Tungsten carbide powder is the intermediate in the line from W powder to cemented carbides.  It can be produced from different raw materials and by different processes.  By far the biggest percentage is manufactured by the conventional method - carburization of tungsten powder - and covers the widest range of powder qualities in regard to average particle size (0.15–12µm).  All other methods in use yield very fine or very coarse powder grades.

The conventional process of carburisation comprises mixing of the respective tungsten powder of desired particle size with high purity carbon (lamp black or graphite) and reacting at temperatures between 1,300 to 1,600°C in hydrogen atmosphere.  The average particle size and particle size distribution of the original tungsten powder determine size and distribution of the WC powder.

The final carbon content of the WC Powder depends on the production mode of the hardmetal producer and is one item of the rigid specification, and varies from slightly sub-stoichiometric to stoichiometric (6.13% C) to slightly over-stoichiometric.  But not only is the carbon content specified but also a series of physical properties including: average particle size, particle size distribution, apparent (bulk) density and homogeneity.

High temperature carburised WC powders (1,700–2,200 °C) are usually coarse 10 to 50µm, but sometimes also 5 to 10µm grades are treated that way. The percentage of high temperature WC is small.

Tungsten carbide powder is packaged in sealed polyethylene-lined steel drums.

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Technical tungsten powder qualities are prepared by hydrogen reduction and are available in average particle sizes from 0.1 (100nm) to 100µm.  The reduction process is, in some respects, unique.  It offers the possibility to produce tungsten powder of any desired average particle size within the above limits only by changes in reduction conditions.

The whole palette of particle sizes finds applications in cemented carbide production.  The main portion of tungsten powder is directed to that manufacture.  Starting tungsten powder for ductile tungsten and powdermetallurgically produced tungsten alloys covers particle sizes between 2 and 6µm.  Extremely coarse powder gained by screening to separate any finer particles has excellent flow characteristics and is used in plasma spraying.

The purity of the tungsten powder is of particular importance in all applications and is mainly influenced by the purity of the original APT. Typical upper limits of foreign element concentrations in µg/g are:

Co, Cu, Mg, Mn, Pb ≤ 2

Al, Fe, Ni, Si, Ca, Cr, Sn ≤10

Mo ≤ 20

The crucial physical properties are average particle size, particle size distribution, apparent, tap and compact or green density, specific surface area, degree of agglomeration and morphology. They are to a certain extent related to each other and can be influenced between limits by the oxide properties and the reduction conditions.

Tungsten powder customers have in most cases very rigid specifications.

Tungsten metal powder is packaged in sealed polyethylene-lined steel drums.

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Intermediates, such as tungsten trioxide, tungsten blue oxide, tungstic acid, and ammonium metatungstate can be derived from APT as shown below, either by partial or complete thermal decomposition or by chemical attack.

Tungsten Trioxide (WO3)

Tungsten trioxide is almost exclusively manufactured by calcination of APT under oxidising conditions (in air).  WO3 is one of the most important, highly pure intermediates for the production of other tungsten compounds including tungsten metal powder.  In the latter application, it was substituted to a large extent by tungsten blue oxide. Because of its bright yellow colour it is used as a pigment in oil and water colours. It is employed in a wide variety of catalysts, most recently for the control of air pollution and industrial hygiene (DeNOx).

Tungsten trioxide particles are pseudomorphous to APT; which means the particles have the same shape and size as the former APT crystals, but consist of very small WO3 grains.  The yellow powder is packaged in sealed polyethylene-lined steel drums and individual packaging (20 to 50kg).

Tungsten Blue Oxide (TBO; WO3-X)

TBO is manufactured by calcination of APT under more or less reducing conditions which vary from producer to producer. TBO is not a chemically defined compound, but consists of various different constituents, like trioxide, tungsten bronzes and different lower tungsten oxides. The relative amount of these compounds in TBO depends on the calcination parameters, (the parameter x typically varies between 0.01 and 0.10).

TBO is the most important precursor in the line from oxide to W and WC powder. The colour varies between deep dark blue to blue, faint blue and green blue. Also the TBO particles are pseudomorphous to the original APT crystals, as described for tungsten trioxide.

Tungstic Acid H2WO4.nH2O

Tungstic acid, formally the most important intermediate in tungsten chemistry, is now exclusively manufactured from APT, in order to make use of the high purity APT level.  For that purpose an aqueous APT slurry is treated with hydrochloric acid and tungstic acid is precipitated, which is then filtered, washed and dried.  Tungstic acid has a very high active surface and is only used in small quantities for special purposes such as the production of ultrafine W and WC powders and tungsten chemicals.

Tungstic acid is a fine yellow powder and is packaged in sealed polyethylene-lined steel drums.

Ammonium Metatungstate (NH4)6[H2W12O40].3H2O

Ammonium Metatungstate (NH4)6[H6W12O40].3H2O has gained increasing usage for a variety of applications, especially chemicals and catalysts, because of its excellent solubility in water. The usual commercial product contains 3 to 4 molecules of water. On an industrial scale, it is obtained by partial thermal decomposition or partial replacement of ammonium ions by hydrogen ions using selective ion exchange and subsequent evaporation.

