Method for concentrating an aqueous solution of ammonium metatungstate, method for producing ammonium metatungstate, use of a reverse osmosis cell for concentrating an aqueous solution of ammonium metatungstate, and apparatus for carrying out the method for concentrating an aqueous solution of ammonium metatungstate.

Abstract

The present invention relates to a method for producing ammonium metatungstate using a reverse osmosis cell, and to an apparatus for carrying out the method according to the invention.

Description

Technical Field

[0001] The present invention relates to a method for producing ammonium metatungstate using a reverse osmosis cell and an apparatus for carrying out the method according to the present invention.

Background Art

[0002] Ammonium metatungstate (AMT) is used, inter alia, for the production of catalysts. There are various methods for producing ammonium metatungstate, which can be classified into the categories of solid-phase conversion and liquid-phase conversion. Generally, solid-phase conversion involves a thermal decomposition step of ammonium paratungstate (APT), while in liquid-phase conversion, decomposition of ammonium paratungstate to ammonium metatungstate is carried out by acidification of ammonium paratungstate.

[0003] German Patent No. 3743267 relates to a method for producing ammonium metatungstate having a loss on ignition of 5.6 to 5.9% by weight by firing ammonium paratungstate in a fired aggregate at a temperature of 150 to 400 °C and then leaching the resulting fired product with water. In order to obtain the desired loss on ignition, fired products having a loss on ignition of less than 5.6% by weight and fired products having a loss on ignition of more than 5.9% by weight can also be proportionally mixed so that the loss on ignition of the mixture falls within the scope of the claims, thereby obtaining a high yield.

[0004] European Patent No. 0193171 discloses a method for producing ammonium metatungstate from ammonium paratungstate, which includes heating ammonium paratungstate at a temperature of 200 to 400 °C, dissolving the heated ammonium paratungstate in water to form an ammonium metatungstate aqueous solution, evaporating the ammonium metatungstate solution to form a concentrated ammonium metatungstate solution, separating insoluble substances from the concentrated ammonium metatungstate solution, and crystallizing ammonium metatungstate from the concentrated ammonium metatungstate solution.

[0005] U.S. Patent No. 7,794,686 describes a method for producing ammonium metatungstate, comprising the preparation of a mixture of solid ammonium paratungstate and water. The mixture is brought into contact with a cation exchange material to lower the pH of the mixture to a range in which the ammonium metatungstate ions are stable and the formation of insoluble tungstic acid is prevented. The mixture is then maintained at this pH until most of the ammonium paratungstate is converted into a solution of ammonium metatungstate. To implement this method on an industrial scale has been found to be very complex depending on the ion exchange material used, as the ion exchange material must be regenerated with acid, and the resulting solution of ammonium salt cannot simply be discharged into the receiving water but must be reused.

[0006] European Patent No. 0200170 describes a method for producing ammonium metatungstate from ammonium paratungstate, the method comprising calcining ammonium paratungstate at a temperature of 275–300°C to form a slurry. The slurry is evaporated to 20% of its original volume to separate insoluble matter and obtain a concentrated ammonium metatungstate solution. As a final step, the described method comprises crystallizing ammonium metatungstate from the concentrated ammonium metatungstate solution. In the described method, it is considered particularly advantageous to leach the calcined material at a very low concentration of less than 12 g / L in order to achieve a high overall yield. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] These known methods for producing ammonium metatungstate share the common characteristic of requiring an evaporation step, which necessitates significant energy consumption. Therefore, while efforts are currently underway to improve the sustainability of existing production methods, there is a need for a method of producing ammonium metatungstate with less energy consumption.

[0008] Numerous alternative concentration methods are described in relation to the production of various tungstate compounds.

[0009] For example, U.S. Patent No. 5,178,848 discloses a method for producing lithium metatungstate by treating an aqueous solution of lithium monotungstate with a cationic extractant to lower the pH of the solution to 3.5-5.0 and obtain a diluted solution of lithium metatungstate. In the next step, it is proposed to concentrate the diluted solution by removing water, using evaporation by heating, vacuum treatment, heating under vacuum, reverse osmosis, or a combination of such methods for this step. The formation of undesirable lithium paratungstate is prevented by saturating the lithium tungstate solution with colloidal tungsten trioxide.

