Economical Options for Handling Kraft Fiberline Rejects Help Recover Fiber Costs

WITH RISING WOOD PRICES, depressed market pulp prices, and new effluent management requirements, some older pulp mills are at a competitive disadvantage from a cost, quality, and environmental standpoint when it comes to handling kraft fiberline rejects. However, there are several potential means of handling rejects that provide an attractive return on investment. This article will discuss the sources and causes of kraft fiberline rejects and present several options for handling them.

What is the best way to handle rejects from a kraft pulp mill? The choices are varied, from disposal of the materials to their reuse, and the answer is always site specific. In addition, improvements in slotted screens have brought new opportunities to purge contaminants from the reject stream while recovering the maximum amount of good fiber. There are also new constraints from outside the mill, such as the cost of wood and compliance with the new Cluster Rule requirements, which can impact the decision.

Despite the number of options to consider for handling rejects, it is clear that if the chip supply is adequately managed, the reject stream from the fiberline is significantly reduced. Mills that currently experience significant fiber losses or upsets to production due to large amounts of rejects in their pulp should first consider opportunities to reduce the source of the problem rather than trying to treat the symptom.

BACKGROUND. When older mills were originally designed, rejects from chemical pulping operations were almost always removed from the process, sometimes with quite a loss of good fiber. Years ago, wood costs were much lower and there were fewer restrictions on purging streams from the mill. At that time, it was practical to have larger fiber losses to ensure that as many contaminants as possible were purged from the system. Often, a majority of the “rejects” were good chips that did not receive adequate impregnation during the cooking process, and were then labeled as “knots.”

Today, fiberlines are designed to remove biological knots prior to cooking, as well as to maintain tight control over chip size. Cooking now involves specific conditions to encourage better chip impregnation. In addition, modern screening systems have sufficient stages to minimize the loss of good fiber from the true “reject” stream. Large reject losses to ensure good quality fiber are no longer the accepted standard.

FIGURE 1: Disposal options include burning the rejects for their fuel value or sending them to a landfill. However, if a large amount of good fiber is sent with the reject stream, the cost is often significant.

WHAT ARE REJECTS? The term “knots” frequently describes the rejects of the first stage of screening after cooking. This is where the hole or opening is around 0.25 in. to 0.5 in. Knots include good chips that did not get impregnated, as well as the “biological” knot (for example, where a branch connected the tree trunk).

Biological knots are very dense and have an extremely high lignin-to-fiber ratio. Therefore, their fiber yield is low. In addition, they require more energy and white liquor consumption than normal wood. Biological knots primarily become dissolved solids in the black liquor rather than pulp. Knots also contain rocks, bullets, and tramp metal.

The term “rejects” includes both the knots and the rejects purged from the brown pulp screens. The more specific term “screen rejects” refers to the fiber bundles purged from the pulp stream prior to bleaching, or to brown papermaking where the screen plate typically has holes (0.05 in. to 0.08 in.) or slots (0.006 in. to 0.018 in.).

Screen rejects may also contain considerable non-wood debris such as pea gravel, bolts, bullets, and wire. When slots are used, it is quite common to find that a tremendous amount of sand is purged from the pulp that the mill did not know was being accepted through the screens.

Changes in equipment away from low-consistency-fed vacuum washers also impact where the debris will settle, especially dense materials like metal and rock. The size and amount of debris that a pulp stream at 3% to 4% consistency can carry is amazing, and it is difficult to remove sand unless the consistency is below 1%. However, much of the new screening technology operates at more than 3% consistency, so diluting to less than 1% is difficult unless it is in a reduced flow stream (for example, after several stages of screening). This encourages mills to add another stage to the screening system for purging the sand and reducing the good fiber content of the reject stream.

MINIMIZING REJECTS. Depending on the source of the wood, biological knots account for a very small amount (typically less than 0.75%) of pulp produced from the digester. If a mill finds it is producing 3% to 4% knots, it means that much of this good wood is not being adequately impregnated during the cooking cycle. Possible reasons for this include oversized chips (primarily thickness), too little cooking liquor volume (liquor-to-wood ratio) or chemical charge, or even dull chipper knives that damage the ends of the chips.

The best technical solution is to separate the true biological knots from the chip system prior to cooking and then adequately control impregnation so that no knots are rejected from the knotter system, except for the stray bolt or rock. Therefore, these knotter rejects should be purged from the system rather than attempting to recook them. Chip thickness screens, air density separators, and chip-conditioning devices have all improved the capability of reducing the traditional “knot” reject stream.

Improper cooking conditions and variations in wood chip characteristics can create screen rejects. Screen rejects are often bundles of fiber that did not have adequate alkali to penetrate the chip. Frozen chips may also impede impregnation during winter months.

