2017年8月10日星期四

How Rare Earth Elements Affect Tungsten Oxide Hydrogen Reduction

In the process of hydrogen reduction of tungsten oxide, the effect of the reduction is not only the metal elements, there are some trace elements, such as rare earth elements.
The effects of trace elements are related not only to quantity, but also to the shape and location of individual elements and the entry stage, for example, the elements that form fine grain particles at grain boundaries are different from the same elements in solid solutions. They are soluble in the bonding phase and different from the tungsten carbide grains.
Some scholars believe that the addition of lanthanum or yttrium (La) (Y) in hydrogen reduction of tungsten oxide, can play a similar role to that of Al.

In the process of research, scholars used the doping method of wet mixing and flow drying to prepare the blue tungsten powder, and push the boat by reverse hydrogen, the decomposition process is performed in two stages, namely at 500 °C, 850 °C, 900 °C, 950 °C, 1000 °C temperature point five 4h reduced with hydrogen. Study found that while maintaining the hydrogen flow rate, the amount of loading the boat, and the holding time, the higher the temperature, the tungsten blue powder particles increases. But with the addition of La2O3, tungsten powder particles gradually became thin, the surface gradually increased, in addition, the addition of 2% Y2O3 is better than the effect of adding 3% La2O3.

 fine tungsten oxide image

The research shows that in the process of blue tungsten oxide hydrogen reduction, the study shows that in the process of blue tungsten oxide hydrogen reduction, the morphology of tungsten powder is changed into quasi regular polyhedron without quasi La or Y by adding rare earth element La or Y; La or Y is present in the form of La2O3 or Y2O3. The La and Y contents determine the distribution of La2O3 or Y2O3in the crystal. When the content of La2O3 or Y2O3 is high, the La2O3 or Y2O3 particles are dispersed distributed.

The results show that the oxides and tungstate forms in the blue tungsten hydrogen reduction tungsten powder exist in the tungsten blue hydrogen reduction tungsten powder, the addition of an oxide film of La or Y on the surface of the tungsten crystal can prevent the precipitation of the vapor phase tungsten oxide on the surface of the tungsten powder particles Or the chemical reaction of the solid phase, and thus inhibit the growth of tungsten powder particles, it is of great help to the preparation and reduction of ultrafine tungsten powder.


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Preparation of Blue Tungsten Oxide - Rotary Calciner

Blue tungsten oxide is one of the most important raw materials for the manufacture of tungsten powder. In China, there are three main methods for preparation of the blue tungsten oxide: APT closed calcination method, APT mild hydric reduction method and intrinsic reduction method. And the APT closed calcination method is the most used method.

APT closed calcination method always keeps the furnace system with positive pressure in the heating process, preventing the air from entering the furnace. By this way, ammonia from APT pyrolysis splits into hydrogen and nitrogen. And in the weak reductive atmosphere, the blue tungsten oxide is produced.

The entire process is confined in a rotary calciner. The usage and control of rotary calciner need cautious operation, as it is the main carrier of APT closed calcination process.

rotary calciner image
Rotary hearth furnace is a rotary calciner with high automation, of which the ring-shaped bottom can be rotated. Generally it is divided into two sections to control the temperature. In different sections, temperature is different due to different configurations of burner. APT is turned from low temperature area to high temperature area by the rotation of the bottom. After a cycle of turning, APT reachs the required temperature and reduce to blue tungsten oxide.

Rotary calciner has simple structure, of which the furnace tube is welded by stainless steel. Baffle in the tube can materials from flowing out to quickly and increase the heat transfer area. Heating cord is embedded in the refractory bricks outside the tube for heating material. The feeding end is connected with the exhaust fan, as the exhaust fan can exhaust away the water vapor and ammonia produced by the decomposition reaction. The recycle of tungsten oxide and APT particles from the exhaust air is carried out by a foam dust collector or an impact dust collector.

In the preparation progress, the temperature in the first section is controlled at about 550℃ and the temperature in the second section zone is controlled at about 600℃. A part of products in the hopper must be retained to seal the discharge screw, preventing air from entering the furnace tube. Throughout the process, the gas pressure is controlled between 49-98Pa greater than the atmospheric pressure. And the positive pressure and the reducing atmosphere are maintained by supplementing a small amount of ammonia and reducing the exhaust air. The product – blue tungsten oxide is Prussian blue. Tungsten recycle rate of rotary furnace can reach 98.5% at the right temperature.

