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How to measure crill – a hundred times smaller than a fiber

Very small particles in a pulp suspension

Most people think of whale food when they hear the word "Crill". For the pulp and paper industry, crill has nothing to do with food, but is a way to measure the very small particles in a pulp suspension.

When measuring fiber properties image analysis is usually being used, as in, for example, L&W Fiber Tester and L&W Fiber Tester Plus. However methods based on image analysis will always be limited by the optical resolution of the system. This particularly affects the possibility to measure small particles as they require extremely high resolution. The crill method is unique in its kind since it is not based on image analysis and therefore not limited by the resolution. Instead the crill method is based on utilizing a particles ability to absorb and diverge light of different wavelengths depending on its diameter.

By leading the pulp suspension through one UV and one IR light source with a detector on the opposite side it is possible to detect if small particles is present in the solution. The more particles the more light is diverged or absorbed. Small particles such as crill will spread and absorb the light from the UV light source whereas the fiber will affect the light from the infrared light source. The contents of crill is presented as a quota, UV/IR detected. The crill method was patented in the early 1980s by Håkan Karlsson, former employee of Lorentzen & Wettre.

Approximately one hundred times smaller than the fiber

During refining the fibers are affected in different ways, for example, the fibrils of the fiber walls are to a certain degree exposed and it is these fibrillary particles on the fiber wall that are called crill. In theory, there are two types of crill free and bound. Entirely separated fibrils are called free crill and those that are still connected to the fiber are called bound crill. The crill particles have a diameter that is approximately one hundred times smaller than the fiber diameter.

The main benefit of analyzing crill is to monitor and control refining and dewatering. As can be seen in figure 1-3, the crill quota is directly proportional to refining energy for three different types of pulp; softwood, CTMP and eucalyptus. It can also is be used as input when modelling paper properties from fiber properties. The crill quota correlates well with fibril perimeter values from L&W Fiber Tester Plus as can be seen in figure 4. The excellent correlation indicates that the same information can be achieved by both methods.

L&W Crill press release

Figure 1 shows the relationship between refining energy and crill quota for softwood pulp.

Figure 2 shows the relationship between refining energy and crill quota for CTMP.

Figure 3 shows the relationship between refining energy and crill quota for eucalyptus pulp.

Figure 4 shows the crill quota versus fibril perimeter values from L&W Fiber Tester Plus.