METHOD FOR CRYSTALLIZATION OF Beta-AMMONIUM TETRAMOLYBDATE

Abstract

A method for crystallization of β-ammonium tetramolybdate includes: performing a stepwise pH-adjusting treatment of an ammonium molybdate solution via zoning to obtain the β-ammonium tetramolybdate. When feeding the ammonium molybdate solution into a reaction system from a first zone and then into second to sixth zones successively, pH.sup.1 of a resultant solution in the first zone is 7.0-6.0; pH.sup.2 of a resultant solution in the second zone is less than 6 and greater than or equal to 4; pH.sup.3 of a resultant solution in the third zone is less than 4 and greater than or equal to 2.5; pH.sup.4 of a crystallized slurry in the fourth zone is less than 2.5 and greater than or equal to 1; pH.sup.5 of a crystallized slurry in the fifth zone is 2.5-4.0; and pH.sup.6 of a crystallized slurry in the sixth zone is less than 2.5 and greater than or equal to 2.0.

Claims

1. A method for a crystallization of β-ammonium tetramolybdate, comprising: performing a stepwise pH-adjusting treatment of an ammonium molybdate solution via zoning, wherein upon feeding the ammonium molybdate solution into a reaction system from a first zone, pH.sup.1 of a resultant solution in the first zone is adjusted to 7.0-6.0, prior to discharge into a second zone; pH.sup.2 of a resultant solution in the second zone is adjusted to be less than 6 and greater than or equal to 4, prior to discharge into a third zone; pH.sup.3 of a resultant solution in the third zone is adjusted to be less than 4 and greater than or equal to 2.5, prior to discharge into a fourth zone; pH.sup.4 of a crystallized slurry in the fourth zone is adjusted to be less than 2.5 and greater than or equal to 1, prior to discharge into a fifth zone; pH.sup.5 of a crystallized slurry in the fifth zone is adjusted to 2.5-4.0, prior to discharge into a sixth zone; and pH.sup.6 of a crystallized slurry in the sixth zone is adjusted to be less than 2.5 and greater than or equal to 2.0, prior to reflux of a portion of the crystallized slurry in the sixth zone to the third zone, with a remaining of the crystallized slurry in the sixth zone being fed into a product zone for a solid-liquid separation, to obtain the β-ammonium tetramolybdate.

2. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein pH.sup.1 is 6-7; pH.sup.2 is 4.0-5.9; pH.sup.3 is 2.5-3.9; pH.sup.4 is 1.0-2.4; pH.sup.5 is 2.5-4.0; and pH.sup.6 is 2.0-2.4.

3. The method for the crystallization of the β-ammonium tetramolybdate according to claim 2, wherein pH.sup.3 is 3-3.9; pH.sup.4 is 1.5-2.4; and pH.sup.5 is 3.8-4.0.

4. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein a concentration of the ammonium molybdate solution is 50-200 g/L.

5. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein an acid used in a pH adjustment at each zone is at least one of an aqueous hydrochloric acid and an aqueous nitric acid.

6. The method for the crystallization of the β-ammonium tetramolybdate according to claim 5, wherein a concentration of the acid used in the pH adjustment at each zone is 5-10 mol/L.

7. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein an alkali used in a pH adjustment is an aqueous ammonia.

8. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein a temperature of the resultant solution during a treatment at each of the first zone, the second zone, and the third zone is 60-90° C.

9. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein a total residence time of the ammonium molybdate solution is 2-6 h.

10. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein a reflux ratio is 5-20 v %.

11. The method for the crystallization of the β-ammonium tetramolybdate according to claim 1, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

12. The method for the crystallization of the β-ammonium tetramolybdate according to claim 2, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

13. The method for the crystallization of the β-ammonium tetramolybdate according to claim 3, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

14. The method for the crystallization of the β-ammonium tetramolybdate according to claim 4, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

15. The method for the crystallization of the β-ammonium tetramolybdate according to claim 5, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

16. The method for the crystallization of the β-ammonium tetramolybdate according to claim 6, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

17. The method for the crystallization of the β-ammonium tetramolybdate according to claim 7, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

18. The method for the crystallization of the β-ammonium tetramolybdate according to claim 8, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

19. The method for the crystallization of the β-ammonium tetramolybdate according to claim 9, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

20. The method for the crystallization of the β-ammonium tetramolybdate according to claim 10, wherein the ammonium molybdate solution is continuously added to the first zone and is continuously subjected to the stepwise pH-adjusting treatment via zoning to continuously provide the β-ammonium tetramolybdate at the product zone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is an XRD pattern of β-ammonium tetramolybdate crystals prepared in Example 1;

[0032] FIGS. 2A-2B are SEM images of β-ammonium tetramolybdate crystals prepared in Example 1;

[0033] FIG. 3 is an XRD pattern of crystals prepared in Comparative Example 1;

[0034] FIGS. 4A-4B are SEM patterns of crystals prepared in Comparative Example 1;

[0035] FIG. 5 is an XRD pattern of crystals prepared in Comparative Example 2;

[0036] FIGS. 6A-6B are SEM patterns of crystals prepared in Comparative Example 2;

[0037] FIG. 7 is an XRD pattern of crystals prepared in Comparative Example 3; and

[0038] FIGS. 8A-8B are SEM patterns of crystals prepared in Comparative Example 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] The present invention is further described below with reference to examples. The following examples are used to further illustrate the present invention, but not to further limit the present invention.

