presented at 4th Pira Wastepaper Conf. 1995

BLEACHING OF WASTEPAPER PULP-CHANCES AND LIMITATIONS

Hans Ulrich Sss, Norbert Nimmerfroh and Bernd HopfDegussa AG, D 63403 Hanau, Germany

1. AbstractThere are narrow limits valid for the possible brightness increase of wastepaper pulp inbleaching processes. Typically the fibers used for paper production have been bleachedpreviously, thus the effect of an additional treatment cannot be tremendous. In addition,bleaching effects can be covered to a high extent by residual printing ink.

On top of this, european wastepaper mixtures normally contain a high level of mechanicalpulp. This sets narrow limits for the application of bleaching chemicals. Chemicals with a highoxidation potential will oxidize and degrade the lignin of the groundwood fibers, with theconsequence of a high effluent load and a low yield. Because of economical reasons neithera low yield nor a high pollution can be tolerated. For chemicals like ozone or chlorine dioxide,acidic bleaching conditions are required. Such conditions are difficult to establish, sincecalcium carbonate is widely used as a filler. Its alkalinity buffers the wastepaper pH into thealkaline region. Thus acidic bleaching conditions require a complete deashing with theconsequence of a lower yield.

Therefore only a small number of bleaching chemicals can be applied with economicalsuccess with standard wastepaper qualities. The chemicals applied in groundwoodbleaching, hydrogen peroxide and sodium hydrosulfite are recommended for the bleaching ofseconary fibers. They compensate brightness losses due to ageing without penalties in yieldor effluent load. Special wastepaper grades, e.g. like woodfree office waste, may be bleachedwith stronger oxidizing compounds to remove impurities. Dyestuffs can be destroyed withreductive bleaching chemicals like hydrosulfite (dithionite) or formamidine sulfinic acid (FAS).The limitations for the brightness increase, as well as the chances to achieve high brightnesslevels with an optimized deinking and bleaching process for different wastepaper qualities willbe described.

2. IntroductionRecycling of paper with printing ink removal has become a very important part of the centralEuropean paper industry. This applies for both, the volume and the economical importance.For years more than 90 % of paper recycling with ink removal were dedicated for newsprintproduction. The amount of virgin fibers in newsprint decreased from about 50 % to levels aslow as 10 % and some paper manufacturers have achieved an input of 100 % wastepaper.

This growing acceptance of secondary fibers now has opened additional sectors of the paperproduction to wastepaper. Higher quality newsprint (super news), magazine paper (SC paper)and LWC paper are produced with several percent of deinked pulp as part of the furnish.Necessarily the quality standards for the deinked pulp had to be improved. Brightness assuch is of lower importance, unless it is not accompanied by cleanliness. The removal of theprinting ink has to reach a very high level.

These high quality levels have to be achieved with low quality wastepaper. Only postconsumer waste is low priced enough to guarantee an economical advantage over primary

2fiber. Higher quality wastepaper not only is more expensive, it is additionally short in supply.Therefore the growth in wastepaper application will be dominantly achieved with postconsumer waste. The high content of mechanical pulp in this type of waste puts severerestrictions on the application of bleaching chemicals.

For the decision which process type is selected two other parameters are of high importance:effluent load and yield. In most central European countries the discharge of effluents isrestricted by the authorities to a narrow range. Even with the inclusion of an expensiveactivated sludge treatment, only limited amounts of COD can be sent into the receivingwaters. Process steps with a high effluent production are therefore counted out. Inconsequence only a small number of bleaching chemicals can be applied.An important factor in wastepaper processing is yield. First of all a high yield means a lowerdemand for the raw material wastepaper and thus a better economical position. Secondly alow yield is identical to losses which have to be disposed. Waste disposal is very expensiveand in some regions nearly impossible, even if the price for it could be afforded.As a result of these limitations, deinking by washing becomes very unfavourable because ofthe yield loss. The preferred deinking process for the production of high quality deinked pulpuses two flotation steps. Between these steps a high consistency dispersion is conducted. Inthe first flotation typically alkaline conditions are applied, only these guarantee an efficientseparation of printing ink and fibers with the lowest losses of fibers and fillers (1). Thethickening to high consistency permits a separation of the water circuits between first andsecond flotation, of which the first one is highly loaded with printing ink. This prevents a re-pollution of the cleaned fibers with dark circuit water. In an ideal situation, the backwater issubjected to a total flotation, to remove dispersed ink particles. If a certain filler removal isdesired or accepted, the second flotation can be conducted under more neutral conditions. Adeinked pulp, prepared with such treatment steps offers nearly ideal conditions for postbleaching.

