Wood Pulping - Papierherstellung

WOOD PULPING

1. Alkaline Pulping Processes

1.1. Kraft (Sulfate) Pulping

The sulfate or kraft process exhibits a higher selectivity of delignification and, for this reason, has almost completely replaced the older soda or sodium hydroxide pulpingg process. Only in the case of annual plants that are easily cooked has the soda process been able to assert itself because the sulfate process offers no distinct advantages.
The predominant position of the kraft process is, above all, due to ist applicability to almost all lignocellulose - containing raw materials, ist low senstitivity to variations in the quality of raw materials, and the excellent technological properties of the resulting pulps. Furthermore, the original disadvantages of the kraft process, such as poor bleachabibility of the pulps and complicated recovery of chemicals have now largely been eliminated.
The terms "sulfate" and "soda" process are derived from the make - up chemicals, sodium sulfate and sodium carbonate, which are added in the recovery cycle to compensate for chemical losses.

Pulping Conditions

The cooking liquor (white liquor) used in the kraft process consits of sodium hydroxide and sodium sulfide as active pulping chemicals, as well as sodium carbonate as residue frrom the causticizing step. Furthermore, small amounts of Na₂SO₄, Na₂SO₃, and Na₂S₂O₃ from side reactions are present in the pulping liquor.
The sodium sulfide added is hydrolyzed almost completely to sodium hydroxide and sodium hydrogen sulfide

Na₂S + H₂O → NaOH + NaSH

and, therefore, only half of the Na2S is effective available alkali.
In comparison with the Sulfite process, the kraft process is a very uniform method that exhibits little flexibility and is well suited to continuous production of a given quality of pulp. In particular, pH adjustments over a wide range are possible. As a result of the large amounts of sodium hydroxide used, the pH value at the start of a cook is between 13 and 14. It decreases continuously during the course of cooking because organic acids are liberated from lignin and carbohydrates during the pulping reaction.
The amount of sodium solution used should be calculated such that the pH does not fall below 10, because otherwise the dissolved lignin would reprecipitate on the pulp fibres. Nevertheless, a drop in pH towards the end of the cooking process is, as a rule, desired because then dissolved xylan fragments reprecipitate on the cellulose fibers.
A very important pulping parameter is the sulfidity of the white liquor. The optimal sulfidity depends on the raw material, the amount of chemicals used, and the pulping temperature. Since the Na2S used in the kraft process improves the selectivity of pulping by accelerating the delignification reactions without simultaneously enhancing the dissolution of carbohydrates, pulping time can be shortened by maintaining a high sulfidity. This raises the pulp yield, and the higher hemicellulose content increases the tensile strengh of pulp.
Kraft pulping is usually carried out at 165 - 175°C. The cooking time at the maximum temperature is in the range of 1 - 2h, depending on the type of wood, the extent of delignification desired, and the cooking temperature. The heating of the digester can be carried out relatively quickly because, as a rule, the impregnation of the chips poses no problem due to the strong swelling of the fiber walls under the strongly alkaline conditions.

Anthraquinone - Catalyzed Pulping

Anthraquinone (AQ) which, when used in catalytic amounts of 0.05 - 0.2% based on o.d. wood in alkaline pulping, causes efficient stabilization of carbohydrates and simultaneous acceleration of carbohydrates and simultaneous acceleation of carbohydrates and simultaneous acceleration of the delignification reaction. The addition of 0,05% AQ to softwood pulping increases pulp yield at a given kappa number by 2 - 3%. Although various AQ derivatives are also effective catalysts for alkaline pulping, they are less cost effective than anthraquinone.
The effect of anthraquinone is based on the AQ/AHQ (AHQ = anthrahydroquinone) redox system. Already in the initial phase of pulping, anthraquinone oxidizes the reducing end groups of arbohydrates to give aldonic acid end groups an thus stabilizes them towards degradation by the peeling reaction.

Prehydrolysisi Kraft Process

The chemical industry requires dissolving pulps with a high α - cellulose content for processing into cellulose products. If the kraft process is used, the wood hemicelluloses must be degraded by acid hydrolysis prior to cooking because the alkaline pulping conditions stabilize the hemicelluloses remaining in the pulp towards further alkaline attack. In contrast, the hemicelluloses in sulfite dissolving pulps can be removed from the fiber matrix by an alkaline purification step.Prehydrolysis can be carried out by using mineral acids (eg 0.25 - 0.5% H2So4 based o.d. wood) at 120 - 140°C, or by steam or water treatment at 160 - 170°C. In latter case, acetyl groups are split off from the xylans of hardwoods or the galactoglucomannans of softwoods, resulting in a pH of 3 - 4. Whereas cellulose is very stable to such acid treatment, hemicelluloses are broken down into smaller, water - soluble chains.

