«ABSTRACT Paper mills and converters produce large quantities of waste paper. Paper trim from the process, roll ends, off-quality product and overruns ...»
Fiber Recovery from Waste Paper:
A Breakthrough in Re-Pulping Technology
Jerry Aue, Energy Center of Wisconsin
Keith Picard, Fiber Recycling Technologies, Inc.
Kevin Grabner, Energy Center of Wisconsin
Alan Button, Buttonwood Consulting, LLC
Paper mills and converters produce large quantities of waste paper. Paper trim from
the process, roll ends, off-quality product and overruns are all sources of waste. Where
feasible, process scraps are re-pulped and blended back into the feedstock. Converting paper to its final product sometimes produces wastes that are difficult if not impossible to re-pulp with conventional technology. For example, many paper products are laminated with plastic.
Many mills either process laminated waste paper into pellets for boiler fuel or pay to have the wastes land filled. This paper describes a new technology that cost-effectively recovers quality fiber from papers currently being sent to landfill.
Fiber Recycling Technologies, Inc. (FRT) has developed a low-energy, mechanical re-pulping method that gently separates fibers while leaving contaminants largely whole, so that fiber can be readily screened from contaminants. Trials were run on a range of recovered papers and landfill waste, including poly-laminated paper trimmings, multilaminated food packaging, and unsorted municipal solid waste. Success from the trials encouraged FRT to start a commercial re-pulping operation using their own technology. This expanding new business re-pulps high-grade landfill waste papers, creating 18 new jobs.
Recovered papers can use considerably less energy in pulping than wood-based pulp, so making secondary fibers competitive with virgin ones can save significant energy. A 20ton per day FRT pulper now operating commercially annually saves about 7,000 tons of waste fiber from being land filled, and requires 11 to 30 kWh per ton for pulping. By comparison, refiner-mechanical pulping of virgin wood requires about 1,972 kWh per ton (Martin, N., et al 2000). Other mechanical and kraft pulping processes have similar or greater energy intensities.
Background The pulp and paper industry converts fibrous raw materials into pulp, paper, and paperboard. The processes involved in papermaking include raw materials preparation, pulping (chemical, semi-chemical, mechanical, or waste paper), bleaching, chemical recovery, pulp drying, and paper forming and drying. The most significant energy consuming processes are pulping and the drying section of papermaking. The pulp and paper industry accounts for over 12% of total manufacturing energy use in the U.S, making it the nation’s third largest consumer of energy in manufacturing. Total energy costs for the Wisconsin pulp and paper industry topped $335 million in 1997, the highest of any Wisconsin industry (Wisconsin Paper Council 2003).
1-1 The U.S. pulp and paper industry is made up of three primary types of producers: 1) pulp mills, which manufacture pulp from solid wood or other materials, primarily wastepaper; 2) paper mills, which manufacture paper from wood pulp and other fiber pulp;
and 3) paperboard mills, which manufacture paperboard products from wood pulp and other fiber pulp. In 1994, virgin wood pulps accounted for 68% of paper production by weight, with used paper covering the remaining 32%, when 82.46 million tons of paper were produced in the U.S. (Martin, N., et al 2000). More than 5.3 million tons of paper and over one million tons of paperboard are produced annually in Wisconsin (Wisconsin Paper Council 2003). Since recovered papers can use considerably less energy in pulping than wood-based pulp, making secondary fibers competitive with virgin ones can save significant energy in the mill (Martin, N., et al 2000).
As the United States paper industry approaches its 50% recovered paper target, getting that last couple of percent has gotten difficult. While there is potentially significant additional recovered paper tonnage, the paper industry faces a difficult challenge to collect and process much of this material in an economically attractive fashion. Among the obstacles to increasing the recovered paper percentage are: (1) papers with difficult to remove and separate contaminants and (2) quantities of particular recovered paper grades that are too small to justify separating and/or marketing.
Difficult to process recovered papers include those with coatings and laminations (such as plastic or foil) that are easily fragmented and difficult to remove in conventional cleaning and screening operations. Some contaminants, such as stickies, become well dispersed, but accumulate in the system and cause problems when they agglomerate. Other recovered papers require significant degradation (fiber damage, fines generation, freeness reduction and fiber shortening) to dislodge and disperse the contaminants.
