CA1112379A - Chromate ion removal from aqueous chlorate solutions - Google Patents

Abstract

CHROMATE ION REMOVAL FROM AQUEOUS CHLORATE SOLUTIONS
(IR 2289) ABSTRACT OF THE DISCLOSURE
Chromate ions present in chlorate rich aqueous solutions, e.g., as obtained from the electrolysis of sodium chloride solutions, are substantially completely removed by passing said solution through a bed of a mixture of an anion exchange resin in the chloride form and a weak cation exchange resin in a conditioned hydro-gen form.

Description

This invention relates to a process for the remvval of chromate ions from aqueous solutions containing such ions and high amounts of dissolved alkall metal chlorates. More particularly, it relates to a method of removing chromate ions from aqueous chlorate rich-chloride solutions by passing the solution through a bed of mixed anion and cation exchange resins each having a specified form.
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. U, S. Patent No. 3, 835, 001 which issued September 10~ 1974 to T. F. O'Brien . discloses a method for the removal of a substani:i~l proportion of chromate ions from an aqueous alkali metal chlorate-chloride solution wherein the solution is~- ~ passed through a bed o~ strong base ion exchange resin in the chloride Eorm at an ~ 10 original solution pH of less than 6. 5 and preferably about 5. While this process ., ef~ects a distinct improvement in the removal of chromate ions from such solutions . over prior methods, the need for further improvement in re~noving even greater pro-portions of chromate ion is evident. That is, it is commercially desirable to pro-vide,a process which will economically remove su~stantially all chromate ions from solutions containing as little as 10 parts per rnillion by weight but generally ., ;.' : ...... Iess than:about 20 grams .per liter of dissolved.alkali metal chromate such that a : single treatment of the solution will provide substantial rernoval and preferably a substantially complete removal of the chromate ions from the solution without the attendant formation of hazardous chlorine dioxide.
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. 20 Accordingly, the present invention involves a mP~hod of removing chromate lons from a solution containing a high amount of dissolved alkali metal chlorateand having a dissolved alkali metal chromate concen~ration which comprises passing said s~lution throu~h a bed consisting essentially oiE an intimate mixture of an anion exchange resin in the chloride form and ~ weak cation exchançle resin , .

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in a conditioned hydrogen form, said cation exchange resin having a number of exchange sites not less than the number of exchange sites present in the anion exchange resin and said anion exchange resin being present in an amount sufEici-ent to provide a number of exchange sites at least about equivalent to the number of chromate ions to be removed from said solution. In a preferred method said -' cation exchange resin is present in an amount not less than 0. 5 parts but not greater than 2. 0 parts by weight for each part of anion exchange resin.

¦ In industrial practice9 the number of exchange sites is commonly specified by the "total exchange capacity" expressed as milliequivalent/milliliter or m~
equivalents/gram. The method of determination oE this quantity differs somewhat arnon~ suppliers of resins. The manuEacturer's technical bulletin (1972) for ~ ~ d e f~ Amberlite/IRC-50 states:
"Total exchange capacity: The max~mum capacity of Amberlite IRC-50 can be determined conveniently by equilibrating a representative sample in the hydrogen form with an excess of 0.1 N sodium hydroxide.
The resin should remain in contact with the excess caustic for 24 to 48 hours. The amount oE sodium hydroxide neutralized is considered equivalent to the maximum capacity of the exchanger". --. ` ..Both cation an~ anion exchange resins useful for this invention may be of : ~o either the gel-type or the macroreticular type, but the macroreticular type is pre-ferred because of its greater stability in multiple cycles of ion exchange opera-tion Ion exchange resins of various types are discussed, for example9 in the ~s Kirk-Othmer Encyclspedia of Chemical Technology~ Second Edition, Volume II9 pages 871 et seq. -~

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The anion exchange resin may be a weak or strong base resin but, from the standpoint of a more quantitative recovery during reseneration, a weak base anion exchange resin is preferred. When employed during the service or chromate ad sorption step of the ion exchange process, the anion exchange resin should be initially in the chloride form The cation exchange resin for this invention is essentîally a weak acid CatiOII exchange resin which rllust be in a ~ondltioned hydrc)gen form at the start of the service step of the process, i~ e~ ~ when beginning chromate removal from ¦ the chlorate rich solution . ,. ''.

