When chemical pulp, mechanical pulp and deinked recycled fibers are used in the production of white papers e. g. printing and writing papers, hygiene paper (tissue) or white top of board, these pulps have to be bleached up to a certain brightness. The bleaching of chemical pulp (see Volume 1) is carried out in several stages, with the elimination of residual lignin as far as possible being the primary objective of the first stage. The subsequent bleaching stages are responsible for brightening the pulp. Bleaching chemicals can be classified according to the particular groups they prefer to react with. Cations or radicals in acid medium react primarily with phenolic structures of lignin, alkaline nucleophiles attack carbonyl groups as fol¬lows:
. • Group I: Cl2, O3, peroxyacids react with all aromatic lignin units, with phenolic groups and their double bonds.
. • Group II: O2, ClO2 react primarily with free phenolic hydroxy groups.
. • Group III: H2O2 in alkaline conditions react mainly with certain functional groups in lignin, e. g. carbonyl groups.
Accordingly, bleaching is divided into pre-bleaching and final bleaching. After the basic chemical pulping process the following bleaching chemicals are added sepa¬rately in several steps, for which certain abbreviations have been introduced [4, 6].
3.3.1
Bleaching Substances
. • Chlorination (C) with chlorine, an effective, selective and inexpensive bleaching agent that reacts very fast with lignin. For environmental reasons during the last ten years chlorine is being substituted by chlorine dioxide, ozone and/or oxygen all over the world. In Europe substitution is already 100 %. In this particular case bleached pulp is called “Elementary Chlorine Free – ECF”.
. • Alkaline extraction (E) with sodium hydroxide, both to extract kraft lignin precip¬itated on the fiber surface and remove degraded lignin moieties originating from the preceding acid bleaching stages (e. g. ozone or chlorine dioxide).
. • Sodium or calcium hypochlorite bleaching (H) is mainly applied to eliminate residual lignin and to control the viscosity in the case of the production of solu¬ble pulp. Today hypochlorite is largely substituted by chlorine dioxide.
. • Chlorine dioxide bleaching (D) – The production of chlorine dioxide is carried out by the reduction of sodium chlorate. The bleaching effect of chlorine dioxide is based on the oxidation of lignin. It has an exceptionally positive effect on pulp brightness. Chlorine dioxide can be applied in several bleaching stages of an ECF-sequence, e. g. as a pre-bleaching stage in connection with alkaline extrac¬tion (DOE), in the first stage in final bleaching to remove the residual lignin and activate the remaining chromophores to be removed in the final bleaching stage (DIE, DIEOP), and in a final bleaching stage to achieve the target brightness (D2, D3).
. • Oxygen bleaching (O) – The treatment of pulp with oxygen under alkaline condi¬tions is a chlorine-free alternative to pre-bleaching. Sodium hydroxide and oxi¬dized white liquor are used as the alkali source. Under these conditions, oxygen also attacks carbohydrates to a considerable extent so appropriate conditions have to be applied to avoid cellulose degradation. Consequently, oxygen bleach¬ing is less effective than conventional pre-bleaching with chlorine. With se¬quences containing only alkaline bleaching stages, a high final brightness is also possible, but these stages are most suitable as oxygen chemical sequences in fiber lines that also bleach pulp with the use of chlorine dioxide.
. • Ozone bleaching (Z) – Because oxygen delignification is limited, additional de-lignifying agents must be used before final bleaching. Ozone is a possibility, but it requires new technology. Like oxygen, ozone is only slightly soluble in water and is also very unstable.
. • Peroxide bleaching (P) – Hydrogen peroxide (H2O2) in an alkaline medium is well suited to the final bleaching of chemical pulps and leads to high brightness stability. Originally sodium peroxide and, since 1950, hydrogen peroxide (as a 50 % solution) were used to bleach mechanical pulps, where they destroy the chromophoric groups present in lignin by cleaving conjugated double bonds by an oxidation process. Hydrogen peroxide bleaching is carried out in the presence of NaOH, sodium silicate and also very often chelating agents (see Section 3.6.3). Depending on the individual process conditions for mechanical pulp and the initial brightness, 1 to 2 % peroxide (100 % active substance) leads to a bright¬ness increase of up to 10 to maximally 18 points, relative to an absolute bright¬ness level of 74 to 80 % measured at TAPPI Standard 457 nm wavelength. In the production of deinked pulp (DIP) from recovered paper hydrogen per¬oxide is the most favorable bleaching chemical. This process requires an alkaline medium obtained by adding NaOH, and also sodium silicate (water glass) to stabilize the hydrogen peroxide. Under these conditions the necessary swelling of the fibers will be achieved, making it easier to detach the printing ink/dye particles from the fibers. Saponification of rosin and synthetic binders/adhesives also occurs. Furthermore peroxide bleaching strongly increases the amount and influences the composition of dissolved colloidal substances (DCS). A high amount of DCS results in a high COD (chemical oxygen demand) load in the process water and in a loss of quality of the mechanical pulp, e. g. strength properties.
