Bleaching is the treatment of pulp with the chemical agents to increase
their brightness. Brightness is a term used to describe the whiteness of
pulp or paper, on scale from 0 % (absolute black) to 100 % (relative
to a MgO standard which has an absolute brightness of about 96 %) by the
reflectance of blue light (457 nm) from paper. Color reversion is the yellowing
of pulp on exposure to air, heat, certain metallic ions, and fungi due
to modification of residual lignin forming chromophores. Mechanical pulp
are particularly susceptible to color reversion, though chemical pulps
may experience this when exposed
to high temperature. Bleaching sequence is a combination of separate bleaching stages (e.g. CEDED, CEHH, CEHD, OC/DEoDED etc.). Each stage consists of a pump to mix the chemicals with the pulp, a retention tower to provide time for the bleaching chemical to react with the pulp, and the washer to remove the bleaching chemicals and solubilized pulp components.
Lignin content measurement is a vital tool to monitor the degree of cook or to measure residual lignin before bleaching and between various stages of bleaching to monitor the process. Important value for the bleaching chemicals charge. Kappa number test is an indirect method for determining lignin by the consumption of permanganate ion by lignin. (Kappa number is the number of milliliters of 0.1 N KmnO4 consumed by one gram of pulp in 0.5 N sulfuric acid after a ten minute reaction time at 25 oC under conditions such that one-half of permanganate remains unreacted). Klason lignin is the residue obtained after total acid hydrolysis of the carbohydrate portion of wood (cellulose, hemicellulose). It is gravimetric method for determining lignin directly in woody materials. Some part of the lignin remains soluble (“acid soluble lignin”) and can be estimated spectrophotometrically in the UV region of spectra.
Lignin preserving (conversion or stablization of chromophores [light absorbing funtional groups])
Bleaching chemicals may be:
Cl2 chlorine , ClO2 chlorine dioxide, O3 ozone , O2 oxygen, Na2O2
sodium peroxide, H2O2 hydrogen peroxide,
NaOCl sodium hypochlorite
Na2S2O4 sodium dithionite, ZnS2O4 zinc dithionite , NaHSO3 sodium bisulfite
A / BLEACHING OF MECHANICAL PULPS
To maintain the yield, lignin preserving bleaching is used. Bleaching of mechanical pulps involves masking the lignin that is present , instead of removing the lignin as is case for bleaching chemical pulps. Bleaching mechanical pulps is often refered to as brightening. Bleaching chemicals may be oxidizing (mainly hydrogen peroxide), reducing (mainly the dithionites) or a combination. Mechanical pulps are bleached with Chemicals designed to alter many of the chromophores. Chromophores are most often conjugated double bonds systems arising in the lignin of pulps.
Fairly limited brightness improvements are realized (6-12 % typically) with a maximum brightness of 60-70 % in a single stage, or up to 75 % in a two stage process. If two stages are used, the oxidative stage is used before the reductive stage or else the oxidant will undo what the reductive compound accomplished.
a/ Dithionite, Hydrosulfite Bleaching
Previously, zinc dithionite was used because is very stable. However, zinc is toxic to fish, therefore, the sodium form has replaced the zinc form. Zinc dithionite was prepared in the pulp mill from zinc and sulfur dioxide as follows:
Zn + 2 SO2 -----> ZnS2O4
Bleaching is carried out at pH 5-6 with chelating agents to prevent
metal ions such as iron (III) from coloring the pulp. Bleaching is often
carried out in the refiners.The reaction time is on the order of 10-30
minutes. The brightness gain
is only 5-8 %. If hydrogen peroxide and dithionite are used in a two-stage process, the dithionite must be the second stage or hydrogen peroxide will re-oxidize those moieties reduced by the dithionite. The reaction of dithionite is :
S2O42- + 2 H2O -----> 2 HSO3- + 2 H+ + 2 e-
b/ Oxidative Bleaching (Peroxide Bleaching)
Some metal ions, such as Fe3+, Mn2+ and Cu2+, catalytically decompose hydrogen peroxide, so peroxide bleaching is carried out with agents that deactivate these metal ions:
H2O2 --------> H2O + ½ O2
Chelating agets, such as EDTA, DTPA or sodium silicate are used. Brightness gain is about 6-20 %. Hydrogen peroxide with sodium hydroxide and/or sodium peroxide (NaOOH) is used to produce the high pH that is necessary to produce the active perhydroxyl ion, HOO-:
H2O2 + HOH => (H3O)+ + HOO- peroxide aninon
Some carbohydrates degradation occurs and is responsible for about half of the peroxide consumed. Color reduction occurs by altering chromophoric groups such as ortoquinones. The pulp is sometimes subsequently treated with SO2 to neutralize OH- and reduce any residual peroxide.
