Managing rejects from recycled fiber plants.
The common attitude about rejects can be summarized as follows: "I don't care about rejects, just get rid of them!" Not too long ago rejects handling was very simple: collection, basic dewatering, loading onto trucks, and transport to a nearby landfill. Costs were low and not much work was involved. As long as there were plenty of options for disposal, rejects handling was a simple process.
Rejects pile up rapidly. A 1000 metric ton/24 hr corrugating medium mill using 100% recycled fiber can generate more than 50 metric tons of rejects per day. Deinking plants can exceed this amount many times over. Landfill costs vary from one region to another. In Germany they can range from [euro]60 to [euro]200 per metric ton. With a recycled fiber consumption in Germany of about 11 million metric tons/yr, 500,000 metric tons of rejects must be disposed of every year.
But what exactly does "disposal" mean today? Taking rejects directly to the landfill will no longer be possible in Europe in the near future. In Germany, for instance, from June 2005 onwards landfill will only be permitted for material with less than 5% organic content. In Austria this ruling has been effective since January 2004.
The "export" of rejects for disposal in countries with lower standards involves tremendous transport costs and is only possible to a very small extent because of regulations on waste shipment. As a result, rejects from recycled fiber operations must generally be incinerated and only the combustion residues can be landfilled.
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Recycle facilities must include the following features:
* Inspection of the incoming raw material will become more and more important
* For baled material, dewiring efficiency will influence the required technology for rejects handling
* Removal of incombustible, inert material such as metal, glass or sand from the rejects is essential
* The metal fraction should be as clean as possible to facilitate its recycling
* Rejects dry content has a decisive influence on suitability for incineration and on the transport costs involved.
What exactly are rejects composed of? There is a basic difference between rejects and sludge. Sludge is homogeneous-for example, the residue from deinking cells, from micro-flotation, or from primary clarification. Rejects, however, are not homogeneous. They can be categorized as either coarse and fine rejects. Coarse rejects are found in pulping and coarse screening, fine rejects in the subsequent process stages of cleaning and fine screening as well as in the approach flow system. Heavy coarse rejects include all kinds of metal of different shapes and sizes, stones, wet strength waste paper that has not disintegrated, and wires.
Light coarse rejects consist of bits of plastic, plastic foils, compact discs, etc. The composition of the coarse rejects depends mainly on the type of pulping system; one can distinguish between systems that discharge heavy-and lightweight coarse rejects separately (such as a TwinPulp System) or systems where these are removed together. With the increasing use of drum pulpers, the focus is on the latter option. With fine rejects, one also distinguishes between light-and heavyweight.
Heavy fine rejects mainly consist of sand, glass, staples and other metallic office waste as removed by high density (HD) and low density (LD) cleaners, as well as via the heavy junk traps of combined screening and cleaning equipment. Light fine rejects from slot screening or lightweight cleaning contain fiber debris, stickies, wax, fillers, etc.
How can we ensure these rejects are disposed of? First, examine-the available options for disposal, considering the increasingly stringent standards set by legislation and requirements for rejects quality.
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Those rejects suitable for incineration must be discharged with the highest possible dry content, that is, with the highest possible thermal value (> 11 MJ). The size of the rejects must also suit the available incineration technology. Additional drying may be necessary. The percentage of organic contaminants in the incombustible, inert material must be reduced to a minimum to enable its disposal. Reusable material such as metal must be removed as clean as possible to make it acceptable for recycling.
Designing a rejects handling system must reflect these disposal objectives as well as the composition of the rejects and where they are removed in the stock preparation process.
The equipment used in the rejects handling system is a key to success. Long life, adaptability to the individual task, and minimum maintenance for ensuring maximum runnability are the basic pre-conditions. One example is the Compax series of reject compactors (Fig. 1). There are several hundred Compax installations worldwide. The new Compax CFX generation will provide intelligent process and performance control, as well as wear monitoring of the machine. Two machine versions for lightweight pulping rejects and fibrous fine rejects are available.
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The Sediphant (Fig. 2) is an example of a rejects handling machine. The patented unit is a combination of sediment separator and thickener, suited to separate out light and heavy fine rejects containing fibers and dewater them. The incombustible heavy particles fraction is discharged with a low moisture content (Fig. 3) and the lightweight rejects are pre-dewatered and can be incinerated after further dewatering (Fig. 4). The filtrate has a uniform quality.