3. Limiting factors for the application of bleaching chemicalsThe removal of the printing ink is of higher importance than the application of bleachingagents. Post consumer wastepaper contains a lot of mechanical pulp. This sets narrow limitsfor the application of bleaching chemicals. Not all bleaching chemicals applied for thebleaching of chemical pulp can be successfully used for the bleaching of deinked pulp. Thehigh level of unmodified lignin and readily soluble hemicelluloses produce a high effluent load,if mechanical pulp is treated with delignifying compounds. Only wastepaper with low levels orwithout mechanical pulp fibers can be treated with delignifying agents like chlorine, chlorinedioxide, ozone or oxygen. Even in those cases one has to take into account that bleaching ofchemical pulp normally requires a multi stage process. Unbleached kraft fibers cannot bebleached to a high brightness with only one or two bleaching stages. On top of this chemicalpulp bleaching produces a lot of effluent. In several countries limits are set by the authoritiesfor the discharge of effluents. In Germany the minimum requirements valid for wastepaperprocessing paper mills ask for COD-levels in the effluent below 6 kg per ton of paper. TheBOD5 is even lower, it must not exceed 1,2 kg/ t paper. These effluent values require anefficient biological treatment.

3.1 Recycling and bleaching of liquid paper boardAs an example different bleaching approaches for the recycling of liquid paper board frombleached and unbleached fibers are described: After repulping, aluminium and polyethyleneresiduals were separated in a screening step. The brightness of the pulp was only 40 % ISO.The Kappa number of the pulp was as high as 39, this indicated the presence of eitherunbleached high yield kraft fiber (Kappa 60 to 80) or of a TMP (Kappa > 140) in a mixture withbleached fiber (Kappa < 1). Liquid paper board on the average has contains about 20 %CTMP and 35 % unbleached kraft pulp. Only about 45 % of the fibers are bleached (2). Thus

3the measured Kappa number indicates the presence of lignin, but tells nothing about itsbleachability. A calculation of the active chlorine requirement is possible, using Kappa number39 and standard Kappa factors. It yields unacceptable high levels for the AOX loads. Ifchlorine dioxide, hypochlorite or chlorine would be applied, the resulting AOX load would bebetween 2 kg (chlorine dioxide) and 8 kg AOX (chlorine). Levels of such an order would not betolerated in Central Europe.

An alternative would be chlorine free bleaching. It would produce no AOX, but still a COD load.Table 1 demonstrates, using three different sequences, the limited bleachability of this pulpproduced from liquid paper board.

Table 1: Bleaching of fibers from recycled liquid paper board.Bleaching conditions: O: 110C, 3 % NaOH, 0.5 MPa O2, 10 % cons.

Z: 30C, pH < 6, 30 % cons.P: 70C, pH 10,8, 2 % Na-silicate, 25 % cons.

sequence chemicals amount,%

Kappa#

COD loadkg/t

brightness% ISO

raw material 39 40,31. P 3 % H2O2 32 18 54,61. O ~ 2 % O2 24 160 45,62. P 3 % H2O2 22 20 63,5

S 1801.O ~ 2 % O2 24 160 45,62.Z 1 % O3 16 21 59,83.P 3 % H2O2 14 37 67,6

S 218

The poor delignification effects achieved are very obvious. Additionally, due to the highintensity of the delignification process the effluent loads are very high. Both effects indicatethe presence of condensed lignin (poor delignification of high yield pulp) and of puremechanical fibers (effluent load). Despite the high amounts of chemicals applied, thebrightness increase is not satisfying. Very likely in practice no one could afford the investmentinto the expensive multistage treatment and deal with the effluent load.The application of ozone in wastepaper recycling was described in a paper where officewaste from North America was used (3) as furnish. In this study ozone was applied after awashing - flotation - dispersing - flotation - washing treatment. The yield is not cited, but itcannot be very high. Because of the disposal problems currently only in tissue production lowyields are accepted. Generally such an expensive process will find only limited interest inEurope. Normally, simply because of the cost of wastepaper, fillers are kept in the process.The high levels of calcium carbonate found in European paper make wastepaper pulpalkaline. The pH of a pulp sample without any chemicals is around 7,2 to 7,6. Since alkalineconditions favour the decomposition of ozone, acidic conditions are preferred for ozonation.An adjustment of the pH of the pulp into the acidic region is nearly impossible. Acidificatione.g. with sulphuric acid, would require a lot of acid and produce a lot of gypsum. Thepresence of calcium carbonate thus excludes any acidic delignification step unless deashingand the simultaneous yield loss are accepted.

YY Aggressive oxidation of old liquid paper board dissolves the pulp. Especially mechanicalfibers are dominantly destroyed, not bleached. There is a very limited chance to push thebrightness without sacrificing the yield and generate a high COD load.

