Chlorine dioxide bleaching of nineteen non-wood plant pulps

Materials

Pulps from non-wood plants were prepared separately at the laboratory by soda-anthraquinone (AQ) process. These 19 non-wood plants had a lot of variation in chemical, anatomical and morphological properties (Ferdous et al. 2020). The chemical characteristics of some of these non-woods were published elsewhere and presented in Table 1 (Ferdous et al. 2020). As for example, jute fiber was characterized with high cellulose, low lignin and longer fiber length (Jahan et al. 2007a). Rice, wheat and kaun straws had high percentage of fines generated from parenchymatic cell (Ferdous et al. 2020a). Therefore, optimum conditions were varied among the raw materials. Bleaching experiment was performed only for the pulp obtained at optimum conditions. The cooking experiment was conducted in an electrically heated thermostatically controlled digester of 20 litre capacity. 1 kg non-wood was used for all experiments. All non-wood plants were cooked under the conditions given in Table 2. Based on our previous studies, the AQ charge and material to liquor ratio were kept constant at 0.1 % and 1:6, respectively (Ferdous et al. 2020a).

Oxygen delignification

Oxygen delignification (OD) was carried out in thermostatically controlled digester, rotating at 1 rpm. OD conditions were 110 °C, retention time 60 min, pulp consistency 10 %, NaOH 2 %, MgSO₄ 0.3 % and O₂-pressure 3.5 kg.cm⁻².

Evaluation of pulp

The kappa number (T 236 om-99), viscosity (T 230 om-99), brightness (T 452 om-92) and HexA (T 282 pm-07) of the resulting pulps from the unbleached and oxygen delignified state were determined in accordance with Tappi Test Methods. Three replicates of all experiments were done, and average reading was taken.

D₀(EP)D₁ and D_HT(EP)D₁ bleaching

Unbleached and oxygen delignified pulps were bleached by D₀(EP)D₁ and D_HT(EP)D₁ bleaching sequences (where D represents chlorine dioxide and (EP) represents peroxide reinforced alkaline extraction). The chlorine dioxide charge in D₀ and D_HT stages were varied by kappa factor 0.15, 0.2 and 0.25. The temperature was 70 °C in D₀ stage for 45 min. Pulp consistency was 10 %. The pH was adjusted to 2.5 by adding dilute H₂SO₄. In the D_HT stage, bleaching temperature was 85 °C and all other parameters remained same. In the alkaline extraction stage, temperature was 70 °C for 60 min in a water solution of 2 % NaOH and 0.5 % H₂O₂ (on od pulp) were used. After (EP) stage, kappa number, viscosity and brightness were determined in accordance with Tappi Test Methods as above.

In the D₁ stage, pH was adjusted to get end pH 4.5. The ClO₂ charge in the D₁ stage was fixed to 1 %. The brightness, viscosity and HexA of the bleached pulp and COD in the mixed effluent collected from D₀, EP and D₁ stages were determined in accordance with PAPTAC Methods H.3.

D_HT(EP) and D₀(EP) pulp properties

The effect of kappa factor on (EP) kappa number, brightness and viscosity of 19 non-wood pulps are shown in Table S1. First ClO₂ stage is denoted as D₀ and D_HT at 70 °C and 85 °C, respectively. As expected, increasing kappa factor resulted in decrease kappa number and viscosity and increase brightness regardless D₀ or D_HT processes. D_HT bleaching is more efficient than D₀ and support chlorine dioxide dose reduction during bleaching (Table S1). The delignification to a lower kappa number is one of the factors to get high pulp brightness. The oxygen delignified pulp showed lower kappa number after (EP) stage. The kappa numbers after (EP) stage in D_HT bleaching were always lower than the corresponding D₀ bleaching. Kaur et al. (2019a) also showed that the kappa number of rice straw pulp after extraction stage was lower for D_HT than D₀ with the same ClO₂ dose. This is related to the acid leaching of lignin as described and reported by (Ikeda et al. 1999, Ikeda et al. 1999a). The highest and lowest (EP) kappa numbers were observed for okra plant and wheat straw, respectively. From the 19 non-wood plant pulps few are shown in Figure 1. As shown in Figure 1, for the pulp bleaching with kappa factor 0.2, (EP) kappa number were 11.76 and 7.78 in D₀ bleaching, which decreased to 9.61 and 5.15 in D_HT for unbleached and oxygen delignified pulp, respectively. The (EP) kappa number for wheat straw pulps decreased from 1.83 and 0.98 in D₀ bleaching to 1.38 and 0.63 in D_HT bleaching. The higher kappa number decreased in the D_HT(EP) stage as compared to the D₀(EP) can be explained by the higher removal of HexA (Lachenal and Chirat 2000). This can also be explained by the degradation of hemicellulose during D_HT treatment, which breaks down the bonds of the lignin-carbohydrate complex (LCC) and increases the extraction of the lignin from the surface of the fiber, which is described by Zhang and coworkers (2018). But according to Ikeda et al. (1999), the temperatures were not high enough to cleave the C-O or C-C bonds that link lignin to carbohydrates in LCC. McDonough et al. (2009) speculated that proportion of HexA contributing to the pulp’s kappa number entering the D₀ stage affected bleachability of a red oak pulp that underwent a hot acid hydrolysis.

