Cyclic Oxidation Method Helps Tungsten Powder Particle Size Rebalance

Tungsten powder is the basic raw material for making cemented carbide.

Most of the cemented carbides in the market do not have much demand for the size of tungsten powder, but they have strict requirements for the particle size uniformity of tungsten powder.

A new method of ammonium tungstate cyclic oxidation reduction method proposed in recent years for producing submicron tungsten powder, it is mainly through a number of ammonium tungstate - tungsten oxide circulation change, submicron tungsten powder and ultimately the average particle size. Therefore, we can also try to use circulating oxidation reduction method to help the common tungsten powder to achieve particle size rebalancing.

It is necessary to prepare blue tungsten for reduction of tungsten powder because of the preparation of ordinary particle grade tungsten powder. The general process of ammonium tungstate cycle oxidation reduction method is: the special pure ammonium paratungstate in argon to slowly raise the temperature rise to 600 °C, holding 10 min, to obtain blue tungsten → with the resulting purple tungsten in the first reduction of hydrogen, preparation of tungsten powder → tungsten powder in the air slowly heated to 500 °C for the first oxidation, when the temperature reached 500 °C for 10 min, to obtain tungsten trioxide → tungsten oxide in hydrogen for the second time to prepare Tungsten powder, end the process, get tungsten powder. 

The tungsten powder was tested with tungsten powder with a particle size of 3~10 and a specific particle size of 0.180 m2/g and a specific surface area of 83.86% m, the main reason is that the composition of blue tungsten is not a single phase, according to the oxidation - reduction deposition mechanism can be seen, blue tungsten in the reduction process is not synchronized to carry out, therefore, the tungsten particle size distribution is very uneven after the first reduction of blue tungsten. When the first reduction of blue tungsten tungsten powder is oxidized into WO₃, and then restore, because the tungsten oxide particle size distribution is uniform, then the reduction process of tungsten trioxide is almost simultaneous, therefore, the particle size distribution of tungsten powder tends to be homogenized. In short, the first reduction process and can not draw tungsten powder with uniform particle size, this should not be too tangled, directly and then melted down a consolidation reduction effect on it.

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Ammonia Purification - Thermal Dissociation Method for Preparing High Purity Tungsten Trioxide

At present, the industrial production of pure tungsten trioxide or APT is mainly purify the crude sodium tungstate solution.

Ammonia purification of a thermal dissociation method is the South China University of Zhang Qixiao et al in the early 21st century proposed purification of ammonium arsenate solution method, it can be achieved by the crude ammonium tungstate solution directly prepared high purity tungsten trioxide.

The use of ammonia purification heat from the main base of high pure tungsten trioxide solution is directly preparing high purity tungstic acid (H₂WO₄), the quality of H₂WO₄ directly affects the quality of the final product and cycle times. The process of ammonia purification by thermal dissociation is:

1.The pulp industry H₂WO₄ slowly adding ammonia, WO₄ will dissolve (NH₄) ₂WO₄ solution, and silica, crude H₂WO₄ of ferric hydroxide, manganese oxide and calcium tungstate are reserved for ammonia dissolution in slag.

2.Filtered (NH₄) ₂WO₄ solution, after heating and removing ammonia, they gradually precipitate the solubility of ammonium paratungstate crystals. 

3.The ammonium paratungstate dissolved in ammonia, is in the temperature range of 250-350 °C ammonium paratungstate for a sufficient period of time to make the properties of ammonium paratungstate crystals change, and then can be dissolved in ammonia to produce (NH₄) ₂WO₄ solution. With the progress of the thermal dissociation process, the impurities in the crystallization of ammonium paratungstate were dehydrated and oxidized. Most of the impurities were insoluble in ammonia, and only aluminum, arsenic and phosphorus still entered (NH₄) ₂WO₄ solution.

4.The obtained (NH₄) ₂WO₄ and then a first evaporation crystallization, to consolidate the effect, in general, the solution and then evaporate the crystallization of ammonium paratungstate will be much higher than the first purity.

The ammonia purification process has simple equipment, high pure tungsten trioxide thermal dissociation method, chemical reagent consumption, low cost, and the purity of WO₃ reached 99.995%-99.999%, reached the requirements of high pure tungsten trioxide.

