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Wood species and raw materials

 

Wood from trees, the primary raw material for pulp and paper products, is a renewable natural resource. Only in North America there are nearly 1000 pulp and paper mills. Some mills produce and sell pulp as a product. Others are integrated mills that produce pulp and then paper. Some paper mills buy pulp as a raw material and make paper. Recycling mills take paper waste and make paper. The pulp and paper industry exists in many parts of the world.

The prime ingredient of wood, which makes it useful as a raw material, is cellulose, a long chain polymer which forms the wall structure of the wood cell, or fiber. Trees grow as a result of the natural process of photosynthesis in which the energy from the sun converts atmospheric carbon dioxide into organic compounds, such as cellulose, releasing oxygen.

The wood cell, or fiber, varies in length from 0.5 mm to over 4 mm. The center of the cell is hollow. Fiber length is a key property for paper making and is determined by the wood species which divide into two main types: softwoods and hardwoods. Softwoods are mainly coniferous trees which have needles instead of leaves, such as pines and firs. These species have long fibers: typically 3 mm long and about 0.03 mm wide. Hardwoods are primarily the deciduous trees with leaves, such as maple, oak, and aspen. Their fibers are short (typically 0.5 mm).

The fiber structure consists of many layers of bundles of cellulose molecules. Wood also contains hemicellulose, another polymer with chain lengths shorter than cellulose. The cell structure is bonded together by lignin which acts like a glue. The process of removing fibers from solid wood is called "pulping." There are two processes for pulping: mechanical and chemical. Mechanical pulping involves "ripping" the fiber out of the solid wood structure by mechanical and thermal means. Mechanical pulp fibers include all of the above ingredients, hence the pulping process has a very high yield (90%+). However, due to the nature of the pulping process there is fiber damage with a loss in strength. Chemical pulping involves chemically dissolving the lignin fraction (and usually the hemicellulose fraction as well). This liberates the cellulose fraction, still in the shape of the original fiber, with relatively little damage and results in high strength and the potential for high brightness after all traces of lignin have been removed and the pulp has been bleached. The yield of chemical pulp is low, < 50% to 60%, because the lignin and hemicellulose have been dissolved. Typically these organic compounds are used as a fuel in a chemical pulp mill.

Pulp and paper products are of different types. There are newsprint, photocopier paper, corrugated boxboard, roofing felt etc.

Each of these products requires certain unique properties which must derive from the raw material. For instance, photocopier paper must have excellent brightness, printing surface and must not curl in the photocopier. Highly bleached chemical pulp is used. This type of paper is made by blending hardwood pulp (for a very smooth printing surface) with softwood pulp (for strength).

 

 

Manufacturing Processes

 

Pulp and paper mills vary greatly in design as they reflect the product being made. The simplest division is to separate pulping from papermaking.

 

1. Mechanical Pulping

Mechanical pulping can be achieved by grinding or refining. Groundwood mills mechanically grind whole logs against an abrasive surface. The pulping is a combination of raising the temperature and mechanically "ripping" the fiber from the wood surface. This is done by feeding logs to the "pockets" of grinders which are powered by large synchronous motors. A typical groundwood mill may have 10 to 20 grinders. The Thermal-Mechanical Pulping process is more modern way of mechanical pulping. It consists of feeding wood chips into the "gap" of rotating pressurized machines, called chip refiners, to produce pulp directly. Normally there are two refining stages. The chips disintegrate inside the refiner as they pass between the "teeth" of the refiner plates. Mechanical pulps can be brightened (bleached) to some degree.

Pulp grinding machinery is heavy, consumes a great deal of power and requires much auxiliary equipment and additional labour for maintenance. All these factors increase the cost of pulp per ton. The constant demand for greater grinder capacity at lower costs was the reason for the development of new types of grinders. When the ring type grinder was placed in operation, it showed quite satisfactory results from the start. This grinder has a massive cast iron ring 8 ft. in diameter which is mounted eccentrically round a standard pulp stone inside rectangular cast iron housing. The ring is rotated by an oil motor, and its speed can be varied to give varying pressures and consequently various grades of pulp. The output of pulp per machine equals that of several pocket grinders and in some cases – the production of 2 ma­gazine grinders. At 200 revolutions per minute, the ring type grinder produces about 25 tons of pulp in 24 hours.

