The Pulmac Lab Master Screen:
A Window on Pulping, Refining, and Screening

W.B. Cowan, CD, MSc

GENERAL

The Pulmac Lab Master Screen is a versatile tool for monitoring particle levels in pulp. It is well established that all stages in the various pulping processes create and/or modify the particle distribution in a given pulp.

The Pulmac Lab Master Screen can produce a highly repeatable and size-based snapshot of this particle distribution determined by the slit width of its screen plate. By creating a baseline of these snapshots at various process locations, a database can be developed that will provide great sensitivity to change in the distributions of particles at any of these process locations.

In virgin pulps shives are the particle of interest. In re-cycle pulps stickies command particular attention. The Pulmac Lab Master Screen gives the capability to accurately monitor both.

• This dependable instrument can be employed as a back-up and calibration tool for on-line shive classifiers.
• It can generate pulp quality benchmarks to label pulp based on shive content.
• It can be used to optimize a recycle mills stickies dispersal strategy.
• It can be employed to quickly troubleshoot process upsets.
• It can assist in refiner and screen room maintenance programs.

The shive baselines established with the Pulmac Lab Master Screen can also make a significant contribution to the knowledgebase necessary in implementing automated process control strategies in new fiberlines.

IN KRAFT PULPING

For obvious economic reasons kraft pulping rates need to be as high as can be safely sustained without seriously damaging fiber quality. The greater the production through-flow in any pulp mill, the lower will be its per ton pulp cost.

But the greater the overall rate of pulping, the greater will be the difference or gradient between the pulping rate deep in the interior and towards the surface of the chips.

This means that fibers near chip surfaces will tend to be overcooked, and fibers remote from the chip surfaces will tend to be under-cooked. Under-cooked fibers are less likely to separate cleanly when blown from the digester, and so appear as the fiber bundles known as shives. The shive content of a kraft pulp will increase with any combination of increased kappa number and increased pulping rate. Maintaining both the kappa number and the shive content of the pulp at nominally constant levels is a significant improvement over relying solely on kappa number. The shives present in kraft pulp frequently receive far less attention than their presence warrants. Shives in the pulp, whether delivered to the papermachine for the production of unbleached paper grades or to the bleach plant for the production of fully bleached pulp, indisputably lower the quality of the final product.

The greater the pulp's shive content, the poorer its quality. Shives present in brown stock paper products such as liner board and bag papers reduce quality:

1. because they reduce visual quality,
2. because they are blemishes which effect the quality of the print images that customers frequently employ for marketing or advertising reasons,
3. because their presence in the sheet contribute to web failures which result in costly downtime.

Shives in pulp going through the bleach plant are more difficult to bleach out than are fibers. The increased concentration of bleaching chemicals needed to ensure complete shive removal is not only expensive but may actually degrade the strength of the bulk fibers.

Incompletely bleached shives, on the other hand, show up as the gray specks adding to a pulp's dirt count.

IN MECHANICAL PULPING

Mechanical pulping mashes pulp logs or chips, producing wood fragments with a very wide distribution of shapes and sizes.

While it is correct to say that chemical pulps contain fibers, it is misleading to say the same of mechanical pulps. Mechanical pulps consist of particles, not fibers. These particles range in size over a very wide domain and contain a large percentage of fines, sometimes exceeding 40%. The particle size distribution for a typical mechanical pulp can only be produced with a shive penalty.

If one tried to operate grinders or primary refiners in such a way as to reduce the shive content to a negligible level, the whole particle size distribution would shift in the direction of more fines and smaller sized structural material. Such a pulp would prove unacceptable as a papermaking furnish. Keeping the shives within a certain range is a useful means to establish coarse control of the grinding or refining operation.

The particle size distribution of mechanical pulps shows particles with dimensions nominally in the wood fiber range merging into a domain where particle size is significantly above wood fiber dimensions. It is impossible to define precisely the transition size beyond which a particle is a shive. Nonetheless, the ideal size distribution from which quality papermaking performance can be expected will initially include a small but significant number of shives occupying the larger particle size end of the distribution.

In fact, the proper positioning of the particle size distribution from grinders or primary refiners can be empirically related to the shive content. Shive measurement can therefore be used as a quality indicator. However, mechanical pulp shives will always show a distribution of shapes defined by aspect ratio, going from squarish to elongated.

