Oxygen has several applications in the pulp and paper industry. A major use of this gas is delignification. Lignin is a polymer found in many plants that makes them rigid, and it must be removed from the pulp to soften the organic material. Oxygen is used in the delignification process to significantly reduce the lignin in pulp, reducing the need for bleach because lignin molecules are easily oxidized. Less bleach also leads to lower toxicity in the wastewater.
During alkaline extraction, oxygen removes additional lignin that survived the delignification process. Using oxygen in this processing stage increases pulp brightness.
Oxygen also plays a role in white liquor oxidation and black liquor oxidation. White liquor oxidation balances sodium and sulfur in pulp, and black liquor oxidation involves oxidizing sodium sulfate into sodium thiosulfate. Using oxygen for this process helps to reduce sulfur losses and odor emissions that occur with bleach.
Yet another application of oxygen is lime kiln enrichment. Lime kilns are used to turn lime mud into lime, which is valuable to the pulping process. When oxygen is added to a lime kiln, it increases production while saving on fuel and reducing sulfur compound emissions.