Repulping

The recovery of fiber from broke (off-specification paper or trim produced in the paper mill) is compHcated by high levels of urea—formaldehyde and melamine—formaldehyde wet-strength resin. The urea resins present a lesser problem than the melamine resins because they cure slower and are not as resistant to hydrolysis. Broke from either resin treatment may be reclaimed by hot acidic repulping. Even the melamine resin is hydrolyzed rapidly under acidic conditions at high temperature. The cellulose is far more resistant and is not harmed if the acid is neutralized as soon as repulping is complete. 

Another technique to reduce the problems caused by stickies is to use additives to reduce the tackiness of these particles. This prevents their later reagglomeration and attachment to paper machine surfaces. These additives are usually added to the pulper. The most common is talc (17) usually added to the pulper in repulpable bags. Emulsified talc is also sometimes added to the pulp just before the pulp encounters high shear. Organic polymers (18) such as a polyvinylpyrrohdinone (PVP) copolymer (19) have also been reported to reduce the tackiness of stickies. 

Most iron-ore-concentrating drums are applied ia a multidmm configuration in order to obtain maximum rejection of nonmagnetic particles. In cobbing service, two dmms are typical but as many as four have been used. These dmm concentrators incorporate a repulping box ahead of each dmm in order to provide the next dmm with a feed that can accomplish nonmagnetic rejection. 

Dmm diameters are typically 762, 914, or 1219 mm, all of which have been used in the various basic appHcations. In some instances, standardization of diameter is specified because of maintenance considerations. Because these separators are constmcted more mggedly and requHe repulping stages, capital costs are higher than heavy-media separators. Costs in 1995 were approximately 10,000/m of width for single-dmm modules and multiples of the single-dmm module. 

Gypsum board paper is a special three-ply paper manufactured from repulped newspapers. The face paper or cream face has the ply against the core unsized so that the gypsum crystals can grow into it, as this is the principal form of bonding between the core and facers. The middle ply is sized and the outer ply is more heavily sized and treated to control paint absorption. For the completed gypsum board system to work, the joint treatment and paper must absorb paint at the same rate. 

When cake washing is involved, it is usually convenient to weigh the wet cake and then repulp it in a known quantity of distilled water or in water at the same pH as the filtrate, if precipitation of... 

For some applications, such as for repulpable type PSAs, it may be advantageous to incorporate high levels of acrylic acid because this makes the polymer more hydrophilic. At the same time, high levels of acid also improve the water-dispersibility of the adhesive, especially at higher pH where the acid groups are converted to the more water-soluble neutralized salt form. Since the high level of acid increases the of the resulting polymer, a non-tacky material results. To make the adhesive pressure sensitive, the polymer can be softened with water-dispersible or soluble plasticizers, such as polyethers [68]. 

The PSA formulator can also take advantage of plasticizers. For example, polyether-based plasticizers have both good low-temperature flexibility and good hydrophilicity. Using these properties, acrylic PSAs have been formulated with these types of plasticizers to obtain high adhesion to food packages stored under refrigerated conditions [102]. Similarly, polyether plasticized acrylics have been used to make repulpable PSAs [103]. 

Several types of adhesive are used for attaching the pages of books to the spine. Traditionally, where repulpability is critical, such as phone books, animal glue... 

Throughout the 1990s a large portion of the research and development effort for hot melt adhesives focused on developing adhesives that are either environmentally friendly or functional [69,81,82]. Environmentally friendly attributes include biodegradability, water dispersibility (repulpability), renewability, and water releasability. Biodegradable adhesives have been developed based on starch esters [83-86] and polyesters such as poly (hydroxy butyrate/hydroxy valerate) [87], poly(lactide) [88-91], and poly(hydroxy ether esters) [92-94]. All but the... 

At 2 30 p.m. the welders were reported missing. Their job card led to No. 3 repulper. It was stopped and drained. Inside were the remains of a rope ladder, welding equipment, and human bones [22]. 

Contaminant removal processes depend on the type and source of secondary fiber to be pulped. Mill paper waste can be easily repulped with minimal contaminant removal. Recycled postconsumer newspaper, on the other hand, may require extensive contaminant removal, including deinking, prior to reuse. Secondary fiber is typically used in lower-quality applications such as multiply paper-board or corrugating paper. 

Deinked secondary fibers are usually bleached in a bleach tower, but may be bleached during the repulping process. Bleach chemicals may be added directly into the pulper. The following are examples of chemicals used to bleach deinked secondary fibers hypochlorite [HCIO, NaOCl, Ca(OCl)2], hydrogen peroxide (H202), and hydrosulphite (NaHS03). 

The most important factors in filter selection are the specific resistance of the filter cake, the quantity to be filtered, and the solids concentration. For free-filtering materials, a rotary vacuum filter is generally the most satisfactory since it has a very high capacity for its size and does not require any significant manual attention. If the cake has to be washed, the rotary drum is to be preferred to the rotary leaf. If a high degree of washing is required, however, it is usually desirable to repulp the filter cake and to filter a second time. 

Surface sizing with cationic starches offers the added property of substantivity to the cellulose fiber. Most mills will use broke in their furnish. Repulping of broke that previously had a starch treatment can mean loss of the starch to the mill effluent and subsequent BOD treatment. Cationic surface sizes are substantive to the fibers and are not lost to the effluent. Thus pollution problems are minimized. 

Rotary Drum Vacuum Filter The underflow from the thickener is the feed to this filter. The filter concentrates the solids (predominately calcium sulfite), which then leave the filter in a cake form and are transferred to a cake repulp tank where they can be reslurried. The liquid generated from the filtering process (filtrate) is collected and pumped to the lime reactor vessel. 

Cake Repulp Tank The cake repulp tank is an agitated vessel whose function is to reslurry the calcium sulfite solids so that they can be oxidized. The solids are mixed with recycled filtrate and fresh make-up water to produce a slurry that is then oxidized. 

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