A waste paper recycling process relates to the treatment of a mixture of waste paper containing non-cellulosic contraries and printing inks, in order to release the contraries from the fibers and further to separate them from the stock in order to produce re-usable pulp for the production of paper and board.
The invention has to do with new and useful improvements in methods for first removing the non-ink contraries from the fibrous mass and second releasing and then removing the ink particles from the said fibrous mass.
The invention is directed to the treatment of the fiber slurry produced during the ink separation stage, after the ink releasing stage has been applied. One aim of the process is to allow both the use of the fibers and the mineral fillers contained in that slurry, for pulp and board making, and the use the solids-free water contained in the same slurry as the washing liquid in the previous ink-separation treatment, thus closing the fibers and the water circuits.
This process includes chemical and thermo-mechanical treatments, starting under alkaline conditions, which may become neutral at the end of the process.
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1. A method of treating a mixture of printed and contaminated waste paper in order to produce a pulp for use in the manufacture of paper and paperboards, said waste paper containing non-ink contaminants including stickies, which method comprises:
(a) forming a first aqueous fibrous suspension of said waste paper at room temperature by applying specific mechanical energy lower that 50 kw.H/Ton to form a pumpable slurry and to release substantially all of the non-ink contaminants including the stickies, from the surface of the paper and without dispersing such non-ink contaminants as finely divided particles throughout the fibrous suspension; (b) removing substantially all of the non-ink contaminants including the stickies, which have been released without dispersal as finely divided particles from the first fibrous suspension by screening and cleaning at room temperature to form a second aqueous fibrous suspension substantially free of the non-ink contaminants including the stickies; (c) after the step of removing the non-ink contaminants softening the ink vehicles and weakening their binding with the surface of the fibers by submitting the second fibrous suspension at a consistency of more than 15% to the simultaneous actions of (A) a high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more than 50 kw.H/Ton applied at the said consistency of more than 15% and (C) at least one deinking agent under strong alkaline conditions having a ph of at least 9; and (d) detaching the ink particles from the surface of the fibers and dispersing them into the second fibrous suspension by submitting the second fibrous suspension to the simultaneous actions of (A) high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more than 50 kw.H/Ton applied at the said consistency of more that 15% and (C) at least one chemical dispersing agent, under strong alkaline conditions having a ph of at least 9 whereby higher specific energy inputs and higher temperatures are used to detach the ink particles from the fibers of the second fibrous suspension after removal of the non-ink contaminants than are used on the first fibrous suspension before removal of the non-ink contaminants; (e) limiting the total duration of the ink softening and detaching steps (c) and (d) to a range between 2 and 10 minutes and (f) removing the detached ink particles from the second fibrous suspension to provide a brightness of at least 59 ISO the final pulp.
19. A method of treating a mixture of printed and contaminated waste paper in order to produce a pulp for use in the manufacture of paper and paperboards, said waste paper containing non-ink contaminants including stickies, which method comprises:
(a) forming a first aqueous fibrous suspension of said waste paper at ambient temperature by applying specific mechanical energy lower than 50 kw.H/Ton to form a pumpable slurry and to release substantially all of the non-ink contaminants including the stickies, from the surface of the paper and without dispersing such non-ink contaminants as finely divided particles throughout the fibrous suspension; (b) removing substantially all of the non-ink contaminants including the stickies, which have been released without dispersal as finely divided particles from the first fibrous suspension by screening and cleaning at ambient temperature to form a second aqueous fibrous suspension substantially free of the non-ink contaminants including the stickies; (c) after the step of removing the non-ink contaminants, softening the ink vehicles and weakening their binding with the surface of the fibers by submitting the second fibrous suspension at a consistency of more than 15% to the simultaneous actions of (A) a high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more then 50 kw.H/Ton applied at the said consistency of more than 15% and (C) at least one deinking agent under strong alkaline conditions having a ph of at least 9; (d) detaching the ink particles from the surface of the fibers and dispersing them into the second fibrous suspension by submitting the second fibrous suspension to the simultaneous actions of (A) high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more than 50 kw.H/Ton applied at the said consistency of more than 15% and (C) at least one chemical dispersing agent, under strong alkaline conditions having a ph of at least 9 whereby higher specific energy inputs and higher temperatures are used to detach the ink particles from the fibers of the second fibrous suspension after removal of the non-ink contaminants than are used on the first fibrous suspension before removal of the non-ink contaminants; (e) limiting the total duration of the ink softening and detaching