1.2. Soda Pulping of Annual Plants

2.. Sulfit Process

The sulfite process is characterized by its high flexibility. In principle, the entire pH range can be used for pulping by changing the dosage and composition of chemicals. In comparison, kraft pulping can be carried out only with highly alkaline cooking liquor. Thus, the use of sulfite pulping permits the production of many different types and qualities of pulps for a broad range of applications.
The sulfite process can be subdivided into the following types of pulping:

acid bislufite pulping bisulfite pulping neural sulfite pulping alkaline sulfite pulping multistage sulfite pulping, and anthraquinone - catalyzed sulfite pulping

All2 these processes are based on the use of aqueous sulfur dioxide and a so - called base, i.e., calcium, magnesium, sodium, or ammonium. In the pulping system, pH - dependent equilibrium reactions occur that depend on the ratio of SO₂ to base. If sodium hydroxide is added to the aqueous sulfur dioxide, first bisulfite and then sulfite are formed:

H₂O + SO₂ → SO₂ · H₂O

SO₂ · H₂O + NaOH → NaHSO₃ + H₂O

NaHSO₃ + NaOH → Na₂SO₃ + H₂O

2.1. Reaction in Sulfite Pulping

In sulfite pulbing, the reactivity of lignin is essentially determined by the pH of the pulping liquor. Fundamental differences exist in the mechanism, rate, and extent of delignification under acidic, neutral, and alkaline conditions.

2.2. The Various Sulfite Processes

Acid Bisulfite Process

In the acid bisulfite process, pulping is performed with a large excess of free SO₂ at pH 1.2 - 1.8. Al of the above - mentioned bases can be employed. No basic differences in their mode of action exist, but the pulping conditions considered to be optimal vary slightly. The course and the result of pulping are influenced by the base selected. For instance, magnesium bisulfite pulping results in the best yields and the highest pulpbrightness. Use of the monovalent bases Na⁺and NH⁺₄ results in stronger pulps and a lower shive content. If ammonium is used as base, the pulping process is very fast, but the pulps are dark in color and cannot be uses in the unbleached state. Pulps of the lowest strength are obtained with calcium as base, and this pulping process must be controlled more carefully because calcium precipitates readily.
The total SO₂ used in acid bisulfite pulping ist in the range 18 - 25 %, depending on the typoe of wood used, degree of cooking desired etc., and the SO₂ to base ratio is adjusted to 4.5 - 6.0. During the cooking phase, the liquor to wood ration is usually 3 - 4:1. The cips are first impregnated with a higher liquor ratio to achieve a homogeneous distribution of the pulping liquor. A part of the pulping liquor (side relief) and the SO₂gas (top relief) ist hen removed after impregnation and returned to the accumulator to prepare fresh cooking liquor. Pulping temperatures of 125 - 140°C are used for the production of paper pulps. Unlike bisulfite pulping, the acids bisulfite process is also well suited to the production of dissolving pulps because the hemicelluloses can be dissolved to a lare extent in the highly acidic pulping process. However, higher maximum tmperatures of 145 - 150°C are employed for this process. The cooking time is usually 2 - 5h at this pulping temperature, but con be extended to more than 7h at lower pulping temperatures. A long heating time is also necessary because the SO₂ penetrates into the wood much faster than the base due to the high vapor presure of the excess free SO₂. Rapid heating can result in lignin condensations due to the high acidity within the chip centers. Since digester relief must also be effected slowly so that the excess SO₂ can be reused in the subsequent digestion, the result is a longer cover - to - cover cycle of 8 - 12 h.

From is first industrial application in 1874 until the end of World War II, the calcium bisulfite process was the principal process for wood pulping used worldwide because of the low consts and good availability of limestone and sulfur dioxide as basic materials for the production of the cooking acid. Today, calcium bisulfite pulping, ammonium bisulfite pulping, and other sulfite pulping, and other sulfite pulping processes based on ammonium are of very little importance.

Bisulfite Pulping

The pulping liquor used in the bisulfite process consists of approximately equal parts of free and combined SO₂, i.e., it contains no excess free SO₂. The pH value is between 3.5 and 5.5, depending on the temperature and the chemical concentration. The use of calcium as base in this pH range ist not possible. Magnesium is usually employed as base (magnefite process) because the recovery of chemicals is easy and inexpensive. Ammonium bisulfite and sodium bisulfite pulping habe also found industrial applications in individual cases.

