JPH0140761B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing chromate ions from aqueous solutions containing chromate ions and large amounts of dissolved alkali metal chlorates. More specifically, the invention removes chromate ions from an aqueous chloride solution by passing it through a mixed layer of anion and cation exchange resins, each having a specific shape. It concerns the method. TFO'Brien, U.S. Pat.
A method is disclosed in which a significant proportion of chromate ions are removed from the aqueous solution by passing it through a layer of strong base ion exchange resin in the chloride form at an initial pH of . Although this method is clearly an improvement over conventional methods in removing chromate ions from the solution, it is clear that further improvements are needed to remove a large proportion of chromate ions. be. That is, it is possible to economically remove virtually all chromate ions from a solution containing dissolved alkali metal chromates in as little as 10 ppm by weight (generally less than about 20 g/m) and to It would be commercially desirable to provide a process in which processing results in significant removal, preferably virtually complete removal of chromate ions from the solution, without the formation of hazardous chlorine dioxide. The present invention therefore provides a method for removing chromate ions from solutions containing large amounts of dissolved alkali metal chlorate and having a concentration of dissolved alkali metal chromate, such as obtained from electrolysis of sodium chloride solutions. The method comprises passing the solution through a layer consisting essentially of a homogeneous mixture of an anion exchange resin in the chloride form and a weak cation exchange resin in the hydrogen form to be conditioned; The number of exchange sites present in the resin is not less than that present in the anion exchange resin, and the anion exchange resin has a number of exchange sites at least approximately equal to the number of chromate ions that are to be removed from the solution. This is a method for removing chromate ions such that they are present in a sufficient amount to donate chromate ions. In a preferred method,
The cation exchange resin is present in an amount of 0.5 parts by weight or more and 2.0 parts by weight or less per part by weight of the anion exchange resin. In industrial practice, the number of exchange loci is usually meq/
It is specified by the "total exchange volume" expressed in ml or meq/g. The method for measuring this amount varies slightly depending on the resin supplier. The technical bulletin (1972) of the manufacturer of Amberlite IRC-50 states as follows: Total Exchange Capacity: The maximum capacity of Amberlite IRC-50 can be conveniently determined by equilibrating a representative sample of the hydrogen form with excess 0.1N sodium hydroxide. It should remain in contact with the excess caustic for 24 to 48 hours. It is observed that the neutralizing amount of sodium hydroxide is equal to the maximum capacity of the exchanger. Both cation and anion exchange resins useful in the present invention can be either gel type or macroreticular type. However, the macroreticular type is preferred in terms of its high stability over multiple cycles of ion exchange operations. For various ion exchange resins, see, for example, the Kirk-Othmer Encyclopedia.
of Chemical Technology, 2nd edition, Vol.
It is described from page 871 onwards. The anion exchange resin may be either a weak base resin or a strong base resin, but a weak base anion exchange resin is preferred since it allows more quantitative recovery during regeneration. When an anion exchange resin is used in the use stage of the ion exchange process, ie the chromate adsorption step, it should initially be in the chloride form. The cation exchange resin for the present invention is essentially a weak acid cation exchange resin, and it is conditioned at the beginning of the use phase, i.e. when it begins to remove chromate from a chlorate-rich solution. Must be in hydrogen form. When the term "conditioned hydrogen form" is used herein, it refers to a cation exchange resin that is initially completely in hydrogen form so that the chlorate-rich solution is not overly acidified during use. refers to a cation exchange resin form in which hydrogen ions are partially replaced with alkali metal (Na ⁺ ) ions. In other words, in order to reduce the "salt-splitting" effect caused by the chromate adsorption process (use stage),
The hydrogen form of the cationic resin is preferably at a neutral pH (6 to
8) Rinse (condition) with alkali metal chloride brine to bring the outlet (brine effluent) pH to 1.0.
It is preferable to adjust the pH to about 2 or less. Salt splitting typically occurs when a solution containing a salt of a strong base and a strong acid is passed through a strong acid ion exchange resin or a strong base ion exchange resin. That is, the cations of the salt easily exchange with the hydrogen of the strong acid resin, and similarly the anions of the salt easily exchange with the hydroxide of the strong base resin. A distinctive feature of weak acid-weak base exchangers as compared to such strong acid-strong base types is that this salt splitting generally does not occur in appreciable amounts. Therefore, the fact that it occurs and that it must be controlled by this method probably means that
This is probably because it contains a very high concentration of salt. The method of the invention, which is advantageously a cyclic process, is outlined as follows: The starting material is provided by treating selected exchange resins, either separately or as a mixture, with a brine solution of an inorganic acid. . This mixture or
By treating the cationic resin with a neutral brine solution, the conditioned hydrogen form of the cationic resin is formed. If the exchange resins have not been mixed prior to their treatment, they are mixed in a manner well known in the art, such as by blowing air in the presence of water. The use stage or chromate adsorption step is then initiated by passing a chlorate-enriched chromate-containing liquid through the mixed resin layer. Depletion of the ion exchange layer is indicated by an increase in the pH to 2 and a simultaneous yellowing of the effluent solution. At that time, the flow of the chromate-containing solution is stopped and the anion exchange resin is treated to recover the chromate ions. In that case, the exchanged resins may or may not be separated in advance. If desired, the resins can be separated by passing a brine solution upward through the resin layer at a flow rate sufficient to separate resins of slightly different specific gravity, as described below. Chromate is easily and completely removed (stripped) by treating the anionic resin or layer with an alkaline solution of an alkali metal chloride, such as about 4% sodium hydroxide in a 12-15% aqueous sodium chloride solution. . After removing the chromate from the anion exchange resin, the ion exchange process is repeated for a number of cycles according to the steps described above until the resin can no longer adsorb enough to produce a useful process. The following table shows theoretical exchange mechanisms for processes with and without resin separation in order to contribute to understanding the various steps of the process and their effects.