steps (c) and (d) to a range between about 1 and 5 minutes; and (f) removing the detached ink particles from the second fibrous suspension to provide a brightness of at least 59 ISO to the final pulp.20. A method of treating a mixture of printed and contaminated waste paper in order to produce a pulp for use in the manufacture of paper and paperboards, said waste paper containing non-ink contaminants including stickies, which method comprises: (a) forming a first aqueous fibrous suspension of said waste paper at a low temperature by applying specific mechanical energy lower than 50 kw.H/Ton to form a pumpable slurry and to release substantially all of the non-ink contaminants including the stickies, from the surface of the paper and without dispersing such non-ink contaminants as finely divided particles throughout the fibrous suspension, the low temperature being sufficiently low to maintain rigidity of non-ink contaminants having a lowest melting point so that the lowest melting point non-ink contaminants will not extrude through screens; (b) removing substantially all of the non-ink contaminants including the stickies, which have been released without dispersal as finely divided particles from the first fibrous suspension by screening and cleaning at the low temperature to form a second aqueous fibrous suspension substantially free of the non-ink contaminants including the stickies; (c) after the step of removing the non-ink contaminants softening the ink vehicles and weakening their binding with the surface of the fibers by submitting the second fibrous suspension at a consistency of more than 15% to the simultaneous actions of (A) a high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more than 50 kw.H/Ton applied at the said consistency of more than 15% and (C) at least one deinking agent under strong alkaline conditions having a ph of at least 9; (d) detaching the ink particles from the surface of the fibers and dispersing them into the second fibrous suspension by submitting the second fibrous suspension to the simultaneous actions of (A) high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more than 50 kw.H/Ton applied at the said consistency of more than 15% and (C) at least one chemical dispersing agent, under strong alkaline conditions having a ph of at least 9 whereby higher specific energy inputs and higher temperature are used to detach the ink particles from the fibers of the second fibrous suspension after removal of the non-ink contaminants than are used on the first fibrous suspension before removal of the non-ink contaminants; (e) limiting the total duration of the ink softening and detaching steps (c) and (d) to a range between about 1 and 5 minutes; and (f) removing the detached ink particles from the second fibrous suspension to provide a brightness of at least 59 ISO to the final
pulp.21. A method of treating a mixture of printed and contaminated waste paper in order to produce a pulp for use in the manufacture of paper and paperboards, said waste paper containing non-ink contaminants including stickies, which method comprises: (a) forming a first aqueous fibrous suspension of said waste paper at a low temperature by applying specific mechanical energy lower than 50 kw.H/Ton to form a pumpable slurry and to release substantially all of the non-ink contaminants including the stickies, from the surface of the paper and without dispersing such non-ink contaminants as finely divided particles throughout the fibrous suspension, the low temperature being sufficiently low to maintain rigidity of non-ink contaminants having a lowest melting point so that the lowest melting point non-ink contaminants will not extrude through screens; (b) removing substantially all of the non-ink contaminants including the stickies, which have been released without dispersal as finely divided particles from the first fibrous suspension by screening and cleaning at the low temperature to form a second aqueous fibrous suspension substantially free of the non-ink contaminants including the stickies; (c) after the step of removing the non-ink contaminants softening the ink vehicles and weakening their binding with the surface of the fibers by submitting the second fibrous suspension at a consistency of more than 15% to the simultaneous actions of (A) a high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more than 50 kw.H/Ton applied at the said consistency of more than 15% and (C) at least one deinking agent under strong alkaline conditions having a ph of at least 9; (d) detaching the ink particles from the surface of the fibers and dispersing them into the second fibrous suspension by submitting the second fibrous suspension to the simultaneous actions of (A) high temperature between 85° and 130°C, (B) high shear forces substantially corresponding to a specific mechanical energy of more than 50 kw.H/Ton applied at the said consistency of more than 15% And (C) at least one chemical dispersing agent, under strong alkaline conditions having a ph of at least 9 whereby higher specific energy inputs and higher temperature are used to detach the ink particles from the fibers of the second fibrous suspension after removal of the non-ink contaminants than are used on the first fibrous suspension before removal of the non-ink contaminants; (e) limiting the total duration of the ink softening and detaching steps (c) and (d) to a range between 2 and 10 minutes; and (f) removing the detached ink particles from the second fibrous suspension to provide a brightness of at least 59 ISO to the final pulp. 2. The method of
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This pulp then remains 5 to 20 minutes in a latency chest (6), at a consistency between 2% and 5%. It may then be deflaked (7) in order to thoroughly separate the fibers bundles one from the other, and thus facilitate the ink removal from the slurry.
