Regulated Preparation Of ?-ammonium Tetramolybdate And Its Effect On Molybdenum Powder

Molybdenum has many excellent physical and chemical properties such as high melting point,good thermal/electrical conductivity,low thermal expansion coefficient,and good corrosion resistance.Now,Molybdenum and its alloys are widely used in aerospace,petrochemical and electronic fields.However,Molybdenum is a non-renewable scarce strategic resource,and its utilization are gradually developing in the direction of diversification and refinement.As the raw material for the preparation of molybdenum and molybdenum alloys,the performance index of molybdenum powder has a vital influence on the quality of various molybdenum products.And different application fields have different requirements for its characteristics.This article is based on the urgent demand for large particles of molybdenum powder in the fields of 3D printing and thermal spraying,in-depth research was conducted on the problems such as the complexity of ammonium molybdate crystal,the difficulty of preparing?-ammonium tetramolybdate single crystal and the heredity of particle size of molybdenum powder due to the grain size in the process of preparing large molybdenum powder.Firstly,the preparation of a single crystal form of?-ammonium tetramolybdate was studied,the crystal size of?-ammonium tetramolybdate was controlled by seed preparation method,and then the heredity of?-ammonium tetramolybdate on the morphology and particle size of molybdenum powder were explored.Finally,on the basis of?-ammonium tetramolybdate heredity,the process was further optimized to prepare large particles of molybdenum powder.In the experiment,X-ray diffractometer(XRD)and scanning electron microscope(SEM)were used to study the phase structure and micro morphology of the powder,and the Fisher particle size analyzer and laser particle size analyzer were used to analyze the particle size and distribution of the powder.The main research contents and results are as follows:(1)The effects of crystallization temperature,solution concentration and pH on the crystal form,particle size distribution and crystal size of ammonium molybdate were systematically studied.And it was concluded that temperature is the main factor affecting the crystal form of ammonium molybdate.When the temperature is 90°C,the pH is between 2.0 and 5.0,and the solution concentration is between 0.3 and 0.7g/m L,the crystalline product is?-ammonium tetramolybdate in a single crystal form.The seed crystal preparation method is adopted to control the crystal grain size of?-ammonium tetramolybdate by controlling the crystallization time.(2)Through a combination of cooling crystallization and slow evaporation crystallization,a single crystal form of?-ammonium tetramolybdate prepared at pH=2 is added as a seed crystal to the hot saturated ammonium molybdate solution with a temperature of 90°C,pH=5.5,and a concentration of 0.5 g/m L in an appropriate amount.Let it stand at room temperature and evaporate slowly to prepare large-size?-ammonium tetramolybdate in the form of single crystals with uniform composition,stable structure and controllable particle size.(3)The genetic analysis of the roasting decomposition of?-ammonium tetramolybdate was carried out,and the conclusions are as follows:during the thermal decomposition of?-ammonium tetramolybdate,N-H bonds and-OH functional groups fracture during roasting decomposition,NH₃ and H₂O were emitted as gas,and Mo=O double bond,Mo-O single bond and Mo-O-Mo bond were retained,forming a layered orthogonal MoO₃ with{MoO₆}octahedron as the basic unit,and the retained chemical bond still retains the structure and performance of the original chemical bond.Therefore,MoO₃ should inherit part of the structure and performance of the raw material?-ammonium tetramolybdate.This innovative discovery is of great value for studying the heritability of?-ammonium tetramolybdate to the particle size of molybdenum powder and the subsequent particle size control of molybdenum powder.The single crystal?-ammonium tetramolybdate of different sizes was roasted and decomposed under the same process,and the study found that the morphology of the product MoO₃ has obvious genetic characteristics.Within a certain range,?-ammonium molybdate has heritability to the particle size of MoO₃,and the particle size of MoO₃ increases with the increase of the particle size of?-ammonium molybdate.When the single crystal particles of?-ammonium molybdate is 100?m,MoO₃ has the largest particle size,which is 17.85?m.But continue to increase the particle size of?-ammonium molybdate,the particle size of MoO₃ decreases.(4)The genetic analysis of MoO₃ hydrogen reduction was carried out,and the conclusions are as follows:during the first-stage reduction,the reaction follows the nuclear rupture model,and MoO₃ has no heritability to the morphology of MoO₂;in the second-stage reduction process,the reaction follows the nuclear reduction model,and MoO₂ has obvious heritability to the morphology of the product Mo powder.MoO₃ has heritability to the particle size of MoO₂ and Mo powder.The larger the particle size of MoO₃,the larger the particle size of MoO₂ and Mo powder.Finally,MoO₃ with a size of 17.85?m undergoes two hydrogen reduction processes to obtain molybdenum powder with a particle size of 4.8?m,which is three times the particle size of the molybdenum powder produced by industrial ammonium molybdate roasting and hydrogen reduction under the same reducing conditions.(5)Using large-particle single crystal?-ammonium tetramolybdate as raw material,the preparation process of large-particle molybdenum powder was optimized through orthogonal experiments.The multi-index range analysis method is used to comprehensively evaluate the particle size of the two-stage reduction product.The optimum process parameters for one-stage hydrogen reduction were obtained as follows:a reduction temperature of 650?,a material layer thickness of 35 mm,a hydrogen flow rate of 150 m L/min,and a wet hydrogen temperature of 40?.The Fisher particle size of MoO₂ prepared under this condition is 7.58?m,which is 2.78?m larger than that of MoO₂ prepared from single crystal large particles of?-ammonium tetramolybdate without process optimization,which is about 3 times of the particle size of MoO₂ prepared from industrial ammonium molybdate.The second-stage hydrogen reduction process plan is that the reduction temperature is1050?,the material layer thickness is 20 mm,the hydrogen flow rate is 150 m L/min,and the wet hydrogen temperature is 40?.The Fisher particle size of the Mo powder prepared under this condition is 6.39?m,which is 1.59?m larger than the size of the molybdenum powder prepared from single crystal large particles of?-ammonium tetramolybdate without process optimization,which is about 4 times of the particle size of molybdenum powder prepared from industrial ammonium molybdate.