NO151999B - PROCEDURE FOR REMOVAL OF CHROMATISTS FROM Aqueous SOLUTIONS WITH HIGH CONTENT OF DISSOLVED ALKALIMETAL CHLORATE AND CHLORIDE - Google Patents

Description

The present invention relates to a method thereof

species specified in claim l's preamble. More particularly, this method relates to the removal of chromate ions from aqueous chlorate solutions containing relatively large amounts of chloride, by passing the solution through a bed of mixed anion and cation exchanger resins, each of which is given a specific shape.

US Patent No. 3,835,001 specifies a method for removing a significant proportion of chromate ions from aqueous alkali metal chlorate chloride solutions by passing the solution through a layer of a strong base ion exchange resin in chloride form, and where the added solution The pH is less than 5.6, preferably approx. 5. Although this method represents a significant improvement in the removal of chromate ions from such solutions compared to known methods, there is a need for a further improvement with respect to removing a larger proportion of the chromate ions. It is thus commercially desirable to provide a method by which one can economically remove essentially all chromatin ions from solutions containing as little as 10 ppm by weight, but generally less than approx. 20 g/l dissolved alkali metal chromate so that a single treatment of the solution will provide essentially complete removal of the chromate ions from the solution without a simultaneous formation of dangerous chlorine dioxide.

According to the present invention, the method for removing chromate ions from a solution containing a relatively large amount of dissolved alkali metal chlorate and containing dissolved alkali metal chromate comprises passing the solution through a bed consisting of an intimate mixture of anion-exchange resin in chloride form and a weak cation-exchanger-resin in conditioned hydrogen form and where cation-exchanger-has-

the pix has a number of yield "points" no less than the number of yield points present in the anion yield resin. The method is further characterized by what is stated in claim 1's characterizing part. According to

the preferred method is the cation exchange resin present

in an amount which. is not less than 0.5 parts by weight, and not greater than 2.0 parts by weight per weight part anion exchange resin.

In industrial practice, the number of yield points is usually indicated as 'total yield capacity' expressed as m-equivalents/ml or ni-efc equivalents/g. The method used to determine this quantity varies somewhat for the different resin suppliers.

In the manufacturer's technical bulletin (1972) for "Amberlite IRC-50"

is indicated:

"Total yield capacity: The maximum capacity of "Amberlite IRC-50" can conveniently be determined by neutralizing a representative sample in hydrogen form with an excess of 0.1 N sodium hydroxide. The resin should remain in contact with the excess of alkali for 24-48 hours. The amount of neutralized sodium hydroxide is considered to be equivalent to the maximum capacity of the extractor."

Both the cation and anion exchange resins useful in the practice of the present invention can be either gel-

type or of the macroreticular type, but the macroreticular type is preferred because of its greater stability to repeated cycles of the ion-exchange operation. Ion-evolving resins of various types are described, for example, in the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Volume II, pages 871, etc.

The anion exchange resin can be a weak or strong basic resin, but from the consideration of a more quantitative recovery during the regeneration, a weak basic anion exchange resin is preferable. When the anion exchanger resin is used in the chromate absorption step of the ion exchanger process, the resin should initially be in chloride form.

The cation exchange resin used in the present invention is

essentially a weakly acidic cation exchanger resin which must

be in a conditioned hydrogen form before the work step of the process, i.e. before chromate is removed from the chloratric solution.

The term conditioned hydrogen form used in the present description means a form of the cation exchange resin in which part of the hydrogen ions therein, which were originally completely in the hydrogen form, have been replaced by alkali metal (Na<+>) ions to prevent a too strong acidification of the chloratric solution during the active stage of the process. In other words, to reduce the effect of "salt splitting" during the chromate absorption step, the hydrogen form of the cation resin is purified (conditioned) with an alkali metal chloride solution, preferably at pH

in the range 6-8 until the outlet has a pH of over 1.0, preferably approx. 2.

Salt cleavage typically 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's cation is readily exchanged with hydrogen from the strongly acidic resin. Similarly, a salt anion will be easily exchanged with hydroxide from the strongly basic resin. A distinguishing characteristic between a weakly acidic or weakly basic yield compared to a strongly acidic or a strongly basic type is that salt cleavage generally does not take place to a significant extent. That it takes place and must be controlled in the present method is probably due to the high salt concentrations present.