AMT is a white crystallised powder. Between 200 and 300°C, it converts to the anhydrous form. Further decomposition leads to WO3. At 80°C, 2,200 g WO3/l are dissolved in water.

AMT is used for the preparation of heteropoly acids, which consist of inorganic oxyacids of phosphorous or silicon and that of tungsten. Such compounds are attractive catalysts for many kinds of organic reactions.

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Ammonium Paratungstate (NH4)[H2W12O42].4H2O is the most important precursor for the majority of tungsten products. Exceptions are only products of melting metallurgy and Menstrum WC produced directly from ore concentrates.

All other intermediates such as tungsten trioxide, tungsten blue oxide, tungstic acid and ammonium metatungstate can be derived from APT, either by thermal decomposition or chemical conversion. 

APT is a white crystallized powder having average crystal size between 30 and 100 µm. Especially crucial for the quality is the purity. Typical levels of todays commercial APT are (upper limits in µg/g):
Al 1-7, As 5-10, Bi 0.5-1, Ca 1-10, Co 1-10, Cr 1-10, Cu 1-3, Fe 3-10, K 2-10, Mg 1-7, Mn 1-10, Mo 5-30, Na 5-10, Ni 1-7, P 5-7, Pb 1-5, S 5-7, Si 1-10, Sn 1-10, Ti 3-10, and U 3-10. APT is packaged in polyethylene-lined drums or woven bags up to 1 ton.

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Marketable ore concentrates of scheelite and wolframite (hübnerite, ferberite) contain typically 65 – 70 % WO3.  Off-grade concentrates having lower WO3 concentrations are seldom on the market and if so, only for a lower price.  In fully integrated companies lower grades (6-40% WO3) are often preferred, because upgrading to high concentrations is mostly combined with a lower yield.

Concentrates are packaged either in large polyethylene “big bags” (1 to 2 tons) or, alternatively, in steel drums and individual packaging (40 to 200kg).

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Recently, Ministry of Commerce has published the first batch of tungsten products export quotas, and announced name lists of 13 export trade enterprises and 16 export supply enterprises. Specifically, all export trade enterprises are the same with the ones in 2012 while the latter are 1 less than 2012.

Tungsten was originally considered a metal that remains stable in soil and did not dissolve easily in water. However, it is now a growing concern to U. S. Environmental Protection Agency (EPA) and the U. S. Department of Defense (DoD) because recent research indicates that tungsten may not be as stable as was indicated in earlier studies. Furthermore, varying soil properties such as pH may cause tungsten to dissolve and leach into the underlying aquifer (ATSDR 2005). Currently, little information is available about the fate and transport of tungsten in the environment and its effects on human health. Research about tungsten is ongoing and includes health effects and risks, degradation processes, and an inventory of its use in the defense industry as a substitute for lead-based munitions. This fact sheet provides basic information on tungsten to site managers and other field personnel who may be faced with tungsten contamination at cleanup sites.

What is tungsten?

Tungsten (also known as Wolfram and represented by the letter W in the periodic table) is a naturally occurring element that exists in the form of minerals or other compounds but typically not as a pure metal (ATSDR 2005; NIOSH 2007).

Wolframite ([FeMn]WO4) and Scheelite (CaWO4) are two common minerals that contain tungsten (Koutsospyros et al. 2006).

Based on its purity, the color of tungsten may range from white for the pure metal to steel-gray for the metal with impurities. It is commercially available in a powdered or solid form (ATSDR 2005; NIOSH 2007).

The melting point of tungsten is the highest among metals and it resists corrosion. It is a good conductor of electricity and acts as a catalyst in chemical reactions (ATSDR 2005; Koutsospyros et al. 2006; Massachusetts DEP 2006).

Tungsten powder is highly flammable and may ignite instantly on contact with air. Tungsten also may cause fire or explosion on contact with oxidants (ATSDR 2005; NIOSH 2007).

DoD has used tungsten as a replacement for lead in bullets and other ammunition since 1999 (Massachusetts National Guard 2006).

Tungsten ore is used primarily to produce tungsten carbide and tungsten alloys, which are used in many general welding and metal-cutting applications, in making drilling equipment for oil wells, and in operations within the aerospace industry. Tungsten metal is also used to produce lamp filaments, X-ray tubes, dyes, and paints for fabrics (ATSDR 2005; Koutsospyros et al 2006).

Under the U. S. Army's Green Bullet program, nearly 88 million tungsten bullets were produced; 33 million remain unfired and available for use at training ranges across the country. Currently, the Army Environmental Center is looking at training ranges to evaluate the presence of various chemicals and other potential contaminants, including tungsten (Inside EPA 2007).

What are the environmental impacts of tungsten?

Tungsten is a common contaminant at industrial sites that use the metal and at DoD sites involved in the manufacture, storage, and use of tungsten based ammunition. It is also found in detectable amounts in municipal solid waste and landfill leachate because of its use in common household products such as light bulbs (Koutsospyros et al. 2006).

