Productividad y empleo en el sector primario

Cellulosic fibers are separated from lignocellulosic materials to produce paper and board. The separation can be mechanical, chemical, or a combination of these methods (Sixta, 2006). In mechanical pulping, fibers are separated by defibrating the wood or wood chips mechanically into fibers and fiber fragments and bundles. The process uses very little or no chemicals, but highly energy consuming. Mechanical pulps, thus, contain the chemical components nearly in the same proportion as they are present in wood. In addition, mechanical pulps contain significant amounts of fines and fiber fragments, which influence the possible pulp applications.

1-9 The fundamental of chemical pulping is to remove the lignin from wood or other lignocellulosic material by chemical reactions to separate the cellulosic fibers containing mainly cellulose and hemicelluloses (Sixta, 2006). This lignin dissolution is conducted at high temperature, but some associated carbohydrate (mainly hemicellulose and some cellulose in soluble form) are also removed.

Chemical pulp typically possesses a high mechanical strength. The most abundantly applied chemical pulping method is kraft pulping, followed by sulphite pulping. Several variations, such as the use of cooking additives, have been introduced to kraft pulping in order to produce pulps with different qualities or to improve the process.

Kraft pulping

Kraft pulping is based on the reactions of hydroxide and hydrosulfide ions (pH^~14) with lignin structures under elevated temperature (Pönni, 2014). Apart from the active species, white liquor also contains small amounts of Na²CO³, Na²SO⁴, Na²S²O³, NaCl and CaCO³. Active alkali content (AA) is an important parameter of kraft pulping; it refers to the hydroxide ion concentration in the cooking liquor taking into account the hydroxide ions originating from the dissociation of sodium sulphide into hydrogen sulphide and hydroxide ions (Sixta, 2006). The alkaline conditions also promote swelling which enhances the cooking chemical penetration. Application of softwood and hardwood chips is possible, but the wood species influences the selection of the pulping parameters due to the differences in the chemical composition (Sjöström, 1993).

Lignin reactions in kraft pulping can be divided into three distinct phases:

initial delignification, bulk delignification, and residual delignification. Initial delignification takes place mainly in the impregnation phase (T< 140 °C) and very little lignin is dissolved (20-25% of total). The rate of delignification increases dramatically when the cooking temperature is elevated above 140 °C, and 70-80% of all lignin dissolves during this phase. The dissolution begins in the S2 layer of the cell wall and progresses into the middle lamella. The bulk delignification phase will continue until about 90% of all lignin has been dissolved. The three precursors of lignin, namely p-coumaryl, coniferyl, and sinapyl alcohols react differently with the cooking chemicals. Generally, the phenolic subunits are more reactive than the non-phenolic units (Sixta, 2006).

Simultaneously to lignin removal, some carbohydrate will be dissolved,

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especially during the initial delignification phase. Due to peeling reactions (i.e.

alkaline induced end-wise degradation) the DP of carbohydrates is reduced by removal of anhydrosugar units from the reducing end of the carbohydrate chain (Sixta, 2006). In the final stages of kraft pulping, the alkalinity of the cooking liquor decreases which leads to the precipitation of hemicelluloses on the fibers.

Cellulose will also dissolve to some extent (10-15%) during alkaline cooking, especially during the residual delignification phase.

A complete lignin removal by kraft cooking is not possible in order to achieve a high yield; therefore, further treatments are conducted with the purpose of reducing lignin content by means of oxygen delignification and bleaching stages. During these processing steps, lignin is removed more selectively by applying oxidants. The bleaching stages use both alkaline and acidic conditions under elevated temperatures (Gullichsen, 1999; Alén, 2000).

Acid sulfite

The acid sulfite (AS) process employs aqueous SO²/M(HSO³)n as its cooking liquor, where the cation M is calcium, sodium, magnesium or ammonium. Acid processes are those in which the pH is 2-3, bisulfite process operate at pH range 3-5, neutral sulfite processes cover the pH range 6-9, and alkaline sulfite processes are above pH~11. Calcium requires pH~2 to stay in solution, while magnesium allows operation up to pH^~4. The sodium and ammonium sulfite solutions can be strongly alkaline without precipitation. The composition of sulfite cooking liquors is generally expressed in terms of total SO² and combined SO². The AS process includes two phases: impregnation at about 110-120 °C in order to uniformly distribute the cooking chemicals within the biomass structure, and the actual cooking which is performed at 125-150 °C (Sixta, 2006).

Lignin reactions in sulfite pulping can be divided into three distinct phases:

sulfonation, hydrolysis and condensation. Sulfonation makes lignin more hydrophilic, and hydrolysis breaks lignin bonds so that new and smaller dissolvable lignin fragments are formed. Lignin condensation darkens the pulp, decreases the homogeneity of delignification, promotes shive formation, and makes bleaching difficult. However, lignin condensation can be avoided by keeping the bound sulfite concentration high enough (Iakovlev, 2011).

1-11 An important drawback of AS cooking is the very high cover-to-cover cooking times (up to 12 hours) due to the very slow (up to 6 hours) low-temperature impregnation stage (Fengel and Wegener, 1989); but a significant advantage with respect to alkaline processes, is the absence of peeling reactions that leads to higher carbohydrate preservation. The common products of acidic biomass fractionation processes are cellulosic fibers, dissolved carbohydrates, lignin, acetic acid, furfural and lignosulfonic acids. Lignosulfonic acids are used in concrete admixtures, road base, oil drilling muds, among other applications.

The dissolved carbohydrates are an especially important feedstock for chemical and biochemical treatment leading to a various valuable products including biofuels (Sixta, 2006).

Kraft process is the dominant pulping system and the main reasons are as follows: excellent pulp strength properties, low demands on wood species and wood quality; well-stablished recovery of cooking chemicals, energy and by-products; and short cooking times. However, the sulfite process is still important at least in certain countries and for some pulp qualities, such as speciality paper grades, tissue and dissolving pulp (Fengel and Wegener, 1989). In terms of fiber quality, sulfite pulps have higher yields at a given kappa number and higher brightness of unbleached pulp due to absence of strong chromophores such as quinones and stilbenes. In addition, it is characterized by covering the whole range of pH (versatility), lower odor problems and lower investment costs (Gullichsen, 1999; Alén, 2000).