Importance of pH Measurement

Hydrometallurgical concentrating and refining applications require pH/ORP sensors to withstand aggressive environments while maintaining accurate measurement.

Processes such as cyanide leaching (e.g gold) and solvent extraction (e.g. copper, nickel, zinc) require pH and ORP to be at very specific values to yield the desired chemical species. pH may need to be controlled as precisely as ±0.05 for some processes. Even where this level of precision is needed, process environments are typically extremely aggressive to the sensor.

Critical Elements of the Sensor

Combination pH sensors having both a measuring and reference electrode are typical for metallurgical applications. Various elements combine to provide its functionality. The ORP sensor is a variation on the pH sensor in that replaces the pH sensing glass and stem with a metal band or tip upon which the redox reaction can take place.

pH Sensor cross section combination

Material Selection

IC Controls pH/ ORP sensors got their start in metallurgical applications and are built to withstand these harsh environments. Materials for all wetted components are carefully selected – including the cable – to ensure sensors will endure the service conditions they will face.
pH Sensor material components

High-performance engineering thermoplastics such as PVDF and PTFE (Teflon) are used extensively. Proprietary epoxy sealants are used to prevent process ingress.

Sensor Size Matters

IC Controls sensors are available in a full 1-inch internal diameter. This increases junction surface area to reduce plugging and ease maintenance. The 1-inch configuration also allows for a substantial increase in electrolyte volume which extends sensor life.

pH Sensor configuration & size

Keeping Contaminants Out

Ions from the process can migrate to the reference cell thereby “poisoning” it causing measurement drift. In addition, particulate such as that found in slurries can plug the reference and thereby prevent measurement. IC Controls has made it a focus to incorporate features that resist these phenomena:

  • Standard wood junction utilizing wood’s natural ability to act as a filter for undissolved minerals while still allowing liquid to flow. This has proven to be highly effective within its working temperature and pH range.
  • Solid polymer junction for use in highly aggressive chemicals that still allows an ionic process connection without the process migrating to the reference cell thereby resisting poisoning

pH Sensor borrow technology from nature

Taking the Heat

Not only do pH and ORP levels drive the yield of extractive processes, but so too does temperature. This means pH sensors may at times be subjected to temperatures above 100 °C (212 °F). To ensure measurement is possible under these conditions, electrolyte and junctions must work together to ensure a good ionic connection to the process. IC Controls provides the following options to meet the needs of high temperature processes:

  • Plasticized electrolyte – prevents loss of contact due to gas formation
  • Porous Hytec junction where wood or solid junctions are beyond their limits
  • High temperature glass pH stem – rated up to 130 °C (266 °F)
  • Preamps rated up to 105 °C (221 °F) (higher possible with special provisions)

Direct Connect versus Preamp

Best practice is to make the connection to either preamp or analyzer/meter within no more than 10 m (33 ft). There are cases though where longer runs are needed – IC Controls pH sensors utilize special cable having multiple shields to protect the extremely low current in the pH signal. This allows cable runs of up to 30 m (100 ft). Where electromagnetic interference (EMI) is prevalent, IC Controls preamps can be integrated into the sensor to improve EMI immunity.

pH Sensor wire connection

Other Options

For inline applications, sensor retraction can facilitate maintenance by allowing removal without having to shut down the process. This is accomplished with a valve and insertion sleeve to isolate the process from the user.

617 SS Ball Valve High Pressure Retractable pH Sensor

An anti-coating treatment is also available to reduce cleaning in applications where scaling or build-up is a problem.

Helpful Resources

pH Sensors mentioned in this blog:

  • Model 642 & 648 pH/ ORP Sensor – wood junction (642 only), Hytec junction, plasticized electrolyte, high temperature glass, preamp
  • Model 605, 606, 617, 643 pH sensors – ball valve retractable
  • Model 647 pH Sensor – solid polymer reference
  • For Hydrometallurgy
  • For all pH/ORP Sensors