The principle of preparing ammonium metatungstate by thermal decomposition can be clearly summarized as follows:
The basic principle of preparing ammonium metatungstate by thermal decomposition method
Thermal decomposition method is based on the thermal decomposition reaction of ammonium paratungstate (APT) at high temperature to produce ammonium metatungstate (AMT) and other by-products. The reaction, by controlling the temperature and time of thermal decomposition, causes ammonium paratungstate to lose part of its ammonium and water of crystallization, and is converted into ammonium metatungstate, which is easily soluble in water.
Detailed process of preparing ammonium metatungstate by thermal decomposition method
Thermal decomposition reaction
Reactants
Ammonium paratungstate, chemical formula (NH₄)₁₀[H₂W₁₂O₄₂]-4H₂O.
Reaction conditions
High temperatures, usually between 200 and 280°C, with some sources indicating a suitable temperature range between 210°C and 265°C. This temperature range is chosen to ensure that ammonium paratungstate can decompose sufficiently while avoiding the formation of insoluble tungsten oxide.
Reaction products
Ammonium paratungstate and other by-products such as ammonia and water vapor.
Thermal decomposition reaction
(NH₄)₁₀[H₂W₁₂O₄₂]-4H₂O → (NH₄)₆[H₂W₁₂O₄₀]-xH₂O + 4NH₃ + nH₂O. where (NH₄)₆[H₂W₁₂₂O₄₀]-xH₂O is ammonium metatungstate hydrate and x denotes the amount of water of crystallization.
Product Conversion
Dissolution
The ammonium metatungstate portion of the pyrolysis product exists in an amorphous form and needs to be converted to a soluble ammonium metatungstate solution by dissolution. This is usually done by dissolving the pyrolysis product with deionized water under heated conditions to convert the amorphous material to ammonium metatungstate in solution.
pH Adjustment
During or after the dissolution process, the pH of the solution needs to be adjusted to a suitable range (e.g., 3.5 to 4.5) to facilitate the conversion of the residual ammonium paratungstate. This step is usually realized by adding appropriate amount of acid or strongly acidic cation exchange resin.
Holding transformation
After adjusting the pH value, keep the solution at a certain temperature (e.g. 90～98℃) and hold it for a certain period of time (e.g. half an hour), so as to make the residual ammonium paratungstate completely transformed into ammonium metatungstate.
Crystallization and drying
Evaporation and crystallization
The converted solution is subjected to evaporation and crystallization, by controlling the evaporation speed and temperature, the concentration of ammonium metatungstate is gradually increased and crystals are precipitated.
Filtration and washing
The precipitated crystals are filtered and washed to remove impurities and residual solution.
Drying
The washed crystals are dried to remove residual water and solvent to obtain ammonium metatungstate hydrate product.
The principle of preparing ammonium metatungstate by thermal decomposition method is to generate ammonium metatungstate and other by-products by controlling the thermal decomposition reaction of ammonium sec-tungstate at high temperature. The thermal decomposition products are converted into high-quality ammonium metatungstate hydrate products through the steps of dissolution, pH adjustment, heat preservation and conversion, as well as crystallization and drying. This method has the advantages of high utilization of raw materials, simple process and stable product quality, and is widely used in industrial production.
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