Chemistry: CaWO4, Calcium Tungstate

Class: Sulfates

Subclass: Tungstates

Uses: An important source of tungsten, rarely cut as gemstones and as mineral specimens.

Scheelite is an important ore of tungsten which is a strategically important metal. Scheelite is named for the discoverer of tungsten, K. W. Scheele. Although most of the world wide production of tungsten comes from the mineral wolframite, scheelite is especially abundant in the US and provides the United States with most of its supply.

Scheelite is a popular mineral for collectors. It forms perfect tetragonal dipyramidal crystals that look very much like octahedrons. These pseudo-octahedral crystals are sometimes truncated with minor pyramids, but only on the top and/or bottom points of the crystal; giving evidence of their true symmetry. Other minerals that form pseudo-octahedral crystals similar to scheelite include wardite, anatase and powellite.

 

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When tungsten carbide cutting tools are employed in machining operation, the entire process of chip formation comprises primary shear, secondary shear and fracture process.

The primary shear of chip formation process is also referred to the workpiece material’s plastic deformation, which is severe and could be found in front of the cutting edge where the chip body is  delineated from a large amount of workpiece material. For the secondary shear, the chip slides over the rack face of the tungsten carbide tool. In the final separation stage, a fracture process takes over.

It is shown that the relative ease of the chip formation process is determined by the workpiece material’s flow properties. In addition, besides the existing conditions, the varied chips’ size and shapes are also under the control of factors like workpiece ductility, inclusion characteristics and etc..


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Chipping

It is known that during the machining operation, when sudden loads or thermal shocks occur, there is a potential for the tungsten carbide inserts to suffer from severe damage due to cracking or chipping.

In some comprehensive model, the researchers have described in details, the toughness of tungsten carbide is strongly influenced by the binder composition. It has been pointed out that the toughness of WC-Co is superior to that of TiC-Ni, in addition, the problem of tungsten carbide inerts failure due to chipping is particularly severe for alumina based tools, but with zirconia additions, the toughness of  tungsten carbide tool could be markedly improved.


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Abrasive Wear

The tungsten carbide inserts’ abrasive wear, as one of the major mircromehanisms, which is referred to the wear that caused by the tool material removal. It has been pointed out that the removal of tool material is result of scoring action of hard phase particles or the workpiece material’s inclusions. In addition, the abrasive wear process can be identified as microchipping of the tool because of rapid increases in force exerted over a localized area.

Specially, in practice use, the details have been described by the researchers, there is a clear correlation between increased tool wear and increased amounts of abrasive particles in the workpiece.


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Under a certain cutting condition, it could be examined that the tungsten carbide inserts’ tool wear is result of variety of mechanisms, and each one of which owns ability to dominant wear mechanism. In general, for the tungsten carbide inserts, the main micromechanisms of tool wear include, adhesive wear, abrasive wear, dissolution wear and oxidation wear, corrosive wear, plastic dominated wear,chipping.

Adhesive Wear

During the chip formation process, it has been pointed out that the chips would slide over the rack face of cutting tool, and it cleans the surface to form an oxides and contaminants free area. But it should be noticed that, for these surface, there is a potential to become one continuous adhesive junction, and when this junction is broken due to the flow of chips, some part of the tool material is removed. It has been found that, especially during steel machining, with more brittle tungsten carbide cutting tools, the adhesive junction is able to result in the whole tungsten carbide particle being plucked from the tool.


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Ammonium metatungstate, also known as AMT, is a tungsten chemical in the form of highly soluble hydrated crystals, solubility is 303.99/loogH:O when 20℃, aqueous fairly stable. It is white or slightly yellow crystals which has no harmful components and slightly acidic.

Neutralization method with secondary ammonium metatungstate and Nitrate as the raw material purchase ammonium metatungstate: secondary ammonium metatungstate deionized water and ammonium citrate with WO3 12%~ 30% concentration of slurry. According to =1:20: nitrate ammonium paratungstate by adding 3% ~ 5% of dilute nitric acid to ammonium paratungstate slurry, control ratio of solid to liquid 1: (3 ~ 4), the Ph value of 2 ~ 4, heating and dissolving solution, the relative density of 1.18 ~ 1.20, heating to 80 ~ 90 ℃, constant temperature and constant volume control Ph value, the stirring reaction for 4H hours, aging, concentrating the supernatant to WO3 content is about 50-, filtration to remove insoluble, liquid concentration, cooling crystallization, continue to 80 ℃ drying.



