Wood bleaching pulp is a pulp wood chemical processing to brighten its color and whiten pulp. The main product of wood pulp is paper, which is white (similar to, but different from brightness) is an important characteristic. This process and chemistry also applies to non-wood pulp bleaching, such as those made from bamboo or kenaf.
Video Bleaching of wood pulp
Paper brightness
Brightness is the amount of incident light that is reflected from the paper under certain conditions, usually reported as the percentage of reflected light, so a higher number means lighter or whiter paper. In the US, TAPPI T 452 or T 525 standards are used. The international community uses ISO standards. Table 1 shows how both systems assess paper with high brightness, but there is no simple way to convert between the two systems because the testing methods are very different. The ISO rating is higher and can be over 100. This is because contemporary white paper incorporates a fluorescent whitening agent (FWA). Because the ISO standard measures only a narrow range of blue light, it is not directly proportional to a white human vision or brightness.
Table 1
Newspaper paper ranges from 55-75 ISO brightness. Writing and printer paper will usually be as bright as 104 ISO.
While the results are similar, the basic processes and chemicals involved in bleaching chemical pulp (such as kraft or sulfite) are very different from those involved in mechanical pulp bleaching (such as stoneground, thermomechanical or chemithermomechanical). Chemical pulp contains very little lignin while the mechanical pulps contain most of the lignin present in the wood used to make the pulp. Lignin is the main source of color in the pulp because of the presence of various chromophores that are naturally present in wood or made in a pulp mill.
Maps Bleaching of wood pulp
Rotate the mechanical pulp
Mechanical pulps retain most of the lignin present in the wood used to make the pulp and thus contain almost as much lignin as cellulose and hemicellulose. It would be very impractical to remove this much lignin by bleaching, and is undesirable because one of the major advantages of mechanical pulp is the high pulp yield based on the wood used. Therefore, the purpose of mechanical pulp bleaching (also referred to as brightening) is to simply remove the chromophores (the color causing group). This is possible because the structure responsible for color is also more susceptible to oxidation or reduction.
Alkaline hydrogen peroxide is the most commonly used whitening agent for mechanical pulps. The amount of base such as sodium hydroxide is less than that used in bleaching chemical pulp and lower temperature. This condition allows basic peroxides to selectively oxidize the non-aromatic conjugate group responsible for absorbing visible light. The decomposition of hydrogen peroxide is catalyzed by transition metals, and iron, manganese and copper are essential in pulp bleaching. The use of chelating agents such as EDTA to remove some metal ions from the pulp before adding peroxide allows peroxide to be used more efficiently. Magnesium and sodium silicate salts are also added to enhance bleaching with alkaline peroxides.
Sodium dithionite (Na 2 ), also known as sodium hydrosulfite, is another major reagent used to brighten mechanical pulp. In contrast to hydrogen peroxide, which oxidizes chromophores, dithionite reduces these color-causing groups. Dithionite reacts with oxygen, so efficient dithionite use requires minimized oxygen exposure during its use.
The traitors may contribute to the strengthening of the brightness by overriding the iron ions, eg as EDTA complexes, which are less colorful than the complex formed between iron and lignin.
The brightness gain achieved in bleaching mechanical pulp is temporary since almost all lignin present in the wood is still present in the pulp. Exposure to air and light can produce new chromophores from the rest of this lignin. This is why yellow newspapers are getting older. yellowing also occurs due to the size of the acid
Recycled pulp bleaching
Hydrogen peroxide and ditionite sodium are used to increase the brightness of the terdeinked pulp. A similar bleaching method for mechanical pulps whose goal is to make the fibers brighter.
Chemical pulp bleaching
Chemical pulp, such as those derived from kraft or pulping sulphite processes, contains less lignin than mechanical pulps, (& lt; 5% compared to about 40%). The goal in chemical pulp bleaching is to basically remove all residual lignin, then this process is often referred to as delignification. Sodium hypochlorite (household bleach) was originally used to whiten chemical pulp, but was largely replaced in 1930 by chlorine. Concerns about the release of organochlorine compounds into the environment encourage the development of Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF) bleaching processes.