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Preparation of Blue Tungsten Oxide - APT Mild Hydric Reduction Method

Blue tungsten oxide is one of the most important raw materials for the manufacture of tungsten powder. Compared with yellow tungsten oxide, blue tungsten oxide with its high activity, large surface area, easy doping, original crystalline shape is seen as an upgraded version of the Yellow tungsten oxide.

Blue tungsten oxide is one of the most important raw materials for the manufacture of tungsten powder. In China, there are three main methods for preparation of the blue tungsten oxide: APT closed calcination method, APT mild hydric reduction method and intrinsic reduction method. And the APT mild hydric reduction method is the most used method.

multi-tube furnace image

The whole preparation process reacts in a multi-tube furnace of which the heating zone is divided into three parts. In the furnace tube, the heated APT•5H2O installed on the heat resistant boat is decomposed. Feeding a small amount of hydrogen into the furnace tube can keep the reducing atmosphere. Decomposed gas product and waste gas are discharged out of the furnace and then fired. The phase composition of hydrogen blue tungsten oxide can be controlled by the flow rate of hydrogen, reduction temperature and speed regulation.

Heating temperature in multiple tube furnace is controlled at the range of 400℃—500℃. In the heating process, a small amount of hydrogen play a catalytic role, increasing the degree of ammonium tungstate out of crystalline water and ammonia and the reduction degree of ammonium paratungstate. Blue tungsten oxide began to form when the temperature reached 450 ℃.

Compared with APT closed calcination method, mild hydrogen reduction method is more environmental friendly. In the process of APT closed calcination, ammonia gas, as a harmful factor to health and environment, is required. For comparison, APT hydrogen mild reduction method only needs a small amount of hydrogen, discharging the environmental pollution problems. And under appropriate parameters, APT hydrogen mild reduction method can prepare blue tungsten oxide with higher quality.

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Preparation of Blue Tungsten Oxide - APT Closed Calcination

Blue tungsten oxide is one of the most important raw materials for the manufacture of tungsten powder. Compared with yellow tungsten oxide, blue tungsten oxide with its high activity, large surface area, easy doping, original crystalline shape is seen as an upgraded version of the Yellow tungsten oxide and used.

There are three main methods for preparation of the blue tungsten oxide: APT closed calcination method, APT mild hydric reduction method and intrinsic reduction method. And the APT closed calcination method is the most used method.

In APT closed calcinations, ammonia escaping by thermal decomposition or a small amount of introduced ammonia gas is used as reducing agent. Characteristics of this method: thermal decomposition temperature fluctuates between 550℃-700℃, high yield and stable quality. Phase composition of blue tungsten oxide is 70% W20O58 and 30%WO3.

APT closed calcination method always keeps the furnace system with positive pressure in the heating process, preventing the air from entering the furnace. By this way, ammonia from APT pyrolysis splits into hydrogen and nitrogen. And in the weak reductive atmosphere, the blue tungsten oxide is produced.

In the process of sealed calcination, ammonium paratungstate(APT) transmutes into BTO by four stages.

1, 220℃, generation of enerateg amorphous APT or amorphous bronze

2, 300℃, generation of ammonium tungsten bronze

3, 420℃, generation of yellow tungsten oxide

4, 480℃-600℃, generation of blue tungsten.

rotary calciner image

The main equipment of APT closed calcination method is rotary calciner. Rotary calciner’s size is decided by production requirements. Generally, the furnace is 6-8 m in long, 300-450 mm in diameter and made of stainless steel.

Temperature, time and specific area are influencing factors which will affect the quality of blue tungsten oxide during calcining process. And temperature is the most important factor.

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Blue Tungsten Oxide Preparation -- Internal Reduction Method

Blue tungsten oxide is one of the important raw materials for making tungsten powder. 

Compared with yellow tungsten oxide, blue tungsten oxide is easy to be doped because of its good activity and large specific surface. It can maintain its original crystalline shape and has been widely used as an upgraded version of yellow tungsten oxide.  There are three main methods for the blue tungsten oxide preparation. APT closed calcination method, APT hydrogen mild reduction method, internal reduction. Internal reduction method was proposed by scholars in Fudan University in 1981. 