[0010] In the paper "Study on new method of the preparation of pure ammonium metatungstate (AMT) using a coupling process of neutralization - nanofiltration - crystallization" by J.-Q. Liu et al., published in Journal of Membrane Science 240 (2004) 1-9, a method for producing ammonium metatungstate by concentrating an aqueous solution of ammonium metatungstate using nanofiltration is described.

[0011] The isopolyanionic properties of tungstic acid salts mean that different metal salts have fundamentally different properties, and therefore, in principle, the experience and knowledge gained in the production of one metal salt can only be applied to a very limited extent in the production of other metal salts.

[0012] Therefore, the objective of the present invention is to provide a method for producing ammonium metatungstate that replaces conventional methods and reduces specific energy consumption. [Means for solving the problem]

[0013] Surprisingly, it was found that this objective could be achieved in the production of ammonium metatungstate by concentrating the ammonium metatungstate solution using a reverse osmosis cell.

[0014] Therefore, the first object of the present invention relates to a method for producing ammonium metatungstate by passing an aqueous solution of ammonium metatungstate (A) through at least one reverse osmosis membrane cell to obtain a concentrate (C) and a permeate (P). [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram illustrating a manufacturing apparatus relating to one embodiment of the present invention. [Modes for carrying out the invention]

[0016] Surprisingly, it was discovered that by using a reverse osmosis cell, a concentrated ammonium metatungstate solution could be obtained without the membrane clogging due to equilibrium shifts caused by the different permeability of various isopolytungstate ions. This eliminates the energy-intensive evaporation process that was essential for the conventional production of ammonium metatungstate. Furthermore, eliminating the evaporation process in the production of ammonium metatungstate removes a major bottleneck, resulting in reduced specific energy demand, shorter throughput times, increased production capacity, and lower manufacturing costs. Reduced energy demand also leads to reduced CO2 emissions, which directly contributes to the sustainability of the manufacturing process.

[0017] The use of reverse osmosis cells is generally known to those skilled in the art. For example, International Publication No. 2004 / 099087 describes a method for treating nitrate-containing wastewater, in which, after preliminary washing to remove solids and suspended matter and separate alkaline earth metals and heavy metal ions by precipitation and ion exchange, CO2 is removed at a lower pH value and the wastewater is passed through at least one reverse osmosis cell and / or electrodialysis cell. Preferably, reverse osmosis in a multi-stage countercurrent method yields a maximum NaNO3 concentration of 200 g / L.

[0018] Within the scope of this invention, it has been discovered for the first time that reverse osmosis membranes can be applied not only to simple inorganic salts in aqueous solutions but also to metals forming isopolyanions, in which case a complex equilibrium may exist between different species, which must not be interfered with by any possible selective ion permeability of the membrane.

[0019] Simple salts such as NaCl, NaNO3, Na2SO4, NH4Cl, NH4NO3, or (NH4)2SO4 dissolve mainly in water to form simple ions. The presence of such ions is independent of the concentration or pH of the solution. Elements that form complex isopolyanions include vanadium, niobium, tantalum, and molybdenum, with tungsten being a particularly unusual case.

[0020] Generally, monomer WO4 2- The formation of isopolytungstates proceeding from this can be formulated according to the following equation. pH + +qWO4 2- =[H p-2r W q O 4q-r ] (2q-p)- +rH2O

[0021] The equilibrium depends on the pH, concentration and temperature of the solution. Due to such complex interrelationships, when using a reverse osmosis cell with isopolymetallate, due to the ion-selective permeability, it has been considered that undesirable compounds such as tungstic acid or ammonium paratungstate are locally precipitated in the cell. The present invention overcomes this prejudice. In the present invention, surprisingly, contrary to the concerns of the prior art, precipitation or clogging of the used membrane was not observed.

