In this stage, the system neutralizes wastewater to promote additional oxidation as well as to meet the discharge pH limits. Sulfuric acid is typically used to lower pH to ap proximately 8.5, where the second oxidation occurs more rapidly. Acid addition must have a fail-safe design because below neutrality (pH = 7), highly toxic hydrogen cyanide can be generated if first-stage oxidation has not been completed.
The system adds hypochlorite either in proportion to that added in the first stage or by separate ORP control to complete the oxidation to sodium bicarbonate (NaHCO3) as follows:
ORP control in the second stage is similar to that in the first except that the control point is near + 600 mV. In the second stage, pH control is more difficult than in the first because the control point is closer to the sensitive region of neutrality. The pH controller can be proportional only.
A subsequent settling tank or filter can remove suspended metal hydroxides although further treatment may be required.
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