Solvent extraction of biomass, its derived ash, or biomass parts such as the seeds has been or is currently used commercially to isolate and separate certain chemicals or groups of related compounds that are present. Inorganic salts are found in some biomass species at concentrations that may justify extraction and purification (Chapter 3). Aqueous extraction of the ash from giant brown kelp and the spent pulp of sugar beet and fractional crystallization of the extract, for example, were commercial processes for the manufacture of potassium compounds in the early 1900s. Examples of some of the organic compounds that are extracted with solvents are triglycerides, terpenes, and lignins. Water and water in mixtures with polar solvents have been used for extraction of several of the low-molecular-weight, water-soluble sugars. Some detail on the extraction of lignins illustrates how solvent extraction processes might be developed.

Aqueous organic solvents are effective for the selective extraction of lignins in biomass. Lignins can also be extracted from biomass by use of dilute aqueous alkali under mild conditions (с/. Lawther, Sun, and Banks, 1996), but aqueous alcohols alone such as 50% ethanol solubilize lignins in wood, leaving relatively pure undecomposed cellulose (Aronovsky and Gortner, 1936; Nikitin et aL, 1962). Deciduous trees are delignified by aqueous ethanol extraction to a greater extent than conifers. Lignin is also readily extracted by mixtures of butanol or amyl and isoamyl alcohols with water. Separation of the lignins from the extracts yields tarlike substances that become brittle on cooling. Since one of the prime objectives of producing chemical pulps from wood is delignification without changing the cellulosic fibers, the data accumulated on the solvent extraction of wood suggests that high-quality paper pulps could be manufactured by solvent extraction of hardwoods and softwoods as well as other biomass species. The lignins in the extracts might provide the starting point for the production of new lignin derivatives and polymers.

Solvent extraction of biomass under relatively mild conditions to remove lignins by a strictly physical process without the addition of other chemicals would seem to offer several advantages over chemical pulping methods. Solvent recoveries approaching 100% should permit solvent recycling with minimal losses. A continuous process for the pulping of wood with aqueous n-butanol, which was found to be the most effective solvent, has been proposed for the pulping of wood and the separation of the lignins (Hansen and April, 1981).

This type of process, which would be expected to be environmentally benign, does not seem to have been commercialized to any extent by the pulp industry (с/. U. S. Dept, of Energy, 1995).