Effluent-Free Manufacture Ammonium Paratungstate
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- Category: Tungsten Information
- Published on Monday, 02 March 2015 14:31
- Written by zsq
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Within the framework of a review of the fundamental processes applied for the manufacture of ammonium paratungstate (APT) the main byproducts and wastes generated in these processes and their environmental impact are summarized.
A subsidiary process based on the electrodialytic recovery of NaOH for the elimination of the salt byproduct, which is generated in the largest quantities, is outlined together with the main aspects to be taken into account for large-scale industrial implementation.
The present stage of development and implementation of an APT production line for the processing of typical scrap material from the lighting filament industry is shown, as well as the oxidizing and dissolving process of compact and lumpy tungsten alloy scrap (Cu-W, Fe-Ni-W) and hard metal scrap (Co-WC, Co-WC-Ta(Nb)C-TiC).
Several as yet unexploited potentials are sketched which could, from scientific and engineering standpoints, be interesting for the development of more efficient intermediate manufacturing processes for APT and tungsten or process operations for a wide variety of raw materials.
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Dsolving of ammonium paratungstate tetrahydrate
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- Category: Tungsten Information
- Published on Monday, 02 March 2015 14:29
- Written by zsq
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Ammonium paratungstate tetrahydrate, APT·4H2O dissolves rather slowly in aqueous ammonia at room temperature. Dissolution at higher temperatures increases the rate of dissolution, but the amount of dissolved tungsten decreases due to the lower solubility of ammonia at higher temperatures which results in a lower pH of the solution. The rate of dissolution was found to be independent of the crystal size. It is therefore concluded that the rate-controlling step occurs in the solution, and is probably H2W12O4210−→2HW6O215−. Calcination of APT·4H2O in air, nitrogen and hydrogen/nitrogen mixtures, into an X-ray amorphous product, is shown to be one way to improve the dissolution rate and to increase the dissolved amount of APT·4H2O. The highest solubilities can be obtained by calcining APT·4H2O in air or nitrogen at temperatures ranging from 250 to 310°C for approximately 2 h. When the temperature is too low or the residence time is too short some crystalline APT·4H2O will still exist; WO3 is formed when the temperature is too high or the residence time too long. It is supposed that the large H2W12O4210− structure is broken during the calcination process.
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Ammonium Paratungstate Tetrahydrate
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- Category: Tungsten Information
- Published on Monday, 02 March 2015 14:22
- Written by zsq
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Ammonium paratungstate tetrahydrate (NH4)10[H2W12O42]·4H2O (APT), a starting material of WO3 and tungsten production, has been subjected to a complex thermoanalytical study in air. The weight loss stages and the various heat effects have been followed by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) in flowing air up to 600 C. Meanwhile evolution of gaseous products has also been analyzed and monitored by both online coupled mass spectrometer (TG/DTA-MS) and infrared gas cell (TG-FTIR), in comparison. Besides that of the previously known H2O and NH3, evolution of two new gaseous products, N2O and NO, which had not been reported earlier, has been detected and traced by both evolved gas analysis (EGA-FTIR and EGA-MS) methods. These oxides of nitrogen are considered as catalyzed oxidation products of the released ammonia in air at 260–350 and 400–450°C. Thus, the two exothermic heat effects observed by DTA at 342 and 443°C have been assigned to ammonia combustion. Furthermore solid intermediate products have been structurally evaluated by both FTIR spectroscopy and powder X-ray diffraction (XRD). Based on significant changes in XRD patterns, a consequent formation of hexagonal ammonium tungsten oxide bronze and monoclinic WO3 are confirmed.
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Ammonium Paratungstate Hexahydrate
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- Category: Tungsten Information
- Published on Monday, 02 March 2015 14:25
- Written by zsq
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The crystallization of ammonium paratungstate tetrahydrate, ((NH4)10OH2W12O42·4H2O, APT·4H2O) from aqueous ammonium tungstate solutions is a key unit operation in current tungsten powder production. Ammonium paratungstate hexahydrate (APT·6H2O) can form under conditions where APT·4H2O forms as well. Based on this information it was postulated that APT·6H2O is metastable. Experiments were carried out to determine whether and under what conditions the hexahydrate is metastable. It was found that the hexahydrate is metastable at a solution concentration of approximately 300-230 g/kg WO3 and from approximately 90° to 96°C. APT·6H2O is not formed when APT·4H2O seeds are added prior to crystallization. It was found that the recrystallization of APT·6H2O into APT·4H2O is prompted by a decreasing water activity in solution during crystallization. The increase in solution temperature during isobaric crystallization also stimulates the recrystallization process. In industrial practice some residual crystals are always present in the crystallizer. Therefore, APT·6H2O will not form during industrial crystallization.
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APT Affect Extracting Tungsten from Scheelite
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- Category: Tungsten Information
- Published on Monday, 02 March 2015 14:15
- Written by zsq
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Ammonium paratungstate(APT) is a white crystalline salt of ammonium and tungsten, with the chemical formula (NH4)10(H2W12O42)·4H2O.A method for extracting tungsten from scheelite, the method comprising the steps of: 1) adding a mixed acid comprising sulfuric acid and phosphoric acid into a decomposition reactor, the mixed acid comprising 150-500g/L of H2SO4 and 15-35 wt. % of P2O5; 2) heating the mixed acid to a temperature of 70-100°C.; adding scheelite to the decomposition reactor and controlling a liquid-solid ratio at 3:1-8:1L/kg; allowing components in the decomposition reactor for reaction for 1-6 h, and filtering to obtain a filtrate; 3) supplementing the filtrate with sulfuric acid consumed in the reaction; 4) crystallizing the filtrate to obtain phosphotungstic acid crystals and mother liquor; 5) dissolving the phosphotungstic acid crystals in water to obtain a phosphotungstic acid solution; 6) transforming the phosphotungstic acid solution to an ammonium tungstate solution for preparing ammonium paratungstate (APT); and 7) supplementing the mother liquor with phosphoric acid and water to an initial level and returning the mother liquor for ore leaching.
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