METHOD FOR CONTROLLING THE CONCENTRATION OF IMPURITIES IN BAYER LIQUORS

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 said layered double hydroxide, wherein the impurities are selected from the group comprising phosphorus, vanadium and silicon.

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 comprise at least one of phosphorus, vanadium and silicon impurities, and wherein the incorporation of the at least one impurity decreases with increasing TA.

2. The method of claim 1, wherein the desired TA is less than 160 gL.sup.1.

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

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

5. The method of claim 1, comprising: measuring the concentration of at least one impurity in a 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.

6. 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.

7. 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.

8. The method of claim 1, comprising: obtaining a Bayer liquor with a desired TA.

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

10. The method of claim 9, wherein the Bayer liquor is treated to reduce the TA.

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

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

13. The method of claim 1, wherein the Bayer liquor is washer overflow, diluted spent liquor, diluted green liquor or lakewater.

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

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

16. The method of claim 1, wherein the Bayer liquor has a TA less than 100 gL.sup.1.

17. The method of claim 1, wherein the Bayer liquor has a TA less than 75 gL.sup.1.

18. The method of claim 1, wherein the Bayer liquor has a TA between 50 and 100 gL.sup.1.

19. The method of claim 1, wherein the impurities are selected from the group consisting of phosphorus, vanadium, silicon, and combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] 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:

[0081] FIG. 1 is a plot showing the effect of TA on P.sub.2O.sub.5 and SiO.sub.2 incorporation into hydrocalumite for the series of runs with 1.sup.st refinery spent liquor shown in Table 1:

[0082] FIG. 2 is a plot showing the effect of TA on P.sub.2O.sub.5 and SiO.sub.2 incorporation into hydrocalumite for the series of runs with 2.sup.nd refinery spent liquor shown in Table 2;

[0083] FIG. 3 is a plot showing the effect of TA on P.sub.2O.sub.5 SiO.sub.2 and V.sub.2O.sub.5 incorporation into hydrocalumite for the series of runs with 3.sup.rd refinery spent liquor shown in Table 3;

[0084] FIG. 4 is a plot showing the effect of TA on P.sub.2O.sub.5 and SiO.sub.2 incorporation into hydrocalumite for the series of runs with 1.sup.st refinery green liquor shown in Table 4;

[0085] FIG. 5 is a plot showing the effect of TA on P.sub.2O.sub.5 incorporation into hydrocalumite for liquors spiked with P.sub.2O.sub.5;

DESCRIPTION OF EMBODIMENTS

[0086] 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.

[0087] 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

[0088] 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.

[0089] 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. The duration of each experiment was one hour.

[0090] Liquors from three alumina refineries (hereinafter the 1.sup.st Refinery, the 2.sup.nd Refinery and the 3.sup.rd Refinery) were used and slaked lime was sourced from the 2.sup.nd Refinery. The slaked lime typically had a solids concentration of 250 gL.sup.1 with an available CaO content of approximately 56%. This lime had been produced by slaking in 2.sup.nd Refinery lakewater.

[0091] 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.

[0092] The concentration of impurity in the original spent liquor and the exit liquor was determined by ICP-OES. The amount of the impurities removed was calculated from a mass balance of the total impurities in the feed streams (liquor and lake water from slaked lime) compared to the total impurities present in the exit liquor. The difference between the feed and exit was assumed to be due to incorporation into the hydrocalumite. Due to a significant volume change during the reaction, an internal standard had to be used to determine volume of the exit liquor. Sodium malonate was used as the internal standard as it is not incorporated into the hydrocalumite.

[0093] The effect of TA on the uptake of P.sub.2O.sub.5, SiO.sub.2 and V.sub.2O.sub.5 was investigated with spent liquors from all three refineries and green liquor from the 1.sup.st refinery.

[0094] The concentration of lime added to the reaction mixture was 100 g CaOL.sup.1 of spent liquor for the 1.sup.st refinery (both spent and green liquors) and the 2.sup.nd refinery experiments, and 125 g CaOL.sup.1 of spent liquor from the 3.sup.rd refinery. The total liquid volume was approximately 2 L (liquor plus distilled water plus lime slurry lake water [88% of lime slurry volume]).

[0095] This sample of 1.sup.st refinery spent liquor had a TA of 262 gL.sup.1 (as Na.sub.2CO.sub.3). This liquor was diluted according to Table 1, to produce a series of liquors with decreasing TA. The actual TA of the reaction mixtures (reaction TA) was less than the water dilution alone due to the extra dilution caused by the lake water contained in the lime slurry. The lime slurry added was proportional to the original feed spent liquor added, which is why the lime slurry volume and lime concentration in the reactor decreases through the experimental runs. The CaO added was relatively constant when proportioned to the feed liquor (approximately 104 gL.sup.1).

