High-calcium ammonium paratungstate means the ammonium paratungstate powder that has excessive Ca content. The phenomenon of excessive Ca content in APT products is likely to occur during the production of APT by ion exchange process. The high-calcium APT must be reprocessed as it will not only affect the quality of tungsten carbides, but also cause a waste of tungsten resources to some extent.
The conventional process to treat ammonium paratungstate with severely Ca content has the disadvantages of long process flow, large consumption and high cost.
A Method for Reprocessing High-calcium Ammonium Paratungstate
1. Principle
(1) Characteristics of EDTA and Ca2+ complex: Good stability; simple complex ratio of 1:1; pHmin=7.6, which can be titrated in neutral or alkaline solutions.
(2) Complexation equilibrium of Ca2+ and EDTA:
Ca+Y=CaY
Stability constant K=[MY]/[M][Y]
At 25°C, the stability constant for CaY in equilibrium of reaction is:
K=4.9 × 1010
LgK=Lg4.9×1010=10.69
(3) At normal temperature, purified water, ATP and EDTA containing high Ca2+ were added to the sample under neutral conditions so that it will not affect other elements.
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