Since the paper cycle cannot be kept up and running without the addition of significant amounts of virgin fibres wood will remain a major raw material source for the paper industry. In recent years, however, wood has become a strongly sought after raw material for many industries which already resulted in rising prices and at least local scarcities. The most responsible and complete utilization of this resource should therefore be given top priority by all stakeholders. As far as the paper industry is concerned mechanical pulping technologies, in particular TMP and CTMP allow – with a yield of well above 90 % – for an almost entire exploitation of the wood mass. These pulps are not only highly resource-efficient but also procure very favourable properties – high opacity and reasonable strength – which make them particularly suitable for the production of lightweight graphic papers.

RPM and Technische Universität Dresden developed a method which aims at net energy savings of at least 30 % for the production of TMP. This is achieved by irradiation treatment of wood chips called Energy Efficient Thermo Mechanical Pulping (ETMP).Please find some results from our recent stuides in the following figures.Figure 1 shows the CSF-values (Canadian Standard Freeness) as a function of the SEC (Specific Energy Consumption). Spruce chips exhibited a strong dependency on the irradiation dose: the higher the dose was, the lower was the SEC needed to achieve a certain CSF.

Figure 1: Drainage resistance (CSF) as a function of SEC and irradiation dose for spruce and aspen

For a target-CSF of 200 ml the energy consumption as compared to non-irradiated spruce chips was 25 % lower for 30 kGy, 40 % for 60 kGy and 6-8 % for 15 kGy. Static strength properties obtained from hand sheets were as well affected by the irradiation intensity to which the chips had been exposed prior to refining. The correlation between tensile index and SEC is shown in Figure 2. The increase in tensile index with increasing refining energy was as expected. The development of the tensile index, however, was faster and steeper for the irradiated wood chips, i. e. a specific target value for the tensile index can be achieved at less energy. This behavior is similar for spruce and aspen, though on a lower level in the case of aspen.

tensile vs

Figure 2: Tensile Index as a function of SEC and irradiation dose for spruce and aspen

Figure 3 shows the development of dynamic strength properties on the example of the tear index. Not unexpectedly tear index values increase with increasing energy input until a maximum after which they drop. The initial increase in tear is steeper than with non-irradiated chips and the maximum is achieved at lower SECs.

Figure 3: Tear Index (Brecht Imset) as a function of SEC and irradiation dose for spruce and aspen

Tear index of a pulp strongly correlates with the mean fibre length. That is the main reason why tear indices for aspen are much lower than those for spruce. The irradiated aspen developed significantly better, in contrast to the reference – a huge advantage compared to spruce.

The energy demand for an irradiation treatment with 30 kGy is 65 kWh/t. An Accelerator inclusive a state of the art shielding needs space compared to a single family house.

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