How does the sulfite pulping process differ from the kraft process?

The sulfite and kraft processes are both chemical methods for producing paper pulp, but they differ significantly in their chemistry, equipment, and capabilities.12

Key Differences

• Chemical Composition: The sulfite process uses an acidic cooking liquor consisting of sulfur dioxide and bisulfite. In contrast, the kraft process uses an alkaline solution of caustic soda and sodium sulfide.132
• Equipment: Because the sulfite process is acidic, it requires digesters with acid-resistant linings. The alkaline nature of the kraft process is less corrosive to iron and steel, allowing for unlined digesters.132
• Wood Suitability: The kraft process can effectively digest resinous woods like pine, as the alkaline liquor dissolves resinous components, which can then be recovered as tall oil. The sulfite process is not suitable for high-resin woods because it does not remove these components as effectively.23
• Pulp Quality: Kraft pulp is generally stronger than pulp produced by the sulfite process. While sulfite pulp is naturally lighter in color, kraft pulp is initially dark and requires advanced bleaching techniques to achieve high brightness.23

Since 1940, the kraft process has become the predominant method in the industry, while new sulfite mills are no longer being constructed.3