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1. A method for purifying biological dust suppressant by two-stage ultrafiltration, comprising fermenting and culturing a biological dust suppressant-producing strain in a culture medium to obtain fermentation broth, refrigerating the fermentation broth at a temperature range of −4° C. to 2° C. for a period of time, taking out the fermentation broth, subpackaging the fermentation broth into centrifuge tubes for centrifugation, and taking supernatant after centrifugation; pumping the supernatant by a peristaltic pump and ultrafiltering the supernatant by a primary ultrafiltration membrane, and leaving a retentate containing a target substance after a primary ultrafiltration concentration in a liquid storage tank, wherein the retentate is a concentrated solution; adding a mixed solution of methanol and ethyl acetate into the concentrated solution for dilution, and simultaneously reversely rinsing the primary ultrafiltration membrane with the mixed solution of methanol and ethyl acetate; penetrating a solution obtained after dilution of the concentrated solution with the mixed solution of methanol and ethyl acetate through a secondary ultrafiltration membrane using a power device, and penetrating the diluted solution, obtained by mixing the concentrated solution with methanol and ethyl acetate, containing the target substance through the secondary ultrafiltration membrane; after cleaning the secondary ultrafiltration membrane repeatedly with clean water, ultrafiltering the retentate containing the methanol and the ethyl acetate through the secondary ultrafiltration membrane again, and repeating this step for three times; and finally placing the retentate obtained from the secondary ultrafiltration membrane, containing the target substance into a rotary evaporator for evaporation at a vacuum degree of 0.5 and a temperature of 50° C., liquefied the evaporated methanol and the ethyl acetate in the condenser tube, and recycled into a recycle bottle, and drying the residue in an evaporation bottle to remove moisture to obtain the high-purity biological dust suppressant powder; wherein a two-stage ultrafiltration purification device comprises a controller, the peristaltic pump, a sanitary fixture, a ceramic ultrafiltration membrane, a drainage tube, a flow valve and the liquid storage tank; wherein the ceramic ultrafiltration membrane comprises the primary ultrafiltration membrane and the secondary ultrafiltration membrane, the primary ultrafiltration membrane with a cutoff molecular weight of more than 20 kD is selected, and the secondary ultrafiltration membrane with a cutoff molecular weight less than 10 kD is selected.
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