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Tungsten and Molybdenum Separation - Paratungstate B Guanidine Salt Precipitation Method

Paratungstate B guanidine salt precipitation method is a method for separating molybdenum and tungsten.

It is known that there are three kinds of orthopolyacids, namely, paratungstic acid A, paratungstic acid B, paratungstic acid Z, the structure of B is the most special, and its chemical formula is Na₁₀[H₂W₁₂O₄₂], containing two H atoms, so the selectivity is strongest.

The key point of B guanidine precipitation method is how to convert A into B. Scholars have found that SiO₂ has a significant catalytic effect on the transition reaction, and can shorten the conversion process to a few hours, which makes the feasibility of B guanidine salt precipitation method.

After the conversion of sec-tungstic acid A to paratungstic acid B, the pH of the solution was increased from 6.7 to 8.1, ensuring that Mo₄^2- could be polymerized in a large concentration range, in order to use guanidine salt precipitation agent can effectively separate and in suitable conditions of pH tungsten and molybdenum, tungsten and molybdenum separation under (pH>7.5), only the formation of paratungstate B can guarantee the high precipitation rate of tungsten.

Recently, some scholars believe that guanidine nitrate is an ideal selective precipitant for B. The concentration of guanidine nitrate was 1.063 times -1.068 times, pH7.70-7.79 was 45 minutes, , stirring speed of 120 minutes and other precipitation conditions, molybdenum containing paratungstate B and guanidine nitrate solution in the precipitation process can fully realize the separation of tungsten and molybdenum, and, in the process of transformation, and the recovery of ammonium paratungstate guanidine nitrate preparation at the same time.

After verification, paratungstate B guanidine salt precipitation method has been confirmed as an effective method for separation of tungsten and molybdenum, as to why there is no large-scale application to the laboratory, reason unknown. But good wine is not afraid of deep alley. In the near future, more scholars will further improve the method of B guanidine salt precipitation. Make it easier to popularize.

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Submicron Tungsten Powder Preparation by APT Cyclic Oxidation Reduction Method

Tungsten powder is mainly used to manufacture tungsten filament for incandescent lamp, inert gas shielded arc welding and atomic hydrogen welding electrode etc..

Submicron tungsten powder has the advantages of fine particle, well-developed surface, high activity, it is not as stringent as the preparation of nano tungsten powder, and the application range is better than ordinary tungsten powder, so the application scope is more and more widely. 

There are many main methods for preparing submicron tungsten powder at home and abroad: they are high-energy ball milling, dry atmosphere reduction, violet tungsten hydrogen reduction, molten salt electrolysis and plasma technology, etc. There are still many shortcomings in the preparation of submicron tungsten powder, such as time consuming, energy consumption, low production efficiency, high production cost, and difficult to realize industrial production. So the next to introduce an innovative submicron tungsten powder preparation method - ammonium paratungstate cycle oxidation reduction method.

The cyclic oxidation reduction method of ammonium tungstate was put forward by scholar Fu Xiaoming. Its main feature is the ammonium tungstate calcined in the air as tungsten trioxide, and then the use of cyclic oxidation reduction preparation of sub-micron tungsten powder, this feature is less pollution, simple and easy process route, easy to control process parameters, simple equipment and suitable for mass production of industrialization and so on, the submicron tungsten powder particle size distribution range is narrow, the structure is spherical, which is conducive to the homogenization of tungsten powder compacts in sintering process, and can achieve good size control.

Ammonium paratungstate oxidation reduction method is carried out under the reduction furnace, the main feature is the cycle of recycling, roughly as follows: the special pure ammonium paratungstate in argon to slow the temperature rise to 800 °C, holding 10 min, the tungsten powder was prepared by the first reduction of the tungsten powder and tungsten powder in the air to 500 °C for the first time. When the temperature reached 500 °C, the temperature was kept for 10 min, tungsten trioxide → tungsten oxide is prepared in hydrogen for the second time. Tungsten powder is also prepared. The second oxidation is carried out using the obtained tungsten powder. The second oxidation is carried out to obtain tungsten trioxide. To obtain tungsten trioxide → the third reduction was carried out again with the obtained tungsten trioxide to prepare submicron tungsten powder.

