Overview
Industries such as metal plating, case hardening of steel, and refining of gold and silver ores utilize cyanide because it dissolves metals easily by forming complexes. While these soluble complexes are beneficial for the industrial processes, it causes significant issues for their wastewater treatment system. When cyanides are present the heavy metals will no longer precipitate with precipitation agents rendering standard treatment options useless. In addition to cyanides interference with precipitation it is highly toxic to humans and aquatic life and must be treated prior to discharge even if only trace amounts are present.
Alkaline chlorination is the preferred treatment method for cyanide bearing wastewater. The process is completed in two stages. In the first stage the pH is adjusted to 11 and sodium hypochlorite is added to oxidize the cyanide to cyanate:
NaClO + NaCN + H2O -> CNCl + 2NaOH
CNCl + 2NaOH -> NaCNO + NaCl + H2O
CNCl + 2NaOH -> NaCNO + NaCl + H2O
The addition of sodium hypochlorite is controlled using an ORP meter with a target between +250 mV and +400 mV.
In the second stage the pH is lowered to 8.5-9 and additional sodium hypochlorite is added to oxidize the cyanate to carbon dioxide and nitrogen:
2NaCNO + 3HOCl -> 2NaCl + H2O + 2CO2 + N2
Again the addition of sodium hypochlorite is controlled with an ORP meter with a target between +300 mV and +600 mV. As the pH is lowered during the second stage it is critical that it remain above 7. Acidic conditions promote the formation of hightly toxic hydrogen cyanide fumes.
A well controlled cyanide destruction system can consistently reduce concentrations to below 1 mg/L. Unfortunately not all metal cyanide complexes are amenable to chlorination and others react slowly so careful considerations should be made during the design of a wastewater treatment system. Salt Creek Technologies and our partners can help guide you while you make these factors through design, selection, operation and troubleshooting.
Following the cyanide destruction process the wastewater can be treated through normal heavy metal precipitation and clarification systems.