METHOD FOR IMPURITY CONTROL

Abstract

A method for controlling the concentration of impurities in Bayer liquors, the method comprising the steps of adding an oxide and/or a hydroxide of a metal other than aluminium to a Bayer liquor with a desired TA forming a layered double hydroxide; and incorporating at least one impurity in the layered double hydroxide, wherein the impurities are selected from the group comprising chloride, fluoride, sulfate and TOC.

Claims

1. A method for controlling the concentration of impurities in Bayer liquors, the method comprising the steps of: adding an oxide and/or a hydroxide of a metal other than aluminium to a Bayer liquor with a desired TA; forming a layered double hydroxide; and incorporating at least one impurity in the layered double hydroxide, wherein the impurities are selected from the group comprising chloride, fluoride, sulfate and TOC; wherein the incorporation of chloride ion and fluoride ions increases with increasing Bayer liquor TA; and wherein the incorporation of sulfate ions and TOC decreases with increasing TA.

2. The A method of claim 1, wherein the impurities are chloride and/or fluoride and the desired TA is greater than 30 gL.sup.1.

3. The method of claim 1, wherein the impurities are sulfate and/or TOC and the desired TA is less than 160 gL.sup.1.

4. The method of claim 1, comprising: monitoring the concentration of at least one impurity in a Bayer circuit.

5. The method of claim 1, comprising: measuring the concentration of at least one impurity in the Bayer liquor with a desired TA.

6. The method of claim 1, comprising: measuring the concentration of at least one impurity in the Bayer liquor with a desired TA; prior to the step of: adding the oxide and/or the hydroxide of a metal other than aluminium to the Bayer liquor with a desired TA.

7. The method of claim 1, comprising: measuring the concentration of at least one impurity in a Bayer liquor with a desired TA; after the step of: incorporating the at least one impurity in the layered double hydroxide.

8. The method of claim 1, wherein the concentration of the at least one impurity in the Bayer liquor after the formation of the layered double hydroxide is less than the concentration of the at least one impurity prior to the step of adding the oxide and/or the hydroxide of a metal other than aluminium to the Bayer liquor.

9. The method of claim 1, comprising: obtaining the Bayer liquor with the desired TA.

10. The method of claim 1, comprising: treating the Bayer liquor to achieve the desired TA.

11. The method of claim 10, wherein the impurities are sulfate and/or TOC and the treating step occurs prior to the step of adding the oxide and/or the hydroxide of a metal other than aluminium to the Bayer liquor, to reduce the TA of the Bayer liquor.

12. The method of 10, wherein the impurities are chloride and/or fluoride and the treating step occurs prior to the step of adding the oxide and/or the hydroxide of a metal other than aluminium to the Bayer liquor, to increase the TA of the Bayer liquor.

13. The method of claim 1, wherein the step of incorporating at least one impurity in the layered double hydroxide results in a reduction of the concentration of the at least one impurity of at least 10%.

14. The method of claim 1 comprising: adding at least one impurity to the Bayer liquor to provide an enriched Bayer liquor; prior to the step of: forming the layered double hydroxide

15. The method of claim 1, wherein the impurities are sulfate and/or TOC and wherein the Bayer liquor is washer overflow, diluted spent liquor, diluted green liquor or lakewater.

16. The method of claim 1, wherein the impurities are chloride and/or fluoride and the Bayer liquor is a green liquor, a spent liquor or an increased TA liquor.

17. The method of claim 1, wherein the metal other than aluminium is selected from the group comprising calcium and magnesium.

18. The method of claim 1, wherein the layered double hydroxide is hydrocalumite and/or hydrotalcite.

19. The method of claim 1, wherein the impurities are sulfate and/or TOC and the Bayer liquor has a TA less than 150 gL.sup.1.

