TUNGSTEN METAL POWDER PRODUCTION - Tungsten Powder - 1

4. Tungsten Powder

Technical tungsten powder grades prepared by hydrogen reduction are commercially available in average grain sizes from 0.1 µm (100nm) up to 100µm. The whole palette of grain sizes find their application in cemented carbides. The main portion of tungsten powder produced (80%) is directed into that production.

The starting powder for the powder metallurgical production of pure tungsten (ductile tungsten) and sintered tungsten alloys commonly covers grain sizes between 2and 6µm. Finer or coarse powder would be unsuitable. Fine powder has too high a sintering activity, not permitting the evaporation of trace impurities during the sintering process, while coarse powder leads to incomplete sintering under usual conditions.

Extremely coarse powder grained by classification (to separate any finer particles) exhibits excellent flow characteristics and is used in plasma spraying.

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TUNGSTEN METAL POWDER PRODUCTION-Rotary Furnace-2

In order to maintain the humidity for a certain grain growth, only a small excess of hydrogen is necessary. This corresponds to the lowest values in pusher furnaces, applied only for very coarse powder. Moreover, the influence of the H₂/WO₃ molar ratio on grain growth is less pronounced.

The hydrogen is cleaned and dried and recycled to the furnace after preheating. Due to the constant motion of the powder bed, fine particles are swept away by the hydrogen flow and form a dust, which must be removed by filtering devices.

Very coarse blue oxide has improved flow characteristics and helps to prevent powder from sticking to the tube wall.

Due to the “dry” conditions, rotary-furnace-produced tungsten powder shows a high degree of agglomeration, lower apparent density, and closer grain size distribution. The upper size limit is approx 6 µm.

Furnaces in use possess different sizes: inner diameter between 300 and 750 mm and heated drum length from 3000 to 8000 mm. The most common equipment has an inner diameter of 600mm and heated length of 6500mm.

Advantages of the rotary furnace compared to a pusher are less specific energy consumption, while automation is less complicated and consequently maintenance is cheaper. Finally, the product is more homogeneous and less contaminated, but the particle morphology may differ significantly from powders produced in pusher furnaces.

TUNGSTEN METAL POWDER PRODUCTION-Rotary Furnace-1

 Rotary Furnace
In contrast to the pusher furnace, where a static powder layer passes the furnace, a dynamic powder flow exists in the rotary kiln. Furnace rotation and the incline continuously move the powder through the hot zone. Feed rate, rotational speed, incline, and lifters inside the tube determine the depths of the dynamic powder layer. High oxide feed, longer retention time (low rotation rate and low inclination), and higher temperature result in coarser tungsten grains.
The powder layer is constantly disturbed by the rotating motion and powder from inside moves to the surface, and vice versa. Therefore, the water-vapor retaining capability of the layer is less compared to a static layer, and the material exchange rate H2O→H2 is of the layer is enhanced. The humidity in a rotary furnace powder layer is thus lower and reduction proceeds under drier conditions as compared to the pusher. Lifters inside the tube combined with stepwise turning motion make sure that a powder bed is maintained for a certain time giving the chance to build up higher humidity both temporarily and locally.

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TUNGSTEN METAL POWDER PRODUCTION-PUSH-TYPE FURNACE-4

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.

TUNGSTEN METAL POWDER PRODUCTION-PUSH-TYPE FURNACE-3

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.

TUNGSTEN METAL POWDER PRODUCTION-PUSH-TYPE FURNACE-2

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).