Magnesium Hydroxide Pulping Applications

Magnesium Hydroxide Pulping Applications

Sulfite pulping was discovered by Benjamin Tilghman in 1857. In the Handbook for Pulp & Paper Technologists, Gary Smook describes how Tilghman learned that the presence of a base could reduce the discoloration of the pulp due to sulfurous acid. Calcium was the first base used. The calcium acid sulfite process was the dominant pulping process for many years until the creation of the kraft process in the 1930s (Smook, 2002). Although the kraft process is now the most common chemical pulping method, sulfite mills and applications requiring sulfite pulp still exist. Sulfite pulp is brighter than Kraft pulp and is easier to bleach to high brightness levels. (Keller & Fahey, 1967). Kraft pulp is stronger (kraft is the German word for strong) and typically has longer and more durable fibers.

In the 1950s, sulfite mills began experimenting with bases other than calcium, such as magnesium. (Smook, 2002). Researchers discovered that magnesium bisulfite could pulp wood species that were not pulped by the conventional sulfite process, such as southern pine (Keller & Fahey, 1967). This magnesium bisulfite cooking process is also known as the magnefite process. Another advantage of magnesium bisulfite pulping is that the spent liquors can be burned to recover heat and chemicals, resulting in less environmental impact than calcium bisulfite pulping. US Patent #3428420 describes using a SO₂ scrubber to form magnesium bisulfite from a magnesium hydroxide slurry (USA Patent No. 3428420, 1969). Lignin foams, derived from spent liquor from the magnefite process, have been investigated as a sustainable material for building insulation (Tondi, 2016). Magnesium bisulfite pulp is also well-suited for dissolving pulps, a source of cellulose, which has a rapidly growing market as a renewable resource (Chen et al, 2019).

Traditional calcium acid sulfite pulping was carried out at a pH of 1.5 due to the insolubility of calcium. Soluble bases, such as magnesium, sodium, and ammonium, allow a higher pH. In bisulfite pulping with magnesia, the pH is typically 4-5 due to the limited solubility of magnesium sulfite. Typical temperatures range from 140°C-160°C and cooking time is controlled to “achieve the desired degree of cooking, as measured by the yield or the lignin content” (Smook, 2002).

The neutral sulfite process is considered a semi-chemical process in which the wood chips are sent through refiners after partial chemical digestion. Semi-chemical pulps have a higher yield than chemical pulps, and NSSC (neutral sulfite semi-chemical), in particular, is a good choice for corrugating media. In NSSC pulping, a sodium sulfite cooking liquor is buffered with soda ash (sodium carbonate) to absorb the organic acids generated during cooking. Sodium sulfite may be prepared by reacting sodium hydroxide with sulfur dioxide from burning molten sulfur. In an NSSC pulping process, sodium hydroxide may be partially replaced with magnesium hydroxide to stabilize pH and provide a buffering effect. Potential benefits of this substitution are chemical cost savings, reduced sodium to the wastewater treatment plant, and improved sludge settling.

Martin Marietta Magnesia Specialties produces CellGuard^®OP, a stabilized, high-purity magnesium hydroxide slurry low in transition metals and ideal for pulping and pulp bleaching applications.

• Chen et al, Z. (2019). Current and Future Markets of Dissolving Pulp in China and Other Countries. BioResources, 14(4) 7627-7629.
• Douglas, H., & Snider, I. (1969). USA Patent No. 3428420.
• Keller, E., & Fahey, D. (1967). Magnesium Bisulfite Pulping and Papermaking with Southern Pine. Madison, Wisconsin: U.S. Forest Service.
• Smook, G. A. (2002). Handboook for Pulp & Paper Technologists, 3rd Edition. Vancouver: Angus Wilde Publications Inc.
• Tondi. (2016). Lignin-based Foams. BioResources, 11(2) 2972-2986.