Wood pulp

Wood pulp is the most common material used to make paper. The timber resources used to make wood pulp are referred to as pulpwood. Wood pulp generally comes from softwood trees such as spruce, pine, fir, larch and hemlock, but also some hardwoods such as eucalyptus and birch.

Contents

1 Manufacture of wood pulp 2 History 3 Environmental impacts 4 Alternatives

Manufacture of wood pulp

Wood pulp is made in several stages:

1. First the bark is removed from the wood. This can be done with or without water (wet stripping). The bark is generally recovered to use as fuel in the pulp and paper making process.
2. The cellulose fibres that keep the wood together are then separated. This can be done in a number of ways:
   • The wood can be crushed with grinders (huge grindstones) and then soaked in water to produce groundwood (GW). Mechanical pulps are used for products that require less strength, such as newsprint and paperboards.
   • The wood can be crushed with refiners using steam at high pressures and temperatures to produce thermomechanical pulp (TMP). TMP differs in quality from groundwood.
   • In additional to the refiners, chemicals can be used to break up the cellulose fibres. Pulp produced this way is known as chemithermomechanical pulp (CTMP). GW, TMP and CTMP are all considered as mechanical pulps. The mechanical pulps tend to turn yellow in time, because of the binding material, lignin, in the pulp.
   • Chemical pulp is produced by combining wood chips and chemicals in huge vats known as digesters. The effect of the heat and the chemicals dissolves the lignin, that binds the cellulose fibers together, without breaking the wood fibres. The fluid that contains lignin and other dissolved material is then dried and used as fuel. Chemical pulp is used for materials that need to be stronger or combined with mechanical pulps to give a product different characteristics. Chemical pulps include kraft pulp (or sulphate pulp).
   • Pulp can also be made out of waste paper and paperboard. Recycled pulp is most often used to make paperboard, newsprint or sanitary paper.
   • Research is under way to develop 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. This could have major environmental benefits in reducing the pollution associated with chemical pulping.
3. 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 oxygen, ozone and hydrogen peroxide.
4. The pulp mixture is now sent to the paper machine, where it is shaped and dried.

History

Using wood to make paper is a fairly recent innovation. In the 1900s, fiber crops such as linen fibres were the primary material source, but a shortage led to experimentation with other materials. Around 1850, a German named Friedrich Gottlob Keller crushed wood with a wet grindstone to obtain wood pulp. Further experimentation by American chemist C.B. Tilghman and Swedish inventor C.F. Dahl enabled the manufacture of wood pulp using chemicals to break down the fibres.

Environmental impacts

The major environmental impacts of wood pulping come from its impact on forest resources and from its waste by-products.

When the paper is bleached with elemental chlorine, byproducts such as chlorinated compounds such as [dioxins] and [furans] are formed, and in high pulping areas such as British Columbia, high concentrations of these contaminates led to the closures of some fisheries in 1992. However, improvements in technology have either eliminated the use of elemental chlorine through Elemental Chlorine Free (ECF) or Totally-Chlorine Free (TCF) technology these technologies reduced the amount of chlorinated compounds released into the environment. Elemental Chlorine Free technology utilies Chlorine Dioxide (ClO2) in place or Chlorine (Cl2). Total chlorine free bleaching utilizes no chlorine in the bleaching process.

The wastewater effluent can also be a major source of pollution, containing lignins from the trees, high biological oxygen demand (BOD) and dissolved organic carbon (DOC), along with alcohols, chlorates, heavy metals, and chelating agents. Reducing the environmental impact of this effluent is accomplished by closing the loop and recycling the effluent where possible, as well as employing less damaging agents in the pulping process.

In the Kraft process the largest volume byproduct from the pulping process is weak black liquor this liquor contains the pulping chemicals and lignin from the trees the lignin is high in heat content so this weak black liquor (about 15% solids) is concentrated into heavy black liquor (usually 68% to 75% solids) by use of multiple effect evaporation. Multiple effect evaporation is a process in which one pound of steam is used to boil 4.5 to 5.5 pounds of water. The heavy black liquor is burned in a recovery boiler and the chemicals fall to the bottom of the boiler in a semi-liquid state called smelt. The smelt then flows out of the boiler and is dissolved in water or weak wash to form green liquor. The green liquor is then clarified. Quick lime (CaO) is added to the clairified green liquor to convert a majority of the Sodium Carbonate (Na2CO3) to Sodium Hydroxide (NaOH). The green liquor with the quick lime is then clarified and the resulting liquid is white liquor. The white liquor is used as pulping chemicals and the process begins again. The spent lime (CaCO3) is then calcined at approximately 1800 degrees F to yield quick lime to be used again in the clarified green liquor.

Paper made from wood pulp can typically be recycled four to seven times before the fibres become too short. To solve this problem recycled paper is usually mixed with virgin wood pulp to ensure a high quality paper.

Alternatives

Today, some people and groups are advocating using field crop fiber instead of wood fiber as being more sustainable.

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