|عدد المساهمات : 541|
|تاريخ التسجيل : 06/12/2014|
موضوع: مراحل صناعة الورق رائع Pulp & Paper مراحل صناعة الورق رائع Pulp & Paper
مراحل صناعة الورق رائع Pulp & Paper
Pulp & Paper
Paper derives from the word "papyrus". Today, Paper includes a wide range of products with very different applications: communication, cultural, educational, artistic, hygienic, sanitary as well as storage and transport of all kinds of goods. It's almost impossible to imagine a life without paper.
Paper has a long history, beginning with the ancient Egyptians and continuing to the present day. After hand-made methods dominated for thousands of years, Paper production became industrialised during the 19th century. Originally intended purely for writing and printing purposes, a wide variety of Paper grades and uses is now available to the consumer.
Paper is a natural product, manufactured from a natural and renewable raw material, wood. The advantage of Paper is that it is biodegradable and recyclable. In this way, the Paper industry is sustainable, from the forest through the production of paper, to the use and final recovery of the product.
Paper is an essential part of our lives and satisfies many human needs. We use it to store and communicate information (newspapers, books, documents and writing paper), for cultural and artistic purposes, to transport and protect food (packaging, sacks, tetra packs), for personal hygiene (tissues, napkins, diapers, etc.) and in medicine (hospital uses).
The manufacturing of Paper is an involved process, from the forest, to the production of paper, to the use and final recovery of the product.
1. Pulp mill
Trees provide the primary raw material for the Paper and board industry. Wood is made from cellulose fibres that are bound together by a material called lignin. In a Pulp mill, the fibres are separated from one another into a mass of individual fibres. After separation, the fibres are washed and screened to remove any remaining fibre bundles. The Pulp may then be used directly to make unbleached papers, or bleached for white papers. Pulp may be fed directly to a Paper machine in an "integrated Paper mill" or dried and pressed into bales to be used as a raw material by Paper mills worldwide.
The pulp-making process
1. Timber and de-barking
2. Chipping machine and pulping process
3. Cleaning and bleaching
4. Bleaching, washing and drying
Chemical Pulp is stronger, but more expensive than mechanical Pulp
Using chemical Pulp is more expensive to produce Paper than mechanical Pulp or recovered paper, but it has better strength and brightness properties. Softwood kraft Pulp is used to provide the required strength when producing light-weight publication papers. Fine papers (e.g. copy papers, writing papers, etc.) are an example of the type of Paper produced mainly from hardwood pulp, which is reinforced by a minor amount of stronger and more expensive softwood kraft pulp. Pine and spruce provide the strongest Pulp (e.g. softwood kraft), while hardwood kraft is produced from birch, eucalyptus, aspen, acacia and many other mixed tropical species.
Today, the fast growing species of tree (such as planted eucalyptus and acacia) are the most rapidly-growing Pulp raw material.
1000 KW/ton of pulp
Yield (from wood material)
Mainly longer fibres
Higher than in case of mechanical Pulp
2. Chemical pulping
In chemical pulping, the wood chips are *****d in a digester with chemicals. *****ng removes lignin, breaking up the wood into fibres. The process results in a slurry where fibres are loose but intact and have maintained their strength.
Generally, chemical Pulp is based on one of two processes: sulphate or sulphite.
Most chemical Pulp is made by the alkaline kraft or sulphate process which uses caustic soda and sodium sulphate to "****" the wood chips. In the unbleached stage, a dark brown, but very strong Pulp results which can be bleached to a high brightness if required. The *****ng chemicals are recovered back to the process through evaporation and burning plants. *****d Pulp is washed and screened to achieve more uniform quality.
The alternative method is the sulphite pulping process. This method is based on an acid *****ng liquor process, and it is best suited for specialty pulp. The sulphite mills produce easily bleached pulps, generally with hydrogen peroxide. These pulps fulfil today's demand for "chlorine free" products in the disposables sector and also in printing and writing papers.
The yield in both chemical processes is much lower than in the manufacture of ground wood, as the lignin is completely dissolved and separated from the fibres. However, the waste lignin from the sulphate and some sulphite processes, can be burnt as a fuel oil substitute. In modern mills, recovery boiler operations and the controlled burning of bark and other residues makes the chemical Pulp mill a net energy producer which can often supply power to the grid, or steam to local domestic heating plants.
