Lignin is exist with cellulose in wood, straw and other plant working as binder of cells walls,so naturally has good binding capacity.

These is a lot of ways to separate the lignin and cellulose.

Most easily way is popular used in China ,they just treated NaOH to separate lignin and cellulose .Cellulose is used as raw material for papers,lignin is found exist in their black wast after filtering cellulose.So based on black liquid they spray it and get lignosulfonate.Usually is called sodium lignosulfonate as they are use Sodium hydroxide to extract it .The problem of extraction is very very simple cheap cost,as in the black wast a lot of cellulose and most of the sugar inside.

Other way is most popular used by sulfite  pulping by acidic calcium sulfite or sodium sulfite and others,the solution is got in acidic for around 10 hour under tempreture of 130.Then non water soluble cellulose and soluble calcium lignosulfonate are separated.Then treat the black liquid by sulfuric acid ,then filtering then spray dry it.