43.2 Bleaching of old corrugated containersIn North America projects exist to upgrade old corrugated containers into high quality fiber. Forexample, a multi-stage bleaching process including the application of oxygen and chlorinedioxide was proposed (4). A transfer of such a process to Europe is impossible, because ofthe totally different fiber furnish applied in the production of corrugated board. In Europe thelevels of mechanical fibers in corrugated board and even in liner are very high. The browncolour of a typical European liner is not an indication of the presence of unbleached kraft fiber.It is the result of brown dyestuff applied on wastepaper with lots of newsprint in it. Table 2compares the bleaching results achieved with two different grades of OCC, one typicalCentral European, one from North America.

Table 2: Deinking and bleaching of old corrugated containers (OCC): Effects of froth flotationand bleaching, standard conditions in flotation ( peroxide,alkali, fatty acid); 1% FASin bleaching

initialbrightness %

ISO

FAS-bleachingbrightness

% ISO

flotation deinkingbrightness

% ISO

FAS postbleachingbrightness

wastepaper OCC(European)

26,5 35,7 31,2 38,8

virgin fiber OCC(US)

17,0 21,8 18,5 25,5

Both process steps, the flotation and the bleaching yield a brightness increase with thewastepaper OCC. Thus, printing ink and dyestuff play an important role in the centralEuropean OCC. On the other hand, the virgin fiber OCC does not respond tobleaching. Thechromophores are very stable. Flotation or bleaching do not really alter the brightness. Thisexplains why small amounts of OCC can be tolerated in European recyling mills, but causeproblems in North America.

The amount of residual lignin in both types of OCC is by far too high to permit a conventionalbleaching procedure. The Kappa number of the wastepaper-OCC was 64. This indicates thepresence of bleached fibers together with unbleached chemical and mechanical pulp. For thevirgin fiber OCC a Kappa number of 96 was determined, an indication for a high content ofultra high yield pulp. One cannot expect to bleach a pulp with a residual lignin level that high tobleach well with only one or two stages. Such Kappa levels would not only need a multi stagebleaching treatment but would in addition require a pulping step. The combined costs forpulping, bleaching and the effluent treatment would never reach an economical basis.

YY Old corrugated containers in Europe have a much higher level of mechanical fibers, whichdissolve under aggressive bleaching conditions. They in addition have a high level ofprinting ink. This makes flotation in Europe effective, in full contrast to North AmericanOCC. Processes for upgrading OCC therefore have to be different in both regions.

3.3 Bleaching of mechanical pulp with oxygen and hydrogen peroxideBecause of the mechanical fiber content post consumer waste is not successfully bleachedwith oxygen. Without a nearly complete removal of the lignin content, oxygen application is notproducing a brightness increase. The removal of the lignin and other readily solublecompounds, e.g. hemicelluloses, produces a significant yield loss and simultaneously aneffluent load. On the other hand the application of oxygen is recommended to decrease oreliminate the content of mechanical fibers (6). Table 3 demonstrates the results of atreatment of mechanical pulp (TMP) with oxygen and hydrogen peroxide on brightness andthe parallel development of the COD, the yield and the freeness. Oxygen indeed has a

5pronounced effect on the development of the effluent load and the yield loss (as a result of thenecessity of caustic soda ) and at the same time only negative effects on the brightnessdevelopment.

Table 3: Combined oxygen/peroxide treatment of TMP and the effect on yield, and COD.Conditions: 75C, 0.5 h with 0.3 MPa O2 pressure 10 % consistency in O-stagesand in EOP-treatment; P-stage at 25 % cons.

stage NaOH%

H2O2%

brightness% ISO

CODkg O2/t

yield%

raw material - - 59,2 - -1. EO 1,0 - 53,2 33 97,02. EO 1,5 - 49,1 35 96,83. EO 2,0 - 45,3 54 95,54. EOP 1,5 1,0 57,3 35 96,75. EOP 2,5 2,0 55,8 57 95,36. P* 1,1 2,0 73,7 34 97,0* with 2 % Na-silicate

YY The use of delignifying conditions on mechanical fibers results in no benefit for thebrightness development. It dissolves the fibers at least in part, generates COD and lowersthe yield.

3.4 Bleaching of deinked office wastepaperOffice wastepaper typically contains high amounts of chemical pulp, while the levels ofmechanical pulp are rather low. Aggressive bleaching with delignification becomes an option.On the other hand, the level of fillers is high in these paper grades and highly bleached CTMPhas become increasingly important as fiber furnish. In Table 3 the results of a deinking andbleaching treatment of a typical North American office waste are described.