At higher speeds such as 350 revolutions per minute, from 45 to 50 tons of pulp are produced in 24 hours.

From 1500 to 2500 hp. motors are used to drive this machine at above mentioned capacities.

 

2. Chemical Pulping

Chemical pulping can be divided into two main processes, sulfite and Kraft. The sulfite process is an acid based cooking process whose use is in decline. The Kraft process is an alkali-based process in which the active chemicals are fully recycled in the Kraft liquor cycle. The Kraft process itself consists of eight individual unit operations. It starts with wood chips being fed to a digester house in which the chips are fed to a digester, impregnated with white cooking liquor and "cooked" at about 175°C for about an hour. In this process the lignin and hemicellulose are dissolved. The spent cooking liquor is then extracted and the pulp is "blown" into the "blow" tank. Modern digesters are continuous vertical columns, with the chips descending down the column. The impregnation, cooking, and extraction processes take about three hours or so. Some digester houses use batch digesters instead, with 6 to 20 batch digesters.

Digesters are designed for periodical cooking of sulfite or viscose pulp with prehydrolysis from different species of wood. They are made either of acidproof steel or of carbon steel. These materials have shown a high corrosion resistance in exploitation.

The digester is a cylindrical vessel with top and bottom conical heads. It is mounted in vertical position on four support columns. The upper charging pipe (загрузочный патрубок) is provided with an easily removable mechanized cover with drive.

For preventing clogging of the circulation system, the digester has three screening zones, i.e. the intake (заборное), relief (сдувочное) and washout (вымывное) screens. The digester equipment also includes a chip packer (уплотнитель щепы), a collector for digester washing, and collector with nozzles (сопла) for pulp washing-out.

From the blow tank the pulp is pumped to the brown stock washers in which the pulp is washed in a multistage countercurrent washing process to remove the spent cooking liquor, including the dissolved organic compounds (lignin, hemicellulose) and the spent sodium compounds which must be removed from the pulp stream and reused. The two output streams consist of 1) washed pulp (which goes to the bleach plant for bleaching or to the paper machines if bleaching is not required) and 2) the spent liquor which is called black liquor and is returned to the Kraft liquor cycle.

The Kraft liquor cycle starts with pumping black liquor from the brown stock washers to the multiple effect evaporators. The black liquor solids are then fed to the recovery boiler in which the black liquor solids are further concentrated, and then the liquor is fired into the recovery boiler as a fuel. The bottom of this boiler contains a large smoldering char-bed of burning black liquor solids below which the sodium compounds form a molten pool of smelt. The smelt pours from the bottom of the recovery boiler into a dissolving tank where it is dissolved in water and becomes green liquor. The upper part of the recovery boiler is conventional in design and produces about 50% of the total steam consumed by the mill. The green liquor is pumped to the causticizing area where burned lime is reacted with the green liquor in order to re-constitute the white cooking liquor. The resulting white liquor is sent to the digester for cooking. The calcium carbonate is precipitated and sent to the lime kiln. The burned lime is used in the causticizing area to reconstitute the white cooking liquor.

 

 

Bleaching

 

After cooking the pulp may need to be bleached. This is necessary for all products which require high brightness or the complete removal of lignin. A typical bleach plant consists of the sequential application of specific chemicals each followed by a reaction vessel and a washing stage. A bleaching sequence, which has been used often in the past, involves pumping unbleached brown stock from the pulp mill and applying the following chemicals in turn: chlorine (to dissolve lignin), caustic (to wash out lignin by-products), chlorine dioxide (to brighten), caustic (to dissolve by-products), and finally chlorine dioxide to provide final brightening. Bleach plant technology is currently in a state of flux as a result of the environmental impact of chemicals, such as chlorine. As a result the industry is moving towards new chemicals, chiefly oxygen and hydrogen peroxide.

The equipment used for bleaching operations consists primarily of closed tanks into which the pulp is pumped in water suspension after being mixed with bleaching chemicals. The pulp is carried by water throughout most of the bleaching operations.

The washing is carried out in rotary drum washers.

Not all pulps are bleached with multistage bleach sequence. Groundwood pulps frequently receive only one stage bleach.

If unbleached pulp is desired, the groundwood pulp will not require any further treatment. It is possible to bleach the groundwood pulp to improve the whiteness and permanence of the paper to be produced, but because of the presence of lignin in the fibers the quality can never be raised to the level of chemical pulps.