Bleaching is a multi-stage process that refines and brightens raw pulp. The objective is to dissolve (chemical pulps) or modify (mechanical pulps) the brown-coloured lignin that was not removed during pulping, while maintaining the integrity of the pulp fibres. A mill produces customized pulp by varying the order, concentration and reaction time of the bleaching agents.
Each bleaching stage is defined by its bleaching agent, pH (acidity), temperature and duration (table 1). After each bleaching stage, the pulp may be washed with caustic to remove spent bleaching chemicals and dissolved lignin before it progresses to the next stage. After the last stage, the pulp is pumped through a series of screens and cleaners to remove any contaminants such as dirt or plastic. It is then concentrated and conveyed to storage.
Table 1. Bleaching agents and their conditions of use
|
Symbol |
Concentration |
pH |
Consistency* |
Temperature |
Time (h) |
|
|
Chlorine (Cl2) |
C |
2.5–8 |
2 |
3 |
20–60 |
0.5–1.5 |
|
Sodium hydroxide (NaOH) |
E |
1.5–4.2 |
11 |
10–12 |
<80 |
1–2 |
|
Chlorine dioxide (ClO2) |
D |
~1 |
0–6 |
10–12 |
60–75 |
2–5 |
|
Sodium hypochlorite (NaOCl) |
H |
1–2 |
9–11 |
10–12 |
30–50 |
0.5–3 |
|
Oxygen (O2) |
O |
1.2–1.9 |
7–8 |
25–33 |
90–130 |
0.3–1 |
|
Hydrogen peroxide (H2O2) |
P |
0.25 |
10 |
12 |
35–80 |
4 |
|
Ozone (O3) |
Z |
0.5–3.5 |
2–3 |
35–55 |
20–40 |
<0.1 |
|
Acid washing (SO2) |
A |
4–6 |
1.8–5 |
1.5 |
30–50 |
0.25 |
|
Sodium dithionite (NaS2O4) |
Y |
1–2 |
5.5–8 |
4–8 |
60–65 |
1–2 |
* Concentration of fibre in water solution.
Historically, the most common bleaching sequence used to produce market-grade bleached kraft pulp is based on the five-stage CEDED process (see table 1 for definition of symbols). The first two stages of bleaching complete the delignification process and are considered extensions of pulping. Because of environmental concerns about chlorinated organics in pulp mill effluents, many mills substitute chlorine dioxide (ClO2) for a portion of the chlorine (Cl2) used in the first bleaching stage (CDEDED) and use oxygen (O2) pre-treatment during the first caustic extraction (CDEODED). The current trend in Europe and North America is towards complete substitution with ClO2 (e.g., DEDED) or elimination of both Cl2 and ClO2. Where ClO2 is used, sulphur dioxide (SO2) is added during the final washing stage as an “antichlor” to stop the ClO2 reaction and to control the pH. Newly developed chlorine-free bleaching sequences (e.g., OAZQP, OQPZP, where Q = chelation) use enzymes, O2, ozone (O3), hydrogen peroxide (H2O2), peracids and chelating agents such as ethylene diamine tetracetic acid (EDTA). Totally chlorine-free bleaching had been adopted at eight mills worldwide by 1993. Because these newer methods eliminate the acidic bleaching steps, acid washing is a necessary addition to the initial stages of kraft bleaching to allow removal of metals bound to the cellulose.
Sulphite pulps are generally easier to bleach than kraft pulps because of their lower lignin content. Short bleaching sequences (e.g., CEH, DCEHD, P, HP, EPOP) can be used for most paper grades. For dissolving-grade sulphite pulps used in the production of rayon, cellophane and so on, both hemicellulose and lignin are removed, requiring more complex bleaching sequences (e.g., C1C2ECHDA). The final acid wash is both for metal control and antichlor purposes. The effluent load for dissolving-grade sulphite pulps is much greater because so much of the raw wood is consumed (typical yield 50%) and more water is used.
The term brightening is used to describe bleaching of mechanical and other high-yield pulps, because they are whitened by destroying chromophoric groups without dissolving the lignin. Brightening agents include H2O2 and/or sodium hydrosulphite (NaS2O4). Historically, zinc hydrosulphite (ZnS2O4) was commonly used, but has been largely eliminated because of its toxicity in effluent. Chelating agents are added before bleaching to neutralize any metal ions, thereby preventing the formation of coloured salts or the decomposition of H2O2. The effectiveness of mechanical pulp bleaching depends on the species of wood. Hardwoods (e.g., poplar and cottonwood) and softwoods (e.g., spruce and balsam) that are low in lignin and extractives can be bleached to a higher brightness level than the more resinous pine and cedar.