Neutral semichemical pulping

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. 

Liquors from Neutral Semichemical Pulping and from Kraft Pulping. In making neutral sulfite semichemical pulp, the black liquors may have about 10 parts of water to 1 part of total solids, of which about one-third is sodium acetate and sodium formate. After evaporation to about 1 part solids to 1 part water, sulfuric acid is added to the concentrate to free the acetic and formic acids. When the concentrate is extracted with acetone, the mixed acids are obtained, the acetone is separated for recycle, and the acids are concentrated and refined. The raffinate is stripped and is passed to the usual furnace to be burned for recovery of the inorganic salt values. This process gives a smelt of sodium sulfate, which may be used in the kraft process as chemical makeup. The loss of the fuel value of the acetic and formic acids is practically negligible. 

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. 

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. 

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 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. 

Figure 3. Ultraviolet-light-induced yellowing of various pulps. Keys NSSC, neutral sulfite semichemical pulp TMP(U), unbleached thermomechanical pulp TMP(B), bleached thermomechanical pulp.

FIGURE 15.6 Preparation of neutral sulfite semichemical pulping liquor via countercurrent contacting of a sulfur dioxide gas stream with aqueous sodium carbonate. 

Semichemical pulping Neutral sulfite semichemical (NSSC) Soda 65-85... 

Semi chemical pulps are usually prepared by the neutral sulfite semichemical (NSSC) process, although modifications of the full chemical processes can be used. Active pulping chemicals are (in the sodium-base NSSC process) sodium sulfite buffered with sodium bicarbonate (pH 7,0-9.0) and (in the ammonium-base NSSC process) ammonium sulfite with ammonium hydroxide used as a buffer. Defiberization is usually accomplished by attrition mills of the disk type. 

Chemical pulping has been performed or proposed with a wide variety of reactants. Today the dominant chemicals used in pulping are sulfur based, although numerous sulfur-free processes have been proposed. The processes available currently include sulfate or Kraft pulping, acid and alkaline sulfite pulping, neutral sulfite semichemical... 

Fig. 8-24. Response of various pulp types to peroxide bleaching. 1, Semichemical (cold alkali) hardwood 2, acid sulfite softwood 3, mechanical spruce 4, semichemical (neutral sulfite) hardwood pulps (Rydholm, 1965).

Wood is converted into pulp by mechanical, chemical, or semichemical processes. Sulfite and kraft (sulfate) are the common chemical processes, and neutral sulfite is the principal semichemical process (NSSC). Coniferous wood species (softwoods) are the most desirable, but the deciduous, broad-leaved species (hardwoods) have gained rapidly in their usage and constitute about 25 percent of pulp-wood. Table 28.2 summarizes the conditions utilized with the various pulping processes,... 

The production of wood pulp from lignocellulosic materials by treatment with various chemical liquors, particularly the neutral sulfite semichemical process and the kraft or sulfate process, gives residual black liquors. These contain salts that carry acetic acid and formic acid equivalent to 5% or more of the dry weight of the wood. 

The effect of various operating conditions on the recovery of the pulping chemicals and heat values in the case of a neutral sulfite semichemical (NSSC) spent liquor has been described recently (I). The thermodynamic equilibria and kinetics of gasification which seem to apply to this same set of experiments were discussed elsewhere (9). Gas compositions and yields examined in the latter study showed that under some of the operating conditions investigated, commercially significant quantities of ammonia and methanol synthesis gases could be produced from NSSC-type spent liquors. 

The dominant process of semiwith sodium or ammonium sulfite as the digestion chemicals. The pulp yield is in the range 70-80% depending on the wood species, most commonly hardwood species like birch, beech, maple, oak and eucalyptus. This pulp is used for fluting production - often as the sole component in furnish. 

Examples Kraft Sulfite Soda Neutral sulfite semichemical High-yield Kraft High-yield sulfite Stone groundwood Refiner mechanical pulp Thermomechanical pulp... 

In chemical pulping, the by-products of wood are more or less eliminated. Semichemical processes use neutral sulfite, sodium carbonate and sulfide and sodium hydroxide in various combinations (yields are 70-80%). 

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