user manual

Appendices
206 UDA2182 Universal Dual Analyzer Product Manual January 2009
Importance of pH control
As shown in Figure 15-4, pH has a direct effect on the ORP potential and must be closely
controlled to achieve consistent ORP control, especially if hypochlorite is used as the
oxidizing agent. Hypochlorite raises pH, which lowers the ORP potential, which in turn
calls for additional hypochlorite -- a runaway situation. To avoid this situation, use close
pH control and locate the ORP electrode at a distance from the hypochlorite addition
point.
Reliable measurement with gold electrode
For this application, a gold ORP electrode gives a more reliable measurement than does a
platinum electrode, because platinum may catalyze additional reactions at its surface and
is more subject to coating than gold. Note that the solubility of gold in cyanide solutions
does not present a problem as it is in contact, primarily, with cyanide. In fact, a slight loss
of gold serves to keep the electrode clean.
Second Stage of Cyanide Destruction
Neutralize and further oxidize cyanate
The wastewater is neutralized in order to promote additional oxidation and to meet the
discharge pH limits. Typically, sulfuric acid is added to lower the pH to about 8.5. At this
pH the second oxidation occurs more rapidly.
WARNING
Failure to comply with these instructions could result in death or serious
injury.
An interlock must be provided to prevent the addition of acid before the
positive oxidation of ALL cyanide. Failure to observe this precaution can
result in the generation of highly toxic hydrogen cyanide.
Additional chlorine or sodium hypochlorite (NaOCl) can be added in proportion to that
added in the first stage, or by separate ORP control to complete the oxidation to sodium
bicarbonate (NaHCO
3
) in the following reaction:
23 2 3
232
NaCNO NaOCl H O NaHCO N NaCl+
+
→
+
+
ORP control in the second stage is very similar to that in the first stage, except that the
control point is near +600 mV. Control of pH in the second stage is more difficult than in
the first stage, because the control point is closer to the sensitive neutral area.
Proportional type pH control is often used.
Removal of suspended metal hydroxides
Following the second stage, a settling tank and/or a filter can be used to remove
suspended metal hydroxides. However, further treatment may be required to lower
concentrations of some metals below their hydroxide solubilities.