The fibrous suspension finally goes through the ink removal process (8) which can be advantageously composed of multi-stage, counter-current, high consistency washing. The number of stages is choosen according to the quantity of ink to be removed and to the desired final brightness. The extraction of the water is conducted through strains of perforated plates, the dimensions of the openings of which will be selected in order to allow for a given quantity of fibers to be carried away together with the effluent, thus ensuring the optimum operation of both the following ink selective separation process, and the final filtration of the recovered satellite secondary pulp.
In case a filler-free secondary pulp is desired, the effluents from the washing step (8) can advantageously be strained again on one or several fine mesh filters (9). By this means, it is possible to remove at each filter stage up to 80% of the mineral fillers contained in that slurry. In such a case, the finest fraction must be sent to a conventional alkaline clarifier (10) and then be disposed of. The clarified fraction is then returned ahead or after the following ink selective separation step (11), according to the operating parameters of this last process (consistency, temperature), and according to the required brightness.
The necessary chemicals are also introduced ahead of this step. In case this process is a selective flotation, ink collectors such as fatty acids or their sodium or calcium soaps can be added, taking care to insure a mixing time of about 5 minutes at a temperature of about 35° to 45°C
It may be worthy to recall that the dispersing agent used during the washing step has a negative effect both on the coagulation produced by the collecting agents during the flotation step, and on the drainability (freeness) of the fibrous suspension during the filtration step. It will be good to inactivate or neutralize these agents for example by precipitation with calcium chloride or calcium hydroxide. The precipitation of the sodium silicate will also contribute to increase the brightness of the secondary pulp through the formation of a precipitated mineral filler. It has also been observed that the quantity of mineral fillers removed together with the foam during the flotation step may vary from 30% up to 70% according to the operating parameters of the process: flotation time, temperature, pulp consistency, dosing and type of chemicals. The rejected foam containing the ink is then pumped to centrifuges or filter-presses and disposed of. The loss of solid particles has been observed to be between 10% and 20% of the flow of secondary pulp, which means about 3% to 6% of the total quantity of pulp feeding the washing step (8).
It has also been observed that the maximum efficiency of the ink removal has been reached at much higher consistencies that the ones recommended by the suppliers of the cells. For example, a cell designed to work at 1% has shown best results between 1,5% and 2%. This peculiarity allows for the treatment of lowest quantities of effluents, using higher consistencies during washing, and larger holes in the extractors perforated plates.
When the requested concentration for the ink selective separation process is higher (say 0,5% or more) than the maximum concentration which can be given to the effluent of the washing step, some heavy stock can be advantageously extracted from the latency chest (6). In this case, the small quantity of long fibers added to the secondary pulp will help in forming the filtering mat in the final filtration step (12).
The selective separation of the ink (11) can also be a process based on adsorption of the ink upon the surface of non-soap solids, as recommended by Ira Puddington et Al. in the U.S. Pat. No. 4,076,578.
The de-inked slurry leaving the process (11) is then filtered on fibrous mat up to at least 4% consistency, possibly above 10% in order to remove from the final secondary pulp the maximum possible quantity of dissolved salts.
In case this pulp contains a very high quantity of ground-wood fines and fillers (such as mixtures of newsprint and magazine paper), the pH ahead of the filtration step has to be dropped down to values below 8, by addition of sulfuric acid (preferably to aluminum sulfate), under intense mechanical agitation (as could be the suction side of a centrifugal pump), and after some long fibers extracted from the washed final primary pulp has been added to the the satellite slurry to be filtered.
