Forward Black Liquor Acid Precipitation: Lignin Fractionation by Ultrafiltration

Lignin recovery from black liquor is an important task for producing valuable chemical products. Acidification processes are currently applied by pulp and paper industries for black liquor treatment, in which two main streams are produced: the precipitated lignin fraction and a lignin-lean black liquor. Membrane filtration is a suitable alternative for lignin recovery from black liquor, but studies on lignin-lean black liquor filtration are scarce. Here, we evaluated the ultrafiltration process for lignin recovery from the both fractions of black liquor acidification. The lignin-lean black liquor presented 22 wt% of total solids with 4.6 wt% of lignin. Lignin retention from the lignin-lean black liquor by the 5 kDa ultrafiltration membrane was equal to 85%, with reduction in total solid concentration from 219.8 to 68.1 g L^-1. Due to the relatively high solid concentration in the lignin-lean black liquor, cake formation was the main fouling mechanism during ultrafiltrations. The precipitated lignin solution presented 4.8 wt% of total solids with equivalent lignin concentration (4.7 wt%). The used membrane was able to retain almost 100% of solids and lignin from the solution prepared from the precipitated lignin. All fouling mechanisms were responsible for flux decay in ultrafiltration of the precipitated lignin solution. Steady state fluxes for lignin-lean black liquor and precipitated lignin solution were 0.9 and 15.9 L h^-1 m^-2, respectively. According to TGA analyses up to 800 °C, precipitated lignin and lignin-lean black liquor presented total mass losses of 63.5% and 44.3%, respectively. Also, the permeate samples presented lower mass losses than their respective feed samples. The ultrafiltration process reduced the average weight molar mass (M_w) of the precipitated lignin solution and lignin-lean black liquor from 1817 to 486 g mol^-1and from 2876 to 1095 g mol^-1, respectively. Thus, the 5 kDa ultrafiltration membrane was efficient for lignin recovery from the lignin-lean black liquor, while membranes with lower cut-off should be proposed for lignin purification from the precipitated fraction.