This invention concerns ion-exchange and chelate-exchange resins and, in particular, novel resins functionalized so as to improve exchange kinetics. The invention also concerns a method of preparing the novel resins and a method for their use in separating chemical species from liquids.
Ion-exchange and chelate-exchange resins are widely employed by industry to separate chemical species from liquids which contain them in solution. These resins are commonly prepared by functionalizing a copolymer bead matrix with functional groups that can associate with chemical species, such as ions or molecules, when the resin is in contact with the liquid. Such resins are generally used in water treatment and purification, food preparation, pharmaceutical manufacturing, chemical processing, metal extraction, and so on, as is generally discussed by R. M. Wheaton et al. in, "Ion Exchange", Vol. 11 Kirk-Othmer Ency. Chem. Tech. pp. 871-899 (2nd Ed. 1966).
A disadvantage associated with such resins, and widely recognized within the art, is slow diffusion into the resin beads for the chemical species being separated. To attain the maximum operating capacity for the resin, it is necessary to use essentially all available exchange sites within the resin bead volume. To do so, substantially all of the available diffusion path length, i.e., the radius for a fully functionalized resin bead, must participate in exchange with the chemical species. Full utilization of the diffusion path length in this instance requires a relatively long time to reach exchange equilibrium. In contrast, resins having short diffusion path lengths reach exchange equilibrium more rapidly than resins having longer diffusion path lengths. A shorter diffusion path length therefore allows for more rapid access to available exchange sites and a quicker approach to exchange equilibrium. This shortened diffusion path ultimately leads to an ability to process relatively large amounts of feed streams without unduly sacrificing operating capacity.
Industry has previously made attempts to shorten the diffusion path length by reducing the size of resin beads. However, small beads lead to larger pressure drops across a resin bed and reduced flow rates for feed streams being processed. As such, substantially reducing the size of the resin beads is not practical for a commercial process.
Macroporous resins, such as those disclosed by Meitzner et al. in U.S. Pat. No. 4,224,415, were developed to improve kinetics by providing a highly porous copolymer bead matrix wherein relatively large pore sizes improve diffusion of chemical species into the interior portions of the beads. However, these resins also have a considerable amount of exchange sites which are relatively inaccessible to diffusion.
Many mining operations generate aqueous streams containing one or more heavy metals, like copper or nickel. Industry typically employs two methods to recover such metals, namely, solvent extraction or the use of chelate-exchange resins. Traditionally, solvent extraction has been used to recover such metals, but due to waste disposal considerations this method is gradually becoming obsolete. Accordingly, chelate-exchange resins are becoming important for these applications.
Improved exchange kinetics are particularly desirable for chelate-exchange resins, since diffusion of chemical species is often limiting with respect to the particular chelation reaction involved. For instance, U.S. Pat. Nos. 4,031,038 and 4,098,867 disclose chelate-exchange resins derived from aminopyridine compounds, such as 2-picolylamines. Although the resins are highly selective for metals like copper or nickel, they exhibit relatively slow exchange kinetics, i.e., the time required to reach equilibrium capacity for metal loading is fairly long. As such, a large amount of the resin is needed or only a portion of the available exchange capacity is used, to maintain a commercially reasonable feed rate for the liquid stream being processed. Further, partial use of the exchange capacity is an uneconomical use of the resin, since it is relatively expensive to produce.
Accordingly, it is desirable to develop resins which (1) exhibit improved exchange kinetics without undesirable increases in bed pressure drop, (2) allow for greater utilization of available exchange capacity, and (3) promote efficient loading and elution of the chemical species being separated. Such resins would result in a more economical and efficient separation process.