20. The method of claim 1, wherein the impurities are chloride and/or fluoride and the Bayer liquor has a TA greater than 50 gL.sup.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0085] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

[0086] FIG. 1 is a plot showing the effect of TA on sodium carbonate incorporation into hydrocalumite for the series of runs with 1.sup.st refinery crystallizer feed shown in Table 1;

[0087] FIG. 2 is a plot showing the effect of TA on impurity incorporation into hydrocalumite for the series of runs with 1.sup.st refinery crystallizer feed shown in Table 1;

[0088] FIG. 3 is a plot showing the effect of TA on impurity incorporation into hydrocalumite for the series of runs with 1.sup.st refinery spent liquor feed shown in Table 2;

[0089] FIG. 4 is a plot showing the effect of TA on the amount of available impurity removed from a 1.sup.st refinery spent liquor;

[0090] FIG. 5 is a plot showing the effect of TA on impurity incorporation into hydrocalumite for the series of runs with 1.sup.st refinery green liquor feed; and

[0091] FIG. 6 is a plot showing the effect of TA on sodium carbonate incorporation into hydrocalumite for the series of runs with 1.sup.st refinery green liquor feed.

DESCRIPTION OF EMBODIMENTS

[0092] Throughout this specification, unless the context requires otherwise, the word comprise or variations such as comprises or comprising, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0093] Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more steps or features.

Experimental

[0094] To further describe the invention, a series of experiments will now be described. It must be appreciated that the following description of the experiments is not to limit the generality of the above description of the invention.

[0095] Experiments were conducted in 3 L stainless steel water jacketed vessels with constant stirring at 1000 RPM. The temperature was maintained at 60 C. and the vessels contained baffles to ensure good mixing.

[0096] Liquor from an alumina refinery (hereinafter the 1.sup.st Refinery) was used and slaked lime was sourced from a 2.sup.nd Refinery. The slaked lime typically had a solids concentration of 250-260 gL.sup.1 with an available CaO content of approximately 56%. This lime had been produced by slaking in 2.sup.nd Refinery lakewater. In some experiments, the lime concentration in the slaked lime slurry was increased to approximately 400 gL.sup.1 by allowing the lime solids to settle in the container and decanting off some of the lakewater.

[0097] The ratios of lime to liquor were kept constant and the TA was varied by changing the amount of distilled water added to the reaction mixture. The total reaction volume was approximately 2 L.

Example 1Crystalliser Feed

[0098] The effect of reaction TA was first investigated using a 1.sup.st Refinery crystalliser feed liquor as the source liquor. A crystalliser feed liquor is a spent liquor that has undergone evaporation to increase its TA (typically by 10%). The TA of the crystalliser feed liquor was 279.4 gL.sup.1. The reaction mixtures examined ranged from 80-230 gL.sup.1 TA. The highest reaction TA was from a reaction mixture with undiluted feed liquor, with subsequent mixtures having more water added to the mixture to lower the reaction TA. The reaction mixture compositions are shown in Table 1. Note that the reactor TA for Run No 1 is approximately 50 gL.sup.1 lower than the feed liquor TA due to the dilution effect of the lakewater contained within the lime slurry. The lime concentration was ratioed to the liquor volume and thus the lime concentration in the reactor dropped with each run. The CaO Conc in Feed column shows the amount of CaO present relative to the amount of feed liquor and this is seen to remain constant. In this experiment a concentrated lime slurry was used which had a solids concentration of 400 gL.sup.1 and an effective CaO concentration of 224 gL.sup.1.

TABLE-US-00001 TABLE 1 Effect of TA reaction mixtures for the experiments carried out with crystalliser feed liquor. Lime Lime CaO Liquor Slurry Water Conc in Conc in Reactor Run Volume Mass Volume Reactor Feed TA No. (L) (kg) (L) (gL.sup.1) (gL.sup.1) (gL.sup.1) 1 1.60 0.71 0.0 122.9 99 228.8 2 1.40 0.63 0.26 109.4 100 199.0 3 1.20 0.54 0.53 94.6 100 168.9 4 1.00 0.45 0.80 79.5 100 139.4 5 0.80 0.36 1.07 64.1 100 110.4 6 0.60 0.27 1.35 48.3 100 81.6

[0099] FIG. 1 displays the amount of sodium carbonate removed per tonne of hydrocalumite produced for the series of runs in Table 1. It is seen that the amount of sodium carbonate incorporated into the hydrocalumite is independent of TA. This is a typical result for all of the liquors examined in this work. There is a small amount of variation in sodium carbonate incorporated between different liquor sources but with a constant liquor source there is no variation in sodium carbonate incorporation.