Bisulfite pulping is normally carried out with 4 - 5% total SO₂ in the cooking liquor and a liquor to wood ratio of 4:1, i.e., the total SO₂used, based on wood, is 16 - 20%. The pulping temperature is ca. 155°C for hardwoods and 165°C for softwoods. The higher temperatures required, compared withe the acid bisulfite process, compensate for the lower rate of delignification causes by the lower acidity of the pulping liquor. However, carbohydrates are also dissolved to a smaller extent during bisulfite pulping. Hence, these pulps have a high hemicellulose content, and yields can reach 60 %. In comparison with acid bisulfite pulping, shorter cooking times of 1 - 3 h are required because of the higher cooking temperatures.
On the whole, the cover - to - cover cycle is much shorter (approx. 6 - 8h), which increases the production capacity.

A serious disadvantage of the bisulfite process is that the extent of delignification attainable is lower than that obtained under highly acidic pulping conditions, since otherwise the losses of yield and strength that occur are excessively high. The higher residual lignin content of the pulp requires the use of more bleaching chemicals and hence produces a greater effulent charge.

Neutral Sulfite Process

The NSSC (neutral sulfite semichemical) process is not suited for making chemical pulp because the degree of delignification attainable is too low. This process uses sodium as base and is widely applied in the production of semichemical pulps, especially in North America. These pulps are employed mainly for the middle layers of corrugated cardboard and in newsprint. Neutral sulfite pulp mills are sometimes uses in combination with kraft mills, which allows the combined recovery of chemicals (cross recovery). The NSSC waste liquor can be used as make - up chemical to compensate for losses of chemicals in krft process.

The neutral sulfite process is a two - stage process with a chemical delignification step followed by mechanical defibration. In the chemical treatment, neutral to slightly alkaline sulfite solutions are used to dissolve hemicelluloses and lignins and, thus, weaken the bonds between the fibers. In this way, the mechanical effort required to expose fiber bundles and individual fibers is reduced considerably.

In the neutral sulfite pulping of hardwood, between 8 an d18 % of sodium sulfite or ammonium sulfite ist used, bases an o.d. wood. To maintain the pH in the range 7 - 9, the cooking liquor must be buffered by the addition of 2.5 - 5% of alkali (NaHCO₃, Na₂CO₃ or NaOH) because acids are released during digestion, particularly acetic acid by the cleavage of acetyl groups form hemicelluloses. The digestion time is 30 - 180 min at the maximum temperature (ca. 170° C).

Alkaline Sulfite Pulping

Alkaline sulfite pulping ist carried out with liqors containing sodium sulfite and sodium hydroxide (sodium carbonate) at initial pH values of 9 - 13. When, as in NSSC pulping, a low chemical chare, low alkalinity, and low temperature are applied, semichemical pulps can be prepared, especially from annual plants. The boundaries between the two processes are fluid. If the pulping liquor consists of approximately equal amounts of sodim sulfite and NaOH, and if strongly alkaline conditions (pH>13) are chosen, chemical pulps can be produced as well. This pulping exhibits characteristics typical of both the sulfite process (no odors due to formation of reduced sulfur compounds; good bleachability pf pulps) and the kraft process (no raw material restrictions, high pulp strengths, low yields, and low brightness). However, the higher chemicals requirement, longer pulping times, higher residual lignin content, and the more complicated chemicals recovers, compared to the kraft process, prevented establishment of this process in the industrial practice.

2.3. Multistage Sulfite Processes

The various modifications that can be achieved in the sulfite pulping system by adjustment of the pH can be advantageously combined by changing the pH during pulping. In this way, it is possible to produce special pulps that are ideally suited to certain fields of application. Sodium, as a soluble base, is particularly suitable for multistage digestions of this tpye. Magnesium can be used as bas within limits. In the two - stage systems, pulpings is either carried out first at a higher pH and the pH is subsequently decreased or vice versa. The Stora process has achieved commercial importance. In this process, a sodium bisulfite - sulfite solution at pH 6 - 8 is used in the impregnation and first cooking stages. Subsequently, excess pulpint liquor is removed and the pH is adusted to ca. 1.5 by introducing SO₂. This is followed by normal acid bisulfite pulping as the main delignification step. The reactive centers of lignin are sulfonated practically completely in the first stage, and thus condensation reactions with phenolic extractives under strongly acidic conditions are blockes, so that even pine heartwood, for instance, can be digested satisfactorily by this method. Furthermore, glucomannan stabilization takes place in the bisulfite - sulfite step such that the acetyl groups are cleaved first, allowing the glucomannans to attach themselves firmly to the cellulose fibrils by increased formation of hydrogen bonds. The restistance to acid hydrolysis is thereby increased.

