Bleaching of Wood Pulps
Concept of Brightness. Brightness is measured as reflectance in the blue portion of the visible spectrum. Complete reflectance provides a white color. Absorption of any part of the visible spectrum by a material will result in the perception of color by the eye. Pulp brightness is measured against a magnesium oxide (MgO) standard on a scale of 0100. Bleached kraft has brightness values ranging from 86-94. Unbleached kraft (brown paper bags) has a brightness of 20-30 and newsprint is around 55.
Source of Dark Pulps. Polysaccharides do not absorb light in the visible range and therefore a pulp composed solely of cellulose and hemicellulose should be white. Pulp darkness is due to lignin and lignin degradation products, and the specific compounds which cause light absorption (and therefore a colored pulp) are termed chromophores.
Methods for Improving Brightness. There are two basic ways to do this. One can retain the lignin and remove only the chromophores. This is termed pulp brightening. This is not a permanent brightness improvement, as UV light and oxygen will create more chromophores and cause yellowing (reversion). Classical bleaching processes are essentially a delignification reaction in which lignin is removed from the pulp. This results in a permanent brightness improvement. Bleaching is typically done in several stages.
Bleaching Operations/Stages. There are several bleaching stages used by the pulp and paper industry. Individual stages are listed below with their one letter abbreviation:
Typical processes involve anywhere from 4-6 stages. Sometimes a Q-stage is used; this is a chelation step used to remove trace inorganics before a peroxide treatment. The industry standard up until a few years ago was a CEDED sequence. A chlorine stage followed by alkaline extraction, a chlorine dioxide treatment, alkaline extraction, and a final chlorine dioxide treatment. More modern processes have shifted away from the use of elemental chlorine (C-stage) as this stage is the major source of toxic polychlorinated aromatics (dioxins and dibenzofurans) found in bleach plant effluents. Examples of new bleaching sequences are DCEDED, ODCEOD and OXED. The subscript implies that a small amount of the other chemical is also added. For example DC implies elemental chlorine is added as part of a D-stage treatment. EO implies that oxygen is added during the alkaline extraction to help remove additional lignin. In general, the first stages remove lignin and the latter stages brighten the pulp.
Chlorine and the Bleaching Process. Environmental problems with chlorine based bleaching processes have led to the search for alternatives. Avoiding C-stages significantly reduces the formation of dioxins, and pulps produced without Cl2 are termed ECF pulps (elemental chlorine free). Pulps which completly avoid chlorine (both C and D stages) are termed TCF pulps (totally chlorine free). The effluent from any chlorine stage cannot be sent to the recovery boiler due to the corrosion problems associated with the chloride anion. Therefore all effluents must be sewered. In contrast, peroxide, oxygen and ozone stage effluents can all be sent to the recovery boiler.
Bleaching Chemistry
Introduction. Removal of lignin left after pulping is the goal of the bleaching process. By doing this one obtains the bright sheet of paper desired by consumers. Chlorine-based processes still dominate, but more environmentally-friendly non-chlorine processes are becoming more prevalent. This trend is likely to continue. In general, it is easier to bleach sulfite pulps compared to kraft pulps, and hardwood pulps relative to softwood due to the nature of the residual lignin. Sulfite pulps still have residual sulfonation which makes them easier to solubilize, while hardwood pulps have less carbon-carbon bonds due to the presence of syringyl units.
Chlorine-Based Processes. To understand chlorine based bleaching, one needs to have an understanding of the oxidation states of chlorine. This element can exist in several oxidation states, ranging from +5 to -1. The most stable state is -1 and is typified by common table salt (NaCl). A listing of the oxidation states of chlorine in relation to common bleaching chemicals is shown below:
Compound Chlorine Oxidation State
Sodium chloride and sodium chlorate are essentially useless for bleaching, due to their inherent stability (filled outer electron shells). The useful forms of chlorine have oxidation states from +4 to 0. In reality, chlorine has one atom of +1 charge and one atom of -1 charge. So one molecule of chlorine wants to obtain 2 electrons to get its +1 charged atom to obtain the -1 oxidation state. One molecule of chlorine dioxide on the other hand wants 5 electrons to reach the -1 state (table salt is more stable than sodium chlorate so a chlorine atom with an oxidation state ranging from +4 to +1 will attempt to reach the -1 state rather than the +5 state). Chlorine dioxide is therefore a more efficient bleaching agent (2.5 times more) on a molar basis than chlorine.
Chlorine is generally obtained at the mill site by electrolysis from NaCl:
2 NaCl + e- ====> 2NaOH + Cl2 + H2
The NaOH can be used in the E-stage and the facility is typically termed a chlor-alkali plant. The chemistry of chlorine based bleaching (C-stage) is based on substitution and oxidation reactions. Below a pH of around 2.0, substitution reactions predominate, and this is the desired reaction, affording a series of chlorinated aromatics. At higher pHs the oxidation reaction predominates and this leads to polysaccharide degradation. These oxidation reactions are the same ones involved in the bleaching of white cottons with Chlorox bleach, which leads to the eventual degradation of your favorite white T-shirt.
Chlorine dioxide is prepared from sodium chlorate and must be prepared at the mill with a sulfuric acid catalyst. Again, one of the products (Na2SO4) can be used at the mill (this time in the kraft pulping operation):
NaClO3 + SO2 ===> 2ClO2 + Na2SO4
Chlorine dioxide is unstable towards light, heat and air. Not something you want to keep around the house.
