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 concentrated by passing the aqueous ammonium metatungstate solution (A) through at least one high-pressure reverse osmosis cell to obtain a concentrate (C) containing AMT and a permeate (P), characterized in that said at least one reverse osmosis is performed under a pressure of more than 50 bar.

2. 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.

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

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

5. The process according to claim 4, characterized in that the permeate obtained is recycled into the process cycle.

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

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

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

9. The process according to claim 8, characterized in that, when the process is operated in a multistage mode, the reverse osmosis cells are operated at different pressures.

10. 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.

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

12. A method of preparing ammonium metatungstate, wherein the method comprises passing an ammonium metatungstate solution through a reverse osmosis cell, characterized in that said at least one reverse osmosis is performed under a pressure of more than 50 bar.

13. The method according to claim 12, characterized in that said reverse osmosis cell contains at least one membrane in the form of a spiral-wound membrane.

Description

(1) FIG. 1 is a diagrammatic view of one embodiment of a device for performing a process for preparing ammonium metatungstate (AMT).

(2) The present invention is described in more detail with reference to FIG. 1 and the following Example, which is not to be construed as limiting the inventive idea in any way, however.

(3) A storage container (1), which is equipped with a stirrer (2) and heat exchanger (3), is at first filled through the valve (4) with diluted AMT solution up to its maximum working volume. After the filling has been completed, the metering pump (5) conveys the solution present in the storage container (1) to a high-pressure pump (6), which forwards the so-called feed solution into an internal cycle driven by the circulation pump (7), in which the feed is mixed with recycled concentrate and supplied to a high-pressure reverse osmosis cell (8), which contains one or more spiral-wound membranes consisting of semipermeable membranes (9) and supporting structures in a pressure pipe. Water passes through the membrane and leaves the total system under gravity as a permeate stream (10). In the internal cycle, the remaining concentrate stream (11) is divided through the pressure control (12) and the control valve (13) into recycled concentrate (17) for the internal cycle and channeled-out concentrate (14). If the plant is operated in batch mode, the concentrate channeled out from the internal cycle maintained by the pump (7) flows back into the storage container (1) under gravity while valve (15) is closed and valve (16) is open. In the storage container, the filling level becomes lower in the batch mode because of the permeate amount being channeled out of the total system over the outer balance limit, so that the AMT concentration increases with time up to a desired preset value, and the batchwise concentration is complete. The internal circulation pump (7) and, in particular, the high-pressure pump supply work to the inner cycle, which leads to significant heating. Part of this excess energy is dissipated as heat with the permeate flow from the total system over the outer balance limit, and the rest is withdrawn from the recycled concentrate (14) through the heat exchanger (3). A constant temperature in the inner cycle is ensured by the temperature control (18), by which the supply of cooling water is controlled.

(4) Alternatively, the system described may also be operated in a continuous mode, in which a feed solution (diluted AMT solution) is permanently supplied to the storage container (1) through the valve (4), and the concentrate formed is led off like the permeate through the opened valve (15) while valve (16) is closed.

EXAMPLE

(5) 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.

(6) 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.