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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¢XC. 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 fÑ-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¢XC, or by steam or water treatment at 160-170¢XC. 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
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