High demand for recycled paper including old newspapers (ONP) and sorted office paper (SOP) has resulted in a combination of higher prices and lower quality. Today's recycled paper market has forced many deinked pulp mill operators to make changes to their original mode of operation to provide acceptable recycled pulp quality at the lowest possible operating cost. This is truly making the best silk purses out of the worst possible sow's ears.
Many deinking plant operators have chosen various blends of lower cost recycled paper grades or lower quality sources of the same grade to reduce raw material costs. Unfortunately, most have discovered that their pulp quality has also deteriorated to unacceptable levels.
Figure 1. A flow diagram of an office paper deinking plant shows how micro screens can be relocated to eliminate some of the agglomeration of micro-stickies.
Some plants have reacted to reestablish their quality by fine tuning their recycled paper quality control and blending procedures with various degrees of success, since recycled paper is an ever-changing raw material. Others have installed and operated sorting lines to upgrade lower quality grades of recycled paper to quality levels required for their process and equipment. This has reduced raw material costs, but increased manpower costs since sorting contaminants from recycled paper is very labor intensive. However, as a result of all this turmoil in the recycled paper market, many of the operating deinking plants have chosen to invest in improvements in their process and equipment required to produce the same high quality from lower quality and lower grades of recycled materials.
SAME OLD PROBLEMS. What are some of the quality problems created by the use of lower grades of recycled paper? Actually, they are the same deinked pulp quality issues the industry has been fighting for many years, only intensified by the lower grades and reduced quality of the recycled raw materials. These include stickies, plastics, color, unbleached fibers, brightness, dirt, and mechanical fibers (in some wood-free grades).
Stickies content in all grades of recycled paper has risen with the continued increased use of pressure sensitive adhesives for everything from post-it notes to non-lick stamps. Lower grades of recycled paper usually contain more of them since they haven't received the same degree of sorting in wastepaper processing plants. Some plants have received truck-loads of recycled paper bales consisting of envelopes covered with adhesive-backed foil stamps from a magazine publisher's promotional mail-in. A bale or two of this material in the pulper makes stickies level go out of sight.
Continuous drum pulpers provide gentle treatment of contaminants, with some mills reporting improvements in contaminant removal after replacing their high consistency batch pulpers.
Plastics, colored paper, unbleached brown paper, and mechanical fiber content of the lower paper grades is also higher due to this same reduction in attention and manpower per ton of paper being sorted on the wastepaper processing line. In fact, many recycled paper processors find it profitable to remove colored paper sorted from high grade sorted white ledger grades and deposit it in lower sorted office paper grades. This practice results in higher colored paper content than unsorted paper in the average office trash can.
Fortunately, a number of tools, discussed below, have been developed for improving deinked pulp quality. Also, a lot of knowledge has been gained in process design through mistakes made and experience gained by startup and operation of the large number of modern ONP and SOW deinking plants installed in the past decade.
PROCESS OPTIMIZATION AREAS. Pulping and detrashing: Pulping has always been a necessary evil in deinking. Recycled paper must be converted to individual fibers in the initial stage of the process so that useful fibers will not be rejected from multiple stages of cleaning and screening. Many deinking plants also utilize washing as the primary method of deinking which benefits from finely dispersed ink particles. Unfortunately, the same violent low consistency repulping action that separates fibers also reduces contaminants to small particles, which can be difficult or impossible to remove from the resulting slushy mixture.
Fine-slotted screens operating at high consistency (3% to 4%) reduce the volume of pulp being pumped through the screens, resulting in a lower installation cost of tanks, pumps, and piping.
Fortunately, technology improvements, such as high consistency batch pulping and continuous drum pulping, have provide a way to reduce the paper to individual fibers, while allowing a kinder and less violent treatment of the contaminants so their size is larger and removal is vastly improved. A number of old low consistency pulpers are being replaced with these modern high consistency pulpers to assist in coping with lower recycled paper quality.
Continuous drum pulpers provide an even gentler treatment of contaminants than high consistency batch pulpers. Several mills are even reporting improvements in contaminant removal after replacing their modern high consistency batch pulpers with drum pulpers.
