THE PULP AND PAPER INDUSTRY
In New Zealand, paper is made from wood using the "Kraft" process. This is a part mechanical, part chemical process that produces a strong pulp. It has veral disadvantages, in terms of complexity and t up costs as well as having a low pulp yield and producing unpleasant-smelling sulfur compounds, but it is still internationally the most widely ud pulp and paper process. The manufacturing process is outlined below. Step 1 - Wood preparation
婴儿便秘怎么办The bark is removed from in-coming logs, and the are then chipped. Sometimes, the wood arrives at the plant already chipped, meaning that this step is unnecessary.
Step 2 - Cooking
The wood chips are heated in a solution of NaOH and Na2S in a pressure cooker, during which time a lot of the lignin (the reinforcing susbstance that make tree cells wood hard and 'woody' rather than soft like tho of other plants) is removed from the wood. The pressure is then relead suddenly, causing the chips to fly apart into fibres.
Step 3 - Pulp washing
The pulp is washed with water to wash out the cooking chemicals and lignin from the fibre so that they will not interfere with later process steps.
Step 4 - Pulp screening
A sieve is ud to remove knots and clumped-together uncooked fibres from the pulp. Step 5 - Bleaching
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This is done in two stages. Firstly the pulp is treated with NaOH in the prence of O2. The NaOH removes hydrogen ions from the lignin and then the O2 breaks down the polymer. Then, the pulp is treated with ClO2 then a mixture of NaOH, O2 and peroxide and finally with ClO2 again to remove the remaining lignin.
Step 6 - Paper making
The fibres are mechanincally treated to make them bond better to each other (strengthening the paper), chemicals added to provide special properties such as colour or water resistance, and then the water is squeezed out and the pulp is rolled smooth and dried.
Various ancilliary process result in the recovery of CaO, NaOH and Na2S, the major chemicals us
ed in the process. Various utilities ensure that such conditions as sufficient reaction times and adequate mixing are met.
On site processing removes the lignin from the liquid wastes, and solid wastes are generally taken to a landfill. Efforts continue to be made to reduce water consumption by recycling, as smaller volumes are easier to process. The most obvious environmental problem continues to be the sulfurous emissions that give Kraft pulping plants their characteristic smell. The are decread by gas incineration, but are not able to be wholly eliminated.
x网INTRODUCTION
Paper is a major product of the forestry industry, and is ud widely in our society. Paper products are ud not only in their obvious applications in the publishing industry and for writing on, but also in a variety of specialty papers, cardboards, brown papers etc. In addition, various chemicals are produced as a byproduct of the pulp and paper industry (e articles).
Paper is made by pulping wood, bleaching this pulp and then spreading it out into sheets to make it into paper. At various stages of the process, chemicals are ud to give the paper particular properties, such as the bleaching chemicals that make paper white (and which also enable it to subs
equently be coloured). The pulping process that is ud in New Zealand is known as "kraft pulping" which relies on a combination of heat, chemicals and mechanical pulping to convert the wood into a smooth, soft pulp suitable for u in paper making.
Kraft pulping is the main pulping process (together with mechanical pulping) ud today, and is the only one discusd below. The kraft process has veral advantages:
• It can be ud with virtually all wood species
• It can easily handle the extractives in most coniferous wood
• The pulp has very good strength (the word 'kraft' means 'strong' in Swedish)abcc的词语有哪些
• The recovery process for the chemicals is well established
However, there are also disadvantages:
• The pulp yield is quite low at about 45 - 50%
• The equipment ud for the chemical recovery is extensive and costly to install
• Sulphurous compounds, which are odorous in the parts per billion range, are formed in the process
• Fairly complicated process are required for bleaching the pulp
Lignin
The main component of wood that needs to be removed to turn it into paper is a compound known as lignin. This name refers to a group of chemicals that are esntially three dimensional polymers of trans-coniferol, trans-sinapol and trans-p-coumarol (e below), along with hemicellulos and aromatic carboxylic acids. Lignin is the reinforcing compound that is deposited on tree cell walls to make the wood strong enough to carry the weight of the tree crown. However, it is also the compound that makes wood pulp brown, so it is removed from all wood pulp except that ud to make brown paper and some cardboards.
HO CH CHCH2OH HO CH2OH CH3O
trans-p-coumarol trans-coniferol
CH3O
HO CH CHCH2OH
CH3O
trans-sinapol
THE MANUFACTURING PROCESS
The process whereby timber is converted into paper involves six steps. The first four convert the logs into a mass of cellulo fibres with some residual lignin using a mixture of physical and chemical process. This pulp is then bleached to remove the remaining lignin and finally spread out into smooth, presd sheets (often with chemicals added to provide particular properties such as colour or water resistance). For some papers (e.g. cardboards and 'brown paper') the bleaching step is unnecessary, but all white and coloured papers require bleaching.
