Regeneration of an ion exchange column

Abstract

The present invention relates to a method of regenerating an ion exchange material loaded with chromate ions and nitrate ions in an ion exchange column, the method comprising subjecting the loaded ion exchange column to a regeneration sequence comprising the following steps: (i) passing a first salt solution through the column forming a first effluent solution; (ii) passing a second salt solution through the column to at least partially remove the chromate ions from the column forming a second effluent solution, wherein the second salt solution has a higher salt concentration than the first salt solution; (iii) passing a third salt solution through the column to at least partially remove nitrate ions from the column forming a third effluent solution, wherein the third salt solution has a salt concentration higher than the second salt solution.

Claims

1. A method of regenerating an ion exchange material loaded with chromate ions and nitrate ions in an ion exchange column, the method comprising subjecting the loaded ion exchange column to a regeneration sequence comprising the following steps: (i) passing a first salt solution through the column forming a first effluent solution; (ii) passing a second salt solution through the column to at least partially remove the chromate ions from the column forming a second effluent solution, wherein the second salt solution has a higher salt concentration than the first salt solution; (iii) passing a third salt solution through the column to at least partially remove nitrate ions from the column forming a third effluent solution, wherein the third salt solution has a salt concentration higher than the second salt solution, wherein the first salt solution comprises from 0.2 to 2.0% w/v of sodium chloride or potassium chloride, wherein the second salt solution comprises from 0.3 to 5.0% w/v of sodium chloride or potassium chloride, and wherein the first salt solution comprises from 10 to 20% w/v of sodium chloride or potassium chloride.

2. The method according to claim 1 wherein the ion exchange material is a nitrate-selective anion exchange resin.

3. The method according to claim 1 wherein the first effluent solution comprises sulphate anions and/or bicarbonate anions which have been removed from the ion exchange material.

4. The method according to claim 1 wherein the salt concentration of the second salt solution passed through the ion exchange material is at least two times the salt concentration of the first salt solution passed through the ion exchange material.

5. The method according to claim 1 wherein the concentration of the third salt solution passed through the ion exchange material is at least two times the salt concentration of the second salt solution passed through the ion exchange material.

6. The method according to claim 1 wherein the first salt solution passed through the ion exchange material has a conductivity of from about 8 to about 20 mScm.sup.1.

7. The method according to claim 1 wherein the second salt solution passed through the ion exchange material has a conductivity of from about 13 to about 150 mScm.sup.1.

8. The method according to claim 1 wherein the third salt solution passed through the ion exchange material has a conductivity of greater than 160 mScm.sup.1.

9. The method according to claim 1 wherein the ratio of the volume of first salt solution introduced into the column to the volume of second salt solution introduced into the column is from 10:1 to 5:1.

10. The method according to claim 1 wherein the ratio of the volume of second salt solution introduced into the column to the volume of third salt solution introduced into the column is from 1:2 to 1:10.

11. The method according to claim 1 further comprising a step of passing water through the column forming an effluent wash water.

12. The method according to claim 11 wherein the effluent wash water is recycled and reused in the regeneration method.

13. The method according to claim 1 further comprising collecting and/or further treating the first effluent, the second effluent and/or the third effluent.

14. The method according to claim 1 wherein the first effluent solution is collected and/or treated separately from the second effluent solution comprising chromate ions.

15. The method according to claim 1 wherein the second effluent solution comprising chromate ions is collected and/or treated separately from the third effluent solution comprising nitrate anions.

16. The method according to claim 1 comprising recycling the first effluent through the ion exchange resin.

17. The method according to claim 1 wherein the second effluent solution comprising chromate ions is subjected to an anion removal treatment to remove the chromate ions.

18. The method according to claim 1 wherein the third effluent solution containing the nitrate anions is subjected to an anion removal treatment to remove the nitrate anions.

19. The method according to 1 wherein at least one of (a) the second effluent solution comprising chromate ions is subjected to an anion removal treatment to remove the chromate ions, or (b) the third effluent solution containing the nitrate anions is subjected to an anion removal treatment to remove the nitrate anions, and the anion removal treatment comprises an electrolytic treatment method.

