La pulpa es un material fibroso lignocelulósico que se prepara separando química o mecánicamente las fibras de celulosa de la madera , los cultivos de fibras , el papel de desecho o los trapos . Mezclada con agua y otros aditivos químicos o de origen vegetal, la pulpa es la principal materia prima utilizada en la fabricación de papel y la producción industrial de otros productos de papel . [1] [2] [3]
Historia
Antes de la invención ampliamente reconocida de la fabricación de papel por Cai Lun en China alrededor del año 105 d.C., los materiales de escritura similares al papel, como el papiro y el amate, fueron producidos por civilizaciones antiguas utilizando materiales vegetales que en gran parte no estaban procesados. Tiras de corteza o material de líber se tejieron juntas, se batieron en láminas ásperas, se secaron y se pulieron a mano. [4] [5] La pulpa utilizada en la fabricación de papel moderna y tradicional se distingue por el proceso de maceración que produce una suspensión más fina y regular de fibras de celulosa que se extraen de la solución mediante una pantalla y se secan para formar hojas o rollos. [6] [3] [7] El primer papel producido en China consistía en fibras de líber de la planta de papel de morera (kozo) junto con trapos de cáñamo y restos de red. [6] [8] [9] En el siglo VI, los agricultores de China domesticaron la morera específicamente con el propósito de producir pulpa para su uso en el proceso de fabricación de papel. Además de la morera, también se fabricaba pulpa de bambú, corteza de hibisco, sándalo azul, paja y algodón. [9] La fabricación de papel con pulpa hecha de fibras de cáñamo y lino de ropa hecha jirones, redes de pesca y bolsas de tela se extendió a Europa en el siglo XIII, y el uso cada vez mayor de trapos fue fundamental para la fabricación y asequibilidad del papel de trapo , un factor en el desarrollo de la imprenta. [1] En el siglo XIX, las demandas de producción de las industrias de fabricación de papel e impresión recientemente industrializadas provocaron un cambio en las materias primas, sobre todo en el uso de madera para pasta y otros productos de árboles que hoy constituyen más del 95% de la producción mundial de pasta. [10]
Paso 1, cosecha
Paso 2, cocinar
Paso 3, hacer hojas
Paso 4, embalaje
Paso 5, usando
El uso de pulpa de madera y la invención de las máquinas automáticas de papel a finales del siglo XVIII y principios del XIX contribuyeron al estatus del papel como un producto económico en los tiempos modernos. [1] [11] [12] Si bien algunos de los primeros ejemplos de papel hecho de pulpa de madera incluyen obras publicadas por Jacob Christian Schäffer en 1765 y Matthias Koops en 1800, [1] [13] [14] papel de madera a gran escala la producción comenzó en la década de 1840 con desarrollos únicos y simultáneos en la fabricación de pasta mecánica realizados por Friedrich Gottlob Keller en Alemania [15] y por Charles Fenerty en Nueva Escocia . [11] procesos químicos seguido rápidamente, primero con J. Roth 'uso s de ácido sulfuroso a la madera se trata, pues por Benjamin Tilghman ' s patente de Estados Unidos sobre el uso de bisulfito de calcio , Ca (HSO 3 ) 2 , a la pulpa de madera en 1867 . [2] Casi una década más tarde, el primer comercial de la planta de celulosa de sulfito se construyó, en Suecia. Utilizaba magnesio como contraión y se basó en el trabajo de Carl Daniel Ekman . Para 1900, la pulpa de sulfito se había convertido en el medio dominante de producción de pulpa de madera, superando los métodos mecánicos de pulpa. El proceso de fabricación de pasta química competidora, el proceso de sulfato o kraft , fue desarrollado por Carl F. Dahl en 1879; la primera fábrica de papel kraft comenzó en Suecia en 1890. [2] La invención de la caldera de recuperación , por GH Tomlinson a principios de la década de 1930, [15] permitió que las fábricas de papel kraft reciclaran casi todos sus productos químicos para la fabricación de pulpa. Esto, junto con la capacidad del proceso kraft para aceptar una variedad más amplia de tipos de madera y producir fibras más fuertes, [16] convirtió al proceso kraft en el proceso de pulpa dominante, a partir de la década de 1940. [2]