Table 4: Bleaching of deinked American office wastepaper with oxygen, formamidinesulfinic acid and hydrogen peroxide. Conditions:O-stage: 3% NaOH, 0,3 MPa, 10% cons., 75C, 1 h.FAS-stage: 0,4%FAS, 0,2% NaOH, 55 cons., 75C, 1 h.P-stage: 1% H2O2, 0,8% NaOH, 2% Na silicate, 25% cons., 75C, 2 h.

trial bleachingstage

Kappa#

brightness%ISO

effluentCOD kg/t

rawmaterial

-- 2,4 70,2 --

1.1 FAS 78,0 9,41.2 P 2,9 86,1 12,3

S.21,71.1 O 71,4 14,92.2 FAS 78,9 14,12.3 P 3,0 86,9 8,3

S 37,3

The contribution of the oxygen treatment to the brightness increase is rather limited. Thedominant effect of oxygen is an increase of the effluent load and thus a lower yield. The effecton the lignin content is poor. The slight increase of the Kappa number very likely is due to the

6decrease in ash content during the thickening steps. These negative results do not becomemuch better, if a well washed deinked pulp is treated. The example listed in table 5 gives dataon the bleaching of a high quality European wastepaper dedicated for tissue production. Theprinting ink was eliminated before bleaching with froth flotation and washing.

Table 5: Postbleaching of a flotation- and wash-deinked wastepaper for tissue productionwith oxygen and/or FAS.Conditions: O-stage: 1 % NaOH, 10 % cons. 75C, 10 min, 0,3 MPa O2;FAS-stage: 0,3 % NaOH, 0,5 % FAS, 5 % cons. 70C, 2 h.

stage Kappa#

CODkg O2/t

brightness% ISO

Colour coordinatesx y

raw material 10,6 - 66,1 0,3291 0,33801. O2. FAS

7,2-

184

64,377,9 0,3117 0,3266

S 221. FAS - 11 81,2 0,3100 0,3242

Again the result of the oxygen treatment is disappointing. It has an effect on the Kappanumber and it produces an additional effluent load, but produces no brightness increase ifcompared to the single stage reductive treatment with FAS. The changes of the colour valuesshow a yellowing effect during the oxygen treatment due to the alkaline conditions. Thesenegative effects of an alkaline oxygen treatment on yield and effluent load are not taken intoaccount in the literature (5). Oxygen application could make sense on a nearly woodfreepaper mixture. On the other hand, what could oxygen achieve on a really woodfreewastepaper? If the lignin content of the paper already is close to zero, nothing much can beexpected from a delignification treatment. One possibility for a positive effect of oxygenaddition is the partial oxidation of impurities, which can decrease mottling caused byunbleached fibers or printing ink. On the other hand the effect of the shear forces applied onthe fibers during oxygen addition has to be clearly separated from the effect of the oxygenitself. Table 6 demonstrates the effect of the shear force applied in an oxygen mixer. The highquality wastepaper was pretreated with flotation and washing and had reached a brightnesslevel of more than 75% ISO without UV. The brightness was as high as 89 % ISO in thepresence of UV light, a clear indication for the presence of optical brightners. On such a brightpulp any bleaching effect can only produce moderate changes. Very small amounts ofhydrogen peroxide increased the brightness to more than 78% ISO, resp. 93% ISO. Theshear force applied in the oxygen mixer decreased the number of dirt points significantly. Asimultaneous addition of oxygen did not alter this result. Certainly a disperser treatment willfurther decrease the dirt count.

Table 6: Postbleaching of mixed office waste with oxygen and hydrogen peroxide. Pulppretreated with flotation and washing. Constant addition: 0,1% NaOH, 1% sodiumsilicate; conditions: 1 h, 70C.

H2O2%

oxygenMPa

consistency%

high shearmixer

brightness% ISO

dirt count /m2

200-250 mm >250 mm0,25 - 20 no 78,8 2900 16500,25 - 10 yes 78,4 1925 9500,25 0,3 10 yes 78,8 2025 850

7Instead of oxygen delignification bleaching with chlorine dioxide can be seen as an alternative.Delignification and bleaching with chlorine dioxide is very efficient with sulfite and kraft pulps.However, the results of bleaching trials with chlorine dioxide on deinked pulp aredisappointing. Table 7 summarizes the results. Office waste containing high levels ofwoodfree paper with ink jet and laser print and some dyed material and in addition some flyersfrom magazines was deinked under neutral conditions. The removal of the prining ink hadonly little effect on the brightness. The analysis of the amount of lignin present gave a Kappanumber of 19,1. The application of chlorine dioxide as active chlorine with the Kappa factors0,2 or 0,4 resulted in a very low brightness gain and a limited decrease of the lignin content.Post bleaching with hydrogen peroxide after the chlorine dioxide treatment increased thebrightness to 76% ISO and had very little impact on the Kappa number. In contrast to theseresults, postbleaching with hydrogen peroxide directly after flotation did lift the brightness tomore than 85% ISO. Thus, bleaching with chlorine dioxide does not remove thechromophores from deinked wastepaper, it only produces COD and AOX.

