【作者】 王桂美；

【导师】 陈建中； 高绍康；

【作者基本信息】 福州大学 ， 物理化学， 2005， 硕士

【摘要】 钼是一种宝贵的稀有高熔点金属,是生产合金刚、不锈钢、耐热钢和合金铸铁的重要合金化元素。目前,国内生产厂家多采用由四钼酸铵焙解成的三氧化钼为原料,经还原制粉、压制成型和高温烧结等工序来生产钼条、钼丝等钼产品。钼制品的性能与原料四钼酸铵的晶型有关。采用酸沉法制备了不同pH 值下的结晶产物以确定四钼酸铵生成的pH 值范围。对产物进行重量分析、热重差热分析及XRD 分析知:2.0<pH<3.0 时,酸沉产物为四钼酸铵;3.0<pH<3.5 时,酸沉产物为三钼酸铵;4.0<pH<4.5 时,酸沉产物为仲钼酸铵。四钼酸铵的结晶形成过程包括两个阶段,即四钼酸铵晶体的成核过程和晶核长大过程。如何有效地控制上述两个过程,将是实现制备不同晶型四钼酸铵产品的关键。鉴于硝酸和盐酸在酸沉工艺中的不同,本文根据经典的均匀成核理论,对硝酸酸沉和盐酸酸沉四钼酸铵过饱和溶液的成核过程分别进行了研究,讨论了过饱和比和温度对成核速率及成核机制的影响。由实验结果可知,无论是硝酸酸沉还是盐酸酸沉,四钼酸铵过饱和溶液的成核速率均随过饱和比和温度的升高而升高;固液界面张力σ、临界成核自由能及临界半径均随温度的升高而减小。对硝酸酸沉和盐酸酸沉四钼酸铵过饱和溶液的成核进行了比较,当用盐酸酸沉时,四钼酸铵过饱和溶液的成核速率要比用硝酸酸沉时过饱和溶液的成核速率大。通过对酸沉法制备四钼酸铵结晶过程的分析,发现影响四钼酸铵结晶的主要因素有:终点pH 值、反应温度、搅拌速度和反应时间。采用正交实验法,对这四个因素进行考察,结果表明这四个因素的影响能力从大到小依次为:终点pH 值、反应温度、搅拌速度、反应时间。为了探索出不同晶型四钼酸铵的生成条件,分别对原料液浓度、pH 值、温度对四钼酸铵晶型的影响进行了讨论。通过控制结晶过程的温度、加酸速度、pH 值等条件,用酸沉法制备了三种不同晶型的四钼酸铵:α型、β型和微粉型。经X 射线衍射分析,确定了各样品的晶型。结合均匀成核的研究结果,初步确定了各种晶型四钼酸铵的生产工艺条件。更多还原

【Abstract】 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.更多还原