While there are many different processes to remove gold from its encapsulating ore, cyanide leaching (cyanidation) is perhaps the most common and well known. A cyanide solution (NaCN or KCN) is elevated to a high pH level (>10.5pH) so that free cyanide (CN
-) will dissolve the gold in the ore.
pH Measurement Challenges
Gold processing operations are highly dependent on online pH measurements for process control as well as safety requirements. All processes involving cyanide must be kept >10.5pH (typically 11-12pH) to avoid the conversion to dangerous HCN gas. Lime (CaO) and caustic (NaOH) are most commonly used to keep the pH elevated. Lime is added to water to create “milk of lime”. In these applications the excess calcium compounds from the milk of lime are well known to coat pH sensors. Beyond chemical addition, fine ground ore and carbon particulates can also coat as well as plug up the porous reference junction used in most pH sensors.
For most submersible and sample line installations we recommend the Barben Performance Series 546 threaded sensor. The 546 uses Barben’s patented Axial Ion Path
® reference design. The reference half cell uses
multiple fi ltering chambers to greatly slow the ingress of process chemicals from poisoning the sensor internals. The large annular Tefl on junction provides a much greater surface area then conventional pH sensors. The increased area is effective in reducing the plugging due to particulate matter and simplifi es cleaning of sensor.