How is chemical pulp made from wood chips?

Chemical pulp is produced by cooking wood chips in chemical solutions within a digester at elevated temperatures and pressures. This process is designed to dissolve lignin—the organic polymer that binds wood fibers together—allowing the fibers to separate as whole, intact units.123

There are two primary methods for this chemical treatment:

• Kraft Process: This is the most common method, utilizing an alkaline solution of caustic soda and sodium sulfide. It is highly effective for various wood species, including resinous pines, and produces strong, durable pulp.142
• Sulfite Process: This method uses sulfite salts with an excess of sulfur dioxide. While historically significant, it is less common today than the kraft process.12

Following the cook, the pulp is washed to remove spent chemicals and may undergo bleaching to increase brightness. Any remaining impurities, such as knots or fiber bundles, are removed through screening. The resulting chemical pulp is valued for its strength and purity, making it suitable for high-quality paper and various cellulose-based products.125