Lignin is unique among biopolymers in having significant aromatic character, which makes it potentially useful for a wide range of applications, including foams, composites, resins, and carbon fibers. However, most of the commercial-grade lignins available today are recovered from the Kraft pulping process and thus have a relatively high metals and ash content, eliminating them from consideration for most high-value applications. Thus, less than 0.2% of the 50 million tons/yr of so-called “Kraft lignins” are being used for nonfuel products. Clearly, cost-effective separations processes for these lignins must be developed if lignin is to achieve its potential as a renewable biopolymer.
We have discovered that mixtures of acetic acid and water at elevated temperatures can be used to selectively extract Kraft lignins in terms of molecular weight. Of equal significance, these fractions are simultaneously purified by the solvent system, with the metals content in the lignin being reduced by almost an order of magnitude with each extraction step. Our process, which we call ALPHA (Aqueous Lignin Purification using Hot Acids), can be operated in two ways: (1) by using increasingly aggressive solvent mixtures to isolate metals-free (i.e., <100 ppm) lignin fractions of medium and high molecular weight, or (2) by “reversing” the process and using solvent mixtures of decreasing strength to isolate metals-free lignin fractions of low molecular weight.