The purpose of pulping is to defibre the paper, detach ink from fibres and produce dispersed ink particles of the size and geometry that can be efficiently removed in subsequent ink removal steps. Def…
The purpose of pulping is to defibre the paper, detach ink from fibres and produce dispersed ink particles of the size and geometry that can be efficiently removed in subsequent ink removal steps. Deflaking of typical papers used for recycled newsprint production is rapid even at neutral pH [1]. In laboratory flotation deinking of a 70:30 ONP:old magazine (OMG) blend, the most significant variables in producing the highest deinked pulp brightness and lowest ERIC values were the highest pulping consistency and pH, the lowest pulping temperature and the highest flotation temperature and longest flotation time [2]. (These conclusions are limited to the range of process variables studied.) Generally consistent results have been reported by other researchers [3].
Preventing excessive ink dispersion during pulping should be avoided through careful choice of pulping conditions and process chemicals [4] for three main reasons:
Smaller ink particles have a greater tendency to redeposit on fibres than larger ones. The very small ink particles produced when pulping paper printed with water-based (flexographic) inks are extremely difficult to remove [5] and can deposit inside the fibre lumen.
The same amount of ink present in deinked pulp as very small particles has a greater adverse effect on brightness than when present as larger ink particles [6]. Ink particles less than 2.5 micrometers in diameter have the most deleterious effect on brightness [5]. Flotation, which is now used more widely than washing as the primary ink removal unit operation, is more effective in removing somewhat larger ink particles (greater than ca. 20 micrometers in diameter).
Process water recycling can return dispersed small ink particles to the process resulting in increased ink redeposition and lower pulp brightness.
Both the rate of ink detachment from fibre and the rate of ink redeposition during pulping are first-order processes [7]. The lower the shear factor during pulping, the slower are both ink fragmentation and ink redeposition and the more ink removed in flotation [8]. Operating at lower shear rates and at higher consistency, the primary purpose of drum pulping is to limit fragmentation of stickies, plastics and other contaminants. Today drum pulpers are used in most new deinking plants and modernization projects [9].
Reducing the pulping pH to near-neutral values can reduce newspaper ink dispersion [5,10]. Image analysis indicates that increasing the sodium hydroxide concentration (and thus pH) increases the formation of very small ink particles. Interestingly, while increased pulp consistency at alkaline pH increases brightness, better brightness is obtained at neutral pH at lower pulp consistency [11]. Sodium sulphite has been suggested for use in neutral deinking replacing sodium hydroxide, sodium silicate and EDTA in the pulper [12].
Deinking becomes more difficult with ONP age as cross-linking of binder resins gradually makes inks more difficult to detach from fibres [13,14]. The autoxidation chemical reaction appears to be responsible for ink chemical changes that reduce deinking efficiency [15]. Addition of antioxidants to offset ink formulations can retard autoxidation. Conventional vegetable-based oil inks are generally more susceptible to autoxidation processes than mineral oil-based inks and exhibit a greater tendency to form very small ink particles [16,17]. However, some vegetable oil-based inks designed to have a low autoxidation rate exhibit deinkability comparable to that of mineral oil-based inks [18].
The primary variables determining ink removal efficiency were pulper conditions: pulp consistency, temperature, and the interacting effects of pulper temperature and consistency and of pulper temperature and water hardness
Paper transportation and storage temperatures can reach 50C (126F) during summer months [15] and accelerate ink autoxidation increasing the associated adverse effects on deinking. Extensive studies in Canadian, U.S. and European newspaper deinking mills have this “summer effect” on ink detachment, fragmentation and removal [17]. Lower deinked pulp brightness appears to be primarily due to higher ageing temperatures causing stronger ink adherence to the cellulose fibres due to deeper penetration of the ink into the fibres, oxidation leading to some chemical bonding of the ink vehicle or binder resin to the fibres, and heat-promoted evaporation of the ink vehicle [19]. To deal with these adverse effects, shorter pulping times at higher pulping temperatures using increased charges of alkaline chemicals and deinking agent in the pulper are recommended [17].
