Solvent Pulping Can Be an Alternative to the Kraft Process, 1993 China Paper Conference Proceedings

Martin C. Schroeter, Ph.D.

For many years the acid sulfite pulping process used to be the preferred method for producing chemical pulps from woody fiber raw materials. Unbleached acid sulfite pulps tend to be light in color, when compared to unbleached Kraft pulps, and can be easily bleached to high brightness levels. Their low strength properties, however, in comparison to Kraft pulps, restrict their use in many fiber products. Kraft pulps, on the other hand, found only limited use in paper products because of their dark color and the difficulty to bleach them to brightness level comparable to sulfite pulps.

The discovery of chlorine dioxide, however, being a powerful bleaching agent, especially for Kraft pulps, diminished the dominance of the sulfite processes in the industry and the Kraft process became the process of choice in the production of fully bleached chemical pulps. This switch from one process to another was further helped by the fact that in the Kraft process virtually any fibrous material can be used as a raw material, including highly resinous pines, while in the acid sulfite process only nonresinous wood species can be used to produce pulps. This made the Kraft process the universally applicable pulping process which it is today.