STUDY OF THE REDUCTION OF TUNGSTEN TRIOXIDE AND TUNGSTEN BLUE OXIDE DOPED WITH NaX (X = Cl, Br, NO2, NO3), Na2CO3 AND KY (Y = Cl, Br)

摘要

The wet or dry doping of tungsten trioxide, WO3, with alkali salts and the subsequent hydrogen reductionof the doped powder is integral to producing coarse tungsten powders. Hundreds of parts per millionalkali are commonly added, yet after reduction, the tungsten powder contains only a minor amount of theadded dopants.Tungsten trioxide, WO3, and Flowable Tungsten Blue Oxide (TBO) were spray-doped with NaX/KY (X= Cl, Br, NO2, NO3, ?CO3; 150 - 900-ppm Na; Y = Cl, Br; 250 - 1500-ppm K). The doped powders werereduced in dry hydrogen in a laboratory furnace either by heating from RT directly to 900oC, or byincluding an intermediate hold at 600oC, followed by the final reduction at 900oC. Alternatively, the boatwas immediately inserted into the 900oC zone.The as-reduced powders were characterized for Bulk and Tap density, BET, as well as Chemical (Na, K;Extractable Cl-, Br-, WO42-) and Sieve analyses. Fisher number and Laser granulometry (Microtrac) weredetermined for both as-is and rod-milled samples. Powder morphology was evaluated via image analysisof suitably prepared cross sections.During the reduction process sodium and potassium monotungstates are formed according to WO3 + MX+ H2O → M2WO4 + 2HX. Under the chosen reduction conditions, the monotungstates are partiallyreduced according to M2WO4 + 4H2 → 2M + W + 4H2O. The alkali metal M is transformed into thevolatile hydroxide MOH.Based on the FSSS numbers, the metal powders made from doped WO3 are distinctly coarser than thepowders made from doped TBO. The rod-milled FSSS numbers are distinctly lower than the as-is data.Thus, doping WO3 produces highly agglomerated powders. Compared to the straight and immediatereduction, the hold at 600oC generates noticeably finer powders with only minor agglomeration. In thiscase, as-is and rod-milled FSSS numbers are nearly identical.Reduced metal particle morphology is similar for both doped WO3- and TBO-based powders, irrespectiveof the dopant used, its concentration, or the reduction temperature profile employed.