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The thermal decomposition of ammonium paratungstate by preparing ammonium metatungstate: ammonium paratungstate and a small amount of citric acid catalyst after mixing into the far infrared, rotary furnace, controlled thermal decomposition temperature 200 ～ 280 ℃, decomposition of about 1 h. The removal of ammonia and water, ammonium paratungstate weightlessness rate was 5% ~ 6%.

The thermal decomposition products immediately after the slurry, adding proper amount of ammonia, Ph value is adjusted in the 3 to 4 range, at 90 ℃ digestion slurry. After evaporation to ammonium metatungstate content of 50% can be used as a liquid product to sell, crystallization, when the evaporation to the original volume of 20% filtering and separating, granulating product. Also can be evaporated and concentrated liquid product is prepared by spray drying. As of 45% ~ 50% ammonium metatungstate by high-speed centrifugal rotary spray dryer, imports of hot air temperature 250 ～ 300 ℃, outlet temperature 90 ～ 95 ℃, the obtained product is a spherical crystal, particle size is 200 mesh accounted for 95%, free water <1.5%.



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Tungsten pentachloride is dark green crystal flash, break down easily when exposed to air. There are two methods for its producing: one is in the hydrogen to restore tungsten pentachloride, and the other produce by six tungsten chloride and tetrachlorethylene reaction.
Using hydrogen reduction to producing tungsten pentachloride and the chemical reaction equation is: 2WCl6 + H2 → 2WCl5 + 2HCl. First, heating tungsten hexachloride at 410 ~ 425 ℃ in quartz tube will be at the same time introducing hydrogen to restore. During the reaction, there generating WClO and cheap WCl4, WCl2 and others, so it needs to sublimation purification in a stream of nitrogen, and then stored in a sealed tube.
By tungsten hexachloride and tetrachlorethylene reaction to obtain tungsten pentachloride the  chemical reaction equation is: 2WCl6 + C2Cl4 → 2WCl5 + C2Cl6. This reaction equipment is same with tungsten tetrachloride’s. Firstly making reaction equipment in vacuum state, and ready for 4 ~ 7g of tungsten hexachloride and tetrachlorethylene 25ml, and irradiation of 100W bulb at 100 ℃ the reaction is carried out in an oil bath for 24 hours.

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Tungsten pentachloride is kind of flash dark green crystal, its physical characteristics as follows: formula: WCI5, molecular weight: 361.2, the relative density: 3.875, melting point: 243 ℃, boiling point: 275.6 ℃, soluble in acetonitrile, benzene, chloroform, carbon disulfide and other organic solvents. Tungsten pentachloride CAS No. is 13470-14-9.
Tungsten pentachloride are sensitive to moisture and water, if placed in the air, which generates green surface film. Hydrolyzed in water will produce blue oxide (W20O58), but in the air can produce oxychloride (WOCl4) by heating. Besides tungsten pentachloride can reacted with amine will easy to generate amino compound. In most polar solvents can decompose, and slightly soluble in carbon disulfide and other non-polar solvents. To produce tungsten pentachloride can use hydrogen reduction tungsten hexachloride at 250 ~ 280 ℃ or use red phosphorus reduction tungsten hexachloride at 380 ~ 400 ℃. Or it can obtain by tungsten tetrachloride disproportionation reaction.

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After hydrolysis VOSO4 and NaSO4 and calcination obtain tungsten-doped vanadium dioxide powder, and then can use vanadium dioxide and tungsten-doped vanadium dioxide powder to producing thermochromism coating. Producing process as follows:
1. First putting VO2 powder, xylene, oil dispersants and ceramic milling beads in a planetary ball mill, 650r/min grinding 20 ~ 30min to obtained VO2 suspension.
2. The suspension was mixed with resin to produce coating with solid content 40% which help VO2 can maintain long-term stable storage.
3. Using a wire bar to make rein form a film and then dried at room temperature. The film thickness is 60 to 70 um.
During the producing process the longer grinding time, grinding grain trend has grown, easy aggregates and VO2 phase transition performance will continue to decline. Comprehensive experimental data found grinding time is about 20 ~ 30min can get the smallest average particle size of 275nm. Prepared coatings below TC without significant spectrum absorption or transmission peaks， at TC above, the coating in near-infrared band (1000 ~ 2500nm) exhibit low permeability, 400 ~ 1000nm band transmission slight increase, this is because when the VO2 powder metal-semiconductor phase transition the band gap change cause transmittance change.

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