Mehr zum Thema "Fixiermittel und Koagulanten"

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.

Among other machines at the same plant the customer owns a two-layer board machine. The raw material is provided by an integrated pulp mill. Depending on grade selection program bleached or unbleached grades are used. According to quality requirements also the ratio of short-and long-fibres varies. The problem especially arose when converting from unbleached top coat (kraft liner) to bleached top coat (top liner).

Our chemical technology division is always interested in offering customers the most advanced solutions. Therefore, over the past years sizing has been converted to ASA. In ASA sizing agents cationic mass starch is used as surface colloid and they contain PAC for deposits reduction.

Our retention system is a single bentonite system, using 1 to 11/2kg BentStar® per ton.

The respective dosing points are ahead of the headbox.

When optimizing sizing deposits increased. Particularly in the top wire area massive problems with the related downtimes occurred. However, it was impossible to identify clearly the source. Even with different operational modes of the sizing agent and its components such as mass starch and PAC it could not be evidenced, whether or not the deposits originate from the ASA. This has been the first explanation of all persons involved. And we were far from getting a grip on the problem technologically.

The main task of our paper technologists was the investigation of the root cause. Therefore, samples from   the top wire suction box were taken and examined in a laboratory by FTIR (infrared spectroscopy). Via IR-spectra it was shown that the deposits were resins from the cellulose fibres used. Now a solution had to be found.

In extensive laboratory screenings various products and procedures were analysed. Besides charge measurement and other methods turbidity measurement is an essential parameter. The best samples from these preliminary tests were further tested. Especially the FCM method provided interesting results to substantiate preliminary tests.

With this test method particles in the filtrate can be detected and additional it is possible to distinguish between bacteria, total counts of particles and hydrophobic particles.

Chemicals are used to reduce the particular hydrophobic particles.

Based on our test results we prerared a machine trial using 1 kg of the fixative FixStar®/ ton cardboard in the thin stock and increased the amount of BentStar® to about 2 kg/ ton cardboard to solve the problem.