The chemistry of C-Stage and D-stage delignification and brightening are different. C-stage reactions can be divided up into substitution and oxidation reactions. Substitution reactions are favored under fairly acidic conditions (<pH 2) and this is the desired reaction. The substition reaction places chlorine atoms on reactive sites of the lignin in the pulp, including the aromatic ring. By substituting chlorine onto lignin one changes the chemical properties of the material, making it easier to eliminate by the solubilization and the subsequent E-stage. Oxidation reactions are favored under more neutral conditions. While this reaction can help eliminate lignin, it also degrades the polysaccharides, which is not wanted. Thus C-stage bleaching is kept acidic to favor substitution.
The C-stage, through substitution, is very effective at eliminating lignin. This is why it is usually the first stage in a bleaching process. Unfortunately, lignin substitution provides compounds such as polychlorinated aromatics and dibenzofurans. These compounds, which include the dioxins, are carcinogenic and toxic to the environment. Therefore the trend in the paper industry is to reduce or eliminate the amount of elemental chlorine used in wood pulp bleaching.
The chemistry of D-stage bleaching involves oxidation of lignin. Contrary to C-stage bleaching, chlorine dioxide oxidation reactions are selective for lignin and the polysaccharides are stable. The reaction is very fast and can be run at a slightly higher pH (3.55). The byproducts include carboxylic acids formed from breakdown of the aromatic ring. Little chlorine substitution occurs which provides a significant reduction in toxic byproducts.
Non-Chlorine Processes. There are several new bleaching stages that do not use any form of chlorine. Many mills have incorporated an oxygen delignification stage directly after kraft pulping. The pulp is mixed with oxygen under alkaline conditions (O-stage) which provides degradation products similar to that of chlorine dioxide bleaching. Fully 30-50% of the lignin can be removed by this process. Additional lignin removal is not possible at present due to significant degradation of the polysaccharides. The beauty of oxygen delignification is that the liquid effluent can be mixed with the black liquor and sent to the recovery boiler.
Hydrogen peroxide (P-stage) is a lignin-retaining reagent that works be modifying chromophores so that they do not absorb light in the visible range. This reagent is typically used under alkaline conditions after most lignin has already been removed. This is a useful stage for final brightening but has the drawback that it is extremely sensitive to the presence of trace metal ions. Metals tend to accumulate in pulp and bleaching mills as systems become more and more closed (no effluent). Therefore most mills have a pretreatment stage to take out the inorganics prior to the P-stage.
Ozone (Z-stage) is used by a few mills, including the Union-Camp facility in Franklin, VA. This is an acidic process with chemistry similar to that of elemental chlorine. It is typically used after an O-stage and the effluent can be sent directly to the recovery boiler. In fact some mills produce TCF pulps via a four-stage OZEP process.
A biotechnological approach to bleaching involves the use of the enzyme xylanase. Fungi naturally produce xylanase to breakdown wood xylans into carbohydrates. The carbohydrates then become the energy source for the organism to grow. Scientists have found fungi that produce alot of xylanase, and they harvest the enzyme and sell it to the mill. The chemistry of xylanas action is not completly clear but it appears to act by chopping up the xylan in wood pulps enough to allow entrapped lignin moieties to escape. It has also been theorized that xylanase breaks lignin-xylan covalent bonds, thereby releasing the lignin.
Alkaline extraction. An E-stage is used to solubilize lignin degradation products. Under alkaline conditions, phenols (Ar-OH) become ionized to form phenolate anions (Ar-O-) which are much more soluble in water than phenols. Thus a filtration removes the degraded lignin from the solid bleached pulp. Oxygen can be added at this stage (Eo) to help reduce the color of the liquid...color is considered a pollutant no matter what it contains.
AOX Content of Pulp and Paper
Detection capabilities in analytical chemistry have reached stages where parts per billion can easily be measured for certain types of compoiunds. With respect to bleaching byproducts, one measures chlorine atoms attached to organic molecules. The value measured is termed "Absorbable Organic Halides," or AOX. A series of samples were recently analyzed and the results are tabulated below:
| Pulp Sample | TCF? | Ave. AOX (mg/kg) |
| No. Amer. Sftwd Kraft 1 | Y | 6.0 |
| No. Amer. Sftwd Kraft 2 | Y | 8.4 |
| No. Amer. Sftwd Sulfite 3 | Y | 2.6 |
| No. Amer. Hrdwd Kraft 4 | Y | 16.4 |
| No. Amer. Sftwd Kraft 5 | N | 125 |
| No. Amer. Sftwd Kraft 6 | N | 133 |
| No. Amer. Sftwd Kraft 7 | N | 225 |
| No. Amer. Sftwd Kraft 8 | N | 230 |
| No. Amer. Hrdwd Sulfite 9 | N | 392 |
| No. Amer. Deinked 10 | Y | 244-418 |
| No. Amer. Deinked 11 | N | 272-303 |
| Paper Sample | AOX Range (mg/kg) |
| European Sources with TCF | 2.0-334 |
| European Sources without TCF | 758-1266 |
| No. Amer. Sources with TCF | 47-399 |
| No. Amer. Sources without TCF | 535-571 |
| Sample | Avg. AOX (mg/kg) |
| Northern No. Amer. Sftwd Chips | 2.5-8.8 (several tested) |
| Southern No. Amer. Sftwd Chips | 5.7 |
| Northern No. Amer Hrdwd Chips | 8.8 |
| Southern No. Amer. Hrdwd Chips | 6.8 |
| Calcium carbonate | 0.0 |
| Cationic Wet End Starch | 25 |
| Ethylated Press Starch | 32 |
Measurable differences occur in AOX contents for TCF and standard grade pulps. The differences disappear upon recycling (deinked pulps). While the raw material has negligible AOX, processing tends to increase the concentration. Data from TAPPI, 79(3) (1996) 111-113.