Lower grades and reduced quality recycled paper have also impacted the operation of high consistency batch pulpers. The reduction of wastepaper quality has generally resulted in higher volumes of trash, which must be removed by the pulper trash screens instead of by wastepaper sorting lines. Many deinking plants have discovered that their trash screens do not have adequate volume to retain the trash now contained in a single pulper batch. Some mills with excess pulping capacity have been able to stop dumping in the middle of the pulper dump cycle. This allows them to empty the trash screen and then empty it again at the end of the dump cycle. Others mills that have recently converted to high consistency batch pulping are opting for larger volume equipment or multiple trash screens.
Reverse cleaning: Many deinking plant operators processing either ONP or SOP have been disappointed with the performance of their reverse cleaners, which were designed to remove lightweight particles including plastics and stickies from the pulp. Problems have been caused to some extent by an increase in the specific gravity of these contaminants.
As a result, fewer of these lightweight cleaning devices are being installed in newer deinking plants. Some existing reverse cleaners have been shut down because they are not considered to be cost effective due to high maintenance costs and low efficiencies. Some reverse cleaners have even been converted to conventional centri-cleaners by adding conversion kits or different cleaner bodies to existing banks of cleaners. The contaminants that were previously expected to be removed by reverse cleaning are now more efficiently removed by other unit operations, such as screening, centricleaning, flotation, and white water clarification.
Centricleaning: While the success of stickies and plastic removal by reverse cleaners has been reduced, many deinking plants have learned that these materials can be removed by modern and efficient centricleaners. Some deinking plants have added additional cleaner systems at several different process points and are reporting varying degrees of success.
Extra sets of cleaners have been installed immediately following the existing primary cleaners as well as in other deinking plant process water loops. In-stallation in a different water loop may be preferred since, in many cases, the stickies agglomerate after a pH change or dilution chest at the beginning of a new wat-er loop and are large enough to be more effectively removed. The separate loop installation can present a problem, since effective cleaning requires low consistency, and additional dewatering equipment may be required.
Screening: The combination of larger contaminant particles provided by improved pulping teamed with modern screening developments has proven to be the major factor in producing high quality deinked pulp today. Even without pulping changes, advancements in screening technology have provided more tools for deinking plant contaminant removal than any other single area in the deinking plant. Screens normally employing baskets with 0.006-in. slots-with some slots as narrow as 0.004-in.-are being used to replace older screens having screen slots of 0.008- to 0.010-in., considered state-of-the-art only five years ago.
Other screening advancements include the location of the screens within the process. When many deinking plants were originally designed, little thought was given to stickies that were too small to be significant or to even detect. Even large stickies were dispersed and assumed to be harmless, even though papermakers complained about deposits that accumulated on paper machine surfaces and clothing.
Now, deinked pulp producers have learned that these small stickies, sometimes called micro-stickies, agglomerate and form measurable stickies. This agglomeration can occur in about any location where the pulp is agitated at lower consistencies, even at the very end of the process in a bleaching tower or in high density storage chest dilution zones. Some plants have combated this tendency to agglomerate by relocating the final set of slotted screens from the front of the process to the end of the process immediately ahead of the pulp dryer (Figure 1).
An additional set of fine screens has been installed ahead of the pulp dryer in a few plants which chose to maintain the existing screen locations to assure that any remaining contaminants are removed. This second set of very fine-slotted screens also provides an extra measure of protection. Since screens only remove a certain percentage of small contaminants, the first set of fine screens always fails to remove a portion of the stickies. The second set has a second chance to remove them, as well as removing any newly agglomerated ones.
Another significant advancement is fine-slotted screens that can be operated at high consistency. Most older, conventional fine screens have had a maximum operating consistency of approximately 1% to1.5%, limiting their process location to a point near the cleaners or at least ahead of a dewatering device capable of being fed at these low consistencies. Now, many of the newer fine screens are capable of operating at 3% to 4% consistency with the same 0.004-in. to 0.006-in. slotted baskets. In addition to improving flexibility in system location, higher consistency operation reduces the volume of pulp being pumped through the screens, resulting in a lower installation cost of tanks, pumps, piping, and building.
Flotation: The flotation deinking operation has undergone revisions that have improved removal efficiency of inks, dirt, and other contaminants, including stickies from the recycled pulp. Many plant operators have improved flotation efficiency by installing upgraded air injectors (photo 5), if they are available for their cells. Others have replaced older, less efficient flotation cells with recently improved cells to cope with the lower and more contaminated grades of recycled paper. Often, plants have reported success in removing stickies and wider ranges of inks and other contaminants through the use of continuing improvements in deinking surfactants with no capital investment.
