Method for Preparing Ammonium Metatungstate

Abstract

The present invention relates to a process for preparing ammonium metatungstate using a reverse osmosis cell, and to a device for performing the process according to the invention.

Claims

1. A process for preparing ammonium metatungstate (AMT), characterized in that an aqueous ammonium metatungstate solution (A) is passed through at least one reverse osmosis cell to obtain a concentrate (C) and a permeate (P).

2. The process according to claim 1, characterized in that said at least one reverse osmosis is performed in a high-pressure reverse osmosis cell, under a pressure of more than 50 bar.

3. The process according to claim 1, characterized in that said aqueous ammonium metatungstate solution (A) is obtained by the calcination of ammonium paratungstate tetrahydrate, and water leaching the calcinated material.

4. The process according to claim 1, characterized in that said solution (A) is subjected to a filtration step before the reverse osmosis is performed.

5. The process according to claim 1, characterized in that said ammonium metatungstate is recovered by cooling the concentrate (C).

6. The process according to claim 5, characterized in that the permeate obtained is recycled into the process cycle.

7. The process according to claim 1, characterized in that the process is operated continuously.

8. The process according to claim 1, characterized in that the process is operated continuously and in a one-stage mode.

9. The process according to claim 1, characterized in that the process is operated in a multistage mode.

10. The process according to claim 9, characterized in that, when the process is operated in a multistage mode, the reverse osmosis cells are operated at different pressures.

11. The process according to claim 1, characterized in that the concentration of ammonium metatungstate in the aqueous solution (A) is from 150 to 550 g/L.

12. The process according to claim 1, characterized in that the concentration of ammonium metatungstate in the concentrate (C) is at least 1200 g/L.

13. A method of preparing ammonium metatungstate, wherein the method comprises passing an ammonium metatungstate solution through a reverse osmosis cell.

14. The method according to claim 13, characterized in that said reverse osmosis cell contains at least one membrane in the form of a spiral-wound membrane.

15. A device for performing a process according to claim 1, characterized in that said device contains at least one reverse osmosis cell.

Description

EXAMPLE

[0037] The storage container (1) was filled with 500 liters of diluted AMT solution having a density of 1.20 g/cm.sup.3 (at 20° C.). The concentration was 242.5 g of AMT/L. At a predetermined pressure of 110 bar, controlled by the pressure control (12), a concentrate having a density of 2.40 g of AMT/L (measured at 35° C.) was prepared. The concentration of AMT was 1682 g of AMT/L. About 427 liters of permeate was separated off. An analysis of the permeate showed a tungsten content of 1.35 g/L (0.64%) without a change of the NH.sub.4/W ratio being observed. The ammonium content as determined by the Kjeldahl method was 0.067 g/L. As the analysis of the permeate shows, the small loss of tungsten did not cause any significant change in the chemical composition. Since the chemical composition has remained unchanged, the permeate could be recycled completely during operation for preparing the diluted AMT solution. The small loss of tungsten through the membrane of less than 1% shows as a further advantage the economic advantages associated with the process according to the invention.

[0038] As can be seen from the Example described, the process control according to the invention causes only a very small loss of tungsten, while high concentrations of ammonium metatungstate can be achieved.