Kraft black liquor as a renewable source of value-added chemicals

Around 1.3 billion tonnes of weak Kraft black liquor is produced every year, and is usually concentrated by evaporation and burned to produce energy. Nevertheless, this strategy implies that many valuable compounds of great industrial interest are destroyed. This article compiles the available information about valorising Kraft black liquor by obtaining chemical products. Although this topic is arousing interest, more research is needed, as only processes which have not yet been validated at industrial scale are described, except for lignin recovery. Due to the heterogeneity of the raw material, complex and integrated processes are being proposed to extract different existing and/or generated compounds. The most common approach is a first stage of lignin recovery by precipitation, followed by the use of the delignified black liquor to obtain principally organic acids, and/or as a fermentation medium, recirculating the by-product streams into the Kraft process to recover the inorganic salts.