Ion Exchange

Technology Description
Ion exchange is a chemical reaction wherein an ion from solution is exchanged for a similarly charged ion attached to an immobile solid particle (i.e., ion exchange resin). Ion exchange reactions are stoichiometric (i.e., predictable based on chemical relationships) and reversible. The resins are normally contained in vessels referred to as columns. Solutions are passed through the columns and the exchange occurs. Subsequently, when the capacity of the resins is reached, the ions of interest, which are attached to the resin, are removed during a regeneration step where a strong solution containing the ions originally attached to the resin is passed over the bed.

The strategy employed in using this technology is to exchange somewhat harmless ions (e.g., hydrogen and hydroxyl ions), located on the resin, for ions of interest in the solution (e.g., copper). In the most basic sense, ion exchange materials are classified as either cationic or anionic. Cation resins exchange hydrogen ions for positively charged ions such as copper, nickel, and sodium. Anion resins exchange hydroxyl ions for negatively charged ions such as sulfates, chromates, and cyanide.

The basic ion exchange column consists of a resin bed which is retained in the column with inlet and outlet screens, and service and regeneration flow distributors. Piping and valves are required to direct flow and instrumentation is required to control regeneration timing. The systems are typically operated in cycles consisting of the following steps (ref. 3):

Service (exhaustion) – Water solution containing ions is passed through the ion exchange column or bed until the exchange sites are exhausted.
Backwash – The bed is washed (generally with water) in the reverse direction of the service cycle in order to expand and resettle the resin bed.
Regeneration – The exchanger is regenerated by passing a concentrated solution of the ion originally associated with it through the resin bed; usually a strong mineral acid or base.
Rinse – Excess regenerant is removed from the exchanger; usually by passing water through it.

The ion exchange process has been commercially available for many years, but early use was primarily for water deionization or softening. Widespread interest in the process for PWB pollution prevention and control is a more recent application that has grown rapidly over the past 10 years.

PWB Manufacturing Applications
Ion exchange is common in PWB shops for several reasons. Among them are:

Several PWB rinse water streams are readily compatible with ion exchange. Simple copper-bearing, low-organic streams from micro-etchant, acid dip, and copper electroplating rinses can usually be sent directly to ion exchange with no pretreatment. With carbon filtration and pH control of the incoming stream, additional rinse streams become ion exchange candidates.
The preponderance of copper as the contaminating metal allows shops to take advantage of the powerful ion exchange/electrowinning combination. Together, these two technologies combine to separate, concentrate and recover copper from rinse streams.
Ion exchange offers shops the ability to close-loop some rinses and reduce the need for downstream treatment.
Due to the large number of rinses potentially amenable to ion exchange and to the ability of ion exchange to produce compliant effluent, some shops (particularly small ones) can employ metal-scavenging ion exchange as a primary end-of-pipe system.
Reducing the quantity of copper entering the waste treatment system greatly reduces the quantity of wastewater treatment sludge generated, which is typically shipped off-site as hazardous waste.

Generally, ion exchange is limited to dilute rinse water streams although scavenging resins can be used to treat more concentrated wastes under certain circumstances. As concentrations increase, ion exchange becomes impractical due to the increasing frequency of regenerations and the declining difference between the concentration of the regenerant which is a constant (typically 5-10 grams/liter) and the concentration of the stream being treated.

Drag-out recovery tanks are used in conjunction with ion exchange systems whenever feasible to reduce the load on the ion exchange system. In operation, the drag-out tanks return the bulk of the plating chemicals directly to the plating bath and an ion exchange unit connected to a subsequent flowing rinse captures only the residual chemicals. The needed size of the ion exchange unit and its regeneration frequency are therefore reduced.

Metal Scavenging Applications. When the sole objective of using ion exchange is to remove metal from a wastestream, a metal scavenging configuration is employed (Exhibit 5-10). This system uses only one type of ion exchange resin, either selective anion or cation, depending on the charge of metal or metal complex being targeted for removal (e.g., a cation-type resin is used for most copper removal applications). Because this system does not have both cation and anion resins, the rinse water will not be fully "deionized" and cannot be reused as rinse water for common rinsing purposes. The primary advantage of metal scavenging is the large capacity (in terms of rinse water treated) vs. a deionizing configuration since only divalent cations are exchanged and common monovalent cations such as sodium and potassium are bumped off the resin and passed. Thus, regeneration cycles are longer, lowering chemical and other operating costs.