Table 7: Neutral deinking and bleaching of european office waste with chlorine dioxide orhydrogen peroxide. Conditions: Repulping at 15% cons., 40C; flotation at 1%cons.,0,05% fatty alcohol ethoxylate, D stages at 60C, 10% cons., 3h; P stages at70C, 20% cons., 2h, with 2% Na silicate.

trial flotationstage

brightness% ISO

bleachingstageKappafactor

bleachingstage

chemicals%

bleaching stagebrightness

% ISO

Kappa#

CODkg/t

rawmaterial

initial brightness67,6

19,1

1 67,4 192 1 D 0,05 1,0 69,9 16,1 72.2 D o,2 3,8 68,4 13,8 152.3 D 0,4 7,6 67,2 11,5 19

2.4 P 1,0 85,4 18,1 15

2.5.1 D 0,2 3,8 68,4 13,8 152.5.2 P 1,0 75,8 12,2 12

YY In consequence following chemicals are of very limited use in the recycling of postconsumer wastepaper:

chlorine, chlorine dioxide, hypochlorite, ozone and oxygen.

YY Yield losses and high effluent loads with COD and AOX with the chlorine containingcompounds are their main drawbacks.

YY In Europe therefore the dominant amount of wastepaper can only be postbleached withchemicals, which first of all attack chromophores by oxidation or reduction and havedelignification only as a minor side effect. This decreases the number of chemicals tohydrogen peroxide, hydrosulfite and formamidine sulfinic acid.

84. Limiting factors for the bleaching effectBefore any bleaching action is taken, printing ink removal has to be optimised consequently.Printing inks are prepared dominantly using pigment particles. These pigments - e.g. carbonblack - cannot be bleached. Conditions and chemicals which oxidise carbon black, would alsodecompose the fibers. Additionally in conventional printing inks the pigment particles arecoated with hydrophobic mineral oils. They are thus not accessible to water soluble agents.

It is of no benefit to overintensify dispersion. Even if every ink particle is destructed belowvisibility, the result only is a uniform grey shade covering the fibers. Thus any bleaching actionwould increase possibly the fiber's brightness, but not the grey residual of inks. Thebrightening effect that could be achieved will be significantly depending on the residual of ink.This disadvantage is intensified by the exponential character of brightness measurement.

Water based printing inks dissolve under alkaline repulping conditions. The larger printing inkparticles disintegrate and the fine particles of the printing ink pigments are set free. This issimilar to the effect of a dispersion with very high intensity. Figure 1 demonstrates the effectof small amounts of fine dispersed carbon black on the brightness of fully bleached pulp. Theshape of the curve demonstrates the negative effect of very small amounts of printing inkresiduals.

Figure 1: Exponential decrease of the brightness of chemical pulp with the addition ofdispersed carbon black (Printex 30)

________________________________________________________________________

Wastepaper consists of a wide variety of fibers. It is obvious how difficult for an unknownmixture any prediction of the bleachability will be. A fully bleached chemical pulp will gain onlysome brightness point. The same will be valid for a highly bleached mechanical fiber. Asignificant increase can only be expected from the bleaching of previously unbleachedmechanical pulp. This type of pulp on the other hand becomes less and less applied inpapermaking. Table 8 gives a comparison of the different types of fibers in post consumerwaste and their reaction to oxidative or reductive treatment.

&

&

&

&

&

&

0 0,5 1 1,5 2

carbon black (%)

0

20

40

60

80

brightness (%ISO)

9Table 8: Presence and bleachability of fibers in post consumer waste. Bleachability withH2O2 resp. FAS or Dithionite +++ very good, +/- nearly no response.

Presence: +++ dominant, + rare.

type of fiber present in bleachabilitymixed waste oxidative reductive

mechanical pulp:bleachedunbleached

+++

+++++

+++

chemical pulp:bleachedsemibleachedunbleached

+++++

++++++

+/-+

+/-

YY In consequence the expectations for the rise of the brightness with a bleaching stage haveto be significantly lower compared with the results for e.g. mechanical pulp. While 2 % ofhydrogen peroxide applied on groundwood typically yield about 15 points of brightness, suchan increase cannot be achieved with wastepaper. On top of this, the grey shade of residualprinting ink limits the bleaching effect.

5. Demand for bleaching chemicals in postbleachingWastepaper consists mainly of previously bleached fibers. Thus an improvement ofbrightness should need only moderate amounts of chemicals. On the other hand, thecompensation of the grey hue of printing ink residuals may require a maximum of brightnessgain. Bleaching with hydrogen peroxide therefore should be conducted with optimizedconditions only. This would require the application of:

high consistency and sufficient retention time and buffered bleaching conditions.

