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Black liquor treatment by acid precipitation produces two main streams: the high purity precipitated lignin and the lignin-lean black liquor (a diluted lignin solution). Here, we evaluated the ultrafiltration process for lignin recovery from lignin-lean black liquor at different pH values (9.0, 9.5 and 10.0). The ultrafiltration process was able to retain 83 ± 0.5% of lignin from the lignin-lean black liquor at pH 9.0. The permeate flux increased with the decrease in pH value (steady state flux of 5.3 ± 0.0 L h −1 m −2  at pH 9.0). Cake formation was the main fouling occurrence during lignin-lean black liquor filtrations. The weight average molecular weight of the lignin-lean black liquor decreased from 1866 to 1208 Da after the ultrafiltration process. The pH value did not significantly influence the molecular weight of permeate stream. Permeate samples presented lower hydroxyl content and higher syringyl(S):guaiacyl(G) ratios than feed samples. Thus, the ultrafiltration process was able to concentrate lignin from the lignin-lean black liquor and to produce a permeate with a narrow molecular weight distribution. The precipitation at more acid conditions is suggested to improve permeate flux lignin retention from lignin-lean black liquor.

Ultrafiltration process for lignin-lean black liquor treatment at different acid conditions