[0022] Within the scope of the method according to the present invention, it has been found that it is particularly advantageous to use a high-pressure reverse osmosis cell. Thus, in one embodiment of the method according to the present invention, reverse osmosis is preferably carried out in a high-pressure reverse osmosis cell under a pressure of preferably 50 bar or more, preferably 90 bar or more, more preferably 100 bar or more, particularly 120 bar or more, particularly 150 bar or more.

[0023] The method according to the present invention further has the advantage that it can be applied to the conventional method of obtaining ammonium metatungstate starting from ammonium paratungstate. Therefore, an embodiment in which ammonium paratungstate tetrahydrate is calcined and the calcined product is leached with water to obtain an aqueous solution (A) of ammonium metatungstate is preferred.

[0024] In order to separate solids and suspended matter from the aqueous solution (A) of ammonium metatungstate, a filtration step can be passed through. Thus, in a preferred embodiment of the process according to the present invention, the solution (A) is subjected to a filtration step before reverse osmosis is carried out.

[0025] By using a reverse osmosis cell in the method according to the present invention, the energy-intensive evaporation step normally required to produce a concentrated ammonium metatungstate solution is eliminated. The reverse osmosis method yields a concentrated ammonium metatungstate solution with significantly reduced energy consumption, and the desired product can be separated therefrom in a further process, achieving energy savings of 10% or more. Therefore, in a preferred embodiment, ammonium metatungstate is obtained by cooling the concentrated solution (C) obtained after reverse osmosis. Furthermore, if the required quality is low, ammonium metatungstate can also be obtained, for example, by spray-drying the solution concentrated by reverse osmosis.

[0026] The method according to the present invention is characterized in particular by its energy efficiency and associated sustainability. This is also reflected in the process control. Therefore, an embodiment in which the obtained permeate is returned to the cycle is preferred. In this way, wastewater generation can be reduced while ensuring high efficiency. Surprisingly, it has been found that if trace amounts of impurities may accumulate in the mother liquor, after many cycles, it is only necessary to discharge a portion of the mother liquor to separate the impurities. The tungsten contained in these mother liquor fractions is completely returned to the process for producing ammonium paratungstate, which is the starting compound for producing ammonium metatungstate. The method according to the present invention can be operated continuously, in batches, or discontinuously. To efficiently utilize the production equipment, the method according to the present invention is preferably carried out continuously.

[0027] The efficiency of reverse osmosis is remarkably high, allowing the process to be carried out in a single step, which is particularly advantageous in terms of cost and time savings. In a preferred embodiment, the method according to the present invention is carried out as a single step. The efficiency of this process is further enhanced by completely reusing the obtained permeate in the manufacturing process, by using the obtained permeate to leach the calcined material, i.e., to produce an aqueous solution of ammonium metatungstate (A). Surprisingly, it has been found that the reverse osmosis cell can be operated at very high pressures of 110 bar or more, thereby obtaining a concentrate with a concentration of ammonium metatungstate exceeding 1200 g / L. Furthermore, it has been found that the method according to the present invention avoids product loss due to, for example, ammonium metatungstate contained in the permeate. By completely using the permeate in the leaching step, there is no need to concentrate the ammonium metatungstate in the permeate or return the permeate to a further upstream processing step to produce ammonium paratungstate.

[0028] To further enhance the efficiency of the method according to the present invention, a multi-stage process is possible, i.e., the solution can be passed through two or more reverse osmosis cells. Therefore, a multi-stage configuration of the method according to the present invention is preferred. Surprisingly, it has been found that the usual procedure of guiding the concentrate flow and the permeate flow in opposite directions is not required in the method according to the present invention. A multi-stage processing control, preferably using multiple reverse osmosis cells connected in series, has the further advantage that the reverse osmosis cells can be individually adapted to corresponding requirements. Therefore, in embodiments of the method according to the present invention, it is preferable to operate the reverse osmosis cells at different pressures when the method is performed in a multi-stage process.

