To achieve an effective reduction of impurities like wood pitch, white pitch und stickies, they should be fixed on fibers and fines in their smallest possible form. This is usually achieved using inorganic and organic fixing agents. For a successful fixation of colloidal materials with minimized agglomeration the type of fixing agent, dosage and dosing point are critical variables. Agglomerates can cause degradation of quality and paper breaks at paper machines.

Nowadays turbidity measurement and charge measurement are used for the selection and optimization of fixing agents. These measuring methods can offer only a simple overview but cannot give information how the colloidal material is fixed and agglomerated.

We want to present a new method to fix colloidal substances on fibers and fines in the desired direction. The advantage of this method is that product selection, determination of dosage and dosage point can be deployed easily in paper mills on site. Following, we show examples for applications and we discuss the different strategies to regulate the dosage of fixing agents.

Impurities and their costly consequences

With different fibers and chemical additives impurities are introduced into paper machine circuits. If they are not treated in their smallest form without undue delay, they later can cause deposits in paper mill circuits and on paper machines. As a consequence paper defects can arise, which often occur if printed and this leads to costly complaints.

Deposits can be formed on different points, even where they are not expected, for example in pumps and screens. In many cases the treatment of impurities with unsuitable additives, treatment on the wrong points as well as the wrong dosages are responsible for these deposits. Changes in temperature and pH-value, shear forces and especially various cationic additives can form not well fixed impurities which later can build agglomerates that can be found as deposits on parts of the paper machine or in the paper leading to loss of production and customer complaints.

Optimisation with the APC (ACAT Pitch Counter)

 Today we have standard methods like measurements of turbidity, charge and COD but they are not able to give information for the selection of the suitable additive, the needed quantity and the dosage point. For example, an agglomeration of impurities cannot be detected with these methods. The APC-method (ACAT Pitch Counter) gives information how impurities are fixed.

Fig. 1 shows the different mechanisms from perfect to poor fixation. In all cases turbidity will be improved.

Due to the compact design the APC can be easily deployed locally in paper and pulp mills.

Interpretation of results

In general, on the x-axis, the particle size of pitch and stickies can be read in μm. On the y-axis the number per ml can be read. In this case the study is made with white pitch particles from coated broke. An ideal fixing agent (yellow and red curve) should reduce the number of particles in all size-ranges compared to the blank sample (black curve) without producing agglomerates with smaller particles (blue curve).

With the APC the different fixing agents and dosages that are responsible for these deposits are measured and evaluated in the terms of cost/benefit factor. Then together with the customer an approach is defined.

Conclusion

With the APC method the optimal fixative, the optimal dosage and the optimal dosing point can be selected. In addition, studies can be carried out directly in the paper mill and so the method is ideally suited to accompany plant tests.

Unfortunately, papermakers often find materials in paper production which entered the system unintentionally and have undesirable effects. They are generally known as impurities. Trends such as increasing closures of paper machine circuits, rising process temperatures and increasingly difficult raw materials (such as DIP, TM) intensify their harmful effect.

The most common unwanted side effects are:

• negative effect on chemical additives,
• negative effect on quality parameters

such as strength, holes etc.

• the formation of deposits and consequently a negative influence on production and quality.

To avoid disruptions in production one tries to remove the impurities or to make them harmless with chemicals. There are different problem solving strategies, which often have to be combined to achieve sustainable success. Of particular interest are:

• Mechanical cleaning with sorters and  cleaners

• Fixation within paper and therefore discharge via  paper

• Masking or removal of stickies
• cleaning of circulating water.

To find the optimal strategy, some preparatory work is needed.

First, the origin and properties of the impurities are determined. In practice, the origin is identified with a system analysis. Samples are taken from the stock  and the impurities potential is determined with different methods. Standardly determined are: turbidity, charge titration, pH- value and conductivity and, if needed, the COD. These values ​​provide indirect information on the system conditions.

The following methods provide a more precise determination:

• Chemical analysis of deposits
• Extraction
• Selective colouring
• Sorting and sheet evaluation in the laboratory
• Laser fluorescence analysis

In general, the main sources can be divided into groups.

The treatment strategy must be selected according to the properties of the substances and their occurrence.

Classification of impurities

There are two main groups of impurities: the so-called „anionic trash”(=real or colloidal dissolved anionic polymers) and the so-called „pitch” (= non-ionic hydrocolloids of different origins).

These substances can originate from various areas, such as:

• Deinking: sodium silicate
• Filler dispersion: poly acrylates, polyphosphates, etc.

• Coated rejects: starch derivatives, binder

etc.

• Pulp: organic acids, hemicelluloses, resins
• Fresh water: humic acids

Also by an improper use of aids „impurity problems” may arise; for example ASA deposits and the like.

The mechanism of the formation of deposits can be summarized roughly described as a problem of hydrophobicity. In water, very hydrophobic particles tend to aggregate into larger particles or to attach to non-polar surfaces. If very large particles are formed, usually they will be easily mechanically separated. On the other hand very hydrophilic particles dissolve molecularly in water. Particularly difficult are substances with both hydrophobic and hydrophilic properties. They will not really dissolve and they can form only small aggregates. Therefore it is difficult to separate them mechanically. Faults in the structure by pH jumps may lead for example to precipitation and deposits. Another important factor is the interaction with various surfaces such as fibres and fillers as well as with screens and other machine parts.

The solution of problems with impurities: A combination of technology and chemistry

 The elimination of impurities results from their physical and chemical properties as well as from their origin and their nature. The most efficient method is always a functional interaction of technology and optimum chemistry.

First and foremost stickies have to be mechanically separated. Macro stickies with sizes over 100 microns are separated by sorting.

With ENESSCO INT., Applied Chemicals offers an appropriate new product, which supports the agglomeration of stickies and improves sorting.

All subsequent separation processes are positively influenced by ENESSCO INT.

Due to various process steps there is still always the risk that secondary stickies are formed or that colloidal systems are destabilized. Therefore, further measures have to be taken to reduce impurities.

An overview of possible procedures is given below:

• Dispersion: of very small particles: risk of  reaggregation and secondary formation of  stickies.

• Micro flocculation: formation of microflocks and subsequent discharge by flotation reduces drastically the impurities potential in cycles

• Adsorption: bentonites may adsorb in particular non-ionic substances, which can be easily  removed

• Fixing: using cationic fixing agents anionic charged substances can be fixed onto fibres and therefore they are discharged with the paper.

Applied Chemicals offers all necessary laboratory work as well as a comprehensive range of additives.

Our application engineers can also provide information on the interactions between the various additives.

We would like to advise you personally. Our application engineers are at your disposal and they also provide on-site support.