In the second stage, the system neutralizes the wastewater to precipitate Cr+3 as insoluble chromium hydroxide, Cr(OH) 3, and to meet the discharge pH limits. The system uses sodium hydroxide or lime [Ca(OH)2] to raise the pH to 7.5 to 8.5, as shown in the following reaction:
In the second stage, providing pH control is more difficult than in the first because the control point is closer to the sensitive region near neutrality. Although the second-stage reaction is fast, a retention time of at least 10 min is usually needed for continuous treatment processes to achieve stable operation. The pH controller is proportional only in this stage.
A subsequent settling tank or filter removes the suspended chromium hydroxide. Flocculating agents can assist in this separation.
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