Existing forms of sodium paratungstate in ammonium paratungstate solution:
The main form of sodium ion (Na) in the ammonium paratungstate crystallization solution is sodium paratungstate. Since sodium paratungstate and ammonium paratungstate are similar in composition and structure, sodium may enter into it in the form of substitution or interstitial impurity ion or surface chemical adsorption in the APT lattice. The source of impurity sodium in ammonium paratungstate has the following two aspects:
- Production water
Using the single-stage double-bed water purification process and controlling the average resistance of pure water to 8*10Ω*cm, the sodium content in the water will be very low, which will not affect the quality of tungsten products.
- Ammonium chloride in desorbent
The sodium content in the APT produced by the ion exchange method is relatively large, generally reaching (3~7)*10%, which is far greater than the highest value of Na content of 0.01, so it will affect the fine tungsten powder. In this case, the sodium content is not only related to its own concentration, but also affected by factors such as APT supersaturation, nucleation rate, crystal growth rate, particle size, shape and surface of existing crystals, and solution viscosity. These factors listed are all affected by the concentration of WO3.
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