TABLE-US-00001 TABLE 1 Effect of TA reaction mixtures for the 1.sup.st refinery experiments CaO Lime Lime conc in Liquor Water Slurry conc in feed Run volume volume volume reactor liquor Reaction number (L) (L) (L) (gL.sup.1) (gL.sup.1) TA (gL.sup.1) 1 1.30 0.00 0.97 110.1 106 165.9 2 1.15 0.24 0.86 98.4 106 147.4 3 1.00 0.48 0.74 85.9 105 129.1 4 0.85 0.73 0.63 73.4 105 109.6 5 0.70 0.98 0.52 60.3 104 90.4 6 0.55 1.24 0.40 47.0 103 70.7 7 0.40 1.52 0.29 33.3 101 50.7 8 0.23 1.72 0.17 20.3 103 30.2

[0096] FIG. 1 shows the amount of phosphorus and silica removed per tonne of hydrocalumite produced for the 1.sup.st refinery spent liquor. As the reaction TAs decrease the uptake by hydrocalumite for both P.sub.2O.sub.5 and SiO.sub.2 increases. For both the P.sub.2O.sub.5 and SiO.sub.2, it is assumed that none of these impurities in the lime solids dissolve under these mild reaction conditions so they are excluded from the input mass balance (XRF analysis show typically 0.94% for SiO.sub.2 and 0.11% for P.sub.2O.sub.5 in the lime solids). At the undiluted TA, with no additional water added (run 1), there was 0 gT.sup.1 uptake for P.sub.2O.sub.5 and there was an increase in SiO.sub.2 into the malonate normalised product liquor (producing the negative uptake), indicating that some of the SiO.sub.2 from the solid lime phase was dissolving to some extent. This means that there may be a higher impurity uptake if there is dissolution of the impurities from the solid phase of the lime, but as this uptake is difficult to quantify, it has been excluded from the mass balance.

[0097] The concentration of P.sub.2O.sub.5 and SiO.sub.2 in the feed liquor was 168 mgL.sup.1 and 715 mgL.sup.1. The percentage removed at the lowest TA was 75% for P.sub.2O.sub.5 and 67% for SiO.sub.2. In the lowest TA run, there were small amounts of P.sub.2O.sub.5 and SiO.sub.2 left in the product liquor at the end of the experiment (4.6 mgL.sup.1 P.sub.2O.sub.5 and 25.7 mgL.sup.1 SiO.sub.2 remaining).

[0098] The uptake of SiO.sub.2 and P.sub.2O.sub.5 was also tested in 2.sup.nd refinery spent liquor (see Table 2 for liquor conditions), showing a similar increase in uptake with decreasing TA (FIG. 2). The initial TA of the liquor was 256 gL.sup.1. Uptake did not appear to change significantly between the two lowest reactions TAs for these experiments.

TABLE-US-00002 TABLE 2 Effect of TA reaction mixtures for the 2.sup.nd refinery experiments CaO Lime Lime conc in Liquor Water Slurry conc in feed Run volume volume volume reactor liquor Reaction number (L) (L) (L) (gL.sup.1) (gL.sup.1) TA (gL.sup.1) 1 1.30 0.00 0.96 108.9 104 163.5 2 1.15 0.24 0.85 97.2 104 145.2 3 1.00 0.48 0.74 85.3 104 126.7 4 0.85 0.73 0.62 72.4 103 107.8 5 0.70 0.98 0.51 59.4 102 88.7 6 0.55 1.24 0.39 46.3 101 69.3 7 0.40 1.52 0.28 33.0 100 49.6 8 0.23 1.72 0.17 20.2 102 29.6

[0099] In this liquor, the concentration of P.sub.2O.sub.5 was 149 mgL.sup.1 and the concentration of SiO.sub.2 was 765 mgL.sup.1 with 70% and 63% of the impurities removed at the lowest TA run. SiO.sub.2 uptake was higher in the 2.sup.nd refinery liquor than the 1.sup.st refinery liquor which agrees with the concentration of SiO.sub.2 in the starting liquors with the 2.sup.nd refinery liquor having a higher SiO.sub.2 concentration (765 mgL.sup.1 vs 715 mgL.sup.1). Uptakes were similar for P.sub.2O.sub.5 where 1.sup.st refinery had a slightly higher P.sub.2O.sub.5 concentration compared to the 2.sup.nd refinery, 168 mgL.sup.1 vs 149 mgL.sup.1.