Significant changes in this distribution can be as important a designator of overall pulp quality change as is the shive content of the pulp. For instance, a significant shift in shape distribution toward low aspect ratio shives is found when the burr on the grinding stone needs sharpening, or the refiner plate needs to be changed. So in order to maximize the utility of shive analysis, rather than using only a weight or length measurement, shape must also be assessed.

IN THE SCREENING ROOM

Screening efficiency can be properly controlled by determining the shive content of pulps throughout the screening system.

Feed Pulp: The fundamental mechanisms that control commercial pulp screening mean that for a given screening system, the shive content present in the accepts pulp leaving the system will be proportional to the shive content of the feed stock entering the screening system. That is, if the feed stock gets dirtier, so will the accepts pulp. This is why it is important that the original pulping process be operated at optimized conditions. This means producing the least quantity of shives commensurate with quality and balanced against the cost of production. For any mill this optimum point means that the shive content should be maintained within reasonably tight limits. If measurement indicates a shive level significantly above or below this optimum range, the pulping process has strayed outside its operating norm and the problem should be investigated.

Accepts Pulp: The shive content of the accepts pulp leaving the screening system should be the lowest that can be achieved by proper operation of the screening system itself. The following points are important for screen system optimization:

• The primary screens should be operated at the accept flow rate needed to supply the production needs of the next process, whether the bleach plant or the papermachine.
• The reject rate should be the highest which can readily be accepted by the secondary screening system.
• The dilution rate should be the lowest that can be safely employed.

The secondary screens are operated quite differently from the primary screens.
Here the goal is to reduce the reject rate and increase the dilution rate to the maximum degree commensurate with two criteria:

• that the accepts shive content from the secondary screens be at the same general level as the shive content of the primary feed stock with which they are to be mixed,
• and that the consistency of the accept flow not be reduced to a point where it excessively dilutes the feed stock to the primary screens. Such excessive dilution might result in a decline in accept pulp cleanliness, or in overloading subsequent dewatering equipment.

Rejects Pulp: The pulp rejected from the secondary system is dewatered and refined to reduce the shive level. The reject refiner should be operated in such a way as to reduce the shive content of the secondary rejects to at least a level below that of the secondary feed, if not the primary feed. Depending upon this level, refined rejects are returned either to the secondary or the primary feed pulp. If refining cannot reduce the shive content to at least the level of the secondary feed pulp, then it is undersized for the system and will increase the shive level of the primary feed stock and ultimately reduce the cleanliness of the accept pulp.

This is an inherent process weakness that should be reviewed as a capital project for upgrading. If the shive level from the refiner is well below the shive content of the feed going to the primary screens, then the operational parameters of primary and secondary screening should be reviewed to assess the feasibility of extracting more rejects from the secondary screen and thus improving primary accept pulp cleanliness.

Recycle Pulp Recycle pulp is generally plagued with stickies that adhere to the papermachine and cause web breaks with associated costly periods of shut down and wash up. There are many approaches to eliminating stickies from recovered paper. However these strategies can be broken into the two main categories of mechanical separation and chemical dispersal. But it is well established that no simple arrangement of equipment and processes will be 100% successful at eliminating stickies. So stickies will continue to remain a cause of paper-quality problems with an associated negative impact on production efficiencies. Any stickies control program must include a means to quantify stickies with precision and accuracy.

The Master Screen can be an invaluable tool for both characterizing stickies- related problems through classification and in assessing the efficiencies of elimination strategies.

CONCLUDING REMARKS

Over the past several decades the Pulmac Lab Master Screen has established itself as a rugged, and extremely reliable window on shive distributions in kraft and mechanical pulps. The data generated can be useful in assessing pulp quality and in monitoring and correcting process efficiencies. In just the last decade it has reaffirmed its reputation in the related application of stickies analysis. Visual displays easily prepared by the Master Screen provide "seeing-is-believing" evidence of the numbers generated by on-line systems and often shift testers will become so attuned that they will instantly recognize minute changes in shape and quantity distributions before either weighing or scanning results for computer-based image analysis.

The Master Screen is a powerful tool for backing up on-line shive classifiers and keeping them in calibration. And when on-line sensors detect a change in particle distribution, the Master Screen can be invaluable in quickly isolating the root cause and supporting process corrections. So whether employed as a stand-alone monitor of shive or stickies content in pulp or as a back-up and reference for on-line fiber analyzers, the Pulmac Lab Master Screen occupies as useful place in the pulp and paper industry now as it did when first introduced over thirty years ago.