Studies On The Nucleation And Crystallization Of Ammonium Tetramolybdate

Molybdenum is the rare metal with high melting point, which is often used to prepare alloy steel, refractory steel and stainless steel. Molybdenum trioxide is the product of the thermal decomposition of ammonium tetramolybdate, which can be changed into molybdenum thread and molybdenum bar through different working procedures, such as deoxidization, et al. The different crystallines of ammonium tetramolybdate have effects on the properties of the products of metal molybdenum. Different polymolybdates can crystallize from aqueous solutions at different pH values of aqueous solution. The products were determined by gravimetric analysis, differential thermal analysis and X-ray powder diffraction. The results showed that ammonium tetramolybdate crystallized from the aqueous solutions in the range of pH values2.0 ～3.0, ammonium trimolybdate in the range of 3.5 ～4.0, ammonium heptamolybdate in the range of 4.0～4.5. The crystal process of ammonium tetramolybdate is comprised of two phases that is nucleation and crystal growth. It will be the key for preparing superfine and different crystallines of ammonium tetramolybdate how to control effectively the process about mentioned. The nucleation rate was studied indicated by the induction period as a function of supersaturation and temperature. The induction period decreased and the nucleation rate increased as the temperature and supersaturation of the solution increased both. According to classical nucleation theory, the interfacial tension, the critical radius and critical Gibbs free energy of formation of the crystal were calculated experimentally. The nucleation rates in supersaturated solutions prepared by HCl and by HNO3 were also discussed. The factors such as the temperature of reaction, the concentration of original solution, the pH values and so on, which can effect the crystal of ammonium tetramolybdate, have been discussed in this paper. Ammonium tetramolybdate with different crystallines were prepared by control of the reaction temperature, speed of acid addition and time of drying. The different crystallines were determined by X-ray powder diffraction.