Chemical Vapor Synthesis (CVS) of Tungsten Nanopowder in Thermal Plasma Reactor Using Ammonium Partungstate

The main sources of tungsten are the high-grade concentrates of wolframite and scheelite ores. Tungsten metal powder is produced from these minerals typically through the intermediate product of ammonium paratungstate (APT). In a subsequent process, tungsten oxides are obtained from APT by calcination in an oxygen bearing atmosphere between 560 °C and 850 °C. Tungsten metal powder is then produced by reducing the oxides with H2. However, it is difficult to produce nanosized tungsten powder with conventional evaporation and condensation methods, due to the high temperature that is needed for evaporation. Nanosized tungsten powder can be produced by various methods such as the electrodeposition, sputtering, ball milling, and complicated chemical methods. But these methods involve multi-steps and have difficulty in establishing commercial application.

The thermal plasma process was applied to produce nanosized tungsten powder using ammonium paratungstate (APT) as the precursor as below:

APT (99.9%) was used as the precursor and hydrogen (99.9%) was used as the reducing agent. The plasma reactor system was set to conduct the experiment, it consistts a plasma generator with a downward plasma torch, a power supply unit, a cylindrical reactor, a cooling chamber, a cooling system, a precursor feeding system, a powder collector, a gas delivery system, an off-gas scrubber solution, and an off-gas exhaust system. The purpose of the scrubbing solution was to remove uncollected powder and to prevent back-flow of the off-gas. The plasma torch consisted of a water-cooled tungsten cathode and a copper anode nozzle operating at atmospheric pressure. The reactor consisted of a vertical water-cooled stainless-steel tube of 15 cm inner diameter and 60 cm length and an inner graphite cylinder of 7.6 cm inner diameter and 60 cm length. Graphite felt was placed between the graphite tube and the inner wall of the water-cooled stainless-steel tube for the insulation of the reactor. The cooling chamber connected to the bottom of the reactor was a water-cooled two-layer stainless-steel box to cool the outgoing gas to a temperature lower than 150 °C. A data acquisition system recorded the temperatures at the reactor exit, the input and output cooling water, and outgoing gas from the cooling chamber. The precursor feeding system consisted of an entrained-flow powder feeder, a vibrator, a carrier gas line, a sample container, and a water-cooled delivery line through which the precursor was fed toward the outside boundary of the visible plasma flame (7 mm diameter) from 15 mm near the exit of the plasma torch.

Argon (99.9%) was used as the plasma gas. Argon was also separately passed through the powder feeder as the carrier gas as well as to keep the atmosphere in the container inert. Before delivering precursor into plasma flame, the reactor was heated by the plasma flame generated until its temperature reached a steady level. A mixture of Ar and H2 flowed through the precursor feeding system to carry ammonium paratungstate powder into the plasma flame. The powder produced was collected using a teflon-coated polyester filter with a pore size of 1 μm. The reactor was purged with an Ar flow of 5 L/min (25 °C, 86.1 kPa total pressure at Salt Lake City) for 10 min before and after each experiment. The detail experimental conditions are discussed in the Section 3 since various experimental conditions were used for specific runs.

Finally, nanosized tungsten powder was obtained from APT by the chemical vapor synthesis (CVS) process in a thermal plasma reactor. The grain size of tungsten was less than 25 nm and was not affected by the plasma torch power and the plasma gas flow rate. But with an increase in the hydrogen flow rate, the grain size of tungsten decreased within the range tested. The produced tungsten powder was treated by hydrogen during which minor amounts of WO2 or WO3 were reduced to tungsten. Finally, nanosized W powder consisting of spherical particles of less than 50 nm was obtained.

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