Bleaching Chemical Pulps
Chemical pulps, such as those from the kraft process or sulfite pulping, contain much less lignin than mechanical pulps, (<5% compared to approximately 40%). The goal in bleaching chemical pulps is to remove essentially all of the residual lignin, hence the process is often referred to as delignification. Sodium hypochlorite (household bleach) was initially used to bleach chemical pulps, but was largely replaced in the 1930s by chlorine. Concerns about the release of organochlorine compounds into the environment prompted the development of Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF) bleaching processes.
Delignification of chemical pulps is rarely a single step process and is frequently composed of four or more discrete steps. These steps are given a letter designation, and these are given in the following table:
| Chemical or process used | Letter designation |
|---|---|
| Chlorine | C |
| Sodium hypochlorite | H |
| Chlorine dioxide | D |
| Extraction with sodium hydroxide | E |
| Oxygen | O |
| Alkaline hydrogen peroxide | P |
| Ozone | Z |
| Chelation to remove metals | Q |
| Enzymes (especially xylanase) | X |
| Peracids (peroxy acids) | Paa |
| Sodium dithionite (sodium hydrosulfite) | Y |
A bleaching sequence from the 1950s could look like: CEHEH . The pulp would have been exposed to chlorine, extracted (washed) with a sodium hydroxide solution to remove lignin fragmented by the chlorination, treated with sodium hypochlorite, washed with sodium hydroxide again and given a final treatment with hypochlorite. An example of a modern totally chlorine-free (TCF) sequence is OZEPY where the pulp would be treated with oxygen, then ozone, washed with sodium hydroxide then treated in sequence with alkaline peroxide and sodium dithionite.
Read more about this topic: Bleaching Of Wood Pulp
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