ION EXCHANGE CHEMISTRY, CHELATE RESINS

Ion exchange is a simple process where an organic molecule has been substituted with acid or base exchange sites. As a solution with exchangeable ions passes through the resin, the ions are captured and exchanged for the ions on the resin. For example, some weak acid ion exchange resins can use the carboxylic acid radical, in the sodium form, R-COO-Na as the exchange site. As solutions of metal ions such as Magnesium (Mg), Calcium (Ca), or Copper (Cu) pass through the resin, they are exchanged for the Na (Sodium) at that site. The molecule would now appear like this R-COO-Ca, for a Calcium exchange.

R-COO-Na + Ca+2 = R-COO-Ca + 2 Na+

These resins are used in water softeners where the alkaline earth metals such as Calcium and Magnesium are exchanged for Sodium. In a water softener Sodium Chloride (salt, NaCl), is used to "regenerate" the resin. The salt solution is passed through the resin in very high concentrations and the sodium displaces the metals attached to the active sites. When the concentration of sodium is low, the sites prefer the alkaline metals again and the resin can be used to "soften" water again.

Chelated Resins

The same principals apply in industrial ion exchangers except that usually a strong acid or base is used to regenerate the resins (except in industrial water softeners). To remove a metal from your waste stream at a low pH using ion exchange, the most cost effective method is to use a weak acid chelating resin. An iminodiacetic acid resin is useful to remove a metal when a chelate such as ammonia or EDTA is present. The pH is adjusted to the correct range remove the metal, but kept under 6 to prevent the formation and precipitation of hydroxides and oxides of metals.

This resin has a hierarchy of preferences, i.e., it prefers Copper more than Zinc. This requires their use mainly in compatible metal systems. If only one metal is being passed through the column, there is no competition for sites and the progress of the system is easier to monitor. Reclaiming the metal is also easier as multiple metal systems are difficult to plate.

The ion exchange resin used in most metal recovery systems is what is called a "weak acid" resin. This system allows low chemical usage, usually about 1.25 times the theoretical amount to regenerate the column. This resin is used in the sodium form. This allows us to regenerate with an acid and recover the metals in a useful form. The resin is then conditioned so that the pH does not drop to low when we start up the column.

The basic chemistry of this system is as follows:
1. The solution is adjusted to a pH which is optimum for the resin but low enough to prevent hydroxides from forming and high enough to ensure the capture of the metals.

2. The solution is passed through a media filter to remove any solids and then through the lead/lag columns where the a metal is exchanged for sodium ions. The capacity of the columns is about 1.8 pounds of metal per cubic foot of resin depending on the competing ions passing through with the metal.

3. When a column is saturated with a metal, "breakthrough" occurs. This results in a greatly increased passage of metal ions to the lag column. This is when you will regenerate the first column.

4. The regeneration chemistry is in 5 steps, they are:
a. Water backwash to remove any broken beads. This recycles to the main holding tank.
b. Acid backwash, to remove the metal. A recirculating acid tank ensures full metal removal.
c. Water backwash to remove residual metal and acid from the column. A timing cycle ensures the backwash goes to the acid tank until the stream is acid free.
d. An caustic soda backwash which removes the acid (recycles to the holding tank for pH adjust before passing throuth the column), and replaces the H+ with Na+ ions. This prevents the pH from dropping when the column goes on line.
e. A final water backwash to restratify the resin and remove the remaining caustic from the system.

W hen setting up a system do not let organic solvents, oil, or grease in the system as these will foul the column.

Feed line segregation is very important for successful utilization of ion exchange. Call for a segregation scheme for your facility.