Example 1

[0040] An ammonium molybdate solution used herein has a concentration of 121.5 g/L and pH of 9.8. The ammonium molybdate solution and 5 mol/L nitric acid solution were added into a first zone respectively, with the solution pH at this zone controlled to be 6.2, to obtain a mixed solution. The mixed solution flowed in the form of overflow and underflow in the direction of second zone.fwdarw.third zone.fwdarw.fourth zone.fwdarw.fifth zone.fwdarw.sixth zone.fwdarw.product collection zone. A 5 mol/L nitric acid solution was added into the second zone (polymerization zone) to control the solution pH at this zone to be 4.8. A 5 mol/L nitric acid solution was added into the third zone (ammonium tetramolybdate crystallization zone) to control the solution pH at this zone to be 2.8. A 5 mol/L nitric acid solution was added into the fourth zone (nucleation zone) to control the solution pH at this zone to be 1.2. A 6 mol/L aqueous ammonia was added into the fifth zone (fine grain redissolving zone) to control the solution pH at this zone to be 3.8. A 5 mol/L nitric acid solution was added into the sixth zone (ammonium tetramolybdate crystal growth zone) to control the solution pH at this zone to be 2.3. The resultant solution flowed into the product collection zone, and the proportion of the solution refluxed from the product collection zone to the third zone was controlled to be 10%. The remaining solution was pumped from the top and then filtered to obtain β-ammonium tetramolybdate crystals. The temperature of the whole system was controlled to be 80° C., the total residence time was 5 h, and the crystallinity was 91.2%. The obtained ammonium tetramolybdate crystal sample was analyzed and detected to obtain the XRD pattern and SEM image shown in FIG. 1 and FIGS. 2A-2B, respectively.

[0041] It can be seen from the XRD pattern that the composition of the sample mainly includes β-ammonium tetramolybdate, with a small amount of α-ammonium tetramolybdate. By analysis, s-ammonium tetramolybdate accounts for 84.4%, and α-ammonium tetramolybdate only accounts for 15.6%. Compared with the conventional crystallization by direct acid precipitation, this example not only resolves the problem that ammonium polymolybdate is prone to be produced during crystallization, but also obtains high-purity ammonium tetramolybdate that mainly contains β-ammonium tetramolybdate accounting for up to 84%, which lays a good foundation for later processing of molybdenum materials.

[0042] It can be seen from the SEM image of the crystallized product that β-ammonium tetramolybdate is mainly in a regular long-strip shape, with a clear crystal boundary, uniform distribution in whole, and is almost formed by accumulation of thin and long particles, with a high purity of crystal form structure, which is very suitable as a raw material for production of molybdenum bars and wires by reduction processing.

Example 2

[0043] An ammonium molybdate solution used herein has a concentration of 121.5 g/L and pH of 9.8. The ammonium molybdate solution and 6 mol/L nitric acid solution were added into a first zone respectively, with the solution pH at this zone controlled to be 7.0, to obtain a mixed solution. The mixed solution flowed in the form of overflow and underflow in the direction of second zone.fwdarw.third zone.fwdarw.fourth zone.fwdarw.fifth zone.fwdarw.sixth zone.fwdarw.central reaction zone. A 6 mol/L nitric acid solution was added into the second zone (polymerization zone) to control the solution pH at this zone to be 5.9. A 6 mol/L nitric acid solution was added into the third zone (ammonium tetramolybdate crystallization zone) to control the solution pH at this zone to be 3.9. A 6 mol/L nitric acid solution was added into the fourth zone (nucleation zone) to control the solution pH at this zone to be 2.4. A 6 mol/L aqueous ammonia was added into the fifth zone (fine grain redissolving zone) to control the solution pH at this zone to be 4.0. A 6 mol/L nitric acid solution was added into the sixth zone (ammonium tetramolybdate crystal growth zone) to control the solution pH at this zone to be 2.4. The resultant solution flowed into the central reaction zone, and the proportion of the solution refluxed from the product collection zone to the third zone was controlled to be 8%. The remaining solution was pumped from the top and then filtered to obtain β-ammonium tetramolybdate crystals. The temperature of the whole system was controlled to be 60° C., the total residence time was 4 h, and the crystallinity was 94.6%. β-ammonium tetramolybdate accounts for 83.6% of the product.