The process and equipment used to prepare submicron tungsten powder by cyclic oxidation of ammonium paratungstate are similar to those of traditional industrial production of tungsten powder. Therefore, the process route is simple and easy, the process parameters are easy to control, the equipment is simple, suitable for industrial production in large quantities.

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Zinc Tungstate / Bamboo Charcoal Composite Photocatalyst

In recent years, tungstate catalysts have attracted more attention for their narrower band gap and larger visible light range.

Zinc tungstate (ZnWO₄) is a very representative tungstate material. As an efficient photocatalyst, ZnWO₄ has excellent catalytic activity, good stability, low energy consumption and no secondary pollution, it has a wide range of applications in environmental protection, water treatment and many other fields. But inevitably, zinc tungstate, like bismuth tungstate, titanium dioxide and other photocatalysts, is easy to agglomerate and difficult to recover in practical applications, therefore, searching for an effective carrier is an effective way to improve photocatalytic performance and reduce the amount of zinc tungstate.

Bamboo charcoal is a three-year-old alpine bamboo as raw material, by nearly 1000 degrees high temperature firing from a charcoal. Bamboo charcoal has porous and porous structure, its molecule is fine and porous, the raw material is easy to regenerate, has strong adsorption ability, can purify the air, eliminate the peculiar smell, is the ideal material to serve as the carrier of zinc tungstate.

The molar ratio of ammonium paratungstate, zinc nitrate and citric acid was mixed with the molar ratio of 1: 1: 2, and the mixture was stirred in a constant temperature water bath at 85 °C. The dispersed bamboo charcoal was poured into the above solution and stirred for 10 min, and then with 7.28 mol / L ammonia water solution pH = 2.0 ~ 3.0 constant temperature stirring to the solution was gel-like, the sol transferred to the beaker, into the microwave oven, microwave oven power 640 W, microwave 10 min, and then in the box-type resistance furnace at 400 °C for 2 h, remove the grinding, the powder over 100 mesh sieve, that was ZnWO₄ / bamboo charcoal composite materials.

After the composite of zinc tungstate / bamboo charcoal composite catalyst showed great photocatalytic activity, some enterprises use composite catalyst degradation rhodamine B, when the content of bamboo charcoal was 20%, the bamboo charcoal exhibited the maximum photocatalytic activity, and the degradation effect reached 92%.

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A New Shortcut for Preparing WC-CO Powder from Ammonium Metatungstate

 Many scholars have tried new methods to reduce production cost. The direct synthesis of tungsten cobalt alloy powder from ammonium metatungstate is an innovative attempt.

By ammonium metatungstate direct synthesis of WC-CO powder the process is roughly using ammonium metatungstate (APT), soluble cobalt salt and organic carbon source as raw materials, using spray conversion method, preparation of tungsten cobalt precursor by direct carbonization in situ synthesis method, the precursor for WC-CO carbide composite powder. The technology greatly simplifies the process of traditional technology, the cost is also reduced in theory.

Some scholars using the preparation of ammonium metatungstate by AMT (WO₃>92.5%), soluble cobalt salts and ultra pure organic carbon as raw materials, according to the wc-6% (mass fraction) CO composition prepared tungsten cobalt mixed solution, and then the preparation of tungsten cobalt precursor powder in spray drying tower, calcined in N₂ atmosphere, finally prepared by in situ synthesis method prepared by direct carbonization of WC-CO composite powder in H₂ atmosphere. For the spray conversion, the calcination process uses fixed parameters: the solution concentration is set to 60% (mass fraction), feed rate 2000ml / min, inlet temperature 240 °C, centrifugal speed 12000r / min, low temperature calcination temperature is set to 200 °C; Set the direct carbonization temperature to 800, 850 and 900 °C. In situ synthesis of WC-CO powder by direct carbonization.

On the set of carbonation temperature test found that W at 800 °C began to slow carbonation, with the temperature rise, carbonization speed, to 900 °C, W has been carbonized more complete, under normal circumstances, W need to be carbonized at 1100 °C, but here the carbonization temperature is reduced by 200 °C, the main reason is that CO on the carbonization of W play a catalytic role.

