Method of Removing Chromate Ions from an Ion-Exchange Effluent

Abstract

The present invention relates to a method of removing chromate ions from an ion-exchange effluent, the method comprising: (i) providing an ion-exchange effluent comprising chromate ions obtained from the regeneration of an ion-exchange material, (ii) admixing the ion-exchange effluent with a source of alkali metal dithionite to form a first precipitate, and (iii) removing the first precipitate

Claims

1. A method of removing chromate ions from an ion-exchange effluent, the method comprising: (i) providing an ion-exchange effluent comprising chromate ions obtained from the regeneration of an ion-exchange material, (ii) admixing the ion-exchange effluent with a source of alkali metal dithionite to form a first precipitate, and (iii) removing the first precipitate.

2. The method according to claim 1, wherein the method comprises regenerating the ion-exchange material.

3. The method according to claim 1, wherein prior to step (ii), the ion-exchange effluent is admixed with a source of calcium chloride to form a second precipitate, and wherein step (iii) further comprises removing the second precipitate.

4. The method according to claim 1, wherein the ion-exchange effluent comprises sodium chloride or potassium chloride, preferably wherein the ion-exchange effluent comprises from 0.3 to 15% w/v of sodium chloride or potassium chloride, preferably from 1.5 to 15% w/v, and more preferably from 2 to 5% w/v.

5. The method according to claim 1, wherein the first and/or second precipitate is removed by filtration.

6. The method according to claim 1, wherein the pH of the ion-exchange effluent is at least 5, more preferably at least 6, most preferably at least 7.

7. The method according to 6, wherein the pH of the ion-exchange effluent is adjusted to from 9 to 10, preferably to from 9.2 to 9.4, prior to the step of admixing the ion-exchange effluent with the source of alkali metal dithionite.

8. The method according to claim 1, wherein the ion-exchange effluent has a concentration of chromate ions of from 100 to 1000 mg/L, preferably of from 200 to 800 mg/L, more preferably from 300 to 600 mg/L.

9. The method according to claim 1, wherein the alkali metal dithionite is added in a mass ratio of from 10:1 to 40:1, preferably from 20:1 to 30:1, more preferably about 25:1, relative to the mass of chromate ions initially present in the ion-exchange effluent.

10. The method according to claim 1, wherein the source of alkali metal dithionite comprises sodium dithionite, preferably wherein the source of alkali metal dithionite consists of sodium dithionite.

11. The method according to claim 1, wherein at least 60 wt % of the chromate ions are removed from the ion-exchange effluent based on the total amount of chromate ions initially present in the ion-exchange effluent, more preferably at least 75 wt %, still more preferably at least 90 wt %, and preferably at most 99 wt %.

12. The method according to claim 1, wherein the ion-exchange material is a strong base anion (SBA) exchange resin, or wherein the ion-exchange material is a nitrate-selective anion exchange resin.

13. The method according to any of claim 2, wherein the ion-exchange material is loaded with chromate ions and nitrate ions and is regenerated by the following steps: (i) passing a first salt solution through the ion-exchange material to form a first effluent solution; (ii) passing a second salt solution through the ion-exchange material to at least partially remove the chromate ions from the ion-exchange material forming a second effluent solution which is the ion-exchange effluent, wherein the second salt solution has a higher salt concentration than the first salt solution; (iii) passing a third salt solution through the ion-exchange material to at least partially remove nitrate ions from the ion-exchange material forming a third effluent solution, wherein the third salt solution has a salt concentration higher than the second salt solution.

14. The method according to claim 13, wherein the first effluent solution comprises sulphate anions and/or bicarbonate anions which have been removed from the ion-exchange material.

15. The method according to claim 13, wherein the salt solution comprises sodium chloride or potassium chloride.

16. The method according to claim 13, wherein the second and/or third salt solution is a more concentrated solution of the same salt as the first solution.

17. The method according to claim 13, further comprising collecting and/or further treating the first effluent and/or the third effluent.

18. The method according to claim 13, wherein the third effluent solution containing the nitrate anions is subjected to an anion removal treatment to remove the nitrate anions, preferably wherein the anion removal treatment comprises an electrolytic treatment method.

19. A method of removing chromate ions from an ion-exchange effluent, the method comprising: (i) providing an ion-exchange effluent comprising chromate ions obtained from the regeneration of an ion-exchange material, (ii) admixing the ion-exchange effluent with a source of alkali metal metabisulfite, preferably sodium metabisulfite, to form a first precipitate, and (iii) removing the first precipitate.

20. Use of a source of alkali metal dithionite to remove chromate ions from an ion-exchange effluent obtained from the regeneration of an ion-exchange material.

21. The use according to claim 19, wherein the source of alkali metal dithionite comprises sodium dithionite, preferably wherein the source of alkali metal dithionite consists of sodium dithionite.

22. The use according to claim 19, wherein the ion-exchange material is a strong base anion (SBA) exchange resin, or wherein the ion-exchange material is a nitrate-selective anion exchange resin.

