Table 7423 Chrome Reduction And Precipitation Reactions

Reducing Agent

Reaction

Ferrous sulfate (FeSOJ

Sodium metabisulfite (Na2S2O5)

Sulfur dioxide (SO2)

2Cr(OH)3 + 3CaSO4

2Cr(OH)3 + 3CaSO4

2Cr(OH)3 + 3CaSO4

Na2S2O5 + H2O ® 2NaHSO3

As shown in Figure 7.42.9, the system monitors and controls the first-stage reaction with independent control loops: acid addition by pH control and reducing agent addition by ORP control. The system adds acid under pH control whenever the pH rises above 2.5; it adds the reducing agent under ORP control whenever the ORP rises above approximately +250 mV.

The ORP titration curve (see Figure 7.42.10) shows the mV range covered when Cr+6 chrome is treated in batches. With continuous treatment, however, operation is main tained in the completely reduced portion of the curve near the nominal +250 mV control point. The exact setpoint for an installation should be at a potential where all the Cr+6 is reduced but without excess sulfite consumption, which is accompanied by sulfur dioxide odor.

Chrome reduction is slow enough that 10 to 15 min can be required to complete the reaction. The reaction time increases if the pH is controlled at higher levels. Variations in pH also affect measured ORP readings; thus, the pH must be stable for consistent ORP control.

FIG. 7.42.9 Continuous chrome treatment.
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