![]() ![]() In practice, hydroxide precipitation is generally carried out at the best compromised pH for metals in the waste stream, normally between 8.5 - 9.5. Such concentrations are obtainable for the individual metal at the corresponding optimum pH. Table II shows typical concentrations achievable by hydroxide precipitating agents. Often, post treatment polishing with scavengers such as dithiocarbamate or sulfide, both of which are noxious and toxic, is required in order to meet discharge standards. The graph indicates that there is no one ideal pH for a multiple-metal system. Theoretical hydroxide precipitation curves for metals normally found in plating baths are shown in Figure 1. However, this method does not ensure total compliance for the various metals present in the waste stream, since all metal hydroxides do not completely precipitate at a single pH. Of the few precipitation methods, hydroxide and sulfide are the two main methods currently used, and hydroxide precipitation is by far the most widely used method. Table I shows current Pretreatment Standards for Existing Sources (PSES) for Electroplating Effluent as well as standards under the proposed MP&M rule (Metal Products and Machinery Effluent Guideline).Ĭonventionally, precipitation has been the method most often used to remove heavy metals. With greater quantities of wastewater produced and discharge standards becoming increasingly more stringent, there is a need for more efficient and cost-effective methods for removing heavy metals. Registration Opens for Electroless Nickel Conference 2022 ![]()
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