Example 3

[0044] An ammonium molybdate solution used herein has a concentration of 82.1 g/L and pH of 9.5. The ammonium molybdate solution and 10 mol/L nitric acid solution were added into a first zone respectively, with the solution pH at this zone controlled to be 6.3, to obtain a mixed solution. The mixed solution flowed in the form of overflow and underflow in the direction of second zone.fwdarw.third zone.fwdarw.fourth zone.fwdarw.fifth zone.fwdarw.sixth zone.fwdarw.central reaction zone. A 10 mol/L nitric acid solution was added into the second zone (polymerization zone) to control the solution pH at this zone to be 4.0. A 10 mol/L nitric acid solution was added into the third zone (ammonium tetramolybdate crystallization zone) to control the solution pH at this zone to be 3.0. A 10 mol/L nitric acid solution was added into the fourth zone (nucleation zone) to control the solution pH at this zone to be 1.5. A 6 mol/L aqueous ammonia was added into the fifth zone (fine grain redissolving zone) to control the solution pH at this zone to be 4.0. A 10 mol/L nitric acid solution was added into the sixth zone (ammonium tetramolybdate crystal growth zone) to control the solution pH at this zone to be 2.0. The resultant solution flowed into the central reaction zone, and the proportion of the solution refluxed from the product collection zone to the third zone was controlled to be 12%. The remaining solution was pumped from the top and then filtered to obtain β-ammonium tetramolybdate crystals. The temperature of the whole system was controlled to be 80° C., the total residence time was 6 h, and the crystallinity was 93.6%. β-ammonium tetramolybdate accounts for 84.2% of the product.

Example 4

[0045] An ammonium molybdate solution used herein has a concentration of 82.1 g/L and pH of 9.5. The ammonium molybdate solution and 5 mol/L nitric acid solution were added into a first zone respectively, with the solution pH at this zone controlled to be 6.0, to obtain a mixed solution. The mixed solution flowed in the form of overflow and underflow in the direction of second zone.fwdarw.third zone.fwdarw.fourth zone.fwdarw.fifth zone.fwdarw.sixth zone.fwdarw.central reaction zone. A 5 mol/L nitric acid solution was added into the second zone (polymerization zone) to control the solution pH at this zone to be 4.6. A 5 mol/L nitric acid solution was added into the third zone (ammonium tetramolybdate crystallization zone) to control the solution pH at this zone to be 2.5. A 5 mol/L nitric acid solution was added into the fourth zone (nucleation zone) to control the solution pH at this zone to be 1.0. A 6 mol/L aqueous ammonia was added into the fifth zone (fine grain redissolving zone) to control the solution pH at this zone to be 3.9. A 5 mol/L nitric acid solution was added into the sixth zone (ammonium tetramolybdate crystal growth zone) to control the solution pH at this zone to be 2.0. The resultant solution flowed into the central reaction zone, and the proportion of the solution refluxed from the product collection zone to the third zone was controlled to be 20%. The remaining solution was pumped from the top and then filtered to obtain β-ammonium tetramolybdate crystals. The temperature of the whole system was controlled to be 70° C., the total residence time was 2 h, and the crystallinity was 92.4%. β-ammonium tetramolybdate accounts for 83.9% of the product.

Comparative Example 1

[0046] Compared with Example 1, the difference mainly lies in that there was no treatment at multiple zones. The details are shown as follows.

[0047] 1 L of ammonium molybdate solution was measured out (same as Example 1). The ammonium molybdate solution was added into a 3 L beaker and stirred in a water bath at 80° C. Then, a 5 mol/L nitric acid solution was gradually added into the ammonium molybdate solution to adjust the solution pH to 2.3, maintained at this temperature for 5 h, and then filtered. The resultant crystallinity was 91.6%. The obtained ammonium tetramolybdate crystal sample was analyzed and detected to obtain the XRD pattern and SEM image shown in FIG. 3 and FIGS. 4A-4B, respectively.

[0048] FIG. 3 is an XRD pattern of the crystallized product by direct acid precipitation. It can be seen from this figure that the product contains most ammonium tetramolybdate, some ammonium dimolybdate, and a small amount of ammonium trimolybdate. By analysis, ammonium tetramolybdate accounts for about 73%, ammonium dimolybdate accounts for 21.7%, and ammonium trimolybdate accounts for 5.3%. β-ammonium tetramolybdate accounts for 34.8% of the ammonium tetramolybdate, indicating that the product has a poor purity. FIG. 4 is an SEM image of the crystallized product. It can be seen from this figure that the product is formed by irregular bulk particles having a diameter of about 1-50 μm, with a clear crystal boundary, uneven distribution in whole. This may be because during the crystallization by acid precipitation, the growth direction of the crystal has been shifted with the change of the pH value of the solution, resulting in growth of the crystal on a plurality of crystalline surfaces. In addition, without a fine grain redissolving stage, some ungrown grains are mixed in the product, resulting in uneven distribution in grain size.

