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REJECT DISPOSAL FROM RECYCLED FIBRE PLANTS
Europe assumes pioneer role in reject handling systems
Today there is still a lack of awareness among the responsible personnel in the paper business of the costs for reject disposal, which may add up to 5 % of the earnings from sales. The "Return of Investment" (ROI) is not any longer the only justification for capital investments in reject handling. Legislation is becoming the other driving force, setting up increasing standards and requirements for re-use, disposal, or incineration of rejects.
I am not interested in reject, just get away with it! Not too long ago reject handling was very simple: Collecting, basic dewatering, loading onto trucks, and transport to a nearby landfill. Low costs and little effort involved with it made the topic a minor issue for the design and operation of a paper mill.
But the fundamental demand was always: Reject has to disappear before it generates problems. The more options there are for disposal, the easier it was to succeed.
Some illustrative numbers: A board mill for 1000 t/day based on 100 % recycled fibre may generate more than 50 t reject per day. De-inking mills are exceeding this value by multiples. Cost for landfill are varying a lot, in Germany typical values range from 60 to 200 € per ton. The recent consumption of recycled fibre in Germany reached approx. 11 million tons per year, resulting in 500.000 tons of reject per year to be disposed off.
This alone motivates investments in the reject handling system, due to very fast R.O.I. of all measures to reduce the amount of reject that has to be disposed off.
1 BDMT of reject at a poor final dryness of 35 % generates almost 3 t of reject that have to be handled, loaded, shipped and finally land filled. Using up to date technology may increase the final dryness up to 65 %, resulting in a total amount for disposal of approx. 1,5 t. The savings per BDMT is almost 1.5 t that do not add any more to the effort, time and costs of reject handling!
Diagram 1: Influence of final dryness on the amount of rejects to be disposed
Depending on the final product the reject rate based on the furnish varies from 5 % e.g. for liner to 25 % and even more e.g. for graphical papers. Based on 60 % final dryness of the reject the amount to be land filled results from 83 kg per ton of paper to 416 kg.
Diagram 2 indicates the possible savings based on the final dryness for different products.
Diagram 2: Reduction of disposal costs with increasing final dryness
Considering average values for the market price the total costs only for reject disposal eat up 3 – 6 % of the total sales!
Diagram 3: Percentage of disposal costs from the total sales for different paper qualities
But what exactly is the meaning of "disposal" today? Putting rejects directly to landfill is not an option any more in several countries, especially in the European Union (EU). Based on EU legislation landfill will be only permitted for material with an organic content of less than 5 %. (Effective in Germany from 01.06.2005; in Austria and the Netherlands already effective since 01.01.2004). Other countries not only from the EU will follow.
Exporting reject to countries with lower standards requires tremendous transport costs, and may be even illegal. The result is: reject from recycled fibre operations has to be incinerated . Only the residuals from incineration may be land filled.
What are the consequences of incineration? Reject has to fulfil certain quality standards! What seems to be a paradox first is going to become the major design criteria for recycled fibre plants:
And nonetheless there is still the fundamental demand: Reject has to disappear! Now! Wherever it appears and whenever it appears!
What is the material, the components, entitled as "reject"? First of all there is the general distinction from sludge: Sludge is homogenous, like the residuals from de-inking cells, the floated sludge from a Dissolved Air Flotation or the settling sludge from a primary clarifier.
Reject however is non-homogenous. The two major categories are coarse and fine rejects. Coarse reject has its origin in pulping and de-trashing, as well as coarse screening. Fine reject comes from the following process stages like cleaning, fine screening and from the approach flow.
There are heavy coarse rejects like metal of all kind, shape and size; stones, not disintegrated paper or wet strength, wires etc.
Light coarse rejects consist from plastic, foils, CDs, etc.
The composition of coarse rejects depends heavily on the type of pulping system: we distinct between systems that discharge heavy and light weight coarse reject separately (e .g. TwinPulp-System) or not. Due to the increasing number of installed drum pulpers the focus shifts towards the second option.
Also fine rejects differentiate into light and heavy weight.
Heavy fine rejects mainly consist from sand, glass, staples and other metallic office waste, discharged from HD- and LD cleaners, as well as from the heavy junk traps of combined screening/cleaning equipment.
