Today, wood chips constitute the main source of fibres used in pulp and paper mills. These chips represent over 70 % of the total supply while only 20 years ago, this raw material was considered to be a residue of the sawmill industry. Other fibre sources include some pulpwood, which is sent directly to the paper mill, as well as recycled paper and paperboard.

Pulpwood Preparation

When the wood is sent directly to the pulp and paper mill, it must be stripped of its bark. This process is often carried out inside enormous drums where the logs are tumbled together. This operation may be done with or without water (wet stripping).

The bark is recovered and used as fuel to produce steam, which the mill needs a great deal of at various stages of the paper making process. Moreover, the water used for wet stripping is usually recycled water as it comes from other sectors of the mill. In addition, water collected at the end of this operation is either reused or sent on to the process water treatment equipment.

Pulp Processing

Wood is composed of cellulose fibres that are cemented together with a substance called lignin. To transform wood into pulp, the fibres must by separated. This separation is done using either a mechanical or chemical process. Depending on the paper or paperboard desired, different types of pulp are produced and mixed into varying proportions.

Mechanical pulp is manufactured by crushing the stripped logs with huge grindstones, which are called grinders, and soaking them with water. When wood chips are ground in refiners using steam at high pressures and temperatures, characteristics of the pulp are different. This is called thermomechanical pulp (TMP). Chemithermomechanical pulp (CTMP) is obtained when chemicals are introduced, in addition to the steam. Generally speaking, mechanical type pulps are used to manufacture products requiring less strength, such as newsprint and paperboards.

In the preparation of chemical pulp, wood chips are cooked in large pressure reactors (batch or continuous digesters) with cooking chemicals (called white liqour: Na2S and NaOH). The combined action of the heat and the chemicals dissolves the lignin, releasing the long wood fibres without breaking them. Chemical pulp is therefore used to manufacture products requiring greater strength, like printing and writing papers. One such chemical type pulp is the kraft pulp, also known as sulphate pulp. Because it holds a brownish color, this type of pulp must often be bleached.

Manufacturing recycled pulp, from waste paper and paperboard begins in huge grinders called pulpers. These are large vats where water, steam and, occasionally, chemicals are added to the paper mixture. First, contaminants such as paper clips, plastic ties and elastic bands must be removed. The pulp is then sifted to eliminate unwanted particles. Recycled pulp is most often used to make paperboard or sanitary paper. On the other hand, recycled pulp is also used to make other types of products where it must be mixed with one or more types of virgin pulp. When the quality of the product requires it, the recycled pulp may also be deinked. Chemicals or soaps are used to dislodge the ink from the wood fibres. The ink is then extracted from the pulp by washing or by flotation.

At the pulp making stage, many measures are taken to ensure the protection of the environment. For example, chemicals used during the Kraft pulp process, which are often referred to as black liquor, are recovered as much as possible. Usually, they are reused in the manufacturing process or for energy purposes. Moreover, steam produced during the cooking of the wood chips is collected and condensed. A significant part of the liquid's pollutants is then removed before being sent on to the wastewater treatment equipment. Also, the heat generated during the pulp making process is recovered as much as possible and reused in the mill. This has allowed pulp and paper manufacturers to considerably reduce their water consumption in recent years.

When waste paper and paperboard is made into recycled pulp, about 15 % to 20 % of the secondary fibres are generally too short to be reused in the manufacturing process. These fibres are thus collected and used for other purposes. The most popular option, because of its economic and environmental benefits, is to use these residues for energy purposes. Several projects are also underway to develop uses for residues generated through wastewater treatment and deinking. Research has been intensified notably in the areas of composting and soil amendment.

Pulp Bleaching

Certain types of paper and paperboard require that the pulp be bleached. Chemicals used for this purpose dissolve and further eliminate the lignin, the natural glue that binds wood fibres. The resulting product is not only whiter but also more resistant and absorbent. Without bleaching, the paper would become brittle and yellow with time, like old newspapers for instance.

In terms of its bleaching practices, the pulp and paper industry has been called upon to clearly demonstrate its commitment to protecting the environment. In the mid 1980s, traces of toxic substances were detected in the effluent of certain mills. An investigation revealed that the source of these contaminants was the chlorine used in the bleaching process. Following intensive research and massive investments, the industry succeeded in considerably reducing discharges of these unwanted substances. This turnaround was made possible by replacing chlorine with other bleaching agents, like chlorine dioxide and hydrogen peroxide, by reducing the use of chemicals, by improving pulp cleaning systems, and by implementing process water secondary treatment systems in several mills.

Sheeting and Drying

When the pulp mixture arrives at the paper machine headbox, its water content is very high (over 99 %). The mixture is therefore sent under pressure onto an immense moving canvas. The action of the canvas combined with that of a suction system serves to extract most of the water contained in the pulp and to gradually form a sheet. The sheet then moves on to the drying section where, coming into contact with immense steam-heated cylinders, it is dried to the desired degree.

As an energy efficiency measure, the steam is collected in the drying section and sent to special equipment that recover the heat. The excess steam forms immense white clouds that we often see floating over mill stacks. To save yet more energy, mills also recover hot water condensed in the dryer-cylinders and convert it to steam.

Finishing and Shipping

Once dry, the sheet of paper passes between immense rolls of heated steel, called calenders. These cylinders compress and smooth the surface of the sheet. This is where the product is given its final features. They will vary considerably depending whether the product is newsprint, fine paper, sanitary paper, glossy paper, paperboard, etc. Each client has specific demands, which require a great deal of flexibility on the part of the mills.

The product generally leaves the mill, by train, van or ship, a few days after it is made.

Paper Finishing.