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OPTIMIZING QUALITY AND YIELD FOR WASTE PAPER BASED PULP IN PRODUCTION OF LINER AND FLUTING
There has been increasing use of OCC (K4) and Mixed Waste (Common Mix) in manufacture of liner and fluting world wide, either to supplement virgin fibre or for use in 100% recycled product. This has placed increasing pressure on mills to maintain strength and appearance of sheet. Control of bale quality, water management, rejects handling and quality control are critical areas in achieving yield, quality and minimizing variable costs. Recycle containerboard plant design to produce medium and liner is discussed. Mixed Waste can have a significant impact on freeness and strength, impacting production and quality. Methods to treat Mixed Waste to minimise these characteristics are described.
Fiber recycling has been increasing since the early 80's and brown fiber recycling to make containerboard has one of the fastest growth rates world wide. The industry grew on the increasing use of Old Corrugated Containers (OCC) from grocery stores and industrial warehouses. In some instances the use of OCC pulp was out of necessity due to the lack of virgin fibre, while in other cases augmenting existing capacity was the aim. Much of this has been due to supply side motivators:
AMERICAN FOREST & PAPER ASSOC. (AF&PA), Washington, D.C., USA, recently released findings of a study that evaluated single-stream recycling programs costs. The study examined costs of single stream versus dual stream recycling and recovered fiber quality. A few of the study's results are noted below1:
According to the study, the total cost per year for U.S. paper mills producing grades that utilize curbside old newspapers and residential mixed paper would increase approximately US$ 50 million dollars, if single stream collection programs were implemented universally. A portion of that cost would be incurred as mills are forced to buy more recovered fiber and ultimately dispose of more rejects, due to the lower quality and higher contaminant levels commonly present in single stream collection.
At present recovered paper on the supply side is heavily influenced by China. There are approximately sixteen paper and board machines that have come on line in the last two years, with the majority being based on recovered fiber, substantial amounts of which are sourced from the US2. Exports to China were close to 13 million tons in 20033. The majority of these mills in China have taken advantage of the single stream recycling programs in the US which are resulting in large quantities of mixed paper at reasonable prices . These mills are designed with cleaning, screening and fractionation to produce a top quality product, as discussed below
During the early 1990's there were a number of containerboard "mini mills" based on OCC, built in the US producing 400-600 T/D such as Corrugated Services, Forney Texas, Rand Whitney , Montville, CT, Cedar River, Cedar Rapids IO, and Visy, Conyers GA. However, in the 21st century there have been a number of machines installed in the EU (Palm - Worth, Prowell, Saica) and China based on all recycle fiber with capacity of over 1,000 T/D, at a width of over 10M and designed for 2,000 m/min. These are definitely state of the art systems. Design emphasis generally has been on the papermachine and controls with little effort placed on the fiber preparation system. Mini-mills or Mega-mills, these plants have to produce consistent quality with a furnish that may have considerable variability. Without an efficient and quality oriented recycle fiber system design, these facilities will not be financial successes - negative impacts of cost, uptime and quality are a constant issue in mills. Today's plants must be able to track fiber from each bale to the roll in the warehouse.
As shown below there are many critical ingredients that make a brown-fiber based mill a success:
The industry shift from box plant clippings to OCC and now to mixed waste has impacted the design and performance of all systems; increase stickies and wax spots, lower strength, higher use of chemicals and poor dimensional stability are typical results.
This report examines techniques to incorporate waste of different qualities, how to deal with typical contaminants and unit operations options and strategies for maximizing performance.
The United States has largely incorporated the specifications laid out by Institute of Scrap Recycling Industries (ISRI), available at www.isri.org . Due to the volumes exported by the US, many countries have adopted these definitions and prices are quoted for these grades. The most common grade for containerboard applications is No.11 as defined below:
Grade No.13 is equivalent to box plant clippings
South African Grades K3 and K4 correspond approximately to No.13 and No.11 respectively. However, there is greater variability in quality with the K3 and K4 found region by region within South Africa than in No.11 and No.13 available in the USA.
In North America, there are no current standard definitions for mixed waste paper. Many of these are locally defined and dependent on the type of curb side recycling practiced in the region and can include the following:
Some areas indicate a maximum 10% wood-containing content with prohibitives not to exceed 0.5% and outthrows not to exceed 3%. Most suppliers create "packs" based on customer needs. Due to this diversity system design for a particular facility is intrinsically related to the furnish to be processed. Conversely, changing furnish after a system is installed can and will dramatically impact throughput, yield and quality.