Only these conditions would result in a completely satisfying effect. The high versatility ofhydrogen peroxide, without any doubt permits also other, less favourable conditions. Thesewould require on the other hand more bleaching agent and/or yield less bleaching gain. This ise.g. valid for the application of hydrogen peroxide in the hot dispersing unit. Temperaturesabove 90C favour thermal decomposition of hydrogen peroxide into hydroxy radicals. Theresult is a rapid consumption of hydrogen peroxide without a bleaching effect. If the disperserhas to be used for peroxide addition, the temperature should not exceed 70C to 80C.

It does not make sense to conduct peroxide bleaching of mechanical pulp or secondary fiberat 5 % to 10 % consistency. The relationship between the amount of fiber and the amount ofwater contaminated with diluted compounds e.g. starch, promotes side reactions. These aree.g. the oxidation of sugars to sugaracids. The technical practice does not offer the amountsof clean water needed to conduct lower consistency treatment with a well washed pulp.

Chemical pulp is treated with hydrogen peroxide in mill scale at medium consistency.However, the load of organic compounds even in a very closed system cannot be comparedto the substances present in a secondary fiber treatment. While there compounds likehemicelluloses and starch dominate, in chemical pulp bleaching a very unreactive watersoluble oxidized lignin is the dominant compound. This lignin does not have reactive sites forside reactions with hydrogen peroxide and can be labelled to be a relative inert COD load.With respect to the highest reaction efficiency also for the bleaching of chemical pulp thehighest brightness gains are achieved at high consistency conditions (7). Thus to use highconsistency conditions (> 20 %) for the bleaching of secondary fiber makes even moresense.

10

There is only a small requirement for alkali to activate the high consistency bleaching ofdeinked pulp. This is first of all the result of the alkaline repulping conditions. Figure 2demonstrates the small demand for caustic soda using 1 % of H2O2 for postbleaching. Inorder to intensify the printing ink removal the pulp was pretreated with flotation - dispersion -flotation.

Figure 2: Postbleaching of a flotation - dispersion -flotation process precleaned mixedwastepaper pulp with 1 % H2O2.Conditions: 80C, 20 % consistency, 1 h, 2 % sodium silicate, increasing amountof caustic soda

________________________________________________________________________

The application of chelatants in wastepaper bleaching does not make sense. Because of thehigh calcium carbonate content of European paper grades calcium concentrations in theprocess water are extremely high. Even the higher chelation constants for transition metalscannot overcome this high excess of calcium ions. Chelation agents do only bind calcium andthus their addition is without positive effect.

For stabilisation and buffering the addition of sodium silicate is recommended. Figure 3demonstrates the positive effect of an increase of the sodium silicate addition in postbleaching of the same deinked pulp already applied in figure 2.

The bleaching temperature using hydrogen peroxide should not be below 50C, because theslow-down of the reaction would cause very long retention times. On the other hand thetemperature should not exceed 80C because the thermal decomposition of hydrogenperoxide into hydroxy radicals, which do not contribute to the brightness development,becomes more and more dominant (8).

Desired reaction: H2O2 + OH- H2O + HOO

- (bleaches chromophores)

Undesired side reactions:(High alkalinity): 2 H2O2 + OH

- 2 H2O +O2 + OH-

(Thermal decomposition or by metals): 2 H2O2 2 H2O + O2

&

&

&

& &

0 0,1 0,2 0,3 0,4

NaOH (%)

0

1

2

3

4

5brightness increase (points)

11

Figure 3: Effect of sodium silicate addition on brightness in post bleaching of deinked pulp.Conditions identical to figure 2. Constant chemicals charge 1 % H2O2 and 0,1 %caustic soda

________________________________________________________________________

Increasing the amount of bleaching chemical does not necessarily produce higher brightness,especially if the printing ink removal was not good enough. Figure 5 demonstrates this for themill scale postbleaching of post consumer waste following two flotation treatments anddispersion.

