Jack,
The following is some general information about exchange resins which while it may be years out of date, will help you and others understand what they are and how they are used.
The resins are synthetic polymers which have the ability to exchange ions which are chemically bonded to them for other ions which are either more strongly attracted or are in such concentration in the solution to which they are exposed that the original ion is displaced.
There are both cation exchange resins and anion exchange resins. Since all metal ions are cations, the resins in this case are cation exchange resins. The resins are fabricated to a close size fraction either by crushing the solid polymer and screening to separate a closely sized portion or by producing them as beads of uniform size, usually about like coarse sand. In use the resins are contained in columns which can be from inches to feet in diameter. The column is constructed so that the solutions added to them will flow through uniformly without channeling.
Resins of the same type are used in water softeners. Some water softeners have salt tanks where the home owner places salt to make a salt solution. The salt solution is passed through the resin column and charges it with sodium ions. When hard water is subsequently passed through the column, the Na ions are replaced by the Ca and Mg ions which are the culprits that cause soap scum and the effluent water contains only Na ions which do not cause soap scum and is thus "softened". Others use resin tanks which are recharged elsewhere and are changed by the supplier such as Culligan.
In the case of metal recovery, the resin column is charged with Hydrogen ions by passing a solution of acid through the tank, usually HCl or H2SO4. When the metal containing solution is passed though the column, the H ions are displaced and the metal ions are retained. Conductivity measurements of the effluent indicates when the column is exhausted of H ions and can hold no more metal ions. The concentration of the metal ions in the solution does not have much to do with the amount of metal that is retained on the column. If it is dilute, a lot will pass through before the H ions are used up. It it is more concentrated, less solution will be required to "fill" the column with metal ions.
While it is unlikely that the resin columns would be used to separate the metal ions from each other it is possible to obtain very complete separation due to the fact that each different metal, be it Au, Pt, Rd, Os, Ag, Rb, Cu or whatever, will be "held" by the resin to a different degree. If a solution containing a mixture of different metals ions, say a mixture of metal chlorides and nitrates, is poured onto the column to establish a metal ion containing zone at the top of the column and then a dilute acid solution is added to the column with the liquid level controlled so that it stays constant, the different metal ions will appear in the effluent at different times, the less tightly held first and the most tightly held last with each of the others appearing according to how tightly they are held by the resin. (Using some non-chemical terminology). This phenomenon was used about 55 years ago during the Manhattan project to separate isotopes of uranium. There was sufficient difference in the attraction to the resin between U235 and U238 to make the separation possible.
Another use of exchange resins is to make de-ionized water. In this case the cation column is charged with acid to place on it Hydrogen ions and the anion column is charged with a hydroxide such as NaOH, KOH, or NH4OH to charge it with OH ions. When water containing dissolved substances is passed through first one column and then the other, the unwanted cations (Ca, Mg, etc.)are replaced by H ions and in the anion column the CO3 and Cl, etc are replaced by OH ions and the result is pure water. Huge amounts are produced for chemical processing, drug production and even in electrical discharge machining which must take place in a non conductive bath. (Very pure water does not conduct electricity easily. Its resistivity approaches 600,000 ohms/sq cm).
Carl Wheeler |
