METHOD FOR PREPARING HIGH PURITY ALUMINUM MONOHYDRATE AND ALPHA ALUMINA

Abstract

The present invention concerns a method of preparing aluminum monohydrate comprising the steps of i) mixing alumina feedstock with ethylenediamine tetraacetic acid, to obtain a feedstock mixture, and ii) subjecting the feedstock mixture to a hydrothermal treatment, wherein the wherein the pH of the feedstock mixture is at least 8. The resulting aluminum monohydrate, although starting from low purity feedstocks, shows an excellent purity and can be calcined to obtain high purity alpha alumina.

Claims

1. Method of preparing aluminum monohydrate comprising the steps of: mixing an alumina feedstock with ethylenediamine tetraacetic acid, to obtain a feedstock mixture, and subjecting the feedstock mixture to a hydrothermal treatment, wherein the pH of the feedstock mixture is at least 8.

2. The method of claim 1, wherein the pH of the feedstock mixture is at least 9, more preferably at least 10, in particular in the range from 10 to 12, and most preferably in the range from 10 to 11.

3. The method of claim 1, wherein the feedstock mixture contains ammonia, and wherein a weight ratio of the alumina feedstock to ammonia is from 100:1 to 10:1, in particular from 50:1 to 10:1.

4. The method of claim 1, wherein the feedstock mixture contains ammonia, and wherein the content of ammonia relative to the content of the alumina feedstock ranges from 1.25 wt % to 3.70 wt %.

5. The method of claim 1, wherein the alumina feedstock is selected from smelter grade alumina, pre-treated (leached) smelter grade alumina, chemical grade alumina, pre-treated (leached) chemical grade alumina, transition aluminas, and aluminum trihydrate.

6. The method of claim 1, wherein a weight ratio of the alumina feedstock to the ethylenediamine tetraacetic acid is from 50:1 to 20:1, optionally from 40:1 to 20:1.

7. The method of claim 1, wherein the feedstock mixture further comprises ammonium hydroxide.

8. The method of claim 1, wherein the feedstock mixture further comprises hydrogen peroxide, sodium hydroxide or potassium hydroxide.

9. The method of claim 1, wherein a weight ratio of the solid compounds of the feedstock mixture to the liquid compounds of the feedstock mixture ranges from 1:1 to 1:2 and/or wherein the temperature during the hydrothermal treatment ranges from 150 C. to 260 C. and/or wherein the hydrothermal pressure during the hydrothermal treatment ranges from 1 bar to 300 bar, in particular from 1 bar to 50 bar and/or wherein the hydrothermal treatment is carried out for 1 to 100 hours, and/or wherein the feedstock mixture is stirred during the hydrothermal treatment.

10. A method of preparing alpha alumina, comprising the method of preparing aluminum monohydrate according to claim 1, and then calcinating the aluminum monohydrate to obtain alpha alumina.

11. The method of claim 10, wherein the purity level of the aluminum monohydrate is from 99.9 wt. % to 99.99 wt. %

12. Use of ethylenediamine tetraacetic acid, in the preparation of aluminum monohydrate, in particular in a hydrothermal treatment.

13. The use according to claim 12, wherein the aluminum monohydrate is obtained from a feedstock mixture which is subjected to a hydrothermal treatment, wherein the pH of the feedstock mixture before the hydrothermal treatment is at least 8, preferably at least 9, more preferably in the range from 10 to 11, and the weight ratio of the feedstock to the ethylenediamine tetraacetic acid is from 50:1 to 20:1, optionally from 40:1 to 20:1.

14. The method of claim 2, wherein the feedstock mixture contains ammonia, wherein a weight ratio of the alumina feedstock to ammonia is from 100:1 to 10:1, in particular from 50:1 to 10:1.

15. The method of claim 2, wherein the feedstock mixture contains ammonia, wherein the content of ammonia relative to the content of the alumina feedstock ranges from 1.25 wt % to 3.70 wt %.

16. The method of claim 3, wherein the feedstock mixture contains ammonia, wherein the content of ammonia relative to the content of the alumina feedstock ranges from 1.25 wt % to 3.70 wt %.

17. The method of claim 2, wherein the alumina feedstock is selected from smelter grade alumina, pre-treated (leached) smelter grade alumina, chemical grade alumina, pre-treated (leached) chemical grade alumina, transition aluminas, and aluminum trihydrate.

18. The method of claim 3, wherein the alumina feedstock is selected from smelter grade alumina, pre-treated (leached) smelter grade alumina, chemical grade alumina, pre-treated (leached) chemical grade alumina, transition aluminas, and aluminum trihydrate.