The term "conditioned h~rogen form" as used herein means a form of cation exchange resin wherein a portion of the hsrdrogen ions of a cation exchange resin which was initially entirely in the hydrogen form have been displaced with alkali ..
metal (Na+) ions to prevent excessive acidification of the chlorate rich solution during the service step of the process. Expressed differently, to reduce the ef-fect of "salt-splitting" by the chromate adsorption (service) step, the hydrogen'' form vf cation resln is rinsed ~conditioned~ with an alkali metal chloride l~rine preferably at a neutral pH (6 8) to bring the outlet (brine eEfluent) pH above 1. 0, preferably up to about 2 . .
Salt-splitting occurs typically when a solution contalning a salt of a strong , acid and a strong base is passed through a strong acid ion exchanger resin or a J ~ stron~ base ion exchange resin. The cation of the salt eXchanges readily with .
the hydrogen of the strong acid resin. Sirnilarly the anion of the salt exchanges readiiy with the hydroxide of the strong base resin. A distinguishing charac~eris-tic 3f weak acid and weak base exchangers compared to the strong acid and i, .

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strong base types is that this salt-splitting does not yenerally take place in asubstantial amount. That it does take place and must be controlled in the pre-sent process is probably due to the very high salt concentrations involved.

The process of this invention, which is advantageously a cyclic process, is broadly described a follows: ~
The selected exchange resins are treated either separately or as a mixture with a brine solution o~ mineral acid to provide the starting rnaterial. Then~ the cation resin ~if separate) or the mixture is treated with a neutral brine solution to provide a cation resin in the conditioned hydrogen form. If the exchange resins have not been mixed before these treatments, they are mixed in a manner weil ,"
known in the art, for example, by air blowing under watbr ~hereafter, the ser-; vice or chromate adsorption step is begun by passing the chromate containing ,,:
chlorate rich liquor through the mixed resin bed On exhaustion of the ion ex-' change bed, as indicated by a rise of pH to 2 and concurrent yellowing of the ~f-fluent solution, the flow of chromate containing solu~lon is terminated and the ,., anion exchange resin- is treated to recover the chromat~ ions with or without prior separation of the exchange resins.

As explained hereinafter, the resins may, if desired, be separated by passin~
~ a brine solution upwardly through the bed at a rate sufficient to cause separation : 20 of the resins which have a somewhat different spec~c gra~

Chromate is readily and completely removed ~stripped) by treatment of the anion resln or bed ~Arith an alkaline solution of alkali~metal chloride e. g. ~ about 4% sodium hydro~de in aqueous 12-15% sodium chlo~ide sc>lution. After the removal :i i~ of the chromate from the anion exchange resin, the process is repeated in a~cord-~ : . ' ' ~ .. `'- . ' :

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ance with the above steps for many cycles Imtil the resins are no longer capable of sufficient absorption to provide a useful process.
To aid in understanding the various steps of the process and their effect, the ollowing table depicting the theorized .
exchange mechanisms for the process with and without resin separation.

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.L~' The flow rates for the chromate contalning solution through the resin ex-change bed in the process of this invention should not be above about l. 0 gallon per minute (g, p. m, 3, preferably 0. 7 5 g, pO m., per square foot due to broa~ening of the adsorption frontr ;
;~ The process of this invention is suitable for any aqueous solution contain-ing alkali metal chlorate in sufficient concentral:ion to decompose upon acidifica-tion to a pH of about 0. ~5 u~ l. 0~ The concentration of alkali metal chlorate or chlorate and chloride should not be so high as to cause salting out under the con-ditions in the exchange column or bed. For example, an acceptable upper limlt 13 for the concentrations of a sodium chlorate rich sodium chloride solution at 80F.
t (26. 7C.) is about ~20 grams per liter of sodium chloride with 550 grams per liter .
of ~odium chlorate.

