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Pollution Prevention and Control Technologies for Plating Operations

Section 4 - Chemical Solution Maintenance


4.5.1 Overview

Acid sorption is a purification technology applicable to dilute to moderately concentrated acid solutions such as anodizing and pickling baths. The term sorption, which includes both adsorbtion and absorption, is a general expression for a process in which a component moves from one phase to another, where it is accumulated, particularly for cases in which the second phase is a solid (ref. 435). Acid sorption is not a widely used technology by the metal finishing industry, although it has been commercially available in North America for approximately 15 years. Of the 318 plating shops responding to the Users Survey, only five (or 1.6%) have employed this technology. As an acid bath maintenance technology, acid sorption competes with diffusion dialysis (Section 4.8).

Acid sorption is one of several processes where resins are used to absorb chemicals present in surrounding solutions and the chemicals are subsequently desorbed with water. These reversible sorption processes include ion exclusion (cation resin), ion retardation (special resin), and acid retardation (anion resin). Of particular interest in metal finishing is acid retardation (ref. 340). This is a separation process where an acid is separated from its salts by using a column containing a strongly basic anion exchange resin of a specific porosity and particle size. This separation occurs because at high concentration the acid crosses the Donnan potential barrier (Donnan invasion) and is taken up by the resin, whereas the salts are excluded from it. The acid is thus "retarded" and the salts pass through the resin. This is not an ion exchange process, because the acid is desorbed from the resin with plain water.

The acid sorption or retardation process is employed to remove dissolved metal contaminants from acid baths. It is most often applied to the purification of sulfuric acid anodizing baths and sulfuric acid and hydrochloric acid pickling baths. When these solutions are contaminated with dissolved metal, the free acid concentration decreases and the anodizing or pickling efficiency drops. Additions of fresh acid are possible up to a point, but eventually, the bath must be either purified or dumped.

Diagrams of the acid sorption process are presented in Exhibit 4-13. These diagrams illustrate the equipment of a particular commercial acid sorption manufacturer. During the sorption step, the acid and metal salt mixture is fed up through the resin bed. Acid is sorbed into the resin while the remaining dissolved metal salts are rejected as mildly acidic solution leaving from the top of the bed. Depending on the metal salt, this solution may be waste-treated or diverted to an electrowinning cell for recovery of the metal. During the desorption step, water flows down through the resin bed. Acid is desorbed from the resin and displaced from the bottom of the bed. City water is typically adequate for this step. The resin is stable under normal operating conditions for many years without the need for regular replacement or any special treatment.

Acid sorption does not recover all of the acid in a treated bath. Rather, it recovers only a percentage (typically 80% to 90%) of the "unused" or free acid (i.e., that acid which is not chemically bonded the dissolved metal). Typically, 40% to 70% of the total acid is free acid. Therefore, if a shop's current method of operation involves dumping and treating spent acid baths and replacing the bath with fresh solution, then acid sorption can be expected to reduce their total acid usage by approximately 30% to 65% (ref. 363, 364).

In addition to reducing acid usage there are several benefits from using acid sorption. These include: (1) reduces neutralization treatment reagent usage (e.g., caustic or lime); (2) reduces interruptions in production (i.e., when used on a continuous basis as opposed to batch purification); and (3) reduces process control variability caused by fluctuations in bath composition (i.e., when used on a continuous basis).

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