In general, a plant to produce medium or liner from OCC can be very similar. Forward cleaners, to remove dirt and other heavy weight contaminants, are often not required for medium. The complications arise with the percentage of mixed waste. If the content of mixed waste in the waste stream is less than 10% its impact will be negligible as long as the wood-containing content is low. Thus a review of the mixed waste quality is required.
If the mixed waste content is less than 30%, it can be combined with OCC and processed together. If it is 30%-50%, it is advisable to treat the mixed waste in a separate system and then combining the two fiber streams. If it is more than 50%, a separate system is recommended.
Process options are discussed below.
The following types of pulpers are commercially available:
There are more low consistency vat pulpers operating at 5-6% consistency than high consistency (14-16%) vat pulpers or drum pulpers operating in brown recycle plants. Drum pulpers are now the dominant pulpers for deink application since they do not break down contaminants exacerbating their removal. There are also potential benefits from lower power and chemical consumption, and less fiber shortening. For brown fiber applications using the drum pulper, the rapid wetting of the furnish can be a problem as well as the complete removal of baling wire. The drum pulper is a more viable option where furnish is delivered loose with a large mixed waste component. All drum pulpers require a series of conveyors to ensure a constant furnish throughput.
High consistency pulpers are uncommon; however Maule has installed several with the HC pulper discharging rejects to an attached trommel screen. The lack of a ragger in the latter precludes the presence of plastic rope and other stringy materials and metal wire in the furnish – generally a difficult proposition. The configuration is always batch.
The most common design is the use of low consistency continuous pulpers with attached detrashers to handle the light weight contaminants. The detrasher is generally operated on a batch basis to clear the pulper of non-fibrous debris, with a cycle time dependent on level of contamination. Bottom mounted rotor pulpers are widespread, however side mounted rotor pulpers manufactured by Voith have a number of installations. A critical item in low consistency pulpers is means to remove large contaminants (plastics, metal etc.) whole – either with a trashwell or a design of the tub to allow these items to be directed to the detrasher. The golden rule is take out contaminants when they are large, the smaller they become the harder it will be to remove. Compromising trash removal early in the system will result in quality and yield issues.
Clearly the higher the mixed waste content the greater the benefit of using a drum pulper. Another market benefit is that contaminants are not broken up and discharged whole as shown below.
Drum pulpers require a steady stream of baling wire-free material resulting in capital costs for an extensive conveyor and dewiring equipment. The latter is particularly challenging in some countries due to a range and techniques for baling.
A large dump tank (45-60 minutes retention time) with consistency control results in good stability in the system.
Equipment and system design must be configured to ensure early removal of non-fibrous contaminants. Operation of ragger and trashwell (if part of the design) to optimal levels are important to both yield and quality. Bale quality and quality control over prohibitives and out-throws cannot be compromised
The unit operations of coarse (2-2.4 mm holes) and fine screening (0.18-2.0 mm slots) are important for all recycle systems. Several installation have found that slotted screens rather than hole screens in the coarse screening have advantages. Due to the cost of slotted screen baskets, forward cleaners are generally installed ahead of the fine screens to remove fine sand and grit. Small diameter forward cleaners have the additional advantage of ink removal. Mixed waste with significant ONP/OMG/MOW content will find these cleaners critical to minimise high speck count in the end product – this will be particularly important if white top is being produced.
Either a single unit operation such as a VSPT Turboseparator or a three stage pressure screening (0.055" hole) system is generally used. The Turbo has a grit cleaner associated with it and lightweight rejects are treated with a VSPT Combisorter. In either case a high-density cleaner precedes the screening operation.
Many mills neglect adequate controls on screens. DCS Flow control and pressure indication on accepts and rejects allow flexibility in reacting to furnish variations. They also provide protection against screen plugging.
Most of the recent 100% recycle fiber installations have installed fractionation system to maximize an engineered sheet in conjunction with the paper machine. There are two system designs possible:
Fractionation into long and short fiber streams for production of recycled two and three-ply linerboard is common world-wide. Another option in a multi-machine mill is to direct fines fraction to the fluting machine. A number of benefits have been noted:
One strategy has been to produce a lower basis weight sheet, as a consequence of engineering the sheet with a target strength property (ring crush for example). An up -charge is then possible, adding further contribution to the machine.
