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Lignin recovery process could improve profits for papermakers

By Gerald Parkinson |

Recovery boilers, in which pulp and paper companies burn spent black liquor from the kraft pulping process to recover and recycle sodium and sulfur, are a bottleneck in many paper mills. The reason is that they operate at their upper limit of heat flux into the boiler tubes, thus preventing an increase in the mills’ output, explains John Blackburn, co-founder of Liquid Lignin Co. (LLC, Charleston, S.C.; www.liquidlignin.com). LLC is developing a process that promises to relieve this situation by removing lignin (a major fuel component) from the liquor. Another benefit is that the lignin, a natural polymer, can be used to replace petroleum-based chemicals such as phenol in wood adhesives — much more valuable applications than boiler fuel, says Blackburn. The process (flowsheet), called Sequential Liquid-Lignin Recovery and Purification (SLRP), is a continuous, three-stage operation. In the first stage, the black liquor’s pH is reduced from 14 to 9 by counter-current contact with carbon dioxide. As the pH drops, lignin precipitates as a dense-phase “liquid lignin” that is phase-separated from the carbonated black liquor. The depleted liquor is recycled to the pulping process and the liquid lignin goes to…
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