La producción mundial de pulpa de madera en 2006 fue de 175 millones de toneladas (160 millones de toneladas). [17] En el año anterior, se vendieron 63 millones de toneladas (57 millones de toneladas) de pulpa comercial (no convertida en papel en la misma instalación), siendo Canadá la principal fuente con el 21 por ciento del total, seguida de Estados Unidos. al 16 por ciento. Las fuentes de fibra de madera necesarias para la fabricación de pasta son "45% de residuos de aserradero, 21% de troncos y astillas y 34% de papel reciclado" (Canadá, 2014). [18] La pulpa química constituía el 93 por ciento de la pulpa comercial. [19]
Pulpa de madera
Los recursos madereros que se utilizan para fabricar pasta de madera se denominan madera para pasta . [20] Si bien, en teoría, cualquier árbol puede usarse para la fabricación de pulpa, se prefieren los árboles coníferos porque las fibras de celulosa en la pulpa de estas especies son más largas y, por lo tanto, hacen un papel más resistente. [21] Algunos de los árboles de madera blanda más utilizados para la fabricación de papel incluyen el abeto , el pino , el abeto , el alerce y la cicuta , y las maderas duras como el eucalipto , el álamo temblón y el abedul . [22] También hay un interés creciente en las especies de árboles modificados genéticamente (como el eucalipto y el álamo transgénicos ) debido a varios beneficios importantes que pueden proporcionar, como una mayor facilidad para descomponer la lignina y una mayor tasa de crecimiento.
Una fábrica de celulosa es una instalación de fabricación que convierte astillas de madera u otra fuente de fibra vegetal en un tablero de fibra gruesa que se puede enviar a una fábrica de papel para su posterior procesamiento. La pulpa se puede fabricar mediante métodos mecánicos, semiquímicos o totalmente químicos (procesos kraft y sulfito). El producto terminado puede ser blanqueado o no blanqueado, según los requisitos del cliente.
La madera y otros materiales vegetales utilizados para hacer pulpa contienen tres componentes principales (además del agua): fibras de celulosa (deseadas para la fabricación de papel), lignina (un polímero tridimensional que une las fibras de celulosa) y hemicelulosas (polímeros de carbohidratos ramificados más cortos). El objetivo del despulpado es descomponer la estructura a granel de la fuente de fibra, ya sean astillas, tallos u otras partes de la planta, en las fibras constituyentes.
La fabricación de pasta química logra esto degradando la lignina y la hemicelulosa en pequeñas moléculas solubles en agua que pueden eliminarse por lavado de las fibras de celulosa sin despolimerizar las fibras de celulosa (la despolimerización química de la celulosa debilita las fibras). Los diversos métodos de fabricación de pasta mecánica, como la fabricación de pasta de madera triturada (GW) y la fabricación de pasta mecánica de refino (RMP), separan físicamente las fibras de celulosa entre sí. Gran parte de la lignina permanece adherida a las fibras. La resistencia se ve afectada porque las fibras pueden cortarse. Hay varios métodos de fabricación de pasta híbridos relacionados que utilizan una combinación de tratamiento químico y térmico para comenzar un proceso de fabricación de pasta química abreviado, seguido inmediatamente de un tratamiento mecánico para separar las fibras. Estos métodos híbridos incluyen el despulpado termomecánico, también conocido como TMP, y el despulpado quimiotermomecánico, también conocido como CTMP. Los tratamientos químicos y térmicos reducen la cantidad de energía requerida posteriormente por el tratamiento mecánico y también reducen la cantidad de pérdida de resistencia que sufren las fibras.