The follovving examples are set forth to demonstrate the process of this i,n-vention.
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Equal parts by weight oi a strong anion exchange resin of the gel-type (Amberlite IRA 400, Product of Rohm and Haas Company) and a weak acid exchange resin of the yel-type (Amberlite IRC 84 - Product of Rohm and Haas Company) wereadded ln an intimate mixture to a glass column having an internal diameter of l. 25 centimeters (cm. ). The mixed resin bed in the column had a height of approximate-ly 80 ce~imeters and a volume of lO0 cublc centimeters (cc. ~ A conditioning step iD~rolving~ the passage of 25 milliequivalents o~ hydrochloric acid throughthe column was made to insure that the exchange resins were partially ln the ~ . .
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chloride and hydrogen forms, respectively. ThiS step was Eo~lowecl by deionized water rinse s, An aqueous chlorate feed liquor containin~ sodium chromate [600 grams per liter (g. p. 1. ) of sodium chlorate and 5, 9 g. p. 1. oE soclium dichromate~ was passed 5 through the column in an upward direction at a rate of 2 cc, per minute. The up-ward direction was desirable because the specific gravity of the chlorate solution was greater than that of the resin, .~ ' ' . .
Successi~e portions (samples) of effluent from the column were analyzed for pH and chromate content usin~ a colorimeter. Progress of the upward moving front 10 of exchange in the column was observable from the color change of the resin, Observations and results of the analysis of successive portions are given in the ;,~i followiny table. Complete recovery of the chromate was achieved by passing al-kaline sodium chloride through the bed on termination oi the chromate removal ' step.
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TABLE I

Sample Bed Vol~me of Chromate in Efflu~nt Height of ExchangeFeed Liquor* Effluent g.p.l. pH Boundary 1 0.92 0.00 1.96 20 cm.

2 1.72 0.00 1.20 36 cm.

3 2.57 0.00 1.35 51

4 3.37 OOO~ 1.58 63 4.16 0.00 1.7~ 6 6 ~.98 0.00 1.96 75 lQ 7 5.33 0.02 2.22 78 (~op~
8 5.78 0.20 2.62 9 6.60 1.05 2.88 -~
7.~5 3.60 2.90 --~
11 ~.29 4.80 2.80 ------ :
Feed ------ 5.90 5.50 ------* Bed volume of feed or effluent is a term that is used for comparing the capacities of ion exchange columns of varying size.
One bed volume i~ the overall volume occupied by -the resin bed (square foot of cross section times height) including the volume occupied by the exchange resin beads themselves~
EX~MP~E :~I
A 1.9 cm. diameter column was packed to a height of 132 cm.
with an intlmate mixture of a weak cati.on exchange resin of the macroreticular type (Rohm and Haas Company's Amberlite IRC 50) and weak anion exchange resin o:E the macroreticular .

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type ~Rohm and Haas Companyls Amberlite IRA 93) in the weight ratio of 60 40.

The mixed resin bed was preconditioned by first passing a 4% aqueous sodium hydroxide solution in a downward direction through the column followed by a 4% aqueous hydrochloric acid solution until the pH dropped below 1. 5. A
final downward wash with nearly saturated, neutral aqueous sodium chloride solution was made to condition the hydrogen form of cation exchange resin until the effluent 3 pH rose to 1. 5.

