There are many impurities like: P, As, Mo, Si, they are easily adsorbed on the active point of the crystal, and hinder the growth of crystal to fine APT crystal, then affect the quality. At the same time due to the Cl-、S concentrating after a long time, they enrich on the surface of the ammonium paratungstate to decrease the activity, so that uniform particles of coarse granular products are difficult to formed.

 

Impurities as P, as, SiO2, Na, Mg, CA, Fe, Mo make influence depending on their interaction properties with paratungstate, the solubility of compounds formed, acid soluble impurities (NH4) H3PW12O40, (NH4) 3AsW12O40, (NH4) 3SiW12O40. These impurities remaining in the mother liquor in the process paratungstate crystallization, concentration more than 1g/L, but the mass fraction no more than 0.005% in the salt. Impurities such as P, As, Si d Si can decrease the crystallization rate of ammonium paratungstate, grain efficiency of other impurities ranking like Mo>Na>Mg>Ca>Fe, depending on the concentration of the original solution.

 

Impurities are easy to absorb into APT crystal to affect its quality, and even lead to defective goods. So, we can take rate control, increase the concentration of NH4Cl, stirring and add reagents to washing to achieve the purpose of removing impurities.

 

The above shows that super saturation closely related to the concentration of ammonium chloride solution, ammonium chloride and impurities. So the solution should be controlled in a reasonable range to make a suitable range of concentration of ammonium. People control from the three aspects: the concentration of WO3 in solution, NH4Cl, impurities.
 

For Super Saturation Affects Ammonium Paratungstate Physical Properties 2/3, please see in
http://news.chinatungsten.com/big5/tungsten-information/81978-ti-10660