Ten Factors Affecting the Ammonium Paratungstate Crystallization(1)
Ammonium paratungstate (APT) is an important tungsten intermediate product, mainly white crystals, there are two kinds of flakes or needle-like, used in the manufacture of tungsten trioxide or blue tungsten oxide tungsten powder.
The quality of tungsten products depends largely on the chemical purity of APT, particle size and particle size distribution.
APT quality standards from the domestic manufacturers, the process and chemical purity are almost the same, but the production of APT products vary widely, this is because APT is a system engineering, not only the purity and process requirements, as well as crystal grain size and crystal distribution requirements, seemingly insignificant on some of the objective factors often play a key role in the final product is often because of these reasons the difference between the least, lost thousands of miles.
There are lots of influence factors of ammonium tungstate, mainly include: the temperature of the solution, the degree of supersaturation, stirring speed, add seed, impurities, pH value, adding ammonium tungstate solution, crystallization time and crystallization liquor density, vacuum degree, etc.
1. The influence of temperature: The temperature is important factors influencing the particle size of ammonium paratungstate.When the solution too full degrees must be under low temperature, rate of crystal nucleus grow up slowly, and nucleation rate faster, so generally get fine grain;And crystallization under high temperature, due to the crystal nucleus to grow up quickly, and the crystal nucleus formation showed relatively slow, so generally get a coarse crystals.
2. The degree of supersaturation: Degree of supersaturation is the most important factors influencing the APT granularity.Because of the formation of crystal nucleus needs a minimum energy, the activation energy, in a certain temperature, the greater the degree of supersaturation, the smaller activation energy is, the faster speed of the formation of crystal nucleus. However, in the production, the production capacity is too small, the production capacity is too small; the degree of supersaturation is too large and the grain size is small, and the crystal quality is poor. In addition, due to the concentration of ammonium tungstate solution and other solution components Of the content are different, the supersaturation of the solution also has a direct impact, therefore, in order to determine the appropriate supersaturation is very important.
3. Evaporation rate and neutralization speed: Adopt evaporation crystallization or neutralize the legal APT, evaporation rate and had a great influence on the granularity of APT and speed, the greater the evaporation speed and neutralization, at the beginning of the crystal form of the greater the degree of supersaturation, the number of crystal nucleus formation, the resulting APT particle size is fine, apparent density is small.