Strong acid hydrolysis of cellulosic and hemicellulosic materials

A method of producing sugars from materials containing cellulose and hemicellulose comprising:

• mixing the materials with a solution of about 25-90% acid by weight thereby at least partially decrystallizing the materials and forming a gel that includes solid material and a liquid portion;
• diluting said gel to an acid concentration of from about 20% to about 30% by weight and heating said gel to a temperature between about 80.degree. C. and 100.degree. C. thereby partially hydrolyzing the cellulose and hemicellulose contained in said materials;
• separating said liquid portion from said solid material, thereby obtaining a first liquid containing sugars and acid;
• mixing the separated solid material with a solution of about 25-90% acid until the acid concentration of the gel is between about 20-30% acid by weight and heating the mixture to a temperature between about 80.degree. C. and 100.degree. C. thereby further hydrolyzing cellulose and hemicellulose remaining in said separated solid material and forming a second solid material and a second liquid portion;
• separating said second liquid portion from said second solid material thereby obtaining a second liquid containing sugars and acid;
• combining the first and second liquids;
• and
• separating the sugars from the acid in the combined first and second liquids by resin separation to produce a third liquid containing a total of at least about 15% sugar by weight, which is not more than 3% acid by weight.