What can cause an inconsistent PH reading?

[ctroyp]

Please excuse me if I lack some information for Chemicals are not my profession. In our waste treatment area, we have two nuetralization tanks with, in each, one ORP controller and one PH probe. The purpose of these tanks is to prepare for removal of metals before entering city sewer. The first tank receives the used Nitric, Caustic, and rinse water solutions. These solutions are pumped from the etching line. The PH, in the first tank is to be maintained in the 3-3.5 range controling with raw nitric acid. The second tank is fed from the first and should maintain somewhere around a PH of 10 and is controlled by adding raw sodium hyroxide and lime. Well, this is where I am having the problem. The PH is fluctuating +/- 3 points. We also cannot seem to get good flock downstream--being the main issue since we cannot control our PH levels. The outputs of the ORP controllers are coming on as configured, but cannot get the PH in the range desired. I have calibrated the controllers and sensors multiple times. I have replaced the sensors, controllers and cables--then recalibrated. I am still having the same problem. I am pumping the same volume of the solutions into the tanks. Over the past several months we have noticed that we have been using more lime than usual. The operator states that this is because the 50/50 Caustic soda could not get the PH up to 10. Is there the possibility that the lime could be affecting the PH reading? Or, is there any other chemical that could affect the PH reading? I have also thought that the tanks are not mixing as needed--the sensor was not seeing the correction factor (Caustic soda and Lime/Nitric, respectively) before it exited the tank. If that was the case, we would never get to our desired PH levels. With that in mind, we have not changed the way we mix the tanks which make me focus more on the solutions entering the tank or the over-abundance of lime or something.

I apologize for the lengthy description, but I wanted to give the most info I could. If I need to add more, please let me know.

Thanks

 

[ctroyp]

Thought I would add that the solutions entering the system (used nitric acid, 50% sodium hyroxide, and rinse water) are used to clean the oxides from aluminum products.

 

[saxon]

ctroyp, There are any number of things that could be upsetting your ability to control pH. The first place to start is to develop the titration curve for the process. Ph is the dependent variable and the ratio of reagent to influent flow is the independent. Once you have established this, you can begin to understand the process and control it.

Also, I would strongly recommend you pickup a copy of "pH Control" by Greg McMillan from the ISA, (

http://www.isa.org).

In order to get into the details of your problem would require going through the entire text.

Sorry I couldn't give you a simple "Do this and you're done"
answer.
saxon

 

[ctroyp]

saxon,
Thank you for the reply. I understand what you are recommending I do and why. I need to educate myself a little more with the system it appears. I may purchase that book--thanks. Fortunately, I believe I have found our problem. I decided it would be best to clean out our neutralization tanks in hopes to get a fresh start. Well, when pumping out the tanks, we found that there was a lot of "sludge" that appears to be from the lime. As I stated before, the operator was using an excessive amount of lime for a long period of time. Since there was so much lime "sludge" in the tanks, the control loop was way out. The nitric acid was not mixing as needed to correct the ph. The system is running beatifully today. It all makes sense to me and hopefully I explained it well enough to you for feedback. Thanks again for you assistance.