The presence of 2% lignosulfonate in addition to the conventional pulping chemicals has been reported to promote ink detachment while limiting ink redeposition onto fibres [20]. In both laboratory experiments and mill trials using 70:30 ONP:OMG, substituting magnesium oxide for some of the sodium hydroxide resulted in slightly lower flotation ink removal efficiency but provided the economic benefits of reduced cationic demand and lower peroxide usage plus reduced process water salinity [21].
In the presence of calcium soaps of fatty acids, newspaper ink particle size first decreases and then increases with long pulping times [5]. Both the fatty acid and calcium ion are required for this ink agglomeration to occur. Soaps do not promote ink detachment from fibre unless nonionic surfactants such as alcohol ethoxylates are also present during pulping.
Steam explosion has been considered as an option to conventional pulping [22]. Because of the extensive ink fragmentation that occurs in steam explosion, washing is important in removing ink from the resulting pulp. The presence of sodium sulphite in the steam explosion step reduced ink content of deinked pulp.
Dispersion after pulping using disk refiners or kneaders is used primarily in office paper deinking to increase ink particle detachment from fibres and reduce ink particle size to that most suitable for removal in subsequent pulp processing steps. Excessive energy input during laboratory disk refining resulted in an increase in residual ink content due to redeposition of the excessively small ink particles form [23].
Screening and Cleaning
In the past several years, the slot size of screens has decreased from the typical 0.008 inch with 0.004 inch becoming common in new and modernised mills [9].
Forward centrifugal cleaners are particularly useful in removing large toner ink particles. Reverse centrifugal cleaners are being less used in deinking mills for two reasons. First, the density of stickies and other contaminants has gradually increased to near that of water. Second, screens are removing contaminants more effectively. Given the limited benefits and the energy cost of running reverse cleaners, some mills have shut them down [9]. The primary exception to this is OCC mills, which have a high concentration of lightweight contaminants such as waxes.
Flotation
Flotation cells designs have evolved to provide such features as air injection nozzles that improve air bubble size distribution. The improved brightness such designs can provide increases operating flexibility by reducing the amount of OMG that must be added to ONP. The advantage of reducing the furnish OMG content is the increased fibre yield that results.
Since relatively high yield losses are associated with washing, reducing yield loss would appear to be a valuable area of research.
When flotation deinking with fatty acids, the presence of calcium ions increased both water-based ink and toner ink removal efficiency by increasing ink attachment to air bubbles [24]. Mill studies confirmed; the single most important parameter affecting ink removal efficiency and filler yield was water hardness [25]. Also improving flotation ink removal effectiveness was a high surfactant dosage to the pulper and a high pH (a high alkaline dosage combined with a low silicate dosage). In laboratory studies, in addition to flotation time, the primary variables determining ink removal eff
iciency were pulper conditions: pulp consistency, temperature, and the interacting effects of pulper temperature and consistency and of pulper temperature and water hardness [26].
Visualisation studies of inks on bubble surfaces can improve understanding of the effects of ink chemistry, flotation cell design, process chemicals and process conditions on ink attachment to and detachment from air bubbles. Visualisation studies using toner inks [27] and models of offset and flexographic inks [28] have been reported. These studies used quite large, stationary air bubbles. Unpublished results indicate that it is possible to visualise ink particles on air bubbles during flotation in laboratory cells (Figure 1) and ink dispersion during pulping (Figure 2) using a Focused Beam Reflectance Measurement (FBRM) instrument [29]. With a longer probe, it should be possible to visualise these phenomena in commercial mill pulpers and flotation cells. Atomic force microscopy is another advanced technique used to study interfacial forces and other interactions in deinking systems [27,30].
In flotation laboratory tests, increasing air flow resulted in a modest increase in ink removal efficiency [10]. Injecting silicone oil with air into flotation cells can result in higher flotation brightness gains [31]. This may be due to either increased gas hold-up resulting in smaller or slower rising air bubbles or increased ink attachment to oil-coated bubble surfaces.
Ink autoxidation associated with the summer effect has adverse effects on flotation ink removal efficiency [32]. The oxidised compounds found in inks after autoxidation are more polar and increase gas hold-up in flotation cells. This inhibits air bubble coalescence; the smaller bubbles increase froth stability resulting in a greater loss of fibres, fines and water in flotation.