19. The method of claim 4, wherein the alumina feedstock is selected from smelter grade alumina, pre-treated (leached) smelter grade alumina, chemical grade alumina, pre-treated (leached) chemical grade alumina, transition aluminas, and aluminum trihydrate.

20. The method of claim 2, wherein a weight ratio of the alumina feedstock to the ethylenediamine tetraacetic acid is from 50:1 to 20:1, optionally from 40:1 to 20:1.

Description

EXAMPLES

Example 1

[0050] In an autoclave 50 g aluminum hydroxide (Al(OH).sub.3) having an impurity (Na.sub.2O, Fe.sub.2O.sub.3, SiO.sub.2, MgO, TiO.sub.2, CaO, Li.sub.2O and ZnO) concentration of about 0.25 wt % (see Table 1 for details) were mixed with 0.5 g EDTA (ethylenediamine tetraacetic acid), 1.5 ml pure NH.sub.3 and 50 g H.sub.2O. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 48 hours. High purity boehmite was obtained in an amount of 41.75 g after filtration of the reaction mixture, washing and drying of the filtrate.

[0051] The results regarding impurity reduction are shown in Table 1 below, wherein the 2.sup.nd line shows the initial amount of each of the impurities. If not defined in another way, the values provided in Table 1 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

TABLE-US-00001 TABLE 1 Example EDTA NH.sub.3 No. [g] [ml] pH Na.sub.2O Fe.sub.2O.sub.3 MgO CaO Li.sub.2O ZnO Feedstock 0.1835 0.0084 0.0008 0.0101 0.0234 0.0255 before treatment 1 0.5 1.5 11 0.0199 0.0044 0.0003 0.0031 0.0044 2 1 3 11 0.0082 0.0022 0.0004 0.0014 0.0015 0.0051 3 1.2 3 10 0.0082 0.0020 0.0001 0.0013 0.0019 0.0042 4 1.5 2 8.5 0.0035 0.0025 0.0001 0.0024 0.0015 0.0067 5 1.5 3 9 0.0059 0.0019 0.0002 0.0012 0.0016 0.0041 6 1.5 3.5 9.5 0.0071 0.0020 0.0002 0.0012 0.0016 0.0052 7 1.5 4 10 0.0059 0.0016 0.0001 0.0011 0.0020 0.0051 8 1.5 4.5 10.5 0.0035 0.0016 0.0002 0.0007 0.0014 0.0042 9 1.5 6 11 0.0059 0.0013 0.0002 0.0007 0.0018 0.0038 10 1.5 10 11 0.0094 0.0013 0.0002 0.0011 0.0024 0.0036

[0052] It can be seen that for impurities Na.sub.2O, Fe.sub.2O.sub.3, MgO, CaO and Li.sub.2O a significant reduction of the impurity level has been achieved.

Examples 2 to 10

[0053] Examples 2 to 10 are prepared in the same manner as Example 1, with the exception that the amounts of EDTA and NH.sub.3 have been adjusted as shown in Table 1 above. It can be seen that for impurities Na.sub.2O, Fe.sub.2O.sub.3, MgO, CaO, Li.sub.2O and ZnO a significant reduction of the impurity level has been achieved.

[0054] From Table 1 above, it can further be seen that an increase in the ammonia content from 6 to 10 ml (see Examples 9 and 10), did not lead to a further improvement, i.e. a further reduction of the impurities, but instead lead to a slight increase of some of the impurities (Na.sub.2O, CaO, Li.sub.2O).

[0055] In FIG. 1 the test results regarding the impurity level of Examples 1, 2, 3 and 5 are compared, thus giving an overview of the influence of the amount of EDTA. Example 1 has the lowest EDTA content (0.5 g EDTA per 50 g alumina feedstockAl(OH).sub.3):weight ratio alumina feedstock:EDTA=100:1). Example 2 contains 1 g EDTA per 50 g alumina feedstock which corresponds to a weight ratio alumina feedstock:EDTA of 50:1. Example 3 contains 1.2 g EDTA per 50 g alumina feedstock which corresponds to a weight ratio alumina feedstock:EDTA of 42.7:1. Example 5 contains 1.5 g EDTA per 50 g alumina feedstock which corresponds to a weight ratio alumina feedstock:EDTA of 33.3:1. It can be seen that a weight ratio of the alumina feedstock to the nitrogen-containing complexing agent which ranges from 50:1 to 20:1 obtains the best test results. Higher amounts of EDTA significantly increase the costs while the impurity level does not further decrease significantly.

