Stock preparation at Palm Worth's PM 6.

In October 2002, Papierfabrik Palm GmbH & Co. KG started up a corrugating medium and testliner machine. The machine is located in Worth, Germany. Palm's newest paper machine is based on Metso Paper's "OptiConcept." PM 6 has a wire width of 11,000 mm and a design speed of 1800 meters/min. Annual production capacity is 600,000 metric tons based entirely on recycled fiber.

Stock preparation was delivered by Metso Paper and is providing 2000 metric tons of brown grade (OCC) recycled fiber daily. The stock preparation system has two lines, each starting with an LC-pulper, two secondary pulpers and one reject drum. It is followed by coarse screening consisting of high-consistency cleaners and deflaking screens. The low-consistency cleaning is followed by fractionation and thickening for short-, medium-, and long-fiber fractions. The long-fiber pulp is further treated in a dispersion stage with subsequent press thickening, a preheater and in a conical disperger, complemented by low-consistency refining.

This paper describes the operating experiences and pulp quality results of PM 6's stock preparation after approximately two years of operation.

PACKAGING PAPER REQUIREMENTS

After newsprint, packaging paper is the most important grade in Germany. In 2003, German production of paper for packaging was 7.48 million metric tons (40% of entire paper production). With a capacity of 820,000 bone dry metric tons/yr (bdt/y), Papierfabrik Palm is one of the main manufacturers for corrugating medium and testliner in Germany.

Standard weight for corrugating medium in the past was around 110 g/[m.sup.2] or even higher, but today the trend is to lower basis weights around 90 g/[m.sup.2]. Weight for testliner was higher than 120 g/[m.sup.2] and here the trend is also toward lower weights.

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Important paper qualities for corrugating medium are such strength properties as Short Span Compressive Test (SCT CD) or Concora Medium Test (CMT MD). For testliner, burst is important and the paper surface must be clean without dirt spots. Good stock preparation performance is required to reach quality targets.

A modern stock preparation system must produce clean pulp without impurities in combination with low fiber loss. Low energy consumption is also very important for economical paper production. For good strength properties, fractionation in long and short fibers--followed by a refining of the long fiber--can be useful.

NEW PM6 IN WORTH

Papierfabrik Palm GmbH & Co. KG was founded in 1872. Today, Palm owns three mills for the production of newsprint and corrugating medium/testliner and 15 converting plants for box production. All of the company's paper is produced using 100% recycled fiber. Newsprint production is located in Aalen and Eltmann and has a capacity of 520,000 bdt/y. Corrugating medium and test-liner is produced in Aalen and since October 2002 in Worth on the new PM 6. PM 6 has a capacity of 600,000 bdt/y corrugating medium and testliner and is designed to produce low basis weights down to 70 g/[m.sup.2]. In 2004, Papierfabrik Palm had 2500 employees and annual sales of about EUR 500 million.

RECYCLED FIBER STOCK LINES

Metso Paper's delivery for the Worth Mill's PM 6 project included complete stock preparation for brown grade recycled fiber (OCC and mixed waste), as well as stock preparation broke and approach systems. Metso Paper's contract covered the main equipment and basic engineering for the pulp mill and short circulation, excluding the pulper feed system, reject system, water handling, and process engineering.

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The stock preparation system for PM 6 has a capacity of 2000 bdt/d and produces three different fractions (short/medium/long fiber). The long-fiber fraction can be treated with a disperger (for clean surface by test-liner) and a refiner (for strength properties).

The key sub-processes in the recycled fiber lines include OptiSlush[TM] pulping, detrashing, cleaning, OptiScreen[TM] coarse screening, LC cleaning, fractionation and fine screening, thickening, OptiFiner[TM] dispersion and engineering services. The machinery units, such as dispersion, are among the largest in the world.

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PULPING

Like other mills, the Palm Worth mill must produce pulp with sufficiently high cleanliness to ensure the runnability of the screening stages. The process needs to disintegrate pulp without excess disintegration of contaminants. It must tolerate varying amounts and types of impurities without excessive problems with wear and remove these impurities without usable fiber loss.

The continuous OptiSlush VC pulper performs the raw material disintegration at a consistency of 4-5%. The holes of the screen plate are 12 mm in diameter. Auxiliary equipment--such as two secondary pulpers with 20 mm holes and an OptiThick reject drum--ensure effective pulping. A two-stage high-consistency cleaning system is positioned after the pulping station to remove the heavy impurities.

COARSE SCREENING

The three-stage coarse screening system features conical OptiScreen CS units, HC cleaners and deflaking OptiScreen units. The first- and second-stage screens feature rotating baskets with 2.0-mm holes. In the third stage, the plate type screens have 2.4-mm holes.

The three-stage coarse screening concept ensures minimal breakdown of impurities with minimized usable fiber losses. Primary and secondary stage hole screens are equipped with rotating baskets resulting in low energy consumption and low wear. HC cleaning after the secondary stage protects the following stage from wear. The tertiary stage is a deflaking type screen to minimize fiber loss.

FRACTIONATION AND FINE SCREENING

Sand removal is done by a forward cleaning system prior to the low-consistency fractionation for the entire production to protect the screen baskets from wear. This reduces the need for sand removal in short circulation.

The four-stage LC cleaning system is followed by two-stage fractionation for short-, medium-, and long-fiber fractions. Fractionation is carried out with 0.15-mm slot baskets in the primary stage and 0.20-mm slot baskets in the secondary stage. The accept from the primary stage is the short-fiber fraction, while the accept from the secondary stage is the medium-fiber fraction. Rejects go to the long-fiber fine screening for further treatment.