Description

FIGURES

[0081] The present invention will be described in relation to the following non-limiting figures, in which:

[0082] FIG. 1 is a graph showing the effect of increasing the amount of added sodium dithionite on the chromate levels in an ion-exchange effluent (without basifying the effluent prior to the addition of dithionite)

[0083] FIG. 2 is a graph showing the effect of increasing the amount of added sodium dithionite on the chromate levels in an ion-exchange effluent (with basification of the effluent prior to the addition of dithionite)

[0084] FIG. 3 is a graph showing the elution profile for an example in which an ion-exchange effluent comprising chromate ions is obtained.

EXAMPLES

[0085] The present disclosure will now be described in relation to the following non-limiting examples.

Example 1

[0086] An ion-exchange effluent was obtained from the regeneration of an ion-exchange material, the effluent having 156.00 mg/L Cr, 27.08 g/L Cl, 8.56 g/L S and 11.07 mg/L V, as determined by ICPMS. This effluent solution was divided into portions. Sodium dithionite powder was added to each portion to give different final concentrations of sodium dithionite in the effluent solution. Upon addition of sodium dithionite, a precipitate was observed to form. In each case, the precipitate was filtered off to leave the supernatant liquid. The levels of Cr, Cl, S, As, Se, U and V were measured for each supernatant liquid. In some cases, the effluent solution was basified with sodium hydroxide to pH 9.3 prior to the addition of sodium dithionite.

[0087] The results are given in the table below and are illustrated in FIGS. 1 and 2.

TABLE-US-00001 NaOH Cr Cl S As Se U V [Na.sub.2S.sub.2O.sub.4] added? Dilutions (mg/L) (g/L) (g/L) (mg/L) (mg/L) (mg/L) (mg/L) 0 g/L No 1484.86 156.00 27.08 8.58 <MDL <MDL <MDL 11.07 1 g/L No 1069.60 59.44 23.24 10.49 <MDL <MDL <MDL 10.06 1 g/L Yes 1058.20 69.25 21.68 9.58 <MDL <MDL 0.81 9.59 2 g/L No 186.89 4.38 25.31 8.33 <MDL <MDL 0.91 0.79 2 g/L Yes 94.09 4.38 24.37 8.38 0.10 0.93 0.76 0.76 5 g/L No 95.53 5.12 25.22 11.70 0.09 0.85 1.62 0.61 5 g/L Yes 97.23 4.67 24.14 11.57 0.07 0.91 0.42 0.56 10 g/L  Yes 94.02 5.06 22.92 21.49 0.06 4.45 5.86 0.54 20 g/L  No 102.53 16.55 22.32 55.06 <MDL 0.79 0.64 1.16 20 g/L  Yes 95.18 8.87 21.43 51.07 <MDL 0.78 0.57 0.69 (MDL = minimum detection limit)

[0088] As can be seen from the table, the addition of sodium dithionite to give a concentration of 1 g/L facilitated around a 60% decrease in Cr levels in the effluent solution. At 2 g/L sodium dithionite and above, the observed decrease was at least 90%. At 5 g/L and 20 g/L sodium dithionite, basification prior to addition of sodium dithionite was found to lower the chromate levels in the resulting supernatant, especially so at 20 g/L. In all experiments, the levels of Cl, As, and Se were found to be relatively constant, indicating that none of these was reduced by the dithionite. By contrast, vanadium levels were found to be significantly lower at 2 g/L sodium dithionite and above, consistent with the reduction of the vanadium ions present in the effluent by the dithionite, leading to precipitation of the vanadium. As expected, sulphur levels were found to increase as the concentration of dithionite increased.

Example 2

[0089] The Purolite A520E (a nitrate-selective ion resin) was loaded with nitrate ions and chromate ions. The following regeneration process was carried out.

[0090] A first KCl solution comprising 3000 ppm of chloride ions was prepared. 5 bed volumes of the first solution were passed through the column at flow rate of 5 bed volumes per hour to provide a first effluent comprising sulphate and bicarbonate anions.

[0091] A second KCl solution comprising 15000 ppm of chloride ions was then passed through the column. Half a bed volume of the second solution was passed through the column at a flow rate of 2 bed volume per hour to provide a second effluent comprising chromate ions.

[0092] A third KCl solution comprising 72000 ppm of chloride ions was then passed through the column. A total of 2.5 bed volume were passed through the column at a flow rate of 2 bed volumes per hour to provide a third effluent comprising nitrate anions.

[0093] FIG. 3 shows that at time t is 0 to 85 minutes HCO.sub.3.sup.− and SO.sub.4.sup.2− are removed as a first dilute salt solution is passed through the column. From time t is 85 to 110 minutes, chromate ions (Cr (VI) ions) are removed as the concentration of chloride ions in the salt solution is increased as the second solution is passed through the column. From t is 110 to 220 minutes, nitrate ions NO.sub.3.sup.− are removed as the concentration of chloride ions in the salt solution is increased as the third solution is passed through the column. From t 220 to the end shows the results as the column is washed.

[0094] This regeneration procedure clearly shows that the chromium can be removed in a small fraction of the waste volume, after the sulphate has been removed and co-incidentally at the beginning of the nitrate removal stage of the process (FIG. 3). In FIG. 3, Cr (mgl.sup.−1) and NO.sub.3, SO.sub.4 (mg/l) are plotted on the secondary Y axis. It can clearly be seen that this offers the potential to separate the chromium into a fraction of the regenerant volume which can then be treated in accordance with Example 1.

[0095] When introducing elements of the present disclosure or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0096] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.