Preparation of Ultrafine Tungsten Powder by Hydrazine Tungstate

Organic amine tungstate, molecular formula is 5(N₂H₅)_2`O₁₂WO₃ (called self reductive tungstate, Auto Reducing Tungstate, as abbreviated as ART). Ultrafine tungsten powder can be prepared by hydrogen reduction by ART. In a solution containing tungstate root, the formation of ART is based on the following reaction:

In the solution containing WO₄^2-, when the WO₄^2- content is 0.5-1.5mol/l, the pH value of the solution is 7-9 and the temperature is 60-90 degrees, adding the hydrazine aqueous solution, the N₂H₄/ WO₄^2- mole ratio is in the 0.8-1.5/l range, which can make ART precipitation rate more than 95%. Under the atmosphere of hydrogen or inert gas, thermal decomposition ART at 420-470 ℃ can be completely decomposed into tetragonal WO_2.90 blue tungsten, and then hydrogenated at a temperature of 600-850 ℃, ultrafine (fine) tungsten powder with a Fei particle size of 0.2-2 um is available.

In the process, the main factor determining the ART precipitation rate lies in the pH value of the solution during the precipitation process and the higher ART precipitation rate when the pH value is low. Through experiments, two kinds of methods to reduce the pH value of the solution of ART precipitation in the ART precipitation process have been found for the solution of ammonium tungstate with a high concentration of free NH₃.

1.The ammonium tungstate solution containing free ammonia was heated and steamed NH₃ before mixing with diamines. When the pH value of the solution was reduced to 7-9, then the hydrazine hydrate was added to the solution. The pH of the solution was stable at 7-9 during the reaction.

2.Before the precipitation operation, add some acid or ammonium salt to the diamines, make the hydrazine acid (or hydrazine ammonium salt) solution of pH7-9, and then mix it with ammonium tungstate mixture, and also make the pH value of the solution ART stabilize at about 7-9. According to the above two methods, the precipitation rate of ART can reach more than 95% at 60-90 ℃ for 4-5 hours, and the product ART is pure.

In the process of reducing tungsten into tungsten powder, in order to ensure the complete reduction of blue tungsten, a reasonable reduction temperature and reduction time should be selected, and the high temperature tends to make the tungsten powder coarse. This ART based blue tungsten oxide as raw materials, preparation of Fisher particle size less than 1 um of ultrafine tungsten powder, the optimum conditions were as follows: five the temperature of 600-800 ℃, the load of each boat 150-200 grams, the reduction of residence time of 4-6 hours.

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