It has been observed that the application of equipment such as Polydisk or Waco Filters to the thickening process (12) has permitted to produce clear filtrate having less that 100 ppm suspended solids and consequently totally re-usable in the pulping (2), cleaning (3) and washing (8) processes without any further clarification.
The final thickened secondary pulp leaving (12) must then be brought to a pH compatible with the following use by addition of sulfuric acid or aluminum sulfate, always under intense mechanical agitation, and can be stored in a buffer chest according to the final use.
The following examples will illustrate three different applications of the general procedure previously described, using different mixtures of waste paper and producing different grades of paper and board. Measurements of brightness were made with an Elrepho meter with 457 mm. light filter, according to I.S.O. standards. Chemicals dosings are expressed in percent by weight of the chemical at 100% concentration relative to the weight of total solids in the line where said chemical is added. Sodium silicate is considered at 38° Be and the Removink F and L as supplied.
The raw material is a mixture of over-issued newspapers and telephone books (white and yellow pages) in a ratio approximately 50/50. The de-inked pulps are used for the production of newsprint and telephone directory papers (white and yellow), on only one high speed paper machine.
In this installation, the pulper has a capacity of 46 m3 containing 2.700 kg of waste paper. Each batch takes 30 min. Dilution water is coming from the effluent of the thickening process (4) and make-up is made using clear filtrate from the Polydisk filter (12). One percent of sodium hydroxide is added in the pulper together with 1% of a de-inking agent such as Removink L 8001 supplied by Chemicarta SPA, Milano. This pulping step typically is performed at ambient or room temperature. When this cold pulping operation is finished, the stock is pumped through turboseparator, screens and cleaners, at consistencies starting around 4% and ending at about 0,6%.
The turboseparator is equipped with a perforated plate having 3 mm. diameter holes and the rejected stock is then sent to a vibrating flat screen also having 3 mm. holes, the rejects of which are disposed of.
The accepted stock from the turboseparator is then diluted from 3% down to 1% before it passes through pressurized slotted screens fitted with 0,30 mm. slot width. The rejected stock is processed through a second stage screen having the same slot size, and rejects of the same go to a vibrating flat screen, rejects of which are disposed of.
The accepted stock from the first stage of screens is then diluted down to 0,6% consistency and processed through a conventional battery of 4 stages of Triclean cleaners. The light and the heavy rejects of the 4th stage are disposed of.
The total loss of both high and low consistency turboseparating, screening and cleaning is varies between 6% and 9% by weight, depending upon the
degree of contamination of the waste paper.
No more stickies or hot melts can be seen in the pulp, and a visual inspection is confirmed by the Sommerville test, which shows less than 0,2% of shives. At that point, the pulp is totally cleaned and the only remaining contaminant is the printing ink. The pulp is then thickened up to 30% consistency in two steps, using a disk filter up to 10-12% and then a screw press up to 30%. Characteristics of the pulp are: brightness=40°-45° ISO, freeness=50°-55° SR, filler content=6-8%, temperature=20°-25°C The ink releasing step (5) is achieved in a kneader under the following operating conditions: temperature=95°-98°C, sodium hydroxide=1,5% , sodium silicate=4% , hydrogen peroxide=1,8% , specific energy=80 KW.H/Ton during 3 minutes. The brightness of the pulp at the end of the treatment is 50°-55° ISO, and freeness is 60°-65° SR. The pulp is then diluted using all the flow of effluents coming from the second stage of washers, then squeezed up to 12% in the first washing stage.
These washers are composed of inclined screws (better known as Rice-Barton or Baker's screws), where the pulp is drained under continuous and vigorous agitation through perforated plates having 1,4 mm. diameter holes, in order to produce an effluent having approximately 0,8-1% consistency.
The thickened stock is then processed through two other similar counter current washing steps and the final usable pulp presents the following characteristics: brightness=59°-60° ISO, freeness=46°-50° SR, filler content 2-3%, consistency=12-14%. This pulp represents 78% by weight of the quantity of pulp feeding the washers (8). The balance 22% is going to the satellite circuit with the first stage effluent which shows brightness=35°-40° ISO, filler content=20-25%, freeness=80° SR.