Yellowing of high-yield pulps
High yield pulps can be bleached, but not permanently. Photoyellowing is mainly due to UV (wavelengths <400 nm) induced free radicals of lignin:
Lignin-OH => . Lignin + . OH
Lignin-O- lignin => Lignin-O. + . Lignin
UV light is mainly absorbed by an alpha-carbonyl group. The alpha-cabonyl
is promoted to an excited state:
Carbonyl groups can also absorb energy and convert oxygen from its triplet ground state to an excited singlet that can oxidize aromatic rings.
Stabilization of high-yield pulps. Objective is the prevention of radical formation methods:
- reduction of alpha-carbonyls to alcohols
- etherification or esterification of phenolic hydroxyl singlet oxygen quenchers (compounds that react more rapidly than the lignin with singlet oxygen)
- hydrogenation of double bonds
- UV absorbers
B/ BLEACHING OF CHEMICAL PULPS (Lignin Removing Bleaching)
The use of three to seven stages increases the efficiency of bleaching by reducing the amount of chemical required. This is due to the complex nature of lignin; each bleaching chemical is going to react differently with lignin. Since chemical pulps are dark to begin with, bleaching increases brightness up to 70 % with a maximum brightness of about 92 %.
Bleached chemical pulps are insensitive to color reversion, but high temperatures may induce some color reversion. Lignin removal is accompanied by significant losses of pulp yield and strength of individual fibers. However, the strength of fiber-fiber bonding increases after bleaching.
Bleaching chemicals are generally more specific to lignin removal than
to carbohydrate degradation compared with the chemicals used in pulping.
Bleaching is much more expensive than pulping for a given amount of lignin
removal. Some of the bleaching chemicals are very specific to lignin removal
while others are much less specific and cause appreciable
carbohydrate degradation and diminished yield.
Oxygen and chlorine are relatively inexpensive, but not particularly selective for lignin removal. These chemicals are used in the early stages of bleaching to remove most of the lignin. Residual lignin is removed in later stages with expensive, but highly selective bleaching agents like chlorine dioxide, hypochlorite, and hydrogen peroxide.
Chlorination Stage (C)
Chlorination is being phased out, due to environmental concerns. Due to its importance for many years, however, lots of what we know about bleaching chemistry is related to the reactions of lignin with chlorine. Elemental chlorine has been used since shortly after its discovery in 1774 by Scheele. Chlorine is manufactured concomitantly with sodium hydroxide by electrolysis of sodium chloride; since these two chemicals are produced together, one often speaks of the chlor-alkali industry. The production of chlorine is summarized as follows:
2 NaCl + 2 H2O + e-. -------> Cl2 + 2 NaOH + H2
Pressurized, upflow reactors are used since the solubility of chlorine in water is low (4 g/L at STP). Chlorine is not overly specific to lignin, and much carbohydrate degradation occurs through its use. The chlorine reacts with the lignin by:
Oxidation includes reactions with both lignin and carbohydrates. Oxidation
of the carbohydrates leads to a decreases cellulose viscosity and decreased
pulp strength. Lignin is not removed to a large degree in this stage, and
the pulp actually gets darker (with a characteristic orange color). Chlorination
produces chlorinated organic materials including a very
small amount of dioxins.
Since these compounds are recognized as powerful toxins and carcinogens, their detection in bleach plant effluents prompted a flurry of investigative activity in Europe and North America to identify point sources and corrective measures. The chlorination/extraction sequence was subsequently found to be the major source of these compounds. Many mills have already replaced up to 50 % of the chlorine (Cl2) with chlorine dioxide (ClO2).
Analytical methods include:
- AOX (Adsorbable Organically bound Halogens),
- EOX (Extractable Organically bound Halogens) and
- TOX (Total Organohalogens – when AOX is analyzed together with volatiles).
Specified load limits for chlorinated material, measured as AOX, have either already been introduced or will be valid in the middle of 1990s for pulp mills in many pulp-producing countries including Scandinavia, Germany, Canada and Australia. These limits vary somewhat, but are in the range of 1 – 3 kg AOX/ton of pulp.
The CD or CD is a modification of C stage bleaching, where some of the chlorine is replaced with ClO2. Substitution of 10 % of the chlorine is used to prevent over chlorination. Substitution of 50 % or more of chlorine with chlorine dioxide at many mills is becoming common to reduce production of dioxins and other chlorinated organic chemicals.
Extraction Stage (E)
The E stage is extraction of degraded lignin compounds, which would otherwise increase the chemical usage in subsequent bleaching stages, with caustic (NaOH) solution. The objective of this step is removal of chromophores from previous steps: chlorinated and oxidized lignin fragments are removed this increases the brightness that can be imparted by subsequent bleaching steps better brightness, opacity, softness & mechanical properties limited removal of polysaccharides.