3. Mechanical pulping
Mechanical pulping uses wood almost down to the last splinter
Wood Pulp was first produced in the mid-19th century by grinding logs against a water-lubricated rotating stone-faced drum. The heat generated by grinding softens the lignin and the mechanised forces separate the fibres to form ground wood This process is still used today, especially for newsprint. In the last two decades or so, newer mechanical techniques using "refiners" have been developed. In a refiner, wood chips. are subjected to intensive shearing forces between a rotating steel disc and a fixed plate. In subsequent modifications to this process, the wood chips. are pre-softened by heat (thermo mechanical Pulp - TMP) to make the fibrillation more effective or by heat and a mild chemical treatment with sodium sulphite (Chemi-thermomechanical Pulp CTMP). After grinding, the Pulp is sorted by screening to suitable fractions. It can then be bleached with peroxide for use in higher value-added products.
Mechanical pulping provides a good yield from the pulpwood, because it uses all the log except for the bark, but the energy requirement for refining is high and can only be partly compensated by using the bark as fuel.
The investment costs for mechanical Pulp mills are relatively low in comparison with other types of Pulp mills.
Mechanical Pulp is well suited for "bulk" grades of paper. Strong softwood (pine, spruce) fibres are abundantly available in Europe, and these fibres, mixed with recovered fibres, are good in the production of publication papers (eg. newsprint, SC-paper, LWC-paper). Softwood kraft Pulp is also used as a component in fine Paper production.
However, mechanical Pulp has lower strength characteristics than softwood chemical pulps and, because it retains most of the lignin which reacts with ultra-violet light, can turn "yellow" when exposed to bright light.
Finally the Pulp is bleached (in 4-7 stages). Quality papers require a Pulp which does not discolour during storage or go yellow when exposed to sunlight, and which retains its strength. Bleaching achieves all three requirements, and has the additional advantage of improving absorption capacity, removing any small pieces of bark or wood left behind as well as giving a high level of purity.
In Europe, ECF-bleaching (elemental chlorine free) and TCF-bleaching (total chlorine free) methods are widely used for environmental reasons
Modern Pulp mills are constantly improving their effluent load and are tending towards totally effluent free plants.
The Production process
Paper and board production involves two steps. First, the fibres need to be produced. This is done in a Pulp mill where Pulp is produced using chemical or mechanical processes. Pulp production can be integrated with Paper production, or produced in a separate Pulp mill. The Paper itself is then produced on a Paper machine from a mixture of fibres (which can be virgin or recovered fibres), chemicals and additives.
Each Paper or board grade is produced on equipment tailored for this particular grade and mill. Tissue machines producing toilet or napkin paper, or paperboard production differ considerably from the newsprint production process and newsprint Paper machine. Production processes are optimised for each grade. There are many variables: raw material composition (mixture of chemical softwood and hardwood pulp, mechanical pulp, recovered paper, fillers, pigments, additives, etc.), machine size (width, speed), type of production equipment, and automation level.
The Paper production line
All Paper and board machines are based on a similar basic process. There are seven distinct sections: head box, wire section (wet end), press section, drier section, size press, calender and reel-up.
Raw material fibres and chemicals (and 99 % of water) are pumped to the head box, which feeds the stock evenly onto the wire section. This is a woven plastic mesh conveyor belt that can be 35 metres long and as wide as the machine. As the Paper stock flows from the head box onto the wire, the water drains away through the mesh leaving tiny fibres as a mat on top of the mesh.
The Paper machine can travel at speeds of up to 2000m/minute and by the time the Paper stock has traveled half way down the wire, a high percentage of water has drained away. By the time the thin mat of fibres has reached the end of the wire section, it has become a sheet of paper, although very moist and of little strength. It then passes to the press section.
This consists of a number of sets of heavy cylinders through which the moist Paper passes. More moisture is squeezed out and drawn away by suction. The Paper then passes to the drier section. This consists of a large number of steam-heated drying cylinders which have a temperature of slightly over 100°c. Synthetic drier fabrics carry the web of Paper round the cylinders until the Paper is completely dry.
Part way down the bank of drying cylinders is the size press, where a solution of water and starch can be added in order to improve the surface for printing purposes. Instead of sizer, a coater can be used which is what happens when coated papers are produced.
At the end of the drying process, the Paper is smoothed using an "ironing" method, which consists of hot polished iron rollers mounted in pairs, one above the other (calenders or soft calenders). This also helps to consolidate, polish and glaze the surface of the paper.
Still traveling at very high speeds, the Paper comes off the machine ready for reeling up into large reels (also called parent reels), which can be cut or slit into smaller ones, according to customer requirement. These large reels are produced and changed without any interruption to the production process.