Internal reduction method by means of an internal component of the tungsten compound to make reduction of W. Intrinsic reduced tungstate was synthesized with APT as raw material (Intramo - lecularly Reduching Tunnstate). It has thermal decomposition in an inert atmosphere, blue tungsten oxide with simple phase composition, large specific surface and high purity has been obtained. 

internal reduction method image

The distinguishing feature of internal reduction method is that the yellow tungstic acid is dissolved in ammonia water first, then add proper amount of urea, the intermediate, which is called the " reduced tungstate IRT ", is obtained by adjusting the acidity with oxalic acid. The dried IRT is then decomposed at 500°C to 700°C in the nitrogen stream. Using the ammonia decomposition of molecule internal component to reduce,  blue tungsten oxide can be prepared. 

The internal reduction method is carried out in a multi-tube furnace. The thermal decomposition temperature of IRT was determined to be 620 ±30°C, and the pyrolysis time was controlled at about 30 minutes.

Two different components of blue tungsten have been prepared by internal reduction method. One is the tetragonal system WO: the particles are very thin, specific surface 22一30m²/g. The other is monoclinic system W20O58, the particles are very crude, specific surface 10m²/g. The blue tungsten oxide prepared by internal reduction method has the advantages of high purity, large specific surface, uniform particle size and simple phase composition. It can be used as a new raw material for tungsten wire, carbide and other products, and its properties and practical use are good.

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Blue Tungsten Oxide and Purple Tungsten Oxide

The performance of tungsten oxide materials will directly affect the reduction behavior of tungsten powder and the final product performance.

purple tungsten oxide image

Therefore, on which material is most suitable for the production of ultrafine tungsten powder has been the subject of much attention in material science workers.

Blue tungsten oxide and purple tungsten oxide is the main raw material for production of ultrafine tungsten powder, blue tungsten oxide originated in the last century 70's, mainly WO2.90, W20O58 and ammonium tungsten bronze (ATB) mixture. Purple Tungsten oxide is a kind of tungsten oxide product developed in recent years in China. Its phase composition is WO2.72 or W18O49.

Blue tungsten oxide is present through hydrogen reduction of raw materials prepared by tungsten powder is the most widely used, as long as the conditions of low temperature, dry hydrogen, high hydrogen flow, thin material layer, producing ultrafine tungsten powder method can restore the blue tungsten oxide by hydrogen.

Purple tungsten oxide is developed in recent years, because of its unique properties, it has shown its superiority in the production of ultrafine tungsten powder.

At present, in the production of ultrafine tungsten powder, purple tungsten oxide and blue tungsten oxide two kinds of raw materials are used simultaneously. Therefore, some scholars have compared the two kinds of raw materials and conducted hydrogen reduction experiments under the same process conditions. The difference of particle size and uniformity of tungsten powder after reduction has been compared. The following conclusions are obtained by comparison:

BTO and PTO reduction image

1. Both blue tungsten oxide and purple tungsten oxide can be used to produce ultrafine tungsten powder under certain reduction conditions. But the excessively high hydrogen flow of the blue tungsten and the excess material will lead to an increase in cost.

2. Purple tungsten oxide  due to its special pore structure, the water vapor can be rapidly discharged from the reducing atmosphere to produce a fine and uniform tungsten powder, and the particle size of tungsten powder is affected by the loading volume and the hydrogen flow rate.

3. Purple tungsten oxide generating hydrogen gas in drying without going through the intermediate stage of reducing WO2, thereby obtaining a fine and uniform tungsten powder.

To sum up, the preparation of ultrafine tungsten powder from purple tungsten oxide has more advantages than blue tungsten oxide.

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Tungsten Oxide Hydrogen Reduction Equipment -- Four Tube Reduction Furnace

Tungsten oxide hydrogen reduction process is generally carried out in a multiple tube furnace, in the tubular reduction furnace, the most used in China are the thirteen tube reduction furnace and the four tube reduction furnace.

four tube reduction furnace image

Four tube reduction furnace is commonly used as a tungsten oxide hydrogen reduction unit, the furnace shell is welded by ordinary steel plates, and the four tubes are placed at the same level on the same hearth. Compared with the old four tube muffle furnace up and down the structure, the temperature uniformity is much better. The automation level of the four tube reduction furnace is higher than the thirteen tube reduction furnace, and it is easier to realize mechanization and electrification, in addition, the capacity of the four tube reduction furnace is large, but the unit energy consumption is lower than the thirteen tube reduction furnace.