Step 1 - Wood preparation
Wood is delivered to the kraft mill in one of two ways: whole logs and sawmill chips (residuals from sawmills). The logs have their bark removed, either by passing through a drum debarker or by being treated in a hydraulic debarker. The drum debarker, which consists of a slightly inclined, rotating drum is best suited to small diameter logs. The hydraulic debarker, which us high pressure water j
三年级上册奥数题ets, can handle large diameter logs. The removed bark is a good fuel, and is normally burnt in a boiler for generating steam.
After debarking, the logs are chipped by multi knife chippers into suitable sized pieces, and are then screened to remove overlarge chips. The thickness of the chips is the most important parameter, as this determines the speed and the thoroughness of the impregnation of the cooking chemicals into the wood chip. Neither debarking nor chipping are usually necessary for sawmill chips.
Step 2 - Cooking
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The "cooking process" is where the main part of the delignification takes place. Here the chips are mixed with "white liquor" (a solution of sodium hydroxide and sodium sulphide), heated to increa the reaction rate and then disintegrated into fibres by 'blowing' - subjecting them to a sudden decrea in pressure. Typically some 150 kg of NaOH and 50 kg of Na2S are required per tonne of dry wood. This process is, like any chemical reaction, affected by time, temperature and concentration of chemical reactants. Time and temperature can be traded off against each other to a certain extent, but to achieve reasonable cooking times it is necessary to have temperatures of about 150 - 165o C, so pressure cookers are ud. However, if the temperature is too high then the chips are delignified unevenly, so a balance must be achieved.
The kinetics of the kraft pulping is quite well understood, but the reaction is heterogenous and therefore difficult to examine. To determine when to interrupt the cooking, a model relating time, temperature and cooking chemical charge is ud. The degree of delignification is the most important parameter for determining pulp quality, and is normally
expresd in what is called a "Kappa number". This number is directly related to the amount of lignin still remaining in the cooked pulp.
There are two different cooking systems; batch and continuous. In batch cooking, chips and white liquor are charged to a pressure vesl and are then heated with steam to a t temperature for a t time. When the correct delignification has been achieved, the cook is "blown" (the pressure is suddenly relead so that the cooked chips disintegrate into fibres). In the continuous process, chips and white liquor are fed continuously to the top of a tall pressure vesl. The chips move down the 'digester' by gravity (as a plug) to be finally blown from the bottom of the vesl. The cooking time cannot be varied in this ca (it is t by the production rate) and only the temperature and the chemical charge can be controlled.
Many developments have taken place during the last decade to improve the 'science' of kraft pulping.
The challenge has been to remove as much of the lignin as possible with out degrading the cellulo and without losing too much yield. It is now well known that the concentrations of NaOH, Na2S and dissolved lignin during the various phas of the delignification are of crucial importance for the pulp strength. Generally speaking, it is desirable to have a high sulphide concentration in the beginning of the cook, a low lignin concentration in the liquid pha towards the end of the cook, and an even alkali concentration during most parts of the cook. How to achieve this in practice under conditions of high temperature and high pressures has been a challenge, and much development is still going on.
Step 3 - Pulp washing
Becau of the high amounts of chemicals ud in the cooking wood in kraft pulping, the recovery of the chemicals is of crucial importance. The process where the chemicals are parated from the cooked pulp is called pulp washing. A good removal of chemicals (inorganic and organic) is necessary for veral reasons:
• The dissolved chemicals interfere with the downstream processing of the pulp
• The chemicals are expensive to replace
• The chemicals (especially the dissolved lignin) are detrimental to the environment
There are many types of machinery ud for pulp washing. Most of them rely on displacing the dissolved solids (inorganic and organic) in a pulp mat by hot water, but some u pressing to squeeze out the chemicals with the liquid. An old, but still common method is to u a drum, covered by a wire mesh, which rotates in a diluted suspension of the fibres. The fibres form a mat on the drum, and showers of hot water are then sprayed onto the fibre mat.
Step 4 - Pulp screening
Apart from fibres, the cooked pulp also contains partially uncooked fibre bundles and knots. Modern cooking process (together with good chip screening to achieve consistent chip thickness) have good control over the delignification and produce less "rejects". Knots and shives are removed by passing the pulp over pulp screens equipped with fine holes or slots. Step 5 - Bleaching
Pulp produced by the kraft process is brown. This prents no problem for certain us, e.g. for sack paper, most corrugated boxes, some bag paper etc. However, a major proportion of the kraft pulp that is made is ud for white or coloured papers such as writing and printing papers, and then the pulp needs to be bleached.
Bleaching involves removing virtually all of the lignin that still remains after cooking, as the lignin contains the chromophoric groups which make the pulp dark. Strictly speaking, bleaching and cooking are both delignification process, and modern developments have tended to blur the difference between the two process. However, traditionally the name
'bleaching' is rerved for delignification that is taking place downstream of the cooking process. In practice, there are two parate "bleaching" process steps: oxygen delignification and final bleaching.
