Ammonium paratungstate was prepared from tungsten alloy swarf employing anodic dissolution in sodium hydroxide using a packed bed configuration followed by chemical conversion into ammonium tungstate through tungstic acid. The alkaline tungstate solution was dialysed to recover sodium hydroxide, which could be recycled to the electrolytic dissolution. The influence of anodic current density and other parameters on dissolution/recovery was studied and discussed. It was observed that a current density of 600 mA/100 g scrap could be applied when the concentration of NaOH was maintained around 2.5 M. The energy consumption for anodic dissolution was 2.14 kWh/kg W with more than 90% recovery achieved. The exhausted scrap contained an alloy of nickel and iron with a very low percentage of tungsten. APT sample prepared through anodic dissolution in NaOH was characterized.

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The ammonium paratungstate is present in the ammoniated aqueous strip solution in an amount of from about 200 to about 300 grams per liter. Further, the ammonium paratungstate is substantially free of the impurities originally present in the brine.

The concentrated, purified ammonium paratungstate can be recovered from the ammoniated aqueous strip solution by any technique known in the art, such as evaporative crystallization, cooling the hot strip solution to allow the ammonium paratungstate to crystallize and the like. The crystallized ammonium paratungstate then can be recovered by filtration.

The filtrate of the filtration can be recycled to the two stage strip circuit of the secondary solvent extraction system where, after pH adjustment, the filtrate is reloaded with ammonium paratungstate at a temperature of from about 45C.to about 55° C. This procedure eliminates the need for additional evaporation stages and thus results in a substantial savings in equipment costs.

The ammonium paratungstate product recovered from the filtration of the strip solution can be dried using any suitable technique known in the art. Further, the ammonium paratungstate will be substantially free of impurities originally present in the brine, namely arsenic, boron, phosphorus, silica and other heavy metals.

The above described process provides an efficient and effective method for recovering concentrated, purified tungsten values (such as ammonium paratungstate) from a brine containing minor amounts of tungsten values (in the form of tungstic oxide) and substantial amounts of impurities without the use of substantial chemical costs, processing costs, or equipment modifications to obtain the desired result.

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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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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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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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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(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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The relative density of the blank (actual density and the theoretical density ratio) is greater than 90% when machinability, billet density at 92 ~ 94% good processing properties. Blooming hot rolling temperature between 1350 ~ 1500 ℃, the deformation process parameters blooming inappropriate choice, will produce stratification blank. Warm rolling start temperature of 1200 ℃, 8 mm thick hot rolled sheet, the warm rolling can reach 0.5 mm. Since tungsten plate deformation resistance, when the rolling roll barrel bending deformation, so that the sheet thickness unevenness in the width direction, or exchange when changing roll mill, the sheet may be due to non-uniform deformation of each part of cracking.

0.5 mm thickness of the sheet of plastic - brittle transition temperature or above room temperature is also, brittle sheets, should be 200 ~ 500 ℃ the sheet rolled into 0.2 mm. Late rolling, tungsten thin long pieces, in order to ensure uniform heating plate, often coated with graphite or molybdenum disulfide, heating plate is not only beneficial, but also the processing of lubrication.

Other Processing

The pipe can be sintered tungsten direct extrusion billets, extruded tube or slip squeeze tube has been spinning sintering. Spinning also produce tungsten shaped products. Large-diameter rods multi-extrusion or rolling production process.

Cutting

Tungsten hard and notch sensitive, cutting difficult, requiring the use of carbide cutting tools. To prevent cutting cracks, often the workpiece is heated to plastic - brittle transition temperature than cutting, and to strictly control the cutting operation program. Grinding tungsten requires a specific type of light grinding wheel, and needs to be cooled, otherwise it will crack. Thickness of 0.2 mm or more in front of tungsten sheet punching and shearing to be pre-heated over a certain thickness of the plate, not cut, often need to use the wheel cutting.




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W–Al₂O₃ nanostructured composite has been produced by mechanical milling of ammonium paratungstate (APT) and Al powder mixture at room temperature. The milled products have been characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The XRD and EDS results confirmed that direct reduction of APT to α-W took place after 25 min milling. It was observed that α→β phase transition took place after 24h mechanical milling. SEM micrographs showed that alumina matrix composite reinforced with W could be obtained by one step MSR (mechanically induced self-propagation reaction) process. X-ray diffraction peak broadening equation confirmed that the mean grain size of these particles was in nanometer range which gradually decreased with increasing the milling time.

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Non-isothermal kinetic (k, A and ΔE) and thermodynamic (ΔH, C_p and ΔS) parameters of ammonium paratungstate decomposition to the onset of formation of WO₃, were determined by analysing thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) curves recorded at various heating rates (1–30°C min⁻¹). Thermal events encountered throughout the decomposition course were due to three endothermic (at 120 ° C (I), 195 ° C (II) and 359 ° C (IV)) and two exothermic (at 265 ° C (III) and 386 °C (V)) processes. The events I–IV were accompanied by weight losses resulting from the elimination of volatile components [(NH₄)₂O, NH₃ and H₂O] leading to the formation of intermediate solid products (metatungstate and polytungstate), whereas event V was due to the crystallization of WO₃ in the immediate vicinity of its formation (at 370 ° C).

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