Dirt is often bark remaining on the log when chipped, and hardwoods are particularly susceptible to inefficient debarking. Because this pepper-like contamination is difficult to remove by bleaching, additional good fiber is rejected at the screens to help purge as much of this contaminant as possible. For this reason, many mills purge screen rejects rather than attempting to recook or refine them. However, adding an additional stage of screening is sometimes cost-effective for recovering good fiber from the reject stream.

In addition, changes to the bleaching sequence increase the risk of having dirt remain in the pulp after bleaching. The Cluster Rule will eliminate most chlorine and hypochlorite bleaching stages that are very effective in removing shives and bark specks. Other bleaching sequences can also remove these contaminants, but they require more attention to how the bleaching conditions are controlled. Consequently, screen operation will become more important.

Depending on mill configuration and what will be done with the rejects, screening can occur before or in the middle of pulp washing or between washing and bleaching. For unbleached mills, most screening is done before washing with the rejects being mechanically treated and returned to the same system.

FIGURE 2: Refining chips and allowing them to stay in the same fiberline results in better wood utilization, but higher operational and equipment costs. In some cases, however, reduced fiber losses may offset these costs.

REJECT HANDLING PRACTICES. The first step in developing a plan to handle fiberline rejects is to quantify the costs associated with the mill’s current reject handling practices. Items to consider are the costs of wood, cooking chemicals, steam, and landfilling. Also, consider that the regulatory requirements of best management practices (BMP) may cause some mills to reevaluate the purging of screen rejects due to fiberand liquor solids associated with contaminants such as sand, plastic, and small pieces of metal. Streams without fiber are easier to assimilate into a practical effluent plan, while those with varying fiber content tend to be more difficult unless they are adequately diluted.

Reject streams from knotters and screens are seldom characterized so that routine daily fiber losses are understood. When an upset occurs, it is easy to increase the amount of good fiber sent out with the purge, thereby increasing the average losses. Once the revised “standard” is set, it becomes accepted as required, often without consideration to the cost of good fiber that is lost.

The efficiency of a screening system is measured by comparing the debris level in the accept stream with the debris level in the feed stream. The debris can be separated from acceptable fiber using a small laboratory flat screen with slots—usually 0.006 in. wide—that is commonly referred to as a six-cut screen. A 10-cut screen is frequently found in brown paper manufacturing systems. The percentage of a sample retained on this screen is the amount of rejectable material present in the sample, and a measure of screening efficiency is made by comparing reject levels before and after a pulp screen.

Material balances that represent normal operation, as well as what happens during the typical upset condition, are essential to evaluate current performance and identify improvement opportunities. Where flow meters are not present or are questioned for accuracy, measuring the rate of change while varying tank levels can sometimes create reliable numbers. Consistency tests are always subject to sampling error, so rigid techniques and duplicate sampling must be employed.

After the existing system has been documented and the costs associated with such have been established, the next step is to examine several options for potential cost savings.

OPTIONS FOR REDUCING COSTS. Once the rejects are purged from the pulp, they can either be treated again and reused or disposed. Reuse options may include another chemical treatment (kraft cook or oxygen delignification), or a downgrade to another fiberline or process. Disposal options include burning the rejects for their fuel value or sending them to landfill, as shown in Figure 1 and described in Options 1 and 2 that follow. Each of the options are described further in the following sections:

FIGURE 3: Refining and moving rejects to another fiberline with different pulp quality allows recovery of some of the fiber’s value. However, this adds complexity between different operating units where adequate surge methods must be evaluated.

1. Dispose in sewer from BSW system. This is the most direct method to install and maintain within an existing fiberline. With the progress made using slotted screens, sand and plastic are purged with a minimum of additional equipment. Some waste treatment facilities will appreciate this fibrous material to help with sludge dewatering.

A disadvantage of this option is the possibility of considerable dissolved organic load that can influence biological oxygen demand (BOD) loadings, especially if the reject stream is purged from the fiberline prior to pulp wash-ing. Another significant disadvantage is the potential to “flush and forget” these rejects, therefore losing sight of the economic value.

2. Dewater sufficiently for hauling to landfill or bark boiler. Rejects can also be dewatered using a screw or three-roll press within the fiberline building, and then hauled to landfill or hog fuel pile. This is sometimes the best economic approach, as it is a definite purge and does not impact the wastewater treatment system. However, the rejects must be washed and adequately drained to avoid material handling problems. In addition, the dissolved inorganic materials such as sodium and sulfur may cause problems with the landfill or with burning if not adequately washed. Also, if a large amount of good fiber is sent with this reject stream, the cost of the lost fiber is often significant.

