The process of preparing ammonium metatungstate hydrate by thermal decomposition can be summarized as follows:
- Preparation of raw materials
Main raw materials
Ammonium paratungstate (APT), which is the common raw material for the preparation of ammonium metatungstate (AMT) by thermal decomposition method.
Auxiliary materials
Some additives, such as strongly acidic cation exchange resin, may be needed for promoting the conversion in the subsequent steps.
- Thermal decomposition process
Equipment preparation
Select appropriate heating equipment, such as a far-infrared rotary furnace or other high-temperature heating device, and ensure that the equipment can accurately control the temperature.
Temperature control
Put ammonium paratungstate into the heating equipment and control the thermal decomposition temperature in the appropriate range. According to the literature, the thermal decomposition temperature is usually between 200～280℃, and there are also some data pointing out that the suitable temperature range is between 210℃-265℃. The temperature should be chosen to ensure that ammonium paratungstate can decompose sufficiently, but without generating insoluble tungsten oxide.
Pyrolysis time
The pyrolysis time depends on the different equipment and operation methods, but is generally controlled at about 1 hour. In this process, ammonium paratungstate will lose part of ammonium and water of crystallization and transform into ammonium metatungstate.
Dissolution and transformation
Dissolving pyrolysis product
Dissolve the pyrolysis product with pre-heated water, the solid-liquid ratio is generally about 1:7. During the dissolving process, the temperature should be raised to 90℃-98℃ to ensure that the pyrolysis product can be fully dissolved.
Adjustment of pH value
During or after dissolution, it may be necessary to add an appropriate amount of additives (e.g., strongly acidic cation exchange resin) to adjust the pH of the solution. According to the literature, the pH should be controlled in the range of 3.5-4.5 to promote the conversion of residual ammonium paratungstate.
Holding transformation
After adjusting the pH value, keep the solution at appropriate temperature (such as 95 ℃) and keep warm for a period of time (such as half an hour) to ensure that the residual ammonium paratungstate is completely converted into ammonium metatungstate.
Crystallization and drying
Evaporation crystallization
The transformed solution is subjected to evaporation and crystallization to gradually increase the concentration of ammonium metatungstate and precipitate crystals. The temperature and evaporation speed should be controlled during the evaporation process to avoid oversized crystals or incomplete crystallization.
Filtration and washing
The precipitated crystals are filtered and washed to remove impurities and residual solution.
Drying
Dry the washed crystals at a suitable temperature (e.g. 80°C) to remove residual water and solvent. The dried crystals are ammonium metatungstate hydrate product.
- Product collection and inspection
Product collection
Collect and package the dried ammonium metatungstate hydrate product.
Quality Inspection
Conduct quality inspection of the product, including the determination of purity, particle size, moisture content and other indicators to ensure that the product meets the requirements of the relevant standards.
The process of preparing ammonium metatungstate hydrate by thermal decomposition method includes raw material preparation, thermal decomposition, dissolution and transformation, crystallization and drying, as well as product collection and inspection. During the preparation process, the conditions and parameters of each link should be strictly controlled to ensure the quality and stability of the product.
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