The Crystallization of Ammonium Paratungstate
Ammonium paratungstate (APT) is an important tungsten intermediate product, mainly with white crystals.
It have flakes, needles and strips three kind, used in the manufacture of tungsten trioxide or blue tungsten oxide, TP, also used for manufacturing ammonium metatungstate and other tungsten compounds.
Because the production of tungsten oxide, morphology and particle size of tungsten powder has genetic relationship with APT, so the production process is not only to the tungsten APT purification, but also to control the crystal morphology and particle size of paratungstate tungstate.
In the 1950s, in the book "Tungsten and Molybdenum Metallurgy" by A.H rickman, used the reaction 5(NH4)2O·12WO3·nH2O to describe the APT formula, used the evaporation crystallization process to get n = 5 flaky crystal, and used the acid neutralization process for n = 11 needle crystal. After that in China are all using this view, that of ammonium tungstate solution producing is a ammonia chemical process of paratungstate tungstate lost, the crystallization process of chemical reaction is:
12(NH4)2WO4===5(NH4)2O·12WO3·5H2O+2H2O +14NH3
(N = 5 temperature> 50 °C to obtain flaky crystals)
12(NH4)2WO4+4H2O===5(NH4)2O·12WO3·11H2O+14NH3
(N = 11, temperature <50 °C to obtain needle-like crystals)
APT crystal water content and crystal shape depends on the crystallization conditions, the current study that APT has three crystalline morphology.
APT·10H2O is needle-like or strip-like orthorhombic structure, it is unstable in the industrial crystallization conditions, at least 50 °C below the crystal can be obtained in this crystal.
2. APT·12H2O is a triad flake crystal, which is a crystal in the state of metastable state in water. It is a metastable phase at the boiling point of (NH4) 2WO4 solution, and only when the concentration of WO3 in the solution is more than 30%, the concentration of NH3 more than 5% of the conditions can be generated, with the extension of storage time, it will happen recrystallization and turn into APT·4 H2O.
APT·4H2O is a monoclinic cubic structure, which is always stable under industrial crystallization, means from WO3 concentration higher than 30%, NH3 concentration is greater than 5% to WO3 concentration less than 10%, in a solution of NH3 was 1%, slightly better in 90-100 °C temperature range as long as there is APT·4 H2O exists, the seed can produce APT·4 H2O crystallization.
If you have any inquiry of tungsten oxide, please feel free to contact us:
Tel.: +86 592 5129696 +86 592 5129595
Fax: +86 592 5129797
评论
发表评论