【table】

Table: In the process of the present invention, the flow rate of the chromate-containing solution through the resin exchange bed is about 1.0 gallons/min per square foot due to the spread of the adsorption front, preferably 0.75
Should not exceed gallons/min. The method of the present invention involves converting alkali metal chlorate into PH
Any aqueous solution containing a concentration sufficient to decompose on acidification to about 0.5 to 1.0 is suitable. The concentration of alkali metal chlorate or the chlorate and chloride should not be so high as to cause salting out under the conditions in the exchange bed or exchange column. for example,
The accepted upper limit for the concentration of sodium chloride-enriched sodium chloride solution at 80㎓ (26.7°C) is about 120 g/d sodium chloride and 550 g/d sodium chlorate. The following examples are provided to illustrate the method of the invention. Example: In a glass column with an inner diameter of 1.25 cm, a gel-type strongly basic anion exchange resin (Rohm & Haas product Amberlite IRA400) and a gel-type weakly acidic exchange resin (Rohm & Haas product Amberlite IRA400) are used.
IRC84) was added in equal parts by weight and homogeneously mixed. The mixed resin bed in this column had a height of about 80 cm and a capacity of 100 c.c. A conditioning step was performed to pass 25 meq of hydrochloric acid through the column to ensure that both exchange resins were partially in the chloride and hydrogen forms, respectively. This step was followed by a deionized water rinse. An aqueous chlorate feed containing sodium chromate (600 g sodium chlorate/5.9 g sodium dichromate) was passed upward through the column at a flow rate of 2 c.c./min. The reason for the upward direction was that the specific gravity of the chlorate solution was higher than that of the resin. The effluent part (sample) from the column is sequentially measured as PH and
It was analyzed for chromate content by colorimeter.
The progress of the upward movement front of exchange within the column could be observed from the color change of the resin. The subsequent observation and analysis results of the effluent portion are shown in the table below. Complete recovery of chromate was achieved by passing alkaline sodium chloride through the bed at the end of the chromate removal step.

