FOCUS:

Calendering technology expands in U.S. as first online coated lines are ordered and two major lines on SC uncoated grades startup this year

By HAROLD M. CODY, Senior Editor

Online Calendering Spreads To Coated Paper Grades in the U.S.

Harold M. Cody
is Senior Editor
for Pulp & Paper
Magazine

OnLINE calendering continues to transform the papermaking process around the world, providing mills with the technology to finish a variety of grades completely on line. Grades currently produced with online calendering—either soft-nip or multi-roll calenders—include almost all printing paper grades and many specialty papers. Board mills are also increasingly looking at new sheet finishing technology including soft-nip calendering and shoe calendering. Pulp & Paper spoke with several top suppliers and other sources in order to assess the current status of calendering and to look at important trends and innovations for the future of papermaking.

ONLINE TAKES OVER SC GRADES. European mills were the first to incorporate online multi-roll calendering, but North American mills are rapidly adopting the technology (Table 1). In North America, the main application to date has been supercalendered (SC) uncoated groundwood grades, where multi-roll calenders have rapidly changed the face of the SC business. These moves have led to an upward shift in quality across the groundwood printing papers spectrum and have put tremendous cost and quality pressure on mills that rely on older technology such as supercalenders, hard roll machine calenders, or even older soft-nip calenders.

Irving installed North America's first online multi-roll calender in early 1999 on an uncoated groundwood machine at the St. John, N.B., mill. Irving's 6-roll Metso (formerly Valmet) OptiLoad unit has two hot intermediate rolls. Irving has been a pioneer in hot soft calendering, reportedly installing the first unit on a North American newsprint machine in 1991.

The world's first online OptiLoad was installed a little earlier, as Stora Enso brought its 6-roll unit online at the Varkaus, Finland, mill in late 1998 on the 354-in.-wide No. 4 paper machine that produces directory and newsprint papers.

In North America, a second multi-roll calender was installed as part of Alliance Forest's revamping of its two printing paper mills in Canada. At the Dolbeau, Que., mill, a soft calender—two 2-roll soft-nip calenders—was installed in 1999. More recently, Alliance installed a multi-roll Janus calender at the Donna-cona, Que., mill in 2000 (Pulp & Paper, April 2001). Voith supplied both units.

The transformation of much of SC paper production from traditional offline finishing via supercalendering to online operation is continuing with two key startups planned for 2001. Both orders went to Metso, and each installation has unique attributes. Abitibi's project at Lufkin, Tex., will utilize a 6-roll OptiLoad on a southern pine furnish for SC-B grades, which offers challenges due to the high pitch content. Great Northern is installing a 10-roll unit on the No. 11 machine at the Millinocket, Maine, mill, which is being converted to printing paper production. The company declined to identify the grade to be produced other than to say it "is an SC paper." However, the 10-roll configuration would certainly appear to provide the capability to produce SC-A grades, or even higher quality products.

LWC GOES ONLINE. In North America, the most innovative news in calendering has been its evolution in coated paper production. Pacifica Papers (formerly MacMillan Bloedel) was one of the world's first—and the first in North America—to use soft calendering to produce coated groundwood papers on line. This innovative installation at the Port Alberni, B.C., mill—which was installed in the mid-1990s and is now somewhat dated—uses two 2-roll soft calenders. Newer machines with multi-roll calenders have more nips and thus can develop higher gloss.

Currently, no mill in the U.S. produces standard coated ground-wood fully on line. All mills use offline supercalenders. In Europe, several coated mills produce lightweight coated (LWC) papers with online coating, including the Rauma mill in Finland and the Haindl mill in Germany, which also has online calendering.

Haindl's Augsburg mill is the current standard bearer for modern LWC technology. The new 400,000-tpy machine, brought on line last summer, operates at a wire width of 411 in. (10.45 m) using an 8-roll calender, of which five rolls are plastic covered and three are steel thermo rolls. The linear load can range from 30 to 450 kN/m and up to 200† C. The sheet is pre-calendered before film coating both sides and is then calendered using an OptiLoad calender. "The mill is making coated offset grades right now and can attain 60 gloss range and a smoothness (Parker Print Surface) range of 1.5 microns," notes Martii Tuomisto, dir., North American calendering, Metso.

However, online technology in coated paper manufacturing is migrating to the U.S. due to two noteworthy LWC projects scheduled for later this year and next. These two major projects will convert existing newsprint machines to LWC paper production.

The first project will convert Madison Paper's Alsip, Ill., mill to a 120,000-tpy LWC mill. The 240-in. machine will be converted via addition of a Voith Speedcoater and Janus Mark 2 (MK2) 8-roll calender. Even more unique is the fact that the mill will utilize significant recycled fiber in the sheet. The company declined to specify the level of recycled content. The stock preparation area will be rebuilt with an EcoCell flotation unit. Startup has been reported to be either in late 2001 or early 2002.

The second major project is also utilizing cutting edge technology to convert a southern U.S. newsprint mill to coated papers next year. This larger project is a major rebuild of the 390-in. machine at Bowater's Catawba, S.C., mill. Voith was unable to confirm the order due to confidentiality restrictions. However, numerous sources report that the project will entail addition of an online Voith SpeedCoater and Janus MK2 multi-roll calender. Metso is supplying a new press section for the machine.

Bowater declined to comment on or confirm this information. However, it would appear likely that the Janus would be an 8-roll unit, based on technology deployed at similar mills. Startup is scheduled for third quarter 2002 at a cost of $106 million, according to published reports.

