Study On The Crystallizing Process Conditions For The Formation Of Single-Crystalline Ammonium Dimolybdate

Ammonium molybdate, the typical isopolyoxometallate with a variety of species,mainly includes ammonium dimolybdate, ammonium tetramolybdate, ammoinumheptamolybdate, ammonium octamolybdate, twelve ammonium molybdate and betaammonium tetramolybdate and so on. The ammonium molybdates used in the mordernindustrial fields are usually the mixture of the above-mentioned molybdates, which caneasily absorb moisture and agglomerate. The purity and physicochemical properties ofthe ammonium molybdates which act as the basic raw material exert a powerfulinfluence over the quality of the molybdenum-based derivatives, especially molyb-denum powders. Furthermore, the crystal forms, grain size, particle size distributionand flowability can restrict the physical properties as well as the post-processingperformance of the resulting molybdenum powders to a great extent. Thus, the studyon the preparation technique of ammonium molybdates with non-agglomeration, welldispersity, uniform particle size distribution and high purity has become one of the hottopics in related field recently. It has been proved that the monocrystal ammoniumdimolybdate, possessing the characteristics of higher purity, more uniform grain size,better dispersity and flowability, higher stability, etc., promises to become thepreferred raw materials for the production of high-quality molybdenum powders.Another advantage of ammonium dimolybdate crystals lies in its unbreakable merit,thorough reduction process and less rawish state, leading to easily controlled roast-dissociation-reduction temperature and higher yield of molybdenum powder. Therefore,how to prepare highly disperse and large size single-crystal ammonium dimolybdate isboth of great application value and important theoretical significance.In this thesis, we have prepared the ammonium molybdate solution through theclassical nitric acid pretreatment-ammonia leaching-sulfide purification unitedprocess using commercial molybdenum oxide as raw material. Meanwhile, the contrastexperiments using sulfuric acid instead of nitric acid, and the measurement of impurity contents as well as the sulfate anion residues in the both solutions obtained by usingthe two acids have been made. The feasibility conclusion for the replacement of nitricacid by sulfuric acid gives the effective way for in situ conversion of sulfuric acid by-produced in the process of commercial molybdenum oxide, resulting in the reductionof comprehensive production costs. In order to make high-end and competitiveammonium molybdate products, we detailedly investigated the further impurityremoval method by selecting and using proper cationic exchange resin. Theexperimental results show that the removing impurity treatment by means of doubleresin columns connected in series is relatively effective for removal of most of themetal ions in the molybdate solutions. Secondly, the ammonium dimolybdatemonocrystal grains have been successfully prepared using ammonium tetramolybdateas reactants. The effects of operation conditions such as temperature, pH value, density,evaporation rate and size of seed crystals on the phase and shape of resulted productshave been systematically studied. Besides, the selected additive is found to be alsoeffective for the control of morphology. The exact measurement results of XRD, FT-IR,Raman, TG-DSC and the analysis of particle size distribution for the ammoniumdimolybdate monocrystals with high purity could provide reliable basic data for thefurther study in related field.Finally, to further optimize the technological parameters for the preparation ofammonium dimolybdate monocrystals and to reduce the material and energyconsumption as well as the costs, the commercial molybdenum oxide was used as rawmaterial to directly prepare ammonium dimolybdate without the classical acid-precipitation crystallization process. The process parameters such as temperature,concentration, acidity, and crystal-seeds have been scientifically verified and thequalified products with desired shape and purity have been obtained. Moreover, thecrystal nucleation and growth mechanism have also been primarily discussed.