HU182557B - Process for removing chromate-ions from mixtures, solutions containing alkalichromates and alkali-chlorates - Google Patents

Abstract

Chromate ions are removed from alkali metal chromate solutions containing alkali metal chlorate by passage of the solution through a mixed bed of ion exchange resins containing a mixture of (a) an anion exchange resin in chloride form and (b) a weak cation exchange resin in conditioned hydrogen form. Substantially complete chromate removal is obtained without attendant formation of chlorine dioxide.

Description

The present invention relates to a process for the removal of a chromate ion from an aqueous solution containing an alkali metal chromate and an alkali metal chlorate and chloride by dissolving the bound chromate ions from the ion exchange resin and regenerating the ion exchange resin. The process of the invention is carried out by passing the solution through a bed of a mixture of anion exchange resin in chloride form and a weakly acidic cation exchange resin in partially hydrogen form, wherein the number of ion exchange sites present in the cation exchange resin is greater than or equal to and the amount of anion exchange resin is sufficient to provide an ion exchange site at least equivalent to the number of chromate ions to be removed from the solution, and then, if desired, recovering the chromate ion from the anion exchange resin and optionally regenerating the ion exchange resin.

No. 3,835,001. U.S. Pat. No. 4,102,125 discloses a process for removing chromate ion from an aqueous solution of alkali metal chloride and alkali metal chloride. According to the known method, the solution is passed through a bed containing a strongly basic aninone exchange resin at the initial pH of the solution to be treated, which is less than 6.5, preferably about 5. The anion exchange resin is present in the chloride form in the bed.

No. 3,980,751. U.S. Pat. No. 4,102,125 discloses a process wherein the chromate ions are also removed from the solution containing the alkali metal chloride and chloride by ion exchange. The operation is carried out in a solution of hydrochloric acid in a strongly acidic medium. (The pH is less than 0.5.) The disadvantage of this solution is that free Cl ₂ gas is easily formed in the strongly acidic medium, which leads to the formation of C1O ₂ , and in the strongly acidic medium, the anion exchange resin has a longer life and consequently a lower capacity.

Although the above known methods are significantly more effective in removing chromate ions than previous known methods, the complete removal of the chromate ion has not yet been achieved.

Thus, an industrial process was needed to economically remove the total amount of chromate ion. The solution to be treated may contain 10 ppm of alkali metal chromate, but may also contain 45 substantially higher amounts. The concentration of the alkali metal chromate in the solution is generally below 20 g / l. An economical industrial process requires one treatment of the solution to remove most, but not all, of the chromate ion without simultaneously producing chlorine dioxide, which is a major hazard.

According to the invention, the chromate ions are removed from aqueous solutions containing a large amount of dissolved alkali metal chromate by passing the solution on a bed containing a mixture of anion exchange resin in the form of chloride 55 and a weakly acidic cation exchange resin in partially hydrogen form, wherein the cation exchange resin is present. is greater than or equal to the number of ion exchange sites in the anion exchange resin, and the amount of anion exchange resin is sufficient to provide an ion exchange site at least equivalent to the number of chromate ions to be removed from the solution.

In a preferred embodiment of the process of the invention, the cation exchange resin is used in an amount of 0.5 to 2.0 parts by weight, based on 1 part by weight of the anion exchange resin.

The advantage of using a mixed ion exchange resin is that this solution increases the life of the anion exchange resin and consequently the capacity of the resin, and also reduces the risk of chlorine dioxide formation when using mixed anion exchange resins. The hydrogen-form cation exchange resin converts chromate ions to dichromate and dichromate ions bind to the anion exchange resin. The cation exchange resin itself does not bind chromate ions. The cation exchange resin is preferably used in an amount greater than equimolar to the anion exchange resin because the hydrogen form is generally present in less than 100%.

