Pulping, kraft

Sulfate or kraft pulping is performed with a solution composed of sodium hydroxide and sodium sulfide, named white liquor. According to the 

Total alkali Titratable alkali Active alkali Effective alkali 

In modern pulping chemistry weight units of NaOH are often replaced by molar units, e.g., moles of effective alkali per liter of solution or kilogram of wood. Table 7-6 shows typical conditions for kraft pulping. The charge of effective alkali (NaOH) applied is usually 4-5 moles or 16-20% of wood. 

The following equilibria are involved in the aqueous solutions containing sodium sulfide and sodium hydroxide  

Since ACi — 10 and K2 10-7, the equilibrium in equation 7-10 strongly favors the presence of hydrosulfide ions and for all practical purposes, sulfide ions can be considered to be absent (Fig. 7-20). The concentration of 

Both neutral sulphite and bisulphite pulping liquor can be reeovered by the Tampella process (Rimpi, 1983). The residual eooking liquor is burnt in a kraft type recovery furnace and the smelt of sodium earbonate and sodium sulphide obtained as described in the seetion on kraft pulping. The dissolved smelt is carbonated with flue gas to form sodium hydrosulphide and sodium bicarbonate and the partially carbonated liquor stripped with steam in order to liberate hydrogen sulphide gas. The gas formed plus make up sulphur is then burnt to sulphur dioxide for the preparation of sulphite eooking liquor. 

Kraft pulping, also termed the sulphate process, is the predominant proeess for the manufacture of ehemieal pulp. The kraft process offers some erueial advantages  

Kraft pulping is carried with a solution of sodium hydroxide and sodium sulphide, the solution being known as white liquor. In the terminology used by the pulp and paper industry, the chemicals are calculated as sodium equivalents and expressed as weight of NaOH or Na20. Sodium sulphide is hydrolysed largely to a mixture of sodium hydroxide and sodium hydrosulphide  

It can be seen that one molecule of sodium sulphide releases one molecule of sodium hydroxide. Thus the effective alkali or the sodium hydroxide available for delignification is the sum of the sodium hydroxide and one-half of the sodium sulphide, when the chemicals are expressed in terms of their sodium hydroxide or sodium oxide equivalents. 

It is general practice to express the chemical concentration used for pulping as a percentage of the effective alkali charge of sodium hydroxide (or Na20) based on oven-dry wood, or as the effective alkali as sodium hydroxide in g 1 . This nomenclature is given here, not because it is necessary to know it in detail, but rather because it is desirable to know its background and use by industry. 

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A cross-sectional view of kenaf is shown in Figure 3d. The ultimate cells are nearly cylindrical with thick cell walls. Kenaf fibers are shorter and coarser than those of jute. Both chemical (kraft) and mechanical pulps have been produced from kenaf, and successful demonstration mns of newsprint have been made for the Dallas Morning Nem, the St. Petersburg Times, and the Bakersfield Californian with a furnish of 82% kenaf chemithermomechanical pulp and 18% softwood kraft pulp. Kenaf fiber is also considered a substitute for jute and used in sacking, rope, twine, bags, and as papermaking pulp in India, Thailand, and the former Yugoslavia. RoseUe bleached pulp is marketed in Thailand. 

Fig. 3. Reactions at the quinonemethide center during pulping (a) sulfite pulping, and (b) kraft pulping.

Salts, eg, alum or calcium chloride [10043-52-4] and cationic polyacrylamides are effective retention aids in bleached and unbleached kraft pulp. 

Fig. 4. (a) Photomicrograph of Douglas fir kraft pulp (b) electron micrograph of Douglas fir pulp collapsed into paper. 

Sulfite pulps have properties that are desirable for tissues and top quaHty, fine papers. Because sulfite pulping is not as versatile as kraft pulping, various options have been developed, and the choice of a specific process is dependent on individual mill situations. The unbleached pulp has high (60+) brightness compared to kraft pulp and is easily bleached. However, it is limited to select wood species. The heartwood of pine, Douglas fir, and cedars are not easily pulped. Additionally, pulps produced from hardwood have limited economic value because of low strength. 

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