Method according to the invention, which is preferably carried out as a cyclic method, can be briefly described as follows: The selected ion exchange resins are treated either separately or as a mixture with a salt solution containing a mineral acid to provide the starting material. Then the cation- the resin (if separate) or the mixture treated with a neutral sodium chloride solution to give a cation resin in conditioned hydrogen form. If the ion exchange resins are not mixed prior to these treatments, they are mixed in a manner well known in the art, e.g. by air blowing under water. Then the chromate absorption_step is started by

pass the chromatin-containing, chloratric fluid through the mixed resin layer. Upon depletion of the ion-resin layer, indicated by a rise in pH to 2 with a concomitant yellowing of the effluent solution, the flow-through of the chromatin-containing solution is terminated and the ion-exchange resin is treated to recover the chromate ions, with or without a prior ion-exchange separation - the resins.

As explained below, if desired, the resin can be separated by passing a sodium chloride solution up through the bed at a rate sufficient to cause separation of the resins having somewhat different specific gravities.

The chromate is completely removed (stripped) by treating the anion resin or layer with an alkaline solution of an alkali metal chloride, for example with a 4% sodium hydroxide solution in an aqueous 12-15% sodium chloride solution. After removing the chromate from the anion exchange resin, the process is repeated according to the step indicated above until the resins are no longer able to provide sufficient absorption to serve the desired purpose.

In order to facilitate the understanding of the different steps according to the method, as well as their effect, the following table shows the assumed yield mechanism for the method, with and without resin separation. The flow rate of the chromate solution through the resin exchanger layer when carrying out the present method should not be more than approx. 0.4 l/min/dm 2 , preferably 0.3 l/min/dm 2 , because spreading of the absorption front.

The present method is suitable for any aqueous solution containing an alkali metal chlorate in a concentration sufficient for the chlorate to be decomposed by acidification to a pH in the range 0.5-1.0. The concentration of alkali metal chlorate or chlorate and chloride should not be so high that salting out occurs under the conditions prevailing in the ion exchange column or layer. E.g. is an acceptable upper limit for a sodium chlorate-rich sodium chloride solution at 27°C approx. 120 g/l sodium chloride and 550 g/l sodium chlorate. The following examples illustrate the invention.

Example 1

Equal parts by weight of a strong gel-type anion exchange resin ("Amberlite IRA 400", (Rohm and Haas Company)) and a weak acid gel-type exchange resin ("Amberlite IRC 84") were introduced into intimate mixture in a glass column with an inner diameter of 1.25 cm. The mixed resin layer in the column had a height of approx. 80 cm and a volume of 100 cm 3. A conditioning step involving passing 25 m equivalents of hydrochloric acid through the column was performed to ensure that the ion resins were partially in the chloride and hydrogen forms respectively. This step was followed by washing with deionized water.

An aqueous chlorate-containing feed containing 600 g/l chlorate and 5.9 g/l sodium dichromate was passed through the column in an upward flow at a rate of 2 cm/min. The upward flow is desirable because that the specific gravity of the chlorate solution was greater than that of the resin.

Successive samples of the effluent from the column were analyzed for pH and the chromate content was determined using a colorimeter. The movement of the upward front for the yield in the column could be observed by a color change in the resin. The observations and the result of the analysis of consecutive samples taken are given in the following table. Complete recovery of the chromate was achieved by passing an alkaline sodium chloride solution through the bed after completion of the chromate removal step.

Example II

A 1.9 cm diameter column was packed to a height of 132 cm with an intimate mixture of a weak cation exchange resin of the macroreticular type ("Amberlite IRC 50") and a weak anion exchange resin of the macroreticular type ("Amberlite IRA 93") in the weight ratio 60:40. The mixed resin layer was preconditioned by passing a 4% sodium hydroxide solution down the column followed by a 4% aqueous hydrochloric acid solution until the pH dropped to below 1.5. A final rinse with nearly saturated neutral aqueous sodium chloride solution was performed to condition the hydrogen form of the cation exchange resin until the effluent pH rose to 1.5.

The chromate absorption was carried out by passing a chlorate solution (450 g/l sodium chlorate) containing 1.02 g/l sodium dichromate up through the resin layer at a rate of 5 ml/min. 1850 ml of the colorless product was collected before the effluent became visibly yellow. During the chromate extraction period, the pH of the effluent rose steadily from 1.6 to 2.05. After further flow through of 150 ml, the effluent contained 24.6 ppm sodium dichromate (^2^20.^) and the pH of the effluent was 2.15.

As in example I, successive samples of the effluent were taken out and analyzed with regard to pH. Each sample's color was noted as a control for chromate content, a colorless sample indicating the substantial absence of chromate. The following table shows the pH

and observational data for each sample.

Initial pH is higher before equilibrium of sodium chloride with the resins.