Commonly used re-pulping methods combine high shear-factor mechanical stress, steam injection, and chemicals to re-pulp the waste paper. A conventional vertical pulper uses a high-horsepower motor driving a blender-like apparatus to shear the material into small pieces to allow rewetting and fiber separation. For example, a conventional vertical pulper of this type may be driven by a 300 hp motor and produce 150 tons per day of pulp.
Waste paper is dumped into the top of the pulper, and blended into a slurry. Pulp exits the bottom of the machine, while contaminants exit out the side. The method is effective for ledger papers and other scrap with limited amounts of contaminants.
When used with laminated paper waste, a conventional blender-like pulper causes plastic and other layers to be ripped into fine pieces and dispersed throughout the re-pulp mixture as a contaminant. Large cleaners and fine screens are required to remove these contaminants before sending to the paper machine. These screens quickly become plugged with fine laminate pieces and become difficult to clean. For this reason, many mills and converters either have laminated waste paper converted into pellets for boiler fuel or pay to have the wastes land filled.
Drum pulpers have shown that slower, lower shear pulping processes can economically recover fiber from many of these difficult to process papers (Patrick, K. 2001).
Drum pulpers are, however, typically quite large with a large footprint and high capital cost.
1-2 A New Re-Pulping Technology Fiber Recycling Technologies, Inc. (FRT) of Neenah, Wisconsin has developed a much smaller modular pulping unit based on low shear. The Airborne Fiber-to-Fiber (AF2F) pulper, shown in Figure 1, has a unique twin rotor design with low connected horsepower.
The twin horizontal rotors (see Figure 2.) turn in opposing directions to provide a simple lifting action and create gentle fiber-to-fiber action for low attrition on both fibers and contaminants. The AF2F pulper rotors, each 10 foot long, are powered by 20 hp motors and operate at 40 rpm. This low energy re-pulping method separates fibers while leaving contaminants largely whole, so that fiber can be readily screened from contaminants.
Figure 2. AF2F Pulper Interior Figure 1.
AF2F Pulper (Drive End) Showing Rotors As part of a bale-to-pulp waste paper processing system developed by FRT, the “10 foot” AF2F pulper shown in Figure 1 can produce 25 tons of pulp per day. The 10 foot model was built as a pilot unit. The AF2F pulper can be extended to 20 feet to increase capacity. The AF2F pulper can be operated in either batch or continuous mode, however, experience to date has been in batch mode.
The baled recovered paper is automatically de-wired, de-densified and metered onto a belt conveyor by the “ABD Debaler” shown in Figure 3. The ABD Debaler was developed by FRT, leveraging experience from Val-Fab Inc, their metal fabricating sister company. In fact, FRT was formed by Val-Fab to expand their fabricating business.1 The loosened waste enters the drive end of the AF2F pulper from a belt conveyor Figure 3. ABD Debaler (Bale Feed Side) immediately above the unit. Process water heated to 100°F to 110°F is also added in this area. Depending upon the furnish, small quantities of caustic soda, bleach, or surfactants may be added to speed the re-pulping process, for example, one gallon per half-ton batch of pulp.
In current batch operations of the 10 foot pulper, about one-half ton of paper is being loaded, pulped, and emptied in 30 minutes. Pulping time
Pulper technology based on patents by Mark Spencer, Inventor.
1-3 can require from 10 minutes to 20 minutes, depending on the furnish. The sight and sound of the pulping process is reminiscent of a commercial horizontal-axis washing machine.
Once the fiber has separated from the contaminant, the AF2F pulper empties the entire mixture to a tank. From there, the pulp mixture is fed into the “C-Screen”, a large, horizontal rotating drum screen that separates the large contaminants from the fiber. FRT developed the C-Screen especially to handle the large contaminants that can be processed in the AF2F pulper. The fiber washes through coarse slits in the drum, while larger contaminants are channeled out the far end of the drum via internal dividers that rotate with a screw action.
