Testing Trace Bismuth in High Purity Ammonium Metatungstate -- Co-Precipitation-HG-AFS Method

Ammonium metatungstate (AMT) is an important intermediate compound of tungsten wet smelting process, with appearance of white and little yellow crystal. In recent years, as the requirement of tungsten product purity being higher and higher, for controlling and accurate detecting of impurity content in tungsten products, the required analysis of impurity element types gas been more and more. A study has put forward a method of co-precipitation-HG-AFS to testing trace bismuth in high purity ammonium metatungstate; HG-AFS refers to hydride generation atomic fluorescence spectrophotometry, which has shown to have advantages of high sensitivity, good selectivity and small matrix interference.

atomic fluorescence spectrometer image

Steps of co-precipitation-HG-AFS method to detect trace bismuth in high purity ammonium metatungstate are as follows:
1. Weigh 0.125g (Accurate to 0001g) of high purity (99.995%) ammonium metatungstate sample  in 50mL beaker, add a small amount of water, suitable amount of coprecipitation agent and KOH (The standard addition test requires adding standard solution in advance);
2. Carry out filtration after heated for 15min at low temperature, and wash the precipitation and the beaker twice by using KOH solution, again to wash twice with water;
3. Then goes on operation of coprecipitation twice, transfer to 25mL volumetric flask and add 5.0mL HCI, 2mL tartaric acid solution after the precipitation dissolved with hydrochloric acid solution (HCl), then diluted with water to scale, shake well to get mixture solution under test;
4. Test the ready solution by atomic fluorescence spectrometer;
5. Prepare series of different concentrations of bismuth standard solution, test the fluorescence intensity according to running program, and process data with computer, draw standard curve using monadic linear regression equation to calculate the content of bismuth.

The aim of choosing hydrochloric acid (HCl) as Current-carrying medium is to obtain bismuth fluorescence with high strength and stability, while, when the solution acidity is changing among 1~6mol/L, bismuth fluorescence value changed little, however the increasing acidity will help to eliminate the interference.

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Ammonium Metatungstate Hydrotalcite Catalyst

In recent years, solid acid alkali catalysis of gas-solid-phase reaction of alkylation of catechols has attracted attention due to the environmentally friendly economy and overcoming the environmental pollution, separation and recovery difficulties and equipment corrosion and other defects, compared to conventional synthesis methods, and has shown significant advantages in saving resources and improving economic efficiency. It is found that Al-Ti-P-O, Al-Ti-Si-P-O and silica-supported ammonium metatungstate (AMT/SiO2) catalyst exhibited high catalytic activity.

AMT hydrotalcite sample XRD spectra image

Hydrotalcite is a kind of anionic clay with layered structure, because of its special layered structure and high catalytic performance in acid-base catalysis, condensation, addition polymerization, hydrogenation and alcohol conversion reaction, hydrotalcite has attracted many concerns. The exchangeable property of hydrotalcite anions make all kinds of anions (such as metatungstate ion) can be introduced into the interlayer of hydrotalcite in succession, thus to form a large class of materials - pillared hydrotalcite layered material.

A study has pointed out a preparation method of ammonium metatungstate hydrotalcite catalyst, and studied its catalytic performance, while the preparation steps are as follows:
1. synthesis of hydrotalcite material, take the raw material of magnesium nitrate hexahydrate, aluminum nitrate and sodium hydroxide, and to control the final pH value among 8 ~ 10 of the system, then go on aging, filtration and washing to get hydrotalcite NO3-MgAl;
2. Weigh a certain amount of fresh prepared hydrotalcite NO3-MgAl to disperse in distilled water, stirring at room temperature for overnight to fully expand;
3. Add dilute nitric acid dropwise to adjust the pH value of hydrotalcite slurry among 5.5~6, then rise the temperature to 348K;
4. Rapidly add ammonium metatungstate solution under the condition of stirring, and to react at 348K for 2 to 3 hours, then pumping wash the sample and dry at 348K to get the ammonium metatungstate hydrotalcite catalyst, i.e. AMT-Mg2Al.
The results showed that after changed with AMT, hydrotalcite will exhibit activity to the reaction of gas-solid phase reaction of catechol and methanol, while the conversion of catechol is 75%, the selectivity of guaiacol is 90%, but the catalytic activity decays.

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Differences among Ammonium Tungstate, Ammonium Paratungstate and Ammonium Metatungstate

The most distinctive difference between ammonium tungstate, ammonium paratungstate and ammonium metatungstate is their solubility in water.

bag of ammonium metatungstate sample photo

Ammonium tungstate is also called ammonium wolframate, its molecular formula is (NH4)6W7O24•6H2O. It is colorless trapezius crystal which is soluble in water and insoluble in alcohol. Under 100℃ it loses four molecular crystal water. Ammonium tungstate can be used as the raw material for ammonium phosphotungstate and other tungstate compound. It can also be applied in producing metallic tungsten and catalyst.