Figure 4: Effect of increasing hydrogen peroxide charges on brightness in mill-scalepostbleaching of post consumer waste.Conditions: 30 % cons., 80C, 1,5 h, const: 1 % sodium silicate, from 0.2% to0.5% increasing caustic soda charge

________________________________________________________________________

The achieved brightness increase demonstrates clearly the limited effect of a very highamount of bleaching chemicals added. As mentioned earlier, this is first of all the result ofprinting ink residuals. An improvement of the effects achieved during bleaching is onlypossible, if more printing ink can be removed. This can be done with a washing stage.Washing the pulp, on the other hand does not only remove printing ink, but simultaneouslyfillers and fines are washed off. The overall yield will be significantly lower.The production of tissue from secondary fiber may be an example for these losses. Typicallythe overall yield of fiber calculated on wastepaper input is less than 70 %. Since filers and

0 0,5 1 1,5 2

Na silicate (%)

0

1

2

3

4

5brightness increase (points)

*

*

*

*

*

0 0,5 1 1,5 2 2,5 3

H2O2 (%)

65

66

67

68

69

70brightness (%ISO)

12

fines are obtained as a aqueous sludge the volume of reject does reach the volume ofproduced tissue paper. Even if the amount of waste can be decreased by drying andcombustion of the fines, the fillers remain and have to be dumped.

The washing stage removes small residual ink particles, which improves significantly thebleachability. Figure 5 gives the brightness increase achieved with hydrogen peroxide on apulp precleaned with flotation - dispersion - washing.

Figure 5: Brightness increase with hydrogen peroxide bleaching of flotation - dispersion -washing cleaned postconsumer wastepaper, residual ash content 1,4 %.Conditions: 25 % cons., 70C, 2 h, 2 % sodium silicate.

________________________________________________________________________

An additional brightness increase can be achieved with a reductive posttreatment using FASor dithionite (9). The pH-level of a deinked pulp normally is slightly alkaline, around pH 7,5,because of the calcium carbonate content. In bleaching of mechanical pulp with hydrosulfitethe pH adjusts itself during the defiberization to the ideal value between pH 5 and pH 6. Withdeinked pulp a drop of the pH value can be achieved for a short time by addition of e.g.sulfuric acid, because the solubilization of calcium carbonate takes some time. On the otherhand this would increase the salt amount in the effluent and decrease the yield.The brightness gain achieved with a two stage postbleaching treatment using peroxide - FASis especially pronounced if the wastepaper mixture contains dyestuff from carbonless copypaper or mass dyed paper. This is especially valid for the recycling of the higher qualitywastepaper, which is used for tissue production. In these recycling processes consequentlythe application of FAS is state-of-the-art in Europe.

In recycling processes based on post consumer waste, the effect of an additional reductivetreatment is less pronounced. The brightness increase in ISO values yields only between 1,5and 2,5 points. For the technical practice the positive effect of postbleaching on yellownessand color is more important. Following a high intensity peroxide treatment, yellowness can bedecreased from values between 12 and 14 down to 10 and 12 with a dithionite stage. Table 9gives examples for the treatment of flotation -dispersion - flotation treated pulp. The dataunderline once again the importance of the previous stages for the bleaching effectsachieved.

,

,

,

,

,

,

,

0 1 2 3 4

H2O2 (%)

62

64

66

68

70

72

74

brightness (%ISO)

13

Table 9: Effects of single stage or two stage postbleaching of flotation - dispersion - flotationdeinked post consumer wastepaper using H2O2 and Na2S2O4.

bleaching stages amount of chemical (%) brightness yellownessH2O2 Na2S2O4 % ISO

raw material 57,4 16,0single stage Ptwo stage P-Y

1,01,0

-1,0

63,665,7

15,613,1

single stage Ptwo stage P-Y

2,02,0

-1,0

65,466,7

13,912,6

single stage Ptwo stage P-Y

3,03,0

-1,0

67,068,5

12,410,5

single stage Y - 1,0 61,5 14,1

YY The increase in brightness with a higher input of chemicals becomes more pronounced ifmechanical cleaning of the pulp is optimized. Buffering with silicate is an advantage, thedemand for caustic soda is limited.

6. Effluent load of deinking (COD load)In order to minimize the effluent load sometimes "neutral" deinking is said to be morefavourable compared with the normal, alkaline process. It is stated, the alkaline process (9)would result in too many dissolved organic compounds in the water circuit. Indeed, thesolubility of a number of compounds used in papermaking is higher under alkaline conditions.To take this as background to ask for neutral conditions means to ignore the big advantage ofalkaline flotation conditions: Because of the higher anionic charge of fibers in the alkalinestage, fiber losses are significantly lower at high pH. The same is valid for fillers, e.g. chinaclay does not flotate under alkaline conditions. Additionally, the separation of fibers and fillersfrom the printing ink is more pronounced under mild alkaline conditions (1). Neutral flotationresults in a lower yield and a lower brightness.

In comparison to the effluent load produced in primary fiber production, the load set freeduring alkaline repulping is comparably low. Table 10 gives a comparison of the effluent load,expressed as COD, set free during repulping of different paper grades.

