Mechanical separation of stickies - the challenge for the new millennium

Hubert Gassmann

Introduction
The use of waste paper which changed and influenced fundamental principles in paper industry in the eighties reached its climax at the end of the nineties. With increasing recycling rates and the growing shortage of waste paper the load of re-preparation and production processes is steadily rising.  Furthermore the constantly increasing economic pressure on the paper industry as well as in particular on the waste paper processing industry is a decisive factor that smallest impurities like stickies become more and more disturbing as the production plants have to increase their production efficiency nearly up to 100%.

This presentation describes the possibilities for the mechanical separation of stickies and gives answers regarding the demands on the new millennium. The use of new technologies has also as goal to avoid the further size reduction of adhesives by less shearing forces. Additionally, new technologies facilitate compact installation possibilities in the process, the avoidance of open systems as well as the reduced demand for auxiliary chemicals. The new technology for the new millennium shows a promising possibility how the almost uncontrolled sticky problem becomes more and more a calculable problem.

A) The Nineties

1. Adhesives / stickies
Before we start thinking about how to eliminate these disturbing particles, we should ask: Where can they really be found?
 
In order to answer this question we study not only the fibre material but also the corresponding water circuits (fig. 2).
 
We gave up analysing stickies a long time ago, referring only to their number or their area. Due to the specific characteristics of adhesives and hot melts we know that it is easy to reduce sticky area. In this case, however, we mostly have to consider the reduction of the number of stickies.
 

Sever al devic es are install ed in the proce ss and their differi ng sheari ng forces have an effect on fibre material and adhesives.  It is no surprise that during the repulping process the size of adhesives particles can be reduced, but the number increases significantly. In this context we would like to emphasise once more that all adhesives are plastic-elastic, flexible and change their form under strong acceleration (fig. 3).
 
As already described with former projects, fig. 4 shows again how a sticky particle of size 180 : 200 µm changes its form by acceleration so far that a typical fiber dimension arises - at least as far as its form is concerned.

 
2. How to separate stickies from fibre material
The separation of adhesives in the screen has been limited to coarsely dispersed particles > 0.04 mm2 or > 100 µm until now .Furthermore it has not been possible to detect finely dispersed stickies. The measuring technique developed allows us to show a part of the microstickies by means of new analytical methods.
 
Due to these analysis techniques it becomes very interesting to compare the separation of stickies in the screening process to that in the fractionating process (fig. 5).A cleaner can separate only finely and coarsely dispersed particles whose specific weight is higher than that of fibre or whose specific weight is less than that of fibre and water.

The measuring of microstickies shows how far the cleaner can operate more efficiently than a screen with a separating element having a slot opening of minimum 0.1 mm (fig. 6).
 
3. Demand on the separation process
Fig. 7 shows different results possible. Goal of the stock preparation plants is and will always be to achieve the highest possible efficiency as well as to separate as many impurities and stickies as possible. Each supplier of preparation, cleaning and screening systems would like to achieve a higher efficiency than his competitors in order to sell his plants.

Here we have to point out once again that the components, screen basket and rotor, as well as their combined effect have considerable influences on the final result. So e. g. it is possible that an existing device separates 50 % or even 90 % of the disturbing particles. In this case we have to take into consideration that the slogan 'smooth operation - smooth process' is on everyone's lips and has become decisive for screen and cleaner in the meantime.

 
Clea ners whic h are judg ed as bein g ineff icien t due to their energy consumption, should be considered from the point of view of specific separation of impurities and adhesives in comparison to their energy supply. The specific energy supply at the cleaner – referring to a percentage separation of stickies – is not higher than the specific energy consumption of the screening process. If the specific values are only considered on the basis of energy supply per ton of fibre, the cleaner would always turn out worse than the screen.

 
As already mentioned in the introduction, paper makers are finding that not only fibres contain disturbing adhesives but also that many of these adhesives circulate in the water circuits. Disadvantage of this circulation is that due to shearing forces exerted by pumps and other devices, the size of adhesive particles is steadily reduced. The dispersing effect of debris and adhesive particles is not desired, and has very negative consequences on the complete process.Deposits in the system, process disturbances and damage of the final product are the results.
 
4. How to sepa rate stic kies from wate r circ uit

Pressure filters as shown in fig. 8 are simple devices, being able to separate coarsely dispersed adhesives > 0.04 mm2 or > 100 µm. So what happens with finely dispersed particles?
 
A pressure filter operating according to function principle (fig. 9) builds up a fibre mat on the surface of the filter element additionally supporting the filtering effect.
 
So not only coarsely dispersed but also finely dispersed particles can be separated. As measuring technique for sticky analysis has been modified, it can be shown that a part of the finely dispersed impurities can efficiently be removed from the process forever at low costs and with small energy input without using any chemicals.
 
