A number of worldwide mechanical pulp mills have found benefits using fine-slotted wedge wire baskets over traditional slotted baskets

Pulp Screening Advancements Reduce Shives, Boost Quality

Screen rooms in many mechanical pulp mills are being rejuvenated today as a result of a significant breakthrough in screening technology that occurred nearly eight years ago–i.e., the advent of fine-slotted wedge wire baskets. When combined with low screen loading and low pulse rotors, this technology can yield significant improvements in shive removal, enhancement of physical pulp properties, and, in some cases, reduction of electrical power.

The design of the wedge wire basket produces a continuous slot over the length of the basket (Figure 1).

Figure 1: The wedge wire basket design results in a continuous slot yield twice the open area of a conventional basket.

Vertical, parallel rods or "wires" are arranged in a circle to form the shape of a cylinder. The wires are held mechanically in place with metal bands or rings. The size of the gap or slot between the wires is precisely set. Typical values are 0.10 mm, 0.15 mm, and 0.20 mm. A cross section of the wire resembles the shape of a wedge, the specific shape varying among suppliers. The angle of the wedge (Figure 2) and the amount that the wedge protrudes into the stock (i.e., the profile) affects the screening performance.

Figure 2: Wedge wire basket design parameters.

By comparison, conventional slotted baskets are built from rolled sheet metal. Parallel slots, each several inches long, are cut into the sheet metal, leaving land areas between each slot to provide mechanical strength to the basket (Figure 1). The wedge wire basket, by virtue of the continuous slot and slender wires, has a much lower land area. This results in approximately twice the open area as the conventional basket for the same slot width. For any given screen, this advantage yields the following benefits:

• Lower passing velocity for a given production rate and slot width.
  
• Higher production rate for a given slot width and passing velocity.
  
• Finer slots for a given production rate and passing velocity.
  

Furthermore, wedge wire baskets facilitate the use of smaller slots than previously possible. Conventional baskets are limited to a minimum slot size of 0.15 mm as compared with 0.08 mm for wedge wire baskets.

Since by definition the width of a Sommerville (6-cut) shive is 0.15 mm, use of slot widths 0.15 mm or smaller results in high shive removal efficiencies approaching that of a barrier screen. Consequently, many integrated mills (e.g., newsprint, directory, SC, and LWC) have shut down the mainline cleaners in the TMP plant in conjunction with upgrading their screen room to narrow-slot wedge wire baskets (Tables 1 and 2).

In such cases the result has been an improvement in pulp quality and lower energy consumption. Elimination of TMP mainline cleaners may not always be appropriate, especially if the paper grade demands very low dirt levels. A prerequisite is a chip supply with low dirt/bark content and an efficient chip wash system.

For maximum performance the use of wedge wire baskets should be accompanied with low passing velocity¹ (less than 0.8 m/s) and the use of low pulse rotors.² Low passing velocities necessarily means large screens. The use of low pulse rotors implies feed consistencies in the range of 1.5% to 2.5%. Higher feed consistencies (i.e., 3.5% to 4.0%) require high pulse rotors to prevent screen plugging. More aggressive rotors have an increased tendency to force "reject material" through the slot.

In some cases, but not all, screen rooms designed to operate at these higher consistencies have significantly lower capital costs. Pipe diameter and pump sizes are reduced. Thickening requirements and volumetric flow of white water may or may not be reduced depending on the particulars of each mill site. If the capital cost savings are significant and there is no penalty in operating costs, and if it is not necessary to obtain the ultimate pulp quality, then screening at 3.5% consistency should be considered. Otherwise, conventional feed consistency should be used. In either case, pulp quality is improved by selecting modern screening technology over conventional screening technology.

NEW OBJECTIVES. The objective of pulp screening has historically been shive removal. With the introduction of narrow-slot wedge wire baskets, the removal of coarse, stiff undeveloped fibers is becoming an additional objective of the screen room. Such fibers have poor bonding potential. They contribute to lower sheet strength, increased linting tendency, and poor sheet consolidation.