Flotation efficiency has been improved at some deinked pulp mills by installing upgraded air injectors.
White water system improvements: Deinking plant operators have continued to learn more about the nearly invisible to completely invisible stickies that lurk in every part of the deinking plant system. Many have discovered that these insidious particles are being transported to every unit operation in the plant by the white water used to dilute and transport the recycled fibers through the process.
The greater stickies population contained in lower, less sorted grades of recycled paper increase the quantities of stickies in the white water, which, in turn, increases their tendency to agglomerate and appear in the final pulp. Many deinking plant operators have started to give more attention to their dissolved air flotation (DAF) clarifiers to aid in removing these small stickies through increased operator attention and improved polymers.
Some mills are also installing clarified water turbidity control systems for continuous monitoring and control of their DAF polymers. These systems provide beneficial cost reduction since they usually decrease the consumption of the costly polymers. A few plant de-signers have even improved pulp quality with methods of white water clarification other than DAF clarifiers.
Flexographic inks, which are nearly impossible to remove by flotation cells, often plague ONP deinking plants utilizing lower cost raw materials. These very small ink particles may be removed from washer filtrate with some success by DAF clarifiers. This removal of flexographic inks can be assisted by turbidity control systems. As these ink particles accumulate in the white water, the turbidity increases, and polymers are automatically increased. By lowering the turbidity set points to improve water clarity, operators are able to improve ink removal without the danger of adding excess polymer, since excess polymer can reduce clarification efficiency.
In many new and existing deinking plants, white water system design has also been carefully scrutinized and revised to prevent contaminated white water from entering cleaner parts of the process.
High white water system temperature can also negatively affect screening efficiency and pulp quality when temperatures become high enough to soften stickies, plastics, and hot melt glues. These materials become more pliable at elevated temperatures, so borderline particles can be extruded through screen slots with accepted pulp. The higher population of these contaminants contained in lower grades of recycled paper requires that screening efficiencies remain at their highest levels to reach tight quality targets.
Higher white water system temperatures are becoming more prevalent due to water loop closure to reduce water and effluent flows and because bleaching systems are being operated at higher temperatures. Some deinking plants have been able to combat the temperature problems by one or more of the following methods:
- High temperature bleaching may be performed at high consistency to reduce heat input to the system
- Cool water makeup to the system is designed to keep the screen temperatures down
- White water cooling towers have been added to reduce screen system temperatures to reasonable levels while maintaining bleaching temperatures at their most efficient levels.
Bleaching: Color, unbleached fibers, and mechanical fibers, prohibited in the higher grades of recycled paper, are an increasing problem when using lower grades of wastepaper. A number of earlier deinking plants using high grade fiber were able to meet acceptable brightness and color standards with fairly simple bleaching systems. ONP plants typically used only sodium hydrosulfite, while office paper deinking typically used oxidative bleaching, such as hydrogen peroxide or sodium hypochlorite.
Now, several newer office paper plants have combined oxygen and peroxide at elevated temperatures and pressures, followed by the reductive stage of sodium hydrosulfite or formamidine sulfinic acid (FAS) also operated at elevated temperatures. Some older plants have even added these newer systems to replace their existing, less effective bleaching systems.
FUTURE NEEDS. As the demand for recycled-content newspapers, magazines, and office papers increases, current trends seem to indicate that the quality of paper being recycled to deinking plants will continue to deteriorate. Since papermakers are reluctant to lower their standards for the quality of pulp they use, the deinking plant operator must continue to search for additional process quality improvements.
Perhaps developments like the ones described here will continue to come to the rescue. Developing "better" stickies, which can be easily removed by existing equipment, seems to offer relief. Ongoing work to create improved pressure sensitive adhesives may soon provide these friendly stickies.
Finally, scientists are almost ready to agree at last on a single laboratory test for measuring stickies content in recycled pulp. This might even provide the deinked pulp industry a tool to determine exactly what their pulp quality is and how each part of the process affects it.
ED GLASS is senior consulting engineer, pulp and paper, Washington Group International, Birmingham, Ala.
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