In industrial practice, the number of ion exchange sites is generally given at the total ion exchange capacity. The latter is expressed in milliequivalents per milliliter or milliequivalents per gram unit; however, the method of determination differs slightly from one resin manufacturing company to another. The Amberlite IRC-50 Resin Technical Bulletin, issued in 1972, states:

Total Ion Exchange Capacity: The maximum capacity of the Amberlite IRC-50 resin is conveniently determined by contacting a representative sample of the hydrogen resin with an excess of 0.1 N sodium hydroxide solution and allowing equilibrium to develop. We allow 24-48 hours for this. The amount of neutralized sodium hydroxide is considered equivalent to the maximum capacity of the ion-exchange resin. "

The cationic and anion exchange resins used in the process of the present invention may be either gel-type or macroretricular, but macroretricular resin-type is preferred because of its greater stability in successive ion-exchange cycles. For example, various types of ion exchange resins are discussed in the following literature:

Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Edition, Volume 11, 871 et seq.

The anion exchange resin used in the process of the invention may be weakly or strongly basic. However, in view of the quantitative recovery during regeneration, the weakly basic anion exchange resin is preferred. In the ion exchange process, the anion exchange resin must initially be present in the chloride form in the chromate ion removal step.

The cation exchange resin used in the process of the present invention is a gj'enge acid, which must be partially hydrogenated upon initiation of chromate ion removal.

The term "partially hydrogen form" means 3

-2192557 that some of the hydrogen ions of the cation exchange resin are replaced by alkali metal ions (sodium ions) so that the chlorate-containing solution becomes too acidic during the removal of the chromate ions. In other words, in order to reduce the effect of "salt decomposition" upon removal of the chromate ion, the cation exchange resin in hydrogen form is suitably treated with an aqueous solution of neutral pH (6-8) such that the pH of the solution leaving the resin is its value should be above 1.0, preferably up to 2.0.

Salt decomposition generally occurs when a solution containing a salt of a strong acid and a strong base is passed through a strongly acidic ion exchange resin or a strongly basic ion exchange resin. The salt cation is readily exchanged for the hydrogen of the strongly acidic resin. Similarly, the anion of the salt is readily exchanged for the hydroxyl ion of the strongly basic resin. In the case of weakly acidic or weakly basic ion-exchange resins, the salt decomposition is generally not significant. However, the fact that the process according to the invention may still undergo salt decomposition and require countermeasures is probably due to the very high salt concentration of the treated aqueous solutions.

The process of the present invention is carried out, preferably in several successive cycles, as follows:

The ion exchange resins, alone or in admixture, are treated with a solution of a mineral acid containing sodium chloride to provide the starting material. The cation exchange resin (if present alone) or the mixture is then treated with a neutral sodium chloride solution to partially convert the cation exchange resin into hydrogen. If the ion-exchange resins had not been blended prior to the above treatments, they would be blended in a manner known per se, such as underwater air blowing. Thereafter, the removal of the cr5 mat ion is started by passing the solution containing the chromate and chlorate ions through the bed of the resin mixture. When the ion exchange bed is depleted, which is indicated by an increase in pH to 2 and a yellow color of the solution leaving the resin, the passage of the chromate containing solution is stopped and the anion exchange resin is treated to recover the chromate ions. If desired, the ion exchange resins are separated before the chromate ions are recovered.

As will be discussed below, the resins may be separated by passing an aqueous solution of sodium chloride from the bottom to the bed at a rate that separates the anion and cation exchange resins of different specific gravities.

The chromate ion can be readily and completely removed from the anion exchange resin by treating the anion exchange resin or bed with an alkaline alkali metal chloride solution. For example, the alkaline alkali metal chloride solution contains 4% sodium hydroxide and 12-15% sodium chloride. After removing the chromate ion from the anion exchange resin, the ion exchange procedure is repeated as above. The resins can be used in a number of cycles as long as their adsorption capacity is not unduly reduced.

The various steps of the process according to the invention and their effects are summarized in Table I, which shows the theoretical ion-exchange steps, both in case of separation of the resin mixture and in case the components of the resin mixture are not separated.

