Chip Thickness Screening and Chip Quality in the Pulp Mill

By Desmond Smith, Ph.D.

Why Screen Chips?
While a large pile of wood chips may seem deceptively homogeneous, the pile is actually composed of large and small chips, light colored and dark colored chips, chips of odd sizes and shapes, and a great number of chips that are more or less uniformly sized with chiseled or angled ends. Some of the chips in the pile are just fine for pulping just as they are, as will be described later in this article. Some require removal entirely, to be selected for another purpose such as feeding a power boiler as boiler fuel. Others are generally acceptable as long as their presence in the chip pile is not too great. But what are these fractions, and how do they impact the pulping process?

It is possible to carefully collect a series of samples and have them classified in such a way that we can develop a picture of the natural distribution of sizes present within a chip pile. Walking up to the chip pile and simply collecting a bag full of chips will likely not produce a sample that will represent the range of chip sizes present throughout the entire pile. In fact, getting a representative sample can be a very aunting and challenging task: specific sampling schemes, sampling locations, specially designed sample collectors, procedures, and many other issues must be addressed before the critical question of representation of a sample has been addressed. Suffice it to say that the combination of natural variability in chips and chip sampling represents a significant challenge, and a potential source of great misunderstanding if not done correctly.

In a nutshell, size does matter. Chips of a large size pulp more slowly than smaller chips, and this difference in pulping rate has a profound impact on pulp yield, pulp quality, digester operation, post digester pulp handling, pulp screening and cleaning, refining, and more.

Chips are also not uniform in chemical composition. Juvenile wood is lower in wood density and cellulose content than mature wood within the same tree, so that chips formed from the core portion of the stem are different than those formed from the outer portion. Wood density has a major impact on pulping yield in mechanical pulping, and thinnings used as raw feed stock for mechanical pulping operations have much lower density that wood that is derived from mature stock. Similarly, wood color has a big impact on pulping chemical usage and bleaching chemical usage in newsprint and TMP operations, so that the use of light colored chips and the preservation of this light color are extremely important to mechanical pulp quality.

Taken together the many chip qualities that exist in a chip pile are profoundly influential in the pulping process. Knowing what this range of variability may be, and having a means at our disposal by which we can affect this variability can make the entire downstream processes run more smoothly and efficiently. The payback for chip quality control is high owing to the large volume and high value of this most important raw material feed stock.

The general phenomenon of chip size distribution and its impact on pulping and the paybacks that can accrue from proper control of chip size are worth exploring here.

Chip Size Distribution
There are four general categories of chip size that are significant to pulping operations.

  • fines compose the smallest portion of our distribution and are usually heavily contaminated with sand, dirt, grit and other mineral contaminants; usually defined as -3 mm round hole 
  • pin chips are marginally larger than fines and represent a wood based, usually contaminant-reduced or even contaminant-free fraction of very thin, small wood pieces; usually defined as +3 mm round hole, -7 mm round hole
  • accepts is that broad classification of chips which represent those that pulp essentially uniformly, are free of  mineral contaminants and whose upper ranges of sizes pulp with about the same uniformity of those in the lower size ranges; usually defined as +7 mm round hole, -8 mm thickness
  • overthick is the fraction of chips whose pulping speed is slow enough that, at the end of the normal cooking cycle, a central core of wood in each chips remains which is incompletely pulped. Overthick chips are usually credited for generating the objectionable materials that are rejected by pulp knotters, and contribute the most to a reduction in pulp yield; usually defined as +8 mm thickness

It is possible for contaminating materials to inhabit each of the four size classifications mentioned. Generally rocks, especially large rocks, will be removed by any of the normal chip sorting and screening methods and are rejected along with the overthick or oversize. Small pebbles in the -8 mm to +3 mm range exist and can make it to a digester, but will usually either be abraded or reduced in size in the digesting process or will collect in the pulp blow tank at the end of the digestion. Their contribution to wear and tear in the interior of the digester certainly must occur, but is generally not considered to be as significant as the fine grit and sand which is imagined to abrade the interior surfaces of a digester much as sandpaper would if it were done intentionally.

