Chip Thickness Screening and Chip Quality in the Pulp Mill
By Desmond Smith, Ph.D.
Why Screen Chips?
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
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
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.
Digesting System Responses to Chip Control
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.
Chip Sourcing for Pulping Operations
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?
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
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.