Honeywell UDA2182 manual First Stage of Chrome Removal, Lower pH and add reducing agent

Appendices

First Stage of Chrome Removal

Lower pH and add reducing agent

Sulfuric acid is used to lower the pH to about 2.5, which promotes the reduction reaction and ensures complete treatment. The reducing agent may be sulfur dioxide, sodium sulfite, sodium bisulfite, sodium metabisulfite, sodium hydrosulfite, or ferrous sulfate. The reaction is given below. The chromate is expressed as chromic acid, CrO3, with a +6 charge on the chromium and the reducing agent is expressed as sulfurous acid, H2SO3, which is generated by sulfites at low pH. The result is chromium sulfate, Cr2(SO4)3, with a +3 charge on the chromium. The reaction is expressed as:

2CrO3 + 3H2SO3 → Cr2 (SO4 )3 + 3H2O

This first stage reaction is analyzed and controlled by independent control loops: acid addition by pH control; reducing-agent addition by redox potential or ORP (oxidation- reduction potential) control. Often an ON-OFF type of control using solenoid valves or metering pumps can be used. The pH controller simply calls for additional acid whenever the pH rises above 2.5. The ORP controller calls for additional reducing agent whenever the ORP potential rises above about +250 mV. (The metal ORP electrode is positive with respect to the reference electrode.)

Titration curve

The ORP titration curve in Figure 15-6 shows the entire millivolt range if Cr+6 chrome is treated as a batch. With continuous treatment, operation is maintained in the fully reduced portion of the curve near the +250 mV setpoint. The ORP setpoint can vary between installations, depending on pH, reducing agent, presence of additional contaminants and dissolved oxygen, and the type of reference electrode used. Determine the exact setpoint empirically. This ORP setpoint should be at a potential where all of the Cr+6 has been reduced without excess sulfite consumption, which can release sulfur dioxide gas. This point can be verified with a sensitive colorimetric test kit or similar check.

REDOX POTENTIAL (mV)

700

600

pH= 2

500

400

pH= 3

300

200

100

0	1	2	3	4	5	6	7
			VOLUME OF BISULFATE ADDED

Figure 15-6 Chrome Reduction - Typical Titration Curve

January 2009 UDA2182 Universal Dual Analyzer Product Manual 209