The installation example at the end of this article illustrates how an overall rejects handling concept with intelligent interplay of the conveyors and their reversing and bypass functions, as well as smart layout, are all important factors for runnability and reliable operation.
This and numerous other installations show how the handling of rejects has developed within only a few years into a complex task that can no longer be accomplished by just installing individual machines. Success requires detailed system know-how.
Filtrates from the rejects/sewer zone represent an important interface with the water system of the overall plant. Standards set by legislation are increasingly influencing the design of rejects handling systems. Specific disposal costs can amount to around 5 % of earnings from paper production (Fig. 5), so the potential savings from an increased rejects dry content alone can be tremendous.
For new facilities, consider the following:
* Include rejects handling in the overall concept early in the design stage
* Define present and future options for rejects disposal
* Allow for future expansion of the rejects handling system from the outset
* Integrate metal separation and rejects size reduction
* Allow good accessibility for operation and maintenance
* Regard rejects handling as an integrated system
The following example describes a state-of-the-art rejects handling system in a newsprint production line using a drum pulper. The objective was to prepare the rejects for incineration in a given particle size and to minimize the incombustible content. The concept must ensure continuous pulper operation even during rejects system shutdowns, or when maintenance work is required.
The mill uses a Dinoscreen to remove coarse contaminants from the sewer channel, dumping them into a separate container. Rejects from HD and LD cleaning and from slot screening are dewatered in a Sediphant. The heavyweight material from the Sediphant discharges into a separate container.
The complete rejects from the drum pulper are collected on a conveyor. A metal detector removes large pieces of metal, which are dumped into a container by reversing the conveyor direction. A magnetic separator, located above the conveyor, removes ferrous metal, also dumping it into a container.
The rejects are then shredded into small pieces in a Lion shredder. Rails allow the shredder to be moved into a maintenance position without interrupting operation of the rejects handling system. A second magnetic separator then removes any remaining ferrous metal from the conveyor feeding the post-shredding stage. Here, two Lion shredders operate one above the other and at 90[degrees] to each other. These reduce the rejects to the required size. The feed conveyor can be swung so that the post-shredding and the subsequent compactor stages can be bypassed.
In normal operation the shredded material is dewatered with a Compax reject compactor, achieving a dry content of more than 60%. After compacting, the rejects are distributed automatically into four containers. A movable conveyor equipped with sensors can fill every container from several positions so that all four containers are used to capacity. The rejects are now ready for incineration.
Most examples of complex rejects handling systems are in Europe, but mills in other countries are demonstrating a new interest in using these systems to reduce costs and improve efficiency. As environmental regulations tighten in all regions, more mills should consider advanced rejects handling.
Market price Total non-paper Amount sent to per ton paper production losses landfill per ton (typical values) (typical values) paper at 60% dry content Newsprint 500 Euro 18 % 300 kg Corrugating medium 300 Euro 5 % 83 kg Graphic papers 1000 Euro 25 % 416 kg Landfill costs Disposal costs per ton paper at as a percentage 100 Euro/t of earnings Newsprint 30 Euro 6.0 % Corrugating medium 8 Euro 2.7 % Graphic papers 42 Euro 4.2 % Figure 5: Example of disposal costs as a percentage of the earnings from paper production for various paper grades.
WHAT YOU WILL LEARN:
* How costs for rejects disposal can amount to 5% of earnings.
* Why rejects handling systems are becoming more complex.
* Case study of a new rejects handling system.
* "China casts large shadow on recycle market," by Alan Rooks, Solutions!, December 2003. Product Code: 03DECS039 (To access article, enter product code in search engine on www.tappi.org).
* "New directions in recycle," by T. Friberg and G. Brelsford, Solutions!, August 2002. Product Code: 02AUGS027.
* Meri web site: www.meri.de.
RELATED ARTICLE: ABOUT THE COMPANY:
Meri Entsorgungstechnik GmbH is a joint venture company between Voith Paper Fiber Systems and Meri Anlagentechnik. The company has installed 30 complex rejects handling systems and has installed more than 600 machines. Meri provides custom subsystems as well as special machine technology.
ABOUT THE AUTHORS:
Gisbert Wunsche (left), Dipl.-Ing. (FH) and Bernhard Niemczyk, Dipl.-Ing., are with Meri Entsorgungstechnik GmbH, Munich, Germany. Contact the authors by email at firstname.lastname@example.org and email@example.com, or by phone at +49 89 545819 0.
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|Publication:||Solutions - for People, Processes and Paper|
|Date:||Jul 1, 2004|
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