Process for recovering lignin

Abstract

There is provided a process for recovery of lignin from a black liquor that contains either soluble or dispersed lignin, generating a 'liquid lignin' at high yield. Soluble lignin at elevated pH is precipitated by reducing the pH of the black liquor stream by countercurrent reaction with carbon dioxide, at elevated temperature and pressure, creating two bulk fluid phases: a heavy lignin-rich phase and a light lignin-depleted phase. The heavy lignin-rich phase is separated and washed countercurrently with a strong acid to displace metal cations from the lignin, creating a low-salt lignin, which is then formed into a low-dust high-bulk-density lignin fuel pellet. If needed, especially for lignin recovered from kraft papermaking black-liquor streams, an oxidation step is included to eliminate negative odor for high-value green-chemistry applications.