[0029] In particular, the method according to the present invention utilizes reverse osmosis to produce a concentrated ammonium metatungstate solution, from which the desired product, ammonium metatungstate, can be obtained. No special requirements are imposed on the production of the aqueous ammonium metatungstate solution. Rather, surprisingly, it has been found that even highly diluted solutions containing only low concentrations of ammonium metatungstate, as described as advantageous in the prior art, can be efficiently converted. In this case, it has been found to be advantageous to first connect multiple reverse osmosis cells in parallel, and then connect such blocks in series in stages. As the number of stages increases, the number of cells connected in parallel per stage can be reduced. The method according to the present invention can also be used for particularly diluted ammonium tungstate solutions, such as those obtained by some production methods. To concentrate such solutions, especially dilute ammonium metatungstate solutions with concentrations of less than 100 g / L, even less than 50 g / L, or even less than 25 g / L, the method according to the present invention, in a preferred embodiment, connects multiple reverse osmosis cells in parallel and connects the resulting blocks in series upstream of a single reverse osmosis cell.

[0030] In a preferred embodiment, the concentration of ammonium metatungstate in the aqueous solution (A) before passing through the reverse osmosis cell is 150-550 g / L, preferably 250-500 g / L, and more preferably 200-300 g / L. Surprisingly, by using the reverse osmosis cell according to the present invention, a particularly high concentration of ammonium metatungstate solution can be obtained, and thus efficient processing control can be achieved. Therefore, an embodiment in which the concentration of ammonium metatungstate in the concentrated solution (C) after passing through the osmosis cell is at least 1200 g / L, preferably at least 1500 g / L, is preferred.

[0031] In the method according to the present invention, it was further found that the presence of small amounts of foreign ammonium salts, such as NH4Cl, NH4NO3, or (NH4)2SO4, in aqueous solution (A), as obtained by several methods for preparing aqueous solution (A), is not significant. Surprisingly, the presence of foreign salts was shown to have no adverse effect whatsoever on the method according to the present invention.

[0032] No special requirements are imposed on the reverse osmosis cell used in the method according to the present invention. However, it has been found to be advantageous when a reverse osmosis cell containing a membrane in a helical winding configuration is used. Therefore, embodiments in which the reverse osmosis cell contains at least one membrane in the form of a helical winding are preferred.

[0033] Another object of the present invention relates to the use of a reverse osmosis cell in the production of ammonium metatungstate. Particularly preferably, the reverse osmosis cell is a high-pressure reverse osmosis cell, which preferably comprises at least one membrane in a helical winding configuration.

[0034] Another object of the present invention relates to an apparatus for carrying out the method according to the present invention, comprising at least one reverse osmosis cell, preferably a high-pressure reverse osmosis cell.

[0035] The present invention will be described in more detail with reference to Figure 1 and the following embodiments, but this should not be construed as limiting the concept of the present invention.

[0036] A storage container (1), equipped with a stirrer (2) and a heat exchanger (3), is first filled with diluted AMT solution to its maximum working volume via a valve (4). After filling is complete, a metering pump (5) sends the solution present in the storage container (1) to a high-pressure pump (6), which transfers the so-called feed solution to an internal cycle driven by a circulation pump (7), where the feed solution is mixed with the recycled concentrate and supplied in a pressure tube to a high-pressure reverse osmosis cell (8) containing one or more helically wound membranes consisting of a semipermeable membrane (9) and a support structure. The water passes through the membrane and is discharged from the entire system without pressurization as an osmotic flow (10). In the internal cycle, the remaining concentrate flow (11) is separated by a pressure control device (12) and a control valve (13) into recycled concentrate (17) for the internal cycle and discharged concentrate (14). When the system is operating in batch mode, the concentrate (14) discharged from the internal cycle, maintained by the pump (7), flows back into the storage container (1) without pressurization, with valve (15) closed and valve (16) open. In the storage container, in batch mode, the fill level is lowered because the amount of permeation flowing out from the entire system exceeds the external balance limit, and as a result, the AMT concentration rises to the desired set value over time, completing the batch concentration process. The internal circulation pump (7), in particular the high-pressure pump (6), supplies work to the internal cycle, leading to significant heating. Some of this excess energy is dissipated as heat through the external balance limit along with the permeation flow from the entire system, and the remainder is taken from the recycled concentrate (14) via the heat exchanger (3). A temperature control (18) that controls the supply of cooling water ensures a constant temperature in the internal cycle.

