Honeywell UDA2182 manual Second Stage of Chrome Removal, ORP Potential a Measure of Status

Appendices

Chrome reduction is slow enough that 10 to 15 minutes may be required for a complete reaction and this time increases if pH is controlled at higher levels. The pH also has a direct effect on the ORP potential as shown in Figure 15-6. Therefore, pH must be controlled to achieve consistent ORP control.

Second Stage of Chrome Removal

Neutralize the wastewater

In this stage the wastewater is neutralized to precipitate the Cr+3 as insoluble chromium hydroxide, Cr(OH)3. Another reason is to meet the discharge pH limits. Sodium hydroxide or lime is used to raise the pH to 7.5 to 8.5 in the following reaction.

Cr2(SO4 )3 + 6NaOH → 3Na2SO4 + 2Cr(OH)3

pH control point close to neutral point

Control of pH in the second stage is more difficult than in the first because the control point is in the sensitive area closer to the neutral point. Although this reaction is fast, for stability, a retention time of at least 10 minutes is usually needed for continuous treatment. Proportional pH control is often used in this stage.

Remove suspended chromium hydroxide

Subsequently, a settling tank and/or filter will remove the suspended chromium hydroxide. Flocculating agents are helpful in this separation.

Batch Treatment

Sequence of steps

Continuous treatment for chrome removal is shown in Figure 15-5. However, all of the reactions can be achieved with semi-automatic batch control. Only a single tank with a pH controller and an ORP controller are required. The steps of the treatment are sequenced, and the pH setpoint is changed to give the same results as for the continuous treatment. Acid is added to lower pH to 2.5; then reducing agent is added to lower ORP potential to +250 mV. After waiting a few minutes to ensure a complete reaction (and possible test for Cr+6), the sodium hydroxide is added to raise pH to 8 as in the second stage of the continuous treatment. The settling period then begins, or the batch is pumped to a separate settling tank or pond.

ORP Potential a Measure of Status

Sulfite is reducing ion

An oxidation-reduction reaction involves the transfer of the electrons from the reducing agent to the ion being reduced. In the chrome removal application, sulfur in the sulfite ion donates electrons to reduce the chromium; simultaneously the chromium oxidizes the sulfur. The ORP potential is a measure of the status of the oxidation-reduction reaction; the platinum or gold electrode detects the solution’s ability to accept or donate electrons. Sulfite (SO3+2), a reducing ion, donates electrons which makes the electrode more

210 UDA2182 Universal Dual Analyzer Product Manual January 2009