Tungsten Metal Powder Production

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. 

The common starting materials are tungsten trioxide (WO3) and tungsten blue oxide (W03 t), the latter being the most widely used material. Tungstic acid (H2WO4) is used only for selected metal grades. 

In principle, APT can also be directly reduced without any prior calcination 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. 

FIGURE 5.19. Average grain influencing reduction parameters. 

Temperature. Temperature influences the rate of all reactions occurring during reduction, hence the dynamic humidity and partial pressure of the volatile [W02(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. 

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The goal of this step—in the classical process achieved by tungstic acid precipitation and successive dissolution by ammonia—is almost total separation of sodium ions, which have been necessary for dissolution but which interfere in the further production steps to tungsten metal powder. Their concentration will be decreased quite effectively from approx. 70g/l to less than lOmg/1. 

Reduction to Metal Powder. The metal powder is obtained from APT by stepwise reduction with carbon or hydrogen. The intermediate products are the yeUow oxide, WO blue oxide, (see Tungsten compounds) and brown oxide, WO2. Because carbon introduces impurities,... 

The APT may directly be reduced with hydrogen to produce tungsten powder. The powder may be pressed, sintered and fabricated to produce tungsten metal and alloy products.The tungsten powder may be heated with carbon to form tungsen carbides which may be converted to cast carbides or certain grades of cemented carbides. Or the tungsten powder may be alloyed with specific metals to form various non-ferrous alloys. 

Production. The classical method is to mix Re and W powders in the desired ratio prior to compaction and sintering. In order to achieve a more even W-Re distribution, restricting the danger of local a-phase formation, mixtures of tungstic acid or tungsten trioxide or tungsten blue oxide with ammonium perrhenate can be used as raw materials. These mixtures are co-reduced by hydrogen to metal powder. 

The products prepared in the above manner are ground as finely as possible and compressed under 2000 kg./cm into rods 3 x 40 or 5 X 40 mm. Successful molding usually requires the addition of 2-5% of metal powder. The rods are embedded in nitride powder (to prevent formation of an oxide coating) and presintered in a small tubular tungsten furnace (cf. Part I, p. 40) at about 2300°C in a stream of Ng the small amount of free metal is converted to nitride in the process. Since the reaction is usually accompanied by considerable shrinkage of the rods and concomitant appearance of porosity, the material must be repulverized, remolded and resintered. This procedure is repeated two to four times, until the presintered rods exhibit some constancy of density. 

The particle size of the metal powder does not depend on that of the starting WO3, but (principally) on the reaction temperature, as well as the heating time and the Hg flow rate. An especially fine tungsten powder is obtained when the above directions are followed a very coarse powder results from reduction with moist Hg above 1500°C. The product is pure if the starting WO3 is also pure. 

Production and quality control of uranium fuel rods used in nuclear power plants are monitored by DC arc emission spectroscopy. Trace elements in high-purity metal powders are measured for quality control purposes. Tungsten powder used to make light bulb hlament can be analyzed for trace elements by arc/spark emission spectroscopy without the need to dissolve the tungsten this eliminates the use of expensive and hazardous hydrofluoric acid. 

Cobalt powder is an important component in cemented carbides (hard metals). The product and its use of cobalt are described in Chapter 26 Tungsten. 

Molybdenum is also recovered as a by-product of copper and tungsten mining operations. The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate. 

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