Blue Tungsten Oxide

Blue tungsten oxide is one of the most important raw materials for the manufacture of tungsten powder. In industry, blue tungsten oxide generally refers to the WO2.90 , W20O58 and ammonium tungsten bronze (ATB) mixture.

In the beginning of 70s, the United States, France, Britain and other countries have successively adopted blue tungsten oxide as raw materials, replacing yellow tungsten oxide in the production of tungsten powder. At the end of 1970s and early 80s, the blue tungsten began to be developed in China. And at the end of 80s and early 90s, it almost replaced the yellow tungsten oxide.

As an upgraded version of the yellow tungsten oxide, blue tungsten oxide is widely used. Compared with yellow tungsten oxide, using blue tungsten oxide is easier to control the particle size and composition, which is beneficial to add other elements during the reduction of tungsten powder. Containing tiny amounts of ammonia, blue tungsten oxide’s true density is 7.16 g / cm. Thus in the production and mixing process, using blue tungsten oxide will have less loss and higher metal absorptivity. Additionally, it is more time-saving as using blue tungsten oxide to produce tungsten powder only by single reduction.

However, it is still unclear about the composition of blue tungsten oxide for industrial use. Blue tungsten oxide is a powder mixture composed of various tungsten compounds. Usually, it is composed by β-tungsten oxide (WO2.9) and γ-tungsten oxide (WO2.72). Some people said that the best phase composition is hexagonal ATB and square ATB. Some people view that the best phase composition is square WO2.9. While others think that the phase composition of monoclinic W20O58 (β-tungsten oxide) is best. A jury is not out on the matter.

There are three main methods for preparation of the blue tungsten oxide: APT closed calcination method, APT mild hydric reduction method and internal reduction method.

ATP closed calcination method always keeps the furnace system with positive pressure in the heating process, preventing the air from entering the furnace. By this way, ammonia from APT pyrolysis splits into hydrogen and nitrogen. And in the weak reductive atmosphere, the blue tungsten oxide is produced.

The APT mild hydric reduction method carries through in a multi tube furnace. After packaging, heating APT in a furnace tube filled with reductive hydrogen. Gas product and waste gas are discharged and then fired. The phase composition can be controlled by the flow rate of hydrogen, reduction temperature and pushing rate.

Internal reduction method dissolves tungstic acid in ammonia with appropriate quantity of carbamide to get intermediate IRT which then decomposes into blue tungsten oxide in ammonia.

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