Cyanide in Effluent

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I have a customer that recently got their water test back and it showed they were close to their cyanide limit (I didn't get actual numbers). This operations welds mild steel, then runs it through an iron phosphate conversion coating and then a liquid dip paint. They have never had this happen before and are looking for a possible cause. Thanks to anyone who can provide some insight. 

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9 Answers

  1. Is it water used in the process or waste water generated from the process?

     If it is process water then you must check the source of water.

    If it is waste water then check the raw materials being processed. There may be change is raw material that adds cyanide in waste water.

  4. I agree with other comments about looking more to the source of this one-off contamination.  Make sure that contaminants are kept out of the welding process, as these vapors can be harmful to personnel.  Furthermore, chlorine with a specified contact time is very effective at eliminating cyanide from wastewater, but this requires super-chlorination above any other chlorine demand.  You may have to then remove the chlorine from the wastewater afterwards.  GAC is one of several methods for that, including sulfite addition.  It seems this would be too much trouble for a problem that has only appeared this one time, and with some minor investigation, should be prevented in the future from appearing.

  5. Cyanide is used extensively in the electroplating and mining industries due to its strong affinity to metal cations. Commercial granular activated carbon (GAC) was used as adsorbent for the adsorptive study of sodium, zinc and iron, cyanide complexes in the present study. The effect of process parameters such as pH, temperature, adsorbent size and dose, contact time on the performance of adsorption was investigated. Optimum pH was found to be 9, 7 and 5 for sodium, zinc and iron cyanides respectively. In the higher temperature range more percentage removal was observed for iron cyanides, whereas, sodium and zinc cyanides were removed optimally at 25–35 °C. Although particle size did not show any major influence on the percentage removal, but optimum size was taken as 2–4 mm. The percentage removal of cyanide compounds increased with the increase in adsorbent (GAC) concentration. However, specific uptake did not increase at GAC concentration above 20–25 g/L. Hence 20 g/L was considered as the optimum dose of adsorbent. Higher removal efficiency was achieved for metal cyanides as compared to sodium cyanide at optimal conditions.

  6. You can treat cyanide with Catalytic process of Hydrogen peroxide here copper is used as catalyst or Chlorination . YOu can also treat with Ferrous sulphate all processes are suitable . Acctivated carbon filter is further recomended for polishing.

  7. You will need to get into the details. There are several small organics that can give false positives in the test. An oil or other chemical they use may have changed formulation. An easy place to start is to have the lab run soluble cyanide to see if the source is solids. Also make sure the lab is using a distillation test method and is not taking a shortcut.

  8. Hello Stephen. It isn't common to find cyanide in wastewater if there are not changes in the process. I recommend you to check if some products are added upstream, mainly in the painting process.  Otherwise, you can check the method used to test the cyanide content, there are some interferences that could affect the result.

  9. The only source of cyanide I can think of in this instance is in the welding vapors when welding mild steel covered with a polyurethan-based paint: you then get diisocyanates in the vapors. How they would go into a water flow is the next question ("water curtain" surrounding the welding area ?)

    1 Comment

    1. Examine every change that has been made upstream, and any changes in suppliers or supplier’s formulations.  We have found chelated cyanides as a component in the metal cleaning solutions: they were being carried through to the waste water.

      We have also confirmed cyanide false positives tests that just did not make sense. The way we worked around it was to spike the sample with a known cyanide solution and then test again.  Do you get more cyanide than the known spike.  Just repeat testing of the same sample will lead to the same results.  

      “How would you distroy cyanides?”  This was a question that one of the EPA managers asked me because he didn’t know. When I indicated that I would start with hydrogen peroxide to break the Carbon Nitrogen bond he thought it was the same reaction as with chlorine - big difference.  Cyanide can also be destroyed by many other methods.  Remember to keep the solution alkaline so that it remains in solution. 

      It is the cyanide complexes that are more difficult to break than the simple salts.