Chemistry of alkaline extraction
Removal of chlorinated lignin. Chlorination appears to give three types of lignin fragments: Small molecular weight fragments-these are water soluble and removed in the wash after chlorination. Larger molecular weight fragments-soluble in alkali, at base concentrations of 0.5- 1% and pH ~ 12. A portion is insoluble even at severe alkaline conditions. These require more oxidative steps. Removal of chlorinated lignin fragments is thought to occur by one of two mechanisms:
physical process-time & temperature control the solubilization
chemical process-base hydrolyzes chlorine in aliphatic & aromatic positions
- Removal of hemicelluloses
- Saponification of fatty acids and resin acids
- Degradation of chain length of polysaccharides
Chlorination and alkaline extraction will remove ~80% of the residual lignin, but the resultant pulp has low brightness due to a relative increase in chromophoric groups. The alkali displaces chlorine and makes the lignin soluble by the reactions such as:
Lignin-Cl + NaOH --------> Lignin-OH + NaCl
The lignin in the E1 effluent gives a dark color that is ultimately reponsible for much of the color of the final mill effluent. Recently oxygen gas has been incorporated into this stage (0.5 % on pulp) at many mills and the term Eo applies. When E stage follows the C stage (E1) it is often used with downflow tower due to the high consistency of pulp (10 - 18 %).
Hypochlorite Stage (H)
The H stage consists of bleaching usually with sodium hypochlorite solution (NaClO). It is important to maintain the pH above 8 because below this pH hypochlorite is in equilibrium with significant amounts of hypochlorous acid (HOCl), which is powerful oxidant of carbohydrates. Since the pH is high, lignin is continuously extracted as it is depolymerized.Hypochlorite reacts principally by oxidation. The chemical is more selective than elemental chlorine, but less selective than chlorine dioxide. Sodium hypochlorite, which is now used since it leads to less scaling, is made from chlorine as follows:
Cl2 + 2 NaOH --------> NaOCl + NaCl + H2O
Chlorine Dioxide Stage (D)
ClO2 is relative expensive, but highly selective for lignin. This makes it very useful for the latter bleaching stages where lignin is present in very low concentrations. It is explosive at concentrations above 10 kPa; hence, it cannot be transported and must be manufactures on site. Downflow towers are used to decrease the risk of gas accumulation. The D stage is useful for reducing shive contents. It reacts by oxidation.
Chlorine dioxide is:
- selective oxidant for lignin
- doesn't degrade carbohydrates
- maintains pulp strength
- free radical (has an unpaired electron)
- unstable, explosive, this probably accounts for its reactivity
- best results obtained in later stages of bleaching
Peroxide Stage (P)
Bleaching with hydrogen peroxide, H2O2 is not
common for chemical pulps. It is usually used for brightening mechanical
pulps, but when it is used to bleach chemical pulps it appears as the last
stage of a sequence such as C-E-H-P or C-E-H-D-P. It is an expensive
bleaching agent, but may be used more frequently as the use of elemental
Oxygen Stage (O)
Oxygen bleaching or pulping is the delignification of pulp using oxygen
under pressure and NaOH. This is an odorless, relatively pollution-free
process used prior to chlorination. This is the most inexpensive bleaching
chemical to use, but also the least specific for lignin removal. A considerable
decrease in cellulose viscosity accompanies this process. Bleaching may
be thought of as extended delignification, that is, an extension of the
pulping process. Some call it oxygen
delignification. It is used before the traditional chlorination first step of bleaching to reduce the Kappa number from 30-35 after pulping (softwood) to 14-18 after oxygen bleaching stage. Bleaching to Kappa numbers below this leads to unacceptable losses of the cellulose viscosity.
The effluent of oxygen delignification can be used in the brown stock washers or otherwise ultimately sent to the recovery boiler because there is no chloride ion (corrosion ).
There are two main methods of oxygen bleaching: medium consistency (10-15%)
and high consistency (30 %). The high consistency method is more common
(in oxygen pre-tower), but with the difficulties, such as the gas in the
reactor (pre-tower) contains high concentration of oxygen and volatile
organic chemicals such as methanol, ethanol, and acetone
that are potentially explosive. Temperature of oxygen bleaching is important to the selectivity of the process with better selectivity at 100 oC than 130 oC.
Ozone bleaching (Z)
Ozone is very strong oxidizing agent and reacts under acidic conditions. Upon reaction, ozone is reduced to oxygen or hydrogen peroxide.
Advantage: Production of ozone can be stopped in the case of
Disadvantage: Acidic conditions (H2SO4) ,
Low solubility of ozone in water (low temperatures used)