Purification of Ammonium Tungstate via Evaporation Crystallization Based on Single Factor Combined with Box-Behnken Method

A combination of single factor and Box-Behnken response surface method was employed to optimize the impurity removal process during the evaporation crystallization of ammonium tungstate solution to prepare ammonium paratungstate (APT) with higher purity. Firstly, to reduce the total content of four impurities (Na, K, S and Mo) in APT, the preferred range of crystallization temperature, stirring speed and initial concentration of ammonium tungstate solution was preliminarily determined by single factor method. Secondly, the impurity removal process during the evaporation crystallization of APT was further optimized by Box-Behnken response surface method, and the interactive effects of three factors on the total amount of four impurities in APT was studied. The results show that the order of influence of three factors on the total content of four impurities is as follows: initial concentration of ammonium tungstate solution>evaporation temperature>stirring speed. The optimum process conditions are an evaporation temperature of 94 ℃, a stirring speed of 1.25 m/s, and an initial ammonium tungstate concentration of 73 g/L. Under the experimental conditions, the total content of four impurities in the prepared APT is reduced to 39.351 mg/L, which corresponds to a relative error of merely 4.110% compared to the optimal prediction value of the response surface method model. Consequently, the purity of APT reaches 4N level. The generated APT crystal is a columnar cuboid morphology with a small amount of broken crystals. The layered structure is obvious, the particle size distribution is uniform, and the grain refinement is obvious.