[0037] Alternatively, the described system can also be operated in continuous mode by continuously supplying a feed solution (diluted AMT solution) to a storage container (1) via a valve (4), and while the valve (16) is closed, the formed concentrate is discharged through the open valve (15) as well as the permeate. [Examples]

[0038] Storage container (1) contains a substance with a density of 1.20 g / cm³. 3 500 liters of diluted AMT solution (at 20°C) were filled. The concentration was 242.5 g AMT / L. A concentrate with a density of 2.40 g AMT / L (measured at 35°C) was prepared at a set pressure of 110 bar controlled by a pressure control device (12). The AMT concentration was 1682 g AMT / L. Approximately 427 liters of permeate were separated. Analysis of the permeate revealed a tungsten content of 1.35 g / L (0.64%), and no change in the NH4 / W ratio was observed. The ammonium content, measured by the Kjeldahl method, was 0.067 g / L. As the analysis of the permeate shows, there was little tungsten loss, and therefore no significant change in chemical composition. Because the chemical composition did not change, the permeate could be completely recycled during the operation to prepare diluted AMT solution. The fact that the tungsten loss through the membrane is less than 1% further demonstrates the economic advantages associated with the method according to the present invention.

[0039] As can be seen from the example described, the method according to the present invention results in very little tungsten loss, while simultaneously yielding a high concentration of ammonium metatungstate.

Claims

1. A method for concentrating an aqueous solution of ammonium metatungstate (AMT), comprising passing an aqueous solution of ammonium metatungstate (A) through at least one high-pressure reverse osmosis cell to obtain a concentrate (C) and a permeate (P).

2. The method according to claim 1, characterized in that at least one reverse osmosis is performed under a pressure of 50 bar or more, 90 bar or more, 100 bar or more, 120 bar or more, or 150 bar or more.

3. The method according to claim 1 or 2, characterized in that the aqueous solution of ammonium metatungstate (A) is obtained by calcining ammonium paratungstate tetrahydrate and leaching the calcined product with water.

4. The method according to any one of claims 1 to 3, characterized in that the aqueous solution of ammonium metatungstate (A) is subjected to a filtration step before the reverse osmosis is carried out.

5. A method for producing ammonium metatungstate, comprising cooling the concentrated liquid (C) obtained by the method according to any one of claims 1 to 4.

6. The method according to any one of claims 1 to 4, characterized in that the obtained permeate (P) is reused in the process cycle.

7. The method according to any one of claims 1 to 4 and 6, characterized in that the method is operated continuously.

8. The method according to any one of claims 1 to 4, 6, and 7, characterized in that the method is operated continuously and in a single step.

9. The method according to any one of claims 1 to 4, 6, and 7, characterized in that the method is operated in multiple stages.

10. The method according to claim 9, characterized in that, when the method is operated in multiple stages, the high-pressure reverse osmosis cell is operated at different pressures.

11. The method according to any one of claims 1 to 4 and 6 to 10, characterized in that the concentration of ammonium metatungstate in the aqueous solution of ammonium metatungstate (A) is 150 to 550 g / L, 250 to 500 g / L, or 200 to 300 g / L.

12. The method according to any one of claims 1 to 4 and 6 to 11, characterized in that the concentration of ammonium metatungstate in the concentrated liquid (C) is at least 1200 g / L or at least 1500 g / L.

13. Use of a high-pressure reverse osmosis cell to concentrate an aqueous solution of ammonium metatungstate.

14. The use according to claim 13, characterized in that the high-pressure reverse osmosis cell includes at least one spirally wound membrane.

15. An apparatus for carrying out the method according to any one of claims 1 to 12, characterized by comprising at least one high-pressure reverse osmosis cell.

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