Tungsten particles may be present in air as a result of mining, weathering of rocks, or industrial applications that involve tungsten. These particles may settle on soil, water, or other surfaces and can be deposited through rain or other forms of precipitation (ATSDR 2005; MA DEP 2006).  Tungsten is considered a “lithophilic” element, based on its strong soil binding capacity and its insolubility in water. Sorption coefficients increaseat lower pH values, indicating lower mobility of tungsten (Koutsospyros et al. 2006).

Increased acidification and oxygen depletion ofsoils from dissolution of tungsten powder have been shown to trigger changes in the soil microbial community, causing an increase in fungal biomass and a decrease in the bacterial component (Strigul et al. 2005).

Studies indicate that an elevated pH in soil at a site may increase the solubility of tungsten by decreasing its sorption coefficient, which may cause it to leach more readily into the ground watertable (ATSDR 2005; ASTSWMO 2008).

Tungsten has been detected at six National Priorities List (NPL) sites (ATSDR 2005).

In 2006, the assumed stability of tungsten in the environment was questioned when tungsten was detected in ground water and above baseline levels in soil at a small arms range at Massachusetts Military Reservation (MMR) where tungsten nylon bullets were being used. The use of these bullets was then suspended (ATSDR 2005; EPA 2010b; Massachusetts National Guard 2006).

What are the health effects of tungsten?

Toxicological information on tungsten and its compounds is limited (Koutsospyros et al. 2006).

Occupational exposure is considered the most common scenario for human exposure to tungsten and its compounds. Inhalation, ingestion, and dermal and eye contact are the possible exposure pathways (ATSDR 2005). Occupational inhalation exposure to tungsten is known to affect the eyes, skin, respiratory system, and blood (ATSDR 2005). Tungsten may cause irritation to eyes, skin and throat; diffuse interstitial pulmonary fibrosis; loss of appetite; nausea; cough; and changes in the blood (NIOSH 2007).

Tungsten was included as part of EPA’s 2008 Integrated Risk Information System (IRIS) agenda. Toxicity is currently being assessed (EPA 2010a).

Studies on rats have shown that oral exposure to tungsten caused post-implantation deaths and developmental abnormalities in the musculoskeletal system. Exposure of pregnant rats to sodium tungstate resulted in fetal death (NIEHS 2003).

Tungsten has not been classified for carcinogenic effects by the Department of Health and Human Services (DHHS), the International Agency for Research on Cancer (IARC), or EPA (ATSDR 2005).

Are there any federal and state guidelines and health standards for tungsten?

A federal drinking water standard has not been established for tungsten.

The National Institute for Occupational Safety and Health (NIOSH) and the American Council of Government Industrial Hygienists (ACGIH) have established a recommended exposure limit (REL) of 5 milligrams per cubic meter (mg/m3) as the time weighted average (TWA) over a 10-hour work exposure and 10 mg/m3as the 15- minute, short term exposure limit (STEL) for airborne exposure to tungsten (ATSDR 2005; ASTSWMO 2008).

The Occupational Safety and Health Administration (OSHA) recommends an exposure limit of 5 mg/m3to insoluble compounds of tungsten and a 1 mg/m3limit of exposure to soluble compounds in construction and shipyard industries (ATSDR 2005).

Massachusetts has established action levels of 1 to 2 parts per million (ppm) in soil and 15 ppm in groundwater (ASTSWMO 2008).

What detection and site characterization methods are available for tungsten?

NIOSH Method 7074 – flame atomic absorption spectroscopy has a detection limit of 0. 1 mg/m3for insoluble forms of tungsten and 0. 05 mg/m3 for soluble forms of tungsten in air (HSDB 2009; NIOSH 2007).

Other NIOSH methods known to be used for tungsten are Methods 7300 and 7301, involving inductively coupled argon plasma, atomic emission spectroscopy, each with a detection limit of 0. 005 mg/m3(NIOSH 2003a, b). Special sample treatment may be required for some tungsten compounds.  ID213 – inductively coupled plasma atomic emission spectroscopy (ICP-AES) has a detection limit of 0. 34 mg/m3 for tungsten in air (NIOSH 2007; OSHA 2007).

Tungsten in soil and water can be measured using the ICP-AES, ICP-mass spectrometry (ICP-MS), and ultraviolet/visible spectroscopy (UV/VIS) methods (ATSDR 2005).

Tungsten is not currently included on the list of recoverable metals using SW-846 Method 3051. However, the digestion method has modified to enhance tungsten recovery from soils (Griggs et al. 2009).

What technologies are being used to treat tungsten?

Treatment technologies to address tungsten contamination in environmental media are currently under development.

According to preliminary studies conducted by various research groups, potential treatment methods involve chemical recovery/soil washing and phytoremediation (Lehr 2004; Warminsky and Larson 2004).

"Ice electrode" is an innovative technology being evaluated for liquid media. This technology is based on the conventional electroplating technique and uses an electrode coated with a thin layer of ice. Tungsten ions adhere to the ice-coated electrode and can be removed by melting the ice (Lehr 2004).

Electrokinetic soil remediation is an emerging in situ technology for removal of tungsten from lowpermeability soils in the presence of copper and lead. A direct current is applied to contaminated soils using electrodes inserted into the ground (Braida et al. 2007).

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