[0100] The experiments were repeated with the 3.sup.rd refinery spent liquor; this time at a higher CaO charge. Experimental liquor conditions are shown in Table 3. The initial TA of this liquor was 272 gL.sup.1. These results also include V.sub.2O.sub.5 as a part of the ICP-OES analysis suite.

TABLE-US-00003 TABLE 3 Effect of TA reaction mixtures for the 3.sup.rd refinery experiments CaO Lime Lime conc in Liquor Water Slurry conc in feed Run volume volume volume reactor liquor Reaction number (L) (L) (L) (gL.sup.1) (gL.sup.1) TA (gL.sup.1) 1 1.30 0.00 1.01 128.1 125.2 171.1 2 1.15 0.24 0.89 114.4 124.7 152.3 3 1.00 0.48 0.78 101.1 125.7 132.8 4 0.85 0.73 0.66 86.3 125.1 113.1 5 0.70 0.98 0.54 71.3 124.3 93.3 6 0.55 1.24 0.43 56.8 126.0 72.8 7 0.40 1.52 0.31 40.7 124.9 52.3 8 0.23 1.72 0.18 24.8 126.1 31.3

[0101] For all three impurities, the uptake into the hydrocalumite increased with decreasing TA (FIG. 3). Compared to the 1.sup.st and 2.sup.nd refinery liquors, SiO.sub.2 uptake turned positive at a lower TA (approximately 130 gL.sup.1, compared to 150 gL.sup.1 for the 1.sup.st refinery liquor and all tests for the 2.sup.nd refinery liquor). This was due to the dissolution of some SiO.sub.2 in the lime and the higher lime charge in the 2rd refinery liquor experiments meant a lower TA had to be achieved before the net uptake exceeded the dissolution. Due to this, although the higher lime charge gave a higher yield per litre of liquor, at a given TA the uptake was less for the 3.sup.rd refinery than the other two. The slopes of the SiO.sub.2 points in FIGS. 1-3 were similar, indicating the change in uptake with TA did not vary with the three liquors.

[0102] The uptake of SiO.sub.2 and P.sub.2O.sub.5 was also tested in 1.sup.st refinery green liquor (see Table 4 for liquor conditions), showing a similar increase in uptake with decreasing TA (FIG. 4). The initial TA of this liquor was 247.5 gL.sup.1, which was lower than the spent liquors from the three refineries.

TABLE-US-00004 TABLE 4 Effect of TA reaction mixtures for the 1.sup.st refinery experiments CaO Lime Lime Conc in Liquor Water Slurry Conc in feed Run volume volume volume Reactor liquor Reaction number (L) (L) (L) (gL.sup.1) (gL.sup.1) TA (gL.sup.1) 1 1.30 0.00 0.96 108.9 104.0 158.3 2 1.15 0.24 0.85 97.2 104.0 140.5 3 1.00 0.48 0.74 85.3 104.0 122.6 4 0.85 0.73 0.62 72.4 103.0 104.3 5 0.70 0.98 0.51 59.4 102.0 85.9 6 0.55 1.24 0.39 46.3 101.0 67.1 7 0.40 1.52 0.28 33.0 100.0 48.0 8 0.23 1.72 0.17 20.2 102.0 28.6

[0103] Phosphorus uptake increased as TA decreased like the spent liquors, but uptake in the green liquor was significantly higher for P.sub.2O.sub.5. SiO.sub.2 uptake also show the trend of increasing uptake with decreasing TA, although SiO.sub.2 uptake was lower in the 1.sup.st refinery green liquor than the 1.sup.st refinery spent liquor.

[0104] The uptake of the three impurities was investigated in 1.sup.st refinery and 3rd refinery lakewaters with TA's of 27 gL.sup.1 and 23 gL.sup.1 respectively. Unlike the previous suite of experiments, where water was added to lower the TA, the amount of lime slurry added was adjusted to 20 gL.sup.1 (based on reactor volume) which was similar to the amount of lime added for the spent liquor experiments at the lowest TA. No additional water was added to the reaction solution. Reaction conditions and the uptakes for P.sub.2O.sub.5 and V.sub.2O.sub.5 are shown in Table 5. Due to SiO.sub.2 levels in the lakewater being close to the detection limit of the ICP-OES, results from SiO.sub.2 were excluded from the impurity removal calculation. Comparing the uptake of P.sub.2O.sub.5 in the 1.sup.st refinery lake water to the lowest dilution spent liquor, showed a lower uptake in the lakewater than the spent liquor. This difference could be due to the lakewater start and end liquor being at the low end of the analytical range of analysis, where the diluted 45E liquor mass balance was calculated based on an analysis of the neat liquor. Results for P.sub.2O.sub.5 and V.sub.2O.sub.5 were more comparable when comparing diluted 3rd refinery spent liquor and lakewater.