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The purpose of fractionation is to optimize the board-making potential of raw materials. Different fiber fractions have varying pulp properties. Fractionation enables a precise focus on the effects of later process stages to gain optimum pulp properties, resulting in maximum fiber potential and energy savings. The need for dispersion and refining can be minimized. Clean and sufficiently strong pulp improves the runnability of PM 6.

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The long-fiber fraction is screened in a three-stage OptiScreen FS fine screening system with 0.25-mm slot baskets. The fine screening system is designed to provide gentle screening action, which reduces the deformable debris from being squeezed through the slots. After fractionation and fine screening, all three fractions are thickened with disc filters and pumped to storage towers.

DISPERSION

Dispersion is used to improve stock quality. It develops pulp properties, reduces speck size, disperses hot melts and stickies, and homogenizes the pulp. At Worth, long-fiber pulp is treated in the dispersion stage, which is equipped with TwinRoll TWA press thickening, a HI-Preheater, and the OptiFiner OFD conical disperger before pumping to the storage tower. The OptiFiner OFD system, with a capacity of 790 metric tons/day, is among the largest dispersion systems in the world.

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The disperger's conical fillings have a large processing area, including tooth-shaped patterns developed specifically for recycled fiber treatment. This ensures high dispersion performance for all types of impurities. Conical technology enables the improvement of fiber strength properties through controlled fiber treatment impact, which facilitates system optimization with respect to both fiber development and the required processing energy.

Stock production requirements depend on PM 6's final paper quality and grammage. Therefore, the stock preparation lines have been designed to produce 700-800 metric tons/day of short fiber, 600-700 metric tons/day of medium fiber and 600-755 metric tons/day of long fiber. The two lines have reached the total design capacity of 2,000 metric tons/day.

PULP CLEANLINESS

Stickies reduction over coarse screening is good (stickies removal efficiency SRE 50-80%), which strongly indicates that the approach used (gentle screening without the disintegration of impurities) is the correct solution for reaching a low sticky content in the final pulp.

The performance of the fractionation and LF screening stages has been very good. In addition to slot size, slot profile and rotor design, fractionation and fine screening efficiencies are controlled through feed consistencies and reject rates. Low-profile baskets and gentle foils have produced good fractionation and screening results.

Stickies removal efficiency for all fractions has met expectations. Short fiber SRE was 99%, medium fiber 98% and long fiber 86%. The stickies level in the LF fraction has been acceptable and dispersion has not been needed all of the time.

Dirt removal (Sommerville 0.15-mm) has also been at an excellent level, reaching 97-99% efficiencies over the stock preparation process.

The LF fraction can be treated by a disperger and by a refiner. Because of the high pulp cleanliness, dispersion has not been needed. Standard corrugating medium does not need any treatment to improve the strength properties.

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For production of premium quality grades, refining of long fiber improves such strength properties as burst or CMT. Long fiber strength improvements have been 30-35%, with energy input of 70 kWh/t. Some 35% of long fiber is used in the final product, which gives about a 10% strength improvement measured in the paper machine.

YIELD

Raw materials entering the lines are brown-grade recycled fiber (1.02 and 1.04). An average line feed capacity of 2,150 t/d is required for the 2000 t/d line accept capacity. This means an average line yield in the range of 92-94%.

Usable fiber loss over the entire stock preparation line, including pulping area, HC cleaners, LC cleaners, coarse and fine screening, etc., is typically 1.3-1.6%.

ENERGY CONSUMPTION

The total installed power in stock preparation is 11,400 kW without dispersion and 14,350 kW with dispersion. The pumps and agitators consume nearly half of the power, 136 kWh/t and 172 kWh/t respectively. The process equipment requires 55 kWh/t without dispersion and 66 kWh/t with dispersion.

CONCLUSIONS

At 2,000 bdt/d, the stock preparation line for PM 6 is one of the largest worldwide and produces a clean stock with good strength properties for the production of corrugating medium with low basis weight. A well-functioning screening concept allows a sticky reduction of 99% in short-fiber fraction, 98% in medium-fiber fraction and 86% in the long-fiber fraction. Because of efficient screening, dirtier and cheaper raw material can be used. In addition to the efficient stickies and dirt removal, the entire stock preparation system has fiber loss of just 1.5%.

The long-fiber fraction can be treated with a refiner and a disperger. Because of the clean stock, it was not necessary to use the disperger. The surface of the testliner is clean without impurities or dirt spots. Production of standard corrugating medium is also possible without using the refiner. The refiner is only required to produce supreme quality. Due to the good strength properties, total starch amount is only 4-5%.

The total energy consumption without dispersion is about 55 kWh/t, which is favorable compared with other stock preparation systems.

Fractionation performance.

Sample  CSF    BMcN  BMcN  BMcN   Fiber
point   (ml)   +16   +30   -100   length
               (%)   (%)    (%)    (mm)

Short   250    1.85  20.3  43.45   1.18
fiber

Medium  281    2.35  22.3  38.55   1.31
fiber

Long    455   13.3   28.8  23.4    1.58
fiber

WHAT YOU WILL LEARN

* The operating experience at Palm's Worth's PM6.

* Pulp quality results from the stock prep system.

* Details from each section of the stock prep system.

ADDITIONAL RESOURCES

* For more information on the production and converting of linerboard and corrugating medium, access Corrugating International online at www.tappi.org. To access CI, click on Publications from the home page.

ABOUT THE AUTHORS:

Tero Jussila, M.Sc. (Tech), is with Metso Paper, P.O. Box 125, 37601 Valkeakoski, Finland, Tel. [phone], Fax [phone], e-mail: [e-mail].

Thomas Welt, Ph.D. and Bernhard Wirth, Ph.D. are with Papierfabrik Palm GmbH & Co. KG, Am Oberwald 2, 76744 Worth, Germany, Tel. [phone], Fax [phone], e-mail: [e-mail], [e-mail].