Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 88 of 707

Fig. 6. Recovery rinsing. wasted plating solution, we needed to drastically increase the rinsewater flow to hold the same 0.1% concentration in the final rinse. This illustrates two points: first, that some of the rinsing goals may be mutually exclusive; second, that it will usu- ally require more than two tanks to effect an efficient recovery and rinsing oper- ation. PERFECT MIXING The rinsing equations discussed above assume perfect mixing, meaning that the surface film of process solution has completely diffused into the general rinse- water, forming a homogeneous solution. If insufficient rinse time is allowed, this may not always be the real-world situation; nor may it be possible and practical to leave the work in the rinse tank long enough for this diffusion reaction to reach completion. Agitation methods that allow convection effects to hasten the diffusion will improve rinsing efficiency. Air agitation, cathode rocker agitation, or other mix- ing methods may help achieve this end (see chapter elsewhere in this Guidebook on solution agitation and mixing). An easy method that is effective in improving rinsing efficiency is "double-dip- ping," simply immersing the work in the rinse, removing it, and reimmersing. This has been demonstrated to be effective with racked work, and would be expected to be even more beneficial in barrel plating. 87

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