Replacing chlorine in the paper bleaching process

1998/03/01 Etxaide, Maider Iturria: Elhuyar aldizkaria

• Ingurumena

The use of chlorine compounds in industrial paper bleaching processes presents important environmental and health problems. This is because part of chlorine reacts with organic molecules that contain woods forming organic chlorinated substances. Organochlorates, in addition to being very toxic, are very stable and have the capacity to accumulate in living beings. These three characteristics make organochlorines very dangerous to the biosphere.

The new demands of the paper market and the growing environmental sensitivity, or even legislation, have led many companies (more than 60 worldwide) to replace chlorine in their processes, some in its entirety others partially. Sweden is the seventh region in the world in paper and cardboard production, but it is also the country with more factories that have applied new systems (20 in total).

Bleaching process

The goal of bleaching is to remove lignin, a resinous substance attached to the cellulose layer. If this is not done the paper comes out weak and with few strips and also ages quickly. Chlorine derivatives have almost always been used for this task, which perfectly separate lignin without damaging cellulose fibers. In the Kraft process, the most common, after removing the skin from the wood, is cooked to 200ºC with caustic soda, sodium sulfate and calcium carbonate. The paste that is extracted is transferred to the bleaching unit where the bleaching and cleaning phases alternate, using the latter a caustic soda. Chlorine gas has traditionally been used for bleaching. Due to its high reactivity, about 10% of this chlorine is transformed into an organochlorine compound, from which 0.5% remains in mass and the remaining 90% becomes chloride ion. Chlorine dioxide and hypochlorite have been used as bleaching agents in the chlorine gas site, but they also do not prevent the problem of organochlorine compounds.

Less polluting alternatives

The application of alternative systems can be initiated in pre-bleaching phases, for example, in order to eliminate lignin before starting the bleaching process, prolonging the cooking time and increasing the pressure, ensuring that the fibers do not spoil during this process. This method is becoming more and more successful today.

Another method of this phase is the so-called Continuous Altered Cooking (MCC). This method consists of alternating steam at high and low pressure with the direction change of the cooking current in half phase. Consequently, lignin is less attached to the cellulose and in the oxygenation phase a greater viscosity is achieved in the mass that facilitates the separation of the rest of the lignin.

Oxygenation is used to reduce lignin shortly before bleaching. This operation is quite delicate because oxygen attacks cellulose. You have to be careful, therefore, if you want to look for profitability. As for the bleaching phase, ozone is a good whitening agent, although due to its instability it tends to degrade to oxygen. This system consists of a closed circuit that allows to recover oxygen and regenerate ozone. To increase the brightness of the mass you can use hydrogen peroxide. In this way, in addition to improving the quality of the pasta, the costs of bleaching are reduced.

Solvopulping is another bleaching process that involves separating lignin fibers from alcohol. This allows recycling alcohol and recovering lignin for other industrial uses. The fibers that are produced are quite pure and require little bleaching.

There are also variants of mechanical processes that use water vapor to soften wood, such as the so-called thermomechanical process (TMP). The so-called chemical/thermomechanical process (CTMP), in addition to water vapor, uses small amounts of chemicals producing a more resistant paste. With this system you can use both hard woods (eucalyptus, maple, birch, beech) and soft woods (pine and spruce). The former provide small fibers with high percentage of cellulose and the latter provide longer fibers that form a more resistant paste, although with more resins. The TMP system applies only to soft woods.

In general, starting with oxygen deslignification and continuing with hydrogen peroxide and ozone bleaching seems to be the most technologically attractive alternative process. This is at least the most marketable alternative.

Experience of a Swedish company

The Swedish company Södra, the world's largest producer of paper pulp, in the face of the growing demand of the chlorine-free paper market, decided to replace all the chlorine of the bleaching process. The craft process that this company uses today consists of long cooking, oxygenated deslignification and bleaching by ozone and hydrogen peroxide. To prevent peroxide from degrading against metals, a chelating agent that has the function of trapping metals is added.

The aim of this company was to be the world's first producer of chlorine-free bleached kraft paste from hard and soft woods. At the beginning of this path, however, it was considered that the non-use of chlorine had another advantage: this method allows closing the wastewater cycle of the entire process. In the traditional process, due to chlorine, it could not be achieved.

And in Euskal Herria what?

After a bad streak in the Basque Country, the new wind has begun to play. Some modifications are already being made in many factories, through the deposition of chlorine in bleaching processes and the introduction of hydrogen peroxide. However, in many areas of paper manufacturing, such as machine cleaning, weaning, turbines and pulp manufacturing, chlorine compounds are still used perfectly. Therefore, there is still way to go in the Basque paper sector.