Analyzer Plays Role in Mine Wastewater Cyanide Treatment

Josue Nduwa & Anil Isaac

Introduction

Cyanide (CN) is commonly utilized in multiple industrial processes in the global mining, metals, chemical, electronics, steel, pharmaceutical and other industries. Five processes have been said to account for most of the CN wastewater streams, which include: refining gold and other metal ores, metal plating, case hardening of steel, neutralizing acid “pickle scum,” and scrubbing stack gases from blast and process industry gas furnaces.

Well-known for its toxic properties, CN is a carefully regulated substance by environmental, health and safety agencies around the globe to protect workers, surrounding communities and local water supplies. When used in the gold mining industry in the form of sodium cyanide (NaCN) or calcium cyanide (CaCN2), the CN compound of choice is mixed with water in a carefully controlled process. Prior to extraction, the ore is crushed and treated with the CN solution, which then becomes a flowing slurry during the gold extraction process. 

After extraction, the residual slurry ore wastewater solution is then transferred and stored for treatment by a mine operator to dissipate the high levels of CN. Due to its toxicity, the wastewater must be treated and evaluated to ensure that the CN has been reduced to a safe level before discharge as effluent into the water system to prevent water supply contamination that could endanger people or wildlife.

The human exposure limits for CN vary by the type and form of CN compound. According to the U.S. National Institute of Occupational, Safety & Health (NIOSH), the recommended exposure limit (REL) for NaCN is 5mg/m3 (4.7 ppm) over 10 minutes. Toxic exposure symptoms can include eye or skin irritation, asphyxia, lassitude, headache, vomiting, breathing difficulty and death.

The global mining industry takes CN process safety and environmental stewardship seriously. Industry CN process safety and environmental protocols are rigidly adhered to by mine management and employees. These protocols include required equipment operation and maintenance procedures. 

The Challenge

Global mining company OceanaGold’s Haile Mine operation, which is located in South Carolina, contacted the applications team at Electro-Chemical Devices (ECD) in Anaheim, Calif., to discuss its CN wastewater treatment process. The CN analyzers supporting the mine’s wastewater treatment process were requiring unusually high levels of maintenance, which also left open the possibility of inaccurate measurement, which is unacceptable.

Gold was discovered in 1827 by Benjamin Haile in South Carolina and his Haile Mine has been in production on and off for nearly 200 years. In the late 19th century, the Haile Mine was the most productive and profitable gold mine east of the Mississippi River. Today, the OceanaGold employees at the Haile Mine are proud of their company history and are deeply committed to the safety and protection of the environment.

According to the U.S. Environmental Protection Agency (EPA), the best available technologies for CN wastewater remediation treatment are oxidation by chlorine, ion exchange and reverse osmosis. The EPA has established a maximum contaminant level (MCL) of 0.2 mg/L and a maximum contaminant level goal (MCLG) of 0.2 mg/L of free cyanide prior to plant discharge of wastewater.

In addition, CN wastewater streams are mobile in soils and have low adsorption to soils with high pH, high carbonate and low clay content. Untreated or ineffectively treated CN wastewater streams can leach to groundwater. Under typical conditions in natural waters (pH less than 9.2), most free cyanide is expected to convert to hydrogen cyanide that is highly volatile.

The operations team at the Haile Mine relies on a three-step oxidation process to treat its CN wastewater. CN analyzers are placed at the starting point of the treatment process to measure CN levels entering the primary process, at the secondary treatment process to measure treatment effectiveness, and they are placed at the end-point of the process to ensure treatment has been completed to required effluent levels and meets regulatory requirements prior to discharge. 

Should the analyzer report CN levels above the legal limit, the water is returned to the system for additional treatment. Failure to meet the U.S. OSHA and U.S. EPA regulations for the safe use and treatment of CN can result in complex investigations, required actions and fines.