Figure 1

Effect of D_HT delignification on kappa number after (EP) stage.

Pulp brightness after D₀(EP)/D_HT(EP) stage is also shown in Table S1. Oxygen delignified pulp showed better pulp brightness than the unbleached pulp regardless D₀ or D_HT bleaching. The (EP) brightness of D_HT treated pulps was better than those treated by D₀ process using an identical kappa factor. Bamboo, dhaincha, jute fiber and jute fiber pulps showed improved brightness, while banana pseudo stem pulp showed the worst brightness. Primarily, unbleached pulp brightness can predict the pulp bleachability. At the staring, banana pseudo stem pulp brightness of was 11.32 % and 15.93 % for unbleached and oxygen delignified pulps, respectively (Table S1). Only (EP) brightness data of a few non-wood plant pulps are shown in Figure 2. The highest brightness advantage in D_HT stage than D₀ stage after (EP) stage was 3 % for dhaincha oxygen delignified pulp. Ventorim et al. (2005) found the first chlorine dioxide stage at high temperature (D_HT) decreased the brightness by 2.5 % ISO and kappa number by 46 % (1.9 units) after extraction stage as compared to the conventional D stage. Their results were also supported by others (Ragnar and Lindström 2004, Eiras et al. 2003). The target of this study was to improve brightness and kappa number reduction at 85 °C for shorter bleaching time, which was achieved in this study.

Figure 2

Effect of D_HT delignification on brightness after (EP) stage.

Figure 3

Effect of D_HT delignification on viscosity after (EP) stage.

As shown in Table S1, oxygen delignification reduced pulp viscosity insignificantly. The highest and lowest (EP) pulp viscosity was for banana leaf and chia pulp, respectively. There was no significant change in post extraction pulp viscosity after D_HT bleaching. As for example, Figure 3 shows that the pulp viscosity of oxygen delignified banana leaf pulp decreased from 19.23 mPa.s in D₀ process to 18.93 mPa.s in D_HT process only. No change of pulp viscosity was observed for chia pulp. But other studied showed that the D_HTE treatment at 95 °C for resulted in significant drop in post extraction pulp viscosity as compared to the DE one (Ventorim et al. 2005, Ragnar 2003). Pulp exposed to high time/temperature reaction and acid pH may undergo slight carbohydrate hydrolysis. The significant viscosity loss is related to the hot acid hydrolysis of the carbohydrates with the 2.5 pH conditions (Ventorim et al. 2005, 2008).

