When Cyanide Solutions Get Darker

In the lab, we recently ran particle size distribution tests on two different ore samples, followed by cyanide shake tests on each size fraction. As expected, sulfide content increased as the particle sizes got smaller. However, we observed something unusual—one sample’s cyanide solution darkened significantly with decreasing particle size, while the other remained relatively clear.

So, what’s causing the difference? While both samples had increasing sulfides, the first sample had a much higher concentration of organic material. This suggests that organic compounds are dissolving or forming metal-organic complexes, leading to the darker solution. Possible causes include:

•      Dissolved organic-metal complexes (e.g., iron-organic interactions)

•      Colloidal organic suspensions, which scatter light and alter solution appearance

•      Changes in cyanide consumption and metal speciation due to organic presence

Further analysis, such as ICP testing and TOC analysis, could help confirm the exact mechanism. In metallurgical processing, high organic content can impact cyanide leaching efficiency by consuming cyanide, interfering with metal recovery, or increasing solution turbidity.

This is why thorough sample characterization is critical in test work—understanding what’s in the ore helps us predict how it will behave in processing!