Detection of polymer induced flocculation of cellulosic fibres by image analysis

The present report summarises work conducted to construct and evaluate a ne w equipment based on image analysis for detection of fibre flocculation. Ap art from calibration measurements the report also contains results from pol ymer adsorption experiments and polymer induced fibre flocculation measurem ents. Attempts are also made to utilise the adsorption measurements to unde rstand the flocculation measurements. The evaluation of polymer induced flocculation of cellulosic fibres has ear lier almost exclusively been based on different light scattering techniques and subsequent data handling of the collected signals. This means that the evaluation of the flocculation has been indirect and to some extent limite d by both the light scattering situation in the measuring zone and by the m ethod chosen for the data treatment. The rapid development of modem compute rs has made it possible to use image analysis for evaluation of different t ypes of processes and it was therefore considered important to investigate if this technique could be used to determine fibre flocculation. In the present work a flow loop was constructed around a narrow flow channe l. A light source was placed under the flow channel and the image of the fi bre suspension was collected with a video-camera, which made it possible to collect images with a very high time resolution. The images were collected by a computer and could then be subjected to analysis with both standard a nd specially developed software. Polymers could be added before measurement in two different stations with a time delay comparable to what is usually found in modem paper machines. Results from experiments with model flocs showed that the equipment and the evaluation principle chosen, Fourier analysis of the collected images, can be used to determine both a degree of flocculation and the size of the flo cs formed. The results from experiments with polymer induced fibre floccula tion, where different types of cationic polyacrylamides were used, showed t hat these types of polymers are efficient through a bridging type of floccu lation. The results furthermore showed that the polymer with a higher charg e is more efficient at lower levels of polymer addition whereas the polymer s with a lower charge are efficient over a larger range of polymer addition s. By comparing the results from the flocculation studies and the results from studies on kinetics of polymer adsorption it was found that only polymer a dsorption measurements at short contact time can be used to predict the flo cculation ability of the tested polymers. It is also recommended that more efforts are directed towards studies on po lymer adsorption kinetics at very short contact times. More realistic model s of polymer adsorption kinetics and bridging flocculation are needed.