Most pH sensors in alkaline Au process slurries need H+ washing (3% dilute HCl) every shift you will need to recalibrate the unit with appropriate buffers. You might attempt calling up an Orion vendor they may have new sensors capable of withstanding lime scale passivation.

We had a similar problem at Lihir Gold in PNG. There was so much lime added that it the extremely viscous and sticky. The pH meter clogged up continually and never worked properly. One solution is to add lime via a timer system (20 seconds on, 20 seconds off) with the on/off ratio coupled to the slurry flowrate (i.e. controlling at a kg/t lime addition assuming the ore mineralogy and process chemistry does not change too much in a short time) and keeping an eye on the downstream pH. Regular manual checks on pH will pick up system drift. Putting the pH meter in the second tank helps too, as most of the precipitation reactions have happened by then, and the probes will last longer. The actual solution to the problem depends on your plant flowsheet, but the above gives a couple of ideas to look at.

Once the lime addition is so high that it exceeds pH 11.5 - 12, the electrodes might as you say not reflect what is essentially a buffered system. Maybe an option might be old-fashioned titrations at regular intervals with oxalic acid to capture prevailing alkalinity rather than the log based pH approach? At least you will get a measure of correlation from that between the two sets of information (and how they might drift apart over time as the electrode clogs up).

This is known issue with pH sensors in the lime based process. There are a couple of issue: external coating of the glass and reference junction (could be prevented by using automatic retractable assemblies (Contact E+H sales folks and they will assist you to select the proper solution, ask for TOPCAL CPC310). The system is capable based on time to remove the pH sensor from the process, clean the probe and even automatically calibrate an entire system. The other issue is contamination of the reference junction. This issue could be resolved by using externally pressurized reference system. The junctions have to be Ceramic (these materials have a higher density in compare to Teflon for example and the process of the contamination will take significantly more extended period of time. The other alternative is to use a special pH sensors with a huge sensing area (CPS341D ) . This sensor is expansive and will also require the cleaning with 3 % HCL or from time to time.

You can try using a conductivity meter, but the pH-conductivity relationship is non-linear, and you would need to calibrate the system to correspond to the actual pH-conductivity relationship in your process stream. Sometimes reasonably accurate over a narrow pH band! Not recommended for fine pH control.

You can find this system in Mettler Toledo. It has an automatic wash probe system with acid solution. If you do not clean your pH probe every shift, it can be scale and not measured.