Ammonium paratungstate is a white crystalline salt of ammonium and tungsten, it is insoluble in water. Its shape is like sheet or needle. Ammonium paratungstate is used as a source for high-purity tungsten oxides, tungsten metal powders, carbides, or as a laboratory reagent. Also used in the manufacture ammonium metatungstate and other tungsten compounds as additives for the petrochemical industry.

Ammonium metatungstate is a tungsten chemical in the form of highly soluble hydrated crystals, solubility is 303.99/loogH:O when 20℃. It is white or slightly yellow crystals which has no harmful components and slightly acidic. Ammonium metatungstate is very versatile and can be used as catalysts, capacitors, nuclear shielding, flame retardants, corrosion inhibitors raw materials, but also excellent raw materials in the preparation of ultra fine tungsten powder, tungsten heavy alloy powder, phosphotungstic acid, arsenic acid, silicon tungstate.

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Difference Between Ammonium Paratungstate and Ammonium Metatungstate

The most apparent difference between ammonium paratungstate and ammonium metatungstate is APT insoluble in water, but AMT soluble in water.

ammonium metatungstate image

Ammonium paratungstate is a white crystalline salt of ammonium and tungsten, it is insoluble in water. Its shape is like sheet or needle. Ammonium paratungstate is used as a source for high-purity tungsten oxides, tungsten metal powders, carbides, or as a laboratory reagent. Also tungsten powder including tungsten trioxide, tungsten blue oxide. Also used in the manufacture ammonium metatungstate and other tungsten compounds as additives for the petrochemical industry.

Ammonium metatungstate is a tungsten chemical in the form of highly soluble hydrated crystals, solubility is 303.99/loogH:O when 20℃. It is white or slightly yellow crystals which has no harmful components and slightly acidic. Ammonium metatungstate is very versatile and can be used as catalysts, capacitors, nuclear shielding, flame retardants, corrosion inhibitors raw materials, but also excellent raw materials in the preparation of ultra fine tungsten powder, tungsten heavy alloy powder, phosphotungstic acid, arsenic acid, silicon tungstate. It can also be used as reagents for chemical analysis such as for chemical diagnosis; source material for the preparation of high purity tungsten trioxide used as a thin film substrate for certain semiconductor devices; high purity source material for research in tungsten chemistry; source material for DENOX catalysts industry; as a precursor to a variety of POM (polyoxometalates).

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Producing Ammonium Metatungstate from Ammonium Tungstate in Silica advantages

ammonium metatungstate product picture

A particular advantage of the ammonium metatungstate (AMT) prepared from ammonium tungstate in silica is that the silica used in digestion and subsequently filtered from the digested solution of ammonium metatungstate (AMT) may be reused repeatedly, that is, its capacity to promote formation of AMT is not diminished with use. While the exact mechanism is not well understood at this time, the role of silica is to prevent the precipitation of insoluble APT and therefore to allow lowering of the pH to the ammonium metatungstate forming region, about 4 to 4.5.
After digestion is completed, the silica is removed, such as by filtering, and the solid AMT may be recovered in the conventional manner, such as by evaporation or spray drying.

The silica used in the invention may be any commercially available silica, provided that its impurities, both in kind and amount, are consistent with the final envisioned application for the ammonium metatungstate. For example, both sodium and phosphorus may be considered objectionable impurities for certain catalyst applications. Residual silica which cannot be removed from the digested AMT solution by filtering is generally below 1 percent by weight, and typically from 0.3 to 0.6 weight percent WO3 basis, and is compatible with many catalyst applications.

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Producing Ammonium Metatungstate from Ammonium Tungstate by Digestion in Silica

At present both ammonium paratungstate (APT) and ammonium metatungstate (AMT) are being used as tungsten sources in the catalyst industry. Since APT has a limited solubility in water (about 2 to 3 percent at room temperature), thus the user should converts APT to a more soluble peroxytungstate form by means of reaction with hydrogen peroxide in order to prepare the catalyst. While AMT is highly water soluble, and thus no need be converted, it is in general more expensive than APT, due to the complexity and/or low yields of processes for producing it.

A process for producing ammonium metatungstate (AMT) from ammonium tungstate (AT) solution is described which involves the addition of about 3.6 percent by weight of silica to an AT solution, digestion for at least about 4 hours at a temperature of at least about 98°C, followed by filtration to remove the silica from the AMT solution. Typically about 0.4 percent by weight of silica remains after filtering. The resulting AMT solution may be further processed to recover solid AMT, such as by evaporation or spray drying.

ammonium metatungstate photo

silica image

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