High-quality printing papers are coated (by using clay and other pigments) several times. In such a case, the coater is off-machine (not integral to the Paper machine). The Paper machine may also have an on-machine coater for pre coating.
These coated papers are often very smooth and glossy and this is achieved by using super calenders (alternate ****l and composition rolls). Unlike calenders, traditionally these have not been an integral part of the Paper machine but they are now sometimes being installed into the drying section of the Paper machine itself, to save production costs.
The board machine often has several wet ends (head boxes and wires) producing multiply sheet, combined on the forming table and press. Basis weight of the boards can be as high as 500 g/sq .m, whereas the printing and writing papers are usually 40-120 g/sq .m.
Paper and board machines are each different - the size of the production capacity and technology varies. Each one is tailored to the specification of the Paper mill. There are currently three worldwide Paper machine suppliers (Valmet, Voith Sulzer and Beloit-Mitsubishi), which deliver Paper machines for all Paper grades.
The Paper machine
Forests and forest products, such as paper, form part of an integrated carbon cycle based on photosynthetic conversions of water, carbon dioxide, nutrients and solar energy into a renewable woody biomass. Once used, most Paper products, when separately collected, can begin a new life as a secondary raw material. They can also be used as a bio fuel if they are not fit for recycling. This continuous process means that the forest provides a renewable source of raw material and that the eco-cycle is balanced
Trees for papermaking
Wood is the primary raw material for the Pulp and Paper industry because it is the main source of cellulose fibre. Non-wood materials, for example cotton and cereal crops such as straw can be used for papermaking, but the advantage of year round wood supply, combined with the product diversity made possible by mixing wood fibres, has made it the most practical and economic option.
The Paper industry once depended almost entirely on softwoods such as spruce, pine, larch, fir and cedar. Now birch, aspen and other hardwoods are used as an ideal raw material for packaging applications like fluting (the middle layer in corrugated cases) as well as printing and writing papers. Eucalyptus, originally cultivated only in Australia and New Zealand, has been successfully cultivated in other warm climates (e.g. South America, Spain and Portugal) and is farmed like a crop to provide high-quality pulp, which is suitable for a wide range of papers. Softwoods provide longer fibres (average 3mm compared with 1 mm for hardwoods) and continue to be used for papers that need the greatest strength.
The Paper maker will often mix a variety of pulps, with different characteristics in any particular type of Paper or board.
Pulpwood used to come from whole, mature trees. Now, the Paper maker is increasingly making more use of small dimension wood, which is unsuitable for other industrial applications such as construction and furniture making. Sawmill waste (chips and sawdust), as well as forest thinnings (trees extracted from the forest at various intervals throughout the growing cycle to enable those remaining to grow to healthy maturity) are also valuable raw material sources.
Chemical / Additives
Chemicals and additives improve the properties of paper
The basic "raw material" chemical used in papermaking is cellulose. Cellulose is a polysaccharid and is the main constituent of the cell wall of plants. It is the most widely distributed organic compound and has a similar structure to sugar. Cellulose can be found in all plants, and is important in papermaking because of its ability to create a sticky mesh on contact with water, which then forms the basis of the sheet of paper. The Paper maker has to extract the cellulose from the plant. As it is not water-soluble, it has to be extracted by using an aggressive, beating method known as pulping.
Typical additives used in papermaking include starch, which gives better surface strength; and China clay, talc and calcium carbonate, which improve the opacity and brightness of paper, as well as the final printing process.
The additives can be applied after Paper production - e.g. onto the Paper surface. In this case, the process is called "coating".
Increasingly, large volumes of used Paper are recovered for recycling. Before the recovered Paper can be used to manufacture new "white" grades of paper, the printing inks have to be removed. A chemical process using alkali and detergents is used. Recovered Paper is dissolved in water and separated from the non-fibre impurities. The fibres are then progressively cleaned in order to obtain the pulp.
For certain uses, e.g. production of graphic papers, the Pulp is also de-inked to increase whiteness and purity. In this process the ink is removed in a flotation process where air is blown into the solution. The ink adheres to bubbles of air and rises to the surface where it is separated.
After the ink is removed, the fibre may be bleached, usually with hydrogen peroxide. The contraries (e.g. pins, staples, adhesives, etc.) removed during the recycling process create a "sludge" which may be burnt for energy recovery, disposed of in landfill or, in some circumstances, used as a land application.
The De inking process