Cyanide Detoxification Treatment using INCO process

[krea0824]

One of the environmental management controls of our mining plant is a detoxification plant for free cyanide management. Process utilizes INCO process, using Sodium Metabisulphite and copper sulfate for the destruction of cyanide complexes to free cyanide. Operation is at 1:8 mixing ratio for CN:Sodium Metabisulphite. We employed two tanks with a retention time of 30 minutes in each tank.
1) Before we discharge, we conduct colorimetry tests using Free Cyanide Test Kit, as concentration is expected to be low. During this time, Free Cyanide concentration at the effluent of the Detox Plant is 0 ppm. The effluent is further conveyed through a 2-km pipeline to its final outfall to a surface water. When we monitored along the 2 km tailings pipeline, we noted increased concentrations of Free Cyanide in the effluent within the tailings pipeline, and much higher concentration when it reaches the surface water... Is the INCO process reversible? Why is it that the free CN concentration increases even when we read it as O ppm at the effluent of our DEtox Plant. We tried titration analysis as well and got similar results. Pipeline is isolated from other potential cyanide contamination.

 

[bimr]

The presence of iron-cyanide complexes is undesirable in the Inco process given their ability to decompose in the presence of sunlight, releasing free cyanide.

It is possible that have a cyanide complex that is difficult to break, like metal-cyanides, ferricyanides, etc. Ferricyanide, if present, is reduced to ferrocyanide and precipitated as an insoluble metal ferrocyanide, M2Fe (CN) 6 where M can be Cu, Ni, or Zn.

"Thus, the cyanide leach solutions would contain numerous metal-cyanide complexes. These complexes are generally much less soluble than are free cyanide (Environment Australia, 1998), and frequently will form solids that drop out of solution. Once these complexes are formed and then released into the near-surface environment, they begin to decompose at varying rates, some quickly, others quite slowly. The decomposition of metal-cyanide complexes releases cyanide into the soil or water. Those complexes that most readily decompose are referred to as weak complexes, those most resistant to decomposition are called strong complexes. The weak and moderately-strong complexes will decompose in weak to moderately-strong acids, releasing environmentally significant concentrations of free cyanide. Examples of each category are:

Weak Complexes--- Zn(CN)4-2, Cd(CN)3-1.

Moderately Strong Complexes--- Cu(CN)2-1, Ni(CN)4-2, Ag(CN)2-1.

Strong Complexes--- Fe(CN)6-4, Co(CN)6-4, Au(CN)2-1.

Some of the strong complexes do not break down in the presence of strong acids, but will decompose when exposed to various wavelengths of light, releasing free cyanide. This is especially true of the iron cyanides, which are often the most common forms in mining wastes. Flynn and Haslem (1995) states that iron, cobalt, and chromium-cyanide complexes normally are not decomposed by acids or bases, but can absorb visible or ultraviolet light and decompose into free cyanide. Clearly this statement is not absolutely true since there are numerous references to such complexes being dissolved in very strong acids."

http://www.mpi.org.au/campaigns/cyanide/cyanide_chemistry/

The Inco reaction will not reverse to cyanide once you have broken it down to cyanate. The reaction is also not reversible by the utilization of a reducing agent.