3. Refine and remain in same fiberline. Knots are typically refined in most brown paper applications. There are assorted mechanical treatment devices for this service, including Solvos and flat and conical disc refiners.

For bleached grades, it is rare to find knots being refined due to the additional lignin content of the raw wood inside the “knot.” Adequate systems for removal of the tramp metal, pea gravel, sand, and plastics must be incorporated for successful system operation. Otherwise, contaminants will accumulate within the system, causing erosion and diminished pulp cleanliness.

The advantage of this third option, which is shown in Figure 2, is higher utilization of the wood and keeping the fiber within the same system. However, this requires more equipment and, therefore, higher capital and operating costs. In some cases, though, reduced fiber losses may offset these costs.

FIGURE 4: Recooking keeps rejects within the same system, but risks reduced pulp production as recirculating knots consume digester volume. Oxygen delignification offers a higher yield than recooking, but capital costs are higher.

4. Refine and move rejects to another fiberline with different pulp quality expectations. This option could include selling the washed and pressed rejects stream to another customer as a supplement to its fiber supply. This is common for brown paper grades, with the rejects being sent to the base sheet. A bleached kraft pulp mill supplying fiber to an integrated fine paper machine might sell its rejects to another company making brown paper or board.

The advantage of this option, which is shown in Figure 3, is to recover at least some of the fiber’s value. However, this adds complexity between different operating units where adequate surge methods must be evaluated.

5. Chemically treat by recooking in same or different line. Knots are frequently recooked in batch digesters. This is economically practical for traditional batch cooking systems if more than 2% to 3% of the digester discharge is uncooked wood. The knots can be sluiced with the black liquor that is added to the digester to maintain a liquor-to-wood ratio during cooking. The flow scheme is relatively simple (Figure 4).

Continuous cooking systems, how-ever, are susceptible to plugging of the cooking liquor screens if the fiber content of the reject stream is variable. Screen rejects are seldom recooked due to the need for a purge of small contaminants, especially hardwood bark.

For traditional batch cooking installations, the advantage of this option is that it keeps rejects within the same system. Disadvantages include possible reduced pulp production as the recirculation of knots consumes digester volume, reducing production capability. Also, the yield may be quite low for the amount of alkali and energy consumed, but this is mill-specific.

6. Chemically treat screen rejects with oxygen and white liquor. Oxygen delignification systems are capable of significantly reducing the shives and dirt counts of pulp, and this is more selective than recooking. However, the capital costs are prohibitive for a stand-alone system. If oxygen delignification is already part of the existing fiberline or is to be added for environmental justifications, this may be an opportunity to reprocess screen rejects as shown in Figure 4.

The advantage of this option is a higher yield than recooking. If oxygen delignification is part of the process, the screening system can follow the reactor, with the rejects returned prior to the reactor.

ECONOMIC CONCERNS. Increasing wood costs are providing a significant incentive for mills to recover as much fiber as possible from reject streams. Based on observations at the specific site, decisions should be made as to what items can be assumed as fixed or ignored as insignificant versus what items are significant, especially if they are variable in frequency. Positive value includes lower manufacturing costs, higher quality (if a value can be attached to the specific change), and increased usable production. These positives are offset by costs or a negative value invested in the rejected material stream.

As an example of how to approach the economics, let’s first take a simple case of sewering screen rejects versus installing a refiner. Assuming $200/ton of rejects as the variable cost for wood, cooking chemicals, and steam, a nominal 1,000 tpd pulp mill that rejects 2% of its production is throwing away more than $1 million. Installing a surge tank, agitator, refiner, an additional stage of screening, and a set of sand cleaners and required pumps is less than $1 million. This means a payback in less than one year while improving the environmental friendliness of the mill. The operating costs associated with this change are not included in this quick economic analysis, and should be evaluated, as should product quality changes.

A system of selling screen rejects to a neighboring fiberline would most likely have an even higher pay- back. Another simple project with high return is to recook knots in existing batch digesters when chip conditioning is not a viable project due to capital constraints.

FURTHER CONSIDERATIONS. When examining potential alternatives to handling fiberline rejects, it is essential that a mill have a clear understanding of its existing steady state processes as well as the impact of upset conditions.

Also, assessing the impact of the new regulatory requirements is critical. The corporation’s philosophy for capital spending versus operating costs, as well as its attitude towards risk management of upset conditions, should also receive consideration. There will always be an interaction between sometimes opposing variables such as avoiding violations of effluent standards, reducing the risk of product quality upsets, and maximizing mill economic objectives. Small projects like redirecting the rejects may have viable economic return in addition to environmental benefits by creating a balance of these optimized variables. **

Wayne Bucher is sr. process engineer, Raytheon Rust Engineering, Birmingham, Ala.