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MNM Serious Accident Alert Surface - Copper Recovery Tank Explosion

Surface – Copper Ore Molybdenum Recovery Circuit – On November 3, 2016, an explosion occurred when incompatible chemicals were mixed in a tank. A miner unloaded a truckload of sodium hydrosulfide (NaHS) solution into a chemical storage tank that contained calcium polysulfide (CaPS) solution. NaHS and CaPS are hazardous chemicals that give off hydrogen sulfide (H2S), a flammable and toxic gas that produces a “rotten-egg” odor. The operator had installed a charcoal filter on the tank’s vent line to control the odors. When these chemicals came into contact in the tank, an explosive concentration of H2S gas was liberated. The concentration of H2S exceeded the filter’s capacity, resulting in a chemical reaction generating enough heat to ignite the filter. H2S vapors in the tank ignited causing a violent explosion damaging the tank, surrounding area, and building. This accident occurred because the operator changed its processes, which resulted in the mixing of incompatible chemicals.

MNM Serious Accident alert Surface – Copper Ore Molybdenum Recovery Circuit
Best Practices: 
• Prior to implementing a process change, such as the introduction of a new chemical, thoroughly evaluate hazards, including chemical compatibility and reactivity.
• Establish procedures for the safe handling and storage of chemicals, ensuring chemical compatibility with the process and equipment.
• Equip chemical storage tanks with properly-designed pressure relief devices that are safely vented.
• Ensure that chemical filters and scrubbers are properly designed and rated for the intended application.
• Conduct continuous H2S monitoring in areas where it may be liberated.
• Do not use pressurized air to off-load NaHS solution.
• Train miners on chemical hazards and emergency response procedures.
• Use proper personal protective equipment during chemical handling.
• For additional safe practices and precautions when handling NaHS, refer to the U.S. Chemical Safety and Hazard Investigation Board’s Safety Bulletin No. 2003-03-B (November 2004) http://www.csb.gov/assets/1/19/SodiumHydrosulfideBulletin.pdf