Light fine rejects from slot screening or light weight cleaning contains fibre broke, spin-ups, stickies, wax, filler, etc.
Now let's come back to the fundamental demand: How can we ensure that reject in its diversity anytime and anyplace will disappear "just in time"?
To find a "sustainable" answer to this question we have to step back and look into the available options for disposal. And here we are back to the standards set by legislation and the reject quality:
The portion of the reject which is suitable for incineration has to be discharged at the highest possible dryness, accompanied by the highest possible caloric value (> 11 MJ). It might be even necessary to apply a drying process in order to achieve this limit. Furthermore the particle size of the reject has to meet the requirements of the available incineration technology. The content of organic contaminants in the non-burnable, inorganic fraction has to be limited to a minimum, in order to allow disposal.
Usable material like metal has to be separated as clean as possible, to make it acceptable for further use.
These targets, combined with the composition of the reject and the locations of origin in the stock preparation process make up the frame for the design of the reject handling system.
The quality of the equipment used within the reject handling system is the key for the success. Sturdiness and adaptation to the individual task, lowest possible maintenance combined with highest availability are the basic pre-conditions.
Another highlight within the various available reject handling machines is the Sediphant, representing a unique, patented combination of sedimentation separator and disc thickener, designed to separate light and heavy fine reject from each other, and dewater both fractions. Non-burnable heavy particles are discharged with very little water content, light weight rejects are separated to be suitable for incineration after further dewatering, and a filtrate of constant quality is generated.
The installation example given at the end of this paper will show the importance of an overall concept for a reject handling system. The interactions between the conveying equipment, reversing and bypass operation, as well as the proper layout are crucial for availability and safe operation.
As one can see in this and numerous other examples proper handling of rejects has developed within only a few years from a side issue to a complex task, which can not be faced with the installation of single machines, but requires detailed system know how.
This is where meri Entsorgungstechnik GmbH, has gathered its experience from 30 installed complex reject handling systems, and more than 600 machines installed in the paper industry regarding tailor-made subsystems as well as machine design.
Not to mention that the filtrates from the reject / sewer area represent an important interface to the water system of a paper mill, and have to be taken into consideration for a complete overall view.
Standards set by legislation are gaining more and more importance for the design of reject handling systems. However also the ROI remains important to justify investments in the reject handling area, considering the fact that a recycled fibre mill may spend up to 5 % of the earnings from sales only for the disposal of reject. The potential savings based only on the final dryness of the reject can be tremendous.
During the design phase of recycled fibre operations the following should be considered:
This recent example indicates one possible up-to-date reject handling system. It was realised for a newsprint mill, using drum pulper technology. The target was to prepare the reject at a given particle size for incineration, and to minimize the non-burnable content. Furthermore the layout has to allow continuous operation even in the event of maintenance or failure, without stopping the pulping system.
To remove coarse material from the sewer, a Dinoscreen travelling screen is used, dumping the reject into a separate container. Reject from HD- and LD and from slot screening are dewatered in a Sediphant. The heavy fraction from the Sediphant is also discharged in a separate container.
The entire reject from the drum pulper are collected onto a belt conveyor. A metal detector allows detection of metal parts that can be dumped into a container by reversing of the belt direction. A metal separator, located above the belt conveyor, removes Fe-metal. Then the reject is shredded into small pieces in the Lion pre-shredding stage. Rails allow shifting the shredder into a maintenance positions without interrupting the operation of the reject handling system. A second metal separator removes remaining Fe-metal from the conveyor feeding the fine shredding stage. Here two Lion shredder are operating in cascade, in order to reduce the reject particles to a given size. The feed conveyor can be swivelled into a bypass position.
In normal operation the shredded material is finally dewatered with a Compax reject compactor, achieving a dryness of more than 60 %. After compacting the reject is distributed into 4 containers by an automatic container distribution system. A rotating and sliding conveyor equipped with specific sensors allows filling every container from several positions, in order to use the complete volume of the container. Now the reject is properly prepared for incineration.
Up to now most of the examples for complex reject handling systems are from Europe, but also in other countries there is an increasing demand to pay more and more attention to reject handling, motivated either by requirements to reduce the operation personnel or to simplify disposal and make it more economical.