Duluth, Minnesota mill is first in North America to make new SCA Plus grade commercially, and targets publication papers market

Lake Superior Paper Industries Adds New Wet End, Moves into SCA Plus Grades

By Harold M. Cody Editorial Director

Lake Superior Paper Industries, a company of Consolidated Papers Inc., started up a new wet end gap former on its 317-inch wide papermachine on February 21, 1998, just 20 days after the rebuild had begun. The new Voith Sulzer Duoformer CFD is the major component of a $32 million project that increased the quality of the supercalendered uncoated groundwood (SCA) grades made at the mill. As part of the project, modifications were also made to the dryer section on the machine, and in the woodyard. To support the new wet end, an increase in vacuum capacity was installed. CP’s central engineering department played a major role in the project, working with mill engineering contractors and suppliers. Primary contractors for the project included Oscar J. Boldt Construction Company for general contracting and Voith Sulzer Inc., which supplied the gap former and new headbox. No change in capacity is anticipated.

New wet-end gap former on the 317-in.-wide machine

The rebuild was designed to produce improved sheet profile, formation and z-direction ash distribution. Improved profiles allow the mil to produce a more uniform sheet, which aids in the manufacture of larger diameter rolls. Improved formation and ash distribution results in improved print quality. The original wet end former on the machine produced a sheet with higher ash on the topside. With the new former, the mill has successfully developed a magazine publication grade referred to as SCA Plus for rotogravure printing in addition to the standard SCA grades that the mill has produced since inception. As noted below, SCA Plus is a grade with improved properties compared to traditional SCA grades, and is designed to compete in the publishing business with lightweight coated (LWC) and other grades.

The Voith Sulzer Duoformer CFD twin-wire former is a roll/blade gap former. The sheet is initially formed on a forming roll. Following this, drainage occurs through the top wire via multi-zone suction boxes, and opposing this under the bottom wire are formation strips (or small foils). Two other vacuum boxes are next before the sheet reaches the couch roll. Here is one somewhat unique aspect for a machine producing SCA grades, in that the wire separates over the top of the suction couch roll. Currently, the machine is running at 4500 fpm on standard 35 lb. grades.

The CFD former is designed for high groundwood content grades and had been pioneered by Consolidated Papers on lightweight-coated grades. The machine is a standard Voith Sulzer design, with some modifications made for SCA. In this type of former, the wrap on the forming roll is varied according to the grade made, e.g. whether it’s newsprint or SCA type grades. A lower freeness furnish requires a greater wrap. In the LPSI mill furnish, they tend to refine a lot to get lower freeness.

This is the third CFD unit for Consolidated Papers, which was one of two considerations for choosing it. The mill did pilot work to verify that their furnish would perform as expected. Three other mills (not company mills) that produce SC grades also use the CFD.

Wire changes on the machine are accomplished without use of a crane. A new wire changing system is used that avoids use of heavy metal poles, using a series of winches and poles.

Other modifications. In addition to installation of the new twin-wire wet end, other modifications were made to the machine. A major one was replacement of the old headbox with a new Voith Sulzer Module Jet dilution control headbox, replacing the previous slice control system. As part of this, new screening, pumping and air removal systems were installed, all supplied by Voith Sulzer. The dilution valve control package is also from Voith.

As noted, one major goal of the rebuild was to improve sheet profile. There has been a notable improvement over the original former in cross direction profiles and basis weight control. Both CD and MD 2-sigma deviations have been significantly lowered (figure 1).

Further down the machine, the press section is largely unchanged, with a DuoCenter II press followed by a conventional straight through fourth press. However, a vacuum roll was installed as the dryer lead in roll to transfer between the press section and dyer section. Also, the first three dryer sections were changed from uni-run to a single tier configuration by adding vacuum to the bottom dryer cans. Ropes were eliminated and the machine now has ropeless threading in the first three sections.

All other major elements of the papermachine were unchanged. As noted below, the mill is a traditional SCA producer, operating 3 supercalenders to handle the output of the machine.

In the groundwood pulp mill, a new jack ladder was added to improve feeding into the grinders. These are the original Tampella pressurized grinders. Also, a de-icing deck was modified for wood coming into the grinders. The bleach plant system was not changed, even though SCA Plus grades have higher sheet brightness than standard SCA grades. Groundwood pulp is bleached with sodium hydro-sulfite.

Recycled content. The mill site itself also incorporates a recycled mill, which is run as a separate business. Superior Recycled Fiber Industries, also a company of Consolidated Papers, is a freesheet recycling business started up in 1993. This facility is used to provide fiber for a recycled SCA grade paper (incorporating post-consumer fiber) LPSI markets under the name Encore. The $80 million recycle facility uses 130,000 tpy of office scrap paper to produce 90,000 tpy of recycled pulp. Most of the pulp is used for high quality freesheet grades both within Consolidated Papers other operations and to others as well.

The lightest weight made at the mill is a 28-lb standard SCA grade. Most production of SCA paper at the mill is 33 and 35 lb. weights, and the majority of production remains standard SCA. The users of SCA Plus are primarily magazines and catalogs. The first official run on SCA Plus was April 8, 1998.

Project Details. While the machine was down for 20 days, every team member remained employed during this period. Production technicians developed lists of project to be done at the mill and scheduled everyone for task assignments that needed to be done. During the downtime for installation, in addition to training the wet end operators, other employees were doing things such as safety training and maintenance. Flexibility was a key in accomplishing this, and it was facilitated by not having problems with work rules, since the mill has always been a non-union mill. LPSI and Superior Recycled Fiber Ind. are Consolidated Papers’ only non-union facility.

Grade Snapshot. LSPI’s main product continues to be standard SCA, made using kaolin coarse filler clay. However, as a result of the rebuild they also make an SCA plus grade. The mill reports it was the first mill in North America to qualify commercially on SCA Plus. The new grade is marketed under the tradename Expedition, and has a higher brightness, gloss, better smoothness and printability than the standard SCA grade.

Profile: LSPI. The original mill was built as a joint venture between Minnesota Power and Pentair Inc. on a 93 acre site that was available from the city of Duluth as an economic development zone. The company was formed in 1985 and the mill started up in 1987. It has been a division of Consolidated Papers since its acquisition in July 1995. The $400 million project to build the mill included the following major components: Tampella pressurized groundwood pulp mill system, Voith Duoformer F twin wire machine, three Valmet Appleton supercalenders, two Jagenberg Vari-top winders, and a Kone wood handling system. The design capacity is 243,000 tpy.

FiGURE 2 and 3: Before and after weight profiles show impact of new wet-end on sheet structure.

Construction began in July 1986. The first paper was made on the machine on Nov. 4, 1987, 21 days ahead of schedule. The mill was located next to a power station that had been mothballed since 1981. The idle plant was retrofitted for new source performance standards to allow it to burn coal and wood. It was restarted and now supplies all process steam for the mill. By utilizing the facility LSPI obtained steam capacity at about half the cost of building new boilers. The existing municipal treatment plant handles effluent from the mill. For details on the original mill start-up, refer to April 1988 Pulp & Paper.

In 1997, the mill shipped 236,236 tons of SC papers, a 14% increase over the 1996 level of 207,139 tons.

FiGURE 1: Major markets for SCA grades in North America show predominance of use in the insert market