Reliable pH/ORP measurement in metallurgical applications
Hydrometallurgical refining applications require pH/ORP sensors to withstand harsh environments while maintaining accuracy. Processes such as solvent extraction (e.g. copper, nickel, zinc) and cyanide leaching (e.g. gold and silver) require pH and ORP to be at specific levels to yield the desired chemical species. With declining ore grades, extractive processes are becoming increasingly aggressive. Under these challenging physical and chemical conditions, pH may need to be controlled as precisely as ±0.05. Because each process is unique and tailored to specific ores, it is critical to scrutinize every application to ensure the instrumentation used in them is appropriately configured to maximize service life and accuracy. IC Controls has designed and manufactured sensors for these applications for more than 35 years – its flagship 642 pH/ORP sensor product, purpose-built for metallurgical applications, illustrates the evolution required to meet the demands of refining applications. Frequently this product achieves two to three times the life of other sensors.
Sensor size influences service life and durability of pH/ORP sensors to a great extent. Designed from the outset for full submersion in metallurgical applications, the 642 sensor has a large one-inch (25.4 millimetres) diameter reference junction to resist plugging. The large sensor diameter in combination with its length (12 inches/305 millimetres) makes for a large internal volume to accommodate electrolyte, as well as sensor options such as an integrated preamp. Since electrolyte depletes over time, electrolyte volume is critical to sensor life. The design flexibility and protection afforded by volume within the sensor has allowed integrated preamps to be developed for use up to 105 °C / 221°F. Incorporating the preamp in the sensor allows for preservation of the measurement signal, especially when transmitted over long distances. Constructed from heavy-gauge extrusion-grade engineering polymer, the large sensor housing acts to protect the sensor internals, as well as guard the exposed pH electrode glass tip.
While heavy-duty construction of the components that house the glass helps shield it from damage, the geometry and the composition of the glass itself also contribute to its mechanical properties. Multiple glass configurations are available in the 642 product and the specific selection is based on process chemistry, the presence of abrasives or debris, as well as desired accuracy. In recent years, the industry has undergone changes to eliminate lead for environmental reasons, which has resulted in new lead-free glass compositions. This move had posed new challenges in the processing of glass for pH sensors which had to be overcome through extensive testing and development. These formulations are now validated in 642 sensors to allow the flexibility to use lead-free glass without compromising service life or accuracy in aggressive environments.
As an electrochemical measurement approach, pH/ORP sensors require electrolyte to operate. For the widest operating temperature range, typically a plasticized, or gel, electrolyte is recommended for the 642 and has the added advantage of inhibiting the ingress of poisoning ions. In some cases though, such as a requirement for high accuracy, IC Controls has found that using a liquid electrolyte can be advantageous. The challenge is to manage the tendency for the liquid to convert to gas as temperature approaches the boiling point. Sensor performance can be pushed to these physical limits by careful attention to the mechanical integrity of the reference assembly.
Moreover, the 642 pH/ORP product family, like the metallurgical processes it is intended for, is tailored for a specific purpose. From its physical size to its material selection and available options, it is designed for long service life and reliable measurements in the most aggressive of processes.