TThe most economical and easily controllable method to remove cyanide is by alkaline chlorination. Chlorine may be added either as a free gas or as bleach (sodium hypochlorite or calcium hypochlorite).
The initial reaction between chlorine and cyanide is the formation of cyanagen chloride.
NaCN + Cl2® CNCl + NaCl
The cyanagen chloride continues to react with sodium hydroxide forming sodium cyanate.
CNCl + 2NaOH ® NaCNO + 2H2O + NaCl
The oxidation of cyanide is usually conducted in what has been called a two stage process.
The second stage is the further oxidation of cyanate to carbon dioxide and nitrogen.
2 NaCNO + 4NaOH + 3Cl2r 2CO2 + 6NaCl + N2 + 2H2O
The initial oxidation to cyanagen chloride is slow below pH 8.0. Above 8.5 the reaction goes to completion in less than 30 minutes, even faster as the pH is raised. Cyanagen chloride is volatile and odorous. When airborne it is a lachrymator (tear producing substance similar to tear gas) and is very uncomfortable to anyone in the area. Since the cyanagen chloride becomes more soluble as the pH increases (less volatile) and the rate of conversion to cyanide dramatically increases with the pH, the “first stage” reaction is usually conducted at pH 11.
The “second stage” oxidation of cyanate to carbon dioxide and nitrogen is also pH dependent. At pH 8.5 the reaction is usually complete in 10 minutes and at pH 10 the reaction usually requires 40 minutes.
Even though the reaction may appear to be a two step process and the rate of reaction number two is slow under reaction number one conditions, cyanate is converted to carbon dioxide and nitrogen at the elevated pH values of reaction one. Stoichiometric requirements of chlorine show that 40% of the total demand would be consumed in step one and 60% in step two. In reality, it has been shown that 70% to 80% of the total chlorine demand will be consumed in step one. Therefore, a significant portion of the cyanate has been oxidized to carbon dioxide and nitrogen in step one. Also, experience shows that the actual amount of chlorine consumed by the reaction may be as much as 200% of the theoretical amount.
If insufficient chlorine is added to the first stage, free cyanide will exit the first stage and in the second stage with the reduced pH, cyanagen chloride will evolve from the solution.
To insure complete oxidation of cyanide, adequate mixing is extremely important. The tank should have a mixing rate of 2.5-3.0 tank turnovers per minute. Poor mixing may produce isolated pockets where the solution alkalinity has been decreased, evolving noxious cyanagen chloride gas.
The reaction is controlled with pH and ORP (oxidation reduction potential) controllers. The control is simple efficient and reliable.
Hoffland Environmental has designed manufactured and installed over 50 cyanide oxidation systems, both continuous flow and batch treatment units. The largest system treats 500 gpm containing over 4,000 pounds of waste cyanide. At peak flow, the system consumed over 30,000 pounds per day of gaseous chlorine and over 50,000 pounds per day of lime for neutralization. Kindly contact HEI for any of the following equipment:
- Chemical treatment modules
- High rate inclined plate clarifiers
- Gravity flow sand filters with automatic back flush
- High pressure filter press
- Turn-key systems