Method for preparing unbleached biomechanical pulp from straws and full utilization of by-products

A method for preparing an unbleached biomechanical pulp from straws and full utilization of by-products, wherein comprising the following steps:

• (1) cutting a physically selected dedusted air-dried straw into a ready-for-use raw material with a length of 3-6 cm;
• and collecting a straw that cannot be used as the ready-for-use raw material;
• (2) placing the ready-for-use raw material into a reactor, treating with an alkaline compound enzyme according to a solid/liquid ratio of 1:4-1:8 W/V, during the treatment, adding KOH to adjust the pH value of the straw to 7-14, and controlling the treatment time within 30-90 min to obtain a treated straw;
• and collecting the wastewater generated during the enzyme-alkali co-treatment;
• (3) taking out the treated straw in step (2), and further treating with hot alkali (KOH) water and steam at a temperature of 80-120° C. to obtain a hot alkali treated straw;
• (4) refining the hot alkali treated straw in step (3) in one or more stages to make the beating degree at 25-55° SR to obtain a refined pulp;
• (5) washing with water the refined pulp to obtain a biomechanical pulp;
• defibrating the biomechanical pulp by a defibrater and mixing evenly to obtain the unbleached biomechanical pulp;
• and collecting the washing water;
• evaporating and concentrating the wastewater generated in the enzyme-alkali co-treatment and the washing water to obtain a residue, and recovering the heat energy for the hot alkali (KOH) water and steam treatment of step (3);
• mixing the residue with the straw that cannot be used as the ready-for-use raw material in step (1), and adjusting the pH to make a fertilizer.