Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 554 of 707

Fig. 9. General schematic for nonrecovery or indirect recovery methods. rejected because of their positive electrical charge. The mechanism of free acid dif- fusion through the membrane, due to the concentration difference between the free acid and DI water, is known as Donnan diffusion. Multiple layers of membrane are arranged in a filter-press-like stack through which both DI water and spent acid flow by gravity. Clean acid is separated from the feed stream by the concentration-driven transport mechanism across the membrane stack to effect a partition and recovery of an acid stream (diffusate) in conjunction with the generation and discharge of a waste stream (dialysate). DD is being utilized for the following applications: recycle of hydrofluo- ric/nitric acids for etching stainless steel; recovery of sulfuric/nitric and sulfu- ric/hydrochloric acids for etching nonferrous metal; reclamation of sulfuric and hydrochloric acids for etching of steel-based materials; recuperation of sul- furic acid from anodizing processes; and regeneration of battery acids. On the positive side DD is a low-energy, low-pressure, continuous process that requires no additional reagent or regeneration chemicals, resulting in less TDS in the plant discharges. On the negative side, for every volume of acid recovered (diffusate), an equal volume of acidic waste (dialysate) is generated for further processing for recov- ery or for waste treatment. While the recovered, clean acid is generally reusable, the operating principle imposes a limit to the achievable concentration for the recovered acid, which can be fortified with concentrated acid as required. Typical maintenance procedures for DD systems include: filtration of the feed stream to remove total suspended solids and to avoid deposition of suspended solids on the membranes; temperature regulation of the feed liquor and DI water supply within a prescribed temperature range to main- tain recovery efficiency; and protection of the membranes against exposure to oxidizing agents such as chromic and nitric acids and to organic sol- vents, lubricants, inhibitors and surfactants. With efficient feed filtration, membrane cleaning is generally required approx- imately twice per year. With observance of the above operating and mainte- nance practices, experience indicates membrane life can be about 5 years. 553

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