 Size distribution, bark content, and moisture content are the most commonly run tests to measure these basic characteristics. Reliable quality control data requires representative samples. If the samples are not representative, there is no reason to do the tests since the biased data will lead to wrong conclusions and decisions. Such samples are possible with careful selection of a safe sampling point, use of a sample scoop specifically sized for the chip testing process, and insertion of the sampling scoop into a free-falling flow of chips. This can be done manually or with an automated sampling device. A well-trained and experienced chip tester knows a good chip when they see it and is they are the most important component of a chip quality program. Once the data is available, it should be reported in a graphical format to everyone in the supply and pulp mill organization that is part of the chip supply and pulping process.
Size distributions, as measured in a chip size classifier, separates chips into fractions that effect pulping and pulp quality. Typically these fractions are:
Fines (dust):
The smallest fraction that passes through a 3 mm round hole. These are often high in bark and dirt.
Pins or thins:
Matchstick sized or thin material, that pass through either a 7 mm round hole or a 2 mm slot. These small chips can cause problems in continuous digesters by plugging the screens through which pulping liquor is circulated.
Accepts:
Chips between 4-8, or 4-10 mm thickness. This is the desired chip size range for most digesters.
Overthick:
Chip greater than 8 or 10 mm thickness. This depends on the wood species and digester system. Overthick chips partially cook leaving the center mostly undercooked.
Overlength:
Chips retained on a 45 mm round hole screen. These chips are too long or wide to use in any pulping system. They contribute to the plugging of bins and feeders and only the outer few millimeters pulp sufficiently leaving the center uncooked.
There are many different types of classifiers, using different configurations or combinations of round holes or slots. It is important that you know what classifier is being used to define chip quality and the combination of screens used in the classifier. Below is a picture of classified chips using one of the common size classification series:
(used with permission of Georgia-Pacific Corporation)
Fines and dust particles have little value in the pulping process. There is little left of them after pulping and they consume excessive pulping liquor, putting an additional load on the recovery boiler. Pulp production at many mills is limited by the capacity of the recovery boiler. Also, fines usually contain a high proportion of bark and dirt that cause pulp cleanliness problems in the bleaching and papermaking processes.
At the other end of the chip size distribution the oversize chips. These chips will not be fully cooked and the undercooked chips require additional processing after passing through the digester. They are usually screened out and sent back to the digester for a second time, resulting in lower yield than before and taking up space inside the digester that good quality chips should be filling. It usually takes several passes through the system before an oversize chip will be fully cooked. These undercooked chips called "knots" might be better disposed of by burning in the hog fuel boiler. Both of these options give a reduced pulp yield. Below are two overthick chips that were pulped in a digester with good quality chips. The overthick chip on the left is one that has partially cooked. The cracks in the chip that were created in the chipping process allowed some liquor penetration into the chip. On the right is an overthick chip from the dense wood around a tree branch. Only the outer 4 to 6 mm were pulped and the center has almost been untouched by the liquor.
Pulp mill operators want an optimal chip size distribution to maximize pulp yield and throughput. Each mill has a different pulping system and therefore, a different chip size specification. For example, if the chips are cooked in continuous digester, a low level of fines and pin and thin chips is needed to maximize liquor circulation and smooth movement of chips down the digester. A batch digester can tolerate more pin chips, but needs chips that are primarily 4 to 8 mm in thickness.
Various types of chip screens can be used ahead of digesters to eliminate chips that are not of the desirable size. The fines end up as boiler fuel. As pulp chips cost more than hog fuel the mill incurs a value loss when it diverts fiber to the boiler. Overlength and overthick chips are reduced in length and thickness before returning to the chip flow. Below is a typical screen system. The larger chips come off the top of the gyratory screen. Not visible is another screen plate below that removes the undersize pin chips and the fines. The second screen is a disk screen that removes the overthick and overlength and sends them to a crusher or slicer that reduces them to pulpable size.
