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Alkaline Chlorination

Environmental legislation in most jurisdictions requires that plant tailings are treated to detoxify cyanide before they are deposited into tailing facilities. To prove compliance, a metallurgical flowsheet showing the steps that will be taken to treat cyanide is required by most financial institutions. SGS is a world leader in all aspects of the cyanide cycle, including alkaline chlorination for cyanide destruction. We help you address this area of technical risk.

SGS regularly develops cyanide destruction flowsheets and conducts the needed batch or pilot scale testing. Data from such tests is used to set technical parameters, and then design operating facilities that will result in compliant cyanide concentrations in your tailings at the lowest possible cost.

Alkaline chlorination yields a number of important advantages:

  • Includes a strong oxidant 
  • Can oxidise thiocyanate (reaction 3)
  • Can oxidise cyanate to N2 and CO2  (reaction 4 in a second stage), avoiding the hydrolysis of cyanate to undesirable ammonia (reaction 6)
  • Requires no copper catalyst 
  • Offers fast kinetics
  • Can oxidise CN-WAD efficiently (hence CN-WAD is also called “CN amenable to chlorination”) leaving low toxic iron cyanide in solution

Limitations to alkaline chlorination include:

  • Process is non-selective, leading to high reagent consumption 
  • Requires high pH (pH 11) to ensure rapid and complete hydrolysis of highly toxic cyanogen chloride gas (reaction 8)
  • Ferrocyanide is not destroyed, but only partially oxidised to ferricyanide (reaction 5). Requires an additional stage and reagent to remove iron cyanide (if required) as a base metal iron cyanide precipitate (such as in reaction 9)
  • May leave residual chlorine in solution (requiring a polishing pond or aeration stage)

Alkaline chlorination is a relatively simple process in which cyanide leachate is treated in a nine-step procedure. During alkaline chlorination, cyanogen (CNCL) is formed and hydrolysed to the cyanide (CNO) at an alkaline pH:

Reagents
          Cl2  +  2NaOH   -->  NaOCl  +  NaCl  +  H2O              

Reactions
          NaOCl  +  CN-WAD -->  CNO-  +  NaCl
          4NaOCl  +  SCN-  +  2OH-  -->  CNO-  +  SO42-  +  4NaCl  +  H2O
          3NaOCl  +  2CNO-  +  2H+  -->  N2  +  2CO2  +  3NaCl  +H2O
          2Fe(CN)64-  +  NaOCl  +  2H+  -->  2Fe(CN)63-  +  NaCl  +  H2O
          CNO-  +  H+  +  H2O  -->  CO2  +  NH3
          NaOCl  +  CN-  +  2H+  -->  CNCl  +  Na+  +  H2O
          CNCl  +  2OH-  -->  CNO-  +  Cl-  +  H2O
          6FeSO4  +  4Fe(CN)63-  -->  2Fe3(Fe(CN)6)2  +  6SO42-

SGS has the technology and expertise to provide you with bankable and effective cyanide management at every stage of your gold recovery operation. Contact us early in the proposal and planning stage of your gold operation to learn if alkaline chlorination is the best solution for destroying the cyanide at your site.

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