Contaminants other than mineral based rock and sand also occur in wood chips and should be considered with some care. Residual wood chip producers seem to be particularly bad about using their chip stream as a general waste stream. Millwrights and other cleanup personnel might, intentionally or inadvertently, toss broken knife pieces, nuts and bolts, bits of chain or other sawmill debris onto the conveyor which ultimately leads to the chip storage bin. Some care must be taken to ensure that these contaminants fail to reach the digester where they can have a devastating effect.

Digester Sensitivity to Chip Fractions
Overthick chips fail to pulp completely during the digesting process, leading to a condition where the central core of the chip passes out of the digester at the end of the pulping process as essentially solid wood. While the smaller chips around them have had the lignin removed from between the individual fibers so that the fibers are liberated from the desired wood pulp, the central core of hard wood remains black, hard, and unsuitable for papermaking. To separate and segregate these chunks and pieces the pulp is passed though a metal screen with small apertures, allowing only the fibers to be extracted. The hard chunks are then rejected and can be passed to a bunker or shunted through a process in which they are ground by refiners and/or re-introduced into the chip flow and passed back to the digester.

Any of these handling processes reduce the overall efficiency of the system. Knotter rejects in a mill with uncontrolled chip size distributions can run as high as 5 - 6 percent. In fact, some digesters are operated with time, temperature, and pressures in the digester so that a target knotter rejects level are maintained. In this last case, you might think that the digester operators expect a certain amount of overthick chips in their flow and operate their digesters in such a way as to accommodate that flow.

The elimination or pre-treatment of overthick chips prior to digesting so that they behave and pulp like accept chips reduces the level of knotter rejects to a very low level. In the case where 5 percent of the pulp produced by the digester may be knotter rejects, this number can be reduced to less than 1 percent, thereby netting the digesting operation more than 4 percent as an overall yield improvement. When the number of tons of pulp produced today reach a thousand and more, the dollar value of this four percent yield improvement can reach the tens of millions of dollars on an annual basis. In terms of return on investment, chip thickness screening may still represent the most profit returning project available to mill process improvement engineers.

Knotter Rejects
In the case where digester operators adjust their pulping process to maintain a certain level of knotter rejects, and then overthick chips are significantly reduced in quantity through a screening and treatment process, it is possible to increase overall pulp yield in the digester by reducing the time, temperature and pressure components of the cooking cycle. The net effect is an improvement in the yield from each and every chip passed through the digester, with similar tens of millions of dollars annual savings possible. Regardless of digester operational strategy, dollars saved by knotter reductions and basic yield improvements are quite dramatic.

Liquor Circulation
Pin chips and small accept chips are significant in the digesting operation through their effect on the circulation of liquor inside the digester. Considering that accept chips are composed of small to large chips (+7 mm round hole to -8 mm thickness, for example), the smallest of the accepts cook quickly and soften rapidly. These are relatively thin, potentially short wood fragments whose centers are quickly penetrated
by the active alkali of pulping liquor. The fibers begin to become liberated and they go to a pulp-like state early in the process. In digesting systems involving the rapid removal and replacement of pulping liquor, it is necessary for the pulping liquor to actually migrate through and between the individual chips in the digester during the cooking process. If the pin chip (-7 mm round hole, +3 mm round hole) percentage is high enough, these rapidly softening small chips then migrate with the moving liquor and accumulate on the surfaces of the extraction screens inside the digester. The net effect is that they restrict the movement pulping process.

Pulp Dirt
The phenomenon of dirt in pulp can be caused by a variety of sources. One is the incomplete cooking of knots which are ground in the process in some fashion and end up as wood chunks in bright pulp, appearing as dark specks. Dirt can also be bark, which has snuck through the process and through the digester. Or dirt can be actual mineral dirt or earth. Any dark spot that discolors the pulp can be classified as pulp dirt. In all these cases, a chip screening system with sufficient design capacity to segregate and handle the oversize and overthick and that can remove the fines will provide excellent control for pulp dirt.

