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.
The furnaces are either gas fired or electrically heated in three or four separate zones. Furnace temperatures range between 600 and 1100℃. For smaller and medium W grain sizes, a temperature profile is preferred in order to decrease the time necessary for the last reduction step from WO2 to W (slow reduction, rate). For larger grain sizes (>6 µm), isothermal reduction conditions are applied.
The reduction is commonly carried out in one stage. Alternatively, a two-stage reduction sequence can be applied instead. In this case, the first reduction stage takes place at lower temperature (500-700℃; formation of brown oxide, WO2) and the second stage at 600-1100℃ (formation of tungsten metal).
In industrial practice, the boats are loaded with a certain oxide weight (layer height) and pushed through the furnace with a given temperature profile and hydrogen throughput. After dynamic equilibrium is reached, the particle size of the metal powder is measured. If the powder does not meet the requirements, parameter adjustments such as change in temperature, boat load, hydrogen throughput, or push time are introduced.
Subsequent to reduction, the powders are screened on 60 mesh (sometimes also on 200 mesh) to eliminate contaminants stemming from furnace or boat materials and are blended to form a homogeneous powder batch. No special atmosphere is necessary for handling, since the powder surfaces are rapidly saturated with oxygen and water vapor. However, below 1 µm, the powders may be pyrophoric and precautions are necessary, in particular below 0.5 µm. Reduction under concurrent hydrogen flow is the most effective method to avoid burning of the fine powders. Already, during the cooling stage in the furnace, the powder is contacted to "wet" hydrogen, and the surface is saturated when the powder leaves the furnace. Under countercurrent flow conditions, the powder has to be slowly saturated with oxygen. This can be achieved either by an inert gas storage (nitrogen or argon containing small amounts of oxygen) or by exposing the powder to the atmosphere in small portions in order to omit local overheating. This can be done by leaving the powder in the boat for approximately 30 min.
It is obvious that the furnace capacity for smaller grain sizes, especially for submicron tungsten powder, is low. Only very thin powder layers can be applied to retain the grains from growing. In order to improve the capacity, a double or triple boat technique was invented. The reduction boat is topped with one or two upper boats in a way permitting hydrogen flow between the boats, so that the capacity also for smaller grain sizes could be increasing considerably.
Modern furnaces are fully automated, which means that all variables can be set and controlled. Loading, pushing, and discharge of the boats is done by machine.
The advantage of the push-type furnace in comparison to rotary kilns is its flexibility in switching from one condition (grain size) to the next and in its high capacity, especially for finer powder qualities. Disadvantages are higher energy consumption, broader grain size distribution, more contamination by the scale from the boats, and higher maintenance costs.
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