The chromate adsorption was accomplished by passing a chlorate solution (450 g. p. l. of sodium chlorate) containing l. û2 g. p. 1. of sodium dichromate (Eeed liquor) upward through the resin bed at a rate of 5 cc. per minute. 1850 cc. of colorless product was collected before the effluent became visibly yellow. The effluent pH during this time rose steadily frorn l 6 to 2. 05. After 150 cc. of ad-ditional flow, the efflue-nt contained 24. 6 parts per million (p. p~ m. ) of sodium dichromate (Na2Cr2O7) and the effluent pH was 2 15.
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s 15 As in Example I, successive samples of effluent were taken and analyzed for ,., - pH. The color of each sample was noted as a check for chromate content, a color-le~ss sample indicating the substantlal absence of chromate. The following tablecontains l:he pH and observation data for each sampleO

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Sampl~ Bed Volume of Chromate in Effluent Heigh-t of Exchange Feed Liqu~r Effluent ~Color pH Bo~dary (cm.) Indicat.ion~
l l.42 colorless t2.3)* 20 2 l.85 colorless l.6 4l 3 3~57 colorless l.67 61 4 4.28 colorless l.77 9l

5.00 colorless l.90

6 5.28 sligh-t yello~ 2.05 ----

7 5.70 dist~nct 2.15 yello~
~24.6 ~pm~

* In;.tial pH is higher preceding equilibration of sodium chlorate w~th resins.
EX~MPLE III
In a 10 inch diameter column especially piped for cycling a continuous ion exchange process, a mixed resin bed was prepared by air mixing under water 1 part by weight of a weak cation resin (~mberlite ~RC 84) with 2 parts by weiyht of a weak anion res.in (Amberlite IRA 94), the mixture reaching a total height of 35 inches in the column.
The bed was treated by passing a 4~ solution of HCl in a ~.
NaCl brine downward until the effluent reached a pH of 1.5. A
: conditioning step was then carried out by passing saturated NaCl ~: brine (pH 9~5) downward. After an ini-tial drop to 0~4 du~ to : ~ salt splltting, the pH rose a~ove 1.0 whereupon the brine rinse was stopped.

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The chromate adsorption step was then begun with an upward flow at 0.25 gallons per minu-te (g~p.m.~ of a liquor contalning 450 g.p.l. o sodium chlorate and 3.1 g.p l. of Na2Cr207. The effluent remained colorless un-til 64 gallons were treated. The e~fluent pH was between 2~05 and 2.2 when the first yellow efflu-ent flow appeared~ 5tripping to desorb chromate from the exchange resin was accomplished by passing 4~ NaOH in half saturated (12%) NaCl brine in a downward flow.

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Sa~ple Bed Yolume~a~ Chrom~te in E~fluent Effluent Feed Liquor ~color indication~ pH

1 1.47 colorless 2.0 2 1.90 colorl.ess 1.8 3 3.50 colorless 1.8 4 5.08 colorle~s 1.95 5.93 colorless 2.05 6: 6.44 slight yellow 2.2 7 6.87 de~inite yellow 2.3 Th foregoing examples de~Qns~-~ate the process of this in-20 vention reIative to the dhromate adsorption step. In Examples 2 and 3 after the chromate adsorption step, the chromate is readily stripped from the weak base anion resin by passing a 4% sodium hydroxide solution in hal~-saturated (12-15% NaCl) brine through the ~ed~ The bed is then regenerated by the passage of a hydro-chloric acid solution therethrough anc~, by means of a slightly alkallne or neutral (NaCl~ sQlution, :
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-- . : ~ .. , ' conditioned, to a pH between 1 and 2. Thereafter, the chromate adsorption step may be repeated.

A general description of another embodiment of this invention follows:

A mixed bed of prescribed ion exchan~e resin is prepared by air-blowing under water about 35 to about 65 weight of an anion exchange resin when in the chloride form and about 65 to a~ut 35 parts by weight of a weak cation exchange resin when in the hydrogen form in an exchange colurnn~

