What Factors Should be Paid Attention to in Producing High Purity APT
High purity quaternary ammonium paratungstate (APT) is a crystalline powder of ammonium paratungstate with a purity of 99.999% to 99.9999%.
At present, the methods of producing high purity APT by controlling impurity content in industrial production are: re dissolution method, ion exchange method, ammonium tungstate method, electrolysis method, recrystallization method, etc.. But because of the different chemical properties of impurity elements, using only one method to achieve the purpose of removing all impurities, so the preparation of high purity APT often need two times of ion exchange, activated carbon adsorption and other means of further purification.
It is found that the following three problems should be paid attention to during the purification of high purity ammonium paratungstate (APT).
(1). The higher the crystallization rate, the higher the impurity content in APT crystallization. When the crystallization rate is more than 65%, the precipitation of impurities is accelerated, and the precipitation rate of the four impurities is sorted as Fe >M o>P >S. Therefore, the ultra high purity APT should be controlled by controlled crystallization, and the crystallization rate should be controlled within 65%.
(2). Increasing the concentration of WO3 in ammonium paratungstate solution is beneficial to improve the purity of APT. The higher the initial concentration of WO3 is, the less impurity content in APT crystal. The decrease of Mo content in APT was faster than that of P, S and Fe in the ammonium tungstate solution with higher WO3 concentration. The precipitation rate of Fe decreased obviously with the increase of WO3 concentration.
(3) With the increase of crystallization temperature, the impurity content in APT decreased. The reason may be that the solubility of impurities increases at high temperature, and the content of APT crystals decreases. At the same time, homogeneous coarse grains are easily obtained at high temperature, and the impurities in the bulk crystals and homogeneous coarse grains are less.
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