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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The thermal decomposition of ammonium paratungstate hydrate was investigated in dynamic air and inert gas (nitrogen and argon) atmospheres by simultaneous thermo-gravimetric analysis (TGA), differential thermal analysis (DTA) and mass spectroscopy (MS). The intermediate and end products obtained during thermal decomposition of ammonium paratungstate hydrate were characterized by X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR).

It was found that the kind of gas was not effective on the thermal decomposition mechanism of ammonium paratungstate hydrate below 665K. In the first decomposition step, anhydrous ammonium paratungstate was obtained by dehydration. In the second decomposition step, only deammoniation occurred and ammonium hydrogen tungstate was obtained. During the third decomposition step, dehydration, deammoniation and reduction of some tungsten atoms to lower valences were observed. At this stage, first, ammonium hydrogen tungsten oxide bronze and later ammonium tungsten oxide bronze were formed by two serial reactions. In the fourth step which occurs after 665 K, ammonium tungsten oxide bronze formed stoichiometric tungsten oxide in air atmosphere and tungsten suboxide in inert gas atmosphere.

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Tungsten metal products are made using a chemically refined intermediate ammonium paratungstate (APT) product. APT is produced by processing tungsten concentrates containing scheelite (calcium tungstate) or wolframite (iron/manganese tungstate). The processing steps usually include several impurity removal steps to make an APT product of the desired purity.

In the process for recovering tungsten from a tungsten concentrate wherein the tungsten content of said concentrate is separated therefrom, purified and converted into solid ammonium paratungstate (APT) by crystallization from ammoniacal solution, of which a portion is off-grade material, the improvement for recovering said off-grade APT with a minimum number of processing cycles which comprises mixing said off-grade APT with an ammonia solution containing about 2.5% to about 12%, by weight, of NH3 and autoclaving said mixture at a temperature above the boiling point thereof for a time sufficient to dissolve substantially all of said APT and recrystallizing solid APT from the resulting solution to yield a solid APT product.

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Ammonium metatungstate(APT) solutions are prepared from relatively insoluble APT by (a) forming an aqs. suspension containing up to 22% wt APT, calculated on the content of WO3; (b) maintaining the pH of the aqs. suspension at 1-75-3.5 by the addition of hydrogen ions, and (c) maintaining the temperature of the suspension at 50-120 degrees C until the APT has been dissolved and converted into metatungstate. The process enables ammonium metatungstate solutions to be more cheaply prepared, as ammonium paratungstate is cheaper than ammonium metatungstate. The process is convenient and efficient, and can be carried out using the commercially available refracting form of ammonium paratungstate. - Concentrated solutions of the metatungstate can be easily prepared.

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(a) providing an aqueous solution of ammonium paratungstate(APT);

(b) adding aqueous potassium hydroxide solution to the aqueous solution of APT as a doping substance, wherein the mole ratio of potassium to ammonium is 0.1 to 10, thereby forming an ammonium potassium paratungstate;

(c) converting the ammonium potassium paratungstate to potassium-doped tungsten blue oxide;

(d) reducing the doped tungsten blue oxide to doped tungsten powder; and

(e) compacting the doped tungsten powder and drawing doped tungsten wire from the compacted powder.

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A process for producing APT from a heavy metal alloy containing tungsten comprising introducing the heavy metal alloy into an electrolytic cell as an anode, said cell comprising an inert cathode and an aqueous electrolyte solution comprising ammonium hydroxide and ammonium nitrate, passing a direct electric current through said cell, said ammonium hydroxide and ammonium nitrate being maintained at a suitable concentration for oxidizing tungsten and forming ammonium paratungstate. After the desired concentration of ammonium paratungstate is reached in the electrolyte solution, the reaction may be stopped and the electrolyte withdrawn. The resulting ammonium paratungstate may be purified by conventional methods known in the art.

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When cutting tungsten carbide cutting tools, tungsten carbide cutting tools for cutting the amount of wear has a significant impact, especially in cutting speed. Different cutting speed on the wear of tungsten carbide cutting tools have different variations. In general, the greater the cutting speed, the greater the amount of flank wear, the more severe tools wear. Uncoated tungsten carbide tools in high-speed cutting, the rapid rise in the amount of flank wear, cutting the distance is very short, but the amount is much larger than the amount of wear and wear low-speed cutting. When the cutting speed reaches 100m / min or more, the coating due to friction during cutting off, the substrate is exposed to participate in a cutting tool. Lead to increased wear of tungsten carbide cutting tools, and ultimately failure.

Different grades of tungsten carbide cutting tools wear is as cutting speed increases. Low speed cutting with adhesive wear as the main wear mechanism, high-speed cutting wear and diffusion wear oxide as the main wear mechanism. This is because with the increase in cutting speed and tools - chip contact surface temperature with increasing pressure, knife - contact debris from sliding contact between the adhesive into contact. Under the adhesion force in the tools and workpiece contact surface friction generated by the process, the tools material fine particles from peeling or cutting. When the cutting temperature reaches 800 ~ 900 ℃, decomposition occurs between the workpiece and the tools element and the diffusion element to each other, the strength of the tools is reduced, resulting in tungsten carbide cutting tools wear or breakage failure.




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