Contrary to what most people think when they hear the word “cyanide,” the term doesn’t just point to a single substance, but instead represents a number of compounds that share in one key detail: they all include a cyano group somewhere in their chemical structure. This cyano group, which consists of a carbon-nitrogen anion, imparts certain properties to cyanide compounds that make them useful for a variety of industrial processes—but also dangerous to humans.
In this article, we’ll define the term “cyanide,” discuss how and why cyanide is used in industry and explore the impact of cyanide on industrial facilities.
What is cyanide and where does it come from?
In basic chemical terms, cyanide (CN) is an anion consisting of a carbon atom and a nitrogen atom that share a triple bond. This cyano group can exist as a free anion (CN–), and it can also bond with other ions or molecules to form various cyanide “complexes” or “species.” There are many cyanide species, each of which has varying degrees of toxicity and reactivity, although all forms of cyanide are considered hazardous to humans. Cyanide species commonly used in industry include hydrogen cyanide (HCN), cyanogen chloride (CNC), sodium cyanide (NaCN), and potassium cyanide (KCN), among others.
Cyanides are produced both through natural and artificial processes. In nature, cyanides are produced by certain types of bacteria, fungi, and algae, and cyanide compounds can be found in over a thousand different plant species, including cassava root, apple seeds, and peach pits. Where manufacturing is concerned, the bulk of cyanide production is focused on HCN, NaCN, and KCN. Some facilities produce these cyanide compounds for their own consumption, while others produce them for commercial sale.
In industrial facilities, cyanides are useful for their ability to enhance the solubility of metal ions in solution, and for their ability to facilitate the bonding of metal ions with other ions or molecules. As such, cyanides are used most extensively in mining, ore refinement, metal plating, steel manufacturing, and pickling brine applications. Additionally, cyanide is used for the production of paper, textiles, plastics, industrial organic chemicals, pesticides, dyes, as well as certain pharmaceuticals and food additives.
Cyanide can enter soil and water through various sources, like vehicle exhaust, pesticide runoff, landfills, and most significantly, through industrial spills and/or improper discharge of industrial wastes. Waste streams resulting from metal mining processes, organic chemical production, iron, and steel manufacturing, and effluent from POTWs typically contain the highest amounts of cyanide comparative to other industries and are typically subject to regulatory restrictions.
How does cyanide affect industrial facilities?
The most significant effect of cyanide on industrial facilities stems from its toxicity to humans. All forms of cyanide are toxic even in relatively low concentrations. Chronic exposure to low levels of cyanide can cause a range of neurological, respiratory, cardiovascular, and thyroid conditions.
Acute exposure to toxic levels of cyanide via food, water, or air will cause severe health impacts such as shortness of breath, chest pain, convulsions, unconsciousness, and death. As a result, facilities that use, handle, or produce cyanide typically must take precautions to prevent cyanide contamination and protect workers and surrounding communities, lands, and waterways from exposure.
To mitigate the health and safety risks of cyanide, many nations and regions around the world have established regulations to protect both the public and the environment from exposure to cyanide. In the United States, the Occupational Safety and Health Administration (OSHA) sets maximum exposure limits on cyanide to protect workers at industrial facilities, while the Environmental Protection Agency (US EPA) sets limits on the amount of cyanide that a facility is allowed to discharge in its gas emissions and/or wastewater streams.
Similar regulatory limitations are imposed for most regions throughout the world, so it is imperative for industrial facilities that handle cyanide to check with their respective local agencies to understand all applicable requirements for handling, disposal, testing, monitoring, and reporting hazardous materials. Below, we’ll discuss these important compliance responsibilities in greater detail.
Compliance with wastewater discharge permit limits
Like any other industrial facility, plants that deal with cyanide must responsibly dispose of their wastes. In the United States, industrial facilities that discharge wastewater directly to lakes, rivers, or other public waterways are known as direct dischargers.
Direct dischargers must obtain a National Pollutant Discharge Elimination System (NPDES) permit and implement any necessary waste treatment technologies to reduce cyanide levels in compliance with the permit. NPDES permits specify the maximum allowable limits for cyanide content in wastewater that is discharged to public waterways, as well as any requirements for testing methods and frequency. Failure to comply with the terms of an NPDES permit can result in substantial fines and legal action.
The specific levels of cyanide allowed under an NPDES permit will vary from one facility to the next based on the industrial sector, type of facility, and production output volume. For facilities engaged in the manufacture of steel or iron, for example, total cyanide content in wastewater is scalable based on a facility’s production volume, and limits are written as kilograms (kg) of cyanide in an effluent stream per thousand kilograms (kg) of the end product.
Facilities within this sector typically must keep average cyanide content in wastewater well below .02 kg total cyanide for each kg of iron or steel produced. For most other industries, such as metal finishers or pesticide manufacturers, cyanide limits are defined differently—typically as a ratio of milligrams (mg) of total cyanide per liter (L) of wastewater. Such facilities usually must keep the average total cyanide content below limits that range between 0.22 and 0.65 mg/L.
It bears mentioning that testing for cyanide can be complicated depending upon which cyanide species are present in a waste stream. Thiocyanate, and cyanide complexes with gold, cobalt, or platinum, are all difficult to recover through strong acid distillation and UV radiation, both of which are commonly used as part of the testing process for total cyanide. As a result, test results may indicate lower-than-actual readings of total cyanide. Facilities that deal with these cyanide species would be well-advised to evaluate testing methods to ensure that they provide an accurate reading of total cyanide content.
Compliance with wastewater treatment facility requirements
Industrial facilities that choose to discharge their wastewater to a public sewer or municipal treatment facility are typically required to manage levels of cyanide in their effluent streams. This is because publicly-owned treatment works (POTWs) may not be properly equipped to treat wastewater with high levels of cyanide and other hazardous materials. In order to assure the safe and reliable operation of POTWs, industrial facilities are usually required to meet certain water quality standards as a condition of their continued authorization to discharge wastewater to the receiving facility.
In the US, centralized wastewater treatment (CWT) facilities have the power to this administer their own wastewater discharge permits and enforcement programs under the US EPA’s National Pretreatment Program. According to US EPA regulations, centralized wastewater treatment facilities are required to keep total cyanide levels in treated effluent below 178 ppm on average, and below 500 ppm on any given day.
Because POTWs are themselves required to meet these stringent limits on cyanide, they often require that industrial facilities pre-treat their wastewater to ensure that cyanide levels are low enough that they do not disrupt the wastewater treatment technology in place at the POTW. Industrial facilities that fail to abide by receiving facility standards risk cancellation of wastewater discharge agreements, and can be liable to legal actions.
Protecting onsite wastewater treatment technologies
Cyanide can interfere with certain types of biological wastewater treatment technologies, causing problems with performance and reliability. As a result, industrial facilities that have an onsite biological treatment system must take steps to monitor and manage cyanide content in wastewater streams in order to avoid system upset.
The specific amount of cyanide that a biological system is able to tolerate depends upon the type of technology used, stream characteristics, process conditions, and other factors. Activated sludge systems, for example, can suffer performance issues from as little as 0.1 mg/L cyanide content, while trickling filters have been shown to tolerate as much as 30 mg/L cyanide. In general, industrial facilities need to operate their biological treatment systems in accordance with recommended specifications, which can mean adopting additional systems to monitor and manage cyanide levels.
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