Neutralizing pH

Q: I am our facility’s environmental, health and safety manager. As part of our wastewater pretreatment system, we have a single-stage, flow-through cyanide oxidation step before the wastewater mixes with the other waste streams for pH adjustment and metals removal. Our cyanide oxidation has a bleach and caustic feed controlled by ORP and pH controllers, respectively. Periodically, we have very high-pH wastewaters that cause the pH within the cyanide oxidation step to be well above our desired upper limit of 11; sometimes the incoming wastewater has pH above 12.5.

This high pH causes our ORP reading to decrease enough to add excessive amounts of bleach into the wastewater. While this bleach is more than enough to treat the cyanide, it creates problems downstream when the waste streams mix. Our operator is afraid to raise the ORP set point since it works fine during normal flow conditions.

Since the controller has the capability, and we already use concentrated sulfuric acid for the pH adjustment stage, our operator has requested that we add sulfuric acid into the cyanide oxidation stage whenever the pH rises above 11. I am very concerned about the release of hydrogen cyanide if something goes wrong and the system accidentally drives the pH well below 7.
Are there any safer acid substitutes? J.W.

 

A: J.W., you are not the first to express concern about using concentrated sulfuric acid in cyanide treatment; however, it has been used safely in this application for many decades. If you do decide to proceed with sulfuric acid, I would strongly recommend that you cover the reactor tank and install a small exhaust to the outside, discharging as high as possible above the roof. You will not need much flow rate, maybe 50–100 cfm, just enough to keep a slight negative pressure in the headspace between the wastewater and cover as well as enough velocity when the access door is opened.

Another consideration is the use of two pH sensors; if the difference between the two readings exceeds a preset limit, an alarm is sounded and the acid feed shut down.

Yes, there are alternatives, but they are very expensive to implement. Some people have recommended the use of saturated sodium bicarbonate (baking soda) which has a pH in the 8s, however, the massive quantities needed for a flow-through system in order to drive the pH from 12.5 to 11, and its slow reaction time at the high pH you are experiencing, makes it infeasible.
Another “safer acid” is carbonic acid, H2 CO3 , formed by dissolving carbon dioxide, CO2 , into water. In your application, the best way to dissolve CO2 into the waste stream is to pump the wastewater through a sidestream pipe, inject the carbon dioxide through a diffuser in the pipe under pressure, and recirculate the wastewater back to the treatment tank. The carbon dioxide can be supplied from 50-lb high-pressure cylinders or 250-lb low-pressure liquefied cylinders. At the high pH that you described, the carbon dioxide or carbonic acid will react quite quickly, and 1/2–3/4 lb of carbon dioxide has the same neutralizing power as 1 lb of sulfuric acid.

Another advantage of carbon dioxide is that, theoretically at least, it cannot drive the pH lower than about 5.7. Again, the disadvantage is the initial high equipment cost for the carbon dioxide storage, feed, and injection system, especially for such a small application.

Some can argue for the use of phosphoric acid, since it behaves more like a weak acid as the pH decreases, however, it still is powerful enough to drive the pH very low to generate hydrogen cyanide.

I was thinking that you could try a dilute solution of acetic acid, a weak acid, however, I am not quite sure of its reaction with bleach or sodium hypochlorite since acetic acid is an organic compound. I am afraid that some odorous byproducts could be generated, at least.

Last, you can contact your chemical supplier to inquire if they have a proprietary “safe” acid product. One of our clients uses a boiler treatment chemical for their pH adjustment acid feed because of its safety; however, this product may still be able to drive the pH very low.

In summary, the challenge here is to use an acidic material that is powerful enough to neutralize the alkalinity in the wastewater fairly efficiently and effectively, but not have the capability to dangerously lower the pH. Based on my experience, carbon dioxide is the best to meet this criteria, but is likely cost prohibitive in your application. If any reader has an additional experience to share, please e-mail me.