Retention and Drainage Aids

Retention and drainage aids are chemicals which are added to the fibre and filler suspension to assist the efficiency of the filtration process. Growth in recent years in the use of retention aids has been greater than that of almost any other paper chemical additive. It has been caused by a combination of factors increased machine speeds, the increased use of filler in alkaline systems, the increased use of recycled paper and the growing tendency to use fillers in newsprint. Retention aids are water-soluble polymers which may be cationic, 

A low consistency suspension of fibres, pigments and chemical additives (thin stock) flows on to the moving wire mesh filtration medium of the paper machine where the wet web is formed. The water which drains away in this process (white water) is then recycled as far as it is practical to do so and is used to dilute the incoming high consistency suspension (thick stock). Retention can be considered either in overall terms or in terms of a single pass of the thin stock across the machine wire (first pass retention). For efficient operation, paper makers aim to achieve as high a first pass retention as possible, which they do by the use of retention aids. This reduces material losses and also assists in minimising the level of suspended solids in the effluent. 

This type of mechanism is likely to be partly operative in systems containing inorganic electrolytes as, for example, in the case of aluminium species. Some polyelectrolytes may also induce flocculation by charge neutralisation but the adsorbed polymer may also be able to bridge from one particle surface to another ( polymer bridging ). 

To determine which mechanism is operative, it is necessary to obtain information on the extent of adsorption of the polymer (adsorption isotherm), the effect of the adsorbed polymer on the 

In some types of flocculation there is no coincidence of the isoelectric point with the point of optimum flocculation and polymer 

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Papermaking additives can be categorized either as process additives or as functional additives. Process additives are materials that improve the operation of the paper machine, such as retention and drainage aids, biocides, dispersants, and defoamers they are primarily added at the wet end of the paper machine. Functional additives are materials that enhance or alter specific properties of the paper product, such as fillers (qv), sizing agents, dyes, optical brighteners, and wet- and dry-strength additives they may be added internally or to the surface of the sheet. 

Increased water system closure and iuCTcased water temperatures are also altering the environment within which the retention and drainage aid has to function, requiring an alternative approach in many cases. 

The paper machine and system chemistry factors which can affect the retention and drainage aid performance are discussed along with some troubleshooting guidelines and examples. 

What does a papermaker expect from a retention and drainage aid ... 

For the supplier, having a wide range of retention and drainage aid products and systems is essential as everything starts with the needs of the papermaker, the products made and the papermaking machinery used. 

The following products form part of the considerations for the retention and drainage systan which will eventually be used and should therefore be looked upon as components of the retention and drainage system - as any change in the product used or the dosage, application point, etc. can have a significant effect on the retention and drainage aid efficiency on the machine ... 

The products normally considered as the effective retention and drainage aid additives are ... 

As machine speeds have increased and wet-end system chemistry has become more complex, the demands on the retention and drainage aid have increased considerably. This has led to the development of an increasing number of retention and drainage aid systems which incorporate two, three or even four components selected from the above list. A very general representation of the time frame for this development is shown in Fig. 3.3. 

Occasionally, two products can be mixed prior to addition or more than one product can be added at the same addition point - out of necessity or because laboratory testing has shown this to be the optimum addition strategy. The main addition points used for the retention and drainage aid components are illustrated in Fig. 3.4 - points 1-6. 

All of the above retention and drainage aid products and systems are in use currently. The system used depends on machine type, machine speed, paper/ board type, production rate, requirements, etc. 

The basic criteria used for selection of the most suitable retention and drainage aid product or system, include the following ... 

Stock approach flow This can be a limiting factor in application of the most efficient products. The papermaking systems need to be able to cope with the changes necessary for implementation of an efficient retention and drainage aid system. Restrictions on pulper capacity, fan pump capacity etc. can reduce the suitability of the most effective system, requiring a less powerful system to be used, as can limited machine speed, water removal capacity and drying capacity. 

Paper/board quality All retention and drainage aids products and systems affect the sheet formation. The need to maintain acceptable formation can limit the type of system that can be applied and also the level of retention and drainage performance which can be sustained. As many trials fail due to an over-performing retention aid as fail due to an under-performing retention aid. 

As the retention and drainage aid is the last chemical addition to the papermaking system, it generally comes under snspicion whenever retention or drainage effect is... 

Additives - all additives have the potential to affect the system response to retention and drainage aids - the main considerations are the additive charge... 

Optical whiteners (OBA, FWA) are anionically charged and can have significant impact on the system charge and effectiveness of the retention and drainage aid. 

All aspects of the stock flow system and the paper or board machine equipment will have an effect on the retention and drainage aid. 

Initial choice of the retention aid/system which can be used may depend on the capability of the fan pump flow or machine speed to increase it may require adjustments to the foil drainage elements or vacuum boxes. Any changes to the system need to be evaluated for impact on retention and drainage aid - machine wires in particular. Changing a wire can inaease or decrease first-pass retention on the machine by as much as 10% in extreme cases. Where drainage is readily achievable, using a more closed wire can be one route to increased retention. 

From DDJ results it was concluded that the retention of a cationic polymer in a paper web will be maximised by allowing sufficient time and agitation for the dissolved polymer molecules to contact and become adsorbed on the surfaces of fines and fibres. Such conditions favour the use of these polymers as dry strength additives. The reverse is true in regard to their use as retention and drainage aids, i.e. thick stock addition for strength, thin-stock for retention drainage [9]. 

With a Miitek DFR-04 lab unit the effect of various defoamers on drainage can easily be tested. For optimum performance of retention and drainage aids a Miitek RET-20 Retention Inline and a Miitek DRA-50 Drainage Analyzer are best installed directly in the headbox and white water. 

Paper Filler retention and drainage aid in the manufecture of paper Flocculant for clarification of effluent water... 

Strom G, Barla P, Stenins P (1982) The effect on pine xylan on the use of some polycations as retention and drainage aids. Svensk Papperstid 85 R100-R106... 

Flocculants are also used as retention and drainage aids in paper production. This is again a solid-liquid separation operation, albeit using very sophisticated and extremely high-speed filtration. Without the use of these reagents it would not be possible to produce paper at the rate that it is today. 

Formation is the appearance of the sheet in its final finished state. The use of retention and drainage aids significantly affects the formation of the sheet leading to a flocc/ or uneven appearance with areas of high and low grammage. Good and bad formation is illustrated in Figure 6.21. 

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