D_HT(EP)D₁ and D₀(EP)D₁ pulp properties

The impact of D₀ and D_HT process on the final pulp brightness and viscosity of 19 non-wood plant pulps are shown in Table S2. A lot of variation of bleachability among these nineteen non-wood plant pulps was observed. Wheat straw pulp showed the highest final pulp brightness. At kappa factor 0.15, final pulp brightness of oxygen delignified was 90.17 % in D₀ process, which increased to 91.30 % in D_HT process. As in initial and D₀/D_HT(EP) low brightness, there was no improvement of final pulp brightness for banana pseudo stem pulp. The final brightness reached to 44 % only at the highest ClO₂ charge. Dhaincha, eggplant plant, jute fiber and jute tick pulps also showed good bleachability. The oxygen delignified eggplant stalks pulp also showed good bleachability. At kappa factor 0.25, the final brightness of eggplant stalks oxygen delignified pulp reached to 82.24 % in D₀ process, while the same brightness was obtained at kappa factor of 0.15 in D_HT process and saved 40 % ClO₂ in the first stage. Kumar et al. (2007) showed that the use of D_HT reduced 15 % ClO₂ requirement and improved the brightness and brightness stability in comparison with the reference sequences D₀.

Figure 4

Effect of D_HT delignification on HexA content in final bleached pulp.

The effect of the D_HT on the residual HexA of final pulps (oxygen delignified) was also studied and shown Figure 4. The residual HexA content in most of the D_HT bleached pulps was much less than the D₀ pulps. Banana pseudo stem, banana leaf, banana peduncle, chia plant and cassava plant pulps did not show significant differences in HexA contents between D₀ and D_HT in final pulps. This result reflected in final pulp brightness. At kappa factor, 0.2, the residual HexA contents were 1.02, 2.37, and 5.31 μmol/g for D_HT pulps and 2.39, 4.34 and 7.21 μmol/g for D₀ pulps from wheat straw, dhaincha and jute fiber pulps, respectively. This is cause of degradation of HexA in D_HT process (Colodette and Henricson 2012). HexA could not react with chlorine dioxide, but react with its intermediates such as hypochlorous acid and molecular chlorine (Tarvo et al. 2010, Lehtimaa et al. 2010), thereby influencing the bleachability and enhancing the consumption of chlorine dioxide. Percentage of HexA removal in D_HT process showed a linear relationship with final pulp brightness (Figure 5). The HexA removal in banana pseudo stem pulp was only 35 %, consequently showed the lowest pulp brightness (44 %) among these 19 non-wood pulps. On the other hand, the highest HexA removal was 96 % for dhaincha pulp, where final pulp brightness was 86 %.

Figure 5

Removal of HexA content D_HT delignification and final pulp brightness.

Final pulp viscosity was also shown in Table S2. The final pulp viscosity was related to the entering viscosity of pulp of unbleached. Chia plant and lentil stalks showed the lowest final pulp viscosity of 5–7 mPa.s, where pulp viscosity entering into the bleaching system was 10–12 mPa.s. The pulp viscosity loss indicates cellulose degradation, ultimately reflected in papermaking properties.

COD in bleach effluent

The environmental impact of 19 non-wood plants pulps bleaching by D₀ and D_HT processes were investigated. As shown in Table 3, the COD value of combined effluent from D₀, (EP) and D₁ stage in D_HT(EP)D₁ bleaching was lower than D₀(EP)D₁. Oxygen delignified pulp had always lower COD load in effluent. The ClO₂ charge in D₀/D_HT stage of oxygen delignified was certainly lower as kappa number was lower, thus released less organic material from the pulp to the filtrate and resulted lower COD load. Similar results were also observed in ECF bleaching by Shin and Mera (1994). As an example of bamboo pulp, the D_HT process at kappa factor 0.2 decreased COD value from 652.92 mg/l to 443.9 mg/l for unbleached pulp and from 479.04 mg/l to 323.72 mg/l for oxygen delignified pulp. Similar COD value reduction in other reported data are available (Kaur et al. 2018, 2019). Kaur et al. (2019a) showed that D_HT based bleaching sequence reduced chlorophenols, chlorocatechols, chloroguaiacols, chlorovanillins, chlorosyringols and bromophenols were reduced by 9 %, 50 %, 34 %, 47 %, 17 % and 31 %, respectively, at the same chemical charge. D_HT based sequence also effectively reduced environmental parameters i. e. COD, BOD, TS, colour, lignin and AOX without compromising optical and strength properties Kaur et al. (2019a). Rolf et al. (2009) showed that the amount of dissolved organics, represented by COD was proportional to kappa number of unbleached pulp for wood and non-wood pulps. But at a given kappa number, the amount of COD was much higher for the non-wood pulp.