A better strategy for achieving chip size uniformity is to make the desired size chips in the first place. Chippers are large, powerful machines containing a disk with multiple knives that can cut logs and wood product residuals into chips at high production rates. They are precision machines and need careful maintenance to obtain low variability in chip size hour-by-hour, day-by-day, and month-by-month. Below are two typical chippers that produce chips from round, low quality logs and from wood products residuals trimmed off in lumber manufacture. On the left is a whole log chipper disk with the hood off for maintenance. On the right is the infeed spout to a sawmill residual disk chipper. A knife is visible as it comes down toward the side anvil where the wood is held as it is chipped.
Significant improvements in chip quality can be made by good maintenance practices. Simple solutions such as replacing knives more often, maintaining close tolerances between the knife and the anvil or replacing worn chipper parts is a good place to start.
There are other things that can be adjusted on a chipper to improve chip quality:
• Changing the knife projection from the disk will optimize the length and thickness of chips. Longer chips are usually thicker chips. Hardwood chips need to be shorter than softwood chips.
• Changing the knife angles will change how the chip is formed. For example, if the knife is tilted from vertical too far, the log will hit the disk and bounce back rather than feeding smoothly.
• Reducing the disc speed will decrease the amount of fines and pin chips produced.
Only only trained employees should carry out these changes. If close tolerances, parts assembly and precise adjustments are not made correctly, the knives can strike the anvil, come out of the knife holder and destroy all the faceplates, knife holders and break the shaft or disk.
Wood characteristics affect the way chips are formed. For example, moisture content will impact the size and uniformity of chips. Moisture will change in stored logs due to season, length of storage and size of the logs. Tree species and trees of different age will also chip differently. Pine thinnings produce less uniform chip size than logs from older stands. Winter conditions, particularly when the wood is frozen can cause a major increase in the proportion of small chips. Winter adjustments that can be made to the chipper include decreasing the knife angle, changing knives more frequently, increasing the knife length, and slowing down the disc speed. To produce the desired chip size all year round, the chipper must be continually adjusted to match the climate and the wood conditions.
Bark is a natural contaminant in wood chips. Even with excellent debarking, at least 0.5% bark will remain in the chips. Mills can tolerate this low level and with state-of-the-art bleaching sequences, bark levels of 1 to 4% can be used and still meet customer specifications. That is not to say that less attention can be paid to debarking, but in extremely cold winter conditions, it is impossible to consistently maintain less than 1% bark content without incurring high wood loss.
Contamination of chips with waste is a problem in almost all mills. Plastic is usually the most serious problem. A single sandwich back can contaminate tons of pulp production. The only way to keep plastic containing trash out of chips is continuous education all along the supply chain. Sawmill workers, for example, must not consider the chips a place to dispose of trash. Photos like this can be used to emphasize the impact of even a small amount of plastic and paper cups with a message can replace Styrofoam cups.
 The definition of chip quality has changed a lot in the last 10 years. It will continue to be modified as pulp mills improve pulping equipment and change pulping liquor flows. As sawmills develop higher lumber yield systems, their chip dimensions and shape will likely change, too. Low variability will certainly be demanded more and more as pulp mills seek ways to produce more pulp with existing equipment. Finally, our chip quality measurement tools should change to reflect the actual impact of chips on individual pulp mills. A more precise definition of the small fractions of chips is needed to relate their impact on pulping liquor flow.
Sample correctly, test only the properties that are really important, talk to the pulp mill and the outside suppliers often, and hire, train and retain the best chip testers you can find. Those are the basics of defining chip quality.
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After a long and successful career at Weyerhaeuser, Bill Fuller became a consultant to the pulp and paper industry worldwide. You can reach him at Consult4FRM@aol.com or +1.253.315.1519
Copyright 2012 Bill Fuller, All Rights Reserved
Used by permission, Paperitalo Publications
As a member of Paperitalo's Consultant Connection group, Bill has exercised his privilege to publish here. Paperitalo Publications is grateful for his contribution.
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