[0100] FIG. 2 shows the amount of several impurities incorporated into the hydrocalumite for the series of runs contained in Table 1. This result shows that the level of each impurity incorporated depended on reaction TA. The amount of sodium sulfate incorporation decreased with increasing reaction TA, whereas the level of sodium chloride incorporation increased with reaction TA. The concentration of sodium sulfate in the crystalliser feed was 23.5 gL.sup.1 and the sodium chloride concentration was 16.7 gL.sup.1.

[0101] These variations of impurity incorporation with TA were unexpected, given that the ratio of lime to feed liquor was constant in each of the experiments and thus the amount of hydrocalumite produced is ratioed to the amount of feed liquor. Thus, one would expect that the level of impurity removal would be constant.

Example 2Spent Liquor

[0102] Spent liquor from the 1.sup.st Refinery was investigated with non-concentrated slaked lime from the 2.sup.nd Refinery. The slaked lime had a solids concentration of 257 g/L and an effective CaO concentration of 141 gL.sup.1. The reaction mixture compositions for the runs with the spent liquor are shown in Table 2. The reaction TA varied from 30-176 gL.sup.1. The highest TA examined in this case is significantly lower than with the crystalliser feed run, because the start liquor TA is lower at 262 gL.sup.1 and because the lime slurry used is not concentrated thus there is more dilution.

TABLE-US-00002 TABLE 2 Effect of TA reaction mixtures for the experiments carried out with a spent feed liquor. Lime Lime CaO Liquor Slurry Water Conc in Conc Reactor Run Volume Volume Volume Reactor in Feed TA No. (L) (L) (L) (gL.sup.1) (gL.sup.1) (gL.sup.1) 1 1.30 0.97 0.0 110.1 106 176.0 2 1.15 0.86 0.24 98.4 106 155.2 3 1.00 0.74 0.48 85.9 105 134.8 4 0.85 0.63 0.73 73.4 105 113.6 5 0.70 0.52 0.98 60.3 104 92.9 6 0.55 0.40 1.24 47.0 103 72.0 7 0.40 0.29 1.52 33.3 101 51.3 8 0.23 0.17 1.72 20.3 103 30.3

[0103] FIG. 3 shows the amount of several impurities incorporated into the hydrocalumite for the runs contained in Table 2, this time also measuring TOC incorporation. The data for the 1.sup.st Refinery spent liquor shows a similar trend in impurity incorporation as that seen for the 1.sup.st Refinery crystallizer feed liquor. There is a trend to increasing sodium fluoride incorporation with increasing TA, as well as a significant increase in sodium chloride incorporation with increasing TA. As previously, sodium sulfate incorporation decreases with increasing TA and TOC is seen to have a similar trend. Sodium carbonate incorporation remains relatively constant over this TA range, with an average incorporation of 110 kgT.sup.1 of hydrocalumite production.

[0104] The liquor composition for the 1.sup.st Refinery spent liquor is displayed in Table 3 along with a 1.sup.st Refinery green liquor for comparison.

TABLE-US-00003 TABLE 3 Liquor composition for 1.sup.st refinery spent liquor along with the composition of a green liquor from the 1.sup.st refinery. TA TC Al.sub.2O.sub.3 Na.sub.2SO.sub.4 NaCl TOC NaF Liquor (gL.sup.1) (gL.sup.1) (gL.sup.1) (gL.sup.1) (gL.sup.1) (gL.sup.1) (gL.sup.1) 1.sup.st Refinery 262 215 95 20.6 15.3 22.0 1.5 Spent Liquor 1.sup.st Refinery 247 200 144 22.0 14.8 22.6 1.4 Green Liquor

[0105] FIG. 4 shows the relative amount of each impurity removed as a function of TA. The same trends in relative impurity removal are still demonstrated, i.e. TOC and sulfate removal decrease with TA, whereas chloride and fluoride removal increase with TA.

