NZ603075A - Method for treating wastewater - Google Patents

Abstract

Disclosed herein is a method for treating wastewater, the wastewater being produced by roasting a starting material in the presence of a sodium salt, leaching soluble sodium vanadate, and precipitating ammonium polyvanadate from an acidic leaching solution with an ammonium salt, the method comprising steps of: a) concentrating the wastewater having a pH value of 5.0~6.5 at a temperature of 90 degree C or higher to crystallize anhydrous sodium sulfate from the wastewater to obtain a first crystal suspension, and separating the anhydrous sodium sulfate from the first crystal suspension at a temperature of 90 degree C or higher to obtain a first solution; b) crystallizing a double salt of sodium sulfate and ammonium sulfate from the first solution at a temperature of 9~20 degree C, and separating the double salt to obtain a second solution; and c) concentrating the second solution by evaporation at a temperature of 70 degree C or higher, crystallising mixed ammonium salts including ammonium sulfate and ammonium chloride from the second solution at a temperature of 60~65 degree C to obtain a second crystal magma, and separating the mixed ammonium salts from the second crystal suspension at a temperature of 55 degree C or higher to obtain a third solution. Preferably wherein: a ratio of a volume of the wastewater before the concentrating in the step a) to a volume of the first crystal magma is 10:3~10:1, and a time during which the anhydrous sodium sulfate is crystallized is at least 40 minutes. the crystallizing in the step b) comprises crystallising the double salt of sodium sulfate and ammonium sulfate from the first solution at a temperature of 9~14 degree C for at least 3 hours. in the step c), a ratio of a volume of the second solution to a volume of the second crystal magma is 2~2.5:1, and a time during which the mixed ammonium salts are crystallized is at least 2 hours.