The molecular formula of APT crystallization is 5(NH4)2O·12WO3·nH2O(n=5, 7, 11), and its solubility in water is very small, which increases slightly with the rise of temperature. Ammonium secondary tungstate crystallization is the main intermediate product for the preparation of blue tungsten oxide, ammonium tungsten bronze and tungsten powder, and its morphological structure greatly affects the morphology and properties of tungsten products. The particle size and loose packing density of tungsten products are dependent on the raw material ammonium secondary tungstate. The properties of ammonium tungsten bronze prepared by ammonium secondary tungstate containing different crystal water are very different. Di Yuying et al. showed that the ammonium tungsten bronze made from pentahydrate and heptahydrate had good properties. However, the production process of ammonium pentahydrate is strict, while the process conditions of ammonium pentahydrate are easy to be controlled, the production process is stable, and tungsten products have been widely used in the industry. Therefore, it is more appropriate to select ammonium pentahydrate as the raw material for the preparation of ammonium tungsten bronze. It is precisely because ammonium secondary tungstate (APT) determines the performance and quality of tungsten products to a large extent, and with the improvement of the requirements for tungsten products, the quality of crystallization of ammonium secondary tungstate also increases accordingly.
1. Preparation of ammonium tungstate solution
Secondary tungstate is crystallized out of ammonium tungstate solution, so the preparation of ammonium tungstate solution is an important step. At present, the preparation of ammonium tungstate solution in China mainly consists of traditional method, solvent extraction method and ion exchange method. The latter two new methods have developed rapidly in recent years. The extraction method and ion exchange method have short process, easy to control production conditions and low energy consumption, but the common weakness of these two methods is the poor separation effect of molybdenum.
1.1 Traditional method
Many manufacturers in China prepare ammonium tungstate solution by traditional method. The advantage of the traditional method is that it is simple and easy to operate. Disadvantages are long process and high energy consumption. The acid of sodium tungstate (Na2W04) solution was decomposed to obtain suitable particle size tungstate, and then ammonia was dissolved. In order to dissolve the tungstic acid, a tungstic acid slurry is first prepared. Dissolving tungsten acid, with 70 ~ 80 ℃ demineralized water first dispatch it into tungsten acid slurry density is 1.60 ~ 1.65, general lOOkg tungstate with demineralized water 135 kg, and slowly add to stir constantly ammonia (25% ~ 28%), the temperature control of 50 ~ 60 ℃, tungsten acid pulp and ammonia press 1: l, the proportion of people, solution of free ammonia control in 20 ~ 309 / l, density control in 1.25 ~ l, within the scope of 30, clarifies, filtration, the filtrate to precipitate the concentrate of para-ammonium tungstate. In this process, a small amount of activated carbon can be added to filter and remove iron hydroxide and silica gel, etc. The Si/W ratio of ammonium tungstate solution was greatly influenced by the amount of ammonia added and the dissolution temperature in ammonia solution, which affected impurity removal.
With a British patent, 150kg tungstate acid and 125kg water were made into a tungstate slurry, which was slowly added with 60L 25% ammonia at a pH of 7 and stirred at 80℃. In the preparation of wolfram, the amount of water added is different, which also affects the subsequent crystallization of ammonium secondary wolfram. The thinner the wolfram, the larger the crystallization particles, the more water-soluble ammonium secondary wolfram and the lower the tungsten recovery.
1.2 Solvent extraction method
In the early 1970s, China began to study the use of organic amine extractant to extract tungsten, and now it has been widely used in industrial production. Solvent extraction has the advantages of short production cycle, stable process, high quality, safe and convenient operation, low cost and so on. It can be applied to a wide range of minerals as well as tungsten slag. Solvent extraction can replace scheelite precipitation, acid decomposition and aqueous ammonia dissolution in classical processes, which can not only improve tungsten recovery rate, but also facilitate the continuous and automatic production process.
After full contact with each other, stand phase separation and filtrate, the tungsten recovery rate reaches 99%, the organic phase load is adjusted with ammonia water, then add sulfuric acid slowly to PH 1.5, stir at 20℃ for 2h, then filter, and then extract with caustic alkali. In this way, the key problem of crystal precipitation in the extraction process can be solved. It can also be extracted with primary, secondary or quaternary amines, a strong base that also extracts tungsten well at a PH of 6 to 8. The process of tertiary amine extract tungsten is similar to that of other countries. This process can not only treat low concentration sodium tungstate solution obtained from low-grade ore, but also can treat higher concentration sodium tungstate solution.
2. Crystallization of ammonium secondary tungstate
Ammonium tungstate solution is unstable, and with the volatilization of ammonia, secondary ammonium tungstate crystals with low solubility are precipitated. Usually there are neutralization, crystallization and freeze – drying. Recently there are new methods such as solvent extraction and spray drying. The specific technology of the new spray drying crystallization method has not been reported, so it is suitable to prepare the pre-alloy powder with low alloying elements.
2.1 evaporation method
Evaporation crystallization has always been favored by industrial production, because of its simple operation, low cost and stable particle size and quality, the purity of secondary ammonium tungstate crystallization can be guaranteed by controlling the evaporation crystallization rate. However, the evaporation crystallization method requires a long time, energy consumption is too long, production cycle is long, and if the product purity is high, the crystallization rate must be controlled low. Ammonia concentration decreases when ammonium tungstate solution is heated. The concentration of the solution increased gradually, and the crystallization of ammonium secondary tungstate was precipitated.
2.2 Combination of neutralization and evaporation
The combination of neutralization method and evaporation method has the advantages of high crystal purity, easy particle size control and fast crystallization speed. Firstly, the solution was vaporized in a constant temperature water bath at 50 ~ 70℃ for 1 ~ 2h, and then neutralized with hydrochloric acid until pH= 7.0 ~ 7.5. Then, the solution was evaporated to a mother liquor density of 1.08 ~ 1.109 / mL, and cooled by feeding. By this method, the secondary ammonium tungstate crystals containing 5 crystal water can be prepared. There is also a direct neutralization of ammonium tungstate solution at 70℃ to pH= 6.5 ~ 7.5, while evaporation and neutralization, resulting in the crystallization of 7 water.
2.3 Freeze drying method
Ammonium tungstate solution: liquid nitrogen = 1:2 ~ 3 (volume), injected into the enamel plate of liquid nitrogen in advance, stir well, and the solution after the injection of paste products are moving rapidly along with enamel tray drying equipment, under the certain vacuum sublimation in addition to the water, then get white amorphous structure and loose the containing water of crystallization of APT. Compared with other crystallization methods, the freeze-drying method has unique advantages, which can obtain uniform fine powder or various uniform composite powder, and can better control the impurities and components of finished products.
2.4 Solvent extraction method
The solvent extraction method is to obtain the secondary ammonium tungstate crystal by controlling the extraction ratio and the acidity of the extraction solution. In addition, the mother liquid can be recycled and used in the human liquid, and the reverse extract can be used repeatedly. This method can greatly shorten the production cycle, reduce energy consumption and reagent consumption, but more one-time investment. During extraction, the pH of ammonium tungstate solution was generally controlled to be greater than 9, and the concentration of W03 was between 100 and 2009 / L. The mixture of material liquid and extractant EPA or ENA 87-1 was oscillated to extract NH4+ from the material liquid, so as to reduce the pH value of the material liquid. PH value of control prostrated liquid was 6 ~ 7. After standing phase separation, the extraction liquid was boiling away at 60℃ for 5h, and then the secondary ammonium tungstate crystal could be prepared, with the crystallization rate up to 96%.