The capability for the ink of being removed from the fibers contained in the effluent has beenverified in the laboratory as follows: an effluent sample has been hyperwashed under fresh water shower on a 200 mesh wire ,and a handsheet has been made, showing a brightness of 56° ISO, which is very similar to the brightness of the final primary pulp. This effluent has then been mixed together with 4% of a special ink-collecting agent purposely designed for this application by Chemicarta SPA, Milano, and kept for 5 min. under agitation at 30°C
The mixture is then processed through one single stage conventional flotation cell, Voith open type, during 15 min.. The loss of weight through the cell is 15-20%, which means only 3-4,5% respect to the total quantity of pulp entering the washers. We have found that addition of 0,5% to 1% of calcium chloride or calcium hydroxide together with the collector, ahead of the flotation, helps controlling the foam and the ink coagulation when low ash content pulps are processed.
The total alkalinity is then dropped down to pH=7-8 with addition of 1% of sulfuric acid on the suction side of the centrifugal pump feeding the disk filter (12). At this point, the pulp shows a brightness=53°-56° ISO, a filler content=15-20% and a freeness=78°-80° SR.
The disk filter (12) is a Polydisk filter sized according to a specific filtering factor=20 liters/min./m2. Besides this unusual value, it is also necessary to feed the mat-peeling showers with air instead of water, in order to reach the maximum possible consistency of the discharged pulp.
Using the above mentioned parameters, a final consistency of 8% to 10% could be obtained and the clear filtrate shown less than 100 ppm average suspended solids, measured on paper filter, black label.
The pulp is then brought to pH=6 and sent to a buffer chest having 8 hours total retention time. From this point, it is then pumped to the mixing chest of the paper machine at controlled flow rates according to the paper grade actually produced and in function of the mean composition of the secondary pulp.
The clear filtrate from the Polydisk filter is then totally recycled in order to dilute the stock ahead of the third washing stage and make-up is provided by fresh industrial water which does not contain aluminium ions.
The application of such a process in a paper mill having one single paper machine offers the following advantages:
(a) possibility to maintain constant freeness and ash content during a grade run, independently from the incoming waste paper characteristics, thus allowing the paper machine to run at maximum speed and efficiency;
(b) possibility to achieve very quick grade change, exactly as whenusing virgin pulp and fillers, without the need to intervene a long time before in the waste paper plant, thus permitting an easier and more constant operation of that plant;
(c) possibility to always use the highest possible quantity of recycled fibers in the paper, by the free disposal of each one of the two fractions and their use in the optimum way.
(d) possibility to produce totally cleaned pulps having the same standards of cleanliness than virgin pulps and thus offering the highest possible runability in the paper machine room, particularly being free of any "sticky" or "hot melt" or ink vehicle free particle.
The raw material is a mixture of printed continuous stationary , old books and office file, in a ratio 50/50.
The de-inked pulp is used to produce, on three distinct paper machines: (a) light weight machine-glazed wrapping papers, (b) fine papers for writing and printing, including wood containing printing grades, (c) stationary and continuous print-out papers.
The operation is similar to example (1) up to the thickening step (4), although it is not necessary to add any chemical agent--caustics or de-inking agent--during the pulping step (2). When entering the ink-releasing step (5), the pulp has a brightness=60° ISO, a freeness=40°-45° SR, and a filler content=20%.
The ink-releasing equipment is the same as for example (1) but operating parameters are as follow: Removink L8001=0,3%; hydrogen peroxyde=0,5%, sodium hydroxyde=1%, sodium silicate=3%. All other parameters remain unchanged. At the end of the process, the pulp has shown a brightness increase of 2° ISO and freeness did not show any appreciable variation.
The pulp is then washed by mean of three washing stages as for example (1), but the design of the perforated plates are different: the first stage is fitted with 2 mm. diameters holes, the second and the third stages are equipped with 1,4mm. diameter holes. Also the feed consistency of the washing stages is different, being 2,5%. With these parameters, the final washed primary pulp has shown following characteristics: brightness=75° ISO, filler content below 3%, freeness 27°-30° SR.
The effluent leaving the first washers has a consistency between 1% and 1,2%, a filler content=60%, brightness=50° ISO, and freeness=70° SR.
