Structure and Thermal Decomposition of Ammonium Metatungstate

Ammonium metatungstate, also known as AMT, is a tungsten chemical in the form of highly soluble hydrated crystals. Global Tungsten & Powders AMT is produced as minus 10-mesh powder to meet stringent demands in the catalyst, medical, semiconductor and other high-technology industries. GTP AMT is also used in materials research and in analytical chemistry. At room temperature, aqueous solutions can be saturated up to 70% by weight of contained WO3. This equates to 1.6 kilograms per liter or 13.3 pounds per gallon.

The structure and morphology of ammonium metatungstate (AMT), (NH4)6[H2W12O40]⋅4H2O, and its thermal decomposition in air and nitrogen atmospheres were investigated by SEM, FTIR, XRD, and TG/DTA-MS. The cell parameters of the AMT sample were determined and refined with a full profile fit. The thermal decomposition of AMT involved several steps in inert atmosphere: (i) release of crystal water between 25 and 200 °C resulting in dehydrated AMT, (ii) formation of an amorphous phase between 200 and 380 °C, (iii) from which hexagonal WO3 formed between 380 and 500 °C, and (iv) which then transformed into the more stable m-WO3 between 500 and 600 °C. As a difference in air, the as-formed NH3 ignited with an exothermic heat effect, and nitrous oxides formed as combustion products. The thermal behavior of AMT was similar to ammonium paratungstate (APT), (NH4)10[H2W12O42]⋅4H2O, the only main difference being the lack of dry NH3 evolution between 170 and 240 °C in the case of AMT.

Synthesis and Interfacial Electron Transfer of a Composite Film of Graphene and AMT

Solar energy is a renewable energy, hydrogen production with that is expected to solve the current common problems of energy shortage and environmental pollution, which has been widespread concerned. In many semiconductor photocatalytic hydrogen production materials, tungsten oxide become a hot research materials in the photocatalytic field due to its stable performance and low cost, but its optical electronic - holes are so easy to combine that restrict the photoelectric device performance. In order to improve the photoelectric conversion performance of the tungsten oxide, the domestic and international researchers carried out a number of explorations. Currently, common methods are semiconductor compound, noble metal deposition, surface sensitization, ion doping.

With ammonium metatungstate as tungsten source, PVP as binding agents, use dip-coating method to prepare graphene - tungsten oxide composite films, using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy methods such as composite structure were characterized by light and current test, electrochemical impedance spectroscopy (EIS), transient photocurrent spectrum and intensity modulated photocurrent spectroscopy methods to study complex film electrode interface at a photoelectric effect carriers transfer process and a charge transport behavior. The results show that the composition of the film and the tungsten oxide nanoparticles are graphene fully complex, significantly improved photoelectric properties; graphene composite after the transient time constant of the film is increased, the electron - hole pairs life expectancy; electron transport time is reduced, as 47.5% of pure tungsten oxide film.

Emulsion Liquid Membrane and APT Production Advantages

Dissolve crude sodium tungstate that obtained after alkali fusion with tungsten fine mud in water as feed solution and adjusted pH to 8-9, after removing impurities of silicon, without removal of phosphorus, arsenic, molybdenum at room temperature for a directly laboratory intermittent film migration, according to the best operating conditions of the orthogonal design, obtain ammonium paratungstate (APT) crystallization directly in inside aqueous phase within 5min . Extraction rate is 99.85%, and the purity level of product is the firsts standards. This article had depth study on various factors, and proposed A-class continuous countercurrent flow with low cost, which is a new approach with great promise.

Advantages of the  first level film production process are: it does not need to adjust the solution to acidic after removing Si, crystalline can be directly produced in  inside aqueous phase by A-class liquid membrane process, acid saving, the process is short, fast, without evaporation and crystallization, energy saving and finer grain, without removing P, As, Mo. Solution containing tungsten material has up to 45g / L when the first extraction. The oil phase and the inner aqueous phase repeatedly reuse, less reagent consumption. According to the economic accounting of film production of mixed rare earth to calculate the operating costs be low, operating costs of liquid membrane method are generally about 1/3 than extraction method, the actual accounts of that is to be tested.

Emulsion Liquid Membrane and APT Production

The concept of emulsion liquid membrane is proposed by N.N.Li in 1968, and it has attracted researchers from many countries for APT production. Because it simulates the activity of migration of biofilms on the migration mechanism, it enables materials highly enriched against the concentration difference. It is the combination of extraction and back-extraction process carried out in a system at the same time, the reaction takes place on the surface area of the extremely thin film interface, and there is carrier on the membrane, so its kinetic advantages are much more than the reaction takes places in solution, also the mass transfer rate, efficiency and selectivity than the extraction process.

