1. Field of the Invention
This invention relates to a method of treating weak base ion exchange resins. More particularly, the invention relates to a method of treating weak base ion exchange resins whereby the regenerated resins are washed with water to remove a portion of the regenerant wastes and then washed with a solution of carbonic acid to remove substantially all the remaining regenerant wastes.
2. Description of the Prior Art
Ion exchange processes generally involve a reversible chemical reaction between a solid ion exchanger and an aqueous solution whereby ions are interchanged between the exchanger and the solution. The most widely used exchangers are ion exchange resins which comprise, for the most part, insoluble organic polymer matrices having attached functional groups which provide mobile ions which may be exchanged for ions in the solution bearing the same charge. To impart increased stability to the resins, the matrices are usually prepared by addition copolymerization reactions which provide varying degrees of crosslinking. Depending upon the nature of the functional group, ion exchange resins are classified broadly as strong acid, weak acid, strong base and weak base. The acid exchange resins are further classified as cation exchangers while the base exchange resins are usually referred to as anion exchangers.
Strong acid cation exchange resins typically comprise sulfonated copolymerized styrene-divinylbenzene products crosslinked to varying degrees. Such resins are widely used in water conditioning, demineralization of sugar and corn syrups, chromatographic separations, and metal recovery.
Weak acid cation exchange resins are commonly prepared by reacting an unsaturated carboxylic acid, e.g., acrylic or methacrylic acid, with a crosslinking agent such as divinylbenzene or ethylene dimethacrylate. Functional groups may comprise phenolic, phosphonous or carboxylic entities, combinations of these and others. Weak acid resins have found application in water conditioning, chromatographic separation, and metal recovery.
Strong base anion exchange resins generally are prepared by affixing quaternary ammonium groups to a polystyrene divinylbenzene matrix. Resins of this type are used principally in water conditioning.
Weak base anion exchange resins contain primary, secondary and tertiary amine groups or mixtures of such groups. Such resins are available in a variety of types including condensation products of amines with formaldehyde, alkyl dihalides, chloromethylated styrene-divinylbenzene, etc. Resins of this type are used principally in demineralization of corn syrups and corn sugar.
Ion exchange resins may be used in batch or column type operations. The latter type of operation is the most widely used in industrial operations. In this case, a column or a vertical cylinder is filled with the resin, and the resin is supported by a porous plate or other means which is permeable to the solution to be treated. The solution may be passed down through the resin (down-flow operation) or up through the resin (up-flow operation). Resin columns may also be operated using counter-current flow techniques whereby the feed solution is passed downwardly and the regenerate upwardly or vice versa.
A typical cycle for the operation of an ion exchange column is as follows:
1. Exhaustion Step
The solution being treated is passed through the column in order to remove ions by exchanging them for an equivalent amount of similarly charged ions on the resin until the resin is essentially exhausted of the latter ions. PA1 A solution containing the necessary ion to recharge the resin is passed through the column. PA1 Water is passed through the column to remove regenerant waste products and excess regenerant. Regenerant waste products are materials which are chemically or physically adsorbed on the resin during the exhaustion step.
2. Regeneration Step
3. Rinse Step
Certain optional steps may also be carried out. For example, when the solution being treated is of significant economic importance, a rinse step may be included before regeneration to preclude mixing of the solution being treated and the regenerant. Also, there may be included a backwash prior to regeneration whereby, the upward flow of water removes insoluble contaminates and the resin bed is loosened.
Weak base resins have found wide application in deionization of corn syrups, including high fructose corn syrups, for the purpose of removing salts and weakly acidic colored bodies. Other types of resins are also used for purification or refining such syrups.
Since ion exchange resins, including weak base resins, contain a finite number of ion exchange sites, the exchange capacity of the resin eventually becomes effectively exhausted. The length of time for this to occur is dependent upon the amount of ionic material removed from the solution and the flow rate of the solution through the resin. The economics of ion exchange refining make it desirable that the resins be reused.
When weak base anion exchange resins are utilized to refine or purify corn syrups and the ion exchange capacity is effectively exhausted, the typical regeneration cycle is:
(1) The ion exchange column is "sweetened off" by displacement of the syrup from the resin with water.
(2) Water is passed upwardly through the column to backwash the resin and thereby remove insoluble debris which was filtered out of the corn syrup and also to reclassify the resin in the column so as to minimize pressure drop and channelling.
(3) The resin is regenerated to the weak free base form by passing downwardly therethrough, for instance, a solution at suitable concentration of NaOH, Na.sub.2 CO.sub.3, ammonia or the like to regenerate the ion exchange capacity of the resin. The regenerant solution may be passed through the bed until typically about one bed volume of regenerant is in contact with the resin.
(4) The regenerant is displaced from the column by slow addition of rinse water which additionally removes a considerable portion of the regenerant waste products. This rinse is referred to in the art as the "slow rinse step" since the rate at which water is introduced into the column approximately equals the rate at which the regenerant was introduced into the column. If the rate the regenerant is passed through the resin column is too rapid, the contact period necessary for satisfactory regeneration to take place may be too short.
(5) More water is passed through the column to remove the remaining regenerant and waste products. This is generally referred to in the art as the "fast rinse step" since the rate at which water is passed through the resin bed is typically about three times faster than that utilized in the slow rinse step. The fast rinse is continued until the resin is essentially free of regenerant solution as determined by measuring the pH or conductivity of the column effluent.
After several regeneration cycles, the weak base anion exchange resins require more rinse water to achieve the same degree of ion exchange capacity. S. B. Applebaum, Demineralization by Ion Exchange, pp. 146-147, Academic Press, New York (1968) attributed this need for more water to oxidative attack by the water which causes the formation of weak acid groups on the resin which combine with sodium or like ions thereby hindering the removal of regenerant waste products by rinse water. One proposed solution to this problem was the utilization of a solution containing relatively small amounts of ammonia for regeneration. However, this approach suffers from the disadvantage that, when dealing with large amounts of regenerant, the ammonia present interferes with the operation of biological waste treatment facilities. Another solution was to use heated water for rinsing, however, this requires energy expenditures and has the tendency to cause physical deterioration of the resin.