【table】

[Table] Example: A homogeneous mixture (weight ratio 60:40) of a macroreticular type weak cation exchange resin (Rohm &Haas's Amberlite IRC50) and a macroreticular type weak anion exchange resin (weight ratio 60:40) was placed in a 1.9cm diameter column with a height of 132cm. It was filled up to the maximum. The mixed resin layer was preconditioned by first passing a 4% aqueous sodium hydroxide solution downward through the column followed by a 4% aqueous hydrochloric acid solution until the PH was below 1.5. Finally, the hydrogen form of the cation exchange resin was conditioned by washing downwards with a nearly saturated aqueous neutral sodium chloride solution until the effluent pH rose to 1.5. For adsorption of chromate, use sodium dichromate.
1.02g/containing chlorate (sodium chlorate
450g/) solution (supply liquid) to the resin layer at 5c.c./min
This was accomplished by passing it through at a flow rate of . 1850 c.c. of colorless product was collected before the effluent turned yellow to the naked eye. During this time, the effluent pH steadily increased from 1.6 to 2.05. Sodium dichromate (Na ₂ Cr ₂ O ₇ ) was added to the effluent after 150 c.c.
It contained 24.6 ppm and the PH of the effluent was 2.15. As in Example 1, take successive samples of the effluent.
I checked the pH. We also noted the color of each sample as an indicator of chromate content. Colorless samples showed virtually no chromate. The following table lists the PH and observation data for each sample:

【table】

[Table] High before equilibrium.
Example: 1 part by weight of a weak cation resin (Amberlite IRC84) and 2 parts by weight of a weak anionic resin (Amberlite IRA94) in the presence of water in a 10 inch diameter column specifically plumbed for circulating continuous ion exchange process. A mixed resin layer was prepared by under-air mixing, and the mixture reached a total height of 35 inches in the column. Add NaCl to the resin layer until the pH of the effluent is 1.5.
The layer was treated by passing down a 4% HCl solution in brine. Then by passing saturated NaCl brine (PH9.5) downwards.
A conditioning process was performed. The pH initially dropped to 0.4 due to salt splitting and then rose above 1.0. Therefore, rinsing with brine was stopped. Next, sodium chlorate 450g/and
0.25 gallons of liquid containing 3.1 g of Na ₂ Cr ₂ O ₇
The chromate adsorption step was started by flowing upward at min. The effluent remained colorless until 64 gallons had been treated. When the first yellow effluent stream appeared, the PH of the effluent was between 2.05 and 2.2. Chromate desorption from the exchange resin by stripping was accomplished by passing 4% NaOH in half-saturated (12%) NaCl brine in a downward flow.

【table】

TABLE The above example illustrates the method of the present invention for a chromate adsorption step. In the Examples and Examples, chromate is removed from a weak base anionic resin by passing a 4% sodium hydroxide solution in half-saturated (12-15% NaCl) brine through the exchange resin layer after the chromate adsorption step. Salt is easily stripped. The layer is then regenerated by passing a hydrochloric acid solution through the layer and 1-2 by using a slightly alkaline or neutral NaCl solution.
Condition adjusted to PH. The chromate adsorption step can then be repeated. Another embodiment of the present invention is outlined as follows: about 35 to 65 parts by weight of an anion exchange resin in chloride form and about 35 to 65 parts by weight of a weak cation exchange resin in hydrogen form placed in an exchange column. A mixed layer of the ion exchange resin described above is prepared by blowing air into 65 to 35 parts by weight of the ion exchange resin in the presence of water. After draining the column, the cationic resin is removed by treating the bed with an inorganic acid solution of alkali metal chloride and then passing through this bed a neutral solution of alkali metal chloride and adjusting the effluent PH as described above. Adjust the condition. An alkali metal chlorate-enriched aqueous alkali metal chloride solution containing less than about 20 g/min of alkali metal chromate is added to the mixed exchange bed at a rate of about 0.5 gallons/min per square foot of said bed. While passing upward at the flow rate, monitor the PH of the effluent stream. When the PH is above about 2 and the effluent turns yellow, stop the chlorate fluid flow. The chlorate solution is drained from the exchange resin bed and a semi-saturated (12-15%) sodium chloride brine, preferably having a neutral PH (7-8), is passed upwardly through the bed. The flow rate at this time is sufficient to separate the two types of exchange resins into an upper layer (anion resin) and a lower layer (cation resin). After stopping the brine flow, the anion exchange resin was charged with 4% sodium hydroxide in 12-15% sodium chloride in a downward direction until the effluent became alkaline, indicating adsorbed chromate removal from the resin. Pass. For regeneration, the exchange resin is then passed with hydrochloric acid in half-saturated sodium chloride brine until the effluent pH drops to 1.0. Conditioning of the cationic resin is accomplished by passing half-saturated sodium chloride brine through the resin upwardly or downwardly until the effluent PH is between 1.5 and 2. The chromate adsorption step is then repeated.

