Ask Bill

To me it has always been "debarking" and "debarker drum," but on two occasions I've seen the unit referred to as a "barker drum" or just plain "barker." It seems that this is a typo that has been propagated - can you clarify this?

Debarking is the correct terminology. In the early days of drum debarking, several major manufacturers used the "barking" term. Fibermaking (no longer in business) actually had a logo with a big bulldog standing inside a "barking drum". Old habits are hard to break and you will hear people refer to barkers, barking drums and ring barkers for a long time. Similarly using correct English, “debarking drum” is correct. Thanks for using the correct terminology.

The pulp mill we supply is starting to complain about plastic in its chip sources. We are not sure how much might be getting into our sawmill chips, but want to start taking action now to maintain our relationships with the chip buyer. What we do to prevent the problem? Isn’t there something inside the pulp mill that can be done to solve the problem?

With the universal use of plastic for packaging and containers, plastic contamination is a serious problem in the industry. In the pulping process, each of the different types of plastic reacts differently. While reverse pulp cleaners and barrier screens are capable of reduction of plastic in pulp, the most effective and economical way to eliminate plastic contamination is in the chip supply system. Unfortunately, a reliable way of removing plastic from chips has not been successfully developed. Air blowing systems, heated air and heating drums have been partially successful in removing large pieces of polyethylene bags, but this is only a fraction of the types of plastic found in chips and large amounts of small chips blow out, too. The most successful method of keeping plastic out of chip supplies is education of all the employees of chip suppliers, the companies that transport chips and finally the pulp mill where chips are received and stored. It must be clearly shown to them that even small amounts of plastic will contaminate and downgrade tons of pulp and paper products. This can be done by showing them samples of pulp with plastic in the sheet. The chips must be viewed as a valuable product, not a place to dispose of even the smallest amount of waste. The education should include signs that clearly state that control of plastic contamination is everyone’s job. More than enough well-marked waste containers must be provided in areas where waste is generated. Waste containers must be regularly emptied to encourage their use. Some mills have the message printed on paper (not plastic) cups in the coffee and soft drink vending machines. The use of plastic is prohibited for packaging of parts and supplies delivered to many mills. Periodically, a group of employees patrols the mill to pick up plastic waste, identify its source and follow-up to get cooperation in plastic control. Landfill disposal sights the mill should be fenced to prevent blowing plastic waste into the mill site. A good article on this subject was written in 1988, but it’s content is still valuable. “Plastic contamination in the pulp mill: An overview” by M.A. Robitaille, Pulp and Paper Canada 89:1 (1988). If you cannot locate a copy, I will be glad to fax it to you. I hope these suggestions are useful.

How much chip breakage occurs in chip pile systems? Specifically, in pneumatic conveyors; from belt conveyors; from chip dozers; and from chip pile deterioration?

Size degradation occurs to some extent in all chip handling systems. There are fragile chips such as dried veneer and a fraction of the chipping headrig (sawmill shaping chippers) chips that will break into pin chips with normal handling. Loosely attached fine particles are adhered to almost all chips and will break off, also. The latter is really not fines generation since they will react the same in pulping either attached or un-attached. They can just be measured after they break off. The spectrum of breakage varies widely depending on the conveying, delivery, storage and reclaim system. Each component contributes to the total. At one extreme is the gentle handling occurring when only belt conveyors and stacker/reclaim storage systems are used. Some mills have not been able to detect any change in chip size through such a system. This does not mean it didn’t occur, but it is small enough that the variability in chip properties masks it. At the far extreme is a system that uses pneumatic handling entirely and the chip pile is managed only with dozers (tracked and rubber-tired). There is some difference in tracked vs. rubber-tired, but the big issue is how far the chips are being pushed and how often the dozer travels over the same chips. Thus, chip roads to the top of the pile and around the reclaim pit suffer the worst damage. Operators that feel it is necessary to “manicure” the top of the pile also create excessive damage. A rule of thumb that has been substantiated by several tests says that pin chips increase 50% and fines double in the latter case of rough handling. Pneumatic damage is worst when the chips are blown long distances (800-1200 feet) through a number of sharp turns and when flatbacks at the elbows are repaired by welding, creating a rough surface that abrades the chips. Some mills have replaced pneumatic systems with belt conveyors based on both the fines generation fiber loss and the 5-10 times higher power cost of blowers. There is some published data in the out-of-print Chip Quality Monograph published by the Joint Textbook Committee of the Pulp and Paper Industry (CPPA and TAPPI) in 1979.

I am interested in pulping Southern pine sawdust fines. Do you have a good ballpark yield number for pulping them along with standard sized chips vs. pulping them separately? Our target kappa number is 95.

