Method of recovering lignin from pulp and paper sludge

Abstract

A thermochemical process is provided for the use of pulp and paper sludge as a source of lignin through extraction and recovery. Pulp and paper sludge contains common components of plant materials as well as minerals employed in papermaking. Sludge is mixed in an alkaline solution (NaOH 0.4% to 4% by weight, preferably 0.5% to 2%, and most preferably 1% NaOH by weight). The solution is heated in an autoclave at a temperature of 150.degree.-250.degree. C. (preferably 180.degree.-220.degree. C., and most preferably 200.degree. C.) for a short time, e.g., from about half an hour to an hour. The extracted lignin, present in the alkaline solution, is precipitated by lowering the pH of the solution to a value of 3-5 with the use of, for example, an inorganic or organic acid. The precipitated solid lignin is then recovered and dried.

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