Metal Finishing Guide Book

2013

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less bath use. In other cases, the use of specialty chemical precipitants, metered into the complexed waste stream or into the neutralization tank, is suitable and effective. Specialty chemical precipitants include dithiocarbamates, dithiocarbonates, starch and cellulose xanthates, poly quaternary amines, and ozone destruction/hydrosulfite reduction. As complexing chemicals are primary reasons for noncompliance in conventional systems, much care and time are necessary to solve the problems created by them. Often significant trial testing in bench scale treatability tests and close work with chemical suppliers are necessary to resolve complexing problems. In some cases involving simple complexed wastewaters, conversion from hydroxide precipitation to sulfide or carbonate precipitation in the neutralization process will achieve necessary reductions in metal solubility. Most metallic sulfides and metallic carbonates have lower solubilities than their hydroxide counterparts. Reaction times required for effective coagulation-neutralization-precipitation vary among wastewater types and complexity. We recommend minimum retention times of 30 minutes, 15 minutes in first-stage reactors. As metal hydroxides tend to reduce in volume the longer they are mixed, the longest practical reaction times are most desirable. Common problems associated with neutralization/reaction tanks, which impede clarifier separation of solids, include soluble complexes caused by chelating agents; charge reversal caused by anionic surfactants, phosphates, and MBS overfeed; solids buoyancy or flotation problems caused by excess oil and grease or gas formation including chemical gassing caused by peroxides, acetates, and carbonates or physical-induced gassing caused by suction leaks on transfer pumps, or significant mixer vortex action; overfeed of dump solutions, particularly alkaline cleaners; and high total dissolved solids (TDS), 7,000 ppm and higher, from overly zealous water conservation practices, or high percentage reuse of treated water. FLOCCULATION/CLARIFICATION PROCESSES The precipitates formed by the proper operation of the coagulation-neutralization stage are commonly removed in conventional wastewater treatment systems by clarification or sedimentation. This process involves solids removal by the efficient settling of solids. Buoyancy caused by oils or floating caused by the entrainment of gas bubbles will prevent efficient settling. Generally, floating problems are controllable in the typical metal finishing wastewater installation. For certain firms, which employ electrolytic/electrochemical pretreatment or ozone generation/air diffusing treatment techniques, dissolved air flotation (DAF) is the preferred unit for solids separation. Solids separation is improved in clarifiers, or DAF units, by polymer (polyelectrolyte) flocculation. As the average charge of metal hydroxides is positive, a negatively charged (anionic) polymer is used in the flocculation process. It is imperative that the wastewater charge remain positive at all times. Coagulants and/or cationic polymers may be necessary in certain wastewater types where charge reversal is common, as in phosphating operations. Nominal flocculation time of 1 minute is recommended for floc tank size. Variable speed mixers are recommended to allow some measure of control of floc size. The size of the clarifier generally varies with the type and style. Basic, open/ empty sedimentation tanks commonly used in low-flow installations should 591

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