 Since I doubt that you would actually be thinking about using sawdust from wood products manufacture in linerboard (the properties are close to hardwood pulp), I am going to assume you are referring to the chip rejects removed by chip screening systems. If you are actually interested in sawdust, contact me and I can provide information on that. Chip fines and sawdust are quite different in particle size and shape.

The composition of chip screen rejects is actually a mixture of both pin chips and fines. For this discussion, pin chips are nearly the same length as normal chips, but have a cross-section that will allow them to pass through a 7mm round-hole classifier tray (match-stick or smaller). Fines are short and thin slivers or small chunks of wood that will pass a 3mm round-hole classifier tray. The fines fraction has little pulping value and is detrimental due to its high content of bark and grit. Therefore, mills that want to recover pin chips for pulping use a pin chip recovery screen that is either a gyratory or roll screen configuration. Use of pin chips is ideal for a batch system such as yours and the payback is usually quite quick. The first step in evaluating such a project is to determine the amount of pin chips and fines in the screen reject flow. If you receive a substantial amount of chips from sawmills, there could be 50-75% pin chips in the screen rejects.

Generally, pin chips have about 90% of the yield and strength of normal accept chips. For high kappa linerboard production, the rapid penetration of pin chips could give you better yield than this. Of course, all of this depends on the composition of the screen rejects, but this has been the experience of the many bleached mills that have implemented such a system.

A presentation on the value of small particles in pulping was presented at a wood yard superintendent workshop (WESTAG) and is available by e-mail if it would be useful.  

I would like information on the optimum chip size distribution and quality requirements for a batch digester system to maximize wood yield. We are a southern mill with about 40% pine and 60% hardwood wood furnish. Do you have a recommendation or where might literature be available?

This is a very complex question and I can only give you a brief summary in this response. Every pulping system has an optimum size distribution that will maximize productivity, quality and uniformity. Most mills get close to this by experience, trials and installing chip screen systems to minimize the extremes in chip size. The best information comes from laboratory cooks using the mills chips and pulping conditions. Some say that such data does not represent the mill situation. That is true. It does represent the best the mill could obtain if the pulping system and the chip size distribution were operating at their optimum. This is the target that the mill can work toward with chipper optimization at their suppliers, chip screen systems, minimizing damage in handling and storage and whenever possible, blending chip types to feed as uniform as possible chip flow to the digester. There is abundant literature data on this subject that I have in my files, on the TAPPI website from conference papers and from woodyard equipment suppliers with strong technical expertise in the relationships of chip quality to pulping performance.  

What type of on-line chip moisture meter would work best for providing data to control continuous pulping? The temperatures get down to -30 to -40 deg. C during the winter. During the winter, chips usually arrive at the mill frozen and the piles sometimes stay frozen for months.

The meter types available on the market use radiation from infra-red, microwave and gamma ray sources. Only the gamma radiation systems are effective in measuring the moisture content of frozen chips. Some suppliers claim that frozen chips can be detected and the meter will adjust the values based on training the meter response to frozen wood. This is not the same as being able to directly sense moisture content of chips that are frozen, partially frozen, blends of frozen and thawed chips, and ice-coated chips.

Chip moisture meters give better data than sampling a chip flow periodically and doing oven-drying tests. Only a few tests can be taken this way and only the seasonal trends are statistically valid. For operational data, the moisture meter can sample every few seconds and calculate a time-weighted moving average that will be far more accurate than any manual sampling and oven-drying test.

Some question the accuracy of moisture sensor data. As with all sensors, chip moisture meters must be routinely maintained and calibrated. It is reasonable to expect =/- 0.5 to 1.0% accuracy from well-calibrated and maintained moisture meters. Part of the calibration process is cross-checking the meter results with laboratory tests on representative chip samples. It is very important that the lab method be correct and reproducible. The oven-drying method is most common, but some mills are using microwave ovens. Microwave ovens are not recommended for chip moisture testing. Only a few grams of chips can be dried in a microwave oven compared to 1000 to 2000 grams in high capacity, forced-air chip ovens. Finding the optimum power setting and length of the drying cycle is impossible using kitchen microwave ovens. Once a scorched wood odor is smelled, the chips have already been over-dried. Then, under-drying often occurs to avoid over-drying. Precision, microwave ovens are available that weigh the sample during the drying cycle, reduces the power as moisture content decreases and turns off the oven when a constant dry weight is achieved. However, the small sample size is an issue even though the correct procedure of drying to constant weight is used.

How can an inclined conveyor belt be safely secured so that gravity won’t cause the belt to run back down the idler rolls and causing damage and possibly serious injury. This has been identified as a Lock-Out/Tag-Out issue due to the potential energy generated by gravity.

