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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.4 ION EXCHANGE
3.4.4 Technology/Equipment Description
3.4.4.1 Ion Exchange Resins and Columns
3.4.4.2 Integrated vs Modular Designs
3.4.4.3 Single vs Duplex Column Operation
3.4.4.4 Counterflow vs Cocurrent Flow/Regeneration
3.4.4.5 Other Equipment/Design Considerations
3.4.4.6 Commercial Equipment
3.4.4 Technology/Equipment Description
The initial part of this section describes some of the more important design elements of ion exchange systems and the latter part presents a description of commercially available equipment.
3.4.4.1 Ion Exchange Resins and Columns
A wide range of ion exchange resins are manufactured, the choice of which depends mainly on the type of metal being recovered and the chemical composition and characteristics of the solution being treated. Properly matching the ion exchange resin and the process chemistry should result in efficient operation, quality byproducts and lower operating costs. Inappropriate selection of resin can result in total system failure.
Resins can be broadly classified as strong or weak acid cation exchangers or strong or weak base anion exchangers. Strong acid resins are so named because their chemical behavior is similar to that of a strong acid. The resins are highly ionized in both the acid and salt form. In a weak acid resin, the ionizable group is a carboxylic acid (COOH) rather than the sulfonic acid group (SO3H) used in strong acid resins. These resins behave similarly to weak organic acids that are weakly dissociated. Because weak acid resins have an affinity for hydrogen ions, they have a limited exchange capacity when used to treat solutions with a pH below 6.0 (ref. 39).
Like strong acid resins, strong base resins are highly ionized and can be used over the entire pH range. Weak base resins are like weak acid resins, in that the degree of ionization is strongly influenced by pH. Consequently, weak base resins exhibit minimum exchange capacity above a pH of 7.0 (ref. 39).
A major advantage of both weak acid and weak base resins is that they can be regenerated much more efficiently than the strong acid and strong base resins. The weakly ionized resins can be regenerated using slightly greater than the stoichiometric reagent requirements, whereas the strongly ionized resins require significantly more regenerant (ref. 39).
Many specialty resins, such as chelating resins, are also in commercial use. Chelating resins that exhibit a high selectivity for heavy metal cations over other cations in solution have been commonly used in metal finishing, especially in the past ten years. Because of their selectivity, they are especially useful for end-of-pipe polishing following hydroxide precipitation. Chelating resins are also used in recovery with electroless copper and electroless nickel plating solutions. Generally, chelating resins cannot be used at low pH (<4) and a pH adjustment step is typically needed before the ion exchange process.
Exhibit 3-25 provides some general guidance on the types and capacities of resins used for common metal finishing chemical recovery applications, the chemicals used for regeneration and the method of recovery.
Most industrial applications of ion exchange used fixed-bed column systems, the basic component of which is the ion exchange column. The column must:
- Contain and support the ion exchange resin
- Uniformly distribute the service and regeneration flow through the resin bed
- Provide space to fluidize the resin during backwash
- Include the piping, valves, and instruments needed to regulate flow of feed, regenerant, and backwash solutions
After the feed solution is processed to the extent that the resin becomes exhausted and cannot accomplish any further ion exchange, the resin must be regenerated. Resin capacity is usually expressed in terms of equivalents per liter (eq/l) of resin. An equivalent is the molecular weight in grams of the compound divided by its electrical charge or valence. For example, a resin with an exchange capacity of 1 eq/l could remove 37.5 gram of divalent zinc (Zn+2, molecular weight of 65) from solution (ref. 39).
The hydraulic loading of resins will vary considerably from application to application, depending on: column design; type of resin employed; concentration of metal in solution; other chemical characteristics of the feed solution (e.g., pH); and the allowable concentration of metal in the column effluent. Typical hydraulic loadings range from 2 to 3 gpm of rinse water per cubic foot of resin.