[0056] In FIG. 2 the test results regarding the impurity level of Examples 4 to 10 are compared, thus giving an overview of the influence of the amount of NH.sub.3. Example 4 has the lowest ammonia content (2 ml ammonia corresponding to 1.5 g ammonia:weight ratio alumina feedstock:ammonia=32.4:1). Example 5 contains 3 ml ammonia corresponding to 2.3 g ammonia:weight ratio alumina feedstock:ammonia=21.6:1. Example 6 contains 3.5 ml ammonia corresponding to 2.7 g ammonia:weight ratio alumina feedstock:ammonia=18.5:1. Example 7 contains 4 ml ammonia corresponding to 3.1 g ammonia:weight ratio alumina feedstock:ammonia=16.2:1. Example 8 contains 4.5 ml ammonia corresponding to 3.5 g ammonia:weight ratio alumina feedstock:ammonia=14.4:1. Example 9 contains 6 ml ammonia corresponding to 4.6 g ammonia:weight ratio alumina feedstock:ammonia=10.8:1. Example 10 contains 6 ml ammonia corresponding to 7.7 g ammonia:weight ratio alumina feedstock:ammonia=6.5:1.

[0057] It can be seen that a weight ratio of the alumina feedstock to ammonia ranging from 100:1 to 10:1 obtains excellent impurity reduction. Higher amounts of ammonia do not further influence the impurity level significantly.

Example 11

[0058] Example 11 was prepared in the same manner as Example 1 using aluminum trihydrate as a starting material having a D50 of 8 m. The aluminum trihydrate was mixed with 1.1 g EDTA and 4.5 ml NH.sub.3. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 24 hours. High purity boehmite was obtained which was filtered, washed and dried.

[0059] The results regarding impurity reduction are shown in Table 2 below, wherein the 2.sup.nd line shows the initial amount of each of the impurities and the last line shows the results of the impurity measurement after hydrothermal treatment. If not defined in another way, the values provided in Table 2 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

TABLE-US-00002 TABLE 2 Na.sub.2O Fe.sub.2O.sub.3 MgO CaO Li.sub.2O ZnO CuO Ga.sub.2O.sub.3 BeO pH (%) (%) (%) (%) (%) (%) (%) (%) (%) Feedstock 0.122 0.0098 0.0003 0.0099 0.0092 0.0170 0.0064 0.0096 0.0014 before treatment After hydro- 11 0.002 0.0021 0.0002 0.0007 0.0006 0.0010 0.0017 0.0062 0.0010 thermal treatment

[0060] It can be seen that for impurities Na.sub.2O, Fe.sub.2O.sub.3, MgO, CaO, Li.sub.2O, ZnO, CuO, Ga.sub.2O and BeO a significant reduction of the impurity level has been achieved.

Example 12

[0061] Example 12 was prepared in the same manner as Example 1 using smelter grade alumina (SGA) feedstock having a particle size of 84 m and having a particle size of 5 m as a starting material. The smelter grade alumina was mixed with 1.5 g EDTA and 6 ml NH.sub.3. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 60 hours. High purity boehmite was obtained which was filtered, washed and dried.

[0062] The results regarding impurity reduction are shown in Table 3 below. If not defined in another way, the values provided in Table 3 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

TABLE-US-00003 TABLE 3 Na.sub.2O Fe.sub.2O.sub.3 MgO CaO pH (%) (%) (%) (%) before 0.133 0.0176 0.0069 0.0216 treatment: SGA 84 m After 11 0.025 0.0060 0.0040 0.0051 hydrothermal treatment: SGA 84 m before 0.488 0.0208 0.0003 0.0294 treatment: SGA 5 m After 11 0.006 0.0044 0.0008 0.0010 hydrothermal treatment: SGA 5 m autoclaved

[0063] It can be seen that for impurities Na.sub.2O, Fe.sub.2O.sub.3, MgO and CaO a significant reduction of the impurity level has been achieved.

Example 13

[0064] Example 13 was prepared in the same manner as Example 12 using smelter grade alumina (SGA) feedstock. Sample A was not leached. Sample B has been leached with HCl/water mixture. EDTA has been added in an amount of 1.5 g and NH.sub.3 has been added in an amount of 6 ml to non-leached smelter grade alumina sample A and leached smelter grade alumina sample B. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 48 hours. The obtained boehmite was filtered, washed and dried.

