Method for preparing unbleached biomechanical pulp by hot steam in coordination with biological enzyme treatment of wheat straw and full utilization of by-products thereof

A method for preparing unbleached biomechanical pulp by treating wheat straw by hot steam in coordination with biological enzyme treatment and full utilization of by-products thereof, wherein the method comprises the following steps:

• (1) taking a waste wheat straw after wheat grain harvest as a raw material, mixing water with the wheat straw at a liquid ratio of 1:3-1:6, adding a trace of KOH to adjust a pH of a mixture of the wheat straw and the water to 10-14, and then charging hot steam whose temperature is 100-120° C. in a hot steam pretreatment stage for 15-60 minutes to obtain a wheat straw treated with the hot steam;
• collecting a waste lye and decaying leaves and straw that cannot be used for processing;
• (2) extruding and splitting the wheat straw treated with the hot steam in an extrusion stage to obtain a split wheat straw;
• collecting wastewater from the extrusion stage;
• (3) reacting the split wheat straw with an alkaline biological complex enzyme at a temperature of 40-60° C. for 30-90 minutes in an enzyme treatment stage to obtain a biologically treated wheat straw;
• collecting wastewater generated in the enzyme treatment stage;
• and
• (4) refining the biological treated wheat straw, and collecting a refining washing water;
• treating the waste lye, the wastewater from the extrusion stage, the wastewater generated in the enzyme treatment stage, and the refining washing water with a multi-effect evaporator to evaporate and concentrate to obtain a residue, and recycling heat energy for the hot steam pretreatment stage of step (1);
• mixing the residue with the decaying leaves and straw that cannot be used for processing in step (1) and adjusting pH thereof for fertilizer production.