Description

(1) The present invention will now be described further, by way of example only, with reference to the following figures, in which:

(2) FIG. 1: is a graph showing the nitrate levels observed in the outlet from an ion exchange column containing nitrate-selective resin.

(3) FIG. 2: is a graph showing the continued removal of chromium after the nitrate capacity of the same resin has been exceeded.

(4) FIG. 3: is a graph showing the elution profile for an example of the present invention.

(5) FIG. 4: is a graph showing the elution profile for Example 3 of the present invention.

(6) FIG. 5: is a graph showing the initial separation of the sulphate, then the chromium, as the chloride ion concentration increases, for Example 4 of the present invention.

(7) FIG. 6: is a graph showing the initial separation of the sulphate, then the chromium, as the chloride ion concentration increases for Example 5 of the present invention.

EXAMPLES

Example 1

(8) A column containing nitrate-selective resin is loaded with water known to contain levels of nitrate exceeding the required standard and levels of chromium, to beyond its accepted service capacity for nitrate and exhibits continued removal of chromium.

(9) FIG. 1 shows the nitrate levels observed in the outlet from an ion exchange column containing nitrate-selective resin. This column would conventionally be taken out of service when nitrate levels in the output rise above a preset threshold, indicated by A on the figure.

(10) FIG. 2 shows the continued removal of chromium after the nitrate capacity of the same resin has been exceeded. It can be seen from FIG. 1 that the flow to two columns, one loaded to a point less than A and one loaded to a point beyond A, can be adjusted to produce the desired output nitrate level.

Example 2

(11) The Purolite A520E (a nitrate-selective ion resin) was loaded with nitrate ions and chromate ions. The following regeneration process in accordance with the invention was carried out.

(12) 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.

(13) 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.

(14) 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.

(15) 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.

(16) 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 could facilitate further volume reduction treatment.

Example 3

(17) Purolite A520e nitrate selective resin loaded to saturation then regenerated with 5 bed volumes of 0.3 to 0.8% potassium chloride solution, followed by 2.5 bed volumes of 14.9% potassium chloride solution [See FIG. 4].

(18) In this example, the outlet of the separated sulphate can simply be slowly dosed back into the outlet water with negligible effect on the water composition. The chromium fraction can be separated from the bulk of the nitrate fraction and treated separately. The nitrate fraction can be treated in an electrochemical cell (see GB 2348209 A and GB 2365023 A). The rinse, at the end of the regeneration process, can be simply retained and re-used as the initial low concentration potassium chloride solution on the subsequent regeneration. The start of the rinse collection can be simply determined by monitoring the conductivity of the solution and setting the collection start point to achieve the desired final rinse concentration.

Example 4

(19) Purolite A600/4149 SBA resin loaded to >20,000 bed volumes then regenerated with 11.7% sodium chloride solution, showing initial separation of the sulphate, then the chromium, as the chloride ion concentration increases.

(20) In this example, the sulphate removed in the initial part of the regeneration, up to 1 bed volume flow, contains very low levels of chloride and chromium and can be simply disposed to sewer, with the chromium fraction (from 1 to 2 bed volumes) separated for treatment into 1 bed volume [See FIG. 5].

(21) From examples 3 and 5, it can be expected that the initial use of a lower concentration of sodium chloride will further improve the separation of the sulphate from the chromium. As with example 3, the rinse can be retained and the concentration adjusted to allow its re-use on a subsequent regeneration. This achieves a considerable reduction in the total volume of regenerant requiring off-site treatment and disposal.

Example 5

(22) Purolite A600/4149 SBA resin loaded to >20,000 bed volumes then regenerated with 5 bed volumes of 1.5% potassium chloride solution, followed by 1 bed volume of 14.9% potassium chloride solution.

(23) This example shows that an initial potassium chloride solution concentration of 1.5% has successfully removed nearly all the sulphate within 5 bed volumes. Levels of chloride and chromium in this fraction are sufficiently low that there is potential to recycle this portion of the regenerant as potassium sulphate, which is a high value fertiliser. The chromium fraction can again be separated into under 1 bed volume for separate treatment and disposal [See FIG. 6].