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From left to right: Ulf Scott, Andritz Product Manager Paper Engineered Services, Matthias Schuchardt, Foreman Mechanic Deinking, Frank Reinboth, Deinking Production Engineer, and Hubert Hermann, Andritz Senior Service Manager Dewatering & Bleaching in front of screw press #5 in Loop 2. In the background you see screw presses #6 and #7 also rebuilt in 2007/2008.
Stora Enso’s Sachsen Mill is a producer of high-quality standard newsprint paper. It is located in Eilenburg, Saxony, in the southeast of Germany, 150 km south of Berlin. It was originally built as a greenfield project in 1994.
In February 2008, the mill’s 320 employees celebrated a milestone – the production of 4,000,000 tonnes of newsprint in 14 years of operation. To put this in perspective, that is the equivalent of a one meter width paper web stretching to the moon and back 120 times.
Blue Angel productsSachsen’s PM1 is a 340,000 t/a machine which incorporates calendering technology from Andritz-Küsters. With a trim width of 9.21 m and an operating speed up to 1750 m/min, PM1 produces “NewsPress” (standard newsprint) and “FlyoPress” (high-gloss uncoated for commercial flyers and newspaper inserts) brands for Stora Enso.
Within only five months after start-up, all of Sachsen’s paper grades received the “ Blue Angel” label for eco-friendly products and services in Germany (see side story). This label is often an important feature for the mill’s printing house customers. But ecology is only part of the story.
“The paper industry is in a difficult situation today as energy and raw material prices are high,” says Ralph Wurlitzer, Sachsen’s Technical Manager. “Technical solutions that help us optimize our processes, improve efficiency, and reduce specific energy consumption are very important.”
Deinking for internal and external customersSachsen is a 100% recycled fiber mill. It converts 500,000 tonnes of household wastepaper per year. About half is collected in the region; the rest comes from its neighbors (e.g. Czech Republic and Poland) and from Nordic countries.
The deinking plant produces pulp for PM1 and also for sale to market. Each day, 1400 tonnes of wastepaper are fed into the two lines of the deinking plant. Large contaminants (such as CD’s inserted in a magazine) are removed in the Andritz FibreFlow® drum pulpers. Finer heavy contaminants (sand and staples) are screened from the stock in the coarse screens. Printing inks and very fine impurities (including stickies) are extracted during the deinking and cleaning stages. The pulp is then stored in a DIP tower for transfer to the paper machine approach flow system or it is dewatered in an Andritz twin-wire press (started up in 1994) before flash drying. The plant has two dewatering loops: Loop 1 has two disc filters and four screw presses, Loop 2 has two disc filters, three screw presses, and one twin wire press.
Dewatering bottleneckLoop 2’s three screw presses were creating a production bottleneck for the whole mill. “We have always had enough pulp to feed the paper machine,” says Frank Reinboth, Deinking Production Engineer, “but the plant could not deliver enough for the production of deinked market pulp, which achieved good prices. The screw presses were the bottleneck,” (The presses were not manufactured by Andritz.) Matthias Schuchardt, Foreman Mechanic Deinking, continues. “The wearing of the screws has always been a problem,” he says. “After several hard facings on the screws during shutdowns, we had reached the limits. Despite the repairs, we did not have optimum performance. The only alternative was to rebuild the screws to their original state or better.”
Long-lasting service relationshipThe business relationship between the Sachsen mill and Andritz started when the mill was built. In 2006, Sachsen asked Andritz to overhaul its screw presses in Loop 2. “To increase production, we wanted to increase the dryness content from each screw press,” says Reinboth. Sachsen also wanted Andritz to upgrade the mechanical back-pressure device on each screw press to pneumatic control and implement the new control strategy in the existing PLC.
Reinboth discussed options with Hubert Riemer, Andritz’s Global Product Manager for Dewatering & Bleaching. “The fascinating thing was that, even though the presses were not supplied by Andritz, Hubert very quickly found out the weak point, which was the geometry of the screw,” Reinboth says. “He immediately proposed a technical solution which included upgrading of the screw geometry in the high-pressure zone of each press so that the new back-pressure control would work efficiently.” Also, the worn-out screw flights needed to be reconditioned and prepared to equip them with Andritz wear protection.
“This solution was very clear and directly addressed the problems we were having,” Reinboth says. “In addition, Andritz guaranteed a capacity increase of 10%, which was much higher than offered by any other supplier. The choice was clear to go with Andritz.”
Sharp capacity increaseThe three screw presses were upgraded during successive maintenance shutdowns. A challenging aspect of the project for Andritz was that the upgrades were done on machines from another supplier. “Each rebuild project has its challenges,” explains Hubert Hermannn, Andritz Senior Service Manager for Dewatering & Bleaching. “Even though we had all the data and machine layouts, there can always be surprises when you actually open the machines.”
Screw press #5 was the first to be rebuilt in April 2007, as it showed the highest wear. After restarting the deinking line, the press immediately delivered the guaranteed 10% capacity increase. “We did some optimization work, for example in the control settings, and then we reached 20% production increase,” Reinboth recalls. “This was enormous.”
The rebuild of screw press #6 came next in September 2007, with screw press #7 seven months later. After the shutdown in April 2008, #7 showed a sharp increase in production from 260 t/d at an outlet consistency of 26-31% to 360 t/d at a consistency of 29-30%. “Now that all the screw presses in Loop 2 have the same excellent dewatering performance, we are in the position to even shut off one press, if paper production goes down a bit,” Reinboth says. “This gives us the potential for energy savings.”
Longer operational lifeAs part of the upgrade, Andritz installed wear plates and shoes to improve the lifetime performance of the presses. Before the upgrade, hard facing of the worn-out screw flights was required every two to three years. However, these machinings were not 100% accurate in restoring the screw diameter to the original dimensions. In addition, the procedure was very labor-intensive.
After the upgrade, changing of the wear shoes in the high-pressure zone of the press can be done in a double shift (compared to three days for a hard facing). “This is really an improvement and makes my life as a maintenance manager much easier,” says
Schuchardt. “The operational life of the screws will be much longer, and repairs are simpler and cheaper.”
Next: disc filters and Loop 1Next in Sachsen’s program to eliminate bottlenecks are the two former Ahlström disc filters located before the screw press in Loop 2. During the September shutdown this year, Andritz installed a new patented “Air Lock” valve on one of the filters. The valve resembles an elephant’s trunk, so is referred to as the “ trunk nozzle.”
“We had concerns that the filtrate quality would suffer – that a higher amount of fibers would be rejected with the filtrate,” says Reinboth. “The Andritz guarantee values were very ambitious. But, we can say that the filtrate quality is better than guaranteed. Andritz also surpassed the stated 5% capacity increase, and there is still room for more. Jürgen Hirschberger (Andritz’s Product Manager for Dewatering & Bleaching) helped us a lot and advised us how to optimize the disc filter operation.”
Also in September, Andritz began up-grading Loop 1 by installing new back-pressure devices on the four screw presses located before the disperger stage. The Sachsen mill has on its “wish list” similar upgrades to the presses as were done for Loop 2 (i.e. changing the screw geometry, reconditioning of the screw and installing the wear plates and shoes) as budgets permit.
“After the very good experience with the upgrades in Loop 2, improvements in Loop 1 have a high priority,” says Reinboth. “Before the first rebuild, we had some questions about Andritz’s ability to upgrade another supplier’s equipment and reaching our requirements for capacity improvement. But, these questions have been answered and we have high praise for the Andritz team on the results achieved.”
