Green and low-carbon preparation of ammonium paratungstate by adding ammonia to ammonium metatungstate solution

Since the current industrial method for ammonium paratungstate (APT) production from ammonium tungstate solutions involves evaporative crystallization to remove the excess ammonium, which discharges large amounts of wastewater and consumes considerable energy, a novel method for preparing APT from ammonium metatungstate (AMT) solutions by adding ammonia was developed and investigated systemically in this study. Thermodynamic calculations indicated that the dominant tungsten-containing anions depending upon the solution pH were: metatungstate [H2W12O406−] at pH 2–4, paratungstate [H2W12O4210−] and [W7O246−] at pH 5–8, and tungstate WO42− at pH > 8. The experimental results showed that the temperature, ammonia dosage, stirring speed, reaction time and different ammonium salts added affected the crystallization efficiency and crystalline morphology. The APT crystallization efficiency reached 91.4% with an average particle size of 492.2 μm under the following conditions: a temperature of 85 °C, a 1.6-fold stoichiometric ammonia dosage, a reaction time 5 h, a stirring speed 150 rpm and a C(NH4)2SO4 1 mol/L. The crystallization mechanism involved the transformations of isopolytungstate ion from H2W12O406− to the intermediate H6W12O426− and then to the final H2W12O4210−, which resulted in APT precipitation due to accumulation and combination of the H2W12O4210− and NH4+ ions in solution. The crystallized product exhibited the two primary phases (NH4)6[H6W12O42]·10H2O and (NH4)10[H2W12O42]·10H2O at 40 °C and 55 °C, respectively, while (NH4)10[H2W12O42]·4H2O was formed at temperatures ≥70 °C. Results showed that (NH4)6[H6W12O42] acted as an intermediate in APT crystallization from the AMT solution, which has not been previously reported. Compared with evaporative crystallization, this method for preparing APT product from AMT solutions by adding ammonia exhibits the advantages of (i) less chemical consumption, (ii) minor amounts of discharged wastewater, (iii) low production costs, and (iv) environmental friendliness.