A lot of work has been done on the use of the extraction stage kappa number to predict pulp bleachability in the following chlorine dioxide stages. (1982) have reported that oxygen enrichment of the first alkaline extraction stage produces brighter pulp in that stage and in the following D 1 and D 2 stages. However, Histed and Vega Canovas (1989) have shown that even without interstage washing, a (DED) sequence produces higher brightness pulp with significant chemical savings compared with a single D stage. Normally the pulp is washed after the D 1 and E 2 stages. Relatively little time (5–10 min) is required for the E 2 stage to be effective ( Axegard et al., 1984). Increasing the temperature of E 2 stage from 40 to 80☌ provides a small increase in D 2 brightness increasing the alkalinity of E 2 stage from pH 7 to 12 linearly increases brightness after D 2 stage. Maintaining a certain concentration of chlorine dioxide throughout the bleaching stage provides a sufficient chemical driving force for the bleaching rate and for particle bleaching. Sulphur dioxide acts as a reducing agent. Sodium hydroxide is added at the end of the tower to increase the pH to 6–7, converting residual chlorine dioxide into sodium chlorite which is nonvolatile and much less corrosive than chlorine dioxide. Sulphur dioxide dissolved in water and sodium hydroxide are used. The antichlor reacts with the chlorine dioxide and removes it. Some mills use antichlor at the end of a chlorine dioxide tower to minimize the negative effects of residual chlorine dioxide.
A 10 mg ClO 2/L residual in the vat corresponds to 1 kg of chlorine dioxide per metric ton of pulp. Normal chlorine dioxide stage residuals are in the range of 10–50 mg/L expressed as active chlorine. Low residual chlorine dioxide bleaching is more economical and produces higher-quality pulp ( Annergren et al., 1987). However, minimizing the residual is very important to reduce the excess chemical use, to avoid washer corrosion, emission of chlorine dioxide into the air, and complications in systems closure. Chlorine dioxide residual at the end of the chlorine dioxide stage is maintained to prevent brightness reversion. The requirement of chemical is much more when only one stage is used. Best use of chlorine dioxide is usually obtained when 20–30% of the total amount is applied in a second (D 2) stage. However, use of alkaline extraction stage and a second chlorine dioxide stage after the first dioxide stage, leads to an increase in brightness ( Table 3.4.3). In spite of the use of higher dose in the D 1 stage, it is not possible to increase brightness further. The brightness increases rapidly with increasing chlorine dioxide charge initially, then the rate of brightness gain decreases to approach an asymptotic limit ( Tables 3.4.2 and 3.4.3) ( Rapson, 1963 Rapson and Strumila, 1979). This suggests that the solution method is promising, but so far this is the only example of an efficient OLED deposited from the solution. However, in 2019, an article was published, in which the authors created a solution-based OLED with having a record high brightness of 6365 cd/m 2 and efficiency EQE = 7.15%, which is brighter than all known diodes, including those made by vacuum thermal evaporation. So, for example, diodes obtained by the spin-coating method based on the non-volatile carboxylates, and, exhibit maximum brightnesses of only 75, 180, and 230 cd/m 2, respectively. With the transition to solution technology, again, the typical values of brightness and efficiency of OLEDs are reduced by orders of magnitude. Key: DPM, 2,2,6,6-tetramethyl-3,5-heptanedione pobz, phenoxybenzoate PO4, di(phosphine oxide). ITO/PEDOT:PSS/Poly-TPD/TCTA:OXD-7: Tb7/Tm3PyPB/LiF/Al ITO/MoO 3/TCTA:MoO 3/TcTa/ Tb3/ Tb2/3TPYMB/LiF/Al