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Additives for Recovered Fiber Processing
For economic and environmental reasons the use of recovered paper as raw mate¬rial has already reached a high level and will grow further. For its preparation specific chemicals are required, depending on the quality of the recovered papers and the necessary properties of the produced paper grades.
184.108.40.206 Additives for Repulping
These chemicals are intended to facilitate the repulping of recovered papers, espe¬cially for mixed and brown grades e. g. packaging material, corrugated boxes. To get an easier and faster defibration the addition of nonfoaming wetting agents
(e. g. nonionic surfactants) and dispersants is used. Wetting agents reduce the surface tension. These products are predominantly surface active, such as sulfonated oils, alkyl sulfates, alkyl sulfonates, alkyl aryl sulfonates, or ethoxylated prod¬ucts based on nonylphenol. Dispersants used for repulping are mainly condensa¬tion products of formaldehyde and a naphthalene sulfonic acid as the sodium salt, or the sodium salt of a polycarboxylic acid. These products have high dispersing capacity for pitch, waxes, bitumen, etc. which otherwise would adversely affect the whole repulping and papermaking process as well as the paper quality.
The appli¬cation of such products preferably takes place in the pulper in undiluted form with amounts of 0.1 to 0.5 %, calculated on oven dry paper stock.
Repulping of wet-strength paper always requires more energy than repulping normal unsized or sized paper. Depending on the type of wet-strength resin used, different methods and chemicals are employed. When UF (urea formaldehyde) resins or MF (melamine formaldehyde) resins have been used, repulping was effected in an acid medium with sulfuric acid and/or alum at elevated temperature above 60 °C in the pulper for 15–30 min. If the wet strength of the recovered paper is based on polyamine-type chemicals, defibering of this paper also requires high-energy pulping at an alkaline pH value >10 by addition of sodium hydroxide. Other useful additives are hypohalous acid and persulfate salts.
220.127.116.11 Additives for Deinking
Newspapers, weekly and monthly magazines, brochures and office papers are used as raw material to produce graphic papers, tissue or the top ply of white board. In the “deinking process” first the printing ink has to be detached from the paper surface then the released ink has to be removed from the pulp slurry, either by flotation or by washing or by a combination of the two. Whereas in Europe flota¬tion is most commonly used and washing is only used for special deinked pulp (DIP) qualities, in North America washing is more common.
The removal of ink from recovered paper is determined by the type of ink binder and the chemicals used during pulping. For wood containing recovered paper grades the most important chemical is sodium hydroxide (NaOH) with addition rates of 0.5–2 % (calculated on oven dry paper stock) to adjust the pH to 10–11. NaOH eases the detachment of the ink particles from the fibers as saponifiable binders in the ink are saponified by NaOH and the fibers swell substantially in this environment. However, sodium hydroxide solution also causes yellowing of the fibers, particularly of mechanical pulp. In order to prevent this, hydrogen peroxide (H2O2) is used as a bleaching chemical, which also has a saponifying effect. In addition, 1–5 % water glass (sodium silicate) is added to stabilize the hydrogen per¬oxide and to prevent the ink particles from redepositing on the fiber.
Additionally 0.1–0.2 % of a nonfoaming wetting agent (e. g. nonionic alkyphenol polyethylene glycol ether) can support the removal of the ink particles from the printed recovered paper. Since hydrogen peroxide is more effective at higher con¬centrations, the pulping process is carried out at stock consistencies of 12–20 % (high consistency pulper or drum pulper). Soaps and fatty acids are used in addition rates of 0.5–1.2 % as dirt collectors and flotation agents. They form calcium soaps with hard water or with the calcium carbonate from the coating or filler of the recovered paper. Calcium chloride must be added if the water is not sufficiently hard. For higher quality papers, further bleaching with sodium dithionite and/or hydrogen peroxide may follow. Coated papers can be deinked more easily because the inks are fixed only on top of the coating layer.
For woodfree recovered paper grades the dosage of deinking chemicals (NaOH, soap, fatty acid, nonfoaming wetting agent) can usually be reduced substantially. Because deinked pulp (DIP) is very often used in paper grades of high brightness, bleaching with hydrogen peroxide, sodium dithionite and/or formamidine sulfinic acid (FAS) is more important and bleaching is often performed in two or more stages.
The effluent/white water quality has to be controlled very exactly. COD increases with increasing pH. For flotation deinking of recovered paper the total chemical costs have a relatively high proportion of 15–20 % of the overall DIP production costs. Yield rates up to 93 % for newsprint production are attainable.
In wash deinking after saponification the ink particles together with pigments and fillers of the recovered paper are removed by washing the pulp slurry through wire supported by the addition of 0.1–0.5 % of a dispersing agent. Here large effluent streams are produced and solid losses are high, with yield rates of only 60 to 70 % being not unusual. So flotation deinking, or a combined flotation-washing process, is also gaining a foothold in North America.
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