The main objective of a pulp mill is to generate fibers to be made into paper. In a chemical pulp mill, this is done by degrading and dissolving lignin away, leaving the fibers. Some examples of chemical pulping are :
1) Kraft
2) Sulfite
3) Soda
In a pulp mill using the kraft process, wood chips are cooked in a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S). The cook time of the process is approximately 2 to 4 hours, depending on the mill. The cooking temperature is between 170 to 180 degrees Celsius. The pH of the cooking liquor is 13+. In the cooking process the alkaline causes the breakdown of lignin molecules which are soluble in the cooking liquor. The following are a few advantages of using the kraft process:
1) produces a number of different pulp grades
2) yields high strength pulp
3) provides means for efficient chemical recovery
4) allows for a various wood types (hardwood, softwood, etc.)
5) handles bark in process
The cooking stage of pulp processing can either be performed in a continuous digester or a batch digester. After the cooking process, pulp is screened to remove large unwanted particles, cleaned to remove residual lignin, and sometimes bleached. Finally, after almost all of the lignin is removed, the pulp is ready to be brought to the paper machines. The paper machines are able to produce many different types of paper, with varying thicknesses and printing characteristics.
In the process of pulping wood and making paper, many undesirable effects are generated. One important issue facing all mills is the generation of food sources for microorganisms which deplete oxygen levels in the rivers. The oxygen depletion occurs if the materials are not removed before the mill effluent reaches the river. This oxygen depletion is known as Biological Oxygen Demand (BOD).
References:
Smook, Handbook for Pulp and Paper Technologists