A three stage fine screening design today has become the focus to eliminate stickies. Virtually all furnishes contain glues, adhesives, toner particle, coatings that become "stickes" during the pulping process. Fine screens equipped with 0.15 - 0.2mm slots are the best defense in their removal. Equipment manufactures have undertaken much work aimed at maximizing removal efficiencies resulting in screen plate design (wedge wire, C bar etc.) and passing velocities (less than 1 m/sec). However, the best method to ensure success is to trial the basket/screen with the normal furnish under the mill conditions.
There continue to be a large number of chemical additives offered to "disperse", agglomerate or modify stickies.
Mechanical dispersion is discussed below
If a Turboseparator or similar device is utilized, light rejects should be treated in a suitable device rather than a trommel type equipment. Adequate control of operating parameters such as pressure, consistency and reject rate is crucial. Tailing stages on all screens are important. Stickies control begins with proper fine screening, while fractionation provides the strength benefits.
Large diameter MD cleaners operating at around 1.4% consistency are common prior to the fine screens primarily for screen wear protection and generally remove a considerable amount of dirt (sand, stones etc.). However, small diameter forward cleaners in three or four stages operating at about 0.8-0.9% in the first stage are generally required to get the fine high density contaminants out. Many operations that produce medium (fluting) do not require this for their quality specifications. Forward cleaners also function to remove ink if mixed waste is a part of the furnish.
Through flow or reverse cleaners are the real "work horse" for the removal of light weight contaminants. These cleaners operate at 0.7% consistency and have very low reject rate by weight. Several configurations are possible depending on the furnish. The most common is a P1+P2+S1 scheme. In this the primary cleaner accepts are re-cleaned by a second cleaner system (P2) – rejects from both Primary stages are cleaner in a Secondary stage. Accepts of the secondary stage are sent to the feed of the P1 stage. Rejects of the S1 stage are sent to the DAF clarifier.
Many 100% recycle mills incorporate throughflow cleaners on the machine as a second line of defense against stickies.
Treating light weight cleaners rejects through a DAF clarifier rather than discharging to sewer is crucial for yield enhancement. Quality is optimized with both small diameter forward cleaners (ink and grit removal) and through-flow cleaners.
Stickies, fines, inks, coatings and other non-fibrous contaminants rejects from through flow/reverse cleaners are processed by a DAF clarifier. There are several different models available which can accommodate different layout configurations. Polymer optimization can be challenging
The sludge from the clarifier is generally dewatered in a belt press together with fine & coarse screen rejects. The underflow from this stage must be discharged to minimize stickies buildup. An option for some mills is to include the dewatered sludge with coal, bark or similar fuels in a fluid bed boiler.
Although older mills continuer to use drum thickeners, capital effectiveness has clearly shown the utility of the disc filter to thicken the furnish from the light weight removal cleaners. Due to the low accepts pressure of the light weight cleaners layout considerations are important to consider.
Early mechanical dispersion systems were aimed at dispersing "asphalt", and consisted of a steaming vessel followed by a high consistency disc refiner. Some versions are often blow line refiners from the pulp plant. These systems have been replaced by sophisticated double screw dispersers and modified HC refiners. The aim is to heat up the stock and then reduce stickies so they are not noticed in the final sheet. This requires about 60 kwhr/t of energy. There is an increase in strength for certain furnishes. In fractionation systems the dispersion system is often applied to the long fiber fraction. Effective light weight cleaning to remove stickes is a more prudent approach to handling them.
Rejects handling is a large challenge in most mills and generally not given adequate consideration. However they often result in high operating cost and a housekeeping problem . There are few devices available that can categorically handle wastes from any mill. There are four types of rejects:
Items 3 and 4 are often combined and dewatered by a ram-type press. Item 2 is often sent to a screw type separator that discharges the dewatered material into a bin. Cleaners have to be set up so that liquid flow is minimal. Item 1 is the most complicated and messy. Rejects from the pulper and detrasher have to be conveyed either to a bin or a trash compactor. All these items are high maintenance and if any of this equipment is out of service, the system has to stop.
