Equipment Description:Retention/Dewatering Aids
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Retention/dewatering aids, as the name implies, are added to speed up the removal of water from the forming section of the papermachine and to increase the amount of the furnish which is retained on the wire. |
retention drainage polymer microparticle |
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Effective retention of papermaking furnish and additives is important due to economic factors to optimise the cost of functional additives and also to minimise the detrimental effects of unretained additives, such as deposit formation. Effective drainage is important because of its effect on drying efficiency and steam usage and hence its positive effect on production efficiency. Retention systems can be conveniently divided into the following main types:- 1. Single component - often a medium to high MW polyacylamide or polyethyleneimine is added, typically before or after the last pressure screen, close to the machine flowbox. In very simple systems alum ,PAC or wet-strength may be the single component. 2. Dual component - Conventionally, a cationic (positively charged) and an anionic (negatively charged) component are added separately. The cationic component is typically starch or polymer (occasionally guar gum) and the anionic component can be an anionic polymer or a microparticle. Addition point summaries will be given in subsequent factsheets. (Level 3) 3. Multicomponent - As an adjunct to the above systems, specific additional chemicals can be added to give a specific benefit e.g. polyaluminium chloride may be added (or polyDADMAC or a cationic resin) to act as an anionic trash catcher. The function of these chemicals is to neutralise anionic substances which may interfere with the retention mechanism. Enzymatic systems based on cellulase or hemicellulase are also available. All the systems work by increasing flocculation of the stock components, which means that larger flocs are formed. This means that the flocs will be more easily retained on the wire, since they will not pass the mesh as readily and water will be moved more readily due to the filtration resistance being lower, due to the floc arrangement. The selection of a retention system depends on several factors, including furnish type (e.g. virgin or recycled), amount of filler and broke, degree of system closure (higher closure means higher conductivity and increased chemical concentration), speed of machine (higher speeds will increase the shear and usually requires higher MW polymer). |
Retention/Dewatering Aids was also mentioned in the following Fact Sheets (23):
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