TABLE-US-00005 TABLE 5 Liquor conditions and impurity uptake for 1.sup.st refinery and 3.sup.rd refinery lake water experiments CaO Lime Conc in Lime Conc in feed Reaction P.sub.2O.sub.5 V.sub.2O.sub.5 Run Liquor Slurry Reactor liquor TA uptake uptake number vol (L) vol (L) (gL.sup.1) (gL.sup.1) (gL.sup.1) (gT.sup.1) (gT.sup.1) 1.sup.st - 1 2.02 0.15 20.2 11.7 27.2 264.3 112.6 1.sup.st - 2 2.02 0.15 20.2 11.7 27.2 308.0 131.6 3.sup.rd - 1 1.50 0.11 20.0 11.5 22.7 415.4 255.8 3.sup.rd - 2 1.50 0.11 20.0 11.5 22.7 453.2 406.4

[0105] To further investigate the uptake of phosphorus, P.sub.2O.sub.5 was spiked into some neat 1.sup.st refinery spent liquor and some diluted 1.sup.st refinery spent (low TA conditions). Three liquor solutions were prepared: 2 litres of neat liquor, 2 litres of liquor with 50 mgL.sup.1 P.sub.2O.sub.5 added and 2 litres of liquor with 100 mgL.sup.1 P.sub.2O.sub.5 added. The P.sub.2O.sub.5 addition was by the addition of 5 or 10 mL of a 20 mgmL.sup.1 P.sub.2O.sub.5 stock solution (107.13 gL.sup.1 Na.sub.3PO.sub.4.12H.sub.2O). These three liquors with 0, 50 or 100 mgL.sup.1 of additional P.sub.2O.sub.5 were used undiluted or diluted to 25% strength with the addition of water (Table 6).

TABLE-US-00006 TABLE 6 P.sub.2O.sub.5 spiking reaction mixtures in 1.sup.st refinery spent liquor CaO Lime Lime conc in Liquor Water Slurry conc in feed Additional Run volume volume volume reactor liquor Reaction P.sub.2O.sub.5 number (L) (L) (L) (gL.sup.1) (gL.sup.1) TA (gL.sup.1) (mgL.sup.1) Notes 1 1.2 0 1.05 136.7 141.0 155.09 0 Neat liquor 2 1.2 0 1.05 136.7 141.0 154.45 50 Neat liquor 3 1.2 0 1.05 136.7 141.0 154.45 100 Neat liquor 4 0.4 1.2 0.35 52.6 141.0 57.31 0 Low TA 5 0.4 1.2 0.35 52.6 141.0 57.07 50 Low TA 6 0.4 1.2 0.35 52.6 141.0 57.07 100 Low TA

[0106] Table 7 shows the concentration of P.sub.2O.sub.5 in the starting liquor (without the dilution due to the lime and the water [for runs 4-6]), P.sub.2O.sub.5 in the end liquor (both raw and corrected back to neat liquor conditions with a malonate normalisation), the difference in concentration and impurity removal based on the mass balance.

TABLE-US-00007 TABLE 7 Liquor results for P.sub.2O.sub.5 spiking experiments. P.sub.2O.sub.5 P.sub.2O.sub.5 in P.sub.2O.sub.5 in in neat product product liquor Start-End Impurity Run liquor liquor (normalised)* difference removal number (mgL.sup.1) (mgL.sup.1) (mgL.sup.1) (mgL.sup.1) (gT.sup.1) 1 150.9 99.5 146.7 4.2 29 2 201.2 122.0 180.2 21.0 8 3 253.3 139.7 207.4 45.9 68 4 150.9 15.5 63.4 87.5 216 5 201.2 12.8 52.4 148.8 379 6 253.3 13.7 57.4 195.9 501 *Normalised back to neat liquor conditions with malonate correction to account for volume changes

[0107] FIG. 5 shows the uptake at the two different liquor strengths for the three P.sub.2O.sub.5 concentrations. Impurity uptake was significantly higher at the lower TA than the undiluted liquor TA. At a given TA, P.sub.2O.sub.5 uptake increased with P.sub.2O.sub.5 addition, but for the higher TA solutions, the additional 50 or 100 mgL.sup.1 P.sub.2O.sub.5 added did not result in an additional 50 or 100 mgL.sup.1 removal. For the three dilute solutions, the remaining P.sub.2O.sub.5 in the product liquor dropped to a level of 12-15 mgL.sup.1 at the three P.sub.2O.sub.5 concentrations, suggesting that at these concentrations, P.sub.2O.sub.5 is almost totally removed despite the initial concentration.