Here's why:
Historically, SO2 was used for:
* Digesting wood: In the kraft process, SO2 was used to break down lignin (the material that binds wood fibers), creating pulp. However, modern processes rely on more efficient and environmentally friendly methods like the alkaline sulfite process, which uses sodium sulfite and sodium hydroxide instead of sulfur dioxide.
* Bleaching pulp: SO2 was also used to bleach pulp, but again, modern processes utilize safer and more effective methods like hydrogen peroxide or ozone bleaching.
The reasons for the shift away from SO2 in papermaking:
* Environmental concerns: SO2 is a significant air pollutant, contributing to acid rain and respiratory problems.
* Process efficiency: Modern methods are more efficient and effective at digesting wood and bleaching pulp.
* Safety: SO2 is a corrosive and toxic gas, posing risks to workers.
Modern papermaking processes are much more environmentally friendly and focus on sustainable practices, making SO2 a relic of the past.