Brightness drops about 1.4 points for each 10% increase of old telephone directories (OTD) in ONP:OMG (33). OTD contains about 20% higher ink concentration than ONP and requires higher water hardness for good flotation deinking efficiency.
Using a series of alcohol ethoxylate surfactants, the surfactant with the highest critical micelle concentration produced the highest flotation ink removal efficiency at surfactant concentrations above the critical micelle concentration [34].
Reducing Flotation Yield Loss
Efficient ink detachment from fibres during pulping or subsequent dispersion operations can increase flotation yield because fibres with attached ink are more readily removed by flotation than completely deinked fibres [25]. Increased OMG content in ONP/OMG furnishes, while it results in higher flotation ink removal and brightness gain, results in higher yield loss [35].
Fibre content in flotation froth is relatively low indicating that the primary means of improving flotation yield would be to reduce the loss of fines and fillers to the froth [25, 36]. However, recovery or retention of these particles could decrease brightness since their high surface area promotes ink deposition and attachment.
Statistically designed experiments indicated that the process variables having the greatest effect on yield loss were flotation time and the interacting variables of pulping time and water hardness and of pulping temperature and flotation time [37]. Mill studies indicated that low water hardness and high silicate dosage favoured low filler yield loss [25]. During flotation, surfactants become concentrated in the froth layer. The release of surfactants present in some inks can cause excessive froth stability increasing yield loss and causing poor ink flotation selectivity [38].
Some mills have installed secondary flotation systems processing primary flotation rejects to improve yield. In a Norwegian mill, a high ratio of primary stage to secondary stage flotation rejects increased ink removal efficiency [25]. Laboratory processing of pilot plant flotation rejects indicated a column flotation cell could recover significant amounts of fibre [39]. Similar processing of washing rejects appears feasible. Washing the top of a steady-state froth column with a water spray has been reported to return fibre from the froth to the pulp [40].
Washing
Since relatively high yield losses are associated with washing, reducing yield loss would appear to be a valuable area of research. With the increased ink removal effectiveness of flotation, vacuum washers, which are less efficient in removing ink but provide significantly lower yield losses, are being increasingly used.
Enzyme Deinking
Enzymes, particularly cellulases, assist ink detachment from fibres and have been used for deinking mixed office papers. Xylanases and lipases have also been evaluated. While enzymes can promote ink particle detachment from fibres, this does not always translate into improvements in flotation response perhaps due to adverse effects of enzyme on ink particle surface chemistry [41].
Increased ink removal has been achieved using an enzyme combined with a flotation deinking surfactant [42]. When compared with a control test using heat-killed enzyme, improved sheet drainage and strength were obtained. Mill use of enzymes requires extensive customisation of the enzyme formulation and process variables to achieve optimum effectiveness [43].
Mill use of enzymes requires extensive customisation of the enzyme formulation and process variables to achieve optimum effectiveness
Office Paper Deinking
One approach to improve toner detachment from fibres has been to incorporate enzymes into the toner formulation [44]. Another is to use modified binder resins that dissolve in the high pH associated with pulping [45].
The combination of toner ink agglomeration and magnetic separation has been proposed for office papers [46]. In laboratory studies, a single pass of agglomerated ink through a magnetic separator resulted in 91% ink removal with a 7% fibre loss.
High quality deinked office paper pulp is finding increased acceptance as a hardwood pulp substitute due to both optical and mechanical properties [47].
Effect of Deinking on Bleaching
Efficient deinking reduces bleaching costs by minimising ink carryover to the bleach plant [48]. The presence of substantial amounts of fines in the pulp delivered to the bleach plant can reduce hydrogen peroxide and sodium hydrosulphite bleaching effectiveness [49]. This is due to both ink attached to the fines and to iron, which has a strong affinity for fine particles. An acid wash or iron chelation is often necessary to achieve adequate bleaching results.
Dr. Borchardt is Technology Manager – Pulp and Paper Chemicals with Tomah Products. He works primarily in the development of deinking agents, digester additives and pitch control agents and is the author of more than 120 peer-reviewed technical papers. He holds more than 80 international patents and recently won the 2003 Henry Hill Award of the American Chemical Society for contributions to the chemical profession.
References
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