The timber resources used to make wood pulp are referred to as pulpwood. Wood pulp comes from softwood trees such as spruce, pine, fir, larch and hemlock and hardwoods such as eucalyptus, aspen and birch. A pulp mill is a manufacturing facility that converts wood chips or other plant fibre source into a thick fibre board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods (kraft and sulphite processes).
The finished product may be either bleached or non-bleached, depending on the customer requirements. Wood and other plant materials used to make pulp contain three main components (apart from water) cellulose fibres (desired for papermaking), lignin (a three-dimensional polymer that binds the cellulose fibres together) and hemicelluloses, (shorter branched carbohydrate polymers). The aim of pulping is to break down the bulk structure of the fibre source, be it chips, stems or other plant parts, into the constituent fibres.
Different pulping processes and types of pulps are presented below:
i. Mechanical Pulping:
Manufactured grindstones with embedded silicon carbide or aluminum oxide can be used to grind small wood logs called “bolts” to make stone ground wood pulp (SGW). If the wood is steamed prior to grinding, it is known as pressure ground-wood pulp (PGW). Most modern mills use chips rather than logs and ridged metal discs called refiner plates instead of grindstones.
If the chips are just ground up with the plates, the pulp is called refiner mechanical pulp (RMP) and if the chips are steamed while being refined the pulp is called thermo mechanical pulp (TMP). Steam treatment significantly reduces the total energy needed to make the pulp and decreases the damage (cutting) to fibres. Mechanical pulps are used for products that require less strength such as newsprint and paperboards.
ii. Hybrid Pulping:
The various mechanical pulping methods, such as Ground Wood (GW) and refiner mechanical (RMP) pulping physically tear the cellulose fibres one from another. Much of the lignin remains adhering to the fibres. Strength is impaired because the fibres may be cut. There are a number of related hybrid pulping methods that use a combination of chemical and thermal treatment to begin an abbreviated chemical pulping process, followed immediately by a mechanical treatment to separate the fibres.
These hybrid methods include thermo mechanical pulping and chemi-thermo-mechanical pulping, also known as CTMP. The chemical and thermal treatments reduce the amount of energy subsequently required by the mechanical treatment and also reduce the amount of strength loss suffered by the fibres.
iii. Thermo-Mechanical Pulping:
It is a pulp produced by processing wood chips using heat (thermo) and a mechanical refining movement (mechanical). It is a two stage process where the logs are first stripped of their bark and converted into small chips. These chips have a moisture content of around 25- 30 per cent and a mechanical force is applied to the wood chips in a crushing or grinding action which generates heat and water vapour and softens the lignin thus separating the individual fibres. The pulp is then screened and cleaned, any clumps of fibre are reprocessed.
This process gives a high yield of fibre from the timber (around 95 per cent) and as the lignin has not been removed, the fibres are hard and rigid.
iv. Chemical Pulping:
Chemical pulp degrades the lignin and hemicellulose into small, water-soluble molecules which can be washed away from the cellulose fibres without depolymerizing the cellulose fibres (chemically depolymerizing the cellulose weakens the fibres). Chemical pulp is produced by combining wood chips and chemicals in large vessels known as digesters where heat and the chemicals break down the lignin, which binds the cellulose fibres together, without seriously degrading the cellulose fibres.
Chemical pulp is used for materials that need to be stronger or combined with mechanical pulps to give a product different characteristic. The kraft process is the dominant chemical pulping method, with sulfite process being second. Historically soda pulping was the first successful chemical pulping method.
v. Chemi-Thermo-Mechanical Pulping:
Wood chips can be pretreated with sodium carbonate, sodium hydroxide, sodium sulfite and other chemicals prior to refining with equipment similar to a mechanical mill. The conditions of the chemical treatment are much less vigorous (lower temperature, shorter time, less extreme pH) than in a chemical pulping process since the goal is to make the fibres easier to refine, not to remove lignin as in a fully chemical process. Pulps made using these hybrid processes are known as Chemi-Thermo-Mechanical Pulps (CTMP).
vi. Recycled Pulping:
It is also called Deinked Pulp (DIP). DIP is recycled paper which has been processed by chemicals, thus removing printing inks and other unwanted elements and frees the paper fibres. The process is called deinking. DIP is used as raw material in papermaking. Many newsprint, toilet paper and facial tissue grades commonly contain 100 per cent deinked pulp and in many other grades, such as lightweight coated for offset and printing and writing papers for office and home use, DIP makes up a substantial proportion of the furnish.
Some other pulps and pulping methods are:
i. Organosolv Pulping:
It uses organic solvents at temperatures above 140° C to break down lignin and hemicellulose into soluble fragments. The pulping liquor is easily recovered by distillation.
ii. Alternative Pulping Methods:
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. 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.
iii. Market Pulp:
It 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. Pulp has high market potential in International market.
iv. Air Dry Pulp:
It is the most common form to sell pulp. This is pulp dried to about 10 per cent moisture content. It is normally delivered as sheeted bales of 250 kg. The reason to leave 10 per cent 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.
v. Roll Pulp or Reel Pulp:
It 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 per cent moisture content. This will be used in further processing.
vi. Flash Dried Pulp:
It is prepared by pressing the pulp to about 50 per cent moisture content and then let it fall through soils 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 per cent moisture (air dry). The bales are not as densely packed as air dry pulp.