[0065] The results regarding impurity reduction are shown in Table 4 below. If not defined in another way, the values provided in Table 4 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

Example 14

[0066] Example 14 was prepared in the same manner as Example 13 using the same smelter grade alumina (SGA) feedstock. Sample C was not leached. Sample D has been leached with HCl/water mixture. EDTA has not been added but NH.sub.3 has been added in an amount of 6 ml to non-leached smelter grade alumina sample C and leaches smelter grade alumina sample D. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 48 hours. The obtained boehmite was filtered, washed and dried.

[0067] The results regarding impurity reduction are shown in Table 4 below. If not defined in another way, the values provided in Table 4 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

Example 15

[0068] Example 15 was prepared in the same manner as Example 13 using the same smelter grade alumina (SGA) feedstock. Sample E was not leached. Sample F has been leached with HCl/water mixture. EDTA has been added in an amount of 1.5 g but NH.sub.3 has not been added. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 48 hours. The obtained boehmite was filtered, washed and dried.

[0069] The results regarding impurity reduction are shown in Table 4 below. If not defined in another way, the values provided in Table 4 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

TABLE-US-00004 TABLE 4 D50 temp Time EDTA NH.sub.3 SGA leached [m] [ C.] [h] [g] [ml] Na.sub.2O Fe.sub.2O.sub.3 CaO before no 84 0.319 0.0136 0.0192 treatment Sample A no 6 190 48 1.5 6 0.006 0.0044 0.0007 Sample B yes 6 190 48 1.5 6 0.002 0.0051 0.0011 Sample C no 6 190 48 0 6 0.034 0.0160 0.0218 Sample D yes 6 190 48 0 6 0.007 0.0181 0.0215 Sample E no 6 190 48 1.5 0 0.022 0.0120 0.0111 Sample F yes 6 190 48 1.5 0 0.032 0.0187 0.0085

[0070] As can be seen from the test results in Table 4, the addition of EDTA and NH.sub.3 (samples A and B) provides the best test results in view of a reduction of the impurities. However, the addition of EDTA only (samples E and F) also provides good reduction of the impurities captioned in Table 4. Merely the impurity level of Fe.sub.2O.sub.3 of sample F is slightly increased. However, the impurity levels of Na.sub.2O and CaO are significantly reduced. Although the addition of NH.sub.3 (see samples C and D) results in a reduction of Na.sub.2O, the impurity levels of Fe.sub.2O.sub.3 and CaO are high.

Example 16

[0071] Example 16 shows test results of a soda impurity level of samples G to V of smelter grade alumina feedstock to which EDTA has been added in an amount of 1.5 g and NH.sub.3 has been added in an amount of 6 ml. Hydrothermal treatment was carried out without steering at 190 C. and 12 bar water vapor pressure. The obtained boehmite was filtered, washed and dried. Samples G to V have thus been treated at the same temperature but for a different period of time. In addition, the D50 of the smelter grade alumina feedstock samples G to V varied as outlined in Table 5 below.

TABLE-US-00005 TABLE 5 Soda Soda d50 [wt. %] [wt. %] time temp. pressure Sample [m] (before HT) (after HT) [h] [ C.] [bar] pH G 2 0.334 0.045 6 190 12 11 H 5 0.087 0.027 6 190 12 11 I 84 0.087 0.051 6 190 12 11 J 5 0.087 0.025 12 190 12 11 K 84 0.087 0.045 12 190 12 11 L 2 0.334 0.015 18 190 12 11 M 5 0.087 0.009 18 190 12 11 N 5 0.087 0.009 24 190 12 11 O 84 0.087 0.036 24 190 12 11 P 5 0.087 0.002 48 190 12 11 Q 9 0.087 0.006 48 190 12 11 R 15 0.087 0.008 48 190 12 11 S 68 0.087 0.018 48 190 12 11 T 84 0.087 0.028 48 190 12 11 U 5 0.087 0.002 60 190 12 11 V 84 0.087 0.026 60 190 12 11

[0072] From Table 5 it can be seen that the addition of EDTA and NH.sub.3 had advantageous effects on the reduction of the soda impurity level. It can also be seen that the higher the D50, the lower the reduction of the soda impurity level was. Particular improvements in the soda reduction could be obtained for a D50 ranging from 2 to 15 m. It can also be followed from the test results of Table 5 that hydrothermal treatment for 18 to 48 resulted in excellent reduction of the soda impurity level, which a treatment period of 60 hours showed no further advantageous effects.