Table I

Each step of the process

Operation Mechanism of ion exchange Separated resins mixed resins anion cation solution anion cation solution chromate ion desorption OH - Cr ₂ O, ² - X 4% NaOH in saline, flow up or down OH - Cr ₂ O ₇ ² - Na + -H ⁺ (unwanted) 4% NaOH in saline, flow down Reclamation Cl - OH H + -Na + 4% HCl in saline, flow up or down ca - oh- H ⁺ -Na ⁺ 4% HCl in saline, flow down Treatment with alkali metal ions Rinse if desired Na + -H + partially Neutral saline, flow up or down rinse Na ⁺ -H + partially Neutral saline, flow down Chromate ion adsorption Cr ₂ O, ^z ~ - Cl- Na + -H ⁺ Solution to be treated, flow upwards Cr ₂ O, ² - Cl- Na ⁺ -H ⁺ Solution to be treated, flow upwards

Notes: The arrow means "displaces". For example, the designation OH ^- - Cr. ₂ O, ² - means that the hydroxyl ion displaces the chromate ion from the resin and replaces it.

XA treated liquid rinse the resin prior to regeneration, to avoid the formation of _two CLU.

-3182557

The chromate ion-containing solution is passed through the process of the invention at a rate of up to 0.5 liters per minute, preferably 0.36 liters per minute, over the 1 m ² surface of the ion exchange resin bed. Because of the widening of the adsorption front, this flow rate is used.

The process of the invention is suitable for the treatment of any aqueous solution which contains sufficient amounts of alkali metal chlorate to decompose when the acidity of the solution is reduced to about 0.5 to 1.0. However, the concentration of the alkali metal chlorate or alkali metal chlorate and chloride must not be such that it can cause salinisation under conditions present in the ion-exchange column or bed, such as an aqueous solution containing sodium chloride and sodium chlorate at an acceptable upper limit of 26.7 ° C. 120 g / l sodium chloride and 550 g / l sodium chloride.

The following examples illustrate the process of the invention in more detail.

Example 1

The same weight amounts of a gel-type, highly basic anion exchange resin (Amberlite IRA 400 from Rohm and Haas Co.) and a gel-type weakly acid cation exchange resin (Amberlite IRC 84 from Rohm and Haas Co.) are well mixed and have an internal diameter of 1.25 cm place in a glass column. The resin bed in the column has a height of about 80 cm and a volume of about 100 cm ³ . 25 milliequivalents of hydrochloric acid are then passed through the column to ensure that the ion-exchange resins are partially chloride or hydrogen. The resin bed in the column was rinsed with deionized water.

An aqueous solution containing 600 g / l sodium chlorate and 5.9 g / l sodium bichromate was passed up the column at a rate of 2 cm ³ / min. The upward passage is justified by the specific gravity of the sodium chlorate solution over that of the resin.

. Samples were taken successively from the column and the pH and chromate content were determined using a colorimeter. The progress of the upward ion exchange front could be monitored by changes in the color of the resin. Our observations and the results of sample analysis are presented in Table II. are summarized in Table. Complete recovery of the chromate ion can be achieved by passing an alkaline sodium chloride solution on the bed at the end of chromate removal.

II. spreadsheet

i5 9 • 3 IFL the c S 3 secs * § · § «Β § • 2 ^ 2 43 2 Ξ '3 K o ε 1 «aig's PH of the storage solution The ion-exchange front is high- , cm, cm 1 0.92 0.00 1.96 20 2 1.72 0.00 1.20 36 3 2.57 0.00 1.35 51

e8 the the 8 Ski g 3 Introduced solution bed volume * 1 chromate ion quantity of the dist 1 tanning solution, 1 2/1 ° © • c jd 53 ¾ sfs w o • 2-S * s 0 w> 4 3.37 0.00 1.58 63 5 4.16 0.00 1.74 68 6 4.98 0.00 1.96 75 7 5.33 0.02 2.22 78 8 5.78 0.20 2.62 (top of the bed) 9 6.60 1.05 2.88 - 10 7.45 3.60 2.90 - 11 8.24 4.80 2.80 - Introduced solution 5.90 5.50

* The volume of the injected or discharged solution is used to compare the capacity of the ion exchange columns of different sizes. One bed volume is the total volume occupied by the resin bed, including the volume occupied by the ion exchange resin particles themselves.

