The present invention relates to a method for preparing an inorganic coagulant and an inorganic coagulant obtained thereby. The method according to the present invention comprises the steps of: 1) reacting polyaluminum chloride with sodium and aqueous sodium carbonate solution to produce hydrogel having an alkalinity of 70-90%; introducing aqueous polymerization initiator solution to the reaction mixture containing the thus produced ultrahigh-alkalinity hydrogel and carrying out polymerization to obtain a polymer blend having a content of Al_13 species of 50 wt% or more in polyaluminum chloride; and 3) carrying out further polymerization while applying ultrasonic waves to the reaction mixture after step 2) to obtain a polymer blend having a content of Al_30 species of 50 wt% or more in polyaluminum chloride. According to the method of the present invention, it is possible to obtain an inorganic polymer coagulant based on Al_30 species, i.e. an inorganic polymer coagulant having a content of AL_30 species of 50 wt% or more, a content of Al_13 species of 20 wt% or less and a content of Al monomers of 30 wt% or less. The inorganic coagulant based on Al_30 species according to the present invention has excellent coagulation activity and storage stability.;COPYRIGHT KIPO 2017
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