Example 17

[0073] Example 17 was prepared in the same manner as Example 1 using smelter grade alumina (SGA) feedstock having a particle size of 4.5 m (Do) as a starting material. 50 g of smelter grade alumina was mixed with 100 ml of water and different amounts of EDTA and NH.sub.3 as shown in Table 6. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 48 hours. High purity boehmite was obtained which was filtered, washed and dried.

[0074] The results regarding impurity reduction are shown in Table 6 below. If not defined in another way, the values provided in Table 6 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

TABLE-US-00006 TABLE 6 pH pH before after Hydor- Hydor- thermal thermal EDTA NH.sub.3 Treat- Treat- Na.sub.2O Fe.sub.2O.sub.3 CaO Sample [g] [ml] ment ment (%) (%) (%) SGA 0.3300 0.0165 0.0218 Feed 17/1 1.5 0.1 7.0 5.9 0.0188 0.0082 0.0071 17/2 1.5 1 9.1 8.6 0.0118 0.0047 0.0013 17/3 1.5 6 10.4 10.3 0.0094 0.0047 0.0014 17/4 0.5 6 10.8 10.7 0.0224 0.0138 0.0021

Example 18

[0075] Example 18 was prepared in the same manner as Example 1 using aluminum hydrate feedstock having a particle size of 8 m as a starting material. 50 g of aluminum hydrate was mixed with 100 ml of water and different amounts of EDTA and NH.sub.3 as shown in Table 7. Hydrothermal treatment was carried out under steering at 190 C. and 12 bar water vapor pressure for 24 hours. High purity boehmite was obtained which was filtered, washed and dried.

[0076] The results regarding impurity reduction are shown in Table 7 below. If not defined in another way, the values provided in Table 7 are given in wt %. In order to be able to compare the purity of the feedstock before treatment (e.g. smelter grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al.sub.2O.sub.3 basis.

TABLE-US-00007 TABLE 7 pH pH before after hydor- hydor- thermal thermal EDTA NH.sub.3 treat- treat- Na.sub.2O Fe.sub.2O.sub.3 CaO Sample [g] [ml] ment ment (%) (%) (%) Aluminum 0.2647 0.0093 0.0034 Hydrate Feed 18/1 1.5 0.1 3.4 5.3 0.0612 0.0079 0.0013 18/2 1.5 0.5 6.1 6.2 0.0129 0.0077 0.0009 18/3 1.5 1 9.1 9.2 0.0082 0.0048 0.0006 18/4 1.5 6 10.3 10.1 0.0082 0.0041 0.0006

[0077] The importance of the pH of the feedstock mixture (i.e. measured before the hydrothermal treatment) for purification of the material can be seen Tables 6 and 7 for SGA and aluminum hydrate, respectively. While mixtures with low pHs of 8 or lower show some degree of purification, pHs of at least 8, preferably at least 9 are necessary to achieve an effective reduction of critical impurities, particularly Na.sub.2O, Fe.sub.2O.sub.3, and CaO. Furthermore, Table 6 shows the effect of the presence of different amounts of the complexing agent. An optimum reduction of all impurities to preferred levels (i.e. for this example <0.010% Na.sub.2O, <0.005% Fe.sub.2O.sub.3 and <0.0015% CaO) is only observed for EDTA additions of 1.5 g, corresponding to a range within 50:1 to 20:1 for the weight ratio of (alumina) feedstock to EDTA, optionally from 40:1 to 20:1.

[0078] The dependence of the impurity concentration as a function of the pH is shown graphically in FIG. 3 for Samples 18/1 to 18/4 of Table 7 above. For the Fe.sub.2O.sub.3 and CaO impurities, due to the lower overall concentrations, a different scale is used (right hand side of the figure). The highly advantageous effect of a pH of above 8 for a feedstock mixture containing EDTA can be readily seen from FIG. 3.

[0079] The effect of different amounts of EDTA was further tested in the additional experiments/samples for the effect on the Na.sub.2O impurity level, as shown in Table 8 below. Thus, the amount of EDTA was only 0.5 g in Sample 18/5 and only 1 g in Sample 18/6. This resulted in an inferior reduction of the Na.sub.2O impurity level, as seen from the comparison to Sample 18/4 having 1.5 g EDTA.

TABLE-US-00008 TABLE 8 pH before pH after EDTA NH.sub.3 hydorthermal hydorthermal Na.sub.2O Sample [g] [ml] treatment treatment (%) Aluminum 0.2647 Hydrate Feed 18/5 0.5 6 10.8 10.6 0.0247 18/6 1 6 10.5 10.4 0.0129 18/4 1.5 6 10.3 10.1 0.0082