Low consistency refining of TMP: who says TMP requires only high consistency refiners? A careful evaluation of LCR options may change your mind--and your process.

For the last seven years, the number of installations of low consistency refiners at thermomechanical pulp (TMP) newsprint mills has increased dramatically. This is an important trend for a pulping process that has traditionally involved only high consistency refiners. This article will examine some driving forces for this phenomenon and explore the rationale for the placement of low consistency refiners within the TMP process flow sheet.

Three distinct and different applications of low consistency refining of TMP are possible. Each application must be evaluated separately with care.

1. Low Consistency Rejects Refiners (LCRR): Low consistency refining of screen rejects can take two different forms. The first scenario relies on low consistency refiners instead of traditional high consistency rejects refiners. In the second scenario, rejects from the mainline screens are refined conventionally with high consistency refiners and then screened at 4% consistency. The long fiber fraction from the rejects screen is then refined at approximately 3.5% consistency with a low consistency refiner.

2. Third Stage TMP Refiners (LCR): The low consistency refiners are after the latency chest and operate at 4%-5% consistency. Typically their role is energy reduction or production increase.

3. Post Refiners: Post refiners are essentially stock preparation refiners installed before the paper machine. Their purpose is to reduce freeness, develop TMP properties for improved sheet surface characteristics, and achieve sheet porosity specifications.

LOW CONSISTENCY REJECTS REFINERS

The use of LCRR on softwood TMP has not yet had commercial use. The author knows of no operating installations. A reduction of 30%-35% in the rejects specific energy consumption (SEC) is possible, but a significant loss of newsprint quality can occur when using insufficient LCRR stages. To replace the high consistency rejects refiner, six LCRR stages in series would probably be necessary. To refine the reject screen rejects without loss of pulp quality, two or three LCRR stages would be necessary. These multiple refining stages may make this approach more costly than LCR or post refiners. Considering the lack of installations, this application of low consistency refining is still at the developmental stage.

THIRD STAGE TMP REFINERS

For newsprint grade TMP, the LCR supplies less than 5% total SEC required in the conversion of wood chips to pulp. The larger, more powerful high consistency refiners provide the critical fiber separation and establish the quality fingerprint of a pulp. Generally, the freeness target of the mainline refiners increases by 50 to 100 points. The LCRs return the final freeness to the original value.

Since 1995, nearly 10,000 air dried (a.d.) metric tons/day LCR capacity has been installed in North America, as Table I shows. Justification for installation of LCR usually cites a 5%-10% reduction in the energy consumption of the main line refiners, an increase in production rate, or both. For mills where the high consistency refiners operate at maximum motor load, the addition of LCR increases the refining horsepower available. This then creates the potential for a production increase. Realization of the production increase may require elimination of bottlenecking downstream, in the chip pretreatment area, or both.

At integrated mills where paper machines have fiber supply limitations, increased production is more important than the energy savings. Published results from Stadacona Papers show a 5%-7% reduction in mainline specific energy and a 10% increase in production rate on northern softwood. The installation was so successful that the company duplicated it on the second TMP line.

Other important benefits of LCR are shive reduction and latency removal. Shive reduction sets the stage for improving density and this gives an improvement in the bonding properties of burst index, tensile index, and tensile energy absorption. Latency in the pulp can be a problem at TMP mills where production increases have reduced the residence time in the latency chest to unacceptably low levels. Installation of LCR usually eliminates this problem.

The use of LCR and post refiners involves a risk of tear loss due to fiber cutting. To minimize this threat, the LCR refining intensity must be low, the gross specific energy application should not exceed 100-150 kWh/oven dried (o.d.) metric ton, and refiner plates must be changed before they become worn. The risk of tear loss has decreased greatly in recent years due to improvements in the refiner design, improvements in plate design, reduction of refining intensity, and careful process design. Although Stadacona Papers reported up to 5% tear loss in the first year of operation, optimization of refiner plate design and operating conditions has reportedly reduced the loss to zero. Worn refiner plates will result in tear loss.

