Pulp Paper & Logistics


ABB 11


of measurements per day – giving much more data that can be used for tighter process control. Freeness measurements are also used to determine whether pulp has been sufficiently developed by refining. The principal objectives of pulp refining are to: ● Increase the flexibility of the cell wall to promote increased contact area


● Fibrillate the external surface of the fibre and generate fines This further promotes the


Figure 2: Bleached softwood fibres, 50 times magnification


flow of water through the screen at the bottom decreases as a fibre pad builds up in the bottom of the chamber. The total volume in millilitres of water that drains from the chamber is the freeness value of the sample. In a manually-operated


freeness test, variation in how the sample is handled each time a test is made means the precision and reproducibility


are not as high as in automatic tests. Another drawback of manual freeness tests is the time needed to collect samples and determine pulp consistency. It is quite common that only a few measurements per sample point can be carried out in a day, and this low frequency of freeness measurements is not suitable for controlling the process with any precision.


These drawbacks are easily


overcome using an online automatic freeness testing system, where the samples are handled the same way every time, both in the preparation stage and in all the tests, resulting in better accuracy and precision. Plus, automatic freeness and fibre analysers can collect samples from multiple locations and process hundreds


formation of hydrogen bonds and increases the total surface area available for bonding. When more strength is desired for a certain paper grade, the specific energy of the refining process can be increased by narrowing the gap through which the fibres are forced in the refiner. Because this results in a lower freeness, the freeness measurement is often used as an indicator of the degree of refining.


Online sensors The initial sensors for online purposes using modern electro- optical and digital processor components (photodiodes and a microprocessor) were developed


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Figure 3: Freeness measurement (manual test in green, automatic test in blue) and Refining specific energy in red at FPC Papeles in Chile July/August 2021


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