Fines from deinked pulp: Effect of contaminants on their ...

BRIGHTNESSTABLE III. ISO brightness (%) <strong>of</strong> the short fractions (fines)obtained after different bleaching sequences (brightnesschanges are in parentheses).BleachingSequence DIP TMP UNP:UMGFIG. 3. ISO brightness <strong>of</strong> DIP and TMP furnishes, consisting<strong>of</strong> the bleached (QPY) long fraction mixed with theirrespective bleached (QPY) fines counterparts, in ratiosranging <strong>from</strong> 70:30 to 100:0 long fraction: fines.Untreated 47.7 52.6 56.9Y 49.4 56.5 —(+1.7) (+3.9)QY or 54.4 59.5 —AY (+6.7) (+6.9)P 45.4 62.6 57.5(–2.3) (+10.0) (+0.6)QP or *50.0* 66.5 67.2AP (+2.3) (+13.9) (+10.3)QPY or *55.7* 70.4 68.8APY (+8.0) (+17.8) (+11.9)*: A 1.2% EDTA charge was applied in the Q-stage and 0.2% MgSO 4added in the P-stage, a departure <strong>from</strong> the procedure described in theexperimental section.tionated <strong>pulp</strong>s differed: DIP had thelowest ISO brightness at 66.9%, followedby UNP:UMG at 72.1% and TMP at74.1%. The same order was seen forthe ISO brightnesses <strong>of</strong> the bleachedfines: DIP, 55.7%; UNP:UMG, 68.8%; andTMP 70.4%.The above results indicate that the lowfinal brightness <strong>of</strong> the DIP after QPY wasa direct consequence <strong>of</strong> the poor bleachabilityand low brightness <strong>of</strong> the DIP fines.To confirm this, we measured the ISObrightness <strong>of</strong> DIP and TMP furnishes,consisting <strong>of</strong> the bleached (QPY) longfractions mixed with their respectivebleached (QPY) fines, in ratios ranging<strong>from</strong> 70:30 to 100:0 long fraction:fines.The results, Fig. 3, show that the 100:0DIP furnish had an ISO brightness <strong>of</strong>75.1%, while the 70:30 DIP furnish had abrightness <strong>of</strong> 66.4%, a drop <strong>of</strong> 8.7 brightnesspoints. Under the same conditionswith TMP, the ISO brightness went <strong>from</strong>76.7% to 74.7%, a drop <strong>of</strong> only 2.0 brightnesspoints.<strong>Fines</strong> Bleachability: The bleachability <strong>of</strong>the fines <strong>of</strong> the three furnishes, obtained<strong>from</strong> the float wash fractionator, wasevaluated for the following bleachingsequences: Y, QY, AY, P, QP, AP, QPYand APY. Again, A and Q treatmentswere equivalent and thus tabulated together.Table III summarizes thesebleaching results.— <strong>Fines</strong> bleaching: DIP vs. TMP: DIP finesdid not respond as well as TMP fines toperoxide or hydrosulphite bleaching. Thehigh 450-ppm ERIC value <strong>of</strong> the DIPfines, compared to only 75 ppm for theTMP fines, explains the low brightnessceiling and partially explains the poorapparent bleachability <strong>of</strong> the DIP fines.The higher level <strong>of</strong> iron in the DIP finesaccounts for the remaining difference inbleaching response.Using the following equations,Brightness= 1 + k/s – (2 k/s + (k/s) 2 ) 1/2k (fines+ink) = k fines + c ink k inks (fines+ink) = s fineswhere k is the absorption coefficient, s thescattering coefficient, c ink the ink concentrationand k ink = 15000 m 2 /kg [5], one cancalculate the effect <strong>of</strong> ink on brightness.The DIP fines had an ISO brightness <strong>of</strong>47.7%, a scattering coefficient, s (fines+ink) ,<strong>of</strong> 100 m 2 /kg and an ERIC value <strong>of</strong> 450ppm. We calculated that k (fines+ink) = 28.7m 2 /kg, k fines = 21.9 m 2 /kg and (c ink k ink ) =6.8 m 2 /kg. If ink was absent, thenk (fines+ink) = k fines = 21.9 m 2 /kg, and theinkless brightness would be 52.2%.During the QPY sequence, k ink <strong>of</strong> blackink remains unchanged and therefore, forthe ink-containing DIP fines to gain 8.0brightness points over-all and reach55.7%, the brightness <strong>of</strong> the DIP finesalone would have to be 63.0% i.e.,(k (fines+ink) would need to be 17.6 m 2 /kgand k fines 10.9 m 2 /kg). For the ink-containingDIP fines to have a brightness <strong>of</strong>68%, the fines alone would have to reacha brightness <strong>of</strong> 90%! The high ERIC value<strong>of</strong> the DIP fines thus limits their brightnessceiling and brightness gain. The finalISO brightness <strong>of</strong> the DIP fines afterbleaching can, therefore, only be furtherimproved by removing additional ink, notby additional bleaching <strong>of</strong> fibres.Other than the difference in ERIC value,the DIP fines contained significantlymore iron than the TMP fines. The exceptionallyhigh concentrations <strong>of</strong> iron in thefines, especially the DIP fines, significantlyreduced the bleaching efficiency <strong>of</strong>hydrosulphite. Hydrosulphite bleachingwas less efficient for DIP fines than forTMP fines (ISO brightness gain <strong>of</strong> 1.7%vs. 3.9%, respectively). As a result, whenthe Y-stage was preceded by a metal-managementstage, hydrosulphite bleachingefficiency was restored giving ISO brightnessgains <strong>of</strong> 6.9 and 6.7 points for DIPand TMP fines, respectively.The high level <strong>of</strong> metals in the DIPfines fraction prevented any benefit <strong>from</strong>peroxide bleaching. Without a metal-managementstage, a P-stage caused a brightnessloss: the peroxide residual was niland the high pH caused alkali darkening<strong>of</strong> the <strong>pulp</strong>. The best P-stage resultsoccurred following either an acid wash atpH 1.5 or a Q-stage using a 1.2% charge<strong>of</strong> EDTA, twice the amount required forthe unfractionated <strong>pulp</strong> and the long fractions.At this dosage, the magnesium inthe <strong>pulp</strong> was also depleted and needed tobe replenished for optimal peroxidebleaching efficiency. The metal-managementstages decreased the iron concentrationby about 100 ppm (<strong>from</strong> 560 to460 ppm), with the subsequent peroxidestage giving an ISO brightness 4.6 pointshigher than the case with no pre-treatment,for a net ISO brightness gain <strong>of</strong>only 2.3 points.The TMP fines, which had an Fe content3.5 times less than that <strong>of</strong> the DIPfines (i.e., 170 ppm vs. 560 ppm), respondedmuch better to peroxide bleachingthan DIP fines. Even without a metalmanagementstage, the ISO brightnessgain was 10 points. A metal-managementstage provided an additional ISO brightnessgain <strong>of</strong> ~4 points, similar to the benefitobserved with the DIP fines.A metal-management stage clearlyimproved the P-stage to a greater extentthan the Y-stage, demonstrating thegreater sensitivity <strong>of</strong> hydrogen peroxidetowards transition metals.A QPY/APY stage brought the finalISO brightness <strong>of</strong> the TMP fines to 70.4%(a gain <strong>of</strong> ~18 brightness points), and <strong>of</strong>the DIP fines to 55.7% (a gain <strong>of</strong> 8 brightnesspoints). The effectiveness <strong>of</strong> the Y-stage decreased when preceded by an efficientP-stage; the two stages do not38 ❘❘❘ 104:8 (2003) T 210 Pulp & Paper Canada