Waste Treatment

Before choosing a wastewater treatment system for mechanical plating, engineering and analytical data must be gathered. The preliminary work should determine:

• Treatment options for the metals in solution

• Present and future flow rates, and batch collection volume requirements

• Water conservation possibilities

• What is required to meet discharge limits

• Availability and type of treatment chemicals

• How sludge will be dewatered, dried, and disposed

In most cases, removal of heavy metals from mechanical plating effluent is accomplished by precipitating metal hydroxides. This is done routinely as long as the metal ions in the effluent are not complexed and the pH is high enough to ensure optimum precipitation. Zinc, for example, reaches minimum solubility at a pH between 9 and 10. A pH of 9.5 provides satisfactory precipitation in about 30 min. Although other metals reach minimum solubility at different pH levels, a pH of 9 to 10 will precipitate other residual metals associated with the process, such as copper, tin, iron, cadmium, and aluminum (but not the chromium present in certain post-treatment chromates). The chemical most commonly used in precipitation is 50% liquid sodium hydroxide. Magnesium hydroxide and lime slurry also can be used as a source of alkalinity. Lime tends to be less expensive but increases the precipitation time.

Chromium presents a special problem when present as a chromate (hexavalent state). Hexavalent chromium must first be reduced to the trivalent state before standard precipitation can proceed. Sodium metabisulfite or ferrous sulfate added at a pH of about 2 will reduce the hexavalent chromium and allow the resultant trivalent chromium to be combined with other wastes for regular precipitation.

Most mechanical plating systems capable of plating 1000 kg (2200 lb) of parts will discharge 22 to 45 1/min (6 to 12 gal/min) of solution; therefore, a batch system with a minimum of two collection tanks should be satisfactory. Each tank should be capable of holding the effluent from one shift, allowing one to fill while the other is being treated.

Flocculation is an important consideration. After pH adjustment of the effluent, it should flow to a tank for flocculation of the suspended metal hydroxides. Addition of a polyelectrolyte causes the small particles to agglomerate into larger, heavier particles, with improved settling characteristics.

After being flocculated and settled, the suspended solids enter a sludge dewatering device such as a filter press. Sludge drying further reduces the volume (by up to 75%) of the solid wastes released from the filter press. A standard mechanical plating and galvanizing system will produce approximately 0.028 m3 (1 ft3) of dewatered and dried sludge per 1000 to 2000 kg (2200 to 4400 lb) of plated parts.

Surface Preparation for Continuously Applied Coatings

Curtiss Dunbar, LTV Steel Company

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