Pre-flocculation of fillers : evaluation of different flocculation conditions

Since January 2009 Innventia AB has been running a research cluster called ‘Future papyrus’. This cluster is aiming to reduce fibre and energy consumption in the papermaking process. One strategy for achieving this aim is to increase the filler content in paper through pre-flocculation. Pre-flocculation of fillers is a technique which means that fillers are flocculated with chemicals (flocculants/retention aids) in a preceding step, before being added to the stock. This enables the formation of larger flocs which are more easily retained in the fibre web and cause less reduction of the paper strength than fillers normally do. Therefore, a larger amount of fibres can be replaced with fillers. Raising the filler content is a way of reducing the consumption of expensive fibres, which is highly prioritized as the paper making industry is trying to reduce manufacturing costs and energy consumption, without lowering the quality of the paper. One of the main difficulties with pre-flocculation is to create stable filler flocs that can withstand the shear forces in a paper machine. It is not necessarily the largest filler flocs that retain the largest size after all the process steps. Therefore, it is important to investigate how different conditions during pre-flocculation affect the floc size and to which extent this size is being kept after shear exposure. The different conditions are for instance the type of retention aid added and its corresponding flocculation mechanism, amount of retention aid (dosage), reaction time, pH and conductivity, amount of fines at preflocculation as well as amount of microparticles added to the system of chemicals. This thesis was carried out within the Future Papyrus cluster. The main purpose was to investigate how pre-flocculation and consumption of retention aid is affected by various parameters such as pH and conductivity as well as concentration of filler, fines and microparticles. The thesis work has also included development of the technique for measuring floc size. As a result of the work with measurement technology, a new method for particle size measurements with OptoPlatform was found. The new method is intended to minimize the shear exposure and is based on a small circulating hose system allowing the sample to pass through the OptoPlatform several times until proper mixing is achieved. The floc size measurements have resulted in some important conclusions regarding pre-flocculation of fillers. One of the most evident results is that the filler concentration at pre-flocculation has an impact on floc size. It was found that high filler concentrations are favourable for build-up of larger filler flocs. Furthermore, the fines content at pre-flocculation affects the particle size vs. dosage correlation. The fines content consumes (neutralizes) some of the retention aid, which results in small flocs at low dosages. However, fines also seem to co-flocculate with the filler, so that the flocs become larger at high dosages. Similarly, the presence of microparticles during pre-flocculation proved to result in larger floc sizes at higher quotients (microparticles/retention aid) and high dosages. When combining fines and microparticles, there was an additive increase in floc size. However, the pH and conductivity conditions in a paper machine proved to have no influence on preflocculation, compared to tap water conditions.Validerat; 20101217 (root