Table

SELECTIVITES OF ION EXCHANGE RESINS IN ORDER OF DECREASING PREEERENCES*'"

Strong acid cation exchanger

Strong base anion exchanger

Weak acid cation exchanger

Weak base anion exchanger

Weak acid chelate exchanger

Barium (+2) Lead (4-2) Mercury (+2) Copper (+1) Calcium (+2) Nickel (+2) Cadmium (+2) Copper (+2) Cobalt (>2) Zinc (*2) Cesium (+Î) Iron (+2) Magnesium (+2) Potassium (+1) Manganese (*2) Ammonia (+1) Sodium (+1) Hydrogen (+1) Lithium (+1)

Iodide (-1) Nitrate (-1) Bisulfite (-1) Chloride (-Î) Cyanide (-1) Bicarbonate (-1) Hydroxide (-1) Fluoride (-1) Sulfate (-2)

Sulfate (-2) Chromate (-2) Phosphate (-2) Chloride(-I)

Magnesium (+2)

' References 63 and 64. 6 Valence number is given in parentheses.

Host industrial ion exchange systems are fixed-column systems consisting of a bed of resin. Batch operations, which mix the resin with a set amount of wastewater, are normally inefficient and therefore seldom used. The fixed-bed systems usually follow a four-step pattern of operation: I) exhaustion, 2) backwash, 3) regeneration, and 4) rinse. In the exhaustion step, the waste solution ions replace the ions bound to the resin until all the exchange sites are filled (a condition coined "column breakthrough"). The bed is then backwashed with water to resettle the resin bed. This resettling redistributes the resin medium to reduce solution channeling (a condition in which the solution passing through the bed follows a least-resistance pattern and bypasses parts of the resin bed). Regeneration occurs by passing a highly concentrated solution of the ion originally associated with the resin through the bed. The resulting low-volume brine, which contains the metal species removed from the wastewater, can be reclaimed (e.g., by electrowinning). The column is finally rinsed to remove excess regenerating material, and the cycle can begin again.

Host ion exchange systems treat wastewater with a cationic exchanger followed by an anionic exchanger, which allows both cationic metal species and anionic metal complexes to be removed from solution. Two sets of columns are often used to provide a continuous, uninterrupted treatment operation. When one set has been exhausted, flow is switched to the other set and the spent columns are regenerated.

The Reciprocating-Flow Ion Exchanger (SHE) is the ion exchange system most frequently used by industry for metals recovery (see Figure 15), This proprietary system operates on the principle that, for the short time period the unit goes off line for regeneration, the buildup of contaminants in the rinse system is negligible. The RFIE units tend to be more practical than fixed-bed systems because the columns use less resin volumes and make more efficient use of regenerating solutions. The RFIE units are used to recover metals from plating rinses of nickel, copper, zinc, tin, and cobalt drag-outs.

Technical Evaluation-Waste Feed Considerations — The following waste characteristics affect the performance of ion exchange processes: (1) the concentration and valence of the contaminant(s), (2) the concentration of competing ionic species, (3) the concentration of interfering inorganics md organics, (4) the concentrations of dissolved and suspended solids and oil and grease, and (5) the corrosiveness of the wastewater relative to the ion exchange resin material. Ion exchange systems can adequately treat metal waste streams containing up to about 2500 parts per million (expressed as calcium carbonate equivalents). Treating concentrations above 2S00 ppm becomes prohibitively expensive. Treatable waste streams should not contain high solids or high organics levels, because solids will foul the resin column and cause treatment inefficiencies due to channeling. Residual organic matter can cause resin beads to mat and bind, which decreases resin efficiency. The pH levels also must be considered in resin selection. Weak acid resins are inefficient for treating wastewaters with pH values less than 6.0. Weak base

(a) Hardware components

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