Categoría de pulpa | Producción [M ton ] |
Químico | 131,2 |
Kraft | 117,0 |
Sulfito | 7.0 |
Semichemico | 7.2 |
Mecánico | 37,8 |
No madera | 18.0 |
Fibras vírgenes totales | 187.0 |
Fibras recuperadas | 147,0 |
Pulpa total | 334,0 |
Cosecha de árboles
La mayoría de las plantas de celulosa utilizan buenas prácticas de manejo forestal en la tala de árboles para garantizar que tengan una fuente sostenible de materias primas. Una de las principales quejas sobre la extracción de madera para las plantas de celulosa es que reduce la biodiversidad del bosque aprovechado. Las plantaciones de árboles de pulpa representan el 16 por ciento de la producción mundial de pulpa, los bosques primarios el 9 por ciento y los bosques de segunda y tercera y más generaciones representan el resto. [24] La reforestación se practica en la mayoría de las áreas, por lo que los árboles son un recurso renovable . El FSC ( Forest Stewardship Council ), SFI ( Sustainable Forestry Initiative ), PEFC ( Program for the Endorsement of Forest Certification ) y otros organismos certifican el papel elaborado a partir de árboles cosechados de acuerdo con las directrices destinadas a garantizar las buenas prácticas forestales. [25]
El número de árboles consumidos depende de si se utilizan procesos mecánicos o químicos. Se ha estimado que, basándose en una mezcla de maderas blandas y frondosas de 12 metros (40 pies) de altura y 15-20 centímetros (6-8 pulgadas) de diámetro, se necesitaría un promedio de 24 árboles para producir 0,9 toneladas (1 tonelada). de papel de impresión y escritura, mediante el proceso kraft (pulpado químico). La pulpa mecánica es aproximadamente dos veces más eficiente en el uso de árboles, ya que casi toda la madera se usa para hacer fibra, por lo tanto, se necesitan alrededor de 12 árboles para producir 0.9 toneladas (1 tonelada) de pulpa mecánica o papel de periódico . [26]
Hay aproximadamente dos toneladas cortas en una cuerda de madera . [27]
Preparación para despulpado
El astillado de madera es el acto y la industria de astillado de madera para pulpa, pero también para otros productos de madera procesada y mantillo . Solo el duramen y la albura son útiles para hacer pulpa. La corteza contiene relativamente pocas fibras útiles y se elimina y se utiliza como combustible para proporcionar vapor para su uso en la planta de celulosa. La mayoría de los procesos de fabricación de pasta requieren que la madera se astille y se tamice para proporcionar astillas de tamaño uniforme.
Pulping
There are a number of different processes which can be used to separate the wood fiber:
Mechanical pulp
Manufactured grindstones with embedded silicon carbide or aluminum oxide can be used to grind small wood logs called "bolts" to make stone pulp (SGW). If the wood is steamed prior to grinding it is known as pressure ground wood pulp (PGW). Most modern mills use chips rather than logs and ridged metal discs called refiner plates instead of grindstones. If the chips are just ground up with the plates, the pulp is called refiner mechanical pulp (RMP) and if the chips are steamed while being refined the pulp is called thermomechanical pulp (TMP). Steam treatment significantly reduces the total energy needed to make the pulp and decreases the damage (cutting) to fibres. Mechanical pulps are used for products that require less strength, such as newsprint and paperboards.
Thermomechanical pulp
Thermomechanical pulp is pulp produced by processing wood chips using heat (thus "thermo-") and a mechanical refining movement (thus "-mechanical"). It is a two-stage process where the logs are first stripped of their bark and converted into small chips. These chips have a moisture content of around 25–30 percent. A mechanical force is applied to the wood chips in a crushing or grinding action which generates heat and water vapour and softens the lignin thus separating the individual fibres. The pulp is then screened and cleaned, any clumps of fibre are reprocessed. This process gives a high yield of fibre from the timber (around 95 percent) and as the lignin has not been removed, the fibres are hard and rigid.[28]
Chemi-thermomechanical pulp
Wood chips can be pre-treated with sodium carbonate, sodium hydroxide, sodium sulfate and other chemicals prior to refining with equipment similar to a mechanical mill. The conditions of the chemical treatment are much less vigorous (lower temperature, shorter time, less extreme pH) than in a chemical pulping process since the goal is to make the fibers easier to refine, not to remove lignin as in a fully chemical process. Pulps made using these hybrid processes are known as chemi-thermomechanical pulps (CTMP).
Chemical pulp
Chemical pulp is produced by combining wood chips and chemicals in large vessels called digesters. There, heat and chemicals break down lignin, which binds cellulose fibres together, without seriously degrading the cellulose fibres. Chemical pulp is used for materials that need to be stronger or combined with mechanical pulps to give a product different characteristics. The kraft process is the dominant chemical pulping method, with the sulfite process second. Historically soda pulping was the first successful chemical pulping method.
