Study On Green Process Preparation Of Ammonium Molybdate In Ammonium Carbonate-Ammonium Bicarbonate System

Ammonium molybdate is used as the main raw material for the production of molybdenum products such as molybdenum powder and molybdenum oxide.Its purity,particle size and morphology will directly affect the quality of subsequent molybdenum products.There are few systematic studies and reports on the morphology and particle size of ammonium molybdate in the traditional ammonium molybdate crystallization process.It is of great significance to study and prepare ammonium molybdate products with large particle size and concentrated distribution.In view of the problems such as large consumption of reagents and long production cycle in the preparation process of ammonium molybdate under the traditional system,this paper prepares ammonium molybdate that meets the chemical composition requirements for MSA-0 grade products in GB/T3460-2017 based on the advantages of high ammonia concentration in alkaline ammonium molybdate solutions in the ammonium carbonate-ammonium bicarbonate system,and further studies on shape and particle size.The main results of the study are as follows:The solubility and mesostable zone of ammonium heptamolybdate in pure water and the solubility of ammonium dimolybdate were measured.The results showed that the solubility of both ammonium dimolybdate and ammonium heptamolybdate in pure water increased with increasing temperature.Apelblat equation and Vant Hoff equation were used to fit the solubility,dissolution entropy and dissolution enthalpy of ammonium dimolybdate and ammonium heptamolybdate,R² all above 0.9990,and the dissolution of both ammonium dimolybdate and ammonium heptamolybdate in pure water is a heat absorption process with increasing entropy.The cooling rate and the stirring rate have a large effect on the metastable zone of ammonium heptamolybdate,at cooling rate of 30℃/h and stirring rate of 100r/min,ammonium heptamolybdate has the largest average mesostable zone width of 14.375℃ and has a good operating space for the preparation of ammonium heptamolybdate by the cooling crystallisation method.The preparation of ammonium dimolybdate from alkaline ammonium molybdate solutions in the system（NH₄）₂CO₃-NH₄HCO₃ was studied.The experimental results show that for 1000m L of ammonium molybdate solution,adding 200m L of ammonium molybdate solution when the negative pressure is 0.07MPa,the oil bath temperature is 140℃,and the solution is turbid,single phase ammonium dimolybdate crystals can be obtained.In the experiment,the temperature of the condensed water during the condensation and reflux process was lowered to 15℃ for the experiment,and the volume of the collected condensed solution could reach about 85%of the volume of the evaporated stock solution.When the crystallization rate is about 70%,the upper limit of the concentration of Na and Si impurities in the ammonium molybdate raw material liquid to be met in order to prepare the ammonium dimolybdate required for the MSA-0 grade product are both 5 mg/L.When the concentration of Na in the solution is 3mg/L and 5mg/L,respectively,the crystallization rate of the product should be controlled below 80%and 75%;when the concentration of Si in the solution is 3mg/L and 5mg/L,the crystallization rate of the product should be controlled.It should be controlled below 75%and around 70%.The analysis results based on impurities show that most Na remains on the crystal surface in the form of physical adsorption,and Si occurs in the crystal in two ways:entering the crystal lattice and physically adsorbing.Based on the small-scale experimental analysis and conclusions,a large-scale glass reactor was used to successfully prepare a large number of ammonium dimolybdate products that meets the chemical composition requirements of MSA-0 grade products.It is proposed to prepare ammonium heptamolybdate by using ammonium dimolybdate as raw material by adding water to dissolve-evaporation concentration removal ammonia-cooling crystallization method.The influence of concentration degree on crystallization product was investigated,and the influence of concentration liquid density,cooling rate,crystal growing time and stirring rate on the particle size and shape of ammonium heptamolybdate was studied.The results show that ammonium heptamolybdate single crystal can be prepared by the evaporation concentration-cooling crystallization method after the dissolution of ammonium dimolybdate with water,in the cooling crystallization process,the density of the concentrate is about 1.38g/cm³,the cooling rate is 30℃/h,the crystal growth time is 1h,the stirring rate is 150r/min,and the final cooling temperature is 22℃,the crystals of ammonium heptamolybdate are complete and prismatic with an average particle size of 337.649μm.The preparation of ammonium heptamolybdate from alkaline ammonium molybdate solutions in the system（NH₄）₂CO₃-NH₄HCO₃ was studied.Under the conditions of negative pressure of 0.01MPa and oil bath temperature of 150℃,the ammonium molybdate solution was concentrated to a density of about 1.38g/cm³ for cooling crystallization,and the cooling crystallization process was set the cooling rate is 30℃/h,the crystal growth time is 1h,the stirring rate is 150r/min,and the final cooling temperature is 22℃.Ammonium heptamolybdate crystals with an average particle size of 341.492μm were obtained.The crystal surface is smooth,the particle size is uniform,the edges and corners are distinct,and there is no agglomeration.The particle size is much larger than that of the ammonium heptamolybdate crystals prepared by evaporative crystallization.