Table 10: Effluent load (COD) produced during repulping of different paper grades with andwithout chemicals. COD values calculated on fiber in kg O2/g.Chemicals applied in standard:0.5 % H2O2, 0.5 % NaOH, 1.0 % sodium silicate, 0.5 % sodium oleate;with high charge: 1.0 % h, 0.9 % NaOH, 1.0 % sodium silicate,0.5 % sodium oleate.

paper grade without chemicals standard charge high chargevirgin fiber newsprint 16 35 37newsprint with > 50 %wastepaper

16 28 33

SC-paper 22 33 41copy paper (woodfree) 34 40 42

The figures demonstrate the influence of the paper production process. The high effluent loadproduced during repulping of the woodfree paper is the result of the application of starch forsurface sizing. Because of the good biodegradability this effluent load does not represent abig problem.

14

Another difference shows up, if figures are compared. The COD values produced with thenewsprint containing wastepaper are lower than those with the newsprint made from virginfiber. The effluent load produced with the recycling of already recycled fiber seems be lower.

The effluent load produced during peroxide postbleaching is given in figure 7. The loadincreases - similar to mechanical pulp bleaching - linear with increasing alkali charge. Thefigure demonstrates that in comparison to virgin mechanical pulp, not only the inclination ofthe straight line is lower, but also the starting level is lower.

Figure 7: Comparison of effluent load (COD) bleaching mechanical pulp (TMP) orwastepaper with hydrogen peroxide

________________________________________________________________________

The background of these lower levels becomes obvious if recycling is repeated several times.The effluent load of mechanical pulp bleaching is primarily the result of the saponification ofacetyl groups and low molecular weight hemicelluloses, e.g. arabinoses. If the acetyl groupsare all saponified and removed, there is less and less material left to produce COD.

Y The effluent load in deinking processes is lower compared with mechanical pulp bleaching.Repeated recycling has a very positive effect on the effluent load.

7. Summary The brightness increase is strongly depending on the quality of the printing ink removal.

Without optimized ink separation the bleaching results will be insufficient.

For the postbleaching of post consumer wastepaper only hydrogen peroxide and dithionitecan be recommended because other, more aggressive chemicals cause yield losses andhigh effluent loads. With mass-dyed papers best results to correct brightness and colorare obtained using formamidine sulfinic acid.

The effluent load produced during the recycling process is lower compared with theproduction of virgin fibers and decreases further with repeated recycling.

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15

8. References1) H. U. Sss, N. Nimmerfroh, H. Jakob, A. Reinold, B. Hopf; Papierrecycling: Flotation von

Fllstoffen und Fasern, Wochenbl. f. Papierfab. 120 (8), 303-307 (1992).2) J. Blechschmidt, A. Strunz, Eigenschaften und Verwertung von Flssigkeitskar-

tonverpackungen, Papier, 47 (11) 650 - 659 (1993).3) V. Gehr, O. Kordsachia, R. Patt, W. Matzke, Die Bleiche von Altpapier mit chlorfreien

delignifizierenden und ligninverndernden Bleichmitteln, Wochenbl. f. Papierfab. 119 (21)839-844 (1991)

4) X. T. Nguyen, A. Shariff, P. F. Earl, R. J. Eamer, Bleached pulps for printing and writingpapers from old corrugated containers, Progress in Paper Recycling, May 1993, 25-32

5) C. Pauli, R. Patt, V. Gehr, O. Kordsachia, ber die Wirkungsweise von Additiven in einerPeroxid- sowie einer peroxidverstrkten Alkali/Sauerstoff-Bleichstufe von Altpapier undHolzschliff, Wochenbl. f. Papierfab. 121 (20) 852 - 857 (1993).

6) Pivi M. Forsberg, Joseph M. Genco, Bleaching of mixed office waste to high brightness,Tappi 77 (3)253-259 (1994)

7) R. Hock, W. Czirnich, Chlorfreie Bleiche von Zellstoff mit Einbindung von Abwasser in denKreislauf der Kochereiabwsser - Konzept und Ergebnisse einer neuen Bleichsequenz,Papier 47 (10A), V 24 - V 29 (1993).

8) V. Hafner, G. Hoevels, B. Hopf, W. Korn, N. Nimmerfroh, A. Reinold, K. Schmidt H.U.Sss, Natriumcarbonat als Alkaliquelle bei der Holzstoffbleiche, Papier 44 (10) 521-529(1990).

9) W. Eul, H. U. Sss; Mglichkeiten und Grenzen des Deinkingprozesses, Wochenbl. f.Papierfab. 116 (6) 224-231 (1988).

10) H. Schmid, K. Schwinger, Betriebserfahrung mit dem Flotationsdeinking von gemischterSammelware im neutralen pH-Bereich, 5. PTS Deinking Symposium, Wochenbl. f.Papierfab. 120 (11/12), 418 (1992).