In the original Janus calender, as installed at Alliance, the rolls were arranged in a vertical stack, but in the Janus Mark 2, the system is inclined at a 45† angle. "This facilitates roll changes and sheet threading and allows much better operator access to both sides of the stack," according to Paul Martin, product manager, Voith.

Voith's first MK2 unit was installed on the No. 5 machine at Lang's Ettringen, Germany, mill. The machine also has a double shoe press, which was the first shoe press on magazine papers, and is a key part of the machine's design speed of 2,000 m/min. The Madison Paper order for its Alsip mill is a Mark 2 Janus calender.

Kuesters is also a major supplier of soft-nip calenders with over 300 units running worldwide, according to Hans Hendrix, vice president, and Mark Sorensen, product manager, Kuesters. The company's multi-roll calender—the ProSoft line—is in use in Europe on coated woodfree grades. There are currently ten ProSoft units running, including one in the U.S. at a confidential location.

COATED WOODFREE. Another calendering development of interest is associated with Metsa Serla's startup of its rebuilt No. 4 paper machine. The machine incorporates several technology innovations. The former copy paper producer has been converted to a 225-in.-wide, 3,937-fpm coated free-sheet machine in a major rebuild that includes a Metso OptiCoater with two jet blade coating stations and an online OptiLoad calender. It is the first machine with online blade coaters and an online OptiLoad multi-roll calender in the world. A new shoe press was also added as part of the project. Several machines produce coated groundwood papers on line, but these use film coaters rather than traditional blade coating.

Metsa Serla's No. 4 paper machine is the first to incorporate this particular combination—using one of the new multi-roll calenders—in the world. However, coated woodfree has been produced on line for sometime, according to various sources. Mills in Europe make coated woodfree using online soft calendering. Sappi, formerly S.D. Warren, pioneered online production in the U.S. in the 1980s with its patented "thermal gradient" calendering. It is reported by several sources that these soft-roll calenders had multiple rolls, but not of the same number or physical arrangement as the current generation exemplified by the OptiLoad or Janus calenders. Both the Skowhegan, Maine, and Muskegon, Mich., mills employed this technology. Exact details on the setup of the calender remain guarded.

SHOE CALENDERING. Other developments in calendering include further application of the use of the "shoe" or "nip" calender. Metso now has two installations running at the Korsnas mill in Sweden, with the original installation now joined by a second unit on the No. 4 paper machine. The company has also received two confidential orders that have been delivered, notes Martii Tuomisto, product manager, Metso.

While applications are currently limited to four operational units, the company has some indication of where shoe calendering will most likely applied. "On high quality board, such as coated SBS, we think shoe calendering makes sense before coating. On less demanding grades, such as uncoated, it could be used as a final calender. It could also be used to replace the Yankee on boxboard machines," explains Mikko Tani, Metso's product manager, calendering.

LINERBOARD APPLICATIONS. Board mills are also increasingly looking at new methods in order to improve sheet properties. "Linerboard mills are taking a hard look at soft calendering here in North America," according to Martin. "We've seen a lot of interest, but, of course, roll cover life has been a major question for board mills. I think we'll see a lot of linerboard mills adding soft calendering in the future." Currently, two Visy mills—Conyers, Ga., and Staten Island, N.Y.—have installed soft calenders.

The advantages of a soft calender over a conventional hard calender on board grades mainly center on the fact that you can get equivalent sheet smoothness, as measured by Sheffield smoothness, while avoiding excessive sheet densification that produces light and dark spots. Soft calendering is already in use on coated board machines. For example, Smurfit Stone Container uses a soft calender on its coated linerboard machine in La Tuque, Que.

ROLL COVER TECHNOLOGY. Soft calendering and multi-roll calendering didn't really develop until polymer roll covers were designed. Polymer covers can withstand the required temperatures and loads, which go up to 400 kN/m and 200† C. All three major roll cover producers—Metso, Voith, and Stowe Woodward—report that they continue to work to develop more durable covers that last longer and keep machines up and running. The business continues to be highly competitive with many mills shopping around and switching suppliers.

Among the important developments in roll coverings are ongoing efforts to find the optimum roll cover material that can supply the longest life at these extreme operating temperatures. "We've developed a new cover named Ivory that has enhanced durability," notes Scott Bowman, composite product manager with Stowe Woodward. "It's a new type of enhanced composite material."

Composite roll covers essentially consists of a polyurethane matrix—or glue—that is reinforced with either fiber or particulate or both. Manufacturers continue to work to find the best combination of materials for providing improved durability.

Rebuilds and retrofits of existing supercalenders continue to attract interest, as well as online applications for multi-roll calendering. Metso is rebuilding three supercalenders at St. Marys Paper in Sault Ste. Marie, Ont., with its multi-roll technology. It also has completed work at Mead Escanaba and has started up two offline OptiLoad units at a mill in Wisconsin on release papers (one a rebuild and one a new unit).

ROLL DEVELOPMENTS. In addition to improved cover performance and continued growth in online applications, key advances in calendering technology center on roll developments. "Caliper control is another major development," notes Martin. "CD caliper control using internal pistons has come a long way, with an increased number of zones across the role for caliper control. Pistons can be bundled together for control in a specific area, and an advantage is that hydraulic pressure control is instantaneous compared to using heat to adjust the roll and, in turn, affecting sheet caliper."