Ten Factors Affecting the Ammonium Paratungstate Crystallization(2)
Ammonium paratungstate (APT) is an important tungsten intermediate product, not only in the purity and process requirements, as well as crystal grain size and crystal distribution requirements.
Seemingly negligible some of the objective factors often play a key role in the final product often For these reasons the difference between the least, lost thousands of miles.
6. The impurities in the solution: As the solution P, As, Mo, Si and other impurities easily adsorbed on the crystal growth of the active point, hinder the crystal growth, but also makes the ammonium paratungstate grain thinning, make the quality of deterioration, and easy to enter the product during the evaporation crystallization process, affecting product quality, and even lead to product failure, so in the crystallization process, should take the control of crystallization rate, improve the concentration of NH4Cl in ammonium tungstate solution, stirring and washing agents and other methods, to achieve the goal of removing impurity.
7. pH value: The nucleation rate and crystal growth rate affected by the pH value, since raw crystal nucleus of pH control in 7.5-8.0 range, is beneficial to reduce the number of spontaneous nucleation and the formation rate, small amount of crystal nucleus can grow smoothly into larger particles; And made a narrow pH range is beneficial to uniform distribution of coarse grain of ammonium paratungstate, only the uniform distribution of particles can be evenly Karl fischer granularity, this performance is particularly obvious in neutralization.
8. Ammonium tungstate solution added: The end of the nucleation period after the addition of ammonium tungstate solution, can not change the particle size distribution conditions, to improve the crystallization of ammonium tungstate ammonium particle size and bulk density, but also to overcome the initial high tungsten concentration , the product size of the smaller defects in the higher equipment capacity, produce the good fluidity of the crude ammonium paraffin.
9. The end of crystallization of the liquor density: Evaporation of the end of the crystallization of the liquor density of ammonium paraquat have a certain impact. Under normal circumstances, the lower end of the crystallization of the liquor density, the higher the crystallization rate, the easier the formation of coarse particles, because the crystallization of a long time can lead to crystal nuclei have sufficient time to grow up; the other hand, the crystallization of the end of the liquor density are larger, the easier the formation of smaller particles. Therefore, on the premise of guarantee the quality of products, can reduce the liquid evaporation at the end of the corresponding density, increase the crystallization rate accordingly.
10. Vacuum degree: With the speed up of the evaporation rate and vacuum degree is reduced, and the increase of the crystal elutriation rate, resulting in a slight increase in the number of microcrystalline precipitation, and the particle size decreases.
In conclusion, the preparation of ammonium paratungstate (APT) is a systematic project with a lot of side details. Each factor has different effects on the preparation of ammonium paratungstate crystals. In most cases, in order to produce a certain physical properties of ammonium paratungstate products, it needs comprehensive control for the various factors.
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