acid type. For this reason, the most harmful reaction in any bleaching system is the oxidation of C2-, C3-, and C6-position of a monomeric sugar moiety to a carbonyl group because these structures create alkali-labile glycosidic bonds in the polysaccharide chains. The generation of carbonyl groups during acidic stages of bleaching lead to a pronounced degradation of polysaccharides during the subsequent alkaline extraction stage (peeling reaction).
A large part of the lipophilic extractives originally present in the wood is
removed during the kraft cooking and the subsequent oxygen delignification as
well as in the washing of the pulps. A minor part of the wood extractives, however,
is carried over to the bleach plant with the unbleached pulp. In bleaching,
such extractives react to some extent with the bleaching chemicals to form oxidized
and /or otherwise modified products 
.
The extractives remaining in the pulps are detrimental to the pulp quality. Most of these extractives are known to be sticky materials which are difficult to remove in washing stages and can leave sticky deposits on pulp and process equipment. For this reason, it is important to perform the bleaching under conditions that result in the lowest possible content of extractives in the fiber product. In addition, modified (mainly chlorinated) extractives have previously been shown to have a strong influence on the brightness stability, specially in birch pulps which normally have a somewhat higher extractives content than softwood pulps.
Major lipophilic components of unbleached kraft pulps are triterpenoids, fatty acids, and fatty alcohols. Among the minor components are resin acids and diterpene alcohols. Although other steroids and triterpenoids exit as well, sitosterol is generally found to be the dominating steroid in both softwood and hardwood pulps.
In general, due to the diversity of compounds and different bleaching stages involved, the bleaching behavior of extractives-based compounds is rather complex and remains not fully understood.
It is known that wood extractives are chlorinated in older types of bleaching sequences. Chlorine typically participates in addition reactions with unsaturated constituents, such as fatty acids, resulting especially in the case of hardwood pulp in dichlorinated compounds, which are extremely difficult to remove from the pulp in the subsequent bleaching stages.
In contrast to Cl2, ClO2 primarily reacts with wood extractives by oxidation. The formation of chlorinated compounds therefore is normally very low in ECF bleaching. However, introduction of carboxyl groups in this case leads to an increased hydrophilicity, thus improving solubility of the reaction products. This is also the reason why replacement of Cl2 with ClO2 in the first bleaching stage results in a lower content of extractives in the bleached pulp. Chlorine dioxide bleaching gives rise to an almost complete removal of unsaturated fatty acids but has a minor effect on the removal of saturated fatty acids.
For the bleaching using H2O2 or O3, the knowledge of the reactions of the different extractives during pulp bleaching is still quite limited.
On the other hand, it is of great importance to avoid bleaching reactions generating toxic or otherwise harmful products of extractives, which are carried over to the bleaching effluents. With regard to this aspect, much attention hat been paid to the analysis of the lipophilic fatty acids. The neutral fraction of the organic material is composed of a large number of different types of organic compounds. Much attention has also been paid to these types of compounds because the lipophilic character of some of them suggests a propensity to bioaccumulate in aquatic organisms. Some of these neutral compounds originate from initial wood extractives, whereas other compounds are degradation products of residual lignin or carbohydrates.
In addition, mainly lignin-derived phenolic compounds especially highly chlorinated phenols, can be environmentally harmful. The analysis of these compounds in bleaching liquors has therefore attracted much attention. In liquors from the conventional chlorine bleaching of pulp, polychlorinated phenolic compounds are typically found in rather high concentrations, whereas the concentrations of compounds in most cases are below the detection limit in effluents from the modern ECF-bleaching. In the latter type of effluents, usually only a minor amount of mono- and dechlorinated phenolic compounds are detected.
The mixing of bleaching chemicals into the pulp slurry is the most important unit operation in bleaching.
Consistent quality of bleached pulp requires effective mixing. If badly mixed, some of the fibers receive too strong chemical treatment and lose their papermaking strength. At the same time some of the pulp is under-bleached so the amount of impurities in the pulp increases. Usually poor mixing leads to overdosage of chemicals, which is uneconomic.
Chlorine dioxide (ClO2) is mixed mainly with a medium
consistency mixer. A medium consistency pump or pump – mixer combination has also been used
for mixing 
.
Alkali or alkali (NaOH) is fed to the pulp web or discharge screw of the washer
of the previous stage. The mixing of the chemicals occurs with the washer discharge
screw, in the steam mixer if there is one, and in the subsequent medium consistency
pump
Peroxide (H2O2) can be fed to the inlet side of an medium
consistency pump to a point where the air has already been removed from the
pulp. Dilute peroxide can also be fed to the discharge screw of the washer.
In the alkali stage if oxygen (O2) is used it is mixed
with a medium consistency pump or with an oxygen nozzle added after the medium consistency
pump.
Ozone (O3) requires special equipment. In a high consistency process, ozone is fed directly into well-fluffed pulp in the reactor. In a medium consistency ozone process ozone is mixed into the pulp with an medium consistency mixer.
In the washing between bleaching stages, residual chemicals and water-soluble
reaction products are removed so that they do not unnecessarily consume expensive
bleaching agents in subsequent stages 
. In addition, washing is used for making the pH and
temperature of the pulp slurry more suitable for the next bleaching stage .
Vacuum drum filters , pressurized drum filters
and wash presses are generally used in washing
between bleaching stages. In the early 1990s new bleach plants were equipped
with vacuum drum filters. In more recent plants wash presses or pressure
drum filters
have been used.
Traditionally the bleach plant has used the most water in the pulp mill. Recently amounts of wastewater have decreased significantly. This has been achieved by re-circulating wash liquors in the bleach plant as much as possible prior to routing them to wastewater treatment. We talk about wash liquor circulation when the wash filtrate from the subsequent stage is used as wash liquor in one of earlier stages.
Another factor which has enabled the reduction of the amount of wastewater
from the bleaching plant is the washer technology used. Since the easy-to-use
vacuum washers restrict this reduction, nowadays more efficient wash presses
or pressurized drum washers are installed in bleach plants .
The more closed the water circulation, the greater the amount of dissolved lignin and other organic materials in the filtrates which consume bleaching chemicals. Efficient post-oxygen washing is the key for low cost bleaching; with one kg of COD/adt pulp consuming the equivalent to 0.085% active chlorine.
Additionally inorganic salts accumulate in the filtrates. They may affect the reactions in bleaching adversely. A tightly closed bleach plant has an inherent risk that significant accumulation of non-process elements will occur. In this respect, chloride, potassium, and calcium are of specific importance to control as these elements can cause considerable problems as corrosion or scaling in machines and pipes. Often it is calcium scaling which determines the limits for closure.
The most troublesome scales formed in the bleaching process are calcium carbonate,
barium sulfate, and especially calsium oxalate. The precipitation takes place
when the solubility of the compound is exceeded due to an accumulation of non-process
elements in the solution, or when mixing acid and alkaline filtrates. The solubility
of the inorganic deposits is affected by several factors, i.e. temperature,
pH value , ionic strength, dissolved organic substances, and
chelating agents.
If a mill should be totally closed, all elements coming into the mill (with the wood, with external cooking and/or bleaching chemicals and with the added raw water) have to identified and given a possibility of being purged from the mill.
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