Example 3Green Liquor

[0106] A green liquor from the 1.sup.st Refinery with the composition displayed in Table 3 was used in this trial. The mixture compositions for the runs are shown in Table 4 with the slaked lime having a solids concentration of 257 gL.sup.1 and an effective CaO concentration of 141 gL.sup.1.

TABLE-US-00004 TABLE 4 Effect of TA reaction mixtures for experiments carried out with green liquor. Lime Lime CaO Liquor Slurry Water Conc in Conc in Run Volume Volume Volume Reactor Feed No. (L) (L) (L) (gL.sup.1) (gL.sup.1) 1 1.30 0.96 0.0 108.9 104 2 1.15 0.85 0.24 97.2 104 3 1.00 0.74 0.48 85.3 104 4 0.85 0.62 0.73 72.4 103 5 0.70 0.51 0.98 59.4 102 6 0.55 0.39 1.24 46.3 101 7 0.40 0.28 1.52 33.0 100 8 0.23 0.17 1.72 20.2 102

[0107] The effect of TA on the impurity incorporation into the hydrocalumite in green liquor is shown in FIGS. 5 and 6. FIG. 5 shows that TOC and sodium sulfate incorporation decreases with increasing TA whereas the degree of sodium chloride incorporation increases with TA. Sodium fluoride incorporation increases with TA, but the effect is not as pronounced due to the low overall level of incorporation (due probably to the low fluoride concentration in the feed liquor). The amount of sodium carbonate removed per tonne of hydrocalumite produced is displayed in FIG. 6. Again there is little variation in the amount of sodium carbonate incorporated within the hydrocalumite and there is no trend in the amount of carbonate incorporated as TA varies. Overall the impurity incorporation trends for the green liquor match those of the spent liquors demonstrating that impurity incorporation is independent of feed liquor source.

Example 4Washer Liquor

[0108] Finally the liquor from a washer was used as a liquor source. The washer liquor was from the last washer at the 2.sup.nd Refinery and was used both neat and diluted 50% with water to compare the effect on impurity incorporation. Each run was undertaken in triplicate and the reaction mixtures are given in Table 5.

TABLE-US-00005 TABLE 5 Effect of TA reaction mixtures for experiments carried out with a last washer liquor. Lime Lime CaO Liquor Slurry Water Conc in Conc in Reactor Liquor Volume Volume Volume Reactor Feed TA type (L) (L) (L) (gL.sup.1) (gL.sup.1) (gL.sup.1) Neat 1.70 0.48 0.0 56.5 40 48.3 Dilute 0.90 0.26 0.9 32.6 41 26.4

[0109] The incorporation results are given in Table 6 both for each mixture and an average of the runs for each liquor type. The results show that the levels of a particular impurity incorporated is reasonably reproducible for a given liquor type. Apart from sodium carbonate, the trends in impurity incorporation with TA are the same for the last washer liquor as the other liquors examined. TOC and sodium sulfate incorporation decrease with increasing TA, whereas the degree of sodium chloride and sodium fluoride incorporation increases with TA.

TABLE-US-00006 TABLE 6 The amount of impurity incorporation in hydrocalumite produced from both a neat last washer liquor and from one diluted 50% with water. Reactor Liquor TA Na.sub.2CO.sub.3 Na.sub.2SO.sub.4 NaCl TOC NaF type (gL.sup.1) (kgT.sup.1) (kgT.sup.1) (kgT.sup.1) (kgT.sup.1) (kgT.sup.1) Neat 48.3 94.4 11.4 5.2 9.8 1.3 Neat 48.3 93.9 11.3 5.3 9.7 1.7 Neat 48.3 92.4 10.0 4.8 9.1 1.4 Average 48.3 93.6 10.9 5.1 9.5 1.5 Neat Dilute 26.4 101.8 14.5 5.0 10.7 1.2 Dilute 26.4 103.1 14.4 4.6 10.9 1.0 Dilute 26.4 102.9 13.4 4.4 10.5 1.0 Average 26.4 102.6 14.1 4.7 10.7 1.1 Dilute