Retention aids

RDAs also act in synergy with other chemical additives such as deaerators, which boosts the overall performance of other wet-end chemicals and reduces their consumption. With improved retention as well as with the fixation of colloidal dissolved substances much cleaner white-water is obtained with a COD reduction and over all less deposits are achieved. Consequently a higher degree of process water closure, less volume of waste water and an improved PM runnabihty can be obtained. The waste water is also more easily treated. 

In the production and processing of chemical, semichemical and mechanical pulps and recovered paper, various inorganic and organic substances are accumulated in dissolved or colloidal dissolved form. Other water-soluble substances enter with the fresh water, fillers, recycled uncoated and coated paper broke and also by chemical additives. As the process water circuits are increasingly closed, the concentration of these water-soluble and colloidal substances and finely dispersed particles increases considerably and an additional contaminant load is thus imposed on the waste water. These substances interfere with the production process by increasing build-ups and deposits, they reduce the efficiency of the chemical additives, and impair the quality of the produced paper. Therefore these substances are also classed as detrimental substances (Fig. 3.18). 

The use of highly cationic polymers is a common possibility to remove the load of dissolved anionic substances by complex formation, fixation to the fibers and subsequent discharge with the freshly produced paper. Besides the dissolved anionic substances there is a variety of nondissolved, hydrophobic substances (particles) mainly coming from the raw materials, e.g. wood extractives, coating additives, adhesives. By fixation of the particles to the fibers they can be removed from the system before they build uncontrolled reactions vdth the paper stock and 

Anionic y Nonionrc Trash and Stickles Fig. 3.18 Sources of detrimental substances. 

Molecular weight [10 g mole ] Polymer Chaise density [meq g ] 

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Starch is a polysaccharide found in many plant species. Com and potatoes are two common sources of industrial starch. The composition of starch varies somewhat in terms of the amount of branching of the polymer chains (11). Its principal use as a flocculant is in the Bayer process for extracting aluminum from bauxite ore. The digestion of bauxite in sodium hydroxide solution produces a suspension of finely divided iron minerals and siUcates, called red mud, in a highly alkaline Hquor. Starch is used to settle the red mud so that relatively pure alumina can be produced from the clarified Hquor. It has been largely replaced by acryHc acid and acrylamide-based (11,12) polymers, although a number of plants stiH add some starch in addition to synthetic polymers to reduce the level of residual suspended soHds in the Hquor. Starch [9005-25-8] can be modified with various reagents to produce semisynthetic polymers. The principal one of these is cationic starch, which is used as a retention aid in paper production as a component of a dual system (13,14) or a microparticle system (15). 

Guar gum [9000-30-0] derived from the seed of a legume (11,16), is used as a flocculant in the filtration of mineral pulps leached with acid or cyanide for the recovery of uranium and gold (16). It is also used as a retention aid, usually in a chemically modified form (14,17). Starch and guar gum are subject to biological degradation in solution, so they are usually sold as dry powders that are dissolved immediately before use. Starch requires heating in most cases to be fully dissolved. 

Retention aid polymers are used in a very high shear environment, so floe strength and the abiHty for floes to reform after being sheared (43) is important. The optimum floe size is a compromise. Larger floes give better free drainage, but tend to produce an uneven sheet due to air breakthrough in... 

Vacuum filters are usually simulated with a Buchner funnel test or filter leaf test (54). The measured parameters are cake weight, cake moisture, and filtration rate. Retention aids are usually evaluated using the Britt jar test, also called the Dynamic Drainage Jar, which simulates the shear conditions found on the paper machine and predicts performance (55). 

The retention of fillers in the sheet during the forming process is important. Both hydrodynamic mechanisms and colloidal or coflocculation phenomena are significant in determining filler retention (7). Polymeric retention aids are used to bridge between filler particles and fibers. Talc is sometimes used with mechanical pulp furnishes in order to reduce the deposition of pitch-like materials onto paper machinery. 

Salts, eg, alum or calcium chloride [10043-52-4] and cationic polyacrylamides are effective retention aids in bleached and unbleached kraft pulp. 

In the paper industry, PEO is widely used as a retention aid and pitch control agent in the newsprint industry (118—135). Typically, a phenol formaldehyde-type resin is added to the substrate before the addition of PEO. The chemical that is added before PEO has been referred to as an enhancer. Recent pubHcations on designing enhancers that work with PEO have resulted in expanding the use of PEO in flocculation of several substrates (128,129). 

Poly(vinyl alcohol) is used as an additive to dry-wall joint cements and stucco finish compounds. Rapid cold-water solubiUty, which can be achieved with finely ground PVA, is important in many dry mixed products. Partially hydrolyzed grades are commercially available in fine-particle size under the name S-grades. The main purpose of the poly(vinyl alcohol) is to improve adhesion and act as a water-retention aid. 

The metering and addition of dyestuff solutions is critical to successhil operation of continuous dyeing. It is also just as critical to meter other components of the process including pulp, broke, fiUers, size, alum, fixing agents, retention aids, wet-strength resins, and other additions that affect dyeing. 

Construction Soil stabilization water absorber water retention aids in cements, grouts, and tiles improvement of mechanical properties of cement and gypsum plasters and water based paints... 

Fillers e.g. calcium carbonate, kaolin, titanium dioxide Sizing agents, starches Natural dyes, brighteners Retention aids... 

By control and substitution of chemicals used in the paper mills, manufacturers of packaging papers and board can contribute themselves directly to a small mineral oil reduction in their products. Some additives contain mineral oil as solvent such as flocculation or retention aids based on polyacrylamide (PAA), resin sizing agents or defoamers. A change of such products to mineral oil-free additives removes their own mineral oil input and reduces the contamination of packaging material. In the area of retention aids based on PAA, this conversion has already mostly taken place. 

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. 

Figure 7.2 Some typical cationic polyelectrolyte retention aids used in paper making.

High shear forces are prevelant in the approach flow system to the paper machine (i.e. as the fibre suspension approaches the point of deposition on the wire), and these have a large impact upon the efficiency of retention aids (Figure 7.8). A study of the effect of shear can often be helpful in establishing the mechanism of retention. Bridging flocculation is irreversibly sensitive to shear (i.e. when the shear forces are removed the suspension does not reflocculate) whereas charge neutralisation is reversibly sensitive to shear. 

Figure 7.8 The effect of shear forces (pre-stirring for different times) on the optimum polymer dose for ionene halide retention aids.

Dry strength additives are usually water soluble, hydrophilic natural or synthetic polymers, the commercially most important of which are starch, natural vegetable gums and polyacrylamides. These polymers are often made in cationic form by the introduction of tertiary or quaternary amino groups into the polymer, and are therefore polyelectrolytes. They are thus also able to function to some extent as drainage and retention aids. 

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