Digesting System Responses to Chip Control
It would be a simple matter if all of the pulping systems responded similarly to chip size control. Unfortunately, each has its own particular sensitivities and requirements. In Kraft cooking, pulping liquor penetrates a chip in a uniform manner in all directions, making the smallest dimension (the thickness) the rate limiting one. Because of the high volume of chips used in most Kraft mills, this process has received the most attention.

In contrast, sulfite pulping occurs by a different mechanism altogether. In a sulfite cook the pulping liquor penetrates the chip primarily through the lumens of the fibers. This makes chip length critical, not thickness. For this reason sulfite chips tend to be shorter than Kraft chips, and the yield and payback benefits for chip thickness control in sulfite mills is less than it is in Kraft mills.

The neutral sulfite semi-chemical (NSSC) process is a modified grinding process in which the chips are pretreated with neutral sulfite pulping liquor. In this rapid process in which the liquor barely has an opportunity to penetrate the chip before it is being physically ground between large refiner plates, the value of chip thickness screening is nil.

In general, mechanical pulping systems such as thermo-mechanical pulping (TMP), bleached chemical thermo-mechanical pulping (BCTMP), and so forth, the major benefit of chip improving overall chip uniformity. By removing overthick chips and especially knots, it is possible to reduce natural chip variability. Uniform feed means uniform processing conditions, and uniform pulp.

Preheating columns soften chips with steam. Compression screws which feed refiners break down the chips to some extent. The center parts of refiner plates do a good job in breaking down oversize chips before they make it into the refining portions, and so reduce the influence of oversize chips on the actual refining process itself. In the case where pulping liquor is used to pre-condition the chips prior to refining, chip thickness can provide some assistance by aiding chemical penetration to the center of the chip.

Pulp Yield
Ultimately the goal of this process is to improve the overall efficiency of pulping. This efficiency manifests itself through improved yield. Typically a chip thickness system in a Kraft process system will improve the overall yield roughly 2% on an absolute basis. This means that if the current yield is 46% (pulp on wood) then the yield improvement would be around 48%. Again, these are profound and dramatic improvements representing millions of dollars of savings to the mill.

Chip Sourcing for Pulping Operations
Chemical pulping has long been a process whose raw material comes as a by-product of normal solid wood processing. Residual chips from sawmilling operations can represent 100% of a pulp mill’s raw material supply. In recent years mills have been moving to a blend of chip sources which include residual chips but also whole log chips and whole log chips produced by chipping operations designed specifically for chip production (satellite chip yards).

The shift to increased use of specifically produced chips has had the benefit of improving chips size uniformity and reducing the overthick chip burden to many pulp mills. Modern whole log chipper designs have the capability of achieving accept chips in excess of 90%, with few fines, thins, and overs produced. Sawmill residual chips from trim ends, cut offs, and waste wood sources may only produce 65-75% accept chips, with a large percentage of sawdust, fines, pin chips and overs included. Residual chips are a natural consequence of dealing with a nonhomogeneous, non-oriented waste wood stream.

What Extent of Chip Size Modification is Possible?
When the raw material supply to a pulp mill may typically contain as many as 15 percent overthick chips (or more), what can a chip thickness screening system be reasonably expected to produce for the digester? In most cases it is possible to reduce the overthick level in the chips to the digester to less than 3 percent on a continuous and consistent basis. In some cases it is possible to get to 2 percent when everything is running in peak operating condition and is well maintained. Below this is very difficult and can be very expensive to achieve.

Of more consequence is the selection of the overthick control point in the process design phase. The next in this series of articles will discuss the relationship between chip size distribution, overthick chips, and pulping response. The relationship between overthick chips and knotter rejects, for example, is an exponentially increasing one, meaning that, as chips increase in thickness by one unit, the knotter
reject levels increase by many times this, above a certain point. Consequently, a process designer might look at this and want to select a thickness control point as low as 5 or 6 mm so that the knotter rejects can be held to an absolute minimum. Unfortunately this has severe implications in the design of the screening system and the use of the overthick processing equipment. What is the right control point? We will discuss this next time.

Desmond Smith, Ph.D., is the Vice President and General Manager of Acrowood Corporation. Known as “Dr. Chips”, he has a doctorate in forestry from the University of Missouri, and has considerable experience in the field of chipping, chip screening and wood quality.

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