Recycled pulp
Recycled pulp is also called deinked pulp (DIP). DIP is recycled paper which has been processed by chemicals, thus removing printing inks and other unwanted elements and freed the paper fibres. The process is called deinking.
DIP is used as raw material in papermaking. Many newsprint, toilet paper and facial tissue grades commonly contain 100 percent deinked pulp and in many other grades, such as lightweight coated for offset and printing and writing papers for office and home use, DIP makes up a substantial proportion of the furnish.
Organosolv pulping
Organosolv pulping uses organic solvents at temperatures above 140 °C to break down lignin and hemicellulose into soluble fragments. The pulping liquor is easily recovered by distillation. The reason for using a solvent is to make the lignin more soluble in the cooking liquor. Most common used solvents are methanol, ethanol, formic acid and acetic acid often in combination with water.
Alternative pulping methods
Research is under way to develop biopulping (biological pulping), similar to chemical pulping but using certain species of fungi that are able to break down the unwanted lignin, but not the cellulose fibres.[29] In the biopulping process, the fungal enzyme lignin peroxidase selectively digests lignin to leave remaining cellulose fibres. This could have major environmental benefits in reducing the pollution associated with chemical pulping. The pulp is bleached using chlorine dioxide stage followed by neutralization and calcium hypochlorite. The oxidizing agent in either case oxidizes and destroys the dyes formed from the tannins of the wood and accentuated (reinforced) by sulfides present in it.
Steam exploded fibre is a pulping and extraction technique that has been applied to wood and other fibrous organic material.[30]
Bleaching
The pulp produced up to this point in the process can be bleached to produce a white paper product. The chemicals used to bleach pulp have been a source of environmental concern, and recently the pulp industry has been using alternatives to chlorine, such as chlorine dioxide, oxygen, ozone and hydrogen peroxide.
Alternativas a la pulpa de madera
Pulp made from non-wood plant sources or recycled textiles is manufactured today largely as a speciality product for fine-printing and art purposes.[10][31] Modern machine- and hand-made art papers made with cotton, linen, hemp, abaca, kozo, and other fibers are often valued for their longer, stronger fibers and their lower lignin content. Lignin, present in virtually all plant materials, contributes to the acidification and eventual breakdown of paper products, often characterized by the browning and embrittling of paper with a high lignin content such as newsprint.[32][33] 100% cotton or a combination of cotton and linen pulp is widely used to produce documents intended for long-term use, such as certificates, currency, and passports.[34][35][36]
Today, some groups advocate using field crop fibre or agricultural residues instead of wood fibre as a more sustainable means of production.[citation needed]
There is enough straw to meet much of North America's book, magazine, catalogue and copy paper needs.[citation needed] Agricultural-based paper does not come from tree farms. Some agricultural residue pulps take less time to cook than wood pulps. That means agricultural-based paper uses less energy, less water and fewer chemicals. Pulp made from wheat and flax straw has half the ecological footprint of pulp made from forests.[37]
Hemp paper is a possible replacement, but processing infrastructure, storage costs and the low usability percentage of the plant means it is not a ready substitute.[citation needed]
However, wood is also a renewable resource, with about 90 percent of pulp coming from plantations or reforested areas.[24] Non-wood fibre sources account for about 5–10 percent of global pulp production, for a variety of reasons, including seasonal availability, problems with chemical recovery, brightness of the pulp etc.[19][38] In China, as of 2009, a higher proportion of non-wood pulp processing increased use of water and energy.[39]
Nonwovens are in some applications alternatives to paper made from wood pulp, like filter paper or tea bags.