The chemical pulp delignification often consists of four or more discrete steps, with each step pointed to by the letters in the Table:
Table 2
The bleaching sequence from the 1950s could look like: CEHEH . The pulp will be exposed to chlorine, extracted (washed) with sodium hydroxide solution to remove chrominated lignin which is fragmented, treated with sodium hypochlorite, washed with sodium hydroxide again and treated with hypochlorite. An example of a chlorine-free modern sequence (TCF) is OZEPY in which the pulp will be treated with oxygen, then ozone, washed with sodium hydroxide then treated sequentially with alkali peroxide and sodium ditionite.
Chlorine and hypochlorite
Chlorine replaces hydrogen in the lignin aromatic ring through aromatic substitution, oxidizing the pendant groups to carboxylic acids and adding to the double carbon carbon bond in the lignin sidechains. Chlorine also invades cellulose, but this reaction occurs mainly at pH 7, where un-ionized hypochlorite acid, HClO, is the main chlorine species in solution. To avoid excessive cellulosic degradation, chlorination is performed at pH & lt; 1.5.
- Cl 2 H 2 O? H Cl - HClO
At pH & gt; The 8 dominant species are hypochlorite, ClO - , which is also useful for lignin removal. Sodium hypochlorite can be purchased or produced in situ by reacting chlorine with sodium hydroxide.
- 2 NaOH Cl 2 ? NaOCl NaCl H 2 O
The main objection to the use of chlorine for bleaching pulp is the large number of dissolved organochlorine compounds produced and released into the environment.
Chlorine dioxide
Chlorine dioxide, ClO 2 is an unstable gas with moderate solubility in water. It is usually produced in an aqueous solution and is used immediately because it is decomposed and explosive in higher concentrations. This is produced by reacting sodium chlorate with a reducing agent such as sulfur dioxide. < 4 SO 2 -> 2 ClO 4 2 3 2 2 NaHSO 4
Chlorine dioxide is sometimes used in combination with chlorine, but is used alone in the bleaching sequence of ECF (chlorine-free element). It is used in moderate acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced. Chlorine dioxide (ECF technology) is currently the most important bleaching method in the world. Approximately 95% of all bleached Kraft slurries are made using chlorine dioxide in the ECF bleaching sequence.
Extraction or washing
All bleaching agents are used to delignify chemical pulp, with the exception of sodium dithionite, breaking lignin into smaller oxygen-containing molecules. The cracking product is generally soluble in water, especially if the pH is greater than 7 (many of these products are carboxylic acids). These materials should be removed between the bleaching stages to avoid excessive use of bleaching chemicals because many of these smaller molecules are still susceptible to oxidation. The need to minimize water use in modern pulp mills has encouraged the development of equipment and techniques for efficient use of water.
Oxygen
Oxygen exists as a state of the base state triplet, which is relatively unreactive and requires free radicals or highly electron-rich substrates such as the lignin deprotonated phenolic group. The production of phenoxide groups requires that delignification with oxygen be carried out under very basic conditions (pH & gt; 12). The reaction involved is mainly a single electron reaction (radical). Oxygen opens the ring and cuts the sidechains giving a complex mixture of small oxygen molecules. Transition metal compounds, especially those of iron, manganese and copper, which have several oxidation states, facilitate many of the radical reactions and the impact of oxygen delignification. While most radical reactions are responsible for delignification, they are detrimental to cellulose. Oxygen-based radicals, especially hydroxyl radicals, HOo, can oxidize hydroxyl groups in the cellulose chain to ketones, and under the very basic conditions used in oxygen delignification, these compounds undergo an inverted aldol reaction that causes cellulose chain division. Magnesium salts are added to the oxygen delignification to help preserve the cellulose chain, but these protection mechanisms have not been confirmed.
Hydrogen peroxide
Using hydrogen peroxide to verify the chemical pulp requires stronger conditions than to brighten mechanical pulps. Both pH and temperature are higher when treating chemical slurry. Chemistry is very similar to that involved in oxygen delignification, in terms of the involved radical species and the resulting product. Hydrogen peroxide is sometimes used with oxygen in the same bleaching stage and this gives the appointment of the letter Op in the bleaching sequence. Metal ions, especially manganese catalyze the decomposition of hydrogen peroxide, so some increase in the efficiency of peroxide bleaching can be achieved if the metal level is controlled.