Ideally all of these undesirable fibers should be removed from the main pulp stream and converted into flexible long fiber with the reject refiner before being recombined with the main pulp stream. Likewise, flexible long fiber should pass unimpeded through the screening and cleaning system into the final accepted pulp to avoid possible fiber breakage in the reject system. Conventional screening technology separates fibers indiscriminately according to fiber length, directing both flexible long fiber and coarse, stiff long fiber to reject treatment.

With relatively large basket openings and high slot velocities the separation is inefficient carrying many stiff fibers into the accepts. As a result, conventional screening technology shows no improvement in the fiber bonding index from feed to accepts.

In contrast, screens equipped with very narrow slots, employing low slot velocities and low rotor tip speeds, do show an improvement in the fiber bonding index from feed to accepts. This suggests that such screens do have a tendency to reject long fiber according to fiber stiffness. Evidence for this exists in the research literature and at mill sites.^3,4,5,6,7 At Enso Publication Papers (Varkaus, Finland), for example, the TMP mainline cleaners were shut down in conjunction with conversion to the new screening technology. It is reported that they obtained the following changes in pulp quality and energy consumption:⁸

• Freeness: 6% decrease
  
• Fiber length: 6% increase
  
• Sommerville shives: 55% decrease
  
• PFI minishives: 36% decrease
  
• Tensile: 7% increase
  
• Tear: 11% increase
  
• Energy: 9% decrease
  

These results suggest that the new screens successfully removed a significantly greater portion of the coarse, stiff fibers and accepted a larger portion of the flexible long fiber.

The foregoing discussion does not suggest that modern screening technology is the ultimate solution to removal of undeveloped fibers. Rather it argues that modern screening technology, as opposed to conventional technology, accomplishes this objective to some degree.

Wedge wire baskets do have one clear disadvantage. They are mechanically weaker than conventional baskets. Early designs failed mechanically until improved manufacturing techniques were developed. The more aggressive the rotor, the higher the likelihood of basket failure and/or wire flexing. Wire flexing, if it occurs, creates a fluctuation in slot width to the detriment of screening performance. The lower mechanical strength of wedge wire baskets favors the use of low pulse rotors and, in some basket designs, it precludes the use of aggressive rotors. Low pulse rotors improve the screening efficiency, lower the screen capacity, and require conventional screen feed consistency (i.e., 1.5% to 2.5%).

UPGRADE JUSTIFICATIONS. It is possible to upgrade existing screens to wedge wire baskets and low pulse rotors. An example of this is the recent upgrade of the TMP screen room at Bowater Mersey in Liverpool, Nova Scotia, performed by NLK. Upgrading existing screens may not be economical if the screen room consists of many small screens (as opposed to a few large screens). In such cases it is often less expensive to buy a few larger screens.

There are many possible justifications for upgrading an existing screen room to wedge wire technology including the following:

• Reduction in shive level equals a reduction in paper machine breaks.
  
• Improvement in pulp quality (e.g., strength, bulk) equals a reduction in kraft content in newsprint furnish and an improvement in paper machine runability.
  
• Reduction in energy for reject refining (i.e., by keeping pulp strength/bulk constant).
  
• Reduction in energy consumption in screen room (e.g., low energy rotors).
  
• Shut-down mainline pulp cleaners equals an energy savings due to lower pumping costs and lower maintenance costs.
  
• Production increase (if screening and cleaning is a bottleneck).
  

The latest development in screening technology is the multi-stage screen. Several stages of fine slotted wedge wire baskets are combined into a single screen. The pulp is diluted between stages to prevent overthickening. For example, the first Valmet MuST screen was put into operation at UPM Rauma's groundwood mill in the spring of 1996.9 It replaced nine screens (three P1, three P2, two S, and one R), reducing the installed motors from 2,225 hp to 330 hp. In 1998 a MuST screen was started up in the groundwood mill of Abitibi Consolidated Inc., Thunder Bay, Ontario. This innovative screening concept greatly simplifies the design and operation of pulp mills and may become the next major trend in pulp screening.

ERIC CANNELL is process specialist, NLK Consultants Inc., Montreal, Quebec.