Some conveyor belt manufacturers market a bar clamp that span the belt width and is strong enough to prevent slippage. The clamps are chained or cabled to a strong structural member in the conveyor structure. Back-up restraints are advisable and for wide belts, redundant clamps may also be needed. Several mills have constructed belt clamps in their mill shops. An experienced woodyard operator has successfully used a one-way drive on the head spool that will also hold the belt in place. A bar or rod through the spool can provide a back-up for the one-way drive or a clamp system. Contact a conveyor belt system supplier for the availability of these safeguards and guidance on making modifications to their installed designs. Once installed, the safeguards must be included on regular safety inspection checklists.

Is there a reliable fire suppression system for chip dozers? We experience compaction of the dry chemical which prevents discharge.

Fire codes specify ABC class fire suppression systems on chip dozers, limiting the choice of extinguishing materials to dry chemical. Foam systems may provide secondary protection, but are not suitable as the primary. Quarterly or more frequent inspection of fire suppression systems is essential to detect equipment failures and chemical caking. Consult the supplier of your systems for their recommended inspection and maintenance procedures. Additives are available that will help prevent caking from settling and moisture pick-up.

Fire prevention is the best approach. Daily inspection of the entire chip dozer will spot fuel, pneumatic fluid and oil leaks. The operator should not hesitate to red-tag the dozer if there is a fire hazard such as dripping on a manifold or exhaust pipes or any other unsafe condition. Electrical connections must be secure and insulated. Smoking must be prohibited when operating the dozer. Accumulations of chip fines and sawdust should be blown away several times a day, if needed on piles with high fines and during dry seasons. Radio communication between the dozer operator and chip storage system operators must be maintained.

How can ice build-ups be prevented on dozer catwalks and steps during the winter?

Catwalk surfaces can be coated with an abrasive material and flat-surface grates replaces with toothed surface grates. In most pulp mills, steam lines are accessible or can be extended to provide removal of thick ice buildups from frozen rain or melting snow. There have been some trials using aircraft deicing chemicals, but no results have been reported. (Let us know if you know anything about these tests.) All chemicals used in pulp mills must be compatible with the pulping systems and this is especially true if it is absorbed into the chips.

The “3 Points of Contact Rule” will prevent most falls, even on slick surfaces. This means that at all times only one hand or foot is not making firm contact with the dozer surfaces. This is good practice at all times and in any weather conditions.

Is there a safe way to clear a chipper chute plug-up without having to go near the chute to hook up tongs, cables, chains and reach down the chipper throat with pike poles?

A product called the “Jam-Ram” marketed by Sun Machinery has been successful in clearing logs bridged in the chute. Below is a photo of a set-up in a gravity feed whole log chipper. One or both of the rams are remotely operated to jostle and push the logs until they feed into the chipper. Since the chipper is not stopped, a jam is cleared much more quickly than would be the case if the chipper had to be shut off, locked out and tagged out.

CEM offers chipper chute modifications to clear plugged spouts. Hydraulic lifting arms move the bottom of the upper part of a chute in an action similar to a dozer blade. This moves all the logs and several such lifts will usually clear the jam. Extreme plug-ups at the bottom of the chute can be cleared with a split chute. The upper half of the chute is hinged for quick opening and relieving the pressure binding an oversize log, for example.

During the winter, chips can freeze into a block that is also frozen to the sides and bottom of railcars. How can this be prevented and how can the chips be dislodged when a freeze-up occurs?

Mills have tried a number of things to prevent freeze-ups and to free them up. The ones that are not effective include spraying the sides of the cars with diesel oil (pulping and environmental issues, also) or an anti-freeze formula. Car shakers/vibrators are marginally successful due to the ability of wood chips to absorb energy so that the vibration does not focus on the interface between the car wall and the chip block.

The coal industry faces this problem frequently and commonly construct warming sheds to heat up the walls of the cars. Waiting until sun warms the car walls also works. An attendee at a recent MOTAG meeting suggested putting flat rubber bladders on the floor of railcars that could be inflated to break the bond. Another person suggested use of a carbon-dioxide blaster. It is a device that directs gas with explosive force into a silo (or rail car). Unlike air cannons with a pressurized reservoir of air, the gas pressure is generated suddenly and released by a chemical reaction. Mounting fittings are permanently installed on silos and it was suggested that such fittings might also be installed in the walls of railcars if frozen cars are a frequent occurrence. The manufacturer of the blaster will be contacted to investigate the feasibility of this solution.

It was also suggested that when a snow-fall occurs, cars be dumped when possible before the snow melts and refreezes during the night. Removing the snow from the top of the car would be possible if shipping rules require the cars be covered during transit.