Currently the main technology key of emulsion liquid membrane is the stability and membrane rupture technology. The requirements are different for the stability of the film in the film extraction and rupture stage, the former one requires high film stability while the latter one requires low stability. Currently it’s difficult to control the film stability and longevity. Membrane rupture technology is related to reuse and recycling of organic phase, which directly affect the economic effects of liquid film method, and different formulations of liquid membrane result in different effects of rupture, therefore the rupture technology also needs to be tested. For the study of surfactant and carriers, it’s also directly affect the technical and economic effects of liquid film. In addition, in the industrial wastewater treatment, since the dissolution of membrane components may cause secondary pollution, these problems can be solved in the research and development of the film.

Emulsion Liquid Membrane in Ammonium Paratungstate Production

Tungsten reserves in China takes the first place in the world, occupying an important position in China's metallurgical industry. 

Ammonium paratungstate is the main raw material of metal tungsten production. Now the factories use extraction or ion exchange process in production, the former one is that the tungsten concentrate with alkali cooking (or alkali fusion) division after removing silicon, phosphorus, arsenic, adding Na2S to make Mo become MoS3 precipitation, and then adjust the pH of supernatant to 2-3, and extracted with a tertiary amine and recovering with NH4OH.

The concept of emulsion liquid membrane is proposed by N.N.Li in 1968, and it has attracted researchers from many countries. Because it simulates the activity of migration of biofilms on the migration mechanism, it enables materials highly enriched against the concentration difference. It is the combination of extraction and back-extraction process carried out in a system at the same time, the reaction takes place on the surface area of the extremely thin film interface, and there is carrier on the membrane, so its kinetic advantages are much more than the reaction takes places in solution, also the mass transfer rate, efficiency and selectivity than the extraction process. This new technology is currently developing forward unit operations. University of Graz in Austria is first one to started the international industrial application, for the recovery of zinc from waste water, followed by the Institute of Environmental Science, South China University of Technology for the removal of phenol from wastewater.

Liquid Membrane Production of Ammonium paratungstate

Tungsten reserves in China takes the first place in the world, occupying an important position in China's metallurgical industry.

Ammonium paratungstate is the main raw material of metal tungsten production. Now the factories use extraction or ion exchange process in production, the former one is that the tungsten concentrate with alkali cooking (or alkali fusion) division after removing silicon, phosphorus, arsenic, adding Na2S to make Mo become MoS3 precipitation, and then adjust the pH of supernatant to 2-3, and extracted with a tertiary amine and recovering with NH4OH. Therefore, the defecator will go through stages of pH = 6. APT will then crystallize resulting in difficulties in normal operation. This problem is difficult to resolve from the formula, it is by strengthening stirring to dissolve the crystals again clarified or improved groove geometry of the internal structure of the practice.

It’s not required in removing P, As, Si in the ion exchange method but molybdenum can not be removed, so the tungsten concentrate with low Mo-containing is often the choice. This method can use alkaline feed solution directly, but due to the amount of exchange is small, the material must first be diluted to WO3 contain 25g / L or less, and the amount of the rinse water is also large, therefore the emissions of alkaline wastewater with a pH10-13 is very large, wastewater treatment has become a burden, and tungsten in waste water of these two methods can not be recovered.

Detinning Process of APT Production- Precipitation Method

Precipitation method in APT production is a common method of separation science. As the tin exists as precipitate forms in the weakly acidic, neutral, weakly alkaline environment, people take a variety of precipitation methods, such as hydrolysis precipitation, tin sulfide precipitation to remove tin of the tungsten solution.

Hydrolysis precipitation method
This is a classic method remove tin. Its basic principle is tin in solution can be hydrolyzed into tin hydroxide precipitate at a pH of about 2 to 11 (depending on the concentration of tin in the solution) to separate tungsten. The main reaction is:
SnO3 2-+3H2O= Sn(OH)4↓+20H-.

Specific operating conditions: add dilute acid(typically sulfuric acid) to sodium tungstate solution (pH is about 14), adjust pH of the solution to 9 to 10, stirring for 10 ~ 15 min, at room temperature (in the case of boiling , the advantage is not very obvious), to stand for aging. The longer the standing time, the higher the tin hydrolysis precipitation rate. It’s believed in the literature that it needs to stand for 96 h.

Hydrolysis detinning rate rises with the tin mass concentration of solution increased. However, when hydrolysing detinning, and the solution is often turbid, the precipitation is difficult to cake filter. Easy to form a tin-containing hydrolysis solution lower gum, gum nucleosome its low concentration, dispersion and strong, easy reunion. It is also above the longer the standing time, the higher the rate of hydrolysis precipitation in addition to tin reasons. Production, the use of tin content in the concentrate within the range of 0.03% to 0.2%, tin in solution concentration of 0.0012 ~ 0.033g / L, therefore, in addition to use of hydrolytic precipitation when tin is difficult to obtain the desired results. Moreover, the operation of acid sodium tungstate were to spend a lot of acid, and the introduction of a large number of impurities anion, seriously affecting the subsequent ion exchange process, difficult to apply to the current tungsten metallurgy most advanced "tungsten concentrate decompose an ion exchange an evaporative crystallization "technology.