Cyanides are chemical compounds that contain a cyano group (-CN), consisting of a carbon (C) and a nitrogen atom (N). They are widely used in various industrial processes, including metalworking, electroplating, mining and the chemical industry, where they are used, for example, to extract gold and silver or for the surface treatment of metals. Cyanides exist in various forms, including inorganic (e.g. sodium cyanide, potassium cyanide) and organic cyanides (e.g. hydrogen cyanide).

Toxicity and environmental relevance

Cyanide is highly toxic to humans and the environment. It inhibits cell respiration by binding to the iron of the enzyme cytochrome c oxidase and preventing oxygen uptake in the cells. Even small quantities can have a lethal effect, which is why cyanides are classified as highly hazardous to the environment. The handling of cyanide and the treatment of cyanide-containing wastewater is therefore strictly regulated.

Applications and sources of cyanide in wastewater

Industries that use cyanide must ensure that cyanide does not enter the environment untreated. Typical sources of cyanide-containing wastewater are

  • Electroplating and metal processing companies: Cyanide is used here for surface finishing, especially for gold plating or galvanizing.
  • Mining: In mining, especially in the extraction of gold and silver, cyanide is used to leach ores in order to dissolve the metals from the rock.
  • Organic syntheses: Cyanide is used as a building block in the chemical industry, e.g. in the production of plastics or pharmaceuticals.
Wastewater treatment plant and precipitation plant for waste disposal companies, metal processing industry and chemical industry.

Photo: CP system ALMA CHEM MCW for the removal of heavy metals, AOX, hydrocarbons, cyanide and chromium (using sodium bisulphite)

Process for cyanide removal from wastewater

Due to the high toxicity of cyanide, effective wastewater treatment is essential. There are various chemical-physical processes that are used to remove or convert cyanide into less toxic compounds:

  1. Alkaline chlorine oxidation (Cl₂/OH-):
    Alkaline chlorine oxidation is one of the most common methods for cyanide treatment. In this process, cyanide (CN-) is oxidized with chlorine or sodium hypochlorite (NaOCl) in an alkaline environment to form harmless compounds such as cyanate (CNO-). The reaction takes place in two stages:

    • CN- + Cl₂ + 2OH- → CNO- + 2Cl- + H₂O
    • Cyanate itself is less toxic and can be degraded to carbon dioxide and nitrogen in the further course of wastewater treatment.

  2. Electrolytic oxidation:
    In electrolytic oxidation, an electric current is passed through the cyanide-containing wastewater, which also oxidizes cyanide to cyanate. This process is particularly interesting for companies that already use electrochemical systems.

  3. UV oxidation:
    Cyanides can also be broken down by UV irradiation and the addition of ozone (O₃) or hydrogen peroxide (H₂O₂). The process is suitable for low cyanide concentrations and can be used effectively in combination with other treatment processes.

  4. Precipitation and adsorption:
    An alternative to oxidation is the precipitation of cyanide compounds. Here, iron(II) or copper salts are added to convert cyanide into insoluble metal cyanide complexes, which can then be filtered out. Adsorption on activated carbon can also be used to reduce the cyanide concentration.

  5. Precipitation and oxidation using hydrogen peroxide (H₂O₂):
    Another effective method for cyanide removal is precipitation and oxidation with hydrogen peroxide. In this process, hydrogen peroxide is added to the wastewater to oxidize cyanide and convert it into cyanate. The reaction proceeds as follows:

    • CN- + H₂O₂ → CNO- + H₂O
      Cyanate can then be further degraded to carbon dioxide and nitrogen. The addition of iron(II) ions as a catalyst(Fenton reaction) accelerates the oxidation. This process is particularly useful for the treatment of cyanide-containing wastewater in metal processing and electroplating.
Filter technology from ALMAWATECH as gravel and sand filters or activated carbon filters

Photo: ALMA FIL AK activated carbon filter for removing residual cyanides after a CP system

ALMAWATECH solutions for cyanide treatment

Our CP plants (chemical-physical plants) such as the ALMA CHEM MCW are specially designed to safely treat highly toxic wastewater, including cyanide-containing wastewater. These plants combine several processes for efficient cyanide removal. In addition to chlorine oxidation, precipitation, UV oxidation and adsorption, we offer customized solutions for precipitation and oxidation with hydrogen peroxide to reliably remove cyanide and other hazardous substances from wastewater.

Sustainability and safety

The treatment of wastewater containing cyanide is not only essential for environmental protection reasons, but is also required by law. Our systems make it possible to reliably comply with the legal limits for cyanide and to operate your wastewater treatment plants safely and efficiently.

Conclusion

The removal of cyanide from wastewater is a highly sensitive and important process in industrial water treatment. With a combination of chemical oxidation processes, precipitation techniques and adsorption, our systems offer a comprehensive solution for the safe and sustainable elimination of cyanides.