The flotation cell used in this application is a high consistency Swemac type, and heavy stock has been pumped from chest (6) and mixed together with the effluent before the flotation, in order to raise the consistency up to 1,5%. In this way, the two lines (primary by washing and secondary by flotation) have exactly the same solids flow rate, or the same capacity in tons/day, but produce two pulps having opposite characteristics. This extraction also procures long fibers which will help the final filtration (12).
This extraction could have been done using washed pulp and this would have increased the brightness of the secondary pulp. But in such a case, the washing equipment would have to be sized for 30% more capacity, which is not a worthy choice in our case.
The flotation is then conducted with only 2% of the same collector (Removink F) and the retention time through the cell is only 10 min., thus producing a loss of weight of 10% (which means 5% of the total pulp).
After acidification at pH=8 ahead of the disk filter, the pulp shows a brightness=70° ISO, a filler content=35-40%, a freeness=65°-70° SR.
The Polydisk filter can be sized using a filtering factor=25 liters/min./m2, and produces an effluent containing 70-100 ppm suspended solids. The other steps of this application are similar to the ones described in example (1).
The application of such a process in a paper mill having several paper machines as in this example is offering the following advantages:
(a) possibility to produce a pulp having physical and cleanliness characteristics similar to the ones of a virgin chemical pulp, thus usable for the production of fine light weight papers, with good Yankee dryer glazing capabilities;
(b) possibility to produce a pulp having physical and optical characteristics of a mixture of fine chemical and/or ground-wood pulp, and mineral fillers, thus usable for the production of printing papers where high opacity and smoothness are requested.
(c) possibility to mix these two pulps together in a ratio which can be very much different from the original one coming together with the raw material.
The raw material is a mixture of low quality printed waste, containing old books, office waste and stationary, and some newspapers and magazines, in variables proportions.
The mill has one multiply board machine, and produces high quality folding box board, which can be on-machine coated and must show an excellent multicolour offset printing aptitude. The white top liner is composed of 100% de-inked primary pulp and the underliner uses the secondary pulp, mixed with other pulp.
Pulping is conducted in a continuous way with the same parameters as for example 1. The cleaning and screening treatment (3) is simplified and composed of centrifugal high-density cleaners, followed by a turboseparator, working at 3% consistency. The following thickening stage is also simplified and composed of inclined screws producing pulp at 15% consistency, followed by a screw press. The finest contaminants will be detached and better dispersed during the ink-releasing step (5) and then carried away with the effluent during the washing stage. They will remain in the secondary pulp thus contributing to add weight and volume to the board, as the underliner does not need to be particularly cleaned.
The pulp entering the ink-releasing and dispersing step shows a brightness=50° ISO, a filler content=25-30%. The operating parameters are the same as for example (1), but the brightness drops down to 46°-48° ISO.
The following washing step has only two stages, which are fed at 2,5% consistency. The perforated plates of the inclined screws have 1,6 mm. diameter holes, and it has been found that the characteristics of the effluent are very similar to the one of example 1.
The washed primary pulp shows a brightness=68° ISO, a filler content=4% and a freeness=45°-50° SR. The fine cleaning of the primary pulp is achieved with the cleaners and the screens installed ahead of the board machine, which is sufficient to reach the desired quality. It must be said that the contaminants have been thoroughly dispersed in the kneader (5) and most of them have left this primary pulp during the washing step.
The satellite circuit is also simplified because the brightness of the underliner has only a third-order influence on the final brightness of the coated board. We have observed that a brightness of the underliner secondary pulp in the 50° ISO range was sufficient to insure the required brightness 80° ISO of the coated board, providing that the top liner primary pulp has 70° ISO. Thus, the flotation time has been reduced below 10 min. and the dosing of the collector has been kept below 2%. We have also observed that it was possible to run without any chemical when lower quality grades are produced, but no compromise can be applied on the dispersion effect, because black spots in the underliner are always visible even through the coated top liner.
The application of such a process to the production of stratified board is offering the following advantages:
(a) possibility to totally replace chemical pulp or high quality selected unprinted waste paper by a low value and large availability raw material;
(b) simplification of the main line by eliminating the fine screening and cleaning equipment;
(c) increase of the total yield, by transferring in the secondary pulp (and then in the underliner or in the middle ply) all finely dispersed contaminants which are not acceptable in the top liner.
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