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Look on the bright side

ST. LAURENT Paperboard was the focus of the first two papers presented at the bleaching session held Thursday afternoon. Jean Bouchard, Paprican, used the mill in a case study on brightness reversion ...


March 1, 1999
By Pulp & Paper Canada

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ST. LAURENT Paperboard was the focus of the first two papers presented at the bleaching session held Thursday afternoon. Jean Bouchard, Paprican, used the mill in a case study on brightness reversion and brightness loss in fully-bleached kraft pulp. The La Tuque, QC, mill produces about 400 000 tonnes per year (t/y) of liner and bleached board. It experienced significant brightness loss with its bleached softwood kraft pulp, used as the top layer of the mill’s white top linerboard. Researchers found that there were two contributors to the brightness loss: blue dye used to eliminate yellowness in the pulp in white water, and, reversion.

An investigation of the white water showed the brightness loss originated from a complex formed from rosin size, lignosulphonate and the blue dye, which then deposits on the white top. By eliminating transition metals, extractives and residual lignin, factors responsible for reversion, the researchers identified carbonyl groups as the source. Borohydride reduction and hydrogen peroxide oxidation of these groups under conditions that can be applied in the last retention tower can increase brightness and decrease reversion.

The researchers recommended that: the rosin-alum complex be injected before the blue colloidal dye to maximize rosin size retention, and; the mill add the blue colloidal dye with an organic coagulant to maximize its retention.

The paper was co-authored by M. Polverari and E. Morelli, Paprican, and P. Gagnon and R. Picotte of St. Laurent Paperboard. It was Gagnon who took the podium next in a paper which complemented Bouchard’s.

At the same time as the work with Paprican was being done, the mill wanted to address upcoming AOX regulations and ECF bleaching as a possibility. A trial that determined the effect of going from 80% to 100% chlorine dioxide substitution was done in the bleach plant that feeds the white top machine. The trials are related.

The mill had suffered a brightness loss of six to eight points from the end of the bleach plant to the machine. Caustic soda and H2O2 were added to the repulper of the last washer. The purpose was to increase the pH of the pulp to the range (9-10) where hydrogen peroxide is most efficient and to have a residence time long enough (minimum two hours) for the reaction to take place.

The trial (different concentrations of caustic and H2O2) was done in six 24-h steps. Assessments were done after each test so optimal conditions could be determined. It was found that for peroxide to be efficient, pulp pH had to be increased. Adding 1.9 kg/t H2O2 to the pulp at the last washer reduced brightness loss from as high as eight points to 3.5 to 4 points.

The ECF bleaching trial was done two months later (April 1998). The new AOX limits take effect at the end of 2000. They will be a 30-day rolling average of 0.8 kg/t and a daily limit of 1 kg/t. The trials determined that it would be possible to achieve the required standards. What about the effect on the pulp’s optical properties? There was some brightness loss and extra pigment had to be added to meet targets. The method developed by Paprican was successfully transferred to the mill. The mill is converting one machine from acid to alkaline papermaking that should make the hydrogen peroxide option more attractive. Gagnon noted that as hydrogen peroxide is added, the mill will have to study what effect it will have on retention and other parameters, and if new chemicals will have to be added.

Barbara van Lierop, Paprican, made a welcome return to the research community after two very successful years as chairman of the Technical Section. Her paper: Using high temperatures in the extraction stage of a bleaching sequence: What’s the advantage? was co-authored by M. Faubert, S. Labonte and D. Pitre, all of Paprican.

As the use of ECF bleaching processes increases, mills still need to produce high-quality pulp that is environmentally friendly while keeping their practices flexible. Many mills use a pressurized oxygen-peroxide stage to allow mills to eliminate dioxin formation while overcoming the inefficiencies of bleaching with ClO2 in the first bleaching stage.

However, one drawback is the high cost of a pressurized reactor. Paprican has developed a short, high-temperature step (PHT) that decreases the size of the reactor. The temperature in the reactor was increased to 110C from 70C.

Applying high temperature s for the first 10 minutes of a 60-minute extraction stage can decrease the kappa number of a softwood kraft pulp by as much as 30%. An EoHT stage at 110C after Do produced kappa number equivalent to an Eo stage at 70C while using 5 kg less chlorine dioxide. The tear tensile curve was not affected. It was found that peroxide is best used at the end of the sequence rather than after the extraction stage because brightness gains obtained after the extraction stage are not retained after the next chlorine dioxide bleaching stage. Using it at the end also helps lower OX levels.

The development is economical, efficient, environmentally friendly and does not affect product quality. In response to a question, van Lierop said she doesn’t believe that pulp yield decreases as a result of using high temperatures but that issue will be the subject of the next study. The work estimated the cost of steam (used to increase temperatures) at $4/t and at this figure, cost savings of $0.5 million/y could be realized.


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