Dispersant using kraft lignin and novel lignin derivative

A lignin derivative comprising a component unit derived from kraft lignin and/or its salt;

• a polyalkylene oxide chain;
• and an anionic functional group,
• wherein the kraft lignin and/or its salt has a functional group,
• wherein the polyalkylene oxide chain and anionic functional group are derived from a radical polymerizable water-soluble monomer,
• wherein the radical polymerizable water-soluble monomer comprises a carboxyl group as the anionic functional group, and
• wherein the lignin derivative is obtained by reacting the functional group of the kraft lignin and/or its salt with:
• an alkylene oxide addition compound of an unsaturated alcohol, which is obtained by adding 2 to 300 moles of alkylene oxides to the unsaturated alcohol, to introduce the polyalkylene oxide chain;
• and
• the radical polymerizable water-soluble monomer comprising the carboxyl group as the anionic functional group,
• followed by polymerization.