Wood pulping with anthraquinone

In 1977, a very promising new application for anthraquinone, with exiting future prospects was introduced in wood pulping. 

A primary objective of wood pulping is to separate lignin from cellulose. Two chemical processes are commonly used to achieve this, namely the sulfite and the sulfate methods (Kraft-process). In the sulfite process, the lignin in wood is sulfo-nated with aqueous solutions of sulfites or hydrogen sulfites, with added SO2, at high temperatures, so that soluble lignin sulfonic acids are produced, which can be dissolved out of the wood. The sulfate method uses sodium hydroxide, sodium sulfide and sodium carbonate as basic pulping chemicals. It derived its name from the small amounts of sodium sulfate which are added to balance out alkali losses. 

The addition of less than 0.1% of anthraquinone can increase the cellulose yield and accelerate the delignification process, leading to better utilization of the 

The reaction of lignin occurs as a result of redox catalysis by the anthraquinone. The mechanism of the reaction between anthraquinone and lignin involves a cleavage in the B-position. 

In addition to anthraquinone, tetrahydroanthraquinone can also be used to accelerate wood pulping. 

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The increase in pulp yield may reach 8% on dry wood basis, but requires high pressures (> 1000 kPa) and a large excess of hydrogen sulfide (ca. 10% of wood). Only a fraction of the hydrogen sulfide (1 -2% of wood) is consumed and the rest is recoverable. Table 7-7 illustrates the influence of some oxidizing and reducing agents on the carbohydrate yield of kraft pulp. Stabilization with anthraquinone is dealt with in Section 7.3.7. 

Considerable interest has been shown in the use of anthraquinone in wood pulping operations. The optimal yield of pulp and pulp viscosity at a given Kappa number was obtained with 0.25 M-NaOH. The rate of delignification was... 

In pulp and paper processing, anthraquinone (AQ) accelerates the delignification of wood and improves liquor selectivity. The kinetics of the liquid-phase oxidation of anthracene (AN) to AQ with NO2 in acetic acid as solvent has been studied by Rodriguez and Tijero (1989) in a semibatch reactor (batch with respect to the liquid phase), under conditions such that the kinetics of the overall gas-liquid process is controlled by the rate of the liquid-phase reaction. This reaction proceeds through the formation of the intermediate compound anthrone (ANT) ... 

One modification that pertains to the present book is the role of anthraquinone (AQ) in the pulping process. Under conditions of alkaline pulping, carbohydrates in the wood reduce AQ into an anion-radical (AQ ). Experiments with lignin quinonemethide as a model compound showed that the AQ anion-radical caused fragmentation of the quinonemethide (Scheme 8.23). 

Semichemieal pulps produced by the sodium bisulphite and neutral sulphite-anthraquinone pulping processes at 75% yields have properties that are rather similar to a lower yield chemical pulp shown in Table 13.4. The pulp eharaeteristies from this proeess are of considerable interest, since it has been shown that ehemical pulps made by the so-ealled ASAM process, in which the wood is delignified with a liquor containing sodium sulphite, sodium carbonate or sodium hydroxide and methanol (c. 40% of the liquor by volume) with a trace of anthraquinone ean be delignified more rapidly and to a lower kappa number (this surrogate measure of the lignin content of pulp is explained in the appendix) than pulps made in the absenee... 

S Saka, RJ Thomas, JS Gratzl, D Abson. Topochemistry of Delignification in Douglas-Fir Wood with Soda, Soda-Anthraquinone and Kraft Pulping as Determined by SEM-EDXA. WoodSci Technol 16 139-153, 1982. 

Solvent-based pulping (Organosolv) is still in a very early stage of industrial development. In this chemical pulping method the dehgnification of the biomass (usually wood) is done in an organic solvent or solvent plus water system. There are a variety of processes, which use different solvents e. g. ethanol, methanol, acetic acid, formic acid, often in combination with sodium hydroxide, alkaline sulfite, and/or anthraquinone. 

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