Research is under way to develop bio pulping (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. In the bio pulping process, the fungal enzyme lignin peroxidase selectively digests lignin to leave remaining cellulose fibres. This could have major environmental benefits in reducing the pollution associated with chemical pulping. The pulp is bleached using chlorine dioxide stage followed by neutralization and calcium hypochlorite. The oxidizing agent in either case oxidizes and destroys the dyes formed from the tannins of the wood and accentuated (reinforced) by sulfides present in it.
Steam exploded fibre is a pulping and extraction technique that has been applied to wood and other fibrous organic material.
Bleaching
The pulp produced up to this point in the process can be bleached to produce a white paperproduct. 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 chlorine dioxide, oxygen, ozone and hydrogen peroxide.
Alternative to wood pulp
Today, some people and groups advocate using field crop fibre or agricultural residues instead of wood fibre as being more sustainable.
There is enough straw to meet much of North America's book, magazine, catalogue and copy paper needs. Agricultural-based paper is a guaranteed way to reduce the stress of paper production on old growth and endangered forests. Some agricultural residue pulps take less time to cook than wood pulps. That means agricultural-based paper uses less energy, less water and fewer chemicals. Pulp made from wheat and flax straw has half the ecological footprint of pulp made from forests.
However, wood is also a renewable resource, with about 90 percent of pulp coming from plantations or reforested areas. Non-wood fibre sources account for about 5–10 percent of global pulp production, for a variety of reasons, including seasonal availability, problems with chemical recovery, brightness of the pulp etc. Non-wood pulp processing requires a high use of water and energy.
Nonwovens are in some applications alternatives to paper made from wood pulp, like filter paper or tea bags.
Market pulp
Market pulp is any variety of pulp that is produced in one location, dried and shipped to another location for further processing. Important quality parameters for pulp not directly related to the fibres are brightness, dirt levels, viscosity and ash content. In 2004 it accounted for about 55 million metric tons of market pulp.
Air dry pulp
Air dry pulp is the most common form to sell pulp. This is pulp dried to about 10 percent moisture content. It is normally delivered as sheeted bales of 250 kg. The reason to leave 10 percent moisture in the pulp is that this minimizes the fibre to fibre bonding and makes it easier to disperse the pulp in water for further processing to paper.
Roll pulp
Roll pulp or reel pulp is the most common delivery form of pulp to non traditional pulp markets. Fluff pulp is normally shipped on rolls (reels). This pulp is dried to 5–6 percent moisture content. At the customer this is going to a comminution process to prepare for further processing.
Flash dried pulp
Some pulps are flash dried. This is done by pressing the pulp to about 50 percent moisture content and then let it fall through silos that are 15–17 m high. Gas fired hot air is the normal heat source. The temperature is well above the char point of cellulose, but large amount of moisture in the fibre wall and lumen prevents the fibres from being incinerated. It is often not dried down to 10 percent moisture (air dry). The bales are not as densely packed as air dry pulp.
Environmental concerns
The major environmental impacts of producing wood pulp come from its impact on forest sources and from its waste products.
Forest Resources
The impact of logging to provide the raw material for wood pulp is an area of intense debate. Modern logging practices, using forest management seek to provide a reliable, renewable source of raw materials for pulp mills. The practice of clear cutting is a particularly sensitive issue since it is a very visible effect of logging. Reforestation, the planting of tree seedlings on logged areas, has also been criticized for decreasing biodiversity because reforested areas are mono cultures. Logging of old growth forests baccounts for less than 10 percent of wood pulp, but is one of the most controversial issues.
Effluents from pulp mills
The process effluents are treated in a biological effluent treatment plant, which guarantees that the effluents are not toxic in the recipient.
Mechanical pulp is not a major cause for environmental concern since most of the organic material is retained in the pulp, and the chemicals used (hydrogen peroxide and sodium dithionite) produce benign byproducts (water and sodium sulfate (finally), respectively).
Chemical pulp mills, especially kraft mills, are energy self-sufficient and very nearly closed cycle with respect to inorganic chemicals.
Bleaching with chlorine produces large amounts of organochlorine compounds, including dioxins.
Odor Problems
The kraft pulping reaction in particular releases foul-smelling compounds. The hydrogen sulfide reagent that degrades lignin structure also causes some demethylation to produce methanethiol, dimethyl sulfide and dimethyl disulfide. These same compounds are released during many forms of microbial decay, including the internal microbial action in Camembert cheese, although the kraft process is a chemical one and does not involve any microbial degradation. These compounds have extremely low odor thresholds and disagreeable smells; a common saying in communities economically dependent on nearby papermills is "Smells like a job" or "Smells like money."
Applications
The main applications for pulp are paper and board production. The furnish of pulps used depends on the quality on the finished paper. Important quality parameters are wood furnish, brightness, viscosity, extractives, dirt count and strength.
Chemical pulps are used for making nanocellulose.
Speciality pulp grades have many other applications. Dissolving pulp is used in making regenerated cellulose that is used textile and cellophane production. It is also used to make cellulose derivatives. Fluff pulp is used in diapers, feminine hygiene products and nonwovens.
Paper Production
The Fourdrinier Machine is the basis for most modern pape rmaking, and it has been used in some variation since its conception. It accomplishes all the steps needed to transform a source of wood pulp into a final paper product.
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