Certain PWB wastestreams are commonly treated with the metal scavenging configuration. Most common are copper, tin-lead and gold rinse systems. Various copper rinses are commonly processed with this technology, including etch, microetch, and copper electroplating. Resins are regenerated using sulfuric acid. In the case of tin-lead, ion exchange scavenging is employed to remove lead from rinse water which is then often discharged. Regeneration may be performed on-site with methane sulfonic acid (MSA) or the resin, when exhausted, can be shipped off-site for processing/disposal. Point source ion exchange treatment of tin-lead rinses may be performed to protect downstream units from lead-bearing streams. Ion exchange may also outperform the primary waste treatment system and this configuration may cost-effectively maintain compliance where lead discharge limits are more stringent than copper limits. Gold rinses are often ion exchanged for the purpose of gold recovery. When exhausted, gold-bearing resin is usually processed offsite to assure efficient recover of the gold.

Deionization. When the objective is to recover metal and recycle rinse water (i.e., closed-loop), a deionization configuration is employed. This configuration uses a combination of cation and anion exchange columns in series to remove all ions from the rinse water (Exhibit 5-11). This strategy may be employed when continuous discharge is impractical due to stringent limits, or where the benefits of water reuse outweigh the cost of installing and operating water recycling ion exchange unit.

A good candidate for deionization is the electroplating copper rinse system. With this application, rinse water containing copper is sent to the cation and anion exchange columns and deionized water is returned as fresh rinse water to the rinse system. The anion regenerant, usually NaOH, can usually be pH treated and discharged. The cation regenerant stream is interesting due to it's similarity to the plating bath make-up--sulfuric acid and copper sulfate. While it is possible to return the regenerant to the plating bath thereby closing the loop for most of the process, this is generally not done for two important reasons: (1) the performance of the PWB through-hole plating in various stress tests is quite sensitive to small variations in bath chemistry making additions of regenerant inadvisable and (2) the copper sulfate plating bath is operated at too low of a temperature to create sufficient evaporative headroom for the regenerant additions. The regenerant is an ideal electrowinning candidate and this is the most common treatment option.

Deionization or metal scavenging can be accomplished using "point-of-source" ion exchange as shown in Exhibits 5-10 and 5-11 or using a central system similar to that shown in Exhibit 5-12. With this system, rinses are first processed through a selective resin for cation removal and subsequently through anion and cation resins for complete deionization and then returned to the rinse system (ref. 36).

Process Residuals. The primary residuals from ion exchange recovery processes are the regenerants and backwash solutions. The regenerants are concentrated wastes and the backwash is dilute. Both solutions are either caustic or acidic, depending on the resin type and application. High metal bearing regenerates (typically cation resin) are sometimes reused directly in the bath, further processed to recover the metal (e.g., electrowinning), waste treated or sent to an off-site recovery facility. Low metal bearing regenerants (typically anion resin) and backwash solutions are typically treated on-site. Waste treatment processes generate sludge that is an EPA listed hazardous waste (F006).

The volume of regenerant produced will depend on the regeneration requirement (e.g., lbs of acid per ft3 of resin) and the concentration of acid used (typically 1 to 5%). The regeneration requirement will depend on the resin type, application (metal or complex being recovered) and the configuration (cocurrent vs counterflow). Typical volumes of regenerant are 20 to 50 gal/ft3 of resin. The volume of regenerant waste is sometimes reduced by reusing the last portion of the regenerant, which will be less contaminated with metal and contains free acid. Backwash volumes depend mostly on the equipment design and the application. Typically, backwashing generates 25 to 75 gal/ft3. The backwash is partly reused by some equipment vendors as make-up water for regenerant, in an effort to reduce the total waste volume generated. Because backwash contains only dilute concentrations of pollutants it is typically not a major concern and is treated on-site and discharged. However, for shops working toward zero discharge, the backwash volume could present a significant problem. Both backwash and regenerant can processed by evaporation to reduce the volume requiring disposal. However, this increases the capital and operating costs of the system. Also, evaporation of hazardous wastes is sometimes regulated as a RCRA technology and may require a permit to operate.