After draining the water from the column, the bed is treated with a mineral acid solution of alkali metal chloride and then the cation resin is conditioned by `
10 ~ passing an neutral solution of alkali metal chloride through the bed to adjust , the eifluent pH as previously explained An aqueous alkali metal chlora~e rich alkali metal chloride liquor containing less than about 20 grams per liter, prefer-~ ably less than 10 grams per liter of alkali metal chromate is passed upwardly - through the mixed excilange bed at a rate of about 0. 5 gallons per minute per ~15 square foot of bed while monitoring the pH of the effluent ~low. When the pH rises above about 2 and the effluent turns yellow, the chlorate liquor flow is terminated.
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The chlorate liquor is drained from the exchange resin bed and half saturated (12-isYo) sodium chloride brine perferably having a neutral pH (7-8) is passed up-wardly through the bed at a rate sufficient to separate the two e~change resins l~o upper (amon resin) and lower (cation res~n) layers. The brine flow is terminatedand 4% sodium hydroxide in 12-15% sodium chloride brine is passed downwardly through the anion exchange resin until the effluent flow turns alkaline in~
dicating removal of the adsorbed chromate from the resin. For regeneration, 4YO

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hydrochloric acid in a half saturated sodium chlorlde brine is then passed through the exchange resins until the pH of the effluent drops to 1. O. Conditioning of the cation resin is accomplished by passing half saturated sodium chloride brine up-wardly or downwardly through the resin until the effluent pH is 1. 5 - 2. The chromate adsorption step is then repeated.

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Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for removing chromate ions from aqueous feed solution comprising (a) passing said feed through an ion exchange resin bed comprising an intimate mixture of an anion exchange resin in the chloride form and a weak cation exchange resin in the conditioned hydrogen form, in which (i) the total amount of exchange sites on the anion resin is sufficient to remove the chromate ions from the feed and (ii) the cation exchange resin has a number of exchange sites not substantially less than the number of exchange sites present in the anion exchange resin (b) monitoring the pH
of the effluent aqueous solution and (c) halting the feed whenever the pH of the effluent rises above about 2.

2. The process of claim 1 wherein said aqueous feed solution is an alkali-metal chlorate solution.

3. The process of claim 2 wherein said aqueous feed solution also includes dissolved alkali-metal chloride.

4. The process of claim 3 wherein said alkali-metal chromate, chlorate and chloride are sodium chromate, chlorate and chloride.

5. The process of claim 1, wherein the absorbed chromate ions are recovered from the anion exchange resin after termination of flow of said feed solution through the mixed exchange resin bed by passing an aqueous alkaline solution through said bed.

6. The process of claim 5 wherein said alkaline solution is sodium hydroxide dissolved in an aqueous sodium chloride solution.

7. The process of claim 5 wherein both the exchange resins are treated after recovery of the chromate ions by passing an aqueous solution of a mineral acid through said resins.

8. The process of claim 7 wherein said aqueous solution of mineral acid is hydrochloric acid in an aqueous sodium chloride solution.

9. The process of claim 8, wherein after treatment with said solution of mineral acid, the cation exchange resin is treated with a substantially neutral aqueous sodium chloride solution to condition said cation resin.

10. The process of claim 9 wherein the treatment is carried out by passing the neutral sodium chloride solution through the mixed exchange resin bed.

11. The process of claim 5 wherein prior to passage of said aqueous alkaline solution through the bed, i) said cation exchange resin is separated from said anion exchange resin, ii) said aqueous alkaline solution is passed at least through said anion exchange resin, iii)an aqueous solution of mineral acid is passed at least through said cation exchange resin to replace alkali metal ions with hydrogen ions,and iv) the exchange resins are remixed.

12. The process of claim 11 wherein said alkaline solution is sodium hydroxide dissolved in an aqueous sodium chloride solution and said solution of mineral acid is hydrochloric acid dissolved in an aqueous sodium chloride solution.

13. The process of claim 12 wherein after termination of the passage of said aqueous solution of hydrochloric acid through said cation:
resin, said cation resin, is conditioned by the passage therethrough of a substantially neutral aqueous solution of sodium chloride.

14. The process of claim 13 wherein said alkaline solution is passed through both anion and cation exchange resins and said mineral acid solution is passed through both anion and cation exchange resins.

15. The process of claim 14 wherein said feed solution is passed through said bed at a rate not exceeding 0.75 gallons per minute per square foot of bed.

16. The process of claim 14, wherein said anion exchange resin is a weak base resin.

17. The process of claim 16 wherein said exchange resins are both macroreticular.