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Objective Achieving efficient rapid demulsification of water-in-oil emulsion constructed of oil with high viscosity was realized. Methods The g-C3N4 was prepared from melamine by thermal polymerization method. The composite filter cake was prepared by mixing super-hydrophilic g-C3N4 with diatomite, and the high viscosity water-in-oil emulsion was quickly demulsified by pumping and filtering. ResultsThe filter cake containing 1.5 wt% g-C3N4 with 0.5 cm thickness can realize completely demulsified, which the emulsion with water content less 6 wt% and droplet size larger than 2.32 μm. The demulsification rate of 20 kPa vacuum degree can reach 3.57 L/min, the demulsification efficiency is 97.36%, and the oil recovery rate is above 99.72%. Thickening the filter cake can be used for demulsification of emulsion with higher water content. Such attractive demulsification effect is caused by the simultaneous action of strong affinity of g-C3N4 to water and the crushing effect by micropores of diatomite. Conclusion The g-C3N4/diatomite composite filter cake can be effectively used for efficient rapid demulsification of water-in-oil emulsion with promising application prospect. [ABSTRACT FROM AUTHOR]