Pulping Trials This section of the paper presents the results from three of the many trials run on a wide range of recovered papers tested in the AF2F Pulper. The financial assistance for six of these trials was provided by the Wisconsin Industries of the Future program. These trials were primarily exploratory and, therefore, no steps were taken to optimize the AF2F Pulper performance on these trial papers. The three recovered papers were: (1) a pre-consumer coated paper plate stock, (2) poly-coated paper trimmings, and (3) unsorted municipal solid waste (MSW) from Chicago.
Trial One – Mill Comparison Trial Using Pre-consumer Coated Plate Stock
Trial number one was run to compare the performance of the AF2F Pulper to an existing conventional production pulper currently processing pre-consumer paper plate stock.
This stock consists trim waste (to get a round plate from square paper) and quality control rejects. The coating that protects the plate from food moisture also makes pulping difficult.
Conventional production pulper. The existing production pulper was a medium consistency pulper operating in a continuous mode. The pulper was a 14 ft. Black & Clawson vertical pulper with a Vokes rotor and a 3/8” extraction plate. The rotor motor was a 300 hp Reliance operating at 4160 volts and 39 amps. An industrial fork lift retrieved the bales from inventory and set them at the top of the pulper. After the baling wires were cut, they were pushed, two at a time, into the pulper. Bale weight was about 1800 pounds each.
The production trial was set up to measure the energy used in pulping the coated plate stock (kWh per ton). The bales were weighed and the consistency was checked throughout the trial. The trial was scheduled to run for one hour of normal recycled pulp production at a rate of approximately 175 t/day of recovered paper. Due to the mill having its second pulper out of service, the trial was actually run at twice the normal production rate to meet the needs of the paper machines. Instead of running 8 bales, as planned, 16 bales (13.9 tons) were processed in one hour. The mill’s standard chemistry and pulping temperature of 175°F were maintained during the trial.
AF2F pulper. Approximately half (855 lbs) of a bale representative of the coated paper plate stock was loaded into the AF2F pilot pulper along with sufficient heated water to bring the pulper consistency to 10% and temperature to 100°F. (maximum possible in the pilot plant).
Trial Two – Recovered Poly-coated Paper Trimmings Trial number two was run on waste paper currently being land filled. The polycoated trimmings raw material, shown in Figure 4., is difficult to process conventionally because it produces thin strings of a very stretchy poly film. Approximately 800 lbs. of polycoated waste paper was run in the AF2F pulper under the conditions shown in Table 3.
A handsheet (or test sheet) is a single piece of paper made in a laboratory setting by draining water from a pulp suspension on a screen-covered sheet mold. This test sheet is used to evaluate paper properties.
Trial Number two results. The trial material was pulped very easily in 10 minutes and the poly film didn’t aggregate into balls or stringers in the pulper. Although it removed the poly film, the pilot plant C screen was not able to separate the poly and fiber materials completely and a significant percentage of the fiber slurry got entrained in the poly film. This resulted in a yield of recovered fiber of 65% (the “accepts level”), which was lower than expected. Two types of handsheets were prepared from the pulp that was collected from the coarse screen.
Paper made on the automatic sheet former is shown in Figure 5. Table 4 contains the results of testing performed on standard TAPPI handsheets.
The fiber recovered from the poly-coated trimmings produced handsheet test results that indicated a high quality fiber is available from this raw material. The tensile and tear strengths were reasonably high for an unrefined recycled fiber pulp. The relatively low brightness came from the presence of black fibers that appear to have originated from the small quantity of printed trimmings.
Trial number three was run on unsorted municipal solid waste (MSW) from Chicago.
As can be seen in Figure 6., the bale of recovered Figure 6. Bale of Unsorted paper contained many non-paper materials. These Chicago Municipal Waste materials included wood, stones, plastics and metal items. A significant portion of MSW is paper and, therefore, potentially a source of fiber for paper products. The gentle twin rotor action and low shear design of the AF2F pulper should be well suited for pulping MSW.
A 700 lb batch of the Chicago MSW material was processed through the ABD debaling unit and conveyed to the AF2F pulper where it was pulped in hot water (100°F.) for 21 minutes to defiber the paper. The trial conditions are given in Table 5.