Example 2

A 1.9 cm diameter column was filled to a height of 132 cm with a macroreticular type weak acid cation exchange resin (Amberlite IRC 50 from Rohm and Haas Co.) and a macroreticular type weak basic anion exchange resin (Amberlite IRA 93 manufactured by Rohm and Haas Co.). ) With a 60: 40 weight ratio.

A 4% aqueous sodium hydroxide solution is first passed down the bed containing the resin mixture, followed by a 4% aqueous hydrochloric acid solution until the pH drops below 1.5. Finally, a nearly saturated, neutral aqueous sodium chloride solution was passed down the bed to partially convert the cationic resin into hydrogen. The latter treatment is carried out until the acidity of the effluent solution

It does not increase to a pH of 1.5.

Thereafter, a solution to be treated containing 1.02 g / l sodium bichromate and 450 g / l sodium chlorate was passed through the bed from below upwards at a rate of 5 cm ³ / min. We receive 1850 cm ^{3 of a} colorless treated solution. Subsequently, the liquid leaving the column will become noticeably yellow. During the treatment, the pH of the solution exiting the column increases from 1.6 to 2.05. An additional 150 cm ^{3 of a} chromate solution was passed through, and the column effluent contained 24.6 ppm sodium bichromate and a pH of 2.15.

In a similar manner to Example 1, successive samples were withdrawn from the column. The acidity of the samples was determined and the color of each sample recorded on the chromate content.

-4182557 as a check. In our estimation, the colorless sample indicates that chromate ion is practically absent. The results obtained are shown in Table III. Table.

III. spreadsheet

"8 § 3 x neither B 3 -S-Sf "FSS «5 © -o O £ os * g t a The leaving solution pH value ea on | l 3 | S • <1 «5 © 1 1.42 colorless (2.3) * 20 2 1.85 colorless 1.6 41 3 3.57 colorless 1.67 61 4 4.28 colorless 1.77 91 5 5.00 colorless 1.90 - 6 5.28 pale yellow 2.05 - 7 5.70 yellow (24.6 ppm) 2.15 - *THE initial pH is higher because not yet

an equilibrium was formed between sodium chlorate and resins.

Example 3

In a 25.4 cm diameter column with a reflux conduit for continuous ion exchange process, a bed of resin mixture is formed by air mixing 1 part by weight of weakly acidic cation exchange resin (Amberlite IRC 84) and 2 parts by weight of weakly basic anion (Amberlite IRC 84). IRA 94). The total height of the mixture in the column is 88.9 cm.

The resin mixture bed was treated with aqueous 4% hydrochloric acid, from top to bottom, until the pH of the effluent from the column was 1.5. A saturated aqueous solution of sodium chloride, pH 9.5, was passed through the column from top to bottom. The pH first decreases to 0.4, due to salt decomposition, and then increases to above 1.0. The brine rinse is then stopped.

The solution containing the chromate ion was passed up the bed at a rate of 1.1 L / min. The solution contains 450 g / l sodium chlorate and 3.1 g / l sodium bichromate. The liquid leaving the column remains colorless until 282 liters of solution are treated and the pH is between 2.05 and 2.2 when yellow liquid begins to leave the column. The chromate ion bound on the ion exchange resin is then removed by passing an aqueous solution containing 4% sodium hydroxide and 12% sodium chloride from the top down.

The results obtained for each sample are shown in Table IV. are summarized in Table.

