Waste Metal Processing

The ion-exchange process using ion-exchange resins is well known, and complete process designs have been developed for many processes including softening brackish waters, condensate polishing, and processing ultrapure water used in the semiconductor industry. Processes for metal recovery have also been developed, but the information is still far from complete.

A complete analysis of a waste stream must be available before any process considerations can be made. An understanding of the solution chemistry for a metal is also required before a test program can be started. Copper is a common waste metal and is an example of a divalent metal. Copper is usually found as a divalent cation having good affinity for cation exchange resins. It also forms complexes with amines. It also occurs as an anion when complexed with EDTA or other chelating agents.

A waste liquor with copper, calcium, and sodium shows that the capacity is reduced by the presence of divalent cations other than copper. A selective resin based on EDTA removes copper from high concentrations of sodium chloride and to a lesser extent from calcium chloride. Cyanide complexes form stable complexes with strong base resins, and the metals can be recovered after the complex is destroyed with acid. In process engineering, environmental engineers should avoid such reactions unless they make provision for handling free hydrogen cyanide.

An experimental program to develop process information is assisted by reference-to-affinity orders that are available for most resins. The affinity series helps to establish

Inlet distributor

Rubber lining

Regenerant inlet nozzle

Regenerant distributor

Service inlet

Inlet distributor

Rubber lining

Regenerant inlet nozzle

Regenerant distributor

Seat plate

Service collector-

laterals

Support plate

Service outlet backwash inlet

FIG. 7.36.11 Internal assembly of an ion-exchange column. (Reprinted, with permission, from Bolto and Pawkowski, 1982, Waste water treatment by ion exchange.)

Seat plate

Service collector-

laterals

Support plate

AuniriJi^

Service outlet backwash inlet

FIG. 7.36.11 Internal assembly of an ion-exchange column. (Reprinted, with permission, from Bolto and Pawkowski, 1982, Waste water treatment by ion exchange.)

the likely displacement of metals when they are present as the metals, but does not give the potential for complex formations particularly when chlorides are involved. For example, Fe3+ forms a complex with chlorides in high concentration so that iron can be removed from concentrated hydrochloric acid as an anion, FeCl—. By dilution, the complex is destroyed, and the iron comes off the resin bed. Other cations including aluminum can also be removed from a concentrated solution. Amphoteric elements also become anions at elevated pH values.

Since there are several possibilities in a mixture of metals in a waste solution, environmental engineers can establish the presence of metals, cations, or anions by performing a single screening test. In this test, they prepare two small columns of anion- and cation-exchange resins. The anion-exchange resins should be in hydroxide form, and the cation exchange resins should be in acid form. The waste and the effluent sample for the metals should be fed into the columns at about 10 bed volumes per hr. Environmental engineers must be careful not to generate cyanide gas.

Once the valence of the metal is established, environmental engineers should start process estimates. Destroying metal complexes before the ion exchange resin is used may be useful. The amount of competing ions should be reduced if possible. If rinse water is deionized prior to the process, economic saving in the recycling of waste rinse waters and the recovery of valuable metals can be achieved.

When precipitation of toxic metals is the preferred process, environmental engineers usually use ion-exchange resins to polish the effluent to meet regulatory requirements. This process is a good place to use chelation resins that can pick up metals with a large salts background. However, precipitation can cause colloidal metal formation which is not easily removed by resins. Adjusting the pH and retention time can prevent this problem.

Metal recovery can be accomplished by electrolysis of the spent regenerant. Since ion exchange only concentrates

Service inlet

FIG. 7.36.12 Simplified ion-exchange operations cycle. The water used for backwash, dilution water, or displacement rinse can be feed water, softened water, de-cationized water, or DI water depending on the ion-exchange resin used and the quality of water produced in the service cycle. (Reprinted, with permission, from Dean Owens, 1985, Principles of ion exchange and ion exchange water treatment, 89.)

FIG. 7.36.12 Simplified ion-exchange operations cycle. The water used for backwash, dilution water, or displacement rinse can be feed water, softened water, de-cationized water, or DI water depending on the ion-exchange resin used and the quality of water produced in the service cycle. (Reprinted, with permission, from Dean Owens, 1985, Principles of ion exchange and ion exchange water treatment, 89.)

trace metals, regeneration conditions are important for economical recovery. For example, using sulfuric acid strips the cation exchange of copper so that electrolysis can be achieved without generating chlorine if chlorides are present.

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