A corresponding two - stage process based on magnesium is calles to HO process (high opacity). Impegnation with a bisulfite solution (pH 3,8 - 4) ist carried out in the first stage. The pH is then adjusted to 6,5 - 7 by the injecition of magnesium oxide milk and cooking is completed at a high pulping tmeperature (170°C) - Since disssolved oranic acids apparently stabilize the pulping liquor, no precipitation occurs in spite of the high pH.

2.4. Antraquinone - Catalyzed Sulfite Pulping

An exceptional feature of sulfite pulping system ist that the catalytic acitvity of anthraquinone is also exerted at slightly alkaline to neutral pH values, whereas effecitve acceleration of delignification in soda puloing requires a pH above 13. This explains how sulfite pulping in a moderately alkaline pH range, which would otherwise permit only inadequate delignification, can be used for the production of chemical pulps if small amounts of AQ (0.1 - 0,2%) are added.
The pulping liquor used in this process consists predominantly of sodium carbonate is added as additional alkali.
This pulping procedure is faster and more selective than alkaline sulfite pulping with approximately equal sodium sulfite and sodium hydroxide charges and without AQ. Better delignification is achieved in spite of the lower chemicals demand and shortened cooking times.

AQ - catalyzed sulfite upling combines the advantages of the sulfite and kraft processes. First, there is no raw material restriction and high pulp strengths are achieved. Second, high yields are obtained, the pulps are easily bleachable and the emission of malodorous sulfur compounds is avoided. The main disadvantage of this process is the relatively high residual lignin content in the pulps, which have a high chemicals demand for bleaching and require the use of chlorine - containing bleaching agents.

3. Other Pulping Processes

Environmental problems and the high investment costs for pulp mills have led to the development of new pulping processes. The aim is to develop a process that con produce high strength pulps with good yields from all lignacellulose - containing raw materials that are suited to fiber production. The lignin content of these pulps must be low and the residual lignin must be highly reactive to permit easy, high - brightness bleaching. An important criterion is also the bleachability with chlorine - free chemicals.

3.1. Nitric Acid Pulping

The first investigations showed that wood can be delignified with nitric acid. The breakdown of lignin proceeds via electrophilic substitution on the aromatic rings and subsequent oxidation reactions. A part from the partial cleavage of methoxy groups and propane side chains, the ether bonds between the lignin units are also partially cleaved. As a result of the oxidation reaction, the nitrated lignin fragments possess mainly a quinoid structure and are highly colored. Alkali treatment is required to dissolve the partially water soluble nitrolignin.

Nitric acid pulping can be accelerated greatly by the addition of sulfuric acid or aluminum sulfate so that the cooking time for pressured digestion at 110°C can be reduced to a few minutes. The alkali extraction step can be enhanced by the addition of oxygen, and ammonium hydroxide can be used istead of sodium hydroxide.

3.2. Pulping with Organic Solvents

Alcell Process

The use of low - bailing alcohols in wood pulping was proposes in the 1930s by KLEINERT; who applied mixtures of enthanol and water to dissolve lignin. In the 1970s, this method, in a modified from, was adopted in many new pulping processes. The Alcell process comes closest to Kleinert's original idea: hardwoods are cooked with a 50:50 ethanol - water ixture. Three extractors in series are used in which the chips are delignified by successive exposure to different pulping liquors. Since the solution applied first has already been used previously in two extraction stages, it has the highest content of dissolved wood components. Fresh solution is employed in the third delignification stage. The exchange of pulping liquor is effected by displacement. The alcohol is recovered by digester after the third extraction stage. The alcohol remaining in the pulped material ist recovered by introducing steam into the extractor.
The waste liquor from the first extraction stept is processed, the alcohol ist distilled off, and the ligning that then precipitates is separated from the carbohydrates remaining in the aqueous seolution by centrifugation.

3.3. Pulping with Organic Acids

Both formic acid and acetic acid, with or without the addition of peroxides, con extensively remove lignin from wood. In all cases, concentrated acids are required, and temperatures range from 100 to 190°C. the use of lower temperatures has the advantage that pressureless systems can bei employed. In this case, however, an acid catalyst or the addition of peroxide is required.Very low kappa numbers can be achieved in acetic acid pulping. The pulps as well as the dissolved lignin and the hemicelluloses are acetylated.

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