ARC. Table - Λ ~ £ co ff <λ τ 'λ <σ'

1 1.47 colorless 2.0 2 1.90 colorless 1.8 3 3.50 colorless 1.8 4 5.08 colorless 1.95

<e g 'g s B 5 The introduced ol- bed volume of dat li Eagle/? · «9 fl a o ff a <5 73 1 1 The leaving solution pH value 5 5.93 colorless 2.05 6 6.44 pale yellow 2.2 7 6.87 yellow 2.3

In the examples above, the process of the present invention was illustrated for chromate ion adsorption. In Examples 2 and 3, the chromate ion is simply recovered from the weakly basic anion exchange resin after adsorption by using a partially saturated saline solution containing 4% sodium hydroxide ( 12-15% sodium chloride) is passed through the bed. The bed is then regenerated by treatment with a solution of hydrochloric acid and then with a slightly alkaline or neutral sodium chloride solution to a pH of 1-2. The adsorption of the chromate ion can then be repeated.

Example 4

Ion-exchange resins are made into a composite bed by mixing 35-65 parts by weight of chloride anion exchange resin and 65-35 parts by weight of hydrogen anion exchange resin under water by blowing air in the ion exchange column.

Allow water to drain off the column and treat the bed with a mineral acid solution of an alkali metal chloride. A neutral alkali metal chloride solution is then passed through the bed to adjust the pH of the effluent as described above.

An alkali metal chloride-rich aqueous alkali metal chloride solution containing less than 20 g / l, preferably less than 10 g / l, is passed up the bed at a rate of about 0.25 l / min / m ² while checking acidity of the liquid leaving the column. When the pH is greater than about 2 and the liquid leaving the column turns yellow, the addition of the chlorate solution is stopped.

The chlorate solution is drained from the bed containing the ion exchange resin and preferably a semi-saturated aqueous solution of sodium chloride (pH 7-8), semi-saturated (12-15%), is passed up the bed at a rate such that the ion exchange resin mixture is separated into an upper (anion exchange resin) and a lower (cation exchange resin) phase, The saline passage is stopped and 12% to 15% aqueous sodium chloride solution is passed down the anionic gel, the effluent does not become alkaline. This indicates that the bound crornate has been removed from the resin.

For regeneration, a semi-saturated sodium chloride solution containing 4% hydrochloric acid is passed through the ion exchange resins until the pH of the effluent is reduced to 1.0. Then the resins are treated with a semi-saturated sodium chloride solution, cut the solution upwards? leading down through the bed until leaving

-518255 will not have a pH of 1.5-2. The adsorption of the chromate ion is then repeated.

Patent claims:

Claims (9)

Patent claims: A process for removing chromate ion from an aqueous solution containing an alkali metal chromate and an alkali metal chlorate and chloride using an ion exchange resin, liberating the bound chromate ions from the ion exchange resin and regenerating the ion exchange resin in the form of an anion and chloride. passing through a bed of a mixture of a weakly acidic cation exchange resin in partially hydrogen form, wherein the number of ion exchange sites present in the cation exchange resin is greater than or equal to the number of ion exchange sites present in the anion exchange resin; and then recover the chromate ion from the anion exchange resin, if desired, and optionally regenerate the ion exchange resin. Second embodiment of the method according to claim 1, characterized in that the kormáttartalmú solution was passed through the bed at a rate of 0.36 up to ² liters / perc'm. 3. The process of claim 1, wherein the anion exchange resin is a weakly basic anion exchange resin. 4. A process according to claim 1 wherein the macroretricular anion exchanger is a cation exchange resin. 5. The process of claim 1, wherein the cation exchange resin is used in an amount of 0.5 to 2.0 parts by weight per 1 part by weight of the anion10 exchange resin. 6. The process of claim 1, wherein the anion exchange resin bound chromate ion is recovered by passing a dilute aqueous alkaline solution through the resin bed. 15 7. The process according to claim 6, wherein the alkaline solution is an aqueous sodium chloride solution containing sodium hydroxide. The method of claim 1, which is an embodiment 20, wherein the ion-exchange resins are regenerated with a dilute aqueous solution of mineral acid. 9. The process of claim 8 wherein the aqueous solution of the mineral acid is an aqueous sodium chloride solution containing hydrochloric acid. Responsible for publishing: Director of Economic and Legal Publishing House 85.4064 40 - Zrínyi Nyomda, Budapest