TUNGSTEN METAL POWDER PRODUCTION-PUSH-TYPE FURNACE-1

2. Push-Type Furnace

Metal-boats are charged with oxide to a height ranging from a few mm up to several cm and are pushed in stages through the furnace in corrosion-resistant steel tubes at specific time intervals. By introducing a new boat into the tube, the row in front is pushed forward by the length of a boat. Hydrogen in excess flows either co- or countercurrent to the tungsten flow direction. The hydrogen is not only responsible for the reduction process itself but serves also to remove the water vapor formed and also acts as protecting atmosphere in the cooling zone. The “wetted” hydrogen leaving the furnace is dried to a desired dew point and recycled to the furnace . As indicated, hydrogen having higher dew points can also be fed into the furnace.

Hydrogen has to be applied in large excess, which guarantees a fast flow over the powder layer. The excess depends on the desired grain size (smaller for coarse and higher for fine powder). The range is somewhere between 2.5 and 40 times stoichiometric.

Multitube furnaces (14 to 18 tubes arranged in two rows) are frequently in use today. The boat material, in most cases, is an iron alloy high in Ni and Cr (lnconel). More seldom, because of the high price, boats are made of TZM (molybdenum alloy with Ti, Zr, and C) or pure tungsten.

The big disadvantage of the iron alloys is that diffusion of the elements occurs into the contacting tungsten powder layer. In this respect, Ni is the most dangerous element although widely used. Ni rapidly diffuses over the tungsten grains, thereby weakening the surface of the bottom and wall of the boats. With time, a Ni, Fe, Cr, and W containing scale is formed. This scale sticks more or less firmly to the boat. After several travels through the furnace, it gets thicker and partly breaks off, contaminating heterogeneously the tungsten powder. Bigger-scale particles can be separated by the always applied screening process following the reduction, but the smaller particles remain in the tungsten powder. The higher the temperature and humidity, the more pronounced the scale formation. Cast alloy material (coarse microstructure) shows enhanced scale formation compared to boats made of rolled sheet. Alloys containing Co instead of Ni are more resistant, but the high price of Co makes them unacceptable for boats. Co containing alloys are only in use as tubes in rotary furnaces.

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Morphology. As noted earlier, low temperature and dry conditions largely suppress any CVT of tungsten and lead to the formation of metal sponges, which are pseudomorphous to the oxide precursor (APT, H₂WO₄). They consist of very fine, polygonal and polycrystalline metal particles. With increasing temperature and humidity, individual tungsten grains form by CVT over comparably large distances. The particles are faceted and commonly exhibit the characteristic shape of the cubic metal. Well-faceted crystals, showing growth steps and being partly intergrown, are characteristic for very humid conditions (high temperature, large powder layer height).

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Agglomeration. One may distinguish between three different types of agglomeration:

Reduction performed at low temperature and dry conditions results in the formation of metal sponges, which are pseudomorphous to the oxide precursor. The very fine crystals(some tenths of a micron or less) stick together in a more or less loose manner. Usually, this type of agglomeration is called pseudomorphology. The finer the crystal size, the higher the strength of these agglomerates.

Bigger tungsten crystals(from approximately 1 to several μm), grown individually grown by CVT, are welded together. This type of agglometeration is looser than the above-described pseudomorphology and occurs mainly under dry reduction conditions. It can be influenced by the dew point of the incoming hydrogen. Higher dew point results in less agglomeration. The strength of this agglomeration type increases with temperature.

Reduction under wet conditions and high temperature (high concentration of [WO₂(OH)₂] in the vapor phase) results in strongly intergrown crystals .

Agglomeration is closely related (inversely proportional) to the apparent density of the tungsten powder. Correspondingly, the apparent density can be influenced within certain limits by the hydrogen dew point. Agglomeration is a prerequisite for good compactability of the tungsten powder.

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Grain Size Distribution. Grain size distribution is to a great extent the consequence of powder layer height. The growth conditions for the individual particles are different and depend on their position within the powder layer. The humidity is higher in the interior and decrease as one approaches the surface. This gradient results in large grain-sized particles inside and smaller grain-sized particles at the surface-neighboring areas. It is easy to understand that the distribution is broader for high powder layers and closer for lower layer. In any case, the distribution can be improved (made closer) by using “wet” hydrogen, since the water vapor gradient from inside to outside the layer will be decreased.

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TUNGSTEN METAL POWDER PRODUCTION-GENERAL-4

Hydrogen Flow Rate. A higher hydrogen flow enhances the material exchange due to the more rapid removal of water vapor. Therefore, the flow is inversely proportional to the average grain size.

Hydrogen Flow Direction. Concurrent hydrogen flow with respect to the tungsten flow generates a higher dynamic humidity at the later part of the reduction, while counter current flow (which is the standard condition) provides higher humidity during the early reduction stages.

Hydrogen Dew Point. The dew point of the incoming hydrogen influences the overall humidity during reduction. More “wet” hydrogen enhances the tungsten particle growth.

It is important to understand that the final average grain size of the tungsten powder is a consequence of combining all the aforementioned parameters. The general rules are:

Small grain size: low temperature, dry hydrogen, high hydrogen flow rate, low dew point, small powder layer height, high porosity, low oxide feed.