With a yield of about 90% in most facilities, a 500 T/D pulp to HD plant will generate about 185 wet tons of rejects daily. With the use of mixed waste yields drop to about 80%, and about 416 wet tons are generated for the same capacity. Thus system design must entail a well operated and maintained rejects handling scheme.
The underflow from the rejects system must be discharged to the effluent treatment system. This stream contains stickies, contaminants which will build up in the water loop if recycled. For a well designed system this will result in about 150-200 lpm for a 500 T/D facility.
Fiber loss from wet strength can be minimized by utilizing unit operations like the Turboseparator – deflakers will tend to degrade plastics exacerbating quality.
Mixed waste and OCC treated separately at the front end of the system and then combined
If more than 10% mixed waste is to be utilized this is a capital saving, hybrid approach that may be appropriate if some of the later unit operations in the system have excess capacity. Typically the mixed waste will be batch pulped at mid consistency, coarse screened and cleaned prior to addition to the OCC system.
Mixed waste and OCC treated in two separate systems
This approach provides the best quality product and allows as high a mixed waste content as required. A typical system for mixed waste, which would augment an existing high quality OCC system, is described below. As can be seen there are only two unit operations that are different from a "traditional" OCC system - medium consistency pulping and washing. The purpose of washing is to wash out the ink and ash associated with the news and other wood-containing paper in the mixed waste. The degree of washing is dependent on the amount of wood containing material in the furnish. The filtrate from the washing is directed to the DAF clarifier together with the rejects from the throughflow cleaners. An important impact of mixed waste is on Freeness as shown below:
The preferred mode of pulping mixed waste if a separate line is utilized is at medium consistency (13-15%). The large amount of mixed papers, ONP and magazines suggests that this method of pulping, which is generally used in MOW deinking systems, be utilized. In addition, this method of pulping prevents large contaminants, particularly "stickies", from breaking up. Extraction through a detrasher would follow after pulping. Large percentages of OCC or wet strength would preclude the use of this method of pulping due to the high reject content and the type of heavy rejects.
A tailing screen such as a VSPT Combisorter would follow a primary stage pressure screen with 0.055-inch diameter holes. An alternate to a pressure screen could be the use of a VSPT (Voith Sulzer Papertech) Turboseparator equipped with 4-mm diameter holes. High-density cleaners would precede the coarse screen.
With a 30% reject rate from the primary coarse screen, accepts from the Combisorter cannot be returned, either prior to or after the coarse screen, due to the fine contaminants in this stream. To remove these lightweight and non-fibrous contaminants an option is to clean this stream either with separate forward and reverse cleaner systems or with combination forward/reverse cleaner system.
A three stage cascaded fine screening system with 0.008-inch or 0.006-inch slots is recommended. The rejects from the third stage are discharged from the system. A mid consistency cleaning system (1.5-2.5%) prior to the screens is recommended to improve the life of the screen basket. Fine screening consistency can take several options - about 1% or about 2.5%; higher consistencies have been recently proposed. There are benefits for each approach and the one implemented is often based on mill preference of a particular vendor and system configuration. Screening at higher consistency can result in a lower capital cost.
A single or two stage series (P1+P2) reverse cleaning system is recommended. The rejects are sent to a DAF clarifier. New cleaner developments, in the last few years, by equipment suppliers have significantly improved contaminant removal efficiencies.
A washing stage follows reverse cleaning with the underflow going to a DAF clarifier. The washing stage is critical in the removal of non-fibrous material and fines, which negatively impact sheet strength. Aggressive washers used for deink systems are best utilized to remove the ink and ash found in mixed waste. Equipment developed in the last few years has demonstrated high removal efficiencies. The filtrate from the washer is sent to the DAF clarifier. The thickened stock is stored in a HD tank. This unit operation adds significantly to the overall capital cost of the project by virtue of the washing and associated clarification requirement. Washing trials with the particular furnish are recommended to ensure the design basis can be achieved
Good design of waste plant can result in high yield and quality comparable to virgin fiber. Careful selection of unit operations and control strategy are important elements in this. Mixed waste use is an ever growing trend due to its low cost; however equipment and design must be modified to accommodate its use if more than about 10% is used.
1. WasteAge May 1 2004
2. Bill Moore, Paper Age June 2003
3. Alan Rooks, Ahead of the Curve 11/19/2003 Tappi