Example III

In a 25 cm diameter column specially designed for recycling in a continuous ion exchange process, a mixed resin layer was introduced by air mixing under water of one part by weight weak cation resin ("Amberlite IRC 84") and 2 parts by weight of a weak anion resin ("Amberlite IRA 94"), the total height of the resin mixture in the column was approx. 90 cm.

The layer was treated by passing a solution of sodium chloride containing 4% HCl down through it until the pH of the effluent was 1.5. A conditioning step was then performed by passing a saturated sodium chloride solution (pH 9.5) down the column. As a result of salt splitting, the pH initially dropped to 0.4, after which the pH rose to above 1.0, after which the treatment was stopped. The chromate absorption step was then initiated by passing through the column 0.9 µl/min of an aqueous solution containing 450 g/l sodium chlorate and 3.1 g/l sodium bichromate. The effluent remained colorless until well over 240 l of the solution had been treated. The effluent's pH was in the range 2.05 - 2.20 when the first yellow color was detectable in the effluent.

Stripping of absorbed chromate from the yield resins was carried out by passing a 12% sodium chloride solution containing 4% sodium hydroxide down through the column.

The preceding examples show the chromate absorption step according to the present process. In examples 2 and 3, absorbed chromate can be easily stripped from the weakly basic anion resin by passing a 4% sodium hydroxide solution in a half-saturated sodium chloride solution down through the layer. The layer is then regenerated by passing hydrochloric acid solution through the layer and is then conditioned using a weak alkaline or neutral sodium chloride solution, to a pH in the range 1-2. Then

the chromate absorption step can be repeated.

A general description of another embodiment of the invention is as follows: A mixed layer of the prescribed ion exchange resins is produced by air blowing under water of 35-65 parts by weight of an anion exchange resin in the chloride form and 65 - 35 parts by weight of a weak cation exchanger resin in the hydrogen form in an exchanger column. After dewatering from the column, the bed is treated with a mineral acid solution of an alkali metal chloride after which the cation resin is conditioned by passing a neutral solution of an alkali metal chloride through the bed to adjust the pH of the effluent

as previously explained. An aqueous alkali metal-chloratric alkali metal chloride liquid containing less than 20 g/l/ preferably less than 10 g/l, of an alkali metal chromate is passed up through the mixed yielder layer in a quantity of 0.2 l/min/dm 2 of the layer while the pH of the effluent is monitored and when the pH rises to above approx. 2, the drain will become visible yellow, after which the flow of the chlorate-containing liquid ends.

The chlorate-containing liquid is then withdrawn from the ion-

exchange resin layer and a half-saturated (12-15%) sodium chloride solution, preferably with a pH in the range 7-3, is passed up through the layer at a rate sufficient to separate the two ion exchange resins into an upper (anion -resin) and a lower (cation-resin) layer. The passage of the sodium chloride solution is terminated and a 12-15% sodium chloride solution containing 4% sodium hydroxide is passed down through the anion exchange resin until the effluent becomes alkaline indicating the removal of absorbed chromate from the resin. For regeneration, a 4% hydrochloric acid solution in a half-saturated sodium chloride solution is then passed through the ion exchange resins until the effluent pH drops to 1.0. Conditioning of the cation resin is carried out by passing a half-saturated sodium chloride solution up or down through the resin until the pH of the effluent is in the range 1.5-2.0, after which the chromate absorption step is repeated.

Claims (5)

1. Method for removing chromate ions from a solution with a high content of dissolved alkali metal chlorate and chloride and which contains dissolved alkali metal chromate by passing the solution through a layer containing a mixture of anion exchanger and cation exchanger resin, characterized in that the solution is passed through an ion exchange resin layer essentially consisting of an intimate mixture of an anion exchange resin in chloride form and a weak cation exchange resin in conditioned hydrogen form and where the total amount of exchange points in the anion resin is sufficient to remove chromat ions from the supplied solution and that the cation exchange resin has a number of exchange points which is not significant less than the number of exchange points in the anion exchange resin and which is conditioned so that the pH of the solution does not fall below 1.0, and that the pH of the solution that has passed the layer is determined and that the supply of the solution is interrupted when the pH of the effluent exceeds 2. 2. Method according to claim 1, characterized in that the solution is passed through the 2' layer in an amount that does not exceed 0.3 l/min/dm of the layer. 3. Method according to claim 1, characterized in that the anion exchanger resin used is a weakly basic resin. 4. Method according to claims 1-3, characterized in that the yield resins used are both macroreticular. 5. Method according to claims 1-4, characterized in that the cation exchanger resin is used in an amount which is not less than 0.5 parts by weight, and not greater than 2.0 parts by weight per weight part anion exchange resin.