The invention relates to a method for extracting undissolved carbonized lignin from black liquor and products produced thereby. The invention further relates to the solid carbon thus extracted and the carbonized black liquor separated from said solid carbon. According to the invention black liquor from an alkaline fractionation process is subjected to a hydrothermal carbonization process, an undissolved carbonized lignin is separated from the carbonized black liquor, the remaining carbonized black liquor is fed back into the alkaline fractionation process, and the undissolved carbonized lignin is separated from the carbonized black liquor is purified.
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