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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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