The process of preparing WC-CO powder from ammonium metatungstate was carried out along the W / CO / C → WCO₃ · CO₆W₆C → W₂C-CO → WC-CO reaction. The WC-CO composite powder with hollow spherical structure can be successfully prepared, the average particle size is about 30 um, and the hollow sphere wall thickness is about 1.8 um; With the increase of carbonization temperature, the reaction system can be directly generated W₂C-CO without further two steps, and then further carbonized into WC-CO composite powder.

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What Factors Should be Paid Attention to in Producing High Purity APT

High purity quaternary ammonium paratungstate (APT) is a crystalline powder of ammonium paratungstate with a purity of 99.999% to 99.9999%.

At present, the methods of producing high purity APT by controlling impurity content in industrial production are: re dissolution method, ion exchange method, ammonium tungstate method, electrolysis method, recrystallization method, etc.. But because of the different chemical properties of impurity elements, using only one method to achieve the purpose of removing all impurities, so the preparation of high purity APT often need two times of ion exchange, activated carbon adsorption and other means of further purification.

It is found that the following three problems should be paid attention to during the purification of high purity ammonium paratungstate (APT).

(1). The higher the crystallization rate, the higher the impurity content in APT crystallization. When the crystallization rate is more than 65%, the precipitation of impurities is accelerated, and the precipitation rate of the four impurities is sorted as Fe >M o>P >S. Therefore, the ultra high purity APT should be controlled by controlled crystallization, and the crystallization rate should be controlled within 65%.

(2). Increasing the concentration of WO₃ in ammonium paratungstate solution is beneficial to improve the purity of APT. The higher the initial concentration of WO₃ is, the less impurity content in APT crystal. The decrease of Mo content in APT was faster than that of P, S and Fe in the ammonium tungstate solution with higher WO₃ concentration. The precipitation rate of Fe decreased obviously with the increase of WO₃ concentration.

(3) With the increase of crystallization temperature, the impurity content in APT decreased. The reason may be that the solubility of impurities increases at high temperature, and the content of APT crystals decreases. At the same time, homogeneous coarse grains are easily obtained at high temperature, and the impurities in the bulk crystals and homogeneous coarse grains are less.

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Tungsten Trioxide Gas Sensitive Performance Test

Tungsten trioxide is a transition metal oxide, with a semiconductor character, has great potential for gas-sensitive materials, sensitive to a variety of gases, especially toxic gases.

In recent years, with the emergence of nanotechnology, it was found that nano-tungsten trioxide has a higher sensitivity and response speed. There are three main methods for preparing nano tungsten trioxide, gas phase method, liquid phase method and solid phase method. In general, hydrothermal method has become the most widely used method for the preparation of nano WO₃ by virtue of its economical, low cost and easy operation.

Commonly used to produce gas-sensing elements of tungsten trioxide mainly nanorods, nano-plate, nano-plate rod-like mixed structure and other forms, as will eventually become what form, are generally in the process of hydrothermal synthesis by adjusting the parameters formed by the results, therefore, even with the same hydrothermal method of nano-tungsten trioxide, often because of different regulators, the temperature is different, and makes the operation of a wide variety of the final nano-structure is also different.

Some scholars use water from tungsten trioxide thermal method, in order to verify the gas sensing properties of the above-mentioned three kinds of structure, using citric acid as acidifier, ammonium metatungstate will be mixed and sodium sulfate, were dissolved into distilled water, according to a certain percentage of the mixed solution by adding citric acid, by adding the amount of citric acid to adjust the reduction of rod, plate, plate-like mixed structure of three kinds of structure of nano-tungsten trioxide.

Through the test of the three structures, the following conclusions are obtained. The specific surface area of BET of nano plate, nano plate rod and nano rod WO₃ is 17.55 m2/g, 11.01 m2/g, 10.57 m2/g, respectively, it can be seen that the specific surface area is nano plate > nano plate rod > nano rod. It can be said that the specific surface area is the key to determine the sensitivity of nano-tungsten trioxide, so the results will be clear at a glance.

In the atmosphere of three acetone, the same concentration of ammonia and formaldehyde, gas sensing properties of tungsten trioxide nano plate is the best, this is mainly due to the nano tungsten trioxide plate has a high specific surface area, can significantly change the effective resistance of the crystal and increase the active site of gas reaction. For gas sensitive reactions, the more active sites of metal oxides are, the more sensitive the gas sensitivity is, and the selectivity and sensitivity of gas sensors are higher.

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