A Greener Preparation Process of Ammonium Paratungstate (APT) by Emulsion Liquid Membrane

Ammonium paratungstate（APT）is white crystalline salt, with the chemical formula (NH₄)₁₀(H₂W₁₂O₄₂)•4H₂O. It is the most important raw material for most of tungsten products. Intermediates such as tungsten trioxide, tungsten blue oxide, tungstic acid, can be obtained from APT, either by thermal decomposition or chemical conversion.

Solvent extraction (SE) and ion exchange (IE) are the two common synthesis method of APT. In the SE method, the raw sodium tungstate (Na₂WO₄) solution from tungsten ore firstly needs to eliminate P, As, and Si by adding HCI and MgCl₂, and Mo by adding Na2S, which is  a pollutant. Further, the acidic waste water needs to be treated and the residual W in it needs to be recovered. In the IE method, although P, As and Si do not need to be eliminated, yet Mo is still needed.

Thus, a greener method is needed. Emulsion liquid membrane (ELM), is considered as an simple, effective method to produce APT. A new process has been described as follows:

 The alkaline feed of crude Na₂WO₄ solution from the caustic fusion of black tungsten ore was neutralized to a pH value near 9 to remove Si as routine, then the resulting feed was directly used as the external aqueous phase without eliminating P, As and Mo.

 Through a single stage batch liquid membrane process at room temperature for 5 rain under optimum operation conditions obtained by an orthogonal test, APT directly crystallizes in the internal aqueous phase with 99.87% yield and the quality attained was the first grade of a national standard; meanwhile the pH value of the effluents was lowered to 6-7 in itself suitable for direct discharge.

In conclusion, direct crystallization in the inner aqueous phase of the liquid membrane is a new method for producing high quality APT, with high efficiency of 99.87% yield.

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