3.4.4.2 Integrated vs Modular Designs
An integrated ion exchange system design is one in which the various components needed to perform the ion exchange recovery and regeneration functions are connected within the one unit. Such systems may also have attached electrowinning units and/or chemical treatment systems for processing the regenerant. The modular or point source design separates the ion exchange column from the regeneration and regenerant processing equipment. With the modular design, the columns are transported to a central station for regeneration (in some cases the modules are hard piped). The regeneration station can be either in the plating shop or at an off-site location (i.e., centralized waste treatment facility). The modular ion exchange strategy can reduce capital costs for small to medium-sized applications where low to moderate regeneration frequency is required. Also, the modular units are considerably smaller and therefore do not occupy as much production area floor space as integrated units (i.e., if the regeneration station is remotely located to a non-production area). However, operating costs are usually higher for modular systems due to the labor needed for transporting the modules and connecting them to the regeneration station (or for changing operating modes and valve positions for hard piped modular systems ) and initiating regeneration. Some commercial ion exchange modules have the appearance of large cans and are referred to as ion exchange canisters. With this type of unit, the canisters can be stacked upon one another to combine anion and cation types or to increase the resin bed volume. Standard column designs are also available.
3.4.4.3 Single vs Duplex Column Operation
Duplex column ion exchange systems are used in many chemical recovery operations, especially where a continuous feed flow is expected. Dual column configurations avoid downtime during regeneration. Two different duplex column arrangements are commonly used. In one arrangement, which is referred to as parallel/standby, the feed stream flows through either one column or the other, but never both. The off-line column is regenerated and then is held in reserve until the other column is ready for regeneration. This is a somewhat inefficient use of the two columns since column switching must take place before breakthrough occurs, which happens before the resin is completely loaded with ions of interest. In the second case, which is referred to as lead/lag, the two columns are placed in series flow. During operation, the majority of metal removal is accomplished in the first column (lead column) until it approaches capacity. The process can continue until the first column is essentially loaded to full capacity with ions of interest, since the second column (lag column) will remove the breakthrough of the first column. After breakthrough is reached, the first column is taken off-line for regeneration and then put back into service as the lag column. The roles of the two columns continue to be reversed following each regeneration. The switching of the columns, initiating of regeneration and other functions of modern ion exchange equipment is usually controlled by a microprocessor.
A special single column design that is widely used is referred to as reciprocating flow ion exchange (RFIE). A commercial application of this design is the RecofloÆ (ref. 39, 349, 364, 365). The RecofloÆ system is characterized by its short resin beds (6 to 24 in. as compared to 40 to 60 in. minimum for standard counterflow design) containing fine particle size resin beads. The RecofloÆ process operates in a counterflow mode (see Section 3.4.1.4) with approximately a five minute cycle. During the cycle, the feed stream is fed for approximately 2.5 minutes, then regeneration occurs for approximately 2.5 minutes. The cycles are controlled by a microprocessor and are continuously repeated. The short cycle time, which is made possible by the short resin bed, emulates a continuous flow process.
3.4.4.4 Counterflow vs Cocurrent Flow/Regeneration
One method of categorizing the operation of different ion exchange systems is by the direction of the service flow (e.g., rinse water) vs the direction of the regeneration flow. The two configurations that can be used are shown schematically in Exhibit 3-26. With cocurrent operation, the service flow and the regeneration cycle flow in the same direction and with countercurrent flow, they flow in opposite directions (as shown, service flow can be either downward or upward). Countercurrent flow is considered by most sources to be the more efficient method (e.g., ref. 42).
With cocurrent flow the hydrogen ions displace metal ions from the top to the lower portion of the bed. Complete removal of these ions can only be accomplished by the use of excessive levels of acid regenerant. With normal regenerant usage, there is a "heel" left at the exit end of the column (i.e., undisplaced metal ions). On the following service cycle, the desired exchange reaction occurs in the upper portion of the bed. However, as the hydrogen ion concentration increases toward the lower section of the bed, some reexchange with previously undisplaced metal ions leads to metal ion "leakage."
After regeneration of the counterflow system, the residual ions are in the top of the bed, with the bottom being fully converted to hydrogen. Thus, there are no residual metal ions present at the bottom of the bed to permit the leakage reaction to occur on the subsequent service cycle (ref. 43).
In addition to reduced ion leakage, counterflow regeneration can increase operating capacities, decrease the need for waste stream pH adjustment, and reduce water rinsing requirements (ref. 43).
3.4.4.5 Other Equipment/Design Considerations
In addition to the basic ion exchange column, auxiliary equipment is employed for various purposes, among which include: resin bed channeling and fouling prevention; pH adjustment of the feed stream; solution pump and flow control; need for regeneration identification; and regeneration cycle control.
