Preparation of metal tungsten powder

Tungsten metal powder is the main raw material preparation of tungsten carbide based cemented carbide and tungsten material, method of preparing the main current tungsten the tungsten oxide powder is a hydrogen reduction method, W0 ₃ by hydrogen reduction reaction of tungsten powder is:

10W0 ₃ +H ₂ ====W0 _2.9 +H ₂ 0

50
—— W0 _2.9 +H ₂ ==== W0 _2.72 +H ₂ 0
9

50 50
——W0 _2.72 +H ₂ ==== ——W0 ₂ +H ₂ 0
36 36

1 1
——WO ₂ +H ₂ ==== ——W+H ₂ O
twenty two


The thermodynamics and dynamics of the process have been thoroughly studied by the predecessors and accumulated a large number of research results, but considering the current particle size and morphology of tungsten powder is a key issue in production, in order to ensure a certain particle size, the reduction process It is often to control the process parameters under the conditions of far-reaching balance and according to the requirements of preparing specific particle size. Therefore, this section focuses on the factors affecting the particle size of tungsten powder and its control. For the thermodynamics and dynamics, refer to the relevant textbooks.
Factors Affecting Particle Size in the Reduction Process of Tungsten Oxide A Mechanism of Particle Growth During Reduction Process The particle size of tungsten powder formed during the reduction process varies with the reduction conditions, that is, under certain conditions, such as high temperature and high humidity. Growing up, there are many opinions about the growth mechanism. The following are two main points.
a Chemical vapor phase migration growth mechanism Hydrated tungsten oxide has a much higher volatility than pure tungsten oxide. During the reduction process, water vapor first reacts with tungsten oxide or fine-grained tungsten powder to form hydrated tungsten oxide, which migrates to other particles through the gas phase and is then reduced, thereby causing the particles to grow. High temperature and wet hydrogen reduction have the most favorable chemical vapor migration conditions.
b Oxidation-reduction mechanism The finer the powder particles, the larger the specific surface area and the surface activity. Therefore, the fine particle powder may be oxidized by water vapor or oxygen in the gas phase to form volatile hydrated tungsten oxide, and then subjected to chemical vapor migration. The particles are reduced to grow the particles.
B Main factors affecting powder particle size and grain shape change a Temperature Increasing temperature accelerates the reduction reaction, correspondingly increases the rate of water vapor formation and promotes chemical vapor migration. Promote particle growth and agglomeration.
b Water vapor partial pressure Water vapor is the basic condition of the chemical vapor phase shift reaction, and the amount thereof includes water vapor contained in the hydrogen and in the reduction reaction. It is not a constant value during the restore process. All the factors that contribute to the reaction rate and all the factors affecting the diffusion process (such as temperature, grain thickness, hydrogen flow direction and flow rate, particle size, boat geometry, etc.) and push speed affect the actual water vapor. The partial pressure affects the particle size and morphology of the powder. The effects of temperature and humidity (dew point of hydrogen) on the relative growth rate of WO ₂ are shown in the table below. [next]

c Properties of raw material powders Studies have shown that the reducing activity of tungsten oxide has a significant effect on the particle size of tungsten powder. It is easy to obtain fine-grained tungsten powder by reducing the active material.
d Impurities and Additives The effect of impurity elements on the variation of tungsten powder particles can be divided into three categories:
The first type is represented by alkali metals, which can act as a carrier for oxygen, prolong the residence time of oxygen in the powder layer, promote the chemical migration reaction, and enhance the particle growth of the tungsten powder.
The second category is represented by calcium, magnesium and silicon, and their effect on the growth of tungsten powder particles is not obvious.
The third type is represented by aluminum , which can form a thin oxide layer with high stability on the surface of tungsten crystals, and suppress the large particles of tungsten powder.
e Operating system Since the particle growth process mainly occurs in the process of reducing WO ₃ to WO ₂ , in order to obtain fine particles, it must be ensured; the original initial stage is in a low temperature, low water vapor partial pressure state. Therefore, the speed of pushing the boat is too fast, on the one hand, the material quickly enters the high temperature zone, which is beneficial to the growth of particles such as W0 _2.9 , and at the same time, the reduction speed is accelerated, and the H ₂ 0 vapor concentration is increased, which is beneficial to the growth of the particles, so to fine Particles generally require slow pushing speed. At the same time, the furnace temperature is lower and the temperature gradient is smaller.
If the loading amount is too much and the material layer is too thick, the internal water vapor will be difficult to discharge, and the internal particles will grow and the upper and lower layers will be uneven.
Process for Hydrogen Reduction of Tungsten Oxide to Produce Tungsten Powder The current reduction process is usually carried out in a rotary tubular furnace, a four-tube muffle furnace and a multi-tube furnace. In contrast, the latter has a temperature-controlled product whose particle size is easily controlled and uniform in particle size. .
The specific processes are:
(1) Yellow tungsten process, that is, W0 ₃ is used as a raw material for reduction.
(2) Blue-tungsten process, that is, blue tungsten oxide is used as raw material. Refers to blue tungsten oxide or APT W0 ₃ at 300 ~ 420 ℃, the resulting two partial reduction furnace product, it is composed mainly of money W0 _2.9 or tungsten bronze (ATB), may also contain minor amounts of tungsten even W0 _2.72 The acid salt, which is characterized by using blue tungsten oxide as a raw material, is easier to control than yellow tungsten.
(3) The process of purple tungsten, that is, using W0 _2.72 (W ₁₈ 0 ₄₉ ) as raw material for reduction, used to prepare ultrafine particle tungsten powder, the essence of which is to firstly APT in rotary furnace, at a certain temperature and weak reducing atmosphere W ₁₈ 0 ₄₉ is prepared. At this time, an aggregate of W ₁₈ 0 ₄₉ rod crystals is formed in the original APT crystal grains. When the original APT crystal grains are 50 to 60 μ m, W ₁₈ 0 ₄₉ is formed in the crystal grains. The diameter of the rod crystal is less than 2 μ m. This W ₁₈ 0 _{49 is} further reduced in a four-tube reduction furnace to obtain ultra-fine tungsten powder with a BET diameter of about 0.08-0.9 μm . These tungsten powders are much larger than yellow W0 _{3 .} The product reduced by blue or tungsten is fine and uniform. At the same time they are further carbonized to obtain WC
It is also difficult to grow in the process. For example, the tungsten powder prepared by the same has a BET particle size of 0.084 μm . After carbonization at 1460 ° C for 2 h, the obtained ultrafine tungsten carbide powder has a BET particle size of only 0.214 μm , which is equivalent to the advanced level in foreign countries. The tendency of particles to grow during carbonization is much smaller than that from blue tungsten.
Tang Xinhe developed an ultrafine powder of tungsten and tungsten carbide from the thermal decomposition of organic amine tungstate. Get very meaningful results. This kind of powder obtained from the so-called "self-reducing tungstate" has excellent performance and has obtained national patents.

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