Published results from Bowater Calhoun show a 6%-8% reduction in mainline specific energy on southern pine and a 4% increase in production rate with no loss of tear. This is an impressive result since fiber coarseness of southern pine requires 30% higher SEC than spruce for newsprint production.

At some installations, mill personnel elect to forfeit the potential SEC savings that LCR can provide. They opt instead to improve the final pulp quality--tensile, burst, and density--at constant total SEC and freeness. If they have sufficient disc filter capacity, other mills choose instead to keep the total SEC constant and lower their final TMP freeness--from 100 mL to 90 mL, for example. The result is an increase in tensile. This approach requires sufficient drainage capacity on the paper machine.

Installation of LCR combined with replacement of old screening technology appears to offer a synergistic effect. The new screens require sizing to yield low passing velocity (<1 m/s) and require fine slotted (0.10-0.15 mm) wedge wire baskets. With this combination, pulp quality--tensile, burst, and density--increases. Shives decrease significantly by as much as 90%. Freeness variations also decrease. Other potential benefits include the following:

* Reduction in off-quality paper due to low strength

* Increased use of lower cost, secondary wood species such as pine in a balsam and spruce furnish

* Reduction or elimination of the kraft component in the paper machine furnish

* Increased paper machine productivity due to less breaks and fewer winder snap-offs

* Improved pressroom performance

* Increased plate life on the high consistency refiners.

POST REFINERS

Post refining of TMP in the stock preparation area of the paper machine develops newsprint properties. For proper control, some continuous measure of the drainage characteristics of the feed to the post refiners is necessary.

Locating the final TMP refining close to the paper machine has several advantages:

* The operation becomes directly coupled to the paper machine, to facilitate immediate response to changing paper properties.

* Small freeness changes in the blend chest pulp effectively disappear.

* Freeness at the TMP disc filter is higher thereby avoiding loss of dewatering capacity.

Table II gives examples of post refining of TMP at the paper machine stock preparation department for newsprint production.

Post refining does not result in TMP refiner energy savings or in TMP production increase. The main objectives are reducing the kraft component of the sheet, improving the bonding properties of the pulp and the quality of the paper--tensile, porosity, and water drop, or both. Freeness and tear index decrease. The specific energy application is approximately 25-50 kWh/o. d. metric ton.

Some people claim that post refining has an advantage over LCR because a portion of the bonding potential developed with LCR is lost in the machine chest (MC) storage. The author has seen no evidence to support this claim. In such cases, the disc filter MC pump has probably induced a slight curl into the fibers. This causes symptoms similar to latency such as a freeness increase and loss of bonding potential. Hot disintegration in the laboratory can restore the original properties. In the mill, process equipment such as pumps, agitators, refiners, and screens between the MC storage and the paper machine removes the curl and restores the fiber properties. In this situation, one could mistakenly attribute the restoration of pulp properties to the post refiners.

CHOICE OF LCR OR POST REFINERS

The selection of which application of low consistency refining to use--LCR, post refiners, or neither--requires evaluation on a mill by mill basis. As with all projects, one must determine the capital cost and the return on investment of each alternative. In addition, the following guidelines will help determine the best alternative for the specific mill situation:

1. If tear loss is intolerable, use LCR, not post refiners.

2. If the paper machine is short of fiber and increased paper production is desirable, consider LCR. Other TMP process equipment may require elimination of bottlenecks to achieve the production increase.

3. If reduction of energy consumption is important, use LCR. This applies especially in locations where the unit cost of electricity is high.

4. If tear loss is tolerable, production increase is not feasible, and energy savings are a minor consideration, consider post refiners. All three conditions must co-exist.

Finally, consider the possibility of using both LCR and post refiners simultaneously on the same mechanical pulp stream. The author is not aware of any mill doing this. While capital cost would be a deterrent, this approach could reduce TMP SEC and lower the freeness variation at the paper machine.

Table I: North American TMP/BCTMP * mills with third state low
consistency refiners.

MILL                                LOW CONSISTENCY        PROCESS
                                        REFNER

Stadacona, Quebec City              2 Andritz Twin-Flo     TMP
QC, Canada                          52/58 in.