Ozone
Ozone is a very strong oxidizing agent and the biggest challenge in using it to whiten wood pulp is to get enough selectivity so that the desired cellulose is not degraded. Ozone reacts with carbon-carbon double bonds in lignin, including those in aromatic rings. In the 1990s ozone was touted as a good reagent to allow the pulp to be bleached without chemicals containing chlorine (completely chlorine-free, TCF). Emphasis has changed and ozone is seen in addition to chlorine dioxide in bleaching sequences not using chlorine (chlorine-free element, ECF). More than twenty five pulp mills worldwide have installed equipment to produce and use ozone.
Chelant Wash
Transition metal effects at some stage of bleaching have been mentioned. It is sometimes useful to remove some metal ions from the pulp by washing the slurry with a chelating agent such as EDTA or DTPA. This is more common in the TCF bleaching sequence for two reasons: acidic chlorine or chlorine dioxide tends to remove metal ions (metal ions are usually more soluble at lower pH) and TCF stages are more dependent on oxygen-based bleach, more susceptible to the detrimental effects of these metal ions. Chelant leaching is usually performed at or near pH 7. Lower pH solutions are more effective at removing transition metals, but also eliminating more beneficial metal ions, especially magnesium.
Other bleaching agents
A variety of more exotic bleaching materials have been used in chemical pulp. They include peroxyacetic acid, peroxyformic acid, potassium peroxymonosulfate (Oxone), dimethyldioxirane, produced in situ from acetone and potassium peroxymonosulfate, and peroxymonophosphoric acid.
Enzymes such as xylanase have been used in pulp bleaching to improve the efficiency of other bleaching chemicals. It is believed that xylanase does this by cleaving the lignin-xilan bond so that lignin is more readily accessible to other reagents. It is possible that other enzymes such as those found in fungi that decrease lignin may be useful in pulp bleaching.
Environmental considerations
The bleaching of chemical pulp has the potential to cause significant environmental damage, especially through the release of organic matter into waterways. The pulp mills are almost always located near large bodies of water because they need a lot of water for the process. Increased public awareness of environmental issues since the 1970s and 1980s, as evidenced by the formation of organizations such as Greenpeace, influenced the pulp-making industry and the government to address the release of these materials to the environment.
Conventional bleaching uses chlorine elements to produce and release into the environment a large number of chlorinated organic compounds, including chlorinated dioxins. Dioxin is recognized as a persistent environmental polluter, regulated internationally by the Stockholm Convention on Persistent Organic Pollutants.
Dioxin is highly toxic, and health effects in humans include reproductive, developmental, immune and hormonal problems. They are known to be carcinogenic. More than 90% of human exposure is through food, especially meat, milk, fish and shellfish, as dioxin accumulates in the food chain in animal fat tissue.
As a result, from the 1990s onward, the use of chlorine elements in the delignification process substantially reduced and replaced with ECF (Elemental Chlorine Free) and TCF (Totally Chlorine Free) bleaching process. In 2005, the chlorine element was used in 19-20% of kraft pulp production globally, down from over 90% in 1990. 75% of kraft pulp using ECF, with 5-6% remaining using TCF. Most TCF slurries are produced in Sweden and Finland for sale in Germany, all markets with high levels of environmental awareness. In 1999, TCF porridge represented 25% of the European market.
Bleaching of TCF, by removing chlorine from the process, reduces chlorinated organic compounds to the background level in pulp mill waste. ECF bleaching can substantially reduce but not completely eliminate chlorinated organic compounds, including dioxins, from waste. While modern ECF plants can achieve chlorinated organic emissions (AOX) of less than 0.05 kg per ton of pulp produced, most do not achieve this level of emissions. In the European Union, the emission of chlorinated organic compounds for ECF plants is 0.15 kg per ton.
However, there is disagreement about the comparative environmental effects of ECF and TCF bleaching. Some researchers found that there was no environmental difference between ECF and TCF while others concluded that among the effluents of ECF and TCF before and after secondary treatment, TCF effluents were the least toxic.
See also
- Johan Richter - Inventor of continuous process to whiten wood pulp
- Pulp & amp; Paper chemicals
References
Source of the article : Wikipedia
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