Comparative Example 2

[0049] Compared with Example 1, the difference mainly lies in that there was no treatment of reflux. The details are shown as follows.

[0050] An ammonium molybdate solution was used (same as Example 1). The ammonium molybdate solution and 5 mol/L nitric acid solution were added into a first zone respectively, with the solution pH at this zone controlled to be 6.2, to obtain a mixed solution. The mixed solution flowed in the form of overflow and underflow in the direction of second zone.fwdarw.third zone.fwdarw.fourth zone.fwdarw.fifth zone.fwdarw.sixth zone.fwdarw.product collection zone. A 5 mol/L nitric acid solution was added into the second zone (polymerization zone) to control the solution pH at this zone to be 4.8. A 5 mol/L nitric acid solution was added into the third zone (ammonium tetramolybdate crystallization zone) to control the solution pH at this zone to be 2.8. A 5 mol/L nitric acid solution was added into the fourth zone (nucleation zone) to control the solution pH at this zone to be 1.2. A 6 mol/L aqueous ammonia was added into the fifth zone (fine grain redissolving zone) to control the solution pH at this zone to be 3.8. A 5 mol/L nitric acid solution was added into the sixth zone (ammonium tetramolybdate crystal growth zone) to control the solution pH at this zone to be 2.3. The resultant solution flowed into the product collection zone and was pumped from the top and then filtered to obtain β-ammonium tetramolybdate crystals. The temperature of the whole system was controlled to be 80° C., the total residence time was 5 h, and the crystallinity was 90.5%. The obtained XRD pattern and SEM image of the ammonium tetramolybdate crystals were shown in FIG. 5 and FIGS. 6A-6B, respectively.

[0051] FIG. 5 is an XRD pattern of the crystallized product prepared without slurry reflux (seed addition) during acid precipitation. It can be seen from the XRD pattern that the product contains most α-ammonium tetramolybdate, and some β-ammonium molybdate and ammonium trimolybdate. By analysis, α-ammonium tetramolybdate accounts for about 62.8%, β-ammonium molybdate accounts for 23.6%, and ammonium trimolybdate accounts for 13.6%. FIGS. 6A-6B are SEM images of the crystallized product. It can be seen from this figure that the product is formed by irregular bulk particles with a small amount of long-strip particles, having a diameter of about 1-20 μm, with a clear crystal boundary, widely distributed in particle size.

Comparative Example 3

[0052] Compared with Example 1, the difference mainly lies in that there was no adjustment at a fifth zone and a sixth zone. The details are shown as follows.

[0053] An ammonium molybdate solution was used (same as Example 1). The ammonium molybdate solution and 5 mol/L nitric acid solution were added into a first zone respectively, with the solution pH at this zone controlled to be 6.2, to obtain a mixed solution. The mixed solution flowed in the form of overflow and underflow in the direction of second zone.fwdarw.third zone.fwdarw.fourth zone.fwdarw.product collection zone. A 5 mol/L nitric acid solution was added into the second zone (polymerization zone) to control the solution pH at this zone to be 4.8. A 5 mol/L nitric acid solution was added into the third zone to control the solution pH at this zone to be 2.8. A 5 mol/L nitric acid solution was added into the fourth zone to control the solution pH at this zone to be 1.2. The solution at the fourth zone flowed through a fifth zone and a sixth zone (without pH adjustment back at the fifth zone and the sixth zone) and then entered the product collection zone. The resultant solution flowed into the product collection zone, and the proportion of the solution refluxed from the product collection zone to the third zone was controlled to be 10%. The remaining solution was pumped from the top and then filtered to obtain β-ammonium tetramolybdate crystals. The temperature of the whole system was controlled to be 80° C., the total residence time was 5 h, and the crystallinity was 88.4%. The obtained ammonium tetramolybdate crystal sample was analyzed and detected to obtain the XRD pattern and SEM image shown in FIG. 7 and FIGS. 8A-8B, respectively.

[0054] FIG. 7 is an XRD pattern. It can be seen from the XRD pattern that the product mainly contains most α-ammonium tetramolybdate and β-ammonium molybdate. By analysis, α-ammonium tetramolybdate accounts for about 36.8%, and β-ammonium molybdate accounts for 63.2%. FIGS. 8A-8B are SEM images of the crystallized product. It can be seen from this figure that the product is formed by long-strip particles with a large amount of fine particles attached to their surfaces, because the product does not undergo the fine grain redissolving stage, and the particle size is distributed widely.