There are various methods for the preparation of Ammonium Metatungstate crystals, and the common preparation methods are as follows.
Thermal decomposition method of ammonium metatungstate crystal preparation
Principle of thermal decomposition method
Ammonium paratungstate (APT) is thermally dissociated by heating, so that it loses part of ammonium and water of crystallization, and is transformed into ammonium metatungstate, which is easily soluble in water, and then the ammonium metatungstate crystals are obtained by evaporation and crystallization.
Steps of thermal decomposition method
Mix ammonium paratungstate (APT) and a small amount of citric acid catalyst fully, and then put it into the far-infrared rotary furnace.
Control the thermal decomposition temperature at 200-280°C, and the decomposition time is about 1 hour.
The weight loss of ammonium paratungstate was 5% to 6% excluding the ammonia and water produced.
The product of thermal decomposition was immediately formed into a slurry, replenished with appropriate amount of ammonia and adjusted the pH value at 3～4.
The slurry is boiled at 90℃, and it can be sold as liquid product when the content of ammonium metatungstate is 50% by evaporation and concentration; when the evaporation and concentration is 20% of the original volume, crystallization, filtration and separation, granulation can be obtained as ammonium metatungstate crystal.
Preparation of Ammonium Metatungstate Crystals by Neutralization Method
Principle of neutralization method
Taking ammonium sec-metatungstate and nitric acid as raw materials, ammonium metatungstate is prepared by neutralization reaction.
Steps of neutralization method
Add ammonium sec-metatungstate to deionized water and ammonium citrate to form a slurry containing WO₃ 12%-30% concentration.
Add 3%~5% dilute nitric acid to the ammonium sec-tungstate slurry at the ratio of ammonium sec-tungstate:nitric acid=1:20.
Control the solid-liquid ratio of 1: (3 ~ 4), pH value in the range of 2 ~ 4, heating and dissolution.
Make the solution relative density of 1.18 ~ 1.20, heating to 80 ~ 90 ℃, constant temperature and volume control pH value, stirring reaction for 4 hours.
After aging for a few hours, the clear liquid was concentrated until the WO3 content was about 50%, and filtered to remove insoluble matter.
The clear liquid continued to concentrate, cooling crystallization, drying at 80 ℃ to produce ammonium metatungstate crystals.
Preparation of Ammonium Metatungstate Crystals Tungstic Acid Method
Principle of tungstic acid method
Utilizing the reaction between tungstic acid and ammonia, ammonium metatungstate is prepared by adjusting the pH value and heating and concentration.
Steps of tungstic acid method
Dissolve tungstic acid with ammonia and add it to boiling deionized water.
Heat the ammonia under stirring to bring the pH to 8.
Add acetic acid while the solution is boiling, and then adjust the pH to 2 to 4 with dilute nitric acid.
Continue to heat and concentrate until the relative density is 1.5 to 2, then stop heating.
After cooling and filtering, the filtrate was added with ethanol and stirred thoroughly, settled and filtered.
Crystals are dried and crushed at 80 ℃ to obtain ammonium metatungstate product.
Preparation of ammonium metatungstate crystals solvent extraction or ion exchange method
Principle of solvent extraction or ion exchange method
Ammonium tungstate solution or sodium tungstate solution is used as raw material, and ammonium metatungstate solution is prepared by solvent extraction or ion exchange method, and then ammonium metatungstate crystals are obtained by evaporation and crystallization. As the specific steps of this method may vary depending on the raw materials and process conditions.
Preparation of Ammonium Metatungstate Crystals Solvent Extraction Method
Steps of solvent extraction method
Preparation of organic phase
Mix ammonium tungstate solution with an organic extractant such as di(2-ethylhexyl) phosphate (D₂EHPA) or tributyl phosphate (TBP) and kerosene solvent to form an organic phase.
Extraction of ammonium ions
The ammonium ions in the ammonium tungstate are extracted by the organic phase to reduce the pH of the solution to 2-4.
Boiling to generate ammonium metatungstate solution
After boiling process, ammonium metatungstate solution is generated.
After treatment, ammonium metatungstate is produced by concentration and crystallization, filtration and separation, drying and other steps.
Preparation of Ammonium Metatungstate Crystals Ion Exchange Method
Steps of ion exchange method
Heating ammonium paratungstate
Heat ammonium paratungstate at 130～140℃ for 1～8 hours.
Blending into slurry and boiling
Blend the product into a slurry and boil it at a temperature of 80～100℃ for 2～6 hours to produce ammonium metatungstate solution.
Boil the solution and control the pH
The acidity of the effluent needs to be controlled at pH 3.5 or so and boiled at 98℃ for 5 hours to convert ammonium tungstate into stable ammonium metatungstate solution.
Post-treatment by ion exchange method
AMT crystals are produced by evaporation and concentration, cooling crystallization or spray drying.
Characteristics of Ion Exchange Method
Ion exchange method can avoid the formation of APT and will not bring in new pollutants due to the relative simplicity of the process and convenient control. The method process can be continuous, so it has a large potential for development.
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