Alberta Newsprint, Whitecourt       3 Metso Conflo JC-04   TMP
Alabama, USA

Bowater, Calhoun, Tennessee         3 Andritz Twin-Flow    TMP
USA                                 52/58 in.

Tembec, Temcell 2, Temiscaming      2 Metso Conflo JC-04   BCTMP
QC, Canada

Bowater Mersey, Liverpool           3 Andritz Twin-Flow    TMP
NS, Canada                          42/48 in.

Norske Canada, Campbell River       4 Andritz Twin-Flow    TMP
BC, Canada                          52/58 in.

Abitibi-Consolidated, Baie-Comeau   3 Andritz Twin-Flow    TMP
QC, Canada                          52/58 in.

Corner Brook Pulp & Paper           3 Andritz Twin-Flow    TMP
NF, Canada                          52/58 in.

Papier Masson, Masson               3 Metso Conflo JC-04   TMP
QC, Canada

Pine Falls Paper, Pine Falls        3 Metso Conflo JC-04   TMP
MB, Canada

Kruger, Trois Rivieres              1 Andritz Twin-Flow    TMP
QC, Canada                          52/58 in.

Total

MILL                                  A.D.     START-UP
                                     METRIC      DATE
                                    TONS/DAY

Stadacona, Quebec City                900       1995 &
QC, Canada                                       1998

Alberta Newsprint, Whitecourt         800        1997
Alabama, USA

Bowater, Calhoun, Tennessee           700+      1997 &
USA                                              1999

Tembec, Temcell 2, Temiscaming        500        1998
QC, Canada

Bowater Mersey, Liverpool             800        1998
NS, Canada

Norske Canada, Campbell River        1800        1998
BC, Canada

Abitibi-Consolidated, Baie-Comeau     900        1999
QC, Canada

Corner Brook Pulp & Paper             930        1999
NF, Canada

Papier Masson, Masson                 660       1999 &
QC, Canada                                       2000

Pine Falls Paper, Pine Falls          700        2000
MB, Canada

Kruger, Trois Rivieres                580        2001
QC, Canada

Total                                9545

Table II: TMP post refiner installations.

MILL                              POST REFINERS            YEAR
                                                         INSTALLED

Norske Canada, Port Alberni       1 Andritz Twin-Flo        1994
BC, Canada                        42/48 in.

Abitibi-Consolidated, Alma        2 Metso Conflo JC-03      1995
QC, Canada

Abitib-Consolidated, Dolbeau      1 Metso Conflo JC-04      1995
QC, Canada

Irving Paper, St. John            2 Metso Conflo JC-04      1995
NB, Canada

Stora Enso, Port Hawkesbury       2 Andritz Twin-Flo        1996
NS, Canada                        52/58"

Abitibi-Consolidated, Donnacona   2 Metso Conflo JC-03      1999
QC, Canada

Great Northern, Millinocket,      1 Andritz Twin-Flo        1999
ME, USA                           42/48 in.

NorskeCanada, Powel River         1 Metso Conflo JC-05      1999
BC, Canada

Spruce Falls, Kapuskasing         2 Metso Conflo JC-03      1999
ON, Canada

Ponderay Newspring, Usk           2 Metso Conflo JC-04      2001
WA, USA

IN THIS ARTICLE, YOU WILL LEARN:

* The three distinct applications of low consistency refining of TMP.

* Effective ways to evaluate each different method for your mill.

* Recent North American mill installations of various low consistency refining processes.

Acknowledgments: The author recognizes the suggestions and contributions of the following reviewers: Gary Harris and Michael Jackson of NLK Consultants Inc., Garth Russell and Anders Mokvist of Metso, and Thomas Pschorn and Brad Cort of Andritz Inc.

ADDITIONAL RESOURCES:

* To participate in TAPPI's Pulp Discussion Board, or to view discussion boards for other topics, go to http//www.tappi.org/index.esp?limit_to=1&ch=6&rc=1&Show_only=57

* To view previous Solutions/and TAPPI Journal articles dealing with pulping, search for "pulping" in the category "all journals" at www.tappi.org