In fig. 10 you can see a twin wire press being installed in front of dispersion for the dewatering of waste paper. 15 % of all stickies being fed to the twin wire press go back to the process again via the filtrate. 

 
Opti mall y swit ched wate r circu its retur n the filtra te into a water loop installed in front. But in practice we do not only find optimally run or switched plants but also plants where this filtrate is used directly for re-dilution after dispersion and for dilution in the screening process. So, however , they try to treat 85 % of the stickies but they absolutely ignore that part in the filtrate. Also  – as with fibre – the maxim is:  separating is better than dividing it finely. By agglomeration afterwards finely dispersed adhesives which have been reduced in size by considerable shear, can combine to larger particles again and continue disturbing the processes.

 
Fig. 11 shows a pressure filter whose reject is re-cleaned by means of two last stage ceramic cleaners. So it is possible to recover fibres and separate debris from the process in a concentrated manner. As with all mechanical equipment the right combination is most important concerning the installation of a pressure filter.
 
In fig. 12 you can see an example for white water preparation with pressure filters in order to use the cleaned filtrate as shower water at the paper machine. Our goals are simple and comprehensible for everyone (see fig. 13): We want to increase production, we want to improve quality and guarantee cleanliness of the system. But the most important factor is: We want to reduce trouble and stress.

 
5. Examples from practice
 
The example fig. 14 shows how far efficiency in an existing screening device can
be improved by a smooth process in comparison with a usual process at the same capacity.
 
A reduction of stickies from 65 % to 83 % in the same device at the same capacity is worth having a good look at.

Fig. 16 shows an example from practice where by means of a pressure filter 80 % of impurities and adhesives contained in the filtrate can be separated from the system in a concentrated manner.
 
In fig. 17 you can see how successes are possible with the installation of a cleaner plant operating efficiently.
 
A separation of 80 % of the particles being responsible for the problems is excellent.

B) The New Millennium
 
At the beginning of the new millennium the paper industry is confronted with many problems. The expectations for the new millennium are influenced by the development of the world market, by the globalisation becoming stronger and stronger as well as by the rapidity of new technological generations.
 
A shortage of raw material is foreseeable. The recycling rates are increasing world-wide whilst waste paper quality steadily reduces. The supply of fresh water becomes more and more a problem in many regions. Environmental protection plays an increasingly important role, more rigorous conditions are the consequence.
 
The costs of energy, water supply and disposal sites are continuously rising. Together with the expenses for raw materials and wages the financial load increases. This forces significant reductions of costs in order to continue to exist on the market. The competition on world market intensifies, the demands on quality of the product delivered have increased correspondingly within the last few years.

Partly, these developments have been known for a long time but we have not been able to gain control of them and the problems have even increased. The consequence is that we as a company prepared for the beginning of the new millennium very early.
 
1. Goals

The saving of expensive energy is one of the goals regarding the new and future development of rotors as well as screens and pressure filters that has been translated into action (fig. 19).
 
The ML Screen shows completely new aspects of a product already established (Bar-Screen basket): The renewable basket gives the chance to decrease costs significantly and take into account environmental protection at the same time (fig. 20).

 
The optimisation of existing technologies is an important goal to meet the diversely increasing requirements effectively, shown in fig. 21.
 
This optimisation concerns the geometries being used in the different products as well as the functionality of the complete system where Fiedler can show excellent successes as specialists for difficult problems e. g. the mechanical separation of stickies.

Complete plants have been optimised. The key here is the improvement of screening systems from stock preparation to headbox. This is carried out with step by step optimisation projects in close co-operation with the customer.

 
Goals already translated into action successfully are:

• improvement of screening results, better separation of debris and shives
• more reliable operation
• increase of capacity (e. g. production increases)
• reduction of sewage and fibre losses
• energy saving and cost reduction by decreasing speed of pressure screens
• removal of disturbing influences on product quality (lower screen pulsations)
• improvement of runnability of paper machine (e. g. reduction of breaks)
• decrease of chemicals, particularly in case of fiber material bleaching

Fiedler have a pressure filter and a cleaner trial plant in order to gain considerable knowledge concerning the screening and cleaning process on your site. The Fiedler owned laboratory gives the opportunity to evaluate your material samples in detail, e.g. the evaluation of debris reduction according to Haindl McNett.
 
Also sticky, debris and sand analyses are carried out in co -operation with PTS (PapierTechnische Stiftung). Hereof experienced paper engineers elaborate an optimisation concept which can achieve the goals that have been set (fig. 22).
 

2.     Prospects
All these developments mean technological know-how which can meet the challenges of the new millennium effectively. But nevertheless this is not yet the end!  Pioneer new developments are necessary to reach a new generation in technology which will make large progress in paper industry.
 
The following goals are mainly pursued in screening processes also in the future:

• removal of light and heavy weight debris
• separation of stickies
• removal of ash
• de-inking

The great challenge is to achieve all these goals with only one single device!