Hazardous chemicals used in pulp and paper mills require careful handling and continuous monitoring to minimize threat to workers, processes, and nearby communities.
Chemical pulping techniques of the Kraft process occur in batch or continuous digesters where heat and white liquor are used to reduce wood chips to pulp. During this process glue holding the woodchips together, known as lignin, breaks down and mixes with the white liquor liberating Hydrogen Sulfide (H2S) gas. The lignin-contaminated liquor is now know as black liquor and is sent to recovery boilers where molten solids are produced. These molten solids are discharged into water tanks that form green liquor and then processed further to convert it back to white liquor for reuse in the digesters.
Extreme heat from the liquor recycling process produces large amounts of steam that are sent throughout the mill. Recovered steam can be a carrier of Hydrogen Sulfide (H2S) and should be monitored continuously.
While the liquor recycling processes is continuous, pulp stock from the digesters is washed, screened, and then sent through to the bleaching process. Bleaching of pulp is achieved through either an oxidation process where Oxygen (O2) is used to dissolve undesired color of pulp stock or chlorination using Chlorine (CL2) or Chlorine Dioxide (CLO2). Although alternate chemicals such as Hydrogen Peroxide (H2O2) and Ozone (O3) are common replacements in some facilities, chemicals used in chlorination and oxidation process produce deadly vapors.
After bleaching, pulp is sent to the burner decks for drying. Natural Gas (CH4) used to heat the burner presents a combustible hazard.