The furnace temperature of the four tube reduction furnace is between 600 °C and 1000 °C centigrade, and natural gas is used to heat the natural gas supply, generally speaking, the loading capacity of a four tube furnace is slightly larger than a thirteen tube reduction furnace, preparation of ultrafine tungsten powder temperature is set to the third gear. But theoretically, four tubes reduction furnace to make ordinary tungsten powder is not bad, the preparation of super tungsten powder is not as handy as the thirteen tube reduction furnace.

In industry, the production of tungsten powder is usually reduced by hydrogen reduction furnace. However, there are also documented that an enterprise adopts the two stage method to obtain fine particles of tungsten powder. When the two stage reduction is adopted, the first stage is carried out at a lower temperature (500 °C - 700 °C), the main function is to produce two suitable tungsten oxide for grain size, using four tube reduction furnace. The second stage was carried out at 600 °C and 1100 °C, using thirteen tube reduction furnace. But the author believes that the best scheme for preparing fine particles of tungsten powder should be to avoid the formation of WO2. In practical work, it is feasible to make a single reduction of hydrogen with blue tungsten or purple tungsten.

Whether it is a four tube reduction furnace or a thirteen tube reduction furnace, the capacity to produce ultrafine tungsten powders from them is not very high, in order to make the tungsten oxide powder uniformly heated, it is inevitable to reduce the amount of tungsten powder loaded into the container, only a thin layer, so the production capacity is obviously not to be seen. But fortunately the four tube reduction furnace is high, some scholars suggest that one or two upper boats should be placed on the bottom of the reduction boat, and the hydrogen can flow in the boat, which can greatly improve the production capacity of ultrafine tungsten powder.

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Hydrated Tungsten Oxide

In the process of hydrogen reduction of tungsten oxide, the vapor pressure of tungsten oxide increases obviously when there is water vapor at high temperature.

electron microscopy hydrated tungsten oxide image

This increase in vapor pressure is due to the reaction of various tungsten oxides with water vapor to form a volatile hydrated tungsten oxide. In the W - O - H system, the hydrated tungsten oxide is much more volatile than other volatile tungsten compounds. Most scholars at home and abroad believe that it is the volatile hydrated tungsten oxide that causes the chemical vapor transfer (CVT) process of tungsten. Moreover, the CVT process runs through the whole stage of the reduction of tungsten oxide and hydrogen. It has a decisive influence on the physical and chemical properties of tungsten powder.

In the CVT process, tungsten and its oxides are contacted with water vapor to produce hydrated tungsten oxide , which is volatilized to the gas phase and undergoes a homogeneous reduction reaction with H2. The reduced product is deposited on the nucleated metal tungsten grains. The reduced product is deposited on the nucleated metal tungsten grains so that the tungsten powder particles grow.

The reaction rate of CVT process is fast, and the morphology of reduction products will change significantly compared with that of raw materials. The actual partial pressure of hydrous tungsten oxide WO2(OH)2 during hydrogen reduction is related to the reduction temperature and the actual humidity of the atmosphere. With the progress of reduction process, the partial pressure (humidity) of oxygen decreases continuously, and at the end of the reduction process, the partial pressure of hydrated tungsten oxide WO2(OH)2 is getting lower and lower.

In addition, hydrous tungsten oxide also occurs during the equilibrium annealing of hydrogen reduction, which causes small particles of oxide to evaporate and large particles grow, which eventually makes the annealed oxide markedly coarser.

To sum up, the key to prepare ultrafine tungsten powder is to reduce the formation of volatile hydrated tungsten oxide WO2(OH)2, and to inhibit the process of chemical vapor migration, under certain temperature conditions, the equilibrium partial pressure of WO2(OH)2 depends mainly on hydrogen humidity, and the greater the humidity is, the greater the equilibrium partial pressure of WO2(OH)2. Therefore, the key to the preparation of ultrafine tungsten powder is to rapidly remove water vapor from the reaction atmosphere.

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