Ultrafine Tungsten Powder Preparation by Hydrazine Tungstate

Organic hydrazine tungstate, the formula is 5(N₂H₅)₂O•12WO₃(also call auto reducing tungstate, abbreviated as ART). Ultrafine tungsten powder can be prepared by hydrogen reduction using ART. In solutions containing tungstate, the generation of ART is based on the following reactions:

the content of WO₄^2- is 0.5-1.5 mol/l in WO₄2-containing solution, the pH value of the solution is 7-9, and the temperature is 60-90°C. An aqueous solution of biuret is added so that the N₂H₄/WO₄^2-mol ratio is In the range of 0.8-1.5/l. The precipitation rate of ART can be more than 95%. Under the hydrogen or inert gas atmosphere, the ART is thermally decomposed at 420°C-470°C, and it can be completely decomposed into tetragonal WO_2.90 blue tungsten, and then under a high temperature of 600-850°C, the blue tungsten can be reduced with hydrogen.  Ultrafine(fine) tungsten powder with 0.2-2 μm Fisher particle size was obtained.

The main factor in determining the precipitation rate of ART is the pH value of the solution during precipitation. A low pH results in a high rate of ART precipitation. There are two ways to reduce the solution pH value during ART precipitation.

Method 1: The NH₄WO₄ solution containing free ammonia is heated and vaporized with NH₃ before being mixed with the biuret. When the pH value of the solution is reduced to 7-9 and the hydrazine is added, the pH of the solution can be stabilized at 7-9 during the reaction.

Method 2: Add appropriate amount of acid or ammonium salt to the bishydrazine before precipitation operation to make a hydrated bishydrazine -acid (or hydrated bishydrazine -ammonium salt) solution with a pH of 7-9, and then mix it with the ammonium tungstate solution. The pH of the solution during the precipitation of ART can be stabilized at around 7-9. Precipitation reaction at 60-90°C for 4-5 hours can make ART precipitation rate more than 95% and ART is relatively pure.

Because the reaction itself emits a large amount of gas to disintegrate the ART particles and form explosive thermal decomposition, these “thinning” make the generated blue tungsten oxide have much smaller particles than the ART blue tungsten, and the ART blue tungsten is further reduced. The phase transformation of β-tungsten into tungsten powder can be “refined” again, so ART blue tungsten is a good raw material for producing ultra-fine tungsten powder. The experimental results show that if the particle size of ART raw material is smaller than the obtained blue tungsten particle size, ART Fisher particle size is 3-11μm, blue tungsten with a Fib particle size of 2-6μm can be obtained accordingly, and the protective gas adopts hydrogen more closely to the industry.

Using ART blue tungsten as raw material, ultrafine tungsten powder with a particle size of 1 μm or less is prepared. The optimum process conditions are as follows: five sections of furnace temperature 600-800°C, loading capacity 150-200g per boat, and reduction time 4-6hour.

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