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As the alkaline cleaner solution is removed from the parts, the concentration of alkali in Station 2 gradually increases. At some concentration, the alkalinity of the water dragged out of the rinse by the parts will have a serious neutralizing effect on the acidity of the descaling solution in Station 3. In order to minimize the effect of this drag-out, fresh make-up water must be fed to Station 2 on a continuous or regular intermittent basis. The volume of water required to establish satisfactory equilibrium must be determined experimentally. Estimates of the volume and flow rate of water required to establish equilibrium can be approximated by using a method described in the MCP process manual. Typical flow rates for rinse tanks range between 1 and 3 gallons of make-up water per rinse tank per minute of operation. The volume of water used in rinsing must be limited to avoid overwhelming the capacity of the waste treatment system. Some common methods of water volume control include the use of counterflows, spray rinses, drag-out tanks, conductivity instruments, flow orifices, intermittent valves, etc. Theoretically, the time required for rinsing is a function of the flow rate of make-up water and the initial and final concentrations of contaminants. The practical choice of a rinsing time may consider other observed factors, such as the clarity of the solution, the color of the rinse water, or the removal of floating foam, to estimate the equilibrium point. In general, alkaline cleaning solutions are difficult to remove completely by rinsing, and often require neutralization to ensure removal. The importance of rinsing to the success of a metal finishing operation should not be underestimated. Inadvertent mixing of solutions through drag-in of incompatible chemicals is a common source of quality problems in metal finishing. |
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