To measure the lignin content in pulp, a number called the "Kappa number" is ud. The Kappa number is directly proportional to the lignin content of the pulp. Pulp from the digester has a Kappa number of 20-35 for softwood and 15-20 for hardwood (hardwood contains less lignin and can therefore be cooked to a lower Kappa number). Oxygen delignification removes about half of the lignin remaining after the cooking process, so that the Kappa number of the oxygen delignified pulp is typically 12-18 for softwood. The final bleaching removes all remaining lignin and decreas the Kappa number to zero.给孩子起名字
Oxygen delignification注意安全标志图片
In oxygen delignification, washed pulp is treated with a highly alkaline solution of sodium hydroxide. The high pH ionizes phenolic groups in the lignin, which are then attacked by molecular oxygen. The aromatic part of the lignin is partly destroyed and it is then depolymerid to lower molecular weight compounds. The are more soluble in water and can be removed from the fibres. It is important that the pulp has been at least partly washed beforehand becau the black liquor solids in unwashed pulp consume oxygen. After the oxygen delignification stage, the pulp has to be washed very well, as otherwi the organics carry over to the final bleaching process, consuming chemicals there and also decreasing the environmental benefits.
The highly alkaline conditions of oxygen delignification also make carbohydrate fractions in the fibres react with oxygen to a certain extent. As the reactions break down the polymer chains of cellulo, and thus decrea the pulp strength, the reactions must be kept to a minimum. It has been found that it is the radical species of oxygen which are particularly harmful to the carbohydrates. The formation of radicals is promoted by the prence of certain metal ions. However, it has been found that magnesium salts inhibit metal ion activity, and magnesium sulphate is therefore normally added as a protector in oxygen delignification.
Oxygen is only sparingly soluble in water, and the controlling factor on the reaction rate is therefore n
ormally the concentration of dissolved oxygen around the fibre. Originally a high pulp consistency (30-40%) was ud to overcome this restriction. However, modern high intensity mixers can distribute the oxygen in very small bubbles on the fibres, and the mixers have made it possible to operate at "medium consistency" (10-12%). Medium consistency has veral advantages: the equipment is simpler and the risk of fire (becau of the u of oxygen) is virtually eliminated.
Oxygen delignification can significantly decrea the water pollution from the final (normally chlorine or chlorine dioxide bad) bleaching. In addition, it is an effluent free process. All dissolved lignin and other organics (as well as the inorganic chemicals) are recovered in the black liquor and returned to the chemical recovery system, rather than being discharged as effluent as they are in chlorine-bad bleaching. Finally, oxygen is a fairly cheap bleaching chemical, although the capital costs are high for an efficient system. On the
negative side, the process has the potential to degrade the pulp strength if it is not controlled properly.
Final bleaching
The final bleaching is always carried out in veral stages to improve the efficiency of the chemicals
ud, and to decrea the strength loss of the pulp. There are quite a number of bleaching chemicals ud commercially, and many more have been tried in the laboratory. The chemicals ud are:
• Chlorine
• Chlorine dioxide
• Sodium hypochlorite
• Oxygen
• Peroxide
• Ozone
Of the chemicals, the first three contain chlorine atoms, whilst the last three u non-chlorine oxidizing compounds. Elemental chlorine (Cl2) was for many years the work hor of the bleaching process. It is efficient in bleaching the pulp and (if properly ud) does not degrade the pulp strength. However, it produces a large amount of chlorinated organic compounds in the effluent, and
strenuous efforts have therefore been made to decrea its usage. For the same reason, the u of sodium hypochlorite (which also tended to affect the pulp strength) is now virtually eliminated.
Modern bleach plants therefore u no elemental chlorine. They are what is called ECF plants: elemental chlorine free bleach plants. Chlorine dioxide, which is ud instead (in addition to non-chlorine compounds), is environmentally much more benign than Cl2. However, while chlorine dioxide is good at prerving pulp strength, it is not as effective as elemental chlorine in delignification/bleaching. ECF plants therefore have to have a rather low incoming Kappa number, and this is normally achieved by using oxygen delignification ahead of the final bleaching.
Most ECF plants u a three step bleaching process of chlorine dioxide followed by a mixture of NaOH, O2 and peroxide (the 'extraction' stage) and then finally chlorine dioxide again. At Kinleith, becau of the efficiency of the oxygen delignification, the peroxide is no longer necessary and a quence of chlorine dioxide then NaOH and O2 followed by more chlorine dioxide is ud. The chlorine dioxide stages normally run at a pH of 3-4.5, and the'extraction' stages at a pH of 10-11. The temperature is kept at 70-80 o C to achieve sufficiently fast rate of reaction.
The amount of chlorinated toxic compounds in the effluent from a correctly operated ECF plant is sm
all (especially after condary treatment) and the effects on the environment appear rather insignificant. However, especially in Europe, there is a perception that using "chlorine" in any form when bleaching is undesirable, and bleaching without using any form of chlorine compounds, so-called total chlorine free bleaching (TCF bleaching) has been developed. In TCF bleaching only oxygen, peroxide and ozone (in addition to caustic and certain chelating agents) are ud. TCF bleached pulp can nowadays reach virtually the same brightness as ECF bleached pulp, but the strength is somewhat lower. Such plants require inevitably oxygen delignification and also, usually, cooking to a lower Kappa number. Chemical costs are also normally higher. TCF pulp is not made in New Zealand.
