Preparation of Pure Ammonium Metatungstate (AMT) From Ammonium Paratungstate

Ammonium metatungstate (AMT, (NH4)6H2W12O40·yH2O) is a polycondensation product of solutions of ammonium tungstate. The applications of it include high distectic point metal, extra fine alloy, petrochemical catalyzer and nuclear energy, space flight, casting titanium, etc.

Traditionally, methods for producing AMT include thermal-decomposition, liquid–liquid extraction, ion exchange. The most common process for preparing AMT is by ammonium paratungstate (APT) thermal-decomposition, which was to remove NH3 and control the content of NH4+ in APT crystal to what was needed by thermal decomposition. Unfortunately, this process has obvious drawbacks, which include long production cycle, complex process, high cost and hard to control.

Thus, A synthesis method of high-quality AMT with simple process, less time consumed, lower cost and less pollution has been invented. The coupling process has been applied to the production and APT is the starting material.

The complete preparation procedures are as follows:

APT, deionized water, HDS-12-2 NF membrane (effective membrane area of 2.5 m2, molecular weight cut-off of 350 Da, RMgSO4 98%, RNaCl 34%, RNaNO3 40%, negative charge nature and spiral wound membrane configuration) was purchase without further treatment.

The complete coupling process for the preparation of pure AMT, which include neutralization, nanofiltration, and crystallization.

(1) Neutralization

A certain volume of deionized water was heated, APT crystal and 3 mol/L nitric acid were added slowly to the deionized water with stirring and pH value of this solution was adjusted to 2–4 until the solution specific gravity reached to 1.18–1.20. This solution was stirred and matured for 4–8 h under 90 °C.

(2) Nanofiltration

Generally, NF membranes have average pore sizes of 10–30 Å, an intermediate range between reverse osmosis (RO) (1–10 Å) and ultrafiltration (UF) (>30 Å). Based on the viewpoint of the transport mechanism of NF membranes, most transport models were divided into sieving and charge effect.

(3) Crystallization

A certain volume of AMT solution was heated and stirred with 200–400 rpm until the AMT solution density reached to 2.19 kg/L. As the stirring rate reduced to 200–300 rpm, the solution crystallizes slowly. Finally, AMT crystal was dried in vacuum dryer to obtain the white powder of pure AMT.

In summary, A synthesis method of high-quality AMT with simple process, less time consumed, lower cost and less pollution has been invented. The results showed that the density of AMT crystallization solution was 2.39 kg/L, APT conversion efficiency to AMT was more than 97.62%. Besides, small-grained, even-proportioned, white-colored and high-purified AMT crystal was be obtained and analyzed. This method has several advantages such as simple technological process, short production cycle, lower environmental pollution, and high quality.

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