Large grain size: high temperature, wet hydrogen, low hydrogen flow rate, high dew point, large powder layer height, low porosity, high oxide feed.

Empirically based equations for calculating the reduction time and average particle size have been derived for rotary furnaces but have been applied industrially. The main difficulty is that the properties of the raw material play a crucial role in the reduction process, and these characteristics are not well enough represented by corresponding equations. This is particularly true for smaller grain sizes. For larger grain sizes, the influence of the oxide precursor is less pronounced.

In industrial practice, the choice of proper reduction parameters is based exclusively on empirical experience. Besides the average grain size, the reduction parameters also influence the grain size distribution, agglomeration, apparent density, and grain morphology.

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TUNGSTEN METAL POWDER PRODUCTION-GENERAL--3

Temperature. Temperature influences the rate of all reactions occurring during reduction, hence the dynamic and partial pressure of the volatile [WO₂(OH)₂] which forms during reduction and which is responsible for the chemical vapor transport (CVT) of tungsten. Temperature and tungsten particle size are directly proportional while temperature and time required for final reduction are inversely proportional.

Oxide Feed. The tungsten mass flow determines the amount of H2O liberated during the entire reduction process. The higher the flow, the larger the grain size.

Tungsten Powder Layer Height. During reduction and accompanying water formation, the powder layer exerts a considerable diffusion resistance against water removal from the layer. The higher the layer, the greater the diffusion resistance and the more slowly the reaction water will be removed. The local humidity is higher at the bottom of growth conditions of the metal particles formed at a particular temperature. The layer height is directly proportional to powder grain size.

Porosity of the Powder Layer. The porosity of the powder layer, and thus its permeability, is determined by the macroporosity (intermediate space between the oxide particles) and by microporosity (porosity of the individual oxide particles). The higher the porosity of the powder layer, the better the material exchange H2O→H2 during reduction and the less the grains of the tungsten particles will grow, resulting in a smaller particle size.

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TUNGSTEN METAL POWDER PRODUCTION--GENERAL--2

In principle, APT can also be directly reduced without any prior calcinations step. The disadvantage of direct reduction is the formation of ammonia which has to be scrubbed, but a certain amount of ammonia cracks and dilutes the hydrogen by nitrogen. Consequently, from time to time, part of the contaminated, circulating hydrogen must be vented, thus increasing costs.

Reduction is carried out in pusher furnaces in which the powder passes through the furnace in boats or in rotary furnaces (see below). Walking beam furnaces or furnaces with internal band conveyors are less often used. Fluidized-bed reactors are still not in commercial use, except for the production of nanophase W or WC/Co powder precursors. Furnaces are provided with several temperature zones controlled between 600 and 1100℃. A large excess of hydrogen is used, which is recycled to the furnace after purification. The flow of hydrogen is usually in a countercurrent direction, more rarely concurrent. The hydrogen acts not only as a reducing agent but also carries away the water formed.

The reduction of tungsten oxides by hydrogen to tungsten metal is, in some respect, a unique process. It offers the possibility to produce tungsten powder of any desired average grain size between 0.1 and 10 µm (and, in the case of doped oxides, even up to 100µm), starting from the same oxide precursor. Individual tungsten particles form during reduction as a result of chemical vapor transport of tungsten (vaporization/deposition process), which is responsible for the final powder characteristics.

By changing the reduction parameters, powder characteristics like average grain size, grain size distribution, etc. can be regulated. Temperature and humidity (i.e., the water vapor partial pressure prevalent during reduction) are the two main parameters in steering the average grain size of the W powder, the latter being related to a number of oxide and process-related variables as indicated in Fig. 5.19 and discussed briefly below. The reason for the strong influence of the humidity on powder grain size originates in the strong dependence of humidity on the nucleation rate of the metal phase and the high mobility of tungsten due to the presence of a volatile tungsten compound ([WO₂(OH)₂]). The lower the humidity, the higher the nucleation rate (under isothermal conditions) and the smaller the grain size.

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TUNGSTEN METAL POWDER PRODUCTION--GEMERAL--1

1.General

The manufacture of tungsten metal powder is a crucial step in tungsten metal and alloy production, since the powder properties significantly affect the properties in subsequent operations, such as pressing, sintering, and metalworking. Between 70 to 80% of tungsten worldwide is produced via powder metallurgy and thus passes through this important stage. In the past, advances in powder technology have greatly contributed to the development of tungsten and its alloys, as well as today’s high standard of product quality. Powder grades are tailormade for the subsequent applications, and the powder industry is facing a competitive market where the stringent fulfillment of exacting demands is an important part of business success.

The powder is characterized by chemical (purity), physical (grain size, size distribution, shape, agglomeration, etc.), and technological properties (fluidity, compaction density, etc.), which are influenced by the production process and which can be controlled—to a certain extent—by the process parameters.

Today, production of tungsten metal powder is accomplished almost exclusively by the hydrogen reduction of high-purity tungsten oxides. Reduction of the oxides by carbon, common in the early years of metal production, is presently used only for the production of tungsten carbide (direct carburization). The hydrogen reduction of tungsten halides (Axel Johnson process) has not become established on a large scale.

The common starting materials are tungsten trioxide (WO₃) and tungsten blue oxide(WO3-X), the latter being the most widely used material. Tungstic acid (H2WO4) is used only for selected metal grades.

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