Ammonium paratungstate

• APW
• Ammonium tungstate

White crystalline powder

Fixed

2.3 g · cm -3

Decomposing above 200 ° C.

Attention

Template: Infobox chemical / molecular formula search available

Ammonium para- tungstate (APT or APT for engl. Ammonium para tungstate ) is a white crystalline tungsten salt with the chemical formula (NH4 ) 10 ( H2W12O42 ) · 4H2O.

Production

Ammonium paratungstate is an intermediate in the preparation of the tungsten-containing ores, and is obtained by precipitation of tungstate ions with ammonium ions. Below 50 ° C the Henahydrat forms, while the " pentahydrate " forms at temperatures in the solution above 50 ° C. The former crystallizes as triclinic platelets or prisms, while the latter pseudorhombic needles forms.

Tungsten-containing ores such as scheelite ( CaWO4 ) or wolframite ( (Mn, Fe) WO4 ) are initially enriched by milling and flotation to concentrations of 10 to 75%. The enriched ore is then calcined in an oxidizing atmosphere at 500-600 ° C to remove impurities, such as additives selected from the Flotationprozeß. Wolframite ores are after, scheelite ores reacted with sodium hydroxide solution with a sodium carbonate solution, with sodium tungstate Na2WO4 forms, which is then purified by a series of reprecipitation. From the resulting purified solution of the ammonium paratungstate is recovered by liquid-liquid extraction or by ion exchange. Tungsten-containing scrap, such as cemented carbide scrap can be converted by oxidation to sodium tungstate, and also thereafter to ammonium tungstate.

APW, is the world's generally customary commercial form of tungsten-containing raw materials and is usually in MTU 's ( metric ton unit) traded, with an MTU contains 10 kg of WO3.

Structure

In recent literature, the anion in the ammonium paratungstate [ H2W12O42 ] 10 -, which has two hydrogen atoms within a cage as described. The tungsten - oxygen cage, which forms the anion center, containing 42 oxygen atoms. Accordingly, the correct molecular formula of ammonium para- tungstate ( NH4) 10 [ H2W12O42 ] · 4H2O and not as previously described in the literature a " pentahydrate " of type ( NH4) 10 [ W12O41 ] · 5H2O with five crystal water. The [ H2W12O42 ] 10 - anion is called Parawolframation type B, in contrast to Parawolframation, type A, which is the sum formula [ W7O24 ] 6 - according to the Paramolybdation, possesses. The existence of para-tungstate - type A anion, so far could not be detected by NMR spectroscopy.

Reactions

Ammonium paratungstate can be converted by thermal decomposition at a temperature of 600 ° C in the tungsten ( VI) oxide WO 3.

The thermal decomposition of APW is done in 4 steps:

Use

APT is used especially as intermediates in the preparation of the tungsten-containing ore to tungsten or tungsten compounds. It is converted by reduction technology in a hydrogen atmosphere with the elimination of water vapor in the elemental tungsten powder, which can be converted by sintering into various forms such as wires or blocks. By carburization you get the above all in the Hartmetallindustie tungsten carbide used.