. • Dithionite (hydrosulfite) bleaching (Y) – In the late 19th century sodium bi¬sulfite (NaHSO3) was developed as the first bleaching agent for mechanical pulp. Because of its very limited brightness increase a few years later sodium dithio¬nite (Na2S2O4) – a further reductive bleaching agent – came to the market. Sodium dithionite is supplied as a white crystalline powder (88 % active sub¬stance) or as a liquid with approximately 13 % active substance. It can also be produced on-site from commercial solutions containing sodium borohydride and sodium hydroxide. Such solutions typically contain 12 % sodium borohy¬dride, 40 % sodium hydroxide and 48 % water (the molar ratio is 3.2 mole NaOH to 1 mole NaBH4). When making dithionite, more sodium hydroxide and sulfur dioxide or sodium bisulfite is added in a continuous process. Hereby sodium bisulfite is then reduced by borohydride and one atom of borohydride reduces eight sulfur atoms. For optimum bleaching efficiency stock consistency should be 5 %, temperature 60 °C and pH 6.
At these conditions addition of 1.2 % (100 % active substance) to dry pulp leads to a brightness increase of 10 to 12 points. This means a brightness of 70 to 76 % (at TAPPI Standard 457 nm) can be reached, depending on the wood quality and on the initial brightness (see also Volume I). There is nearly no increase in dissolved colloidal substances (DCS) with dithionite bleaching. Dithionite is also very often used for the final bleaching of deinked pulp (DIP). For maximum brightness levels of the mechanical or deinked pulp,
e. g. above 72 or 74 %, a two-stage bleaching process is applied with hydrogen peroxide in the first stage and sodium dithionite in the second one. In most cases this combination is also the most economic one.
• Formamidine sulfinic acid (CH4N2O2S) (FAS) is another strong reductive bleaching agent, produced from hydrogen peroxide and thiourea. FAS is a white to slightly yellowish crystalline powder with a purity of 99 %. The reaction condi¬tions are in a wide range of pulp temperature (60 to 120 °C), pulp pH (6.5 to 10.0) and pulp consistency (4 to 30 %), therefore FAS is more often commer¬cially applied in pulp bleaching of recovered papers than in bleaching of me¬chanical pulps.
3.3.2
Bleaching Auxiliaries
These products enable a more efficient bleaching of the chemical, mechanical and deinked pulps and lead to increased whiteness. They are generally based on sur¬face active agents, i. e. wetting agents (see Section 3.6.7.) which are resistant to bleaching chemicals as well as chelating (complexing) agents (see Section 3.6.3.). Some of the transition metals (e. g. Fe, Mn, Cu) in pulp can reduce the bleaching effect due to catalyzed decomposition reactions of dithionite and the re-oxidation of reduced chromophore groups. Chelating agents also prevent transition metal ions from catalyzing peroxide decomposition. Since the beginning of peroxide bleaching of mechanical and deinked pulps, sodium silicate (Na2O 3–4 SiO2) has been used as an additive. Addition of 0.5 to 3 % (41 °Be’ solution calculated on dry pulp) stabilized peroxide, acts as a buffer and has the ability to form metal com¬plexes. Small amounts (0.05–0.1 %) of magnesium sulfate (usually in the form of Epsom salts, MgSO4 7H2O) are commonly included in the peroxide bleach liquor or added separately. Magnesium acts as a stabilizer of peroxide, probably together with sodium silicate.
Similar Articles :
- Paper Functional Chemicals Paper and Board Coating General Aspects The paper coating process...
- Systems for Fiber Stock Preparation The target of fiber preparation systems is to modify the...
- Economic Aspects of Paper In 2002 the world production of paper and board was...
- Stock Preparation Unit Processes and Equipment Fiber Materials Feeding The objective of fiber material feeding is...
- Paper Coating Color Preparation Paper Coating Color Preparation 7.7.1 General Aspects of Coating Kitchen...
- Solid Waste Solid Waste The different processes within the pulp and paper...
- Management Information Systems Management Information Systems 9.3.1 Importance of Information Systems Information systems...
