Wastes from a metal-finishing plant originate in at least three principal ways; rinsewaters are discharged, spray fom chromium plating mayfind its way into the atmosphere through exhaust fans; cleaning regularly, and plating solutions occasionally, must be dispersed of when it is no longer possible to adjust them to operable limits.
The first step in waste treatment is to decrease, to the extent posible, the need for it. This entails efficient rinsing, installation of drain stations ahead of all tanks, use of spray rinses, and use of plating and other solutions in as dilute a form as possible.
In spite of many efforts to dispense with cyanide-containing plating baths because of environmental restrictions, no practical substitute for the cyanide copper bath has been developed. The cyanide-containing compounds are oxidized by chlorine to remove it.
Oxidation of cyanide ion by chlorine is generally regarded as taking place in three steps.
1. CN- + Cl2 ® CNCl + Cl-
2. CNCL + 2OH- ® CNO- + Cl- + H2O
3. 2CNO- + 4OH- + 3Cl2 ® 6Cl- + 2CO2 + 2H2O + N2
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