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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.4 ION EXCHANGE
3.4.3 Applications and Restrictions
Ion exchange is used for a variety of purposes in the metal finishing shop, including: treatment of raw water; recovery of plating chemicals from rinse water; purification of plating solutions; wastewater treatment; and wastewater polishing. The following discussion of applications and restrictions focuses on use of this technology for chemical recovery from rinse water. The other applications, with the exception of raw water purification, are discussed in other interim reports.
Ion exchange is a useful technology for recovering plating chemicals from dilute rinse waters. Two common configurations are shown in Exhibit 3-22, application IX-1 is referred to as metal scavenging. It uses only one type of ion exchange resin, either anion or cation, depending on the charge of metal or metal complex being recovered. 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. In AE-2, both anion and cation resins are employed and the rinse water can be recirculated in a closed loop. With both of these configurations, rinse water containing a dilute concentration of plating chemicals is passed through an anion and/or cation column (or dual columns of the same type) and the metals are removed from the rinse water and held by the ion exchange resin. When the capacity of the unit is reached, the resin is regenerated and the metals are concentrated into a manageable volume of solution. Depending on the chemical nature of the process, the regenerant (eluate) solution can be returned directly to the plating tank for reuse, further processed and returned, or the metals can be recovered by another technology such as electrowinning (see Section 3.5). The most common applications for these configurations of ion exchange are with the recovery of copper, nickel, and precious metals.
Application IX-3 (Exhibit 3-23) is a bath maintenance configuration that is only applicable to chromic acid solutions. The chromium bearing rinse water is passed through a cation column to remove trivalent chromium, and tramp metals such as iron, nickel and aluminum and is then used as make-up for evaporation in the bath. This application prevents the build-up of contaminants in the bath, a process that is hastened by recovery rinsing. Since hexavalent chromium is an anion, it is not removed by the cation resin.
Drag-out recovery tanks (see Section 2) are used with ion exchange systems whenever feasible. In operation, the drag-out tanks return the bulk of the plating chemicals to the plating bath and an ion exchange column captures only the residual quantities of chemicals. The needed size of the ion exchange unit and its regeneration frequency are therefore reduced.
Some of the respondents to the Users Survey misapplied the ion exchange technology for chemical recovery. For example, PS 261, operates a Watts nickel plating line with a 140°F bath and a four stage counter flow rinse. This shop feeds the ion exchange system from the first rinse, which is the most concentrated, and returns the treated water to the last rinse. Instead, this shop should utilize the configuration shown in Exhibit 3-22. Application IX-1. The use of a recovery rinse will greatly reduce the nickel load on the ion exchange columns.
As a recovery technology, ion exchange should be applied to dilute rinse waters. It is not applicable to concentrated drag-out solutions or plating baths (although it can be used as a bath maintenance technology to remove tramp metals, see Section 4). A major limitation of this process is that many plating baths are more concentrated than the ion exchange regenerant. Therefore, it should not be used in a "bleed and feed" system, where spent bath is bled to the rinse water. The result in these cases is that the chemicals are diluted in the rinse water, collected by ion exchange, regenerated (using costly chemicals), and recovered in a lower concentration than they started (ref. 40).
Ion exchange is applicable to a wide range of plating processes. Exhibit 3-24 shows the applications identified by the Users and Vendors Surveys.
Ion exchange is used much more frequently for metal recovery from non-cyanide solutions than for cyanide solutions (e.g., cadmium cyanide, copper cyanide, zinc cyanide), with the exception of gold cyanide. When applied to cyanide solutions, the task of recovery is more difficult due to the nature of the cyanide complex. As explained by Reinhard (ref. 342), when a cation followed by an anion column arrangement is used, the cyanide complex in the rinse water is decomposed into free metal cations and cyanide anions and these cations and anions are exchanged for H+ and OH- ions in the corresponding resin beds. When the cation resin bed becomes exhausted there is insufficient acidity left to decompose the cyanide complex which is an anion and it will be exchanged for an OH- ion in the anion resin bed. This presents a significant problem since the cyanide complex will accumulate in the anion resin and because of its high affinity to the resin matrix, regeneration will not remove it. If this occurs, the resin may require replacement. This problem is not a concern with gold cyanide applications because in most cases the resins are incinerated during the recovery process, rather than regenerated. Incineration of spent gold resin is an economically acceptable practice because of the relatively small quantity of resin used and its low cost in comparison to the gold contained on the resin.
Another operational problem with cyanide solutions can be caused by excessive free cyanide in the rinse water. If this condition exists, the free cyanide will remove the heavy metal from the cation resin and form a new metal-cyanide complex.