Pretreatment of the feed stream is usually performed. Filtration is a basic requirement for nearly all ion exchange applications. If solids are permitted to enter the ion exchange bed, they will often create an uneven film on the top of the bed that acts as a plug. The solids will impede flow and cause channeling through the bed. Channeling of the feed solution will result in incomplete usage of the bed and inefficient processing (ref. 349). Most commonly, cartridge filtration is used for this purpose. Multimedia filters are sometimes used in high flow applications, where changing of the cartridge filters would be too time consuming (ref. 348). Other types of pretreatment include pH adjustment and carbon filtration. The adjustment of pH is used for certain applications where resin capacity can be enhanced by increasing or lowering the pH. Carbon filtration is used to remove certain organics such as oils that can become irreversibly sorbed by ion exchange resins and oxidants such as peroxide that can oxidize and ruin the resins (ref. 348).
The means for identifying the point at which regeneration should be initiated varies among commercially available equipment. The methods employed depend on the overall design of the system (e.g., a lead/lag unit may be able to tolerate some ion leakage from the first column whereas a single column may not), the tolerable leakage concentration, the variability of the feedstream, the ion(s) of concern, and the solution chemistry. Some equipment uses direct measurement methods to identify breakthrough. Examples of applicable methods and instruments include: conductivity meters, sometimes used in conjunction with pH meters; specific ion probes; and colorimetric analytical methods. A different strategy is to regenerate a column based on elapsed time or flow. These latter methods are applicable to feed streams with relatively constant parameters.
Other design features of ion exchange systems, such as controls, are discussed in Section 3.4.1.6.
This subsection contains a description of commercially available ion exchange equipment that is manufactured and/or sold by vendor survey respondents. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.
Kinetico Engineered Systems, Inc. (Kinetico) manufactures various ion exchange equipment including that applicable to chemical recovery, bath maintenance, raw water treatment, and end-of-pipe polishing. Since 1980, they have sold more than 300 units. The Kinetico Complete Ion Exchange (CIX) series is applicable to metals recovery and water recycling with configurations similar to that shown in Exhibit 3-22 (applications IX-1 and IX-2). The CIX series are integrated packaged units with standard sizes ranging from 30 to 500 gpm. All units are dual column design for continuous operation (lead/lag configuration) with counterflow regeneration. Regeneration is initiated automatically by a patented control valve that monitors flow or by a manual override (push button). The control valve, which is non-electric, also directs the flow of solution for the regeneration cycle. Power for the valve comes from the flow and pressure of the water.
Kinetic Recovery Corporation offers modular ion exchange and regeneration systems for metals recovery and water recycling (see Exhibit 3-22, applications IX-1 and IX-2) as well as bath maintenance and end-of-pipe polishing. The modular ion exchange systems, which are dual column, are packaged with a cartridge filter system, flow meter, level sensor, feed pump, valve assembly, conductivity monitoring system and pump controls. The separate regeneration system (Exhibit 3-27) is capable of regenerating cation and anion exchange resins simultaneously. It consists of two chemical feed pumps, a valve assembly/control board with electrically actuated ball valves, flow meter and globe valve and a regeneration controller with a PLC and an operator interface. The valve assembly board allows the adjustment of the ion exchange into several modes of operation (parallel, series) and regeneration. With the operator interface unit, the regeneration mode can be selected and all operating conditions (mode, time elapsed, alarm condition) are displayed.
Memtek Corporation manufactures integrated ion exchange equipment for metal recovery and water recycling and end-of-pipe treatment. Their metals recovery/water recycle series (Rinse water Maintenance System or RMSô) is a dual column (lead/lag) packaged system with prefiltration, conductivity monitors and alarms, regenerant make-up tanks with level controls, and microprocessor controller. Memtek markets specific ion exchange systems for electroless copper and lead applications (see Exhibit 3-28). These two system include a preconditioning pH adjustment step.
Ionics International Ltd. (i3) offers both a semi-automatic integrated ion exchange system and a modular system with separate ion exchange and regeneration stations. Both the integrated and modular ion exchange systems are dual column (lead/lag) systems used for metals recovery and water recycle. The integrated systems typically include two anion and two cation columns containing approximately 6 ft3 of resin per column. Regeneration is manually initiated and automatically controlled. The modular systems are hard piped to the regeneration station rather than transported. The modular units typically permit a service flow rate of 10 gpm.