Method for preparing weathered coal-derived humic acid/urea-formaldehyde slow-release fertilizer granules by reactive extrusion process with depolymerization-activation-polycondensation trinity and fertilization method

A method for preparing a weathered coal-derived humic acid/urea-formaldehyde slow-release fertilizer granule by reactive extrusion process with depolymerization-activation-polycondensation trinity, comprising:

• (1) adding urea, paraformaldehyde, and water to a reactor followed by adjustment to pH 8-12 and reaction at 30-80° C. for 1-3 h to obtain a hydroxymethylurea solution;
• wherein a mass ratio of urea to paraformaldehyde is 2-4:1, and the water is 10-50% of a total mass of urea and paraformaldehyde;
• (2) adding a weathered coal-derived humic acid and an alkaline activator to the hydroxymethylurea solution obtained in the step (1) followed by reaction at 30-80° C. for 5-30 min to obtain a reaction mixture;
• wherein the weathered coal-derived humic acid is 10-50% of the total mass of urea and paraformaldehyde;
• and the alkaline activator is 0-10% of a mass of the weathered coal-derived humic acid;
• (3) sealing a die orifice between a reaction unit and an extrusion unit of a reaction-extrusion integrated machine;
• and adjusting the reaction mixture obtained in the step (2) to pH 3-5 followed by feeding to a twin-screw reactive extruder of the reaction unit of the reaction-extrusion integrated machine;
• (4) starting screws of the twin-screw reactive extruder, and simultaneously starting a vacuumization-devolatilization device of the twin-screw reactive extruder to remove moisture from the reaction mixture, and reacting the reaction mixture at a temperature of 50-90° C. and a screw rotation speed of 50-150 rpm for 10-30 min;
• during this process, depolymerization and activation reactions of the weathered coal-derived humic acid, a condensation reaction of the hydroxymethylurea to generate urea-formaldehyde, and a polycondensation reaction between the generated activated weathered coal-derived humic acid and the generated urea-formaldehyde are carried out simultaneously;
• (5) opening the die orifice between the reaction unit and the extrusion unit, and starting a twin-screw extruder of the extrusion unit such that a reaction product obtained in the step (4) is delivered by the twin-screw reactive extruder of the reaction unit to the twin-screw extruder of the extrusion unit;
• and extruding the reaction product at an extrusion temperature of 50-90° C. and a screw rotation speed of 50-150 rpm through the twin-screw extruder to obtain a strip product;
• and
• (6) drying the strip product obtained in the step (5) at a preset temperature followed by granulation to obtain the weathered coal-derived humic acid/urea formaldehyde slow-release fertilizer in a cylindrical particle form, wherein weathered coal-derived humic acid molecules are inserted into a main chain of urea formaldehyde.