Recovery and Disposal of Wastes

Chromic acid wastes may be either recovered or destroyed, and the choice of process should be based on a comparison of initial costs, labor costs for operation and maintenance, chemical costs, space requirements, and utility costs. The volume of wastes and the value of the chromic acid and rinse water saved can greatly influence the choice.

Local, state, and federal authorities are constantly increasing their attention to antipollution programs. Strict regulations are being enforced regarding the allowable limits for chromic acid wastes that leave the plating plant in any form. The prevailing limits for chromic acid contamination of waste water range from about 0.05 to 5 ppm. These limits vary for each locality, depending on the uses of the receiving body of water, supplementary water flows that affect dilution, and the ability of sewage plants to handle wastes. A new plater must check with federal, state, and local authorities to determine what limits are applicable at that time.

Preventive Measures. The problem of waste disposal can be greatly minimized if suitable measures are taken to minimize the amount of wastes produced. The following practices contribute to minimizing wastes:

• Extend drainage periods to permit more solution to return to the tank. In hand operations, this is made possible by providing a drainage bar over the tank to hold racks.

• Provide drip boards to return solution lost when going from tank to tank.

• If possible, rack parts in such a way as to eliminate cupping action.

• Use reclaim rinse tanks. The rinse solution can be used to maintain the level of liquid in the processing tank. Sometimes concentration methods may be profitable to facilitate use of rinse waters.

• Control drag-in of water to permit use of reclaim rinse tanks.

• Complete recovery should be used only in conjunction with technologies for the removal of metallic impurities.

Disposal of chromic acid wastes is most commonly based on reduction of hexavalent chromium to the trivalent form and, in either a batch or a continuous operation, precipitating the trivalent metal hydroxide by means of an alkali. The actual chemicals used vary from locality to locality, depending on cost and availability. Chromic acid is first neutralized to a suitable pH and is then reduced with one of the sulfite compounds (sodium sulfite, sodium metabisulfite), sulfur dioxide, ferrous sulfate, iron, copper, or brass. After completion of reduction, trivalent chromium is precipitated as hydroxide with alkali. The amount of chemicals required to complete reduction can be governed by laboratory analysis, or, because the reaction is solely one of oxidation-reduction, it may be controlled automatically by use of electrodes.

The most commonly used reducing agent for large plants is sulfur dioxide gas. It can be obtained in liquid form in cylinders of various sizes, is comparatively inexpensive, and can be fed directly into the treatment tank. The rate of addition is easily controlled and gas is delivered from the cylinder under its own pressure. A lower initial acidity is required because the gas forms sulfurous acid when dissolved in water. The operating pH is 2 to 3, and the ratio of sulfur dioxide to chromic acid used commercially is slightly under 3 to 1. The sulfur dioxide method lends itself readily to an automatic system because the gas feed can be controlled by a flowmeter, and the reaction can be controlled by oxidation-reduction potentials.

Ferrous sulfate also is a widely used reducing chemical, especially in localities where large quantities are available from pickling plants. The quantity required can be easily determined by titration. The ratio of ferrous sulfate to chromic acid varies between 5 and 16 to 1. Reduction of chromium is followed by neutralization with lime or caustic. Above a pH of 7, the metals precipitate as hydroxides, together with calcium sulfate. The main disadvantage of the ferrous sulfate method is the large volumes of sludge that have to be handled.

The sulfite-containing compounds generally are slightly more expensive than sulfur dioxide or ferrous sulfate. In addition, several difficulties are involved in sulfite treatment, such as solubility, loss of hydrogen sulfide through hydrolysis, slightly lower pH, and, occasionally, the need for additional treatment to complete the process.

Regardless of the chemical treatment selected, all chromic acid disposal systems require collection, treatment, and settling tanks. The operating procedure consists of chemical additions, mixing, separation of precipitated metal, clarification, and sludge disposal. Variations in equipment design affect economy, time and labor requirements, and equipment costs.

In recent years several new companies have been formed that recycle chromium plating wastes into new products, thereby avoiding the long-term potential liabilities of landfill operations. No matter what method of waste disposal is selected, the plater is well advised to know what happens to the wastes and what the liability could be.

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