[Claims]

1 A carbon molded body mainly composed of glassy carbon and graphite, with residual hydrogen of 0.02% by weight.
An impermeable carbon molded article having the following properties and a true specific gravity of 1.9 or less.

Claims (1)

[Scope of Claims] 1. In removing chromate ions from a solution containing a large amount of dissolved alkali metal chlorate and having a concentration of dissolved alkali metal chromate, the solution is treated in the chloride form. A method comprising passing through a layer consisting essentially of a homogeneous mixture of an anion exchange resin and a weak cation exchange resin in hydrogen form to be conditioned, wherein the number of exchange sites present in the cation exchange resin is less than that present in the anion exchange resin and in an amount sufficient for the anion exchange resin to provide a number of exchange sites greater than the number of chromate ions that are to be removed from the solution. A method for removing chromate ions. 2 The solution is 0.75 gallons per square foot of bed.
2. The method of claim 1, wherein the bed is passed through the bed at a flow rate not exceeding min. 3. The method according to claim 1, wherein the anion exchange resin is a weak base resin. 4. The method according to claim 3, wherein both of the two exchange resins are macroreticular. 5. The method of claim 4, wherein the cation exchange resin is present in an amount of not less than 0.5 parts by weight and not more than 2.0 parts by weight per part by weight of anion exchange resin. 6. The method of claim 1, wherein the PH of the solution effluent stream passing through the mixed ion exchange resin bed is in the range of no less than about 1 and no more than about 2. 7. The method of claim 5, wherein the solution contains a high proportion of dissolved sodium chlorate and sodium chloride. 8. The method of claim 7, wherein the adsorbed chromate ions are recovered from the anion exchange resin by passing a dilute aqueous alkaline solution through the mixed ion exchange resin bed after passing the solution stream through the bed. . 9. The method according to claim 8, wherein the alkaline solution is sodium hydroxide in an aqueous sodium chloride solution. 10. The method of claim 8, wherein both ion exchange resins are treated by passing a dilute aqueous solution of an inorganic acid through the mixed ion exchange resin after recovery of the chromate ions. 11. The method according to claim 10, wherein the aqueous solution of inorganic acid is hydrochloric acid in an aqueous sodium chloride solution. 12. The method of claim 10, wherein hydrogen ions are replaced to some extent with sodium ions by treating at least the cation exchange resin with a substantially neutral aqueous sodium chloride solution. 13. The method of claim 12, wherein the treatment is carried out by passing a neutral sodium chloride solution through a bed of mixed ion exchange resins. 14 Separate the cation exchange resin from the anion exchange resin before passing the dilute aqueous alkaline solution through the bed, so that the alkaline solution is passed only through the anion exchange resin, and the cation exchange resin is filled with neutral sodium chloride. 11. The method according to claim 10, wherein after passing through the aqueous solution, both ion exchange resins are remixed after recovery of the chromate ions.