The Paper Making Process, Part 3: The Process

Waste paper

Compared with producing a tonne of paper from virgin wood pulp, the production of one tonne of paper from discarded paper may use half as much energy and water. It results in 74% less air pollution, saves 17 pulp trees, reduces solid waste going into landfill sites and creates 5 times more jobs (Earth Care, 1988).

Thirty four per cent of the worlds pulp is derived from reclaimed paper (WRF, 1997), and it is estimated that it could contribute 30% of the needs of developing countries.

Recycling in the Paper

Industry Have you ever wondered what happens to a piece of paper when you recycle it? The paper industry is responsible for most of the recycling now taking place. 1993 was the first year in history in which more paper was recycled than was buried in landfills. But recycling is not as simple as it may seem…

Paper can be recycled only 5 to 8 times before the fibres in the paper become too short and weak to be reused. Old newspapers are commonly used to make tissue and cardboard, while magazines are often recycled into newsprint. Interestingly, the clay originally added to the paper to make it glossy will help to separate the ink from the paper during recycling.

How Paper is Recycled

First, the waste paper must be collected. One of the most expensive parts of recycling is the collection, sorting, baling, and transportation of waste paper. The next step in the recycling process is re-pulping. The bales of sorted waste paper are disintegrated in a hydro pulper, where they are reduced to individual fibres. Chemicals are added at this point so that ink particles, coatings and additives, and extremely small contaminants such as fillers start to separate from the paper. Depending on the required level of improvement, the pulp is sent through several stages, where heat, chemicals, and mechanical action may be used to further improve the pulp.

Finally, the pulp mixture enters a flotation device, where calcium soap and other chemicals are added. Air bubbles in the mixture float the remaining ink and contaminants to the surface as a scum, where it is skimmed away. The pulp is now sent to the stock preparation area, where it is treated and loaded into the flowbox* of a paper machine. From this point, the pulp is treated just the same as if it had been freshly made from any other raw material rather than recycled. At the end of the recycling process, a new paper product has been produced from material that might otherwise have been dumped in a landfill. Recycling is an important way for consumers and papermakers to work together for a cleaner environment.

Additives

Below we will look at some of the chemical additives used in the small-scale paper making industry. Many other chemicals are used for dying, tinting, cleaning and quality improvement.

Chemical and Application

  • Caustic soda (NaOH) – Used in the cooking or digestion* process in small mills.
  • Lime (CaOH) – Used for the cooking of low quality materials such as jute or old
  • rags.
  • Ammonia and calcium sulphate – Other chemicals used for the digestion of raw materials to form a pulp.
  • Chlorine – Used for bleaching paper. Chlorine is losing favor due to environmental pressures and is being replaced by other agents, such as hydrogen peroxide (H2O2), ozone or enzymes.
  • Hypochlorite – Also used for bleaching paper.
  • Alum – For pH correction, which is necessary for many of the finishing processes.
  • Rosin – Used for sizing* paper. Normally used in conjunction with alum as a “sizing system”.
  • Alkile ketene dimer – Now used as a sizing agent in place of the alum and rosin system.
  • Starch – To improve stiffness of paper and board.
  • China clay /chalk – A filler used to improve opacity, brightness, quality and finish of paper. Up to 20% clay is used for some grades of paper. Fillers are often cheaper than fibre and used liberally.
  • Talc – Can be used instead of clay where the where the pH is close to neutral. Also used to reduce “stickiness” of pulp.

The Paper Making Process

The process of making paper is based on the fact that wet cellulose fibers bind together when dried under restraint. The processing of paper usually involves the initial separation of the cellulose fibers to form a wet pulp, some form of treatment, such as beating and refining, while in the pulped state, to enhance the quality of the final product, then forming of the sheet paper by hand moulding or by paper making machine, and drying. Some further processing is often carried out before or during drying to acquire the desired finish.

It is worth remembering that paper production and the related technologies are often complex and sophisticated and only a brief overview with a few examples can be given in this document. In this section we will look at the stages involved in transforming raw materials into paper in a small-scale mill. The process is similar, whatever the raw material (or mixture of raw materials), and at whatever scale of paper production, but the complexity of the technology involved may vary considerably.

Delivery and Preparation

Depending on the size of the plant and the arrangements for procuring raw materials, deliveries will be made either by truck, or by collectors who deliver small quantities of recycled material. It is important to ensure that there is sufficient storage capacity for the raw material. This is particularly important where seasonally available raw materials, such as straw or bagasse*, are used and a large supply will have to be stored for later use. The storage requirement can be considerable. For example, a mill producing 10 tonnes of paper per day with a mixture of 70% straw pulp and 30% imported pulp, with 3 months storage capacity, will require an area of 2,000m3 and 16m high.

Straw preparation, for example, requires that the straw be cleaned to remove dust and cut into short lengths. Bagasse, on the other hand, will have been reduced to a suitable size at the sugar mill, but the pith will need to be removed. Wood will be chipped to an appropriate size. Specialized equipment is required for this kind of preparation. The material will then usually be transported to the pulping area on a conveyor belt or by hand.

On a global scale there is a large market in pulp. The world trade in ‘market pulp’ is enormous and large-scale plants will often use a mixture of market pulp and pulp from locally sourced raw materials.

source: www.practicalaction.org

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