Semichemical pulping

Sheet density, kg m Brightness (unbleached), % Scattering coefficient, m kg  

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 

Historically chemical pulping was carried out in large batch digesters (200-400 m ). However, the trend is towards the use of continuous digesters, now accounting for about 65% of kraft production. The largest in operation have production capacities of more than 2000 tonnes of pulp a day. 

Steam to pack chips and relief for air. steam and condensible gases (turpentine) 

In kraft pulping it is possible to delignify to a lower kappa number without undue carbohydrate loss by a process known as extended delignification. This involves adding alkali in two or more stages so as to maintain a fairly constant alkali concentration during the cook. The process is important as it reduces the extent of bleaching required subsequently and reduces the environmental impact of the bleach plant. 

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Semibulk containers Semicarb azones Semichemical pulps Semichemicals Semicoke... 

NSSC Pulping. The characteristics of semichemical pulps are especially suited to the production of cormgated medium, which is the raw material for the fluted center ply of cormgated boxes. Neutral sulfite semichemical pulping was developed specifically as a semichemical process for cormgation medium and lends itself to small mills having minimal capital investment. For many years, this was the only semichemical pulping process. 

Semichemical pulping can be accompHshed by a sulfur-free system of sodium hydroxide and sodium carbonate. The sodium carbonate is recovered by simple incineration, and sodium hydroxide is added as makeup. Advantages in recovery operation are obtained if potassium hydroxide is added occasionally to maintain ca 20 mol % potassium carbonate (66). Lastiy, semichemical pulping can be accompHshed by using kraft white Hquor of low strength. 

Pulp Manufacture. Sodium sulfite is utilized in neutral semichemical pulping, acid sulfite pulping, high yield sulfite cooling, and some kraft pulping processes (339). Many pulp mills prepare their own sulfite and recycle as much as possible, but use of merchant sodium sulfite by pulp mills is substantial. Much of the by-product sodium sulfite from resorcinol manufacture goes into pulp appHcations as well as a substantial fraction of the lower assay manufactured sodium sulfite. 

Semichemical pulping comprised 6% of U.S. pulp production in 1993.1 Semichemical pulp is often very stiff, making this process common in corrugated container manufacture. This process primarily uses hardwood as furnish. 

Semichemical pulps are typically bleached with hydrogen peroxide in a bleach tower. 

NSSC [Neutral sulfite semichemical pulping] A papermaking process in which wood chips are digested in an aqueous solution of sodium sulfite and sodium carbonate at 140 to 170°C for several hours. 

TMP, thermomechanical pump CTMP, chemi-thermomechanical pump NSSC, neutral sulfite semichemical pulp. Source Refs. 3, 10, and 12. 

Wood chips are processed in mild chemical liquor and subjected to mechanical refining using disc refiners. Semichemical pulping liquors have variable composition ranging from sodium hydroxide alone, alkaline sulfite (sodium sulfite - - sodium carbonate), mixtures of sodium hydroxide and sodium carbonate, to Kraft green or white liquors [3]. Sodium sulfite/ sodium carbonate liquor is most commonly used and the pulp product obtained thereafter is referred to as neutral sulfite semichemical (NSSC) pulp. 

Thermomechanical Pulp (TMP), CTMP, and Semichemical Pulping Liquors... 

Semichemical pulp of bagasse prepared by the acid sulfite cooking was obtained from Edfuo Company of Egypt. 

Figure 6. Grafting rate of semichemical pulp. Conditions initiator, ceric ammonium sulfate (0.08 g/100 mL, 1% HsSO 30°C. Key (monomer) O—O, acrylonitrile — , ethyl acrylate. Grafting rate of extracted bagasse. Conditions monomer, ethyl acrylate 60°C. Key (initiator, ceric ammonium sulfate) X—X, 0.08 g/100 mL, 1% H,SOi ----------------------b 0.10 g/100 mL, 1% H2SOt.

Semichemical pulp of lower lignin content (1.89%) showed higher rate and yield of grafting (Figure 8). 

Grafting polymethyl methacrylate successfully onto partially delignified bagasse or semichemical pulp on using sodium bisulfite alone, i.e., in the absence of soda lime glass or china clay may not be contributed to the inorganic matter, particularly silica, present in bagasse, since ... 

Silica is less efficient than soda lime glass in initiating the grafting of polymethyl methacrylate in presence of sodium bisulfite (9). The results achieved with semichemical pulp show very high rate and yield of graftings (Figure 8). 

Figure 7. Ceric ammonium sulfate consumption for cotton linter O—O, 30°C — , 60°C and for semichemical pulp - ---------30°C X—X 60°C.

Again, it is noteworthy that the yield of grafting was practically the same at equilibrium on carrying out the grafting process on semichemical pulp in presence and absence of soda lime glass (Figure 8). 

There are several modifications of the sulfite method which are designated according to the pH of the cooking liquor (Table 7-1). For the production of chemical pulps, delignification is allowed to proceed until most of the lignin in the middle lamella is removed after which the fibers can be readily separated from each other. Semichemical pulps are often produced by the neutral sodium sulfite method followed by mechanical fiberization of the partially delignified wood. 

Figure 2D. Integral thermograms of sulfite and sulfite semichemical pulps. Rate of heating 40°C/min.

Figure 2D compares a number of sulfite pulps. Two of them, an unbleached spruce sulfite, USS, and an unbleached mixed hardwood sulfite, HS, have remarkably similar degradation curves but show different final residues. A bleached neutral sulfite semichemical pulp, FB, and an unbleached semichemical birch sulfite, SBS, have similar and much steeper initial degradation curves than the other two pulps. The semichemical birch sulfite pulp, SBS, has a degradation curve which crosses over those of the other three pulps shown in Figure 2D. 

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