Sulfur dioxide Chromic acid Chromic sulfate Water
Most hexavalent chrome wastes are acid, but the reaction rate is much faster at low pH values. For this reason, pH control is essential. Sulfuric acid is preferred because it is less expensive than other mineral acids. The setpoint of the pH controller is approximately 2.
As in the treatment of cyanide, the chemical reaction is not reversible, and the control of sulfur dioxide addition is by electrode potential level, using ORP instrumentation. The potential level of hexavalent chromium is +700 to +1000 mV, whereas that of the reduced trivalent chrome is +200 to +400 mV. The setpoint on the ORP controller is approximately +300 mV.
The control system consists of feedback loops for both pH and ORP. The common and preferred design is for fixed-flow systems.
The system removes the trivalent chromic sulfate from solution by subsequently raising the pH to 8.5, at which point it precipitates as chromic hydroxide. The control system for this step can be identical with the one used in Figure 7.42.11. Table 7.42.2 summarizes the critical process control factors.
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