Today, the use of recycled paper is established in many areas and the volume produced around the world is increasing all the time. This is mainly because optimized manufacturing processes have led to an improvement in the quality of the end product: it is now possible to produce high-quality products from almost 100 per cent waste paper. This is a positive development, because recycled paper has the clear advantage that it conserves resources. In contrast to paper produced from virgin fibers (primary wood pulp), recycled material requires almost no trees to be felled and in some cases none at all. Energy and water consumption is therefore also lower.
Everything would seem to be perfect
When paper is made from virgin fibers, the lignin is usually first separated in a chemical process using sulfates or sulfites. These substances are salts of sulfuric acid or sulfurous acids, which are able to break down the cell walls of the wood fibers, removing the lignin. Lignin occurs naturally in the fibers and prevents the paper from becoming bright white. If it is not removed, the paper also turns yellow more quickly.
In the sulphate process, the wood chips are boiled in an alkaline solution before being shredded and turned into paper pulp. Breaking down lignins produces hydrogen sulfide and mercaptans. In the less common sulfite process, the chips are boiled in an acidic solution, but this only works with spruce, beech, and eucalyptus. This process creates a flue gas that contains sulfur dioxide.
In both processes, the contaminant gases produced must be reliably filtered out of the air, otherwise they lead to corrosion. They are harmful to human health and should not be inhaled.
Although it disposes of this first energy-intensive process, recycling paper also produces contaminant gases during the bleaching process, which removes unwanted discoloration from the pulp.
Paper is usually bleached using dithionite or chlorine dioxide, which produces sulfurous gases and chlorine gases. Bacterial starch degradation in the waste paper produces hydrogen sulfide, which is highly corrosive. This means that in this case too, solutions are required to prevent corrosion, protect staff, and ensure safe operation.
Gas phase filtration provides an economical solution by using specially adapted pellets to separate the exact gases that are produced in the specific process on site by means of adsorption, absorption, and chemisorption.
Monitoring systems such as the compact ChemWatch S can be used to continuously monitor the corrosive potential of the ambient air.
The level of corrosivity is represented according to corrosion class using replaceable copper and silver sensors. Since corrosiveness is influenced by the concentration of the contaminant gases and parameters such as temperature and relative humidity, these are recorded and shown separately as additional indicators. The values measured are clearly displayed on an online dashboard, allowing access from anywhere. The large number of variables measured provides a comprehensive picture of the conditions in the control rooms. If the parameters change or the corrosion class rises critically, it is possible to react quickly in order to achieve the best possible protection for the electronic components from corrosion. This results in a sustainably fail-safe, operationally reliable production environment for the pulp and paper industry.