Component | Wood | Nonwood |
---|---|---|
Carbohydrates | 65–80% | 50–80% |
| 40–45% | 30–45% |
| 23–35% | 20–35% |
Lignin | 20–30% | 10–25% |
Extractives | 2–5% | 5–15% |
Proteins | <0.5% | 5–10% |
Inorganics | 0.1–1% | 0.5–10% |
| <0.1% | 0.5–7% |
Pulpa de mercado
Market pulp is any variety of pulp that is produced in one location, dried and shipped to another location for further processing.[41] Important quality parameters for pulp not directly related to the fibres are brightness, dirt levels, viscosity and ash content. In 2004 it accounted for about 55 million metric tons of market pulp.[41]
Air dry pulp is the most common form to sell pulp. This is pulp dried to about 10 percent moisture content. It is normally delivered as sheeted bales of 250 kg. The reason to leave 10 percent moisture in the pulp is that this minimizes the fibre to fibre bonding and makes it easier to disperse the pulp in water for further processing to paper.[41]
Roll pulp or reel pulp is the most common delivery form of pulp to non traditional pulp markets. Fluff pulp is normally shipped on rolls (reels). This pulp is dried to 5–6 percent moisture content. At the customer this is going to a comminution process to prepare for further processing.[41]
Some pulps are flash dried. This is done by pressing the pulp to about 50 percent moisture content and then let it fall through silos that are 15–17 m high. Gas fired hot air is the normal heat source. The temperature is well above the char point of cellulose, but large amount of moisture in the fibre wall and lumen prevents the fibres from being incinerated. It is often not dried down to 10 percent moisture (air dry). The bales are not as densely packed as air dry pulp.[41]
Preocupaciones ambientales
The major environmental impacts of producing wood pulp come from its impact on forest sources and from its waste products.
Forest resources
The impact of logging to provide the raw material for wood pulp is an area of intense debate. Modern logging practices, using forest management seek to provide a reliable, renewable source of raw materials for pulp mills. The practice of clear cutting is a particularly sensitive issue since it is a very visible effect of logging. Reforestation, the planting of tree seedlings on logged areas, has also been criticized for decreasing biodiversity because reforested areas are monocultures. Logging of old growth forests accounts for less than 10 percent of wood pulp,[24] but is one of the most controversial issues.
Effluents from pulp mills
The process effluents are treated in a biological effluent treatment plant, which guarantees that the effluents are not toxic in the recipient.
Mechanical pulp is not a major cause for environmental concern since most of the organic material is retained in the pulp, and the chemicals used (hydrogen peroxide and sodium dithionite) produce benign byproducts (water and sodium sulfate (finally), respectively).
Chemical pulp mills, especially kraft mills, are energy self-sufficient and very nearly closed cycle with respect to inorganic chemicals.
Bleaching with chlorine produces large amounts of organochlorine compounds, including polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/Fs).[42][43] Many mills have adopted alternatives to chlorinated bleaching agents thereby reducing emissions of organochlorine pollution.[44]
Odor problems
The kraft pulping reaction in particular releases foul-smelling compounds. The hydrogen sulfide reagent that degrades lignin structure also causes some demethylation to produce methanethiol, dimethyl sulfide and dimethyl disulfide. These same compounds are released during many forms of microbial decay, including the internal microbial action in Camembert cheese, although the kraft process is a chemical one and does not involve any microbial degradation. These compounds have extremely low odor thresholds and disagreeable smells.
Aplicaciones
The main applications for pulp are paper and board production. The furnish of pulps used depends on the quality on the finished paper. Important quality parameters are wood furnish, brightness, viscosity, extractives, dirt count and strength.
Chemical pulps are used for making nanocellulose.[citation needed]
Speciality pulp grades have many other applications. Dissolving pulp is used in making regenerated cellulose that is used textile and cellophane production. It is also used to make cellulose derivatives. Fluff pulp is used in diapers, feminine hygiene products and nonwovens.
Paper production
The Fourdrinier Machine is the basis for most modern papermaking, and it has been used in some variation since its conception. It accomplishes all the steps needed to transform pulp into a final paper product.
Ciencias económicas
In 2009, NBSK pulp sold for $650/ton in the United States. The price had dropped due to falling demand when newspapers reduced their size, in part, as a result of the recession.[45]
Ver también
- Nanocellulose
- Paper chemicals
- Pulp mill
- Pulpwood
- Versuche und Muster ohne alle Lumpen oder doch mit enem geringen Zusatze derselben Papier zu machen by Jacob Christian Schäffer on Google Books
- Johan Richter, developer of the process for continuous cooking of pulp
- World Forestry Congress
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Bibliografía
- "Pulp and Paper", Environment Canada, Government of Canada, 2014.[dead link]